2002_handbook of polymers in electronics

Handbook of Polymers in Electronics

Bansi D. Malhotra

Handbook of Polymersin Electronics

Rapra Technology Limited

Shawbury, Shrewsbury, Shropshire, SY4 4NR, United KingdomTelephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118

http://www.rapra.net

Bansi D. Malhotra

First Published in 2002 by

Rapra Technology LimitedShawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2002, Rapra Technology Limited

All rights reserved. Except as permitted under current legislation no partof this publication may be photocopied, reproduced or distributed in anyform or by any means or stored in a database or retrieval system, without

the prior permission from the copyright holder.

A catalogue record for this book is available from the British Library.

Typeset by Rapra Technology LimitedPrinted and bound by Polestar Scientifica, Exeter, UK

ISBN: 1-85957-286-3

i

Contents

1 Charge Transport in Conjugated Polymers...................................................... 3

1.1 Introduction ........................................................................................... 3

1.2 The Electronic Ground State .................................................................. 4

1.3 Charge Transport Carriers ..................................................................... 6

1.3.1 Soliton ....................................................................................... 7

1.3.2 Polaron and Bipolaron ............................................................... 9

1.4 Transport Properties of Polymers ......................................................... 12

1.5 Factors Influencing the Transport Properties of Polymers .................... 14

1.5.1 Disorder ................................................................................... 14

1.5.2 Doping ..................................................................................... 15

1.5.3 Interchain Coupling ................................................................. 17

1.6 Models of Charge Transport in Conducting Polymers ......................... 18

1.7 Conclusions ......................................................................................... 27

Acknowledgements ....................................................................................... 28

References ..................................................................................................... 28

2 Electrical Properties of Doped Conjugated Polymers .................................... 37

2.1 Introduction ......................................................................................... 37

2.2 Metallic State ....................................................................................... 39

2.2.1 Conductivity ............................................................................ 40

2.2.2 Magnetoconductance ............................................................... 50

2.2.3 Thermoelectric Power .............................................................. 55

2.2.4 Magnetic Susceptibility and Specific Heat................................ 56

2.3 Critical and Insulating States ............................................................... 58

Polymers in Electronics

ii

2.4 Summary.............................................................................................. 63

References ..................................................................................................... 65

3 Non Linear Optical Properties of Polymers for Electronics ........................... 69

3.1 Introduction ......................................................................................... 69

3.2 NLO Polymer Issues for Device Applications ...................................... 70

3.3 Properties of Third-Order NLO Polymers ........................................... 71

3.3.1 Background of Third-Order NLO Polymer Research .............. 71

3.3.2 Poly(arylenevinylene), PAV ...................................................... 72

3.3.3 n-BCMU-PDA ......................................................................... 75

3.3.4 PT ............................................................................................ 76

3.3.5 Processible π-Conjugated Polymers .......................................... 76

3.3.6 Third-Order NLO Polymer Waveguides .................................. 82

3.4. Properties of Second-Order NLO Polymers ......................................... 84

3.4.1 Azo-Dye-Functionalised, Poled Polymers for Second-OrderNon Linear Optics ................................................................... 84

3.4.2 EO Polymers ............................................................................ 87

3.4.3 Serially-Grafted Polymer Waveguides ...................................... 88

3.4.4 Refractive Index Grating Fabrication into Azo-Dye- ...................Functionalised Polymer Waveguides ........................................ 90

3.5 Future Targets of NLO Polymers for Optical Device Applications ...... 93

3.6 Conclusions ......................................................................................... 94

Acknowledgements ....................................................................................... 94

References ..................................................................................................... 94

4 Luminescence Studies of Polymers ................................................................ 99

4.1 Introduction ......................................................................................... 99

4.2 Basic Photophysical Deactivation Processes ....................................... 100

4.2.1 Luminescence ......................................................................... 101

4.2.2 Bimolecular Photophysical Processes ..................................... 103

iii

Contents

4.2.3 Quenching Processes .............................................................. 104

4.3 Methods for Fluorescence Studies ...................................................... 105

4.3.1 Time-Correlated Single-Photon Counting Studies .................. 105

4.3.2 Quantum Yields ..................................................................... 105

4.4 Fluorescence of Polymers, Excimer Fluorescence ............................... 106

4.4.1 Fluorescence of Polymers in Solution ..................................... 110

4.4.2 Fluorescence of Polymers in Gel State .................................... 122

4.5 Conclusions ....................................................................................... 136

Acknowledgements ..................................................................................... 136

References ................................................................................................... 136

5 Polymers for Light Emitting Diodes ............................................................ 141

5.1 Introduction ....................................................................................... 141

5.2 The Physics of Electroluminescent Devices ........................................ 142

5.2.1 The Physics of Conjugated Polymers ..................................... 142

5.2.2 The Physics of the Device....................................................... 144

5.2.3 LED Characterisation ............................................................ 147

5.3 Polymeric Structures for LED ............................................................ 148

5.3.1 Polyphenylenes ...................................................................... 148

5.3.2 Polythiophenes ....................................................................... 164

5.4 Recent Developments ......................................................................... 168

5.4.1 Polarised Electroluminescence ............................................... 168

5.4.2 Lifetime and Degradation in LEDs ........................................ 170

5.4.3 Microcavities ......................................................................... 170

5.5 Concluding Remarks ......................................................................... 171

References ................................................................................................... 172

6 Photopolymers and Photoresists for Electronics .......................................... 185

6.1 Introduction ....................................................................................... 185


iv

6.2 Microlithography Process .................................................................. 187

6.2.1 Resist Coating ........................................................................ 188

6.2.2 Exposure ................................................................................ 188

6.2.3 Development .......................................................................... 189

6.2.4 Post Baking ............................................................................ 190

6.2.5 Etching .................................................................................. 190

6.2.6 Resist Removal (Stripping) .................................................... 190

6.2.7 Doping ................................................................................... 190

6.3 Resist Requirements ........................................................................... 190

6.3.1 Solubility ............................................................................... 191

6.3.2 Adhesion ................................................................................ 191

6.3.3 Etching Resistance ................................................................. 191

6.3.4 Sensitivity and Contrast ......................................................... 191

6.4 Resist Materials ................................................................................. 193

6.4.1 Conventional Photoresists ..................................................... 193

6.4.2 Deep-UV Photoresists ............................................................ 197

6.4.3 Electron-Beam Resists ............................................................ 202

6.4.4 X-Ray Resists ........................................................................ 206

6.4.5 Special Resists ........................................................................ 208

6.5 Conclusions ....................................................................................... 212

References ................................................................................................... 212

7 Polymer Batteries for Electronics ................................................................. 217

7.1 Introduction ....................................................................................... 217

7.2 Ionically Conducting Polymers .......................................................... 218

7.2.1 Lithium Polymer Electrolytes and Lithium Batteries .............. 218

7.2.2 Proton Polymer Electrolytes ................................................... 239

7.3 Electronically Conducting Polymers .................................................. 242

7.3.1 Lithium-Doped Conducting Polymer and Lithium-Polymer Batteries ................................................................... 243

v

Contents


References ................................................................................................... 245

8 Polymer Microactuators .............................................................................. 255

8.1 Introduction ....................................................................................... 255

8.2 Sample Preparation and Measurements of Electrolytic Deformation . 257

8.3 Electrochemistry and Expansion Behaviour in Polyaniline Film......... 259

8.4 Dependencies of the Expansion Ratio on the Degree of Oxidationand Dopant Ions ................................................................................ 260

8.5 pH Dependence of Electrolytic Expansion ......................................... 262

8.6 Time Response of the Electrolytic Expansion..................................... 265

8.7 Anisotropy of Electrolytic Expansion in Polyaniline Films ................. 266

8.8 Contraction Under Strain in Stretched Polyaniline Films ................... 267

8.9 Electrolytic Expansion in Other Conducting Polymers ...................... 267

8.10 Applications of Electrolytic Expansion .............................................. 268

8.11 Conclusions ....................................................................................... 269

References ................................................................................................... 269

9 Membranes for Electronics ......................................................................... 271

9.1 Introduction ....................................................................................... 271

9.2 Plasma Polymerisation ....................................................................... 276

9.2.1 History .................................................................................. 277

9.2.2 General Characteristics .......................................................... 277

9.2.3 Synthesis of Plasma Polymers ................................................ 278

9.3 Characterisation of Plasma Polymers ................................................. 282

9.3.1 IR Spectroscopy ..................................................................... 283

9.3.2 XPS ........................................................................................ 283

9.4 Applications of Plasma Polymers ....................................................... 283


vi

9.4.1 Packaging .............................................................................. 284

9.4.2 Insulator ................................................................................ 284

9.4.3 Semiconductive Films............................................................. 285

9.4.4 Conductive Films ................................................................... 286

9.4.5 Resist Films ............................................................................ 286

9.4.6 Ultrathin Polymer Films ......................................................... 286

9.4.7 Chemical Sensors ................................................................... 287

9.4.8 Biosensors .............................................................................. 287


References ................................................................................................... 291

10 Conducting Polymer-Based Biosensors ........................................................ 297

10.1 Introduction ....................................................................................... 297

10.1.1 Biosensors .............................................................................. 298

10.1.2 Construction of Biosensors .................................................... 299

10.1.3 Transducers ........................................................................... 301

10.1.4 Biological Component ........................................................... 301

10.1.5 Importance of Conducting Polymers to Biosensors ................ 302

10.2 Preparation of Electrodes ................................................................... 304

10.2.1 Synthesis of Conducting Polymers ......................................... 304

10.2.2 Conduction Mechanism in Conducting Polymers .................. 305

10.3 Immobilisation of Biomolecules/Enzymes .......................................... 305

10.3.1 Methods of Immobilisation.................................................... 305

10.3.2 Advantages of Immobilisation ............................................... 309

10.4 Characterisation of Enzyme Electrodes .............................................. 309

10.4.1 Determination of Enzyme Activity ......................................... 309

10.4.2 Effect of pH ........................................................................... 310

10.4.3 Effect of Temperature ............................................................ 311

10.4.4 Effect of Storage Time ........................................................... 312

10.4.5 Response Measurements ........................................................ 313

vii

Contents

10.5 Types of Biosensors ............................................................................ 313

10.5.1 Optical Biosensors ................................................................. 31410.5.2 Electrochemical Biosensors .................................................... 315

10.6 Biosensors for Healthcare .................................................................. 318

10.6.1 Glucose Biosensor .................................................................. 318

10.6.2 Urea Biosensor ....................................................................... 320

10.6.3 Lactate Biosensor ................................................................... 321

10.6.4 Cholesterol Biosensor ............................................................ 323

10.6.5 DNA Biosensor ...................................................................... 324

10.7 Immunosensor ................................................................................... 326

10.8 Biosensors for Environmental Monitoring ......................................... 326

10.9 Conclusions ....................................................................................... 326


References ................................................................................................... 327

11 Nanoparticle-Dispersed Semiconducting Polymers for Electronics .............. 341

11.1 Introduction ....................................................................................... 341

11.2 Material Preparation Methods ........................................................... 344

11.3 Photophysics of Charge Separation Nanoparticle-Polymer Systems ... 346

11.3.1 TiO2-Conjugated Polymer Composites .................................. 34811.3.2 Nanoparticle Semiconductors-Polymer Systems ..................... 35311.3.3 Gold-Polythiophene Blends .................................................... 357

11.4 Summary............................................................................................ 360


References ................................................................................................... 361

12 Polymers for Electronics .............................................................................. 367

12.1 Introduction ....................................................................................... 367

12.2 Polymer Electroluminescence ............................................................. 368

12.3 Conduction in Polymers ..................................................................... 375


viii

12.4 Molecular Electronics ........................................................................ 379

12.5 Polymer Deposition Technologies ...................................................... 379

12.6 Summary............................................................................................ 388


References ................................................................................................... 388

13 Conducting Polymers in Molecular Electronics ........................................... 393

13.1 Introduction ....................................................................................... 393

13.2 Synthesis of Conducting Polymers ..................................................... 397

13.3 Preparation of Ultrathin Conducting Polymer Films .......................... 399

13.3.1 Langmuir-Blodgett Films........................................................ 39913.3.2 Self-Assembly Monolayers ..................................................... 404

13.4 Characterisation of Conducting Polymers .......................................... 404

13.5 Molecular Devices Based on Conducting Polymers ............................ 406

13.5.1 Diodes ................................................................................... 40613.5.2 Field-Effect Transistor............................................................ 40913.5.3 Biosensors .............................................................................. 41113.5.4 Electronic Tongue .................................................................. 41413.5.5 Electronic Nose ...................................................................... 41513.5.6 Nanowires ............................................................................. 41813.5.7 Electroluminescent Displays .................................................. 41913.5.8 Microactuators ...................................................................... 423

13.6 Conclusions ....................................................................................... 424


References ................................................................................................... 425

Abbreviations and Acronyms............................................................................. 441

Contributors ...................................................................................................... 449

Index ................................................................................................................. 453

1

Preface

There is a global effort towards the applications of polymers in electronics. The demandfor new polymeric materials that can replace the widely used semi-conductor silicon inmicroelectronics has recently intensified. This has essentially been due to the continuingdrive towards higher circuit density of the micro-electronic components and the much-needed very high speed processing of the data being continuously generated in variousresearch, manufacturing and commercial establishments located worldwide. It isanticipated that polymers may perhaps offer viable solutions to the problems presentlybeing confronted by the modern electronics industry.

Among the various polymeric materials, conjugated polymers have been projected tohave innumerable applications in electronics and are thus presently at the centre-stage ofresearch and development. Conducting polymers have been found to have applicationsin a wide range of emerging areas such as light-emitting diodes, photonics, micro-actuators,light-weight batteries, biosensors and molecular electronics. However, it may be notedthat development of polymers for electronics is still an open field wherein polymers areused not only as insulators but can also be tailored for the desired electronic propertiesfor specific applications. It was thus thought that a Handbook dedicated entirely to thepreparation, characterisation and potential applications of polymers coupled with thefundamentals of the electrical, optical and photo-physical properties will go a long wayin bridging a long-felt industrial need and motivate the dedicated and younger researchersto venture into new experiments.

‘Handbook of Polymers in Electronics’ has been designed to discuss novel ways polymerscan be used in the rapidly growing electronics industry. Recent developments inmicroelectronics have prompted enhanced interest towards the search for new molecularmaterials that can be utilised for increased density of packaging. Vibha Saxena and cowriters(Chapter 1) discuss the phenomenon of charge transport in electrically conducting polymers,considered to be a direct consequence of conjugation, i.e., chemical un-saturation of thecarbon atoms in the polymer chain. It is indicated that an improved understanding of themechanism of charge transport in these materials is likely to unravel new hidden phenomenahaving implications in polymer electronics. Reghu Menon (Chapter 2) discusses the role ofeasily polarisable delocalised p-electrons in determining the electrical properties of conductingpolymers. Toshikuni Kaino (Chapter 3) focuses on the transmission and processing of digitalinformation using conjugated non-linear optical devices based on polymers. Barbara Wandelt(Chapter 4) in her extensive coverage of the luminescence properties of polymers reveals howfluorescence probes can provide an insight into the nature of intermolecular interactions inthese systems. Alberto Bolognesi and cowriters (Chapter 5) reveal that polymers offer a

2


unique possibility of working with cheaper technology giving flexible films that can be usedto emit light. Jean-Claude Dubois (Chapter 6) discusses the technological developments ofphotopolymers and photo-resists presently being used in microelectronics industry. BrunoScrosati (Chapter 7) has provided an interesting insight into the potential of polymericelectrodes for lightweight batteries for applications in electronics. Keiichi Kaneto and cowriters,in their outstanding appraisal (Chapter 8), have shown that the changes in molecularconformations arising due to the localisation of p-electrons and electronic repulsion betweenthe polycations influence the operation of a conducting polymer micro-actuator. Isao Karubeand cowriters (Chapter 9) have given an excellent review of the preparation, characterisationof the plasma-polymerised membranes for application in electronics. Conducting polymershave been predicted to play decisive role towards the fabrication of third generation biosensors.Keeping this in view, Asha Chaubey and cowriters (Chapter 10) have shown that redoxpolymers can be advantageously used to combine both the role of protein immobilisationmatrices and the physical transducer resulting in improved response characteristics andminiaturisation. K.S.Narayan (Chapter 11) reveals a synergistic approach towards the nano-particle dispersed particles semi-conducting polymers for application in miniaturised electronicdevices. Tim Richardson (Chapter 12) deals with the various options available to a deviceengineer associated with the technological development of polymer based electronic devices.Chapter 13 contains a comprehensive review on molecular electronic applications ofconducting polymers.

‘Polymers in Electronics’ is the result of the invaluable contributions of many celebratedresearchers who have been active in their respective fields for many decades. I am gratefulto all of them for their active participation in this important project. Special thanks aredue to Ms Frances Powers of Rapra for her timely suggestion that this project should beundertaken. Ms Claire Griffiths, Dr Arshad Makhdum and Dr Sarah Ward of the editorialstaff at Rapra have worked extremely hard to check that everything in the Handbook iscorrect and that the project is completed in time. Mr Steve Barnfield is thanked for thetypesetting and the excellent cover design of the Handbook. I would also like to extendmy thanks to Geoffrey Jones of Information Index who so skilfully produced the index.

I am thankful to all the members of my research group (Biomolecular Electronics &Conducting Polymers) of the National Physical Laboratory (NPL), New Delhi for themany discussions and suggestions during the operation of the project. The Handbookwould not have been possible without the invaluable advice received on many occasionsfrom a number of eminent scientists including Professor S.K. Joshi, Dr R.A. Mashelkar,FRS, Dr W. Hayes, Professor A.P.F. Turner, Professor E.S.R. Gopal, Professor S.Slomkowskii, Dr A.K. Raychaudhri, Dr Krishan Lal and Dr Howard H. Weetall. I amthankful to all my colleagues especially Dr K.K. Saini, Dr. S.S. Bawa and Dr SubhasChandra of NPL, New Delhi for many discussions held during the implementation ofthe project. Finally, it would have been difficult to complete the project without theemotional support I received from Shashi (wife), Aditi (daughter) and Rajat (son).

Bansi D.Malhotra

3

1 Charge Transport in Conjugated Polymers

V. Saxena and B.D. Malhotra

1.1 Introduction

Since the early 1950s, polymers have been used extensively as passive components inelectronic devices because of their light weight, flexibility, corrosion resistance, highchemical inertness, electrical insulation and ease of processing. In 1975, an inorganicconjugated polymer, polythiazyl, (SN)x, was discovered, which possesses metallicconductivity and becomes a superconductor at 0.29 K [1]. However, the idea of usingpolymers for their electrical conducting properties actually emerged in 1977 withthe findings of Shirakawa and co-workers [2], that the iodine-doped trans-polyacetylene, (CH)x, exhibits conductivity of 103 S cm-1. Since then, an active interestin synthesising other organic polymers possessing this property has been initiated.As a result, other polymers having a π-electron conjugated structure, such aspolyaniline (PANI), polypyrrole (PPy), polythiophene (PT), polyfuran (PFu), poly(p-phenylene) (PPP) and polycarbazole (PCz) [3-6] have been synthesised and studied.Some important conducting polymers and their energy gaps are shown in Table 1.1.Since the beginning of the last decade, these polymers (hereafter called conducting orconjugated polymers) have been extensively investigated for an understanding oftheir physical and chemical properties.

sremylopgnitcudnoctnatropmI1.1elbaT

remyloP )Ve(egdenoitprosbalacitpO

snarT enelytecaylop- 4.1

siC enelytecaylop- 0.2

elorrypyloP 5.2

enehpoihtyloP 0.2

(yloP p )enelynehp- 0.3

(yloP p )enelynivenelynehp- 4.2

enilinayloP 6.1

4


The charge transfer process is one of the most intriguing properties of conducting polymersbecause the electrical conductivities of this class of polymers vary over many orders ofmagnitude due to chemical or electrochemical doping. It is understood that a wide varietyof phenomena are involved in charge transport in this group of materials. A major sourceof this phenomenon originates from the quasi-one-dimensional (q-1D) nature of thematerials. A polyconjugated chain can be considered as a q-1D metal, having one chargecarrier per carbon atom. It is a well-established fact that such a half-filled system givesrise to Peierls instability by opening up an insulating gap at the Fermi level. This leads toa band structure responsible for the important electronic properties in polymers andthereby results in the existence of a non linear excitation called a soliton. This excitationand other excitations, such as polarons and bipolarons found in non degenerate ground-state systems, are produced due to the chain relaxation or deformation that results fromadding/removing an electron from the polymeric chain. Under the influence of an appliedelectric field these non linear defects become mobile, resulting in an increased electricalconductivity. Each of these particles possesses its own characteristic transport properties.A clear understanding of the intrinsic excitations in doped and undoped conjugatedpolymers is still lacking [7-8]. A considerable amount of work has been carried out byseveral researchers focusing on this fundamental problem. The charge transfer propertiesas a function of temperature, pressure, magnetic fields, etc., for various polymeric sampleshave also been reported in the literature. The collective contributions from variousparameters, such as electron-phonon interaction, electron-electron interaction, quantumlattice fluctuations, interchain interactions, etc., make it difficult to estimate thecontribution from individual parameters quantitatively. Moreover, the contributions fromdisorder and doping, etc., make it rather difficult to envisage a microscopic mechanismfor charge transport in doped conducting polymers. Therefore, the theoretical modellingof transport properties in conducting polymers is still a challenging problem due to theextreme complexity of the system. However, recent developments in reducing the extentof disorder have explained many phenomena regarding charge transport in dopedconducting polymers. In this chapter, an overview of the past few years, a study of chargetransport in conducting polymers is presented.

1.2 The Electronic Ground State

It is well known that the accessible energy levels of an electron in a crystal are groupedinto bands, which may be visualised as originating from the electronic levels of the atom.The bands form by the splitting of the atomic levels when the atoms approach one anotherand obtain their equilibrium positions in the crystal. The bands are separated by forbiddenenergy ranges called the energy gap. In a semiconductor, this gap separates the bandwhich is completely filled (valence band) from the lowest energy band which is completelyempty at absolute zero (conduction band) and accounts for the conduction processes in

5

Charge Transport in Conjugated Polymers

this class of materials. In metals, the conduction band is partially filled, implying that afinite density of states exists at the Fermi level.

Conjugated polymers differ from crystalline semiconductors and metals in several aspectsand are often treated theoretically as a one-dimensional system. The formation of theband gap is explained taking into account either electron-phonon interactions or electron-electron interactions among π-electrons. If electron-phonon interaction dominates inreal π-conjugated polymers, these systems could be treated using Peierls theory. In contrast,when electron-electron interactions dominate, the Hubbard model could be used to explainthe physical properties of polymers.

The Peierls model explains why a chain of unsaturated carbon atoms with one conductionelectron per atom does not exhibit metallic properties. If all the atoms are spaced atequal distance, a, the basic cell in reciprocal space is the Brillouin zone in the interval–π/a<k<π/a (k is the wave vector). With one electron per atom, the band would be half-filled and hence the chain would exhibit metallic behaviour. A periodic distortion of thechains, commensurate with the original structure, generates an n-fold super-structureand reduces the Brillouin zone to –π/na<k<π/na, with n being the number of atoms in thenew unit cell. The effect of the distortion is to open a gap at the boundaries k = ±π/na ofthe new Brilliouin zone (Figure 1.1). Therefore, if only states below the new gap are

Figure 1.1 One-dimensional electronic system with a half-filled band; band structure(a) before and (b) after Peierls distortion, where E is energy and EF is the Fermi energy

6


occupied with electrons, a reduction of the energy will occur and the distorted state willbe more favourable, implying that the semiconducting state is more stable than the metallicstate. The size of the single-particle gap (δ) is proportional to the amplitude of the latticedistortion (u). However, in a three-dimensional array of one-dimensional chains, thequantum lattice fluctuations and the interchain coupling tend to reduce the Peierls gap.The Peierls model clearly indicates that a one-dimensional chain of unsaturated carbonatoms leads invariably to a semiconducting state [9]. However, Frohlich showed that thePeierls state is semiconducting only if the periodic lattice distortion is commensuratewith the lattice [10]. If it is incommensurate, the phase of the periodic lattice distortioncan move through the lattice carrying a charge density wave. In such a case, the Peierlsstate is conducting. The increased Frohlich conductivity in the Peierls state is rathersensitive to various extrinsic features such as disorder, chain interaction, pinning-depinningprocesses, etc. A considerable amount of work has been reported in this area [11-13].

The Peierls model completely neglects the coulomb repulsion for an electron that istransferred to a state already occupied. In the simple Hubbard model, electroncorrelation is taken into account, but electron-phonon interaction is assumed to benegligible [14]. This model yields a gap in the absence of a Peierls distortion. For anexactly half-filled band this model gives insulating behaviour. However, for the casedeviating from the half-filled band condition, the conductivity behaviour is observed.This model provides an excellent intuitive framework and analytic benchmark forunderstanding the role of electron-electron interactions in a conducting polymer.However, to obtain a realistic description of these systems one needs to consider amodel incorporating both electron-phonon and electron-electron interactions. ThePeierls-Hubbard model incorporates both the coulomb interaction among π-electronsand their coupling to lattice degrees of freedom [15]. These models have been appliedto simple polymers and found to agree with the experimental findings [16-19]. However,none of these models was found to satisfactorily explain all the physical properties ofconducting polymers. Some other important models are the Su-Schrieffer-Heeger model[20, 21], the Pariser-Parr-Pople model [22], and the Mott-Hubbard model, which havebeen used extensively to explain the features of these materials.

1.3 Charge Transport Carriers

The nature of the charge carriers in conducting polymers is not very well understood. Itis believed that intrinsic excitations, such as solitons, polarons and bipolarons, do playthe role of charge carriers [23, 24]. Considerable theoretical and experimental work hasbeen carried out in this area [25-27]. The properties of these excitations are discussed indetail in the following sections.

7


1.3.1 Soliton

The probability of finding this excitation for conduction holds true for conjugated polymerswith degenerate ground state structures such as trans-polyacetylene (PAc) (Figure 1.2a). Ina degenerate ground state, the system has isoenergetic regions. For a neutral chain, thesoliton can be thought of as a 2pz unhybridised orbital of a sp2 hybridised carbon atom,which is a non bonding orbital occupied by a single electron. Neutral, negative and positivesolitons in a trans-PAc chain are shown in Figure 1.3. This excitation, called a soliton, hasa 1/2 spin with zero charge, which can move along the chain without a distortion [25]. Thenon bonding orbital corresponds to a soliton level at the middle of the forbidden gap. Itmay contain zero, one or two electrons. The addition or removal of one electron to theneutral state corresponds to a negative or positive soliton with zero spin (Figure 1.4).

Further oxidation of the polymer creates a dication. However, because of the two-folddegeneracy of PAc, these cations are not bound to each other by any lattice distortionand can freely separate along the chain. In the case of PAc, it is reported that solitons aredelocalised over about 12 (CH) units [26]. Therefore, solitons are isolated, non interactingcharged defects that form domain walls separating two phases of opposite orientationwith identical energies.

Figure 1.2 Ground state energy (E) as a function of the configuration co-ordinate (Δ)for a system with a (a) degenerate and (b) non degenerate state

8


Several experiments have been carried out to confirm the physical properties of solitonsin trans-polyacetylene [27]. Lately, this excitation has also been studied in anotherdegenerate ground state conjugate polymer, poly(1,6-heptadiene) [28]. The one-dimensional spin diffusion and associated spin dynamics are verified from electronmagnetic resonance spectroscopy, nuclear magnetic resonance (NMR) spectroscopyand electron nuclear double resonance (ENDOR) measurements [13]. The density ofneutral solitons has been estimated by Motsovoy and co-workers [29]. For more detailson the physical properties of solitons, the reader is referred to a review article byHeeger and co-workers [13]. However, more theoretical and experimental work is

(a)

(b)

(c)

Figure 1.3 Formation of solitons in trans-polyacetylene: (a) neutral, (b) positiveand (c) negative soliton

So

(a)

S-

(b)

S+

(c)

Figure 1.4 Energy levels and localised level occupations of the (a) neutral, (b) negativeand (c) positive solitons

9


required in order to understand the influence of chain interaction, coulomb interaction,disorder, etc., on the physical properties of this excitation.

1.3.2 Polaron and Bipolaron

In the case of a non degenerate ground state, the energies of the two chains on either sideof the defect are different (Figure 1.2b). Therefore, a single bond alternation defect insuch a chain cannot behave as a charged soliton. A charge injected on a chain isaccompanied by a distortion in the chain and forms a polaron [30]. The removal of anelectron from a polymeric system creates a free radical and a positive charge. The radicalcation is then coupled by a local bond rearrangement and a quinoid-like bond sequenceis formed. However, because of the higher lattice energy of quinoid compared to benzenoid,these distortions are limited. In the case of PPy, the lattice distortion is believed to extendover about four pyrrole rings. This combination of charged site coupled with a freeradical via a local lattice distortion is called a polaron (Figure 1.5).

A polaron may be a radical cation (oxidation) or a radical anion (reduction). Like a freecarrier, a polaron has a spin of 1/2 and a charge of ±e. Polaron formation creates newlocalised electronic states in the band gap (Figure 1.6). Theoretical studies indicate thatthe polaron states of PPy are symmetrically located about 0.5 eV from the band edges[31]. The lower energy states are occupied by a single unpaired electron.

Figure 1.5 Formation of a polaron and a bipolaron in polypyrrole

10


In many cases, it has been found that the conductivity of the system is spinless, whichsuggests that charge carriers other than polarons would be appropriate in these cases [32,33]. Therefore, it was proposed that polaron interaction would produce a new chargecarrier with no spin and 2e charge corresponding to a positive bipolaron (Figure 1.5).

When a polymeric chain having a polaron is subjected to further oxidation, an electronis removed from either the polaron or the rest of the chain. In the former case, a polaronradical is removed and two new positive charges result, which are coupled through latticedistortion. In the latter case, two polarons are formed. However, the formation of abipolaron causes a further decrease in ionisation compared to two polarons, indicatingthat bipolaron formation is thermodynamically more favourable. It has been reportedthrough quantum chemical calculations that bipolaron energies are lower than those ofpolarons by 0.4 eV [13].

Bipolaron states are located symmetrically within the band gap, about 0.75 eV awayfrom the band edges in case of PPy (Figure 1.7). Continuous doping of the polymercreates additional localised bipolaron states, which overlap to form continuous bipolaronbands. During doping, the polymer band gap also increases, but bipolaron bands tend tomerge with the conduction band (CB) and valence band (VB), resulting in metal-likeconductivity. The experimental evidence of formation of polarons and bipolarons hasbeen given by Bredas and co-workers by examination of the optical spectra of manydoped conjugated polymers [32]. The spectral and spin signatures of polaron pairs in π-conjugated polymers have been reported by Lane and co-workers using photoinducedabsorption and optically detected magnetic resonance (ODMR) [34]. Later, explicit

Figure 1.6 Energy levels and localised states for the (a) positive and(b) negative polarons

Q = + e

S = 1/2

(a)

Q = - e

S = 1/2

(b)

11


evidence for bipolaron formation in conducting polymers has been provided by Ramseyand co-workers using ultraviolet photoelectron spectroscopy and electron-energy-lossspectroscopy [35]. A number of experimental and theoretical studies have dealt with thephysical properties of polarons and bipolarons [36-40]. The effect of an electric field onpolarons and bipolarons in a single polymer chain has been investigated by Magela [41].It is found that dynamic effects do not reverse the stability relation between polaronsand bipolarons. Conversely, as the electric field intensity gets stronger, the moving polaronstructure is destroyed faster than the structure of moving bipolarons. The stability of abipolaron as a function of the strength of the long-range coulomb interaction with andwithout impurities has been studied in detail [42]. It is found that in a free state a bipolaronis stable only when the coulomb interaction is small (weak coupling limit) and it becomesunstable for strong coulomb interactions. Being bound to a dopant, the bipolaron becomesstable in a wide range of coulomb interactions. Mizes and co-workers reported a studyon the stability of polaron in trans-polyacetylene and poly(phenylene vinylene) [43].They pointed out that many defects and the short conjugation length in these materialswould tend to stabilise the polaron [43]. Three-dimensional band structure calculationsof these polymers indicate the destabilisation of polaron. Nevertheless, the existence ofchain endings and other conjugation breaks are found to stabilise the polaron. However,later, Vogl and Campbell concluded (from local density functional calculations forpolyacetylene) that the effect of interchain interaction is sufficient to destabilise a polaron,thus making the electron into a conduction band electron of the kind usually found inconventional semiconductors [44]. In contrast, Emin and Nagai suggested that defectand disorder would tend to localise the polaron. The effect of conjugation is apparently

Q = 2 e

S = 0

(a)

Q = - 2 e

S = 0

(b)

Figure 1.7 Energy levels and occupied localised states for the (a) positive and(b) negative bipolaron

12


sufficient to prevent such localisation [45]. However, the roles of disorder, interchaininteraction, dopants, etc., in pinning and stabilising these excitations are not preciselyknown. A detailed study of polarons and bipolarons is given by Conwell and Mizes [46].The reports of these studies have implications in the emerging field of molecular electronics(Chapter 13).

1.4 Transport Properties of Polymers

The strong variation of the electrical conductivity of conjugated polymers upon doping,as observed in the metal-insulator (M-I) transition, was first observed in the case ofpolyacetylene [47]. Since then, a great amount of work has been devoted to elucidatingthe conduction mechanism involved. The charge transport in these materials has beenexplained by dividing these materials into three regimes according to their reducedactivation energy, defined as:

W(T) = -T[dlnρ(T)/dT] = d(lnσ)/d(lnT) (1.1)

where σ is the conductivity, T is the temperature and ρ is the resistivity.

(i) when W(T) is greater than 0 at low temperature, the system is near the metallic sideof the M-I transition.

(ii) when W(T) is independent of temperature for a wide range of temperatures, thesystem is on the critical regime of the M-I transition (i.e., where the M-I transition isfeasible).

(iii)when W(T) is greater than 0 at low temperatures, the system is near the insulatingside of the M-I transition.

Another important parameter characterising the M-I transition is ρr, the resistivity ratio,ρr~ρ/(1.4 K)/ρ/(300 K). Being amorphous materials, the M-I transition in conductingpolymers is determined by the extent of disorder. In early publications, the data analysiswas not focused on the precise identification of the metallic, critical and insulating regimes.This hindered the clear understanding of the charge transport phenomenon in conductingpolymers. Recent reports on the improved quality of the polymeric samples havesubstantially reduced the dominant role of the disorder. Therefore, the current emphasisis on the metallic properties of these systems. The transport properties of doped conductingpolymers are given in Chapter 2 of this book. We herein give a brief account of themetallic conductivity observed in these polymers.

13


Metallic polymers are defined as polymers having a finite conductivity at temperaturesapproaching absolute zero and a room temperature conductivity in the range of that ofconventional metals, such as copper [48]. Apart from this, temperature independentPauli susceptibility down to 10 K, linear temperature dependence of the thermoelectricpower down to 10 K, linear term in the specific heat at low temperatures, and largemetallic reflectance in the infrared are other features observed in metallic polymers [49].These features show the presence of a continuous density of states with a well-definedFermi energy. Although a positive temperature coefficient of resistance (TCR) is desirablefor a good metal, it is not an essential criterion for metallic behaviour. The extensivestudies conducted on disordered metals in the past few years have shown that the absenceof a significant positive TCR does not necessarily imply the system being non metallic.Another alternative method to determine the metallic state of polymers is the reducedactivation energy as defined above. The conductivity in the disordered metals at lowtemperature is given by [50, 51]:

σ(T) = σ(0) + mT1/2 + BT ρ/2 (1.2)

where B is a constant depending on the localisation effects, and m is a constant. σ(0) isthe conductivity at absolute temperature.

Here the second term in the equation arises from the electron-electron interaction andthird term is the correction to σ(0) due to localisation effects.

The value of ρ is determined by the temperature dependence of the scattering rate of thedominant dephasing mechanism.

In the clean limit: ρ = 3 for electron-phonon scatteringρ = 2 for inelastic electron-electron scattering

In the dirty limit: ρ = 3/2

Among the various metallic conducting polymers studied so far, oriented trans-polyacetylene has been studied the most [48, 52-56]. The maximum room temperatureconductivity parallel to the chain axis obtained by Naarman and subsequently by othersis approximately 105 S cm-1, being roughly one-fifth of that of copper [57-60]. Themaximum value of intrinsic conductivity and stretchability of iodine-doped trans-polyacetylene is very much dependent on the film thickness; the thinner the film, thehigher the conductivity in both stretched and unstretched films [61]. The main differencebetween polyacetylene synthesised by Shirakawa and Naarman is the higher density anddegree of chain orientation in the latter case, the spin dynamics being the same for bothkind of polymers.

14


The transport properties of heavily doped Naarman polyacetylene have been reviewed byPaasch [62]. The sign of temperature dependence indicates that the conductivity is limitedby material imperfections such as tunneling/hopping regions and disordered metal regions.Heavily doped and highly disordered polyacetylene is almost metallic (the Peierls distortionis suppressed) resulting in a mean free path of about 100 Å, consistent with the observedcoherence length. Recently, Park and co-workers have found a metallic positive TCR atlow temperature in ClO4

--doped polyacetylene [63]. The temperature dependence of highlyoriented iodine-doped polyacetylene has been compared with that of ClO4

-- and FeCl3--doped polyacetylene and it has been found that a positive TCR in conducting polymers ishighly influenced by the extent of disorder present in the sample [64]. Thummes and co-workers have emphasised that the conductivity of doped polyacetylene at low temperaturesfollows a T1/2 law indicating that electron-electron interactions play a dominant role atvery low temperatures [65 ,66]. Similar T1/2 dependence was found in the case of Naarmanpolyacetylene [67]. For an intermediate temperature range (4-40 K), inelastic electron-phonon scattering is found to be the dominant scattering mechanism. These facts suggestthat both interaction and localisation play dominant roles in determining conductivity atlow temperature in metallic polyacetylene.

1.5 Factors Influencing the Transport Properties of Polymers

It may be remarked that the actual charge transport process in a conducting polymer isdependent on several parameters such as disorder (e.g., presence of vacancies, clusters,inhomogeneities), interchain coupling, the degree of doping, and the distribution andnature of dopant ions, etc.

1.5.1 Disorder

It is known that disorder is an inherent feature of polymeric systems. The different sourcesof disorder in a polymer include inhomogeneous doping that arises due to the nature ofthe catalyst used and also the processing routes. Besides this, the process of preparationmodifies the morphology and may lead to partial crystallinity, sp3 defects, chaintermination, crosslinks, cis-segments within trans-chains and impurities, etc. Chemicaldefects such as non conjugated carbon atoms inserted in the chain or impurities mayresult in localised and strong potential, i.e., strong disorder. On the other hand, staticfluctuations in the conformation of chain along its length are spread over some distance,leading to weak disorder. Both types of disorder may limit the conjugation length andhence the transport properties of polymers. Disorder effects are known to produce tailsin the bands of regular systems and this usually reduces the energy gap between valenceand conduction bands [68].

15


Several studies on the effect of the disorder have been based on either the Huckel or Su-Schrieffer-Heeger model with perturbation added in the form of either a screened coulombpotential representing a charge impurity or a completely localised potential acting on asingle site [69]. It has been argued in literature that disorder can either act as a barrier tothe quasi-particle movement, or trap it. The rotation of rings or bonds owing togeometrical fluctuation is another disorder and has been shown to reduce the conjugationlength [70]. The effect of disorder in interchain hopping has been investigated by Wolfand Fesser [71]. They showed that dimerisation decreases and the density of states in thegap increases as a function of increasing disorder in interchain coupling. The perfectlydimerised Peierls ground state breaks down towards a ‘metal-like’ state at a critical randomdistribution of interchain coupling. The random interchain coupling changes the propertiesof the band gap to a pseudo gap, with a small but non zero density of states. Harigayaand co-workers have reviewed the doping induced disorder in conducting polymers.Both theoretical and experimental studies suggest that it is difficult to reduce the Peierlsgap to zero without taking account of the effects of disorder [72]. Lately, the effect ofspatial disorder and anisotropy on the mobility of charge carriers has been reportedusing a dynamical Monte Carlo simulation. It is found that small disorder decreases themobility of low external fields whereas a considerable increase in mobility is found whenspatial disorder reaches value about 5%. Such studies are essential to optimise theperformance of devices using these materials. [73]. A model was proposed by Levy andco-workers for understanding the experimental features observed due to disorder nearthe M-I transition in a q-1D conducting polymer. The polymer is modelled as a compositemedium consisting of spherical regions of ordered polymers, randomly distributed in amuch more disordered polymer host. Within each spherical region, the polymer chainsare highly oriented, but the axis of orientation varies randomly from sphere to sphere[74]. It can be concluded that, in general, disorder is detrimental to transport phenomenain solids. The higher the disorder, the lower will be the dimensionality of the system.

Although a considerable amount of work has been devoted to investigating the role ofdisorder in transport properties, systematic investigation of the physical properties bycontrolling and varying different types of disorder has not been accomplished yet.Therefore, more extensive studies are required in order to understand the effect of differenttypes of disorder on structural, electronic and transport properties of conducting polymers.The main problem in understanding the conduction mechanism pertains to the correlationof disorder with that of the charge carrier transport.

1.5.2 Doping

During the doping process, the charge is injected or removed from the polymer chainand dopant ions sit in the polymer matrix in order to maintain charge neutrality. Ingeneral, the doping process is inhomogeneous and the distribution of dopant ions in the

16


polymer matrix is not uniform. This is mainly due to the complex morphology of thepolymer matrix, which consists of both crystalline and amorphous regions [75, 76]. Thedopants easily diffuse into the vacancies in the amorphous region till saturation level isreached, after which they slowly migrate into the crystalline regions. According to theKivelson and Heeger model [77], after adding or removing electrons to a chain, thedopant ions have a negligible effect on the electronic properties of the system. This hasbeen attributed to high conductivity within the metallic regions and their weak dependenceon the dopant species. Taking this fact into account, doped polymers can be describedwithin an idealised π-electron model with variable electron density.

However, neglecting the effect of impurity potentials arising out of dopant ions is adrastic assumption because random weak potential can sometimes produce bound statesin one dimension. A theoretical study has revealed that disorder effectively quenches thePeierls distortion [78]. The influence of dopant ions on interchain coupling may eitherweaken it by placing chains farther away or vice versa. Cohen and Glick found that inthe neighbourhood of a dopant ion the intrachain hopping is enhanced by 10% and theinterchain hopping is enhanced by 100% [79]. Thus, the dopant may act as a bridge forthe interchain transport. The interchain hopping strength at sites in the presence andabsence of dopant ions are estimated to be nearly 0.17 eV and 0.10 eV, respectively. Theelectron spin resonance (ESR) measurements in conducting polymers studied by Bernierand co-workers have demonstrated that the dopants play a major role in the chargetransport process [80]. Salkola and Kivelson suggest that the counterions affect the energygap [81]. It is suggested that a minor change in the arrangement of counterions relativeto the polymer chain is capable of influencing the energy gap of the polymer lattice. If theposition of the soliton is taken as centre, then it is observed that if counterions arelocated near to odd and even sites, then the gap is enhanced. Conversely, if the counterionsare located near to even or odd sites then the gap is reduced.

Yamashiro and co-workers have estimated the dopant-chain interaction and its role ininterchain transfer [82]. They showed that the dopants mediate the largest interchaintransfer of about 0.3-0.1 eV with five to seven carbon atoms in another chain that is incontact with a common dopant column. The interchain transfer via dopants has littleeffect on interchain states but yields a modification of the orbital energy spectrum.

It has also been reported that the electronic gap between a soliton and the conduction bandis decreased due to coulomb potential of the dopant ions and vanishes at a sufficiently highdopant concentration. A random distribution of dopants can have a strong effect ondisordering. The aggregation of dopants into an ordered structure suggests that finite metallicsegments exist which increase in length upon doping. As a result, the density of states increasessmoothly as a function of dopant concentration. Increase in dopant concentration beyond acritical level leads to the formation of a superlattice in many doped polymers [83].

17

1.5.3 Interchain Coupling

In contrast to the extensive efforts devoted to the studies of a one-dimensional model ofπ-conjugated polymers, no attention has been paid to account for the effect of the threedimensions of the real materials, being isotropic in nature. The interchain couplingshave considerable influence on major sources affecting the solid state of polymers andthereby lead to substantial differences between the exact form of conducting polymersand their oligomers. For example, in an infinite idealised one-dimensional polymermaterial, bond alteration can exist in the ground state, but it does not persist at finitetemperatures. Moreover, the domain walls (kink/solitons) connecting the two degeneratebond alteration regions will always be generated by thermal fluctuations at finitetemperatures and even a small density of these kinks will destroy the long-range order inone dimension. Nevertheless, in real materials, even a weak interchain coupling is sufficientto sustain the long ranges order up to a certain temperature.

Three-dimensional coupling is characterised by the transfer integral, t, which is the ratioof electron-phonon coupling to the electronic intersite coupling. Generally, only nearestneighbour interactions are taken into account, so only one or two transfer integrals arerelevant. Another parameter, tanisotropy, can be expressed as the ratio of t⊥ to t//, t⊥ and t//

being the interchain and on-chain transfer integral, respectively. This helps in determiningthe magnitude of the band structure anisotropy. It has been found that the three-dimensional effect dominates if t⊥/t// > 10-2 [84]. Three-dimensional band calculationsperformed on trans-polyacetylene and polyphenylvinylene yield similar results if t⊥/t// isat least 3 x 10-2 [85].

In an anisotropic Landau-Ginzberg model, it has been argued that interchain couplingcan be detrimental to polarons and bipolarons [86]. The result is confirmed by theoreticalcalculations with the Su-Schrieffer-Heeger (SSH) model. It is found that a polaron isdestablised and charge is spread over the whole crystal if the energy gained by the latticerelaxation of a single chain is of the order of 4t. Since the polaron binding energy issmall, a small t is sufficient to destabilise the polaron. Bipolarons are stable with respectto transverse delocalisation as long as coulombic effects are negligible. Ab initiocalculations in crystalline polyacetylene also confirm the possibility of instability ofpolarons and bipolarons [87].

The role of interchain interaction in the case of oriented trans-polyacetylene has beeninvestigated extensively by Leising and co-workers [88, 89]. Their study has shown thatfor a highly conducting polymer there is no gap around the Fermi energy consistent withthe metallic properties of polymers. The effect of interchain coupling on excitation hasbeen studied by Shi-Jie using a tight-binding model [90]. A new spinless charged polaronicexcitation (q=±e, s=0) was obtained due to the transverse interchain coupling, which isdifferent from the general magnetic polaron (q=±e, s=1/2) obtained in the perfect 1-D


18


model. A polaron is more easily stimulated energetically in a longer chain than in ashorter one. Additionally, interchain coupling decreases the energy of creation of a polaron.The effect of interchain coupling on the electronic structure of both doped and undopedtrans-polyacetylene has been studied by Conwell and co-workers [91]. They suggestedthat the interchain coupling is energy dependent, decreasing at a constant rate from amaximum value at the bottom of the valence band. They suggested that the interchaininteraction by itself is not enough for transport into the metallic state in doped conductingpolymer samples. Although, in principle, the interchain coupling is sufficient to shrinkthe energy gap between the soliton band and the conduction band to zero, in reality thecoulomb interaction with dopant ions plays a significant role in giving rise to the metallicdensity of states. The detailed theoretical modelling is carried out only for trans-polyacetylene and it should be extended to other polymeric systems in order to delineatea comprehensive picture about the effect of interchain interaction in both doped andundoped conjugated polymers [92, 93].

Recently, Prigodin and Efetov have developed a theoretical model for interchain interactionat the M-I transition in a random network of coupled metallic chains [94]. In this model,the interchain disorder due to intrinsic defects and the randomness in the distribution ofinterchain contacts induce localisation. The M-I transition in such a system is determinedby the critical concentration of interchain crosslinks, which in turn depends on thelocalisation lengths and interchain coupling. Moreover, a metallic state can exist in sucha random network of coupled metallic chains only if the concentration of interfibrilcontacts is large enough to overcome the percolation threshold.

As mentioned above, the interchain coupling plays a significant role in chargedelocalisation, screening and coulomb interaction and thereby determines the natureand stability of intrinsic excitations. The transport properties of conducting polymersare highly dependent on the way the chains are organised and arranged with respect toeach other. In spite of innumerable theoretical and experimental work, a quantitativeestimation of the effect of interchain coupling on transport properties of various polymericsystems has not yet emerged.

1.6 Models of Charge Transport in Conducting Polymers

As discussed in previous sections, the mechanisms for the formation of metallic state andcharge conduction in conducting polymers have been the subject of intensive study sincethe occurrence of an insulator to metal transition was reported upon doping. It is proposedthat non linear defects such as polarons, solitons and bipolarons have a major role inthese systems [36, 95-97]. In earlier synthesised conducting polymers, inhomogeneitiesoften dominated the transport properties, and metallic island models were proposed to

19

explain such features. Most of the transport measurements in such conducting polymerswere in the insulating regime. However, recent advances in synthesising ordered andhighly conducting polymers have signalled the onset of a new generation of polymers[57]. Improved homogeneity and a reduced degree of disorder in new kinds of polymershave provided a new opportunity for investigating metallic features through transportand optical measurements [49]. More recently, experimental results obtained in polyanilineand polyaniline derivatives were interpreted by invoking a q-1D variable range hoppingmodel of Nakhmedov, Prigodin and Samukhin [98, 99]. In spite of the rapid progress inobserving high quality samples of conducting polymers, the subtle details of the mechanismof charge transport are not yet understood. Firstly, the role of solitons, polarons andbipolarons in the charge transport is not very clear. The Kivelson model pertains to aphonon assisted hopping between soliton states in the case of lightly doped polyacetylene[100, 101]. However, the interpretation of experimental data in the framework ofalternative models cannot be ruled out [102, 103]. Secondly, the difference betweenmicroscopic and macroscopic transport properties has to be understood clearly. Thirdly,the connection between the measured conductivity and structural parameters of the samplehas to be sorted out. The present section deals with the various models reported so far inaddressing these problems.

In Kivelson’s model [100, 101], it is suggested that the low-energy charge excitationsintroduced into polyacetylene by light doping are charged solitons because of the largebinding energy. At reasonable temperatures the population of solitons is extremely smalland consequently thermally activated conduction due to free, charged soliton movement isdifficult. At low temperatures, electron hopping between solitons is a less strongly activatedconduction mechanism and it is a dominant process at low temperatures (Figure 1.8a). Forexample, the activation energy for phonon-assisted hopping or transition of an electronfrom the charged soliton to the neutral soliton is small if the neutral soliton happens to benear another impurity. The hopping conductivity is determined by the rate at which anelectron hops between a pair of solitons. Because of the disorder (namely the randomdistribution of impurities), the conduction pathways are essentially three-dimensional, withinterchain hops. Kivelson’s model predicts a steep power law dependence for the conductivity,σ=T9, which is close to that observed experimentally. The conductivity versus temperaturedata can be explained reasonably well in terms of the variable-range hopping (VRH) model,especially in the case of iodine-doped polyacetylene [104]. The temperature and frequencydependence of conductivity is found to be in agreement with VRH among soliton-likestates in the lightly doped regime [105, 106].

Similar to Kivelson’s model, Chance and co-workers proposed interchain hopping forspinless conductivity in doped polyacetylene, doped poly(p-phenylene) and other dopedpolymers [102]. The mechanism accounts for the observed dopant concentrationdependence of the conductivity in trans-polyacetylene and the observation of anomalously


20


low magnetic susceptibilities in the highly conducting regimes of several doped polymers(Figure 1.8b). However, this model does not explain the transition from spinless carriersto carriers with spins upon doping. Pietronero suggested that the main contribution toresistivity in conjugated polymers is due to scattering between the conduction electronand the phonon of the conjugated polymer chains [107]. The observed high conductivityof heavily doped polyacetylene has been explained using the one-dimensional model.The only possible scattering for polymers is from kF (the fermi vector) to –kF or viceversa involving large momentum (2kF) phonons of high energy (hν~0.2 eV for

Figure 1.8 Interchain transport of solitons and bipolarons in (a) polyacetylene and(b) poly(p-phenylene)

21

polyacetylene-type systems). With first-order scattering one obtains a strong conductivityincrease even at room temperature, because of phonon freezing effects. High-orderscattering processes with low energy phonons may then become less important. In thecase of elastic scattering (kBT >> hν), where kB is the Boltzmann constant, the electricalconductivity is estimated as 1.6 x 105 S cm-1 which is nearly an order of magnitude lessthan the value estimated for graphite intercalation compounds. However, this estimationis not realistic since, in polymers, only 2kF phonons scatter, i.e., hν0 >> kBT. At roomtemperature this analysis gives high intrinsic conductivity, σ = 1.4 x 107 S cm-1.

Kivelson and Heeger carried out a detailed study on the intrinsic conductivity of conductingpolymers [77]. They described the expression for the conductivity as:

2 02 2

02

2 2

π να

νn e a t

h

hk TB

exp⎛⎝⎜

⎞⎠⎟

(1.3)

where α (~4.1 eV/Å) is the electron-phonon coupling constant, ν0 is the phonon frequency,t0 is the electron hopping matrix element [107], h is Planck’s constant, n is the conductionelectron density and a is the C-C distance in the chain direction. A large t0, and a smallnumber of phonons gives the conductivity at room temperature as 107 S cm-1. This valueincreases exponentially at low temperatures. Since the charged ions are spatially removedfrom the q-1D conduction path, the usual scattering of phonons is reduced. This is becausephonons of wave vector 2kF are required to backscatter electrons. Since these phononsare thermally excited only at higher temperatures, the resistivity is very small at lowtemperatures, with a rapid rise when the thermal energy kBT approaches the energy ofthe 2kF phonons. At 0.12 eV, the rapid increase in resistivity can account for the changeto metallic temperature dependence, which is a prominent feature of the conductivity inconducting polymers. As discussed earlier, the interchain couplings are necessary to prevent1D localisation. Even a small interchain coupling, t⊥ ~0.1 eV, may give rise to thesuppression of the Peierls distortion. The condition necessary for the system to remainthree-dimensional is

L/a >> 2t0/t⊥ (1.4)

where L characterises the distance between the chain interruptions or sp3 defects or crosslinks.

When the concentration of chain interruptions is sufficiently high such that the left handside of equation 1.3 is small, then the wave function will be localised. The possible limitsfor the conductivity arise from the chain interruptions and/or phonon scattering. All theabove factors suggest that in high-quality conducting polymers the electronic mean freepath could be much larger than the structural coherence length and real metallic featurescould be observed.


22


Similar charge transport models have been proposed by Prigodin and Firsov [108, 109].According to their model, an abrupt transmission from extended to localised states isexpected at the critical interchain exchange integral t⊥c~(3h/2πτ), τ being the scatteringtime. Therefore, the delocalisation in a q-1D metallic chain appears only if t⊥ is largerthan the threshold value, t⊥~0.3/τ. Later, Nakhmdeov and co-workers carried out a studyof hopping transport in q-1D systems near the M-I transition with weak disorder [98].They set the cut-off temperature regimes in which band transport crosses over to hoppingtransport. In a high temperature regime, the band transport is governed by phononscattering and disorder while at low temperatures, the hopping has been ascribed toMott’s VRH. In the intermediate temperature region, the temperature dependence ofconductivity shows a power law behaviour. However, these temperature regions aredifferent in a q-1D system when there is a weak intrachain coupling. In these cases, thetemperature dependence of conductivity gradually varies from 1D behaviour at hightemperatures to that of an isotropic 3D behaviour at low temperatures. In the intermediaterange, the conductivity follows exp(-T0/2T)exp[-(T0/2T)]1/2 dependence. Joo and co-workers extended this model, suggesting that the effect of finite temperature emergesthrough phonon scattering, which is expected to be highly anisotropic [110-112]. Therole of phonon forward scattering is to break the phase coherence of the impurity scatteringand thereby destroy the weak localisation. Moreover, the conductivity versus temperaturecurve exhibits a characteristic maximum at temperatures where the phonon backwardscattering time becomes comparable with the impurity scattering time.

Epstein and co-workers have widely used q-1D models to interpret transport propertiesin both metallic and insulating polymer systems [113, 115]. Conducting polymers wereconsidered as an inhomogeneous system, which consists of partially crystalline andamorphous regions. The overlap of π-orbitals gives rise to crystalline regions whereasamorphous regions emerge from weak chain interactions. When the size and volumefraction of the crystalline region increases with respect to the amorphous region, thesystem is expected to undergo a transition from insulator to metal. The charge carriersare subjected to 1D localisation while passing through the amorphous region in betweenthe crystalline region and thereby the movement of charge carriers occurs through the1D localised regions and often dominates throughout the 3D extended states in crystallineregions. When the volume fraction of 3D extended states increases, the probability ofmovement of charge carriers is through the path of least resistance and therefore, apercolative metallic transport is expected. Microscopic properties in crystalline andamorphous regions are different and hence the usual Anderson localisation in thehomogeneous disorder limit is not appropriate for conducting polymers.

More recently, Samukhin and co-workers proposed [99] a q-1D fractals model in order toexplain the experimental data obtained for poorly conducting polymers. It is found that atlow temperatures, the VRH conductivity obeys a q-1D Mott’s law, σdc ∝ exp-(T1/T)1/2, but

23

the characteristic temperature T1 is greater than T0 for a 1D chain by a factor 1/D-1, Dbeing the dimensionality of the system. Similar temperature dependence was obtainedfor dc conductivity. Low frequency conductivity is entirely controlled by the weak chargetransfer between clusters, each cluster being very dense and well isolated. In contrast toq-1D models, Qiming and co-workers proposed a granular-rod model for the metallicstate of the conducting polymer [116]. In this model, the metallic islands correspond tosingle strands of polymer. The macroscopic conductivity results from anisotropic three-dimensional VRH in the network of metallic rods. This model explains very well thetemperature dependence of the conductivity, σ = σ0exp(-T0/T

1/2), the doping dependenceof T0, the anomalous 1/T dependence of the thermoelectric power as well as the linearincrease of Pauli susceptibility with dopant concentration. A temperature range, wherethe variable range hopping is valid, is decreased below the experimentally observedtemperature range (over which the above equation holds) if the metallic islands correspondto 3D bundles of the polymer strands.

Sheng’s fluctuation induced tunnelling (FIT) model was used extensively to interprettransport properties in metallic polymers. This model was originally developed for granularmetal [117, 118] and polymers filled with carbon black or alumina flakes. The polymersystem is described in terms of highly conducting regions separated by much lessconducting or insulating areas. The electrical conduction is dominated by electron transferbetween large conducting segments. Since the electrons tend to tunnel between conductingregions at points of their closest approach, the relevant tunnel junctions are usuallysmall in size and are therefore subject to large, thermally activated voltage fluctuationsacross the junction. By modulating the potential barrier, the voltage fluctuations directlyinfluence the tunneling probability and introduce a characteristic temperature variationto the normally temperature independent tunnelling conductivity. A non metallic featureof doped polyacetylene was explained in the framework of this model [119]. A similarmodel was used to interpret the transport properties of emeraldine polymer as a functionof the protonation level, x [120]. At no composition level does the conductivity appeartruly metallic. For all compositions, the conductivity behaviour is similar to that of agranular metal and this data fits transport via charging energy limited tunnelling betweenconductivity islands. The data were found to be consistent with percolation among theseislands for x ≥ 0.3 with the presence of an insulating layer surrounding each island abovethe percolation threshold. The size of these islands is estimated to be 200-300 Å. Luxand co-workers reported scanning electron microscopy (SEM) and transmission electronmicroscopy (TEM) pictures of highly conducting polyaniline in support of the conductingisland concepts [121]. Experimental measurements of TEM, X-ray diffraction (XRD),temperature dependence of dc conductivity and magnetic susceptibility indicate theapplicability of this model. However, electron paramagnetic resonance (EPR) spectroscopyand magnetic susceptibility studies also suggest that pristine and doped polyaniline containat least two types of spin carriers. It was suggested by Conwell and Mizes that the


24


conduction mechanism is not due to FIT dependence in conducting polymers since themetallic regions in the FIT model have negligible temperature dependence of conductivity.Also, the phonon scattering, scattering due to imperfections, and defects in the metallicregions are not considered in the FIT model. They determined the band motion versusdiffusive hopping transport in oriented metallic conducting polymers to understand theeffective dimensionality of the system. They showed that for band motion t⊥τ// is muchgreater than h/2π, while for diffusive motion t⊥τ// is much less than h/2π, τ// being theaverage time for the electron scattering along the chains (τ//~10-14 s from σ// = ne2τ//m*, ifσ~105 S cm-1, n = 1022 cm-3, m* = free electron mass = 9.1 x 10-28 g, e = 1.6 x 10-19 C). Ifthe system consists of high conducting regions mixed with low conducting regions, thenσ⊥ is the mixture of band motion and hopping. However, the measurements σ// /σ⊥ as afunction of temperature in oriented metallic trans-polyacetylene shows that σ// /σ⊥ is nearlytemperature independent and hence it is impossible that σ⊥ is due to diffusive hopping inboth cases [122, 123]. In contrast, experimental results demonstrated by Park and co-workers [124] indicate that σ// /σ⊥ increases with σ// and therefore, the issue of transportin highly oriented metallic conducting polymers is still debatable.

Voit and Buttner examined the FIT model critically and it was concluded that physicalparameters obtained from this model do not allow consistent description of highly dopedpolyacetylene [125]. Kaiser and Graham extended the FIT model for heterogeneoussystems by introducing geometric factors to the insulating barriers. Such barriers couldbe due to material imperfections that dominate the total resistance of the sample [48]. Ifthe intrinsic conductivity is very large and barriers form only short segments in theconduction path, the temperature dependence of the measured conductivity reflects thatof the barriers, but the magnitude is very much larger than the barrier conductivity dueto geometrical factors. Furthermore, if the barriers are reasonably good heat conductors,the temperature differences across them (and therefore, their contribution to thermoelectricpower) will be small [126]. In contrast to conductivity, the thermoelectric power couldthen follow the intrinsic metallic behaviour.

In addition to the tunnelling transport across the insulating barrier, a parallel phononassisted hopping transport was included in the Kaiser and Graham model. The bulkconductivity is the combination of Kivelson and Heeger q-1D transport, hopping/tunnelling transport and 3D disordered metallic transport (Figure 1.9). This heterogeneousmodel was reviewed by Kaiser for understanding the experimental data obtained onNaarman-polyacetylene. Localisation effects can appear at low temperatures despite thehigh conductivity. These could be due to charging energy effects in interchain transfer atlow temperatures or to quantum corrections. The linear thermoelectric power behaviourobserved indicates a smaller interaction between electrons and phonons than in normalmetals, which is consistent with the remarkably high conductivities observed [127]. Paaschwas of the opinion that this model contains seven independent data and so may describe

25

the smooth temperature dependence. He argued that multiparameter fits of simpledependencies sometimes make the picture seem unambiguous whether the relativeinfluences of the different contributions are reliable or not [128]. He modified the FITmodel, arguing that one of the main barriers for the tunneling process is the chain segmentswith residual dimerisation. These segments exhibit a dimerisation gap which acts as atunnelling barrier for the charge carriers. In general, however, the multiple parameterfitting procedure in FIT model has not been found satisfactory to explain the physicalproperties of conducting polymers.

The exponential dependence of conductivity in the insulating regime of conductingpolymers has been usually attributed to VRH. However, a wide range of exponent(d = 0.25-1) has been observed [49, 129, 130]. Moreover, T–1/2 behaviour of conductivityis often observed in granular metallic systems. Schreiber and Grussbach [131] suggestedthat the fluctuations in mesoscopic systems could give a wide range of values of theexponent due to the fractal nature of wavefunctions near the mobility edge. Thetransition of polymer system from q-1D to 3D was studied as a function of dopinglevel [132].

Figure 1.9 Schematic representation of the conduction processes in (a) microscopic or (b) macroscopic pictures that are in agreement with transport data in

Naarman-polyacetylene


26


Stafstrom [133, 134] reported that the enhancement in interchain interaction withincreased dopant concentration can induce 3D localisation of the electronic states. Heused the many-channel Buttiker-Landaur conductance formula [135] to study theconductance as a function of length of the system. Each channel contains several polymersegments represented by chain interruptions. The chain interruptions along the channelscan be caused by sp3 defects, crosslinking between chains, etc. The hopping acrosssuch a chain interruption should, therefore, be reduced considerably compared to theinterchain hopping. He showed that the most relevant parameter for causing localisationin the number of chain interruptions is the number of chain interruptions and not thechannel length. The conductance of the system is unaffected by the presence of chaininterruptions up to a critical value and, therefore, the critical chain length for whichthe conductance begins to drop should be used to characterise the transition betweendiffusive and non diffusive conductance. Moreover, if the interchain hopping term isequal to the interchain hopping strength (3D case) or if the number of channels is verysmall (1D case), the usual type of disorder-induced exponentially localised wavefunctions appear. Therefore, the q-1D nature of conjugated polymeric systems providesan example of a class of materials that differ from previously studied materials in theway the conductance responds to disorder in the form of chain interruptions. Recently,Schon and co-workers suggested the possibility of band-like transport in oligothiopheneas a result of strong observed dispersion of the valence band based on band structurecalculations [136].

Phillips and co-workers proposed a random dimmer model (RDM) [137-139] whichhas a set of delocalised conducting states, even in 1D, that initially allow a localisedparticle to move through the lattice almost ballistically. They showed that any disorderedbipolaron lattice can be mapped onto a RDM. The model is found to be applicable tothe M-I transition in a wide class of conducting polymers, such as polyaniline andheavily doped polyacetylene. Calculations performed on polyaniline demonstrateexplicitly that the conducting state of the RDM is coincident with a recent calculationof the location of the Fermi level in the metallic region [140, 141]. A RDM analysis onpoly(p-phenylene) also indicates the presence of a set of conducting states in the vicinityof the band edge. In highly disordered conducting polymers, the usual exponentialdependence of temperature is explained by phonon assisted hopping or tunnelling orboth. Zuppiroli and co-workers have proposed a model which describes adiabatically,the dimensionality, homogeneity, coulomb interactions and multi-phonon character inthe framework of hopping conduction [142, 143]. It was shown that the electrontransport in these materials is due to correlated hopping between polaronic clusters.They showed, both theoretically and experimentally, that the charging energy is theprincipal barrier to hopping. Polaronic clusters which originate from fluctuations inthe dopant concentration function as metallic grains in the granular metal hoppingmodel. The final temperature dependence is the same as in the case of models of Sheng,

27

Abeles, Arie or Efros and Shklovskii [144, 145]. Nagashima and co-workers recentlydescribed ac transport studies in polymers by using a statistical model of resistor network[146]. The model takes into account the polydispersiveness of the material as well asintrachain and interchain charge transport processes. The real and imaginary part ofthe resistivity was determined using a transfer-matrix technique. At low frequencies,interchain processes are more important and determine the transport mechanism. Onthe other hand, at high frequencies charge transport should be restricted along thepolymer chains, as interchain processes should be dominant. Both regimes describedby the model reproduce the experimental results in a remarkable way.

Among various models proposed by several researchers for conducting polymers in theinsulating side, the theoretical work by Ovchinnikov and Pronin [147] and Lewis [148]are slightly different from other models. In the former model, a q-1D percolation modelwas proposed for explaining the conductivity. According to this model, an impuritycaptures an electron from one of the adjacent chains and forms a charged impuritycentre. Such a carrier can detrap by an activated process and diffuse along the chain.This polaron can recombine with another impurity centre near the chain and thenescape to an arbitrary chain adjacent to the second impurity centre. Thus, conductionby percolation is possible in such a system if an infinite cluster of chains can be connectedby impurity centres. Lewis and co-workers suggested that the charge transport occursby tunnel transitions between localised states and lattice fluctuations, and electron-lattice coupling tends to broaden and reorganise the energy levels at each site. Theyestimated the electric field, frequency and temperature dependences of charge transportin both conducting and non conducting polymers. Moreover, in this model the localisedstates correspond to the Urbach states in amorphous systems since the optical absorptionfrom a distribution of states extend out from the fundamental absorption band edge.

1.7 Conclusions

It can be concluded that interchain interaction and disorder play a leading role ingoverning the transport properties of polymers. Nevertheless, the theoreticalunderstanding of charge transport phenomenon is not yet completely developed inthese polymers. None of the models for charge transport in these polymers is able toexplain all the features of the conducting polymers. The major hurdle is in quantifyingthe intrinsic and extrinsic parameters, for example, interchain and intrachain interaction,coulomb interaction, electron-phonon interaction, charge delocalisation, extent ofconjugation length and disorder, etc., whose contributions to charge transport are greatlyintermixed. Therefore, better understanding of the charge transport in these materialsis essential to determine the various physical phenomena operating in the polymer-based electronic devices.


28


Acknowledgements

The authors are grateful to Dr. K. Lal, Director, NPL, New Delhi, India, for his interestin this work. Viba Saxena thanks CSIR, India, for the award of a Research Associateship.

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37

2 Electrical Properties of DopedConjugated Polymers

R. Menon

2.1 Introduction

In the past three decades, several types of π-electron systems have shown very interestingfeatures in electrical transport properties [1-4]. Charge-transfer complexes, intercalatedgraphite, conjugated polymers, carbon-60, carbon nanotubes, etc., are some of the well-known π-electron systems. Polymeric materials were considered as insulators before thediscovery of metallic poly(sulfur nitride), [SN]x, and the enhancement of conductivity indoped polyacetylene, (CH)x, by several orders of magnitude [4, 5].

The polyconjugated chains -(C=C-C=C-C=C)n- consist of alternating single (σ-bonds)and double bonds (π-bonds). The π-electrons are highly delocalised and easily polarisable,and these features play important roles in the electrical and optical properties ofpolyconjugated systems. It also makes the latter rather different from conventionalelectronic systems [6-8]. Moreover, the intrinsic q-1D nature and the extent of bothintra- and interchain delocalisation of π-electrons play significant roles in the structural,electrical and optical properties of polyconjugated systems. Nevertheless, the complexmorphology of polymeric systems, which are partially crystalline and partially amorphousin nature, plays a crucial role in the physical properties. In general, the conjugationlength, the strength of the interchain interaction and the extent of disorder are some ofthe significant parameters that govern the physical properties of polyconjugated systems.

The electrical and optical properties of (CH)x, PANI, PPy, PT, poly(p-phenylenevinylene)(PPV), PPP and polythienylene vinylene (PTV) are some of the extensively studiedconjugated polymers [7, 9]. In first-generation conducting polymers (1976-1986), themaximum possible values of electrical conductivity were limited due to the presence ofstructural and morphological disorder, disorder-induced localisation, etc. The metallicfeatures were rather weak. This was mainly due to the presence of strong structural andmorphological disorder, as a result the π-electrons were not very well delocalised tofacilitate intra- and interchain charge transport [10]. In the past decade, significantimprovement in reducing the structural and morphological disorder has helped to createthe new generation of conducting polymers in which the metallic features arepredominantly observed in transport measurements. For example, in iodine-dopedTsukomoto (CH)x, the conductivity was around 105 S cm-1 [11].

38


By the early 1990s, several groups had started making high quality materials of PPV,PPy, PANI and polyalkylthiophene (PAT) [9]. In doped, oriented PPV samples theconductivity is of the order of 104 S cm-1 [12]. In high quality, PF6-doped PPy and PTsamples (prepared by low temperature electrochemical polymerisation), theconductivity is nearly 500 S cm-1 [13]. With the development of counterion-inducedprocessibility of PANI by dodecylbenzoyl sulfonic acid (DBSA) and camphor sulfonicacid (CSA) dopants, the conductivity was enhanced to nearly 500 S cm-1, and itstemperature dependence showed a significant metallic positive TCR in the range150-350 K [14-16]. The conductivity was enhanced to 103 S cm-1 in the case ofregioregular PAT [9, 17].

The conductivity of undoped polyconjugated systems is 10-6-10-10 S cm-1, hence it canbe considered at the semiconductor-insulator boundary [18]. The band gaps of knownpolyconjugated systems vary from 0.8 to 4 eV [9]. The charge carrier density inconducting polymers can be varied by several orders of magnitude (nearly 8 orders) bydoping. In fully doped systems, the carrier density could be as high as 1022/cm3. Thecarrier mobility in doped conducting polymers is much lower with respect to that ininorganic semiconductors, and this is largely due to the presence of strong disorder inpolymeric systems. Transient charge carriers can be generated by photoexcitation inconjugated polymers [7, 8, 19]. The maximum level of doping in conjugated polymerscould be as high as 50%, and that corresponds to one dopant per two monomers.

In conducting polymers the doping process can generate various types of charge carrierslike polarons, bipolarons, solitons, free carriers, etc., and this to a large extent dependson the doping level, the structure of the polyconjugated chain, interchain interactions,disorder, etc., [7, 8, 19]. In degenerate systems like (CH)x, solitons are formed, especiallyat doping levels below 6%. However, in non degenerate systems like PPy, PT, etc., bothpolarons and bipolarons are formed depending upon the energetics. However, as theinterchain interactions and the carrier density increases and the extent of disorderdecreases, these excitations could behave more like free carriers.

The M-I transition in doped conducting polymers is mainly governed by the extent ofdisorder, interchain interaction and doping level [18, 20]. The main source of disorderin conducting polymers are the sp3 defects in the chain, chain ends, chain entanglements,voids, morphological and doping induced defects, etc., [9]. In fibrillar morphology,the chains are extended, hence it is possible to have delocalised states along the chainlength direction. In globular morphology, the chains are coiled up, and this tends tolocalise the electronic states. In unoriented conducting polymer systems, the chains arerandomly dispersed and the physical properties are isotropic. However, by orientingthe polymer chains by mechanical stretching it is possible to enhance the conductivityalong the orienting axis, and an anisotropy of conductivity of the order of 100 can be

39

Electrical Properties of Doped Conjugated Polymers

easily achieved. Since conducting polymers are partially crystalline and partiallyamorphous, the volume fraction of crystalline regions and the size of the crystallinecoherence length play dominant roles in the charge transport. In general, the disorder-induced localisation plays a dominant role in the M-I transition and in the transportproperties of conducting polymers.

2.2 Metallic State

The metallic state in doped conducting polymers is inferred from the following: a largefinite dc conductivity as the temperature (T) goes to 0 K, temperature independentPauli spin susceptibility down to 10 K, linear temperature dependence of thethermoelectric power down to 10 K, linear term in the specific heat at low temperatures,free carrier absorption and large metallic reflectance in the infrared, etc., [20]. Thisevidence indicates the presence of a continuous density of states with a well-definedFermi energy. In some conducting polymers, the typical metallic positive TCR wasobserved from 300 to 150 K, and in some others it was only below 20 K. However,recently Park and co-workers reported a metallic positive TCR in doped (CH)x, from300 to 1.5 K, which is quite exceptional [21]. Although the typical negative TCR inhigh quality conducting polymers is indicative of non metallic behaviour, its temperaturedependence was rather weak so that the logarithmic derivative of conductivity, σ, (W= dlnσ/dlnT) has a positive temperature coefficient. This implies a finite value ofconductivity and a finite density of states at the Fermi level at very low temperatures,as expected in the case of disordered metallic systems. This evidence indicates that inspite of the disordered q-1D nature of polymer chains, it is possible to have a metallicstate in these systems [22]. In the metallic state, the average size of the delocalisedstates is considerably larger than that of the structural coherence length, hence thecarrier transport is slightly hindered by the presence of disorder potentials in theamorphous region. Although conducting polymers are intrinsically q-1D electronicsystems, the interchain coupling can be sufficiently large to enable the formation ofthree-dimensional metals.

The critical parameter in the M-I transition is the statistical average of a wide range ofvalues of the correlation/localisation length, Lc. If Lc is greater than the average structuralcoherence length (which characterises the size of the crystalline regions), then thedisorder can be considered as within the weak limit, which means the system sees onlyan average of the random fluctuations of the disorder potentials. Conversely, if Lc isless than the average structural coherence length, then the extent of disorder isconsiderably higher. The values of Lc and structural coherence length can be determinedby transport property measurements and X-ray diffraction, respectively [9, 20].

40


2.2.1 Conductivity

The electrical conductivity is mainly determined by the carrier density, n, relaxationtime, τ, and effective mass, m, of the carrier (electrical conductivity, σ = ne2τ/m, where eis the electron charge). According to the Ioffe-Regel criterion, the interatomic distance isconsidered as the lower limit for the mean free path, λ, in a metallic system. Hence, fora metallic system kFλ is greater than 1, where kFλ = [h(3π2)2/3] / (e2ρn1/3), kF is the Fermiwavevector and ρ is the electrical resistivity [23-25]. In highly doped conducting polymers,n ≈1021 per cm3, λ is around 10 Å, and kFλ ≈1-10, at room temperature. The detailsabout metallic conducting polymers are shown in Table 2.1 [26].

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desnednoC:scisyhPfolanruoJ,noneM.R,gokslhA.MmorfnoissimrepybdecudorpeR.1717,33-13,23,01,8991,gnihsilbuPPOI,rettaM

Pietronero suggested that in a one-dimensional chain the only possible source of scatteringfor charge carriers is from +kF to –kF, involving the high-energy 2kF phonons [27]. Moreover,due to phonon freezing effects, possible even at room temperature, the first-order scatteringshould induce a strong enhancement in conductivity in one-dimensional chains. Theconductivity in the chain direction, σ⎜⎜, in a one-dimensional chain is given by:

σ⎜⎜ = (ne2a/πh)vFτ = (ne2a2/πh) / (λ/a) (2.1)

41

where a is the carbon-carbon distance, vF (=2t0a/h) is the Fermi velocity, t0 (= 2-3 eV) isthe π-electron hopping matrix element and τ is the backscattering lifetime. In the limit ofelastic scattering for a half-filled band system, σ⎜⎜(300 K)≈105 S cm-1. However, since themain scattering involved in a conducting polymer chain is only due to the 2kF phonons,and by including the inelastic scattering process below the characteristic temperature(kBT∼hω0/4 and T∼600 K), another two orders of magnitude of enhancement inconductivity should be possible, i.e., ≈ 107 S cm-1. Similar estimates of conductivity havealso been obtained by the Kivelson and Heeger model [28]. In later models, theconductivity is expected to increase exponentially at low temperatures. Hence, thetheoretical studies suggest that the intrinsic conductivity in one-dimensional models ofconjugated polymers is expected to be even larger than that of conventional metals.

The low temperature conductivity measurement is a simple and sensitive method to geta qualitative level of understanding about the extent of disorder present in the system.Since conductivity is directly related to the mean free path (which is rather sensitive tothe presence of any disorder) the variation in low temperature conductivity is quitedramatic as disorder varies. The characteristic behaviour of the temperature dependenceof conductivity can be understood in detail by defining the reduced activation energy, W,as the logarithmic derivative of the temperature dependence of conductivity, i.e., W =d(lnσ)/d(lnT) [29, 20]. If the system has a finite value of conductivity with a negativeTCR, then W shows a positive temperature coefficient at low temperatures. Moreover,this ensures that there is a finite conductivity as T→0. In general, as the resistivity ratio,ρr [(= ρ(1.4 K)/ρ(300 K)], increases the temperature dependence of W gradually movesfrom a positive (metallic) to a negative (insulating) temperature coefficient at lowtemperatures. The approximate values of ρr for various conducting polymers in the metallic(M), critical (C) and insulating (I) regimes are shown in Table 2.2 [30].

The conductivity in the disordered metallic regime is expressed by [25, 31]

σ(T) = σ (0) + m´T1/2 + BTp/2 (2.2)

where m´ = α[4/3 – γ Fσ/2], α is a parameter depending on the diffusion coefficient, γFσis the interaction parameter, p is determined by the scattering rate (for electron-phononscattering, p = 3; for inelastic electron-electron (e-e) scattering, p = 2 in the clean(weakly disordered) limit or 3/2 in the dirty (strongly disordered) limit). The secondterm in Equation 2.2 results from the e-e interactions and the third term is the correctionto σ(0) due to the localisation effects. In disordered metals, e-e interactions play animportant role in the low temperature transport. Usually, the sign of m is negativewhen γFσ > 8/9, and this results in the change of sign (from negative to positive) in theTCR at low temperatures.


42


Iodine-doped polyacetylene I-(CH)x is one of the most extensively studied systems amongdoped conducting polymers [11, 32]. The maximum room temperature conductivityparallel to chain axis in highly oriented I-(CH)x is nearly 105 S cm-1, and the anisotropyof conductivity is 100-200. The stretchability and the maximum obtainable conductivityin I-(CH)x is very much dependent on the film thickness. The conductivity is higher inthinner films (below 10 μm), in both stretched and unstretched films. The structural andphysical properties of I-(CH)x have been reviewed by Tsukamoto [11]. The averagecrystalline coherence lengths, parallel and perpendicular to the chain axis, are 150 and50 Å, respectively. In highly conducting samples, the carrier density is nearly 1022 cm-3

and the mean free path is around 500 Å. The density of states at the Fermi level isapproximately 0.3 states (eV C)-1, (1.8 x 1018 states (J A S)-1).

Recently Park and co-workers [21] have observed, for the first time in conducting polymers,a metallic positive TCR, from 300 to 1.5 K, in ClO4-doped (CH)x; although in earlier workthe positive TCR was observed in FeCl3-(CH)x and PANI-CSA down to 180 K. Theconductivity for a ClO4-(CH)x sample at 300 K is nearly 40,000 S cm-1. Its conductivityincreases by a factor of two at 1.5 K, as shown in Figure 2.1. For samples with conductivityin the range 2,000-20,000 S cm-1, the positive TCR was observed at temperatures above 150K. The room temperature conductivity of highly oriented I-(CH)x is nearly a factor of two

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ρr σσσσσ )K003( a ρρρρρr σσσσσ )K003( ρρρρρr σσσσσ )K003(

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43

Figure 2.1 Normalised resistivity (ρ) versus temperature for ClO4-doped polyacetylenesamples doped by: (1) ClO4 (Fe), ρ(300 K) ~ 2.41 x 10-5 (Ω cm); (2) ClO4 (Fe), ρ(300K) ~ 5.01 x 10-5 (Ω cm); (3) ClO4 (Fe), ρ(300 K) ~ 6.58 x 10-5 (Ω cm); (4) ClO4 (Fe),

ρ(300 K) ~ 8.84 x 10-5 (Ω cm); (5) ClO4 (Cu), ρ(300 K) ~ 5.88 x 10-5 (Ω cm); (6) ClO4

(Cu), ρ(300 K) ~ 1.67 x 10-4 (Ω cm).(Reproduced with permission from Y.W. Park, E.S. Choi, and D.S. Suh, Synthetic

Metals, Elsevier Science SA, 1998, 96, 1-3, 81)

larger than that of ClO4-(CH)x and FeCl3-(CH)x; yet, it does not show any metallic positiveTCR [30]. The temperature dependence of resistivity of I-(CH)x samples is shown in Figure2.2a [30, 33]. The metallic samples have a rather weak negative TCR, and it shows a largefinite conductivity at temperatures below 1 K. The absence of a positive TCR in the case ofI-(CH)x suggests that the doping induced disorder is higher with respect to that in ClO4-(CH)x and FeCl3-(CH)x systems. The sample B2 is systematically aged up to B6, so that itcontinuously moves from the metallic to insulating regime. The W versus T plot of the ρ(T)is shown in Figure 2.2b. The metallic B2 shows the expected positive temperature coefficientof W(T), and upon ageing it tends towards a negative temperature coefficient, as expectedfor insulating systems. This indicates that the positive TCR in conducting polymers is quitesensitive to subtle variations in the extent of disorder present in the system.


