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Simple Acid-Base Hydrolytic Chemistry Approach
to Molecular Self-Assembly
Chi Ming Yam
A thesis submitted to the Facu fty of Graduote Srudies and Research of
McGill University in pamhl firijïllmenr of the requirementsior
the degtee of Doctor of Philosophy.
January 1999 Department of C hemistry McGill University Mon treal, Quebec
Canada O Chi Ming Yam
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This thesis is dedicated to my parents. Wah Yam and Yuet Yee Tarn
for iheir infinite love and support.
Acknowledgments
1 would iike to deeply thank my research supervisor Prof. Ashok K. Kakkar for his
kind guidance and helpful suggestions throughout my study at McGili University.
1 would also like to thank all rny lab-mates, especially Hongwei Jiang, Maria G. L.
Petmcci and Samuel S. Y. Tong for creating an enjoyable environment to work in.
1 would like to express my gratitude to:
Y.-K 1, Leung and Adam Dickie for proof-reading my thesis.
Frederic Chaume1 for translation of the abstract to French.
Mr. Nadim Saadeh for running the mass spectra.
Mr. Michel Boulay for his assistance with the infrared spectrometers.
Dr. F. Sauriol for technical assistance in NMR spectrometry.
Dr. Georges Veiileux a iNRS-Energie Varenne and Ms. Suzie Poulin at École
Polytechnique (Université de Montréal) for the use of their X-ray photœlectron
spectrometers.
AU members of the support staff in the Department of Chemistry, especiaily Ms.
Renée Charron for her help throughout rny stay at McGill,
AU my friends in the Otto Maass building and Montreal.
Financial support from the Department of Chemistry, McGili University in the forrn
of a teaching assistmtship.
Finally, 1 would like to thank my parents, my sisters and my best friend, Sau Ling
Cheung for their love and encouragement,
Foreword
In accordance with guideiine I-C of the "Guidelines for Thesis Reparation" (Faculty
of Graduate Studies and Research), the following text is cited:
"Candidates have the option of uicluding, as part of the thesis, the text of one or more
papers submitted, or to be submitted, for publication, or the cIear1y-duplicated text of one
or more published papea. These tex& must conform to the "guidelines for Thesis
Preparation" with respect to font size, line spacing and ma@n sizes and mus< be bound
together as an integral part of the thesis. The thesis must be more than a me= collection of
manuscripts. All components must be integrated into a cohesive unit with a logical
progression from one chapter to the next. In order to ensure that the thesis has continuity,
connecting texts that pmvide logical bridges between the different papers are usudly
desirable in the interest of cohesion. The thesis must include the foliowing: a table of
contents, an abstract in English and French, an introduction which States the objectives of
the research, a review of the literature (in addition to that covered in the introduction to each
paper), a final conclusion and summary . Where appropriate, additional material must be
provided (e.g. in appendices) in sufficient detail to allow a clear and precise judgment to be
made of the importance and originality of the research reported in the thesis. Whcn co-
authorized papers rn included in the thesis the candidate must have ma& a substantiai
contribution to ail papers included in the thesis. The candidate is required to make an
explicit statement in the thesis as to who contributed to such work and to what extent."
Chapters 5 ,6 and 7 are published papers and manuscripts written by the author and
were used in preparation of this thesis. Foliowing normal procedure, dl the papers have
ken published. accepted for publication or submitted for publication in scientific joumals.
A list of papers is given below.
Chapter 5: Simple Acid-Base Hydrolytic Chernistry Approach to Molecular Self- Assembly: Thin Films of Long Chain Alcohols Terminated with Alkyl, Phenyl,
and Aœtylene Groups on Inorganic Oxide Surfaces.
Yam, C. M.; Tong, S. S. Y.; Kakkar, A. K- Langmuir 1998.14, 6941-6947.
Chapter 6: Molecular Self-Assembly of Dihydroxy Teminated Molecules via Acid-Base
H ydrol ytic C hemistry on Silica Surfaces: Step-by -Step Mu1 tilayered Thin Film
Construction- Yam, C. M.; Kakkar, A. K. Langmuir 1999, in press.
Chapter 7: Molecuiar Self-Assembly of Diaikynyl Teminated Chromophores via Acid- Base H ydrol ytic C hemistry on Inorganic Oxide Surfaces: Sîep- b y -Sep
Multilayered Thin film Construction. Yam, C. M.; Kakkar, A. K. Langmuir 1999, submitted.
Al1 the papers include the research director. Dr. Ashok K. Kakkar, as CO-author. 'Ihe
manuscript in chapter 4 includes Samuel S. Y. Tong (McGiü University) who CO-authored
this paper, and assisted in the study of the chree-step deposition process. Other than the
supervision, advice and direction of Dr. Ashok K. Kakkar, ail of the work presented in this
thesis was perfonned by the author.
Abstract
A new mute to molecular self-assembly using a simple acid-base hydrolytic appmach
on silica based surfaces, is reported in this thesis. Based on this methodology, a number of
corn pounds containing terminal groups with acidic protons, such as alcohols, thiols.
carboxylic acids, and terminal alkynes, can be easily deposiied on silica surfaces. 'Ihe
quality of the thin fdms was monitored by contact angle goniometry. ellipsometry, XPS,
FTIR-ATR and UV-Vis absorption spectroscopies. The deposition conditions were
optimized to produce ordered and densely packed mono- and multilayers. Using the two-
step process. self-assembled monolayen (SAMs) of a variety of long cha i . alcohols
containing terminal alkyl, phenyl and acetylenc groups on silica surfaces were successfully
prepared. The newly forrned monolayers were found to be relatively ordered and densely
packed. They showed comparable stabilities to OTSBiO at ambient and high temperatures.
and upon matment with acids and bases. A layer-by-layer constmction methodology,
based on acid- base h ydrol y sis of aminosilanes and dih ydrox y terminated molecules
containing rigid-rod type and alkyldiacetylene backbones, led to multilayers with higher
stability under various conditions compared to monolayers. The thin Fdm assemblies were
subjected to topochemical polyrnerization, and upon UV-Vis exposure, the formation of a
blue film was observed.
Using the acid- base h ydrol ytic c hemistry apprc. ach, silica surfaces func tionalized with
Sn-NEb groups can be easily modified using a number of terminal aikyne molecules with
varied backbones. SAMs of a variety of rigid-rod allcynes on silica surfaces were
successfully prepared. The n-x interactions in the molecules lead to ordered and densely
packed thin film structures with a surface coverage of 2-7 molecules/ 100 A2. The thin fh
assembly with diacetylene backbone was also subjected to topochemical polyrnerization,
and upon W-Vis exposure, the formation of a blue film was observed. Furthemore, a
vii
layer-by-layer construction methodology using aniinostannanes and dialkyne renninated
molecules containing allryl or aromatic type backbone led to multilayered structures on siiica
surfaces without increasing disorder in the thin films with the ir-zrease in number of layers.
The acetylene groups in the thin film assemblies were found to coordinate with wbalt
carbonyl, corroborated by the observation of A- at 277 m.
Résumé
Une nouvelle méthodologie pour l'auto-assemblage mo~éculaire utiiisant une simple
hydrolyse acide-base sur des surfaces de silice. est rapport& dans ceae t h e . Basée sur
cette technologie. un nombre de composés contenant des groupements terminaux avec des
protons acides. tel que des alcools. thiols, acides carboxyliques et alcynes terminaux,
peuvent être aisément déposés sur des surfaces de silice. La qualité des f h s minces a été
contrôlée par goniomktrie à angle de contact 6Ilipsométrie. XPS. FTIR-ATR a
spectroscopie d'absorption üV-Vis. Les conditions de deposition ont kté optimisées pou
produire de simples e t muiticouches 0 r d 0 ~ h et uensémment entas&. Utilisant k
procéd6 à deux étapes. une varSté de monocouches auto-assemblées (SAMs) d'alcool à
longue chaine contenant des groupements alcyls. phényls et act5tyiènes sur des surfaces de
silice ont été préparées avec succès. Ces nouvelles monocouches ont ddmonûé une grande
densité et organisation. Elles ont démonué des stabilitées comparables à 0TS/Si02 à
température ambiante et klevée et aux traitements aux acides et aux bases. Une
méthodologie de construction couche par couche. basée sur l'hydrolyse acidebase des
aminosilanes, de molécules de type barre rigide terminées par un groupement dihydroxy et
sur une chaîne akyldiacétylène. résulta en des multicouches de stabiliîé supérieure. sous
di ffdrentes condi tions, comparativement aux monocouches. Les réactifs des fiims minces
ont été soumis 1- polymt5r;sation topochimique. et a ~ r è s exposition à des rayons UV-Vis.
la formation d'un f h bleu a dté observée.
Utilisant l'approche de l'hydrolyse acide-base. les surfaces fonctionalisées de silice
avec des groupes Sn-NEs. peuvent être facilement modifiées utilisant des alcynes
ceminaux avec differentes chaînes. Une grande vari6té de SAMs comprenant des barres
rigides d'alcynes ont 6té successivement préparées. Les intéractions K-K entre les molécules
ont permis la formation de füms minces ordonnés et denses avec une couverture de 2-7
mol&ules/100 A'. Le füm mince assemblé avec une chaîne de diacétylène a aussi éîé
soumis ii la polym&isation topochimique. et sous exposition UV-Vis. la formation d'un
film bleu a B t é observée. De plus. une m&hodologie de construction de couche par couche
utilisant des aminostannates et des molécules terminées par des groupements dialcynes et
contenant une chaîne de type alkyl ou aromatique ont men6 à des structures multicouches
sur des surfaces de siiice sans augmenter le désordre dans le film mince avec
l'augmentation du nombre de couches. Il a tté observé que les groupements acétylènes dans
les Actifs des films minces pouvaient coordoner avec le cobalt carbonyl, corroboré par
l'observation du k, à 277 nm.
Table of Contents
Acknowledgments Foreword Abstract Résumé Table of Contents List of Schemes List of Tables List of Figures List of Abbreviations List of Publications Originated from Research at McGill University
Chapter 1 General Review of Molecular Self-Assembly and Scope of Thesis
1.1 Organic Thin Films 1.2 Lanpuir-Blodgert&B)Films
1 -3 Self- Assem bled Monolayers (SAMs)
1.4 SAMsoCAUcanoic AcidsonMetalOxide Surfaces \ 1.4.1 SAMs of Alkanoic Acids on Silver, Copper and Aluminum Oxide
1.4.2 SAMs of Alkanoic Acids with Ammatic Chromophores
1.4.3 StabilityTests
1.4.4 S A M s of Hydroxamic Acids 1 -4-5 SAMs of Dioic Acids
1 -5 S AMs of Orgmosulfur Compounds on Metal Surfaces
1.5. 1 S AMs of Thiols on Gold 1.5.1.1 Deposition Proçess 1.5.1.2 Monolayer S tmctures 1.5.1 -3 Thermal and Chernical Stabilities
Page ... m
iv
1 -5.2 SAMs of ~Substituted Thiols on Gold 15
1 -5.3 SAMs of Aryl Thiols on Gold 1.5 -4 SAMs of Chelating Aromatic Dithiols on Gold
1.5 -5 SAiMs of Conjugated &oDithiols and Acetylthiols on Gold
1 -5.6 SAMs of Diaikyl Sulfides on Gold 1.5.7 SAMs of Diakyl Disulfides on Gold
1 -5.8 SAMs of Branched Thiols and Disulfides on Gold
1 -5.9 SAMs of Thiols on Copper and Silver
1.6 SAMs of Organosilicon Derivatives on Hydroxylated Surfaces
Deposition Aocess
Reproducibility Monolayer Swtures Chernical and Thermal Stabiities SAMs with Aromatic Chromophores
SAMs with Second Order Nonlinear Optical Properties Surface Modification
Multilayer Formation
L -6.8.1 Hydroboration-Oxidation of a Terminal Vinyl Group
1.6.8.2 LiAiHQ Reduction of a Surface Ester Group 1 .6.8 -3 Photolysis of a Nitrate-Bearing Group
1.6.8.4 Hydrolysis of a Boronate-Pmtecting Group 1.7 Alkyl Monolayers on Silicon
1.8 S AMs with Alternate "Inorganic/Organic** S ystems
1.8.1 Layered Phosphonate Thin Films
1.8.2 Cobalt-DiisocyanideThin Films
1.9 Monomenc and Polymerized Diacetylene LB and Self-Assembled
Thin Films
1.10 Limitation of Traditional Approaches to Molecular Self-Assembly 1.1 1 Acid-Fase Hydrolytic Chemistry
1.1 1.1 Synthesis of Aminosilanes 1 -1 1.2 Chemistry of Aminosilanes
1.1 1.2.1 Reaction with Water
1.1 1 -2.2 Reactions with Alcohols, Phenols and Silanols
1.1 1 -2.3 Reactions with Thiois and Carboxylic Acids
1.1 1.2 -4 Reaction with Acetylene Cornpounds 1.1 1.3 Synthesis of Aminostannanes 1.1 1 -4 Chemistry of Aminostannanes
1.1 1 -4.1 Reactions with Water and Air
1.1 1 -4.2 Reactions with Protic Species 1.1 2 Acid-Base Hydrolytic Chemistry Approach to Molecular Self- Assembly
on Inorganic Oxide Surfaces 1.13 Scope of Thesis 1.14 References
Chapter 2 Methods for Surface Characterization 2.1 Contaçt Angle Goni0rnet.y 2.2 Fourier Transform lnfrared Spectroscopy in the Attenuated
Total Refleçtion Mode (FïIR-ATR) 2.3 Eilipsometry 2.4 X-ray Photoelectron Spectroscopy W S ) 2.5 W-Visible Spectroscop y 2.6 Referençes
Chapter 3 A Novel Route to Etncient tnorganic Oxide Surface Modification v i a Simple Acid-Base Hydrolytic Chemistry
Introduction Acid-Base Hydrolysis S urfaœ F w tionalization 3.3.1 Si-NEbApproach 3.3.2 Sn-NE4 Approach Optimization of Deposition Conditions Surface Properties of Thin Films Conclusion Expehental Section 3.7.1 MateRals 3.7.2 Substrate Preparation 3.7.3 Si-NEt, Approach to Surface Functionalization
3.7.3.1 Two-Step Deposition Process 3.7.3.2 Three-S tep Deposition Process
3.7.4 Sn-NE& Approaçh to Surface Functionalization 3.7.5 Contact Angle Measurernents 3.7.6 EUipsometq Re ferences
Chapter 4 Links between Materiil Presenteâ in Chapters Five to Seven 92
Chapter 5 Simple Acid-Base Hydrolytic Chemistry Approach to Molecolar Self-Assembly: Thin Films of Long Chain Alcohols Terminated with Alkyl, Phenyl and Acetylene Croups on Inorganic Oude Surfaces
Introduction Acid-Base Hydrolysis
Surface Func tionalization Three-Step vs Two-Step Deposition Process
Stabiiity of SAMs
Canclusion Experirnental Section 5-7.1 Materials
5 -7.2 Substrate Reparation 5.7.3 Two-S tep Deposition Process 5.7.4 Three-S tep Deposi tion Process 5.7.5 Contact Angle Measurements 5 -7 -6 Fourier Transfomi Infrared Spectmscopy in the Attenuated
Total Reflection Mode
5.7.7 Ellipsornetry 5.7.8 X-ray Photoelectmn Spectroscopy Re ferences
Chapter 6 Molecular Self-Assembly of Dihydroxy Terminated Molecules via Acid-Base Hydrolytic Chernistry on Inorganic Oxide
Surfaces: Step-by-Step Multilayered Thin Film Construction
6.1 Introduction 118
6.2 Acid-Base Hydrolysis 120
6.3 Mondayers of Diols 120 6 -4 Multilayered min Film Assem blies of 2.4-Hexadiyne- 1 5-di01
and 5.7-Dodecadiyne- l,12-di01 126
6.5 Stabiiity of Mono- and Multiiayers 136 6.6 UV-Vis Exposure of Mono- and Multilayers 140
xiv
143 144
6.7 Conclusion 6.8 Experimental Section
Materials Substrate Preparation Freparation of SAMs
Preparation of Multilayers Contact AngIe Measuremen ts
Fourier Transfomi Infrared Spectroscopy in the Attenuated Total Reflection Mo& EUipsomecry X-ray Photoelecuon Spectroscopy üV-Pol ymerization
6.9 Re ferences 150
Chapter 7 Molecular Self-Assembly o l Alkynyl Terminated Chromophom on Inorganic Oxide Surfaces via Acid-B~se Hydrolytic Chemistry: Monolayer and Sbp-by-Step Multilayered Thin Film Construction
lntroduc tion Acid-Base Hydrolysis Monolayers of Akynes Estimation of Surface Coverage of Rigid-Rod -ne Chromophom UV-Vis Exposure of a SAM ofp-Bis(butadiyny1)bemne Multiiayer Thin Film Assembiies of 1.9-Decadiyne and p-D ieth ynylbenzene Cobalt Carbonyl Adsorption on Monolayers and Multïlayers of 1.9-Decadiyne and p-Diethynylbenzene Conclusion Experirnen ta1 Section 7.9.1 Materiais 7 -9.2 Substrate Pteparation 7.9.3 Preparation of S AMs 7.9.4 Preparation of Multilayers 7 -9.5 Cobalt Carbonyl Adsorption 7.9.6 Contact AngleMeasurernents
7 -9.7 Fourier Transfomi Infrared Spectmscopy in the Atîenuated Totai Reflection Mo& 182
7.9.8 Ellipsomew 183
7 -9.9 X-ray Photoelecuon Spectroscopy 183 7 -9.1 0 UV-Polymerization 184
10 Referiences 184
Chapter 8 Conclusions, Contribution to Original Knowledge and Suggestions for Future Work
8.1 Conclusions 8 -2 Contribution to Original Knowledge 8.3 Suggestions for Future Work 8.4 Refe~nces
List of Schemes
AU the schemes shown in the thesis depict a 'cartoon representation' of the various steps involved in the thin-fdm construction process.
Scheme 3.1 Scheme 3.2
Schemc 5.1
Scheme 5.2
Scheme 6.1
Scheme 6.2
Scheme 7.1
Scheme 7.2
Si-NE5 approach to sudaœ functionalization
Sn-NE& approach to surface functionalization
Surface functionalization using two different reaction methodolo@es: two-step and three-step processes 95 Molecular self-assembly of a series of short-to-long chah length
alcohols terminated with aikyl, phenyl and acetylene groups on
glass, quartz and single crystal silicon 97
Molecular self-assembly of a series of dihydroxy molecuIes on
glass, quartz and single crystal silicon 121
A step-by-step reaction methodology for fabrication of multilayers of dihydroxy molecules on glas , quartz and single crystal silicon t 29
Molecular self-assembly of a series of alkynyl chromophores
on glass, quartz and single crystal silicon 157
A step-by-step reaction methodology for fabrication of multilayers of dialkynes on glass, quartz and single crystai siiicon 158
List of Tables
Table 1.1 Peak positions for CH,(C&),SH CH, siretchhg modes in crystalline
and liquid States and adsorbed on gold 14
Table 1.2 Advancing contact angles OF water (CA,d and HD (CAfm) on thioi monolayers adsorbed on gold 16
Table 1.3 Physicai propenies of monolayers on silicon, oxidized silicon and gold 34
Table 1.4 Acid strength of some protic species 43
Table 3.1 The effect of silanation tune on surface properties (Cho and Te) of
octadecanol thin f i s using a two-step process 75
Table 3.2 Static contact angles of water (CA,& ellipsometric CI;) and theoreticai thicknesses (Tt) for monolayers on Si(100) substrates 80
Table 5.1 Static contact angles of water (CA,d, theoretical fl,) and ellipsomeiric thicknesses (Te). and Fm-ATR data of alcohol thin füms self-assembled
on Si(100) by the two-step process 99
Table 5.2 Static contact angles of water ( C w , theoretical (Tt) and ellipsomeiric thicknesse. (Te). and FIiR- ATR data of alcohol thin füms self-assem bled on Si(100) by the three-step process 107
Table 5.3 Results of the stability tests on the octadecanol thin film or. a Si( 100)
substrate 108
Table 6.1 Static contact angles of water (Cho), theoretical 0,) and ellipsometric
thicknesses (Te), and XPS data for SAMs prepared from dihydroxy terminated molecules on Si(100) substrates 123
Table 6.2 FTIR-ATR data for SAMs prepared from dihydroxy teminated molecules on Si(1ûû) substrates 124
Table 6.3 Static contact angles of water (Cb . , ) . ellipsometric thicknesses (Te). and FTIR-ATR data for the multilayers prepared from 2.4-hexadiyne- 1.6-di01
on Si(100) substrates 132
Table 6.4 Static contact angles of water (Cho), ellipsometric thicknesses (Te), and F ï i R - A m data For the multilayers prepared from 5,7-dodecadiyne- 1.12-
di01 on Si(100) substrates 134
Table 6.5
Table 7.1
Table 7.2
Table 7.3
Table 7.4
Table 7.5
Table 7.6
Table 7.7
(a) Results of the stability studies on SAMs of 2.4-hexadiyne-1.6-di01 and S,7-dodecadiyne- 1.12-di01 137
(b) Results of the stability studies on a SAM of octadecanol and a thin fdm of 2.4-hexadiyne- l,6-di01 capped wiih OTS 138
(c) Results of the stability studies on multilayers of 2.4-hexadiyne- 1 &-di01 capped with OTS 139
(d) Results of the stability studies on rnultilayers of 5,7-dodecadiyne- l,12- di01 capped with OTS 140
Static contact angles of water and HD, theoretical (TJ and eliipsometric
thicknesses (TC), and XPS data for SAMs of alkynes on Si(100)
su bsuates 159 FTIR-ATR data for SAMs of alkynes on Si(100) subsmtes 160
(nm). absorption (A), absorption coeficient (E ) and surface
coverage (8) of rigid-rod alkyne thin films on quartz 168
Static contact angle of water (CA,), ellipsometric thickness (Te), and
FTIR-ATR data of a SAM of p-bis(butadiyny1)benzene on a S i (10)
substrale upon exposure to UV-lamp for 30 min 17 1
Static contact angles of water ( C h o ) and HD (CA,), ellipsometric
thicknesses (TJ and FTIR-ATR data for the multilayers of 1 9-decadiyne
on Si(100) substrates 176
Static contact angles of water (Cb,) and HD (CA,), ellipsomeuic thicknesses (Te) and FTIR-ATR data for the rnultilayers of pdiethynylbenzene on Si(100) substrates 177
Statiç contact angles of water (C&& eiiipsomeuic thicknesses (T,) and FIVbATR data for the thin films of y-diethynylbenzene and
19-decadiyne on Si(100) substrates aftcr reaction with cobalt carbonyl 180
List of Figures
Figure 1.1
Figure 1.2 Figure 1.3
Figure 1.4
Figure 1.5
Figurz 1.6
Figure 1.7 Figure 1.8
Figure 1.9 Figure 1.10 Figure 1.11
Schematic diagram of a Langmuir-Blodgeti trough for deposition of
rnonolayers 3 A schematic view of the forces in a self-assembled monolayer 4
A S A M of octadecanoic acid on silver 7
A SAM of octadecanoic acid on copper and alurninum oxi& 8
A S A M of oçtadecanethiol on gold 12
A S A M of 1.2-bis(mercaptomethy1)-4.5-di(tetradecy1)ben~me on gold 18
A S A M of dioctadecyl suMi& on gold 20
A S A M of dioctadecyl disufide on gold 21
A SAM of a branched thiol on gold 22 A SAM of a branched disulfide on gold 22 A SAM of octadecyltrichlorosilane on SVSiO, 24
Figure 2.1 (a) A typical water contact angle on a methyl surface (b) A typical hexadecane contact angle on a methyl surface 63
Figure 2.2 A schematic description of an optical setup for ATR rneasurements 64
Figure 2.3 A schematic description of an eliipsometer 66
Figure 3. 1 The effect of silanation cime on contact angles of water of oçtadecanol
thin fdms using a two-step process 76
Figure 3.2 The effect of silanation tirne on ellipsomeuic thickness of octadecanol
thin films using a two-step process 77
Figure 5.1 Static contact angles of water for monolayers of octadecyltnchlorosilane,
octadecanol, hexadecanol, tetradecanol, decanol and hexanol on
Si(100) substrates 100
Figure 5.2 Static contact angles of water for monolayers of propynol, butynol.
pentynol, hexynol and undecynol on Si(100) substrates 103
Figure 5.3 FTIR-ATR (nonpolarized) spectra for monolayers of undecynol,
hexynol, pentynol, butynol and propynol on Si(lO0) substrates in the region 2800 - 3000 cm" 105
Figure 5.4
Figure 6.1
Figure 6.2
Figure 6.3
Figure 6.4
Figure 6.5
Figure 6.6
Figure 7.1
Figure 7.2
Figure 7.3
Figure 7.4 Figure 7.5
Figure 7.6
Figure 7.7
F ï R - A I R (nonpolarized) spectra for monolayers of -O-(C&),,-CH, on Si(100) substraw in the region 2800-3000 cm-': A. before any treamient; B. after heating at 150 "C for 1 h: C, trcatment with aq. 2.5 M MSO,. 25 OC, 1 h; D. boiling CHCI,. 2 h; E, boiling methanol. 2 h; F. aq. 1 M NH,OH, 25 "C, 1 h 109
Eiiipsometric thickness of the 1 to 10 layered thin films of
2,4-hexadiyne- 1,6-di01 130 Eilipsometric thickness of the 1 to 1 O layered thin films of 5,7-dodecadiyne- 1.12-di01 131
FïXR-ATR (nonpolarized) spectra for the 1 to 10 layered (top to bottom) thin füms of 2.4-hexadiyne- 1.6-di01 on Si(100) substrates in the region
2800-3000 cm-' 133
FIiR-ATR (nonpolarized) spectra for the 1 to 10 layered (top to bottom)
thin films of 5.7-dodecadiyne- 1.12-di01 on Si(100) substrates in the region 2800-3000 cm-' 135
UV-Vis spectra of the 1 to 10 layered thin films of 2,4-hexadiyne- 1.6-di01 on quartz. The inset shows the UV-Vis spectra of the 1 and 2 layered thin films
UV-Vis spectra of a 10-layered thin film of 2.4-hexadiyne- 1.6-di01 upon exposure to UV-larnp for a period of 0.5. 15. 30.60 and 120
min (top to bonom)
Static contact angles of water for monolayers of hexadiyne. octadiyne. nonadiyne and decadiyne on Si(100) substrate Ellipsometric thickness for monolayers of hexadiyne. octadiyne. nonadiyne and decadiyne on Si( 100) substrate
FITR-ATR (nonpolarized) specua for monolayers of hexadiyne. octadiyne. nonadiyne and decadiyne on Si(100) substrates in the
region 2800-3000 cm*'
UV-Vis spectrum of a monolayer of p-diethynylanthcene on quartz
UV-Vis absorption of a monolayer of p bis(butadiyny1) benzene upon
exposure to UV-lamp for a period of 0.5. 15,30 and 60 min
Eliipsometric thickness of the 1 to 5 layered thin f h s of 19-decadiyne Ellipsornetric thickness of the 1 to 5 layered thin fdrns of
p-dieth ynylbenzene 174
Figure 7.8 W-Vis absorption of the 1 to 5 layered thin films of 1.9-decadiyne 175
Figure 7.9 UV-Vis specmim of a thin f h on quartz of p-diethynylbenzene with
adsorbed cobalt carbonyl 179
List of Abbreviations
A
ATR
br
CA,,
C A , d Et
FTIR h
riD
.J
LB m
min
Me MS NMR OTS Ph R, R', R S
SA
SAM
t
Te Tt THF UV-vis
XPS
hm,
Absorbance Attenuated total ~flecrion Broad Contact angle of water Contact angle of hexadecane Doublet Ethyl group Fourier tram form in frared spectroscopy
Hour Hexadecane Coupling constant Langmuir-Blodgen Multiplet Minute Methyt group Mass spectmmetry Nuclear magnetic resonance Octadecyltnchlorosilane Phenyl group
Alkyl F O U P
Singlet Self-assembly/self-assem bled Self-assembled monolayer Triplet EUipsometric thickness Theoretical Thickness Te trah ydrofuran Ultra violet and visible X-ray photoelectron spectroscopy
Position of maximum peaks on W - V i s spectra
Asymmeûic stretching
Symmetric stretching
List of Publications Originated from Research at
McGill University
(1) Yam, C. M.; Kakkar, A. K, J. Chem Soc., Chem- Commun. 1995,907.
