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Masters Theses 1911 - February 2014

January 2007

Integration Of Organic Electronic Materials With Semiconductor Integration Of Organic Electronic Materials With Semiconductor

Nanoparticles Nanoparticles

Tongxiang Lu University of Massachusetts Amherst

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INTEGRATION OF ORGANIC ELECTRONIC MATERIALS WITH SEMICONDUCTOR NANOPARTICLES

A Thesis Presented

by

TONGXIANG LU

Submitted to the Graduate School of the University of Massachusetts Amherst in partial fulfillment

of the requirements for the degree of

MASTER OF SCIENCE

September 2007

Chemistry

INTEGRATION OF ORGANIC ELECTRONIC MATERIALS WITH SEMICONDUCTOR NANOPARTICLES

A Thesis Presented

by

TONGXIANG LU

Approved as to style and content by:

__________________________________________ Vincent M. Rotello, Chair __________________________________________ Kenneth R. Carter, Member

________________________________________ Bret Jackson, Department Head

Department of Chemistry

iii

ACKNOWLEDGEMENTS

I would like to thank my advisor Professor Vincent M. Rotello for his guidance

and support during the past three years. It has been a great learning experience for me

and I really appreciate his advice and support. I would further like to thank Professor

Kenneth R. Carter for his support and guidance.

I wish to express my appreciation to the members of the Rotello group, those

here and those who have already graduated. Special thanks go to Yuval, Palaniappan,

Michael, and Changcheng for their support.

I would particularly like to thank my family and friends who are responsible for

me making it to this point.

iv

TABLE OF CONTENTS Page ACKNOWLEDGEMENTS.................................................................................................... iii LIST OF FIGURES ................................................................................................................. v LIST OF SCHEMES............................................................................................................... vi CHAPTER 1. INTEGRATION OF ORGANIC ELECTRONIC MATERIALS WITH SEMICONDUCTOR NANOPARTICLES...................................................... 1

1.1 Introduction...................................................................................................... 1

1.1.1 Semiconductor and metallic NPs building blocks ................................. 1 1.1.2 Organic semiconductor building blocks ................................................ 3

1.1.3 Hybrid organic-inorganic materials ....................................................... 3 1.2 Synthesis of semiconductor quantum dots....................................................... 5

1.3 Synthesis of tetrathiafulvalene (TTF) derivatives and functionalization of quantum dots ............................................................................................... 6

1.4 Manipulation of cadmium selenide quantum dots to facilitate the “On/Off” switching of fluorescence ................................................................ 8

1.5 Fabrication of hybrid organic-inorganic FETs based on the modified QDs.. 10 1.5.1 Quantum dots Assembly...................................................................... 10 1.5.2 Device platform fabrication ................................................................. 10 1.6 Conclusions.................................................................................................... 12 1.7 Experimental methods ................................................................................... 12

1.7.1 Syntheses.............................................................................................. 12

1.7.1.1 4, 5-bis-(benzylthio)-1, 3-dithiole-2-one (1).......................... 12 1.7.1.2 4,5-bis(3-Picolylthio)-1,3-dithiole-2-one (2) ......................... 13 1.7.1.3 4,5-Bismethylthio-4′,5′-(3-picolylthio)tetrathiafulvalene

(3)……………………………………………………………13 1.7.1.4 Preparation of CdSe NPs ........................................................ 14 1.7.1.5 Preparation of CdSe NPs coated with compound 3................ 14

1.7.2 Conditions for fluorescence spectroscopy............................................15

REFERENCES.......................................................................................................................16

v

LIST OF FIGURES

Figure Page

1-1. CdSe quantum dots single electron transistor device...................................................2

1-2. OIFET based on CdSe coated with electron-donor TTF derivatives ...........................5

1-3 TEM image of CdSe nanoparticles coated with pyridine which have an average

size of 6nm....................................................................................................................6

1-4. Fluorescence of CdSe-TOPO (red line) and CdSe-TTF (blue line) in THF.

Excitation at 380 nm. Both of them have the same optical concentration ...................8

1-5. Titration of PbClO4 (9.65 mM) against CdSe-TTF in THF. Excitation at 380 nm .....9

1-6. The recovery of fluorescence of CdSe-TTF with addition of NOBF4 in THF.