44


The σ(T) of I-(CH)x samples at various stretching ratios (l/l0) is shown in Figure 2.3 [18]. Theunstretched sample has a σ(300 K) ∼ 800 S cm-1, and it shows a strong negative TCR. Theanisotropy of conductivity increases upon increasing the stretching ratio. Even in highlystretched samples the number of misaligned and criss-crossed chains is rather high so that theσ(T) is nearly identical for both parallel and perpendicular directions to the chain axis, andthe interchain transport plays an important role in both cases. Hence, further enhancementin chain orientation (σ⎜⎜/σ⊥ >103) is of considerable importance in order to observe the intrinsicanisotropic features in the charge transport properties in conducting polymers.

As described in previous works [18, 21], the σ(T) below 4.2 K follows a T1/2 dependence(see Equation 2.2), both parallel and perpendicular to the chain axis in oriented I-(CH)x

samples. The T1/2 dependence indicates that the contribution from e-e interactions plays

Figure 2.2 (a) Resistivity versus temperature for an iodine-doped polyacetylene sampleaged from metallic B2 to insulating B6; (b) W versus T for the same data. The dotted lines

indicate the power law regime.(Reproduced by permission from M. Ahlskog, R. Menon, A.J. Heeger, Journal of Physics:

Condensed Matter, IOP Publishing, 1997, 9, 20-22, 4145)

45

a dominant role at very low temperatures. This is also consistent with the enhancednegative contribution to magnetoconductance (MC), as explained in detail in Section2.2. For an intermediate temperature range (4-40 K), where σ ∝ T3/4, the inelastic electron-phonon scattering (p = 3/2) is the dominant scattering mechanism for both parallel andperpendicular directions to chain axis [20, 32]. This is also consistent with the enhancedpositive contribution to MC at temperatures above 4 K. This suggests that both interactionand localisation play dominant roles in σ(T) at low temperatures in metallic (CH)x samples.

The temperature dependence of resistivity of a sulfuric acid doped PPV (PPV- H2SO4) sample(A) as it gradually aged to sample number (L) is shown in Figure 2.4a [34, 35]. Free-standingfilms of PPV (thickness 4-8 μm) were stretch-aligned to 10:1 ratio [12]. The optical anisotropyis nearly 50, as obtained from the dichroic ratio measurement at 1520 cm-1. This indicatesthat the PPV chains are quite well oriented after tensile drawing. The W versus T plot of thesame data is shown in Figure 2.4b. The metallic samples follow a positive temperaturecoefficient of W at low temperatures; as ρr increases, W gradually moves towards the criticaland insulating regimes, similar to that observed in the case of I-(CH)x.

Figure 2.3 Conductivity (both parallel and perpendicular) versus temperature foriodine-doped polyacetylene samples at various stretching ratios (l/l0)

(Reproduced by permission from C.O. Yoon, R. Menon, A.J. Heeger, E.B. Park, Y.W. Park,K. Akagi, and H. Shirakawa, Synthetic Metals, Elsevier Science SA, 1995, 69, 1-3, 79)


46


Figure 2.4 (a) Resistivity versus temperature for a PPV-H2SO4 sample from metallic(highly doped) C to insulating (less doped) L and (b) W versus T for the same data.

(A-E on the metallic side, G-L on the insulating side)(Reproduced by permission from M. Ahlskog, R. Menon, A.J. Heeger, T. Noguchi, and

T. Ohnishi, Physical Review B, American Physical Society, 1997, 55, 11, 6777)

(a) (b)

In PPV-H2SO4 samples, the low temperature conductivity of metallic samples in bothparallel and perpendicular directions to the chain axis can also be fitted to Equation 2.2[34, 35]. However, at temperatures below 4 K, the T1/2 fit is rather good even in thepresence of a magnetic field, as shown in earlier works [34]. Hence, in metallic PPVsamples, the localisation-interaction model is valid at low temperatures, as observed inthe case of metallic (CH)x samples. Moreover, the MC data in both systems are consistentwith the localisation-interaction model, as explained below. Although the anisotropy ofconductivity in metallic, oriented (CH)x and PPV samples is nearly 100, the σ(T) is rathersimilar in both parallel and perpendicular directions to the chain axis, indicating that ananisotropic three-dimensional model is appropriate in these systems.

The temperature dependence of conductivity of PPy doped with PF6 (PPy-PF6) is shownin Figure 2.5a [36]. The W versus T plot of the same data is shown in Figure 2.5b. The

47

metallic sample (M1) shows a positive TCR below 12 K, and m′ in Equation 2.2 isnegative. The behaviour of W(T) is consistent with that observed in metallic (CH)x andPPV samples. The temperature and field dependence of the resistivity of metallic PPy-PF6

samples at temperatures below 1 K shows the presence of a large finite conductivity(∼150 S cm-1) at 75 mK and 8 T field [37]. This suggests that the three-dimensionaltransport is quite robust and that the intrinsic one-dimensional nature of disorderedpolymer chains is not causing any severe localisation, although the conductivity at mKtemperatures in these systems is around the minimum value of conductivity of a metallicsubstance according to Mott’s theory [24]. Hence, the interchain interactions aresufficiently large enough to prevent any severe localisation in polymer chains. Furthermore,PPy-PF6 samples can be stretched by a factor of two, and their conductivity increases upto 3000 S cm-1 [13]. This is one of the best examples of conductivity in a doped conjugatedpolymer that has a long-term stability at ambient conditions.

Figure 2.5 (a) Normalised resistivity versus temperature for various PPy-PF6

samples (M→ for metallic, I→ for insulating, and ‘c’ for critical) and(b) W versus T for the same data

(Reproduced by permission from C.O. Yoon, R. Menon, D. Moses, and A.J.Heeger, Physical Review B, American Physical Society, 1994, 49, 16, 10851)

(a) (b)


48


The σ(T) of 2-acrylamido-2-methyl-1-propane-sulfonic acid (AMPSA) samples are shownin Figure 2.6 [38, 39]. These samples show a positive TCR at temperatures above 80 K.Although the σ(300 K) of PANI-AMPSA samples is slightly lower with respect to PANI-CSA, its positive TCR is observed down to much lower temperatures. The wet spunPANI-AMPSA fibres have shown an impressive room temperature conductivity of nearly2000 S cm-1 [40]. The difference in σ(300 K) and σ(T) between CSA- and AMPSA-dopedPANI samples is possibly due to the variations in microstructure and its contribution todisorder-induced localisation. This shows that the charge transport in doped conductingpolymers is rather sensitive to slight variations in the morphological features.

Figure 2.6 Conductivity versus temperature for PANI-AMPSA samples at variousdoping levels mentioned in subscripts

(Reproduced by permission from P.N. Adams, P. Devasagayam, S.J. Pomfret,L. Abell, and A.P. Monkman, Journal of Physics: Condensed Matter, IOP

Publishing, 1998, 10, 37-38, 8293)

49

Figure 2.7 Normalised resistivity versus temperature for various doped PEDOTsamples (from metallic PF1 to insulating PF6, from metallic BF1 to insulating BF5)

(Reproduced by permission from A. Aleshin, R. Kiebooms, R. Menon, andA.J. Heeger, Synthetic Metals, Elsevier Science SA , 1997, 90, 1-3, 61)

The σ(T) of doped poly(3,4-ethylenedioxy-thiophene), PEDOT, samples are shown inFigure 2.7 [41, 42]. In some doped PEDOT samples, a positive TCR have been observed attemperatures below 10 K. Similar positive TCR and weak σ(T) values have been observed inpoly(3-methyl)thiophene (PMeT)-PF6 samples having σ(300 K) ≈ 200 S cm-1. In these systems,Masubuchi and co-workers have observed a pressure tuning of the M-I transition [43, 44].In doped PEDOT samples, the σ(T) can be exceptionally weak, as shown in Figure 2.7. Forexample, even for samples with σ(300 K) ≈10 S cm-1 (i.e., much below the Mott minimumvalue), the conductivity at 1 K can be around 4 S cm-1. One possible explanation is that thestructural disorder in PEDOT samples is considerably less with respect to other systems;since the β-positions in the thiopene rings are blocked by an ethylenedioxy group, the chainextension is possible only through the α-positions. A large finite conductivity [~150 S cm-1]has been observed in metallic PEDOT and PMeT systems too.

In summary, the conductivity and its temperature dependence in doped (CH)x, PPV, PPy,PANI and PEDOT samples suggests that by reducing the role of disorder-inducedlocalisation in charge transport, it is possible to observe the intrinsic metallic positiveTCR in doped conducting polymers.


50


2.2.2 Magnetoconductance

It is well known that MC is a sensitive local probe for investigating the microscopic transportparameters (e.g., scattering process, relaxation mechanism, etc.) in metallic and semiconductingsystems [18]. The quantitative level of understanding of MC for disordered systems obtainedusing the localisation-interaction model is rather useful to check the appropriateness of thismodel for metallic conducting polymers. Moreover, the consistency of using this model canbe verified by comparing the results from the temperature dependence of conductivity andMC measurements.

It is well known that in an ideal one-dimensional conductor the transverse orbital motionis restricted, thus the carriers cannot make circular motion in the presence of a magneticfield. Hence, one could hardly expect any MC in an ideal one-dimensional conductor.However, in the presence of any finite interchain transfer integral, as in several q-1Dconductors, the MC can be used as a powerful tool to investigate the intrachain versusinterchain transport. Nevertheless, the fine features in anisotropic MC can be madeinconspicuous in the presence of disorder.

In disordered metallic systems, it is well known that the quantum corrections due to weaklocalisation (WL) and e-e interactions contribute to the MC at low temperatures [18, 32].Usually, the WL contribution (positive MC) dominates at temperatures greater than 4 Kand low fields (below 3 T) and the contribution from e-e interaction (negative MC)dominates at temperature below 4 K and higher fields (above 3 T). However, as the extentof disorder increases, both WL and e-e interaction contributions decrease, and the hoppingcontribution to MC (large negative MC) increases. Although the theoretical estimate forthe upper limit to the quantum corrections to MC in conventional disordered systems isbelow 3% (i.e., Δσ/σ < ± 3%), the observed MC in oriented metallic conducting polymersis slightly higher, which is probably associated with the anisotropic diffusion coefficient,anisotropic effective mass, etc.

There are several detailed studies of MC in conducting polymers [18, 32, 45]. The behaviourof MC in oriented and unoriented conducting polymers shows a clear difference. Highlyconducting, oriented samples (e.g., (CH)x and PPV) exhibit a positive MC due to the WLcontribution (especially, when the field is perpendicular to the chain axis) at temperaturesgreater than 2 K; whereas, in unoriented samples (e.g., PPy, PANI, PEDOT) the MC is observedto be negative at all temperatures and fields [31]. In oriented samples, the MC is an interplaybetween WL (positive MC at low fields and temperatures greater than 2 K) and e-e interaction(negative MC at high fields and temperature less than 2 K) contributions; the sign of the MCdepends on the angle between the magnetic field and chain axis. This anisotropic MC inoriented samples is due to the anisotropy in the WL contribution, since the positive MC dueto the WL contribution is maximised when the field is perpendicular to the chain axis, and itis minimised when the field is parallel to the chain axis [32, 34, 35].

51

The MC for oriented metallic I-(CH)x and PPV-H2SO4 samples are shown in Figure 2.8a[32] and (b) [34, 35], respectively. In both systems, the anisotropy of conductivity isnearly 100. As shown in these figures, when the field is perpendicular to the chain axis,the sign of the MC is positive. However, when the field is parallel to the chain axis, thesign of the MC is negative. The experimental results in both systems show that the WLcontribution is negligible when the field is parallel to the chain axis. Although the agedsample remains metallic down to 1.4 K, the positive MC due to the WL contributionvanishes owing to the increasing extent of disorder caused by the aging. This indicatesthat even a marginal increase in the extent of disorder in metallic conducting polymers

Figure 2.8 (a) Magnetoconductance versus field for a metallic I-(CH)x sample at 4.2 K(dot), 2 K (square) and 1.2 K (triangle), upper is transverse and lower is longitudinal

(Reproduced by permission from R. Menon, K. Vakiparta, Y. Cao, and D. Moses, PhysicalReview B, American Physical Society, 1994, 49, 23, 16162)

(b) Magnetoconductance versus field for a metallic PPV-H2SO4 sample, upper is transverseand lower is longitudinal

(Reproduced by permission from M. Ahlskog, R. Menon, A.J. Heeger, T. Noguchi, andT. Ohnishi, Physical Review B, American Physical Society, 1996, 53, 23, 15529)

(a) (b)


52


can suppress the quantum transport involved in the WL contribution to a positive MC.Hence, the coherent interchain transport in intrinsically q-1D polymer chains can beeasily affected by minute variations in the interchain alignments, disorder, etc.

The MC due to the e-e interaction can be distinguished from the WL by scaling the totalvalue [34, 46]. The scaling plot for a metallic oriented PPV-H2SO4 sample is shown inFigure 2.9. The expected universal scaling behaviour in the longitudinal MC, due to thedominant contribution from e-e interaction, is clearly shown in Figure 2.9b. However, inthe case of transverse MC, the scaling behaviour deviates, as shown in Figure 2.9a. Thisclearly shows the importance of the WL contribution to the transverse MC. Thisanisotropic MC can be used to probe the microscopic anisotropy at the molecular length

Figure 2.9 Universal scaling plot of magnetoconductance for a metallic PPV-H2SO4

sample: (a) transverse (b) longitudinal(Reproduced by permission from M. Ahlskog, R. Menon, A.J. Heeger, T. Noguchi and

T. Ohnishi al., Physical Review B, American Physical Society, 1996, 53, 23, 15529)

(a)

(b)

53

scale which is usually masked by the morphological features in the bulk conductivitymeasurements. Hence, modelling the anisotropic MC can provide a quantitative estimateof the number of misaligned chains in oriented samples. In slightly less conducting (~100 S cm-1) metallic samples of unoriented PEDOT a universal scaling of the MC hasbeen observed, as shown in Figure 2.10 [41, 42]. In this case, since the chains are notoriented, the scaling behaviour is nearly identical in both transverse and longitudinalMC, indicating that the e-e interaction is the dominant contribution to the MC.

Although the anisotropy of conductivity in metallic oriented-(CH)x or PPV-H2SO4 samplesis nearly 100, the behaviour of the MC is identical whether the current is parallel orperpendicular to the chain axis. This suggests that high quality oriented conducting polymersbehave as anisotropic three-dimensional systems in which the charge transport mechanismis nearly identical in both parallel and perpendicular directions to the chain axis.

The MC for a metallic PANI-CSA sample, at low and high fields, is shown in Figure 2.11[15]. The MC shows H2 and H1/2 dependence at low and high fields, respectively. Similarresults have been observed for metallic PPy [36] and PEDOT [41] samples. This fielddependence is consistent with the localisation-interaction model. These systems are just

Figure 2.10 Universal scaling plot of magnetoconductance for a metallic PEDOT- PF6

sample at various temperatures (dots 1.48 K; diamonds 2.02 K; open triangles 3.03 K;closed triangles 4.24 K)

(Reproduced by permission from A. Aleshin, R. Kiebooms, R. Menon, F. Wudl, andA.J. Heeger, Physical Review B, American Physical Society, 1997, 56, 7, 3659)


54


on the metallic side of the M-I boundary, and their values of conductivity are not highenough to observe any positive MC due to the WL contribution. Hence, in PANI, PPyand PEDOT systems, the MC remains negative at all temperatures and fields. When theextent of disorder increases and the system moves to the critical and insulating regimesthe negative MC increases dramatically.

In summary, the MC in metallic conducting polymers is a sensitive probe for investigatingthe microscopic charge transport mechanism. In oriented metallic conducting polymers, theanisotropic MC depends on the angle between the chain axis and the applied field, and thiscan give qualitative information regarding the molecular scale anisotropy in these systems. Inless conducting and unoriented systems, the magnitude of the negative MC increases as ρr

increases, and this can be used to discover the extent of disorder present in the system.

Figure 2.11 Magnetoconductance versus field for a metallic PANI-CSA sample: (a) lowfield fit and (b) high field fit

(Reproduced by permission from R. Menon, C.O. Yoon, D. Moses, A.J. Heeger, and Y.Cao, Physical Review B, American Physical Society, 1993, 48, 24, 17685)

(a)

(b)

55

2.2.3 Thermoelectric Power

In doped conducting polymers, the thermoelectric power is not as sensitive to disorder aselectrical conductivity, since the mean free path involved in the electrical transport isvery much affected by the extent of disorder present in the system [18]. Although themetallic positive TCR is rather unusual in highly conducting polymers for a wide rangeof temperatures, the metallic linear temperature dependence of thermoelectric power,S(T), is quite usual in all conducting polymers for a wide range of temperatures (10-300K). The quasi-linear temperature dependence of thermoelectric power is observed topersist well into the insulating regime.

The remarkable linearity of S(T) and the negligible non linear contribution tothermoelectric power in high quality metallic (CH)x indicate that the lattice contributionto thermoelectric power due to phonons is less significant. The S(T) is quite linear evenin the case of samples on the insulating side of the M-I transition [18].

Kaiser [47] has proposed a heterogeneous model to explain the apparent difference inthe behaviour of S(T) and σ(T). In such a model, the less conducting regions that limitthe motion of charge carriers, determine the bulk transport properties. If the thermalcurrent carried by phonons is impeded less by thin insulating barriers than the electricalcurrent carried by electrons or holes, then the system indeed shows a metallicthermoelectric power. In other words, the majority of the temperature gradient occursacross the highly conducting regions and the majority of the electrical potential dropoccurs across the thin insulating barriers. A large enhancement of thermoelectric poweris expected if the barriers play any significant role to the total value of the thermoelectricpower. However, in conducting polymers, on the metallic and critical regimes of the M-I transition, no such enhancement has been observed; this indicates that the barrierscontribute little to the thermoelectric power.

The S(T) of various PPy-PF6 samples on both sides of the M-I transition is shown inFigure 2.12 [36]. Similar results have been observed in PANI-CSA samples too [48]. Inthese systems the quasi-linear thermoelectric power is relatively insensitive to the variationsin ρr near the disorder-induced M-I transition. The density of states estimated from S(T)is around one state per eV per two rings for PANI-CSA, and nearly one state per eV perfour rings for PPy-PF6, for samples near the M-I transition.

The conventional notion suggests that the sign of the thermoelectric power depends on thesign of the charge carrier. However, Park and co-workers [49] have observed a positivethermoelectric power for both p- and n-type doped (CH)x. Since the sign of the thermoelectricpower depends on the band structure, etc., it may not be that straightforward in complexsystems such as doped conducting polymers. More work is required to fully understandthis anomaly regarding the sign of thermoelectric power in conducting polymers.


56


2.2.4 Magnetic Susceptibility and Specific Heat

In metallic systems, the temperature-independent Pauli susceptibility (χP) is a characteristicfeature for delocalised carriers [18]. The Pauli susceptibility is directly proportional to thedensity of states at the Fermi level, i.e. χP = μ2

BN(EF). where μB is the Bohr magneton andN(EF) is the density of states at the Fermi level. Hence, it is possible to determine the N(EF)from the temperature-independent χP for metallic systems. Usually, in disordered systems,the measured χP is the sum of both Curie and Pauli terms; the Curie term gives an estimate ofthe localised spins present in the system, and this in turn is a measure of the extent of disorder.

A small Curie term has been observed in all metallic conducting polymers at very lowtemperatures (T < 20 K) [18]. This indicates the presence of localised spins due toimpurities, defects, etc. The χ(T) of PANI-CSA samples near the M-I transition show thisbehaviour [50]. The density of states at the Fermi level for metallic PANI-CSA and PPy-PF6 samples are one states per eV per two rings and three states per eV per four rings,respectively [51]. These values are rather similar to that obtained from the thermoelectricpower measurements. The Curie term at low temperatures is lower for metallic samplesthan for insulating samples. The magnetic properties and spin dynamics in dopedconducting polymers are described in recent review articles [51].

Figure 2.12 Thermoelectric power versus temperature for various PPy-PF6 samplesfrom metallic side (dot) to insulating side (square)

(Reproduced by permission from C.O. Yoon, R. Menon, D. Moses, and A.J. Heeger,Physical Review B, American Physical Society, 1994, 49, 16, 10851)

57

There are few reports of thermal property measurements (e.g., thermal conductivity,specific heat, etc.) [52, 53]. The linear term in specific heat at low temperatures is evidenceof the continuous density of states with a well-defined Fermi energy for any metallicsystem. The low temperature specific heat, C, for a metallic PPy-PF6 sample and for aninsulating PPy-p-toluenesulfonate (TSO) sample is shown in Figure 2.13 [54]. The datafor both samples fit to the equation C/T = γ + βT2, where γ and β are the electronic andlattice contributions, respectively. From the values of β and γ, the calculated density ofstates for metallic and insulating samples are 3.6±0.12 and 1.2±0.04 states per eV perunit, and the corresponding Debye temperatures are 210±7 and 191±6.3 K, respectively.These values are comparable to those obtained from the spin susceptibility data.

Although the resistivity ratio of the insulating PPy-TSO sample is an order of magnitudelarger with respect to the metallic PPy-PF6 sample, both systems show a linear term inspecific heat, and the density of states of the insulating system is only a factor of threelower with respect to that in the metallic sample [51]. Moreover, from the specific heatdata [54] it seems that both systems have a finite density of states at the Fermi level,hence the insulating system can be considered as slightly less metallic (less density ofstates due to localisation) with respect to the ‘real’ metallic system (i.e., finite conductivityas the temperature tends to 0 K). The comparison of specific heat and conductivity data

Figure 2.13 Heat capacity (C/T) versus temperature for doped PPy samples frommetallic PPy-PF6 (circles) to insulating PPy(TSO) [triangles]

(Reproduced by permission from T.H. Gilani and T. Ishiguro, Journal of the PhysicalSociety of Japan, Physical Society of Japan, 1997, 66, 3, 727)


58


indicates that the latter is more sensitive for determining the extent of disorder, and fordiscerning the metallic and insulating systems near the M-I transition. Furthermore, bothsystems do not show any anomalies due to glassy behaviour that can occur due to thelocalisation of charge carriers in the amorphous regions, one-dimensional localisation,etc. Surprisingly, the linear term in the specific heat is strongly present in both metallicand insulating systems down to 2 K, although a Curie term in the spin susceptibility wasobserved for all conducting polymer samples at low temperatures. This indicates that forconducting polymer samples near the M-I transition, the extent of disorder is not severeenough to induce any drastic localisation of charge carriers, contrary to the conventionalview that all states are localised in a disordered one-dimensional conductor.

2.3 Critical and Insulating States

The intrinsic metallic state in high quality doped conducting polymers is suppressed bydisorder-induced localisation [18, 20]. As the extent of disorder increases, the effectiveconjugation length and the interchain transport decreases; the system gradually movesfrom the metallic to the insulating state via the M-I transition. From the previous sectionit is known that even slight variations in the extent of disorder have dramatic effects inthe temperature dependence of conductivity near the M-I transition. However, in othertransport property measurements like thermoelectric power, specific heat, etc., the effectof disorder is not that conspicuous near the M-I transition, especially in conductingpolymer systems.

The power law behaviour of conductivity is universal for systems at the critical regime,and the temperature dependence of conductivity is given by σ(T) ∝ Tβ, where β isbetween 0.33 and 1 [20]. In the power law regime (temperature-independent W(T)regime) W(T) = β. The value of β and the temperature range of the power law regimecan be determined from the log-log plots of W versus T. As a typical example, the Wversus T plot of various conducting polymer systems near the critical regime is shownin Figure 2.14 [55]. Usually, the value of β is lower for systems with a smaller value ofρr, and as ρr increases the system moves towards the insulating regime in which σ(T)becomes exponential, i.e., σ(T) ∝ exp(-T0/T)γ, where γ = 1/(d +1) and d is thedimensionality of the system.

The W versus T plots for various conducting polymers are shown in Figures 2.2b, 2.4band 2.5b. In all these figures, the temperature coefficient of W(T) varies distinctly formetallic (positive), critical (temperature-independent) and insulating (negative) regimes.These figures clearly show that the critical regime is rather robust in conducting polymers.Moreover, the critical regime can be easily tuned to the metallic and insulating regimesby pressure and magnetic fields, respectively, as shown in case of a PPy-PF6 sample in

59

Figure 2.15 [55]. The enhanced interchain transport under pressure is driving the systemtowards the metallic state. Alternatively, the field-induced transition to the critical andinsulating regimes occurs when the localisation length is comparable to the magneticlength, and the field shrinks the overlap of the wavefunctions of the delocalised states.The field-induced transition, from metallic to critical regime, for a doped PPV sample isshown in Figure 2.16 [35]. Although this sample has a large finite conductivity in zerofield, it approaches zero at 8 T. Another typical example of a field-induced transition,from power law to exponential law behaviour of conductivity at low temperatures, isshown for a PANI-CSA sample in Figure 2.17 [15, 16]. These field-induced transitions inconducting polymer systems near the M-I transition show that the mobility edge and theFermi level are situated rather close to each other, and slight variations in the overlap ofthe wavefunctions of the delocalised states by magnetic field, disorder, etc., could alterthe electronic properties of the system.

The negative magnetoresistance (MR) in metallic (CH)x and PPV samples is quite sensitiveto the extent of disorder [18, 20]. Even in the case of oriented metallic samples of (CH)x

Figure 2.14 W versus temperature plot for various conducting polymers in the criticalregime of M-I transition: unoriented I-(CH) x (open circle), oriented I-(CH) x (open square),

oriented K-(CH) x (dark square), PPy-PF6 (dark triangle) and PANI-CSA (dark circle)(Reproduced by permission from R. Menon, C.O. Yoon, D. Moses, Y. Cao and A.J.

Heeger, Synthetic Metals, Elsevier Science SA, 1995, 69, 1-3, 329)


60


Figure 2.15 W versus temperature plot for a PPy-PF6 sample in the critical regime: pressure-induced transition to the metallic side and the field-induced transition to the insulating side

(Reproduced by permission from R. Menon, C.O. Yoon, D. Moses, Y. Cao and A.J. Heeger,Synthetic Metals, Elsevier Science SA, 1995, 69, 1-3, 329)

Figure 2.16 Field-induced transition from metallic to critical regime: (a) W versustemperature for a metallic PPV-H2SO4 sample (E from Figure 2.4) at 0, 5 and 8 T

fields and (b) conductivity versus T0.1 for same data

(Reproduced by permission from M. Ahlskog, R. Menon, A.J. Heeger, T. Noguchi, andT. Ohnishi, Physical Review B, American Physical Society, 1997, 55, 11, 6777)

61

and PPV, a marginal increase in disorder can easily suppress the quantum interferenceprocess involved in the WL contribution to negative MR [35, 45]. As disorder increasesand the system moves towards the critical and insulating regimes, the positive MRcontribution increases due to the shrinkage of the overlap of the wave function of thelocalised states. In conducting polymers, as the chain orientation decreases, thecontribution to positive MR increases. The MR is positive at all temperatures and fieldsin PANI, PPy, PMET, PEDOT, etc., since the chains are less oriented in these systemswith respect to (CH)x and PPV [20]. In these systems, as ρr increases the positive MRincreases, as shown in the case of PPy-PF6 samples in Figure 2.18 [36]. Hence, themagnitude of the positive MR can be used to obtain a rough estimate of the extent ofdisorder present in the system.

Usually the magnitude of the positive MR at very low temperatures (T < 4 K) is assensitive as the temperature dependence of conductivity for probing the extent of disorderpresent in the sample. The typical values of the magnitude of positive MR at 1.4 K and8 T field, in the metallic, critical and insulating regimes are less than 5%, between 5%and 20%, and larger than 20%, respectively [18]. The large positive MR in the VRHregime is due to the shrinkage of the overlap of the tails of the wavefunctions of thelocalised states, and the hopping transport becomes more difficult at higher fields and

Figure 2.17 Field-induced transition from critical to insulating regime for PANI-CSAsamples: resistivity versus T–1/4 plot. H = 0 T (dark dot) and H = 10 T (dark diamond)for a sample with ρ(T) ∝ T–0.26 and H = 0 T (dark square) and H = 8 T (dark triangle)

for a sample with ρ(T) ∝ T–0.36

(Reproduced by permission from R. Menon, C.O. Yoon, D. Moses, A.J. Heeger, andY. Cao, Physical Review B, American Physical Society, 1993, 48, 24, 17685)


62


lower temperatures. In the VRH regime, the low field MR follows a H2 dependence, andthis can be used to determine the localisation length in the insulating regime. If thelocalisation length is of the order of few hundreds (few tens) of Angstrom then the extendof disorder is relatively low (high) [56]. Hence, the MR data is complementary to thelow temperature conductivity data in probing the extent of disorder present in criticaland insulating systems.

Although transport property measurements, like thermoelectric power, specific heat, etc.,are important in the critical and insulating regimes, they are not as sensitive as the lowtemperature conductivity and MR. The linear temperature dependence of thermoelectricpower and a linear term in the specific heat (which are typical for metallic systems) havebeen observed in the insulating regime too. As the system moves into the insulating sidethe hopping contribution to thermoelectric power (Shop ∝ T1/2) dominates over the metallicdiffusion thermoelectric power (Sdif ∝ T) [56]. Usually the negative hopping contributioncan be estimated by subtracting the diffusion contribution, and this gives an estimate ofthe extent of disorder present in the macroscopic level. The gradual variation of S(T)from the positive linear temperature dependence to the negative temperature dependencecan be correlated with the microstructure, as the system moves to the highly disorderedgranular, metallic islands. A typical example of this feature is shown in Figure 2.19 [20,56]. These features in S(T) can be used to obtain a qualitative picture about themacroscopic scale disorder present in conducting polymers.

Figure 2.18 Magnetoresistance versus resistivity ratio (ρr) for various PPy-PF6 samplesfrom metallic to insulating side

(Reproduced by permission from C.O. Yoon, R. Menon, D. Moses, and A.J. Heeger,Physical Review B, American Physical Society, 1994, 49, 16, 10851)

63

The Curie term in χ(T) is rather dominant at low temperatures for systems in the criticaland insulating regimes, and the temperature independent Pauli term is usually observedat temperatures greater than 100 K. However, χ(T) is not as sensitive as σ(T) and MR foridentifying the metallic, critical and insulating regimes near the M-I transition [17].

2.4 Summary

This brief overview of the electrical transport properties in doped conducting polymershighlights the following points:

(1) Conducting polymers are rather complex systems, in which the structural andmorphological features influence the electrical and optical properties significantly. Thecharge transport properties, the M-I transition, etc., are strongly governed by bothmicroscopic and macroscopic level structural disorder, doping-induced disorder, etc.

Figure 2.19 Hopping contribution to thermoelectric power for various doped PANI samples(Reproduced by permission from C.O. Yoon, R. Menon, D. Moses, A.J. Heeger, Y.

Cao, T.A. Chen, X. Wu and R. D. Reike, Synthetic Metals, Elsevier Science SA, 1995,75, 1-3, 229)


64


(2) In general, the effective conjugation length, interchain interactions and morphologyare the important parameters that influence the physical properties, disorder-inducedlocalisation, charge transport mechanism, etc.

(3) In doped conducting polymers, both localised and delocalised states coexist as aninterpenetrating network. In oriented materials, the electronic states are delocalisedalong the chain direction, as a result the conductivity, the carrier mobility, etc., arehigher with respect to unoriented systems. In unoriented globular or granularconducting polymer systems, the localised states determine the charge transportproperties.

(4) The molecular structure, the doping level, interchain interaction, the extent of disorder,etc., determine the stability of solitons, polarons, bipolarons, free carriers, etc., indoped conducting polymers.

(5) A metallic state has been observed in high quality samples of doped (CH)x, PPV,PANI, PPy, PMeT and PEDOT. The experimental evidence indicates the following:finite conductivity at mK temperatures, linear temperature dependence ofthermoelectric power, linear term in specific heat, temperature independent Paulisusceptibility, quantum corrections (weak localisation and e-e interaction) to MC,metallic reflectance and free carrier absorption in the infrared.

(6) The metallic positive TCR has been observed from 300 to 1.5 K in ClO4-doped(CH)x. In PANI-AMPSA (CSA) the positive TCR is observed at temperatures greaterthan 70 (150) K. In several metallic conducting polymer samples a positive TCR hasbeen observed at temperature below 20 K. These features show the intrinsic metallicnature of doped conducting polymers.

(7) The √T dependence of conductivity, at low temperatures, in metallic conductingpolymers indicates that the e-e interaction contribution is significant. The universalscaling behaviour of the MC confirms the dominant role of the e-e interactioncontribution.

(8) In oriented metallic systems, the anisotropic MC due to the interplay of WL and e-einteraction contributions (i.e., the sign of MC is positive (negative) when the field isperpendicular (parallel) to the chain axis) can be used to probe the misaligned chains.

(9) The behaviour of σ(T) and MC is nearly identical in both parallel and perpendiculardirections to the chain axis in highly oriented metallic (CH)x and PPV. This suggeststhat the charge transport mechanism is nearly identical in both parallel andperpendicular directions to chain axis, and the system behaves more like an anisotropic3D system.

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(10) The metallic, critical and insulating regimes can be identified from the W versus Tplots. The positive, temperature-independent and negative temperature coefficientsof W(T) corresponds to metallic, critical and insulating regimes, respectively.

(11) In the critical and insulating regimes, the resistivity ratio and positive MR increasesas the extent of disorder increases. The field-induced transitions, from metallic tocritical and from critical to insulating regimes, show that the mobility edge andFermi level are situated rather close together. Hence, due to interchain transportand disorder, conducting polymers are at the M-I boundary.

(12) From the hopping contribution to the S(T) and the Curie term in χ(T), a semi-quantitative level of information about the extent of disorder can be obtained.

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3 Non Linear Optical Properties of Polymersfor Electronics

T. Kaino

3.1 Introduction

The upcoming information technology (IT) era of the 21st century requires processing oflarge amounts of information accurately at fast speed. Signal processing by opticaltechnology will be a key for that requirement because high-quality and high-speed digitalsignals can be processed effectively using optical systems. Optical technologies are notonly for long haul signal transmission and processing applications, but also for shortdistance mutual communication applications, like local area networks. Within a coupleof years, individual subscribers will receive information transmitted via optical fibrecable, moveable terminal unit, satellite, and cable network. To process such information,high-speed optical switching devices and parallel signal processing devices are required.So, there is a growing interest in the transmission and processing of digital informationusing non linear optical (NLO) devices. In these optical systems, optical interconnectionand optical device installation in an integrated substrate is important to overcomebottlenecks. For practical optical signal processing applications, optical devices shouldbe gathered together in an integrated substrate with optical waveguide structure to controloptical signals with low input power.

Among the many types of NLO materials, NLO polymers are the most attractive becausethey may be used to fabricate optical waveguides by standard photoprocesses and it ispossible to enhance optical non linearities by synthesising a variety of NLO chromophores(dyes) and attaching them to a polymer backbone. NLO polymer devices can be fabricatedon planar passive waveguides, such as glass or polymer waveguides, since these can beused to process optical signals with a high data transmission rate. NLO polymerwaveguides offer the potential to create highly complex integrated optical devices andoptical interconnections on a planar polymer substrate because their optical propertiescan be tailored by using different types of passive materials and NLO active materials[1]. The other advantage of using polymers is that they are low in cost and it is possibleto fabricate monolithically integrated optical circuits where all the optical devices arecombined in one step.

The potential of the combination of a signal processing function (active function) usingNLO polymers with a signal transmission function (passive function) of transparent

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polymers is the most appealing prospect for waveguides made from polymers. For example,a silicon wafer area network using polymer-integrated optics has already been described[2]. This device is based on a combination of an active reconfiguration function with apassive transmission function.

In this chapter, the development of NLO polymers with thin film or channel waveguidestructures directed toward practical optical devices will be discussed. All optical signalprocessing polymers and electro-optical polymers are presented. Hybrid polymer opticaldevices for future applications will also be presented.

3.2 NLO Polymer Issues for Device Applications

The chemical and physical properties of polymers, the type of optically active materials(i.e., chromophores or dyes) and the polymer device fabrication processes used decidethe applicability, advantages and limitations for creating practical optical devicesusing NLO polymers. If NLO polymers are used as waveguide device materials, thereexists a trade-off between their optical non linearity, the optical transmission lengthand their processibility [3]. Since a high concentration of NLO dyes in a polymerbackbone are needed to achieve a large NLO function, optical loss will increase dueto π-π* transitional absorption of the dyes. Hence, the reduction of transmissionloss of the dye-functionalised polymer must be balanced against the enhancement ofoptical non linearity. Processibility of the NLO polymer will also be influenced bythe concentration of the dye, and a highly NLO dye-functionalised polymer usuallybecomes difficult to process.

Spin coating or casting techniques on substrates such as silicon, glass and transparentpolymer are typically used to create polymer thin films for waveguides. The inherentchemical and physical properties of the matrix polymer decide the fabrication process.To fabricate polymer channel waveguides with optical non linearity, thephotolithography technique is a basic method to define device patterns in the polymer.After patterning, reactive ion etching (RIE) is an excellent example of waveguidefabrication technology that can be compared to the solvent etching process. Waveguideswith buried structure created via these techniques comprise of two types of compositions:(1) an etched groove backfilled with a NLO polymer, and (2) an NLO ridge waveguidesurrounded by a lower refractive index cladding polymer. The problem of trying toreduce the waveguide fabrication cost is a highly complicated process, because manysteps are required, including a mask fabrication process. To solve the problem, directpatterning of the waveguides by using electron beam or laser beam has already beeninvestigated [4]. The embossing process is also used to fabricate polymer waveguidesthrough an inexpensive moulding technique [5].

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Non Linear Optical Properties of Polymers for Electronics

High transparency with excellent processibility is needed for NLO dye-functionalisedpolymers for waveguide applications, though they suffer from high optical loss in thenear-IR region. This is because NLO chromophores usually have π-π* transitionabsorption at visible wavelength which influences the near-IR absorption of thewaveguides. When discussing the transparency of a polymer, not only the absorption ofdyes should be considered, but also the vibrational higher harmonics of the polymer IRabsorption [6]. Almost all the passive polymer optical waveguides developed so far havean optical loss of higher than 0.5 dB/cm in this region (except for deuterated and/orfluorinated polymer waveguides) [7]. The deuteration and fluorination of the hydrogenin the polymer is very effective for reducing the vibrational higher harmonic absorptionloss of the polymer in the visible to near-IR region [8]. Transparency in the near-IRregion rather than in the visible region is sometimes important because wavelengths of1.3 and 1.55 μm are used in optical telecommunication systems. Thus the reduction ofthe loss in the near-IR is a critical issue for the telecommunication applications of theNLO polymer waveguide devices.

The stability of the NLO polymer waveguides in high-temperature conditions is also amain issue for practical applications. As will be discussed in Section 3.4, the alignmentof the dye in the polymer matrix which induces the second-order optical non linearitywill be lost in high-temperature conditions. Reliability, reproducibility, acceptable costperformance, and compatibility with other optical systems are also important factors forthe development of a NLO polymer waveguide device.

3.3 Properties of Third-Order NLO Polymers

3.3.1 Background of Third-Order NLO Polymer Research

Liquid crystal devices and thermo-optical polymer devices are already being used as switchingcomponents with a millisecond response time. Practical optical switching devices withnanosecond to picosecond response times have not been developed so far. For pico- to sub-picosecond all-optical signal processing systems, the development of highly efficient third-order NLO materials with processibility is required. Delocalised π-electron conjugatedpolymers are of great interest because of their potentially fast response time: faster thantens of femtoseconds. In view of practical use, polymers are expected to overcome thedisadvantages of molecular crystals in mechanical properties and processibility. Variouskinds of π-electron conjugated polymers have been energetically studied to achieve higherthird-order NLO efficiency because many π-conjugated structures are possible and henceabsorption wavelengths can be controlled to achieve high efficiency via resonant effect.Highly efficient third-order NLO polymers can be applied to many optical devices such asthin film and fibre waveguides. In particular, thin films with large optical non linearities

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have many useful applications in integrated optics. These include optical switching andoptical signal processing that show applied optical field intensity dependent refractive indices.Among the most studied films are polyacetylene and polydiacetylene (PDA), which havebeen reported to show third-order NLO susceptibilities, χ(3), of the order of 10-10 to 10-9

esu [9, 10]. The non linear electronic polarisation of π-electron conjugated polymersoriginates from mesomeric effects that depend on the size of the π-conjugated systems. Thelength dependence of χ(3) for polyenes and polyynes has already been discussed and it hasbeen revealed that the hyperpolarisabilities increase rapidly with chain length up to 10repeating units and approach an asymptotic value after about 15 repeating units [9].Although large χ(3) values have been observed in the crystalline states or uniaxially orientedfilms of these polymers, the high crystallinity or uniaxial domain results in large lightscattering losses. These polymers are usually difficult to process because they are rarelyfusible and are insoluble in almost all solvents. For example, one of the representative π-electron conjugated polymers, poly [bis(p-toluenesulfonate) of 2,4-hexadiyne-1,6-diol] (PTS-PDA), shows a large χ(3) of around 10-9 esu [10]. However, it has problems in processibilityand stability. High crystallinity and low processibility prevent its use in optical waveguidedevices. Therefore, other types of amorphous π-conjugated polymers with large χ(3) andgood processibility are needed. Jenekhe and co-workers have reported several novel mainchain χ(3) polymers with aromatic Schiffs base structure [11]. Spangler and co-workershave also reported several main chain χ(3) polymers with vinyl structures containing π-electron conjugations in their backbones [12].

By considering this previous research, several amorphous or low crystallinity conjugatedpolymeric NLO materials are presented in the following sections.

3.3.2 Poly(arylenevinylene), PAV

PAV electrically conductive, tough, good optical quality, thin films with good transparencyare easily obtained from the precursor polymers. PAVs exist with variations in chemicalstructure: PPV, PTV and poly (2,5-dimethoxy-p-phenylene vinylene) (MOPPV) [13, 14].PAVs are of special interest as electroluminescent polymers because, by changing theirchemical structure, the luminescent wavelength can be dramatically controlled from greento red [15]. NLO properties are also an interesting feature of the PAVs because their precursorpolymers are soluble in organic solvents or water. Hence, processibility for fabricating thinfilms by spin coating can be achieved without any modifications of the conjugated length.

Good optical quality thin films of PAV have been prepared through a new precursorpolymer that is soluble in organic solvents [16]. For example, PTV thin film can befabricated as follows. Polymerisation of a sulfonium monomer, 2,5-thienylenebis(methylene-dimethyl-sulfonium chloride), is carried out in a methanol-water mixture

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at –2 °C by adding a methanol solution of tetramethylammonium hydroxide. The reactionis quenched by the addition of hydrochloric acid. A yellow precipitate (precursor polymer)appears as the solution is warmed to room temperature. A precursor polymer thin film isobtained by spin coating of the dichloromethane solution of the precursor polymer ontoa substrate under inert atmosphere to prevent oxidation with air. The film is heated at200-250 °C in a vacuum of 10-2 Torr for 5 hours, to give a tough, flexible PTV film. Theresulting PTV thin film is chemically stable in air below 10 °C.

The thermal conversion process of PAV precursor polymers to fully π-conjugated finalstructure is shown in Figure 3.1, where A is an arylene unit and X is a halogen. Theconversion proceeds from a non π-conjugated precursor polymer to a fully convertedpolymer via partially converted copolymers. At the initial conversion level, short linearsequences are formed. These copolymers have both π-conjugated and non π-conjugatedsequences for various conversion levels. At the higher partial conversion level, the shortsequences have developed into long rigid sequences. At the final conversion level, thedevelopment of the long rigid π-conjugated sequences is completed, and even the highlybent non π-conjugated parts between the rigid π-conjugated sequences are converted.Controlling temperature and heating time can vary the π-conjugation lengths in PAV.The conversion dependence of the MOPPV χ(3) spectra superimposed onto that of theabsorption spectra is shown in Figure 3.2 [17]. As the conversion proceeds from A to F,an increase in absorption intensities and red shift of the absorption maxima is observed.Conversely, the χ(3) peaks are fixed over all conversion levels. The wavelengths of the χ(3)

peaks are 30 nm longer than the absorption peak wavelength of the fully converted film.The steep enhancement of χ(3) from D to F associated with the development of π-conjugatedsequences is notable. The magnitude of the χ(3) values of the fully converted MOPPVfilm (F) reaches 1.6 x 10-10 esu. The χ(3) increases due to the three-photon resonant effectat the final conversion level where the long rigid π-conjugated sequences are formed.The position of the absorption peak is decided by the maximum sequence present, butthe χ(3) peak always appears at the same wavelength. The χ(3) peak and the absorptionpeak match each other only when the fully converted state is obtained.

Figure 3.1 Thermal conversion process of PAV precursor polymers to fully π-conjugated

final structure


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The optical absorption maxima are revealed to be at 520, 500 and 420 nm for PTV,MOPPV and PPV films, respectively. X-ray diffraction patterns indicate that the PAVfilms have low crystallinity. The χ(3) values of these PAV films evaluated by third-harmonic generation (THG) measurement at 1.85 μm are found to be 5.85 x 10-11 esu,3.2 x 10 -11 esu, and 7.8 x 10-12 esu for PTV, MOPPV and PPV films , respectively.These values are non resonant χ(3) values and are almost the same as that reported fora processible polydiacetylene (n-butoxycarbonylmethyl urethane) (n-BCMU-PDA,n = 4,3) thin film [18].

The incident light wavelength dependence of χ(3) revealed that as the wavelength decreases,χ(3) increases. This was caused by three-photon resonant enhancement effects due to theoverlap of the third harmonic wavelength with a polymer absorption band. The maximumχ(3) of PPV film, 1.4 x 10-10 esu at 1.475 μm, is about one order higher than that of nonresonant values. The χ(3) values are almost the same for PPV, MOPPV and PTV in theabsorption wavelength region. Thus, dispersion measurements are required in theabsorption energy range to clarify the nature of the resonance and to determine themaximum resonantly enhanced χ(3). The evaluated three-photon resonant values of χ(3)

of the PAV films are almost of the same order of magnitude as that of crystalline PDAfilms. This result indicates that PAV thin films will be effective for NLO device application,due to their ease in processing films as well as possible good optical quality.

Figure 3.2 Conversion dependence of the MOPPV χ (3) spectra superimposed on their

absorption spectra

(Reproduced with permission from [17], copyright 1991, Elsevier Science Publishers)

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3.3.3 n-BCMU-PDA

PDAs have been widely investigated as potential materials for all-optical switching devices.For these purposes a large non linear refractive index (n2) and processibility are two ofthe most important features. Soluble PDA, such as 4BCMU-PDA and 3BCMU-PDA areof special interest for the application of polymeric waveguides with high optical nonlinearity. The χ(3) values of amorphous and crystalline films of 4BCMU-PDA have beencompared using THG measurements. The orientation effect due to the crystallisation ofthe polymer is revealed to be approximately a factor of five, i.e., the χ(3) values of thespin coated film is about 10-11 esu at 1.06 μm and that of the crystalline film is 5 x 10-11

esu [19]. Using a degenerated four wave mixing (DFWM) technique with backwardinteraction geometry, χ(3) values of the red form and the yellow form of 4 μm thick4BCMU-PDA film are revealed to be 4 x 10-10 esu and 2.5 x 10-11 esu, respectively. Thedifference of these values is explained to be due to a conformational transition from anextended rigid coil in the red form to a random coil in the yellow form [20].

Using an inverted rib design, single mode channel waveguides of 4BCMU-PDA and 3BCMU-PDA spun films have been fabricated. Guided light is predominantly in the PDA layer withthe glass channels, which provides lateral confinement [21]. Single-mode planar waveguidesmade from these films have propagation losses as low as 1 dB/cm. Using this technique,directional couplers and grating couplers are fabricated, which are potentially attractive forapplications in all-optical signal processing. At a wavelength near the two-photon absorbance,the large imaginary component of n2 dominates the response of the directional coupler [22].Assanto and co-workers fabricated an efficient grating guided-wave coupler using 4BCMU-PDA slab waveguides, and a coupling efficiency of 45% at 1.064 μm was obtained [23].Using this grating coupler, waveguide losses measured by the total waveguide throughputover given distances (longer than 1 cm) were revealed to be larger than 5 dB/cm for 0.65 μmthick film at 1.064 μm. This large loss may be due to non uniformities in film thickness andrefractive index. Minimisation of the loss is needed for actual device applications.

Strong self-lensing effects have been observed through thin film slab waveguides formedwith 4BCMU-PDA using a 1.0 μm mode-locked Nd:YAG laser which provides single 35 pspulses. The estimated value for n2 is approximately 10-13 cm2/W [24]. Using a pump andprobe technique for measuring n2 of thin films of spun 4BCMU-PDA at 630 nm, the real partof n2 is reported to be negative with a magnitude of around 6 x 10-12 esu. If the alignment ofthe polymer chain is attained, the value is expected to be almost the same as PTS-PDA. Themechanism for the non linear index is thought to be phase space filling by excitons [25].

Channel waveguides of PTS-PDA single crystal on a Si/SiO2 substrate have been fabricatedby a photolithography technique [26]. The value of n2 in the PTS-PDA chain axis directionis found to be 3 x 10-11 cm2/W at 1.06 μm. There will be a considerable contributionfrom two-photon resonance in this system.


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3.3.4 PT

Among the class of π-conjugated polymers, PT is of considerable interest because a sulfur-containing heterocyclic backbone increases the rigidity of the polymer and hence the π-electron localisation. Soluble PT is synthesised by adding alkyloxy or alkyl groups into athiophene ring, such as 3-alkyloxymethoxy thiophene or 3-methyl-4´octyl-2,2´-bithiophene-5,5´-diyl (3MOT). THG measured as a function of wavelength for poly(3MOT) is revealedto be 4.4 x 10-11 esu at 2.4 μm, i.e., in the preresonant region [27]. The NLO properties ofpoly(3-butylthiophene), which is soluble in common organic solvents, were investigatedusing DFWM at 1.06 μm. The response time was less than 70 ps, limited by the laser pulsewidth. In the case of the solution of the polymer in chloroform, the position of a three-photon resonance was revealed to depend on the concentration of the polymer [28].

Prasad and co-workers studied poly(3-dodecylthiophene) using a 60 fs pulse DFWMand obtained a resonant χ(3) of 5.5 x 10-11 esu at 660 nm, which is about six timessmaller than that obtained with 400 fs pulses [29]. The effective χ(3) is less when theexcitation pulse duration is shorter than the relaxation time of the excitation which givesrise to the non linear response. They concluded that the short non linear response obtainedwith pulses less than 200 fs is derived from initial photogenerated excitons. Jenekhe andco-workers reported an organic polymer superlattice using a soluble conjugatedpolythiophene derivative [30]. From DFWM measurements in a phase conjugate geometryfor the sulfuric acid solution of the polymer, a χ(3) of 2.7 x 10-7 esu at 532 nm wasestimated. For the experiment 250 mJ pulses with 25 ps full width at half-maxima wereused. The response time of the polymer is said to be faster than the normal instrumentresponse of 25 ps.

3.3.5 Processible π-Conjugated Polymers

Apart from typical π-electron conjugated polymers such as PAV, n-BCMU-PDA and PT,processible main-chain polymers with different types of π-conjugation structure havebeen investigated. Three types of main-chain polymers will be discussed: symmetricallysubstituted benzylidene aniline with chloride (SBAC) polymer, azo-dye polyester, andheteroaromatic polymers. The main chains of these polymers possess donor substituentsat both ends of their π-electron conjugated unit [31], a donor-acceptor structure in theirπ-electron conjugated unit [32], and a π-deficient aromatic ring that is alternatelyconnected with a π-rich aromatic ring [33], respectively. They are easy-to-fabricateamorphous solids made by conventional spin coating or casting methods from solution.By using THG measurements, the wavelength dependence of χ(3) for these main chainpolymers was investigated and the resonant enhancement of χ(3) revealed. Waveguidingproperties of the polymers were also investigated.

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SBAC polymer is a novel symmetrical molecule with a large χ(3). SBAC is a benzylideneaniline derivative in which donor groups with nitrogen atoms symmetrically substituteboth ends of the π-conjugated unit. This novel molecule was designed to control thetransition dipole moment, P, between the excited states and enhance the molecularhyperpolarisability, γ, as shown in Figure 3.3. The transition dipole moment between the1Bu and 2Ag excited states is enhanced by symmetrical donor substitution of the relativelyshort π-conjugation of benzylidene aniline [34]. This idea is different from donor-acceptorsubstitution based on a two-level model, such as the azo-dye polyesters. In this donor-acceptor system, the χ(3) of short π-conjugation molecules depends on dipole momentchange between the ground state (G) and the 1Bu excited state due to the intramolecularcharge-transfer effect from donor to acceptor.

Figure 3.3 Transition dipole moment between the 1Bu and 2Ag excited states of SBAC

A strong χ(3) enhancement of the vacuum-deposited SBAC polymer thin films was observedwhen the incident wavelength is three times the wavelength of the lowest energy peak in theabsorption spectra. This three-photon resonant χ(3) reached approximately 2 x 10-10 esu. Evenin the off-resonant regions, the χ(3) was 4 x 10-11 esu. These values are comparable to those ofnon crystalline or non oriented π-conjugated polymers. The control of transition dipole momentsby donor/acceptor substitution or heteroatom introduction into π-conjugated systems is aneffective way to enhance the χ(3) of the materials. However, it is difficult to obtain thin films ofthe vacuum-deposited SBAC polymer with good optical quality. Therefore, SBAC polymerswere synthesised where SBAC molecules are used as a part of the polymer main chain [35]. Thesynthetic scheme for the SBAC polymers is shown in Figure 3.4. SBAC polymers are easilyobtained by polycondensation reaction from terephthalic aldehyde and N, N-dibutylaminoderivative of SBAC. As a result, the SBAC molecule was combined with an alkyl chain or with


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an ester chain. Since the SBAC polymers are soluble in organic solvents, good optical qualitythin films were obtained. Since SBAC polymers have an absorption in the third-harmonicwavelength region (l = 0.50 to 0.60 μm) of the laser beam used here, the χ(3) values in theresonant region were evaluated. All polymers exhibit χ(3) values of 10-10-10-11 esu in thisresonant region. The wavelength dependence of χ(3) of polyester SBAC(II-c) thin film is shownin Figure 3.5. The maximum χ(3) value obtained is about 7 x 10-11 esu at 1.57 μm.

Figure 3.4 SBAC polymer synthetic scheme

Figure 3.5 χ(3) wavelength dependence of SBAC polymer (II-c)

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In an azo-dye-introduced main-chain polymer, the dye possesses an electron donorand an acceptor along the backbone (electric dipoles are arranged in a head-to-tailconfiguration) [36]. A mono-azo dye may easily be incorporated into the mainchain of a polymer system. The synthetic scheme of such a polymer is shown inFigure 3.6. The polymer is polyester with N, N-diethylaminonitroazobenzeneintroduced into its backbone. The polymer is soluble in common organic solventsand fabricated into a thin film using a spinning technique. The χ(3) spectrum of thepolymer is shown in Figure 3.7 along with its absorption spectrum. The largest χ(3)

is in the resonant region with a value of 2.0 x 10 -11 esu. For the polyimide (PI)system with π-electron conjugation units in the backbone without a donor/acceptor,χ(3) was of the order of 10-11 esu even for the four benzene ring π-electron conjugationsystem [32]. The value thus obtained for the azo-dye-introduced main-chain polymeris very high even though the dye used is a short π-electron conjugation system (i.e.,a two benzene ring mono-azo dye). This is because the main-chain polymer has avery high dye density, as recognised from its chemical structure. An intramolecularcharge transfer derived from a substituted donor and acceptor also contributes tothe increase of χ(3). A value of χ(3) larger than 10-10 esu was achieved for anintramolecular charge transfer dye-attached three benzene ring polymer system [37].Thus, it should be emphasised that a large χ(3) is possible not only in π-electronconjugated polymers, but also in short π-electron conjugated systems like an azo-dye-introduced main-chain polymer by modification of molecular structure. Thesemain-chain polymers show promise for the development of processible materialswith a large χ(3).

Figure 3.6 Azo-dye-functionalised main-chain polymer synthetic scheme


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A different type of processible π-conjugated polymer is a heteroaromatic polymer withπ-rich (electron donating) heteroaromatic rings and π-deficient (electron accepting)heteroaromatic rings which are alternately combined to form the polymer [33]. Theseheteroaromatic polymers are periodically sequenced copolymers with well-defined polarmonomer units, and include polythiophenediyl-pyridinediyl (PTPY), polythiophenediyl-bipyridinediyl (PTBPY) and polyphenylenediyl-pyridinediyl (PPPY). Their chemicalstructures are shown in Figure 3.8. The most important feature of these polymers is theintramolecular charge transfer from the π-rich rings to the π-deficient rings [38]. Thecharge transfer between heteroaromatic rings in the π-conjugated sequences is thoughtto influence certain excited states that contribute to the χ(3) enhancement. PTPY, in

Figure 3.7 χ(3) wavelength dependence of azo-dye-functionalised main-chain polymer

Figure 3.8 Molecular structure of heteroaromatic polymers

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particular, shows a red shift of the absorption maximum compared with poly(thiophene-2,5-diyl) and poly(pyridine-2,5-diyl), which suggests the occurrence of an intramolecularcharge transfer from thiophene rings to pyridine rings. The orbital distortion throughheteroatoms in the polymer breaks electron-hole symmetry to contribute to the χ(3)

enhancement. The existence of different heteroaromatic rings that induce the chargetransfer interaction on the π-conjugated sequences was thought to influence the propertiesof certain excited states, such as 1Bu and 2Ag, which contribute to the χ(3) enhancement.These heteroaromatic polymers are synthesised by nickel(0) complex polymerisation asreported elsewhere [39]. The synthetic scheme of PTPY is shown in Figure 3.9. Dissolvingthe polymers in formic acids and spin coating on a glass substrate allowed amorphouspolymer films to be prepared. Typical film thickness was 0.01-0.3 μm. THG measurementsof the polymer films were performed between 1.25 and 2.1 μm. The χ(3) value was fourtimes larger than the resonance value of amorphous 3BCMU-PDA and was comparableto those for PPV and the promising π-conjugated polymers for optical switchingapplication. The absorption and χ(3) spectra obtained for PTPY are shown in Figure3.10. A steep enhancement of χ(3) was observed at the fundamental wavelengths close tothree times the wavelength of the absorption maximum. In the off-resonance region,3hv/Eg, where hv is the photon energy of the fundamental wavelength and Eg is theoptical band gap energy, the χ(3) values for PTPY and PTBPY are larger than those ofPPPY, which has no π-rich thiophene ring. The maximum χ(3) value of PTPY is 2.8 x 10-10

esu at 1.525 μm fundamental wavelength (hn= 0.813 eV). Poly(1,4-di(2-thienyl)benzene),PDTB, another type of heteroaromatic polymer with no π-deficient pyridine ring, wasreported to have a χ(3) value of 1.1 x 10-11 esu in the three-photon resonant region and avalue of 0.2 x 10-11 esu at the non resonant wavelength [39]. This result, where χ(3)

values of PDTB are one order smaller than those of PTPY, suggests that the orbitaldistortion due to the intramolecular charge transfer between the π-rich thiophene ringand the π-deficient pyridine ring enhances the non resonant χ(3). The resonance χ(3) valueof PTPY is larger than that of PTBPY. Usually, the χ(3) value of π-electron conjugatedpolymers depends on the π-conjugation length, and elongation of the π-conjugation

Figure 3.9 Synthetic scheme for PTPY


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sequence reduces the optical band gap energy. The χ(3) value has been reported to beproportional to the minus 6th power of the optical band gap energy. Taking into accountthat Eg = 2.7 eV and 3.0 eV for PTPY and PTBPY, respectively, the χ(3) value for the(PTPY)/(PTBPY) system has been calculated to be 1.9. This value is consistent with theexperimental value of 1.8 at the near resonance wavelength corresponding to 3hv/Eg =0.825. Bipyridine units on the PTBPY polymer chain, which are twisted more stronglythan the periodic sequence of the five-membered thiophene ring and the six-memberedpyridine ring in PTPY, are considered to shorten the conjugation length and result in adecreased χ(3) value.

3.3.6 Third-Order NLO Polymer Waveguides

Among the many types of processible NLO polymers, polyurethane with symmetricalsubstituted tris-azo dye with fluorinated alkyl units (PSTF for short) was selected as awaveguide material. PSTF is a novel third-order NLO main-chain polyurethane withtris-azo dye incorporated into the main chain with a fluorinated alkyl backbone [40].This polymer is similar to the SBAC polymer family whereby a symmetrically substitutedπ-conjugated molecule was incorporated in a main-chain polymer. Azo dye is an effectiveNLO dye investigated by many researchers and the tris-azo dye is very special for NLOapplications. Fluorinated alkyl units are expected to be effective for reducing the polymer

Figure 3.10 χ(3) wavelength dependence of heteroaromatic polymers

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waveguide loss. The chemical structure of PSTF is shown in Figure 3.11, where Rrepresents fluorinated alkyl units. The refractive index of the NLO polymers should bewell controlled when single-mode waveguides are to be fabricated using the NLOpolymers. Although third-order NLO polymers generally have higher refractive indicesthan those of transparent polymers for the cladding layer, the PSTF has a moderaterefractive index because of its fluorinated alkyl chain. As a cladding material, UV-curedfluorinated epoxy resin was selected. It has an appropriate refractive index for fabricatingPSTF single-mode waveguides [41]. The channel waveguide of the PSTF was fabricatedapplying standard photolithography and RIE techniques. By using a silicone negative-type photoresist and a mask aligner, the mask pattern was transferred to the photoresistand a ridge waveguide was fabricated through oxygen plasma etching. By applying theUV-cured epoxy-cladding polymer on top of the ridge, a buried PSTF waveguide wasfabricated. The far-field pattern of the guided mode at 1.3 μm reveals that it is a quasisingle-mode waveguide. The losses of the waveguide at 1.3 and 1.55 μm were slightly large, 3.5and 4.5 dB/cm, respectively, even though a fluorinated alkyl chain was used as part ofthe matrix polymer. This was due to the high tri-azo dye content of the PSTF, 44 to 66mol percent, to the backbone polyurethane units. The non resonant χ(3) of the PSTFpolymer film measured by THG was approximately 2 x 10–11 esu at 1.55 μm. The nonlinear optical effect of the waveguide was also confirmed by detecting self phasemodulation (SPM) of the waveguide at this wavelength. From the result, the non linearrefractive index, n2, of the polymer was calculated to be 2.8 x 10-14 cm2/W [42]. Thisvalue was approximately an order of magnitude smaller than the value obtained byTHG measurement. This result was due to the strong two-photon absorption of thepolymer at 1.55 μm.

With respect to optical devices using NLO materials, an optical Kerr switch that uses thethird-order optical non linearity of the material is a simple method to attain ultrafastresponse optical switching. In this device configuration, switching gate power, Pπ, forthe π-phase shift of the signal beam at the signal wavelength λ, in the NLO waveguidewith the core area A, and the medium length L, is expressed as:

Pπ = 3 λ A/4Ln2 (3.1)

Figure 3.11 Molecular structure of PSTF polymer(Reproduced with permission from [17], copyright 1992, American Institute of Physics)


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It is important to switch optical signals with small gate power. To reduce the gate power, thecore area should be decreased, or the medium length and n2 of the medium should be increased.The characteristics of all-optical switching materials and the required optical switching powerfor the materials are shown in Table 3.1. Optical switching using the PSTF waveguide hasnot been attained so far owing to its strong linear and non linear absorption. This absorptionlimits the wavelength at which optical switching can be operated. It is thus important tothink about the trade-off between linear and non linear absorption and the χ(3) value ofpolymer waveguides when selecting the wavelength for optical switching.

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In the case of chalcogenide glass fibre with an n2 value of 2 x 10-14 cm2/W, a peak switchingpower of less than 1 W was obtained using a 3.2 metre fibre [43]. To realise the all-optical switching using polymer waveguides with less than 1 W switching power, thepolymer waveguide should have an n2 value two orders of magnitude higher than that ofthe PSTF waveguide or it must have one-order lower absorption loss as well as one-order higher n2. Much effort should be concentrated in developing NLO polymers withhigher optical non linearity with low loss for all-optical switching with allied voltagesimilar to that of glass optical fibre switches.

3.4. Properties of Second-Order NLO Polymers

3.4.1 Azo-Dye-Functionalised, Poled Polymers for Second-OrderNon Linear Optics

Azobenzene-dye-, stilbene-dye-, or polyene-dye-functionalised polymers have been investigatedas second-order NLO materials (χ(2) materials), showing efficient NLO characteristics withlow absorption loss [44]. In this case, the dye in the polymer should be aligned in a specificdirection to possess second-order optical non linearity. For that purpose, a high electric field

85

will be applied to the dye-functionalised polymers, i.e., electric field poling is necessary toobtain χ(2) polymers. The non linear electronic polarisation of the dye-functionalised polymersoriginates from mesomeric effects that usually depend on the size of the π-conjugated systemsof the dye. The π-conjugation length dependence of the second-order NLO hyperpolarisability,β, has been discussed by many researchers [45]. It has been revealed that β of the poledpolymer increases rapidly with chain length if the molecular planarity of the dye is maintainedand there will be no aggregation of dye in the polymer network [46]. These dye-functionalisedpolymers are easy to process as high-quality thin films for fabricating optical waveguides anda large χ(2) value (~10-7 esu) is easily obtainable. From these points, dye-functionalised polymersare promising χ(2) materials.

The polymer investigated here is a polymethylmethacrylate (PMMA) copolymerised withmethacrylate esters of a dicyanovinyl-terminated bisazo dye derivative. A nitro-terminatedversion of the bisazo dye derivative and a typical monoazo dye, Disperse Red 1 (DR1), derivativeis also discussed in [47]. These azo dyes are hereafter referred as 3RDCVXY, 3RNO2, and2RNO2, respectively. The molecular structure of 3RDCVXY is shown in Figure 3.12a.

Figure 3.12 (a) Chemical structure of 3RDCVXY bis-azo dye (b) Chemical structureof deuterated 3RDCVXY polymer


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Each dye molecule contains a dicyanovinyl group or a nitro group as an electron acceptor,and an ethylethoxyamino group as an electron donor. This donor-acceptor charge transferwill greatly contribute to β. The 3RDCVXY and 3RNO2 dye contain three benzene ringsconnected with azo groups, so their conjugated structure is longer than that of 2RNO2.

The glass transition temperatures (Tg) were 135, 100 and 80 °C for 3RDCVXY, 3RNO2

and 2RNO2, respectively. The maximum absorption wavelength of 515 nm (2.41 eV) for3RDCVXY was longer than those of 3RNO2 and 2RNO2, which were 500 nm (2.48 eV)and 470 nm (2.64 eV), respectively. The methyl substitution of the 3RDCVXY was veryeffective for increasing the dye content in the copolymer and as a result the dye contentwas nearly 2 times higher than those of the 3RNO2 and 2RNO2. These polymer films onglass substrates were poled using a parallel electrode poling technique. The poling directionwas vertical to the film surface. The χ(2) was determined according to the standard procedureand its wavelength dependence is shown in Figure 3.13. The χ(2) increases as the fundamentalwavelength decreases, corresponding to the absorption spectrum. This is caused by the χ(2)

enhancement effect of two-photon resonance near the absorption band. It was revealedthat the maximum χ(2) of the 3RDCVXY reaches 1.0 x 10-6 esu. This is 3 and 7 times largerthan those of the 3RNO2 and a typical inorganic NLO crystal, LiNbO3, respectively.

Figure 3.13 χ(3) wavelength dependence of 2R, 3R, and 3RDCVXY polymers

A thermal ageing test shows that the χ(2) of 3RDCVXY is stable even when kept at 80 °Cfor more than 6 months. The thermal and temporal stability make 3RDCVXY a viablesubstitute for the current inorganic electro-optical (EO) materials. The stability of these

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EO polymers at high temperatures is an important area of NLO polymer research. Oneway to increase the thermal stability of NLO polymers is a crosslinked polymer systemwhere NLO dye is covalently attached to the polymer network at more than one site. Abifunctional molecule, such as a molecule with an amino group and a (N-ethyl, N-hydroxyethyl)amino group, is one example, which reacts with a trifunctional isocyanuratecomonomer. This polymer is reported to be stable at 75 °C for more than three months.

Deuteration or fluorination will be needed even for NLO waveguides. It is necessary tofabricate channel waveguides with low loss to make NLO polymer devices such as phasemodulators that can be driven with low input power. To this end, a deuterated 3RDCVXYpolymer was developed for device application; its chemical structure is shown in Figure3.12(b) [48]. This polymer is composed of 3RDCVXY-attached PMMA and a transparentPMMA where all the hydrogen atoms are deuterated.

3.4.2 EO Polymers

One of the important features of second-order NLO materials for obtaining opticallyactive devices is an EO characteristic. EO polymer waveguide devices such as modulatorsand couplers are of special interest because response time faster than a nanosecond willbe obtained using the device, more than 5 orders higher than conventional liquid crystaloptical devices or thermo-optical waveguide switching devices. For such applications,azo-dye-functionalised poled polymers are promising materials and have been investigated[49]. Using an electric-field poling technique, efficient EO property with moderateabsorption loss is attained.

By using the deuterated 3RDCVXY polymer, a channel waveguide was fabricated. Thewaveguide fabrication process is shown in Figure 3.14 where standard photolithographyand RIE is used. The selection of cladding layer is important to obtain flatness of thewaveguide surface where the metal electrode for supplying the driving voltage will bedeposited. Through this process, a Mach-Zehnder (MZ) interferometer was fabricated.The EO coefficient (r-coefficient) of the waveguide was 26 pm/V at about 70 MV/mpoling voltage, and a half-wave driving voltage of 12 V of the interferometer was obtained[50]. The value is low compared to the LiNbO3 interferometer, whose r-coefficient wasapproximately 32 pm/V, though it is possible to enhance the value by applying a higherpoling electric field to the waveguide or by using a longer waveguide. From a practicalviewpoint, the value should be increased to more than 40 pm/V to reduce the drivingvoltage with transistor-transistor logic (TTL) level. By using the deuterated 3RDCVXYpolymer, a vertical coupling MZ interferometer was also fabricated [51]. These azo-dye-functionalised poled polymers have the potential to be applied to a variety of opticaldevices due to their processibility and low potential.


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3.4.3 Serially-Grafted Polymer Waveguides

To realise frequency conversion devices by using a second-order NLO process, the quasi-phase-matching (QPM) technique is an attractive method as it is easy to achieve frequencyconversion, such as second-harmonic generation (SHG), with high efficiency and to obtaina collimating light beam from the device [52]. Almost all the QPM devices reported todate use inorganic materials and there have been very limited reports on QPM frequencyconversion devices using second-order NLO polymers [53]. This is because the latter aredifficult to fabricate into a specific structure with precise periodic poling pitches, whichis an important factor in realising high efficient frequency conversion [54]. To obtainhigh efficiency from the QPM devices, a channel waveguide structure is desirable.

A serially grafted waveguide fabrication technique, where two types of polymer core arealigned by using photolithography and RIE techniques, is an effective method forfabricating a QPM waveguide. The fabrication process and fabricated polymer QPMwaveguide structure are shown in Figure 3.15. Electric poling is necessary to make thepolymer film NLO active. For fabrication of the QPM waveguide, deuterated 3RDCVXY(as NLO and active part) and a UV-cured epoxy resin (as a transparent part) are used asa waveguide core. The deuterated 3RDCVXY polymer exhibited a d33 (= 0.5 χ(2)) valueof 80 pm/V at 1.3 μm [49]. Changing the fluorination content of the resin can easilycontrol the refractive index of the UV-cured epoxy resin. This controllability made iteasy to form a periodically aligned structure composed of a poled 3RDCVXY polymer

Figure 3.14 Fabrication method for a core channel waveguide based on 3RDCVXY

89

and a UV-cured epoxy resin with almost the same refractive index. This refractive indexmatching between two polymers is important to reduce the Fresnel reflection at theinterfaces between two polymer cores.

The process shown in Figure 3.15 was used to fabricate the serially grafted polymerwaveguide. On top of a silicon substrate, a low refractive index UV-cured epoxy resinwas first spin coated as an undercladding layer of the waveguide. Then, after fabricatinga thin film layer of the NLO polymer and patterning it by photolithography, a core ridgeof NLO polymer was fabricated by an RIE process. After spin coating of the UV-curedepoxy resin on top of the NLO polymer core, the epoxy resin core ridge was fabricatedusing photolithography and a RIE process. The refractive index difference between theUV-cured epoxy resin and the NLO polymer was controlled to within 0.001. Then, aburied channel waveguide with periodic structure was fabricated by covering these coreridges with a low refractive index UV-cured resin. Although the oxygen plasma etchingrates of these two polymers are different, this overcladding layer covered the grooveunder the NLO polymer core ridge that was formed during the fabrication of the seriallygrafted waveguide structure. Finally, the NLO polymer in the waveguide was poled bysupplying a high electric field by using parallel electrodes. This poling process providedlarge second-order optical non linearity only to the periodical parts of NLO polymer inthe core layer. The periodic length (Λ) in the waveguide was controlled easily by changingthe mask pattern for the photoprocess. Using this technique, a QPM frequency conversionpolymer waveguide was successfully fabricated.

The loss of the NLO polymer and UV-cured resin of this QPM waveguide were 0.67 dB/cm and 0.53 dB/cm, respectively. The Fresnel reflection loss of the grafted interface was

Figure 3.15 Schematic of polymeric QPM waveguide fabrication method


90


0.005 dB/point [55]. By changing the periodic length, in the 10 to 90 μm range at intervalsof 1 μm, the phase-matched wavelength for SHG was controlled. SHG experiments wereperformed with 5 mm long QPM waveguides using a colour centre laser with 1.48-1.65μm as fundamental wavelengths. When the fundamental wavelength was 1.586 μm, theSHG intensity was strongest in the waveguide with a periodic length of 32 μm. Therelationship between the applied voltage and the SHG intensity at the wavelength isshown in Figure 3.16. The QPM waveguide had an efficiency of 4x10-1%/W/cm2, thelargest ever reported for a QPM polymer waveguide [56].

This hybrid waveguide fabrication technique can be applied for an optical samplingdevice and a compact all-optical switching device. The application of hybrid waveguideswill be key in optical signal processing technologies.

3.4.4 Refractive Index Grating Fabrication into Azo-Dye-FunctionalisedPolymer Waveguides

Waveguide grating (an important device in optical communication and signal processingsystems) has functions such as input-output coupler, wavelength filter and wavelengthdivision multiplexer. A variety of polymer waveguide gratings have already been proposed[57, 58]. An azo-dye-functionalised polymer is one of the candidates for fabricating

Figure 3.16 Relationship between laser power and SHG efficiency of the QPM waveguide(Reproduced with permission from [57], copyright 1996, American Institute of Physics)

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gratings using the photochemical reaction of the azo dye in the interface region of twolaser beams. This reaction can be explained by following two mechanisms.

The first mechanism is the reversible trans-cis-trans isomerisation of the dye where astrong interference beam was irradiated [59, 60]. A surface relief grating can be fabricatedthrough the movement of the azo-dye-functionalised matrix polymer through dyeconformational change due to isomerisation. These surface relief gratings based on theisomerisation mechanism were optically and thermally erasable. The second mechanismis irreversible photobleaching of the dye that occurs under higher energy irradiation. Inthis case, a π-conjugated system in the azo dye is broken and a permanent refractiveindex change of the polymer occurs [61, 62].

By the two laser beam interference process using 532 nm light from SHG of a continuouswave (CW) Nd:YVO4 laser as a light source, a high output efficiency, 35% or more, wasattained using 3RDCVXY polymer thin film, as shown in Figure 3.17 [64]. The efficiencywas monitored using a 0.5 mW 633 nm He-Ne laser by measuring the first-order diffractedlight power. Using atomic force microscopy, the thickness of the relief structure of thefilm was evaluated to be about 30 nm. This grating has a very thin relief structure, one-tenth or less than those of typical surface relief gratings with the same diffraction efficiency[59, 60]. So, the fabricated structure was not a relief grating but a refractive index grating.From the IR absorption measurement of the 3RDCVXY polymer film before and afterlaser irradiation, it was revealed that 532 nm laser light could effectively induce thephotobleaching of 3RDCVXY. The refractive index changes of the film byphotobleaching were measured at 633 nm, 830 nm, 1300 nm and 1550 nm wavelengths.

Figure 3.17 Diffraction efficiency of the grating with 1.6 μm spacing


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The refractive index decreased with increase of irradiation time at each wavelength. Therefractive index difference (Dn) of the film before and after 20 minutes irradiation was0.022, 0.009, 0.005, and 0.005 at 633 nm, 830 nm, 1300 nm, and 1550 nm, respectively.

To fabricate a waveguide grating, two simple methods were applied [63]. One waswaveguide fabrication by using an etched groove of an epoxy resin substrate shaped byusing a dicing saw. An azo-dye-functionalised PMMA solution was spin coated into thisgroove, then a UV-cured resin was spin coated on top of the waveguide, and a buriedwaveguide of the azo-dye-functionalised polymer was fabricated. The other method wasgrating formation on the buried waveguide through photobleaching by using two beaminterference light from a 532 nm SHG laser. A periodical refractive index grating structurewas fabricated onto this buried channel waveguide. In designing and fabricating thegrating, the combination of these simple techniques is an easier method than that oftypical fabrication processes such as photolithography and RIE method.

PMMA was used as a host polymer of the refractive index buried waveguide core andDR1 was added to functionalise this polymer. PMMA and DR1 were dissolved inchlorobenzene and the solution was filtrated through a 0.2 μm filter. A channel waveguidewas fabricated using the above-mentioned method. The thickness of the substrate wasapproximately 90 μm and the groove size was 41 μm in width and 44 μm in height.PMMA containing 10 wt% DR1 (DR1(10%)/PMMA) solution was spin coated on thegroove-shaped substrate. A UV-curable epoxy resin of equal composition to the substratewas spin coated on this DR1(10%)/PMMA buried waveguide layer, about 80 μm indepth. The refractive indices of the waveguide materials measured using a MetriconModel 2010 Prism Coupler at 1.3 μm wavelength were 1.500 and 1.466 for DR1(10%)/PMMA and the UV resin, respectively.

To evaluate the optical propagation loss of the fabricated buried waveguide a cutbackmethod was applied. A schematic of the waveguide endface cut by a dicing saw is shownin Figure 3.18. Light from a 1.3 μm laser diode was coupled to optical fibre, the core/cladding of which was 50/125 μm in diameter, and the fibre was butt-jointed to thewaveguide end face. A Ge photodiode was used as a detector. By varying the waveguidelength from 0.7 to 1.6 cm, the loss was evaluated to be 1.3 dB/cm. This loss was practicallyavailable for a few centimetres application of the waveguide, with a loss budget of ~3dB/cm. A refractive index grating was formed in a DR1 (10%)/PMMA thin film on aglass substrate. The light source and its intensity were the same as those of the 3RDCVXYphotobleaching reaction. The diffraction efficiency of the grating with 1.6 μm periodand saturated after irradiation for 20 minutes reached 6.2%. The refractive indexdecreased with increase of irradiation time at each wavelength measured. The change inrefractive index of the film before and after 20 minutes irradiation was 0.014, 0.006,0.005, and 0.004 at 633 nm, 830 nm, 1300 nm, and 1550 nm, respectively.

93

By using the above procedure, a refractive index grating was inscribed into the buriedwaveguide. Irradiation intensity, time and grating period were the same as those for thethin film grating. To detect output-coupled light via the grating, 1.3 μm laser diode lightwas coupled into the waveguide through optical fibre. By rotating a photodiode fromthe position perpendicular to the grating (0 degrees) to the end face where light comesout (90 degrees), output-coupled light angle and its intensity were measured. The output-coupled light angle was detected at approximately 70 degrees and the output-couplingefficiency was 10.2%, which was defined as the intensity ratio of light output from thegrating to light from the end face. The input-coupling characteristic was also measuredand the coupled light was detected at approximately 70 degrees. The coupling efficiencywas less than 1% because the waveguide surface flatness was not good.

The polymer waveguide grating was fabricated by the combination of using a groove-shaped substrate for buried waveguide fabrication and a photobleaching technique forthe grating fabrication. These simple fabrication methods are widely applicable to avariety of polymer systems in which refractive index change can be induced byphotobleaching.

3.5 Future Targets of NLO Polymers for Optical Device Applications

Currently, several technical problems exist in second- and third-order NLO polymers.As mentioned above, the trade-off between χ(2) and χ(3) values of polymer waveguides,linear and non linear absorption, processibility, and reliability should be carefullyconsidered. To address these issues, novel hybrid waveguides, i.e., a waveguide with thecombination of a signal processing function of NLO polymer and a signal transmissionfunction of a transparent polymer, should also be considered. Hybrid waveguides can befabricated by constructing the waveguide such that many optical functions are serially

Figure 3.18 A schematic of the end-face of a buried waveguide


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grafted or vertically integrated. The vertically integrated polymer device fabricated todate was a vertically stacked second-order NLO polymer directional coupler [54]. Thenon linear optical material used was a 3RDCVXY polymer. Standard photolithographyand RIE techniques were applied to fabricate the vertically integrated waveguides. Usingthe vertically stacked directional coupler, optical coupling characteristics similar to thatof an in-plane directional coupler were obtained. These hybrid techniques may be extendedto many types of functional device fabrication. The hybrid waveguide design can beapplied not only for second-order NLO waveguides but also for all optical switchingwaveguides by considering the role of the integrated polymers.

3.6 Conclusions

NLO waveguides for optical switches and optical modulators have been discussed, mainlyfocusing on the azo-dye-functionalised NLO polymers. To make the most of polymerprocessibility, a hybrid waveguide structure is proposed. It is anticipated that this newapproach of the application of hybrid waveguides will be a key for future advances inoptical signal processing technologies. These hybrid waveguide fabrication techniquescan be applied for EO waveguides and for χ(3) waveguides. The successful developmentof polymeric optical waveguiding devices will require much effort not only by NLOmaterial and device researchers, but also by optical system researchers.

Acknowledgements

The author would like to thank S. Tomaru, T. Kurihara, M. Amano, T. Watanabe, M.Hikita, Y. Shuto, M. Asobe, T. Hattori and T. Shibata for their contribution to theresearch work.

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4 Luminescence Studies of Polymers

B. Wandelt

4.1 Introduction

When radiation is incident on a material some of the energy absorbed by a chromophoreis re-emitted as light of a longer wavelength, which is in agreement with Stokes law [1-3]. There are many pathways for the deactivation of the absorbed energy. Two importantdomains characterise the radiation: intensity and frequency. The intensity depends onspectroscopic selection rules: the efficiency of competing non radiative, quenching andenergy transfer processes. The frequency depends on the nature of the emitting states,i.e., whether it is a singlet or a triplet state, and whether this is a complex formatted byan interaction with some other chromophore. Most of the significant features inluminescence studies have their origin in the fact that the electronically excited states areaffected by molecular interaction and molecular motion.