(2) Yarn, C. M.; Tong, S. S. Y,; Kakkar. A. K. Langmuir 1998,/4,6941.
(3) Y am, C . M.; Dickie, A.; Malkhasian, A.; Kakkar, A. K.; Whitehead, M. A. Cm. J.
Chem. 1998. 76. 1766.
(4) Yam, C . M.; Kakkar, A. K. iungmuir 1999, in press.
(5) Yarn, C. M.; Kakkar, A. K. Langmuir 1999, subrnitted.
Chapter One
General Review of Molecular Self-Assembly
and Scope of Thesis
1.1 Organic Thin Films
Optoelectronics and molecular electronics have recently emerged as an important area
in material science.' hie to the Limitations of inorganic materials in both areas, ordered
organic materials are becomuig increasingly signifcani. This may be the main reasonl for
the growing uiteiest in both Langmuir-Blodgett (LB), and self-assembled (SA) thh films.
Both SA and LB films are fomed from molecular assembiies, the former from solution and
the latter at the airlwater interface. They allow che chemist to construct thh f h s with
desira ble properties b y incorporating sui tabl y oriented chromo phores with different
functional groups. These thin films offer significant potential in technolog#" including. for
example, thin-füm op tic^,^ sensors and tramducers.' protective6 and patternable materials?
surface preparation and modification. chemically modifeû electmdes: and biological thin
films of proteindo
Although the advance in thin film technology seems exciting and promising, there are
still many unsolved problems.' Organic thin films suffer from fragility, impurities, and
defects, multing in difficulties in producing films with good mechanical. thermal. and
chernical stability. In order to improve the latter properties, films can be polymerized and
cross-linked. interchain interactions, such as van der Waals forces. x-lc interactions and
covalent bonding (such as Si-O-Si cross linkages in allryltrichlorosilane monolayers on
silica), rn important factors in the molecular engineering of thin film consuuction.
Although the= are many ways to consmct thin films. assemblies with chromophores
con taining desired backbones and the optim ization of the overall propertîes of ihe organized
structures in the targeted domain still await further investigation.'
1.2 Langmuir-Blodgett (LB) Films
Much effort has been devoted for the construction of thin f h s by the LB technique
(Figure l.l).11-12 Blodgett fmt reported the preparation of ultrathin organic f h s using this
deposition method in 1935." During the process of LB f h deposition, arnphiphilic
compounds spiead at the air-water interface to produce monoiayers. Usuaiiy an amphiphile
consists of two parts, a hydrophilic headgmup and a long-chain hydrophobie tail. The
molecules are oriented at the interface upon compression. The monolayer can then be
transferred ont0 the substrate by dipping the substrate perpendicularly through the interface.
Several layers can be built on the substrate by repeating the dipping process. By conirolling
the number of dipping cycles, highly ordered ultrathin füms with appropriate thickness can
be obtained. LB films with different rnoleçular orientations, such as X- (tail-to-head), Y-
(tail-to-tail and head-to-head) or 2- (head-to-taii) structures = prepared by aUowing the
deposition to occur either d u ~ g the down- and upstroke, or oniy during the downstroke or
the upstroke. In addition. "mixed" (two or more amphiphiles per layer) and "aitemate"
fdms (two or more amphiphiles in successive layers) can also be easily prepared using a
specially designed "aitemate layer" trough.14
In recent years. molecules of completely different stmcture such as porphyrins.15
p hthalocyanines.16 oligothiophenes." and polyc yclic ammatic quinones." have also been
used to form stable LB films. They have gained increasing importance due to their
improved mechanical, rhemal, and chernical stabilities. Many of the recent activities on LB
füms an: focused on photocherni~al~~ and thermal reac t i~ i ty ,~ electrical cond~ctivity.~'
pyroelectric a ~ t i v i t y , ~ and nonlinear optical proper t ie~ .~~ These füms can be adopted to
electronic applications," such as infmed sensors, frequency converters and information
storage.
Moving su bstrate
t H ydrophobic
tail
Moving barrier to control pressure
I H ydrophilic Water head 1
Figure 1.1 Schematic diagram of a Languir-Blodgett trough
for deposition of monolayem.
Although the LB technique is effective in f h formation. it has sorne drawback~.'~'~
LB f h s are usually unstable under a variety of chernical or physicd conditions because the
interactions between the films and the substrate surface are weak. As a consequence. these
surface layers can easily reanange. Under large compressive or ensile stress. surface
reorganization of the f h s occurs. It becornes highlv difficult to control domain structures
in the films since f h quality depends on mechanical manipulation. Furthermore. planar
substrates are requinxi for chin füm deposition. These Limitations prevent the extension of
the k c hnique to more sophisticated systems such as three-dimensional molecular structures
and multilayer as~emblies.~' Therefore. it is necessary to develop other molecular assernbly
methods. although the chemistry involveci may ofien not be s t r a igh~a rd .~
1.3 Self-Assembled Monolayers (SAMs)
The field of self-assembled monolayers (SAMs) has attxacted growing aaention over
the pas t 15 years or SO."~" in 1946 Zisman Fust pubbshed the preparation of monolayers
by adsorption of a surfactant onto a clean metai but the potential of self-assembly
was not recognized und N u v o and AUara successfully prepared SAMs of diallcyl
disulfides on gold more recently."
A self-assembling molecule c m be divided into three parts,12 the head group. alkyl
chab and the terminai functional group (Figure 1.2). The head group pmvides
chemisorption on îhe substrate surface, and results in the formation of a covalent S i 4
bond for alkyltrichlorosilanes on hydroxylated surfaces; a covalent, but slightly polar, Au-S
bond for alkanethiols on gold; or an ionic CO,'Ag+ bond for carboxylic acids on AgOfAg.
The alkyl chains provide interchah van der Waals interactions which help in the formation
of an ordered and closeiy packed assembly. When a polar buiky group is substituied in the
alicyl chah, stronger long-range electrosiatic interactions m invoduced in the assembly.
The terminal functional group may be modifïed to ailow adsorption of another layer. The
SA monolayer is chemically bonded to the surface. while the LB monolayer is only
physicaliy adsorbed. Thus. the SAMs are expectcd to be stronger and more resistant han
the LB thin films?"
Alkyl , or derivatized- &y1 group 1
Surface-ac tive headgroup Chemisorption at the surface
Substrate
e
Figure 1.2 A schematic view of the forces in a seif-assembled monolayer."
3 Interchain van der Waals and electrostatic interactions
Moreover, the SA technique aliows the preparation of highly ordered and stable
monolayers on various surfaces."."-" By t a i l o ~ g both head and tail groups of the
molecules, SAMs provide excellent systems for a more fundamental understanding of such
phenornena as ordering and growth. wetting. adhesion. lubncation. and corro~ion. '~ These
studies may be important in designing assem biies of three-dimensional stnic tures. In
addition, the âensely packed, highly stable and ordered SAMs offer great potential in the
areas of corrosion prevention, Wear protection and the preparation of electrooptic devices
and sensor arrays?'
There has been extensive investigation on many self-assembling systems, including
organosilkon cornpounds on hydroxylated surfaces (e-g.. SiO, on Si and 40, on Al)
38-54 alkanethiolS on gold,31.34.S5-63 silve? and ~opper.~%iialkyl suifides on gold." diakyl
disulfides on g ~ l d . ' ~ " ~ carboxylic acids on silver" coppeP7 and alurninurn 0xide.6 '~~ 1-
alkenes on h ydrogen-tenninated silicon ,7G71 and films with alternate "inorganidorganic"
s y ~ t e r n s ~ " ~ (including layered phosphonate f h s on silica and goldmn). in ail these cases,
the mo1ecdes attach to the surface of the substrates via strong chernical bonds.
1.4 SAMs of Alkanoic Acids on Metal Oxide Surfaces
The molecular self-assembly of longchah n-alkanoic acids on metallic oxides, such
as silver, copper and aluminum oxides, has ken stddied in the past 10 years. The driving
force for these thin films is the formation of a surface salt between the carboxylate anion
and a surface metai cation based on acid-base reactions. AUara and N U Z Z O ~ ~ and Ogawa et
al6' have snidied the adsorption of n-alkanoic acids on aluminum oxide. and reported that
under appropriate conditions, long-chah alkanoic acids adsorb on oxidized aluminurn
surfaces to form closely packed and ordered monolayers, displaying advancing contact
angles of water and hexadecane (HD) of -1 10' and -XI0, respectively.
Besides thin films on aluminum oxide, Schlotter et al? have also studied the
adsorption of arachidic acids on silver. The latter SAM exhibits an eliipsomeûic thickness
of 27.6 + 0.8 A, which is comparable to the calculated thickness of 28.0 A for a hilly
extended c h a h From the FT-IR spectra. they observed that the acid head group
dissociatively chernisorbs on the surface with a am zigzag conformation wiih a tilt angle
of - 10' to the surface normal. ûn the othx hand, Sarnant et al." reported similar results,
that an ordered SAM of docosanoic acid was fonned on siiver with a tilt angle of -27O to
the surface normal.
1.4.1 SAMs of Atkanoic Acids on Silver, Copper and Aluminum Oxide
It has been found that there are differiences in the chemisorption of a h o i c acids on
different metal oxide sudaces, such as Ag, Cu, and Al.6' On the surface of Ag, the n-
alkanoic acid dissociatively adsorbs in a very ordered way and fuily occupies al1 the
available sites, and the two oxygen atoms of the carbxylate sit nearly symmetrid on the
surface (Figure 1.3). This binding geometry is not ~ i g ~ c a n t l y affected by the chain-chain
interactions. A similar packing density with a t r a zigzag conformation and a similar chain
tilt was observed for both long and short chah acids. The "odd-even" e k t observed in
contact angle and Fï-IR data is closely tied to the orientation of the teminal methyl group.
For even nurnbered carbon chains, the symmetnc vibration mode for the methyl group is
stronger, and the asymrnetric vibration mode is weaker because the teminal methyl group
points closer to the surface normal, resulting in a higher contact angle, especiaily for HD,
on such a "methyl" surface. For odd nurnbered carbon chains, the symrnetric vibration
mode is weaker and the asymmetric vibration mode is stronger because the terminal methyl
group points away from the surface normal, resulting in an exposw of the methylene unit
next to the methyl group to the surface and exhibithg a lower contact angle. For acids of
sufficient length (n > 1 1), al1 the even chah acids give an identical structures but aU the odd
chain acids display sligh tly different struç tures.
Figure 1.3 A SAM of octadecanoic acid on silver."
On the surface of Cu and Al, n-alkanoic acid also completely dissrniates, but the head
group carboxylate coordinates to the surface with a tilt (Figure 1.4). For shorter chah
acids, the film is disordered due to insufficient cohesive :nteractions which result in a less
dense film. As the chah length increases, the cohesive force becornes strong enough to pull
îhe molecular chains into a "normal" orientation for optimal interaction energySs6 Moreover,
some twisting or strain of the chains has to devell~p to achieve the ultimate perpendicular
onenration. Consequently, acids with odd or even chains have the same "outer surface
structure". Only those methylene units near the head group contribute to most of the
absorption intensity of the methylene modes. The rest of the groups have only linle
contribution because of the perpendicular orientation. For the chains that are beyond a
certain threshold length, they stand up straight.
Figure 1A A SAM of octadecanoic acid on copper and aluminum oxide."
Although it has been suggested that long alkane chai. acids are aligned normal on
both Cu and Ai surfaces, there are still some differences in the band shape for the
methylene stretches. vJCH,) and v,(CH,). On copper. the va(CH2) band around 2917 cm-'
and the v,(CH,) band ai 2849 cm-' are more or less symrneûically shaped. ûn durninum.
the v,(CHJ band has broad multiple peaks. and the vXCH~) band is shifted to a slighly
higher frequency. It was suggested that the strain cr twist of the molecdar chains is more
serious on aluminum than on copper. leading to a more or&red system for the latter. It is
amibutable to a difference in the binding geomeüy/strength of the carboxylate on each metal
as weii as the surface lattice of the metal oxides.
Upon exposure to air, ai i three metal surfaces forrn oxides with different basicities.
The peak frequency for symmetric carboxylate of the SAM on the metal oxide inmeases in
the order: 1402 cm" on silver < 1440 cm" on copper < 1478 cm-' on durninum. The shifi is
parallei to the stabiiity of the monolayer on metal in the order of Al < Cu c Ag. However,
Allara and NUZZO,~ Smith and Porter," and Soundag et aLa2 concluded that the monolayer
of steak acid on A@ surface is less ordered than that on A.40, Differences between the
results in the two groups may be due to the differences in the preparation condition^.^
1.4.2 SAMs of Alkanoic Aads with Aromatic Chromophores
In their study of the effect of inuoducing aromatic chromophores dong the aikyl
chah of aikanoic acid films on the monolayer structure. Tao et al." reported that a variety
of aromatic chromophores, except for large moIecules, can be incorporated in a monolayer
assembly, resuiting in a weli-ordered SAM on the surface of silver. These large
chromophores, such as 1,4-substituted naphthyl and 9.10-substituted anthracenyl groups,
require a more expanded lattice for packing, resulting in a wiâer separation between the
akyl c h a h above the chromophores, Thus they need to tilt further in a less ordered and
tram zigzag way to make molecular contact with neighboring chains. However, it is still
uncertain as to whether the disorder of alkyl c h a h is due to a disorder in the arrangement
of the chromophores or whether it is a mere result of the larger spacings between the
chains.
1.4.3 Stability Tests
In a study of the stability of the alkanoic acid films," it was found that the SAMs of
allcanoic acid dkolved in chloroforrn, resulting in a significant decrease in contact angles
for both HD and water. It is attributable to a weak bonding of carboxylate to the metal
surface. The low stability of the acid f i s limits their applications in thin film technology.
1.4.4 SAMs of Hydroxamic Acids
In order to improve the stabiiity of the above films on metallic oxides, monolayers of
hydroxarnic acids (R(CH,),CONHOH) on native oxides of me& wcre e ~ a m i n e d . ~
Hydroxamic acids fonn more stable monolayers on basic metal oxides such as silver and
copper, but fonn less stable monolayers on acidic metal oxides such as aluminum, iron,
titmium and zirconium. Furthexmore. hydroxamic acids fonn more stable monolayers than
carboxylic acids or phosphonic acids on these substrates. There an: two advantages of
using hydroxamic acids compared to thiois on copper and silver? Fin& the thiols will etch
the surface of copper oxi& during thin film formation, but the hydroxamic acids do not.
Second. hydroxamic acids are more inert to onidalion, and ~IE thus monz stable in air for a
longer Lime." Monolayers of thiolaies photooxidile on the surface ESU~M~ in the
formation of rnetal-su1fonates.'* nerefore. hydroxamic acids can be used as an alternative
to thiols as protective coatings. Other potential uses of these monolayers comprise
lithography, corrosion resistance. and vibology.""
On the native oxides of aluminum, zirconium. and iron, hydroxamic acids also form
more stable monolayers than carboxylic acids or phosphonic acids. Owing to the srnailer
size of the hydroxamic acid than the phosphonic acid, more ordered monolayers of
hydroxamic acids on zirconium oxide can be for~ned.~'" They may have similar techniça1
applications as the SAMs of the phosphonic acids on zirconium oxide. Finally. Fokers et
al." found that phosphonic acids fonn more stable monolayers than hydroxamic and
carboxylic acids on titanium oxide.
1.4.5 SAMs of Dioic Acids
We now turn to researçh on the thin films of &twalkanedioic acids on Ag which may
be used to build multilayers of carboxylic acids on metallic oxides. AUara and A&
reported the fmt exarnple of a SAM of a folded dioic acid, 1,32-dotriacontanedioic acid. on
silver. The monolayer structure consists of folded chains of loop shape bound by the
carboxyl groups on the substrate. resulting in a theoreticcal thickness of -22 A which is
consistent with the measured value of 20 I 2 k Exposure of CH, groups to the surface
was confirmed by the wetting measurements on the monolayer and c l a n polyethylene. A
lower water contact angle of 103" was obtahed in both cases. The similarity of these thin
fiims to the polyethylene surfaces was further supporteci by having a contact angle for HD
of -O0. Besides the symrnetric and asymmetric CH, stretchùig frequencies at -2850 and
-2920 cm-', a shoulder at the higher frequency side of the 2928 cm-' band, representing a
component with a low degree of confocmational disorder." appears in the FT-IR spectra.
Furthemore, the intense peak œntered at 1400 cm-' and the weaker bands at 1537 cm'' are
attributed to the s ymmetriç and asymmetric stretching modes, respec tively , of a carboxy late
g r o ~ p . 6 ~ Thus, the looped structure of dioic acid makes it impossible for fabrication of
multilayered thin film assem blies.
1.5 SAMs of Organosulfurs Compounds on Metal Surfaces
In 1983. N u v o and ~ l l a r a ' ~ fust reporced thaî dialkyl disulfides (RS-SR) cm fonn
ordered monolayers on gold surfaces. Later, it was found that sulfur compounds coordinate
very strongly to gold, 31.3455-63 silverVM ~opper .~* and platinurn surfaces." Since gold does
not fonn a stable oxide, it can be handled in ambient conditions. Thus, most of the work
has been done on goid surfaces. SAMs on gold have potential applications in industrial
technologies, such as electrode modifi~ation:~ corrosion res is tan~e .~~ biomaterial
c ~ a t i n g s , ~ and biosensor ~echnology.~~ This is due to a s m n g interaction of sulfur with the
gold surface (-40-45 kcal mol-'), and thiols on gold can form highly ordered SAMs with
the thickness in angstrorns sca1e.l' However, SAMs of alkanethiois on gold are ielatively
fragileg2 and decompose on moderate heatingY (e.2.. 80 OC in hexadecane). Hence. the
thiol thin füms have Iimited applications in adhesion, lubrication, and optœlectronic device
fabri~ation.~
1.5.1 SAMs of Thiols on Gold
SAMs of organosulfur compounds includmg diallryl ~ u l f i d e , ~ ~ dialkyl d i su l f ide~ .~~
thiophenols~' mer~aptopyridines,~ rner~aptoanilines,~~ thiophenes.% cysteinesP7
xanthates,g8 thiocarbaminate~.~~ thiocarûamates,'m thiourea~.~"' and mercap toirnidaz01e-s~~~
on gold have k e n reponed in m e n t years. However. the SAMs of alkanethiols on gold are
the most weli-studied ones (Figure 1.5). Arnong the thiols, octadecanethiol monolayer has
been shown to form a proiecting coating of the metal surface against oxidation-"
Figure 1.5 A SAM of octadecanethiol on gold.
1.5.1.1 Deposition Process
It has k e n reporied that even with very dilute solution (e.g. 5 pM) of longchain
alkanethiols ( e.g. C22) the main process of film formation is quite fast (- I min). 'O3 During
the adsorption process, small and disordered clusters grow. followed by an ordered domain
f ~ r m a t i o n . ~ From kinetic studies. a very fast step takes a few minutes to reach 80-90% of
the lirniting values of the contact angles and thickiess. followed by a slow step which lasts
several hours before they ~ a c h their fmal values of thickness and contact angles?"
1 -5. 1.2 Monolayer Structures
The presence of a closely packed, methyl-tenninated monolayer is often indicated by
an advancing water contact angle of -1 10°. and an advancing HD contact angle of
-45 0,75.29.3L.3C36.U.67-68.104-105 A lower vaiue of the contact angle, relative to a weU-
characterizai system such as alicanethiols on gold, indicates disorder and/or lower surface
covemge in the monolayer. However. a value comparable to that of the ordered system
does not necessarily indicate the same structure or a corresponding degree of order.
Porter et al." observed that there was a significant drop in the ellipsornetric thicbiess
for short chahs of alkanethiols on gold, Together with the ET-LR data, they indicate
increasing disorder with lwse packing. Bain et al? also saw a sirnilar trend in the contact-
angle data. For n > 10, advancing contact angles. CA, and CA, were found to be 1 1 1-
1 14O and 45-48O, respectively. The contact angles becarne iower for shorter chains. This
trend could have k e n caused by the probe liquid sensing thc underlying goldY or
increasing disorder in short-chaîn monolayers. For the partidiy formed monolayers, and
monolayers where disorder has been inuoduced intentionaliy, lower contact angles were
observed?" The oddeven effect, as mentioned before in the case of the alkanoic acids on
silver, is l e s prominent for short (n < 1 1) chains of thiols on goId.
As the length of the alkyl chains increascs, the fquency for the v3(CH,-) and vS(CH2)
modes decreases." From Table 1.1,'' it was observed that the frequencies of vJCH,) and
vs(CH,) decreased from 2921-' to 2918 cm", and 2852 cm'' to 2850 cm-', respectively, as
the length of the alkyl chain increased from n = 5 to n = 2 1. This suggests that for shorter
chains, the thin films are more disordered and iiquid-like; while for longer chains, the thin
films become more ordered and crystalline-like.
In addition, the measured intensities of the methyiene svetches can be used to
caiculate average tilt of the chah axis from the surface normal. Tbe details for this type of
calculation have b e n ceported el~ewhere.'~ and involve the cornparison between the
measured intensities and those which are calculated for a hypotheticd isotropie monolayer.
It was found that for n = 15-2 1, the monolayers of aikanethioh have a tilt angle of 20-30'
from the surface normal and 5@5S0 rotation about the molecular axis?
Table 1.1 Peak positions for CH,(CH,),SH CH suetching modes in crystailine and
Liquid states and adsorbed on gold"
Peak positions for
crystalline and liquid
states. cm-'
1.5.1.3 Thermal and Chemical Stabili ties
SAMs of alkanethiols on gold have been found to be inert to air, water and ethanol at
room temperature, consistent with the results that there is no change in contact angle or
thickness under such conditions for several months? 'Ihe monolayers desorbed upon
heating to temperatures above 70'. Qualitatively, desorption was the slowest in air. faster in
ethanol and the fastest in a hydrocarbon solvent. As compared b amine monolayers on
Cr'" and carboxylic acid monolayers on silver.8' thiol monolayers on gold are more stable;
however, they are less stable than silane monolayers on silica Furthemore, monolayers of
the long-chain thiols are thennaliy more stable than those of the short-chain thiols.