Excitation at 360 nm .................................................................................................... 9

1-7. The synthetic “On/OFF” fluorescence switch composed of CdSe nanoparticles

coated with TTF-derivatives ......................................................................................10

1-8. Examples of a device platform produced by a) NIL replication of electrodes.

b) Photolithographically fabricated interdigitated electrodes .................................... 11

vi

LIST OF SCHEMES

Scheme Page

1-1. Synthesis route of TTF derivatives ................................................................................7

1-2. The place exchange of TTF derivatives onto CdSe nanoparticles.................................8

1

CHAPTER 1

INTEGRATION OF ORGANIC ELECTRONIC MATERIALS WITH SEMICONDUCTOR NANOPARTICLES

1.1 Introduction

There is growing demand to fabricate low cost and/or high density electronic

components, which has led to exploration of new materials as alternatives to more traditional

inorganic materials (silicon based). These new materials generally fall into three classes:

a) Semiconductor and metallic nanoparticles (NPs) building blocks.

b) Organic semiconductors building blocks.

c) Hybrid organic-inorganic materials.

1.1.1 Semiconductor and metallic NPs building blocks

Current research on nanoparticles can be classified into two main categories. The first

involves manipulation and exploration of single nanoparticles in devices, for example, single

electron transistors (SET) (Figure 1-1).1 The second category focuses on the application of

nanoparticle assemblies to produce a new generation of devices, such as optoelectronics and

photonics devices.2 Moreover, the nanoparticles can also be divided into two classed based

on the surface capping ligands: first class composed of metallic NPs with electronically

active organic ligands on the surface – NPs in such systems act as a conducting scaffold. The

second class is composed of semiconductor nanoparticles that are stabilized with long alkyl

chains (which are electronically insulating) ligands and are used for construction of

supperlattices and glassy solids.

2

Communication, coupling, and coherence between semiconductor nanoparticles, also

known as quantum dots (QDs) are central themes for fabrication of electronic devices.

Recent investigation of two coupled quantum dots has shown coulomb blockade3 and

electron pairing.4,5 Furthermore, it has been demonstrated that electronic energy transfer in

mixed size quantum dot solids arises from dipole-dipole interactions. For example,

quenching of the luminescence (lifetime) of the small dots accompanied by enhancement of

the luminescence (lifetime) of the large dots is consistent with long-range resonance energy

transfer of electronic excitations from the more electronically confined states of the small

dots to the higher excited states of the large dots.6 Recently, Talapin and Murray have

showed that initially poorly conducting PbSe NPs solids assembled on a field effect transistor

(FET) platform can be chemically activated, by exposure to hydrazine vapors, for fabricating

n- and p-type channel transistors.7 The original NP arrays were insulating because of the 1.5

nm oleic acid ligands coating the particles, and showed a 10 fold increase in conduction after

thorough washing that removed some of the ligands.

Figure 1-1. CdSe quantum dots single electron transistor (SET) device. Adapted from reference 1a.

3

1.1.2 Organic semiconductor building blocks

Organic electronics, or plastic electronics, is a branch of electronics that deals with

conductive organic molecules. This is as opposed to traditional electronics which relies on

inorganic semiconductors such as copper or silicon. There are three major classes of

conjugated organic materials: (a) organic charge-transfer complexes, (b) various conjugated

polymers derived from polyacetylenes, polyphenylenes, polypyrroles and polyanilines, and (c)

pentacene and sexithiophene of the small conjugated molecules or oligomers.

Organic conductive materials are lighter and more tunable than inorganic conductors.

In addition, the flexibility of organic synthesis enables the formation of organic molecules

with useful luminescent and conducting properties and thus their usage as materials in

organic field-effect transistors (OFETs),8 organic light emitting diodes (OLEDs)9 and

photovoltaic cells.10 However, in general, the organic conductive materials have a higher

resistance and therefore conduct electricity poorly and inefficiently as compared to inorganic

conductors.

1.1.3 Hybrid organic-inorganic materials

Hybrid organic-inorganic materials take advantage of the merits of both organic and

inorganic components. One approach to achieve the composite materials is to fill conductive

polymer matrices, such as polyaniline or polythiophenes with semiconductor nanoparticles:

CdS, CdSe or CuS NPs, thus providing access to peculiar morphologies, such as

interpenetrating networks, p-n nanojunctions, or “fractal” p-n interfaces, not by traditional

microelectronic technology. A significant feature which makes the composite materials

different to fully organic or inorganic based materials is the improved long-term stability. For

example, recently Bawendi and Rubner reported the tunable electroluminescence of CdSe

4

poly (phenylene vinylene) heterostructures, 11 while Alivisatos has shown incorporation of