The photochemists [3, 4] suggested classifying polymers to distinguish between those inwhich the repeat unit contains a chromophore and those in which isolated chromophoresare attached to a polymer chain as an end group or minor component of a copolymer.That is important, since the main deactivation processes are bimolecular complexesthrough dimer formation, which is dependent on segmental motion of the polymer chain.In the solution state, the translational and segmental motions are very extensive but aremuch reduced in concentrated or bulk polymer systems. The macromolecules that becomeexpanded in a good solvent tend to contract in a poor solvent below the value of itsunperturbed dimensions, defined by Flory [5]. However, the translation and segmentalmotion of a polymer chain are temperature dependent, so that the thermally activateddynamics of the polymer chain can affect the mobility of the probe molecule.

It is one of the purposes of this chapter to demonstrate, by reviewing and summarisingthe data already available in the literature, that, due to photophysical features of polymers,fluorescence investigations are the methodologies suited to the study of environmentaleffects in a polymeric network. Since the fluorescence properties of a probe molecule,such as decay kinetics, are strongly affected by microenvironments, the fluorescenceprobe method can be used to probe the local mobility of polymer chains both in solutionand in the solid state. Fluorescence probe methodology can be applied for studyingmicrostructures and the morphology of a polymeric medium [6-7]. It can also be applied

100


in microheterogeneous systems, imaging processes, microlithography [8], latex beads[9], silica [10] and imprinted polymers [11]. Various fluorescent species (such asmonomers, ground-state and excited complexes) have been observed owing to thedifferences in the formation of molecular interaction between moieties of polymers, andonly some of these will be discussed here. Obviously the most important molecular probein polymer fluorescence is an excimer: an excited complex of two identical species, oneof which is in the excited state prior to complexation.

The present review begins with a brief description of the basic principles of luminescence[1-3], after which it deals with the fluorescence studies of polymers in solution and energytransfer in polymeric systems. Spectral and time dependent studies of polymers in the gelstate are discussed later, with an emphasis on phase transition and phase separation studies.

4.2 Basic Photophysical Deactivation Processes

The excess of energy taken up by light absorption is known to dissipate throughphotochemical and photophysical processes. Some of these are unimolecular whereasothers are bimolecular. Some of the unimolecular processes of energy dissipation followingthe absorption of light are depicted in a Jablonski diagram (Figure 4.1).

Figure 4.1 Jablonski diagram of the important unimolecular photophysical processesof excitation energy dissipation: absorption (A), fluorescence (F), phosphorescence

(P), internal conversion (IC), intersystem crossing (ISC), vibrational relaxation (VR).S0 is the singlet ground state, S1 and S2 are excited singlet states and T1 is the excited

triplet state.

101

Luminescence Studies of Polymers

The initial absorption raises the molecule to an excited singlet state (S2). However, theinternal conversion (IC) between excited singlet states is usually very rapid, so that themolecule relaxes to the ground vibrational level of the first excited singlet, S1, in times ofthe order of a picosecond. The energy may then be emitted as fluorescence, or intersystemcrossing (ISC) may take place to the triplet state, T1. The triplet state may then lose itsenergy and return to the ground state either by emitting phosphorescence or byradiationless transition to the ground state or by ISC from T1 back to S1 when the energydifference between them is small enough to allow the thermal activation. These processes,involving changes in multiplicity, are generally forbidden and require times frommillisecond to seconds.

The relative content of all the processes is dependent on the molecular and supermolecularstructure of the material as well as on the molecular motions, which in turn are affectedby temperature. Obviously, there will be higher energy singlet and triplet states, but incondensed phases these are more involved in photochemical reactions.

4.2.1 Luminescence

Fluorescence is defined as the emission from a transition between states of the samemultiplicity, usually from the lowest vibrational level of the first excited singlet state tothe ground state. A mirror-image relationship usually exists between absorption andemission spectra; the longest absorbed and shortest emitted wavelength usuallycorresponds to the 0-0 transition. If the spontaneous emission of radiation of theappropriate energy is the only pathway for return to the ground state, the average statisticaltime that the molecule spends in the excited state is called the natural radiative lifetime,τ0, which relates to the rate constant, k0, of the fluorescence in the following way:

τ0

0

1=k (4.1)

Each process competing with the spontaneous emission reduces the observed lifetime, τ,relative to the natural lifetime:

τ =+ ∑

1

( )k kF ii

(4.2)

where kF is the rate constant of fluorescence and

kii

∑ is the sum of the rate constantsfor all processes competing with emission.

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Equation 4.1 can be approximated to a useful expression relating τ for a molecule to themaximum extinction coefficient εmax (λ), determined from the absorption spectrum as afunction of wavelength (λ):

τ ε λF =

−10 4

max( ) (4.3)

This equation predicts an approximate value of 10-9 s at εmax ≈ 105 l mol-1 cm-2. Themeasured fluorescence lifetime is usually less than the predicted value. This is attributedto the presence of quenchers like oxygen and many other impurities occurring at aconcentration of about 10-3 M in the commonly used spectroscopic solvents. Fluorescenceis a comparatively fast process and the radiative fluorescence lifetime is usually in therange of 10-6 to 10-12 s.

Delayed fluorescence differs from ‘normal’ fluorescence in that the measured rate ofdecay of emission is less than that expected from the transition giving rise to the emission.Spectral distribution of the delayed fluorescence is similar to that of ‘normal’ fluorescence.However, its lifetime corresponds to the excited triplet state.

Phosphorescence emission is the result of a transition between states of differentmultiplicity (typically T1 to S0) that has a much smaller rate constant than that forfluorescence. Consequently, the natural lifetime, τ0

P , of the triplet state is long, varyingbetween 10-6 s and seconds. The natural lifetime can be formulated according toEquation 4.4.

τ0

1P

Pk= (4.4)

Vibrational relaxation (VR) from a vibrational level of a higher electronic state such asS2 to the vibrational ground state is very rapid. Before the molecule can react, thevibrational energy is quickly distributed among the various vibrations and can be partiallyor completely dissipated by collisions into heat.

IC takes place as a transition between two isoenergetic vibrational levels of differentelectronic states of the same multiplicity, which may have quite different energies at theirequilibrium geometries. The IC is used in a wider sense, encompassing vibratrionalrelaxation. It denotes radiationless transitions from the first excited singlet state into thevibrationally equilibrated ground states. The radiationless processes from the higher tolower excited states are usually very fast. The lifetimes of the higher excited states arevery short and quantum yields of emission from higher excited states are negligible.

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ISC differs from IC in that the transition takes place between states of different multiplicityas can be seen in Figure 4.1. IC and ISC processes are similar in that they involve aconversion of electronic energy into vibrational energy, which is followed by rapidrelaxation to the lowest vibrational level of the lowest excited state. These all are knownas radiationless processes.

4.2.2 Bimolecular Photophysical Processes

Considering the excitation originating from the singlet ground state of a molecule, whichis able to interact with a similar or a different molecule, there is a further possibility ofenergy dissipation involving the other molecule. This may result in the energy transferbetween the molecules. Additionally, it may cause interaction of the excited state of amolecule with the ground state of the other, resulting in the formation of an excitedcomplex. The competition between the processes of energy dissipation can be controlledby either thermodynamic or kinetic parameters, but the latter are of prime importance ina consideration of molecular motion and luminescence. As significant competition toradiation can come only from processes capable of occurring in the appropriate timescales,changes in fluorescence are likely to be caused primarily by the fastest molecular motions,whereas phosphorescence can be significantly affected by a very wide variety of relativelyslower processes.

Excimers are complexes/dimers of electronically excited molecules with molecules of thesame type in their ground state. They only exist in the excited state and they dissociate intomonomers upon radiative or non radiative deactivation in agreement with scheme shownin Figure 4.2. This phenomenon of association of chromophores is called concentrationquenching. Since the discovery of the pyrene excimer by Förster and Kasper in 1954 [12],

Figure 4.2 Birks’ scheme [1] of excitation and deactivation by bimolecular processes:kEM and kME are the rate constants of excimer association and dissociation,

respectively, kFM and kFE are the rate constants of fluorescence emission of excitedmonomer chromophore and excimer, respectively, and kIM and kIE are the rate

constants for non radiative energy decay of the monomer end excimer, respectively.


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these complexes were observed frequently with aromatic hydrocarbons. Excimers ofaromatic molecules adopt a sandwich structure with a separation distance of 0.30-0.35nm between them. Their fluorescence emission spectrum from the broad and structure-lessband has been found to be shifted to the lower energy (relative to the molecular emission)by 50-60 nm. Excimer fluorescence can be observed in solutions and in solids, if the crystaland/or material structure allow a close overlap of the molecular planes.

Exciplexes are complexes of two different molecules usually of 1:1 stoichiometry. Theirfluorescence phenomena are similar to those described for excimers, but their formationis not restricted to aromatic systems. If the sum of effective rate constants of the nonradiative processes is so high such that the lifetime of emission is undetectable, thesemolecules do not necessarily luminesce. In contrast to the excimers, which are non polar,the exciplexes are polar entities. It was shown by Beens and co-workers [13] that exciplexesfrom aromatic hydrocarbons and aromatic tertiary amines demonstrate the charge-transfercharacter of the complexes as reported by Knibbe and co-workers [14], and their dipolemoments were greater than 3.3 x 10-29 C m (10 D).

Birks [1] and Klessinger and Michl [2] have given a more detailed discussion of thephotophysics of organic molecules.

4.2.3 Quenching Processes

There are many quenching processes. Quenching by photochemical reaction refers to organicphotochemistry and will not be discussed here. Photophysical quenching, which does notlead to a new chemical compound, includes self-quenching through excimers and exciplexformation. The last photophysical process includes exciplex formation, but there are otherpossibilities like quenching by electron transfer or by energy transfer due to presence ofheavy atom. In complicated systems like polymers, the competition between the possibleprocesses is controlled by either thermodynamic or kinetic parameters, and naturally bothare important and interactive. Rate constants for energy transfer can be measured by excitinga chromophore with a fast light pulse followed by monitoring the dynamics of the excitedspecies. It can be measured from the competition between emission and the deactivationby an added quencher or acceptor. The Stern-Volmer equation (Equation 4.5) relates theexperimental values of the quantum yields of the process, Φ0 and Φ, in the absence andpresence of acceptor respectively, to the molar concentration of the quencher Q as:

ΦΦ

0 1= + [ ]k Qqτ (4.5)

where τ is the lifetime of the donor in the absence of additives, and kq is the quenchingrate constant.

105

4.3 Methods for Fluorescence Studies

The phenomenon of fluorescence spectroscopy contains two domains: spectral studiesand time-dependent studies. Although fluorescence spectroscopy cannot be classified asa ‘finger print’ type, the spectral studies are widely used as analytical applications offluorescence. Although the time-dependent studies of fluorescence pertaining to thefluorescence lifetime measurements are not sufficiently specific, they are used for manypurposes, such as determination of reaction kinetics, rates of competitive processes, andto probe local environments. There are essentially two types of methods for measuringfluorescence lifetimes: pulse fluorometry (relating to measurements performed in thetime domain), and phase and modulation fluorometry (relating to the frequency domain)[15]. Among the several publications devoted to the methods of studies of time dependentfluorescence, the reader is referred to the book edited by Lakowicz [15].

4.3.1 Time-Correlated Single-Photon Counting Studies

A single photon fluorometer consists of the following subsystems: an optical spectrometer(including a pulsed light source), a data acquisition system (consisting of timing electronics)and a data analyser (composed of a computer with re-convolution software). Birch andImhof [16] have widely published on instrumentation design and analytical methods.

The data analysis software is used to extract the kinetic information in a given system fromthe experimental intensity decay curve. The recorded decay curve of emission intensity, I(t),is generally synthesised by means of an exponential re-convolution function of the form:

I t B tn

nn( ) exp( / )= −∑ τ (4.6)

with n = 1,2,3,..., where τn and Bn are the lifetime parameter and pre-exponential functionof the nth component in the decay, respectively. An important feature of the equation isthat the existence of n decay components suggests the existence of n excited states.Moreover, it is usually recommended that the number of decay components chosen forleast-squares analysis should always be the minimum necessary to give a satisfactory fit.The last-squares analysis uses a quantity χ2 as a measure of discrepancies between dataand fitted function and, for a good fit, χ 2 less than 1.2 is recommended [16].

4.3.2 Quantum Yields

A useful quantity in the description of photophysical and photochemical processes is thequantum yield. The quantum yield, Φi, of a process ‘i’ is defined as the number, nA, of molecules,


106


A, undergoing that process divided by the number, Q, of light quanta absorbed [2]:

Φi

AnQ

= (4.7)

For practical use, we measure the quantum yield relative to the quantum yield of aknown standard compound. The following materials can be used as standards:

• Rhodamine B in ethanol at 22 °C; ΦF = 0.69 with λexc. = 366 nm [17];

• Quinine sulfate in 1N H2SO4, 25 °C; ΦF = 0.546 with λexc. = 328 nm [18];

• 2-Aminopyridine in 1N H2SO4, 25 °C; ΦF = 0.60 with λexc. = 290 nm [19].

It will be useful for many purposes to distinguish between quantum yield related to theabsorbed radiation and efficiency, ηi, related to the number of molecules in a givenstate [2]:

ηi

i

A

nn

= (4.8)

which is the ratio of the number, ni, of molecules undergoing a specific reaction to thenumber, nA, in the excited state. If only processes that obey a purely exponential rate lawsuch as fluorescence are involved in deactivating the singlet state, S1, the quantum yieldof fluorescence may be written as the ratio of the observed to the natural lifetime:

ΦF = τ

τ0(4.9)

where the natural lifetime, τ0, and the observed lifetime, τ, of the singlet state are givenby equations (4.1) and (4.2).

4.4 Fluorescence of Polymers, Excimer Fluorescence

The typical synthetic vinyl polymers prepared by the free radical polymerisation of vinylmonomers are shown in Figure 4.3. They have a linear chain structure in which thesubstituents R and R´ are separated by three carbons. If the substituents R are aromaticchromophores, the vinyl polymers will be rich in excimer structures and strong bimolecularquenching of the fluorescence will occur. This is in agreement with the Hirayama rule[20], which says that if the chromophores are separated by 3 carbon atoms, the probabilityof excimer formation is the highest, assuming that the chain is flexible enough to rotate

107

around the carbon-carbon bond. The majority of literature in polymer studies in the lasttwo decades has originated due to excimer emission and formation. Excimer emissionhas been detected from a number of aromatic vinyl polymers [21-24]. The aromaticchromophores have included phenyl, naphthalene, pyrene and carbazole. The vinylpolymers and copolymers that have been widely studied are: polystyrene (PS) [22],polyvinylnaphthalene (PVN) [23], polyvinylcarbazole (PVCZ) [24]. Many photophysicalprocesses in polymers depend on whether the process occurs in the solution or solidstate. This is because the rotational diffusion in the polymer chain can control the kineticsof the processes. Therefore, the conformation of the chain backbone required to bringneighbouring aromatic chromophores into sandwich geometry is one of unfavourablyhigh energy. This is almost entirely absent from these systems, where the low-temperaturedistribution of chain conformations becomes trapped in a rigid medium, but it can havea transient existence when segmental motion and rapidly rotating chain backbonesundergo a variety of conformation states. According to this model, the polymer chainsdissolved in fluid solution should have a Boltzmann distribution of suitable excimer-forming sites, the concentration of which will be the factor determining excimerfluorescence intensity. Harrah [25] has illustrated these dependencies for poly-2-vinylnaphthalene. The emission characteristics of a solid-state polymer will then dependon the conformational equilibrium in solution existing at the temperature solidification.A demonstration of this phenomenon in solid solutions of poly-2-vinylnaphthalene andpoly-4-vinylbiphenyl, as a relation between the ratio of excimer to monomer emissions


Figure 4.3 Schematic representation of vinyl polymers with substituents R and R´

R

R'

R

R'

R

R'

R

R'

R

R' = H

R' = H

R' = H

R = H

R = CH3

R =

R = C OCH3

O

R = C O

O

R' = CH3

polyethylene (PE)

R' = CH3

polypropylene (PP)

polystyrene (PS)

polymethyl methacrylate (PMMA)

polymethyl methacrylate (PMMA)

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and temperature of solidification, has been shown by Frank and Harrah [26] in agreementwith the following equation:

II

ERT

E

M

E

s

∝ −⎛⎝⎜

⎞⎠⎟

expΔ

(4.11)

where ΔEE represents the energy barrier between the normal and excimer creatingconformations at the temperature at which the film was cast, and Ts is the temperatureof solidification.

The analysis of excimer and monomer emission in fluid systems is complex. A generalkinetic scheme for excimer formation, including the inter- and intramolecular complexformation in terms of a singlet ground and excited states, and the various photophysicalprocesses occurring in polymers, is given in Table 4.1. The three routes of energy

sremylopnignirruccosessecorplacisyhpotohP1.4elbaT

sessecorplacisyhpotohP seicepslaitinI stcudorP etaRtnatsnoc

noitprosbaygrenE 1 h+M ν 1 *M Ia

ecnecseroulF 1 *M 1 h+M ν MF k MF

CI 1 *M 1M k MI

CSI 1 *M 3 *M k IT

gnihcneuqremonoM 1 Q+*M 1 *Q+M k MQ

ecnecserohpsohP 3 *M 1 h+M ν MP k MP

CSI 3 *M 1M k TI

noitargimtelgniS 1 +*M 1M 1 +M 1 *M k MM

noitamrofremicxE 1 +*M 1M 1 *)MM( k ME

aivnoitamrofremicxEpartdemroferp

1 +*M 1 )MM( 1 +M 1 *)MM( k ME

noitaicossidremicxE 1 *)MM( 1 +M 1 *M k EM

ecnecseroulfremicxE 1 *)MM( 2 h+)M1( ν EF k EF

CSI 1 *)MM( 3 *)MM( k EI

ssecorpevitaidarnoN 1 *)MM( (2 1 )M k EI

gnihcneuqremicxE 1 Q+*)MM( (2 1 *Q+)M k EQ

109

deactivation illustrated in Figure 4.1 are fluorescence, phosphorescence and non radiativetransition, and represent the competitive relaxation processes if the system is unimolecular.The situation becomes rather more complex when bimolecular interactions are included.There are further possibilities involving both the molecules. As an example, somecompeting radiative, migration, and energy transfer processes, which can occur aftersinglet excitation in a polymer chain, are shown in Figure 4.4.

Figure 4.4 Schematic illustration for competing radiative, transfer and migrationprocesses which can occur after singlet excitation. Energy quantum absorbed by the

chromophore can be dissipated by single chromophore, monomer emission, hνM, or bydimer chromophore, excimer emission, hνE, or it can migrate along the polymer chain,

and/or the excitation energy can be transferred to an acceptor molecule.

When two identical chromophores are brought into proximity in a suitable relativeorientation, there is a possibility that excitation energy may move as an exciton fromone to the other by a non radiative process. In a polymer chain containing a large numberof chromophores, the exciton can migrate down the chain until a suitable trap appearsor it can be transferred to a chemically different molecule (quencher). Much of the evidencefor energy migration comes from observations of excimer emission. In many polymers,the intensity of excimer emission is greater than can be explained by absorption at thepre-formed sites, as would be necessary in a solid, and neither by conversion of theabsorption site to excimer geometry by conformation change of a flexible chain. Theexplanation is that absorption occurs at monomer sites, and then the energy migratesuntil the appropriate exciton trap appears. Among the many papers devoted to energy


110


migration studies, a wide description of particular mechanisms of energy migration andenergy transfer can be found in [3]. The competition between the photochemical processesin polymers, as shown in Figure 4.4, can be controlled by either thermodynamic orkinetic parameters. The latter are of prime importance if we consider the molecularmotion in the polymer. The various photochemical processes occurring in polymers andthe related rate constants are given in Table 4.1.

Since IM = kFMM* and IE = kFEE*, inserting a stationary-state of excited monomer andexcimer allowed the following expressions for monomer and excimer emission intensities(IM and IE, respectively) to be obtained by David and co-workers [23, 24]:

I k

I k k Q k

k k M k Q k k Q k k MkM FMa E QE ME

M EM QM E QE ME EM ME

=+ +

+ + + + −( )

( )( ) (4.12)

I k

I k Mk k M k Q k k Q k k MkE FE

a EM

M EM QM E QE ME EM ME

=+ + + + −( )( ) (4.13)

From the practical point of view, the ratio of monomer to excimer intensity has beenused very frequently:

II

kk

k k k Q

k MM

E

FM

FE

E ME QE

EM

=+ +⎛

⎝⎜⎞

⎠⎟(4.14)

Application of these equations and the scheme of photophysical processes to fluorescenceobserved under variety of conditions have been used in a number of papers on processesin polymer solutions and in solids [3, 23, 24, 33].

4.4.1 Fluorescence of Polymers in Solution

4.4.1.1 Effect of Conformation of the Polymer Chain

In solution, the rate of the many bimolecular photoprocesses due to a polymer chainmay be limited by the rate of mutual diffusion of the interacting species, but the mostimportant factor in the case of a polymeric system is the chain conformation and flexibility.Guillet and coworkers [27, 28] studied photophysical properties for naphthyl-substitutedpolymethacrylate (PNMA) solutions very extensively. The spectral properties of absorptionand emission of PNMA studied using different solvents under different conditions areshown in Figure 4.5. Somersall and Guillet [27] observed delayed fluorescence, involving

111

the triplet state, with a lifetime of 0.1 s, when PNMA in tetrahydrofuran-ether glass at77 K was excited at 313 nm. Similarly, Cozzens and Fox observed delayed fluorescencein glassy PVN [29]. These studies provide evidence for complicated chromophoreinteractions involving the excited states. Somersall and co-workers [27] studied thefluorescence behaviour of polynaphthyl methacrylate in different solvents and they haveshown that the ratio of excimer to monomer emission varied with the solvent quality,being high in poor solvents and low in good solvents, as illustrated in Figure 4.6.Fluorescence spectra of PNMA at 298 K contain two main bands, a mirror image of theabsorption band due to monomer emission and a broad band shifted about 60 nm tolonger wavelengths which is attributed to excimer fluorescence. When chloroform isused as a solvent for PNMA, the monomer emission is predominant, being visible withbenzene as a solvent. However, when the solvent is ethyl acetate, monomer emissionalmost disappears and the excimer emission rises. Aspler and co-workers [28]demonstrated that the fluorescence properties correlate with a change in the effectivevolume of the random coil in solution. An increase in excimer emission was observed ifa non solvent was added to the solution to compress the polymer random coils. Theseobservations did not depend on whether they used a polar non solvent (such as alcohol),or a non polar solvent (such as cyclohexane). In Figure 4.6, one can observe an increasein the ratio of emission intensity from excimer to monomer bands when PNMA was inchloroform and then cyclohexane was added to a 1:1 mixture of the solvents; the ratio,IE/IM, changed from 1.44 to 2.81 when the viscosity decreases from 0.184 x 10-3 to0.125 x 10-3 N s m-2 [28]. The relationship between the fluorescence and viscosity wasexplained according to the classical Flory theory [5] of intrinsic viscosity:

Figure 4.5 Ultraviolet absorption and emission spectra of polynaphthyl methacrylatein chloroform at 298 K: (1) absorption, (2) fluorescence, (3) and (4) delayed emission

in tetrahydrofuran-ether at 77 K and phosphorescence(Reprinted with permission from Macromolecules, 1973, 2, 219, copyright 1973,

American Chemical Society)


112


[ ]η φ= < >63 2 2 3 2/ /S

M or

[ ]η φ= < >h

M

2 3 2/

(4.15)

where ø is constant, 2.1 x 1021, and < S2 > is the mean square radius of gyration, < h2 >1/2 isthe root mean square end-to-end distance and M is the molecular weight. The averagedensity of polymer segments, ρ, is proportional to M / < S2 >3/2 and the relationship is:

ρ

η∝ 1

[ ](4.16)

The rigidity of the chain has been characterised by the value of the steric factor, σ, whichis defined as:

σ2

2

2= < >

< >h

h f(4.17)

where < h2 >f is the theoretical distance between chain ends with completely free rotationaround all carbon-carbon bonds. Chain length is important in providing a sufficient

Figure 4.6 Fluorescence of PNMA in different solvents at 298 K: (1) in benzene, (2) inethyl acetate, (3) in chloroform, (4) in chloroform-cyclohexane (1:1). The intensities of

the spectra are not comparative(Reprinted with permission from Macromolecules, 1973, 2, 221, copyright 1973,


113

number of chromophore interactions in intramolecular excimer formation behaviour.Equation 4.16 can be used to explain molecular weight effects on excimer formation asa chain densification effect. Aspler and co-workers [28] observed the influence of molecularweight of PNMA in the range from 40,000 to 360,000 on increase of excimer emission.Similarly, Nishijima and co-workers [30] found a strong effect of molecular weight onincrease of excimer formation when they studied low molecular weight polyvinylnaphthalenes series, range 1,400-6,000.

The conformation of a polymer chain can be affected by the temperature of the polymersolution. The conformation change of poly(phenyl methacrylate) (PPMA) intetrahydrofuran (THF) solution was observed in dilute solutions using fluorescenceemission, viscosity, density and dipole moment measurements by Wandelt and Szumilewicz[31]. The fluorescence spectra of PPMA in THF solution when the temperature risesfrom 266 K to 318 K is shown in Figure 4.7. We can observe the shift of the maximum

Figure 4.7 Fluorescence spectra of poly(phenyl methacrylate) in THF, 4.8 x 10-3 M, atdifferent temperatures: (1) 266 K, (2) 273 K, (3) 279 K, (4) 289 K, (5)295 K, (6) 308

K, (7) 318 K: Excitation at 265 nm.


114


of fluorescence from λmax ≅ 335 nm to emission centred at λmax ≅ 358 nm. The emissionspectrum of PPMA becomes very broad at 295 K where two bands are visible andcomparative. Similar behaviour of fluorescence spectra was observed by Somersall andco-workers [27] for other methacrylate polymers PNMA, for which the monomer emissionis relatively strong in comparison with vinyl polymers under the same conditions. Thefluorescence emission spectrum of PPMA at 266 K fits 80% of the fluorescence spectrumof phenyl methacrylate, as is shown in Figure 4.8. Abuin and co-workers [32] reportedsimilar fluorescence spectra in phenyl-containing methacrylate polymers solutions in ethylacetate and in dichloromethane. As a first approximation, the phenyl chromophore inPPMA can be considered as isolated, because these polymers do not conform to theHirayama 3 carbons rule. But we can observe considerable excimer emission from otherpolymers where the chromophores are separated by more than three atoms, for examplein the spectra of naphthyl-containing methacrylates by Somersall and co-workers [27].The spectrum of PPMA obtained at 318 K fits the Gaussian distribution function, as isshown in Figure 4.9, which would suggest that the majority of emission is excimeremission. Although the monomer and excimer bands from PPMA are strongly overlapping,the bands are distinguishable. The steric factor values obtained from viscosity studies[31] correspond to the values obtained for polyvinylcarbazole which was established asa stiff polymer [23, 24]. These studies emphasise the role of chain conformation andexcimer binding energies in determining the extent of excimer formation. PS and PVNare examples of polymers where excimers can exist at nearly every point of the chain andthe concentration of excimer sites could be quite high, especially when the mobility of

Figure 4.8 Fluorescence spectra of poly(phenyl methacrylate) in THF, 4.8 x 10-3 M, at266 K (1) and of phenyl methacrylate in THF (2)

115

the chain allows the necessary rotation to form an excimeric structure. Naphthyl estersare polymers where the probability of adjacent excimer formation is low and the highintensity of excimer emission would be due to the high efficiency of energy migration.

The effect of temperature on excimer formation in a polymer chain is mostly due tothermally-activated conformational changes in the polymer chain. It was establishedthat in dilute polymer solutions the singlet excimer formation is an intramolecular processsince the ratio of excimer to monomer emissions intensity is independent of polymerconcentration [23, 24]. However, it was shown by David and co-workers that in vinylpolymers, excimers usually occur between neighbouring chromophores and theirformation is activated by temperature. Starting from 77 K, the intensity of excimerfluorescence from vinyl polymers is initially constant when it is in the glass state, andthen increases, whereas the monomer intensity decreases. This is due to the fact that therate constant for excimer dissociation, kME, is much lower than the rate constant forexcimer formation, kEM, in the low temperature range. At a temperature higher than 202K, excimer dissociation is very efficient for the vinyl polymers, and the intensity of excimerfluorescence decreases. This observation corresponds to a minimum which was observedby David and co-workers [23, 24] at 202 K in a graph of ln(IM/IE) versus 1/T. Values ofenergy of activation of excimers formation in poly-1-vinylnaphthalene andpolyacenaphthylene in methyl tetrahydrofuran (MTHF) solution, obtained by Davidand co-workers [23, 24], were respectively 11.3 and 3.4 kJ mol-1. But the energy value of14.7 kJ mol-1 was reported for activation of excimer formation in poly-2-vinylnaphthalenein solution by Al-Wattar and Lumb [33] and 15.1 kJ mol-1 by Harrah [25]. Chandrossand Dempster [34] reported values of energy activation 13.8, 16.8 and 12.6 kJ mol-1 for


Figure 4.9 Fluorescence spectrum of poly(phenyl methacrylate) in dilute solution inTHF, 4.8 x 10-3 M, at 318 K (1) and the fitting Gaussian curve (2)

116


excimers association in, respectively, 1,3-bis-1-naphthylpropane, 1,3-bis-2-naphthylpropane and 1-methylnaphthalene.

The fluorescence behaviour of PVCZ solutions is quite different from that of the othervinyl polymer solutions. The emission spectrum of PVCZ results from two distinctexcimers: a low-energy sandwich type structure with emission centred at 420 nm and ahigh-energy excimer centred at 375 nm. Itaya and co-workers [35] have proposed thatthe high-energy excimer has a structure in which only one pair of phenyl rings from thetwo-carbazole chromophores overlap. In the temperature-dependent curve for ln(IM/IE)as a function of 1/T, two minima corresponding to the formation and dissociation of twoexcimer structures were observed for PVCZ by David and co-workers [23, 24]. Thesetwo different excimer emissions have been assigned to different conformations of thechain, and a change in the conformation occurs between 143 and 295 K. Furtherinformation on these structures was obtained with use of time-resolved fluorescencespectroscopy. Time-resolved fluorescence spectra for PVCZ in benzene obtained by Hoyleand co-workers [21] are presented in Figure 4.10. This experiment showed that the highenergy excimer, in a partially eclipsed form, appeared at the earliest times of observation,

Figure 4.10 Time-resolved fluorescence spectra of polyvinylcarbazole, 5 x10-4 M, indegassed benzene at 296 K, excitation wavelength 313 nm. Spectra obtained with timeintervals from the lamp maximum: (1) 0-0.23 ns, (2) 8-9.4 ns, (3) 19-35 ns, (4) 182-

323 ns. Spectra are adjusted to the same intensity scale.(Reprinted with permission from Macromolecules, 1979, 5, 957, copyright 1979,


117

(i.e., at a 0.2 ns interval from a photon absorption) and the emission from the low-energy sandwich excimer was absent at that time. The low-energy excimer due to thenormally eclipsed form appeared after 8 ns to be present after 182 ns as well. Thesestudies suggest that the chain of PVCZ requires significant rotational time to form thesandwich excimer. The PVCZ chain was reported [23, 24] as a stiff polymer for whichthe steric factor from Equation 4.17 was 2.8, whereas it was 2.2 for PS [23, 24], and hasa dielectric segmental rotational relaxation time of about 180 ns for a chain of Mw

greater than 4 x 104 [36].

An interesting experiment was done by Cuniberti and Perico [37]. They synthesised anumber of monodisperse polyethylene oxide samples terminated with pyrene groups.This experiment, which was related to the cyclisation of the polymer chain as it isschematically presented in Figure 4.11, showed that intramolecular excimer fluorescenceincreases inversely proportionally to the molecular weight of the polyethylene oxidechain separating the two chromophores. Similar experiments were continued by Winnikand co-workers [38] and they obtained the particular rate constants for cyclisation forPS and polydimethyl siloxane chains terminated by pyrenes. The cyclization activationenergy, 14 kJ mol-1, was three times lower than that for excimer dissociation, 44 kJmol-1, in pyrene-labelled PS. The difference corresponds to the binding energy of theexcimers for solution of pyrene in cyclohexane, but approximate calculations haveshown that the probability of cyclisation is strongly sensitive on the length of PS chainseparating the two chromophores forming the excimer [38, 39]. It was shown that theaddition of non labelled PS to pyrene-labelled polydimethyl siloxane reduces the rateof cyclisation. These effects were interpreted in terms of the decrease of theconformational mobility of pyrene-labelled polydimethyl siloxane chain throughinteraction with PS molecules.


Figure 4.11 Intramolecular excimer formation through end-to-end cyclisation ofpolymer chain

118


It has been demonstrated that the excimer emission intensity from chromophoresincorporated into the vinyl polymer chain have been correlated with a change in the effectivevolume of random polymer coil in solution, and the volume of random polymer coil wascorrelated with viscosity, which is dependent on temperature. But a temperature change ofa polymer solution does not always lead to expansion or contraction of the chain coil; itcan cause conformation change to a more or less packed polymer chain and to a more orless excimer forming conformation. Both of these structural changes in a polymer chaincan affect the fluorescence, but each polymer needs to be considered individually.

4.4.1.2 Energy Transfer and Migration

The energy transfer and migration processes play an extremely important role in thephotochemistry and photophysics of polymers. But, there is no direct measurement thatallows conclusions to be made about these processes. Three types of energy transfer canbe distinguished. Firstly, there is a transfer from the chromophore absorbing the incidentradiation to the photochemically active site. A second type is collisional energy transfer,where the electronic excitation energy localised on the absorbing chromophore istransferred to another group on the same or different molecule as a result of the overlapof electronic charge clouds which occur during a collision. As an indicator of energytransfer efficiency we can use the Smoluchowski equation [3]:

k

r Dq

AB AB= 4

103

π(4.18)

where rAB is the energy capture radius and DAB = DA + DB is the diffusion coefficient forrelative movement of energy donor and acceptor.

The third type of energy transfer takes place by resonance transfer over an extensiverange of space, as a result of dipole-dipole interaction, as was proposed by Förster [40].Migration of energy along polymer chains can occur by two mechanisms. When theelectronic interactions between adjacent chromophores are weak, and the orientationcorrelations are low, the energy undergoes a statistical sequence of jumps from onechromophore to another as schematically shown in Figure 4.4. Each of the jumps has thecharacteristics of non radiative resonance transfer as proposed by Förster [40] and Dexter[41], and characterised by an effective energy diffusion distance. When there are stronginteractions between neighbouring chromophores, and orientation correlations areappropriate, the energy may be delocalised as a wave over a number of chromophoreunits. The most convincing quantitative evaluation of down-chain energy migration comesfrom efficiency of polymer luminescence quenching by acceptor species present inconcentrations which would be most inefficient if the absorbed energy were localised.

119

When a system contains two fluorescing chromophores such that the emission spectrum ofthe donor overlaps the absorption spectrum of the acceptor, excitation energy from the donorcan be transferred to the acceptor over a considerable separation distance, R. The efficiency,E, of this energy transfer would be governed by the equation proposed by Förster [40]:

E

R

R R=

+06

06 6 (4.19)

where R0 is a characteristic distance for a rotationally averaged pair for which half ofthe excitation energy is transferred. This characteristic distance depends on the overlapintegral between the emission spectrum of the donor and the absorption spectrum ofthe acceptor [40].

There is always uncertainty if we adopt this equation for polymeric systems; the mostimportant consideration seems to be the inhomogeneity of the polymer solution whenthe polymer segments are constrained into local high and low concentrations by thedistribution of macromolecules.

An incident photon absorbed by a chromophore is then transferred to the site of chemicalactivity. This is frequently followed by chain scission reactions, which in photooxidationdepends upon the solubility and diffusivity of oxygen and the onset of localised molecularmotion in the solid state [42-44]. To protect the polymer from the photooxidation andphotodegradation, some additives are frequently used. Kryszewski and co-workers [45,46] presented fluorescence intensity and lifetime studies as indicators of the efficiency ofenergy transfer between two polymers: PS and poly-2,6-dimethylphenylene oxide (PPO)and correlation between energy transfer and photochemical stabilisation. Thus transferof energy from PS to PPO can be seen in Figure 4.12 in the diminution of PS monomeremission at about 285 nm, and the replacement of PS excimer emission with a maximumat 330 nm by PPO emission with a maximum at 315 nm. Overlap of the PPO emissionwith PS emission makes estimates of energy transfer from intensity measurementsunreliable; time-resolved studies of fluorescence decay and resulting lifetime data havebeen used for this purpose. The decay curves have been fitted by two exponential functionin agreement with Equation 4.6. The fluorescence lifetime data of the mixture of PS withPPO in solution are presented in Table 4.2.

We can observe emission from both PS excimer and PPO. The τ1 = 12 ns due to pure PS insolution decreases with PPO increase and the relative intensity of the short-lived PPOcomponent increases in the presence of PS. The decrease in the lifetime of PS subjected toStern-Volmer analysis gave the slope 5.2 x 103 l mol-1. Semi-quantitative analysis, assumingthe mutual diffusion coefficient of 2 x 10-11 m2 s-1, gave the distance of interaction in theorder of magnitude 103 nm. Quenching efficiencies of this magnitude would be possible only


120


noitartnecnoc,K892tanarufordyhartetniSPfosretemarapemitefiL2.4elbaT01x9.2 3- llom 1- ;OPPybdehcneuq,stinuenerytsfo λ .cxe ,mn562= λ .me mn033=

noitartnecnocOPPllom000,01( 1- )stinuremonomfo

τ1 )sn( τ2 )sn( χ2

00.0 0.21 - 9.0

42.0 3.11 1.1 3.1

03.1 4.8 2.1 5.1

00.2 4.6 1.1 6.1

08.3 9.4 1.1 2.1

)SPon(21 - 9.0 9.0

Figure 4.12 Fluorescence of polystyrene quenched by poly-(2,6-dimethyl-p-phenylene oxide) in tetrahydrofuran at 298 K, PS concentration 2.7 x 10-3 mol l-1

(styrene units): (1) no PPO, (2) 0.43 x 10-4, (3) 0.83 x 10-4, (4) 4.3 x 10-4,(5) 1.74 x 10-4, (6) 2.61 x 10-4 mol l-1

(Reprinted from Polymer, Volume 23, M. Kryszewski, B. Wandelt, D.J.S. Birch,R.E. Imhof, A.M. North and R.A. Pethrick, Photo-energy transfer in

polystyrene-polyphenylene oxide blends, 926, copyright 1982, with permissionfrom Elsevier Science)

121

if the donor energy is effectively available over the whole of the PS chain coil and all themonomer segments of the PPO chain are able to quench the excited PS. However, studies offluorescence in the solid state of the system [45] have shown that energy transfer from PS toPPO is efficient in the solid blend, and at 15 wt.% of PPO, almost all the emission is fromPPO. However, the magnitude of PS stabilisation in the presence of PPO, calculated by analysisof the polymer chain scission processes of the polymer mixture in solution, is smaller than itwould be from the energy transfer characteristics by fluorescence lifetime measurements.This suggests that chain scission arises from energy more localised than the mobile excitonsin the quenching of fluorescence. Jensen and co-workers [47] reported that the decrease in PSexcimer fluorescence is greater than can be explained by absorption of PPO at the excitationwavelength. Kryszewski and co-workers [46] showed the molecular compatibility of PS-PPO blends is important. After studies of the fluorescence intensity and fluorescence lifetimemeasurements during storage and annealing of the polymers and blends, they suggested thatannealing and storage permit chain packing rearrangements that favour non radiative energyconversion and transfer processes, and in the case of blends lead to the PS to PPO transferwhich quenches the PS emission and photodegradation.

Amrani and co-workers [48] have used fluorescence measurements of energy transferfrom donor to acceptor for studies of polymer compatibility. They labelled methylmethacrylate-ethyl methacrylate copolymer and/or methyl methacrylate-butylmethacrylate copolymer with donor-naphthalene and polymethyl methacrylate withacceptor-anthracene. The variation in the ratio of donor to acceptor fluorescence wasplotted as a function of butyl methacrylate and ethyl methacrylate in the copolymer andgradual increase of the ratio corresponded to gradual transition from two-phase to aone-phase system. The fluorescence technique was found to be more sensitive to smallchanges of compatibility of the polymers.

Winnik [49] used fluorescence measurements of transfer of the electronic excitationbetween donor-naphthalene and acceptor-pyrene chromophores attached to the samepolymer chain for studies of thermoreversible phase separation of aqueous solutions ofpoly(N-isopropylacrylamide) (PNIPAM). Dilute solutions of the doubly labelled polymerPNIPAM were heated from 277 K to 313 K, and the fluorescence emission intensity ofpyrene (integrated spectrum) was measured when the system was excited with 290 nm,donor excitation, and when excited with 328 nm, acceptor excitation. Non radiativeenergy transfer between excited naphthalene and pyrene occurred in aqueous solution ofthe polymer. The increase in intensity of pyrene fluorescence when the solution wasexcited at 290 nm, shown in Figure 4.13, is due to a phase separation process at lowercritical solution temperature (LCST). When the LCST was reached, the phase separationinto polymer-rich and polymer-lean phases occurred. It was concluded that the collapseof the polymer chain leading to densification of polymer phase is followed by dominationof intramolecular contributions to the energy transfer process.


122


4.4.2 Fluorescence of Polymers in Gel State

4.4.2.1 Effect of Chain Conformation on Excimer Emission

Excimer formation can be explained in terms of a combination of local rotational isomericand longer range diffusional motions of the chain. In polymer solution we observed anddiscuss the role of both; in solid film any type of movement is generally restricted, andthe mobility of a polymer in a gel state is rather between the solution and solid, a moreprecise description of which was published by de Gennes [50].

The freely-rotating chain of a polymer in dilute solution is represented by a statisticaldistribution of conformational structures of the chain. The resultant excimer fluorescenceemission is a broad Gaussian band. The excimer fluorescence spectrum from a solidpolymer results in a very broad spectrum which is characteristic of the distribution ofconformations adopted by the polymer in the preparation process. This distributiondepends upon the thermal history of the sample and the conditions used for the casting;these dependencies were presented by Frank and co-workers [26]. The complexity ofphotophysical processes accompanying the excimer formation and the inherent complexity

Figure 4.13 Plot of pyrene emission intensity as a function of temperature foraqueous solution of poly-(N-isopropylacrylamide) (PNIPAM) labelled with

naphthalene(N)-donor and pyrene(Py)-acceptor; PNIPAM-Py/366-N/50, 44 ppm inwater. Wavelength excitation at 290 nm due to N excitation, at 328 nm due to

Py excitation(Reprinted from Polymer, Volume 31, F.M. Winnik, Phase transition of aqueouspoly-(N-isopropylacrylamide) solutions: a study by non radiative energy transfer,

2132, copyright 1990, with permission from Elsevier Science)

123

of polymer systems (e.g., tacticity) together with the conformational sensitivity of thestructures make for analytical problems. For some purposes, we can stimulate some ofthe local movement by photo and/or thermal cure [6, 50].

Isotactic polystyrene (iPS) in the gel state was used by Wandelt [7] to obtain a selected,homogeneous helical conformation of the vinyl polymer chain. A solution of iPS in benzylalcohol (BA) quenched from 443 K to 273 K, with a rate of 0.42 K s-1, leads to theformation of a gel [51]. BA appeared to be a good solvent for the gel formation andtransparent in the infrared regions where the characteristic absorption of the helicalconformations occur; additionally it is not photoactive in the region of PS excimer emission[51]. The iPS chain of the fresh gel exists in the extended conformation defined by Atkinsand co-workers [52] and Sundararajan and co-workers [53, 54], which can easily beconverted to the three-fold helical conformation by annealing at a temperature higherthan the Tg [52, 54], i.e., between 383 to 393 K. The sample of crystalline iPS wasobtained from iPS/BA gel by heating at 293 K in the presence of nitrogen. The sampleshowed 68% crystallinity obtained from differential scanning calorimetry (DSC)thermograms. Characteristic three-fold helical conformation of the chain and spheruliticmorphological forms were observed by infrared spectroscopy and microscopy [7]. Thefluorescence emission spectra from the 68% crystalline iPS, and from atactic PS (aPS)film cast from chromophore, both excited at 257 nm are shown in Figure 4.14. One cansee that the emission band from crystalline iPS is narrower then that of atactic PS, andthe crystalline iPS band is shifted a few nm to the red. Examples of time-resolvedfluorescence decays from the crystalline iPS are shown in Figure 4.15. We can observesimilarities of the decays obtained for the 330 and 350 nm wavelengths of emission theyare both due to excimer emission. Some differences in the decay can be observed foremission wavelength 310 nm, where possibly some monomer emission at the shortertime of the decay was collected. The recorded decay curves of emission intensity fromthe crystalline and atactic PS were analysed using a non linear least-squares analyticalmethod [16], and the resulting photophysical parameters of the decays are shown inTable 4.3.

A two-exponential fitting function was used for fluorescence decay from the crystallineiPS sample. Some amount of short-lived component in the parameters fitting thefluorescence decays from crystalline iPS correspond to some presence of monomeremission, which strongly overlaps the excimer band. The long-lived component of 21 nsis in the majority, more than 80% of the emission intensity is due to the component.However, this lifetime parameter does not change with the wavelength. There is evidencefor homogeneity of the excimer structures in the crystalline polymer, especially if wecompare the data with the same from aPS film, gathered in Table 4.3. A three-exponentialfunction, in agreement to Equation 4.6, was found to be necessary to fit the fluorescencedecay from aPS film. Considerable changes in the excimer lifetime parameters and their


124


Figure 4.15 Fluorescence emission decays of crystalline iPS with excitation at 257 nmand different emission wavelengths: (1) 310 nm, (2) 330 nm, (3) 350 nm, (4) lamp pulse

(Reprinted from Polymer, Volume 32, B. Wandelt, Correlation of photophysicalparameters with conformational structure of crystalline iPS and comparison with data

of atactic PS, 2709, copyright 1991, with permission from Elsevier Science)

Figure 4.14 Steady-state fluorescence emission spectra of crystalline iPS obtained from gel andatactic PS film cast from chloroform at room temperature, excitation wavelength 257 nm

(Reprinted from Polymer, Volume 32, B. Wandelt, Correlation of photophysicalparameters with conformational structure of crystalline iPS and comparison with data

of atactic PS, 2708, copyright 1991, with permission from Elsevier Science)

125

contributions with emission wavelength were observed in the case of aPS. This suggeststhat the excimeric band is very complex. The aPS film was cast from chloroform solutionat room temperature, and it included microstructural domains with frozen polymer chainsof different conformations. Some examples of fluorescence lifetime measurements for PSfrom different laboratories are gathered in Table 4.4.

The lifetime data of PS differ from laboratory to laboratory, because the fluorescencedecay curve reflects not only the polymer type and solvent, but the processes ofpreparation, i.e., temperature, quality of the solvent, etc.

Basically, in the extended conformation as well as in the three-fold helical conformationof the iPS chain, the phenyls do not form excimer states, because the distance betweenparallel phenyls in the three-fold helical conformation is 0.665 nm, as reported by Natta[62] and Sundararajan and co-workers [54]. This distance is too large for excimerformation of the sandwich type (which have a distance of 0.3-0.35 nm). In this situation,the excimer can be formed only outside the crystalline region, e.g., in the region of thelamellar borders, because in these areas some deformation of the helical conformation ofthe PS chain makes the excimer structure formation more probable. The excitation energycan effectively migrate along the helical structure [60, 63] to the lamellar border, where

SParofdnaSPienillatsyrc%86rofatadyacedecnecseroulF3.4elbaT

λ me

)mn(τ1 )sn( τ2 )sn( τ3 )sn(

B1

)%(B2

)%(B3

)%(χ2

SPi

013 7.3 ± 4.0 6.91 ± 2.0 - 2.72 8.27 - 72.1

023 3.4 ± 3.0 5.02 ± 3.0 - 7.61 3.38 - 61.1

033 2.5 ± 3.0 0.12 ± 3.0 - 9.11 1.88 - 99.0

043 1.5 ± 9.0 3.12 ± 4.0 - 9.9 0.09 - 11.1

053 7.4 ± 6.0 6.12 ± 2.0 - 9.6 1.39 - 90.1

SPa

013 6.1 ± 4.0 4.7 ± 6.0 4.91 ± 4.0 4.51 5.14 1.34 81.1

023 8.1 ± 6.0 5.8 ± 9.0 3.12 ± 4.0 2.21 1.93 7.84 22.1

033 6.2 ± 3.0 4.31 ± 9.0 2.92 ± 9.0 2.41 5.45 3.13 72.1

043 1.2 ± 3.0 8.31 ± 9.0 0.03 ± 9.0 7.21 8.15 5.53 91.1

053 1.2 ± 3.0 0.51 ± 2.1 8.23 ± 2.1 2.61 0.35 8.03 32.1


126


the pre-excimer sites are formed. Although the energy migration coefficient in PS is lowerthan, for example, in PVN [64, 65], only high effectiveness of energy migration wouldexplain the strong increase of the excimer fluorescence emission from the crystalline iPSin comparison with the amorphous aPS observed by David and co-workers [23, 24].However, only higher effectiveness of the energy migration in the three-fold helicalhomogeneous conformation than in the extended conformation, can explain the 10 timeshigher excimer emission from the crystalline state than from the fresh gel of iPS (extendedconformation of the chain) [51].

4.4.2.2 Effect of Phase Separation and Collapse Transition

The phase separation process was observed in iPS gel [66, 67]. The gel was formed bycooling a solution of iPS in BA (0%-20% w/w) from a temperature of 443 K, in whichclear solution is obtained, to 273 K, at a maximum cooling rate of 0.42 K s-1, which lead

seirotarobaltnereffidmorfatademitefilSP4.4elbaT

etatslacisyhPSPfo

tnevloS λ cxe , λ me)mn(

semitefiL)sn(

χ2 ecnerefeR

noitulos HC 2 lC 2 582,662 3.1 55

noitulos HC 2 lC 2 563,662 7.21 55

mlif HC 2 lC 2 563,662 0.22 55

noitulos eneulot 043,752 3.51 4 65

noitulos eneulot 092,752 6.410.29.0 3.1< 65

noitulos eneulot 092,752 2.512.43.1 3.1< 65

noitulos HC 2 lC 2 072,752 5.3127.0 3.1< 75

noitulos HC 2 lC 2 583,752 9.8120.1 85

noitulos C2H4 lC 2 033,552 5.51 95

noitulosMw 008,3=

C2H4 lC 2 063,552 71 95

noitulossremogilo

enaxeholcyc 063,052 12 06

noitulos C2H4 lC 2 333,352 5.7158.1 16

noitulos HC 2 lC 2 333,352 5.511< 16

127

to a meta-stable state in which the chains adopt an extended conformation [52-54].Generally, a polymer gel is a network of flexible chains, for which a two-dimensionalview of the structure is similar to a fish net, but it is filled with solvent. We consider a gelto be a series of chains which are associated by a physical process and where the crosslinksare not strong, thus under any weak and finite stress the crosslinks will eventually split,and the long-term behaviour of the material will always be liquid-like. Thus, in this case,gelation is conceptually similar to a glass transition. It is not an equilibrium process, butit corresponds to the freezing of a certain number of degrees of freedoms [50]. There areno strict physical parameters in such a system, with respect to temperature or time of theprocess. In the case of the iPS/BA system, the BA is a poor solvent for iPS, and hence,phenyl-solvent interactions are unfavourable in comparison with intramolecular phenyl-phenyl interactions. Under these interactions, some segregation into small regions highin chain concentration and others which are solvent rich (i.e., polymer-rich and leanphases) may occur [50]. Then the polymer chain structure in the polymer-rich regionscan be converted to the more stable form. The rate at which this process occurs dependsupon the temperature and time used. Moreover, it has been found [51-54] that the solventplays an important role in determining the conformational structure of the gel. Annealingallows the iPS chains to undergo phase separation into PS-rich regions, which can betransformed into a crystalline phase with a characteristic, three-fold helix conformation[7, 52, 54]. The phase separation step is determined at a fixed concentration by twofactors: temperature and time. The annealing measurements were carried out at 318 K.This temperature was chosen because it lies sufficiently close to the Tg, reported to be323 K [52], and allows effective production of the heterogeneous two-phase system.After annealing for 30 minutes at 318 K, characteristic morphological structures wereobserved using polarised optical microscopy and a crystallinity value of 23% was obtainedfrom DSC thermograms [66]. The change of the steady-state emission spectrum of iPSgel during annealing at 318 K over a period of 30 minutes is shown in Figure 4.16. Thechanges observed indicate an increase in the intensity of a red-shifted excimer emissionwith time of heating. The fluorescence emission decays of a sample of iPS annealed forhalf an hour required two exponential functions to obtain a good fit of the experimentalcurve. The photophysical parameters obtained at a series of wavelengths are presentedin Table 4.5 together with the same for freshly prepared gel.

Changes in the lifetime parameters with emission wavelength suggest that the excimeremission band is rather complex and more than one type of excimer structure may becontributing to the emission spectrum. The lifetime parameter of about 26 ns in amountover 43%, at the emission wavelength 340 nm, shows that the long-living componentsof the decay correspond to the less energetic, red-shifted side of the fluorescence band.For comparison the lifetime parameters of the freshly prepared gel suggest clearly thatone type of excimer structure is present: similar contribution at the emission wavelengths330 and 350 nm, and the same lifetime of 19 ns (if we take into account the error for


128


K813taruoh5.0rofdelaennalegSPirofsretemarapyacedecnecseroulF5.4elbaT

λ me )mn( τ1 )sn( τ2 )sn( B1 )%( B2 )%( χ2

K813tagnilaennafonim03retfalegSPi

013 6.8 ± 5.0 5.81 ± 6.0 6.95 4.04 53.1

033 3.11 ± 3.0 5.42 ± 2.0 1.06 9.93 89.0

043 7.21 ± 4.0 9.52 ± 6.0 8.65 2.34 62.1

legSPihserF

013 4.5 ± 3.0 3.81 ± 2.0 6.13 4.86 71.1

033 9.5 ± 5.0 5.81 ± 2.0 8.91 2.08 51.1

053 6.11 ± 3.1 6.91 ± 5.0 4.81 6.18 91.1

both emission wavelengths). It was suggested by Atkins and co-workers [52] andSundararajan and co-workers [54] that the extended conformation of iPS chains existsin the fresh gel and that it is stabilised by the solvent. If the quenching process of thepre-gelled solution is comparatively fast (of the order of minutes) the elimination of

Figure 4.16 Changes of the fluorescence emission spectrum of iPS gel (10% w/w) with timeof annealing at 318 K: (1) 0 min, (2) 4 min, (3) 9 min, (4) 14 min, (5) 20 min, (6) 30 min(Reprinted with permission from Macromolecules, 1991, 24, 5144, copyright 1991,


129

solvent cannot occur and the polymer is frozen in a themodynamically non equilibriumstate. The rate at which the conformational transition occurs will depend on theconditions of the gel. When the polymer chain undergoes the volume phase transition,the excess of solvent is expelled from the polymer chains. Density measurements of theiPS gel during the volume phase transition were performed and correlation with thefluorescence lifetimes for the long-term storage process of the iPS gel at 283 K waspresented by Wandelt and co-workers [67]. Changes in the lifetimes for excimer emissionat the red side of the spectrum (at 330 and 350 nm) are shown in Figure 4.17. After400 hours of storage, the gel exhibits an emission spectrum that contains a red-shiftedexcimer component, curve 2, with a fluorescence lifetime of 27 ns. After 528 hours ofstorage, a fluorescence lifetime of 29 ns is achieved. The long-lived fluorescenceexcimeric structure vanishes after longer periods of storage and a majority of componentof 20 ns can be observed after 864 hours. This suggests that the long-living excimercomponent is due to an unstable transition state.

Figure 4.17 Dependence of the excimeric fluorescence lifetimes of iPS gel on timeof storage at 283 K: (1) from emission decays at 330 nm and (2) at 350 nm:

Excitation at 257 nm.

The equation for observed lifetime (Equation 4.2) can be rewritten:

τ = + −( )k kf n1 (4.20)


130


where kf is the fluorescence rate constant (which is independent of temperature), andkn is the rate constant of the radiationless processes. The latter appears to be responsiblefor the lifetime changes. The lifetime increase accompanies the decrease of the nonradiative deactivation rate constant at around 400 hours of storage, when the new red-shifted excimer emission appears. From measurements of time-resolved emission spectrafor iPS/BA gel stored for 240 hours, shown in Figure 4.18, it may be concluded thattwo phases exist in the system. A high-energy excimer band with a maximum at 325nm obtained with no delay after excitation and with a time interval of 9 ns can beidentified with one phase; the same spectrum was obtained with time interval from 6ns to 11 ns. A close correspondence of the band with the steady-state fluorescencespectrum from the fresh gel, the dotted curve, suggests correlation of the band with ashort-lived component in the excimer spectrum. The red-shifted emission band withmaximum at about 345 nm in Figure 4.18 was obtained using a time interval of 79 to

Figure 4.18 Time-resolved fluorescence emission spectra of iPS/BA gel stored for 240hours at 283 K; λexc.=257 nm; slit width = 20 nm; obtained with time intervals: (1) 0-9ns, (2) 6-11 ns, (3) 79-152 ns; dotted curve is steady-state fluorescence spectrum for

iPS/BA gel after preparation.(Reprinted from Polymer, Volume 33, B. Wandelt, D.J.S. Birch, R.E. Imhof, R.A.

Pethrick, Time-resolved excimer fluorescence studies as a probe of the coil collapsetransition and phase separation in isotactic PS/BA gel, 3561, copyright 1992, with

permission from Elsevier Science)

131

152 ns. This red-shifted band appeared after phase separation into polymer-lean andpolymer-rich phases, and it can be ascribed to the polymer-rich phase. After longertimes of storage, the red-shifted excimer structure of lifetime 29 ns was transformedinto an excimer structure of lifetime 20 ns, as seen in Figure 4.17. To observe the threetime-resolved bands, due to different excimeric structures in an iPS gel, a model iPS gelwas prepared [51] by placing a crystalline, previously nucleated material into a hotsolution of iPS in BA, which was immediately quenched to a solid gel. Time-resolvedemission spectra shown in Figure 4.19 indicate the existence of three excimer states.The high-energy band 1 obtained with no delay from excitation pulse, with a maximumat about 325 nm, corresponds to spectra 1 and 2 in Figure 4.18. This band, obtainedwith time interval 0-5 ns, corresponds to a short fluorescence lifetime, 17 ns, which ischaracteristic for polystyrene in solution. The band 2 with a maximum at 330 nm,obtained at a time interval of 7-16 ns, is due to an excimer structure clearly identifiedas characteristic for crystalline iPS [7] of 21 ns lifetime. This was completely invisibleafter 240 hours of storage but it was observed [67] after 744 hours of storage when the


Figure 4.19 Time-resolved fluorescence emission spectra for modelled fresh iPS gelwith suspended crystalline material. Spectra obtained with time intervals: (1) 0-5 ns,

(2) 7-16 ns, (3) 71-149 ns(Reprinted with permission from Macromolecules, 1991, 24, 5144. Copyright 1991,


132


polymer chain was transformed to the three-fold helical conformation. Figure 4.19pertains to the crystalline material suspended in the last step of gel preparation havinga memory of chain conformation. The band (3) in Figure 4.19 was obtained with atime interval of 71-149 ns, with a maximum at about 345 nm, and is due to the red-shifted excimer fluorescence of 29 ns lifetime. The red-shifted fluorescence componentwas identified as an unstable excimer structure which finally undergoes transformationinto a stable excimeric structure with emission at 330 nm, represented by band 2 inFigure 4.19. The time-resolved spectrum of the component 2 was shown [66] to be thesame as the steady-state fluorescence spectrum of the 68% crystalline iPS gel.

A linear combination of components 2 and 3 using the pre-exponential parameters obtainedfrom the analysis of the decay at 330 nm emission wavelength in Table 4.5 has been usedto predict the theoretical spectrum for the two-phase excimer emission [66]. The totalintensity in Figure 4.20, curve 1 was calculated in agreement with Equation 4.21.

Figure 4.20 The spectrum estimated using the equation I = 0.60(comp.2 +0.40(comp.3) is shown as curve 1; the steady-state fluorescence emission spectrum

of iPS gel after annealing for 30 min at 318 K as curve 2; comp.2 and comp.3correspond to spectra 2 and 3 in Figure 4.19.

(Reprinted from Polymer, Volume 33, B. Wandelt, D.J.S. Birch, R.E. Imhof and R.A.Pethrick, Correlation of steady state and time-dependent studies of a two-phase iPS gel,

3556, copyright 1992, with permission from Elsevier Science)

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IE = 0.6(comp.2) + 0.4(comp.3) (4.21)

where comp.2 and comp.3 corresponds to spectra 2 and 3 in Figure 4.19. This wascompared with the experimental spectrum of the annealed iPS gel. Comparison of theexperimental and theoretical curves in Figure 4.20 indicates that differences are observedat short wavelength due to the monomer and to some degree the extended conformationcontributing to the spectrum, contributions which are not included in the theoreticalcurves. The generally good agreement between the theoretical and experimental curvesupports the assumption that the excimer emission arises from a two-phase system.

Description of the excimer photophysics for a two-phase system presented by Wandeltand co-workers [66] are based on both the two-phase model assumptions and theexperimental results. The two-phase model describes the results of the experimental studiesof photoenergy migration in heterogeneous solid-state polymer blends by Frank andcollaborators [68, 69]. Tao and Frank [69] used three-dimensional electronic excitationtransport to interpret the ratio of excimer to monomer fluorescence for poly-2-vinylnaphthalene with polycyclohexyl methacrylate. The assumptions of the two-phasemodel are:

• There is no energy transfer between the different domains, and

• The exciton can migrate inside a particular phase but becomes trapped at interfaceswithout escaping.

The two-phase photophysical process described by Wandelt and co-workers [66] wassimilar to that previously used for a single phase [3]. It is worth considering that theemission arises from not only different states but also different phases, and the processesare simultaneous and independent. Thus the two-phase system excimeric fluorescencecan be calculated:

IE (two-phase) = IE (1) + IE (2) (4.22)

where 1 and 2 refer to particular phases. The generally good agreement obtained inFigure 4.20 between the experimental spectrum and that arising from the two-phasemodel calculated by Equation 4.22 supports the model assumptions. Investigation of thefluorescence emission from the crystalline iPS shows that it is very effective, but thethree-fold helical conformation of the chains ensures that the parallel phenyl groups areseparated by a distance of 0.665 nm [54] and packed into a lamellar structure about0.57 nm long. The distance between phenyl groups is large for excimer formation, butthe very regular and periodic chain microstructure incorporated into the lamellarmacrostructure gives the chance for efficient energy migration. The structure of thecrystalline phase favours exciton energy migration along the helix to the lamellae boundary


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where it is finally trapped in the interface region. It has been established [7] that all ofthe chromophores associated with the crystalline (and only 20% of those in theamorphous) state participate in the excimer emission.

Fluorescence studies of volume phase transition of polyacrylamide (PAAM) in mixed acetone/water solvent, with incorporated dansyl group and pyrenyl probe were reported by Huand co-workers [70, 71]. They observed an increase in the fluorescence lifetime of theprobe with increase of acetone content in the solvent. They reported that gradual increaseof fluorescence lifetime accompanies the volume phase transition of the PAAM gels fromthe swollen to collapsed state with increasing hydrophobicity of the microenvironment.

Picosecond fluorescence studies were applied by Winnik and co-workers [72] for studiesof temperature-induced phase transition of pyrene-labelled hydroxypropylcellulose (HPC-Py) in water. Temperature dependence of the fluorescence emission ratio of excimer tomonomer emission (IE/IM) showed a significant increase of excimer emission in atemperature range 283-313 K, then a decrease to a constant value at 319 K. Two excimerbands were observed when time-resolved spectroscopy was used: i) a broad, structurelessband with a maximum at 420 nm and a corresponding lifetime of 250 ps and ii) the well-known band of pyrene excimer, with a maximum at 470 nm and a lifetime of 68 ns. Inthe initial time region, 0-150 ps, monomer emission was observed, with a simulation bya superposition of three components (377, 398 and 421 nm). They observed only oneexcimer emission above the LCST and that was with a maximum at 470 nm. Theyconcluded that the LCST implies a complete disruption of the ordered microstructures,which were created in cold water.

An interesting application of fluorescence studies was reported by Huang and co-workers[73]. They studied intermolecular complex formation between mesogenic terphenyldiimidemoieties of a thermotropic liquid-crystalline (LC) polyimide (P-11TPE). The temperaturedependence of the complex fluorescence peak wavelength is shown in Figure 4.21. Itshifts slightly to a shorter wavelength below the Tg and then shifts to a longer wavelengtharound crystallisation, then again shifts to shorter wavelength during a complicatedcrystal-crystal transition around 200 °C, and around the phase transition from crystallineto smectic phase at 240 °C. An Arrhenius-type plot for changes in fluorescence lifetimeof the complex during heating is shown in Figure 4.22. The lifetime decreases withtemperature until the Tg after which it slightly increases to again decrease gradually withtemperature until the material reaches the smectic phase. The decrease of lifetime withtemperature seen in Figure 4.22 indicate that the radiationless deactivation process whichaccompanies the observed fluorescence is in agreement with Equation 4.20 and is involvedin the complicated phase transitions of LC polyimides. It was also concluded that theseapparent changes in wavelength of fluorescence and lifetime behaviour are dependent ontemperature and indicate the complexity of the nature of intermolecular complexes andradiationless deactivation processes in various phases of the thermotropic LC polyimides.

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Figure 4.21 Temperature dependence of the fluorescence peak wavelength duringheating of thermotropic liquid-crystalline polyimide. Excitation at 320 nm.

(Reprinted from Polymer, Volume 40, H.W. Huang, T.I. Kaneko, K. Horie and J.Watanabe, Fluorescence study on intermolecular complex formation between

mesogenic terphenyldiimide moieties of a thermotropic liquid-crystalline polyimide,3826, copyright 1999, with permission from Elsevier Science)

Figure 4.22 Arrhenius-type plot for the change in lifetime of fluorescence of liquid-crystalline polyimide during heating

(Reprinted from Polymer, Volume 40, H.W. Huang, T.I. Kaneko, K. Horie and J.Watanabe, Fluorescence study on intermolecular complex formation between

mesogenic terphenyldiimide moieties of a thermo-tropic liquid-crystalline polyimide,3826, copyright 1999, with permission from Elsevier Science)

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4.5 Conclusions

Many of the significant features observed in the photophysics and photochemistry ofpolymer systems have been shown to have their origin in the way the behaviour ofelectronically excited states are affected by molecular motion. Most of the phenomenainvolve a competition between physical processes leading to non radiative deactivationand emission of the excitation energy, and thus are most easily observed in thecharacteristics of fluorescence. The competition between processes can be controlled byeither thermodynamic or kinetic parameters. Which of the parameters is of primeimportance depends largely on: (i) the environment of the luminescent species, (ii) if theluminescent species is attached to the polymer or detached from the polymer, or (iii) ifthe luminescent species is randomly distributed along the polymer chain or not. Forexample, measurement of the relative intensities yields information concerning the orderingof the polymer chain and similarly studies of the kinetics of fluorescence decay yieldinformation on energy migration and segmental motion. The changes in fluorescenceintensity and fluorescence lifetime around phase transition temperatures can yieldinformation on packing pattern and variation with thermal treatment of the polymerduring the phase transition.

The excited state properties, principally fluorescence, of probe molecules incorporatedin the polymer chain can be used to a large extent to study molecular motion, order, andenergy migration in polymeric systems. The present review has revealed how fluorescencemethodologies can provide an insight into the nature of the intramolecular andintermolecular interactions, which are responsible for thermally initiated variations aswell as the formation of microstructures and the morphology of a polymeric medium.

Acknowledgements

This work was supported by KBN grant No. 3 T09B 058 14 (Poland).

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5 Polymers for Light Emitting Diodes

A. Bolognesi and C. Botta

5.1 Introduction

Polymer science, which is a young branch of chemistry, has been the subject of greatdevelopment both as a basic and applied science in the last 30 years. In the first instance,the great interest in polymers was due to the good mechanical properties associated withmaterials having low density and low processability costs. Later the possibility ofcombining new chemical functions in a backbone opened new fields of applications formacromolecules. So structures with specific uses were developed creating new interfaceswith technological fields that, at the beginning, were unusual for polymers: pharmacology,biology, medicine, optics, electronics.

In particular, the development of polymers for electronics is still an open field wherepolymers are used not only as insulators, but where the electronic properties of conjugatedmacromolecules can be tailored for specific applications.

The problem which confined the use of polymers only in the area of insulators wasovercome in 1977 with the discovery by McDiarmid and Shirakawa [1] that iodine-doped polyacetylene possesses metallic conductivity.

After this discovery, many polymeric structures were synthesised with the aim of improvingboth the electrical conductivity and the stability of the materials. At the end of the 1980s,work by Tang and Van Slyke [2] on the electroluminescence (EL) of aluminium 8-hydroxyquinoline (Alq3) stimulated a great amount of work worldwide on the generationof light by electrical excitation in organic materials. It was in 1990 that Friend and co-workers [3] presented the first electroluminescent polymeric device able to emit at 2.2eV with an external quantum efficiency of 0.05%. The light emitting diode (LED) wasformed by a single layer of PPV sandwiched between an indium tin oxide (ITO)-coatedglass and an aluminium cathode which was vacuum evaporated on the top of the polymerfilm. Injection of electrons takes place at the cathode, while holes are injected at theanode. In the following years, the number of scientific contributions on polymeric LEDsincreased consistently. Different strategies were developed, all addressed to understandingthe basic mechanism of the injection of opposite charges in the polymers, their travellingin the thickness of the layer, and their radiative recombination, all these factors beingfundamental for the increase of efficiency and lifetime.

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The great effort that is still pursued in this field is responsible for both the optimisationof new polymeric structures and for the preparation of new and more reliable devices.The fast development of this branch of polymer science has stimulated the interest of theindustrial world and nowadays there are several small, medium and big enterprises, bothin the chemistry area and in the microelectronics sector, developing LED prototypes. Ithas been recently reported that Philips will be able to produce backlighting and segmenteddisplays very soon [4], Seiko-Epson and Cambridge Displays Technology are developinga technology using ink-jet printing of electroluminescent material for colour patterning.The development in this area is extremely fast; in reference [5] some of the web sitescontinuously updating in this field are reported. It is suggested that the reader visit thesesites for the very latest news.

5.2 The Physics of Electroluminescent Devices

5.2.1 The Physics of Conjugated Polymers

Several reviews have been reported on the electronic properties of conjugated polymers [6-9] as well as on the physical mechanisms occurring in LED devices [10-12]. Here, a fewaspects of this very wide field are summarised, focusing on the properties of the electronicexcitations which are more relevant in the physics of an electroluminescent device.

Conjugated polymers possess strongly anisotropic electrical and optical properties sincethe π-electron delocalisation occurs along the chain direction. An ideal conjugated polymerchain possesses a one-dimensional structure which, due to the Peierls instability, leads tothe electronic structure of a monodimensional semiconductor with an energy gap typicallyin the 1.5-3.5 eV range. The Su, Schrieffer and Heeger theory, often referred to as SSH,[6, 13], where electron-electron (e-e) interactions are neglected, describes the propertiesof the polymeric semiconductors by introducing a strong electron-phonon interaction.According to this theory, for non degenerate ground state polymers, injection of charges(by chemical doping, through an electrode or by photoinduced interchain electron-hole(e-h) separation) leads to the formation of singly charged polarons or doubly chargedbipolarons. Neutral species (exciton-polarons) can be generated by photoexcitation viae-h separation or by the fusion of two polarons with opposite charges, obtained viacharge injection from the electrodes, and are responsible for photoluminescence (PL)and EL, respectively. Both the charged and the neutral excitations are stabilised througha local modification of the bond alternation. This produces two localised electronic stateswithin the forbidden energy gap. The energy position of these levels, which aresymmetrically displaced from the midgap, depends on the spatial extent of the defect(polaron or bipolaron) [14]. The more extended the defect, the closer the levels to themidgap. The optical transitions occurring between these levels and the conduction and

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Polymers for Light Emitting Diodes

valence bands (see Figure 5.1) account for the sub-gap optical absorption bands observedby chemical doping, by photoinduced absorption, and by charge injection. According tothis model, the PL is due to the spin-allowed transition from the upper to the lower ofthe polaron-exciton levels.

However, the PL emission is generally observed at energies higher than the spacing of thetwo polaronic levels, measured with photoinduced absorption techniques. Indeed, thereare several areas of the physics of conjugated polymers which cannot be explained withouttaking into account the e-e interaction effects. In order to describe the phenomena relatedto the neutral excited states of conjugated polymers (which are primarly involved in thephysics of LEDs), models generally applied to the physics of molecular organic crystals[15] are more appropriate. In conjugated polymeric systems, the coulombically bound e-h couple can be described as a Frenkel exciton (localised within a molecule) or a Wannierexciton (extended over many molecular units), according to its binding energy or degreeof localisation [16-18]. When interchain interactions are strong, charge transfer excitonsare formed, as excimers and exciplexes [15, 19, 20].

Only singlet excitons, S, are created by photon absorption (direct triplet generation isforbidden from the spin selection) and recombine radiatively with a certain probability,expressed by the PL quantum efficiency, ηPL (the ratio between the number of photonsemitted and the number of photons absorbed). When the exciton is created by thecoalescence of two polarons, the probability of forming a singlet exciton is 1/4 while

Figure 5.1 Scheme of the generation and radiative recombination of the singlet excitonS by photoexcitation (a), and by charge injection (b) mechanisms

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for a triplet exciton (which does not contribute to the EL emission) it is 3/4 [21, 22].After an exciton has been created, it moves in the material by hopping or by resonanttransfer processes [15, 23-26]. If more than one material is used (blends orheterostructures), by a proper selection of the materials, the exciton migration processesmay red shift the emission and also increase the emission efficiency [25-27]. If onlyone polymer is used, exciton migration effects decrease the ηPL of the material since theprobability of exciton quenching by impurities is increased. The degree of order andcrystallinity of the material plays an important role in these processes as well as indetermining the charge mobilities [28, 29].

5.2.2 The Physics of the Device

In this section, some basic information on operative guidelines is provided for readerswho are new to this field. LEDs are devices that transform electrical signals into opticalsignals. The typical structure of a single-layer polymeric LED is shown in Figure 5.2. Aglass substrate coated by ITO is used as a transparent (positive) electrode. A polymerlayer is deposited onto the substrate by a spin coating technique. On top of the polymeris deposited, by vacuum evaporation, a metallic layer which forms the negative electrode.By applying a bias voltage at the electrodes, light emission is obtained through thetransparent electrode.

The working mechanism of the device is shown schematically in Figure 5.3, where thelevels of the electrodes and the HOMO (highest occupied molecular orbital) and LUMO(lowest unoccupied molecular orbital) of the polymers are reported, together with theintragap levels of the polarons and the exciton-polaron. The position of the HOMO and

Figure 5.2 Scheme of a single layer LED

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LUMO of the polymer, relative to the Fermi levels of the electrode, are determined by theionisation potential (IP), the electronic affinity (EA) of the polymer and the workfunction(φ) of the metals, as shown in Figure 5.3. The working mechanism of a LED consists offour main steps:

(1) By applying a bias voltage, charges of opposite signs are injected in the active materialand form positive and negative polarons.

(2) The charges P+ (P-) move in the material towards the negative (positive) electrode,driven by the applied electric field.

(3) Polarons of opposite sign couple to generate the excitons, S.

(4) S recombine radiatively by photon emission.

The efficiency of an EL device (ηEL) is the ratio between the number of the emittedphotons and the number of injected charges. It can be expressed as

ηEL = γ ηEX ηPL (5.1)

where γ is a factor dependent on the double charge injection (γ = 1 for perfect chargebalance), ηEX is the efficiency of singlet exciton generation (maximum ηEX is 1/4) andηPL is the measurable PL quantum efficiency [30].

Figure 5.3 Working mechanism of a LED: P+ and P- are positive and negative polarons;φ is the metal work function; EA is the electronic affinity; IP is the ionisation potential;

Δe and Δh are potential barriers for negative and positive charges; HOMO is thehighest occupied molecular orbital; LUMO is the lowest unoccupied molecular orbital.


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Some considerations can be made on each of the four steps in order to increase the deviceperformances:

(i) Charge injection. The number of injected positive and negative charges must besimilar. This is obtained if the electrode metals have φ such that the potential barriersfor positive (Δh) and negative (Δe) charge injection are similar (Δe≈Δh). Positiveelectrodes generally used are ITO, PANI [31, 32] and silver, with φ in the 4.5-4.8 eVrange [33-36]. Recently, the introduction of a conducting polymer interfacial layerbetween the ITO and the active polymer has been reported to provide φ determinedby the intermediate polymer [37]. For the negative electrode, calcium (2.9 eV), indium,aluminium (4.2-4.3 eV), silver, copper and several alloys are used [38]. Calcium usuallygives the best results since the device efficiency can be increased by reducing Δe,which controls the injection of the minority carriers [33, 34, 39].

(ii) Charge transport. A good balance of positive and negative charge transport isnecessary. Unfortunately, conjugated polymers are better hole than electrontransporters, since P- are easily trapped. The high trapping of negative charges isresponsible for bias-dependent efficiencies with low efficiency near the onsetvoltage. Improvement of charge transport can be obtained by inserting an electrontransporter layer (ETL) and a hole transporter layer (HTL) between the activematerial and the electrodes. The introduction of an ETL also has the advantageof reducing the hole currents, as it acts as a hole blocking layer, thus increasingthe charge balance [40, 41].

iii) Exciton generation. The efficiency of exciton generation can be optimised byincreasing the probability of polaron coalescence, through a good balance of positive/negative charges in a restricted active volume (heterostructures) [40].

(iv)Radiative recombination. The efficiency of radiative recombination of the S excitonis directly related to ηPL, and can be maximised by reducing the quenching centresand by localising the exciton (polymer blends, conjugated copolymers, conjugated-non conjugated copolymers [42, 43]). The active area of the polymer must be farfrom the electrodes, where a high concentration of quenching defects are present[44-46]. This is generally obtained by confining the active layer at the interface betweentwo materials (heterostructures). Another way to increase the radiative recombinationprobability is to transfer the excitation from one material to another (more efficient)material (lower gap polymer or dye) by resonant energy processes such as Foerstertransfer. These processes also red shift the emission. Factors which limit the radiativerecombination are an excess of density of charges (exciton-polaron quenching) andquenching by the electric field [47-49]. These effects can be reduced by improvingthe charge balance and by reducing the working bias voltage (V).

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5.2.3 LED Characterisation

In order to characterise a device, the I-V (current intensity versus bias voltage) characteristictogether with the EL-V (EL intensity versus bias voltage) curves are measured, and,through their ratio, the external efficiency of the device is deduced taking into accountthe geometrical factors [50]. In Figure 5.4 a typical behaviour of a LED is reported.When a good charge balance is obtained in the device, the two curves are similar andpossess the same onset voltage, Von. If the current onset is at lower voltages than the EL(as in the case of Figure 5.4), an excess of positive charges is usually responsible for bias-dependent efficiencies. The spectral shape of the EL is then compared with the PL of theactive material. The same spectral shape is expected when the active region is in the bulkof the material, while emission influenced by chemical degradation of the polymer [44]or by interference effects [45] is often observed when the emitting material lies at thepolymer-electrode interface.

Many LEDs have been studied in order to gain information on the detailed physicalmechanisms involved in their operation. The studies on fluorescence quenching inducedby electric fields, as well as internal electric field distribution analysis, have provideddeep insight into these phenomena [16, 21, 47, 51-59].


Figure 5.4 Typical I-V and EL-V curves for a single layer LED based onpoly(3-alkylthiophene). The onset voltage (Von) is shown.

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5.3 Polymeric Structures for LED

The tailoring of polymeric structures plays a significant role in the development of thisnew technology. Polymer chemists have synthesised from the beginning new structuresin order to obtain the desired properties of conjugated macromolecules. The role ofmany factors are almost completely understood, but there is still a lot of work to dobecause improvements both in lifetime and in electroemission efficiencies are possible.The structural control can be extended to several physicochemical parameters:

• The band gap of a conjugated polymer is responsible for the PL and EL peak position,

• The EA and the IP of the polymer strongly affect the charge injections from theelectrodes into the polymers, and this is related to the PL and EL efficiency [33, 34],

• The solid state packing of the macromolecules influences the stability and the emissionefficiencies [42],

• The surface polarity of the polymer is responsible for the adhesion between the activepolymer and the electrodes, which is an important factor in charge injection, and

• Resistance to oxidation and to temperature of the active polymeric layer arefundamental to the lifetime of the device.

Optimisation of all the above parameters is extremely difficult on a chemical synthesisbasis, but the great efforts provided by many groups in this area have lead to excitingresults for both improvements in macromolecular chemistry and in technological application.

5.3.1 Polyphenylenes

5.3.1.1 PPV

PPV (Figure 5.5) was one of the first polymers studied, for its good PL and EL efficiencies.Several synthetic routes have been reported for the preparation of this polymer. Itsinsolubility is a great problem in the preparation of high molecular weight materials. Forexample, step growth polymerisation, such as Wittig condensation betweenterephthaldicarboxaldehydes and arylene-bisphosphylidenes gives very low molecularweight polymers, because when the growing chain is formed by 6-10 repeating units, itbecomes insoluble and chain growth stops. The low molecular weight polymers synthesisedin this way cannot be used for thin film preparation because they are insoluble. Toovercome these difficulties, a soluble form of a precursor was synthesised. A thin film ofthe precursor was formed by spin coating techniques; a subsequent thermal treatment ofthe film lead to the insoluble conjugated thin film of PPV.

149

The Wessling method, consisting of the preparation of a sulfonium precursor polymer,was one of the first reported [60]. In Figure 5.6 the procedure followed to prepare PPVaccording to [61] is reported. Several modifications of this general procedure have beenintroduced, consisting of changing the conditions of the transformation process [62-64]and/or the nature of the chemical species to be eliminated [65-67]. Another procedure,introduced by Gilch [68] in 1966, consists of the polymerisation of dichloro-p-xylenewith potassium tert-butoxide in organic solvents.