From the contact angie and ellipsometxy data. monolayers of octadecanehi01 on gold
were resistant toward 1 M HCl or 1 M NaOH at mom temperature for 1 day. but were
Peak positions for CH,(CH,),SH adsorbed on gold. cm"
CH,
, Va
v,
crystalline
2918
285 1
n=21
2918
2850
liquid
2924
2855
n = i 7
2917
2850
n = 1 5
2918
2850
n = l 1
2919
2851
n = 9
2920
2851
n = 7
2921
2852
n = 5
2921
2852
atîacked after a month? Over ihis period, the water contact angle was l o w e d by 3' in a
base. and 8' in an acid. In addition, the surface of the gold was visibly piüed in the latter
case. The deterioration became more severe in boiling acid and base. Other chernicals that
attack either the gold surface, such as aqua regia mercury and 1,-, or the thiol monolayers
themselves such as halogens (1,. Br,), strong oxiduing agents (peroxide, ozone). and
ethereal solutions of borane and phosphocus pentachlaride. mus t be avoided?
1.5.2 SAMs of o-Sustituted Thiols on Gold
SAMs of fùnctionalized allcanethiols arie important for surface modification. A
num ber of functional groups can be in troduced in the chromophore structure provided that
(i) they do not react with thiols; (ü) they do not compte strongly with the thiol to adsorb
ont0 the gold; and (iii) they are not too large to prevent close packing of the hydrocarbon
chahs. As the length of the hydrocarbon chah becornes shorter, the perturbations of the
structure of the monolayer by interactions between the tail groups increase."
Table 1.ZsS sumrnarizes the contact angles of water and HD on various monolayers,
from HS(CHJ&OOH having CA, of (P to HS(CH,)(CF,),CF, having CA, of 118'.
A cornparison of the nitrile and methyl ester surfaces provides an example of the Iength
scales detemi ining îhe wetîing intera~tion.~~' Both surfaces have comparable CA, of 4 5 O .
but hexadeçane only wets the nitrile surface. On the methyl ester surface, hexakane
interacts with the exposed methyl group mainly by a London force, whereas water interacts
with the underlying polar ester group by long-range dipole-dipole interactions-lm The
contact angle data for the thiol monolayers suggest that they are closely packed to expose
the tail group on thc surface.
On the other hand, functiondkd alkanethi01 SAMs are essential tools for surface
engineering. 'lhe OH and COOH groups aie very useful teminal groups for surface
modification. A carboxylic acid-temùnated thiol can react wiih the acid chloride to produce
the comsponding thioester. Based on this reaction, Kim et al.''' syntheskd polymeric
self-assembled monolayers and multilayers from the âketylene
HS(CH,),,C-CCS(CH7)10C00H, which are dixussed in Section 1.9. SAMs of OH-
terminated alkanethiols have been used in many surfa= rnodif~cation reactions, for
example, a second monolayer of OTS cm be chemisorbed on the monolayers of 11-
hydroxyundecane-1-thiol (HUT) on gold surfaces,"' resulting in a highly ordered and
closely packed bilayer. Thus, a combination of these two technologies seems to be
promising to c o n s r n t stable multiiayers on gold surfaces.
Table 1.2 Advançing contact angles of water ( C k ) and HD ( C h ) on thiol monolayers adsorbed on go1dss
1.5.3 SAMs of Aryl Thiols on Gold
Besides the aikmethiol SAMs, organosulfur monolayers containhg aromatic rnoieties
in the backbone on gold have also k e n ~tudied."~-"~ The thiolate headgroup of alkanethi01
SAMs may be susceptible u> oxidation under ambient conditions, resulting in ihe formation
of s ~ l f o n a t e s . ~ ~ * ~ ~ ~ These o x i d k d monolayers become highly ~ n s t a b l e . ' ~ ' " ~ ~ It has been
suggested that the thiolate is less subject to oxidation under ambient conditions when the
mercapto group is direcdy attached to an aryl."' Sabatani et aLg3 prepared SAMs of the
RSH 1
H S(CH2)2(CFz)5CF3
HS(CH,),, CH,
HS(CH, )17CH=CH,
HS(CH,), ,OSi(CH,),-
(C(CH,),)
HS(CH,),, Br
HS(CH.,)I ]Cl
CAipp CAw
1 18 7 1
112 47
107 39
104 30
83 O .
83 O
- R S H
HS(CHJllOCH,
HS(CH7)17SCOCH3
HS(Ch?)lnCOICH,
HS(CH,),CN
_ HS(CH,),,OH
- HS(CH,),,COIH
CA," CAm
74 35
70 O
67 28
64 O
O O
O O
aromatic compounds thiophenol (TP), pbiphenyl mercaptan (BPM), and pteiphenyl
mercaptan (TPM) on gold BPlWAu and TPM/Au have either higher contact angles of water
(BPM) or have a srnalier hysteresis (diffefence between advancing and receding contact
angles) V M ) than TP/Au. suggesting a beaer packhg or different orientation of the
former monolayers. The ellipsomemc thickness of a TP monolayer is signifrçandy smaiier
than the theoretical value. possibly due to a nonperpendicular orientation of the phenyl ring
with respect to îhe surface123 ando r poorer packing of the molecules in the monolayer.
Rowever, the eliipsometnc thicknesses of BPM and TPM monolayers are comparable to
the theoretid values. Furthemore, BPM and TPM films are comparatively more stable
than TP layers under a variety of condition^?^ in addition. Kolega and ~ c h l e n o f f " ~
reported that well-ordefed SAMs can be praduced by linearly aüaching more phenyl groups
to the thiophenol. On the other hand. Tao e t have studied the surface properties of
the SAMs generated from various aromaticconiaùiing thiols on gold, siiver and copper,
indicating that these thiols produce densely packed and weii-or&red f i without
intmducing steric intluence in the monolayer assernbiies. Cygan et al.''-' reported that
SAMs of 4-(2'-ethyl-4'-(phenylethyny1)phenylethynyl)- 1 -phenylthioiate, a representative
of a family of linear conjugated oligorners having a phenylene ethynylene backbone. on Au,
have a potential to act as molecular wires.
1.5.4 SAMs r t f Chelating Aromatic Dithiols on Gold
In order to increase the thermal stability of thiol films. Gary et al.'26 prepared SAMs
of chelating aromatic dithiols, denvatives of 1.2-bis(mercaptomethy1)-4.5-dialkylbenzene.
on gold (Figure 1.6). The advancing contact angles of water and HD were found to be 1 14
+ 2' and 48 + 2". respectively. on the SAMs of the long-chain chelates. The wenabiiities of
the chelating SAMs do not refiect the "oddeven" effect as observed for the nomal hi01
SAMS.'~' The d m suggest that the thin f h s of the longchain chelaihg aromatic dithiols
are densely packed and highiy oriented, exposing terminal methyl rather than mzthylene
groups?' In contras& SAMs of the shorter chain analogues exhibited lower contact angles
for both water and HD, Preluninary thermal stabilities of the films suggested that the
chelahng aromatic SAMs are more thermaily stable than their nomal alkanethiol analogues-
Au Au
Figure 1.6 A SAM of 1.2-bis(mercaptomethy1)4,5-di(tetradecyl)bn on gold?
1.5.5 SAMs of Conjugateâ a,a-Dithiols and Acetylthiols on Gold
Flexible a,wdithiols have been shown to form either multilayers via disulfide
linkages or looped structures with both ends of the molefule atiaching to the s u r f a ~ e . ~ For
rigid a,o-dithiols, for example, 1 &phenyldithiol and 4.4'-biphenyldithiol, there was no
indication tow ards looped structures. Multilayer thin films wem consuuçted b y adsorbing
one thiol end to the surfxe while freeing the other end for oxidative S-S c o ~ p l i n g ~ ~ ~ ' ~ ~
resulting in densely pa~ked s t r ~ c t u r e s . ~ ' ~ The use of acetyl-protected thiols. for example.
1,4-phenyldithiolacetyl and 4.4,-biphenyldithiolacetyl. is a convenient method for the in
situ, base-promoted Liberation of the thiol. Moreover, the acetyl-protected thiols can adsorb
directly on gold without the use of exogenous base. Thete a mculties in oxygen-
promoted mdtilayer formation with the aromatic dithioh but they can be overcome by the
direct adsorption of the acetyl thiols. These aromatic a,o-dithiol monolayers may be usefui
in the design of molecular wires which are capable of bridging proximate gold surfaces.
1.5.6 SAMs of Dialkyl Sulfides on Gold
Systems based on dialS.1 sulfides (such as R(CH,),S(CH,),Rg. where R and R '
represent different functional groups such as CH, or COOH) on gold6" (Figupe 1.7) ane
attractive because the structures of the two alkyl groups connected to sulfur can be varied
independently by straightforward synthetic methods. This variation ailows a degree of
control of the local structure of the adsorbed monolayer that is not easily possible with other
organosulfur compounds or simple fatty acid derivatives?' SAMs of dialkyl sulfides were
shown to be resistant to air, water, dilute acid and ethanol, but highly non-resistant to
suong base. Furthemore, they are not stable at high temperature (-80 "C) and to sorne
reagents including 30% hydrogen peroxide and ethereal solutions of diborane and
phosphorous pentachloride. It was found that the methyl-terminated dialkyi sulfides
exhibited slightly lower contact angles with water than the methyl-termhted thiols. Ihe
carboxylic acid tenninated dialkyl sulfides showed much higher contact angles with warer
than the carboxylic acid teminated thiols due to the fact that the dialkyl sulfides fdms are
less tightly packed and ordered on gold than those of the analogous alkanethiols.
Au
Figure 1.7 A
- SAM of dioctadecyl sulfide on gold.'
1.5.7 SAMs of Diaîkyl DisuIfides on Gold
Nuzzo et a1.29.65 reported that monolayers prepared from a variety of substituted
dialkyl disulfide molecules (such as (X(CH,),S),. where X = CH,, COOH. or NH,) on
gold surfaces are stable and densely packed (Figure 1 -8). The surface properties. as well
as the stabilities of these SAMs have been shown to be sirnilar to SAMs of thiols. Enhanced
stabilities have also been observed in S AMs prepared from pol ydisul fides. "' The detailed
stnictures of the assemblies involve intra- and intermolecular interactions sirnilar to those of
the bulk crystalline phases. ï k SAMs of diaikyl disulfides provide a lot of significant
applications in electrochemistry, adhesion and wetting, biology, micmelectronics, and
material science?
Figure 1.8 A SAM dioctadecyl disulfide on g01d.~'
1.5.8 SAMs of Branched Thiols and Disuifides on Au
Besides SAMs of non-branched thiols and disulfides, SAMs of branched thiols,
[CH,(CHJl ,CH,]2-CH(SH) (Figure 1.9). and disulfides, ([CHJCH,) lCH2]2-CH-S)2
(Figure 1.10). can also be deposited on gold.I2' rIhe sulfbr ar~ms in the disufide groups
are still available for interaction with the gold surface. resulting in a tilt angie of - 15" to the
surface normal, This is significantly smailer than the tilt in allcanethiol monolayers (-30")
because the total ami occupied by two alkane chains in SAMs of branched thiols is larger
than that occupied by a sulfur atiaching onto the gold surface. The SAMs of branched
disulfides have been shown to be Iess weli-packed han the corresponding thiol
monolayers; but SAMs with polar fwtional groups were more closely packed. These
branched disulfide monolayers have a lower surface coverage than SAMs of Linear
disulfides because when a branched disulfide molecuie seif-assembles on the gold surface,
two Au-S bonds form but four allrane chains become imrnobiiizeû.
Figure 1.9 A SAM of a branched thiol on g ~ i d . " ~
S-S +
Figure 1.10 A SAM of a branched disulfide on gold.'"
1.5.9 SAMs of Thiols on Copper and Silver
n-Alkanethiols can also self-assemble on surfaces of copper and s i l ~ e r . ~ ~ " ~ Contact
angle measurements of long-chah (n 1 12) monolayers indiate that they are also densely
packed and well-orderied. The structure of the thin films on silver and copper m different
from that on gold in the foiiowing ways: (i) the hydrocarbon chah tiits more closely to the
surface normal; (ü) thin films have a lower population of gauche conformations at m m
temperature; and (iü) the "oddeven" effect is offset by one methylene group. compared to
monolayers on gold. The structural differences between a thiol monolayer on Ag/Cu and
that on Au are due to a difference in the bonding between the head group and the metal. As
compared to thiols on Au, the thin films on AgICu are more densely packed. However,
since Ag/Cu are more reactive than Au, it is more Micult to prepare reproducible thiol
monolayers on AglCu than on Au. Thus. it requires more effort to prepare hi$-quality thin
films of thiols on AgKu. Nevertheless, they offer the opponunity to explore how the
structural differences affect properties, such as weuing, intercalation, and capricitance.
1.6 SAMs of Organosilicon Derivatives on Hydrox yiated Surfaces
SAMs of Iong-chain organosilane compounds (R-SiCl,, R-Si(OCH,),. R = alkyl
group with > 10 carbon atoms) on hydroxylated surface^^^-'^ (Figure 1.1 1 ) have attracted
growing attention since their discovery by Sagiv et al? in 1980, although this r a t ion of
silanization was deviscd more than 40 years ago for chromatographic applications. These
S AMs have been success full y prepared on substrates of silicon O xi de. 25.".399.44 aluminum
o ~ i d e , ' ~ , ~ ~ quartz. 131-132 glass,"' r n i ~ a , ~ ~ ~ ' ~ zinc se~enide.~~*'" gemanium oxide,lJ3 and
oxidized g ~ l d . " . ~ ~ ~ SAMs of trichlorosilane on silica appeared to be very auracave because
of the availability of large süica-like substrates, such as g las and sihcon wafers.lN
Amongst the silanes, octadecylvichlomsilane (OTS) is the most popular one for generating
SAMs on different substrates. These SAMs can be widely employed in the preparation of
reverse phase HPLC columns for the chromatographic separationktndysis of a range of
organichio-organic molecules. ' Furthemore, the S AMs of OTS on inorganic oxide
surfaces offer significant applications in environmental anal ysis, l" biomedical studies, '" the formation of an~thrombogenic biomaterials.'" !~bricants.'~' glass-reuiforced
comp~sities, '~~ the investigation of polymer interfacial pr~penies,"~ chernical
sensors/biosensors, la as well as eleçtroc hemical s tu die^.'"^
1.6.1 Deposi tion Process
Substrates with hydmxylated surfaces are required for the formation of SAMs of
alkylchlorc~ and alkylalkoxysilanes. The SAMs of the silanes adsorb on these substrate
surfaces via strong Si-O-Si bonds to the surface siianol groups (-Si-OH) .26 Akoxysilanes
are more stable to hydrolysis than chlorosilanesl" because (i) the bypmduct from the
hydrolysis of chlorosilanes (acid), caialyzes further reactions and quickly leads to gel
formation; and (ü) the hydrolysis of chlorosilanes is more exothermic than that of
alkox ysilanes. 14'
Figure 1.1 1 A SAM of octadecyltrichlorosilane on SüSiO,.
The deposition mechanism is assumed to dinde into three partsY (i) the
trichlorosilane moleniles approach the clean silica surface by polar-polar interactions; (ü)
since the silica surface is hydratedlu, the trichlorosilane groups of the silane molecules are
hydrolyzed when they get close enough to the sudace, followed by (üi) water elimination
with the surface silanols and theù close neighbors via strong. covalent Si-O bonds. It has
k e n r e p ~ r t e d ' ~ ~ lhat oniy some of the OTS chains. about one in five, form bonds to the
surface. Thus. a completely h y d d surface is not necessary for complete surface
coverage. But trifunctionality of the silane molecuies is necessary to form densely packed
monolayers. In addition, curing at 150 promota cross-linking of organosilane
molecules and covalent bond formation to the silica surface, resulting in highly stable and
well-ordered layers. lS'
However, i t is difficul t to produce highquali ty S AMs of tric hlorosilanes because the
aiky luic hlorosilanes are highl y moisture sensitive. "J'J~' Whiie incomplete monolayers are
fomed in the absence of ~ a t e r , ~ ~ " ' ~ excess water results in facile polyrnerization in
solution and deposition of polysiloxane on the surface."' Angst e t al.150 reported that the
SAMs of OTS on h ydrated thermal oxide are ordered and densely packed, w h e m they are
disordered and loosely packed on dry oxide. McGonem et al.lS3 suggested a moisture
quantity of 0.15 mg/100 ml of solvent to be optimal for the formation of ciosely packed
monolayers. Under such condition, XPS studies confirm the complete surface reaction of
the -SiCl, gro ii ps. lS4 Recently, Tripp and Hair 15' demonstrated that mcthylchlorosilanes
were completely hydrolyzeâ to methylsilanols at the solid-gas interface by surface water on
a hydrated silica
Temperature is another important parameter in the monolayer formation. It was found
that the threshold temperature below which an ordered monolayer is formed increases with
the chain length- For example 18 OC and 10 OC is preferred for the deposition of octadecyl
and tetradecyl chain. respectively." There is competition between the polyrnerization of the
hydrolyzed silanes in solution and the reaction of such groups with surface Si-OH groups
to form a SAM. As temperature decceases. surface reaction is preferred and miction
kinetics decrease, resulting in an or&red system. This is confmned by ment solid-state
13C NMR studies of OTS monolayers depositeâ on fbmed silica panicles.'"
1.6.2 Reproducibili ty
A problem arises in the reproducibility of alkyltrichlorosilane monolayers because the
quality of the monolayers is very sensitive to reaction conditions. For example, Silberzan et
al." argued that 2 minutes is enough for the formation uf a monolayer. while Banga et al."
suggested 90 minutes. and Wasserman et al? 24 hours. Sagiv et al.'" reported that partial
OTS monolayets have heterogeneous island structure, which is confumed by cecerit AF'M
studies. However. Ohtake et al.." Maihauser and Frank.'" Wasserman et a1.L54.1s8 and
Ulmanl2 all reported chat the monolayers are homogeneous and disordered. These
conflicting results might have simply corne from different substrate properties andlor
different experimental conditions for the tilm preparation.lS9 Furthemore. the competition
between polymerization and surface a n c h o ~ g is a major source of the reproducibility
problerns because small differences in water content and in surface Si-OH group
concentration may cause significant differenccs in rnonolayer q ~ d i t y . ' ~ Nevertheless.
owing to high stability of the akylsilane monolayers, they are still considered as ideal
materials for surface modif~cation and funçtionalization applications, such as adhesion
p r ~ r n o t e n ' ~ ~ and boundaxy lubricants.16'
1.6.3 Monolayer Structures
Wasseman et aLu reported that monolayers prepared from a homologous senes of
methyl-termïnateû alkyltrichlorosilanes display CA,, of -llOO. consistent with that
reported by Sagiv et al.;39 and C A , of -40'. consistent with the values of Ulman et al.42
However. values of C A , were 3 - 6 O lower than the corresponding values of SAMs of n-
alkanethiols on gold.% and an "odd-even" eflect of CA, was not observed. The
thicknesses measured by ellipsometry and by low-angle X-ray retlection share a hi&
correspondence with each other, suggesting that the alkyl chahs are in the dl-@mrs
conformation and orient nearly normal to the surface."
In the FT-IR study of a homologous series of alkyltrichlorosilane~.~~~~ the frequency
of the asymmetric methylene strecdi, v,(CH,) was found to decrease as the chain length is
increased. For example. v,(CHJ decreases from 2922 cmd1 to 29 18 cm" as the chain length
increases from tetradecyltrichlorosilane (TTS) to octadecyitrichlorosiIane (OTS).
Furthemore. the peak at 2920 cm-' is much bma&r in ihe 'ITS monolayer (band width =
2 1 cm-') than in the OTS monolayer (band width = 16 cm-'). Both frequency shift and peak
broadening suggest that the monolayer becomes more disorkred and liquid-like as the
chah length decreases.
Allara et dS3 found that the tilt angles of the alkyl chains in OTS monolayers on SiO2
and oxidized gold are 10 + 2 O from the surface nomal with a monolayer coverage of -96 + 4%. Biernbaum et used mar-edge X-ray absorption fuie structure spectroscopy and
X-ray photoelectron spec troscop y to stud y SAMs of OTS. octadec y ltrimethoxysilane
(OTMS). and ( 17-aminoheptadecy l)vimethoxysilane (AHTMS). They found that (i) the rilt
angles of the chains in OTS SAMs are 5 + 5 O ; (ii) SAMs of OTMS exhibit a higher tilt angle
of 20 f 5' because of the difference in the adsorption rnechanisms of trichlorosilane and
uimethoxysüane groups; and (üi) the inmdwtion of a polar amino group at the chain
terminus mults in a more disordered monolayer.
1.6.4 Chernical and Thermal Stabilities
Monolayers with high stability bear significant con tributions to rnolecular
electr~nics. '~~ Wassennan et al? reporced that monolayea of methyl-iemiinated siioxanes
were resistant to a variety of conditions such as air, 1 % detergent solution, hot water and
hot organic solvents. Moreover, the wettability or thickness of the monolayer was
unaffected by mbbing the surface vigorously with a tissue or Cotton swab. However, when
the monolayers were exposed to aqueous base at room temperature for 80 min,
approximately 504b of the monolayer had been removed. After 160 min, their surfaces were
visibly etcheù because the Si-O bonds hydrolyze under basic conditions.lH The high
stability of these monolayers is due to the formation of the covalent Si-O bond , as well as
the maximization of van der Waals interactions between adjacent alkyl chahs.
The thermal stability of OTS monolayers on aluminum was studied by Cohen et al."'
They heated the monolayer to -125 OC and then cooled it back to room temperature. A
partial (60%) SA monolayer of OTS on aluminum, a SAM of arachidic acid (AA) on
aluminum, a LB monolayer of cadmium arachida?c (CA), and a trilayer having a LB bilayer
of cadmium arachidate on O T W wene employed for cornparison. From the contact angles
and FMR data, the CJTW monolayer was shown io be more stable than the other
systems. For exam pie. aU the systems, except the OTS/Ai monolayer. showed a decrease in
both CA, and CA, under such conditions. In the FT-IR spectra, ihey observed apparent
randomization between 100 OC and 130 OC, in both the C-H and the C=O stretching
regions for the CA bilayer on OTS (van der Waals intrraction between monolayer and
substrate). For tfie SA monolayer of AA and LB monolayers of CA (ionic monolayer-
substrate interaction), there were changes only in the C-H, but not in the C=O stretching
region." For the OTSlAl system, k r e were only slight changes. suggesting that it is the
most stable system. Therefore, it could be concluded that the polysiloxane backbone in the
SAM of OTS/Ai provides extra stability to the system.
1.6.5 SAMs with Aromatic Chromophores
'Ihin films with useful aromatic and other functional groups have growing
prominence in industry because of their optical and electronic uses." Incorporation of
aromatic or heteroaromatic rings into the alkyl chah might inuoduce disorder into the
monolayer because such grou ps possess s ymme tries diffefeient from c y lindncal, longchain
alkyl groups. Tillman et al.'2 found lhat the construction of SAMs of trichlorosilanes
containïng aromatic groups, CH,-(CH,)mC,H,-O-(CH2)n-SiC13, requires alkyl chah
lengths greater than at least 13 carbons, unless forces otber than van der Waals attractions
of alkyl c h a h (such as dipole-dipole interactions) have b e n introduœd into the systerns to
produœ ordered thin f h s . For example, monolayers prepared h m 1 -(trichlorosilyl)- 1 1 - (pn-nony1phenoxy)unâecane showed that the phenoxy group can be introduced into such
monolayers without losing order and close packing, as cornpamd with OTS. These systems
become more disorderd and liquid-like as the alkyl lengths decrease.
1.6.6 SAMs with Second Order NonJinear Optical Properties
Based on the above akyltrïchlorosilane chemistry, SAMs of compounds with second
order nonlinear optical pmperties can be easily deposited on silica surfaces. Synthesis of
molecular materials with large second-order optical nonlinearities have extensive uses in
second harmonic generation. electmoptic, and photorefractive dev iced Poled p ~ l y r n e r ' . ~ ~ ~
and LB film tramfer approachesl" have been employed in the fabrication of
noncentrosymmetric assem biies in m e n t years. 'ihere are, ho wever, ce- difficufties
which persist in the s y n t h e s i ~ . ~ . ~ ~ ~ - ' " An aiûactive alternative approach is to build the
S AMs with second order nonlinear optical (NLO) materials via molecular self-assem bl y ,16'
resulting in photochemicaliy and thennally stable multilayers.'"
Marks et al.169 have developed a SA strategy by attaching the -Sic13 group to s m d
molecules, and introducing a monolayer of NLO-active dyes via an SN^ reaçtion with the
SAM. For example, SAMs of [2-(pchloromethylphenyl)ethynyl]silane were aliowed to
react with [2-[4-[N,N-bis(3- h ydroxypropyl)arnino] phen yl]ethynyl]-4'-pfidine which
possesses second order NU) properties. They consuiicted highly ordered SAMs fmm a
dilute solution of [[4-~,N-bis(3-hydroxypropyl)amino]phenyl]azo]-4'-pyridine on a
benzyl chloride SAM surface, foilowed by annealing at 1 10 OC. B y repeating this process.
they could prepare multilayered thin füms with simcant second order NLO proper t ie~ . '~
1 A.7 Surface Modification
Surface modification can be achieved by using SAMs of alkyltrichlorosilanes with
terminal func tional grou ps, for example, halogen, 170-171 cyanide. 172 thiocyanides. ln methy 1
ether,'" acetate,l7' thioa~etate."~ a-hal~acetate,"~ ~ iny l ,~ '~" ' (~imeth~lsil~l)eth~nyl,~~~
methyl ester, 43.176 and p-chloromethylphenyLL" Surface modircation reactions are
important for providing active surfaces for the attachent of rnoleçules with different
properties. For example. pyridine surfaces can be produœd by reacting bromo-terminated
akylsilane monolayers with the Lithium sdt of 4-methylpyridine.171.176 Such surfaces react
with pal~adium.~'' r h e n i ~ r n , ' ~ ~ and osmium c~mplexes"~ to provide imrnobilization of
organometallic rnoieties.