CdSe nanorods composites for fabrication of photovoltaic cells.12 In both cases there is no

real electronic coupling, achieved through covalent bonding, between the polymer matrixes

and the NPs and control of phase segregation and morphology are very difficult. The second

approach is to directly coat the NPs with electronically active ligands. Alivisatos and Fréchet

have reported the synthesis of a pentathiophene phosphonic acid ligand that facilitates

electron transfer between a CdSe NPs and an organic semiconductor polymer for the

construction of a composite organic-inorganic LED.13 Following this report a number of

papers appeared in the literature showing the exchange of insulating ligands with conjugated

ligands. Advincula and colleagues have synthesized and used oligothiophene dendrons with

varying size to coat CdSe NPs for construction of photovoltaic cells.14 In another example, a

CdSe–tetraaniline hybrid influences the band structure of the nanocrystal and in particular

shifts the band ascribed to the HOMO level towards higher potentials and that originating

from the LUMO level towards lower potentials15. These recent studies mark the first attempts

in achieving real molecular-NP hybrids but lack addressing problems of NPs assembly and

segregation and deserve further research.

Here, we will explore the fabrication of hybrid organic-inorganic field effect

transistors (OIFETs) by assembly of quantum dots coated with electronically active organic

ligands. (Figure 1-2) Towards this goal, the semiconductor nanoparticles (QDs) will be

synthesized first which will then be coupled to the organic semiconductor such as

tetrathiafulvalene (TTF).

5

Figure 1-2. OIFET based on CdSe coated with electron-donor TTF derivatives.

1.2 Synthesis of semiconductor quantum dots

In recent years semiconductor quantum dots have found many new applications and

play an important role in interdisciplinary research. Quantum dots have unique physical and

photoemission effects; interest has arisen for a functional application of these nanocrystallites.

First, CdSe is a highly fluorescence material and is interesting because it shows no

photobleaching. This is an important property because in most organic fluorophores there is

photobleaching that occurs. Since CdSe does not photobleach they can be used as

biomarkers. The ability to tune the fluorescence properties by external stimuli (energy

transfer or electron donation) allows one to custom design a sensor for a specific application.

Second, QDs also have tunable band gaps. These tunable band gaps lead to opportunities for

harvesting light energy in the visible and infrared regions of solar light.16 Owing to these

special properties, the research of quantum dots is focused on two aspects:

6

a) Addressing the large potential for using luminescent colloidal QD as labeling

reagents in biosensing, biomarkers, and biotechnological applications.17

b) Designing and developing QD based solar cells and light emitting devices.

The quantum dots, in general, can be synthesized using various methods including a)

arrested precipitation in solution18 b) molecular precursor route19 and c) synthesis in a

structured medium.20 For device application crystalline material with ordered structure is

preferred and the molecular precursor route is an ideal choice as the particles synthesized

using this method are, in general, nanocrystalline. The use of coordinating solvents and high

temperature leads to the synthesis of nanocrystalline materials with well defined sizes and

shapes, while making them soluble in either non-polar or polar solvents.21 (Figure 1-3)

Figure 1-3. TEM image of CdSe nanoparticles coated with pyridine which have an average size of 6nm.

1.3 Synthesis of tetrathiafulvalene (TTF) derivatives and functionalization of quantum dots

To fabricate efficient, low operating voltage OIFETs based on quantum dots, the

capping ligand of the QDs needs to be exchanged from electronically insulating to

electronically conductive or at least semiconductive. The ligand chosen for this purpose is

7

derived from tetrathiafulvalene (TTF).22 Tetrathiafulvalene (TTF) is a well studied redox

material, and the ability to electron donation allows for the modulation of the TTF by

chemical and electrochemical methods.23 TTF also has the ability to respond to external

stimuli in the presence of different ligands. TTF is the important component of charge

transfer complexes that can be applied in numerous fields of scientific research and

applications. The ability of TTF moieties as redox centers have been demonstrated and also

extensively researched.

Scheme 1-1 shows the synthesis of TTF derivative,24 a ligand that consists of TTF

and two pyridine “legs” that will allow binding to the surface of quantum dots. Next, through

one step place exchange with TOPO ligands the electronically active ligands were coated

onto quantum dots.

S

SS

S

S

Hg(OAc2) NaOMe

S

SO

S

S

SS

H3CS

H3CS

(i-pro)3P+

SNCl

HCl.Ph

Ph

O

OS

SO

S

SPh

Ph

O

O

N

N

S

SH3CS

H3CS S

S S

S

N

N

S

SS

S S

S S

S

N

N

N

N

Scheme 1-1. Synthesis route of TTF derivatives.

8

Scheme 1-2. The place exchange of TTF derivatives onto CdSe nanoparticles.

1.4 Manipulation of cadmium selenide quantum dots to facilitate the “On/Off” switching of fluorescence

The fluorescence of quantum dots coated with TOPO was taken and demonstrated a

significant fluorescence on its own. Then the fluorescence of CdSe with electron donation

ligands-TTF was recorded (Figure 1-4). The complex demonstrated 2-fold decrease in the

fluorescence due to electron transfer between CdSe and TTF.