Figure 5.5 Structure of PPV

n

Figure 5.6 Synthetic procedure for PPV(Reprinted from Synthetic Materials, 41-43, 261-264, P.L. Burn, D.D.C. Bradley,

A.R. Brown, R.H. Friend, A.B. Holmes; Studies of the Efficient Synthesis of PPV andDimethoxy-PPV, 1991, with permission from Elsevier Science)

XH2C CH2XS

H2C CH2

X =

Cl- +

X

or X = S(CH3)2

S

+ Cl-

(0.4 M / MeOH)

1. -OH (0.4 M / H2O), 1h, 0°C, N2

2. H+ (0.4 M / H2O), to pH = 5-7

3. Dialysis, 3 days (dist. H2O), r.t., N2

n

X =

Cl- +

(II) X = S(CH3)2

H2C CH

+ Cl- (I)

OMe

Solution in MeOH

n

Solution in CHCl3

(III)

300 °C, 1 h

vacuum

200 °C, 2 h

HCl (g)

n

PPV

150


The EL spectrum of a LED prepared with PPV is reported in Figure 5.7, exhibiting anemission centred at 525 nm that corresponds to yellow-green light. The external efficiencyof the simple LED formed by ITO, PPV, and aluminium (usually reported as ITO/PPV/Al), was 0.05% [3]. This value increases by one order of magnitude if calcium is usedinstead of aluminium [69]. The use of a calcium electrode increases the efficiencies, butthis metal is very reactive to humidity and its use may be allowed only in an inertatmosphere or when the electrode is well covered with a protecting layer.

The first EL from PPV stimulated the work of chemists to develop new alternative routesto improve the preparation of this polymer. The idea of having a polymer soluble inorganic solvent already in its conjugated form led to the introduction of solubilisinggroups in the PPV backbone. Some of the most studied structures that were developedare reported in Table 5.1. A detailed description of the synthesis for their preparation isnot given; references are reported for each structure [70-85]. Some of the general criteriafollowed to prepare these materials are discussed.

Highly ordered structures (i.e., highly packed in three dimensions) have the tendency todecrease the PL and consequently the EL efficiency. In fact, strong interchain interactions,which are much more relevant in the solid state than in solution, increase both excitonmigrations to quenching sites [42, 43] and the formation of low energy excited states, mainlydecaying non radiatively [86]. Consequently, it must be noted that polymer PL efficienciescan be very high in solution, but generally decrease by one order of magnitude in the solidstate. The introduction of branched side chains, rather than linear residues, was found to beeffective in lowering the degree of order of the material, preventing the tight packing of themacromolecules. This is the case of structures 1a, 1b, 2a, 2b, 4a, 4b in Table 5.1. For example,

Figure 5.7 Electroluminescence spectrum of PPV

151

Son and co-workers [87] found that the introduction of cis double bonds in a PPV interruptsthe conjugation and disturbs the polymer chain packing, resulting in the formation of analmost amorphous PPV with an EL efficiency of 0.22% for a simple ITO/PPV/Al LED.According to these findings, a simple ITO/MEH-PPV (1a)/Ca LED was reported to give anexternal quantum efficiency of 1% at 620 nm [70]. Changing substituents also has the effectof modulating the gap, resulting in a change in colour emission, as shown in Table 5.1.

It is worthwhile mentioning that in [70] a simple flexible device of PANI-polyethyleneterephthalate (PET) is described. PANI is a soluble conducting polymer which acts as ahole injecting electrode substituting ITO; on the top of the PANI layer a film of MEH-PPV(poly[2-(2´-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene]) was deposited. The electroninjecting electrode was formed by calcium. The advantage of this LED with respect toother ones (where glass is covered by ITO) is that PET and PANI are flexible, hence thedevice can be completely bent without failure. This example shows the potential of polymericmaterials with respect to inorganic compounds which cannot give large flexibleelectroluminescent areas. The preparation of a suitable film by spin coating or by castingfrom solution is the key step for development of new interesting devices. An example of aneedle-like LED is described in [88], where the MEH-PPV active layer is directly formedonto a thin aluminium wire. The cathode is then formed by deposition onto the activelayer of a thin transparent film of PANI thus obtaining an electroluminescent wire.

Soluble electroluminescent polymers can be dissolved with non electroluminescentpolymers; by spin coating the mixed solution, a blend film is obtained. This is a reliablemethod for improving the efficiencies both in PL and EL. In fact, blending allows adecrease of the interchain interactions. The drawbacks of blending mainly consist of thehigher working voltage and a decreased lifetime of the devices [89]. Moreover, the mixingof two different polymeric materials often leads to phase segregation which has to beavoided if a decrease in the interchain interactions is required.

The synthesis of conjugated copolymers is another approach followed to prepare newstructures useful for device preparation. Moreover, the copolymeric approach is a suitablemethod for further tailoring the electrooptical properties of the active layer. In fact, thecombination of different monomeric units, in different ratios, in the same structure,allows a fine modulation of the properties of the materials [90].

In Table 5.2 some of the many soluble copolymers based on the PPV structure are reportedas examples. The structures so far prepared are mainly amorphous; the glass transitionshave to be taken into account for the preparation of devices where the dimensionalstability is very important. Some of the materials are block copolymeric structures (9, 12and 16), while in other structures, although a repeating unit can be identified, the termcopolymer is preferred because more than one chemical function appears in the repeatingunit. As in the previous table, for each structure references are reported [69, 91-101].


152


Table 5.1 PPV derivatives used as active materials in LED. For somestructures the external and/or internal efficiencies are reported together with

the kind of cathode used and wavelength of the EL emission

OR

OCH3

(a) MEH-PPV R =

Elmax = 620 nm; ηel = 1% (Ca cathode) [70, 71, 72, 73]

(b)

[74, 75, 76]

OC1C10 R =

(c)

[77]

R =

n

OR

OR

(a)

Elmax = 560

[78]

R =

(b)

Elmax = 560

[79]

R =

n

1

2

153


Table 5.1 Continued

OR

OR

Si C6H12

CH3

CH3

(a) R =

Si CH3

CH3

CH3

R' =

(c) R = Si C8H18

CH3

CH3

[80]

R =(b)

Elmax = 540 nm; ηel = 0.0003% (Al cathode) [81]

R' = H

R' = H

n

Elmax = 564 nm; ηel = 0.05% (Al cathode);

0.08% (Ca cathode) [82]

R

(a) R =

R

R'

n

R' = –(CH2)nH n = 6, 8, 10

(b) R =

[83]

Elmax = 510 nm; ηel = 0.1% (Mg cathode); [84]

0.002% (Al cathode)

3

4

154


Table 5.1 Continued

Elmax = 540-560 nm; ηel = 0.01% (Al cathode) [85]

n

(a)

Z

OR

Z = H

(b) Z = SO2CH3

R =

5

Table 5.2 Examples of complex polymeric structures used as active materials in LED

nS

CN

C12H25

OC6H13

OC6H13 CN

Elmax = 750 nm; ηel = 0.2% (internal, Ca cathode); 0.07% (internal, Al cathode) [91]

OC6H13

NCC6H13O

C6H13O

OC6H13

CN

n

Elmax = 710 nm; ηel = 0.2% (internal, both Ca and Al cathode) [92, 93]

6

7

155


Table 5.2 Continued

N N

N

C10H21

C10H21

Elmax = 540 nm; ηel = 0.015% (Al cathode) [94]

OCH3

OCH3

nm

ηel = 0.02-0.3% (Al cathode) [69]

n

OCH3

OCH3

OC7H15

OC7H15

Elmax = 590 nm [95]

n

R

R

OCH3

O(CH2)6

OCH3

O

OCH3

OCH3

Elmax = 455 nm [96]

8

9

10

11

156


Table 5.2 Continued

n

Si

*

C8H17

O

OCH3m

Elmax = 576-618 nm; ηel = 0.1-0.02% (internal, Al cathode) [97]

n

[98]

[99]

n

NCN

Si

R

R

n

[100]

12

13

14

15

157

Structure modulation, through a proper choice of the monomeric units forming thecopolymer, has a great influence on its electronic properties; altering the copolymercomposition allows tuning of the band gap, i.e., the control of the emission colour extendingfrom the red, near IR region (700-1100 nm, peaked at 850 nm) of copolymer 6 in Table 5.2[91], to the orange/red of the cyano-PPV [92] (structure 7) or of its derivatives [93] and tothe blue of poly(1,20-(10,13)-didecyl)distyrylbenzene-co-1,2-(4-(p-ethylphenyl))triazole(TRIDSB) [94] (structure 8). Another example of modulation of the gap is shown incopolymers 11 and 14. The insertion of a non conjugated segment or of a silicon atom,respectively, interrupts the conjugation path giving emission peaked at around 450 nm.

One of the factors that induces a decrease in the EL efficiency is the poor suitability ofpolymers to transport one of the injected charges, resulting in an imbalance in therecombination. A suitable chemical approach to increase the injection of electrons consistsof increasing the electron affinity of the polymer for the minority charge carriers. Asmost of the semiconducting polymers are easily p-doped and not n-doped, the increaseof the electron affinity through the introduction of suitable electron withdrawingsubstituents has been found to be very promising. This is the case of structure 7 (Table 5.2)showing emission at 710 nm; internal quantum efficiencies up to 0.2% were reachedboth with a calcium and aluminium cathode in a single layer device [92].

It is important to remark that, due to the presence of cyano groups, the lowest unoccupiedorbitals, forming the conduction band, are lowered, so that good injection of electronsboth from calcium and aluminium occurs and equivalent efficiencies are obtained eventhough the two metals have different work functions. However, for PPV single-layerdevices, it was reported [69] that the quantum efficiency with the calcium electrode isone order of magnitude higher than the corresponding device where aluminium is used.

Table 5.2 Continued

n*

OC10H21

*

OC10H21

OC10H21

*

OC10H21

m p

Elmax = 515-567 nm [101]

16

158


Figure 5.8 Hole transporter (tetraphenyl diaminobiphenyl (TPD)) and electrontransporter (2-(4-biphenyl)-5-(tert-butylphenyl-1,3,4-oxadiazole (PBD))

An even better balance in the injection of the opposite charges was found when anotherlayer more suitable for hole transport, such as unsubstituted PPV, is used [70]. A double-layer LED, formed by ITO/PPV/cyano-PPV/Al has an internal quantum efficiency ashigh as 4% with emission at 610 nm [70]. The cyano-PPV approach was extremelyuseful and other withdrawing substituents were introduced in the PPV backbone withthe aim of increasing the electron affinity; other electron-deficient nitrogen containinggroups such as oxadiazole [102, 103], triazole [94], pyridine [104, 105] and quinoxaline[106] were also introduced in the polymer structure.

Copolymer 10 was found to give bright EL either with a dc forward bias or with areverse bias voltage, and also with alternate field. The EL spectra are almost equivalentfor a simple ITO/10/Al configuration.

As previously reported, the balance between the two opposite injected charges is one of themain goals of this area. Apart from the increase in the electron affinity of the material, theadding, in the proper position (i.e., near the cathode or near the anode) of an additionallayer formed by polymeric material and/or small molecules as electron transporters or holetransporters was found to be very useful. An example has already been reported above:PPV has been used as a hole transporter in a double-layer configuration LED [92]. Thenumber of low molecular weight molecules synthesised as hole or electron transporters isextremely high. Two of the simplest molecules are shown in Figure 5.8. Good electrontransporting molecules include 1,3,4-oxadiazole derivatives, while triphenylamine andrelated structures (TPD, Figure 5.8) are good hole transporters. A simple method to obtaina better electron injection consists of the deposition of a layer of 1,3,4-oxadiazole on thetop of the polymeric emitting layer.

N N

H3C

CH3

CH3

NN

O

TPD

PBD

159

It was found, however, that devices having 1,3,4,-oxadiazole derivatives, or other lowmolecular weight molecules, as electron transporters in the form of a vacuum evaporatedthin film, have a short lifetime, a feature probably associated with recrystallisation oraggregation phenomena [107].

Incorporating hole or electron transporting molecules in the polymer backbone is a generalprocedure that has also been applied to other emitting polymers [108-110]; polymerichole or electron transporters have also been prepared [111, 112].

There are examples in the literature where the balance between holes and electrons isreached by inserting a hole blocking material between the ITO and the active polymers.For example, a simple polymethylmethacrylate (PMMA) monolayer prepared by meansof the Langmuir Blodgett technique was reported to give a four-fold efficiency increase ifintroduced between the ITO and the active layer [113]. Greenham, Nüesch and Yang[114-116] followed similar approaches to control the performance of a MEH-PPV baseddevice and a polyparaphenylene-based device respectively.

PANI was reported to improve the performance of a PPV LED [116]. The high interfacearea between the PANI film and the active luminescent layer enhances the local electricfield increasing charge carrier injection [117].

Other materials were found to be very efficient if introduced between the ITO and theactive layer. Many authors [118, 119] reported that poly(3,4-ethylenedioxythiophene)(PEDOT), mixed with polystyrene sulfonated acid, PSSA, (Figure 5.9) has a meaningfuleffect both on the lifetime and in the EL efficiency if spin coated between ITO and theactive polymeric layer. Very recently this subject has been studied by means of theultraviolet photoelectron spectroscopy (UPS) technique [37].


O O

S n SO3H

n

PEDOT PSSA

Figure 5.9 Poly(3,4-ethylenedioxythiophene) (PEDOT) and polystyrene sulfonatedacid (PSSA)

160


The problem of the interfaces between the conjugated systems and the electrodes hasbeen deeply studied, indicating that chemical reactions limiting the charge injectionsoccur at the interfaces [120]. Degradation of the polymer near to the surface occurs andthis is one of the limiting factors reducing the lifetime of the devices.

5.3.1.2 Other Polyphenylene Derivatives

The deep and detailed work performed by different groups on the many PPVderivatives is only a part of the huge amount of other structures which wereinvestigated with the aim of reaching valuable performances of polymeric LEDs. Asthe double bonds in PPV are reactive chemical functions which can be oxidised, thepolyparaphenylene (PPP) structures were intensively investigated with the aim ofobtaining more robust materials.

PPP (Figure 5.10a) is insoluble and only the introduction of alkyl side chains(Figure 5.10b), linear or branched, has enabled the polymers to be solubilised, obtainingthin homogeneous films suitable for LED applications, even though one of the first LEDsemitting in the blue region was prepared from conversion of unsaturated PPP [121].

Figure 5.10 Polyparaphenylene (PPP) (a) and substituted polyparaphenylene (b)

n n

(a) (b)

PPP, substituted or unsubstituted, is not planar; the band gap is relatively high andemission is shifted towards a higher energy with respect to PPV. So, while the emissionof PPV is mostly centered in the yellow-green region, PPP emissions can reach the blueregion of the spectrum.

161

Soluble PPP obtained through the Suzuki coupling, as reported by Schlüter and Wegner[122], gives high molecular weight compared to previously reported soluble PPP synthesis[123] and an LED prepared with an alkoxy PPP with a two-layer configuration, ITO/PVCZ/alkoxy PPP/Ca reaches a quite respectable EL efficiency of up to 3% [124]. Withthe introduction of alkoxy-branched side chains, it was possible to prepare the active thinlayer by means of the Langmuir Blodgett technique [125]. During the transfer process,orientation of the macromolecule axis in the dipping direction occurs. LEDs having theseoriented layers as the active film give polarised EL with a polarisation ratio between thetwo perpendicular directions of about 5 and an external emission efficiency value of 0.05%.

A block of substituted phenyl sequences connected with double bonds has allowed for thepreparation of copolymer structures that can be used as a thin active layer in an LED, withemission in the 460 nm region and internal quantum efficiencies from 2% to 4% [126].The Mullen and Scherf approach [127, 128] to ladder PPP (L-PPP) provided a good structuralimprovement for the preparation of stable and tuneable light emitting polymers.

An example of an L-PPP is reported in Figure 5.11. As the introduction of the bridgesbetween adjacent monomers induces planarisation of the backbone, a remarkable shifttowards higher wavelengths was observed for both PL and EL that reached a value of600-620 nm at the maximum of the peak. This shift was also attributed to the formationof excimers due to interchain interactions in a highly planarised, conjugated segment[128]. The introduction of suitable substituents has led to an L-PPP [129] having blue-green emission with external EL efficiencies of 4% in a single-layer configuration withaluminium as the cathode. It was also reported that a certain modulation in the emittedlight through the applied field variation was possible in the blue-green range [129].


HR

HRC6H13C6H13

C6H13

n

Figure 5.11 An example of an L-PPP

An interesting result in preparing devices with red-green-blue (RGB) emission was shownby Leising and co-workers [130] using parahexaphenyl (PHP) as an active emitting layer.The blue emission of PHP was converted by means of a suitable dye layer absorbing inthe blue region into green light which excited another dye layer absorbing green andemitting red. The light was also spectrally purified with a suitable dielectric mirror/filter.A representation of the RGB LED architecture is reported in Figure 5.12, as from [130].

162


5.3.1.3 Poly(phenylene ethynylenes)

The desire of polymer chemists to test new structures in the family of phenylenes has ledto the preparation of polymers having triple bonds in between two phenyl rings;poly(phenylene ethynylenes), alkoxy or alkyl substituted, have been prepared and studiedfor their electrooptical properties.

PL emissions in the 550-600 nm region were found [131] in polymeric structures havingan average degree of polymerisation between 23 and 28.

Alternate copolymers containing thiophene units [132] having PL efficiencies up to 38%in CHCl3 solution at 460 nm were also described.

These polymers were recently studied for their liquid crystalline behaviour [133] and forthe possibility to be oriented and give polarised EL [134, 135].

5.3.1.4 Polyfluorenes

Another class of semiconducting polymers containing the phenylene ring has recently attractedthe attention of many researchers for its promising properties. In Figure 5.13 the structure ofpoly(9,9-dihexylfluorene) is shown as an example of this class of polymers [136, 137]. Thepossibility of introducing, as in the case of substituted PPV, alkyl side chains of differentchemical nature and branching level without seriously affecting the electronic properties ofthe backbone is one of the advantages of this polymeric family [138].

Figure 5.12 RGB LED architecture [130](Reproduced with permission from Advanced Materials, 1997, 9, 1, 35, published by

Wiley-VCH, 1997)

163

Polymerisation of 2,7-dibromo-9,9-bis(3,6-dioxaheptyl)fluorene with Ni(0) catalyst isreported to be efficient, giving polymers with molecular weight of 215,000 [139]. Othersynthetic approaches through the Suzuki coupling gave lower molecular weight [140].Pei and Yang introduced 2-methoxyethoxyethane in the 9 position of a fluorene monomer[139]. The corresponding polymer, obtained by polymerising the 2,7 dibromoderivativesof the monomer by Ni(0) catalyst, poly[9,9-bis(3,6-dioxaheptyl)-fluorene-2,7-diyl](BDOH-PF) had a PL efficiency in solution of 77%, which remains almost constant(73%) in the solid state, with two sharp peaks in the blue region and a broad intensepeak in the green region attributed to interchain excimer emission. A simple single-layerLED, ITO/BDOH-PF/Ca has an external quantum efficiency of 0.3%. This low ELefficiency, almost unexpected on the basis of the PL quantum yield, was attributed to thepoor electron injection. The introduction in the active polymeric layer of lithium triflate[139], in a given ratio with respect to the polymer, increased the EL up to an externalquantum efficiency of 4% in the blue-green region.

Addition of inorganic salts, as described by Pei and co-workers [141, 142] induces, uponapplying the field, a migration of the cations and anions in opposite directions causing n-doping at the cathode and p-doping at the anode. This doping is responsible for a moreefficient injecting contact at the interfaces between cathode/anode and the active layer.The polar nature of the side chains solvates the ions of lithium triflate that were introducedin the polymer and promote their transport. By adding polyethylene oxide in the matrix,the device gave a white emission [142].

The copolymerisation of fluorene units with other phenylene monomers was studied byMiller and co-workers [143]. Copolymers having molecular weight ranging from 24,000up to 204,000 were obtained with a Ni(0) polymerisation. High PL efficiencies in solutionand the solid state were found for these copolymers, while external EL efficiencies werearound 0.15% at 525 nm when tetraethyl ammonium tetrafluorate was added (2%) inthe active layer. Introduction of different amounts of anthracene units in poly(di-n-hexylfluorene) did not have a meaningful improvement in the device performance [144].

Soluble polyfluorenes were found to give a liquid crystalline behaviour. For example,poly(9,9-di(ethylhexyl)fluorene (PDF2/6) (Figure 5.14) was found to have a transition intoa birefringent fluid phase [145] which can be obtained in a highly oriented arrangement by


Figure 5.13 Poly(9,9-dihexylfluorene)

C6H13H13C6

n

164


Figure 5.14 Structure of poly(9,9-di(ethylhexyl)fluorene) – PDF2/6

n

deposition onto a highly preoriented polyimide substrate [145]. The degree of orientationwhich can be reached [145] seems to be dependent on the kind of side chain, (linear orbranched) or on the different packing reached by changing the side chains.

5.3.2 Polythiophenes

The wide field of research in the area of electroluminescent polymers is not limited tophenyl-like structures. Polythiophene derivatives have also been the subject of manystudies for their interesting electrooptical properties. Polythiophene was one of the firstconducting polymers to be studied, but only when its soluble form was obtained byintroduction of alkyl side chains. The chance to prepare thin film by solution casting orby spin coating drastically increased the interest in these organic semiconductors. Fromthe very beginning it was clear that the modulation of the band gap was easy through aproper choice of the substituents. Poly(3-alkylthiophenes) (PATs) in a head-to-tailconfiguration are almost planar; the so called trans conformation (Figure 5.15) minimisesthe steric hindrance between adjacent monomer units. However, the introduction ofhead-to-head connections leads to a distortion from the planarity of the conjugated system;bulky substituents [146] in the 3 position of the thiophene ring or 3,4-dialkyl substitutedthiophene [147] inserted in a regular head-to-tail connection have the same effect. Withthis simple method it was easy, for example, by copolymerising 3,4-disubstituted thiopheneand 3-alkyithiophene in different ratios [147], to obtain copolymers with differentabsorptions from 320 nm to 450 nm. Berggren [148] used a series of polymeric structuresto prepare LEDs covering all the visible range (Figure 5.16). By blending the differentcopolymers in a matrix such as PMMA, (which acts as an energy transfer blockingmaterial), the colour emission could be varied by a proper applied voltage [148] fromorange to blue (Figure 5.17).

Tuning of the EL was also obtained by preparing copolymers of 3-alkylthiophene andunsubstituted thiophenes in different molar ratios [149].

165


Figure 5.15 Structure of a trans conformation of a regioregularpoly(3-alkylthiophene)

SS S

R

R R

R

Figure 5.16 Polymeric and copolymeric structures emitting from 450 nm to 800 nm(Reproduced with permission from O. Inganäs, Nature, 1994, 372, 444,

published by MacMillan)

S n S n

Sn

S

S n

IV. PCHMT III. PCHT II. PTOPT I. POPT

166


Figure 5.17 Electroluminescence at two different applied field of a blend of differentPAT structures

(Reproduced with permission from O. Inganäs, Nature, 1994, 372, 446,published by MacMillan)

Polarised luminescence was obtained with a simple stretched film of poly(3-(4-octylphenyl)thiophene) as an active film. The anisotropy (the ratio of the intensity of theemitted light parallel versus the intensity of the emitted light perpendicular to the drawingdirection) was up to 3.1 [150]. A suitably functionalised PAT was found to give polarisedelectroemission in the yellow/orange region when the active layer of the LED was preparedby means of the Langmuir Blodgett technique [151].

In many works, the tailoring of the electronic properties of PAT was achieved bycontrolling the regioregularity of the backbone. Barta and co-workers [152] prepared apoly(4,4´-dialkyl-2,2´-bithiophenes) (PDABT) having both head-to-tail connections andtail-to-tail connections, where different lengths of the side chains do not drastically affectthe optical properties. EL was observed at 541 nm with an efficiency of 0.5%, which istwo orders of magnitude higher with respect to previous results on regioregular PAT[153, 154]. The higher EL in PDABT was attributed to the head-to-head connectionbetween two 3-alkylthiophones forming exciton traps thereby hindering their migrationto quenching sites [155]. It was suggested [155] that in PDABT, the head-to-headconnection, which is responsible for a reduced planarity in the backbone, influences therelative position of singlet and triplet states. The non planar situation favours the creationof singlet exciton with respect to triplet states leading to an improvement in EL.

The photophysics of many substituted polythiophenes has been recently summarised byTheander and co-workers [156] who showed that PL efficiency, in the solid state, as highas 24% can only be obtained with the polymer shown in Figure 5.18 at 599 nm.

167

Heeger and co-workers [157] found that high EL efficiencies can be reached by dispersingsurfactant-like additives in the active layer. They reported that a device prepared withsurfactant-like compounds in poly(3-octylthiophene) gives, with an aluminium electrode,0.03% external efficiency, much higher than the corresponding device without additivesbut with calcium as the cathode. The procedure reported in [157] has been successfullyapplied to other electroluminescent polymers by the same authors.

PATs are available by three synthetic procedures:

(i) Electrochemical synthesis allows the preparation of thin doped films on an electrode.The subsequent reduction of the polymer gives the neutral state [158].

(ii) FeCl3 polymerisation [156] is a simple method to obtain the polymer without manyproblems. Careful cleaning of the polymer by many precipitations in methanol hasto be done in order to avoid the presence of the iron impurity (acting as dopant).The starting point is the monomer, 3-alkylthiophene, which is easily obtained. Theregioregular control with this kind of polymerisation is not extremely high. However,when the substituent in the 3 position has a high steric hindrance, a polymer havinghead-to-tail regularity higher than 90% may be obtained [156].

(iii) With Ni catalyst polymerisation a higher control of the regioregularity can beobtained [154].

A very simple method for reaching high values of regioregularity has recently been reportedby McCullough [159] and Bolognesi [160]. Regioregularity as high as 98%-100% hasbeen reached, giving emission in the 700-720 nm range. Other synthetic procedures forhighly regioregular polymers based on the Suzuki coupling [161], Stille reaction [162]and using Rieke zinc [163] have been reported.

As in the case of polymers of the phenyl family, a great amount of work has been doneconcerning the introduction of electron/hole transporter molecules in the main conjugatedbackbone and by modifying the architecture of the devices [164-167]. Introduction of


Figure 5.18 PAT structure exhibiting PL at 599 nm with ηpl = 24%

n

C8H17C8H17

S

168


Alq3 between the PAT thin film and the cathode increases the EL efficiency up to 0.05%,using aluminium as the cathode [167]. It was also observed that a certain tuning of theEL by applying different electric fields may be reached [168].

5.4 Recent Developments

5.4.1 Polarised Electroluminescence

The opportunities offered by polymeric structures for LED fabrication are manifold. Besidesthe tuning of the properties through proper tailoring of the structures and the cheap techniqueused to prepare thin and large area films, polymeric materials have the advantage, withrespect to other low molecular weight material, that they can be stretched to give highlyoriented structures. Since conjugated polymers are monodimensional semiconductors, theirelectrical and optical properties are strongly anisotropic and the chain orientation leads toextremely high electrical and optical anisotropy. Stretched (CH)X was reported [169] tohave highly anisotropic electrical behaviour with a ratio between the conductivity paralleland perpendicular to the drawing direction in excess of 100. Besides the electrical anisotropy,optical anisotropy can be observed in oriented absorbing polymers.

Dyereklev and co-workers [150] and Ohmari [164] showed that an LED with an orientedelectroluminescent polymer as an active layer emits polarised light.

The development of polarised electroemission is extremely important for optical applications;in backlighting liquid crystal displays (LCDs) the light passes through a set of polarisers inthe front and in the back of the active LCD layer [170]. These polarisers absorb 50% of thelight so that higher light intensities are reached by increasing power consumption. For thisreason, polarised EL has recently been the subject of attention for many research groups.

Different methods can be applied to reach a high orientation in the polymers to be usedin an LED.

It has already been mentioned that the Langmuir Blodgett technique allows orientation ofrod-like systems during the transfer process of the monolayer onto the substrates [125,151]. So a multilayered structure directly deposited onto the ITO electrode can be used asthe active layer of a LED. The highest dichroic ratios in EL were around 5 with a PPPderivative [125]. Though the Langmuir Blodgett technique can be used to prepare verywell-defined architecture suitable for fundamental studies, [171], this technique has thelimit of requiring long preparation procedures before reaching an appropriate thickness.

The mechanical stretching of a thin film which has to be assembled between the cathodeand anode is the simplest method of orienting polymeric materials. The work by Dyereklev[150] is an example of this approach. External efficiencies between 0.1% and 0.01%

169

were obtained with an EL dichroic ratio of 2. Hamaguchi was able to orient alkoxysubstituted PPV through the rubbing procedure [172], reaching a polarisation ratio upto 4. More recently, Jandkhe and co-workers [173] were able to gain, through a particularprocedure applied to a partially converted PPV precursor, a dichroic ratio in EL of about12 at 511 nm with a brightness of 200 cd/m2. This result is very promising and near tothe value required for technological applications. However, the authors mentioned thatlifetimes of the devices are strongly decreased.

In a series of very recent works, Grell [145] and Grell and Bradley [174] reported verypromising results on the polarised EL obtained with dialkylsubstituted polyfluorenederivatives. High orientation was reached [175] by heating the polyfluorene films, depositedonto a preoriented polyimide (PI) film, at the temperature where it exhibits a birefringentfluid phase. PI film orientation is achieved by rubbing its surface with a cloth as describedin literature following a well-established procedure [176]. PI is a good insulator, so theoriented PI film, deposited on ITO, contained a good hole transporter, 4,4´,4´´-tris(1-naphthyl)-(N-phenyl-amino)triphenylamine to promote hole injection from the ITO to thepolyfluorene. It has been reported that the procedure followed to reach the high orientationof the macromolecules [174] produces a highly oriented monodomain structure [177] whosemorphology has been described as a glassy liquid crystalline monodomain. In Figure 5.19,the polarised EL of a device is shown. The EL anisotropy for the peak at 477 nm is 15.


Figure 5.19 Polarized EL from a LED having as active layer a highly orientedpolyfluorene [145]. Perpendicular and parallel refer to the chain orientation(Reproduced with permission from Advanced Materials, 1999, 11, 8, 671,

published by Wiley-VCH, 1999)

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The research in this field is very active and even higher polarisation ratios could beobtained in the near future.

5.4.2 Lifetime and Degradation in LEDs

All the ‘components’ of a device are prepared with materials that can be chemicallyreactive. Metals constituting the cathode electrodes may react with the active layer causinga change in the structure of the conjugated system. Moreover, charge injection in polymersintroduces reactive species in the backbone and creation of crosslinks between adjacentchains may not be excluded.

Lifetime is strictly related to this degradation phenomena. In addition, the presence ofimpurities or traces of oxygen has a strong effect in decreasing the overall EL efficiencyas well as the lifetime [178]. In fact, oxidation of conjugated polymers in the presence ofsinglet oxygen is reported by many research groups [179, 180].

Parker and co-workers recently studied the mechanism of degradation in a PPV LED [181],in a single-layer device packaged with a glass cover in a nitrogen atmosphere. They were ableto measure lifetimes of around 20,000 hours at luminance greater than 100 cd/m2 whenoperating in constant current mode. However, during continuous operation the luminanceof the devices changed in a non monotonic way and the operating voltage increased in alinear fashion. The obtained data were explained attributing the main degradation processof the polymer to the passage of electrons, while hole currents seem not to lead to degradation.

5.4.3 Microcavities

Apart from the nature of the polymeric active material, the structure and size of the LEDdevice play an important role since they determine the coupling of the polymer excitationsto the electromagnetic resonant modes. In fact, a device of the type reported in Figure 5.2acts as a microcavity, and simply by adding a mirror (metallic or dielectric, distributedBragg reflector) between the glass and the ITO layer, a good resonant microcavity isobtained for typical polymer thickness of the order of 100 nm [182-185]. The secondmirror of the cavity is already formed by the top metal contact (Al or Ag) [186]. Theeffect of the microcavity is both to modulate the intensity of the PL or EL of the polymer,according to its spectral position, and to modify the spatial distribution of the emittedlight. In fact the polymer emission is enhanced at the wavelengths corresponding to theresonant modes of the cavity, while it is quenched at the wavelengths which are notcompatible with the cavity modes. The result is a sharp narrowing of the emission spectrum(see Figure 5.20). For a planar structure the spatial distribution of the emissionconsiderably narrows, increasing the intensity of the light emitted in the forward direction[185]. The dependence of the emission wavelength on the viewing angle [185, 187] can

171

be suppressed by introducing proper cavity optical length dispersion [188]. The spectralposition of the enhanced emission can be varied (within the spectral range of the freespace emission) by varying the optical parameters of the cavity (thickness, refractiveindex, and quality factor), thus obtaining different colours with the same material [185].Thus, microcavity structures have been fabricated in order to control the emission energy,linewidth, intensity and directionality of many polymeric LEDs.

Microcavities have been very intensively studied because they are good structures foroptically pumped lasers [189-196]. In fact, many polymers (see Table 1 in reference[195]) showed gain narrowing (spectral narrowing of the PL above an energy thresholdof the optical pump excitation) in solution, in blends, and also in undiluted sub-micronfilms [197] for microcavity and planar waveguide structures [198]. Further efforts areneeded in order to obtain electronically pumped lasers due to the high current densitiesand problems related to excited state absorption from injected charges [199].

5.5 Concluding Remarks

The rapid growth of this new area of polymer science has stimulated the work of chemists,physicists, and engineers. The exciting results that have been obtained are due to a strongcooperation among different group of scientists used to working in their own field. Nowthere is a common area and this is the reason why this field is growing very rapidly.

Figure 5.20 Narrowing of the emission spectrum in a microcavity.(Reproduced by permission from Journal of Applied Physics, 1996, 80, 1, 209,

published by the American Physical Society, 1996)


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The research presented in this chapter is only a part of the field of EL in organic materials. Alot of work has been done with low molecular weight molecules that have been usedsuccessfully to prepare good LEDs with very respectable performances. High vacuumevaporation techniques are generally used for the preparation of the active layer. Polymersoffer the possibility of working with a cheap technology giving flexible films that can be bentwithout breaking and that can be oriented to emit polarised light. These are the real advantagesof working with polymers and in the near future even better results will be reached.

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185

6 Photopolymers and Photoresistsfor Electronics

J.-C. Dubois

6.1 Introduction

There has been tremendous progress in integrated circuit technology since its beginningabout 25 years ago. This has given rise to a continuous increase in chip size and in thedensity of components combined with a reduction in their cost. In addition, othercharacteristic parameters, such as power consumption, switching speed and reliability,have been improved. Integrated circuits consist of patterned, thin films ofsemiconductors, metals and dielectrics on a semiconducting substrate such as siliconor gallium arsenide.

Today the so-called very large scale integration (VLSI) technology is used, permitting themanufacture of integrated circuits with several millions of discrete transistors workingat frequencies reaching GHz. This trend to miniaturisation is continuing, leading to ademand for sub-micron device geometry. This evolution of the minimum size of the VLSIis known as More’s law and can be represented by Figure 6.1.

Figure 6.1 More’s law showing the evolution of the minimum size of VLSI versus year

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Such resolution cannot be reached by using conventional (or near-UV) photolithographybecause of diffraction phenomena. Here other technologies with potentially higher resolutionsuch as deep-UV photolithography, electron-beam lithography, X-ray lithography and ionbeam lithography are under development. Electron-beam lithography is already usedcommercially for fabricating master masks for photolithography. The formation of thedesired patterns on the substrates requires the use of a resist coating in which the pattern isfirst generated. Depending on the wavelength of the irradiation, different minimum linewidthare obtained. The different techniques of lithography are summarised in Table 6.1. Onecan expect the development of deep-UV lithography techniques at 193 nm giving linedimensions as small as 0.12 μm, necessary for the latest VLSI.

The resist layer, usually a polymeric film (0.5-1.0 μm thick) is spin-coated onto thesubstrates and then exposed to radiation. One of the key points for obtaining highresolution is to obtain polymers with sensitivities well adapted to the chosen type ofradiation and with response speeds which allow high production rates.

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Photopolymers and Photoresists for Electronics

This chapter describes some aspects of polymeric resist materials for microlithographyallowing the necessary dimensions of VLSI to be reached.

6.2 Microlithography Process [1, 2]

In order to have a better understanding of a resist’s requirements, a description of the differentprocessing steps used in microlithography is necessary. In the example shown in Figure 6.2,the resist is applied as a thin film to the substrates, consisting of SiO2 on Si [3, 4].

Figure 6.2 Microlithography principle

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6.2.1 Resist Coating

Polymeric resists are usually deposited on the substrates by spin coating, followed by bakingin order to eliminate the residual solvent and to suppress mechanical stress. Some attemptshave been made to deposit polymeric films by the plasma deposition technique [5].

6.2.2 Exposure

In this step, the resist layer is exposed, generally through a mask, to one of the followingtypes of radiation: UV light (including recently deep-UV), ions, electrons or X-rays.

The lithographic method could be contact, proximity or projection mode [6], Figure 6.3. Inthe contact mode the mask is in contact with the substrate to be treated. In the proximitymode there is a gap between the mask and the substrate, while in the projection mode theimage of the mask is projected on the substrate. The projection mode is the best method forVLSI production since it allows more precision and involves less deterioration of the mask.

The source of radiation is generally a mercury lamp or a mercury-rare gas (xenon)discharge lamp. This gives a maximum of radiation in the 350-450 nm range. The steppersuse classically a high-pressure mercury lamp with two radiations: i-line (436 nm) and g-line (365 nm). The recent development of excimer lasers emitting in the deep-UV regionhas introduced the use of deep-UV radiation (150-300 nm), and especially the 248 and193 nm wavelengths [7, 8, 9].

Figure 6.3 The different lithographic methods: contact, proximity or projection mode

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The resist contains radiation-sensitive groups which undergo chemical reactions whenexposed, leading to the formation of a latent image which closely matches the pattern ofthe mask.

Organic-based resists have been classified according to whether they consist of one ortwo components. In one-component systems, the structure of the polymer contains theradiation-sensitive groups either in the main chain or in the side chain. In two-componentsystems, the resist consists of an inert matrix resin and a radiation-sensitive moleculecalled a sensitiser, which is usually, a low molecular weight compound.

6.2.3 Development

Development is an important step of the process, since it is one of the keys for obtaininga well-defined pattern on the substrates. Development transforms the latent image intoan image serving as a mask for etching of the substrates. Two technologies are nowavailable: wet development, which is widely used in circuit manufacture and drydevelopment, which is still under study.

Traditionally, resists have been divided into two classes depending on their behaviourupon irradiation: positive and negative resists (Figure 6.2).

Positive resists become more soluble in the irradiated area relative to the unexposedarea whereas negative resists become less soluble in the irradiated area relative to theunexposed area.

6.2.3.1 Wet Development

Wet development can be based on three different types of radiation-induced changes:(i) variation in molecular weights of the polymers (by crosslinking or by chain scission),(ii) reactivity change, and (iii) polarity change of the polymer. However, the use of asolvent may cause swelling and may lead to a lack of adhesion of the resist to thesubstrate. These problems may be solved by using dry development techniques whichhave recently been introduced.

6.2.3.2 Dry Development

Dry development may be achieved using either a vapour phase process or a plasma(usually oxygen plasma).


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6.2.4 Post Baking

Post baking is used as a post-development step to eliminate the residual developmentsolvent, to improve adhesion between the resist and the substrate and chiefly to increasethe resistance of the resist to the subsequent etching process.

6.2.5 Etching [1, 3]

The most common method of pattern transfer to the substrate is by wet chemicaletching. However, all semiconductor wet etching processes exhibit the same basiclimitation. This limitation is due to the isotropy of the process, which makes linewidthcontrol difficult for features less than 2 μm when thick substrate layers are used. Theneed to transfer fine features in thick substrates has led to the development ofanisotropic etching techniques such as plasma etching, reactive ion etching (RIE)and sputter etching.

Dry etching is more economical, gives rise to less pollution and leads to integrated circuitswith higher densities than does wet etching. As a rule, aromatic polymers resist plasmaetching better than aliphatic polymers.

6.2.6 Resist Removal (Stripping)

The main goal of this step is the complete removal of the resist without affecting thewafer surface. Two methods are used: wet stripping (the use of either solvents, whichdissolve the resists, or oxidisers that transform the resist into carbon dioxide and waste)and plasma resist stripping.

6.2.7 Doping

During this step very small amounts of ‘impurities’ (e.g., boron, arsenic) diffuse into thestripped regions of the semiconductor substrates. The cycle described in this section hasto be repeated several times to obtain the integrated circuit component.

6.3 Resist Requirements

The main requirements of a resist are solubility, adhesion, etch resistance, sensitivity toradiation and contrast.

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6.3.1 Solubility

Resists are generally deposited onto substrates by spin coating; therefore, solubility inorganic solvents is necessary.

6.3.2 Adhesion

The resist must possess good adhesion properties to various substrates, such as metal,silicon dioxide, silicon nitride or semiconductors, throughout the various steps ofintegrated circuit manufacture. Poor adhesion leads to loss of resolution.

6.3.3 Etching Resistance

Wet chemical etching requires good adhesion and chemical stability of the resist towardsacidic or basic etching solutions. Most dry etching techniques involve a high radiation fluxand temperatures often higher than 80 °C. Polymers exhibiting high Tg values and containingradiation stable groups (e.g., aromatic structures) have higher dry etch resistance.

6.3.4 Sensitivity and Contrast

The sensitivity, σ, is related to the ability of a polymer to undergo structural modificationon irradiation.

Sensitivity is said to increase as the dose required to produce the lithographic image decreases.The sensitivity of a positive resist, σ0, is the dose required to achieve complete solubility ofthe exposed region under conditions where the unexposed region remains completelyinsoluble. The sensitivity of a negative resist is conventionally defined as the dose at which70% of the original film thickness has been retained after development, σ0,7. The requiredsensitivity varies with the type of irradiation and is expressed as energy/surface (e.g., J/m2).

The contrast, γ, is related to the ability of a polymer to give vertical sidewalls. Resolution(defined as the smallest linewidth which can be achieved) depends on the contrast. Theparameters are determined from the sensitivity curve of the resist, which expresses thenormalised film thickness, er/e0 (er is the thickness after development and irradiation, e0 isthe initial thickness of the resist), as a function of log10 (Dose) (Figures 6.4(a) and 6.4(b)).

For a negative resist:

γ = (log10 σ1/σ0)-1 (6.1)


192


and for a positive resist:

γ = ⎜(log10 σ1/σ0)-1⎜ (6.2)

where σ1 is the dose required to reach the required thickness after development.

For narrow-UV radiation, values of the doses for the i- and g-line vary from 10-50 mJ/cm2

and for deep-UV, from 10-20 mJ/cm2. For electron beam lithography, this dose is expressedas a current density around 5-10 mC/cm2.

Negative resists are in general more sensitive than positive resists but they exhibit alower contrast (γ<1) (Table 6.2). A contrast higher than 3 is generally required for thehigh circuit density of new generation technologies such as the 256 Mb DRAM.

Figure 6.4 Typical sensitivity curves for negative and positive resists

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6.4 Resist Materials

6.4.1 Conventional Photoresists

Photolithography using light in the wavelength range 350-450 nm is the technologycurrently used in the manufacture of integrated circuit devices.

6.4.1.1 Positive Photoresists

All near-UV positive photoresists are two-component systems; the polymeric material isa low molecular weight novolac polymer (Figure 6.5) and the sensitiser is a derivative ofa 1,2-diazonaphthoquinone (DNQ) (20%-50% by weight). DNQ forms a complex withthe phenol groups of the novolac resin and prevents the dissolution of the latter in anaqueous base.

Exposure of the resist to UV light results in photodecomposition of the sensitiser to anunstable ketocarbene. This reacts with water to produce the base-soluble indene carboxylicacid, which no longer inhibits dissolution of the novolac polymer in aqueous base [4](Figure 6.6).

There are many commercially available positive photoresists, which differ slightly fromone another. The most significant advantages of this family of photoresists are the lackof swelling during the development step, a good resolution in thick coating and well-known film forming properties.

Since swelling does not take place during the development of positive photoresists,several process variations have been reported aimed at reversing the tone of the imageso that the resist would act as a negative resist [11, 12]. The most interesting process isprobably that described by Moritz and Paal [13] and known as the Monazoline process.

Figure 6.5 Novolac polymer


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The process is outlined in Figure 6.7; it is based on the thermal decomposition ofindene carboxylic acids in the presence of amines such as monazolines, a range ofcommercially available detergents (1-hydroxyethyl-2-alkyl-imidazolines).

6.4.1.2 Negative Photoresists

6.4.1.2.1 One-Component Systems

In this type of photosensitive system, UV sensitive groups, such as chalcone, cinnamate,styrylacrylate, diphenyl-cyclopropene carboxylate, cinnamilidene malonate, p-carboxy-cinnamate, p-phenylene-bis acrylate and styrylpyridinium, etc., are included in the mainchain or in the side chain of the polymer. UV irradiation gives rise to crosslinking. Thecrosslinking of poly(vinylcinnamate) [14], which is the key process occurring in theKPR resist (Kodak), is shown in Figure 6.8.

Figure 6.6 Photodecomposition of 1,2-diazonaphthoquinone

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Figure 6.7 Monazoline process: the thermal decomposition of the carboxylic acid inpresence of the monazoline base transforms the acid into an insoluble indene

derivative leading to a negative resist (reverse tone process)

Figure 6.8 The KPR resist, a negative resist which crosslinks through the double bondsunder irradiation


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6.4.1.2.2 Two-Component Systems

Negative near-UV resists are derived mainly from photoinduced crosslinking of partially cyclizedcis-1,4-polyisoprene with bisazido sensitisers [15]. Cyclised cis 1,4-polyisoprene is obtained byheating 1,4-polyisoprene in the presence of a Lewis acid (ZnO2, AICI3, SnCl4) (Figure 6.9).

Figure 6.9 Formation of cyclised cis-1,4-polyisoprene

The photochemical transformations of a bisazide sensitiser leading to the crosslinking ofa cyclised cis-1,4-polyisoprene are shown in Figure 6.10. An insoluble three-dimensionalcrosslinked network is obtained.

Figure 6.10 Photochemical transformations of a bisazide sensitiser in a carbene leadingto the cross-linking of cyclized cis-1,4-polyisoprene. There are several different

possible reaction mechanisms.

197

These resists are commercially available and are able to meet the requirements of mostintegrated circuit designs. Their sensitivity is high, allowing good masking speed. Forexample, KTFR (made by Kodak) is a negative resist of this type.

The main limitation of negative resists of this type is their dependence on organic solventdevelopers, which cause resist image distortion as a result of swelling. However, for highresolution circuits, positive resists are usually preferable.

6.4.2 Deep-UV Photoresists

Interest in deep-UV photolithography (150-280 nm range) lies in the possibility ofobtaining higher resolutions than cannot be achieved with conventional optical technology,since the diffraction-limited resolution of optical projection printing tools is proportionalto the exposure wavelength. Resolution limits in the 0.5-0.25 mm range may be expected.One of the problems of deep-UV photolithography is the low brightness of mercury arclamps, which are the most common exposure sources. That is to say that new exposuresources and/or very sensitive resist materials must be developed. That is why there isnow a great interest in excimer lasers [1, 16, 17] as intense deep-UV sources. The excimerlasers are mainly lasers based on KrCI, KrF and ArF, which have output wavelengths of222, 249 and 193 nm, respectively.

KrF exposure systems (step and repeat) already exist commercially and are used for0.25 μm linewidth generation. But most emphasis has been recently put on the 195 nmexposure systems for 0.18 μm linewidth generation. The next generation of componentswill require linewidths below 0.12 μm.

6.4.2.1 Positive Deep-UV Resists

6.4.2.1.1 One-Component Systems

One-component positive resists are essentially copolymers or terpolymers derived fromPMMA. Copolymer structures are chosen in order to increase the low value of theabsorption coefficient of PMMA at 215 nm. Comonomers are selected either to absorbin the 230-280 nm range or because they contain a photosensitive chromophoric group.Poly(methyl methacrylate-co-3-oximino-2-butanone methacrylate-co-methacrylonitrile),p(PMMA-OM-MAN) (Figure 6.11), sensitised with t-butyl benzoic acid requires anexposure dose of less than 30 mJ/cm2 [18].

The sensitivity of this terpolymer is 170 times that of PMMA and is capable of sub-micronresolution. Poly(aromatic sulfones) may also be used as positive resists [19]. Their quantumefficiency is low, however. Aromatic moieties ensure a useful plasma etch resistance.


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6.4.2.1.2 Two-Component Systems

Several attempts have been made to design two-component deep-UV positive systemswith increased sensitivity. An example is a system based on a novolac polymer and 5-diazo Meldrum’s acid as a dissolution inhibitor [20]. The Meldrum’s acid derivativeprovides a bleachable chromophore at 250 nm, which is converted to volatile compoundson irradiation according to the scheme shown in Figure 6.12.

Figure 6.12 Meldrum’s acid is converted to a volatile compound under irradiationat 250 nm

The resist exhibits reasonable sensitivity. However, the novolac resin itself has a strongabsorption around 310 nm, which is bad for the sensitivity.

Wilkins and others [21] have described another system. In this case, the resin is a copolymerof methyl methacrylate and methacrylic acid (which is transparent above 260 nm andwhich is soluble in aqueous alkali) and the dissolution inhibitor is an o-nitrobenzylcarboxylate. This system exhibits a good photosensitivity (100 mJ/cm2 in the 230-300nm range) with a very high contrast (g>5).

Figure 6.11 Terpolymer of PMMA sensitive to 240 nm radiation

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6.4.2.1.2.1 t-Butyl Carbonate (T-BOC) Photoresists

The progress in two-component systems is due to the combination of two things:

• an acid photogeneration and

• a polymer with labile groups sensitive to the acid (e.g., T-BOC) leading to positiveresists [22]. The i-line and g-line resists, such as novolac resists, absorb too much andgive bad contrast when used as deep-UV resists in the 248-193 nm domains.

Very good results are obtained with the use of a different approach to photoactivationusing the t-butyl carbonate (T-BOC) group. This group is fixed on a polymer and usedwith an acid photogenerator. The acid generated decomposes the T-BOC group, leadingto a soluble polymer.

6.4.2.1.2.2 Acid Photogenerators

A typical acid photogenerator is triphenyl sulfonium triflate (Figure 6.13), whichgenerates trifluoromethane sulfonic acid. Onium salt photoinitiators of this type caninduce cationic photopolymerisation under photoirradiation. The reaction is complexand several products of this type are used with Lewis acids, such as AsPF6

–, AsF6– and

BF4–. See, for example, [44].

Figure 6.13 Example of an acid photogenerator

Other acid photogenerators are used, such as diphenyliodonium hexafluoropropanesulfonate, or bis-(diphenylsulfonyl)diazomethane. The latter (shown in Figure 6.14)generates benzene sulfonic acid.


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6.4.2.1.2.3 T-BOC Polymers

Different types of T-BOC polymers have been synthesised and are used in associationwith the photoacid generators. A resist made with poly(p-tert-butoxycarbonyloxystyrene)(T-BOC-PHS) and an onium salt was first described by IBM Laboratories [49]. The T-BOC-PHS is decomposed by the acid generated by the photoacid generator, CO2 andisobutylene is evolved (Figure 6.15).

Figure 6.14 Bis(diphenylsulfonyl)diazomethane

Figure 6.15 Decomposition of T-BOC-PHS in an acid medium

The resolution of the system is moderate and the stability after irradiation is poor.However, it is used for the 243 nm domain due to its sensitivity. Derivatives of this resistsystem have also been proposed, e.g., a copolymer of 4-hydroxystyrene with a t-butylacrylate and a phenyloxysuccinimid as the acid photogenerator. This resist is more stableduring processing (Figure 6.16).

An evolution toward the lower wavelength is in progress with the use of the 193 nm ArFlaser source. Optical absorption requirements demand new resist polymers with no

201

aromatic structures. Polyacrylates seem adapted to this wavelength, but the lack ofaromatic structure gives poor resistance to plasma etching. For this reason alicyclicradicals, such as adamantine, are used on the polymer [49-56].

AZ Electronic Materials, a subsidiary of Clariant Corporation, has proposed a resistwith alicyclic radicals for use in 193 nm microlithography (Figures 6.17a and 6.17b).

Figure 6.16 Copolymer of t-butyl acrylate and 4-hydroxystyrene used as a deep-UV resist

Figure 6.17a Mechanism of degradation of the photoresist AX 1000

6.4.2.2 Negative Resists

Microresists for shorter wavelengths (MRS) are developed in an aqueous base, whichavoids the swelling phenomena. Resolution of 0.16 μm lines and spaces has beendemonstrated. The MRS is composed of a phenolic resin and a bisazide [24].


202


Many other deep-UV negative resists have been reported. Examples are summarisedin Table 6.3.

6.4.3 Electron-Beam Resists

The major advantages of electron-beam lithography over conventional photolithographyare a higher potential resolution and the possibility of direct beam writing on the resistsurface. In addition to the usual requirements discussed earlier, positive or negativeelectron-beam resists have to possess the following properties:

• Be able to exhibit sub-micron resolution (resolution being affected more bybackscattering of electrons than by diffraction effects since the de Broglie wavelengthof electron beams is of the order of a few tenths of an Angstrom),

Figure 6.17b Lithography results with AX 1000 at 193 nm.The resolution is better than 0.15 μm.

(Picture courtesy of AZ Electronic Materials, a business unit of Clariant Corporation)

203

• Have a sensitivity of the order of 10-6 C/cm2 (at 10-30 kV), and

• Be sufficiently stable to withstand dry etching as well as wet etching.

6.4.3.1 Positive Electron-Beam Resists

PMMA was one of the first positive resists to be investigated. When subjected to electron-beam irradiation, PMMA suffers extensive main chain scission, owing to the presence ofquaternary carbon atoms. PMMA is an excellent positive resist from the viewpoint ofadhesion and resolution, but it exhibits rather low sensitivity (50 μC/cm2 at 15 kV) andpoor resistance to dry etching techniques.

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204


One way to increase the sensitivity of PMMA in electron-beam lithography is to weakenthe main chain stability of the polymer. Substitution either on the quaternary carbon(using polar electronegative substituents) or in the side chain may do this. Some examplesare given in Table 6.4.

In order to improve the thermal stability of PMMA, Roberts [32] has described a two-component electron-beam resist.

By copolymerising methacryloyl chloride and methacrylic acid with methyl methacrylate,it is possible to form intermolecular crosslinks (acid anhydride bonds) in situ on theappropriate substrate by heating the resist after spin coating (Figure 6.18).

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205

Electron-beam irradiation breaks the acid anhydride linkage, thus restoring solubility tothe irradiated areas. Several systems derived from that of Roberts have been studied [33,34]. Resistance to plasma and to developer solvent is improved by the anhydridecrosslinking.

Poly(olefin sulfones), alternating copolymers of sulfur dioxide and an olefin, are anotherimportant class of positive resists, which exhibit a very high sensitivity [35] owing to theweak carbon-sulfur bond (see Figure 6.19). Poly(butene-1-sulfone) shows the bestproperties (σ = 1.6 μC/cm2 at 20 kV) and is commercially available.

Figure 6.18 Crosslinking through an anhydride [32]

Figure 6.19 Mechanism of the degradation of polysulfone by electron-beamirradiation

Unfortunately, poly(olefin sulfones) are not sufficiently stable towards dry etching. Inorder to improve the etch resistance, a two-component system consisting of a novolacresin (known for its excellent dry etching resistance) and poly(2-methyl-1-pentene) (actingas a dissolution inhibitor) has been designed by Bowden and others [35, 36] at LucentTechnologies, USA. Poly(2-methyl-1-pentene) undergoes spontaneous depolymerisationduring irradiation leading to gaseous compounds, and the exposed regions become soluble.

Resists having the structure [37] shown in Figure 6.20 have good oxygen plasma resistance,presumably due to the conversion of Si to SiO2.


206


Figure 6.20 Poly(olefin sulfone) resistant to plasma etching

6.4.3.2 Negative Electron-Beam Resists

The chemistry of negative electron-beam resists derives from the ability of epoxy, episulfide,vinyl(allyl) and halogen (particularly chlorine) groups to promote crosslinking. However,the inherent sensitivity of these reactive moieties incorporated into the polymer is wellwithin the required exposure range of most electron beam exposure machines.Incorporation of aromatic groups into the polymer improves the dry etch resistance andthe resolution of negative electron-beam resists. Characteristics of some examples ofnegative electron-beam resists, including poly(acrylate) and polystyrene derivatives, aregiven in Table 6.5. The different structures of these polymers are given in Table 6.6.

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6.4.4 X-Ray Resists

X-ray lithography is an extension of near-contact lithography for replication of 1 μmand smaller geometry. X-ray masks are difficult to fabricate with sufficient yields.Resolution in X-ray lithography will never do better than the resolution of the electron-beam exposures used to make the masks, since X-rays cannot be focused or deflected.

207

For technological reasons, the energy flux deposited at the wafer is low, so highly sensitiveresists (σ<10 mJ/cm2) are required. A typical sensitivity curve of fluorinatedpolymethacrylate (FPB) in comparison to the curve of PMMA is shown in Figure 6.21.

Figure 6.21 Comparison of the sensitivity curve of a FPB resist (σ = 70 mJ/cm2) withthat of PMMA (σ = 360 mJ/cm2) at X-ray irradiation of 13.34 Å [48]


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PVMS

208


Figure 6.22 Bis-acryloxybutyl tetramethyl disiloxane (BABTDS) used for photolocking

It can be said that positive or negative resists developed for electron-beam lithographycan be used for X-ray lithography. However, the sensitivity of conventional electron-beam resists is not sufficient from an economical point of view.

The highest X-ray resist sensitivity has been obtained by using an elegant resist techniquereferred to as ‘photo-locking’ [47]. The resist consists of a plasma-degradable acrylicpolymer (poly(2,3-dichloropropyl acrylate)) and a volatile silicon-containing acrylate(bis-acryloxybutyl tetramethyl disiloxane, Figure 6.22) which can be readily polymerisedby the absorbed radiation.

The sensitivity of this system is 1.5 mJ/cm2. Highly sensitive positive X-ray resists are notreally well adapted at this time since they do not possess all the required sensitivity andmasking properties. However, they offer interesting advantages for including inorganicatoms which could be transformed into oxide (e.g., silica). The X-ray masking techniqueitself suffers from problems such as the difficulty of mask manufacture.

6.4.5 Special Resists

6.4.5.1 Multilayer Resists

The multilayer resist (MLR) strategy was introduced by Lin and co-workers [16, 17] andcalled ‘portable conformable masking’ (PCM). The aim of the multilayer resist technologyis the simultaneous achievement of good linewidth control, high resolution and goodstep coverage. Generally, these requirements cannot be met simultaneously since goodstep coverage and leveling of topography variations on the wafer surface require a thickresist while high resolution is obtained with a thin resist. The main feature of multilayersystems is the separation of imaging and step-coverage functions in different layers. High-resolution patterns are generated in the thin top layer followed by pattern transfer intothe thick bottom layer (bilayer resist systems). Sometimes, a third layer, called the isolationlayer, is introduced between the two, preventing mixing of these products. This processis referred to as a tri-layer resist system (Figure 6.23).

209

Some resists that have been previously described may be used in MLR technology, e.g., anovolac-type resist for the imaging layer and PMMA for the planarising layer. However,most polymers do not possess sufficient etching resistance to withstand oxygen plasmaetching of the thick hard-baked resist layer during the process of a multilevel system. Thishas led to an extensive use of silicon-containing polymers since these materials are knownto convert into SiO2 in oxygen plasma and thereby to exhibit high tolerance to oxygen-reactive plasma etching. Examples of some silicone resists are shown in Table 6.7.

6.4.5.2 Polyimide-Based Photoresists [57, 58]

Polyimides are interesting thermally-stable polymers which could resist at hightemperatures (more than 300 °C). These are used as insulation layers in microelectronicsdue to their good dielectric properties. Photoresists based on polyimides are very attractivebecause they allow direct patterning of the polymer.

6.4.5.2.1 Negative Polyimides

Most of the negative polyimides are based on the principles of the pioneering work ofRubner [58]. A soluble polyamic acid precursor carries a photo-reactive group sensitivein the 250-450 nm range able to crosslink under irradiation. After development, thecrosslinked patterns are converted into polyimides by heating, as shown in Figure 6.24.In the system of Rubner, the sensitive group R allowing photocrosslinking is an acrylateester. This product is now developed by CIBA (Probimide, Selectilux HTR) or DuPont.The acrylate is eliminated during subsequent heating.

Figure 6.23 Multilayer resist system


210


Other types of photosensitive groups have been used. Yoda and Hiramoto [60] reporteda negative photopolyimide having photosensitive groups introduced to polyimideprecursors through acid-amine ion linkage. Toray developed this product.

6.4.5.2.2 Positive Polyimide Resists

Positive polyimide resists have also been developed. One example starts from a solublepolymer [60] with a formulation analogous to a novolac positive resist; o-naphthoquinonediazide was used as a photoreactant linked to the polymer (Figure 6.25).

The different characteristics of polyimide-based photoresists are represented in Table 6.8 [57].

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maeb-nortcelE MBI 34

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maeb-nortcelEyar-XVUpeeD

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VUraeN TTN 64

211

Figure 6.24 Formation of polyimide, the polyamic acid is crosslinked byradiation through the acrylate and cyclised into polyimide by heating at

200 °C after development

Figure 6.25 Example positive photopolyimide resist material


212


6.5 Conclusions

It is clear that the progress in VLSI technology is closely linked to lithographic techniques,as the size of the circuit decreases, the speed and the power consumption decreases.These techniques are dependent on the photochemistry and the resists. The minimumlinewidth permitted is already around 0.1 μm or less with sources like electron-beam, X-rays or the 193 nm excimer laser. No doubt these limits will progress and one alreadyspeaks of 157 nm projection lithography. The requirements placed on photoresists willprobably increase in the near future.

References

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]75[stsiserotohpdesab-edimiylopfoselpmaxE8.6elbaT

rerutcafunaM kramedarT epyT )1( esoDmcJm( 2- )

egaknirhS)%( )2(

ssenkcihT(μ )m

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3-XI

EEE

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840504

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2272IP2372IP1472IP

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050554

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343dimiborP004dimiborP

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002007

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yaroT)eceenotohP(

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SS

052-051 04-0326-05

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retse=S,edimiylopdesilcycerp=IP,rehte=EepyT)1(gnikabgnirudssenkcihtmlifehtfoesaerceD)2(

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6. M. King in VSLI Electronics Microstructure Science, Volume 1, Ed., N.G.Einspruch, Academic Press, New York, NY, USA, 1981, Chapter 2.

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8. Introduction to Microlithography, 2nd Edition, Eds., L.F. Thompson, C.G. Willsonand M.J. Bowden, American Chemical Society, Washington, DC, USA, 1994.

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10. J. Pacansky, Polymer Engineering and Science, 1980, 20, 1049.

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12. W.G. Oldham and E. Heike, IEEE Electron Device Letters, 1980, 1, 217.

13. H. Moritz and G. Paal, inventors; International Business Machines Corporation,assignee; US Patent 4,104,070, 1978.

14. K.G. Clark, Microelectronics, 1971, 3, 11, 23.

15. J. Sagura and J.A. van Allan, inventors; US Patent 2,940,853, 1960.

16. K. Jain, C.G. Willson and B.J. Lin, IBM Journal of Research and Development,1982, 26, 2, 151.

17. K. Jain, C.G. Willson and B.J. Lin, IEEE Electron Device Letters, 1982, 3, 3, 53.

18. E. Reichmanis and C.W. Wilkins, Journal of the American Chemical Society,Division of Organic Coatings and Plastics Chemistry Preprints, 1980, 43, 243.

19. M.J. Bowden and E.A. Chandross, Journal of the Electrochemical Society, 1975,122, 10, 1371.

20. B.D. Grant, N.H. Clecak, R.J. Twieg and C.G. Willson, IEEE Transactions onElectron Devices, 1981, 28, 1300.

21. C.W. Wilkins, Jr., E. Reichmams and E.A. Chandross, Journal of theElectrochemical Society, 1982, 129, 11, 2552.


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23. T. Iwayanagi, T. Kohashi and S. Nonogaki, Journal of the ElectrochemicalSociety, 1980, 127, 2759.

24. T. Iwayanagi, T. Kohashi, S. Nonogaki, T. Matsuzawa, K. Douta and H.Yanazawa, IEEE Transactions on Electron Devices, 1981, 28, 1306.

25. H. Nakane, A. Yokota, S. Yamamoto and W. Kanai, Proceedings of the RegionalTechnical Conference on Photopolymers: Principles, Processes and Materials,Mid-Hudson Section, SPE, Ellenville, NY, USA, 1982, 43.

26. S. Imamura and S. Sugarawa, Japanese Journal of Applied Physics, 1982, 21, 5, 776.

27. M. Kakuchi, S. Sugawara, K. Murase and K. Matsuyama, Journal of theElectrochemical Society, 1977, 124, 1648.

28. S. Sugawara, O. Kogure, K. Harada, M. Kakuchi, K. Sugegawa. S. Imamura andK. Miyoshi, presented at the 9th International Conference on Electron and IonBeam Science and Technology, Extended Abstract, 680. (Part of theElectrochemical Society Spring Meeting, St. Louis, MO, USA, 1980)

30. T. Tada, Journal of the Electrochemical Society, 1979, 126, 9, 1635.

31. S. Matsuda, S. Tsuchiya, M. Honma and G. Nagamatsu, inventors; MatsushitaElectrical Co., Ltd, assignee; US Patent 4, 279, 984, 1981.

32. E.D. Roberts, Applied Polymer Symposia, 1974, 23, 87; Journal of the AmericanChemical Society, Division of Organic Coatings and Plastics Chemistry Preprints,1973, 33, 1, 359; 1975, 35, 2, 281; 1977, 37, 2, 36.

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34. J.H. Lai and L.T. Shepherd, Proceedings of the 10th International Symposium onElectron and Ion Beam Science and Technology, 1983, 82-83, 185.

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40. M. Gazard, A. Eranian, F. Barre and C. Duchesne, inventors; Thomson CSF,assignee; FR Patent 7,615,520, 1976.

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42. J.M. Shaw. M. Hatzakis, J. Paraszczak. J. Liutkus and E. Babich. Proceedings ofthe Regional Technical Conference on Photopolymers: Principles, Processes andMaterials, Mid-Hudson Section, SPE, Ellenville, NY, USA, 1982, 285.

43. M. Hatzakis, J. Paraszczak and J. Shaw, Preprints from Microcircuit Engineering,Lausanne, Switzerland, 1998, 1, 386.

44. Photoinitiation, Photopolymerisation and Photocuring, Fundamentals andApplications, Ed., J.P. Fouassier, Hanser, Munich, Germany, 1995.

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217

7 Polymer Batteries for Electronics

B. Scrosati

7.1 Introduction

Conductive polymers are a class of materials that play a key role in the modern technologyfor energy storage and conversion. This prominent position results from the latestdevelopments in the field that have produced a series of new polymers with exceptionalelectric properties. Depending on the nature of the electric carriers, these polymers canbe broadly divided into two classes, namely the electronically and the ionically conductingpolymers. Both classes of materials are of interest in the energy area. The electronicpolymers may find use as electrodes in advanced-design devices, such as high-energybatteries and super capacitors. The ionic polymers are widely studied as electrolytes forthe development of high performance fuel cells and of high energy density batteries, withparticular focus on lithium batteries.

Indeed, novel-design, high energy density batteries are in increasing demand. The growingmarket for portable electronic products, and the stringent environmental necessity forzero emission vehicles, e.g., electric vehicles (EV), has motivated research on thedevelopment of electrochemical power sources characterised by high energy density, longcyclability, reliability and safety. A recent breakthrough has been the commercialisationof rechargeable lithium batteries, the so-called lithium-ion batteries [1-6] that are nowproduced at a rate of a million units per month for the consumer electronics market.Another important innovation in the field is the development of redox supercapacitors[7, 8]. These high power devices are of increasing importance in the EV technologydesigned as a support to the energy power sources, such as the batteries or the fuel cells,to assure well-balanced vehicle operation. In this combined action, the supercapacitorprovides the peak needs for the vehicle acceleration while the battery assures its long-range running.

Most of the lithium-ion battery and supercapacitor research and development projects arefocused on the fabrication of prototypes using liquid electrolytes. An important step forwardin this technology is the replacement of the liquid electrolyte with an ionic membrane and,eventually, of the common inorganic-type electrode materials with advanced electronicallyconducting polymers, in order to produce novel devices having a full polymeric configuration.This is an interesting concept since it provides the prospect of a favourable combination of

218


the high energy and long life typical of lithium or lithium-ion cells with the reliability andeasy manufacturing typical of all-polymer structures. The practical exploitation of this conceptrequires the availability of polymer membranes having ionic conductivity approaching thoseof the conventional liquid solutions and of polymer films having charge storage capabilitiesapproaching those of the conventional electrode materials. This chapter attempts to describethe various classes of polymers presently under investigation to meet these requirements. Theionically conducting polymer membranes, which are designed as electrolyte separators, arefirst discussed. After this, the properties and the application potential of the electronicallyconducting membranes, which are designed as innovative electrodes, are reviewed.

7.2 Ionically Conducting Polymers

These polymers have been studied and developed in view of their application as electrolyteseparators in energy conversion and storage devices, such as batteries and fuel cells.Accordingly, these materials have been generally termed ‘polymer electrolytes’. Withrespect to batteries, the development of polymer electrolytes has been mainly confined tolithium ion conducting membranes, i.e., membranes compatible with the electrochemicalreactions driving high-energy lithium batteries. Indeed, these are the batteries in continuouscommercial evolution as ideal power sources for the consumer electronics market. Modernfuel cell technology has relied on polymer electrolytes with proton transport, i.e., onmembranes capable of assuring simple yet efficient structures to power sources mainlydirected to EV applications. Some recent advances in the R&D of these two classes ofpolymer electrolytes will be discussed in the following sections.

7.2.1 Lithium Polymer Electrolytes and Lithium Batteries

Historically, the first types of lithium polymer electrolytes to be considered for batteryapplications were those formed by blending high molecular weight poly(ethylene oxide)(PEO), with a lithium salt LiX, where X is preferably a large anion [9-12]. Various booksand extensive review articles have been published on these polymer electrolytes [5, 13-16] and thus only their essential properties will be briefly recalled.

The PEO-LiX membranes are typically prepared by casting an acetonitrile solution ofthe two components or by directly hot-pressing their intimate mixture. The polar oxygenatoms in the sequential oxyethylene groups of the PEO chains coordinate the Li+ cations,separating them from their X- counteranions. Accordingly, the structure of the PEO-LiXcomplexes may be broadly discussed as a sequence of polymer chains coiled around thelithium ions while the anions are more loosely coordinated [14]. A pictorial model ofthis structural sequence is shown in Figure 7.1.

219

Polymer Batteries for Electronics

This oversimplified picture progressively diverges from reality depending upon the relativeconcentration of the two components. As the concentration of the LiX, expressed as theratio between the oxygen atoms in PEO and of the lithium ions in LiX, increases, theoverall structure becomes more and more complicated due to ion-ion and associationphenomena [17].

As for all conductors, the conductivity of the PEO-LiX polymer electrolytes depends onthe number of the ionic carriers and on their mobility. The number of the Li+ carriersincreases as the LiX concentration increases, but their mobility is greatly depressed bythe progressive occurrence of ion-ion interaction which may even lead to large ion clusterformation [14].

Due to their particular structural position (Figure 7.1), the Li+ ions can be released totransport the current only upon unfolding of the coordinating PEO chains. Thus, highconductivity and fast Li+ ion transport are restricted to the amorphous state of the PEOcomponent, which on average occurs at temperatures above 70 °C. This is shown byFigure 7.2, which illustrates the phase diagram and the ionic conductivity Arrhenius plotof a typical electrolyte in the PEO-LiClO4 system.

It may be clearly noticed that at ambient temperature, when the system is in its crystallinestate, the conductivity is very low, i.e., in the 10-8-10-7 S cm-1 range. However, at around70 °C, i.e., at the crystalline to amorphous PEO transition, the conductivity increases by

Figure 7.1 Schematic structural arrangements of the PEO chains coilingaround Li+ cations

220


several orders of magnitude to reach values of practical interest (i.e., around 10-3 S cm-1)at 100 °C. This implies that the use of the PEO-LiX electrolytes is restricted to batteriesfor which a relatively high temperature of operation does not represent a major problem,e.g., batteries designed for EV traction. Indeed, various R&D projects aimed at theproduction of PEO-based, EV lithium batteries are in progress worldwide [18-20]. Thesebatteries typically use a lithium metal anode and a Li-intercalation cathode. The lattermay be basically described as a compound with an ‘open structure’, i.e., a layered (e.g.,TiS2, V2O5, LiCoO2,) or a tunnel (e.g., V6O13, LiMn2O4) structure, which provideschannels for the reversible insertion-deinsertion of lithium ions [21-23]. It is possible toname this intercalation cathode by the general AyBz notation. A schematic diagram of anelectrochemical process of the battery reported in the literature [2, 20] is given inFigure 7.3. Upon discharge, the Li+ ions, produced at the Li metal anode, travel acrossthe electrolyte to reach the AyBz cathode and are inserted into its structure, while theelectrons travel through the external circuit to reach the cathode and modify its electronicdensity of states [23]. The charging process is the exact opposite and thus the overallprocess may be written as shown in Equation 7.1.

xLi + AyBz ⇔ LixAyBz (7.1)

Figure 7.2 Phase diagram and conductivity Arrhenius plot of the PEO-LiClO4 polymerelectrolyte. x is percent fraction of LiClO4.

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The open circuit voltage of this battery is associated with the difference in the Fermilevels of the two electrodes. Accordingly, if the overall process does not induce phasechanges in the host AyBz cathode, the discharge voltage decreases upon increasing lithiumintercalation level (x in Equation 7.1). A typical example, where AyBz is TiS2, is given inFigure 7.4.

Some selected examples of R&D projects presently in progress for EV lithium polymerbatteries [24-26] are listed in Table 7.1. Although these projects are very relevant, the hightemperature operation of the batteries obviously limits their practical output. Accordingly,many studies have been carried out with the goal of improving the conductivity of thePEO-based polymer electrolytes at ambient temperature. Various approaches have beenconsidered, e.g., the use of modified PEO polymer architectures to achieve low crystallinityat room temperature. These include block copolymers, crosslinked polymer networks andcomb-shaped polymers having short oligooxyethylene chains attached to the polymer

Figure 7.3 Schematic diagram of the Li-AyBz lithium battery(Reproduced with permission from B. Scrosati, La Chimica e l’Industria, 1997, 5,

464, published by Societa Chimica Italiana)


222


Figure 7.4 The Li/TiS2 cell. Open circuit voltage (a) and voltage discharge profile (b)upon development of the xLi + TiS2 ⇒ LixTiS2 discharge process. Derived from [23].

DOS = density of states, Eg = band gap, EF = Fermi level.

backbone [14, 27-29]. Other approaches have considered the addition of plasticisers, suchas organic liquids, e.g., propylene carbonate (PC) or ethylene carbonate (EC) [30, 31] orlow molecular weight ethylene glycols [32]. However, these modified PEO electrolytes (inparticular the ones with added plasticiser), although reaching high levels of conductivity,

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suffer from serious drawbacks such as the loss of mechanical stability and, particularly,reactivity toward the lithium metal electrode [20, 33, 34] that may rule them out frompractical applications. Indeed, to our knowledge, no practical lithium battery using aplasticiser-added polymer electrolyte has so far reached large-scale production.

Therefore, the ideal solution in this field would be the use of ‘solid plasticisers’, namelyof solid additives which would promote amorphicity at ambient temperature withoutaffecting the mechanical and the interfacial properties of the electrolyte. A result thatapproaches this ideal condition has been obtained by dispersing selected ceramic powders,such as TiO2, Al2O3 and SiO2, at the nanoscale particle size, in the PEO-LiX matrix [35-41]. The conductivity behaviour of a selected example of these ‘nanocomposite’ polymerelectrolytes is shown in Figure 7.5.

It may be seen that the related Arrhenius plot does not break around 70 °C, as typicallyexpected for PEO-based electrolyes. It clearly suggests that the added ceramics preventPEO crystallisation, with a resulting enhancement in conductivity which at roomtemperature may reach values of the order of 10-5 S cm-1, compared to the 10-7 S cm1 ofthe standard, ceramic-free electrolytes.

This outstanding effect is explained by assuming that, once the composite electrolyte isannealed at temperatures higher than the PEO crystalline to amorphous transition (i.e.,above 70 °C), the ceramic additive, due to its large surface area, prevents local PEOchain reorganisation with the result of freezing a high degree of disorder which favoursthe Li+ ion transport [38, 42, 43]. It has been proposed [44] that the Lewis acid characterof the added ceramic may compete with that of the lithium cations for the formation ofcomplexes with the PEO chains. Thus, the ceramics may act as crosslinking centres forthe PEO segments, lowering the polymer chain reorganisation tendency and promoting

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Figure 7.5 Conductivity Arrhenius plots of ceramic-free and composite PEO-based,polymer electrolytes. w/o = weight percentage.

Li+ conducting pathways at the ceramic surface [44-46]. Therefore, according to thismodel, the structural modifications at microscopic levels promote consistent enhancementin the transport properties of the electrolyte. In addition, the all-solid configuration (noaddition of liquids) gives to these nanocomposite electrolytes a high compatibility withthe lithium metal electrode [47-50], all these properties making them suitable for use assafe and efficient separators in rechargeable lithium batteries [51].

The fabrication and test of prototypes based on the preferred composite electrolyte (aPEO-LiCF3SO3 system with dispersed γ-LiAlO2 ceramic powders) has been demonstrated(Table 7.1). The cathode was LiMn2O4, i.e., the lithium manganese spinel operating inits medium (3V vs. Li) voltage range [52, 53]. The voltage profile of this battery is shownin Figure 7.6. Long cycle life and an energy density of the order of 110 Wh kg-1 areexpected for this battery [26].

Peled and co-workers [54-56] have reported another interesting battery application ofthe PEO-based composite electrolytes in a cell of the following structure:

Li/(PEO)20LiI-EC-Al2O3/FeS2

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The cell operates at 135 °C on the basis of the overall process shown in Equation 7.2.

4 Li + FeS2 ⇔ Fe + 2Li2S (7.2)

The overall process is associated with a theoretical energy density of 800 Wh kg-1 and avoltage slope between 2.5 V and 1.8 V.

The need to extend the use of polymer electrolytes to devices capable of operating at ambientand sub-ambient temperatures has motivated the search for materials capable of offeringvalues of conductivity even higher than those of the PEO-LiX composites. The most successfulresult has been obtained with the development of electrolyte membranes formed by trappingliquid solutions (e.g., solutions of a lithium salt in organic solvent mixtures) in a polymer,e.g., polyacrylonitrile (PAN), PMMA or polyvinylidene fluoride (PVdF) matrices [20, 57-59]. The immobilisation procedure varies from case to case and includes UV crosslinking,casting and gelification, the latter being the most commonly adopted. The gelificationprocedure may involve a sequence of steps including: (i) the dissolution of the lithium salt inthe given organic solvent mixture, (ii) the addition of the polymer component and its dispersionin the solution, (iii) the short-time heating of the slurry at around 90-100 °C for promotingcomplete homogenisation, and (iv) the cooling of the resulting solution to room temperatureto promote gelification [60]. These gel-type membranes will be hereafter simply indicated bylisting their components in sequence. For instance, the gel electrolyte formed by immobilisingan ethylene carbonate-dimethyl carbonate lithium hexafluorophosfate solution in PAN, willbe here indicated as LiPF6-EC-DMC-PAN.

Strictly speaking, the gel membranes cannot be classified as ‘true’ polymer electrolytes,but rather as hybrid systems where a liquid phase is contained within a polymer matrix.A schematic view of this structure is represented in Figure 7.7.

Figure 7.6 Voltage profile of a typical charge-discharge cycle of a Li /PEO-LiCF3SO3 +γLiAlO2 / LiMn2O4 polymer battery at 94 °C and at C/10 rate. Derived from [26].

ΔV is the charge (upper curve) and discharge (lower curve) voltage.


226


Figure 7.7 Schematic representation of gel-type polymer electrolyte configuration

227

However, there has been some discussion as to whether these gel electrolytes are indeedsimple two-phase materials where the polymer is a passive component acting as a rigidframework for regions of liquid solutions or whether they are integrated systems wherethe polymer provides the stability of the gel network down to the immediate vicinity ofthe Li+ ions [61]. Recent papers reporting NMR [62, 63] and Raman [64-66] spectroscopystudies show that some interactions do occur between the polymer backbone and theelectrolyte solution, to an extent that depends upon the nature of the two components.

For instance, in the case of electrolytes using a PAN matrix, clear evidence of coordinationbetween the polymer backbone and both the solvated ions and the solvent molecules,has been shown, whereas in those using the PMMA matrix the interaction is weak [65].Thus, while the latter can be regarded as hybrid systems where the liquid solutions areimbedded in a ‘passive’ host polymer, the former are systems characterised by an ‘active’polymer experiencing strong interactions with the solutions. These structural differencesmay be of importance when a selection has to be made in view of battery application. Ingeneral, the PAN-based membranes, having characteristics which approach those of a‘true’ polymer electrolyte entity, are expected to be more stable than the PMMA-basedcounterparts and thus to offer a higher reliability as practical battery separators.

Feullade and Perche [67] originally introduced gel-type electrolytes. However, interest inthese materials has increased recently with the development and the characterisation ofmany new types of membranes having different properties [59, 68-74]. Some examplesof these membranes with their composition and their electrochemical properties are listedin Table 7.2 [72].

Clearly, these gel-type electrolytes have quite promising properties in terms of conductivity,approaching that of liquid solutions. This can be seen in Figure 7.8, which shows theArrhenius plots of some selected examples, and Figure 7.5 which compares theconductivity of gels with that of PEO-based membranes.

However, a high ionic conductivity, while obviously an important feature, is not sufficientto make a given electrolyte suitable for battery applications. Additionally, the chemicaland electrochemical stability are key parameters. The time evolution of the conductivity ofa LiPF6-EC-PC-PAN electrolyte obtained at 25 °C, the storage temperature, is shown inFigure 7.9 [74]. The trend shows that the conductivity is quite high, approaching 10-2 Scm-1 and remains stable for many days, indicating that the membrane does not degrade bycrystallisation or phase separation phenomena either upon thermal or time excursions.

In general, the electrochemical stability of an electrolyte is typically established bydetermining its breakdown voltage. This test can be carried out by running sweepvoltametry on cells using the given membrane as electrolyte, a ‘blocking’ (i.e., not reversibleto the membrane’s mobile ion) working electrode and a third reference electrode. Under


228


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Figure 7.8 Conductivity Arrhenius plots of various gel-type electrolytes. The plot of atypical liquid solution is also reported for comparison purposes.

Figure 7.9 Time evolution of the conductivity of the LiPF6-EC-PC-PANelectrolyte at 25 °C


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these conditions, the voltage at which the current starts to flow through the cell may beassumed as the decomposition of the electrolyte. In the case of the gel-type membranesunder discussion here, the test has been run by using a Ni blocking electrode and a Lireference electrode. Results obtained for the LiPF6-EC-PC-PAN electrolyte are shown inFigure 7.10. The current onset is detected around 4.3 V vs. Li, this being high enough toallow the safe use of the electrolyte membrane with a large selection of electrode couples.