1.6.8 Multilayer Formation
In order to transform thin films into practical devices with technologid uses,
mu1 tilayers of appropriate thickness should be reproduci bl y constructed with minimum
disorder." However, most recently published reports suggest that the quality of
trichlorosilane fdms rapidly degrades as the thickness of the films increases. 104.178-179
In multilayer construction. the rnonolayer surface is first rn-ed to a hydroxylated
one by a chernical reaction, for example, the hydroboration-oxidation of a terminal vinyl
group* 25.107.178 the LiALH, reduction of a surface ester g r o ~ p , 4 ~ . " ~ the photolysis of a
nitrate-bearing group.180 and the hydrolysis of a boronate-protecting gro up. lg' The
hydroxylated surface then allows another rnonolayer to adsorb on top of it. Upon the
repetition of this process, multilayered films may be constructed.
1.6.8.1 Hydroboration-Oxidation of a Terminal Vinyl Group
Sagiv et a1.25.104,178 reported chat an olefm-teminated SAM cm be converted into an
alcohol-bearing surfafe on which self-assembleci multilayer assem blies can be built.
However, olefm hydroboration did not satisw the quantitative yields required to optunize
such a suategy. Hence. the water contact angle of the alcohol surfice was higher than
expecteû. Nevenheless, the OH-bearing surface was a suitable base for th construction of
multilayer assem blies of averagequali ty.
1.6.8.2 LIAIH, Reduction of a Surface Ester Group
An improvement in mulrilayer construction based on the L w reduction of methyl
esters to alcohols was reported by T i et al? They created îhe construction of
multilayer films of methyl23-(mchlomsilyl)tricosanoate (MTST) of -O. 1 pm thickness on
oxidized silicon substrates. The advancing water contact angle on the hydroxylated surfaces
was found to increase as the number of layers increased. For example, the contact angle for
water on the reduœd 20th layer had increased to -50° from the initial value of -30".
suggesting chat there is an exposure of CH, groups to the surface. The reasons for this
disorder may include (i) the imperfect packing of the ester gmup; (ii) the introduction of
disorder by the chernical reaction of the ester gmup with LiAiH,, or by the polysiloxane
network; and (üi) the s d a œ reorganization4'~" by burying the OH groups as much as
possible. to expose more CH, groups to the surface. In a sirnilar way. contact angles for
HD. on the third layer of the unreduced ester surface. d e c m to 12' h m the initial value
of 28", reflecting a tendency towards incfeasing disorder of the füms with increasing layer
num ber. FT-IR data indicated that the alkyl chahs in ik multilayer samples are more ated
and disordered. and a small amount of ester groups ~rnained unrûacted. The primary
disadvantage of thû method is the requirement of a very aggressive and airlmoisture
sensitive reagent for L N H 4 reduction. IeaWig some inorganic residues on the substrate
surface after each reduction cycle. Nevertheless, it is possible to prepare a rnultilayer h
with a thickness of 4 . 1 pm."
1.6.8.3 Photolysis of a Nitrate-Bearing Group
Analogous to previously reported phototrarssfomations of SAMs,l8' a new approach
to rnultilayer formation by the photolysis of a nitrate-bearhg SAM was reported by Collins
et al.''' The success of this methodology was supported by (i) the decrease of the water
contact angle from 8 1' to 3 1°, (ii) the lack of substantial change in the methylene stretching
region of the Fï-IR spectmn (2900-3000 cm-'). and (üi) the smali defresase in
ellipsometric thickness ( 1.5 A), upon photolysis.
1.6.8.4 Hydrolysis of a Boronate-Protecting Group
Another appmach to multilayered thin film construction. based on the hydrolysis of a
boronate-protecting group, has been developed by Kato et al.''' They p ~ p a r e d a SAM of
5-(2-methyl- 1,3,2-dioxabo~an-5-yl)penty1trich~orosi~ane (MBPS) on alurninized silicon.
with an eiüpsometric thickness (Te) of 15 + 3 A. which is comparable to the thmreticai
thickness. Upon hydrolysis by waterlethanol, the boronate protecting group of the SAM
converted to the diol-terminad group which further reacted with another MBPS to result in
a two-layered füm with a rhickness of 29 f 3 A. By repeating the process, a three-layered
film of MBPS with a thickness of 42 f 4 A was cmstructed. If the hycirolyzed two-layered
film was treated with OTS, a the-layered assembly with a reasonable thickness of 54 k 5
A was formed. This method provides a route to consuuct multilayen with high rnolecular
density and effective surface coverage. Furthemore, the surface of the mdtiiayers can be
functionalized with designed silane compounds.
1.7 Alkyl Monolayers on Silicon
The surface of silicon has k e n cornprehensively studied for many years because of
its importance in modem t e - ~ h n o l o g ~ . ~ ~ A stable and densely-packed SAM covalently
bonded directly to the silicon surface can lead to a new area in film technology.
Linford and Chidsey70 recently reported that diacyl monolayers can be covalently
bonded directly to the hydrogen-tenninated silicon surface upon pyrolysis of neat diaçyl
peroxides. They have comparable monolayer thickness, chah packing, and we&g
properties to the monolayers of long-chah aikyI thiols on gold or trichlomalkylsilanes on
oxidized silicon. However, Fï-IR spectra suggested the presence of some carbnyl groups
which indicated ihat these monolayers did not d e i j comprise & y 1 chahs.
The physicai properties, in term of vJCH,), CA,,, C A , and Te. of the three
monolayer s ystems, diacyl peroxides on silicon, alky ltrichlorosilanes on oxidized silicon
and alkanethiols on gold. are shown in Table 1.3.'" It was found chat the propenies of the
monolayers containing 17 and 18 ca rbns are nearly identicai, suggesting that al l arie
closely-packed and highly ordered. For shorterchain species, [CH,(C&),,C(O)O]fl-
Si(111) and CH,(CH2),,SiCl/oxidized Si are shown to be less ordered than
CH,(CH,),,SWAu. A lower surface coverage of 10-25% was observed in the case of
stearic acid and octanoyl chloride on silicon. The films prepared from the longer-chain acyl
peroxide showed comparable stabilities with those fmm the longer trichlomsilane. For
example. after 2 h in boiling chioroforrn and 1 h in boiling water, the frequency of the
asyrnmeaic CH;? stretch had shifted from 2917.5 cm-' to 2919.9 cm-' for
[cH,(cH2)i6C(0)O]fi-Si(l 1 1); and from 29 17.3 cm" to 29 19.9 cm" for
CH,(C~), ,SiCl~oxidized Si. On the other hand, for CH,(CH,),,SWAu, the asymmeuic
stretching frequency of CH, Ulcreaîed from 29 17.9 cm-' to 2920.9 cm", and the thickness
decreased by 30%. after only 30 min in boiling chloroform.
Table 13 Physical properties of monolayers on silicon, oxidized silîcon. and pold7O
In order to improve the quality of the newly formed rnonolayers. allcyl monolayers on
System
[CH,(CHJ,,COO]JH-Si(111)
CH,(CH,),,SH/Au
silicon have been prepared from 1-alkene and hydrogen-terminaîed Si(l1 1). upon free-
radical initiation with diacyl per~xides .~ ' They have been s h o w to be densely packed and
highly ordemd, exhibiting CA, of 1 13O, CA, of 45". and va(C&) at 292 1 cm-', similar to
vJCH,), cm-'
29 17.5
29 17.9
those for the SAMs of u i ch lo ro~ l s i i anes on oxidized silicon and allcanethiols on
g ~ l d . ~ ' . ' ~ The monolayers were highly resistant to boiling chloroform. boiling water.
CA, O
f 12
114
boiling acid and boiling base.
1.8 SAMs with Alternate 461norganic/Organic" Systems
Alternate "inorganic/organic" systems, based on transition-metal coordination
chemistry, have b e n developed for preparing self-assembled multilayers with potential
uses as active components in charge-separated assem blies. l u and matenals with selective
chernical responses for sensor applications." Metal-ligand coordination chemistry.
including p y d n e s with RU," dithiols wiih diamines with RU." and diamines with
~i-P~(CN).," have been involved in multiiayer deposition. Moreover, Mailouk et a1.77-79
CA, O
46
50
Te. A
25
28
developed layered phosphonate f h s and Anseil et al." a new self-assembly system based
on the cobalt-diisocyanide system.
1.8.1 Layered Phosphonate Thin Films
Yamanakaet aLLsS and Aiberti et al.1g6 initiated the study of the chemisûy of y-metal
0 phosphonates in the mid-1970s. These phosphonatesl" are good for the mo1ecula.r
design of structures with specirc properties because a nwnber of organic groups can be
anached to the y-layers. Momver , ahost any organic molecule can be converted into a
phosphonic acid denvative. and crystallkd as a highly ordered and stable layered metal
phosphonate? These layered materials have a wide range of applications,1a8 including
chernical sensing, nonhear optics, cataiysis, dielectric coatings, and ion exchmgers,
Highiy ordered and stable multilayers of metai-bis(phosphonate) (MBP) are datively
easy to make. Maüouk et a . P 9 first demonstrated that this could be achieved by adsorbing
the appropriate metal salt (e-g. ZrO(=lJ and a,wbis(phosphonic acid) altemately ont0 a
suitably prepared surface, such as siiicon and gold substrates. Usually. an "anchoring"
molecule (thiol, e.g. [S(CH,),PO,H,],, or silanol. e.g. HO(CH,)ZSi(CH,),PO,HZ). which
possesses a surface active group at one end and a phosphonate group at the other. is first
adsorbed, followed by binding a layer of meiai ions through metal-phosphonate bonds. A
monolayer of a,cikbis(phosphonic acid) then attaches one end to the adsorbed metal ion
layer, freeing the other end to attach another layer of metal salt. Multilayers can be prepared
by repeating these adsorption steps. For example. the Mallouk g r ~ u p ~ ' ~ ' has successfuUy
prepared a 12-layered thin film of zirconium 1-10-decanebisphosphonate.
1.8.2 Cobal t-Diisocyanide Thin Films
Cobalt- 1.4-diisoc yanobenzene (Co-DiCNB) rnultilayer i i s can be prepmd on an
amine-functionaiized surface. Anseil et al." have previously found that Co2+ ions could
bind strongly to the amine-terminal surfaces. However, it is difficult to bind another amine
rnolecule to the surface-anchored cobalt. As s h o w from the FT-IR and eiiïpsometry dafa
for the Co-DiCNB systern, isocyanide ligands bind strongly to the surface-bound cobalt
ions. Multilayers of Co-DiCNB on be constructed by repeated altemate deposition of
cobalt and diisocyanobenzene on an amine-functionalized substrate. This system provides
an alternative to MBP chernisûy for multilayered self-assembly. In addition, when we
combine both MBP and Co-DiCNB systems together, "hybndn structures with alternating
metal-bisphosphonate and cobalt-isocyanide layers can be fabricated6
1.9 Monomeric and Polymerized Diacetylene Langmuir-Blodgett and Self-
Assembled Thin Films
The major disadvantage of Langmuir-Blodgeti films is their limiteci stability towards
various chernical or physical conditions. The stability problem can be solved by the two-
dimensional polymerization of the LB films. Suice diacetylenes are monorneric amphiphiles
with two triple bonds in the hydrocarbon tails of the molecules, they can undergo
topochernical polymerization upon W-inadiation to fom extended polymers both on solid
supports and at îhe air-waier i n t e r f a ~ e . ~ ~ ~ " ~ ~ Polymerization of the monolayer hel ps to
increase its mechanical strength. Thus, a highiy stable film of polydiacetylene (PDA) can be
used as a protective surface coating for biomedical or optoelectronic applications."
Furthemore. the PDA films have potential use as stabilizers of Lipid membranes,192
supponing matrices for biosensing r n o l e ~ u l e s . ~ ~ ~ liposomes for drug-deli~ery.'~' and
optical corn ponents in nonlinear optical d e ~ i c e s . ' ~
Wegner fust reporied polymeritation of diacetylenes in the solid state.lgl Sheth and
Leckband2'" found that the polperization of 10.12-pentacosadiyonic acid (PCA)
monolayers helps to irnprove the chemicai and mechanical stabiiity, and the aging behavior
of the film. However, in the preparation of the two dimensional polyrner from PCA and its
derivatives, unpolymerized domains and &fects are introduced in the polymer Nms."
These consuauits on fabrication must be overcorne before high quaiity. homogeneous f h s
can be prepared.
Although the factors a f f e c ~ g the stability of a SAM are not weli understood, its
stabiiity may be xelated to the interactions between individual molecuies and various types
of molaular defects.ls and defects between ordered d ~ r n a i n s . ~ ~ ' ~ SAMs are fairly stable
under ambient conditions, but they becorne fragile at extnzme pHs." in many nonaquwus
sol~ents . '~ ' in the presence of Cl-. CN-. and t t ~ i o l s , ' ~ ~ at high t e r n p e r a t ~ r e s , ~ " ~ ~ ~ and af
extreme elecmde potentials."' Thus, they are not applicable in corrosion passivation and
inhibi t i ~ n , ~ lubrication," nor adhesion? In order to improve the durabüity of S AMs, Kim
et al.''' have prepared diacetylenic SAMs (HS(CH,) ,,C=CC-C(CH2) loCOOH, DA-
COOH) that can be polymerized in a plane paraiiel to the substraie upon UV exposure.
They compared the stability of unpolymerized (hexadecanethiol, HDT) and polymerized
S AMs (PDA-COOH), b y exposing them to an especiall y aggressive solvent, a 1 : 1 mixture
of ethanol and 1.0 M aqueous KOH at 1 0 OC. The entire SAM of HDT desorbs while
there is only a litde change in the FT-IR spectnim of the SAM of PDA-COOH after solvent
exposure, thereby attesting the impmvement in the stability of the PDA SAM.
From the Fï-IR data of the HS(CH~),,C~CC~C(CH,),~COOH,'~~ it was found chat
the absorùance in the hydrocarbon region increased with the increase in number of iayers.
However, upon UV-polyrnenzation the magnitude of these peaks decreased signfiantly,
suggesting that the methylene bonds arie o r i e n ~ d more parailel to the substrate a k r
p~lymerization."~ The UV-Vis data indicate that polyrnerization is complete within 5 min;
afterward the intensity or position of the absorption maximum (620 nm), does not change
further. Yet, two absorption maxima are usually observed in studies of polydiacetylene LB
one is found between 600 and 640 nm, which corresponds to tk "blue polymer".
and the other is between 500 and 550 nrn, which corresponds to the "red polymer". The
result suggests that the self-assernbly approach only gives rise to the more highly
conjugated %lue polymer". These materials have potential uses in such areas as u l t r a h
photoresisis. and mgged adhesion layers for grafting of bilayers. and multilayers. 1 10102105
1.10 Limitation of Traditional Approaches to Molecular Self-Assembly
Although the above described approaches to rnolecular self-assembly have thek own
advantages, they have their individual limitaiions also. The self-assembly of organosulfur
compounds on gold depends on the specific interaction between the sulfur head group and
the gold surface. Moreover, these compounds do not adsorb on the surfaces of many other
metal oxides." Carboxylic acids and phosphonic acids do adsorb on various metal
surfaces, but the adsorption is weak.666g.7'-80 It is impossible to selectiveiy adsorb a
carboxylic acid in the presence of a terminal phosphonic acid group on metal oxide
surface^.'^ These systems can neither be used to incorporate polar groups into a
r n o n ~ l a ~ e r . ' ~ ~ intmducing difficul ty in laycr- by-layer construction of multilayers with
complicated polar and interesthg planar buky x-systems.' Moreover. phosphonic acids
have large head groups which are moce inconvenient to synthesize? in the case of
aLkyltrichIorosilane rnonolayers. although (i) they are physically and chemicaüy robust
because of the presenœ of 3-dimensional polymer network;"' and (ü) they can self-
assemble on a large number of silica-like s ~ b s t r a t e s . ~ ~ it is difficult to synthesize a nurnber
of silane coupling agents with designed suiface functional groups because most of hem are
highly moishire sensitive. There is also a difficulty in depositing alkyi monolayers with
desircd functional groups on silicon via free-radical initiation with diacyl peroxides.
Multilayer formation using long chah alkanes on silicon is also a significant issue.
An ideal thin film consmt ion methodology should be able to fulfül the following
requirernents: (i) it should involve easily accessible reagents to construct self-assembled
monolayers and multiiayers on indusvially important subsuates; (ü) be versatile in
i n c o r p o r a ~ g a variety of compounds with different surface functional groups; and (üi) in
the resulting monolayers or multilayers it should be feasible to carry out topochernical
plymerization of suitably oriented diacetylene groups. We have developed an alternative
approac h to molecular self-assem bly on inorganic oxide surfaces using acid- base hy drolytic
chemistty of amino-silanes and -s<annanes with molecules containhg terminal acidic
moieties. It addresses some of the issues mentioned above in constnicting self-assernbled
thin films.
1.11 Acid-Base Hydrolytic Chemistry
1.11.1 Synthesis of Aminosilanes
The reaction of ammonia or an arnine wiih a halosilane resuits in the formation of a
silicon-nitrogen bond.'" The bromo- and iodosilanes appear more reactive toward a given
amine than the c hlorosilanes. Owing to their availability. however, ihe chlorosiianes ati=
most frequently employed. The hali&s released during the course of the reaction are
precipitated as the ammonium or amine salts?
Similady, amines nact with tetrachlorosilanes via stepwise substitution of chlorines
to give S ~ ( N R ' ~ ) ~ . ~ ~ '
Trialkox ychlorosilanes reac t wi th am ines in the sarne way as trialkylc hlorosilanes and
the alkoxy groups are inert under these condition^.^^
1. 1 1.2 Chemistry of Aminosilanes
1.11.2.1 Reaction with Water
Aminosilanes react with water, resulting in the cleavage of the silicon-nitrogen bond.
The first step of the hydrolysis is the formation of a silanot. Depending upon its stability
and the reaction conditions, the silanol may either be isolated or undergo condensation with
another silanol or aminosilane to yield the d i s i l ~ x a n e . ~
1.11.2.2 Reactions with Alcohols, Phenols and Silanols
Aminosilanes react with alcohols, phenols, and silanols to form alkoxysilanes.
phenoxysilanes. and disiloxanes, respe~tively.~~' The extent and rate of reaction aie
dependent on (i) hindrance around both the Si-N bond and the hydroxyl group of the
alcohol or siianol and (ü) the acidity of the atiacking alcohol.
1.11.2.3 Reactions with Thiols and Carboxylic Acids
With thiols and carbxylic acids, aminosilanes react to yield silylthiols and
silylcarboxylates. ~spec t ive ly .~ '
1.1 1.2.4 Reaction with Acetylene Compounds
However, aminosilanes do not react with protic species with lower acidity, such as
acetylene ~ o r n ~ o u n d s . ~ ~ ~ Because of its lower metai-to-nimgen bond suength, and the
higher basicity of nitrogen in aminostannanes than the corresponding aminosilanes, the Sn-
N bond c m be easily cleaved by such protic ~pecies.'~' Acid-base hydrolytic chemistry of
aminostannanes is discussed below.
1.1 1.3 Synthesis of Aminostannanes
The simple aminostannanes were not synthesized und about 196 1.209 In a
comparative study of the behavior of group N ha.!;& mwards amines.'0g Si and Ge were
found to react as:
whereas Sn and Pb as:
However, if a protic reagent (HA) is pcesent together with an amine, the following reaction
can o ç ~ u r . 2 ~ ~
In order to rnake aminostannanes directly and more effiçiently, Lithium salts of
secondary amines are used to react wiîh organotin halides."gm
For example. t i n o chlorides react with Lithium salts of secondary amines to give
tetraaminostannanes-m8
1.11.4 Chemistry of Aminostannanes
1.11.4.1 Reactions with Water and Air
In general. the aminostannanes are very sensitive to moisture and carbon dioxide and
must be protecd from the atrno~phere.~'~ Water is oniy one of a wide range of protic
species (HA) which attack the aminosiannanes as indicated by the general e q u a t i ~ n . ~ ~ ' ~
For example. aminostannanes are easily hydrolyzed to the hydroxide or oxide.
Exposure of aminostannanes to the atmosphere actually affords the carbonates.
1.11 A.2 Reactions wi th Protic Species
The reactions of aminostannanes with pmtic species. appear to require chat HA
should have a p y v a l u e of I 25 (Table 1.4).'08 Among the more interestkg examples are
those where HA is an alcohol. wboxylic acid. thiol, indene, cyclopentadiene. phosphine.
and acetylene (Eq. 17). The reaction of aminostannanes with pmtic species becomes the
basis of many preparative procedures and can be applied to the preparation of
~tannosiloxanes.~~~
Table 1.4. Acid strengih of some protic spec ie-~~~'
PH, 1 26
1 .12 Acid-Base Hydrolytic Cheniistry Approach to Molecular Self-
Assembly on Inorganic Oxides Surfaces
The surfaçes of inorganic oxides such as si'ica g l a s and quartz axe acidic in nature,
and contain surface hydroxyl gro~ps.~" Thus, the acid-base hydrolytic chemistry described
above can be easily applied to these surfaces to give a versatile chemisorption method for a
variety of organic microstnictures (RH), according to following equations, where E = Si,
Sn.
Cyclopentadiene
ROH
RSH
16
16- 19 1
le11
SiCl, NR',H RH {Si }-OH - {Si ) -O-SiCl -{Si )-O-Si-NR2 -{Si }-O-Si-R (2 1)
- HCI - NR',H-HCI - NK2H
SnCl, NRt2H, RH ( Si 1 -OH -{Si 1-O-SnCl -{ Si} -O-Sn-R (22)
- HC1 - NR'2H-HCl
E(NR'2)4 RH {si) -OH -{Si J -O-E-NK2 - {Si} -O-E-R (23) - NRe2H - NRt2H
1.13 Scope of Thesis
Using acid-base hydrolytic chemistry described above for amino-silanes and -
stannanes, a new approach to molecular self-assembly has been developed. Treatrnent of
surface hydroxyl groups on glas , quartz and single crystal silicon with commerciaiiy
available group (IV) chlorides, ECl, (E = Si, Sn), and then NEbH or directly wiîh
Si(NEt,), or Sn(NEtJ, yields surface-anchored NE& moieties, which mict with several
organic molecules containing temùnal acidic protons. such as alcohol. carboxylic acid.
thiol, phosphine, cyclopentadiene, indene, and alkynes, via acid-base hydrolysis. leading
to rnolecularly self-assembled monolayers. This new approach is elaborated in Chapter 3
and optimal conditions for thin film construction are discussed.
Using the simple %id-base hydrolysis route, silica surfaces functionalized with
aminosilanes cm be easily modified with a variety of long alkane chain alco hols terminating
with different functionalities. In Chapter 5, the generality of the new acid-base hydrolytic
approach to seif-assembly of a series of lhin films containing short-to-long chah alcohols
terminated with alkyl, phenyl and acetylene groups on SVSiO, surfaces and a cornparison
of a two-step thin film construction pmcess involving the reaction of Si(NEt.J, with surface
hydroxyl groups foliowed by the reaction with ROH, with the three-step sequence using
SiCl,, NEt.,H and ROH, are discussed.
The traditional piepariition meihodologies for building multüayered structures have
iimitations associated with c o n m h g film ihickness and individual layer compositions of
the resulting thin film assemblies. Based on the hydrolysis of surface-anchored
aminosilanes with molecules coniaining terminal bifunctional groups, silica sudaces can be
modifïed with a variety of multilayered f i s . In Chapter 6, the possibility of building
multilayered assemblies in a hyer-by-layer fashion using bifunctional chromophores, is
explored. The multilayened thin films are consuucted via the riesiction of Si(NEtJ, with
hydroxyl groups of the inorganic oxide surfaces. followed by the reaction with dihydroxy
chromo phores incorporating acety lene, diacetylene and aromatic moieties in the backbone.
Upon repetitive reactions with Si(NEt..J, foUowed by R(OH),, multiiayered thin nIm
assern blies can be fabricated on Si/SiO,. The possi biii ty of topochernical pol ymerization of
the diacetylenic moieties in the mono- and multilayeried assemblies upon UV-Vis exposure
is also investigated-
There have b e n a number of attempts made to introduce other useful rnoieties into the
alkanes while retaining the basic long chah structure that leads to self-organization via van
der Waals forces of attraction. investigations of the organic microstructures with extensive
conjugation in the backbone which will involve purely intemolecular R-K interactions for
molecular self-assembly, have just begun. Because of its lower metal-to-nitrogen bond
strength. and the higher basicity of nitmgen in arninostannanes ihan the corresponding
aminosilanes, the Sn-N bond can be easily cleavcd by protic species. Owing to the lower
acidity of acetylene than alco hol. S AMs of chromo phores with t eminahg acetylenic
hydrogen on SiOJSi can only be consuucted via aminostannane approach rather than
aminosiiane approach. in Chapter 7 . molefular self-assembly of a number of rigid-rod
aikynyl chromophores on inorganic oxide surfaces anchored with arninostannanes as weU
as the possibility of topochernical polymerïzation of the diacetylene moieties in the lhin
fdms upon UV-Vis exposure are discussed. Using this methodology. step-by-step
multilayered thùi mm construction by the reaction of surface-anchored aminostannanes with
dialkynyl tertninaied chromophores becomes possible. Thus, a two-step thin füm
construction process involving the reaction of Sn(NE\), with hydroxyl groups of the
inorganic oxide surfaces, followed b y the reaction wi th diallryne c hrornophores
incorporathg alkyl and armatic moieties in the backbone is also discussed in Chapter 7 .