Figure 1-4. Fluorescence of CdSe-TOPO (red line) and CdSe-TTF (blue line) in THF. Excitation at 380 nm. Both of them have the same optical concentration.

Fluorescence studies were done on the CdSe quantum dots upon addition of

numerous oxidants. The oxidants used were FeClO4, PbClO4, and NOBF4.25 The spectra did

9

not show significant change after the mixture of FeClO4 with CdSe-TTF in THF. However,

the intensity of fluorescence increased partially after the addition of PbClO4. (Figure 1-5)

More importantly, the best recovery was seen with 2-fold increase in the fluorescence

intensity when NOBF4 was incubated with CdSe-TTF. This is due to its ability to withdraw

electrons from the present TTF molecule (Figure 1-6). Therefore, the quenching and

recovery of fluorescence of quantum dots leads to a synthetic “ON/OFF” fluorescence switch.

(Figure 1-7)

Figure 1-5. Titration of PbClO4 (9.65 mM) against CdSe-TTF in THF. Excitation at 380 nm.

Figure 1-6. The recovery of fluorescence of CdSe-TTF with addition of NOBF4 in THF. Excitation at 360 nm.

10

Figure 1-7. The synthetic “On/OFF” fluorescence switch composed of CdSe nanoparticles coated with TTF-derivatives.

1.5 Fabrication of hybrid organic-inorganic FETs based on the modified QDs

1.5.1 Quantum dots Assembly.

Having achieved the synthesis of the semiconductor NPs with the appropriate

electronically active ligands, we will optimize and address the issues of NPs assembly on the

device platforms. Assembly and superstructure of NPs on surfaces/platforms can be

achieved using conventional techniques such as spin coating, dip coating, drop casting, inkjet

printing or the Langmuir-Blodgett technique (LB). We will employ the drop casting and

controlled evaporation techniques. These techniques are simple but still allow very controlled

assembly of the QDs. Our goal will be to make the structured assembly as high a degree as

possible.

1.5.2 Device platform fabrication

Device platforms will be fabricated using two major techniques – standard

photolithography (PL) following metal deposition and lift-off, and nanoimprint lithography

(NIL). Although the standard PL is the work horse of the semiconductor industry, the

11

simplicity and diversity of the NIL technique is much more appealing, especially for

constructing devices on flexible plastic substrates. For this project we have chosen to work

simultaneously with PL and NIL in order to gain more diversity of structures and electrode

materials. Recently, we have been testing our ability to replicate available device electrodes

platforms. Processes include the fabrication of a negative mold, a positive mold and the final

imprinting of the pattern followed by metal deposition and lift off. The photo-polymer

formulation used in the NIL process is a combination of 61% ethoxylated bisphenol A

dimethacrylate, 18.5% 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate, 18.5% N-vinyl

pyrolidone and 2% 2,2-dimethoxy-2-phenylacetophenone. Figure 1-8a shows preliminary

results of a two step NIL replication process of devices and figure 1-8b shows PL prepared

devices. Devices are designed to have a variety of channel width (W) and length (L) allowing

the understanding of grain size effect and optimization.

Figure 1-8. Examples of a device platform produced by a) NIL replication of electrodes. b) Photolithographically fabricated interdigitated electrodes.

12

1.6 Conclusions

Quantum dots coated with electronically active ligands-TTF derivatives were

synthesized through one step place exchange reaction. The fluorescence of quantum dots can

be manipulated through electron transfer between quantum dots and ligands. Therefore, a

functional “ON/OFF” fluorescence switch was achieved. Furthermore, quantum dots coated

with TTF derivatives will be assembled on device platforms, which will bridge between

inorganic semiconductor nanoparticles and organic conductors/semiconductors, with the

ultimate goal being the fabrication of devices.

1.7 Experimental methods

1.7.1 Syntheses

1.7.1.1 4, 5-bis-(benzylthio)-1, 3-dithiole-2-one (1)

A boiling solution of mercuric acetate (2.1g, 6.56mmol) in 20ml of acetate acid was

slowly added with stirring to a red solution of 4, 5-bis-(benzylthio)-1, 3-dithiole-2-thione (1g,

2.6mmol) in 80ml of chloroform. After vigorous stirring for 15min at room temperature, the

white precipitate was filtered off and washed with chloroform (3*10ml). The filtrate was

washed with water (4*50ml), and a red precipitate was filtered off and then washed with

water again to neutralize any acetate acid in the organic phase (chloroform). The solution

was dried over MgSO4 overnight. After filtration and removal of the solvent, a yellow

crystalline solid was obtained.