One may then conclude that, the gel-type electrolytes, and the PAN-based ones in particular,have electrochemical properties that in principle make them suitable for application in versatile,high-energy lithium batteries. In practice, their use may be limited by the reactivity towardsthe lithium electrodes induced by the high content of the liquid component. Indeed, severepassivation phenomenon occurs when the lithium metal electrode is kept in contact with thegel electrolytes [60, 69]. This confirms the general rule that if from one side the wet-likeconfiguration is essential to confer high conductivity to a given polymer electrolyte, from theother it unavoidably affects its interfacial stability with the lithium metal electrode.

On the other hand, the high conductivity of the gel electrolytes may be exploited in aneffective way by directing them to the development of new-design, plastic-like batteries wherethe lithium metal anode is replaced by a lithium-accepting compound, such as a carbon orgraphite [75]. These are the so-called ‘rocking chair’ or, more commonly ‘lithium-ion’ batteries[76]. Basically, these batteries operate on the cyclic transport of lithium ions from one lithium-

Figure 7.10 Sweep voltammetry of a Ni electrode in a LiPF6-EC-PC-PAN electrolytecell at 25 °C. Li counter electrode. Scan rate: 0.5 V s-1.

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rich cathode (e.g., LiCoO2 or, alternatively, LiNiyCo1-yO2 or LiMn2O4) to a lithium-pooranode [1, 6, 20] (e.g., C), according to the general overall process shown in Equation 7.3

6C + LiCoO2 ⇔ LixC6 + Li1-xCoO2 (7.3)

These lithium-ion batteries are quite effective and are presently produced at a rate ofseveral millions of units per months, prevalently by Japanese manufacturing companies[20] and directed to the consumer electronics market where they have now assumed aprominent position [2].

These commercial batteries are commonly prepared in the so-called ‘bobbin-type’configuration, by layering in sequence a thin layer of the carbon anode (backed on ametal, usually copper) foil collector, a microporous separator felt and the composite(blend of active material, conductive carbon and binder) cathode (backed on a metal,usually aluminum) foil. In a typical cylindrical design (Figure 7.11), the three contactedlayers are coiled one on top of the other to obtain a spirally wound geometry having ahigh surface area. This assembly is then housed in a suitable container which, aftertapping with the liquid electrolyte, is hermetically sealed to prevent either leakage or

Figure 7.11 Schematic representation of a cylindrical lithium-ion battery(Reproduced with permission from B. Scrosati, La Chimica e l’Industria, 1997, 5,

465, published by Societa Chimica Italiana)


232


contact with the external environment. This procedure is commonly used to produce aseries of different geometries that includes prismatic as well as cylindrical cases.

The most common, commercially available lithium-ion batteries use a liquid electrolyte,generally formed by a solution of a lithium salt (e.g., LiPF6) in an organic solvent mixture(e.g., EC-DMC). The next important step in this technology is the replacement of thefibre separator embedded with the liquid electrolyte with a polymer membrane that canact both as the separator and the electrolyte. This is expected to be an importanttechnological progress because it provides the prospect of a favourable combination ofthe high energy and long life typical of the lithium-ion concept, with the reliability anddiversified design that the plastic configuration may offer.

Accordingly, many attempts to reach this goal are presently underway. One of therequirements for a successful result is the availability of polymer electrolyte membraneshaving electrical and chemical properties comparable with those of the common liquidelectrolytes. An approach in this direction is based on the use of an elasticised electrolytemembrane separator formed by a copolymer of vinylidene fluoride and hexafluoropropylene[77, 78]. This membrane is capable of absorbing large quantities of liquid electrolytes andthis feature has been exploited for a battery fabrication process which initially involves thelamination of three battery components, i.e., the anode film, the membrane soaked with alow vapour pressure plasticiser and the cathode film. A second process in which the plasticiseris eliminated to precondition the separator into a highly porous, liquidphilic membranefollows this first step. Finally, activation is accomplished by spreading a suitable lithium-ion liquid electrolyte solution (e.g., the LiPF6-EC-DMC solution) throughout the separatormembrane and the electrode films [79].

Alternative routes to obtain lithium-ion plastic batteries have considered the use of PAN-based gel-type polymer electrolytes as separators. These electrolyte membranes, althoughmacroscopically solid, contain in their structure the active liquid electrolyte (Figure 7.7).Therefore, they have a configuration which in principle allows a single lamination processfor the fabrication of the lithium-ion battery, i.e., a process that avoids intermediateliquid extraction-soaking activation steps.

The feasibility of the gel electrolytes for lithium-ion batteries development has been testedby first examining their compatibility with appropriate electrode materials, i.e., thecarbonaceous anode and the lithium metal oxide cathode. This has been carried out byexamining the characteristics of the lithium intercalation-deintercalation processes inthe electrode materials using cells based on the given polymer as the electrolyte andlithium metal as the counter electrode.

As an example, the voltage response of a graphite electrode cycled in a LiClO4-EC-DMC-PAN electrolyte cell is shown in Figure 7.12. The response follows Equation 7.4.

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xLi+ + 6C ⇔ LixC6 (7.4)

It has been seen that the response approaches that observed in liquid electrolyte cells,namely a voltage profile which, during the intercalation process, decreases along a seriesof distinguishable plateaus corresponding to the progressively occupied staging graphitephases [80, 81]. A similar trend is reproduced upon the reverse, lithium deintercalationprocess, although with an apparent loss in capacity. This is also expected on the basis ofthe results obtained in liquid electrolytes, which have demonstrated that the formal excesscapacity during the initial cycles is due to the side reactions involving the decompositionof the electrolyte with the formation of a passivation layer on the graphite electrodesurface [82, 83]. It may be noted that, in contrast with the case of the lithium metalelectrode, the occurrence of this passivation layer, which is electronically insulating butlithium ion conducting, is essential for assuring the proper cyclability to the graphiteelectrode [1]. The layer, often called the solid electrolyte interface (SEI) [84] prevents thedecomposition of the electrolyte, thus allowing the continuation of the electrochemicalintercalation process down to very low voltage levels, i.e., around 50 mV vs. Li(Figure 7.12) and with a reversible capacity which cycles around 300 mAh g-1. Thesetwo features are important to maintain high battery voltage and a long cyclability,respectively, when the graphite anode is coupled with a lithium metal oxide cathode.

As in the case of the graphite anode, the electrochemical response of the cathodes canalso be evaluated by following the lithium intercalation-deintercalation processes, again

Figure 7.12 Typical voltage response of the Li intercalation-deintercalation process ofa graphite electrode in a LiClO4-EC-DMC-PAN electrolyte cell. Temperature: 25 °C.

Lithium counter electrode. Cycling rate: C/4.


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using cells with a gel electrolyte and a lithium metal counter electrode. The results obtainedin the case of a lithium manganese spinel cathode cycled in a LiClO4-EC-DMC-PANelectrolyte cell are reported in Figure 7.13, the data collected to promote and evaluatethe process as shown in Equation 7.5.

LiCryMn2-yO4 ⇔ Li1-xCryMn2-yO4 + xLi+ + xe (7.5)

Instead of the stoichiometric LiMn2O4, a Cr-added spinel has been used in this test. Theuse of a metal-doped manganese spinel is common in the lithium-ion technology since ithas been demonstrated that a partial substitution of Mn (III) with M (III) metals, such asCr, may consistently stabilise the spinel structure, conferring to the cathode a good capacityretention upon cycling [53, 85-87]. The trend seen in Figure 7.13 demonstrates that thevoltage profile and the cycling capacity (about 130 mAh g-1) match those expected forthese cathode materials in liquid electrolyte cells.

Once the compatibility of the gel-type electrolyte with both anode and cathode materialsis ascertained, one can proceed with the combination of the two for the fabrication ofpolymer-based lithium-ion battery prototypes. A few examples of these prototypes havebeen reported at the laboratory level scale. One is provided by a battery of the type C/LiClO4-EC-PC-PAN/LiCryMn2-yO4.

This battery was fabricated and tested under a coin-type cell configuration [80, 81]. Theoverall process of this battery is shown in Equation 7.6.

Figure 7.13 Typical voltage response of the Li intercalation-deintercalation process ofa LiCryMn(2-y)O4 electrode in a LiClO4-EC-DMC-PAN electrolyte cell. Temperature:

25 °C. Lithium counter electrode. Cycling rate: C/4.

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6C + LiCryMn2-yO4 ⇔ LixC6+ Li1-xCryMn2-yO4 (7.6)

A typical charge-discharge cycle is shown in Figure 7.14, and confirms the feasibility ofthe PAN-based gel electrolytes as separators in lithium-ion batteries by showing that thecell can indeed be cycled with a good capacity delivery.

Another lithium-ion polymer cell recently tested as a laboratory-scale prototype [88] hasthe configuration KC8/LiClO4-EC-PC-PAN/LiMn2O4.

In this case a potassium-graphite (KC8) electrode has been used as the carbonaceous anodematerial. Upon anodic polarisation this electrode irreversibly deintercalates potassiumresulting in a graphite-like compound, which on subsequent cycles performs with fastkinetics of its lithium intercalation-deintercalation process [89, 90]. Accordingly, the firstcharging process of the battery may be written as shown in Equation 7.7 at the anode.

KC8 ⇒ K+ + e- + 8C (7.7)

The reaction at the cathode is shown in Equation 7.8.

LiMn2O4 + e- + Li+ ⇒ Li2Mn2O4 (7.8)

The charge balance in the electrolyte is assured since the amount of Li+ intercalated inLi2MnO4 is compensated by the amount of K+ deintercalated from KC8. The discharge


Figure 7.14 Typical voltage profile of the charge-discharge cycles of theLi/LiClO4-EC-DMC-PAN/LiCryMn(2-y)O4 polymer battery at 25 °C.

Cycling rate: C/10.

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process and all the other cycles involve the cycling of the Li+ ions between the twoelectrodes, according to the typical lithium-ion process shown in Equation 7.9.

8C + Li2Mn2O4 ⇔ 4/3LiC6 + Li2/3Mn2O4 (7.9)

In this system, the maximum achievable specific capacity is 372 mA g-1 referred to theanode. These processes have been confirmed by the experimental response of the cell [8]which showed a voltage profile developing around the 3V characteristic of the lithium-rich manganese spinel phase [91] and delivered about 80% of the theoretical capacity at0.1 mA cm-2 cycling rate [88].

Another interesting application of the lithium-ion battery concept has been applied tothe SnO2/LiNi0.8Co0.2O2 electrodic couple [92]. Convertible oxides, and tin oxide inparticular, first proposed as alternative anode materials by the Japanese Fuji Photo FilmCompany [93, 94], are presently the object of considerable attention in the lithium-ionbattery community [95-98]. When negatively polarised in a lithium cell, tin oxide firstundergoes an irreversible reaction shown in Equation 7.10.

SnO2 + 4Li ⇒ Sn + 2Li2O (7.10)

This results in metallic Sn particles that remain finely dispersed in the Li2O matrix. Thisproduces a favourable geometry and facilitates a subsequent reversible reaction asindicated by the lithium alloying-dealloying process shown in Equation 7.11.

Sn + 4.4Li ⇔ Li4.4Sn (7.11)

The lithium oxide, surrounded by the tin particles, creates a sufficient amount of freevolume to accommodate the mechanical stresses experienced by the metal particles duringthe course of the process. This greatly improves the cyclability of the electrode [94]. Theinterest in this electrode lies in its high specific capacity, which reaches values approaching700 mAh g-1, i.e., almost double that offered by the more conventional graphite electrode.This feature is somewhat contrasted by a large, initial irreversible capacity associatedwith reaction 7.10 and by a certain tendency to lose capacity upon cycling, although thelatter issue can be largely controlled.

A Ni-Co mixed compound can be used as the cathode. This material is presently consideredas a valid alternative to the more common LiCoO2 cathode, both in terms of cost andenvironmental compatibility [103, 104]. The electrochemical process is similar in bothcases, i.e., the reversible release and uptake of lithium shown in Equation 7.12.

LiNi0.8Co0.2O2 ⇔ xLi+ + Li1-xNi0.8Co0.2O2 (7.12)

237

Considering the reactions of the two electrodes, the overall process of the SnO2/LiNi0.8Co0.2O2 cell may, under steady conditions, be written as shown in Equation 7.13.

LiNi0.8Co0.2O2 + Sn ⇔ LixSn + Li1-xNi0.8Co0.2O2 (7.13)

This process has been exploited for running a laboratory prototype polymer lithium-ioncell based on the LiClO4-EC-DMC-PAN electrolyte.

The voltage profile of a typical discharge-charge cycle of this cell is shown in Figure 7.15,along with anode and cathode voltage variations. It may be seen that the SnO2 anodecycles with a trend comparable to that usually obtained in more conventional liquidelectrolyte cells [105], delivering a reversible capacity approaching 300 mAh g-1. Thiscapacity level is maintained upon further cycling, although with a certain progressivefade, this again being experienced in liquid-electrolyte cells as well.

Figure 7.15 Typical voltage profile of a charge-discharge cycle of the SnO2 / LiClO4-EC-DMC-PAN / LiNi0.8Co0.2O2 polymer battery at 25 °C. Cycling rate: 0.25 mA cm-2.


238


Peramunage and Abraham have recently reported an advanced lithium-ion polymer cell[106, 107]. In this case, a material of the Li [Li1/3Ti5/3]O4 family [108, 109], e.g., theLi4Ti5O12, intercalation compound, has been used as an anode. The lithium intercalation-deintercalation process in this compound is shown in Equation 7.14.

Li4Ti5O12 + 3Li ⇔ Li7Ti5O12 (7.14)

This system evolves around 1.5 V versus Li and is accompanied by very little change inlattice dimension. Thus, as opposed to the lithium-metal alloy cases discussed above, theabsence of structural deformation makes Li4Ti5O12 an almost ‘zero strain’ electrode materialcharacterised by a very good cyclability and by very little capacity fade upon cycling. Thisimportant feature has been experimentally confirmed [107] by determining the responseof a lithium-ion polymer prototype cell such as Li4Ti5O12/LiPF6 -EC-PC-PAN/LiMn2O4.

Some charge-discharge cycles obtained for the cell are shown in Figure 7.16. An apparentdrawback of this cell is its relatively low overall voltage, which stabilises around 2.5 V,which is about 1.5 V less than that of more conventional lithium-ion systems based ongraphite anodes. The difference is in the voltage levels of the two anode materials, i.e.,1.5 V versus Li for Li4Ti5O12 versus the 0.050 V vs. Li for graphite.

Figure 7.16 Voltage profile of a charge-discharge cycles of the Li4Ti5O12/LiPF6-EC- PC- PAN/LiMn2O4 polymer battery at 25 °C. Cycling current densities are shown in the figure.(From reference 107, reproduced by permission of the Electrochemical Society, Inc.)

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However, this may not be an issue in the near future considering that the trend in electronicscircuitry, wherein the use of the batteries is directed, is that the voltage-powering request willbe progressively reduced from the initial 4 V to 3 V and, in the future, to an even lower range.This may somewhat eliminate the need for high-voltage power sources and thus, the constraintof choosing low-voltage anodes. This in turn opens new avenues for electrode materialsoperating within the stability window of the electrolyte, such as the Li4Ti5O12 discussedabove, with important advantages in terms of the cyclability and safety of the battery.

The results so far described, although demonstrating the feasibility of the lithium-ion conceptin polymer electrolyte batteries, are somewhat limited to cases involving laboratory cells.More recently, various battery manufacturer companies worldwide have announced theirinvolvement in the large-scale, commercial production of lithium-ion polymer batteries [110].However, the level of released information is very scarce and, it is not possible at this stage toevaluate the effective status of the development of these advanced types of batteries. One canonly cite that the Japanese Matsushita Battery Company has announced the completion ofthe facility for the large-scale production of thin-film lithium-ion polymer batteries [111].However, no information is available on the nature of the polymer electrolyte or on the typeof electrodes selected for this development. Other Japanese companies, including Sony EnergyTechnology and Toshiba, have announced similar activity, with even less details of the typeof electrode and electrolyte materials used for their products [110]. Shipments of lithium-ionpolymer electrolyte batteries have also been reported by Ultralife Batteries in the USA [112].According to the released information, these batteries use a graphite anode, a Li1+xMn2-xO4

cathode and a polymer electrolyte which has not yet been disclosed.

Apparently, these commercial lithium-ion polymer batteries have characteristics, such asreduction in thickness and improvements in safety, which make them very appealing forthe modern consumer electronics markets, particularly the new generation of cellularphones. This may lead to the conclusion that the evolution of the lithium-ion batterytechnology will be focused on polymer configurations with an output that is expected tosoon experience a substantial share in the electronics market.

7.2.2 Proton Polymer Electrolytes

The research into proton conducting polymer electrolytes has consistently increased inrecent years due to the transport characteristics which make them promising for variouselectrochemical applications of interest for the electronics market, including sensors and,particularly, fuel cells [113]. Nevertheless, the proton conductivity of the known polymersystems still remains below the upper limit of proton conductivity in liquids. The majorproblems arise from the numerous additional requirements, other than protonconductivity, which must be met for any specific application.


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Nafion is presently the material of choice, because it acts as a good, chemically inert andstable polymer electrolyte [114]. As is well known, the hydrophobic part of this polymerprovides a relatively good mechanical stability even in the presence of water, while the hydratedhydrophilic domains provide high proton conductivity (i.e., higher than 10-3 S cm-1 at ambienttemperature) at water contents exceeding 30% wt. In fact, due to the hydrophilic nature ofthe sulfonic groups attached to the polymer backbone, the conductivity strongly dependsupon relative humidity [115].

Although widely used, Nafion is somewhat affected by some operational problems andthus, new, optimised proton conducting membranes would be highly welcome. Indeed,intense research efforts are presently directed to reach this goal. Accordingly, many studieshave been performed on proton conducting electrolytes based on polar polymers havingbasic sites. These sites form compounds with strong acids, such as H2SO4 or H3PO4.Particularly, polybenzimidazole (PBI), polyvinylpyrrolidone (PVP) and polyacrylamide(PAAM) form complexes with inorganic acids [116, 117]. As is well known, the amidegroup has basicity comparable to that of water and thus it can be protonated at either theoxygen or the nitrogen atoms. Indeed, the high conductivity (around 10-2 S cm-1) observedat room temperature for the PAAM-H2SO4 electrolyte has been attributed to the protonationcapability of the amide groups [118].

It may be noted that it is not only important to adjust the polymeric host to the requirementsof a particular application as described above, but also to optimise the environment of theproton. In fact, adding protonated and unprotonated solvent species can increase protontransport. As suggested by Kreuer [119], these species result in the generation of protonicdefects in the non polar gel environment. In the same way, the inclusion of basic nitrogengroups in branched polyethyleneimine-H2SO4 or imidazole sulfonated polyaromaticmembranes is expected to produce an increase in conductivity. This effect could be attributedto the proton exchange between two amine groups.

In general, the ionic transport in linear or crosslinked swollen polymers containing a lowmolecular weight polar or ion-chelating additive mainly occurs in the solvent phase [118,120]. This concept has been applied to develop proton conducting polymeric gel or hydrogelmembranes [121-123] which reach conductivity values around 10-3 S cm-1 at room temperatureand are not destroyed or dissolved even at high humidity levels.

The question is whether the procedure successfully used for the fabrication of gel-type,lithium conducting polymer electrolytes discussed in the previous paragraph may besuccessfully extended to other ion conducting systems. In this respect, one may considerthat ionic transport in crosslinked swollen polymers containing a low molecular weightpolar or ion-chelating additive may indeed occur in the solvent phase and thus, that thegel concept can be extended to the proton conducting systems. Effectively, protonconducting polymeric gel or hydrogel membranes with conductivity values around 10-3

S cm-1 have been developed in the past [122]. Recently, a new approach aimed at

241

developing proton conducting gel membranes having transport properties not dramaticallydependent on the humidity level has been reported [124]. These novel types of protongel electrolyte membranes have been obtained by incorporating salicylic acid (SA) orbenzoic acid in a highly plasticised PMMA matrix. The synthesis involved the mixing ofsolutions of organic acids in protophobic solvents, such as PC and EC, with low contentsof protophilic solvents (such as dimethylformamide (DMF), methylformamide orformamide) into a highly plasticised PMMA matrix [124, 125]. Here the carboxylicgroups are expected to be able to act as proton donors and to show low levels of hydration,while the ring itself is rather non polar. In addition, the melting point of these acidmolecules is generally higher than the boiling point of water, which makes them interestingcandidates for supporting high proton conductivity at room temperature. Thesemembranes feature as robust electrolyte systems with a thermal stability that extends upto 70-90 °C [124]. It is also important to point out that the conductivity values of themembranes were found to be higher than those of the plain liquid solutions. This effectwas in part attributed to a specific interaction between the polymer and the non polarfragment of the SA molecule, thus enhancing ionic transport. In addition, it was assumedthat the presence of DMF induces a more polar environment inside the gel matrix due totheir strongly polar amide groups [124].

Figure 7.17 Conductivity of proton membranes as function of the humidity content.(From reference 124, reproduced by permission of the Electrochemical Society, Inc.)


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The effect of the environmental humidity on the conductivity of SA-based PMMA membranescompared with that of a typical Nafion membrane, the presently most used proton-conductingmembrane, is shown in Figure 7.17. The high conductivity of the membranes confirms thattheir transport mechanism is much less influenced by the humidity level then that of Nafion.

As for most of the other advanced systems, these new protonic gel-type membranesappear to be still under evaluation. Presently, Nafion and related membranes still dominatethe fuel cell market. The limited length of this chapter and its focus on batteries does notallow inclusion of a description of the present status of the polymer electrolyte fuel cells.The interested reader is referred to the many books and review articles available on thesubject [e.g., 113, 126, 127].

7.3 Electronically Conducting Polymers

The discovery in the late 1970s that certain types of polymers, though intrinsically poorconductors, can acquire electronic conductivity approaching that of metals followingchemical or electrochemical treatment has triggered intensive research interest in thefield. The essential structural characteristic needed by polymers to attain this significantconductivity change is a conjugated π-system extending over a large number of monomericunits, a trait common to polyheterocycles, such as polypyrroles and polythiophenes, andto polyanilines. The processes that switch conjugated polymers from the insulating tothe conducting state are redox reactions, whether chemically or electrochemically driven.They are called doping processes, p-doping or n-doping, in relation to the positive or thenegative sign of the injected charge.

Many extensive review articles have been published on the synthesis procedure of theseconducting polymers, as well as on their electronic structure and its evolution upon thedoping processes [129-131]. The reader is referred to these references for detailedinformation. The most important feature of conducting polymers is in the reversibility ofthe doping process, which involves the formation of charged complexes which includepolycations or polyanions, and to maintain the electrical balance, the correspondingstructural insertion of adversely charged ions from the redox medium. Thus, conductingpolymers can be switched repeatedly between their doped and undoped states byelectrochemical oxidation and reduction processes that may involve a relatively large amountof electronic and ionic charge. This makes conjugated polymers an interesting class of highcapacity electrode materials where the charge injected or released is accompanied by acorresponding ion motion within their structure. Thus, conducting polymers may beregarded as ion-insertion electrodes that act during the charge-discharge (i.e., doping-undoping) process as mixed electronic-ionic conductors, somewhat similar to the insertioninorganic compounds discussed in Section 7.2.1. Consequently, much effort has been devotedto their use as cathodes in batteries, with particular attention to lithium batteries.

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7.3.1 Lithium-Doped Conducting Polymer and Lithium-Polymer Batteries

Although, in principle, conducting polymers can be used either as anodes (by exploitingtheir reduction or n-doping process) or as cathodes (by exploiting their oxidation or p-doping process), most battery applications have been confined to the latter. Considerableinterest was devoted in the early 1980s to the use of polyheterocycles and polyanilines ascathodes in lithium batteries. The initial efforts were mostly concentrated in cells using aliquid electrolyte, i.e., a solution of a lithium salt in an organic solvent.

More recently, attention has been focused on polyheterocycles. In the typical case of acell based on a PPy cathode, a LiClO4-PC electrolyte and a lithium metal anode, theelectrochemical process can be written as:

(PPy)n + nyLiClO4 ⇔ [(PPy)y+ (ClO4-)y ]n + nyLi (7.15)

The charge process, schematically drawn in Figure 7.18, involves the p-doping (oxidation)of the polymer with the formation of a polycation whose positive charge iscounterbalanced by the ClO4

- electrolyte dopant anion (depicted as A– in Figure 7.18)which diffuses into the polymer matrix. The PPy oxidation at the cathode is accompaniedat the anodic electrode by the reduction of lithium ions, which deposit as lithium metalon the given substrate. In the discharge process the electroactive polymer cathode releasesthe ClO4

- anions and the Li+ cations are stripped from the metal anode to restore theinitial electrolyte concentration. The extent of the process is defined by the term y, generallycalled ‘doping level’ which, representing the percentage of moles of the dopant anionover the moles of pyrrole monomer units, is proportional to the charge involved andthus to the battery capacity.

Considerable effort was paid in the early 1990s to the marketing of lithium-polymer batteries,especially of prototypes using PPy or polyaniline as the cathode [132-135]. However,although benefiting from some interesting features, such as a reasonably high voltage anda low cost, these batteries suffered from some major drawbacks, including a relatively lowenergy density, limited power density and, particularly, severe self-discharge [135-137].

The self-discharge issue has been addressed by considering fully solid-state configurationswhere the liquid electrolyte was replaced by a polymer electrolyte, such as the previouslydiscussed PEO-LiX blends or the PAN- or PMMA-based gels. This important conceptwas first exploited using the PEO-based polymer electrolytes to fabricate thin-film Li/PPy or Li/PT batteries [138, 139]. Significant results have also been obtained using thehighly conducting, gel-type electrolyte membranes described in Section 7.2.1. Cells formedby laminating a lithium metal anode, a PMMA-based electrolyte membrane and a PPycathode have been successfully assembled and tested [140, 141]. The most relevant featuresof these cells were a very high charge-discharge coulombic efficiency and a long cyclability.


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It is interesting to use the polymer cathodes in lithium-ion cell types. This concept hasbeen exploited by assembling cells having the structure C/LiClO4-EC-PC-PMMA/PPy.

Laboratory prototypes have been fabricated by preparing the electrodes in the form ofthin films backed on metallic substrates and separating them by the polymer electrolytemembrane [142, 143]. By charging the cell, Li+ cations enter the graphite structure andClO4

- anions simultaneously inject into the PPy structure:

n6C + (PPy)n + nyLiClO4 ⇔ [(PPy)y+ (ClO4-)y ]n + nLiyC6 (7.16)

where n is the number of pyrrole units corresponding to one positive doping charge andy is the lithium intercalation level in graphite. Since both electrodes experience intercalationfrom different electrolyte species, this particular cell has been named a ‘dual lithium ion’battery [142]. The cell is characterised by good cycling performance and by a total energydensity of 300 Wh kg-1. Other advantages of the dual battery are the low cost, thecompatibility with the environment and high power capabilities. Drawbacks in respectto the standard C/LiMO2 polymer lithium-ion batteries are in the lower capacity andlower operational voltage [144].

Figure 7.18 Schematic representation of the doping process of heterocyclic conductingpolymers, e.g., polypyrrole

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The results reported above indicate that a proper choice of the battery components enablesthe intrinsic potentialities of the polymer electrode and electrolyte materials to be exploitedfor the development of revolutionary electrochemical devices. Accordingly, a number oflaboratories are currently seeking to enhance the electrochemical properties of conductingpolymers by designing suitable materials, the final goal being to optimise their responsein advanced, plastic-like batteries. Undoubtedly, this will be the type of batteries thatwill dominate the electronic market in the new millennium.

Acknowledgements

I would like to thank my students and collaborators, Giovanni Battista Appetecchi, FrancoBonino, Fausto Croce, Simona D’Andrea, Alessandra D’Epifanio, Stefania Panero, LuigiPersi, Priscilla Reale, Paola Romagnoli, Fabio Ronci and Giuseppe Savo, for theirimportant and dedicated research work, some results of which are reported in this chapter.

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116. J.S. Wainright, J.T. Wang, D. Weng, R.F. Salvinell and M. Litt, Journal of theElectrochemical Society, 1995, 142, 7, L-121.

117. M.F. Daniel, B. Desbat and J.C. Lassegues, Solid State Ionics, 1988, 28-30, 632.

118. E. Zygadlo-Monikowska, Z. Florjanczyk and W. Wieczorek, Journal ofMacromolecular Science A, 1994, 31, 9, 1121.

119. K.D. Kreuer, Chemical Materials, 1996, 8, 610.

120. D.G.H. Ballard, P. Cheshire, T.S. Mann and J.E. Przeworski, Macromolecules,1990, 23, 5, 1256.

121. J. Przyluski and W. Wieczorek, Synthetic Metals, 1991, 45, 323.

122. J.R. Stevens, W. Wieczorek, D. Raducha and K.R. Jeffrey, Solid State Ionics,1997, 97, 1-4, 347.

123. G. Vaivars, J. Kleperis, A. Azens, C.G. Granqvist and A. Lusis, Solid State Ionics,1997, 97, 1-4, 365.

124. A.M. Grillone, S. Panero, B.A. Retamal and B. Scrosati, Journal of theElectrochemical Society, 1999, 146,27.

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125. S. Panero and B. Scrosati, Journal of Power Sources, 2000, 90, 13.

126. J. Przyluski, W. Wieczorek, Z. Poltarzewski, P. Staiti and N. Giordano, in RecentAdvances in Fast Ion Conducting Materials and Devices, Eds., B.V.R. Chowdari,Q.G. Liu and L.Q. Chen, 1990, World Scientific Publishing, River Edge, NJ,USA, 1990, 307.

127. A.J. Appelby and F.R. Foulkes, Fuel Cell Handbook, Van Nostrand Reinhold,New York, NY, USA, 1989.

128. T.A. Skotheim, Handbook of Conducting Polymers, Volumes 1 and 2, MarcelDekker, New York, NY, USA, 1986.

129. B. Scrosati in Solid State Electrochemistry, Ed., P.G. Bruce, Cambridge UniversityPress, Cambridge, UK, 1996, 229.

130. M.G. Kanatzidis, Chemistry and Engineering News, 1990, 68, 49, 36.

131. C. Arbizzani, M. Mastragostino and B. Scrosati in Handbook of OrganicConductive Molecules and Polymers, Volume 4, Ed., H. S. Nalwa, John Wiley &Sons Ltd., New York, NY, USA, 1997, 595.

132. N. Furukawa and K. Nishio in Applications of Electroactive Polymers, Ed., B.Scrosati, Chapman & Hall, London, UK, 1993, 150.

133. K. Nishio, M. Fujimoto, N. Yoshinaga, O. Ando, H. Ono and T. Suzuchi, Journalof Power Sources, 1991, 34, 153.

134. H. Mustedt, G. Kohler, H. Mohwald, D. Naegele, R. Bittin, G. Ely and H.Meissner, Synthetic Metals, 1987, 18, 259.

135. M. Mastragostino, A.M. Marinangeli, A. Corradini and C. Arbizzani,Electrochimica Acta, 1987, 32, 1589.

136. S. Panero, P. Prosperi and B. Scrosati, Electrochimica Acta, 1987, 32, 1461.

137. P. Novak, O. Inganas and R. Bjorklund, Journal of Power Sources, 1987, 21, 17.

138. C. Arbizzani, M. Mastragostino, S. Panero, P. Prosperi and B. Scrosati, SyntheticMetals, 1988, 28, C663.

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140. S. Kakuda, T. Momma, T. Osaka, G.B. Appetecchi and B. Scrosati, Journal of theElectrochemical Society, 1995, 142, 1, L1.


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141. T. Osaka, T. Momma, H. Ito and B. Scrosati, Journal of Power Sources, 1997,68, 2, 392.

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143. A. Clemente, S. Panero, E. Spila and B. Scrosati, Solid State Ionics, 1996, 85,273.

144. G.B. Appetecchi, S. Panero, E. Spila and B. Scrosati, Journal of AppliedElectrochemistry, 1998, 28, 12, 1299.

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8 Polymer Microactuators

K. Kaneto and M. Kaneko

8.1 Introduction

Actuators that generate movements and forces, such as bending, expansion andcontraction driven by stimulation of electrical, chemical, thermal and optical energies,are different from rotating machines such as electric motors and internal combustionengines. There are many sorts of soft actuators made of polymers [1-3], gels [4] andnanotubes [5]. Particularly, biomimetic actuators are interesting because of the applicationto artificial muscles that will be demanded for medical equipment, robotics andreplacement of human muscle in the future.

In natural muscles, nowadays, the mechanism of actuation has being revealed to someextent; however, the details are not known. In fact, the muscles are constructed in anordered structure from molecular level to macroscopic level [6]. The driving forceoriginates from the conformational change of peptide molecules by the chemical energycycles. Similarly, various sorts of stimulating energies can change the conformation ofmolecules. For example, the cis-trans photoisomerisation in azobenzene, as shown inFigure 8.1a, is a well-known phenomenon [7]. The cis form converts to the longer transform upon illumination by UV light, and the trans form reverts to the cis form uponillumination by visible light.

The electrical conductivity in polyacetylene (Figure 8.1b), the representative of conductingpolymers, dramatically increases from insulator to conductor upon chemical orelectrochemical oxidation [8]. This results from the delocalisation of π-electrons and inthe change of polymer conformation. At the pristine stage (or in the reduced state), thepolyacetylene is flexible because of the single bond in the bond alternation. The singlebond has more freedom in the rotational and bending modes than that of the doublebond. In the oxidised state, the bond alternation is reduced and the molecular structurebecomes more planar than that of the reduced state. Similarly, polyaniline, shown inFigure 8.1c and discussed mainly in this chapter, changes its bond alternation from thebenzenoid form to quinoid form upon oxidation [9, 10]. This results in the deformationof polymer conformation.

In fact, films, fibres or blocks of conducting polymer expand and contract uponelectrochemical oxidation and reduction [1, 2, 11-16], respectively. This process is

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tentatively named ‘electrolytic deformation’ or ‘electrolytic expansion’ in this chapter.The mechanisms of electrolytic deformation have been classified by three principalmechanisms [17, 18]: (1) the insertion and removal of bulky ions, (2) conformationalchange of polymer structure due to the delocalisation of π-electrons, and (3) electrostaticrepulsion between likely charged polycations (polarons and/or bipolarons). In mechanism(1), the insertion is induced by the neutralisation of polymers for oxidation and reduction.The magnitude of expansion or contraction depends on the volume of ions and cannotbe larger than the total volume of the inserted ions. For mechanism (2), the expansion ofthe polymer is associated with an expanding spring and depends on the morphology ofpolymer structure. The electroexpansion of hydrogels [4] is explained by mechanism (3).To accomplish the larger expansion ratio in the electrolytic deformation of conductingpolymers, it is desirable to utilise mechanisms (2) and (3).

In this section, the behaviour of the electrolytic expansion in conducting polymers,especially polyaniline and poly(o-methoxyaniline) (PMAN) are described, with discussionof the basic redox reaction of polyaniline, the dependence of the expansion ratios onoxidation levels, the kind of anions, strain, the pH of the electrolyte and anisotropy.

Figure 8.1 Conformational changes of molecular structure, (a) photoisomerisation ofazobenzene, (b) and (c) extension and contraction of polyacetylene and polyaniline,

respectively, upon oxidation and reduction

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Polymer Microactuators

8.2 Sample Preparation and Measurements of Electrolytic Deformation

Conducting polymers are prepared by either chemical or electrochemical oxidation ofmonomers, like pyrrole, thiophene and aniline, following the methods described in theliterature [9, 19]. For the measurement of electrolytic deformation, it may be preferableto use soluble polymers, such as polyaniline (soluble in N-methyl-2-pyrrolidinone (NMP)).Others are hardly soluble in usual organic solvents. However, polymers with substitutedlong alkyl chains have been found to be soluble in organic solvents. Usually, the conductingpolymers prepared by the electrochemical methods are obtained as thin films. Suchconducting polymers are, however, difficult to process. The polyaniline prepared by thechemical oxidation is a powder and the base form is named emeraldine base and issoluble in NMP. The emeraldine film can be prepared by casting the NMP solutioncontaining its concentration of 2-10 wt.% on a glass plate. The cast film obtained by thismethod can be stretched mechanically to more than 3 times the original length. Thefilms have been examined along the stretch direction, the perpendicular direction [14]and also the thickness direction [20] to investigate the anisotropic behaviour.

For a measurement of electrolytic deformation, a bimorph actuator [11] has beenfabricated, and the ratio of expansion is estimated from the bent curvature of the actuators.This method is effective for qualitative measurement and demonstration, since theexpansion is tremendously magnified. Even with an expansion rate at the level of 1%,the bending is clearly observed. For the direct measurement of the displacement, a balance[12], an Instron testing machine [17, 21] and a special cell with a pinhole at the bottom[13, 14] were employed. In this last method, the change of the film length is picked up bya thread through the pinhole as shown in Figure 8.2. The results obtained using the cellwith the pinhole will be discussed.

From the simultaneous measurements of the redox current (cyclic voltammogram, CV)and the change of film length (Δl) by the application of linear voltage sweep cycles, therelationship between the degree of oxidation and the rate of expansion (Δl/l0, l0 is theoriginal length) is obtained. By the application of a stepwise voltage, the diffusion constantof ions in the film is estimated from the time response of the electrolytic deformation.The diffusion constant is estimated [12] from the initial time dependence of injectedcharges after the voltage application:

f = 4D 1/2 t1/2 π-1/2 d-1 (8.1)

where f is the injected charge normalised to the saturated value, d is the thickness of thefilm, D is the difussion coefficient and t is the time. The diffusion constant obtainedfrom Equation 8.1 is based on the model that ionic species diffuse from the surface of thefilm. Equation 8.1 may also be applicable to the time response of the deformation.

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Figure 8.2 Schematic diagrams for the measurement of electrolytic expansion along (a)the film length and (b) the thickness direction. WE, RE and CE are the working

electrode, reference electrode and counter electrode, respectively.

(a)

(b)

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8.3 Electrochemistry and Expansion Behaviour in Polyaniline Film

Polyaniline in an aqueous acid solution takes three typical redox stages [9] depending on thedegree of oxidation, as shown by the cyclic voltammogram curve in Figure 8.3. The halfredox potential, E1/2 is defined as E1/2 = (Ea + Ec)/2, where Ea and Ec are anodic and cathodicpotentials at the peak currents for oxidation and reduction, respectively.

Figure 8.3 Typical cyclic voltammogram (upper), redox behavior in chemicalstructures (middle) and expansion and contraction of polyaniline film along the

stretched direction (lower). Ea and Ec are the anodic peak and cathode peak,respectively, E1/2, LS-ES = 1/2 (Ea + Ec).


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The most electrically conductive state is the emeraldine salt (ES) that is between pernigranilinesalt (Pas) at the high potential side and the leuco-emeraldine salt (LS) at the low potentialside. The LS and the Pas are the most reduced and oxidised states, respectively, and havebeen found to be insulating. In an oxidation process from the LS to the ES, two electrons arewithdrawn and two chloride ions are doped for every four benzene units. For the oxidationfrom the ES to the Pas, two electrons are withdrawn and two protons are released. The LS/ES reaction is reversible, however, the hydrolysis occurs at the higher oxidised Pas states.

The bottom of Figure 8.3 shows a typical expansion behaviour in a polyaniline filmalong the stretched direction. By the potential sweep from the LS to higher potentials,the film starts to expand and shows the maximum expansion at the ES. Then the filmcontracts slightly. When the potential was returned from the Pas state, the film expandsslightly, and then contracts below the potential of E1/2, LS↔ES and returns to the originallength. This extension and contraction behaviour is very similar to the weight changemeasured by a quartz crystal microbalance [22]. The result indicates that the electrolyticexpansion closely relates to the insertion and exclusion of dopant ions in the film. It maybe noted that this does not violate the mechanisms (2) and (3) described in Section 8.1.

The unstretched film and the stretched film perpendicular to the stretched direction showmonotonous expansion during the oxidation LS→ES→Pas and vice versa [14]. The resultpossibly indicates that the dopant ions settle between the polymer chains. In the expansionof the film perpendicular to the stretched direction, irreversible expansion, namely, acreeping effect, was observed even under light load during the redox cycles [14].

8.4 Dependencies of the Expansion Ratio on the Degree of Oxidationand Dopant Ions

The dependence of the expansion ratio [13] on the level of reduction, y, for various kindsof electrolytes in polyaniline films at pH = 0 are shown in Figure 8.4. The level of reductionwas determined from the amount of electrically injected electrons in the cyclic voltammogramprocess of test materials, and the definition is shown as the inset of Figure 8.4. Here, y = 0is the basis of the ES state, and y = –0.5 and 0.5 are the LS and Pas states, respectively. Thepolyaniline film contracts 2%-3% by the reduction from the ES state to LS state at aroundy = –0.2. The expansion ratio of polyaniline films at pH = 0 strongly depends on the kindof negative ions as seen in Figure 8.4. The hysteresis of the curve originates from the nonequilibrium condition of the system due to the slow rate of diffusion and also due tothermodynamics. Though the radius of benzenesulfonic acid (BSA) is extremely large thereis no large contraction since the molecule is too large to penetrate into the film.

The anion radius dependence of the contraction ratio and the diffusion coefficientsin polyaniline film at pH = 0 are shown in Figure 8.5, showing that the larger the ion

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Figure 8.4 Dependency of the expansion ratio on the level of reduction, y, for various kindsof electrolytes in polyaniline films at pH = 0. The definition of y is shown by the inset, and y

= 0 is taken as the ES state. l0 is the length of film at y = 0. BSA is benzene sulfonic acid.

Figure 8.5 Anion radius dependence of the contraction ratio in polyaniline film andpoly(o-methoxyaniline) at pH = 0.


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the more it expands and the smaller the diffusion coefficient. Sulfuric acid is consideredto be a divalent negative ion. The results proved that the electrolytic expansion issurely due to the doping and dedoping of bulky ions. However, in the PMAN, theexpansion ratio does not show any dependence on the kind of anions at pH = 0,which will be discussed later. It is important to note that the expansion ratio at theextrapolation to the zero ion radius is a finite value of about 1% and not zero forboth films. The finite value results from the conformational change of polymerstructure and the electrostatic repulsion.

8.5 pH Dependence of Electrolytic Expansion

The observed shift of the redox peaks with change in pH of the solution in an aqueouselectrolyte solution indicates that protons are sometimes involved in electrochemicalreaction. The pH dependence of the E1/2 and electrolytic expansion in polyanilineand poly(o-methoxyaniline) [20] are shown in Figures 8.6a and 8.6b, respectively.As is evident from these results, E1/2 is pH dependent at pH below 0 for polyanilineand below 1.5 for poly(o-methoxyaniline). The gradient of the pH dependence isapproximately 60 mV/pH for both curves, indicating that the oxidation takes placeby the ejection of a proton per one electron [9]. On the other hand, at pH greaterthan 0 for polyaniline and greater than 1.5 for poly(o-methoxyaniline), the E1/2 isindependent of the pH, suggesting that anions are injected by the oxidation. Takingthis into account, the electrolytic expansion at pH < 0 (polyaniline) or < 1.5 (poly(o-methoxyaniline), where protons are ejected by the oxidation, results from theconformational change and electrostatic repulsion. The difference of the magnitudein electrolytic expansion rates at lower and higher pH regions may be attributed tothe inserted bulky anions plus protons.

The dependence of the electrolytic expansion rates in poly(o-methoxyaniline) filmon the type of anions is shown in Figures 8.7a and 8.7b for pH = 0 and pH = 2,respectively. It should be noted that at pH = 0, the expansion rate scarcely dependson the kind of anion, whereas at pH = 2 the remarkable dependence is observed. Theresult indicates that at pH = 0 or pH < 1.5 in poly(o-methoxyaniline) the electrolyticexpansion and contraction are certainly driven by the change of polymer conformationand/or the electrostatic repulsion.

263

Figure 8.6 The pH dependency of the E1/2 and electrolytic expansion in (a) polyaniline,and (b) poly(o-methoxyaniline)

(a)

(b)


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(b)

(a)

Figure 8.7 The dependency of electrolytic expansions in poly(o-methoxyaniline) filmson the kind of anions at (a) pH 0, and (b) pH2. TSA is toluene sulfonic acid.

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8.6 Time Response of the Electrolytic Expansion

Usually the diffusion constant in a solution is much larger than in a solid, as the rate ofoxidation and reduction are determined by the diffusion of ions in the film. The typical timeresponses [13, 23] of an applied step potential, current and the expansion and contraction inpolyaniline film at pH = 0 are shown in Figure 8.8. The larger the applied potential, the fasteris the response of expansion. However, the response of the expansion is slower than thecurrent response, since the film expansion takes place after the oxidation of film.

From the response of the injecting charge normalised to the saturated value based onEquation 8.1, the diffusion coefficient of dopant ions can be estimated. The diffusionconstants in polyaniline [13] and poly(o-methoxyaniline) film [23] in various electrolytesat pH = 0 have been summarised. The diffusion constants of poly (o-methoxyaniline) arealways larger than that of polyaniline, as the oxidation occurs by ejection of protons inpoly(o-methoxyaniline), while in the case of polyaniline the oxidation occurs by injectionof bulky anions as discussed previously.

Figure 8.8 Typical time responses of an applied step potential (upper), current (middle)and the expansion and contraction (bottom) in polyaniline film at pH 0


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8.7 Anisotropy of Electrolytic Expansion in Polyaniline Films

Polymers are quasi-one-dimensional; therefore, anisotropic behaviour of the electrolyticexpansion is expected [14, 24]. Uniaxial pulling can stretch the polyaniline films by castingfrom the NMP (used as plasticiser) solution. In the stretched polyaniline film, chains alignin the stretched direction. The anisotropic expansion of uniaxially stretched film paralleland perpendicular to the stretched direction and of unstretched film is shown in Figure 8.9[14]. The electrolytic expansion perpendicular to the stretched direction is a roughestimation, because of the creeping effect caused by the continuous elongation arising as aconsequence of repeated electrochemical oxidation and reduction. The electrolytic expansionfor the perpendicular direction is larger than that of the unstretched film, resulting fromthe fact that the dopant ions are intercalated between the main chains.

Figure 8.9 The anisotropic expansion of uniaxially stretched film

The electrolytic expansion for the thickness direction in polyaniline cast film [20] showsan extremely large expansion ratio of more than 25% as shown in Figure 8.10, and iscomparable to that of natural muscles [6]. A similar result was also obtained in the castfilm of poly(o-methoxyaniline) for the thickness direction. The large expansion ratio forthe thickness direction is conjectured to relate to the condensation process of the castfilm. It may be remarked that the evaporation of NMP solution results in shrinkage onlyin the thickness direction, but not in the area. Therefore, the cast film has more freedomto expand in the thickness direction than that parallel to the film surface.

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8.8 Contraction Under Strain in Stretched Polyaniline Films

The contraction ratio of polyaniline film as a function of strain obtained in the stretchedpolyaniline film is shown in Figure 8.11 [25]. The result indicates that the contraction forceis of the order of 1-2 MPa, which is about 10 times larger than that of natural muscles. Undersmaller strains, the contraction ratio for the perpendicular direction is slightly larger thanthat observed in the stretched direction. At larger strains, however, the contraction ratiodecreases more rapidly in the stretched direction than in the perpendicular direction, indicatingthat the mechanical strength is weaker in the direction perpendicular to polymer chains.

8.9 Electrolytic Expansion in Other Conducting Polymers

Apart from polyaniline, other conducting polymers that are being studied for electrolyticexpansion include polypyrrole [11, 15-17], poly(alkylthiophene) [26] and carbonnanotubes [5]. For example, electrochemically prepared polypyrrole films were used tostudy the qualitative movement of electrolytic expansion by fabricating a bimorphactuator. The movement of bending and stretching of the actuator was demonstrated inelectrolyte solution [15]. Actuators fabricated by electrodeposition on gold-coatedpolyethylene films were studied [11] for the evaluation of expansion ratio and responsetime. Also, a microactuator of several tens of microns made from two layers of gold and

Figure 8.10 CV curve (upper) and electrolytic expansion (lower) ratio for the thicknessdirection in a polyaniline cast film


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polypyrrole [16] has been demonstrated. Using polypyrrole films [17], the expansionratio and the force were evaluated by an Instron pulling test machine. The expansionbehaviours of bimorph actuators using poly(alkylthiophene) solids and gels [26] havealso been studied. In carbon nanotube actuators [5], backbone types [24] were fabricatedfor demonstration of the bending motion. In these actuators even electrolyte solutionswere used; the origin of actuation was explained by the mechanism of non faradic chargingand discharging on the enormous surface area of the carbon nanotubes.

8.10 Applications of Electrolytic Expansion

Actuators fabricated by conducting polymers are soft, flexible, and lightweight, with lowvoltage drive and strong contraction force. Tweezers, microvalves, and directors of opticalfibre [27] are some of the technological applications. Two types of bimorph actuators havebeen proposed [24]. One is the backbone type, which consists of two conducting polymerfilms stuck together on a double-sided adhesive tape, which can also be replaced by a solidpolymer electrolyte. The other is the shell type, in which conducting polymer films stuckon adhesive tapes are sandwiched onto a sheet of electrolyte media. The shell-type actuatoris self-standing and works in air. There are some advantages in the use of these bimorphstructures. One of these relates to the oxidation and the reduction of the polymer filmprocess resulting in the bending force doubling. Since dopants transfer from one film to the

Figure 8.11 The typical contraction ratio of polyaniline film against the strain for thestretched direction (//) and the perpendicular direction (⊥)

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other film through the electrolyte media, the electrolyte can be minimised. The electrolyticactuator can also be used both for positioning and rechargeable battery.

8.11 Conclusions

The fundamentals of electrolytic expansion in polyaniline films have been discussed. Ioninsertion and exclusion by electrolytic oxidation and reduction are the primary mechanisms.However, it is also evident that the changes in molecular conformations, arising due to thedelocalisation of π-electrons and the electrostatic repulsion between the polycations, areother mechanisms operating in a conducting polymer microactuator. By investigating themolecular structure and the higher order structure to optimise the electrolytic expansion, itshould be possible to improve the expansion ratio and the force for practical usage.

References

1. R.H. Baughman, Makromolekulare Chemie, Macromolecular Symposium, 1991,4, 277.

2. K. Oguro, Y. Kawami and H. Takenaka, Journal of Micromachine Society, 1992,5, 1, 27.

3. K. Kaneto, M. Kaneko, Y. Min and A.G. MacDiarmid, Synthetic Metals, 1995,71, 1-3, 2211.

4. Y. Osada, H. Okuzaki and H. Hori, Nature, 1992, 6357, 242.

5. R.H. Baughman, C. Cui, A.A. Zakhidov, Z. Iqubal, J.N. Barisci, G.M. Spinks,G.G. Wallace, A. Mazzoldi, D.D. Rossi, A.G. Rinzler, O. Jaschinski, S. Roth andM. Kertesz, Science, 1999, 284, 1340.

6. R.M. Alexander, Nikkei Science, 1992, 13 (in Japanese).

7. Y. Hirshberg, Journal of the American Chemical Society, 1956, 78, 2304.

8. C.K. Chiang, M.A. Drug, S.C. Gau, A.J. Heeger, E.J. Louis, A.G. MacDiarmid, Y.W.Park and H. Shirakawa, Journal of the American Chemical Society, 1978, 100, 1013.

9. W.-S. Huang, B.D. Humphrey and A.G. MacDiarmid, Journal of the ChemicalSociety, Faraday Transactions, 1986, 82, 2385.

10. L.W. Shacklette, J.F. Wolf, S. Gould and R.H. Baughman, Journal of ChemicalPhysics, 1988, 88, 6, 3955.


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11. Q. Pei and O. Inganäs, Synthetic Metals, 1993, 55-57, 1, 3718.

12. T.H. Herod and J.B. Schlenoff, Chemisty of Materials, 1993, 5, 7, 951.

13. K. Kaneto, M. Kaneko and W. Takashima, Japanese Journal of Applied Physics,1995, 34, 7A, L837.

14. W. Takashima, M. Fukui, M. Kaneko and K. Kaneto, Japanese Journal ofApplied Physics, 1995, 34, 7B, 3786.

15. T.F. Otero, J. Rodriguez, E. Angulo and C. Santamaria, Synthetic Metals, 1993,55-57, 1, 3713.

16. E. Smela, O. Inganäs and I. Lundström, Science, 1995, 268, 1735.

17. M.R. Gandhi, P. Murray, G.M. Sprinks and G.G. Wallace, Synthetic Metals,1995, 73, 247.

18. K. Kaneto, K. Kudo, Y. Ohmori, M. Onoda and M. Iwamoto, IEICETransactions, 1998, E81-C, 7, 1009.

19. K. Kaneto, S. Hayashi, S. Ura and K. Yoshino, Journal of the Physical Society ofJapan, 1985, 54, 3, 1146.

20. M. Kaneko and K. Kaneto, Synthetic Metals, 1999, 102, 1350.

21. P. Murray, G.M. Spinks, G.G. Wallace and R.P. Burford, Synthetic Metals, 1997,84, 1-3, 847.

22. H. Daifuku, T. Kawagoe, N. Yamamoto, T. Ohsaka and N. Oyama, Journal ofElectroanalytical Chemisty, 1989, 274, 313.

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27. R.H. Baughman, L.W. Shacklette, R.L. Elsenbaumer, E.J. Plichta and C. Bechtinin Molecular Electronics, Ed., P.I. Lazarev, Kluwer Academic Publishers,The Netherlands, 1991, 267.

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9 Membranes for Electronics

I. Karube and A. Hiratsuka

9.1 Introduction

In materials science, thin films are designed to have a desired molecular order. The filmsshould also have several different material properties in a restricted geometry. Organisedfilms are now being designed to perform new and special functions. In the past, organicfilms were considered to be too fragile with insufficient purity to give reliable and consistentproperties to make thin films for practical use.

Extensive scientific studies have generated a wide variety of useful knowledge for thedevelopment of new thin film materials that exhibit specifically desired behaviour. Anumber of books, reviews, and general articles have been published about organic thinfilms, such as Langmuir-Blodgett (LB) films, self-assembled films [1, 2], conducting films[3, 4] and imprinted polymers. Other related topics include polymer surfaces and interfaces[5]. However, this trend is changing with the discovery of new materials. Recently, manynew compounds and polymers have been synthesised and made into thin films by avariety of techniques. These organic films are carefully constructed to avoid the commonproblems. Although the preparation and study of LB films are still very active areas ofresearch, polymeric films fabricated by either oxidative deposition or polymerised in situare rapidly becoming more popular.

Many intensive scientific investigations are focused on the preparation and characterisationof new polymeric organic films. Many newer surface science techniques, designed anddeveloped for semiconductors and dielectric materials, address specific details about thestructure and morphology of these organic films.

Fundamental studies are also in progress for the characterisation of the optical,spectroscopic, and electrical properties, including energy transfer between molecules inthe film and between layers. Clearly, knowledge of the morphology of the substrate isneeded. Significantly, cooperative effects can change the behaviour of any film. The extentof interaction, both laterally and vertically between the layers and substrates, couldinfluence behaviour. With this base of knowledge, it is hoped that this leads to ourunderstanding of intermolecular interactions, energy transfer, and dynamic behaviourfor best optimising a film for a specific research study or specific application.

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Another field of scientific research is ‘interface materials’. These materials interface betweentwo layers of different materials. This is often studied between two different polymeric layers.In this regard, surface pre- and post-treatment creating specific interactions are active areasof research. This leads to wetting phenomena, adsorption, adhesion, and lubrication. Thenature of the interfacial organic thin films contributes to the orientation of the molecules andtheir packing. It also affects molecular motion, including lateral diffusion, gas or ionic diffusionthrough the film, phase transitions, melting and other processes.

Polymeric materials for the purpose of biomedical applications are presently of growingconcern. One can find applications in such diverse fields as tissue engineering, implants,therapeutic devices and diagnostic assays. Polymers can be prepared with severalcompositions and modifications while their physical properties including morphology canbe regulated by variation in their composition and modifications. Polymer surface-basedchemistries for biosensors are gaining importance and will continue to see growth in demandin the near future. Polymers are used to enhance the speed, sensitivity and versatility ofbiosensors and in medical diagnostics to measure vital analytes. Today’s diagnostic medicineis placing demands on technology for new materials and specific applications. Polymersare thus finding ever-increasing use as diagnostic medical reagents [6].

One example of the application of polymers in coatings is use as a binder, which is necessaryto integrate the system chemistry. The reagent matrix must be carefully selected to mitigateor eliminate non uniformity in a reagent’s concentration due to improper mixing, settingor non uniform coating thickness. Therefore, aqueous-based emulsion polymers and water-soluble polymers are being extensively used. Polymers must be carefully screened and selectedto avoid interference with the chemistry. The properties of polymers, e.g., solubility, viscosity,solid content, surfactants, residual initiators, film forming temperature and particle sizeshould be carefully considered. In general, the polymer should have good adhesion to thesupport substrate and should show little or no change during handling or manufacture offilms. The coated matrix must have the desired pore size and pore distribution to allowpenetration of the analyte being measured as well as having the desired gloss, swellingcharacteristics and surface energetics. Depending on the system, swelling of the polymerbinder due to the absorption of the liquid sample may or may not be advantageous. Emulsionpolymers have a distinct advantage over soluble polymers due to their high molecularweight, superior mechanical properties and potential for adsorbing enzymes properties ina restricted geometry. Polymeric binders used in multilayered coatings include variousemulsion polymers, gelatine, polyacrylamide, agarose, polyvinylpyrrolidone, polyvinylalcohol, copolymers of vinylpyrrolidone and acrylamine, and hydrophilic cellulosederivatives such as hydroxyethylcellulose and methyl cellulose.

Biosensors have been widely researched and developed as a tool for medical andenvironmental monitoring. They are designed to produce a digital electronic signal thatis proportional to the concentration of a specific chemical or a set of chemicals.

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Membranes for Electronics

The biological recognition element is generally chosen from the enzymes, antibodies, receptors,tissues and microorganisms due to their excellent selectivity for target substances. Thetransducers could be electrodes, photon counters, thermisters, quartz crystal microbalancesor semiconducting devices. Ion-selective, field-effect transistors (ISFET) and surface plasmonresonance (SPR) techniques have also been utilised. An essential technique in developingbiosensors includes immobilisation of the biological components to the surface of thetransducers. The performance of the biosensors is governed by the techniques used incombining these two components. The recent trend in biosensors is towards miniaturisationwith semiconductor microfabrication or micromachining techniques [7-9]. The immobilisationmatrix (interfacial design) should feature a thin film in order to maintain the desired sensorcharacteristics such as response time, sensitivity, reproducibility and reusability.

There has been an increasing utilisation of organic thin films in many new electronic, opticaland mechanical devices. Organic photoconductors are used in copiers and printers. The firstorganic photoconductors were charge transfer polymers that can generate and transportcharges. In later versions, these functions were separated into different polymer layers, eachof which could be handled easily. Polymers as liquid crystal displays are now ubiquitous inwatches and flat panel displays. Not only have the liquid crystals been improved, but also thesurface treatment and manufacture processes have been significantly advanced. This has ledto the surface modification of organic materials for specific applications.

Photoresists and electron-beam resists are the key to the success of VLSI electronic circuits.Without these resists, most electronic equipment would not exist. These polymers are spunonto the semiconductor and exposed to the circuit pattern leading to main chain scissionor crosslinking. Subsequently, unpolymerised sections are removed. This process is employedeither in wet or in dry conditions. This is known as the photolithographic process, whichis part of the semiconductor fabrication technology. Further treatment includes diffusionof various semiconductor elements and metallisation for conduction lines. Layer by layer,the total package is developed. Current research is now directed toward finer features inthe patterns and changes in the surface characteristics for subsequent layers.

High temperature polymers, such as polyimide and other related polymers, are also usedas insulating layers or for packaging. Further, sputtered or thermally oxidised SiO2 layerswere used. This process required a large sputtering chamber and the films obtained werenot uniform resulting in the development of cracks. However, polyimides did not causeproblems. The adhesion of metal layers to polyimide was at times not so good. Both ofthese problems now seem to be under control.

In the case of magnetic disks, storage density is demanded. Efforts to reduce the bit sizeby manoeuvring the read-write head closer to the disk surface tends to generate newproblems such as stick or friction. To overcome these problems, lubricants were added

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to the disk surfaces. The most popular have been fluorinated polymeric ethers, usuallyon top of an amorphous carbon coating, allowing the read-write head to be much closerto the magnetic storage disk surface. A number of groups are studying friction, lubrication,and adhesion from a fundamental point of view [10-12].

Sanyo introduced electrolytic capacitors in 1983 based on tetracyanoquinodimethane(TCNQ). Since that times several other capacitors have been introduced into themarketplace. Polypyrrole, polyaniline and polythiophene have all been used. Thesecapacitors range in values from 0.1 μF to 200 μF and have low equivalent series resistanceand high frequency impedance with good reliability and lifetimes.

In 1967, Updike and co-workers investigated a promising approach to glucose monitoringin the form of an enzyme electrode [13]. Glucose was oxidised to glucono-lactone by theenzyme of glucose oxidase (GOD); the enzyme was incorporated and immobilised in thepolymer gel matrices. This application of polymeric gel materials is growing very fast.Polymers have found applications in such diverse biomedical fields as tissue engineering,implantation of medical devices and artificial organs, prostheses and many other medicalfields. Various approaches are used for diagnostic applications of these biomaterials.

In the first approach, enzymes are used. As for GOD, oxygen consumption is measured.An enzyme-immobilised electrode is not essential in this system but an oxygen electrodeis required for measuring changes in the dissolved oxygen concentration.

The second approach uses an enzyme-immobilised electrode and the commonly usedimmobilising matrices are polymers. The enzyme is immobilised close to the electrode,and so only a small amount of reaction product is needed. Thus response time andsensitivity are improved. This system is polarised at a suitable redox potential.

In the third approach, the polymers are functionalised not only as immobilised matricesbut also as electron mediators. In this approach, the nature of the enzyme reaction isexploited. For example, in the case of GOD, the enzyme is reduced (or oxidised) by D-glucose (substrate) and then this reduced (oxidised) enzyme is oxidised (reduced) by anacceptor. The mediator could also be a redox polymer. Direct electron transfer betweenGOD and the electrode can also occur.

In the past decade, much work has been done on the exploration and development ofredox polymers that can rapidly and efficiently shuttle electrons. Several research groupshave wired the enzyme to the electrode with a long chain polymer having a dense array ofelectron relays. The polymer penetrates and binds the enzyme to the electrode. Gregg andHeller have done extensive work on osmium-containing polymers. They have made a largenumber of such polymers and evaluated their electrochemical characteristics [14]. Theirmost stable reproducible redox polymer was poly(4-vinyl pyridine) to which Os(bpy)3Cl2

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(tris-(2,2´-bipyridyl) osmium chloride) had been attached to the pendant pyridine group.The resultant redox polymer was water insoluble. To make it water soluble and biologicallycompatible, Heller and co-workers partially quaternised the remaining pyridine pendantswith 2-bromoethylamine. This polymer is water soluble and the newly introduced aminegroups can react with a water-soluble epoxy, e.g., polyethylene glycol diglycidyl ether, andGOD to produce a crosslinked biosensor coating film. Such coating films produce highcurrent densities and a linear response to glucose up to 600 mg/dL.

In contrast, Boguslavsky and co-workers used a flexible polymer chain to put electronrelays [15]. Their polymers provide communication between redox centres in glucose oxidase(GOD) and the electrode. No mediation occurred when ferrocene was attached to a nonsilicone backbone. Their ferrocene-modified siloxane polymers are stable and non diffusing.Therefore, biosensors based on these redox polymers give good response and stability.

In order to transfer electrons directly between the electrodes and enzymes, an electron relaythat transfers redox equivalents (electrons) from the active site of GOD’s cofactor to theelectrode surface is needed. The choice for an artificial electron relay depends on a molecule’sability to reach the reduced flavin adenine dinucleotide, FADH2 (in close proximity to theGOD active site), undergo fast electron transfer, and then transport electrons to the electrodesas rapidly as possible. Surridge and co-workers have carried out electron-transport rate studieson an enzyme electrode for glucose using interdigited array electrodes [16].

For integrating biosensors with electronics, the enzyme electrode should be fabricated ina mass fabrication oriented technique and should be capable of being miniaturised.Integration of enzymes and mediators simultaneously should improve the electron transferpathway from the active site of the enzyme to the electrode. Conductive polymers suchas polypyrrole, polyaniline and polythiophene are formed at the anode by electrochemicalpolymerisation [17]. For integration of bioselective compounds and/or redox polymersinto conductive polymers, functionalisation of conductive polymer films is essential. Thereis a pressing need for an implantable glucose sensor for optimal control of blood glucoseconcentration in diabetics. A biosensor providing continuous readings of blood glucosewould be most useful at the onset of hyper- or hypoglycaemia, enabling a patient to takecorrective measures. Furthermore, incorporating such a biosensor into a closed-loopsystem with a microprocessor and an insulin infusion pump could provide automaticregulation of the patient’s blood glucose. Johnson and co-workers used two noveltechnologies in the fabrication of a miniature sensor for implantation in the subcutaneoustissues of humans with diabetes [18]. They developed an electrodeposition technique toelectrically attract GOD and albumin onto the surface of the working electrode. Theresultant enzyme/albumin layer was crosslinked by butraldehyde. They also developeda biocompatible polyethylene glycol/polyurethane copolymer to serve as the outermembrane of the sensor to provide differential permeability of oxygen relative to glucoseto avoid the oxygen deficit encountered in physiological tissues.


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Recently, commercial electrochemical microbiosensors, e.g., Exactach (Medisense), andsilicon-base arrayed systems (I-Stat) have appeared in the market. These new technologieswill certainly have an impact on rapid chemical analysis by the turn of this century. Atypically important example is that of blood glucose determination on very small bloodvolumes (<5 μL) obtained by a finger pricker. This is possible as detection instruments canbe designed compactly. Certainly, there is a market for small, disposable electrochemicaltests in the emergency room, surgical and critical care units as well as in homes.

Fabrication of polymers for these small and integrated sensors should be by the newprocessing technologies, which can produce accurate, mass reproducible and thinpolymers. The polymers fabricated by conventional methods may have potentialproblems such as the difficulty of preparing thin (<1 μm) and homogeneous films. Aplasma-polymerised film offers a new alternative [19]. The plasma-polymerised film isachieved in a glow discharge or plasma in the vapour phase. Such films are thin (< 1μm), pinhole-free, flat-surface structures and are chemically and mechanically stable.

In the following sections, the characterisation techniques, properties and applicationsfor both electronical and biological aspects of plasma-polymerised films are described.

9.2 Plasma Polymerisation

The polymers employed for integration between biomaterials and electronics should bemass-producible, very thin, and stable. In general, organic thin films have received agreat deal of interest due to their extensive applications in the fields of mechanics,electronics and optics [20, 21]. Applications also include chemical, physical and biologicalsensors, microelectronic devices, non linear optical (NLO) and molecular devices [22].In spite of a large number of studies in this area, only a few of these materials have beensuccessfully used, even for electronic and optic-based applications. This is mainly becauseorganic films often show poor thermal and chemical stability and poor mechanicaltoughness [23]. Therefore, it is of interest to develop polymer thin films of high qualityfor a variety of industrial applications.

Ultrathin polymer films can be prepared using two kinds of technology. The first includeswet processes like LB, spreading, dipping or solvent casting methods. The other is dryprocessing, such as physical vapour deposition (PVD) and chemical vapour deposition(CVD). Of these methods, the CVD methods, such as plasma polymerisation, arefrequently used to make polymer thin films [24-26].

Comparing these two technologies, dry processing is more advantageous, mainly becauseits technology originated from semiconductor or VLSI processes. VLSI and related

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technologies are very compatible with mass-production miniaturisation and integrationprocesses. Among all the VLSI technologies, plasma polymerisation is a process ofpreparing thin polymer films. For the time being, there have only been a few examples ofplasma polymerisation compared with other ordinary polymer processing techniquessuch as chemical or electrochemical synthesis. This is partly because of the difficulty ofcontrolling the polymeric reactions. In spite of the difficulty, extensive work has beendone on characteristic studies and improvement of instrumental and processingtechnologies. Hence, applications to electronic fields have been carried out and severalpractical examples for surface coatings and corrosion protections can be found. Plasma-polymerised films for chemical and biochemical applications are still rare, but the numberof reports related to this field is currently on the increase.

9.2.1 History

Organic chemical reactions induced by discharge have been used and studied since the 1800s[27, 28]. In 1874, Thenard [29] and Wilde [30] reported that solid materials were producedon the reaction wall after discharging in hydrocarbon vapours. In 1931, Brewer and co-workers [31] reported that insoluble yellow coloured materials were produced when applyingthe glow discharge to methane at room temperature. Stewart [32] produced insulating filmfrom hydrocarbon gas under a high-pressure condition of 10-5 Torr. However, most of thesematerials were not intended to control the character or the structures of the reaction products.The initial study on the nature of well-defined adhesion and uniformity structures for metalcoating films was reported by Goodman [33]. Following this work, plasma polymerisationhas been utilised for the processing of pinhole-free and sub-micron films [34].

9.2.2 General Characteristics

Plasma polymers are deposited as a thin film and/or as a powder on surfaces contactinga glow discharge of organic or organometallic feed gases. Plasma polymerisation is aspecific type of plasma chemistry, which involves reactions between plasma species,between plasma on the surface [35].

Among the several possible mechanisms which have been expressed, a free-radical reactioncould be a dominant process for plasma depositions [36, 37]. Hence, two types of reaction,i.e., plasma-induced polymerisation and plasma-state polymerisation are presumed. Theplasma-induced polymerisation is the conventional free-radical induced polymerisation ofmolecules containing unsaturated carbon-carbon bonds. The plasma-state polymerisationdepends on the presence of ions, electrons and other species which are energetic enough tobreak any bond. The resulting decomposition products of the plasma recombine by a free-


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radical termination reaction. This allows the polymerisation of unconventional startingmaterials, such as saturated alkanes or benzene [38]. The most notable aspect of plasmapolymerisation is that the thin polymer film can be prepared from almost any kind oforganic vapour. Unlike conventional organic polymers, plasma polymers do not consist ofchains with a regular repeat unit, but tend to form an irregular three-dimensional crosslinkednetwork. The chemical structure and physical properties may be quite different from theconventional polymer derived from the same starting materials. Plasma-polymerised filmsare in general chemically inert, insoluble, mechanically tough, and thermally stable. Sothese films have been used in a variety of applications such as permeability selectivemembranes, protective coatings, and electrical, optical and biomedical films. There areseveral advantages of plasma-polymerised films over conventional polymers:

• The starting feed gases used do not have to contain the type of functional groupsnormally associated with conventional polymerisation.

• Films are often highly coherent and adherent to a variety of substrates, includingconventional polymers, glasses, and metals.

• Polymerisation may be employed without the use of solvents.

• Plasma-polymerised films can be easily produced with thickness of 10 nm to 1 μm.

• Ultrathin, pinhole-free films may be prepared.

• With careful control of the polymerisation parameters, it is possible to fabricate achemically functionalised surface with a desired thickness.

9.2.3 Synthesis of Plasma Polymers

The physical and chemical properties and the deposition rate for plasma-polymerisedfilms depend on the following factors:

• Reactor type,

• Feed gas composition,

• Frequency and power of the excitation signal,

• Flow rate of feed gases,

• Plasma temperature, and

• Substrate position.

Several review articles have already discussed the effect of such properties. The plasmapolymerisation reactions are especially influenced by the monomer gases and the power supplied.

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9.2.3.1 Reactors

Many authors [39-42] have provided detailed studies of experimental configurations.The most widely used reactor configurations for plasma polymerisation can be dividedinto the following types (see Figure 9.1):

Figure 9.1 Reactors for plasma polymerisation

Internal electrode reactor with dcpower supply

Internal electrode reactor with RFpower supply

External electrode reactor withRF power supply

Electrodeless microwave(2.45 GHz) reactor


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• Internal electrode reactors with direct current (dc) power supply

Internal electrode reactors have several names, e.g., flat bed, parallel plate, planar,diode, etc. However, this system is rarely used to develop the plasma polymerisationbecause it breaks the insulation between the electrodes due to polymerisation on thecathode by the dc glow discharge.

• Internal electrode reactors with radio frequency (RF) power supply

The RF power supply is coupled to the system by means of a blocking capacitor(capacitive coupling). An applied electrode potential oscillates around a cathode self-bias potential, which is prone to being negative. The working conditions and apparatusgeometry can significantly influence the extent of ion bombardment on the substrate,the electron energy distribution and the production of active species [43]. Metalplate electrodes are aligned parallel with the reactor. Either an alternating current(ac) (1-50 kHz) or RF field is used. The vacuum chambers can either be made of glassor conductive materials, such as metals. In the case of bell jar reactors, no particularcare is essential about the grounded electrode, but the design and arrangement of thecathode requires special attention. On the other hand, the metallic shield reactorsurrounding the electrode improves the glow confinement inside the inter-electrodespace. However, metallic reactor material can be sputtered, contaminating the target.

• External electrode reactors with RF power supply

External electrode reactors can be used either capacitively or inductively coupled.Many experimental arrangements have been reported. Each differs in power supply,reactor geometry, and sample position. The working frequency of the most commonlyused power supplies is 13.56 MHz. Power is transmitted from the power supply tothe monomer material by a capacitor and a coil. The tube geometry is variable [44-46] and the position of the sample stage may vary from upstream to downstream ofthe monomer gas flow in order to obtain different polymer composition and properties.Insulating tubular reactors may be made of glass, quartz, ceramics or alumina asreactor materials. Usually inductively coupled tubular reactors cannot be uniformlycoupled to the power supply while operating at low pressure (<< 1 Torr). However,coupling uniformity increases with increasing working pressure [47].

• Electrode-less microwave or high frequency reactors

The name of these reactors implies that no impurities can be sputtered off and incorporatedinto the growing films. A microwave (MW)-powered system is characterised by tubularquartz or Pyrex reactors and by a resonant cavity coupled with a power supply typicallyin the resonant cavity. The polymer is generally collected outside the glow region [48].

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The differences between the MW discharge and the RF discharge have been discussed[49]. The paper said that the large differences between the two systems could be theproduction of different active species and the different roles of ions and electrons.

9.2.3.2 Gas Parameters

The main parameters of the gas are flow rate and pressure.

The flow rate is controlled by either a mass flow or a needle valve, which connectsbetween a gas reservoir and the reaction chamber.

The deposition rate is limited by the supply of the feed gas. At a high flow rate, thedeposition rate decreases and the activated species may be prevented from reaching thesubstrate. D’Agostino and co-workers showed that unsaturation of the feed gases alwayscauses an increase of the maximum deposition rate by one order of magnitude and ashift to higher flow rate [50, 51].

The effect of the direction of flow on deposition rate distribution has received someattention. The effect of the position of the monomer inlet in a capacitively coupled bell-jar reactor has been studied [41, 52]. In the latter work, the position of the inlet wasfound to affect the efficacy of the resultant plasma polymer as a reverse osmosis membrane.

The effects of the pressure on the plasma polymerisation process include:

• The effect on residence time: the residence time is directly proportional to the pressure.

• The effect on average electron energy: for RF plasma the average electron energy isproportional to E/pg, where E is the electric field activating the plasma and pg is thepressure in the plasma. The chemical affects associated with low power plasma areobserved at high pressures.

• The effect on mean free path: the mean free path of a molecule, λ, in a gas isexpressed as:

λ = (π r2 N) / 4 (9.1)

where r is the radius of the molecule and N is the gas density [53]. The pressure canaffect the mean pathway by affecting the gas density.

The polymer powders are formed between 200 mTorr to 2 Torr under relatively low flowrate conditions. The exclusion of powder formation will occur at above 2 Torr in whichlow mean free path, long residence time and relatively high electron energy are required.


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However, inhomogenity in deposition rate is caused by an increase in pressure. Thusmost plasma polymerisation is carried out at pressures below 1 Torr, in order to obtainthe increased interaction of the plasma with a polymer surface to obtain homogeneousfilms [54, 55].

9.2.3.3 Power Parameter

Increase of power will result in an increased density of energetic electrons and morefrequent ion bombardment on the electrode. At constant pressure and flow rate, thedeposition rate of the film increases with power at first and then becomes independent athigher values of power [43].

For a dc or ac glow discharge, an increase of power is obtained by both an increase inpotential drop between electrodes and an increase in current density on the electrodes.Both effects result in an increased density of energetic electrons and increase bombardmentof the electrode by energetic ions.

For an RF discharge, the power has been reported to increase with an increase in current,which results in an increase in the energetic electrons.

9.3 Characterisation of Plasma Polymers

Plasma polymer films are generally considered to be amorphous. The chemicalcomposition and the nature of the internal bonding of plasma polymers are usuallydifferent from conventional polymers synthesised from the same monomer units. Plasmapolymers may have a high degree of crosslinking and may contain unsaturated bonds.The relationship between the deposition condition and the resulting structure of thecompositions has been investigated and reviewed [56, 57]. In most plasma polymerisationexperiments, the quantities of solid plasma films are very small, typically milligrams,and the polymers are generally insoluble in organic solvents due to their high degree ofcrosslinking. These factors are essential for the characterisation of plasma polymers.Therefore, sophisticated tools should be used instead of the analytical methods generallyused for conventional polymers. The structures of plasma polymer films have been studiedby numerous modern analytical techniques. These include IR and Fourier Transform IR(FTIR) spectroscopy, UV absorption spectroscopy, electroluminescent spectroscopy, Augerelectron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) or electronspectroscopy for chemical analysis (ESCA), secondary ion mass spectroscopy, ionscattering spectroscopy, solid state NMR spectroscopy, chromatography, combinedpyrolysis/gas chromatography/mass spectroscopy (P/GC/MS), atomic absorption

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spectroscopy, neutron activation analysis, nuclear elastic recoil detection, elemental analysis,electron spin resonance (ESR) spectroscopy, X-ray diffraction, reflection high energy electrondiffraction, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA),transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomicforce microscopy (AFM). Among these analytical tools, IR and FTIR spectroscopy andXPS are frequently used to investigate the chemical and physical structures of films [54].

9.3.1 IR Spectroscopy

IR spectroscopy can be used for the identification of functional groups in plasma polymerfilms. Although primarily a qualitative analytical tool, it has been used to quantitativelymeasure the concentrations of functional groups and the crosslinking density of plasmapolymer films [42-44]. IR spectra might be obtained by depositing the plasma polymer onIR transparent substances or on other surfaces by using attenuated total reflectance (ATR).

9.3.2 XPS

XPS (or ESCA) is a powerful tool for analysing the surface of plasma polymer films. Sinceplasma polymers are deposited as extremely thin films, XPS is ideally suited for thedetermination of the chemical properties of these films. XPS is a technique in which thenumber and energy of core level electrons expelled from atoms are analysed on their absorptionof X-rays. The penetration depth of the X-ray is about 5 nm and this yields the relativepopulation of elements at the surface. The electron energy observed for a given atom willalso be influenced by the electron withdrawing power of the nearest neighbour atom. Thus,XPS is also suitable for functional group identification. This method is most effective whenthe nearest neighbour atom is highly electron withdrawing. Consequently, fluorinated plasmapolymers are most widely studied by XPS [38]. XPS can also be used to obtain informationregarding variations in depth by means of angle-resolved spectra and ion milling.

9.4 Applications of Plasma Polymers

Both electronic and medical applications of plasma polymers have been reported [54-61]. Most of these investigations are on the interface between polymers and inorganicmaterials, for instance, metal/polymer interfaces in structural adhesive joints, and cationdiffusion along polymer/metal interfaces under an applied electric potential. In anotherreference, more specific aspects for electrical and electronic applications [59] were treated,wherein protective films for microcircuitry, and for wettability were explained. The useof such film for surface treatment has also been examined.


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9.4.1 Packaging

Due to their excellent electrical and thermal properties, polysiloxanes are some of themost prevailing materials used in electronic encapsulation applications. Elastic thin filmshave been developed using some proprietary silicon-containing compounds [60]. Mostof the conventional polysiloxanes are either gel-like or rubbery materials, and have limitedapplications in areas demanding high mechanical strength of the coating material inaddition to passivation properties. However, plasma-polymerised films have goodmechanical strength and resist cracking up to 25 microns thickness. These thin films areuseful for conformal coatings in hermetic packages and as dielectrics for integrated circuitsor multichip modules.

Another packaging material is plasma-polymerised hexamethyldisilazane, which has acharacter of moisture impermeability and which could be used for protection of thinfilm nichrome resistors [61]. Plasma polymerisation/vapour deposition (PP/VD) wasapplied to provide functional hermetic encapsulation of integrated circuits [62]. Theplasma-polymerised films combine the features of surface cleaning, surface treatment,and hermetic encapsulation into a series of in-situ vacuum processes, functional hermeticencapsulating polymer films with a total thickness of 5 μm have been produced,independent of the composition, dimensions or geometry of the substrates. Complementarymetal oxide semiconductor (CMOS) circuits fabricated by a plasma process immersed inRinger’s solution were tested for a year. The result showed unchanging device performanceparameters characteristic of hermetically encapsulated integrated circuits.

9.4.2 Insulator

Thin films of high thermal resistivity and electrical insulation were prepared by plasmapolymerisation of silazane and subsequent pyrolysis in air. The film had strong adhesion,high thermal resistivity, and very good insulating properties in a broad range oftemperatures. The author described how the chemical constitution of the films transformedinto a silicon oxide type and a polycrystalline structure after the pyrolysis process [63].

The hybrid films for the pinhole-free electrical insulators were prepared usinghexamethyldisiloxane (HMDSO) and silicon monoxide (SiO) [64]. The HMDSO hybridfilms were prepared on the substrates by evaporating SiO during HMDSO plasmapolymerisation in RF discharge. SiO was evaporated by heating in RF plasma consistingof HMDSO and oxygen at a pressure of 10-4 Torr.

The films for insulators can be fabricated not only by inorganic compounds as mentionedabove, but also by organic compounds. Plasma-polymerised films with uniform thin

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organic layers [65] can be prepared from almost any organic compounds. It has beenreported that a plasma polymer copolymerised from allylamine and ethylene could beused as an orientation layer for a liquid crystal [69]. The synthesised films had goodresistance to heat and chemicals. These plasma polymers have been considered aspassivation or insulation layers of ICs and other electronic devices.

Parylene C (poly(chloro-p-xylene) was used as an electrical insulator for the polymerisation[66]. A novel reactor was designed to deposit thin adhering and insulating films to commonsemiconductor materials. The reactor incorporates an electrode-less or inductively coupledpower source and a thermally activated sublimation and pyrolysis chamber for vapourdeposition of Parylene C. This design allows for the in-situ application of a thin adherentcoating of plasma-polymerised methane followed by a thicker coating of Parylene C aswell as serving as a ‘bio’ interface. Adhesion between the plasma polymer of methaneand Parylene C could be improved by incorporating an intermediate layer of plasmapolymerised para-xylyene film.

9.4.3 Semiconductive Films

A doped material was prepared by plasma polymerisation using 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) [67]. Electrical conductivity of the iodine-doped material decreased by five orders of magnitude and the iodine doping caused aserious structural change.

A structural and electrical survey for conductive phenomena was performed usingacetonitrile plasma polymerised film [68]. This polymer was discovered to have twostates of electrical conduction. Electrical conduction in the high conduction state wassensitive to the nature of electrode material. Electrical conduction in the low conductionstate was unaffected by the electrode material.