B y repetitive reactions with Sn(NEi,), followed by H-Cg-R-CS-H, rnultiiayered thin
film assemblies were fabricaied on SilSi02.
The evolution of thin film structures was routinely monitored by surfkce weüability
measurements, FlTR-ATR, ellipsometry. X-ray photoelectron spectroscopy and UV-Vis
spectroscopy. These surface characterization techniques probe the structure of the newly
formed thin films by different physicd processes, and can provide complementary and
definitive information- A summary of these surface characterization techniques is provided
in Chapter 2. The links between material presented in Chapters 5 to 7 aie provided in
Chapter 4. Finally, conclusions, contributions to original knowledge. and suggestions for
future work are discussed in Chapter 8.
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(200) (a) Li. J . ; Liang, K.; Camilione. N.; Leung, T.; Scoles. G. I. Chem Phys. 1995.
102, 5012. (b) McCarley, R. L.; Dunaway, D. J.; Willicut, R. J . Langmuir 1993,
9, 2775.
(201) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Larnp, B. D.; Chung, C.;
Porter, M . D. Langmuir 1991, 7, 2687.
(202) Kim. T.; Chan, K. C.; Crooks, R. M. J. Am Chem. Soc. 1997, 11 9, 189.
(203) Porter, M. D. Anal. Chem. 1988,6, 1143A
(204) Bloor, D.; Chance, R. R. Polydiacetylenes; Martinus Nijhogg: Pordrecht, 1985.
p363.
(205) Chan, K. C.; Kim, T.; Schoer, J. K.; Crooks, R. M. 1. Am Chem Soc. 1995,
II 7, 5875.
(206) N a r a DL.; Atm, S. V.; Eliiger. C. A.; Snyder. R. G. J. Am Chem Soc. 1991,
113, 1852.
(207) Fessenden, R.; Fessenden, J. S. Chem. Rev. 1%1,61, 361.
(208) Jones. K; Lappert, M. F. Orgmotin Compounds, Vol. 2. Sawyer, A. K., Ed.;
Marcel Dekker, Inc.: New York, 197 1, p5 10.
(209) Van Der Kelen, C. D.; Van Der Berghe, E. V.; Verdonck, L. Organotin
Compomds, Vol. 1, Sawyer, A. K.. Ed.; Marcel Dekker, Enc.: New York. 197 1.
p96.
(2 10) Poller, R. C. The Chemistry of Organotin Compounds; Academin Press: New
York, * 970, p94.
(2 11) (a) iier, R. K. The Chemistry of Silicon; Wiley: New York, 1979. (b) Pintchovski,
F.; Prïcew, I- B.; Tobin, P. L.; Peavy, J.; Kobold, K. J. Electrachem. Soc. 1979,
126, 1428.
Chapter Two
Methods for Surface Characterization
In the study of thin films, we are interested in both their surfaœ and bulk properties.
The evoiution of chin fW svucnires'" can be monitored by contact angle goniometry.
eilipsometry, FIIR-ATR, X-ray photoelectron and UV-Vis spectroscopies. Contact angles
with different liquids. such as water and hexadecane. are measured to evaiuate weaing
propenies. surface energy. and uniformity. and to obtain information on surface orded
Fourier transfonn infrared (Fï-IR) specuoscopy, in the attenuad total reflection (Am)
mode.' is used to &termine the identity, molecular packing and orientation of
chromophores. Ellipsome*' is used to measure the thickness and uniformity of freshly
prepared films. X-ray photoelectron spectroscopy (XPS)' is used to study surface
composition and rnonolayer structure, and UV-Vis spectroscopf for chromophore
identification and estunation of surface coverage. A combination of these techniques
provides a useful indication of the quality of the thin f i s . A brief introduction to these
techniques is given beiow.
2.1 Contact Angle Goniometry
The quality of monolayer and multilay~r f h s can be estirnated from contact angle
measurements. nie shape of a liquid drop on a piane, homogeneous surface is highly
dependent on Ihe free energy of both the liquid drop and the surface.* The contact angle is
the angle at the contact point on the surface. The contact angle of a liquid is the result of the
mechanical equilibrium of a drop on a solid surface under the action of surface tensions.
~ s v . n v . and YSL, according to Young's equation:10
yLv cos0 = ysv - YSL
where, y is the surface interfacial tension, and LV, SV, and SL refer to iiquid-vapor, soiid-
vapor, and solid-liquid interfaces, respectively.
If there is no interaction between the soiid and the liquid (ideal case), the contact angle
will be 180°.' As the interaction between the solid and iiquid increases, the iiquid spreads
until8 = 0'. Since real surfaces seldom display a true, unique, thermod ynamic equilibrium
contact angle, a Werent contact angle is measured when the drop has advanced (83 or
receded (03 on the surface pnor to rneasurementl An advancing contact angle is measured
using the following steps: while the d e is still in the drop (captive drop), a small volume
of liquid is added to the &op, and the contact angle is measured before the boundary of the
drop has moved, A similar procedure is employed for m e a s u ~ g d g contact angle:
while the needle is in the &op, a i i e d volume of liquid is withdrawn, and the contact angle
is rneasured before the boundary of the drop has moved. For easier and rnoE convenient
operation, contact angle rneasurements of a frœ-standing (sessile) drop arie established.
However, if the drop is allowed to fa11 from the needb to the surface. srnalier contact angles
are usually obiained because of mechanical vibrations.' Normally, the contact angles are
measured on both sides of the droplet, and a number of readings are coilected at different
places on the surface, to pmvide a staiistical meaning to the value.
Contact angle goniornetry is also dependent on the relative hydrophilicity or
hydrophobicity of the thin Glm surface and the wethg liquid. For exmple, a ciean and
unfunctionalired g l a s surface produces a contact angle of -18O with water. When a non-
polar liquld such as hexadecane is deposited ont0 a hydrophilic surface such as
unhnctionalized g l a s slide, an inmase in the forces of repdsion at the solid-iiquid
interface results in a higher contact angle. Conversely. when hexadecane is deposited onto
an organic monolayer, an increase in the forces of attraction results in the l o w e ~ g of ihe
contact angle. Typical water and hexadecane contact angles on a methyl surface are 110-
1 15O and 40-4S0 respectively (Fi- 2.1). ' Thus, contact angle goniometry is a sensitive
tool to examine surface composition and the structure of organic thin films.
Figure 2.1 (a) A typical water contact angle on a rnethyl sufiace, where 8 = 110 - 1 LSO.
and (b) a typical hexadecane contact angle on a methyl surface. where 8 = 40 - 45O.'
2 .2 Fourier Transform Infrared Spectrosropy in the Attenuateà Total
Refîection Mode (FTIR-ATR)
Fï-IR spectroscopy is a common tool For the study of molecular packing and
orientation in organic füms.' However, a number of problems arise in the case of self-
assembled thin f h s . The intensity of the signal is too weak for measurement since the film
is only monornolecuIarly thick Thus, FTIR-ATR, has been ernployed to increase the
sensitivity for surface analysis. nie intensity of the signal will also be dependent on the
coverage. thickness and density of the T i . A typical setup for an FTIR-ATR experiment is
shown in Figure 2.2.' This method rnakes use of a prism that aliows the incoming beam co
hit the intemal surfaces and then reflect, which occur a number of rimes before exiting.
Such multiple reflections intensify the signal to a reasonable detectable level. The prisms
employed are generally made up of silicon. germanium. KRS-5 or ZnSe. Under conditions
of total interna1 reflection, a decaying evanescent wave appears outside the sides of the
prism.' When a thin f h of sample is pressed against the crystal with a f m contact, the
evanescent wave wiii peneûate the film approximately a micrometer before reflecting and
infrared spectra of the surface region can be obtained. It offers an advan tage by limiting the
depth region of the surf' studied, and not the entire buik of material- Furthemore, when
a polarized light is employed, the orientation of molecular chah in the surface region can be
determinedO2
A .L 4 s o .
CI 0 0 %
0 S O O * 0
4 s .. 4 s O i 0 O 0
0 0 - 0 - % i 0 W ='' 0 0 - = 450 '., ** @
0
s h W @
v .r % 0
Sample KRS-5 crystal
Figure 2.2 A schematic description of an optical setup for ATR measurements.
ATR is a useful tool for the determination OC the packing and the crystailinity of
organic thlli film.' The deiection of the [R bands depends on the orientation of the
molecular chah itself. Vibrations that ive parallel to the surface of the crystals are mosi
readily detected due to the polarizattion of the IR beam. Those perpendicular to the surface
are difficult to detect. For example, for alkyl groups that are trm molecularly-oriented and
perpendicular to the surface, methylene C-H vibrations assoçiated with them wiil be parallel
to the surface. On the other hand, for disordered, cis oriented alkyl groups, it is diificuit to
deteçt the C-H vibrations which are perpendicdar to the surface.
2.3 Ellipsometry
Film thickness can be ktermineû from the eliipsometric parameters using standard
classical electromagnetic theoryl* togelher with a paralel layer model consiscing of an
air/€dm/su bstrate structure. Ln this model, we assume the system to be reasonably described
as a flat sample consisting of an infinite substrate with a parallel ovedayer of thickness (d).
AU phases are considered to be of homogeneous composition and isouopic. with the optical
behavior of each phase king accurately described by a single dielectric function. A more
complete description of this approach is a v e n elsew here.13 but specifc relevant &tails are
a v e n below. Ti= measured analyrer and polarized angles, A and P. respectively. are
relaied to the complex elecaic fields of incident and reflected Light via the following
equations:
The E's denote the complex electric fields of the incident (+) and reflected (-) light h s of
p (parallel to the plane of incidence) and s (perpendicular to the plane of incidence)
polarkation. nK terms r,, and r, stand for the standard Fresnel coefficients (reflection
coefficients) and are functions of both the parameters of tan ty (the amplitude raiio) and A
(the optical phase shift). which are calculated direftly from the raw ellipsometry data and
rdr, can be ~la ied to the optical properties of the samp1e.l' With this approach. it is
possible to determine the refractive index of a film. The refractive index of a "clean"
substrate acts as a reference value. For the sme substrate, but with an overlayer of
refractive index (nJ. thickness (d) and n, can be calculated by numencal iteration
techniques. However, errors exkt in calculating n, for monolayer samples which concem
the impossibility to mutinely obtain reiiable values of both n, and d; moreover. the actual
value of n, for a close p a ~ k e d monolayer differs only slighdy from the bulk material. in
principle. ellipsomeiry can detemine both the thickness and the refractive index if the f i
has a thickness greater than 50 A.
In a typical eliipsometer (Figure 2.3).' monochromatic light (He-Ne laser) is plane
polarized. hits the surface. and is then reflected. A compensator changes the latter
elliptically polanzed reflected beam to plane-po le& The analyzer then detemines the
angle of polarization by which tbe compensator polarized the beam. Practicaliy, one has to
estimate the refractive index of ihe organic film. Usually a value of n, = 1.45 - 1.50" is
used for monolayers of simple alkyl chains of, for example, alkanethiols on gold and
alkyltrichlorosilanes on sika.
Source
1\ Plane- polarized light
Compcnsator
Figure 2.3 A schematic description of an ellipsometer.'
2.4 X-ray Photoelectron Spectroscopy (XPS)
XPS has been intensively employed for studying chernical composition of organic
surfaces. It offers chemists an opportunity to study such interfacial phenornena as wetting,
adhesion and friction, and how they relate to the physioçhemical parameters of functional
groups.8 Furthemore. XPS is an essential tool to study the stability of an organic f h on a
surface by foilowing the surface composition of the film as a function of tirne.
In an XPS experiment8 a sample is exposed to X-ray radiation and the propenies of
inner-shell elecîrons are probed. If E,, is the energy of the X-ray. and Ej is that of a core
electron in the atom (S. p. etc.), E, - E, will be the energy of the ejected elecmn. Since the
value of Ej is characteristic of an elecmn in a paiticular atom, the detennination of 5 provides atomic identification. Surface composition can also be determined quantitatively
because the number of elecirons ejected is proportional to the number of atoms present.
Chemical scaie information can often be determined by rneasuring the smaii shifts
("chernical shifts") in Ej. Special techniques are employed, owing to the small depths of
photoelectrons, in or&r to stress on the contribution from atoms in different deph
positions in a film. For exunple, a gazhg electmn taLeoff geornetry emphasizes the
contribution h m surface atoms, and a study of photœlectron spectroscopy as a function of
the takeoff angle provides an excellent way to study compositional depth distribution in a
filrn,l6
2.5 UV-Vis Spectroscopy
Chromophores (with rs electron deloçalization) give high optical absorption in the
UV-Vis region. Using the Beer-Lambert law A = E- 1-c, (where A stands for the
absorbance, ê, 1, and c the extinction coefficient, the thickness of film, and the
concentration of the chromophores within the film, respectively), we can calculaie the
surface coverage, d, = A-€-' (moVcm2)? Assuming lhat the extinction coeficient of îhe
chromophore in solution is the same as that of the film, it provides an estimation of the
surface coverage. The UV-Vis spectra are coUected h m a quartz slide functionalized on
both sides. Thereforie, to calculate surface coverage, absorbance is divided by 2 to obtain
the value for each individual monolayer. Quartz siides are c hosen as the su bstrate because it
does not absorb in the UV region as glas does-
2.6 References
(1) Ulman, A. An Inroduction to Ultrathin Organic Filmsfrom hgmuir-Blodgen to
Self-Assembly ; Academic Press: Boston, 199 1 -
( 2 ) Ulrnan, k Characte rizution of Organic ntin Film; B utterworth-Heinemann: Boston,
1995.
(3) Barraud, A.; Vandevyver, M. Growth Md Characterimtion of Organic Thin Film
(Langmuir-Blodgett Fi&ns) in Nonluleor Optical Propenies of Organic Molecules and
Crystals, Vol. 1; Chemla. D. S.; Zyss. J.. Ed.; Academic Press: Orlando. 1987.
(4) Fowkes, F. M. Contact Angle, Wedi l i t y and Adhesion, Advances in C hernisîry
Series 43; Amencan Chernical Society: Washington, D. C., 1964.
(5) Bubeck, C.; Holtkamp, D. Adv. Mater. 1991.3, 32.
(6) McMarr, P. J.; Vedarn, K. I. Appl. Phys. 1986. 59, 694.
(7) Flory, F. R. m in Films for Optical Systems; M. Mekker: New York. 1995.
(8) Nefedov, V. 1. X-ray Photoelectron Spectroscopy ofSolid Su$aces; VSP: Utrecht,
1988.
(9) Li, D.; Swanson, B. 1.; Robinson, J. M-; Hoffbauer, M. A. f. Am, Chem. Soc.
1993,115, 6975.
(10) Young, T- Miscellaneous Works; Peacock, G.. Ed.; Murray: London, 1855, Vol. 1.
(1 1) Sagiv. J. J. A m Chem Soc. 1980, 102, 92.
( 12) Azzam. R M. A.; Bashara, N. M. Ellipsometry and Polarïzed Lighr. North-Holland:
Amsterd; ni, 1977.
(13) Ailara, D. L.; Nuzzo, R. G . Langmuir 1985. 1. 52.
(14) Aspnes, D. E. Optical Properties of SolidF; Seraphin. B. O., Ed.; North-Holland:
Amsterdam, 1975.
(15) Wassennan. S. R.; Whitesides, G. M.; Tidswell. 1. M.; Ocko, B. M.; Pershan, P.
S.; Axe, J. D. J. Am. Chem Soc. 1989,111, 5852.
(16) Tillman, N.; Ulman, A.; Elman. J. Lungmuir lm, 6, 1512.
Chapter Three
A Novel Route to Efficient Inorganic Oxide Surface
Modifications via Simple Acid-Base Hydrolytic Chemistry
3.1 Introduction
Robust and highly ordered two-dimensional thin f h assemblies, incorporating
organic molecules of a diverse nature. represcnt materials with poiential applications in
areas suc h as photonics, sensors and heterogenizd homogeneous catal y sis. ' Manipulaiing
the cooperative forces which cause molecuiar self-assembly and dictaie the spatial and
energetic aspects in the resulling thin films is a chalienging task Fabrication of dirathin
films on solid substrates via moiecular self-assembly requires molecules with suitable end
grou ps to effect surface anchoring by covalent bond f~rmation.~' Tradi tional m olecular self-
assembly mutes indude alkyltrichlorosilanes on ~ i / S i 0 , , ~ alkanethïols on gold3, silver.'
and coppe8 and carboxylic acids6 on silver, copper and aluminum oxides. These
methodologies are end-grouplsubstrate dependent For example, ihiols adsorb ont0 gold;'
the adsorption of carboxylic acids on metal oxide surfaces is weak;' and although
aikyltrichlorosilanes adsorb stmngly on inorganic oxide surfaces. it is diffcult to synchesize
a number of silane coupling agents with desired backbone structures. We have developed
an alternative approach to molecular assem bly on inorganic oxides using simple acid-base
hydroiytic ~hernistry?-'~ This new approach employs commercially avaihble or easily
synthesized reagents, Ieading to the formation of s&ace-anchored-NEs moieties. which
can react with a variety of organic compounds with tmiinal acidic groups. Using this
simple acid-base hydrolysis route, the surfaces of inorganic oxides such as glass, quartz
and single-crystal silicon containhg hydroxyl groups that axe acidic in nature," can easiiy
be convertexi into surface-anc hored-NEt, moieties u pon reac ting with Si(NEtJ, or
Sn(NE\),. Treaunen t of the surface-anchored-NEt, moie ties with several organic molecules
containing terminal açidic protons. including rigid-rod akynes. via acid-base hydrolysis,
can lead to densely packed molecularl y self-assern bled monolayers.
3.2 Acid-Base Hydrolysis
The chemistry of organosilicon-I2 and organotin-Ritr~gen'~ (Me,E-NR',, E = Si, Sn:
R' = CH,, C2H4 cornpounds towards various protic species including alcohols. thiols.
carboxylic acids. cyclopentadiene. indene, phosphines and temiinal alkynes. has been weU
documented. Because of the lower metal-to-nitrogen bond strength, and higher basicity of
Ntrogen in stannylamines than the corresponding aminosilanes, the Sn-N bond in the
former complexes can be more e a d y cleaved by a variety of protic species.13
The formation of Si-N bond is extremely facile. When we ueated a solution of Me,Si-
Cl in dierhylether with excess diethylamine. a white solid (NEbH-HCl) was precipitated
upon contacf at r w m temperature. me resulting Me,Si-NE5 compound reacted with one
mole equivalent of REH (R = alkyl or aryl; EH = OH, SH. COOH) yielding the
corresponding Me,Si-ER compounds almost quantitatively (Equation 3. 1).12 According to
the pK, values of various protic species as shown in Table 1.4. -OH is more acidic than -
-H. Thus, Me3Si-NEt, reacts with one mole of equivalent of HCg-(CH,),-OH (e-g.,
NE&. which was prepared by ceacting (CH,O),Si-Cl with diethylamine, reaçted with one
mole equivalent of ROH (e.g.. ROH = CH,(CH,),,-OH. HCS-(CH2)2-OH. HC<-
(CH,),-OH and C,H,-CC-C,H,-OH) to give the corresponding (CH,O),Si-OR (Equation
3.2)-l2
Equation 3.1
- NR2H*HCl REH (CH&Si-Cl+ 2 NR,H -(CH3)3Si-NR2 *(Cfi&Si-ER + NKH
E = O, S, CO0
Equation 3.2
'The Me,Sn-NR', (R' = CH,. C,H,) compounds are convenientiy prepared from
Me,Sn-Cl with LiNRV2 (R' = CH,. GH,) and easily react with ROH. RSH. RCOOH (R =
alS.1 or aryl group). cyclopentadiene (C,H,H). indene (C,H,H). Ph,PH, and alkynes
(e-g., C6H5-CgH, CH,(CH,),,-CSH) via acid-base hydrolysis D give (CH,)$n-OR. -
SR. -OC(O)R. -C,H,. -C,H, Ph2P. -Ce-C,H, and -CS-(CH2),&H, mpectively in
quantitative yields (Equation 3.3).13 Simiiarly. the reaction of two quivalents of
(CH,),Sn-Nb with H C S - R - C e - H (R = p-C6H,. p-C6H4-C,H4. pC,,H,. -CS-C,H4-
C g - and (CH,),, n = 2. 4. 5 and 6) , yields the correspondhg trimethyltin substituted
alkpes with the elhination of diethylamine (Equation 1.4).13
Equation 3.3
- tic1 REH = ?OH, RSH, RCOOH, (CH3 $n-Cl+ NR,Li - (CH3),Sn-NR2 , (CHd3 Sn-ER + NR2H
RMH, CSHsH, C9H7H, PhZPH
Equation 3.4
pGC-C,H@C-, (CH,),
3.3 Surface Functionalization
As discussed above, the surfaces of inorganic oxides such as silica, glass and quartz
are acidic in nature. and contain suxface hydroxyl groups. ' ' Thus the acid-base hydrolytic
chemistry described above can be easily applied to these surfaces to construct SAMs for a
variety of organic molecules with tenninal acidic protons.
3.3.1 Si-NE5 Approach
The surface functiondhtion was carried out by two different strategies (Scherne
3.1): one is called "two-step process"; and the other a "three-step process". In the two-step
process, Si(NEtJ,, prepared" and isolated by reacting SiCl, with NEt$, was reacteâ with
clean substrates and then with the deskd protic species (REH). In the three-step process,
clean substrates were fmt functionalized with SiCl, foiiowed by NEt7I-i to prepare surfixe
anchored-NE& moieties which then reacted with the b i red REH.
Scheme 3.1 Si-NE\ approach to surface functiondization
Si Si \ @ I N 0 \ ' . si. ,si. o o o g o 0'8 O * 0'
i HER __II_) l - i / . * p - l 7 - NEt2H
- -
CI CI I I I I I c l NEt2 NEt2
\ 07'. ,y\ \ O l . Si 0 Si . / Si Si o 0 ooo' o. 080 1 I 'O/ 1-17 Nu2H tr HER
O O -7 - r i 1 7
- NEt2H-HCl - NEt2H
3.3.2 Sn-NE(, Approach
'The reaction of SnCI, direcdy with amines does not lead to the formation of
aminostannanes.'%owever, the reaction of R',SnCl with REH in the presence of amines
(NR",H) does lead to the formation of R',SnR with the elimination of NR"~H-HCL." In
this study, only a two-step Sn-N'EL, appmach is applied for the surface functionalitation
(Scheme 3.2). Sn(NEb),, p r e p a r e d l b d isolated h m the reaction between SnCI, and
LiNEt,, was used to fùnctionalize the surface of clean subsîrates with a SnNEt, layer, and
then reacted with the desired protic species (IZEH).
Scheme 3.2 Sn-NEs approach to srrfaçe functionalization
HER
3.4 Optimization of Deposition Conditions
The deposition of E(NEL,), (E = Si, Sn) on silica leading to a monolayer of [Si-]-O-
E-NEt, depends on the chernical reactions between E(NEG), and the hydroxyl groups on
the silica surface. The physical factors including water content in the system, temperature,
nature of solvent, concentration of adsorbates, and reaction lime which a f f s t these
reactions, had to be o p t i m ï d in order to produce high-quality and densely packed thin
films.
Due to the high reac tivity of E(NEL,), towards water, the NE&-func tionalized surface
of Si/SiO, is also expected to be highly sensitive to moisture. However, a molar arnount of
water k required to form a polymeric network on the surface. Excess physisorûed water in
the system was removed by (i) putting the c l a n substrates in an oven at -150 T before
deposition, (ii) using dry solvent, and (iii) performing thin film deposition under nitrogen
in a self-assembly apparatus. When either a wet solvent or a wet substrate was employed, a
thick white füm on the surface was observed, since too rnuch water gives rise to
muitilaye~d polymerized siloxane (-Si-O-Si-) or stannoxane (-Sn-O-Sn-) s tmctures. This
is consistent with the eariier studies1"17 of OfS deposition on SVSiO, in which excess
water results in facile polymerization in solution and polysiloxane deposition of the surface.
Temperaaire is another imporîant parametef7 because it can affect the rate of
deposition as well as the rate of polymerization of E(NEs), in solution. It was found that a
high temperature favors polymerization of ECl, and E(NEQ,, and gives rise to turbidity in
solution and a thick layer of siloxane or stannoxane on surface. Thefefore, room
temperature is sumien t for the silanaiion/stannation process. However, 70-80 T is
preferred for amination of Cl-funçtionalized surfaces to give surface-anchored-NE\
moieties and to remove the byproduct of NEsH-HCl from surface, which is soluble in hot
toluene (three-step process: Schemes 3.1). Owing to the diffemnce in teaçtivity of a variety
of protic species towards NEb-functionalized surfaces, and the fact that reaction on the
surfaœ is more retarded than that in solution due to steric effects, an optimized temperature
of 70-80 T was found to be ideal for alkynyl thin €dm deposition, and 40-60 T for
akohol, thiol and acid film deposition.
As mentioned above, a wide range of temperatures is preferred for deposition of
various protic species. Since toluene has a comparatively high boiiùig point, it satisfies the
temperature requirement. In addition, since E(NE\), and a number of alkyl and aromatic
protic species are soluble in it, dry toluene was chosen as the solvent for thin f h
deposition. In the case of more polar protic species such as diols, dry THF was prefemd
since they are highly soluble in THF.