Yield: 0.7g, 70%

1H-NMR (400 MHz, CDCl3, TMS): δ=7.45-7.98 (m, 10H); UV-Vis (CH3OH, λmax) spectra:

248.7nm. MS (EI): m/e 390.

13

1.7.1.2 4, 5-bis (3-Picolylthio)-1, 3-dithiole-2-one (2)

4, 5-bis (Benzoylthio)-1, 3-dithiole-2-one (1) (500mg mg, 1.28 mmol) was dissolved

in 20 ml of anhydrous methanol under argon at room temperature. A solution of sodium

methoxide prepared from sodium (58.8 mg, 2.56 mmol) in 5 ml of anhydrous methanol was

added to the above solution slowly within 20 min under N2 atmosphere. With the reaction

proceeding, the color of the reaction mixture changed from pale yellow to reddish due to the

generation of air-sensitive dithiolate ions. To the above reaction vessel a mixture of 3-picolyl

chloride hydrochloride (419.9 mg, 2.56 mmol) and K2CO3 (177.9 mg, 2.56mmol) in 21 ml of

anhydrous methanol was added. Then, the reaction mixture was stirred at room temperature

for 6 h. After removing the solvents, column chromatography of the crude reaction mixture

on silica gel with dichloromethane/methanol (30:1, v/v) afforded compound 2 as brown solid.

Yield: 250 mg, 53.5%

1H NMR (400 MHz, CDCl3, TMS): δ=3.83 (s, 4H, -SCH2), 7.17-7.15 (m, 4H, pyridine ring),

8.54–8.56 (m, 4H, pyridine ring).

1.7.1.3 4, 5-Bismethylthio-4′,5′-(3-picolylthio) tetrathiafulvalene (3)

Compound 2(250 mg, 0.69 mmol) and 4, 5-bismethylthio-1, 3-dithiole-2-thione (468

mg, 2.1 mmol) were suspended in triisopropyl phosphite (3 ml) under argon. The resulting

mixture was heated over an oil bath up to 120–130 °C, and kept at this temperature for 3 h.

Evaporation of the excess of triiospropyl phosphite from the mixture afforded an oily black

residue, which was subjected to chromatography on silica gel with

14

dichloromethane/methanol (30:1, v/v) as the eluant. The compound 3 was obtained as red

solid.

Yield: 85mg, 26%

1H NMR (400 MHz, CDCl3, TMS): δ=2.43 (s, 6H, -SCH3), 3.79 (s, 4H, -SCH2), 7.17–7.19

(m, 4H, pyridine ring), 8.55–8.57 (m, 4H, pyridine ring).

1.7.1.4 Preparation of CdSe nanoparticles

A typical synthesis for CdSe nanocrystals: 0.0514 g of CdO, 0.2232 g of

tetradecylphosphonic acid (TDPA) and 3.7768 g of trioctylphosphine oxide (TOPO) were

loaded into a 25 mL flask. The mixture was heated to 300-320 °C under Ar flow, and CdO

was dissolved in TDPA and TOPO. The temperature of the solution was cooled to 270 °C;

selenium stock solution (0.0664 g of selenium powder dissolved in 2 g of trioctylphosphine

(TOP)) was injected. After injection, nanocrystals grew at 250 °C to reach desired size.

1.7.1.5 Preparation of CdSe nanoparticles coated with compound 3.

The CdSe nanoparticles (7 mg) and compound 3 was dissolved in 15ml

dichloromethane under argon. The mixture was heated around 40 °C for 6 days. After

removal of excess dichloromathane, the mixture was precipitated into 30ml of methanol. The

precipitate was collected after centrifugation. The ratio of compound 3 to TOPO is around 1

to 2 according the integral of 1H NMR.

15

1H NMR (400 MHz, CDCl3, TMS): δ=1.25 (bs, 104H, TOPO) 2.43 (bs, 6H, -SCH3), 3.79 (bs,

4H, -SCH2), 7.17(bs, 4H, pyridine ring), 8.55 (bs, 4H, pyridine ring); UV-Vis (DCM, λmax)

spectra: 508 nm.

1.7.2 Conditions for fluorescence spectroscopy

The fluorescence spectroscopy studies were performed at Jasco FP-6500

Spectrofluorometer. The experiment conditions in figure 1-4 are given below.

Excitation: 380 nm, emission range: 450-700 nm, response: 2 sec, sensitivity: high,

band width (Ex) 1 nm, band width (Em) 3 nm, data pitch: 0.5 nm, scanning speed: 200

nm/min.

16

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