Organometallic compounds were used to fabricate semiconductive thin films on differentsubstrates by glow discharge polymerisation [69]. Tetramethyltin (TMT) and diethylzinc(DEZ) were deposited on several substrates such as polypropylene, SnO2, quartz andglass. The physicochemical properties of the deposited films were characterised by FTIR,XPS, SEM and X-ray diffraction.

Plasma-polymerised thiophene for passivating the surface defects on GaAs has beenemployed [70]. The paper showed the passivation of GaAs surface was made possible bysulfur present in an overlayer, provided by the thin film of plasma-polymerised thiophene.The deposition of polythiophene lowered the barrier height, reduced the surfacerecombination velocity and increased diffusion length.


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9.4.4 Conductive Films

A conducting plasma polymer was processed using TCNQ with quinoline [71]. Theresults showed that the plasma-polymerised quinoline and TCNQ had conductivities of10-12 Ω-1 cm-1 and 10-9 Ω-1 cm-1, respectively. However, a high conductivity of 10-5 Ω-1 cm-1

was measured for the plasma polymers mixed with TCNQ and quinoline.

p-Xylene polymer was fabricated by plasma polymerisation and investigated for high-field conduction and photoconduction [72]. The electronic properties of the plasmapolymer were similar to those of semicrystalline poly-p-xylene, influenced by the short-range molecular order, while the charge transport influenced by the long-range order didnot occur in these systems.

An organic conductive plasma polymer was produced from fumaronitrile and p-aminobenzonitrile in a heated system [73]. The films were very smooth and pinhole-free,with electrical conductivity ranging from 10-6 to 10-7 Ω-1 cm-1.

9.4.5 Resist Films

Lithography is indispensable for the fabrication of integrated circuits. Dry resist coatingis a key technology to replace a conventional wet lithography. There have been severalpapers and reports about plasma-polymerised resists [76-78]. PMMA has been used as aplasma-polymerised resist [74, 75]. Copolymerised resists for high sensitivity have beenproduced [76-78], using metal-containing monomers blended in the resist polymer asresist sensitiser. Multilayered resist film was produced by forming alternating multiplelayers. The resist properties were improved by including the plasma etching resistivelayer in the multilayerd resist system [79].

9.4.6 Ultrathin Polymer Films

It is well known that thin polymer films with thickness about 100–1000 nm can beprepared by plasma polymerisation or electron beam and UV irradiation of organic vapour.However, modern electronic applications require films of 30 nm or even thinner. In 1992,a new system was developed [80] which could plasma polymerise a pinhole-free ultrathinpolymer film (2-10 nm) in a RF glow discharge. Ultrathin hydrocarbon films (C2H2 asfeed gas) were deposited on electronic grade n-type silicon wafers. A capacitively coupledRF discharge was initiated in a bos-type reactor in a flow of pure hydrocarbon vapour,or a hydrocarbon/argon mixture. The deposition conditions included a discharge frequencyof 13.56 MHz and a pressure range of 0.5-0.8 Torr.

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9.4.7 Chemical Sensors

Integration of sensors on silicon and/or glass is an emerging technology for fabricationof an intelligent sensing system. Low temperature plasma polymerisation processes areuseful for the integration and show various unique characteristics of the sensors.

ISFETs are used for pH measurement with a reference field-effect transistor. Sensingelements of the ISFETs were covered by the plasma polymerised styrene and werecharacterised [81]. An NO2 gas sensor was fabricated using plasma polymerisation ofcopper phthalocyanines. The materials processed were copper phthalocyanines without(CuPc) and with a chlorine substituent (CUPc-Cl), and with hydroxymethyl (CuPc-CH2OH) and phthalimidomethyl substituents [82]. It has also been reported that thetetramethyl plasma-polymerised films can be used for sensing propane gas [83].

Plasma-polymerised humidity sensors were proposed using styrene (PPS) [84]. The sensorwas constructed with a 12 nm Au - 164 nm PPS - 12 nm Au sandwich structure. Theresponse time of the capacitance was less than one minute.

9.4.8 Biosensors

The promising areas for applying plasma deposition technology to biomedical devicefabrication are sensor electrodes and the packaging of implantable integrated sensors.Insulation and packaging materials on implantable electronic devices must bebiocompatible. They must have good mechanical durability and good cohesive andadhesive properties, to prevent leakage from the implant surroundings.

9.4.8.1 Enzyme Support

Plasma-polymerised HMDSO film was used to produce a biocompatible surface and anenzyme support system [85]. The adsorption of urease onto a well-defined solid support,petroleum-based activated charcoal, has been achieved to provide the enzymatic hydrolysisof urea. The adsorption of urease, and the activity and stability of the enzyme on thesupport were studied and optimised, improving its availability for clinical applications.

9.4.8.2 Catalytic Biosensors

The most popular catalytic biosensor in clinical analysis is a glucose sensor, used in monitoringthe blood sugar of diabetes patients. The biological component often used GOD.


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GOD specifically catalyses the oxidation of glucose:

D-glucose + O2 � D-glucono-1,5-lacton + H2O2 (9.2)

An amperometric oxygen electrode or a noble metal electrode (such as platinum, silver and

gold) detects the biochemical reaction event. The electrode current increases with an increasein the concentration of glucose. This event is primarily an electron transfer process mediatedby a specific enzymatic reaction. In order to achieve the best sensor performance, thedistance between the electrode and GOD should be as short as possible, with rejection ofinterfering compounds. The plasma-polymerised film is an alternative material for theimmobilisation matrix that allows for such operation. There have been several typicalexamples shown in papers. A plasma-polymerised glucose microsensor using acrylic acid,methacrylic acid and 2-amino-benzotrifluoride was prepared [86]. The plasma-polymerised

films were deposited on the electrode, which was fabricated on a silicon substrate.Subsequently, oxygen and ammonia plasma treatment was carried out to introducecarboxyl and amino groups, respectively, which were coupled to GOD. A large amount

of GOD was immobilised onto the surface of the film. Due to the thickness of the film,the diffusional resistance for the analyte and reaction product is low and a response timeof about 4 seconds was obtained. The calibration curves of the sensor were linear from0.05 mmol/l glucose to 2 mmol/l glucose and the variance for a series of 10 injections of2 mmol/l glucose was found to be 2%. Another sensor using plasma-polymerised filmshas been reported [87]. This overcomes the poor adhesion on slide glass or silicon substrateswithout a chromium layer (often used between a glass slide and platinum to promoteadhesion but causes undesirable electrochemical side reactions with alloy formation).The calibration curves of the sensor were linear from 0.5 mmol/l glucose to 100 mmol/lglucose and a response time of about 2 seconds was obtained. Perfluoroallylphosphonicacid plasma-polymerised film was prepared as a charge rejection membrane [88]. Thefilm deposited on a working electrode showed the ability to reject a negative organicinterfering material such as ascorbate (vitamin C). This overcomes the problem with wireelectrodes, which were found to be difficult to homogeneously coat with Nafion films.Encapsulation of GOD by plasma-polymerised film has also been proposed [89, 90]. GODis encapsulated between a Millpore membrane filter and a propargyl alcohol plasma-

polymerised layer. The pores of the plasma-polymerised matrix are small enough to preventloss of GOD but large enough to allow the diffusion of glucose and electron. Thepolymerisation condition needed relatively low power discharge, and hence, thedenaturisation of GOD frequently caused by the high energy of plasma species wasminimised. The film also played a role as a barrier for interference from Cu2+. Nylonmembranes have been widely used in such a sensor assembly for conventional applications.These sensors, however, have slow response times because the thickness of the nylonmembranes was more than 50 μm whereas those of the layers of plasma-polymerisedfilms were less than 100 nm.

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9.4.8.3 Affinity Biosensors

Affinity biosensors take advantage of biomolecular interactions such as antigen-antibody,

receptor-ligand and nucleic acid base pair association, collectively referred as ‘biospecificinteraction’ in the biochemical and medical field. Transducers detecting the binding eventsare mostly quartz crystal microbalances (QCM) and surface plasmon resonance (SPR)devices. Affinity biosensors employing QCM are principally based on the measurementof the change in resonant frequency of a QCM according to mass changes on its surface.The QCM enables one to detect the mass change with a biospecific-binding event.Ethylenediamine plasma-polymerised films formed on gold electrodes covering the surfaceof quartz crystals are extremely thin and homogeneous. A QCM coated with such a filmshowed stable oscillation in phosphate buffer saline (PBS) compared with those coatedwith polyethylenimine (PEI) and (3-aminopropyl) trimethoxysilane (APTES). The noiselevels of the plasma-polymerised film, PEI, and APTES were 12, 54 and 50 Hz, respectively[91]. This indicated that the plasma-polymerised film on QCM was more homogeneous

than APTES and PEI. Moreover, there are a large number of amino groups, which enabledantibodies to be immobilised on the surface of the film. Therefore, sensors producedusing this method were more reproducible from sample to sample and exhibited lowernoise and higher sensitivity than sensors made using conventional APTES and PEIimmobilisation methods. Standard deviations (sample amount = 10) in the frequency dueto antigen-antibody bindings were investigated for three kinds of coating methods. Thevalues found for coatings made from plasma-polymerised film, APTES, and PEI were5%, 47%, and 38%, respectively. The time-course profile of the sensor responsedemonstrates a good reusability. The calibration plot of frequency change against

concentration of the antigen, human serum albumin (HSA), was linear for concentrationsranging from 10-2 to 10 mg/l with 15 minutes of incubation. They also reported on the‘red tide’ sensor [92]. Antibodies to the red tide-causing plankton, Alexandrium affine,were immobilised onto a piezoelectric device coated with the plasma-polymerised film.Since the sensor is often used in seawater, the plasma-polymerised film matrix played arole as a coating against the seawater. Affinity biosensors using SPR detection have beenwidely reported [93, 94]. BIAcore (Uppsala, Sweden) has developed BIAcore 2000 BiosensorAB into a widely used commercial instrument [95]. A surface plasmon wave is a nonpropagating evanescent wave formed at a metal-coated (mainly gold-coated) surface whenlight is directed toward the interface at a very specific reflection angle. It extends fromthe metal surface into the sample solution, decaying exponentially as a function of distance.Refractive index changes localised near the glass/gold interface resulting from the interactionbetween biomolecules perturb the evanescent wave and alter the propagationcharacteristics of the surface plasmon. Therefore, the resonance angle changes. Since theevanescent wave decays exponentially as a function of distance, the interaction ofbiomolecule interactions should be carried out close to the gold surface to obtain high

sensitivity. A typical penetration depth of the evanescent wave from the gold surface is


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several hundred nanometres. When antibodies are directly immobilised on the gold surface,it is often difficult to attach a large amount of antibodies with good adhesion. Therefore,antibodies are bound to be denatured and the sensor surface suffers from low surface coverageas well as non specific adsorption. Plasma-polymerised films might be suitable for the coatingmatrix between the antibodies and the gold. The sensor chip made with the film showed abetter sensor response (higher sensitivity) than a conventional design, e.g., carboxylateddextran designed by BIAcore AB. The reason is that the antibodies immobilised onto theplasma-polymerised films are two-dimensional and are present at a high density whereasantibodies immobilised onto carboxylated dextran are three-dimensional and are packed ata lower density. The calibration plot of change of resonance angle against concentration ofHSA was linear for concentrations ranging from 10-2 to 50 mg/l with 7 minutes of incubation[96]. Similarly, another type of sensor chip was also reported [97] using a plasma-polymerised

film and it was shown that it was effective for use in detection of the pesticide, etfenprox,which is a low molecular weight molecule. SPR detection for low weight molecules (molecular

weight < 3000) is difficult with conventional sensor chips.

9.4.8.4 DNA Sensor

In spite of the usefulness of plasma-polymerised film in biosensors, only a few cases havebeen reported on how such materials are used. Recently, a gene chip has been reported[98]. This chip is a high density, oligonucleotide probe array for easy detection oridentification of genetic information from a virus. Due to the demand for immobilisingthousands of different kinds of oligonucleotides in mm2 areas, the processes for fabricatingthese chips have employed semiconductor process technology. Interface materials betweenoligonucleotides and substrates need to be well characterised in terms of structuralproperties such as thickness, pinhole density, strong adhesion to a variety of materials,chemical resistivity and hydrophobicity. Miyachi and co-workers deposited a plasma-polymerised film of HMDSO as a support layer for deoxyribonucleic acid (DNA)oligonucleotide arrays [99]. Non specific DNA was avoided due to the thin, hydrophobicproperties of plasma-polymerised film. Hence, the background signal of the DNA arraywas lower than that of a poly-L-lysine-coated [100] using a conventional interface layer.DNA array detection systems are widely used. Plasma-polymerised films and/or the plasmaprocess as an interfacial design between substrates and DNA may thus help towards theincreased the use of both biosensors and DNA arrays.

Acknowledgements

The authors are grateful to Dr. Hitoshi Muguruma for many helpful discussions. We alsoacknowledge Dr. Scott J. McNiven and Dr. Kyong-Hoon Lee for their helpful suggestions.

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10 Conducting Polymer-Based Biosensors

A. Chaubey, M. Gerard and B.D. Malhotra

10.1 Introduction

Current research and development in bioanalytical problems aims at improved stability,selectivity and sensitivity of sensing devices [1-6]. With the advent of computerisationand increasing sophistication in many technological areas, dramatic changes are beingseen in the way scientific experiments are being controlled. Industrial applications ofbiochemical and morphological processes, in fields such as production of pharmaceuticals,food manufacturing, wastewater treatment and energy production, are on the increase.This has led to the development of biosensors. It is now widely accepted that sensingdevices such as biosensors are likely to revolutionise several areas of analyticalbiotechnology, such as healthcare, veterinary medicine, agricultural, petrochemical andpollution monitoring, by providing valuable real-time information [7-12]. It has beenobserved that about 80% of the non medical biosensor application market (Figure 10.1a)is covered by food industries. The rest of the market (20%) involves environmental,agriculture, defence, veterinary medicine and general industrial processing, etc. Themedical diagnostic biosensor application areas (Figure 10.1b) involve centralised testing(75%), decentralised testing (20%) and consumer testing (5%). Clark and Lyons werethe first to demonstrate that an enzyme could be integrated into an electrode to constructa biosensor [13]. Updike and Hicks described the first functional enzyme electrode basedon glucose oxidase deposited onto an oxygen electrode [14]. Technical developments ofbiosensors for medical care have demanded the greatest attention.

Research on biosensors has been motivated by a strong practical instinct with clearapplications in sight. The key attraction of biosensors for the applied sciences is thatthey offer the prospect of simplified, virtually non destructive analysis of turbid biologicalfluids. There is an increasing demand of biosensors for the determination of substancesin biological fluids such as blood, urine, serum, etc. [15, 16]. A common requirement ofall these is the need for on-site chemical information on dynamic or rapidly evolvingprocesses, preferably on a real-time basis. There is a distinct trend in clinical analysisfrom a centralised laboratory to a doctor’s clinic and to a patient’s self testing at home. Amedical practitioner requires correct information about various biological parameters,such as glucose, urea, haemoglobin, albumin, creatinine, amylase, lactate, etc. Manycompanies, including MediSense (USA), Boehringer (Germany) and Yellow Springs

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Figure 10.1 The biosensor market: (a) non medical, (b) medical

Instrumentation Company (USA), have marketed a hand-operated blood glucosebiosensor. The National Physical Laboratory (India) has also patented a device that canbe used for estimating glucose in blood.

10.1.1 Biosensors

A biosensor is a synergic combination of analytical biochemistry and microelectronics.Biosensors have recently been considered as a highly potential field of scientific research.This is because development of biosensors is necessary for the realisation of implantable,integrated and intelligent devices for biochemical information [17]. Biosensors areanalytical devices that respond selectively to analytes in a given sample and convert their

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Conducting Polymer-Based Biosensors

respective concentration into an electrical signal via a combination of a biologicalrecognition system and a physicochemical transducer. The schematic of a biosensor isshown in Figure 10.2.

10.1.2 Construction of Biosensors

In analytical devices, the biologically active molecules, such as enzymes, cells and antibodies,are used repeatedly. These are, therefore, fixed to the carrier materials. There are severaladvantages for immobilising the enzymes for application in analytical chemistry. Theseinclude stabilisation of enzymes and retention of enzyme activity for long periods of time.

Biosensors provide a powerful and inexpensive alternative to conventional analyticalstrategies for assaying chemical species in complex matrices. A biosensing deviceincorporates a biological molecular recognition component connected to a transducer.The main aim of a transducer is to produce a continuous or discrete electronic signal,which is directly proportional to the concentration of an analyte. In biosensors, thefollowing sequence of events take place:

• Specific recognition of the analyte,

• Transduction of the physicochemical effect caused by the interaction with the receptorinto an electrical signal, and

• Signal processing and amplification.

Figure 10.2 Schematic of a typical biosensing device comprising of immobilisedbiomolecule and a transducer

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Depending on the level of integration, biosensors can be subdivided into four generations(Figure 10.3).

In first-generation biosensors (Figure 10.3a), the biocatalyst is either bound to or entrappedbetween the membranes, which in turn are fixed on the surface of the transducer [13,14].The second-generation biosensors (Figure 10.3b) involve the adsorption or covalentfixation of the biologically active component to the transducer surface and permits theelimination of the semipermeable membrane [18, 19]. These were based either on thepotentiometric or amperometric methods of detection. The commercially available kitsfor estimation of blood glucose are based on amperometry. In the third-generationbiosensors (Figure 10.3c), conducting polymers, such as polyaniline, polypyrrole, etc.,have been used as the immobilisation matrix [20, 21]. These biosensors are found to becost effective, easily available and more stable. The fourth-generation biosensors (Figure10.3d) are based on direct incorporation of the biological elements into the matrices [22-23]. The most commonly used matrices are the conducting polymers and silicon. Thesebiosensors are expected to involve the application of the micro-electrode technology forin-vitro as well as in-vivo applications.

Figure 10.3 Different generations of biosensors (a) first-generation (b) second-generation (c) third-generation (d) fourth-generation

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10.1.3 Transducers

A transducer converts the biochemical signal to an electronic signal. The biochemicaltransducer or biocomponent gives the biosensor selectivity or specificity. The transducerof an electrical device responds in such a way that a signal can be electronically amplifiedand displayed. The physical transducers vary from electrochemical, spectroscopic, thermal,piezoelectric and surface acoustic wave technology [24, 25]. The most commonelectrochemical transducers being utilised are based on amperometric and potentiometrictechniques [26-28].

Amperometric biosensors measure the current produced during the oxidation or reductionof a product or reactant usually at a constant applied potential. Such sensors have fastresponse times and good sensitivity. However, the excellent specificity of the biologicalcomponent can be compromised by the partial selectivity of the electrode. This lack ofspecificity requires sample preparation and separation, or some compensation for interfacingsignals. Potentiometric biosensors relate electrical potentials to the concentration of analyteby using an ion-selective electrode or a gas-sensing electrode as the physical transducer[29-33]. These are selective, have large dynamic ranges and are non destructive.

10.1.4 Biological Component

Biocomponents are typically enzymes, tissues, bacteria, yeast, antibodies/antigens, liposomes,organelles, cell membrane components, etc. [23, 33-35]. Although the biomoleculeincorporated within a biosensor possesses an exquisite level of selectivity, it remains astructurally weak component of the system and is vulnerable to extreme conditions such aspH, temperature and ionic strength [36]. Most of the biological molecules have a very shortlifetime in solution phase and thus have to be fixed in a suitable matrix. The immobilisationof the biological component decreases its activity, but imparts stability to the biologicalcomponent against the environmental conditions [37, 38]. For the economic utilisation ofthe biocatalysts, their immobilisation in a suitable matrix is an important practice in biomedical,industrial and basic enzymology for repetitive and continuous processes. The reactionconditions and the methodology chosen for immobilisation are important in determining theactivity of biomolecules. The activity also depends on the surface area, porosity and hydrophiliccharacter of the immobilising matrix. Immobilisation of the biological component can bedone in a number of ways, depending on the type of the component. Techniques such asphysical adsorption, crosslinking, gel entrapment, covalent coupling and electrochemicalentrapment, etc., have been used for this purpose [39-42].

A number of matrices have been used for the immobilisation of enzymes on various matrices,such as polymeric films, membranes, gels, carbon and silica, etc. [43-46]. To overcome theproblems of slow electron transfer reactions of biological molecules at ordinary electrodes,


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various modified electrodes have been designed to facilitate direct coupling of biologicalredox reaction to the electrodes [40, 47, 48]. These include electrodes modified by depositionof polymer species and electrodes based on conducting salts or conducting polymers [49-51].

10.1.5 Importance of Conducting Polymers to Biosensors

Conducting polymers have recently evolved as an important research area with diversescientific problems of fundamental investigations and a potential for commercialexploitation [52-55]. This important development has implications in interdisciplinaryresearch. Conducting polymers contain π-electrons in the backbone and these areresponsible for their unusual electronic properties such as electrical conductivity, lowenergy optical transition, low ionisation potential and high electron affinity. The electricallyconducting polymers contain extended π-conjugated systems with single and double bondsalternating along the polymer chains. The higher values of the electrical conductivityobtained in such organic polymers have led to their being called ‘synthetic metals’. Theapplication of conducting polymers in the field of analytical chemistry and biosensingdevices has been reviewed by various researchers [43, 48, 56-58].

Conducting polymers have attracted much interest as a suitable matrix for the entrapmentof enzymes [59, 60]. The techniques for incorporating enzymes into electrodepositableconducting polymeric films permit the localisation of biologically active molecules onelectrodes of any size or geometry and are particularly appropriate for the fabrication ofmultianalyte amperometric microbiosensors [61]. Electrically conducting polymers haveconsiderable flexibility in the available chemical structure, which can be modified as required.By chemical modeling and synthesis, it is possible to modulate the required electronic andmechanical properties. Moreover, the polymer itself can be modified to bind proteinmolecules [62-64]. Another advantage offered by conducting polymers is that theelectrochemical synthesis allows the direct deposition of the polymer on the electrode surface,while simultaneously trapping the protein molecules [65, 66]. It is thus possible to controlthe spatial distribution of the immobilised enzymes and the film thickness, and to modulatethe enzyme activity by changing the state of the polymer. The development of any kind oftechnology in this field heavily depends on the understanding of the interaction at themolecular level with the biologically active protein, either as a simple composite or throughchemical grafting. For efficient relay of electrons from the surface of an electrode to theenzyme active site, the concept of ‘electrical wiring’ has been reported [67, 68]. Conductingpolymers are likely to provide a 3-D electrically conducting structure for this purpose.Conducting polymers are also known to be compatible with biological molecules in neutralaqueous solutions. They can be reversibly doped and undoped electrochemically, processeswhich are accompanied by significant changes in conductivity and spectroscopic propertiesof the films that can be used as a signal for the biochemical reaction [69, 70]. The electronicconductivity of conducting polymers changes over several orders of magnitude in responseto changes in pH and redox potential of their environment [71].

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Conducting polymers have the ability to efficiently transfer the electric charge produced bythe biochemical reaction to the electronic circuit [72, 73]. Moreover, conducting polymerscan be deposited over defined areas of the electrodes. This unique property of conductingpolymers along with the possibility of entraping enzymes during electrochemicalpolymerisation has been exploited for the fabrication of amperometric biosensors [74-79].Besides this, conducting polymers exhibit ion-exchange and size-exclusion properties due towhich they are highly sensitive and specific towards the substrate [43, 80-82]. Numerouspublished papers conclude that conducting polymers can be used as a medium for the

Figure 10.4 Modes of electron transfer in a conducting polymer-based (CP)amperometric biosensor: (a) enzyme and mediator immobilised on conducting

polymer, (b) enzyme linked with conducting polymer through mediator, (c) enzymedirectly linked to conducting polymers without any mediator


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immobilisation of enzymes [11, 43, 83, 84]. They provide good detectability and fast responseas the redox reaction of the substrate, catalysed by an appropriate enzyme, takes place in thebulk of the polymer layer. A schematic of the different pathways suggested for electrontransfer in the conducting polymer-based amperometric biosensors is given in Figure 10.4.

This chapter provides an overview of the importance of conducting polymers and theirapplication to biosensors. In this context many new conducting polymers have beensynthesised and studied. These include PPV, PT, PPy, PFu, PANI and PVCZ.

10.2 Preparation of Electrodes

10.2.1 Synthesis of Conducting Polymers

Various methods have been reported in the literature for the synthesis of conducting polymers[85-87]. However, the most widely used method is oxidative coupling, which can be eitherchemical or electrochemical. The electrochemical technique has recently gained popularitydue to the ease with which it is possible to produce homogenous and coherent films, toachieve uniform doping and to control the thickness of the film [88]. The methods ofelectrochemical polymerisation of conducting polymers generally employed are (a) constantcurrent, or galvanostatic, (b) constant potential, or potentiostatic, or (c) potential scanning/cycling or sweeping [89]. The charge consumption accompanying the rate of polymerformation is linearly dependent on the time and is independent of concentration of monomer.Generally, potentiostatic conditions are recommended to obtain these films; galvanostaticconditions are employed to obtain thick films. Electrodes should be chosen carefully sothat they are not oxidised during the electrochemical oxidation process. Metals usedsuccessfully as anodes are chromium, gold, nickel, palladium, platinum and titanium.Platinum plate and glass coated with a conductive ITO layer are the two most popularelectrodes. Metals such as aluminium, indium, iron and silver are unsuitable forpolymerisation of PPy and PT as they will get oxidised before the polymerisation occurs.Polymer films have also been grown on a number of semiconducting materials such as n-doped silicon [90], gallium arsenide and cadmium sulphide [91] and the semimetal, graphite[92]. Since Diaz and co-workers reported the electrochemical synthesis of PPy [93], manyconducting polymers, such as PANI, polyindole, PT, polyazulene, PVCZ, polypyrene, PFu,etc., have been reported in the literature [54, 74, 94]. Polyaniline can be electrochemicallygrown in the form of a blue-green powdery pigment on a platinum anode during electrolysisof a solution of aniline in sulphuric acid [95-97]. This powdery product was known as‘aniline black’. Genies and co-workers have presented a historical survey of PANI [94].Many researchers have presented detailed reports on the electropolymerisation mechanism,properties and applications of PANI. Details relating to the various methods for the synthesisof conducting polymers have been given in Chapter 13.

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10.2.2 Conduction Mechanism in Conducting Polymers

On doping, conducting polymers show enhanced electrical conductivity by several orders ofmagnitude [98]. To explain the electronic phenomenon in these polymeric systems, a newconcept involving solitons, polarons and bipolarons has been used [99]. These are structuraldeformations produced as a consequence of doping. These defects are delocalised over 4-5monomeric units and are mobile, resulting in the enhanced conductivity. The conduction inPA having two-fold degeneracy has been understood in terms of the motion of chargedsolitons [100]. However, for highly conducting PPy and PT, which are energetically nondegenerate, the conduction occurs via spinless positive charge carriers termed as bipolarons[101]. At lower concentrations, a polaron is envisaged which has a positive charge at one endand a free electron at the other end. The coalescence of polarons may lead to the formationof bipolarons with increasing charge density. Conductivity in conducting polymers is influencedby a variety of factors including polaron length, the conjugation length and the overall chainlength and by the charge transfer to adjacent molecules [102].

10.3 Immobilisation of Biomolecules/Enzymes

Stable immobilisation of macromolecular biomolecules on conducting microsurfaces withcomplete retention of their biological recognition properties is a crucial problem for thecommercial development of miniaturised biosensors. Various conducting polymers havebeen utilised for immobilisation of enzymes at an electrode surface including PPy [74-76, 103-106], polyindole [79], PANI [77, 107, 108], poly (N-methylpyrrole) [65] andcopolymers of N-substituted pyrroles [34].

10.3.1 Methods of Immobilisation

Most of the conventional procedures for biomolecule immobilisation (such as crosslinking,covalent binding and entrapment in gels or a membrane) suffer from low reproducibilityand a poor spatially controlled deposition. A few biosensors based on insulatingelectropolymerised films of polyphenol, poly(o-phenylenediamine) and overoxidisedpolypyrrole have been reported. Malitesta and co-workers [109], Bartlett and Caruana[110], Groom and Luong [111] and Ramanathan and co-workers [112] have studied thechanges in the dielectric constant in polypyrrole immobilised glucose oxidase films as afunction of glucose concentration. Among the conducting polymers, PPy and its derivativesplay a leading role due to their versatile applicability and the wide variety of molecular(redox) species which may be covalently linked to a pyrrole group. Many recent reportshave been published on the immobilisation of biological reagents [3, 113-115]. Thedifferent methods which can be utilised for immobilisation of the enzyme/biomoleculeon conducting polymers are shown in Figure 10.5.


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Figure 10.5 Different methods of enzyme immobilisation on conducting polymers (a)physical adsorption (b) crosslinking by bifunctional reagents (c) covalent bonding to

the matrix (d) entrapment within the polymer matrix (e) immobilisation in aconducting polymer polyvinyl carbazole (PVCZ) / stearic acid (StA) monolayer using

Langmuir-Blodgett film technique

10.3.1.1 Physical Adsorption

A number of biomolecules have been physically immobilised on conducting polymers[66, 112, 116-119]. This is the simplest method of enzyme immobilisation. Since thebinding forces involved are hydrogen bonds, van der Waals forces, etc., porousconducting polymer surfaces are most commonly used. The pre-adsorption of anenzyme monolayer prior to the electrodeposition of the polymer, [120] and two-stepenzyme adsorption on the bare electrode surface and then on PPy film [121] havealso been investigated.

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10.3.1.2 Crosslinking

The crosslinking of the biomolecules via bifunctional reagents, e.g., glutaraldehyde andbovine serum albumin, has been utilised to stabilise the biomolecules. In this case, acrosslinked network of enzyme is formed resulting in the formation of a bigger moleculeon the polymer surface [122, 123]. Enzymes have also been immobilised on the surfaceof electrodeposited polymer by applying the bovine serum albumin and glutaraldehydeprocedure followed by the electrodeposition of the polymers [1, 124, 125]. This methodhas certain limitations since it may cause a drastic loss in the activity of the biomolecules.

10.3.1.3 Covalent Bonding

The biomolecules are attached directly to the matrix by chemical/covalent linkage, whichis not reversed by pH or changes in ionic strength. Carbodiimide coupling to form peptidebonds has been extensively used for the covalent coupling of the enzyme with polymers[126-128]. Since chemical modification is involved, this method results in the drasticloss of activity of the enzymes/biomolecules. Covalently attached redox biomolecules onpolymers have been utilised as highly electron transfer mediators in flavin adeninedinucleotide (FAD) centres of oxidases [73].

10.3.1.4 Electrochemical Entrapment

A number of enzymes may be incorporated into conducting polymer films duringelectrochemical deposition on appropriate electrodes. This is a mild procedure whichdoes not affect the activity of the biomolecules. However, it creates a diffusion barrier tothe transport of substrate/products in and out of the matrix. Conducting polymers providea facile means for ensuring proximity between the active site of an enzyme and theconducting surface of the electrode with considerable potential for biosensor construction.This method provides a facile and controllable method for the deposition of biologicallyactive molecules to defined areas on the electrodes. For electrochemical entrapment, asolution of the electropolymerisable monomer and the aqueous buffer containing theenzyme is used for the deposition process. The polymer film can be depositedelectrochemically using a potentiostatic or galvanostatic method. It has been establishedthat glucose oxidase can be successfully entrapped in PPy films [74, 75] and themorphology of the film alters to a more fibrillar form.

Polymerisation based on the electrochemical oxidation of a given monomer from a solutioncontaining the enzyme is the simplest method of enzyme immobilisation in a polymer atthe working electrode surface and results in the formation of conducting or non conducting


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polymer layers containing entrapped enzyme molecules. The electropolymerisation is oftendone in aqueous solution at a pH close to neutral in order to immobilise an enzyme withoutloss of its activity [1, 66, 74, 75, 107]. During electropolymerisation, mediators can beimmobilised along with an enzyme [107, 129, 130] or can be conjugated with the monomerbefore polymerisation [131]. This well-controlled procedure of enzyme immobilisation isof great significance in the fabrication of microsensors [107, 124, 125, 130, 132] used inthe preparation of multilayer devices [130, 133, 134] and multienzyme biosensors [125,135-138]. Many theoretical models have been proposed with the electrochemical entrapmentof enzyme for the evaluation of the role of the thickness of polymeric layers, the enzymelocation, the enzyme loading, etc., on the functioning of a biosensor [65, 139, 140, 141].Foulds and Lowe [75], Umana and Waller [74] and Bartlett and Whitaker [77] have describedthe entrapment of glucose oxidase within the growing network of conducting PPy. Thismethod is inconsistent with the application of an appropriate potential to the workingelectrode soaked in aqueous solution containing the enzyme and monomer molecules.

10.3.1.5 Immobilisation by the Langmuir-Blodgett Technique

It has been observed that the surface of the conducting polymer plays an important rolein the effective immobilisation of the desired enzyme. The Langmuir-Blodgett (LB)technique can be successfully applied to deposit a desired monolayer with the desiredorientation of the biomolecules/enzymes [142-145]. Ramanathan and co-workers [146]have utilised the polyemeraldine base LB films for the immobilisation of GOD. Thesefilms have been shown to function as amperometric glucose biosensors and have a linearrange from 5 to 50 mM. LB films of PT immobilised with GOD and urease have alsobeen prepared for application to respective biosensors [147, 148].

The electrochemical entrapment stabilises the enzyme in a polymer layer [149] byprotecting against environmental humidity and biological contaminants. However,covalent binding of an enzyme to a suitably modified polymer with the formation ofpeptide bonds after carbodiimide activation has widely been utilised [126-128, 150]. Ithas also been observed that the original biocatalyst sensitivity remained unchanged whenthe electrode surface was renewed after five months of controlled storage [151]. Artificialmediators such as ferrocene carboxylic acid or quinones have been utilised to reoxidisethe enzymes in conducting polymers [76, 103-104, 152].

The most popular ways to immobilise enzymes on conducting polymers are either toentrap the enzyme within the growing polymeric films [56, 59, 153, 154] or to use atwo-step procedure based on the formation of a functionalised conducting polymer filmfollowed by the covalent binding of the enzyme at the functional groups at the polymersurface [125, 155, 156].

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10.3.2 Advantages of Immobilisation

Biosensors require highly active enzymes/biomolecules; therefore, the immobilisation methodsmust be chosen in such a way that they can achieve a high sensitivity and functional stability.This is important for economic reasons also. The measurable activity gives an idea about thebiocatalytic efficiency of an immobilised enzyme. The rate of substrate conversion shouldrise linearly with enzyme concentration. The measured reaction rates depend not only on thesubstrate concentration and the kinetic constants Km (Michaelis Menten constant) and Vmax

(maximum velocity of the reaction) but also on the immobilisation effects. The followingeffects have been observed [157] due to the immobilisation process:

• A decrease in the affinity of the enzyme to the substrate (increase in Km) is observeddue to conformational changes of the enzyme on immobilisation.

• A partial/complete inactivation of a part of the enzyme molecule may occur(decrease in Vmax).

• Ionic, hydrophobic or other interactions between the enzyme and the matrix(microenvironmental effects) may result in changed Km and Vmax values. These effectsare reversible and are caused by variations in the dissociation equilibria of chargedgroups of the active centre.

• A non uniform distribution of substrate/product between the enzyme matrix and thesurrounding solution affects the measured (approximate) kinetic constants.

• In biosensors the biocatalyst and the signal transducer are spatially combined, i.e.,the enzyme reaction proceeds in a layer separated from the measuring solution. Insuch a system, the substrate and products are transformed by diffusion. Thus diffusionand the enzyme reaction cannot proceed independently of one another; they arecoupled in a complex manner [158].

It has been predicted [159] that, in the future, the immobilisation techniques compatiblewith microelectronic mass production processes will be utilised for the fabrication ofamperometric enzyme electrodes.

10.4 Characterisation of Enzyme Electrodes

10.4.1 Determination of Enzyme Activity

During the course of immobilisation, some portion of an enzyme gets inactivated.Therefore, it becomes essential to determine the portion of enzyme remaining active


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after immobilisation. The determination of the initial rate of product formation orsubstrate consumption in the measuring cell using the sensor gives the measure of theenzyme activity taking part in the measuring process.

The following formula is used to calculate the apparent enzyme activity (U, units percm2) of the enzyme electrode:

U = AV / ∈st (10.1)

where A is the difference in the absorbance at a particular wavelength before and afterincubation, V is the total volume of assay, ∈ is a millimolar extinction coefficient, s is thesurface area of the film and t is the incubation time. One unit can be defined as theamount of enzyme, which converts 1 μM of reactant into product per minute.

10.4.2 Effect of pH

It has been observed that the enzymes are active over a limited range of pH and most ofthe enzymes have a definite optimum pH. The optimum pH is obtained due to the effecton the affinity of the enzyme with the substrate; stability may be irreversibly destroyedon either side of the optimum. The pH profile in the linear measuring range of a biosensoris a diffusion controlled phenomenon and, therefore, is less sharp than those of therespective enzymes in solution [159] and it shifts towards higher pH in solution due tothe decrease in local pH [160].

Usually, the enzymes in dilute solution exhibit a bell-shaped pH activity curve. If theimmobilised enzyme is studied in essentially buffer-free conditions, considerable changein pH activity profile may be observed. These changes will depend partially on the intrinsiccatalytic activity of the enzyme and on the substrate concentration, the reaction rate andthe rate of H+ liberation for acid releasing reactions. Reactions catalysed by GOD will below and the internal pH of the matrix will be a bit different from that of the bulk solution.With relatively high substrate concentrations, the reaction rate will be high enough tocause a marked difference between the pH of the internal matrix and the bulk phase. Forsuch a case, the change in pH of a microenvironment of the enzyme will also affect thereaction rate and the increase in substrate concentration. Thus if the bulk phase pH ishigher than the optimum pH of the enzyme, these factors will act synergistically (positivecooperation) or if the bulk phase pH is less than the pH optimum, they will actantagonistically (negative cooperation). A schematic representation of the pH/activityprofile of an enzyme (urease) in dilute solution and in the immobilised state in PVCZfilms is shown in Figure 10.6.

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10.4.3 Effect of Temperature

The overall enzyme-catalysed reaction succeeds in three steps: formation of the enzyme-substrate complex, conversion of the complex to an enzyme-product complex, anddissociation of the enzyme-product complex to products and free enzyme. There are atleast 18 thermodynamic parameters for the forward reaction, i.e., the heat, free energyand entropy of activation, and the heat, free energy and entropy of the process for eachof these stages.

The increase in the enzyme activity with temperature follows the Arrhenius relation:

log k = log A - Ea / 2.303 RT (10.2)

where k is the reaction rate constant, A is a numerical constant, Ea is the activationenergy, R is the thermodynamic gas constant and T is the temperature (in K).

The slope of the Arrhenius plot (log k against 1/T) is given by

Slope = Ea / 2.303 R (10.3)

Figure 10.6 Activity profile of an enzyme (urease) in (a) native state (■) and (b) afterimmobilisation in the polymer matrix (PVCZ) at different pH in phosphate buffer (●)


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It has been observed that the rate of enzyme reaction rises with temperature up to acertain maximum above which, thermal inactivation of the enzyme takes place. Theinactivation of enzymes by heat is due to the denaturation of the enzyme protein. Theeffect of the instability of the enzyme, free and in the immobilised state, can be studiedby exposing the enzyme to thermal treatment for a defined period prior to measuring itsactivity at a temperature at which it is stable. Chaubey and co-workers obtained 40 °Cas the critical temperature of PPy-polyvinyl sulfonate films immobilised with crosslinkedlactate dehydrogenase [123]. The activation energies below and above the criticaltemperature were found to be 93.3 and 22.4 kJ/mole, respectively.

10.4.4 Effect of Storage Time

A biosensor should have excellent stability, reproducibility and a long-term storage time.Biosensors with an electron transfer mechanism based on shuttling of the free diffusingartificial mediators between the enzyme and electrode surface have poor long-term stabilitydue to continuous loss of mediators [163]. In order to increase the long-term stability, it isessential to utilise a new electron transfer pathway between the enzyme and the electrodesurface [164]. In this context, conducting polymer based biosensors have been reported toundergo direct electron transfer between the enzyme and the electrode surface [165, 166].

Figure 10.7 Effect of storage time on the response of a conducting polyanilinebased lactate biosensor: ■ LOD in solution phase and ■ LOD in immobilised state

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It has been observed that enzymes are highly unstable in the solution phase and lose theiractivity rapidly with time. In the immobilised state, the enzyme is observed to lose itsactivity very slowly. The immobilisation of enzymes on conducting polymers providesincreased stability but decreased electron transfer in the medium resulting in the reducedresponse of the biosensor. The response of the enzyme lactate oxidase (LOD) to the analytein solution and in the immobilised state as an enzyme electrode after storing at 4-10 °C isshown in Figure 10.7.

10.4.5 Response Measurements

Use of electropolymerised conducting films in the development of small sensing devicesis found to be important because electropolymerisation allows control over the thicknessand spatial location of electrode modification. These interfacing electroactive materialshave inherent oxidation-reduction abilities. These materials are important for direct andrapid electron transfer at the electrode surface [73, 119, 161-163]. Physical and chemicalproperties can be modified by appropriate polarisation and doping with counter ions.

When a biologically active material interacts with an analyte, a physicochemical changetakes place that is converted into an electrical output signal using a suitable transducer.Based on various types of transducers, biosensors may be classified into optical,calorimetric, piezoelectric and electrochemical biosensors. Cooper and Hall have reportedthe electrochemical response of a GOD loaded PANI film [84]. Mu and co-workers havestudied the bioelectrochemical response of the PANI-uricase electrode [162].

The factors affecting the overall response of amperometric biosensors involve currentlimiting processes. There is a diffusion of substrate from the bulk solution to the outersurface of the membrane, enzyme diffusion of the product through the enzyme layer, andquiescence of the solution (if the enzyme layer is thin). For thick enzyme layers andslower diffusion of substrate through the enzyme layer compared to external mass transfer,simultaneous limitation of higher enzymatic reaction and diffusion must be considered.The electrochemical synthesis of conducting polymers allows the direct deposition of thepolymer on the electrode surface, while simultaneously trapping the problem molecule.It is thus possible to control the spatial distribution of the immobilised enzyme and thefilm thickness, and to modulate the enzyme activity by changing the state of the polymer.

10.5 Types of Biosensors

Based on the type of transducer used, biosensors can be classified as optical,electrochemical (amperometric or potentiometric), piezoelectric or thermal. Among allthese the electrochemical biosensors are most widely used.


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10.5.1 Optical Biosensors

Optical biosensors are based on the measurement of light absorbed or emitted as a consequenceof a biochemical reaction. In such biosensors, light waves are guided by means of opticalfibres to suitable detectors. Such biosensors have been used for the detection of pH, O2 andCO2. Gerard and co-workers have optically measured the pyruvate concentration (Figure 10.8)by immobilising lactate dehydrogenase (LDH) onto a PANI electrode [163]. These electrodesare stable for about 15 days and have a response time of about 90 seconds. Chaubey and co-workers have immobilised LDH on polypyrrole-polyvinyl sulfonate (PPy-PVS) electrodes onITO plates and have estimated the lactate concentration by optical measurements [123].Singhal and co-workers immobilised glucose oxidase on polyhexyl thiophene by the LBtechnique and fabricated an optical glucose sensor using UV-visible spectroscopy [147]. Ureaseand glutamate dehydrogenase (GLDH) have been co-immobilised in electrochemicallyprepared PPy-PVS films for application in a urea biosensor [66]. Quantification of urea bykinetic methods was carried out by measuring the oxidation of NADH in the presence ofglutamate dehydrogenase and urease at 350 nm:

Urea + H2O → NH4+ + 2HCO3

– (10.4)

α-ketoglutarate + NH4+ + NADH → L-glutamate + NAD+ + H2O (10.5)

Figure 10.8 Calibration graph of an optical pyruvate biosensor based on polyaniline

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10.5.2 Electrochemical Biosensors

The interaction between the enzyme and substrate produces an electrochemical signalthat can be detected by an electrochemical detector. These are based on mediated orunmediated electrochemistry.

10.5.2.1 Conductometric Biosensors

The conductometric biosensors measure the changes in the conductance of the biologicalcomponent arising between a pair of metal electrodes. Contractor and co-workers constructedbiosensors for glucose/GOD, urea/urease, neutral lipid/lipase and haemoglobin/pepsin bymonitoring the change in the electronic conductivity, which is a result of a change in redoxpotential and/or the pH of the microenvironment in the polymer matrix [167]. The responseof a urea conductometric biosensor is shown in Figure 10.9. Ramanathan and co-workersstudied the application of polyaniline LB films as glucose biosensors [146]. They have alsoinvestigated the dielectric spectroscopic measurements of a PPy/glucose biosensor. Conductivitybiosensors based on conducting polymers have been developed for penicillin [168].

Figure 10.9 Response curve of a conductometric urea biosensor

10.5.2.2 Potentiometric Sensors

Potentiometry is a rarely used detection method employed in biosensors, with enzymesimmobilised in an electrodeposited polymer layer, although certain advantages over


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amperometric detection for a PPy-based electrode with immobilised GOD have beendemonstrated [84].

Potentiometric biosensors with conducting polymers can be produced, using the pHsensitivity of polymers [169]. The sensitivity of PPy to NH3 was used to producesuch biosensors [170, 171]. LB films of poly(N-vinyl carbazole) have been utilisedfor the fabrication of a potentiometric urea biosensor (Figure 10.10). Conductingpolypyrrole molecular interfaces have also been used to modulate the biologicalfunction of enzymes and living cells at the electrode surface by the adjustment ofelectrode potentials. Often, additionally obtained discrimination of electrochemicalinterfaces, by using conducting polymers as matrices for enzymes, allows the use ofsuch biosensors for analysis of natural samples, e.g., flow injection determination oflactate in whole blood. Apart from the advantage of a well-defined formation of thepolymer layer, this methodology has some difficulties associated with a significantchemical and electrochemical activity of the polymer matrix due to the similarity ofthese materials towards ion-exchange processes and redox equilibrium. Karyakinand co-workers have described a potentiometric pH response of electrodes modifiedwith PANI for application as a glucose biosensor [172]. These electrodes exhibit afully reversible potentiometric response of about 90 mV/pH in the range of pH 3-9.This biosensor has a response of about 5-6 minutes and has been shown to be useful

Figure 10.10 Potentiometric response of PVCZ/urease electrodes with different ureaconcentration

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for estimation of glucose in the range of 0.1-30 mM. A creatinine electrode wasfabricated by co-immobilisation of creatininase, creatinase and sarcosine oxidase ina PPy matrix [173]. Difficulties concerning the immobilisation of enzymes in theelectrodeposited polymer layer or the mechanism of the entrapment and dynamicseffects on a number of biosensors have been widely discussed. However, no conclusivereports on the comparison of different polymer matrices for immobilisation of thesame enzyme are currently available.

10.5.2.3 Amperometric Biosensors

Amperometric biosensors measure the current produced during the oxidation orreduction of a product or reactant usually at a constant applied potential. The mostimportant factor affecting the functioning of amperometric biosensors pertains tothe electron transfer between catalytic molecules, usually oxidase or dehydrogenase,and the electrode surface, this transfer most often involving mediation or a conductingpolymer. Although the role of the electrodeposited conducting polymer films is notfully understood and explained, various polymers used for the enzyme immobilisationmay significantly affect the response of biosensors sensitive to given species. Muchwork has been done in recent years on the application of electrochemically grownconducting polymer layers in amperometric biosensors.

A theoretical model describing the operation of a conducting polymer based biosensorhas been reported. In such a model, two different mechanisms have been suggested.An enzymatic reaction may take place at the polymer/solution interface without anydiffusion of the substrate molecules into the polymer, or diffusion of substrate intothe bulk of the polymer layer due to its porosity may occur [174]. It is shown thatthe quinone-hydroquinone reaction at a PPy modified electrode is accompanied bythe diffusion of the substrate into a porous polymer layer. A general opinion is thatno electron transfer occurs between the working electrode surface and the enzymemolecules entrapped in the polymer layer. However, de Taxis du Poet and co-workershave suggested a direct electron transfer between the redox centre of GOD immobilisedin poly(N-methylpyrrole) and a gold electrode surface [73]. The immobilisation ofGOD via manipulation of pore size in PPy to enhance enzyme loading, has also beenreported [175]. Verghese and co-workers have conducted similar experiments in PANI/GOD films [176].

Biosensors based on conducting polymers have found potential applications inhealthcare, veterinary medicine, environmental monitoring, immunosensing, etc.


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10.6 Biosensors for Healthcare

10.6.1 Glucose Biosensor

Selective determination of blood glucose is of utmost importance for the screening andtreatment of diabetes. About 4% of the world population is presently suffering fromdiabetes. Normal blood glucose concentration varies from 4.2-5.2 mM. A number ofglucose biosensors are available in the market for blood glucose estimation, but none ofthese is based on conducting polymers. Numerous reports have been published for theimmobilisation of GOD in conducting polymers for glucose estimation [75, 110, 175].GOD catalyses the oxidation of glucose in the following sequence of reactions:

D-Glucose + GOD (FAD) → Gluconolactone + GOD (FADH2) (10.6)

GOD (FADH2) + O2 → GOD (FAD) + H2O2 (10.7)

Conducting PPy is one of the first conducting polymers to be used for the fabrication ofa glucose biosensor [75, 84]. These studies have indicated that up to 125 mU of GODactivity can be incorporated. Umana and co-workers have fabricated a PPy-based glucosebiosensor for detection of hydrogen peroxide [74]. The response current of these electrodeswas found to be diminished after about one week indicating gradual leaching of theGOD from the PPy matrix. This has been attributed to the electrostatic repulsion betweenthe cationically charged enzyme and the polycationic PPy as well as the porosity of thePPy. It has been investigated if the polymers containing para- and ortho-quinone groupsas electron-transfer relay systems for oxido-reductases can effectively catalyse theelectrooxidation of glucose [177, 178]. Ramanathan and co-workers have immobilisedGOD after manipulation of the pore size in PPy to improve the loading of the enzyme[175]. The incorporation of large size dopant anions such as paratoluene sulfonate (PTS)and ferricyanide into PPy films during electropolymerisation and subsequent exchangeof these ions with a smaller ion like chloride in solution, ion self-exchange orelectrochemical switching renders the PPy more porous and the immobilisation of GODmore facile. The response of such polypyrrole-GOD electrodes is shown in Figure 10.11.

L-B films of polyemeraldine base have been deposited on ITO glass substrates by injectinga solution of 60% CHCl3 in N-methyl phenazine containing 100 μl of GOD. The activityof GOD immobilised in these polyemeraldine base films determined by the o-dianisidineprocedure has been found to be 5 IU cm-2 [146].

A few reports are available wherein conducting polymers have been incorporated in materialssuch as graphite [179] and carbon paste [180]. However, these have not yet beencommercialised. A glucose biosensor utilising covalently coupled GOD to poly(o-aminobenzoic acid) (PAB, a carboxy group functionalised polyaniline) has been described [42].

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Amperometric response measurements conducted via unmediated and mediated (withferrocene carboxylic acid and tetrathiafulvaline) reoxidation of GOD have shown thatglucose can be detected over a wide range of concentrations. An enzyme-conducting polymer-mediator model provides for better charge transport in a biosensor. The screen-printedelectrodes consisted of two silver tracks with an active working area of 4 mm x 2 mm(Figure 10.12), printed on a polyvinyl chloride (PVC) substrate. On one of the tracks, theAg/AgCl reference electrode was screen-printed and on the other electrode, the PAB/GOD

Figure 10.11 Amperometric response of GOD immobilised PPy based glucose biosensorsfor PPy-ferricyanide unexchanged (▲) and PPy-ferricyanide exchanged with Cl– (■)

Figure 10.12 A two electrode set-up used for an amperometric glucose biosensor basedon poly(o-amino benzoic acid). Each electrode consists of two screen-printed tracks ofAg paste. On one of the tracks, Ag/AgCl (reference electrode) was printed and on theother, a PAB/GOD complex was adsorbed (working electrode). The non working area

was covered by the PMMA mask.


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complex was adsorbed. The area of the electrode exposed to the solution was always keptat 8 mm2 by masking it with a PMMA layer. A nylon membrane shielded the working areaof the screen-printed PAB/GOD electrode. The optimal response obtained at pH 5.5 at 300K lies in the range from 1 mM to 40 mM of glucose. The operational stability of the PAB-based glucose biosensor has been experimentally determined to be about six days.

10.6.2 Urea Biosensor

Urea is the most important end product of protein degradation in the body. Itsconcentration in blood depends on the protein catabolism and nutritive protein intakeand is regulated by renal excretion. Thus the estimation of blood urea nitrogen is importantin the assessment of kidney failure. The normal level of urea ranges from 3.6 mM to 8.9mM. All enzymatic methods for urea determination are based on the principle of ureahydrolysis by urease:

(NH2)2CO + 3H2O → 2 NH4+ + OH- + HCO3

– (10.8)

Most biosensors described in the literature for the determination of urea are potentiometricbased on NH4

+ or HCO3- sensitive electrodes [181, 182]. Osaka and co-workers

constructed a highly sensitive and rapid flow injection system for urea analysis with acomposite film of electropolymerised inactive PPy and a polyion complex [183]. Pandeyand co-workers fabricated a urea biosensor based on immobilised urease on the tip of anammonia gas electrode (diameter: 10 μm) made from a PPy film coated onto a platinumwire [170]. The enzymatic response was achieved in the wide range of 0.001-0.05 Mwith a stability of more than 32 days. Cho and co-workers [184] developed a procedurefor urea determination by crosslinking urease onto PANI-Nafion composite electrodes,which could sense the ammonium ions efficiently. Such a urea biosensor has a detectionlimit of about 0.5 μM and a response time of 40 seconds.

Immobilisation of urease and glutamate dehydrogenase enzymes in electrochemicallyprepared PPy/PVS film has been carried out using physical adsorption and electrochemicalentrapment techniques [66]. Hydrolysis of urea is catalysed by the enzyme as in Reaction10.4. The ammonium ion released in the first reaction is coupled with α-ketoglutarate inthe presence of GLDH; NADH acts as a cofactor (Reaction 10.5). A kinetic method wasutilised for the quantification of urea (Figure 10.13) by measuring the oxidation of NADHin the presence of glutamate dehydrogenase and urease at 350 nm. The leaching wasfound to be 5%-10% in the case of entrapped enzymes over a period of one hour insolution. However, in the case of adsorbed enzymes, this was found to be 15%-20%.The half-life of this electrode was found to be about 30 days for adsorbed enzymes and35 days for the entrapped enzymes.

321

10.6.3 Lactate Biosensor

L-lactate is the intermediate product of carbohydrate metabolism. The rapid, accurateand selective assay of L-lactate and pyruvate is necessary in clinical biology where it isimportant in the growth of certain cells and in fermenters. L-lactate also plays an importantrole in food industries engaged in fermentation of wine and dairy products. The lactateconcentration level in blood indicates various pathological states, including shock,respiratory insufficiencies and heart diseases, and finds immense importance inneonatology and sports medicine.

Different lactate oxidising enzymes use different co-substrates and, therefore, a varietyof electrochemical indicator reactions in biosensors can be utilised. Most of the lactatebiosensors are based on enzymes like lactate oxidase (LOD) and lactate dehydrogenase(LDH). A needle-type lactate biosensor has been recently developed by Yang and co-workers who fabricated poly (1,3-phenylenediamine) electrodes immobilised with LODfor continuous intravascular lactate monitoring [185]. In the enzyme electrodes basedon LDH, the biochemical reaction has been coupled to the electrode via NADH oxidation,either directly [119, 123, 163], or by using mediators [186] or additional enzymes [119].This may lead to a shift of the unfavourable reaction equilibrium by partial trapping ofthe reduced cofactor. Direct oxidation of NADH requires potentials of more than 0.4 V;

Figure 10.13 Lineweaver-Burke plot for the urea biosensor based on PPy/PVS filmsadsorbed with urease


322


other interfering substances get oxidised at this potential. In order to avoid this, mediatorsor pretreatment of the electrode have been utilised. Potassium ferricyanide has beenextensively used for spontaneous oxidation of NADH in potentiometric LDH electrodes[186]. Yao and co-workers observed that the co-immobilisation of LOD and LDH inpoly(1,2-diaminobenzene) films show a highly sensitive detection of L-lactate due toamplification of the signal by substrate recycling [187].

LOD and LDH have been co-immobilised on electrochemically prepared PANI filmsby physical adsorption [119]. Regeneration of L-lactate by substrate recycling(Figure 10.14) is shown to provide an amplification of the sensor response. The linearityfor the LOD/LDH immobilised electrodes has been found to be from 0.1 mM to 1 mMof lactate with a detection limit of 5x10-5 M. These enzyme electrodes are stable forabout 3 weeks at 4-10 °C implying that these can be used for estimation of L-lactate incells and fermentation. A comparison of the results obtained with the PANI electrodesimmobilised with either LOD or LDH, indicates that the co-immobilisation providesthe possibility to detect L-lactate at lower concentrations. Chaubey and co-workershave reported the results relating to the electrochemical preparation and characterisationof the PPy-PVS/LDH electrodes. NAD+ is reduced to NADH as a result of its reactionwith L-lactate in the presence of LDH [123]. NADH in turn is oxidised at the workingelectrode releasing two electrons at 0.2 V (bias voltage). The PPy-PVS film acts as anelectron acceptor and hence gets reduced. An attempt has also been made toexperimentally investigate the effect of film thickness, pH, temperature and enzymeconcentration on the activity of the PPy-PVS/LDH electrodes.

Figure 10.14 Substrate recycling of lactate/pyruvate in PANI/LOD-LDH electrodes

323

10.6.4 Cholesterol Biosensor

Cholesterol and its fatty acid esters are important components of nerve and brain cellsand are precursors of the biological materials such as bile acids and steroid hormones.Accumulation of cholesterol in blood leads to fatal diseases such as arteriosclerosis,cerebral thrombosis and coronary diseases. Kajiya and co-workers immobilised cholesteroloxidase (ChOx) and ferrocene carboxylate in PPy electrochemically to describe thesensitivity of the resulting films [188]. The response was proportional to the cholesterolconcentration up to 0.05 mM. It has been demonstrated that ferrocene attached to polymerchains can mediate electron transfer from horseradish peroxidase (HRP) to a conventionalelectrode surface [189]. In the case of immobilised HRP and ChOx the sensor yields0.35 μA to 10 mM of cholesterol whereas 3 μA was obtained in the case of free ChOx.It was therefore suggested that the sensor response is limited by the interfacial transportor reaction rate of H2O2. The sensor response was also found to be independent of theapplied potential between –100 and 100 mV.

Trettnak and co-workers prepared a cholesterol biosensor by electropolymerising pyrrolein solutions of 0.1 M phosphate buffer containing NaClO4 (10 mM) and 15 to 55 IU/ml of ChOx at 0.8 V vs SCE (standard calomel electrode) [190]. The sensor showshigh reproducibility in the range 0 to 250 μM with the detection limit of about 5 μMof cholesterol. The amperometric cholesterol biosensor based on PPy/ferrocenecarboxylic acid electrodes is shown in Figure 10.15. Kumar and co-workers have used

Figure 10.15 Response curve for the polypyrrole-ferrocene carboxylic acid basedcholesterol biosensor


324


dodecylbenzene sulfonate (DBS) doped PPy films for immobilisation of ChOx byphysical adsorption [191]. These ChOx/DBS-PPy/ITO electrodes exhibit a responsetime of about 60 seconds, linearity from 2 to 8 mM of cholesterol and stability ofabout three months at 4 °C. The cyclic voltammetry studies were performed in 0.1 Mphosphate buffer (pH 7.0) using enzyme immobilised DBS-PPy/ITO films with andwithout ferricyanide ion mediator as a working electrode, Ag/AgCl as reference electrodeand Pt as the counter electrode.

Cholesterol + ChOx(ox) → Cholestenone + ChOx(red) (10.9)

ChOx (red) + Fe3+ → ChOx + Fe2+ (10.10)

0.4V

Fe2+ → Fe3+ + e- (10.11)

The oxidation peak at 0.75 V (without ferricyanide) was observed to shift cathodicallyby about 350 mV (observed at 0.4 V in the presence of ferricyanide). In such a case theoxidation of other elements at 0.75 V, which could result in an increase in the resultingcurrent, can be avoided.

10.6.5 DNA Biosensor

DNA biosensors are considered to be important for the clinical diagnosis of inheriteddiseases, rapid detection of pathogenic infections and screening of complementaryDNA colonies required in the field of molecular biology research. Present methodsof genetic analysis are dependent upon the ability to detect specific DNA sequencesin a heterogeneous mixture. More recently, DNA integrated electroactive polymers(thin films or two-dimensional L-B monolayers) have created a novel class of intelligentmaterials, which possess superior intelligent material properties of self assembly, selfmultiplication, self repair, self degradation and self diagnosis, etc. [192, 193].

Very few reports related to the interaction of DNA with conducting polymers areavailable [194, 195]. Livache and co-workers have described one-stepelectrodeposition of a PPy film functionalised by a covalently linked oligonucleotide[196]. Earlier they had described electrochemical copolymerisation of pyrrole andoligonucleotides having a pyrrole moiety. Introduction of a pyrrole moiety to the 5´end of the oligonucleotide was explained by phosphoramidite chemistry [197]. Mostof these rely on measuring changes in the peak current of a redox active marker thatpreferentially binds to the duplex formed in the hybridisation [198]. Marazza andco-workers have described detection by differential pulse voltammetry and

325

chronopotentiometric stripping analysis by the use of an electroactive indicator,daunomycin hydrochloride, which interacts with the double-stranded DNA [199].Doping of DNA probes within electropolymerised PPy films and monitoring thecurrent changes provoked by the hybridisation event is another possibility, as describedby Wang and co-workers [200].

Immobilisation of DNA on a conducting polymer matrix facilitates detection of asignal (amperometric/potentiometric) generated as a result of interaction of proteinsor drugs with DNA. Possibilities of detecting the signal generated as a result ofhybridisation of DNA strands can also be explored. Gambhir and co-workers haverecently described adsorption characteristics of DNA on electrochemically generatedPPy-PVS films as a function of pH. Adsorption on PPy doped with an anion proceedsby anion exchange by the adsorbed molecule. DNA possesses a fixed negative chargedue to the presence of PO4

-. Therefore, PVS displacement favours strong binding dueto energetic interactions with PPy [201]. Characterisation of adsorbed DNA on thePPy-PVS films has been carried out by UV-visible spectroscopy, FTIR spectroscopyand cyclic voltammetric studies (Figure 10.16).

Figure 10.16 Adsorption of DNA onto conducting PPy-PVS films as a function ofpH.6.0 (◆), 6.5 (■), 7.0 (▲), 8.0 (●) on ITO glass plates


326


10.7 Immunosensor

Immunosensors are small, portable instruments for analysis of complex fluids and are designedfor ease of use by untrained personnel, for rapid assay with sensitivity comparable to that ofenzyme linked immunosorbant assay (ELISA). During the past decade, a number of methodsfor analysing microorganisms, viruses, pesticides and industrial pollutants have been developed.Immunosensors are based on immunochemical principles that can automatically carry outestimation of the desired parameter. Barnett and co-workers have detected thaumatin usingantibody containing PPy electrodes [202]. In the recent past, immunoassays have relied oncomplex indirect enzyme methods in which the resultant product of the enzyme immunoreaction can be measured [203]. Recently, antibodies have been raised against the conductingpolymer, carbazole as a hapten, which may react to modulate the polymer electrochemistry.It has been observed by cyclic voltammetry that the reaction of the antiserum influences thepolymer matrix electrochemistry by an amperometric response. This system has been shownto form the basis of a direct sensor for immunoassay [204].

10.8 Biosensors for Environmental Monitoring

Environmental monitoring of hazardous pollutants has become important to regulatoryagencies and the general public. A number of biosensors for environmental pollutants suchas sulfite [205], nitrite [206], phenolic compounds [207], etc., have been fabricated. Caoand co-workers have reported amperometric gas sensors that can be used for the estimationof gases such as CO, NO2, and O2 [208]. Responsive chemoresistors for gas analysis havebeen reported where the organic polymer modified electrodes were processed by patternrecognition [209, 210]. Yadong and co-workers have performed studies on the ammoniagas sensitive properties and sensing mechanism of PPy [211]. The redox enzyme polyphenoloxidase has been utilised to oxidise phenolic compounds in a two-step reaction to theelectrochemically reducible quinone [212]. Pesticide contamination in developing anddeveloped countries is rapidly growing due to the increasing and indiscriminate use ofthese organic chemicals in agricultural fields. Since pesticides remaining in the food cropsare transferred through the food chain system, the residues have been cumulatively buildingup in the human body to alarming proportions necessitating drastic control measures.Accurate and efficient monitoring of these residues is therefore of vital concern to thesurvival of our society.

10.9 Conclusions

Marked progress has been made in the last decade towards the application of conductingpolymers to biosensors. In this context, the researchers working in this field of research

327

have extensively investigated the immobilisation of enzymes in conducting polymer films.The self-assembly of redox enzymes on the surface of an electrode followed by theelectrochemical deposition of conducting polymers can be used as a molecular interfaceto enable the redox enzyme to communicate electronically with the electrodes. Thusconducting polymers are advantageous in the fact that they combine the role of theenzyme entrapment matrix and chemicophysical transducer, resulting in substantialminiaturisation and reduced response time. The structure of the polymer affects thesensitivity and the detection limit of the biosensor obtained, as well as the ion-exchangeand size-exclusion properties leading to improved detection. Conducting polymers arehighly effective in minimising the effect of interferents when used for entrapment ofredox enzymes and as protection against electrode fouling by non specific adsorption ofhigh molecular compounds from natural samples. It has also been revealed that theconducting polymer matrix serves as an electron shuttling medium between the enzymeand the electrode, and is thus capable of functioning as an electron transfer mediator.Simultaneous integration of enzymes and mediators may improve the electron transferpathway from the active site of enzyme to the electrode. Functionalised conductingpolymers will play an important role in the integration of biological molecules fordevelopment of improved biosensing devices.

Acknowledgements

The authors are thankful to the Director, NPL, for his interest and constant encouragement.A. Chaubey gratefully acknowledges CSIR, India, for the award of a Senior ResearchFellowship.

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11 Nanoparticle-Dispersed SemiconductingPolymers for Electronics

K.S. Narayan

11.1 Introduction

A composite, a blend or a bilayer consisting of a conjugated semiconducting polymerand a nanoparticle component combine to form an interesting system due to a varietyof features. In these binary systems, profound changes are observed in the structural,optical and electronic properties with the degree of interaction between the twocomponents varying from a weak electrostatic interaction to a strong chemical link.Studies of such hybrid systems add further insight into the individual characteristics ofthe different components.

The distinctiveness of the nanoparticle properties arises from size-dependent fundamentalproperties such as ionisation potential, melting point, optical fluoroscence and absorbance[1-4]. The typical range of size for observing variation in these properties depends uponthe solid being periodic over a nanometre lengthscale. In the case of nanoparticles ofsemiconductors such as cadmium sulphide, CdS, and cadmium selenide, CdSe, when theparticle size is smaller than that of the exciton in the bulk semiconductor the lowestenergy optical transition significantly increases due to quantum confinement [5]. Byprecisely controlling the size and the surface of a nanocrystal, its properties can be tuned.However, due to the large surface to volume ratio, the radiative luminescence from thesenanoparticles is significantly reduced through non radiative processes mediated by thesurface states [6]. These non radiative losses may be minimised by an improved methodof core-shell synthesis [7]. By enclosing a core nanocrystal of one material with a shell ofanother having a larger bandgap, one can efficiently confine the excitation to the core,eliminating non radiative relaxation pathways and preventing photochemical degradation[7, 8]. In fact, nanocrystals prepared using these routes were demonstrated to be usefulas fluorescent probes in biological staining and diagnostics [9]. Compared withconventional fluorophores, the nanocrystals have a narrow, tunable, symmetric emissionspectrum and are photochemically stable [9]. These nanocrystal probes are thuscomplementary to and in some cases better than existing fluorophores. Such nanoparticleswhen dispersed in a non conductive polymer matrix show improved long-term stability.This is due to the fact that the polymer matrix to some extent prevents interdiffusion ofinorganic substituents, which is observed to be a drawback in heterojunction-basedinorganic semiconductors.

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Conjugated polymers on the other hand derive their semiconducting properties by havingdelocalised π-electron bonding along the polymer chain. The π (bonding) and π*(antibonding) orbitals form delocalised valence and conduction wavefunctions, whichsupport mobile charge carriers. The classic model polymers which have been studiedextensively from this perspective of electrical transport are polyacetylene, PANI and PPy.Conjugated polymers such as PPV, PT and their derivatives have been studied for theirlight-emitting and photodetection properties, and can be classified as second-generationpolymers. They have recently been shown to be a viable option for large area displaydevices and image sensors [10-13].

The tunability of the properties can be achieved by attaching pendant groups to the mainchain or by control of conjugation length. There has been considerable effort in estimatingthe different lengthscales and the timescales of the different photo-induced excitationsprior to culmination as free carriers in these polymers. This knowledge is especiallyrequired to fully exploit the potential and obtain highly efficient photoactive materials[14, 15].

The concept of size-dependent effects has also been investigated in conjugated copolymerheterostructures, exclusively based on polymers without any nanoparticle component[16]. Theoretical studies of polydiacetylene-polyacetylene-polydiacetylene triblockpolymer chains led to prediction of results similar to those of multiple quantum wells ininorganic low-dimensional semiconductors [17]. Discrete split-off exciton states as wellas localisation of electronic states were predicted. Theoretical studies of various aspectsof the electronic and excitonic properties of quasi-one-dimensional semiconductingpolymer super lattices, which are periodic copolymers, have been reported [18]. Theelectronic structure was systematically studied as a function of copolymer compositionand block sizes, AmBn. However, the experimental efforts to test these concepts have notshown evidence of such quantum effects. Recent results have been reported on blends ofABA triblock conjugated copolymer, poly(2,5-benzoxazole)-block-poly(benzobisthiazole-2-hydroxy-1,4-phenylene)-block-poly(2,5-benzoxazole). The higher energy gap poly(2,5-benzoxazole) shows quantum confinement effects at room temperature based on PLemission and excitation, electric field-modulated PL and electric field and picosecondabsorption spectroscopy results [16].

This chapter focuses on the blends and multilayers of a variety of nanoparticles andconjugated polymers. However, it must be mentioned that there has been a large amountof research in the last two decades on nanocomposites of conventional polymers [19].Polymer nanocomposites in this context are generally defined as the combination of apolymer matrix resin and inorganic particles that have at least one dimension, i.e., length,width, or thickness, in the nanometre size range. Typical of this class of materials is thenanocomposite which researchers at Toyota Co., discovered in the 1980s: polyamide 6

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Nanoparticle-Dispersed Semiconducting Polymers for Electronics

(from caprolactam), with dispersed ion-exchanged montmorillonite as the reinforcement.Several benefits of such a nanocomposite that have been identified include:

• Efficient reinforcement with minimal loss of ductility and impact strength,

• Heat stability and flame resistance,

• Improved gas barrier properties,

• Improved abrasion resistance,

• Reduced shrinkage, and

• Residual stress.

The shapes of the particles used in nanocomposites can be roughly spherical, fibrillar orplatelets, and each shape will result in different properties. For maximum reinforcement,platelets or fibrillar particles would be used, since reinforcement efficiency is related tothe aspect ratio (length/diameter, L/d). The most extensive research has been performedwith layered silicates, which provide a platelet reinforcement [19].

The blends with conducting polymers go beyond the physical aspects and tap in to thequantum electronic processes. Dispersion of electron acceptor nanoparticles such as TiO2

in these second-generation conducting polymers has proven to be an effective methodologyfor enhancing photoinduced charge separation in a layered device [20]. Pristine conjugatedpolymers with absorption in the visible range do not have good photoconducting properties,primarily arising from the constraints in both the yield of free carriers due to the neutralexcitation as a dominant species and the low carrier mobility prevailing in these disorderedsystems. Since this neutral excitation can be dissociated at an interface between the polymerand an electron accepting species, charge separation is often facilitated via inclusion of ahigh electron affinity substance such as C60 [21, 22]. Device fabrication with composites ofconjugated polymers and C60 as the active layer, with efficient photoinduced charge transferpreventing the initial e-h recombination, was a significant advancement in this field [21,22]. A prerequisite for such an enhancement are materials with high electron affinity witha distribution in the polymer matrix such that the interparticle distance is of the order ofthe exciton diffusion length. In addition, the charge separation process must be fast enoughto compete with the radiative and non radiative decay pathways of the singlet excitonwhich is in the range of 100 ps - 1 ns [23]. The large surface area to volume ratio of thenanoparticle enhances the probability for these charge separation processes.

The first-generation conducting polymers have been used as the supporting matrix indifferent composites for intercalation of catalytically important nanoparticles so that thecatalytic activity can be retained in the composite. A few such hybrid materials havebeen synthesized by Qi and Pickup, who have incorporated Pt and PtO in a matrix of

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PEDOT as the conducting polymer component. This has been shown to work well as anelectrode of a fuel cell and its performance is comparable to the commercial carbonsupported catalyst at a higher Pt loading [24-26]. PPy and PANI have the ability toreduce and precipitate some metals from their respective acid solutions. PPy-SiO2 andPANI-SiO2 hybrid nanocomposites offer the advantage of a material with large surfacearea. These two important characteristics have enabled the above mentionednanocomposites to be an attractive host for entrapping a substantial amount of metalparticles with important catalytic activity and these metal-rich composites can be usedfor subsequent catalytic application, as postulated recently [27-29].

The nanocomposites of metal nanoparticles in a matrix of a passive non conductingpolymer matrix have a percolation threshold for transport when the nanoparticleconcentration reaches values high enough to provide conduction along the chain ofconnected nanoparticles [30]. At the threshold values of the filler concentration, theconductivity of the composite changes by several orders of magnitude analogous to themetal-insulator transition. However, it has been shown that there are subtle size-dependentpercolation thresholds, with the nanoparticle systems of sizes 5 nm and 12 nm havingmuch lower threshold values in terms of relative concentration compared to micron-sized nanoparticles [31]. Metallic levels of conductivity have been achieved in the pastby filling polymers with conductive particles (5-10 μm). However, the loadings requiredfor percolation are large enough to seriously compromise the weight and flexibilityadvantages of polymers, while intrinsically conductive polymers are not able to reachthe necessary levels of conductivity. The decrease in the loading density without sacrificingthe conductance behaviour of the composite can be achieved if the surface area of theunit weight of the filler is increased. On the other hand, insertion of insulatingnanoparticles (oxides) in an active conducting polymer matrix also leads to variousinteresting properties. Introduction of these insulating particles has shown to increaseconductivity in certain cases, such as PPy-ZrO2, PPy-Fe2O3, and PANI-TiO2. Modelsbased on macroscopic structural rearrangements and packing density have been used toexplain these properties. PPy and PANI nanocomposites using SiO2, latex bead andpolyethyelene oxide have been fabricated. Intensive efforts are also being made to develophybrid materials with appreciable magnetic and electrical properties, such as Fe2O3

containing doped PANI and PPy-silica coated magnetic Fe2O3 particles (5-30 nm) [34].However, at this stage, very few examples are available regarding the mechanism ofelectrical transport in the nanocomposites.

11.2 Material Preparation Methods

A brief summary of the synthesis adopted for nanoparticles, conjugated polymers andcomposites is discussed in this section. The last decade has seen an explosion of activity in

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the preparation of various kinds of nanoparticles. No attempt is made to exhaustivelycover the various strategies used for fabricating these structures but a few representativeexamples are provided. Common techniques for preparing semiconductor nanoparticlesinclude arrested precipitation of colloidal particles from homogenous solution by controlledrelease of ions or forced hydrolysis in the presence of surfactants [2-4]. For example,controlled mixing of Cd2+ with sulfide ions yields colloidal particles of CdS. Surfactantsare often used to stabilise the particles. For metal nanoparticles, chemical reduction ofmetal ions is a common approach. To narrow the particle size distribution resulting fromthe homogenous nucleation step, the colloidal suspension is typically fractionated usingsize-selective precipitation. A homogeneous monodispersion of semiconductingnanocrystallites has been obtained after pyrolysis of organometallic reagents by injectioninto hot coordinating solvents [35]. This method provides discrete nucleation and permitscontrolled growth of macroscopic quantities of the nanoparticles [35].

Physical approaches are also used for synthesis of metal oxide nanoparticles. In physicalvapour synthesis, a plasma is used to heat a precursor metal. The metal atoms boil off,creating a vapour. A gas is introduced to cool the vapour, which condenses into liquidmolecular clusters. As the cooling process continues, the molecular clusters are frozeninto solid nanoparticles. The metal atoms in the molecular clusters mix with oxygenatoms, forming metal oxides, such as aluminum oxide, smaller than 100 nm.

Another commonly used application-specific method is discrete particle encapsulation(DPE). In this method, selected chemicals are used to form a thin polymeric shell aroundeach nanoparticle providing the characteristics a user needs. Then a second thin-shellcoating is added, so the nanoparticle will disperse in the best needed format. This shellcontains spacer molecules that prevent the nanoparticles from coming into contact witheach other. The result is steric stabilisation for nanoparticles in non liquid solvents andpolymers, and electrosteric stabilisation for those needing to disperse in a fluid.

Methods for preparing soluble PPV and PT are standardised to a great extent and arecommercially available. The issue of structural homogeneity and purity of conjugatedpolymers becomes particularly important when the target structure is obtained as a difficultto characterise solid. An example which demonstrates these difficulties is the Wessling-Zimmermann route to synthesise PPV [36, 37], where the elimination of small moleculesfrom a so-called precursor polymer leads to a molecular structure with extended conjugation.Failure to perform this polymer-analogous reaction quantitatively will leave sp3 carboncentres within the chain, which would thus interrupt the π-system. Selectivity is a generalconcern within chemical synthesis that will, of course, raise questions at different levels ofsophistication. Oxidative coupling of 3-alkyl-substituted thiophenes leaves one with anambiguity since the coupling can occur in different fashions producing sequences such as2,5′ (head to tail), 2,2′ (head to head) and 5,5′ (tail to tail) [38]. More subtle procedures


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have been devised to regioselectively synthesise head-to-tail polyalkylthiophenes Manysuch routes for obtaining high purity polymers with regioregularity are now well established.

Inorganic nanoparticles can be introduced into the matrix of a host conducting polymereither by some suitable chemical route or by an electrochemical incorporation technique.An example of this route for hybrid nanocomposite materials is to polymerise aniline orpyrrole in the presence of some preformed inorganic particles, using an appropriate solubleoxidant. PPy (colloidal) gold nanocomposites have recently been introduced by Marinakosand others [39]. Although the work starts with template-guided polymerisation, it ultimatelyprovides template-free nanocomposite particles, tubes or wires. Gold nanoparticles werearranged within the pores of an Al2O3 membrane using a vacuum filtration technique.Fe(ClO4)3 is passed through the membrane and encounters rising pyrrole vapour withinthe membrane. PPy is grown within the pores of the respective film supporting the goldparticles and the Au-PPy composite grows inside. The template membrane can then be gotrid of by dissolving in a solvent [39]. PPy-coated gold nanoparticles were also synthesisedwithin the microdomain of a diblock copolymer, providing an excellent means of formationof such dispersions [40]. Diblock copolymers, owing to their ability to form microdomainsand to associate in solution in the form of micelles, can provide small compartments insidewhich particles of a finite size can be generated and stabilised [40]. Since this reviewessentially focuses on optical and electronic properties and not the structural or chemicalaspects of the hybrid nanoparticle-polymer composites, the reader is referred to a recentreview which exhaustively covers recent efforts from this perspective [41].

A crucial factor in processing films and the hybrid composites is obtaining homogenoussolutions before utilising the various options for deposition of these solutions [42]. Forexample, spin coating of a nanoparticle with poor dispersion properties in a polymersolution can produce a thin film of small, randomly ordered nanoparticle crystallitesinterspersed with amorphous areas. Introduction of stabilisers has been known to improvethe homogeneity of these systems. For example PNVC has been combined with SiO2 toform a nanocomposite in the presence of a polymeric stabiliser, PVP [43]. Key parametersfor the coating are concentration, solvent evaporation rates, and the spinning rates. Phaseseparation always exists for multicomponents but if the lengthscale over which this phaseseparation occurs is more of the order of the relevant lengthscales of the charge carriers/excitons diffusion processes, then the hybrid system can be considered to be fairly uniformfrom the perspective of charge separation strategies.

11.3 Photophysics of Charge Separation Nanoparticle-Polymer Systems

The mechanisms for charge generation and separation upon optical excitation and lightemission are different for the spherical nanoparticles and essentially linear chain polymers.

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Hybrid systems based on these different class of materials reveal interesting electronicand optical properties and add further insight into the individual characteristics of thedifferent components. The photovoltaic properties rely on efficient charge separationupon photoexcitation. Pristine polymers have a problem with large amounts of disorderand low mobilities, in spite of large absorption. Many of the properties of polymersystems will be examined with three different varieties of nanoparticles:

• High dielectric, insulating TiO2,

• Semiconducting CdS, and

• Metallic Au.

The various properties of these systems are explored and provide a glimpse of the richnessin terms of photophysical and electronic processes.

Charge generation and charge transport processes have been extensively studied in pristinePPV and its derivatives [44]. Based on the coincidence of the onsets of thephotoconductivity and absorption in MEH-PPV, it was concluded that photoexcitationof PPV leads to a direct generation of mobile charges through an interband π-π* transition[45, 46]. On the other hand, based on a variety of complementary experiments, it isargued that the photocurrent (Iph) originates from secondary processes, where the initialintrachain excitons dissociate to free charges [44]. These two points of view differ in tworespects. Whereas the band model [45] assumes a small binding energy for photoexcitedelectron-hole pairs and immediate charge separation, the exciton model requires largerexciton binding energy and extrinsic charge separation mechanisms [47]. Excitondissociation could arise from exciton dissociation at defect sites [48], field and thermalionisation [49], exciton interaction with trapped carriers, or exciton-exciton interaction.The evolution of Iph upon photoexcitation and the subsequent decay is, therefore, avaluable tool for exploring various processes and developing a deeper understanding ofphenomena such as electroluminescence in these systems. Extrinsic mechanisms are mostlikely to affect the relatively long (>nanosecond) transient photoconductivitymeasurements [50]. This relatively slow decay process is interpreted as being extrinsicdue to dissociation of polaron pairs (or interchain excitons) through interaction withoxygenated defects to create positive polarons. The slow component has been modelledin terms of a recombination limited dispersive decay.