Concentration of adsorbates is another parameter to be considered since too much
E(NEtJ, c m result in polymerization leading to thick layers of siloxane or stannoxane on
the surface. According to published reports," a 2" diameter surface of standard SilSiO,
substrate contains -10i6 OH groups. The concentration of ECI,, E(NEtJ., or RH (protic
species) we amployed was 0.1-0.5 % solution by volume
to the concentration'-s for OTS (or thiol) deposition.
monolayer conversion,
(or weight), which is comparable
and is in excess for cornpiete
Fiaily, reaftion time c m also affect the quaiity of thin films in such a way that
insufficient tirne would lead to an incomplete rnonolayer, detected by surface wettability
measurements and ellipsometry. However. excessive t h e could result in polymerization in
the solvent and introduçe a thick film of siioxane or stannoxane at the surface. Using
octadecanol and Si(NEtJ, as a model the tirne effect on two-step silanation was examined.
The data is presented in Table 3.1 and Figures 3.1 and 3.2.
Table 3.1. The effect of silanation time on surface properties (CA, and Te) of
octadecanol thin füms using a two-step processa
I Silanation Time, min I
a Parameters for thin hùn deposition: concentration of Si(NEt.,),, 0.5% by volume;
concentration of octadecanol, 0.1% by weight; temperature for silanation. x t ; temperaime and reaction tirne for octadecimol deposition, 50 OC and 24 h, respectively . CA, = contact angle with water in degrees; Te = eliipsomeaic ihickness in A
O 200 400 600 800 1ûûû 1200 Reaction Cie, min
Figure 3.1. The effect of silanation time on contact angles of water of octadecanol thin films using a two-step process.
O 200 400 600 800 lûûû 1200 Reactioo tirne. min
Figure 3.2. The effeçt of silanation time on ellipsometric thickness of octadecanol chin films usine a two-step process.
The fornation of a goodquality and densely packed monolayer of octadecanol on
Si/Si02 should result in a water contact angle of - 1 10 O and thickness of 27 A.'' Both data
for contact angle of water and eliipsomeüy indicated chat incomplete monolayer formation
resulted below 4 h and polymerizaiion oçcurred above 8 h. Therefore, 8 h is optimal for
silanation proçess, resulting in a thin film of octadecanol on Si/SiO, with water contact
angle of 1 10" and ellipsometric thickness of -25 A In the same way, the optimum tirne for
deposition of Sn(NEQ, on Si/Si02, and its funher reaftion with the protic species was
found to be 8 h and 24 h, respectively. In the latter case, incomplete monolayer formation
was observed below 18 h, while any increaçe in the period beyond 24 h did not riesuit in
irnproved quality of the thin films.
3.5 Surface Propetties of Thin Films
The generality of the new simple acid-base hydrolytic chemistry approach to
molzcular self-assembly under optimal conditions was examined using surface wetiability
and ellipsometriç thicknesses of the thin frlms of a variety of chromophotes on Si/Si02.
Contact angle goniometry and eilipsometry were used to determine the hydrophobicity and
the thickness of the thin füms, respectively. These two methods togerher provide a
complementary means of comparing the surface coverage in the newly formed thin films.
An investigation of the wening characteristics of thin films was h e d out by
measuring staîic contact angles of deionized water on monolayer surfaces. The data
presented in Table 3.2 is consistcnt with wetting characteristics of thin fdms:'klean
Si( 100) (Si/SiOJ surface, 15'; and E-R. 80-8S0 (E = Si or Sn) for R = aryl o r alkynyl. and
100-1 10' for R = alkyl. The lower contact angles of water on the acetylene terminated
surfaces (80-82') han the corre-siponding long alkyl c h a h (paraffins, - 1 IO0) may be
caused by the fact chat CH groups (sp character) adhere more smngly to water than CH,
(sp3) groups.'O Sirnilarly, potential factors responsible for the lower contact angles of water
on the phenyl terminami surfaces include (i) interactions between water and the nclouds of
the benzene ringstO and (ü) disorder in the aromatic monolayer a s ~ e r n b l ~ . ~ ~ The long alkyl
chains also help to shield the water molecules fmm the polar surface of s~/s~o,? When a
long chah of alkyl groups (e-g. -(CH,),,-CH,) is inserted into the backbone, the type of
bonding at the surface: Si-S, Si-O, and Si-O-C(0) groups, also effects the wetting behavior
of these thin films. On Si-NEb funçtionalized surface, a thin film of octadecanol gave a
higher contact angle of water (1 10°) than those of octadecanethiol and octadecanoic acid
(lOOa), suggesting that the thin film of octacîecanol is more ordered, and/or it has a higher
coverage on surface than those of octadecanethiol and octadecanoic acid. It is probably
attributable to the difference in binding geomeûy andor binding strength of hydroxylate,
thiolate and carboxylate on the surface of silice23 Similady, on Sn-NE5 functionalized
surface, a thîn film of octadecanol gave a higher water contact angle (105") than those of
octadecanethiol and octadecanoic acid (95').
Measurernent of the thickness of thin films by ellipsometry was made by asswing a
refractive index of the organic fiIm of 1.46 which is comparable to Literatuxe valuesz4 of
OTS film on silica (1.45- 1.50). A detailed calculation of f h thickness by eUipsornetxy has
ken reported el~ewhere."~ The thickness of both funccionalkd and non-fûnctionalued
substrates were measured to &termine the monolayer thickness. Calcuiation of the
theoretical thicknesses was based on typical of bond lengths between elements
projected on the surface. As shown in Table 3.2, the measured thicknesses, which ane
comparable to the theoretical values, suggest the formation of a densely packed monolayer
for each sample with a tiit to normal. However, for thiols and carboxylic acids, they have a
much smaiier measured thickness than the theoreticai value, suggesting that they are more
tiIted andor they have a lower coverage on surface. A fidl characterization of the surface
properties of SAMs of alcohols and aikynes on siiica surfaces is presented in the following
c hapters.
Table 3.2. Static contact angles of water (CA,& eüipsometric (TJ and theoretical thicknesses (Tt) for monolayers (Schemes 3.1 and 3.2) on Si(100) subsmtes
R Group on the Substrate
Te Cr,). A CA,, O
85
3.6 Conclusion
The acid-base hydrolysis of surface bound silyl- or stannylarnines. obtained using
commercially avaiiable or easily synthesized E(NEQ, (E = Si. Sn). is a generai and
promising approach to the functionaiization of inorganic oxide surfaces. leading to the
formation of densely packed thin füms of a number of cornpounds with terminai aOdic
protons. Using a two-step route, E(NEtJ4 was allowed to f i s t react with hydroxyl groups
of the inorganic oxide surfaces to produce surface-anchored NE4 moieties which m e r
react with a variety of organic chromophores with teminal acidic protons. The reaction
conditions were op t i rn id for the formation of a relatively densely packed thin füm via the
two-step process: (1) for silanation/stannation reaction. an ideal concenaation of amino-
silane or -stannane solution was 0.5% by volume; reaction tune, 8 h at room tempe-;
and (2) for the deposition of compounds with tenninal aciâic protons, the best
concentration of reagents was found to be O. 1% by volume or weigh$ reaction tirne, 24 h at
40-60 'T for alcohols. thiols and carbuxylic acids. and 70-80 sC for alkynes. A detailed
investigation of the structural and physical pmpertirs of the alcohols and alkynes on
Si/SiO,, and M e r synihetic elaboraiion to rnultilayered structures. will be discussed in
Chapters 4-6.
3.7 Experimentzll Section
3.7.1 Materials
Silicon tetrac hloride, tin(IV) chloride, p-methylthiophenol, thiophenol. p henol.
benzoic acid, 1 -oc tadecanethiol, 1 -hexadecanol, 1 -0ctadecano1, octadecanoic acid,
cyclopentadiene, indene. diphenylphosphine. phenylacetylene, 1 *8-nonadiyne, 2-propyn- 1 - 01, 3-butyn- 1-01 and 5-hexyn- 1-01 were pwhased from Aldrich and used as received. 4-
Pentyn- 1-01. 1 ,5-hexadiyne, 1 -7-octadiyne, 1.9-decadiyne and octade- 1 -yne were
purchased from ChemSarnp and used as received. Si(NEtJ4 was ~ r e p a r e d ' ~ from SiCl, and
excess NEsH. Sn(NEt,), was preparedl' from SnCl, and LiNEi,. Olher rigid-rod alkynes,
H-CC-R-CC-H (R = p-C,H,. p-C,HIC6H,, p-C,,H, and p-CS-C,H,-CS-) ,
em ployed in this stud y were convenien tly prepared ushg fiterature procedures."
HOC,H4C-CC6H, was synhesized by modirxcations of published procedures." Toluene
was distilled over sodium. Diethylarnine was distilled over KOH. The substrates were
placed in a plastic carousel fiüed with a magnecic stir-bar at the bottom. Thin tilm
82
depositions were perfonned un&r nitrogen in a self-assembly apparatus sirnilar in design to
a Schlenk flask but with a flat bottom.
3.7.2 Substrate Preparation
The glass. quartz or Si wafers were cleaned by (i) soaking in soap solution and
sonicaiing for 1 h; (ii) repeated washings with d e i o n i d water. (üi) ireatment with a
solution mixture containing 70% conc. H,SO, and 30% H202 (piranha solution) at 100 OC
for 1 h. Cuutwn: Piranha solution is highly explosive, a d care should be uken while using
this mixture; (iv) ~peated washing with deionized water. and (v) f d y heathg in oven at
150 OC for 5 min and vacuum drying for 5 min to removed physisorbed water before taking
into a nitrogen dry box.
3.7.3 Si-NE& Approacb to Surface Functiondization
Clean silica surfaces can be functionaikd with protic species by fo!lnwing either
two-step or three-step proçess.
3.7.3.1 Two-Step Deposition Process
The clean substrates were ûeated with (i) 0.5% solution by volume of Si(NEt,),,
which c m be conveniendy prepared" by the hection of SiCl, and excess dry NEbH. in dry
toluene for 8 h at room temperature, followed by (ii) REH in dry toluene at 40-60 T for 24
h after sonicating in dry toluene for 5 min to remove excess Si(NEt.J, physisorbed on the
surface. A k r thorough washing with toluene, the substrates were dried at 120 qC for 5
min,
3.7.3.2 Three-Step Deposition Process
The clean substrates were treated with (i) 0.5% solution by volume of SiCl, in dry
toluene for 18 h at m m temperature foilowed by sonicating in dry toluene for 5 min to
remove excess SiCl, physisorbed on the surface. (ü) 0.5% solution by volume of dry
NEtH in dry toluene for 18 h at 70 O C , foiiowed by (fi) REH in dry toluene at 40-60 OC for
24 h after sonicating in dry toluene for 5 min to remove e x c e s NEbH physisorbed on ihe
surface. After thorough washing with toluene, the substrates were dried at 120 sC for 5
min.
3.7.4 Sn-NEt, Approach to Surface Functionalization
Similarly, clean silica surfaces can be functionalized with protic species by the
following two-step process. The clean substrates were m t e d with (i) 0.5% solution by
volume of Sn(NEt.J,. which can be convenientiy preparedls by the reaction of SnCl, and
LN+,. in dry toluene for 8 h at room temperature, foiiowed by (ü) REH in dry toluene a
70-80 OC for 24 h &sr sonicating in dry toluene for 5 min to remove excess Sn(NEt,), on
ihe surface. After thorough washing with toluene, the substrates were dried at 120 qC for 5
min.
3.7.5 Contact-Angle (CA,,,) Measurements
The static and advancing contact angles were measured with a Rame-Hart NRL 100
goniorneter. On average, 8 drops of water were meaured on different areas of the polished
side of a silicon wafer for each sarnple, and the vaiues reported are the mean values with a
maximum range of El0. The advancing contact angles of captive drops were found to be
roughly 5" above the static values of sessile (free-standing) drops. if the drop was allowed
to fall from the needle of the s y ~ g e to the surface, smaiier contact angles were usually
obtained because of the mechanical vibrations.'"
3.7.6 EMpsometry
A Gaertner Scientüic eilïpsometer operating with a 633 nm He-Ne laser (A = 6328 A)
was employed. 'Ihe angle of incidence was 70'. and the compensator was set at 45O. An
reported values wiih a maximum range of +2 A are the average of a least six measurements
taken at different locations on the sarnple. The thickness was caiculated by comparing data
from the same substrate before and after funçtionalization, and using a value of 1-46 for the
refractive index This value is based on the assumption that the monolayer is similar to bulk
p d f m s with a refractive index of 1.45." If the monolayer is more crystailine-like, sirnilar
to polyethylene, the refractive index thus should be within 1.49- 1.55." It was found that an
increase of 0.1 in the &active index from 1.45 to 1.55 resulted in a decrease in the
measured thkkness by -2
N(CH,CH,),H-HCI: 'H NMR (270 MHz. CDCl,) 6 1.41 (t, J = 7.3 Hz, 6H. CH,),
2-97 (q, j = 7.3 Hz, 4H, CH,), 9.44 (b, S. H-HCl); Mass Spec. (EI): 110.
Si(NE&),: 'H NMR (200 MHz, C6D,) 6 0.95 (t, J = 7.0 Hz. 24H. CH,). 2.90 (q, J = 7.0
Hz. 16H, CH,); M a s Spec. (EI) 3 16.
Sn(NEt3,: 'H NMR (200 MHz, C6D,) 6 1.16 (1. I = 7.0 Hz, 24H. CH,), 3.16 (q, J =
7.0 Hz, 16H, CH2); Mass Spec. (EI) 407.
(CH,),Si-N(CHFH,),: 'H NMR (270 MHz, C6D6) 6 0.1 1 (s, 9H. (CH,),Si). 0.96 (t, I
= 7.0 Hz, 6H, CH,), 2.76 (q, J = 7.0 Hz, 4H, CH& Mass Spec. (EI): 145.
C,H5-O-Si(CH3),: 'H NMR (270 MHz, C E 1 3 6 0.30 (s, 9H. CH,), 7.17, 7.54 (m.
5H, C,H,); Mass Spec. (Eu 166.
~,H,-C(O)O-S~(CH,)~: 'H NMR (270 MHz, CDC13 6 0.32 (s, 9H. CH3), 7.33. 8.01
(m, 5H. C6H,); Mass Spec. (El) 194.
CH3-C,H,-S-Si(CH3)3: 'H NMR (270 MHz. Cm!,) 6 0.18 (S. 9H. (CH,),Si). 2.23 (S.
3H, CH,), 7.00, 7.29 (m, 4H. C6HJ; Mass Spec. (EI) 196.
c H 3 ( C ~ 3 , , - 0 - ~ i ( ~ ~ , ) , : 'H NMR (200 MHz. C,D6) 6 0.15 (S. 9H. (CH,),Si). 0.94 (t.
J = 6.1 Hz, 3H. CH,). 1.35 (m. 30H. CH,). 1.60 (m. 2H, CH,), 3.57 (t, J = 6.3 Hz. 2H.
CH,); Mass Spec. (EI) 342.
CH3(CH3,7-S-Si(CH3): 'H NMR (200 MHz. C6D6) 6 0.16 (S. 9H. (CH,),Si). 0.92 (t.
J = 6.2 Hz, 3H, CH,), 1.25 (m. 30H, CH,), 1.61 (m. 2H. CH,), 2.46 (t, J = 6.2 Hz. 2H.
CH,); Mass Spec (EI) 358.
CH,(CH,),,-C(O)O-Si(CH,),: 'H NMR (200 MHz. C,DJ 6 0.14 (S. 9H. (c~, ) ,S i ) .
0.94 (1, J = 6.0 Hz, 3H. CH,). 1.34 (m. 28H. CH,). 1.87 (m. 2H. CK), 2.50 (t. J = 6.3 Hz, 2H, CH;); Mass Spec. (ET) 356.
(CH,),Si-O-(CH3,-CICH: 'H NMR (200 MHz, C6D6) 6 0.1 1 (S. 9H. CH,), 1.79 (t, J
= 2.6 Hz, 1H. CsH), 1.51 (m. 4H. CH,), 1.98 (m, 2H. CH,), 3.42 (t, J = 6.0 Hz, 2H,
CH,); Mass Spec. (EI) 170.
(CH,),Si-O-(CH2)3-C-CH: 'H NMR (200 MHz. C6D6) 6 0.1 1 (S. 9H. CH,). 1-78 (t, J
= 2-6 Hz. IH. C S H ) , 1-58 (m, 2H, CH,). 2-15 (m. 2H, CH,), 3.51 (t, J = 6.0 HZ, 2H,
CH2); M a s Spec. (EI) 156.
(CH,),S~-O-(CHJ~C=CH: 'H NMR (200 MHz, C6D6) 6 0.1 1 (S. 9H. CH,). 1.77 (t, J
= 2.5 HZ. 1H. CiCH), 2.27 (m. 2H. CH2), 3.55 (t, J = 7.0 Hz. 2H, CH,); Mass Spec.
(EI) 142.
(CH,)3Si-O-CH2-C=CH: 'H NMR (200 M H z C,D,) 6 0.10 (s, 9H. CH,), 2.04 (t. J =
2.2 Hz, lH, CeH), 4.07 (d, J = 2.4 Hz, 2H. CF&); Mass Spec. (El) 128.
(CH30)3Si-O-(CH,),,CH,: 'H NMR (200 MHz, C,D,) 6 3.54 (s, 9H. OCH,), 1.01 (t,
J = 7.2 Hz. 3H, CH,). 1.34 (m. 26H. CH,), 1.65 (m, 2H. CH,). 3.89 (i, J = 6.4 Hz, 2H. CH& Mass Spec. (En 362.
CH,(CH,),,-O-Sn(CH,),: 'H NMR (200 MHz. C6Dd 6 0.25 (S. 9H, (CH,),Sn), 0.93
(t, J = 6.4 Hz, 3H. CH,), 1.35 (m. 32H, CH,), 3.38 (t, J = 6.4 Hz, 2H, CH,); M a s Spec. (EI) 433.
CH,(CH,),,-S-S~(CH,),: 'H NMR (200 MHz C6D6) 6 0.28 (S. 9H. (CH,),Sn). 0.91
(t, J =6.6 Hz, 3H, CH,), 1.34 (m, 32H. CH,), 2.18 (t, J = 6.9 Hz, 2H, CH,); Mass Spec. (EI) 449.
(cH,),Sn-C,H,H: 'H NMR (270 MHz, C6D6) 6 0.24 (S. 9H. CH,), 2.22 (S. 1H. H),
5.50, 6.07 (m, 4H, C,H,); Mass Spec. (EI) 229.
(CH,),Sn-C,H,H: 'H NMR (270 MHz, C6DJ 6 0.24 (S. 9H. CH,). 3.05 (S. 1 H. H).
6.26, 6.75, 7.17, 7.3 1 (m. 6H, C,H,); Mass Spec. (EI) 279.
C,H,-CS-Sn(CH,),: 'H NMR (270 MHz C,D,) 6 0.22 (s, 9H, CH,), 6.78. 7.28 (m.
SH, C,H,); Mass Spec. (Ei) 265.
CH3(CH,),5-C=C-Sn(CH3),: 'H (200 MHz. C6D6) 6 0.25 ( S . 9H. (CH,),Sn),
0.93 (t, J = 6.3 Hz. 3H. CH,), 1.34 (m. 28H. CH,), 2.20 (t, J = 6.4 Hz, 2H. CH,); Mass Spec. (EI) 413.
(CH,)3Sn-C=C-(~~2)2-~=~-~n(~~,),: 'H NMR ( 2 0 M H z C6DJ 6 0.16 (S. 18H.
CH,). 2-41 6, 4H. CH,); Mass Spec. (EI) 403.
(CH,),Sn-C=C-(CH,),-C=C-s~(cH,),: 'H NMR ( 2 0 MHz, C,D$ 6 0.17 (s, 18H.
CH,). 1-58 ((. J = 6.0 Hz. 4H. CH,). 2.13 (t, J = 5.6 Hz, 4H, CH,); Mass Spec. (ET) 431.
(CH,) ,Sn-c~C-(cH,)~-c~c-sn(CH,) , : 'H (200 MHz, C6D6) 8 0.19 (S. 18H.
CH,), 1.40 (m. 6H. C4) . 2- 12 (t, J = 5.5 Hz, 4H. C 4 ) ; Mass Spec. (EI) 445.
(CH3)3Sn-C=C-(CH2),-C=C-Sn(CH3)3: 'H NMR (200 M H z C,DJ 6 0.19 (S. 18H.
CH,), 1.25 (t, J = 4.6 Hz, 4H. CH,), 1-40 (m. 4H. CH,), 2.14 (t, J = 4.8 Hz. 4H, CH,); Mass Spec. (Er) 459.
HCGC-C,H,-CgH: 'H NMR (200 MHz, CDCI,) 6 3.17 (S. 2H. C e H ) . 7.45 (s, 4H.
C6H4); Mas Spec. (El) 126; IR (KBr) v,: 3040. 1494, v,,: 3263. v,: 2103.
HCg-C, ,H,-CSH: 'H NMR (200 MHz. CDCI,) G 3.14 (s, 2H. CeH). 7.55 (S. 8H.
C,,H,); Mass Spec. (EI) 202; [R (KBr) v,: 3034. 1488. v,: 3272, v,: 2105.
HCiC-C,,H,-CICH: 'H NMR (200 MHz. CDCl,) 8 3.56 (S. 2H. CnCH). 7.62. 8.62
(m. 8H. C,,H,); Mass Spec. (EI) 226; IR (KBr) v,: 3058. 1593, v,: 3281, v,,: 2093.
HCGC-C=C-C,H,-CEC-C=CH: 'H NMR (200 MHz, CDCI,) G 2.48 (S. 2H. CgH).
7-39 (s, 4H, C6H4); Mass Spec. (EI) 174; IR (KBr) v,: 3063, 3041, 1493, vc-: 3275,
v,: 2191.
C,H,-C*-C,&-OH: 'H NMR (200 MHz, CDCl,) G 6.82. 7.34 (m, SH. C6H,), 7.44,
7.5 1 (m. 4H. C,H,), 1.90 (S. br, OH); Mass Spec. (Ei) 194; IR (KBr) v,: 3076. 3053.
3018, 1512, v,: 3419, v,: 2221.
(CH,),-Sn-C=C-C,HrCIC-Sn(CH3)3: 'H NMR (200 MHz, CDCLJ 6 0.36 (S. 18H.
CH,), 7.37 (S. 4H. C,HJ; Mass Spec. (EI) 451; IR (KBr) v,: 3042. 1490. v,~,: 2963.
2920. v,: 2 130.
(CH,),-S~-C=C-C,,H,-C=C-S~(CH~)~: 'H NMR (200 MHz, CsDJ 8 0.28 (S. 18H.
CH,). 7.07. 7-43 (d, J = 8.5 Hz, 8H. C,,H,); M a s Spec. (El) 527; IR (KBr) v,: 3034,
1488. v,-,: 2985,2916, v,,: 2135.
(CH3),-Sn-CEC-C,,Hb-C3C-Sn(CH,),: 'H NMR ( 2 0 MHz, C,D,) 6 0.32 (S. i8H.
CH,), 7.32, 8.98 (m. 8H, C,,H,); Mass Spec. (En 551; IR (KBr) v,: 3076. 3060, 3040.
1518, v,-,: 2961,2899, v,,: 2147.
(CH,),-Sn-C=C-C=C-C,H,-C=C-Cd-Sn(CH,),: 'H NMR (200 MHz, C,D,) 8
0.42 (S. 18H, CH,), 7.35 (S. 4H. C6HJ; Mass Spec. (EI) 499; IR (KBr) v,: 3079. 1549.
v,-,: 2963,2923.2855. v,,: 2203.
(CH,O),Si-N(CH,CH,),: 'H NMR (200 MHz. C,D6) 6 1.01 (t, J = 7.2 Hz. 6H. CH,),
2.46 (q, J = 7.2 Hz. 4H, C b ) , 3.49 (S. 9H, OCH,); Mass Spec. (EI): 193; IR (CCl,) v,,,:
2967,2942, 2868, 284 1 cm", v,,,: 1080 cm".
(CH,O)3Si-O-(CH2),-CICH: NMR (200 MHz. CDCl,) G 3.59 (s, 9H. OCH,),
1-95 (5 J = 2.0 Hz. IH, C S H ) . 1.65 (m. 4H. CH,). 2.23 (m. 2H, C&), 3.82 (t, J = 5.5
Hz. 2H. CH2); Mass Spec. (En 218; IR (CCl,) v,,: 2964, 2944, 2869, 2843 cm-', v,:
2 1 19, vSi,: 1086 cm".
(CH,O),Si-O-(CH2),-CrCH: 'H NMR (200 MHz, CDCI,) 6 3.60 (s, 9H. OCH,),
1.98 (t, J = 2.7 Hz, IH, C K H ) , 2.48 (m, 2H, CH,), 3.90 (t, J = 6.9 Hz, 2H, CH,);
Mass Spec. (En 190; IR (CCIJ v,-,: 2965,2945,2844 cm", v,: 2 124. v,,: 1088 cm-'.
(CH,O),-Si-O-C,H,-C=C-C4Hs: 'H NMR (200 MHz, CDCl3 6 3.49 (s, 9H. OCH,).
6.88, 7.3 1 (m. 5H. C,H,), 7.39, 7.47 (m. 4H. C,H,); Mass Spec. (Ei) 314; IR (KBr) v,:
3037, 15 12 cm", v,,: 2947,2846 cm-', v,: 2218, L,,: 1098 cm-'.