A typical configuration for the studies of these properties involves an active layer (polymer,polymer-nanoparticle blend) in a sandwich configuration, as shown in Figure 11.1,between a transparent electrode and a metal electrode of suitable work function. ELrequires the injection of electrons from one electrode and holes from the other, the captureof oppositely charged carriers (so-called recombination), and the radiative decay of the


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excited electron–hole state (exciton) produced by this recombination process. Thecomplementary process of charge separation upon photoexcitation is evaluated on thebasis of the charge transport and transfer to the electrodes to constitute a photocurrent,which depends on the incident photon rate, the charge generation efficiency, therecombination rate and parameters which control the trap-limited transport in thesedisordered polymer systems. Routes for exciton and charge transfer processes in thesemixed systems are shown in Figure 11.2. In most cases, the polymer acts as the hole-transporting medium, with the nanoparticles as a sink for electrons. In addition it isnormally required to synchronise the charge generation rate, the transfer rate, the transittime to the electrodes (which is governed by the transport parameters) and the durationfor the decay process for the electron acceptors to come down to an uncharged state.

11.3.1 TiO2-Conjugated Polymer Composites

Studies of charge separation at the interface between organic molecules and nanocrystals,particularly systems of organic dyes adsorbed on TiO2 nanocrystalline films, as a basisfor efficient photovoltaic devices have generated considerable interest [51]. It has beenshown that in a nanocrystalline TiO2/PPV composite, excitons photogenerated in thepolymer could be dissociated at the interface between the components with the electronstransferred to the nanocrystals [52] (Scheme b in Figure 11.2). Considerable attentionhas been directed towards dye-sensitised nanocrystalline semiconductor films sinceO’Regan and Grätzel [51] reported a high-efficiency dye-sensitised solar cell.Nanocrystalline semiconductor films are highly porous, thus having a large internal surfacearea. Only the first monolayer of adsorbed dye results in efficient electron injection into

Figure 11.1 Typical device measurement for (a) electroluminescenceand (b) photocurrent

(a) (b)

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the semiconductor, but the light-harvesting efficiency of a single dye monolayer is verysmall. In a mesoporous film consisting of nanometre-sized TiO2 particles, the effectivesurface area can be enhanced about a thousand-fold theoretically, thus making lightabsorption efficient even with only a dye monolayer on each particle. Recent work ondye-sensitised solar cells is centred on ruthenium-bipyridine complexes sensitisingnanocrystalline TiO2 films, which has proven to be highly efficient in photon-to-electronconversion. However, in order to study the photoelectric conversion mechanism anddevelop a more efficient dye-sensitised solar cell, it is necessary to probe the sensitisationof other kinds of dyes, such as organic molecules, which are easily modified [53-55].

TiO2 particles have been traditionally interesting to study due to a wide variety ofapplications. TiO2 occurs as the mineral rutile, anatase, octahedrite, ilmenite, and brookite,and exhibits distinct size effects. Anatase TiO2 is used widely for welding-rod coatings,acid-resistant vitreous enamel, specific paints, etc. Below a critical size, TiO2 clusters can

Figure 11.2 Scheme for efficient charge separation in a nanoparticle-polymer composite(adapted from [13]): (a) absorption in the polymer is followed by an electron transfer

process to the nanoparticle (electron acceptor), (b) absorption in the polymer is followedby an exciton transfer process to the nanoparticle, followed by the hole transfer to the

polymer, (c) absorption in the nanoparticle is followed by hole transfer to the polymer [23]


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absorb the energy of UV light to release electrons and radicals by oxidation. So it is alsoused to protect against external irradiation and sunlight. The absorbed organic compoundson TiO2 clusters can be decomposed by oxidation due to the presence of a radical releasedby irradiation. Therefore, such nanoclusters of TiO2 are known as photocatalysts. Becauseof the unique properties, anatase TiO2 nanoclusters have high potential for applicationsin diverse areas of environmental purification, such as purification of water and air.TiO2 has also been studied for preparation of composites with conducting polymers,such as poly(3-methylthiophene) supported on TiO2, for solid-state photoelectrochemicaldevices [56]. Because of the combination of electrical conductivity of PANI and the UVsensitivity of anatase TiO2, such nanomaterials are expected to find applications inelectrochromic devices, NLO systems, and photoelectrochemical devices.

Another class of application of TiO2 dispersed polymers that has been explored is solid-state polymer laser diodes and photonic crystals. The high refractive index TiO2 particlescan scatter the emitted photons in the active polymer medium such that the gain exceedsloss above a critical excitation threshold. Scattering from the randomly distributed highrefractive index nanoparticles greatly increases the pathlength traversed by the emittedlight. Solid-state lasing has been observed from freestanding films of MEH-PPV andTiO2, with greatly reduced threshold pump powers [57]. The tunable photonic crystals,TPCs, consist of periodic particle arrays in which either the particles or a matrixcomponent is either optically or electrically tunable, thereby enabling tunability for theproperties of photonic crystals, particularly the width and position of the photonic bandgap. The basic PC will be a three-dimensionally periodic array of spherical nanoparticles,which are structurally similar to those of naturally occurring opals. Conducting organicpolymers have been heavily used as the tunable component. TPCs are expected to combinethe advantageous properties of conventional PCs with the tunability of such materials asconducting polymers, photoresponsive materials, and ferroelectric materials.

A significant property of nanocrystalline TiO2/PPV composites is that the excitonsphotogenerated in the polymer could be dissociated at the interface between thecomponents with the electrons transferred to the nanocrystals as shown in Figure 11.2.This feature has been exploited to form efficient photodiodes. TiO2, even in dilutequantities, acts as a charge separator, with the primary photogeneration and carriertransport essentially in the polymer backbone. TiO2 at such low levels of concentrationcan be introduced with a fair degree of uniformity in an MEH-PPV matrix as observedin the SEM images shown in Figure 11.3. Another important aspect which has notbeen addressed in these devices is that photodiodes are typically used in the reversebias in the high rectification ratio range for maximum sensitivity, and under these biasconditions the spectral range of interest should not be a limitation [20, 58]. In pristineMEH-PPV based devices, the photocurrrent spectral response Iph(I) in the reverse biaspeaks at the absorption edge, (indicative of an antibatic response) where Iph(I) is

351

asymmetric with respect to the absorption response, with practically no photocurrentin the absorbing region, and this was explained qualitatively on the basis of excitonquenching at the Al interface coupled with the low electron mobility of the polymer.Parameters such as exciton/free carrier diffusion lengths, barrier depths, and filmthickness come into play to model the spectra [58]. With the presence of electronacceptor moieties such as TiO2, the spectral range can be controlled by the magnitudeof the bias voltage [20]. The results also highlight important differences in the switchingresponse of the devices in the forward and reverse bias and its implication inphotodetector devices [20].

An Iph/Idark (photocurrent/dark current) ratio as high as 104 at a reverse bias of –4.5 V anda responsivity of ~50 mA/W at –9 V with a dark current of 10 nA has been observed inthese blended materials [23]. The open circuit voltage (as shown in the bottom part ofFigure 11.4) was in the range of ~0.8 V with a short circuit current density of ~5 nA/cm2

for a photon density of 10 μW/cm2. The intensity dependence of Iph for input power 0.1 μW/cm2 to 1 mW/cm2 is linear in the entire voltage range. In the case of TiO2 dispersed samples,the spectral features are sensitive to the magnitude of the reverse bias as shown in Figure11.4. TiO2 nanoparticles play the role of electron acceptors since the interparticle distances,with large surface area, are in the same order of magnitude as the singlet exciton diffusionlength (50 Å-150 Å) observed in MEH-PPV [58]. At higher absorption, λ < 3400 Å, wheree-h pairs are generated in TiO2 nanocrystallites, there may be a hole transfer process ontothe polymer from TiO2.

Figure 11.3 100 nm TiO2 particle uniformly dispersed in matrix of MEH-PPV


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(a)

(b)

Figure 11.4 (a) Spectral response of TiO2 dispersed in MEH-PPV in forward and reversebias. Note that the response at –3.3V is scaled down by a factor of 10. (b) Typicalphotodiode characteristic Iph – V, with Iph measured using the lock-in technique.

The lengthscales of the device, which may decide the spectral width of the differentregions, are the thickness of the polymer layer, l, 1/α(λ) where α(λ) is the absorptioncoefficient, the exciton/free carrier diffusion length, 1/β, and the barrier width, lb.

The bias dependendency of Iph(λ) can be qualitatively understood in terms of the modelby Ghosh and others [59]:

I ae e dx ae dxph

ax l lb xl lb axl lb

l .( ) ( ) ( )∝ ∫ + ∫− − − −− −−

β0

353

Iph in this model depends on the ability of the minority carriers (electrons generated inthe bulk) to reach (diffuse) the interface and is characterised essentially in terms of 1/βand lb. This argument is also applicable for excitons which diffuse and undergodissociation. The barrier normally in these sandwich devices is primarily at the metal-polymer interface and is estimated to be ~200 Å [58]. For illumination from the sideopposite the barrier, i.e., the ITO side, Iph results in an antibatic response; the numericalestimates of Iph(λ) for different diffusion length yield profiles which are remarkably similarto the experimental results [20]. It is seen that the experimental Iph(λ) at different voltagesin the reverse bias can be explained by the diffusion length having a stronger dependencyon the voltage than the barrier length within the framework of this model. An improvementin processing methods whereby a higher concentration of nanoparticles can be dispersedin the polymer matrix without a phase separation could lead to the realisation of the fullpotential of these systems for attractive technological applications.

Dye-sensitised TiO2 particles blended with different hole-transporting polymer matriceshave also been investigated [55]. The dye-sensitised layer acts effectively as an intermediaryfor the charge transfer processes. Host polymers with different HOMO levels have beenused to prove that the Iph enhancement requires hole transfer from the dye to the polymer[55]. TiO2 can also be introduced as a porous nanocrystalline layer into which the polymerpenetrates. A monolayer of TiO2 can be self-assembled onto the ITO surface usingactivation of the ITO surface with 3-aminopropyltriethoxysilane [53]. The forward-biascurrents and open-circuit voltages are determined by the conduction band energy ofTiO2 [53]. Apart from a single-layer film containing dispersed TiO2 particles, aheterojunction of dye-sensitised TiO2 with an amorphous organic hole transport material,2,2´,7,7´-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9´-spirobifluorine, has beendemonstrated to be an efficient solar cell with a high yield of 33% [60].

11.3.2 Nanoparticle Semiconductors-Polymer Systems

Semiconductor nanoclusters, such as CdS, CdSe and ZnSe, have also been demonstratedas active layers for emission and detection devices with the optical range of interesttuned by size selection of the nanoparticles. The nanoparticle has shown to reveal efficientphotocurrent [61] and EL properties with the spectral response governed by the particlesize. A clear systematic blue shift in the photocurrent onset with the decrease in theparticle size has been observed, see Figure 11.5a. Reports on a heterostructure devicewith an inorganic layer of CdS and a polymer light emitter show the advantage of thehigh charge transporting properties of an inorganic semiconductor and the enhancedemission efficiency of the polymer emitter [62]. It was found that the work function ofthe nanoparticles could be matched with that of the metal contacts improving the electroninjection efficiency and it was also observed that the emission colour governed by the


354


recombination zone could be controlled by the magnitude of external voltage. At lowvoltages, emission from the nanoparticle layer occurs, and at higher voltages the emissionfrom the polymer was reported to predominate [62].

The possibility of synergistically combining the efficient photoconducting, PC propertiesof the inorganic component and the EL properties of the polymer in a single device wasexplored recently [63]. Results on such hybrid devices, where photoconductivitycharacteristics are dominated by the quantum size of the inorganic semiconductor andthe enhanced EL spectral feature is attributable predominantly to the polymer component,were reported [63]. The potential of such devices, where windows for the detection andemission can be selected by nanoparticle-size engineering and/or modifying the chemicalstructure of the polymer, was demonstrated [63].

The EL response of such hybrid devices can essentially highlight the emissive polymercomponent, for example, a maximum at 6200 Å, 5400 Å, and 5250 Å for CdS-44/P3HT(polyhexylthiopene), CdS-44/MEH-PPV and CdS-44/6FPBO (6-fluoro poly(benzoxazole-bioxydecyl)) devices, respectively. (CdS-44 = 44Å CdS nanoparticle.) The EL spectra of

Figure 11.5 (a) Nanoparticle photocurrent spectral response for different particle sizes,and (b) Typical electroluminescence response of 6FPBO and MEH-PPV

(a)

(b)

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the pure polymer devices are shown in Figure 11.5b. The EL spectra in the multilayerhybrid devices are closer to the pure polymer responses as shown in Figure 11.5 andFigure 11.6 with efficiency nearly an order of magnitude higher than CdS-only devicesand polymer-only devices. 6FPBO is an electron transporting emitter [64] while MEH-PPV and P3HT are predominantly hole transporting. The recombination in the 6FPBOdevice is closer to the 6FPBO-hole injecting layer interface with a minimum trap emissioncontribution from the nanocluster layer, resulting in a sharper emission.

It has been observed in LED devices based on nanocrystals, which have high band-edge PLyield, that the insertion of a hole transporting polymer layer enhances the nanocrystalband-edge emission characteristics [62]. In the present case the results can be interpreted interms of the CdS-44 layer enhancing the polymer emission characteristics. It is expectedthat the absorbance and the emission from the polymer layer will modulate the emissionfrom the CdS-44 layer. The issues of PL quenching, charge separation and transport innanocrystal CdS-44/MEH-PPV composite blends have been studied in detail [23, 65]. Theresults correlate quenching with improved quantum yield for charge separation. In thepresent case of bilayers with completely segregated phases, the possibility of Förster tranferof the exciton to the nanoparticles will be effective only at the interface leading to a possibleemission quenching. Initial results on PL with excitation energies corresponding to that ofthe polymer in CdS-44/P3HT do not show substantial quenching effects. Quenching effectscan be minimised by reducing Förster transfer rates, by choosing polymers of appropriateelectron affinity values and by using suitable capping materials for the nanocrystals. TheEL efficiency can also be enhanced by optimising device parameters, such as thickness,with prior information on values of mobility and electron-hole pair diffusion length, tohave radiative recombination away from the interface [63, 65].

In stark contrast, the short circuit photocurrent spectral response in all the three devicesis identical to that of a CdS-44 layer independent of the type of the polymer. The Iph(λ)and EL response of the CdS-44/P3HT device is shown in Figure 11.6. The Iph(λ) responseshown is representative of any CdS-44-based hybrid devices while the EL(λ) response isrepresentative of any P3HT based hybrid device. The polymer signature is observed inthe photocurrent, but the Iph(λ) in the non absorbing region of CdS-44 and absorbingregion of the polymer is less than 3-4 orders in magnitude than the Iph(λ) in the CdS-44absorption region. A significant separation width of nearly 2000 Å (2.0 eV) in the visiblerange between the emission maxima and the photocurrent region is observed for theCdS-44/P3HT device as shown in Figure 11.6. The external zero bias photocurrentefficiency in all the hybrid devices is in a similar range as that of pure nanoparticledevices with a responsivity of ~100 mA/W. It is expected that if the polymer thickness isincreased to be comparable to the absorption depth, the spectral response would bemodified. The large photocurrent primarily reflects the high efficiencies commonlyobserved in CdS-based detectors due to high absorption and efficient charge carrier


356


Figure 11.6 Depiction of a bifunctional device with a spectral window for detectionand emission controlled by independent components with photocurrent and EL of amultilayer device ITO/PVCZ/P3HT/CdS/A1: The nanoparticle size is 4.4 nm, which

corresponds to a band edge of ~450 nm.

357

separation. The novelty of such a dual functional bilayer device with independent controlof spectral windows for emission and detection response is depicted in Figure 11.6.

The tunability of a Shottky diode based on a semiconductor/conjugated polymer (doped)interface has been explored electrochemically manipulating the work function of theconjugated polymer [66]. In the above case, the work function along with the chargecarrier concentration and the nature of interface decide the LED characteristics.

Another concept that has been explored is that of ‘diffused’ and ‘fractal’ p-n junctions innanoparticle-polymer composites with both p- and n-type nanoparticles. Diffused junctionsare obtained upon converting p-type particles into n-type. One can obtain an area consistingof pure p-type and pure n-type particles close to the electrodes, with an intermixed regioninbetween the electrodes. Peculiar effects are observed in the intermixed layers since thenanoparticles sizes are similar or smaller than the Debye screening length [30, 31].

Results similar to that of the TiO2 dispersed MEH-PPV have also been observed forCdSe dispersed in MEH-PPV [23, 64]. In this case, a clear case of PL quenching is alsoobserved as an evidence of charge transfer. Results have shown that the electron affinityof even the smallest CdSe nanocrystals is sufficient to allow electron transfer from MEH-PPV. A general rule of thumb, which has been followed to enhance transfer process, isEa

nanocrystal – Eapolymer > Upolymer – Vcharge transfer, where Upolymer is the binding energy of the

singlet exciton on the polymer and Vcharge transfer is the coulombic energy associated withattraction between the electron and hole in the final, charge separated state. At highconcentration of the nanocrystals, where both the nanocrystals and polymer componentsprovide continuous pathways to the electrodes, quantum efficiency up to 12% has beenreported [64].

11.3.3 Gold-Polythiophene Blends

Another class of materials, alkanethiol-stabilised metal nanoparticles, display electronic,optical and structural features that are tunable via particle size [67]. The theme of thissection is to demonstrate the effects of interfacial chemistry and material heterogeneityon electronic and optical properties of luminescent conjugated polymers at metalinterfaces.

Hybrid systems comprising of these materials reveal several interesting features. Reportson blends of gold nanoparticles and conducting oligomers have demonstrated conceptssuch as the construction of mesoscopic oligomer bridges between metal nanoparticles[68]. Gold nanoparticles have surface reactivity amenable to immobilisation at chemicallyfunctionalised surfaces and can bind to positively charged polymers via electrostaticinteraction [69] or covalently to amine, thiol and phosphine functional groups [70]. The


358


electronic structure of the polymer chains is also expected to be strongly perturbed in anenvironment of these metal nanoparticles. The striking features in this case are the complexstructures that spontaneously appear upon film formation of gold nanoparticles andthiophene-based polymers [71, 72]. Fluorescence centres of ~1 μm diameter, containingthe pure polymer, are dispersed throughout the film, see Figure 11.7. At the periphery ofthe fluorescent centres, a blue shift of ~120 nm in the fluorescence is observed, tunablevia the gold nanoparticle concentration.

The different component-specific features present in the film can be distinguished by thecontrasting PL properties along with the direct TEM observation of the nanoparticles. Thesource of the PL in the polymer region is known to arise from the radiative decay of theintrachain singlet exciton level. Fluorescence microscopy and near-field scanning opticalmicroscopy (NSOM) have been used to study the PL variation, with TEM for morphologicalstudies of the phase separated system. NSOM is used to acquire spectra from regions ofsize less than that of the wavelength of the light source employed and the results demonstratethe capability of this technique to differentiate the components in the sub-micron lengthscaleof these multiphase systems. In fact, Au-P3OT (polyoctylthiophene) systems can be viewedas model systems for probing using NSOM due to the spatial gradient of the spectral shiftsoccurring at nanometre lengthscales. The shear-force feedback technique in NSOM providesa topographic image in parallel with the optical image.

A typical confocal fluorescence image of the Au-P3OT blended film is shown in Figure 11.7.Randomly distributed circular features with sharp intensity contrast are present throughoutthe film, indicating distinct phases. The circular features are essentially a signature of the

Figure 11.7 Confocal fluorescence microscope image of gold nanoparticles dispersedin P3OT

359

differences in the evaporation/dewetting rates of the two systems during the film formation,resulting in the creation of local domains. The emission of these circular regions resembledthe emission from a pristine polymer film which has a PL maximum centred at ~700 nm.

NSOM fluorescence images are consistent with the simultaneously obtained topographicimage for these PT/gold nanoparticle films. As the proportion of polymer is increased, thesize of the circular regions increases and the contrast between the bright and dark regionsdecreases. It is also to be noted that these polymer/Au nanoparticle film patterns, as shownin Figure 11.7 and Figure 11.8 are obtained only with polythiophene derivatives, such as

Figure 11.8 NSOM fluorescence contrast image (1 μm x 1 μm) of a bright spot/sphereon the Au blended P3OT film (a) along with the emission spectra at different distances

away from the centre of the spot (b)

(a)

(b)


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P3OT and P3HT. The PPV-based polymers such as MEH-PPV do not show such distinctphase formation. The high density of the nanoparticles in the periphery can be explainedby the non uniform evaporation rate and a finite mobility of the nanoparticles, which getpinned at the rim. Another factor which comes into play is the aggregation of polymerchains, which can act as an effective drag on the nanoparticle motion during the dryingprocess, giving rise to the inhomogenous radial distribution of the nanoparticles. The ring-like structures have been observed in alkanethiol-coated gold nanoparticles [74]. Thenanoparticle mobility in that case was high enough to allow most particles to accumulatein the ring during the receding process of the contact line created by the interface betweenthe solvent and the substrate [74]. Spatially-resolved PL spectra are obtained with the fibretip held at several positions in the vicinity of a typical fluorescent centre of the blendedsystem [72]. The emission spectra from different regions are shown in Figure 11.8. Spectraare blue-shifted as the tip moved from the centre towards the periphery. The PL from thesefluorescent centres can be interpreted in terms of a crystalline (maximum ~700 nm) and anamorphous phase (maximum ~565 nm) of the polymer. The macrophase separation ofthese nanoparticles and the polymer in the films arises due to the non formation of ahomogenous solution. An important issue which needs to be resolved is the determinationof whether the electronic structural changes of the polymer in the blended system aresecondary effects caused by a nanoparticle-induced physical process such as rearrangementof the polymer chains or by a nanoparticle-specific, weak metal-polymer interaction as thedriving factor for the appearance of different phases [72].

It is noted that a typical nanoparticle stochastically dispersed in a polymer forms a systemwhich is disordered. These disordered systems are generally parameterised by the presenceof a percolation threshold, a change in structure and the dependence of topology oninterconnected chains. In terms of optoelectronic properties, the entire advantages ofamorphous silicon technology are applicable here with a much greater flexibility, a farwider range of control and better functionality, such as spectral distribution, switchingspeed and substrate choices.

11.4 Summary

In summary, nanoparticles and polymers can form two distinct components in a compositewith a diverse set of properties. The origin of these properties is distinctly different in thetwo components. One can tailor properties in composites by exploiting the variousattributes. Nanoparticle-polymer composites demonstrate a synergistic approach toenhance efficiency in a variety of phenomena and in certain instances to achieve a completeset of novel properties. Considerable efforts to sort out issues such as processibility andstability should lead to the use of such combinations of materials as potential routes tomany nanotechnology-based device applications.

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Acknowledgements

K.S. Narayan thanks A.G. Manoj, Th.B. Singh, A. Alagiriswamy, G.L. Murthy, N. Kumar,K. Vijaya Sarathy and collaborators - Professor D.D. Sarma, Dr. J.W. White, Dr. R.J.Spry, Professor S. Ramakrishnan and Professor C.N.R. Rao. He also acknowledges theDepartment of Science and Technology and Council of Scientific and Industrial Research,Government of India, for partly funding the project.

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12 Polymers for Electronics

T.H. Richardson

12.1 Introduction

Electronics at the beginning of the third millennium is still massively dominated bytraditional inorganic semiconductors, metals and ceramics. However, the last 20 years,and in particular the last decade, have seen enormous developments in the use of organicmaterials that can process electric charge or photons [1-4]. In one area at least, that ofelectroluminescent light-emitting diodes, organic materials are poised to make a hugeimpact on consumer electronics. This has arisen as a result of three main factors:

• The global research effort towards the realisation of cheap, lightweight, flat-panel,flexible, low power displays,

• The insatiable consumer demand for ever-increasing visual display quality, and

• The information technology revolution that has seen the enormous increase in the numbersof computer monitors, mobile phones and paging devices in the last 5 years alone.

It is noteworthy here, at the beginning of this chapter, to mention the awful truth aboutlight-emitting polymers – their discovery was entirely serendipitous! Whilst measuringthe breakdown voltage of a conjugated polymer [5], researchers noticed the emission offaint green light from their device. So was born the modern era of organicelectroluminescence. More surprising still, perhaps, is the fact that organicelectroluminescence was actually first observed as early as 1963 in single crystals ofanthracene [6]. Whilst certainly taking the limelight, luminescent organic compoundsare by no means the only group of materials that are continuing to attract researchinterest in electronics and photonics.

The most used organic component in the electronics industry has been around for severaldecades already: the humble polymer photoresist [7]. This is the shy, retiring partnerthat is essential for the success of every integrated device in every circuit board. As devicefeature size has gradually decreased since the invention of the first transistor, refinementsin photoresist technology have had to keep pace [8-9]. However, the real excitement willbegin in the next decade or two when polymer transistors become commonplace, andwhen these transistors eventually comprise only a few tens of molecules! In a world in

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which a surgeon can transplant hearts, lungs, livers and kidneys, not to mention therecent arm transplants, the idea of molecular-scale transistors and other devices is reallynot far-fetched.

This chapter will include an introduction to some of the topics mentioned above withthe intention of giving the reader some insight into the world of molecular electronics. Itis less meaningful to discuss complex theories and device designs without devoting somethought to the deposition techniques available for the fabrication of polymer membranesfor electronics devices. Ordered polymer layers of well-defined thickness and orientationdo not appear by magic and so this chapter is closed with a survey of the fabricationtechnologies at the disposal of the organic device engineer.

12.2 Polymer Electroluminescence

Liquid crystal materials [10] currently dominate the electronic display market since theyare cheap and easy to produce and are extremely reliable passive devices. They produceno light of their own but utilise ambient light that is reflected from the displays in whichthey are housed. Whilst such displays are extremely effective in a wide range of single-user applications (e.g., wristwatches, lap-top computers, miniature TVs, video cameradisplays), there are two major drawbacks for multi-user applications (e.g., full-size TVs,bulletin boards), because they process incident light from the surroundings, their brightnesseffectively is dependent on ambient lighting conditions (except for slight enhancementsmade possible with backlighting), and the display image quality is highly dependent onthe viewing angle of the observer.

Polymer electroluminescent devices [11] offer the possibility of efficient full-colour, low-voltage displays with an improved brightness and viewing angle dependence over liquidcrystal displays. Such devices contain several layers of organic materials interspersed betweena metal cathode and a transparent (most often ITO) electrode to allow the luminescence toexit the device. Arguably the most important breakthrough in organic LEDs came in 1987when Tang and co-workers [12] used monopolar charge transporting layers alone or eitherside of an emitter layer, as depicted in Figure 12.1. Although much of the early organicLED work focused on low molar mass organic molecules [13-16], researchers at Cambridgemade the serendipitous discovery that the conjugated polymer PPV was electroluminescent.Whilst studying the insulating properties of the polymer in an Al-polymer-Al deviceconfiguration, they noticed green light emerging from the device. They subsequently realisedthat one of their electrodes had partially oxidised and was acting as a good hole-injectingcontact. Thus holes were being favourably injected at one side of the polymer and electronsfrom the metal contact. Radiative recombination was occurring within the PPV, and theresearch field of polymer electroluminescence was born [17]!

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Polymers for Electronics

The advances since this first discovery are summarised in Figure 12.2 which indicates howthe drive voltage has been gradually reduced from around 100 V to a few, whilst the luminousefficiency has increased to around 100 lm/W. Many advanced display devices are expected tobe marketed during the first decade of the new millennium, but the Pioneer Corporation wasthe first to produce a working 64 x 256 pixel device commercially, in 1996 [18].

Figure 12.1 Typical organic electroluminescence device configurations showing thearrangement of electrodes, carrier transport and emitting layers. (ETL and HTL refer

to electron and hole transporting layers, respectively.)

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The simple two-layer device shown in Figure 12.1a contains two organic layersbetween a metal cathode and an optically transparent anode. The cathode is normallya low work function metal such as calcium, magnesium or aluminium. Calcium andmagnesium, and to a lesser extent aluminium, are relatively unstable in air and oxidisequite rapidly leading to short device lifetimes. This problem is overcome in the researchenvironment by carrying out device testing in an evacuated glove box with very low

Figure 12.2 (a) Increase in luminous efficiency and (b) reduction in drive voltage sincethe beginning of research into organic electroluminescence

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levels of water vapour and oxygen (typically sub-ppm). Clearly, efficient encapsulationof devices will be extremely important for obtaining long-life, stable operation incommercial devices. The almost universal anode material is ITO owing to its opticaltransparency and high conductivity. Furthermore it can be easily patterned usingstandard photolithography and a hydrochloric acid etchant.

The application of a voltage across the device results in the injection of electrons andholes (at the electrodes) which then move towards the centre of the device. Excitedstate organic molecules are formed as a result of the recombination of electron-holepairs; visible light is emitted as these excited species decay to their ground state. Thewavelength of the emitted light depends on the region in which recombination takesplace, which in turn is governed by the respective electron and hole mobilities of theelectron-transport and hole-transport layers. A typical set of device characteristicsfor a device containing an emitting polymer material is shown in Figure 12.3. Thecurrent-voltage characteristic is similar to that of a traditional diode except that theturn-on voltage is much higher. The luminance output of the device is seen to beproportional to the current (density) through the device and the electroluminescencespectrum indicates that the emitted radiation is green in colour and is relativelybroadband. A selected range of emitter materials as a function of the peak emissionwavelength is shown in Figure 12.4 and demonstrates that polymers are availablefor red, green and blue light outputs. Also shown are some common low molar massmaterials that have been developed in parallel with their polymeric counterparts. Itis tempting to believe that organic molecules exhibiting high photoluminescentefficiencies in solution (the often preferred screening medium) will therefore be usefulcandidates for electroluminescent devices. Although this is sometimes true, there aremany examples of little or no electroluminescence being produced. This is due toconcentration quenching in the solid state through the formation of exciplexes and/orquenching due to interactions with oxygen or the electrode materials [19].

One disappointing feature of most electroluminescent polymers is that the emissionspectrum tends to be relatively broad (full wave half maximum (FWHM) ~100-150nm). Ideally for display applications, high colour purity is desirable. Severalapproaches to sharpening the emission spectrum of electroluminescent light outputhave been made. These include the use of rare earth containing materials in order toutilise the atomic levels in the metal ions themselves as the electronic transition sites[20-21]. More success, however, has been achieved using microcavities in which bothspectral sharpening and luminance enhancement can be achieved simultaneously [22].A typical microcavity structure comprising a dielectric mirror at one end of the usualdevice configuration is depicted in Figure 12.5. It must be noted here that the thicknessof the transport and emitting layers are crucial in tuning the peak emission wavelength


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Figure 12.3 Device characteristics for a polymer light-emitting device showing (a) thecurrent-voltage characteristic (open circles) and the luminance-voltage characteristic

(filled squares) and (b) the absorbance (Abs) and photoluminescence (PL) spectra. Theelectroluminescence is very similar to the PL.

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Figure 12.4 A selected range of organic emitter materials as a function of the peakemission wavelength

and intensity. Spectral bandwidths have been reduced typically to 25 nm using thisapproach. One present difficulty is the angular dependence of the emission wavelength;the apparent colour of the emitted light changes slightly depending on the viewingangle as a result of the modified pathlength of the cavity. One possible way ofovercoming this problem is to move from a planar to a radial geometry in whicheach layer of the device is deposited onto a substrate containing an array ofhemispherical lens-type features.

The future for electroluminescent devices is certainly bright.


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Figure 12.5 (a) Typical microcavity-based device structure for colour purityenhancement and (b) the resulting narrow spectral output

(b)

(a)

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12.3 Conduction in Polymers

Conjugated polymers have attracted great interest for a number of devices includinglight-emitting diodes, photovoltaic cells, field-effect transistors and photocopiers [23].The common property linking all these applications is that the polymer must be able tosupport the transport of electrical charge and accept or relinquish this charge at its interfacewith other media. The density of this current, J, is given by:

J = q {n(x)μn(E) E(x) + Dn δn(x)/δx} (12.1)

where q is the electronic unit charge of the carriers involved in conduction, n(x) is thedensity of the carrier, μn(E) is the carrier mobility, E(x) is the electric field and Dn is thecarrier diffusion coefficient. This equation is shown for a single carrier but in thegeneral case where electrons and holes are involved in the conduction process, termscan be added such that both carriers are represented. Equation 12.1 shows that thecurrent density is spatially dependent but the steady-state bulk conductivity, σ, isindependent of position:

σ = J/E = q n μn (12.2)

The density of the charge carriers and their mobilities therefore determine the conductivityof a material.

The majority of conjugated molecular materials have energy gaps in the range 1.5-3.0eV and are thus viewed as wide band gap semiconductors. They are very different totraditional inorganic semiconductors, the main difference being their much lower carriermobility (typically 10-5-10-6 cm2 V-1 s-1 compared to 103-104 cm2 V-1 s-1 for GaAs). Insilicon, for example, the atoms are held together by covalent bonds formed by theoverlapping electron sp3-hybridised orbitals. These electrons are delocalised resulting ingiant extended ‘orbitals’ referred to as ‘energy bands’ that are responsible for givingcharge carriers their freedom to move easily through the solid under the influence of anelectric field. For a small organic molecule, the electrons do not have it so easy. Carbon-carbon single bonds (σ bonds) form as a result of the overlap of two sp3 orbitals, onefrom each carbon atom. The electrons in these bonds are strongly localised between thetwo atom centres and cannot effectively participate in any conduction process. Carbon-carbon double bonds, on the other hand, involve a 2pz orbital from each carbon atom aswell as two overlapping sp2 orbitals. The overlapping 2pz orbitals are referred to as a πbond; the σ bond and the π bond together are termed the ‘double bond’. Electrons in πbonds are much more delocalised and can roam over an extended region far beyond thevicinity of the σ bond. In a conjugated molecule, an alternating sequence of carbon-carbon single and double bonds exists; this leads effectively to another giant (although


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not as giant as in the case of silicon) orbital stretching along the length of the conjugatedregion of the molecule. This region is effectively a ‘cavity’ along which π electrons canmove relatively easily and contribute to conduction. As will be mentioned later, however,the conductivity of a conjugated material is not merely governed by the length of themolecule; indeed, the main bottleneck to the process arises from the large energy barrierfor intermolecular charge transport. The energy bandgap reduces as the length ofconjugation increases as seen in Figure 12.6. This explains why small organic moleculesare usually colourless (appearing as white crystals due to scattering) whereas longerconjugated molecules are coloured; hence carrots are orange and grass is green due totheir principal component molecules being β -carotene and chlorophyll, both moleculeshaving regions of extended conjugation [24]. As the conjugation length increases, theenergy difference between the HOMO and the LUMO decreases according to:

Egopt (n) ~ Eg

opt (infinity) - K/n (12.3)

where Egopt (n) is the band gap (optical exciton or carrier energy gap) for a conjugated chain

of n repeat units, Egopt (infinity) is the energy gap for an infinitely long conjugated chain and

K is a constant. It should be noted, however, that very long conjugated chains twist, bend anddistort quite easily; this disrupts the π-electron system so the molecule is said to possess aneffective conjugation length that is dependent on the particular polymer chain environment.

Figure 12.6 The effect of conjugation length in polymers on the optical exciton gap

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Electrons and holes on conjugated polymer chains transfer from chain to chain (moleculeto molecule) in response to an applied field at a rate that determines the carrier mobilityand hence the conductivity of the material. Many models have been developed to explainthe detailed conductivity mechanisms for molecular materials but their description isbeyond the scope of this chapter. The interested reader should refer to the excellentreview by Campbell [25]. High conductivities are usually achieved by doping the polymerin order to add carriers that will subsequently be able to move through the polymermaterial under the influence of an electric field. If an electron accepting dopant is used,then the p-type doping results in oxidation of the polymer and the formation of a positivepolaron, also known as a radical cation. Further doping results in a positive bipolar orradical dication. This process is shown in Figure 12.7 for PPP. These polarons or bipolaronscan move along the polymer chain relatively easily and can hop from one polymer chainto another as a result of redox (polaron-transfer) reactions between neighbouring chains.The bulk electrical conductivity is primarily dependent on the product of the numberand mobility of the charge carriers. Doping increases the carrier density, long (but nottoo long) conjugation regions enhance the intrachain transport, and alignment of thepolymer chains (by stretching or poling for example) augments the interchain transportwhich is usually the limiting step of the conduction process.

Figure 12.7 Polaron and bipolaron formation as a result of doping


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Even when doped, most ‘conducting’ polymers do not show metallic-level conductivity.An exception is polyacetylene whose conductivity has been measured to be as high as~105 S cm-1 [26]. PPV is commonly labelled as a conducting polymer but when undopedtypically has a conductivity of around 10-14 S cm-1 and is insulating. Most conductingpolymers have conductivities in the range 0.1-1 S cm-1 when fully doped [27]. A selectedrange of well-known conducting polymers is shown in Figure 12.8. The research field isadvancing rapidly and this selection represents only the best-known families of polymersrather than the most conducting individual examples.

Figure 12.8 Well-known conducting polymers

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12.4 Molecular Electronics

It is important to understand the driving forces behind the elucidation and developmentof the optical and electrical properties of polymeric materials. These are varied and fallinto two main categories: low-technology and futuristic high-technology opportunities.Imagine being able to produce large, thin sheets of a highly conducting polymer exhibitingconductivity comparable to that of copper. There are many applications for such a material,the most obvious ones being coatings for electromagnetic shielding (instead of usingexpensive, heavy metal boxes) and electronic circuit interconnecting tracking (to replacethe miles of copper or gold currently used). Speculate further about circuits in whicheach silicon transistor is replaced by a polymer version; again this is not farfetched.Organic field electron transistors (FETs) have been developed in research groups alreadywith mobilities around 0.1-0.5 cm2 V-1 s-1 [28-30]. Their performance is poor comparedto silicon transistors but it must be noted that it took just 40 years to progress from thediscrete silicon transistor to a global microelectronics revolution in which the transistorgate geometry is currently 0.18 μm and photolithography occurs using deep ultravioletradiation of around 190 nm.

The fascination with organic electronic devices stems from the lure of miniaturisation.Consideration of the gradual reduction in transistor dimensions since the 1960s suggeststhat molecular scale transistors may be achievable shortly. Researchers have minimisedthe z-dimension (thickness) for devices such as unimolecular rectifiers [31]. Metzger’sand Ashwell’s research in this area has shown that charge-transfer TTF:TCNQ-type(tetrathiafulvalene:tetracyanoquinodimethane) molecules indeed show directionalconduction [32-33]. Little progress has been made in shrinking the x- and y-dimensionsto similar values to the z-dimension (~2-10 nm) in order to achieve a truly molecularscale device. Furthermore, the question of making contacts at this scale has received littleattention. In the humble opinion of this author, however, the switch to organic-basedelectronic diodes and transistors is unlikely; the more probable event will be the inventionof completely novel electronic devices that utilise complex biological molecules as theiractive ingredients. Computing based on DNA [34] is a growing discipline far removedfrom present-day electronic circuit approaches.

12.5 Polymer Deposition Technologies

As mentioned earlier, it is misleading to describe advances in the properties of polymerswithout also describing how such polymers are processed into thin films or crystals.Moreover, many of the physical properties of polymers are inextricably linked to theirstructural and orientational order. An excellent example of this is the piezoelectric andpyroelectric coefficients of the well-known polymer, polyvinylidene difluoride (PVdF)


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[35]. When this polymer is processed into thin sheets, these coefficients are close to zero,yet after stretch aligning (which causes the ordered alignment of the long polymer chains)the coefficients rise to the highest known values for organic materials, rendering PVdFextremely useful for pressure and heat sensing applications.

Whilst single crystals of polymers can sometimes be grown, the most common geometricalconfiguration of a polymer in an electronic device is a thin planar film. This results fromthe advanced planar microelectronics and integrated optics technologies that have grownup over the last 30 years. A number of techniques exist for the deposition of polymers inthin film form, the most well-known and well-used method being polymer spin coating.Spin coating is used most often for depositing layers of photoresist as part of the patterningprocess in integrated circuit fabrication. This simple, yet effective, technique is illustratedin Figure 12.9. A relatively viscous polymer solution is placed onto the substrate to becoated which is then rotated at a fixed angular speed in the range 500-4000 rpm.

Figure 12.9 The polymer spin-coating process

The polymer solution flows radially outwards to form a thin solution layer that subsequently‘sets’ as the solvent evaporates. The uniformity of the layer depends on a number of factorsincluding the initial acceleration of the substrate and the rate of solvent evaporation, bothof which can be easily controlled. The film thickness, d, depends on the solution viscosity,η, rotation speed ω, solution density ρ and spinning time t and is given by:

d = {η /4π ρ ω2 t}1/2 (12.4)

More involved models to relate the thickness to the spinning conditions have beendeveloped [36]. Generally, the polymer molecules within spin-coated films are relativelydisordered and order has to be induced after deposition via stretch aligning or morecommonly via electrical poling at elevated temperature [37].

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A favoured technique by those who require highly ordered or partially ordered polymerfilms is LB deposition [38]. An LB film is an ultrathin organic assembly formed by thesequential transfer of floating Langmuir layers from a water surface onto a solid substrate.In its conventional form, it comprises two-dimensional solid sheets of well-packed organicmolecules deposited in a predetermined, controllable sequence. The first materialsinvestigated by Pockels, Langmuir and Blodgett were generally rod-shaped amphiphilicmolecules such as octadecanoic acid [39]. There is today a huge variety in the shapes andtypes of molecules that can be deposited as LB films, many of them being polymers [40].

Before the LB film can be fabricated, a floating Langmuir film must first be created at anair-water interface. Octadecanoic acid (Figure 12.10a) is an amphiphilic molecule, whichillustrates the LB process nicely. The polar carboxylic group is attracted to other polarsubstances such as water, whereas the long alkyl chain is hydrophobic and is repelled bywater. This amphiphilicity is responsible for giving octadecanoic acid its uniqueorientational properties; the polar headgroup is effectively dissolved in water, yet thealkyl chain protrudes from the water surface in a near normal direction. The carboxylfunctionality is so strong that many alkanoic acids will spontaneously spread from abulk crystallite placed in contact with a water surface. Usually, however, the material isdissolved in a solvent such as chloroform, minute droplets (each containing ~2μl) ofwhich are then dropped carefully on the water surface. The solution spreads over theavailable water surface rapidly, the solvent evaporating fully over a period of a fewminutes, leaving behind the randomly distributed octadecanoic acid solute molecules.Indeed, even at this point in the procedure, a monolayer film exists, albeit aninhomogeneous one. Uniformity in the surface density and thickness of the monolayer isimproved by now reducing the water surface area available to the floating solute molecules,thus compressing the initially expanded monolayer to form eventually a closed-packed,two-dimensional sheet of octadecanoic acid. Typically its molecular surface density inthe compressed state is ~5 x 1014 cm-2 and its thickness is ~2.5 nm.

In practice, an apparatus known as a Langmuir trough is used to perform the compression.There are many different commercial designs of Langmuir troughs available, the mostwell-known being the NIMA [41] version. Each provides a means of confining the floatingmonolayer within an accurately controlled surface area that can be varied from amaximum value (at which the spreading process is usually performed) to a minimumvalue corresponding to maximum compression. After spreading and evaporation of thesolvent, the area available to the monolayer can be continually reduced in order togradually compress the molecules. The presence of a full or partial layer of molecules ata pure water surface dramatically modifies the surface tension. A change in the surfacetension, known as surface pressure, Π, can be measured easily using a sensor that probesthe water surface. A highly wettable plate (known as a Wilhelmy plate) is suspendedfrom the microbalance using a fine, light and inextensible thread. A surface pressure-


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Figure 12.10 (a) Octadecanoic acid, a typical amphiphilic molecule, (b) a schematicsurface pressure-area isotherm for octadecanoic acid and (c) the orientation of

octadecanoic acid molecules in a monolayer

(a) (b)

(c)

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area isotherm, which describes the change in surface tension of the water surface as thefloating monolayer is compressed, is shown for an alkanoic acid in Figure 12.10b.Inspection of this isotherm is useful to obtain an insight into the orientation of themolecules within the monolayer. At large area in its expanded state, the surface pressureis close to zero indicating that only weak interactions are occurring between molecules.As the confinement area of the monolayer is decreased further, the surface pressure beginsto rise – this region is referred to as the liquid-condensed region (L2) and corresponds tophases in which the molecules are tilted relative to the plane of the water surface. Furthercompression results in the formation of the superliquid (LS) phase in which the alkylchains protrude almost orthogonally from the plane of the water surface. The LS regionrepresents a state of high incompressibility such as found in a traditional three-dimensionalsolid. Indeed the LS region used to be referred to as the ‘solid’ region of the isotherm[42].

The area occupied on the water surface by a single alkanoic acid molecule within themonolayer is about 0.2 nm2. This value corresponds closely to the cross-sectional area ofthe carboxyl end of the molecule as shown in Figure 12.10c. This implies that the floatingfilm is indeed monomolecular in dimension. Such measurements have been made for ahuge number of different materials, providing insight into the relationships between thedetailed molecular structure and the molecular orientation at the air-water interface.

The transfer of floating Langmuir films onto solid supports is in principle a straightforwardprocess involving the successive insertion and withdrawal of a substrate through themonolayer film at the air-water interface. Its success depends on the optimisation of anumber of controlling variables such as the deposition surface pressure, the depositionrate (rate of withdrawal and/or rate of insertion), substrate cleanliness and surfacetreatment, monolayer fluidity, pH and ion content of the sub-phase and temperature.The most fundamental requirement is that of a highly stable feedback mechanism betweenthe surface pressure monitors and the motors, which drive the confinement beltsresponsible for confining the monolayer. This ensures that the small decrease in the surfacepressure that results when part of the monolayer is transferred onto a solid substratestimulates further compression of the monolayer in order to restore the original presetdeposition surface pressure. This ensures that the molecular surface density within thetransferred monolayer remains constant. A schematic diagram of a typical Langmuirtrough is shown in Figure 12.11. The substrate insertion and withdrawal is normallyachieved using a motor-driven micrometer screw to which is attached the clamp for thesubstrate. The sequential transfer process for a hydrophilic substrate withdrawn andinserted through an alkanoic acid monolayer is shown in Figure 12.12. The transfer ofthe first monolayer occurs due to the strong interaction between the carboxylic acidgroups of the alkanoic acid molecules and polar sites (usually taking the form of freehydroxyl groups) on the hydrophilic substrate surface.


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Figure 12.11 A typical Langmuir trough for the fabrication of LB films showing thesurface pressure monitor (P), the wettable plate (W), the substrate (S) attached to the

deposition mechanism (D) via the clamp (C), the moveable constant perimeter barriers(MB), the barrier drive motors (BD) and the water bath (B).

Figure 12.12 The sequential monolayer transfer process for the formation of LB filmsshowing the formation of Y-type assemblies. Inset: Y, X and Z-type architectures.

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Reinsertion of the monolayer-coated substrate results in the transfer of the secondmonolayer. There is a strong hydrophobic interaction between the alkyl chains withinthe first monolayer (the new substrate surface for the second layer) and those protrudingfrom the water surface in the floating monolayer. A second layer is thus adsorbed andthe surface of the coated substrate becomes hydrophilic again and is thus ready to acceptthe third monolayer upon its next withdrawal through the Langmuir film. This processcan be repeated over many cycles and is referred to as Y-type deposition. Most rod-shaped amphiphilic molecules follow this mode of deposition as do most pendant-chainpolymers, like polysiloxanes [43-44]. This deposition mode results essentially (exceptfor the first monolayer) in a non centrosymmetric structure. Such assemblies are generallyvery stable over time owing to the strong stabilising interactions between hydrophobicchains from adjacent monolayers, and between hydrophilic carboxyl groups that canform hydrogen bonds across the interface between layers and in-plane sideways dimersthat serve to strengthen the layers laterally.

Certain materials, such as many phthalocyanines [45], can only be deposited duringeach upstroke (Z-type deposition) and others only during each insertion (X-type). Thetype of deposition followed by a particular material affects the properties of the resultingLB films due to symmetry restrictions for second-order physical effects such aspiezoelectricity or optical second-harmonic generation. The bilayer unit (equivalent tothe unit cell in a crystal) is symmetric for Y-type deposition but non centric for Z- and X-type modes, as indicated in Figure 12.12. Unfortunately, several researchers have foundthat Z- and X-mode multilayers are often temporally unstable, although the zwitterionicdyes of Ashwell prove to be one of the exceptions [46].

One solution to achieving non centrosymmetry within stable multilayer films is to usethe alternate layer LB deposition technique. This employs a special kind of Langmuirtrough known as an alternate layer trough that is effectively two single-compartmenttroughs hybridised into one unit. Such an apparatus is depicted in Figure 12.13 and thisshows how two independent water surfaces are used to carry two different monolayers,A and B. The deposition sequence is ABABABA…. so that the resulting architecture ispseudo Y-type. Thus, alkyl chains from adjacent monolayers interact hydrophobically,as do the hydrophilic, polar groups; the difference here is that the molecules withinlayers A and B are inherently different. Therefore, dipoles μ1 and μ2 would not cancelcompletely and indeed, if the molecules have been designed such that μ1 and μ2 act inopposite directions with respect to their alkyl chains, then an overall electric polarisationresults which is an additive function of μ1 + μ2. The resultant stability of this system isclearly a trade-off between the structural stability arising from the pseudo Y-typearchitecture and the energetic stability that is expected to gradually reduce as the numberof transferred layers increases owing to the large build-up in electric polarisation [47].However, the alkyl chain regions act as screening layers meaning that multilayer films


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Figure 12.13 Alternate layer Langmuir trough showing the substrate attached to therotating drum (R). All other symbols have the same meaning as in Figure 12.11.

containing up to ~200 layers can often be deposited without structural degradation.Alternate layer LB films have been adopted most commonly for studies concerning nonlinear physical phenomena such as piezoelectricity, pyroelectricity and optical second-harmonic generation.

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Although the LB approach is applicable to an ever-increasing range of materials,water-soluble compounds are excluded. In a much lower technology, yet elegant,deposition process, water-soluble polymers can be deposited layer by layer to formsimilar molecular assemblies to LB films [48]. This method, generally known as thepolyelectrolyte layer-by-layer self-assembly process, makes use of the ionic attractionbetween opposite charges on anionic and cationic electrolytes. A solid substrate ispositively charged by chemical cleaning methods and is placed in a solution of ananionic polyelectrolyte, such as poly (sodium 4-styrenesulfonate) [49]. The depositionprocess is depicted in Figure 12.14. During a period of around 10 minutes, a monolayerof the polymer is adsorbed onto the substrate surface via the electrostatic attraction.The process is self-regulating since a second monolayer of the anion cannot beadsorbed due to electrostatic repulsion since there are many unpaired anionic chargeson the polymer which remain electrostatically unsatisfied. Therefore, after monolayeradsorption, the coated substrate is washed thoroughly in water and then placed in asolution of a cationic polyelectrolyte, such as poly(alylamine hydrochloride [50].Again ionic attractions result in the adsorption of a cationic monolayer. This procedurecan be repeated many times to build up alternate layer polymer films. The advantagesof this method over LB deposition are that it is very easy to perform and applicableto water-soluble polymers; the drawback, however, is that the rate of deposition islimited to around 5 monolayers per hour compared to conventional LB depositionrates of 20 monolayers per hour. Recent developments in LB technology have led tomuch higher rates approaching 30 monolayers per minute [51].

Figure 12.14 The layer-by-layer polyelectrolyte self-assembly process forcharged polymers


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12.6 Summary

Much progress has been made in the development of polymers for electronics applicationsover the last 20 years. The next few years will see the impact of flat-screen displays basedon organic materials and the continued development of biological electronics forbiosensing and biocomputing applications. The amazing developments in themicroelectronics sector over the last few decades has been driven by the insatiable demandfor technology to improve mankind’s quality of life and to accelerate further progress. Asimilar development in the fabrication of organic circuits may or may not occur dependingon the perceived advantages of moving away from silicon. Certainly if it were to happen,it would take much less than 40 years. Undoubtedly, this research area is set to attractincreasing attention and to produce devices that are as of yet totally inconceivable.

Acknowledgements

The author would like to thank Al Campbell, Dave Lidzey, Rob Fletcher, Colin Dooling,Pierre Oliviere and O. Worsfold for helpful discussions during the preparation of this chapter.

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20. J. Kido, K. Nagai, Y. Okamoto and T. Skotheim, Chemical Letters, 1991, 1267.

21. M. Weaver, S. Martin, D.D.C. Bradley, M. Pavier, T. Searle and T.H. Richardson,Synthetic Metals, 1995, 76, 1-3, 91.

22. D.G. Lidzey, M.S. Weaver, T.A. Fisher, M.A. Pate, D.M. Whittaker, M.S. Skolnickand D.D.C. Bradley, Synthetic Metals, 1996, 76, 1-3, 129.

23. M. Stolka in Special Polymers for Electronics and Optoelectronics, Eds., J.A.Chilton and M.T. Goosey, Chapman and Hall, London, UK, 1995, 284.


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51. T.H. Richardson, C.M. Dooling, O. Worsfold, L.T. Jones, K. Kato, K. Shinbo,F. Kaneko, R. Tregonning, M.O. Vysotsky and C.A. Hunter, Colloid andSurfaces A, 2002, 198-200, 843.


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13 Conducting Polymers in MolecularElectronics

B. D. Malhotra, V. Saxena, A. Gambhir, R. Singhal,S. Annapoorni and A. Mukhopadhyay

13.1 Introduction

Electronic technology has developed enormously over the past decades. The trend is tomake better, faster and smaller electronic devices for use in our daily living. Most of theelectronic devices are fabricated directly on semiconductor silicon. Today’s advancedsilicon chip can store sixteen million bits of information within an area less than 1 cm2.However, there is a practical limit to the capacity of the storage of information withinthe chip. It is being conjectured that as chip density increases, crosstalk between themtends to degrade their performance. If the components are pushed further, they mayshort-circuit. This inherent technical difficulty has led to the evolution of the field ofmolecular electronics.

Molecular electronics (ME) is so named because it uses molecules to function as ‘switches‘and ‘wires’. ME is a term that refers both to the use of molecular materials in electronicsand to electronics at molecular level. It is as yet not very clear how molecular electronicdevices will operate, but it is conjectured that active molecules are needed, either inisolation or becoming active by association with other molecules. It is thought thatelectronics is likely to imitate some of the basic functions of macroscopic devices such asmemories, sensors and logic circuits.

Organic molecules such as conducting polymers, proteins and pigments are being consideredas alternatives for carrying out the same functions that are presently performed bysemiconductors (e.g., silicon) and metals. Among them, conducting polymers (or conjugatedpolymers) have been considered as highly promising for molecular electronics [1-5]. Theseconducting polymers offer a unique combination of properties that make them attractivematerials for use in molecular devices (Figure 13.1). The conductivity of these polymerscan be tuned by chemical manipulation of the polymer backbone, by the nature of thedopant, by the degree of doping and by blending with other polymers. In addition theyoffer lightweight, processibility and flexibility. Because of these advantages, the use ofconducting polymers in molecular electronics is rapidly evolving from physics, chemistry,biology, electronics and information technology. These molecular electronic materials differfrom conventional polymers by having a delocalised electronic structure that canaccommodate charge carriers such as electrons and holes. Besides this, these organic materials

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exhibit Peierl’s instabilities due to the built-in high anisotropic interactions and undergosubstantial geometric modifications due to electronic excitations. This results in variouscharge transfer processes and a substantial degree of disorder leading to various localisedstates in the forbidden gap due to localisation. Conducting polymers exhibit the behaviourof both metal and semiconductor. Polyacetylene, the first and simplest electrically conductingpolymer, doped with iodine (I2) has been shown to have an electrical conductivity of 1000S cm-1 [2]. Some of the conducting polymers that have recently generated much interest areshown in Figure 13.2. It can be seen that conducting polymers behave as anisotropicsemiconductors with band gaps in the range 1.4-3.2 eV. It has been suggested that electricalconduction in the conjugated polymers occurs via non linear (or topological) defects (solitonsor polarons/biploarons) generated either during polymerisation or as a consequence ofdoping [6, 7]. Solitons can be produced as a result of an interruption in the structure of adegenerate conducting polymer such as trans-polyacetylene (Figure 13.3a). In otherconducting polymers such as polypyrroles, polyanilines and polythiophenes, etc., the charge

Figure 13.1 Applications of conducting polymers in molecular electronics

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Conducting Polymers in Molecular Electronics

Figure 13.2 Structures and band gaps of some importantconducting polymers

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carrier is a polaron [8], a non linear defect that occurs as a result of localisation of chargein a polarised lattice of a conjugated polymer (Figure 13.3b). Due to increased doping, theconcentration of non linear defects (solitons/polarons) increases resulting in an enhancedelectrical conductivity. It has been shown that increased electrical conductivity in highlyconducting polymers is spinless which results due to the formation of bipolarons (abipolaron is a bound state of two polarons) [9, 10]. The concentration of bipolarons canbe increased as a consequence of external stimuli (doping/heat/irradiation) on a conductingpolymer. Solitons and polarons have recently been shown to have implications in thetechnical development of molecular electronic devices [11].

The development of molecular electronics is dependent upon the synthesis and tailoring of‘active molecules’ and is a great challenge to researchers. Conducting polymers can beprepared both by chemical and electrochemical techniques. The nature of a monomer

Figure 13.3 Energy band diagram of (a) soliton (b) polaron and (c) bipolaron

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governs the selection of a method. For example, polyacetylene is prepared by polymerisationof acetylene using a Ziegler-Natta catalyst at low temperatures [12]. Heterocyclic polymers,such as polyaniline, polypyrrole and poly(p-phenylene), have been synthesised using theelectrochemical technique [13-15]. Due to ease of handling and control of thickness, theelectrochemical technique has been extensively used for technical development of molecularelectronic devices. By varying the nature of the groups, specific interactions with externalphysical and chemical phenomena can be developed in these materials, leading to moleculardevices such as transducers, memories and logic operators.

Characterisation of conducting polymers is very important for investigating the electronicprocesses occurring in molecular electronic materials. A variety of techniques (electrochemical,optical, ESR, SEM, AFM, gel permeation chromatography (GPC)) have been widely used todelineate the physical properties of the conjugated polymers [16-20]. For example, the changesin the optical spectra accompanied with doping have been considered very significant inelucidating the mechanism of structural changes in the polymer chains. Information onmorphological changes has been found very helpful towards the fabrication of lightweightbatteries [21, 22]. Electrochemical characterisation provides information regarding redoxbehaviour, the number of electrons in the redox reaction and diffusion coefficient estimationof conducting polymers [23-25]. Thermal techniques such as DSC and TGA reveal valuableinformation on the thermal stability and degradation of these organic molecular electronicmaterials [26-28]. Time-of-flight (TOF) measurements have recently been used to estimatethe magnitude of charge carrier mobility in conducting polymer systems [29]. It is emphasisedthat the experimental data accumulated as a result of characterisation plays a significant rolein the application of a desired conducting polymer to molecular electronic applications.

Conducting polymers have been shown to have a number of potential technological andcommercial applications in optical, drug delivery, memory and biosensing devices [30-34]. Among these, application of conducting polymers to molecular electronics hasattracted the maximum attention. The major challenge confronting the materials scientists,including biochemists and physicists, is how the properties of these electronic materialsdiffer from those of conventional semiconductors. Our group has been actively engagedin the research and development of conducting polymer-based molecular electronic devicesfor the past fifteen years [35-39]. It was, therefore, thought that a review based on therecent research findings will prove helpful to the researchers venturing to enter this highlyfascinating field of molecular electronics.

13.2 Synthesis of Conducting Polymers

Conducting polymers have been prepared by both chemical and electrochemical techniques.Conducting polyheptadiene has been obtained by cycling 1,4-heptadiene in the gas phase


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by selective deposition on polyethylene and Teflon. Polyacetylene has been prepared byallowing acetylene to polymerise on the wall of a reaction flask on which the catalystsolution containing Ti-[O-n-C2H9]4 and [C2H5]3Al is coated [40]. Polymerisation on thewall of a reaction flask depends on the catalyst activity and preparative conditions such asthe Al/Ti ratio, the polymerisation temperature, the acetylene pressure and the ageingcondition. Polyacetylene can also be polymerised by the Durham route [41, 42], in which7,8-bis(trifluoromethyl)tricyclodeca-3,7,9-triene is used as a monomer and CuCl6:(C6H5)4Sn(1:2) and TiCl4:(C2H5)3Al (1:2) are used as catalyst. Synthesis of poly(arylenevinylene)involves preparation of a water-soluble dimethyl sulfonium salt as a precursor monomer,its base-catalysed polymerisation and subsequent thermal elimination to yield the conductingpoly(arylenevinylene). A solid-state polymerisation method makes use of monomer crystalsusing heat or light. Polydiacetylene and polysulfur nitride with perfect stereoregularityhave been prepared using this technique. Besides this, polyaniline can be prepared usingaqueous and non aqueous routes using chemical technique [43].

Photopolymerisation utilises photons to initiate a desired polymerisation reaction in thepresence of photosynthesisers. Polypyrrole has been reportedly synthesised from pyrroleusing a tris (2,2´-bipyridyl ruthenium) complex as a photosynthesiser. The advantage ofthis technique lies in the easier control of the polymerisation reaction to a desired surface.The oxidative coupling method is one of the best methods for obtaining high molecularweight poly(paraphenylene) [44]. Plasma polymerisation makes use of molecules occurringin various plasma environments. This technique results in very thin but uniform polymericlayers that strongly adhere to a desired substrate. It has been found that the plasma-polymerised films are usually highly crosslinked and are resistant to higher temperaturesand chemicals. The method makes use of monomers to form cations followed by theircoupling to form dications, and repetition of this process produces a conducting polymer.Polythiophene has been synthesised by both electrochemical [45] and chemical techniques[46]. Pandey and co-workers [47] have reported the chemical synthesis of poly(aniline-co-orthoanisidine) that has been found to be soluble in common organic solvents such asacetone, chloroform and n-methylpyrrolidone. Wang and co-workers [48] latersystematically investigated the effect of temperature on the synthesis and properties ofpoly(aniline-co-orthoanisidine). It has been revealed that this processable conductingpolymer has application in molecular electronic devices.

Electrochemical oxidative polymerisation is known to be an effective method for obtainingconducting polymers [49]. There are three methods: galvanostatic, potentiostatic andpotentiodynamic (or cyclic sweep). In galvanostatic mode, a conducting polymer isgenerated by supplying a constant current between a working electrode and a counterelectrode. Films produced by this method are smooth and adherent to the anodic surfaces.In the potentiostatic mode, the conducting polymer is grown at a predefined constantpotential maintained between the counter and working electrodes. The selection of the

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constant potential for the synthesis of a conducting polymer (e.g., polyaniline) by thismethod is obtained from polarisation studies. It can be remarked that this method producespowdery deposits that are non adherent to the electrode surface. In the case of the potentialcycling (i.e., cyclic voltammetry) method, the potential is cycled between two predefinedpotentials with a definite sweep rate. It has been found that this method gives homogeneousconducting polymer films that strongly adhere to the electrode surface. It can be seenthat the electrochemical method is an advancement over chemical techniques becausethe resulting product does not need to be extracted. The flexible and smooth films can beprepared by the judicious selection of the conducting salt acting as an electrolyte. It hasbeen ascertained that the charge and the geometry of the anions greatly affect the propertiesof a given conducting polymer. Apart from these, some general considerations such asthe choice of the solvent, counterions [organic/inorganic] and also the substrate play akey role in the mechanical and electrical properties of conducting polymers preparedusing electrochemical techniques.

13.3 Preparation of Ultrathin Conducting Polymer Films

Ultrathin films of conducting polymers have been projected to have applications inmolecular electronics [50, 51]. The approach lies in fabricating devices starting fromatoms and molecules. This has been considered to be an attractive alternative if orderedstructures are required. It is thought that an understanding of the physical propertiesof conducting polymer monolayers will prove very valuable in the evolution of thearea of molecular electronics. It has been shown that it is possible to obtain by thesetechniques a system of ‘wires’ and ‘switches‘ comprising of conducting polymermonolayers [52, 53].

13.3.1 Langmuir-Blodgett Films

The preparation of polymeric Langmuir-Blodgett films with different characteristics isof great scientific and technological interest. Materials that form monolayers on thesurface of water comprise of molecules that contains both hydrophilic (water-attracting)and hydrophobic (water-repelling) chemical groups. These materials are known asamphiphiles. Langmuir-Blodgett films are formed by first depositing a small quantity ofan amphiphilic material (stearic acid) dissolved in a volatile organic solvent onto thesurface of purified water (sub-phase). On evaporation of the solvent, the pressure-areaisotherm (Figure 13.4) is recorded by compressing the monolayer mechanically [54, 55].This results in the formation of the floating organic material into a ‘two-dimensionalsolid’. And since the amphiphile has hydrophobic and hydrophilic ends, the compressioncauses the molecules to be aligned in the same way on the surface of water (Figure 13.5).


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Figure 13.4 Pressure-area isotherm of an Langmuir-Blodgett film ofpoly(3-hexylthiophene)

Figure 13.5 Possible orientations of a molecule deposited byLangmuir-Blodgett technique

The molecules in their closest packed arrangement (solid phase) are removed from thesurface of water by suitably dipping and raising a suitable plate (substrate) through theair/water interface in three ways (Figure 13.6). If the substrate is hydrophilic the firstmonolayer is transferred as the substrate is raised through the sub-phase, these stack ina head-to-head and tail-to-tail configuration. This deposition mode is referred to as Y-type deposition. This results in an odd number of monolayers being transferred onto thesolid substrate. However, if the solid substrate is hydrophobic a monolayer will bedeposited as it is first lowered into the sub-phase, thus a Y-type film containing an evennumber of monolayers can be fabricated (Figure 13.7). If a layer is deposited on thesubstrate only when the solid substrate enters the sub-phase, this is called X-typedeposition. On the other hand if a layer is deposited on the substrate when withdrawn

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X Y Z

Figure 13.6 X-, Y- and Z-type deposition in Langmuir-Blodgett film deposition

Figure 13.7 Schematic of Y-type deposition in a Langmuir trough. (a) Compressedmonolayer of molecules on water surface, (b) Deposition of first monolayer on withdrawal

of hydrophilic substrate, (c) Deposition of second monolayer on insertion of hydrophilicsubstrate, (d) Deposition of third monolayer in a head-to-head and tail-to-tail configuration


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from the sub-phase, it is called Z-type deposition. Film deposition can be characterisedby knowing the deposition ratio, τ, as given by Langmuir and co-workers [56]:

τ = Al / As (13.1)

where Al is the decrease in the film area of the sub-phase and As is the contact area ofthe substrate.

Various deposition modes can be obtained using a parameter φ given by Hoing and co-workers [57]:

φ = τu / τd (13.2)

where τu and τd are the deposition ratio on the upward and downward passage, respectively.For the Y-type deposition, φ =1, for X-type, φ = 0, and for Z-type deposition, φ = ∞.

Many factors such as temperature, humidity and pH (aqueous phase) can influencemeasurements on interfacial films. Langmuir-Blodgett films offer ways of studyingmolecular conformational changes and providing information on molecular packing,crosslinkage and denaturing. Langmuir-Blodgett films are ideal systems for spectroscopyof complex monolayers. Energy transfer between excited molecular states and modelmembranes has been used to mimic photosynthetic systems. It has recently beendemonstrated that the Langmuir-Blodgett technique can be utilised to process a varietyof conductive polymeric systems into multilayer films. The conjugated polymer backboneprovides a route for the flow of electrons either along the molecule or between nearestneighbours, the latter being by way of charge-transfer interactions.

Langmuir-Blodgett films of a number of conducting polymers such as polypyrroles,polyanilines and poly-(o-anisidines), etc., have recently been prepared. Hoing and co-workers[58] have shown that electrically conducting polypyrrole films can be formed at the air-water interface of a Langmuir-Blodgett trough using a solution containing a surface-activepyrrole monomer and a large excess of pyrrole on a sub-phase containing ferric chloride(FeCl3). They showed that the chemistry initiated at the air-water interface and the propertiesof the resultant polymer are strongly influenced by the type of the surface-active monomerused. Ultrathin films of 3-octadecylpyrrole (3ODP) and 3-octadecanoylpyrrole (3ODOP)were subsequently obtained using Langmuir-Blodgett technique. The pyrrole/3ODOP filmwas found to be highly aniosotropic with conductivity in the plane being 107 times greaterthan the conductivity across the film thickness. Bardosova and co-workers [59] haveinvestigated the formation of polythiophene Langmuir-Blodgett films on silicon. The resultsof atomic force measurements conducted on a five-layer polythiophene film revealed thatthe rearrangement of molecules occurs resulting in a fibrous structure.

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Aghbor and co-workers have obtained films of deposition of preformed polyemeralidinebase (PEB) by dissolving PEB in a N-methylpyrrolidone/CHCl3 mixture in an aqueous sub-phase containing acetic acid [60]. Ram and co-workers [61, 62] subsequently showed that itis possible to obtain quasi-ordered Langmuir-Blodgett films of emeraldine base withoutincorporating fatty acid tails in the molecule. However, the results of cyclic voltammeteryand chronopotentiometry indicate that irregularities begin to form in the film and the orderednature of the films is lost, resulting in its reduced electroactivity. Dabke and co-workers [63]studied the electrochemistry of polyaniline Langmuir-Blodgett films using cyclic voltammetrycoupled with a quartz microbalance. It was found that the multilayer films exhibit poorelectrochromic response. These results have implications in the fabrication of molecular devices.

Recently, ultrathin films of poly(o-anisidine) and poly(ethoxyaniline) have been fabricatedfor application in nanotechnology [64-66]. Matsura and co-workers [67] have fabricatedmonolayers of β-carotene using a Langmuir-Blodgett film technique together with theflow-orientation method. They have utilised XRD, UV-visible and FTIR techniques toelucidate the film-structure of β-carotene indicating that β-carotene orients perpendicularto the air-water interface. It was found that the films are, however, well-ordered both inthe stacking direction and the in-plane direction.

The ability to tailor properties of conducting polymer films has been of advantage forseveral applications. The best-known example is of liquid crystalline polymers that havepotential applications in electro-optic applications such as calculators, wristwatches,message boards, flat panel televisions, waveguide switches, real-time optical dataprocessing systems, etc. The molecular order existing in Langmuir-Blodgett multilayersfacilitates the formation of liquid crystalline polymer substances since some of theconditions are easily met. These conditions are:

• The volume contraction during the chain formation should not be in the direction ofchain growth,

• The overlapping volumes of monomer units in the polymer chain should not muchdiffer from that of the monomer molecule, and

• The chain period of the polymer must coincide with a transitional period of themonomer crystal lattice, which are required for a polymerisation.

Conducting polymer Langmuir-Blodgett films find applications in insulation, adhesionand encapsulation. The molecular electronic applications of conducting polymerLangmuir-Blodgett films include microactuators, high-density information storage, high-definition television, Schottky devices, biosensors and chemical sensors. It may beemphasised that the industrial use of each of these applications is closely linked withmanipulating the architecture of a conducting polymer via ‘molecular engineering’. It is


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appropriate to mention here that the Langmuir-Blodgett technique provides an opportunityto control the thickness and supermolecular organisation of electroactive polymers atthe molecular level for application to the potential field of molecular electronics.

13.3.2 Self-Assembly Monolayers

One of the methods of formation of organised monolayer assemblies (OMAs) on thedesired solid surface has been described in the preceding section. Another process is themethod of self-assembly wherein the molecules are transferred to the surface of a solidfrom the liquid phase by a dipping process. These self-assembled monolayers have potentialfor technological and scientific applications ranging from microelectronics to biologicalsensors [68]. Monolayers of polythiophene and substituted polythiophene have beenformed on gold substrates by a dipping method [69]. Multilayered thin films with preciselycontrolled thickness and layer sequences can also be obtained by this technique [70-76].

Another approach to fabricating self-assembled monolayers is by depositing alternatinglayers of p- and n-type polymers on a suitable substrate. Here the molecules are held by theelectrostatic attraction of the alternatively charged polymers. Converting the polymer intoa polycation or a polyanion by using the appropriate acidic solution produces the charges.For example, this technique has been utilised for preparing multilayers of polyaniline andpolypyrrole samples with uniform ordering [68]. Different conjugated and non conjugatedpolymers in the form of bilayers have been used to make heterostructures.

13.4 Characterisation of Conducting Polymers

Characterisation of a conducting polymer is important before it can be used for anytechnological application. Physical and chemical properties of conducting polymers havebeen investigated by a number of experimental techniques. The molecular weight of aconducting polymer has been estimated by GPC. Thermal techniques such as DSC/DTAhave been utilised to check the stability of conducting polymers in air. ESR measurementshave been found to reveal information on the phenomenon of doping in electricallyconducting polymeric systems. It has been shown that the origin of magnetic propertiesin a conjugated polymer lies in its π-electron system, which has also been considered asthe reason for the observed chemical properties [77]. Dielectric relaxation and lowfrequency conductivity measurements in the frequency range from 100 to 107 Hz haveproven valuable in giving information on the mechanism of conductivity that cannot beobtained with dc conductivity data alone [78]. The morphology of conducting polymerfilms has been investigated using optical microscopy, SEM/TEM, scanning tunnelingmicroscopy (STM) and AFM techniques [79-82]. Studies conducted on the morphology

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of conducting polypyrrole films containing different anions such as NO3–, F–, ClO4

–,BF4

– and CH3C6H4SO3–, respectively, have revealed that the topology of the growing

conducting polypyrrole surface is influenced by the nature of the electrolyte [83-84]. Forinstance, it has been shown that the fibrillar structure of polyacetylene is oftenadvantageous since it can store up to about 7% of electrical charge [85]. AFM and STMmethods [86] have recently provided valuable information on the presence ofmicrodomains in poly(3-hexylthiophene)/stearic acid films (Figures 13.8 and 13.9).

Figure 13.8 Atomic forcemicrograph of conducting

poly(3-hexylthiophene)

Figure 13.9 Scanning electronmicrograph of conducting

poly(3-hexylthiophene)


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Spectroscopic methods have been used to yield information on the charge transportbehaviour in conjugated polymers. On interaction of a photon with a conductingpolymer, electrons get excited to a higher potential with concomitant creation ofelectron-hole pairs. The application of spectroscopic techniques in the IR and UV-visible regions to conducting polymers has brought interesting information on theconfigurational changes arising as a consequence of doping. Photoelectronspectroscopy methods have been used as probes to investigate electronic and chemicalstructures of conducting polymers such as polypyrroles and polythiophenes. 13C NMRstudies have been conducted on polyazulene, polybithiophene and polyfuran in theirrespective doped and undoped states [87]. The 13C NMR data obtained on theelectrochemically prepared polypyrrole indicate the presence of α-α′ bonding inconducting polypyrrole films [88].

The ellipsometric technique has been used for the estimation of thickness, refractiveindex and optical dielectric constants of a conducting polymer film [89]. These conductingpolymer films have been used for the fabrication of electrochromic and glucose biosensingdevices [90, 91].

13.5 Molecular Devices Based on Conducting Polymers

13.5.1 Diodes

There has been an increased interest towards the possible applications of conductingpolymers as the active elements in electronics [92-95]. The characteristics of conductingpolymer/inorganic semiconductor interfaces have been considered as very important sinceit has been indicated that restrictions on Schottky barrier devices can be overcome byusing conducting polymer contact layers. Besides this, the ability to manipulate theinterface characteristics by changing the polymer dopant allows switchable devices to befabricated. Semiconducting polymers such as polyacetylene, polypyrrole and polyanilinehave recently been used for the fabrication of Schottky barrier diodes, like metal-insulator-semiconductor (MIS) diodes and p-n junction diodes [96-98]. A schematic diagram for aSchottky device is shown in Figure 13.10. Schottky diodes formed between metallic AsF5

–

doped (CH)X and n-type GaAs indicate high electronegativity [99]. The polyacetylenehas been found to exhibit p-type behaviour when it is used for fabrication of Schottkydiodes with low work function metals.

Heterojunctions have recently been fabricated using electrochemically preparedpolypyrrole and metal (indium, titanium, aluminium and tin). It has been revealed thatcarrier concentration, estimated as 1.5 x 1020 and 5 x 1017 in doped and undopedpolypyrrole samples, respectively, plays an important role in controlling the junction

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characteristics and hence the performance of these Schottky devices [100]. The electricalcharacteristics of the junctions have been found to be dependent on the work function ofpolypyrrole, estimated as 4.42-4.49 eV.

It has been recently revealed that it is possible to fabricate all vacuum deposited metal(Pb, Al, In, Sn)/polyaniline/metal Schottky devices [101]. It has been shown that thebarrier height and the ideality factors determined are dependent on the work functionof the metal used in the fabrication of these devices. The improved ideality factorobtained as 1.2 for an Al/polyaniline/Ag device has been attributed to more intimatecontact of the metal with the vacuum deposited polyaniline electrode.

Poly(3-methylthiophene) was prepared using an electrochemical technique to fabricatesilicon-based devices [102]. From the results obtained using current-voltagemeasurements, chronopotentiometery, SEM and FTIR techniques, it has been seen thatthe rectifying behaviour was induced by covalent bond formation between poly(3-methylthiophene) and the silicon.

Schottky devices have recently been fabricated by thermal evaporation of indium onpolyaniline, poly(o-anisidine) and poly(aniline-co-orthoanisidine), respectively [103].The values of the rectification ratio, the ideality factor and the barrier height of anindium/poly(o-anisidine) have been experimentally determined as 300, 4.41 and 0.4972,respectively. The observed deviation from the Schottky behaviour for these devicesseen at higher voltages has been explained in terms of either the Poole-Frenkel effect ordue to the presence of a large number of defects containing the trapped charges existingat the indium/poly (aniline-co-orthoanisidine) interface.

Figure 13.10 Schematic diagram of a conducting poly(3-alkylthiophene) basedSchottky device


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Recently, the junction properties of Schottky devices using films of chemically synthesisedpoly(3-cyclohexylthiophene) and poly(3-n-hexylthiophene) units and metals have alsobeen studied [104]. Electrical properties of the poly(3-cyclohexylthiophene)/metaljunctions were compared with those of the poly(3-n-hexylthiophene)/metal junctions(Figure 13.11). Better rectification properties of the poly(3-cyclohexylthiophene)/metaljunctions were attributed to the decreased conductivity that perhaps results due to sterichindrance in the thiophene ring.

Figure 13.11 Current-voltage and capacitance-voltage characteristics of metal/conducting polymer junctions

MIS structures were recently fabricated [105] by thermal deposition of metals (indium,aluminium and tin) on Langmuir-Blodgett films of cadmium stearate (CdSt2) obtainedon polypyrrole films electrochemically deposited onto ITO glass. Junction parameters,like barrier height, rectification ratio and work function, of these devices wereexperimentally determined. The ideality factors of the CdSt2 layer/semiconductingpolypyrrole structures have been estimated as 6.63, 6.57 and 6.54 for tin, aliminiumand indium, respectively, in comparison to the values of 8.85, 8.82 and 8.20 obtainedwith a semiconducting polypyrrole interfaced with the same elements. It has beenconcluded that the passivation of the semiconducting polypyrrole results in the lowervalue of the ideality factor.

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13.5.2 Field-Effect Transistor

In conventional transistors, ‘field effect’ has been used to improve device characteristics[106]. This ‘field effect’ controls the current through a ‘gate’ electrode and thereby opensthe possibility of transistor action without requiring the existence of p-n junctions. Thisphenomenon is useful not only for fabricating devices but also a very useful tool forstudying semiconductor and surface states [107]. The application of field effect to fabricateconjugated polymer based device was first demonstrated by Koezuka and co-workers[108] using electropolymerised polythiophene. When charge is induced in the polymerlayer by applying a voltage to the gate, the conductivity of polymer changes, which inturn controls the drain current flowing between ‘source’ and ‘drain’. After Koezuka,many researchers applied various conducting polymers to construct field-effect transistor(FET) devices [109-111]. Simultaneously, they exploited thin film fabrication methods[112]. Tsumura and co-workers have fabricated a molecular electronic device based onan electrochemically prepared semiconducting polythiophene thin film [113]. They haveshown that this solid-state FET device is normally off (enhancement type) and the source(drain) current can be modulated by more than 102 by varying the gate voltage. Thetransconductance and the carrier mobility have been estimated as 3 nS and 10-5 cm2 V-1 S-1,respectively, using electrical measurement technique.

MIS field-effect transistors (MISFETs) and enzyme field-effect transistors (ENFETs) basedon conducting polymers have also been fabricated. A schematic diagram of such a deviceis shown in Figure 13.12. Janata and co-workers have investigated the electrical properties

Figure 13.12 Schematic diagram of an ion-selective field-effect transistor,VG = gate voltage, VD = drain voltage, ID = drain current


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of insulated gate field-effect transistors (IGFETs) using a chemically prepared polyanilineas a gate electrode [114]. The response of the polyaniline insulated gate field-effecttransistors (PANI-IGFETs) to step function potential has been experimentally determined.It has been found that the dc behaviour of these devices is the same as that of thecorresponding MOSFETS (metal oxide semiconductor field effect transistors). Saxenaand co-workers [115] reported an ion-selective microelectrochemical transistor (ISMET)specifically responsive to Cu(II) ions using electrochemically prepared polycarbazole films.The device turns on by adding 2.5 x 10-6 M Cu(II) ions and reaches a saturation regionbeyond 10-4 M Cu(II) ions (Figure 13.13). In the above concentration range, the deviceresponse is linear. The increase in drain current, ID, with increase in Cu(II) ionconcentration was attributed to the conformational changes in the polymer matrix whicharise due to the occupation of Cu(II) ions in the polymer matrix.

Figure 13.13 ID-[conc.] plot of an ion-selective microelectrochemical transistor

The field-effect mobilities in these FET devices are found to be around 10-5 cm2 V-1 s-1

depending on the applied voltages and the nature of the gate insulator. These values aresignificantly lower than that of inorganic semiconductor devices in which mobilities arefound to be in the range of 0.1-1 cm2 V-1 s-1 [116]. Work on sexithienyl based FETsyielded more promising values, the field-effect mobility reaching 3 x 10-2 cm2 V-1 s-1

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[117], which is about one order of magnitude lower than that of an amorphous Si-basedFET [118]. In the FET configurations, the interface of the gate insulator and organicsemiconductor layer plays a crucial role in charge transport [110, 119]. It was also foundthat an oligothiophene (sexithiophene) showed a mobility of 0.46 cm2 V-1 s-1 incyanoethylpullulan (CYEPL) which is used as a gate insulator, the mobility being almosta thousand times higher than that measured using SiO2 as the gate insulator [120]. Waragaiand co-workers reported a similar trend; the highest mobility recorded was 3 cm2 V-1 s-1

for dimethylsexithiophene (CMSxT) on CYEPL, which was a hundred times more thanthat measured using SiO2 [121]. It is to be noted that these values of the mobility arecomparable to or even surpass that of amorphous silicon [122]. Further, Garnier and co-workers have reported fully plastic FET by using a printing technique [123]. A recentlyproposed device comprising of ‘polymer grid triodes’ by Yang and Heeger is worth noting[124]. The performances of conjugated polymer based FETs are quite encouraging andcould be used to control a pixel in a liquid crystal display. Lately, Friend and co-workersdemonstrated a high mobility conjugated polymer FET driving a polymer LED [125]. Itcan be seen that conjugated polymers show a clear superiority over amorphous siliconeif the cost is taken into account. However, lifetime and low response of the device restrictthe competition of these materials with silicon-based devices. Nevertheless, the possibilityof making a flexible panel with conjugated polymers opens a different field of large-area,low-cost plastic electronics.