3.8 References
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Series, ACS, 1964, vol. 43, p.52.
(23) Tao, Y. J, Am Chem. Soc. 1993,115,4350.
(24) Wasserman, S. R,; Whitesides, G. M.; Tidswell, 1. M.; Ocko, B. M.; Pershan, P.
S.; Axe, J. D. J. Am Chem Soc. 1989,111, 5852.
(25) Auam, R M. A.; Bashara, N . M. Eilipsomtry and Polarized Lighr, North-HoLiand:
Amsterdam, 1977.
(26) Dean, J. A. Longe's Hondbook of Chemisny; McGraw-Hill: New York, 1992.
(27) (a) Lewis, J.; Khan, M. S.; Kakkar, A. IC; Johnson, B. F. G.; Marder, T. B.; Fyfe,
H. B.; Wittmann, F.; Friend, R H.; Dray, A. E. J. Organomet. Chem. 1992,425,
165. (b) Jiang, J.; Kobayashi, E.; Aoshirna, S. Polym J. 1990.22, 274. (c)
Talcahashi, S.; Kuroyama, Y .; Sonogashira, K. ; Wagihara, N. Synthesis Cornmwi.,
1980, 627.
(28) Wityak, J.; Chan, J. B. Synthetic Commun. 1991, 21, 977.
Chapter Four
Links between Material Presented in Chapters Five to Seven
Chapters 5-7 are written based on the fotlowing manuscnpts,
Chapter 5: Yam. C. M.; Tong. S. S. Y.: Kakkar. A. K. Lnngmuir 1998.14. 6941-6947.
Chapter 6: Yarn. C. M.; Kakkar, A. K. Langmuir 1999, in press.
Chapter 7: Yam. C. M.; Kakkar, A. K. Langmuir 1999, submitted.
Chapter 5 describes the molecular self-assembly of a series of alcohol compounds
terminaîed with alkyl. phenyl and acetylene groups on Si/SiO surfaces using the acid-base
hydrolytic chemisüy approach which was discussed in Chapter 4. The two-sep process
based on the reaction of Si(NEt,), with surface hydroxyl groups followed by the reaction
with ROH is compared with the rhree-step methodoloEy involving the reaction of surface
hydroxyl groups with SiCl,, NEt.,H and ROH. in sequence. The former is found to be
more efficient than the three-sep process. The two-step thin film construction method, is
then elaborated using dihydroxy chromophores containing acetylene, diacetylene and
aromatic backbone structures to fabricate mono- and multilayered interfaces, as discussed in
Chapter 6.
Thin films containing chromophores with exclusive n-conjugation in the backbone
offer significant potential in the fabrication of electronic and photonic based devices. In
Chapter 7. construction of mono- and multilayered thin f h s on Si/Si02 based on the
rûaction of surface anchored Sn-NE& groups. which are much more basic than Si-NEb.
with acetylenes. is reported. Such thin füms help to examine the role of intermolecular K-K
interactions exclusively in molecular .self-assembly.
Chapter Five
Simple Acid-Base Hydrolytic Chemistry Approach to
Molecular Self-Assembly: Thin Films of Long Chain Alcohols
Terminated with Alkyl, Phenyl, and Acetylene Groups
on Inorganic Oxide Surfaces
5.1 In traduction
"Molecular sel f-assembly" constitutes a prominent area of research due to its poten tiai
applications in the fabrication of materials with novel properties. ' Sel f-assem bled
monolayers (SAMs) c m provide the desired molecular level contml. and offer an attnctive
route to molecular engineering of solid state devices."' Much of the current emphasis has
been placed on the preparation and characterization of the long alkane chah assemblies,
since these studies arie fundamental to the understanding of the self-organization
phenornenon. Some of the cornmon methodologies employeû in the preparation of the
"fit-generation" sel f-organized thin f i i s include c hiorosilanes on siiica su~faces,~
alkanethiols on g01d.'~ silver7 and copper.' diaUcyl s-lfides on gold: diallcyl disulfides on
gold.1° carboxylic acids on aluminum oxide and silver,ll and 1-alkenes on hydrogen
terminated Si( 1 1 1 ).lZ The combination of spontaneous adsorption. strong molecule-
substrate and van der Waals interactions, leads to highly ordercd and densely packed
monolayers using the above mentioned routes.
Hydroxylated silica based surfaces such as glass. quartz and single crystal silicon are
of great teçhnological importance." However, fabrication of thin f h assemblies
incorporahg chromophores with desired functional groups Ma traditionai molecular self-
assembly mutes requires molecules with suitable end-gmups to affect surface anchoring by
covalent bond formation. Difficulties exisr in synthesizing chromophores with appropriate
end-group functionaiities. We have developed a new and versatile approach to molecdar
self-assem bly based on the hydrol ysis of surface-anchored aminosilanes wi th organic
chromophores containing acidic end-gro~ps . '~ Using this simple acid-base hydrolysis
route. silica surfaces such as g l a s , quariz and single crystal Si can be easily modifie& In
addition. the advantage of using acid-base hydrolytic chemistry for molecular self-assembly
is the ability to incorporate a variety of functionalities on a single subsmte. In this chapter,
we discuss molecular self-assembly of long alkane chah aicohols containing terminal alkyl.
phenyl and acetylene moieties on inorganic oxide surfaces. and compare a two-step thin
film construction process involving the reaction of Si(m-), with surface hydroxyl groups
foilowed by the reaction with ROH, with the three-step sequence using SiCl,, NE-H and
ROH. A detaded characterizaiion of the r e s u l h g SAMs using surface wettability
measurements, FI'IR-ATR, eiiipsometry and X-ray photoelectron spectroscopy (XPS) etc.,
indicates that the NO-step process produces monolayers that are more densely packed and
ordered than the k - s t e p process. The long c h a h alkane SAMs prepared using our new
two-step acid-base hydrolytic approach are of sirnilar quality as those p r e p a ~ d from
traditional surface functiondization routes-
5.2 Acid-Base Hydrolysis
The chemistry of organosilicon-mines (R9,Si-NR,, R' = Me, OMe. R = Mc. Et)
towards hydrolysis has k e n well d~cumented . '~ The formation of Si-N bond is extremely
facile. For example, the reaction of R',Si-Cl in diethylether with excess diethylamine at
room temperature gives R' ,Si-NEb in a quantitative yield. The resulting aminosilane.
R',Si-NE- reacts with 1 mol equiv. of ROH to give the corresponding silylated alcohol,
R',Si-OR. We were intrigued by the behavior of the surface-irnmobiiiled Mt2 moiety
towards R-OH leading to surface modifications based on this simple acid-base hydrolysis.
5.3 Surface Functionalization
The surface functionalization was camied out using two different m t i o n
methodologies. In a two-step process (Scheme 5.1). the aminosilane, Si(NEk),, was
preparedl6 and isolated by reacting SiCl, with NEbH. It was then used to leact with ciean
substrates, foUowed by the reaction with the desireâ chromophore. In a three-step process
(Scheme 5.1), surface anchored amino groups were p~pared by reacting ciean substrates
first with SiCl, foliowed by NEL,H. Substrates functionalized with amino groups were then
reactcd with the desired alcohol.
Scheme 5.1 Surface functionalization using two different reaction methodologies: two-step
and three-step processes
- - . -
CI CI I I
I I I 1 NEt2 NEt2 O O
Si Si I I \ e l \ H I \ f Si, Si Si, ,si, 00000
bOi oO~'oO O O '0'6 O 0 O'
r l - 1 7 WH 1 - 1 7 ZIOR - r i - i 7 - NEt;?H-HCI - NEt2H
[Th ree-S tep Proces j
In order to explore the generality of the new acid-base hydrolytic approach to self-
assembly, a series of thin films containing short-to-long chah length alcohols tecminated
with alkyl. phenyl and acetylene groups were self-assembled on glass. quartz and single
crystal silicon (Scheme 5.2). The evolution of thin film structures was routinely monitoried
by surface wettabiüty measurements. FT-IR. ellipsomeuy and X-ray photoelectmn
spectroscopy. This multi-technique approach probes the structure of the newly fomed thin
i ims b y differen t ph ysical processes. and can provide complementary and deffitive
inf~rrnation.~'-'~
X-ray photoelectron spectroscopy is a useful technique in determinhg the surface
composition of the anchored species in the molecularly self-assembled thin films. We
employed this technique for the analyticai evaluation of the various organic thin films on
silicon wafers, and the survey spectra for these monolayers showed the presence of three
elements: silicon (2p. 99 eV. 2p, 103- 104 eV). carbon (1s. 285 eV). and oxygen ( 1s. 532-
533 eV). The molecules containing conjugated backbones showed a peak at 29 1 eV for C , ,
corresponding to. for example. aromatic carbon. The data confims the identity of the
molecules in the thin film structures. and is consistent with the one obtained from OTS
films."
The presence of a close-packed methyl-teminated monolayer is o k n indicated by
characteristic advancing contact angles of water and hexadecane. and asymmetric and
symmerric CH, stretching frequencies in their m - A T R spectra? Although contact
angles do not provide any suuctural information about the monolayers. they are useful
indicators of their q ~ a l i t ~ . ~ ~ The contact angles of water. t - h thicknesses. syrnmetric and
asymmevic sueiching frequencies of the methylene groups of well characterized thin films
prepared from ociadecyltnchlomsilane on silica are 1 10'. 23-28 A and 2850. 2915 cm"
respectively." As the chah length decreases from -(CH2),,-CH, to -(CH2),-CH,. the
contact angle drops from 110 to 90°, and there is a definite tend towards higher peak
frequencies of v,(CH..J and v,(CHJ stretches?
The data obtained from sulface wettability rneasurements. ellipsometry. and F ï i R -
ATR for thin films of alcohols prepared using the two-step process (Scheme 5.1) on
Si(100) substrates are presented in Table 5.1. Upon comparing this data with that from
SAMs prepared using other well-known meth~ds. '~ it is apparent chat the new acid-base
hydrolytic approach is capable of producing thin films of high quality and order. For
exarnple. the static contact angle of water for a thin f h of octadecanol is similar in value
( 1 10") to that for the SAMs prepared fmm OTS on silica based surfaces and thiols on gold.
and is well in accord with densely packed and orientai SAMs. However. the advancing
contact angle with hexadecane ( C b ) was found to be about 5-10° lower for the
octadecanol SAM (30°) on Si(100) than those reported for the monolayers prepared from
OTS on silica based surfaces (35-40'). This may k due to a number of factors including (i)
the higher hydrophilicity of [Si]-O-R in our SAMs than [Si]-CH,-R in OTS. and (ii) the
Si( 100) surface employed in our studies. It has been reported that CA, of' the thin fdms of
CH,-(CH,),-CO0 on Si(100) are LOO lower as compared to those on Si(l11) surface.
although both gave similar contact angles with &O (1 10°)." In order to confinn this
hypothesis. we prepared a SAM of OTS on a Si( 100) surface. and a similar lowering of the
CA, angle tc 30°was observed,
As the chai length of the alkylalcohol decreases from -(CH,),,-CH, to -(CH,),-C H ,.
CA,,, drops from 110" to 95O (Figure 5.1). It may be due to the scnsitivity of the probe
liquid to the underlying substrate.' as well as the structures becoming incrcasingly
disordered with lower packing density and coverages in small chah length molecu l
For shorter chains, the film is Iess ordered and dense for hck of cohesive interactions. As
the c h a h length increases. the cohesive forces become suong enough to puil up the
molecular chains into 'normal' orientation. for optimal interaction energy." Studies of
long-chain alkanethiols adsorbed on gold and silver surfaces showed similar re~ults,~%.nd
the same arguments apply to the lowering of CA,,, of alkynyols from undecynol to
propynol (Table 5.1).
Table 5.1. Static contact angles of water (CAm,), theoretical (TJ and ellipsomevic
thicknesses (T,), and FTIR-ATR data of alcohol thin films self-assembled on
Si(100) substrates by the two-step proces
Thin Fiim on CA, Tt. A Si(1ûû) (Si) WO)
1 1
OTS I l 1 0 1 2 6
-OCH,CSH 60 9
- O ( C H J p e H 10
-O(CH,),CCH 85 1 1
-O(CHZ),CSH 5W) 13
-O(CH,),CSH 90 19
clean Si wafer 10
FTIR- ATR
v,(CH2) cm"
2920,2850
2918, 2849
2921,2851
2922,2853
m- ATR
v(C,HJ cm-'
Thin Film
Figure 5.1. Static contact anges of water for monolayers of octadecyltrichlorosilane (OTS), octadecmol (ODA). hexadecanol (HDA), tcmdecanol (T'DA). decanol (DA), and hexanol (HA) on Si(1ûû) substrates.
The contact angles of water and hexadecane from our thin tilms terminated with
phenyl and acetylene groups are comparable to those obtained from thio-alkynyl and thio-
aromatic thin films on g~ld. '~." but lower than the molecuiarly self-assembled long chah
alkanes. The lower contact angles of water (70-90") and hexadecane (-15') on the phenyl
or acetylene tenninated surfaces than the corresponding long alkyl chahs (paraffins, ca.
1 L O O (water) and 35-400 (hexadecane)) may be due to a number of factors including the fact
that the -Cc-H (sp hybridization) or -C,H, (sp' hybridization) groups adhere mon:
suongiy to waler than CH, (sp3) groups.14 and the introduction of any surface functionality
into the alkane chain reduces monolayer order." It has alsu been suggested that in paraffm
tilms. the hydrogen atoms f o m a protective coating preventing atiractive forces in a highly
polar inorganic oxide surface from contributing to the spread of wdter drops." As in the
case of alkane thin films, the contact angle with water for the monolayers teminated with
alkynyl groups increases as the chain 1engt.h increases (Figure 5.2). and this supgests that
Oie thin films become increasingly ordered and tightly packed as the chah Iength is
increased.
Ellipsomeuy is a commonly employed technique to measun: thickness of the newly
developed thin films. A detailed evaluation of this technique has b e n discussed
else~hcre. '~ . '~ However, to calculate the thickness of an interface. one needs to compare
data from i he same r?ibstrate kfore and &ter functionali:ation. The thickness of thc SiOl
layer introduced from the reaction of aminosilane with surface hydroxyl groups and
physisorbed molecular amounts of water was measured for background subtnction. and an
assumption was made for the refractive index of the organic phase. For long chah alkanes,
a typical value of the refractive index employed for calculating thickness is in the range of
1.45 - 1 .SO.'~ For the self-assemblai organic thin Fiirns reportcd in this study, we used a
value of 1.46. The typical values of bond lengths between elements projected on the surîacc
were used to obtain theoretical thicknesses of such rnolecules.31 For a uans-extended
chain. the projection of the C-C bond onto the surface normal (z mis) is 1.26 a. and for the
C-Si and Si-O bonds. the projections are 1.52 and 1.33 A. mspectively. Including an
additional 1.92 A for the terminal methyl group. we expect a monolayer prepared from
occadecanol to have a thickness of -26 A. As shown in Table 5.1. the eiiipsometric
thicknesses for our monolayers provide strong evidence for the formation of a fiim one
rnolecular layer in thickness. Upon comparing these values with the calculated thicknesses.
the data points to the presence of a densely packed array of chains with a tilt. The
ellipsometry data here probably at k s t pmvide a qualitative indication chat monolayers of
alcohols with different terminating groups are k i n g self-assembled on Si/SiO, with a
certain orientation. similar to those of thiols on gold and aikyltrichlorosilanes on Si/Si02.
'The p& positions of the frequencies for v,(CHI) and v,(CH.) modes in the FlTR-
ATR spech provide insight into the intrmolecular environment or the akyl chains in ihin
film assernb~ies.~ They are also ideal for the alkynyl and phenyl ieminated thin films due to
the minimal overlap of their absorption bands with those of other modes. Previous FT-IR
studies have shown that the location of these peaks are sensitive indicators of the extent of
the lateral interactions between long n-alkyl chains." A typical spectrai patem observed for
an ordered hydrocarbon assembly is that of the p d c positions for asyrnmetnc and
symmetric CH, suetching frequencies at -2920 and -2850 cm". The latter are characteristic
of a closely packed hydrocarbon cnvironment." and occur at -2926 and -2856 cm" in
liquid-iike disordered ~ h a i n s . ~ ' The v,(CH,) and v,(CH2) stretching frequcncies of thc
octadecanol SAM on the Si(100) sudace wcre observed at 2918 and 2849 cm". The= an:
consistent with previously publishcd resulis for octadecyluichlorosilane on silica bascd
surfaces." and octadecanethiol on gold." In addition. the monolayer of octadecanol
showed film thickness and static contact angle of water (Table 5.1 ) reasonably in accord
with prcviously published results for those weii characarizd OTS films.'3 These results
suggest that the octadecanol SAM is tightly packed and relatively well-ordercd.
Propynol Butynol Pentynol HexynolUndecynol
Figure 5.2. Static contact angles of water for monolayers of propynol. butynol. pentynol. hexynol. and undecynol on Si(1ûû) substrates.
As the chah length decrcases from -(CH,),,-CH, to -(CH2),-CH,. the vJCH,) and
v,(CHJ stretching frequencies in the FIIR-ATR spectra of these thin films movc to highcr
wave numbers (from 2918. 2849 cm" for -(CH2),,-CH, to 2924. 2856 cm" for -(CH2),-
CH,). A similar trend was observed for hydroçarbon thin films prcpared tiom
trichlorosilanes on siiica and thiols on go~d, '~ and suggests that the structure of the
mono la yen becomes increasing ly disordered and Liquid like as the chah length dccreases,
possibly duc to lower packing density and chromophore coverage.
The peak positions in the FRR-ATR spectra of the phenyl and acetyiene teminated
SAMs (Table 5.1) were observed at ca. 3 3 0 , 3000 (1500). 2 100. 2920 (2850) cm". and
car. bc assigned to acctylenc hydrogen. phenyl, acetylene and methylene groups.
respectively." The peak positions of phenyl and acetylene groups in the monolaycrs on
substrates are slightly shifted from the reference spectra obtained frorn a KBr pcilct of the
pure compound; however. they are similar to those reçently reported for thio-aromatic and
thio-alkynyl films on gold.'0-'5 The slight shifts in Peak positions on the surface can be
atlnbuted to orientation effects in the SAMS." A broad band in the l<WX)-IL50 cm-' region
is assigncd to vibrations of the siloxane (-Si-O-Si-) bridge and of Si-O- surfacc
The v,(CH,) and v,(CH,) stretching frequencies for the alkyne tenninated alcohols arc
shown in Figure 5.3, and fa11 in the region 2920-2921 and 2850-2853 cm*'. respcctively.
Figure 5.3. FTIR-ATR (nonpoluized) spectra for monolayers of undecynol (A). hexynol
(B). pencynol (C). butynol (D) and propynol (E) on Si(100) substrates in the region 2800-3000 cm-'.
5.4 Three-S tep vs Two-Step Deposition Process
The wetting characteristics. eUipsometric thicknesses and v,(CH,) and v,(CH,)
frequencies of the thin frlms prepared from alkanols by a three step process are s h o w in
Table 5.2. A simiiar trend of decreasing contact angles of water and incrcasing CH2
frequencies as in the thin fiims prepared using the two-step process. is observed as the
chain length decreases. The contact angle with water from an octadecanol SAM prepared
using the three step process (Scheme 5.1) is lower (105') han the one prepared using the
two sep process ( 1 1 0°) as well as rhat from OTS SAMs on silica surfaces ( 1 10"). The
ellipsometric thickness for the octadecanol monolayer prepared tiom the three-step process
is approximateiy 2A smder than the one from the two-step process. Simiiarly, the
asymmetric and syrnmetric stretching frequencies of the rnethylene groups at 2920 and
285 1 cm" arc slightiy higher than the SAM prepared using the two-step process. These
results suggest that the film is not as densely packed as that from thc two-sep proçcss. and
there may be some disorder in the structure. The latter may have been introduced during thc
multistep deposition process (Scheme 5.1)- The reaction of SiCl, with surface hydroxyl
groups may lead to poor surface coverage. and here may be incomplete conversion of the
r-esulti.int [Si]-CI moieties to [Si]-NE<, upon reacthg with NEkH. It is evident then that the
quaiity of the thin fiims can bc substantialiy enhanced by using the two-step process which
climinates thc m û n e n t fust with SiCl, followed by NELH LO produce surfiîcc anchored
arnino groups. and employs Si(NEt,), directly. As a whole, the two-step process produces
more ordered SAi is than the three-step proçess.
Table 5.2. Static contact angles of water (CA,,), thcotetical (Tt) and ellipsomctric
thicknesses (T,), and FITR-ATR data of alcohol thin films self-assembled on
Si(100) substrates by the three-step process
Thin Film on Si(lO0) CAw,, 1 1 Tt 1 Tc. A ( FTIR-ATR
5.5 Stability of SAMs
The stabilities of the thin films prepared by our new acid-base hydrolytic approach
werc tested under a variety of conditions including temperaturc. organic solvents. acid.
base and water. The experiments wcrc monitored by contact angle goniometry, eilipsometry
and FTIR-ATR, and the results are prcsented in Table 5.3 and Figure 5.4.
Upon repeated washing with water at room tcmperaturc and wiping off the rnoisture
with a Kimwipe, the contact angle of a monolayer of octadccanol with water was lowered
by about 5'. but remained stable tiom thereon. The ellipsometric thickness did not change
much (24 A). and the asymmelnc and syrnmetric rnethylenc smtches wcrc only slightly
shifted to higher frequencies (2920, 2850 cm"). When the thin film was placed in boiling
water. the contact angle with water decreased to 55'. the thickness reduced to 13 A, and the
v,(CH2) and vS(CHZ) frequencies werc increascd to broad pcaks at 2923 and 2850 cm".
These results suggest that the thin film has been hydrolyzed. The hydrolysis of the thin film
under these conditions c m be undentood by considering the nature of Si-ioelement bond
at the interfaces, and the Si-OR bond is expected to be more sensitive to hydrolysis chan the
Si-CH$ bond in the monolayers of octadecyluichlorosilane (OTS). The marnent" of a
monolayer of OTS on glass with boiling water for 1 h. lai to a decrease in ellipsomevic
thickness to 23 A from an initiai value of 25 A, as well as an increase in the v,(CHZ)
suetchhg frequency to 29 19.1 cm'' (initially at 29 17.6 cm").
Table 53. Results of the stability tests on the octadecanol thin film on a Si(100) substraie
Aq. 10% 2.5M H,SOJ908 55
dioxane, 25 OC, 1 h, sonicate
CHCI, 25 OC, 1 h, sonicate 110
Boiling CHCI,, 2h 70
CH30H 25 OC. I h, sonicate 1 05
Boiling CH30H, Sh 105
Aq. 10% 1M NH,OH/W% 30
Figure 5.4. FTIR-ATR (nonpoluized) spectra for monolayers of -O-(CH&-CH, on
Si( 100) substrates in the region 280-3000 cm-' : A. before any marnent; B, after heating at 150 O C for 1 h; C. ueatrnent with aqueous 2.5 M H,SO,, 25
O C . 1 h; D. boilinp CHCI,, 2 h; E. boilinp methanol, 2 h: F. aqueous 1M
NH,OH, 25 O C . lh.
The oçtadecanol SAM was found to be stable at room temperature for long periods of
timc. However. when it was heated to 150 O C for lh. the contact angle decreased to 100".
the thickness to 2 1 A. and the v,(CH2) and v,(CH2) stretching fquencies were increased to
2921 and 2850 cm-' (Figure 5.4 B). When an octadecanol SAM was sonicated in methanol
for 1 h, the contact angle with water was found to be s t U relatively high at 105'. but the
thickness was reduced to 15 A. Sirnilarly. when it was placed in boiling methanol for 2 h.
the contact angle decreased only to 105O. but the lhickness was reduced to 10 A. and the
v,(CH2) and v,(CH,) stretching frequencies increased to 2923 and 2852 cm-' (Figure 5.4
E). These rirsults suggest that the oçtadecanol thin t - h is solvolysed in methanol, The
Iarger contact angles upon hydrolysis in methanol could possibly bc èxplained by
considering the conversion of some of the [Si]-O-(CH,),,-CH, to [Si]-O-CH, on the
surface. The monolayers exposing methy 1 groups to the surface. [S iO] 2-Si(CH3), and [Si]-
O-Si(CH,), give contact angles with water of - lûûO.
Ln a sirnilar expenment, an octadecanol SAM was placed in boiling chlorofonn for 2
h. and the contact angle with water was reduced to 70'. the thickness to 13 A. and the
v,(CHI) and v,(CH,) stretching lrequencies weni increased to 2923. 2851 cm*' (broad
peaks. Figure 5.4 D). A sirnilar behavior for the monolayers prepared using long chah
thiols on gold and carboxylic acids on silver. has been obscrved p rcv ious~y . "~ .~~ For
example. a monolayer of octadecanethiol on gold gives a thickncss of 28 a. and v,(CH,)
stretching frcquency at 2917.9 cm". Upon maiment with boiling chloroform for 0.5 h. the
thic kness decreases to 20 A, and the v,(CHJ lrequenc y increases to 2920.8."
The octadecanol monolayers were fourid to be highly unstablc in 10% aqueous 2.5 M
H,SO, or 1M NH,OH in a 90% dioxane by volume. The films wen: significantiy damagcd
atier 30-60 min in boiling chloroform, and 1 h in boiling acid or base. The surfaces on
these films were found to be visibly etched by boiling in 90% dioxane/lm aqueous 1M
NH,OH. Under these conditions. the stability of the octadecanol thin film is similar to that
of SAMs prepared using traditional self-assembly routes." It has been reported lhat the
monolayers of OTS on glas and octadecanethiol on gold are also badly damaged. and the
surfaces pitted in boiling 90% dioxane/lû% aqueous IM NH,OH mixture."