13.5.3 Biosensors

One of the most exciting areas of research in molecular electronics lies in the developmentof biosensing devices (usually called biosensors or receptrodes). The term has been definedas an analytical device incorporating a biological or biologically derived sensing elementeither integrated or intimately connected with a physicochemical transducer. The aim isto produce either discrete or continuous electronic signal(s) that is (are) specific to asingle analyte (or a related group of analytes). A biosensor can be considered as acombination of an electrochemical or an electrical sensing device and a miniaturisedreactor containing an immobilised biomolecule, and is used in most cases to measure theconcentration of a substrate. A number of biomolecules such as enzymes, antibodies,organelles, cells and receptors have been used as sensing probes for the fabrication ofbiosensors. The transducer can either be an electrical, optical, thermal or piezoelectricdevice. The electrochemical method of detection has been at the centre of biosensordevelopment. The potentiometric technique relates to the dependence of the potential onthe analyte concentration, whereas in amperometric biosensors current is based onheterogeneous electron transfer reactions, i.e., the oxidation and reduction of electroactivesubstances. A schematic of a biosensor is shown in Figure 13.14.


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Much interest in biosensors has been attached to the potential application of this highlyexpanding field of research to the solution of a variety of problems that occur in clinicaldiagnostics, food industry, agriculture and environment industry. Some of the other reasonsthat have made this area very attractive include those (i) that provide quick, selective andreliable information on the measurement species, (ii) that yield measurable signal, (iii)that require minimum pretreatment of the samples, (iv) that are inexpensive and can beused repetitively.

Various publications in recent years indicate organic conducting polymers as a convenienttool for the immobilisation of enzymes at the electrode surface and its interaction withmetallic or carbon electrode surfaces. The application of conducting polymers in analyticalchemistry has recently been reviewed [126-130]. Some other reviews have been devotedto their use in design of biosensors [131, 132].

Immobilisation of biomolecules on the surface of an effective matrix with maximumretention of their biological recognition properties is a crucial problem for the commercialdevelopment of a biosensor. Different methods of immobilisation have been used. Onesuch method is electrochemical entrapment. Several conducting polymers can be depositedelectrochemically and, in the process, a biological molecule can be entrapped. This processis also useful in the fabrication of microsensors in preparation of a multilayered structurewith one or more enzymes/biomolecules layered within a multilayered copolymer foranalysis of multiple analytes [133-135]. A number of reports have appeared onimmobilisation of biomolecules using electrochemical entrapment [130, 131, 136-143].

Another procedure for the immobilisation of biomolecules is the covalent binding of enzymeto a conducting polymer film. This is essentially a two-step procedure based on the formationof a functionalised conducting polymer film followed by the covalent binding of the enzymeat the functional groups on the polymer surface. The major advantage associated with thisprocedure lies in the independent optimisation of conditions required for the synthesis of thepolymer with respect to solvent, electrolyte salts, film thickness and side reactions. This is

Figure 13.14 General principle of a biosensor

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followed by immobilisation of the desired biomolecule (e.g., enzyme) using an appropriateprocedure in order to preserve the active state of the enzyme. Covalent coupling of an enzymewith a polymer can be carried out using carbodiimide coupling with the formation of peptidebonds [126, 144-146]. Bovine serum albumin and glutaraldehyde coupling [147] and variousbifunctional reagents have been used to form a crosslinked network of enzyme, with thecrosslinking reagent on the surface of a conducting polymer [148-151]. Others report chemicalgrafting or affinity of the biomolecule at the functional group [152-155].

Conducting polymers such as polypyrrole [127] and its derivatives [156, 157], polyaniline[158-164], polyindole [137] and poly-o-aminobenzoic acid have recently been used for thefabrication of biosensors. A few biosensors based on insulating electropolymerised filmslike polyphenols, poly(o-phenylenediamine), poly(dichlorophenolindophenol) andoveroxidised polypyrrole have also been elaborated [165-167] .

The glucose biosensor remains the most extensively studied biosensor. Lowe and co-workers [156] entrapped glucose oxidase in a polypyrrole matrix electrochemicallydeposited on a printed platinum electrode. Cooper and co-workers have reported theelectrochemical preparation of a glucose oxidase loaded polyaniline film [157]. It hasbeen found that polyaniline films exhibit enhanced loading of glucose oxidase after aself-exchange and hence can be used for the fabrication of third-generation glucosebiosensors [37]. Some recent reports on polyaniline-glucode oxidase interaction includethe work of Ozden and co-workers [168] and Karyakin and co-workers [169].

Morphological studies have revealed that the surface of a conducting polymer is criticallydependent on the method of preparation [170] and plays an important role in the effectiveimmobilisation of desired enzyme [171, 172]. The Langmuir-Blodgett technique formonolayer deposition could be very successful in achieving the desired orientation of themolecule. Few reports have appeared on this subject [173]. Okhata and co-workers haveused Langmuir-Blodgett films of GOD-stearic acid monolayers on a platinum electrode tofabricate an ultrathin glucose sensing membrane [174]. Besides this, direct preparation ofLangmuir-Blodgett films of GOD without the need to add a lipid layer has also beendemonstrated by crosslinking with glutaldehyde. Ramanathan and co-workers [171] havesuccessfully used a physical adsorption technique to immobilise glucose oxidase in Langmuir-Blodgett films of polyemeraldine base (PEB) deposited onto ITO glass. These GOD-PEBfilms have been seen to result in the linear increase of anodic current as a function ofglucose concentration (5 to 50 mM).

Other biomolecules, such as antibodies [175-177], DNA [178-180], etc., have attractedmuch interest in the development of biosensors useful for detection of viruses andgenetically transmitted diseases. Few reports have appeared on immobilisation inconducting polymers. A full range of optical, electrochemical and piezoelectrictransduction modes aimed at detecting the base pair hybridisation between the immobilised


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cDNA probe and the target DNA have been developed. Mediated electron transferreactions of DNA for detecting PCR (polymerase chain reaction) amplified genomicDNA and ferrocene mediated oligonucleotides for a sandwich-based electrochemicaldetection of DNA hybridisation have been described. Immobilisation of DNA onto glass,carbon and gold electrodes has also been reported [181, 182].

Chaubey and co-workers have given a detailed account of the application of conductingpolymers to biosensors in Chapter 10.

13.5.4 Electronic Tongue

Taste is produced when interactions between molecules and biological membranes arenot specific to or characteristic of each molecule. The five kinds of basic taste qualitiesreflect the differences among these interactions. What is important in recognition oftaste is not discrimination of minute differences in amounts of molecules but rather thetransformation of molecular information contained in interactions with biologicalmembranes into several kinds of groups, namely taste intensity and quality. Taste comprisesof five basic qualities:

• Saltiness produced by NaCl,

• Sourness produced by the hydrogen ions of HCl, acetic acid, citric acid, etc.,

• Bitterness produced by quinine, caffeine, L-tryptophan and MgCl2,

• Deliciousness produced by monosodium phosphate and disodium inosinate in meatand fish, and by disodium guanylate in mushrooms, and

• Sweetness due to glucose, fructose and sucrose.

Substances producing taste are received by the biological membrane of gustatory cells intaste buds on the tongue. Taste is perceived when the information on the substances istransformed into electrical signals that are transmitted to the brain via nerve fibre. Thetaste sensor employs the concept of global selectivity, implying the ability to respond tomany kinds of chemical substances at the same time. Wine has both taste and odourqualities due to the aromatic molecules in the liquid and vapour phases being different.Most wines contain about 8%-85% water and 500 other substances, some of which arevery important to the flavour in spite of low concentrations.

The overall perception of taste is known to be due to a combination of taste senses andsmell, and also called flavour or trigeminal sensor. Sussi and co-workers have designed

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an odour-sensor array of different conducting polymers [183]. Polymerisation occurswhen the monomer (25 mg) and the oxidising salt is sprayed onto four interdigitedelectrodes fabricated onto the alumina substrate. The electrical resistance measuredbetween different inner electrodes has been found to vary from 1 to 100 kΩ when volatilemolecules are adsorbed at the surface of the conducting polymer film. The averageintensity, defined as the ratio of the resistance change to the base resistance value, wasestimated to be less than 2% for elements related to wine sensing. Toko [184] hasfabricated a multichannel taste sensor, with global selectivity comprising of several kindsof lipid/polymer membranes for transforming information about substances producingtaste into electrical signals. The electronic tongues or taste sensors are based on theprinciple of potentiometry, i.e., the charging of the membrane is measured. A voltammetrytechnique has several advantages due to features such as very high sensitivity, simplicity,versatility and robustness. An electronic tongue, working on the principle of pulsedvoltammetry based on an array of five working electrodes, has recently been designed tofollow the deterioration in the quality of milk due to microbial growth when milk isstored at room temperature [185]. Depending on the purpose and on the object to bemeasured, it is possible to miniaturise the taste sensor. Accordingly, a taste-sensing field-effect transistor (TSFET) has recently been designed [186] for estimation of chemicalsubstances in foodstuffs and organisms.

A number of sensors for the measurement of odour and deliciousness based on conductingpolymers are currently at various stages of technical development. It appears that we aregradually entering into a new age of ‘food culture’.

13.5.5 Electronic Nose

The complicated olfactory system in humans and animals can detect and differentiatethe presence of an odour even at trace levels [187]. Sensory evaluation is one of theimportant parameters for environmental monitoring, quality assessment for food, wineand beverages, and clinical diagnosis, as well as for the control of many cosmetics andfermentation processes [188-190]. Typically, sensory evaluation in odour as well as food/wine testing is performed by a panel of well-trained professionals based upon their senseof smell, taste, experience and mood. However, the human olfactory system is very sensitivebut not selective.

The human nose neither tries to break the aroma into different constituents nor to quantifythe constituents. Unfortunately, there is no effective instrumental analysis to replace thehuman sense. Analytical instruments such as gas chromatography (GC)/mass spectrometry(MS), high pressure liquid chromatography (HPLC) and NMR spectroscopy could be usedto monitor the particular compounds present in a variety of samples. However, their use is


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impractical for monitoring unknown odorous compounds. The processes suffer fromtedious, time-consuming and expensive works, and more importantly, a lack of a directcorrelation between the instrumental results and human perception. These instrumentsprobably cannot detect threshold (10-9 ppb) levels of some odorous compounds, e.g., α-terpinethiol in water [191]. Consequently, there is an enormous demand for an electronicinstrument that can mimic the human sense of smell and provide low-cost, rapid, sensoryinformation. The earliest work on the development of an instrument specifically to detectodour dates back to 1961, when Moncrieff reported a mechanical nose. The first report onan electronic nose was from Wilkens and Hatman in 1964 [192]. However, the term‘electronic nose’ appeared around the late 1980s when one was demonstrated at a conferencein 1987 [193]. The electronic nose can be used to monitor and measure odour anywhere,such as in environmental monitoring, mining, clinical diagnosis, food and drinks, householdproducts, healthcare or pharmaceutical products, perfumery, tobacco and smoke.

An electronic nose is often a bench-mounted instrument comprising of an array ofelectronic chemical sensors with partial specificity and an appropriate pattern-recognitionsystem, capable of recognising simple or complex odours [194]. Several technologiessuch as tin oxide, quartz resonator and surface acoustic wave (SAW) devices have beenproposed for the technical development of electronic nose for the analysis of vapoursand different gases. These devices operate at elevated temperatures (e.g., 100-600 °C).They are quite sensitive to combustible materials such as alcohols but are generally poorat detecting sulfur- or nitrogen-based odours. Although there are some oxide materialsthat show a good specificity to certain odours, there are several potential advantages toemploying organic materials in an electronic nose. Of these materials, the most interestingare conducting polymers, which are sensitive but not selective.

Conducting polymers are highly suited as odour sensing devices:

• These sensors exhibit rapid adsorption and desorption kinetics at room temperature.

• The polymer used can be made highly specific to chemical substances.

• They are highly insensitive to poisoning from sulfur-containing compounds.

• There is low power consumption, unlike tin-oxide sensors, and so no heater is required.

• They are easier to process than oxides, and spin casting, electrochemical, screen-printing and Langmuir-Blodgett methods could be employed to fabricate sensors.Organic polymers like polypyrrole can be readily patterned using standard integratedcircuit technology.

• These sensors can be operated close to room temperature.

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Different conducting polymers produce different responses when exposed to complexvapours [195, 196]. The conducting polypyrrole is, however, the most commonly used,although some work on polyaniline has also been reported [197]. To date several electronicnoses are available commercially or close to it. An example is the Odour Mapper (UMISTVentures, UK), which consists of an array of twenty conducting polymer chemoresistors.Similarly, Neotronics, UK, has designed an electronic nose comprising of two conductingpolypyrrole electrodes separated by 10 μm [198]. The anions diffuse between the chainsdue to the relatively weak interchain bonding of the polypyrrole whereas the cations formpart of the chain. The size of the anion and the spacing between the polymer chains definehow the solvent is dispersed around the chains. The resistance of the sensor (arising due tothe motion of charge carriers along the chains) varies with the changes in the concentrationof the exposed vapours. The variation of change in resistance recorded from 10 sensorswhen exposed to a 4% ethanol solution is shown in Figure 13.15. Attempts have also beenmade to use the polypyrrole nose to detect several other vapours such as isobutyl-3-methoxypyrazine, trichloroanisole, vanillin, isoalleraldehyde and methanthiol [199]. Therestriction to the electron flow when a trifluoromethyl molecule is attached directly to thepolypyrrole chain is shown in Figure 13.16. It may thus be remarked that the polypyrrolesensors respond to vapours in a similar way, though not as effectively as the human nose.

These electronic noses are for specific purposes and there is presently no universal nosethat can solve all odour sensing problems. There is thus a need to develop specific electronicnose technology appropriate for the application. This means developing sensors, materialsand appropriate pattern-recognition methods. There is thus a wide scope for the developmentof an artificial nose, based on conducting polymers, that can mimic the human nose.

Figure 13.15 Profile of the variation in resistance with 10 different sensors for 4%ethanol solution


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13.5.6 Nanowires

Nanomaterials have been found to have several technological and commercial applicationsincluding use in electronic, optical, drug-delivery and biosensing devices. The applicationof conducting polymers as nanomaterials for electronic communication between redoxenzyme and electrode surfaces is one of the active areas in biomolecular electronics.Martin and co-workers have explored the electronic, optical and electrochemical propertiesof electrochemically synthesised polypyrrole within the pores of nanoporouspolycarbonate filtration membrane walls [200]. Besides this, these researchers haveaccomplished the template synthesis of a number of polymers on a nanoscale [201, 202].It has been suggested that organic microtubules can perhaps serve as a useful replica ofvarious biological systems. In this context, Malbers and co-workers [203] have shownthat heteroarene oligomers comprising of two pyridinium groups, linked by thiopheneunits of variable length, thienoviologens, are promising candidates for molecular wires.Glucose oxidase and choline oxidase exhibit strong adsorption to these conductive layersobtained on gold electrodes. Entrapment of conducting polymers such as polyacetylene,polypyrrole, polyaniline and polythiophene in sol-gel films has recently been proposed[204]. It has been shown that it is possible to entrap conducting polymers in porousstructures such as alumina, sol-gel films and polycarbonate membranes. In this context,it has recently been demonstrated that polyaniline can be electrochemically entrappedinto the tetraethylorthosilicate (TEOS) matrix obtained on an ITO glass [205]. Cyclicvoltammetry, UV-visible and IR spectroscopy and SEM studies have been used to detectthe presence of electroactive polyaniline in a TEOS-derived sol gel matrix. The results ofthese studies indicate that conducting polymers can be utilised as ‘molecular wires’.

Figure 13.16 The restriction of electron flow in polypyrrole when trifluoromethylmolecule is attached directly to the polymer chain

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13.5.7 Electroluminescent Displays

The current interest in exploring the semiconducting properties of conducting polymersis for application to electroluminescence, i.e., using these materials as an emissive layerin LEDs [125, 206-211].

Electroluminescence is the emission of light by electrical excitation. Pope and co-workers[212] observed emission in single crystals of anthracene using silver paste electrodes at400 V. Subsequently, it was established that the phenomenon of electroluminescencenecessitates the injection of electrons from one electrode and holes from the other, thecapture of one by the other (recombination) and the radioactive decay of the excitedstate (exciton) produced by the recombination process.

Current flat-panel display technology primarily revolves around inorganic LEDs, backlitliquid crystal displays (LCDs) and vacuum fluorescent displays. Although thesetechnologies are trim and efficient compared with cathode ray tubes, they can be bulkierand much more power consuming than required for many applications. In many battery-operated devices, such as laptop computers, cellular telephones and other hand-heldinstruments, the illuminated display is the primary energy consumer. Furthermore, LEDdisplays are expensive to fabricate because of the many individual diodes required tomake up an alphabet, each with its own contacts and interconnections. These problemsforced researchers to look for other materials.

Conducting polymers have been considered to have outstanding potential for replacinginorganic light-emitting materials such as used in large area, lightweight, flexible displays.As compared to conventional fluorescent materials, conducting polymers offer thefollowing advantages:

• The characteristics of conducting polymers can be altered either by modifying theelectron structure or by chemically altering the polymer backbone.

• They operate with low dc voltage and are less power consuming compared toconventional LEDs or LCDs.

• They can be processed in the form of thin films and have potential for the productionof flexible devices.

• They can be uniformly illuminated over a large area.

• Output colours can span the whole visible spectrum.

• The polymeric materials are available at low cost.


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Owing to the above advantages, a number of conducting polymers have been producedthat emit light across the visible spectrum and which may be used to fabricate deviceswith greater quantum efficiencies [31, 213-215].

Poly (f-phenylene vinylene) (PPV) gives emission in the yellow-green region. Red emissioncan be observed by the substitution of electron-donating groups at the 2- and 5-positionson the phenyl ring [216]. Electroluminescence has been reported in different conjugatedpolymers such as poly (vinyl carbazole) (PVCZ), polyalkylfluorene [217, 218] andfluorinated polyquinoline [219]. Many efforts have been made to improve luminescenceyields in conjugated polymers both through the use of copolymers formed with segmentsof the chain having different π-π* gaps and by the synthesis of higher purity polymers.Saxena and co-workers reported LEDs based on copolymers of substituted thiophenes[220]. The structures of the copolymer have been related to their electroluminescenceproperty (Figure 13.17). The devices emit greenish-blue light in the wavelength region of550-580 nm (Figures 13.18 and 13.19), which is easily visible in a poorly lighted room.The quantum efficiencies are in the range of 0.002% to 0.01% (photons per electron) at

Figure 13.17 Relation between the structure of copolymers of 3-cyclohexyl thiopheneand 3-n-hexyl thiophene, and the quantum efficiency of light-emitting didoes based on

these copolymers

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room temperature, significantly higher than the corresponding values for poly(3-cyclohexylthiophene)-based LEDs. Another approach to improve the performance is theuse of additional semiconductor layers. These layers separate the emissive layer from theelectrodes where undesirable non radiative recombination may occur, and they alsotransport both carrier types to the recombination zone more efficiently [221].

Figure 13.18 Photoluminescence spectra of copolymers synthesised using ratios of 3-n-hexyl thiophene: 3-cyclohexyl thiophene (I) 1:9, (II) 2:3, (III) 1:1, (IV) 9:1

Figure 13.19 Electroluminescence spectra of copolymers synthesised using ratios of 3-n-hexyl thiophene: 3-cyclohexyl thiophene (I) 1:9, (II) 2:3, (III) 1:1, (IV) 9:1


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To date, these conjugated polymers have been reported to exhibit light emission in theentire visible spectrum [222], with quantum efficiency over 4% [223], response time of1 μs [224], brightness of about 106 cd/m2 [225] and device lifetime of 1000 hours [226].Recently, it has been shown that polymer displays can be fabricated using methods suchas screen printing and ink-jet printing [227]. The six processable polymers covering thewhole visible spectrum are summarised in Table 13.1.

Considerable progress has been made towards the understanding of the electronicprocesses that control the properties of the conducting polymer based electroluminescentdiodes [228-231]. One of the problems still confronting the technologists relates to thered-shifted blue emission in solid films of large-gap polymers including PPV-based laddercopolymers. It is expected that many other problems like durability under drive andunder storage conditions, degradation at the polymer-metal interface and the formationof dark spot defects, etc., will soon get solved resulting in speedy commercialisation ofthe polymer electroluminescent displays.

Owing to their advantages, the potential for electroluminescent devices is enormous in,for example, radio receiver, toys, small hand-held devices, large panel displays, stereoequipment and automobile dashboard, notebook computer screens, etc. Many majorcompanies such as Eastman Kodak, Hewlett Packard and Phillips, along with severalsmall entrepreneurial ventures, such as Uniax Corp. (USA) and Cambridge DisplayTechnology (UK), are attempting to grasp the huge market for electroluminescent displays.Large-scale flexible displays, roll-up TV screens, full colour polymer displays andluminescent room lighting are still a long way off. One hurdle is that different polymersare needed to emit light in different colours, and they come with a range of properties.However, in the near future it appears that the two technologies will coexist until themarket chooses the winner. Bolognesi and Botta have discussed in detail in Chapter 5 theapplication of polymers for light-emitting diodes.

13.5.8 Microactuators

There is a considerable scope for the development of materials suitable for the fabricationof electromechanical actuators having very small dimensions. Microactuators are devicesthat convert electrical energy to mechanical energy. It has been suggested that conductingpolymers offer attractive alternatives for actuators that function more analogously tonatural muscle. Conducting polymer electromechanical actuators are based on the largedimensional changes that occur from the redox doping of any of the host conductingpolymers such as polypyrrole, polyaniline and polyalkylthiophene [232]. Thesedimensional changes are largely due to the volume required to accommodate cations oranions, together with the co-intercalating solvating species. The electrochemical actuators


424


have direct similarities to both electrochromic displays and conducting polymer batteries,and the electromechanical cycle corresponds to both the chromatic switching cycle of anelectrochromic display and the charge-discharge cycle of a battery.

Reversible electrochemical actuators comprise of an anode, a cathode and a separatingelectrolyte. Either electrode or both can be conducting polymers. However, other redoxmaterials such as graphite have also been used as electromechanical electrodes. The natureof redox processes for conducting polymer electrodes has been found to be rate dependent.For instance, an electrode that is anion doped can be reduced either during the additionof cations or the removal of anions.

Conducting polymers have also been utilised for fabrication of hydrostatic and extensionalactuators. The hydrostatic actuators provide mechanical work using net volume changeduring electrochemical redox processes. The net volume change is the overall volumechange of the anode, the cathode and the electrolyte. The extensional actuators makeuse of either linear or biaxial changes in conducting polymers to obtain mechanicalwork. The dimensional changes can either be individual or relative changes in thedimensions of two or more elements. Compared to alternative actuator materials, thedevelopment of conducting polymer actuators is still in its infancy. The discovery thatconducting polymer actuators can be operated at lower voltages indicates that these canbe utilised for medical applications. Another advantage of conducting polymer lies in thefact that it is possible to use a conducting polymer actuator without an external source.However, subsequent cycles would require external recharging.

The performance of conducting polymers is critically dependent on molecular diffusion,which restricts the response of these actuators. This is because fast response time canonly be achieved using very thin electromechanical elements. Thus conducting polymersare of greatest interest for either large actuators or microactuators using parallel arraysof thin electromechanical elements. It may be emphasised that many of the proposedapplications of microactuators, such as microrobotics for exploration and repair of thehuman body to microscopic machines for the manipulation and alteration of microndimensional objects, are futuristic. Some of the shorter term goals, however, includemicrovalves, microtweezers and micropositioners. Kaneto and co-workers [233] havegiven (Chapter 8) an excellent review of conducting polymer based microactuators.

13.6 Conclusions

It has been shown that electrically conducting polymers are versatile electronic materialsthat display a unique range of properties for application to molecular electronics. However,there are several problems associated with these materials, namely reproducibility, stability

425

and processability. A better understanding of the relevant principles is gradually emergingto accelerate the drive towards the wide range of practical applications [234-239]. It maybe remarked that the field is still expanding as new advances are transferred to commercialapplications. Recent reports promise the use of regioregular conducting polymers andcomposites with nanoparticles in optoelectronic devices. Besides this, the use of conjugatedpolymers in integrated biosensors is another area which needs to be exploited.

Acknowledgements

We are grateful to Dr. K. Lal, Director, NPL, India, for his interest in this work. VibhaSaxena is thankful to CSIR, India, for the award of Research Associateship.

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440


441

Abbreviations and Acronyms

ηEL Electroluminescence efficiency

ηPL Photoluminescence efficiency

3MOT 3-methyl-4′-octyl-2,2-bithiophene-5,5′-diyl

3ODOP 3-Octadecanoylpyrrole

3ODP 3-Octadecylpyrrole

ac Alternating current

AES Auger electron spectroscopy

AFM Atomic force microscopy

Alq3 Aluminium 8-hydroxyquinoline

AMPSA 2-Acryloamido-2-methyl-1-propane sulfonic acid

aPS Atactic polystyrene

APTES (3-aminopropyl)trimethoxysilane

ATR Attenuated total reflectance

BA Benzyl alcohol

BABTDS Bisacryloxybutyl tetramethyl disiloxane

BDOH-PF Poly(9,9-bis(3,8-dioxaheptyl-fluorine-2,7-diyl)

BSA Benzenesulfonic acid

CB Conduction band

ChOx Cholesterol oxidase

CMOS Complementary metal oxide semiconductor

CSA Camphor sulfonic acid

CTFE Chlorotrifluoroethylene

CV Cyclic voltammagram

CVD Chemical vapour deposition

CYEPL Cyanoethylpullulan

442


DBP Dibutylphthalate

DBS Dodecylbenzene sulfonate

DBSA Dodecylbenzoyl sulfonic acid

dc Direct current

DEC Diethylene carbonate

DEZ Diethylzinc

DFWM Degenerate four wave mixing

DMF Dimethylformamide

DNA Deoxyribonucleic acid

DNQ 1,2-Diazonaphthoquinone

DPE Discrete particle encapsulation

DR1 Disperse Red 1

DSC Differential scanning calorimetry

EA Electron affinity

EC Ethylene carbonate

e-e Electron-electron

e-h Electron-hole

EL Electroluminescence

ELISA Enzyme linked immunosorbant assay

ENDOR Electron nuclear double resonance

ENFET Enzyme field-effect transistor

EO Electrooptical

EPR Electron paramagnetic resonance

ES Emeraldine salt

ESCA Electron spectroscopy for chemical analysis

ESR Electron spin resonance

ETL Electron transporter layer

EV Electric vehicle

FAD Flavin adenine dinucleotide

FADH2 Flavin adenine dinucleotide (reduced form)

443


FBP Fluorinated polymethacrylate

FET Field-effect transistor

FIT Fluctuation induced tunnelling

FTIR Fourier transform infrared

FWHM Full wave half maximum

GC Gas chromatography

GLDH Glutamate dehydrogenase

GOD Glucose oxidase

GPC Gel permeation chromatography

HMDSO Hexamethyl disiloxane

HOMO Highest occupied molecular orbital

HPC-Py Pyrene-labelled hydroxypropylcellulose

HPLC High pressure liquid chromatography

HRP Horseradish peroxidase

HSA Human serum albumin

HTL Hole transporter layer

I(t) Emission intensity

IC Internal conversion

IEEE Institute of Electrical and Electronics Engineers

IGFET Insulated-gate field-effect transistor

IP Ionisation potential

IPS Isotactic polystyrene

IR Infrared

ISC Intersystem crossing

ISFET Ion-selective field-effect transistor

ISMET Ion-selective microelectrochemical transistor

IT Information technology

ITO Indium tin oxide

KC8 Potassium graphite

LB Langmuir Blodgett

444


LC Liquid crystalline

LCD Liquid crystal display

LCST Low critical solution temperature

LDH Lactate dehydrogenase

LDM Laser displacement meter

LED Light-emitting diode

LOD Lactate oxidase

L-PPP Ladder PPP

LS Leuco-emeraldine salt

LUMO Lowest occupied molecular orbital

MAdMA 2-Methyl-2-adamananol methacrylate

MC Magnetoconductance

ME Molecular electronics

MEH-PPV Poly[2-(2´-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene]

M-I Metal-insulator

MIS Metal-insulator semiconductor

MISFET Metal-insulator semiconductor field-effect transistor

MLMA Mevalonic lactone methacrylate

MLR Multilayer resist

MOPPV Poly(2,5-dimethoxy-p-phenylene vinylene)

MOSFET Metal oxide semiconductor field-effect transistor

MR Magnetoresistance

MRS Microresists

MS Mass spectrometry

MTHF Methyltetrahydrofuran

MW Microwave

MZ Mach-Zehnder

NAD Nicotinamide adenine dinucleotide

NADH Nicotinamide adenine dinucleotide (reduced form)

n-BCMU-PDA n-Butoxycarbonylmethyl urethane

445


NLO Non linear optical

NMP N-methyl-2-pyrrolidinone

NMR Nuclear magnetic resonance

NSOM Near-field scanning optical microscopy

ODMR Optically detected magnetic resonance

OMA Organised monolayer assembly

P(PMMA- Poly(methyl methacrylate-co-3-oximino-2-butanone methacrylate-co-OM-MAN) methacrylonitrile)

P/GC/MS Pyrolysis/gas chromatography/mass spectroscopy

PAAM Polyacrylamide

PAB Poly(o-aminobenzoic acid)

PAN Polyacrylonitrile

PANI Polyaniline

PaS Pernigraniline salt

PAT Polyalkylthiophene

PAV Poly(arylene vinylene)

PBI Polybenzimidazole

PBS Phosphate buffer saline

PC Propylene carbonate

PCM Portable conformable masking

PCz Polycarbazole

PDA Polydiacetylene

PDABT Poly(4,4-dialkyl-2,2′-bithiophene)

PDF2/6 Poly(9,9-di(ethylhexyl)fluorine

PDTB Poly(1,4-di(2-thienyl)benzene

PEB Polyemeraldine base

PEDOT Poly(3,4-ethylenedioxythiophene)

PEI Polyethyleneimine

PEO Polyethylene oxide

PET Polyethylene terephthalate)

446


PFu Polyfuran

PHP Para-hexaphenyl

PI Polyimide

PL Photoluminescence

PMAN Poly(o-methoxyaniline)

PMeT Poly(3-methyl)thiophene

PMMA Polymethyl methacrylate

PNIPAM Poly(N-isopropylacrylamide)

PNMA Naphthyl-substituted polymethacrylate

PP/VD Plasma polymerisation/vapour deposition

PPMA Poly(phenyl methacrylate)

PPO Poly(2,6-dimethylphenylene oxide)

PPP Poly(p-phenylene)

PPPY Polyphenylenediyl-pyridinediyl

PPV Poly(p-phenylene vinylene)

PPy Polypyrrole

PS Polystyrene

PSSA Polystyrene sulfonated acid

PSTF Polyurethane with symmetrical substituted tris-azo dye withfluorinated alkyl units

PT Polythiophene

PTBPY Polythiophenediyl-bipyridenediyl

PTCDA 3,4,9,10-Perylene tetracarboxylic dianhydride

PTPY Polythiophenediyl-pyridinediyl

PTS p-Toluene sulfonate

PTS-PDA Poly[bis(p-toluenesulfonate) of 2,4-hexadiyne-1,6-diol]

PTV Polythienylene vinylene

PVC Polyvinyl chloride

PVCZ Polyvinylcarbazole

PVD Physical vapour deposition

447


PVdF Polyvinylidene fluoride

PVN Polyvinylnaphthalene

PVP Polyvinylpyrrolidine

PVS Polyvinylsulfonate

q-1D Quasi one-dimensional

QCM Quartz crystal microbalance

QPM Quasi-phase-matching

RDM Random dimmer model

RF Radio frequency

RGB Red green blue

RIE Reactive ion etching

SA Salicyclic acid

SAW Surface acoustic wave

SBAC Symmetrically substituted benzylidene aniline with chloride

SCE Standard calomel electrode

SEI Solid electrolyte interface

SEM Scanning electron microscopy

SHG Second-harmonic generation

SPIE International Society for Optical Engineering

SPM Self phase modulation

SPR Surface plasmon resistance

SSH Su Schrieffer and Heeger

StA Stearic acid

STM Scanning tunnelling microscopy

T-BOC t-Butyl carbonate

T-BOC-PMS Poly(p-t-butyloxycarbonyloxystyrene)

TCNQ Tetracyanoquinodimethane

TCR Temperature coefficient of resistance

TEM Transmission electron microscopy

TEOS Tetraethylorthosilicate

448


Tg Glass transition temperature

TGA Thermogravimetric analysis

THF Tetrahydrofuran

THG Third-harmonic generation

TMT Tetramethyltin

TOF Time of flight

TPD Triphenylamine derivative

TRIDSB Poly(1,20-(10,13)-didecyl)distyrylbenzene-co-1,2-(4-(p-ethylphenyl))triazole

TSFET Taste-sensing field-effect transistor

TSO p-Toluene sulfonate

TTF Tetrathiafulvaline

TTL Transistor-transistor logic

UPS Ultraviolet photoelectron spectroscopy

UV Ultraviolet

VB Valence band

VLSI Very large scale integration

Von Onset voltage

VR Vibrational relaxation

VRH Variable-range hopping

WL Weak localisation

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction

?BL ?-Butyrolactone

449

Contributors

S. AnnapoorniDepartment of Physics and AstrophysicsUniversity of DelhiDelhi – 110007India

Alberto BolognesiIstituto di Chimica delle MacromolecoleConsiglio Nazionale delle RicercheVia E. Bassini 1520133 MilanoItaly

C. BottaIstituto di Chimica delle MacromolecoleConsiglio Nazionale delle RicercheVia E. Bassini 1520133 MilanoItaly

Asha ChaubeyBiomolecular Electronics & Conducting Polymer Research GroupNational Physical LaboratoryDr K.S. Krishnan RoadNew Delhi-110012India

Jean-Claude DuboisUniversité Pierre et Marie CurieLaboratoire de Physico-chimie des PolyméresTour 44.1er et Case 1854 Place Jussieu.75252 Paris CedexFrance

450


Anamika GambhirBiomolecular Electronics and Conducting Polymer ResearchNational Physical LaboratoryDr. K.S. Krishnan RoadNew Delhi – 110012India

Manju GerardDepartment of ChemistryAllahabad Research Institute (Deemed University)NainiAllahabad-210007India

Atsunori HiratsukaHealth Care GroupLiving Environment Development CenterMatsushita Electric Industrial Co., Ltd.3-1-1 Yagumo-naka-machiMoriguchi CityOsaka 570-8501Japan

Toshikuni KainoInstitute for Chemical Reaction ScienceTohoku University2-1-1 KatahiraAoba-kuSendai-shi, Miyagi980-77 Japan

Masmitsu KanekoDepartment of Computer Science and ElectronicsKyushu Institute of TechnologyIizuka, Fukuoka, 820-8502Japan

Keiichi KanetoDepartment of Computer Science and ElectronicsKyushu Institute of TechnologyIizuka, Fukuoka, 820-8502Japan

451

Contributors

Isao KarubeKarube Laboratory Biosensor DivisionResearch Centre for Advanced Science and TechnologyUniversity of Tokyo4-6-1 KomabaMeguru-kuTokyo 153-8904Japan

Bansi D. MalhotraBiomolecular Electronics & Conducting Polymer Research GroupNational Physical LaboratoryDr K.S. Krishnan MargNew Delhi-110012India

Reghu MenonDepartment of PhysicsIndian Institute of ScienceBangaloreIndia – 560012

Amalesh MukhopadhyayDepartment of Science and TechnologyNew Mehrauli RoadNew Delhi – 110016India

K.S. NarayanChemistry and Physics of Materials UnitJawaharlal Nehru for Advanced Scientific ResearchJakkurPO Bangalore 560064India

Tim H. RichardsonApplied Molecular Engineering GroupDepartment of Physics & AstronomyUniversity of SheffieldHounsfield RoadSheffield S3 7RHUK

452


Vibha SaxenaBiomolecular Electronics and Conducting Polymer ResearchNational Physical LaboratoryDr. K.S. Krishnan RoadNew Delhi – 110012India

Bruno ScrosatiDepartment of ChemistryUniversity ‘La Sapienza’00185 RomeItaly

Rahul SinghalBiomolecular Electronics and Conducting Polymer ResearchNational Physical LaboratoryDr. K.S. Krishnan RoadNew Delhi – 110012India

Barbara WandeltTechnical University of LodzFaculty of ChemistryDepartment of Molecular PhysicsZeromskiego 11690924 LodzPoland

453

Index

Main Index

A

absorbance 372absorption 100, 111acetone 398acid-amine ion linkage 210acid anhydride bonds 204-205acid photogenerators 1992-acrylamido-2-methyl-1-propane-

sulfonic acid (AMPSA) 48acrylate ester 209actuators 255, 267affinity biosensors 289-290Ag/AgCl electrode 319Alexandrium affine 289alkanethiol-stabilised metal nanoparticles

357alkanoic acid 383alkoxy substituted PPV 1693-alkyloxymethoxy thiophene 763-alkyl-substituted thiophenes 3453-alkylthiophene 164Alq3 see aluminium 8-hydroxyquinolene

(Alq3)alternate layer Langmuir trough 385,

386alternate layer LB deposition technique

385alternate layer LB films 386aluminium 8-hydroxyquinolene (Alq3),

electroluminescence 141aluminium 3703-aminopropyltriethoxysilane 3523-aminopropyltrimethoxysilane (APTES)

2892-aminopyridine 106

amperometric biosensors 301, 303, 313,317electron transfer in 303-304response measurements 319

anisotropyconductivity 53electroluminescence 169electrolytic expansion in polyaniline

films 266etching techniques 190

anode 370, 371anthracene 121, 367antibodies 413ArF 197, 200aromatic hydrocarbons 104Arrhenius relation 311artificial muscles 255arylene-bisphosphylidenes 148atactic PS (aPS) 123-125atomic force micrograph (AFM) 404,

405attenuated total reflectance (ATR) 283Au-P30T 358, 360AX 1000 photoresist 201AZ Electronic Materials 201azo benzene 255, 256azo-dye-functionalised main-chain

polymersynthetic scheme 79χ(3) wavelength dependence 80

azo-dye-functionalised poled polymers84-87

azo-dye-functionalised polymerwaveguides 90-93

azo-dye polyester 76


454

B

backscattering lifetime 41band gap 5, 148, 394, 395band structure 4, 5batteries

C/LiMO2 polymer lithium-ion 244high energy density 217Li-AyBz lithium 221Li/LiClO4-EC-DMC-PAN/LiCryMn(2y)

O4 235Li/PEO-LiCF3SO3 + çLiAlO2/LiMn2O4

225lithium 218-239, 243-245lithium-ion 217, 230-231rocking chair 230-231SnO2/LiClO4-EC-DMC-PAN/

LiNi0.8Co0.2 O2 237BDOH-PF 163benzene 112, 116benzenesulfonic acid (BSA) 260benzyl alcohol (BA) 123benzylidene aniline 77β-carotene 376, 403bias voltage 144, 146bifunctional device 356bifunctional reagents 307bimolecular photophysical processes

103-104bimolecular photoprocesses 110bimorph actuator 257binder 272biocomponents 301-302biological fluids, analysis 297biological parameters 297biomedical applications 272biomimetic actuators 255biomolecules, immobilisation 305-309,

412-413biosensors 272-273, 275-276, 287, 309,

411-4141st generation 300

2nd generation 3003rd generation 3004th generation 300amperometric see amperometric

biosensorsblood glucose 298cholesterol 323-324classification 313conducting polymer-based 297conductometric 315construction 299-300design 412development 297, 298effects of storage time 312-313electrochemical 313, 315fabrication 413general principle 412generations 300glucose 318, 413healthcare 318importance of conducting polymers

302-304key attraction 297market 298medical diagnostic application 297microbiosensors 276non medical application market 297optical 314potential applications 412potentiometric 301, 315-317research and applications 297-300response measurements 313schematic 299, 411selectivity 301sequence of events 299-300specificity 301types 313-317

2-(4-biphenyl)-5-(tert-butylphenyl-1,3,4-oxadiazole (PBD) 158

bipolaron 4, 6, 9-12, 142, 377, 396energy levels and occupied localised

states 11

455

Index

formation 9, 11instability 17interchain transport 20negative 11

1,3-bis-1-naphthylpropane 1161,3-bis-2-naphthylpropane 116bis-acryloxybutyl tetramethyl disiloxane

(BABTDS) 208bisazide 196, 201blood glucose biosensor 298bobbin-type configuration 231Boltzmann constant 21Boltzmann distribution 107bovine serum albumin 413branched side chains 150breakdown voltage 227Brillouin zone 52-bromoethylamine 275buried waveguide, end-face 92-93butraldehyde 275Buttiker-Landaur conductance formula

26

C13C NMR studies 406C60 343cadmium selenide (CdSe) 341, 353, 357cadmium sulphide (CdS) 341, 353-355calcium 370camphor sulfonic acid (CSA) 38capacitance-voltage characteristics 408carbazole 107carbon-carbon bonds 112carbon-carbon distance 41carbon-carbon double bonds 375carbon-carbon single bonds 375carbon nanotubes 267carboxylic acid 195carrier density 377carrier mobility 375carrier transport 369

catalytic biosensors 287-288cathode 370centralised testing 297chain conformation effect on excimer

emission 122-126chain densification effect 113chain interaction 9chain interruptions 21chain length 112-113chain orientation 13, 169chain scission reactions 119chalcogenide glass fibre 84channel waveguides 75, 87charge carriers 375charge-discharge cycle 238charge-discharge process 242charge generation and separation 346-

347charge injection 143, 146, 148charge separation 349charge separation nanoparticle-polymer

systems, photophysics 346-360charge transfer excitons 143charge transfer process 4, 348, 394charge transfer TTF:TCNQ-type

molecules 379charge transport 3-36, 146, 406charge transport carriers 6-12charge transport models 18-27chemical doping 4, 143chemical sensors 287chemical vapour deposition (CVD) 276chloroform 112, 124, 398

polynaphthyl methacrylate in 111chloroform-cyclohexane 112chloromethylated polystyrene 203chlorophyll 376cholesterol biosensors 323-324chromophores 99, 107, 113, 115, 118,

119association of 103

(CH)x-FeCl3 42


456

(CH)x-I2 42cis-polyacetylene 3, 395cis-1,4-polyisoprene 196cis-trans photoisomerisation 255Clariant Corporation 201C/LiClO4-EC-PC-PAN/LiCryMn(2-y)O4

234C/LiClO4-EC-PC-PMMA/PPy 244C/LiMO2 polymer lithium-ion batteries

244ClO4-(CH)x 43ClO4-doped polyacetylene, normalised

resistivity (ρ) versus temperature 43ClO4--doped polyacetylene 14coatings 272coin-type cell configuration 234collapse transition 126-135collisional energy transfer 118colour components 376colour purity 371, 373

enhancement 374complementary metal oxide

semiconductor (CMOS) circuits 284concentration quenching 103conducting films 271, 286

applications 403properties 403

conducting polymers 1, 6, 217, 255, 257,343, 375-378analytical chemistry 412applications 397as odour sensing devices 416-417band gap 395biosensors 297-239characterisation 397, 404-406characteristics 419charge transport models 18-27conduction mechanism 305electrical transport properties,

summary 63-65electrolytic expansion 267-268

in molecular electronics 393-439applications 394, 397

magnetic susceptibility 56-58molecular devices based on 406-424specific heat 56-58structures 378, 395synthesis 304, 397-399see also specific types and applications

conducting properties 3-4conduction band 4, 5, 10, 16, 18, 142-

143conductivity see electrical conductivityconductometric biosensors 315configuration co-ordinate 7conformation change of PPMA 113conformation effect of polymer chain

110-118conjugated polymers 1, 5, 342, 367, 375

anisotropic electrical and opticalproperties 142

band gap 148charge transport 3-36light emission in entire visible

spectrum 422-423physical properties 397physics of 142-144preparation methods 344-346synthesis 151

conjugation length 376consumer testing 297contraction ratio 260, 261contraction ratio of polyaniline films 267COP 206, 207core channel waveguide 88core-shell synthesis 341Coulomb interaction 9, 11Coulomb potential 16covalent binding 308, 412covalent bonding 307critical states 58-63crosslinking 307

through anhydride 204-205

457

Index

crosslinking centres 223crystal-crysal transition 134crystalline iPS 123-125, 131, 133Cu(II) ions 410current density 375current-voltage characteristics 371, 372,

408cyano-PPV 157cyanothylpullulan (CYEPL) 411cyclic voltammogram (CV) 257, 259,

260cyclised polyisoprene 2033-cyclohexyl thiophene 420, 421

D

data acquisition system 105data analyser 105deactivation, Birks’ scheme 103decentralised testing 297deep-UV photoresists 197degenerate state 7delayed emission 111delayed fluorescence 102, 110density of states 16, 57DFWM 761,2-diazonaphthoquinone (DNQ) 193

photodecomposition 194diblock copolymers 3462,7-dibromo-9,9-bis(3,6-

dioxaheptyl)fluorene 163dichloro-p-xylene 149dielectric materials 271dielectric relaxation 404N,N-diethylaminonitroazobenzene 79diethylzinc (DEZ) 285differential scanning calorimetry (DSC)

123, 397diffraction efficiency of waveguide grating

91diffusion coefficient 41, 257, 260, 262diffusion constant 257

dimethylsexithiophene (CMSxT) 411diodes 406-408dipole-dipole interaction 118discharge-charge cycle 237discrete particle encapsulation (DPE) 345disorder 9, 12, 41, 50

influence on transport properties 14-15

disordered metallic regime 41DNA 413-414

immobilisation 325DNA biosensors 324-325DNA sensor 290dodecylbenzene sulfonate (DBS) 324dodecylbenzoyl sulfonic acid (DBSA) 38dopant-chain interaction 16doping 4, 143, 190, 242, 377, 393, 396,

397, 404influence on transport properties 15-

16double-layer LED 158down-chain energy migration 118DR1 92drive voltage 369, 370dual lithium ion 244

E

elastic scattering 41electrical conductivity 21, 40-49, 141,

305, 394, 396anisotropy 53Arrhenius plots 224, 229power law behaviour 58temperature dependence 41, 46

electrical properties of doped conjugatedpolymers 37-68

electrical transport properties ofconducting polymers, summary 63-65

electric field 377intensity 11poling technique 87


458

electric vehicles (EV) 217electrochemical biosensors 313, 315electrochemical doping 4electrochemical entrapment 307-308,

412electrochemical oxidative polymerisation

398electrochemical stability 227electrochemical synthesis 167electrode-less microwave or high

frequency reactors 280-281electrodes 369

preparation 304-305electroluminescence 166, 347

aluminium 8-hydroxyquinolene (Alq3)141

anisotropy 169efficiency 145, 150, 157, 159, 161,

166, 168, 170little or no 371measurement 348organic 367, 369, 370polarised 168-170polymer 368-374

electroluminescence response 354electroluminescence spectra 421electroluminescent devices

physics of 142-147potential for 423

electroluminescent displays 419-423electrolytic capacitors 274electrolytic deformation 256

measurements of 257electrolytic expansion 256

anisotropy of, in polyaniline films266

applications 268-269measurement of 258pH dependence of 262time response of 265

electrolytic expansion in conductingpolymers 267-268

electromechanical actuators 423electron-beam irradiation 205electron-beam lithography 186electron-beam resists 202-206, 273electron correlation 6electron-electron (e-e) interactions 4-6,

13, 52, 53, 142electron-energy-loss spectroscopy 11electron-hole (e-h) pairs, recombination

of 371electron-hole (e-h) separation 142electron-hole (e-h) transporter 167electronically conducting polymers 217,

242-245electronic encapsulation 284electronic excitation energy 118electronic ground state 4-6electronic nose 415-417electronic tongue 414-415electron magnetic resonance

spectroscopy 8electron nuclear double resonance

(ENDOR) 8electron paramagnetic resonance (EPR)

spectroscopy 23electron-phonon coupling constant 21electron-phonon interactions 4, 5, 6, 142electron-phonon scattering 13electrons 24electron spectroscopy for chemical

analysis (ESCA) 282, 283electron spin resonance (ESR) 16, 404electron transfer in amperometric

biosensor 303-304electron transporter 158electro-optical (EO) materials 86-87electropolymerisation 308electropolymerised conducting films 313ellipsometric technique 406EL-V (EL intensity versus bias voltage)

147emeraldine salt (ES) 260

459

Index

emission efficiency 148emission spectrum 371emission wavelength 373emitter materials 371, 373emitting layers 369emulsion polymers 272encapsulation 371end-to-end cyclisation 117energy band diagram 396energy bandgap 376energy gap 3, 4, 375, 376, 395energy migration coefficient 126energy migration processes 118-121energy migration studies 109-110energy storage and conversion 217energy transfer 110, 118-121

efficiency 118fluorescence measurements of 121types 118

environmental monitoring 326enzyme acitivity, effect of temperature

311-312enzyme activitiy, determination of 309-

310enzyme activity profile 311enzyme electrodes 309-313enzyme field-effect transistors (ENFETs)

409enzyme immobilisation 304-309enzyme-immobilised electrode 274enzyme linked immunosorbant assay

(ELISA) 326enzymes 302, 303

optimum pH 310-311enzyme support system 287etching 190, 191ethyl acetate 112ethylene carbonate (EC) 222excimer dissociation 115excimer emission 113

chain conformation effect on 122-126excimer fluorescence 106-135, 111

excimer fluorescence lifetimes, iPS gel129

excimer formation 115effect of temperature 115molecular weight effects on 113

excimer photophysics for two-phasesystem 133

excimers 103, 104, 143exciplexes 104, 143excitation, Birks’ scheme 103excitation energy 119excited singlet state 100, 101excited triplet state 100exciton generation 146excitons 347-348exciton trap 109expansion ratio, dependencies of 260-

262extensional actuators 424external electrode reactors with RF power

supply 280

F

FeCl3-(CH)x 43FeCl3--doped polyacetylene 14FeCl3 polymerisation 167Fe2O3 344Fermi energy 13Fermi level 4, 5, 56, 57, 145Fermi velocity 41ferrocene carboxylic acid 319ferrocene-modified siloxane polymers

275field-effect mobilities 410field-effect transistors (FETs) 379, 409-

411field-induced transition 61

W versus temperature 60films see conducting films; thin films;

ultrathin films and specific typesflavin adenine 307


460

flavin adenine dinucleotide (FADH2) 275Flory theory 111fluctuation induced tunnelling (FIT)

model 23-25fluorescence 100, 101, 109, 111

decay 125definition 101delayed 102, 110investigations 99lifetimes 105‘normal’ 102of polymers 106-135

in gel state 122-135in solution 110-121

study methods 105fluorescence emission spectrum for iPS gel

132fluorescence measurements of energy

transfer 121fluorescence probe method 99-100fluorescence spectra 111

of phenyl methacrylate 114of poly(phenyl methacrylate) 113-115

fluorescence spectroscopy 105fluorinated polymeric ethers 274fluorinated polymethacrylate (FPB) 207

sensitivity curve 207fluorinated polyquinoline 420fluorosphores 341food industries 297Frenkel exciton 143Fresnel reflection loss 89-90Frohlich conductivity 6

G

GaAs 285gas chromatography (GC)/mass

spectrometry (MS) 415gel electrolytes 230, 232gel state, fluorescence of polymers in

122-135

gel-type electrolytes 227-229configuration 226

glass transition temperatures 86glow discharge 277glucose biosensors 318, 413glucose oxidase (GOD) 274, 275, 287-

288, 297, 317, 318glutamate dehydrogenase (GLDH) 314glutaraldehyde 413GMC 206, 207GOD-PEB films 413GOD-stearic acid monolayers 413gold nanoparticles 346, 357-360gold-polythiophene blends 357-360GPC 404graphite anode 233ground state energy 7

H

1,4-heptadiene 397heteroaromatic polymers 76

molecular structure of 80χ(3) wavelength dependence 82

heterojunctions 406hexamethyldisilazane 284hexamethyldisiloxane (HMDSO) 284,

287, 2903-n-hexyl thiophene 420, 421high energy density batteries 217high pressure liquid chromatography

(HPLC) 415hole transporter 158, 169HOMO (highest occupied molecular

orbital) 144-145, 376hopping/tunnelling transport 24Hubbard model 5, 6Huckel model 15humidity content 241humidity sensors 287hybrid waveguides 93-94hydrochloric acid etchant 371

461

Index

hydrostatic actuators 4248-hydroxyquinolene (Alq3) 168

electroluminescence of aluminium 1414-hydroxystyrene 200, 201

I

I-(CH)x 43, 44, 45immobilisation

biomolecules 305-309, 412-413DNA 325enzyme 304-309methods 305-309

immunosensors 326imprinted polymers 271impurity potentials 16indium tin oxide see ITOinelastic electron-electron scattering 13,

41inelastic electron-phonon scattering 45inelastic scattering 41information technology 69, 367inorganic nanoparticles 346insulated gate field-effect transistors

(IGFETs) 410insulating states 58-63insulators 284-285interchain coupling, influence on

transport properties 17-18interchain hopping 19interchain interactions 4, 143interchain transfer 16interface materials 272internal conversion (IC) 100-102internal electrode reactors

with dc power supply 280with RF power supply 280

intersystem crossing (ISC) 100, 101, 103intramolecular excimer formation 117iodine 394iodine-doped polyacetylene 19, 42, 141

conductivity (both parallel and

perpendicular) versustemperature 45

resistivity versus temperature 44iodine-doped trans-polyacetylene 3, 13iodine-doped Tsukomoto (CH)x 37ion-beam lithography 186ionically conducting polymers 217-242ionic attraction 387ion-selective field-effect transistor (ISFET)

273, 287, 409ion-selective microelectrochemical

transistor (ISMET) 410iPS/BA gel 126

time-resolved fluorescence emissionspectra 130

iPS gel 123crystalline 123-125, 131, 133density measurements 129excimeric fluorescence lifetimes 129extended conformation 128fluorescence decay parameters 128fluorescence emission spectrum 128steady-rate fluorescence emission

spectrum 132time-resolved fluorescence emission

spectra 131IR spectroscopy 283ISC see intersystem crossing (ISC)isobutylene 200isotactic polystyrene see iPSITO 144, 151, 159, 170, 353, 371, 413,

418ITO/BDOH-PF/Ca 163ITO-coated glass 141ITO/PPV/Al 150ITO/PPV/cyano-PPV/Al 158ITO/PVCZ/alkoxy PPP/Ca 161I-V (current intensity versus bias voltage)

147

J

Jablonski diagram 100


462

K

Kerr switch 83Kivelson and Heeger model 16KPR resist 194, 195KrCl 197KrF 197

L

lactate biosensors 321-322lactate dehydrogenase (LDH) 312, 314,

321-322lactate oxidase (LOD) 312-313, 321-322lactate/pyruvate substrate recycling 322ladder PPP (L-PPP) 161Landau-Ginzberg model 17Langmuir-Blodgett deposition 381, 401

X-type 400-402Y-type 400-402

Langmuir-Blodgett films 271, 318, 381,384, 385, 387, 399-404, 408, 413applications 403-404pressure-area isotherm 400

Langmuir-Blodgett technique 159, 168,308, 413

Langmuir films 381, 383Langmuir trough 381, 383-385, 401laser power and SHG efficiency 90layer-by-layer polyelectrolyte self-

assembly process 387LCST see lower critical solution

temperature (LCST)least-squares analysis 105LEDs 141-183, 355, 357, 367, 411, 419-

421characterisation 147-148complex polymeric structures used as

active materials 154-157degradation 170double layer 158lifetime 170mechanisms involved 147

microcavities 170-171narrowing of emission spectrum in

microcavity 171needle-like 151physical mechanisms occurring in 142physics of 144-146polymeric structures 148PPV derivatives used as active

materials in 152-154prototypes 142recent developments 168-171single layer 144structural control 148working mechanism 145

leuco-emeraldine salt (LS) 260Lewis acids 196, 199, 223Li+ cations 219Li+ ion 223Li-AyBz lithium battery 221LiClO4 220LiClO4-EC-DMC-PAN 232-234, 237LiClO4-PC 243LiCoO2 231LiCoO2 cathode 236LiCryMn(2-y)O4 electrode 234lifetime parameter 105, 127light emitting diodes see LEDslight-emitting polymers 367Li/LiClO4-EC-DMC-PAN/LiCryMn(2-y)O4

battery 235Li2MnO4 235LiMn2O4 231lineweaver-Burke plot 321LiNiy Co1-yO2 231Li/PEO-LiCF3SO3 + γLiAlO2/LiMn2O4

battery 225Li/(PEO)20LiI-EC-Al2O3/FeS2 cell 224-225LiPF6-EC-DMC 232LiPF6-EC-DMC-PAN 225LiPF6-EC-PC-PAN 227, 229, 230liquid crystal devices 71liquid crystal displays (LCDs) 168, 419

463

Index

liquid crystalline polyimide, change inlifetime during heating 135

liquid electrolytes 217, 232lithium batteries 218-239lithium battery 243-245

R&D projects 223lithium-doped conducting polymer 243-245lithium intercalation-deintercalation

processes 232-234, 238lithium-ion battery 217, 230-231

schematic representation 231lithium metal anode 243lithium metal oxide cathode 233lithium polymer electrolytes 218-239lithium triflate 163lithographic methods 188

contact mode 188projection mode 188proximity mode 188

lithography 286Li4Ti5O12/LiPF6-EC-PC-PAN/LiMn2O4

cell 238Li/TiS2 cell 222LiX 218-219lower critical solution temperature

(LCST) 121, 134luminance enhancement 371luminance output 371luminance-voltage characteristic 372luminescence 101-103

basic principles 100luminescence quenching 118luminescence studies 99-140luminescent organic compounds 367luminous efficiency 369, 370LUMO (lowest occupied molecular

orbital) 144-145, 376

M

Mach-Zehnder (MZ) interferometer 87magnesium 370

magnetic disks 273magnetic susceptibility of conducting

polymers 56-58magnetic susceptibility studies 23magnetoconductance (MC) 45

in conducting polymers 50-54maximum extinction coefficient 102medical diagnostic biosensor application

297MEH-PPV 151, 159, 347, 350-352, 354,

360Meldrum’s acid 198membranes 271-296mercury lamp 188mercury-rare gas (xenon) discharge lamp

188metal/conducting polymer junctions 408metal-insulator (M-I) transition 12metal-insulator-semiconductor (MIS)

diodes 406metal-insulator transition 15, 18, 38-39,

55, 58, 59metallic I-(CH)x, magnetoconductance

versus field 51metallic oriented-(CH)x 53metallic polymers 13metallic state in doped conducting

polymers 39-49metal oxide nanoparticles 345methacrylate esters, PMMA

copolymerised with 85methacrylic acid 204methacryloyl chloride 2043-methyl-4′octyl-2,2′-bithiophene-5,5′-

diyl (3MOT) 76methyl methacrylate 204methyl methacrylate-butyl methacrylate

copolymer 121methyl methacrylate-ethyl methacrylate

copolymer 121methyl tetrahydrofuran (MTHF) 115microactivity-based device structure 374


464

microactivity structure 371microactuators 255-270, 423-424

applications 424microbiosensors 276microlithography

development 189dry development 189exposure 188-189processing 187-190techniques 186wet development 189

microresists for shorter wavelengths(MRS) 201

microscopic charge transport mechanism54

miniaturisation 379MIS field-effect transistors (MISFETs)

409molecular devices based on conducting

polymers 406-424molecular diffusion 424molecular electronics (ME) 368, 379

conducting polymers in 393-439applications 394, 397

use of term 393molecular interaction 99molecular motion 99molecular scale anisotropy 54molecular scale transistors 367-368, 379molecular structure

conformational changes 256of heteroaromatic polymers 80

molecular weight effects on excimerformation 113

Monazoline process 193, 195MOPPV see poly-(2,5-dimethoxy-p-

phenylene vinylene (MOPPV)More’s law 185MOSFETS 410Mott-Hubbard model 6MRS see microresists for shorter

wavelengths (MRS)

multilayer resists (MLR) 208-209, 209silicon-containing polymers 210technology 209

muscles 255

N

Naarman polyacetylene 14, 24, 25NADH 321-322Nafion 240nanocomposite polymer electrolytes 223nanocomposites 342

benefits 343metal nanoparticles 344particle shape 343preparation methods 344-346

nanocrystal probes 341nanocrystals 341nanoparticle components 341-365nanoparticle photocurrent spectral

response 354nanoparticle properties 341nanoparticles 342

preparation methods 344-346size-dependence 344

nanoparticle semiconductor-polymersystems 353-357

nanotubes 255nanowires 418naphthalene 107, 121naphthyl-substituted polymethacrylate

(PNMA) 110n-BCMU-PDA 75-76, 76n-butoxycarbonylmethyl urethane) (n-

BCMU-PDA) 74n-doping 242near-field scanning optical microscopy

(NSOM) 358-359needle-like LED 151negative bipolaron 11negative deep-UV resists 201-202

examples 203

465

Index

negative electron-beam resistscharacteristics 206formulae 207

negative magnetoresistance 59negative polaron 10negative polyimides 209-210negative soliton 8Neotronics 417neutral soliton 8nickel catalyst polymerisation 167NIMA 381n-methylpyrrolidone 398N-methyl-2-pyrrolidinone (NMP) 257NMP solution 266non degenerate ground state 7, 9non labelled PS 117non linear optical (NLO) chromophores

69, 71non linear optical (NLO) devices 69non linear optical (NLO) materials 69non linear optical (NLO) polymers 69

future targets for optical deviceapplications 93-94

third-order 71-72, 82-84non linear optical (NLO) properties 69-

98non radiative energy transfer 121non radiative resonance transfer 118non radiative transition 109‘normal’ fluorescence 102novolac resin 193nuclear magnetic resonance (NMR)

spectroscopy 8, 227, 415

O

octadecanoic acid 381, 3823-octadecanoylpyrrole (3ODOP) 4023-octadecylpyrrole (3ODP) 402Odour Mapper 417odour sensors 414-417one-dimensional chain 40

one-dimensional electronic system 51-methylnaphthalene 116optical absorption edge 3optical biosensors 314optically detected magnetic resonance

(ODMR) 10optical properties, non linear 69-98optical second-harmonic generation 386optical signal processing 72optical spectrometer 105optical switching devices 71, 72

materials characteristics 84optical technology 69organic electroluminescence 367, 369,

370organic molecules 393oriented I-(CH)x, W versus temperature

59oriented K-(CH)x, W versus temperature

59osmium-containing polymers 274oxadiazole 1581,3,4-oxadiazole 158, 159oxidation 10

resistance 148oxidative coupling method 398

P

PAB/GOD 320packaging materials 284PAN 225, 227, 230, 232, 243PANI 3, 37, 38, 40, 42, 54, 159, 243,

256, 257, 274, 314, 316, 342, 344, 394, 395, 397, 402, 413, 423

electrolytic expansion 263thermoelectric power 63

PANI-AMPSA 48conductivity versus temperature 48

PANI-CSA 55, 56, 61magnetoconductance 53magnetoconductance versus field 54


466

W versus temperature 59PANI-Nafion composite electrodes 320PANI-PET 151PANI-SiO2 344PANI-TiO2 344PANI-uricase electrode 313parahexaphenyl (PHP) 161Pariser-Parr-Pople model 6parylene C (poly(chloro-p-xylene) 285Pauli susceptibility 13, 56PAV see poly(arylenevinylene) (PAV)PC see propylene carbonate (PC)PCHMT 165PCHT 165PCMS 206, 207PDABT 166p-doping 242PDTB 81PEDOT 53, 54, 344

normalised resistivity versustemperature 49

PEDOT-PF6, magnetoconductance 53Peierls distortion 6, 14, 16, 21Peierls-Hubbard model 6Peierls instability 4, 142, 394Peierls model 5, 6PEO see poly(ethylene oxide) (PEO)PEO-LiCF3SO3 224PEO-LiClO4 219, 220PEO-LiX 220, 223, 225, 243

membranes 218-219pernigraniline salt (Pas) 2603,4,9,10-perylenetetracarboxylic

dianhydride (PTCDA) 285P(ETMA-co-MMA) 206, 207PF6-doped PPy 38phase and modulation fluorometry 105phase separation 126-135pH dependence of electrolytic expansion

262pH effects 302, 310-311phenyl 107

phenyl methacrylate, fluorescence spectraof 114

phenyloxysuccinimid 200phonon freezing 21, 40phonons 20, 24phonon scattering 21phosphorescence 100, 102, 109, 111photobleaching 91photochemical processes 105photoconductors 273photocurrent/dark current ratio 351photocurrent measurement 348photoelectron spectroscopy 406photoexcitation 142, 143photoinduced absorption 10, 143photoisomerisation 256photolithography 75, 92, 186, 273, 371photo-locking 208photoluminescence (PL) 142-143

efficiency 166quantum efficiency 143, 145spectra 372, 421

photon absorption 143photonic crystals 350photonics 367photons 119, 188, 367, 398, 406photooxidation 119photophysical deactivation processes

100-104photophysical processes 105, 108photophysical quenching 104photopolyimide 210photopolymerisation 398photopolymers 185photoresists 185, 273, 367

characteristics 192conventional 193negative 194-197one-component systems 194-195positive 193-194t-butyl carbonate (T-BOC) 199two-component systems 196-197

467

Index

see also polimide-based photoresists;specific types

photosynthesisers 398photovoltaic properties 347pH sensitivity 316phthalocyanines 385physical adsorption 306physical vapour deposition (PVD) 276physical vapour synthesis 345π-bond 375π-conjugated final structure 73π-conjugated polymers 72

processible 76-82π-electron bonding 342π-electron conjugated polymers 71π-electron conjugated structure 3π-electron conjugation 79π-electron delocalisation 142π-electron hopping matrix element 41π-electrons 5, 37, 255, 256, 376piezoelectric coefficient 379-380piezoelectricity 386Pioneer Corporation 369plasma deposition technique 188plasma etching 190plasma polymerisation 276-282, 398

advantages over conventionalpolymers 278

gas parameters 281-282general characteristics 277-278history 277power parameter 282reactors 279-281synthesis 278-282

plasma polymerisation/vapour deposition(PP/VD) 284

plasma polymersapplications 283-290characterisation 282-283films 282-283

platelet reinforcement 343PMAN see poly(o-methoxyaniline)

(PMAN)PMMA 92, 164, 197, 203, 207, 209,

225, 227, 243, 286, 319, 320sensitivity curve 207

PNIPAM 122PNMA see polymethacrylate (PNMA)PNVC 346polarisation ratio 170polarised electroluminescence 168-170polaron-exciton levels 143polaronic clusters 26polarons 4, 6, 9-12, 27, 377, 396

energy levels and localised states 10formation 9instability 17negative 10

polyacenaphthalene 115polyacetylene 12, 14, 19, 20, 37, 40, 72,

255, 256, 342, 394, 397polyacrylamide (PAAM) 240, 241-242

volume phase transition 134polyacrylonitrile see PANpoly(3-alkoxycyanoterephthalylidene)

422polyalkylfluorene 420polyalkylthiophene (PAT) 38, 147, 267,

346, 422, 423, 1164structure 166, 167synthetic procedures 167trans conformation 165

polyamic acid 211polyamide 6 342-343poly-o-aminobenzoic acid 413polyaniline (PANI) see PANIpoly(aniline-co-ortho anisidine) 398polyaniline film 265

anisotropy of electrolytic expansion in266

contraction under strain 267electrochemistry 259-260expansion behaviour 259-260expansion ratio 260-261


468

polyaniline insulated gate field-effecttransistors (PANI-IGFETs) 410

poly(o-anisidine) 402, 403poly(aromatic sulfones) 197poly(arylenevinylene) (PAV) 72-74, 76, 398poly(2,5-benzoxazole) 342poly(2,5-benzoxazole)-block-

poly(benzobisthiazole-2-hydroxy-1,4-phenylene)-block-poly(2,5-benzoxazole) 342

polybenzimidazole (PBI) 240poly[bis(p-toluenesulfonate) of 2,4-

hexadiyne-1,6-diol] (PTS-PDA) 72polycarbazole (PCz) 3, 395polyconjugated chains 4, 37polycyclohexyl methacrylate 133poly(3-cyclohexylthiophene) 408polydiacetylene (PDA) 72, 398polydiacetylene-polyacetylene-

polydiacetylene triblock polymerchains 342

poly(9,9-di(ethylhexyl)fluorene (PDF2/6)163, 164

poly(9-9,dihexylfluorene) 162, 422poly-2,6-dimethylphenylene oxide (PPO)

119, 121poly(dimethyl-

tetrafluoropropylmethacrylate) 204poly-(2,5-dimethoxy-p-phenylene

vinylene (MOPPV) 72-74poly-(2,6-dimethyl-p-phenylene oxide)

120poly(3-dodecylthiophene) 76polyemeraldine base (PEB) 318, 403, 413poly(ethoxyaniline) 403poly(3,4-ethyelene dioxythiphene) 40poly(ethylcyanoacrylate) 204polyethylene (PE) 107, 398poly(3,4-ethylenedioxythiophene)

(PEDOT) 49, 159polyethylene glycol/polyurethane

copolymer 275

polyethyleneimine-H2SO4 240poly(ethylene oxide) (PEO) 218, 219,

221-224see also PEO

polyethylenimine (PEI) 289polyfluorene 162-164, 169polyfuran (PFu) 3polyheptadiene 8, 397polyheterocycles 243poly(hexafluorobutyl-methacrylate) 204poly(3-hexylthiophene) 400, 405poly(3-n-hexylthiophene) 408polyimide 209-210, 273

formation 211polyimide-based photoresists 209-210

characteristics 210examples 212

polyindole 395, 413poly(N-isopropylacrylamide) (PNIPAM)

121poly-L-lysine 290polymer chain 10

conformation effect of 110-118polymer compatibility 121polymer deposition technologies 379-387polymer electroluminescence 368-374polymers, electronics applications 367-391polymer science 141polymer spin coating process 380polymer transistors 367polymethacrylate (PNMA)

fluorescence in different solvents 112naphthyl-substituted 110

poly(o-methoxyaniline) (PMAN) 256,261, 262electrolytic expansion 262-264

poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] 422

poly(2-methyl-1-pentene) 205poly(methyl-isopropenyl-ketone) (PMIPK)

203polymethyl methacrylate (PMMA) 107,

469

121, 159copolymerised with methacrylate

esters 85poly(3-methylthiophene) 40, 407polynaphthyl methacrylate in chloroform

111poly(olefin sulfones) 205, 206polyparaphenylene (PPP) 37, 159, 160,

377, 398poly(o-phenylenediamine) 305polyphenol 305polyphenylene derivatives 160-161polyphenylenediyl-pyridinediyl (PPPY)

80, 81poly(phenylene ethynylenes) 162polyphenylenes 148-164poly(phenylene vinylene) 11poly(phenyl methacrylate) (PPMA)

conformation change of 113fluorescence spectra of 113, 114, 115

poly(p-phenylene) (PPP) 3, 20, 395, 397,422

poly(p-phenylenevinylene) see PPVpolypropylene (PP) 107poly(pyridine-2,5-diyl) 81polypyrrole see PPypolysiloxanes 284poly(sodium 4-styrenesulfonate) 387polystyrene (PS) 107, 119

fluorescence 120lifetime data 125, 126lifetime parameters 120

polystyrene sulfonated acid (PSSA) 159polysulfone, degradation 205polysulfur nitride 398polythiazyl (SN)x 3polythienylene vinylene (PTV) 37, 72, 74poly(thiophene-2,5-diyl) 81polythiophenediyl-bipyridinediyl (PTBPY)

80-82polythiophenediyl-pyridinediyl (PTPY)

80-82

polythiophene (PT) 3, 37, 38, 76, 164-168, 274, 285, 342-345, 394, 395,398, 402, 404, 406

poly-4-vinylbiphenyl 107polyvinylcarbazole (PVCZ) 107, 116,

306, 420time-resolved fluorescence spectra for

116time-resolved fluorescence spectra of

116polyvinylcarbazole (PVCZ)/urease

electrodes 316polyvinylchloride (PVC) 319poly(vinylcinnamate) 194polyvinylidene difluoride (PVdF) 379-380polyvinylidene fluoride (PVdF) 225polyvinylnaphthalene (PVN) 107, 115,

126, 133polyvinylphenol (MRS) 203poly(4-vinylpyridine) 274polyvinylpyrrolidone (PVP) 240, 346poly-p-xylene 286Poole-Frenkel effect 407POPT 165portable conformable masking (PCM) 208positive bipolaron 11positive deep-UV photoresists

one-component systems 197two-component systems 198

positive electron-beam resists 203-205positive MR 61positive photopolyimide resists, example

211positive polaron 10positive polyimide resists 210positive soliton 8post baking 190potassium tert-butoxide 149potentiometric biosensors 301, 315-317power law behaviour of conductivity 58p(PMMA-OM-MAN) 197PPO see poly-2,6-dimethylphenylene


470

oxide (PPO)PPP see polyparaphenylene (PPP)PPPY see polyphenylenediyl-pyridinediyl

(PPPY)PPV 3, 17, 37, 38, 40, 72, 74, 81, 141,

148-160, 342, 345, 347, 420, 422cis double bonds 151derivatives used as active materials in

LED 152-154electroluminescence spectrum 150soluble copolymers based on 151structure 149sulfuric acid doped 45synthetic procedure 149

PPV-AsF5 42PPV-based ladder copolymers 423PPV-based polymers 360PPV-H2SO4 42, 53, 60

magnetoconductance 52magnetoconductance versus field 51resistivity versus temperature 46

PPy 3, 9, 10, 37, 38, 40, 42, 53, 54, 244,267, 268, 274, 305, 316, 317, 318,320, 324, 326, 342, 344, 346, 394,395, 397, 398, 402, 406, 408, 413,417, 418, 423cathode 243doped with PF6 (PPy-PF6) 46films 307heat capacity versus temperature 57

PPy-Fe2O3 344PPy-ferrocene carboxylic acid based

cholesterol biosensor 323PPy-PF6 56, 58

heat capacity versus temperature 57magnetoresistance versus resistivity

ratio 62normalised resistivity versus

temperature 47S(T) 55W versus temperature 59, 60

PPy-polyvinyl sulfonate films 312

PPy-PVS/LDH electrodes 322PPy-SiO2 344PPy-p-toluenesulfonate (TSO) 57PPy-ZrO2 344pre-exponential function 105propylene carbonate (PC) 222proton membranes, conductivity 241proton polymer electrolytes 239-242PSTF 82

molecular structure 83waveguide 84

PT see polythiophene (PT)PTBPY see polythiophenediyl-

bipyridinediyl (PTBPY)PtO 343-344PTOPT 165PTPY see polythiophenediyl-pyridinediyl

(PTPY)PTV see polythienylene vinylene (PTV)pulse fluorometry 105PVCZ see polyvinylcarbazole (PVCZ)PVMS 206, 207PVN see polyvinylnaphthalene (PVN)PVP see polyvinylpyrrolidone (PVP)p-xylene polymer 286pyrene 107, 121

emission intensity 122pyrene-labelled PS 117pyridine 158pyridine rings 81pyroelectric coefficient 379-380pyroelectricity 386pyrrole/3ODOP film 402pyrrole 398pyruvate biosensor, calibration graph 314

Q

q-1D 4conduction path 21electronic systems 39fractals model 22

471

metallic chain 22models 22, 23Mott’s law 22percolation model 27transport 24variable range hopping model 19

quantum lattice fluctuations 4quantum yield 105-106

definition 105-106quartz crystal microbalances (QCM) 289quasi-one-dimensional material see q-1Dquasi-one-dimensional (q-1D) material 4quasi-phase-matching (QPM)

polymer waveguide 90technique 88waveguide fabrication method 89

quenching processes 103, 104, 118quinine sulfate 106quinone-hydroquinone reaction 317quinoxaline 158

R

radiation 99frequency 99intensity 99types 188

radiationless processes 103radiative recombination 146Raman spectroscopy 227random dimmer model (RDM) 26rate constant 101, 103, 104, 108, 110,

115, 130reactive ion etching (RIE) 89, 92, 94,

190reactors for plasma polymerisation 279recombination

of electron-hole pairs 371radiative 146

recombination process 347-348red-green-blue (RGB) emission 161redox behaviour 259

redox current 257redox potential 302redox reaction 304refractive index grating fabrication 90regioregularity 167resistivity, temperature dependence 43,

45resistivity ratio 12, 57resists

adhesion 191coating process 188contrast 191etching resistance 191films 286materials 193-210negative 189, 192positive 189, 192removal (stripping) 190requirements 190-192sensitivity 191solubility 191special 208-211see also multilayer resists (MLR)

reversible electrochemical actuators 424rhodamine B 106RIE see reactive ion etching (RIE)rocking chair battery 230-231

S

salicylic acid (SA) 241sample preparation 257sandwich configuration 347SBAC polymer 76-78

synthetic scheme 78transition dipole moment between 1Bu

and 2Ag excited states 77wavelength dependence 78

scanning electron microscopy (SEM) 23,405, 418

scanning transmission microscopy (STM)404


472

scattering rate 41Schottky devices 406-408Schottky diode 357Schrieffer-Heeger model 15second-harmonic generation (SHG) 88, 91

efficiency and laser power 90second-order NLO polymers 84-87self-assembled films 271self-assembly monolayers 404self-phase modulation (SPM) 83semiconducting polymers 341-365semiconducting state 6semiconductive films 285semiconductor nanoparticles 345semiconductors 4, 11, 271, 273, 276-

277, 393sensing devices 297sequential monolayer tranfer process 384sequential transfer process 383serially-grafted polymer waveguides 88-

90sexithienyl based FETs 410σ bond 375signal processing by optical technology

69signal processing function 69-70signal transmission function 69-70silicon 375

SiO2 on 187silicon chip 393silicon-containing polymers, multilayer

resists 210silicon monoxide 284silicon transistor 379single crystals 380single layer LED 144single photon fluorometer 105singlet excitation, processes occurring

after 109singlet exciton 143singlet ground state 100SiO2, on Si 187

SiO2 fibre 846FPBO 354, 355Smoluchowski equation 118SnO2/LiClO4-EC-DMC-PAN/LiNi0.8 Co0.2

O2 battery 237SnO2/LiNi0.8 Co0.2O2 cell 237SnO2/LiNi0.8Co0.2O2 electrodic couple

236solid electrolyte interface (SEI) 233solid-state polymer laser diodes 350soliton band 18solitons 6-9, 16, 19, 38, 394, 396

energy levels and localised leveloccupations 8

formation 8interchain transport 20physical properties 8

special resists 208-211specific heat of conducting polymers 56-

58spectral bandwidths 373spectral sharpening 371spectral studies 105spectroscopic methods 406spin coating 151, 188, 346, 380sputter etching 190sputtering 273stability 141, 148steady-rate fluorescence emission

spectrum for iPS gel 132Stern-Volmer equation 104Stokes law 99structure modulation 157sub-gap optical absorption bands 143substituted benzylidene aniline with

chloride polymer see SBACsubstituted polyparaphenylene 160sulfonium precursor polymer 149sulfuric acid doped PPV 45supercapacitor research and development

217surface plasmon resonance (SPR) 273, 289

473

surface polarity 148Su-Schrieffer-Heeger (SSH) model 6, 17,

142sweep voltametry 227, 230

T

T1/2 law 14, 44taste-sensing field-effect transistor

(TSFET) 415taste sensors 414-415T-BOC-PHS 200

decomposition in acid medium 200T-BOC polymers 200t-butyl acrylate 200, 201t-butyl carbonate (T-BOC) photoresists

199Teflon 398temperature coefficient of resistance

(TCR) 13, 14, 38, 39, 41-43, 47, 48,49, 55

terephthaldicarboxaldehydes 148terephthalic aldehyde 77tetracyanoquinodimethane (TCNQ) 274,

286tetraethylorthosilicate (TEOS) 418tetrahydrofuran-ether 111tetrahydrofuran (THF) 113, 1202,2′,7,7′-tetrakis(N,N-di-p-

methoxyphenyl-amine)9,9′-spirobifluorine 353

tetramethylammonium hydroxide 73tetramethyltin (TMT) 285tetraphenyl diaminobiphenyl (TPD) 158TGA 397thermal ageing test 86thermal property measurements 57thermoelectric power, temperature

dependence 55thermoelectric power (S(T)) of conducting

polymers 55thermo-optical polymer devices 71

thermotropic liquid-crystallime polyimide135

thermotropic liquid-crystalline (LC)polyimide (P-11TPE) 134

2,5-thienylene bis(methylene-dimethyl-sulfonium chloride) 72-73

thin films 71-74, 77, 91, 148, 271, 284applications 273new materials 271

thiophene, plasma-polymerised 285thiophene rings 81third-harmonic generation (THG) 74, 76,

83third-order NLO polymers 71-84

research background 71-72waveguides 82-84

three-dimensional band calculations 17three-dimensional coupling 17three-fold helical conformation 132tight-binding model 17time-correlated single-photon counting

studies 105time-dependent studies 105time-of-flight (TOF) measurements 397time-resolved fluorescence spectra

iPS/BA gel 130iPS gel 131PVCZ 116

time-response of electrolytic expansion265

TiO2 343TiO2-conjugated polymer composites

348-353TiS2 221toluene sulfonic acid 264trans-cis-trans isomerisation 91transducers 299, 301, 313transistors

molecular-scale 367-368polymer 367

transistor-transistor logic (TTL) 87transmission electron microscopy (TEM) 23


474

trans-polyacetylene (PAc) 3, 7, 8, 11, 13,17, 19, 394, 395

transport properties of polymers 12-14factors influencing 14-18

transport property measurements 39triazole 158TRIDSB 157trifluoromethane sulfonic acid 199triphenyl sulfonium triflate 199triplet exciton 144triplet state 101, 111tris (2,2′-bipyridyl ruthenium) complex

3984,4′,4′′-tris(1-naphthyl)-(N-phenyl-

amino)triphenylamine 169tunable photonic crystals (TPCs) 350two-phase system, excimer photophysics

for 133two-photon resonance 86

U

ultrathin films 286, 399-404ultraviolet photoelectron spectroscopy

(UPS) 11, 159unoriented I-(CH)x, W versus temperature

59unsubstituted PPV 158unsubstituted thiophenes 164urea biosensor 320

V

vacuum-deposited SBAC polymer 77valence band 4, 10, 143variable-range hopping (VRH) model 19,

22, 23, 25, 61-62

very large scale integration see VLSIvibrational relaxation (VR) 100, 102vinyl polymers with substituents R and R′

106-107VLSI, minimum size versus year 185VLSI electronic circuits 273VLSI technology 185, 212, 276-277voltage profile 233volume phase transition of PAAM 134

W

Wannier exciton 143water-soluble polymers 387waveguide grating 92

diffraction efficiency of 91waveguides 69, 71

third-order NLO polymer 82-84weak localisation (WL) 50, 52, 54Wessling method 149Wilhelmy plate 381Wittig condensation 148

X

X-ray diffraction (XRD) 23, 39, 74X-ray lithography 186, 206X-ray photoelectron spectroscopy (XPS)

282, 283X-ray resists 206-208

Z

Ziegler-Natta catalyst 397ZnSe 353Z-type deposition 401-402

2002_handbook of polymers in electronics

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