5.6 Conclusion
Self-assem bled monolayers of a variety of organic c hromop hores terminated with OH
groups on Si(100) (Si/SiOJ surfaces have been succrssfully prcpared by a simple acid-
base hydrolytic chemistry route. The two-step process involving the reaction of surface
hydroxyl groups first with Si(NEt,), followed by ROH. is more efficient than the three-step
process involving the reaction of SiCl,. NEt,H and ROH in sequence. and produces
closely-packed and well ordered thin films. The new simple acid-base hydrolytic chemistry
two-step route. is able LO produçe monolayers of sirnilar quality as the traditional self-
assembly routes such as deposition of thiols on gold and alkylrrichlorosilancs on inorganic
oxides. When placed in hot water and oqanic solvents. these films are more susceptible to
hydrolysis than the corresponding OTS monolayers on silica. as expected: however. they
show comparable stabilities ar arnbient and high tempemures, and upon treatment with acid
and base.
5.7 Experimental Section
5.7.1 Materials
Ociadecyltrichlorose (OTS). HO(CH,),CH, (x = 5 , 7. 9. 1 1. 13. 15 and 17).
HOC6H,. HOC,H&HS, HOCH,C-CH. HO(CH,),C=CH and HO(CH,),C=CH were
purchased in high purity frorn Aldrich. and used as received. HO(CH2),C6Hs.
HO(CHZ),CXH and HO(CH,),CgH were purchaxd
HOC,H,C<C,H, was synthesized by modifications of pub
from ChemSamp Co. Inc.
was verif id by routine characterizarion methods. Solvents were dried and distilled from
appropriate reagents: tolwne over sodium: diethylamine over potassium hydroxide. Single
side polished Si(100) substrates were purchased from Nova Electronic Materials.
HOC,H,C*C,H, 1.8 g (8.1 mmol) of HOC6H,I was dissolved in 50 ml of dry NEbH.
and L ml of HwC6H, (8.9 mmol), 0.1 g of Pd(PPhl)ZCl, and 0.03 g of CUI were added.
The reaction mixture was stirred at room temperature undcr nitrogen overnipnt. Solvent
was removed under vacuum, and yellow crystals were obuined by extraction with hot
hexane. After recrystallization from hexane, 0.5 g of the compound was obtained in 32%
yield; Mass Spec (Ei): 194. IR (KBr) u (OH): 3419 cm", u (C,H,): 3076. 3053. 3018.
15 12 cm'', u (CS) : 2221cm-'; 'H NMR (200 MHz. CDCI,) 8 ppm 6.82. 7.34. 7.44.
7.51 (m. 9H, Ph), 1.90 (s, 1H. OH).
5.7.2 Substrate Preparation
Glass, quartz and single crystai silicon substrates were first cleaned (i) by soaking in
soap solution and soriocating for 1 h; (ii) rcpeaied washing with deionixd water: (iii)
trcatment with a solution mixture containing 70% conc. H,SO, and 30% H 2 0 2 (piranha
solution) at 100 O C for 1 h. Caution: Piranha solution is highly explosive. and c m should
be taken while using this mixture; (iv) repeated washings with deionized water; and (v)
finally heating in ovcn at 150 OC for 5 min and vacuum drying for 5 min-
5.7.3 Two-step Deposition Process
The clean siiicon wafers were treated with (i) 0.5 % solution by volume of Si(NEtJ,.
which can be conveniently preparedI6 by the reaction of SiCl, and excess dry NE&H. in dry
toluene for 8 h at room temperature; followed by (ii) ROH in dry toluene at 50 OC for 24 h.
5.7.4 Three-step Deposi tion Process
The general synthetic strategy involved the treatrncnt of clean subsu-atcs with (i) 0.5
55 solution by volume of SiCl, in dry toluene for 18 hr at room temperature, (ii) 0.5 5%
solution by volume of dry NE&H in dry toluene for 18 h at 70 OC, followed by (iii) ROH in
dry toluene at 50 OC for upto 24 h,
5.7.5 Contact-Angle (CA) Measurements
The static and advancing contact angles were measured with a Rame-Hart NRL 100
goniorneter. On average, 6 drops of water and hexadecane were measured on different
areas of the polished side of a silicon watèr for each sample, and the values reported are the
mean values with a maximum range of Ho. The advancing contact angles of captive drops
were found to be roughly 5' above the static values of scssile (free-standing) drops. If the
drop was alloweû to fd i from h e needle of the syringe to the surface, smdcr contact
angles were usually obtained kcausc of the mecha iical vibrations."
5 . 7 -6 Fourier Transform Infrared Spectroscopy in the Attenuated Total
Reflection Mode (FTIR-ATR)
The organic thin films were grown on the <100> surfaces of the singie side polished
silicon wafers. A KRS crystal was sandwiched between the reflective faces of two silicon
waîers (1.2 x 4.0 cm), and the angle of incident light was set at 45'. AU spectra were run
for 4000 sans at a resolution of 4 cm'', using a Bruker ES-48 spectrometrr. A spectrum
of two clean silicon wafers with a sandwiched KRS crystal was measured as a background
corrzction.
5.7.7 Ellipsometry
A Gaertner Scienmc ellipsomeier equipped with a 633 nm He-Ne laser (A = 6328 A)
was employed. The angle of incidence was set at 70.0°. and the compensator angle ai
45 .O0. Al1 reported values with a maximum range of El A are the average of at least six
measurernents taken at different locations on the sample. The thickness was calculated by
comparing data from the sarne substrate belote and after functionûLization and using a vaiue
of 1.46 for the refractive index. This value is based on the assumption that the monolayer is
similar to b u k p d f m s with a refractive index of 1-45." If the monolayer is more
crystalline-like, similar to polyethylene, the refractive index thus should be within 1.49-
1.55." It was found that an increase of 0.1 in the refractive index from 1.45 to 1.55
resulted in a decrease in the measured thickness by -2
5.7.8 X-ray Photoelectron Spectroscopy (XPS)
The XPS spectra were obtained by using a VG Escalab M M specuometer with
monochromatized MgK,X-ray source to produce the photoemission of electrons from the
core levels of the surface atoms. About 50 A of depth was pmbed for a detector
perpendicular to the surface. Thc analyzed surface was 2 x 3 mm. AU pe& positions were
corrected for carbon at 285.0 eV in binding energy to adjust for charging efl'ccts. The
power of the source was 300 watts and a pressure of 10" mbar.
Binding Energies (eV): Ociadecyllrichlorosilane: C. 1s 285.0: 0. 1 s 532.0: Si,, 98.6.
102.6. (Si)-O-(CH,),,CH,: C, 1s 285.0; 0, 1s 533.1; Si, 99.0, 104.0; (Si}-O-
(CH,J5CH,: C, 1s 285.0; 0, Is 533.2; Si2, 99.1. 103.8; (Si}-O-C6Hs: C. 1s 285.0.
29 1.0; 0. 1s 533.9; Sizp 98.8. 103.5; (Si}-O-(CH2),-C6Hs: C. 1s 285.0. 291.1; 0. 1s
532.9; Si,, 99.2, 103.7; (Si}-O-C6H,-C6H,: C. 1s 285.0, 291.1; O. 1s 533.2: Si2, 99.3,
104.1; { Si)-O-C6H4-C-=C-C6Hs: C. IS 285.0. 291.1; O. IS 532.9; Si2, 98.7, 103.6; (Si}-
285.0; 0, 1s 532.9; Si,, 98.7. 103.2: (Si}-O-(CH,),-Cg-H: C. 1s 285.0; 0. 1s 533.1 ;
Si2, 99.3, 103.8; {Si}-O-(CH,),-Cg-H: C. 1s 285.0: 0, 1s 532.7; Si,, 99.2. 103.5;
{Si } -O-(CH,),-Cg-H: C, 1s 285.0; 0. 1s 53 1.1 ; Si., 99.2. 104.0.
5.8 References
(1 ) Wu, C. G.; Chen, J. Y. Chem Murer. 1997, 9, 399.
(2) Nuzzo. R. G.; Allara, D. L, J. Am, Chem. Soc. 1983, 105, 4481.
(3) (a) Jackman, R- J.; Wilbur, J. L,; Whitesides, G. M. Science 1995,269, 664. (b)
Chidsey, C. E. D. Science 1991,251, 9 19.
(4) Sagiv, J. J- Am. Chem. Soc. 1980, 102, 92.
(5) Bain. C. D.; Troughton, E. B.; Tao, Y.; Evall, J.; Whitesides, G. M.; NU~ZO. R. G.
J. Am. Chem. Soc. 1989, 111, 321.
( 6 ) Porter. M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C- E. D. J. Am. Chem- Soc.
1987,109. 3559,
(7) Walczak, M. M.; Chung, C.; Stole. S. M.; Widrig, C. A.; Porter, M. D. J. Am.
Chem. Soc. 1991,113, 2370.
(8) Laibinis, P, E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.; Parikh, A. N.; Nuno, R.
G . J. Am, Chem. Soc. 1991.113. 7152.
(9) Troughton, E. B.; Bain. C. D.; Whitesides, G. M.; Nuzzo. R. G.; Allara. D. L.:
Porter. M. D. Langmuir 1988 4 , 365.
( 10) Nuzzo, R, G.; Fusco, F. A,; Allara. D. L, JI Am. Chem, Soc. 1987, 109, 2358.
(1 1) (a) Tao, Y. J. Am. Chem, Soc. 1993. 115, 4350. (b) Schlottcr, N. E.; Porter. M.
D.; Bright. T. B.; Allara, D. L. Chern. Ph.. LRtts. 1986, 132. 93.
(12) Linford, Mm R.; Fenter, P.; Eisenberger. P. M.; Chidscy, C. E. D. J. A m Chem
Soc- 1995.11 7 , 3 145.
(13) Silberzan, P.; Leger. L.; Ausserre, D.: Benattar, J. Langmuir 1991. 7. 1647.
(14) (a) Yam, C. M.; Kakkar, A. K. J. Chem. Soc., Chem Commun- 1995.907- (b)
Yam, C. M.; Dickie, A.; Malkhasian, A,; Kakkar, A. K.; Whitehead, M. A. Can. J.
Chem. 1998. 76, 1766.
(15) Fessenden, R.; Fessenden, J. S. Chem. Rev. l % l , 6 I , 36 1
(16) Anderson, H. H. J. Am. Chem. Soc. 1952, 74, 1421.
(17) Wasseman, S. R.; Tao, Y-; Whitesides, G. M. Langmuir 1989, 5. 1074.
(18) Evans, S. D.; Sharrna, R.; Ulman. A. Langmuir 1931, 7. 156-
(19) Tao. Y.; Chang, S.; Ma, L. J- Chin. Chem Soc. 1Y95.42, 659.
(20) Dhirani, A. A.; Zehner, R. W.; Hsung. R. P.; Sionnest, P. G.; Sita, L. R. J. Am.
Chern. Soc. 1996,118, 3319.
(2 1) Frydman. E.; Cohen, H.; Maoz, R.; Sagiv, J. Lungrnuir 1997. 13. 5089.
(22) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S,; Whitesides G- M.;
Nuzzo, R. G . Langmuir 1995. 11. 8 13.
(23) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988.
110,6136.
(24) Linford. M. R.; Chidsey, C . E. D. J. Am. Chem Soc. 1993, 115. 12631.
(25) Sabatini, E.; Moulakia, J. C.; Brueninp. M.; Rubinstein, 1. Langmuir 1993. 9.
2974.
(26) Ulman, A. Chem. Rev. 1996, 96, 1533.
(27) Adam. N.K. Contact Angle. Wenubiliy and Adhesion. Advances in Chernistq
Series; ACS: 1964, Vol. 43, p.52.
(28) Azzarn. R.M.A.; Bashara N.M. Ellipsometry and Polari~ed Light; Nonh-Holl~d
Publishing Company: Amsterdam. 1977.
(79) Ulman, A. An Introduction to Ultrathin Filmfrom Lungmuir-Blodgett to Self-
Assernbly, Academic Press: Bos ton. 199 1 .
(30) Wasserman. S. R.; Whitesides. G. M.; Tidswell. 1. M.; Ocko, B. M.; Pershan. P.
S.; Axe, J. D, J. Am. Chem. Soc. 1989.111, 5852.
(31) Dean. J.A. Longe's Handbook of Chemis-; McGnw-Hill: New York. 1992.
(32) (a) Synder. R. G.; Strauss. H. L.; Elliger, C. A. y. Phys. Chem. 1982. 86, 5145.
(b) Synder, R. G.; Maroncelli, M.: Strauss. H. L.; Hallmark. V. M. J- Phys. Chem.
1986, 90. 5623.
(33) Tour. J. M.; Jones. L., II; Pearson. D. L.; Lamba. J. I. S.: Burgin. T. P.;
Whitesides. G. M.; Allara D. L-; Parikh, A. N.; Aue, S. V. A m Chem. Soc.
1995, 1 17, 9529.
(34) Maoz. R.; Sagiv J. l. Colloid lnte>face Sci. 1984. 100. 465.
(35) Wityak. J.; Chan. J. B . Sythet ic Commun, 1991. 21, 977.
Chapter Six
Molecular Self-Assembly of Dihydroxy Terminated Molecules
via Acid-Base Hydrolytic Chemistry on Inorganic Oxide
Surfaces: Step-by-Step Multilayered Thin Film Construction
6.1 Introduction
Self-organization of molecules on interfaces is an intriguing and highly promising
approach to construct ordered and structurally stable thin films. and offers trernendous
opportunities in the fabrication of advanced materials for a variety of applications.' Much
effort has been devoted to the deposition and full charactcrization of monohyers of
organosilicon compounds on hydroxylated surfaces' such as silica. alumina etc..
alkancihiols on g~ld.~^' silver.' and copper.6 dialkyl sulfides on gold.' dialkyl disulfidcs on
gold.~arûoxylic acids on silver and aluminurn oxide.' and 1-alkenss on hydropn
terminated Si(l1 l)? It has b e n demonstrated that due to suong intermolecular van der
Waals forces of attraction. such monolayers an: densely packed and highly ordercd.ll-"
Howcver. thin f h based technology requires the construction of multilaycrcd interfaces
with rnoicculcs that contain useful moieties in the backbonc. To preparc multilayered
supramolecular structures. the surfacc of a monolayer must bc moditied so that another
layer could be adsorbed on top. In silica based chernistry. the tcrminal group must be
converied to a hydroxyl group. and the latter cm be achieved by h e reduction of a surfacc
ester group." and hydrobontion-oudation of the terminal double bond." Once a
subxqucnt monolayer is adsorbed on the "activated" monolayer. multilayered films may bc
built by repctition of this proccss.
The preparation methodologies for building mullilayered structures have limitations
associatcd wi th conmlling film thickness and individual laycr com positions of the resulting
thin film assemblies. The design requiremenis for the construction of covalentiy bound
multilayer assemblies on inorganic oxide surfaces, require that an effective synthetic
methodology be developed for the quantitative creation of ihe hydroxylated self-assembled
monolayer (SAM) surface using bifunctional molecules, and an efficient rcûction sequence
of the surface with bifunctional molecules under miid reaction condition^.'^
We have discussed a new syntheuc route to molecular sclf-assembly based on the
hydrolysis of surface anchored aminosilanes with molecules containing terminal OH groups
in Chapter 4. Using this r n e t h o d ~ l o g ~ , ~ ~ silica surfaces can be modified with a variety of
SAMs. We were intrigued by the possibility of building multilayercd assemblies in a layer-
by-layer fashion using bifunctional chromophores. In this chapter, we discuss a two-step
thin film construction process involving the reaction of S~(NEL), wilh hydroxyl groups of
the inorganic oxides surfaces, foiiowed by the reaction with dihydroxy chromophores
incorporating acetylene, diacetylene and aromatic moieties in the backbone. Upon repetitive
reactions with Si(NEtJ, followed by R(OH)? multiiayered thin film assemblies were
fabncated on glass, quartz, and single crystal Si. The characterization of the mono- and
multilayercd thin films was carried out using surface wettability measurements.
clIipsometry. FT-IR, UV-Vis and X-ray photoelectronic spectroscopies. It has btxn
suggcsted that the quality of the rnonolayers obtained 'rom trichlorosilanes, o r dcrivativcs
then: of. rapidly degrades as the thickness of the rilm increascs. Our resulis indicati: chat
thin films of appropriate thicknesses can be adsorbed using acid-base hydrolytic chcmisiry
and dihydroxy terminated chromophores, for considerably more than a fcw layers, without
losing ordcr.
An understanding of the factors govcrning stability of sclf-assembled supramolecular
structures is just beginning to emerge. In Chapter 5, we have observed that SAMs prepared
using our acid-base hydrolytic approach may be susceptible to hydrolysis in boiling watcr
due to the higher sensitivity of the Si-OR bond to water than the Si-C bond in SAMs
prepared from uichlorosilanes. From a detailed stability study under varied conditions, out
results suggest that the multilayer thin film construction enhances stability of the thin films-
We have dso investigated topochernical polymerization of the diacetylcnic moieties in the
mono- and multilayered assemblies. Upon exposure of the diacetylenic thin films to UV-
light. the formation of a blue film was observed.
6.2 Acid-Base Hydrolysis
Aminosilanes (Me,Si-NR,. R = Me. Et). which cm bc easily prepared from their
corresponding chlorosilanes and amines in qumtitativc yields. undergo acid-base
hydrolysis with molecules containing a terminal OH group. to yield the silylated a l c o h ~ l s . ~ ~
We have recenrly demonsuated a similar behavior of thc surface-immobilized Si-NEs
groups towards molecules containing terminal OH groups and varied backbones. lcading to
well ordered thin f i s based on simple acid-base hydrolytic ~hernistry.'~" The surfaces of
glass, quartz and single crystal Si(Si0,) contain hydroxyl groups. The aminosilanes. which
are much more basic than alkoxysilanes, undergo a facile acid-base hydrolytic rcaction with
silica based surfaces and molecules containing terminal OH groups. The simplicity and
versatility of this approach prompted us to consuuct mono- and multilayered
suprarnolecular structures using bifunctionai chromophorcs (HO-R-OH) with desired
chernical framework i and Si(NEQ,, in a two-step reaction rncthodology.
6.3 Monolayers of Diols
The tetra(diethylamino)silane, Si(NEs), was prcpared" and isolated in cxceilent
yields by reacting SiCl, with excess NE-H. It was reacted with dean single crystal silicon
(SiOJ substrates to yield surface anchored Si-NEt, moicties, followed by their waction
with a scnes of dihydroxy chromophores of rigid-rod type. llexiblc or a mixture thereof.
backbones (Scheme 6.1). 5.7-Dodecadiyne- 1.12-di01 was prepared via CuCUO, catalyzed
coupling of teminal alkync.19 Oiher dihydroxy chromophores were synthesized Ma
palladiurn/copper cataiyzed coupiing of hydroxyaryl halides with terminal a l k y i e ~ . ~ ~
The monolayers were chanctenzed by surface wettability measurements.
ellipsometry. XPS. and FTIR-ATR specuoscopy. These are commonly used chin f h
characterization techniques.21-23 and provide information about the evolution of the
structure at interfaces. Ihe data obtained from the thin films on Si(100) substrates axe
presented in Tables 6.1 and 6.2. The contact angles of water for the thin fdms from the
dihydroxy chromophores were found to be in the range of 1 5-60', and are similar to those
for the SAMs from HOC,,Si/Si" and HOC,,SWAU." The contact angles of water from the
thin films prepared from dihydroxy molecules containing only phenyl. biphenyl and
phenylacetylene type backbones (15-35O) suggest that there is no looping of the terminal
hydroxy groups toward the surface due to rigidity of their structures. since the thin Films
will then become more hydrophobic by exposing phenyl o r acetylene groups to thc surface.
The molecules containing CH2 groups in the backbone gave contact angles with water
(50-60") that were higher than those obiained frorn thin films containing purely rigid-rod
backbones. It is possible that surface reorganization happens more aasily in these films. and
the monolayers crsûucture and minirnize free anergy of the system by burying the hydroxyl
groups as much as possible, given the restrictions irnposed by covalent bonding to the
surface, and thus expose CH, groups to the surface. ""' Such "surfacc reconstruction" has
also been postulated for the oxidimd polyethylene thin films. the surfaces of which con k t
largcly of cxposed carboxylic acid groups with small amounts of ketones and possibly
aldchydc g r o ~ p s . ' ~
Static contact angles of walrir (CA,,), theoretical (Tt) and ellipsometric (T,) thicknesses, and XPS data for SAMs prepared from dihydroxy terminated
molecules on Si(lO0) substrates.
Thin Film {Si} XPS (C 1s. O 1s. Si-Si 2p. Si-O
2p) 285.0, 532.0, 98.6, 102.6
285.0, 291-1, 533.5. 98.8, 104.2
285.0. 29 1-2, 532-9, 99.1. 103-9
285.0, 533.2. 99.0. 103.7
533.2. 98.9, 103.3
124
Table 6.2. FïiR-ATR data for SAMs prepared fiom dihydroxy wrrnùiated molecules on
Thin Film (Si) "on; 1 cm- Va (CH,). v CS' "si09 v(C,H,). cm" cm v, (CH.). cm'' cm-'
29 18, 2849 1 089
EllipsometrJ6" is commonly used to measure thickness of the newly developed thin
films, and in general, one compares data from the samri substratc beforc and aftcr
functionalization. The thickness of the S i 0 layer is measured for background subtraction.
and an assumption W made for the refractive index of the organic phase. For long chah
alkanes. a typical value of the refractive index employed for calculating thickness is 1.45 -
1.50." and we used a similar value of 1.46. The typical values of bond length between
elements projected on the surface were used for obtaining theoretical thicknesses of such
mole~ules. '~ For a tram-excended chain. the projection of the C-C bond onto the surface
normal (z mis) is 1.26 A. For the Si-O and C-O bonds. the projections are 1.33. and 1.17
a, respectively. For the CH2-CS. C g - C S and C S bonds. the projections are 1.46.
1.38 and 1.18 A. respectively. including an additional 097 a for the teminal -OH group.
we expect a monolayer prepared [rom 2.4-hexadiyne- 1.6-di01 and 5.7-dodecadiyne- 1.12-
di01 to have a chickness of 14 and 22 A. As shown in Table 6.1. the ellipsomevic
thicknesses provide strong evidence for the formation of a film one molecular layer in
thickness, and suggest a densely packed array of chains with a slipht tilt, The ellipsomeiq
data gives a qualitative indication that monolayers of dihydroxy alco hols with different
backbones are being self-assembled on SilSi02, sirnilar to hose of HOC,,Si/Sin and
HOC,, S H / A ~ . ' ~
X-ray photoelectron spectroscopy is a well known technique io determine the surface
composition of the anchored species in the moleçuIuly self-asscmhled thin film.'6 We
cmployed b i s technique for the analytical evduation of the V ~ ~ O U S dihydroxy organic thin
films on silicon wafers. and the data presenied in Table 6.1 conlirms the composition of
the thin film structures. The survey spectra for these monolaycrs show only h e elcments:
silicon (2p. 99 eV. 2p. 103 cV). carbon (1s. 285 eV), and oxygen (1s. 533 eV). The
molecules containing conjugated backbones showed a peak at 29 1 eV for C,, corresponding
to. for example. aromatic carbon. These values arc comparable to those obtained from thin
Tilrns prepared using other methodologies, and serve to confimi the identity of the
rna te r i a l~ .~ '~~"
The peak positions of the frquencies for asymmetric and syrnmctric stretching of
methylene groups in the FIIR-ATR spectra can provide insight into the intertnolecular
environment of the backbone c h a h s in these assemblied They were selected for structural
interpretation owing to the minimal overlap of their absorption bands with those of other
modes. A typical spectral pattern for an ordered hydrocarbon assembly has peak
frequencies for asymmetric and symmetnc CH? stretching at -2918 and -2849 cm-'. The
latter arr= characteristic of a closely packed hydrocarbon environment. and at -2928 and
-2856 cm" for Liquid-like disordered ~hains."'~' The v,(CH,) and v,(CHJ stretching
frequencies of the dihydroxy SAMs with CH, groups in the rigid-rod backbones wcrc
observed at -2920 and -2850 cm". These are consistent with previously published resulis
for HOC,,S~/S~'' and HOC, ,S WAU." and suggests that these ihin films an: relatively well
packed.
The peak positions in the FTIR-ATR specua for the hydroxy. phenyl. and acetylenc
groups of the dihydroxy SAMs (Table 6.1 ) were observed at ca. 3300, 3000 ( 1500). and
2100 cm" respectively. Thc peak positions for the phenyl and acetylene groups in ihc
monolayers are slightly shifted from the reference specua obtained from a KBr pellet of ihc
pure çompound. but are similar to those recently reported for the thio-aromatic and thio-
alkynyl films on gold." These changes are atvibuted to orientational effects in SAMS." A
broad band in the 1050- 11M cm-' region is aitributable to vibrations of the siloxane (-Si-O-
Si-) bridge and of Si-O- surfâce bonds."
6.4 Multilayered Thin F i lm Assemblies of 2,4-Hexadiyne-1,6-di01 and
5,7 - Dodecadiyne- 1,12-diol
As discussed above. the dihydroxy cornpounds form SAMs exposing hydroxyl
groups to the surface, and by rcaçting these OH groups with Si(NEQ,. followed by the
reaction with the dihydroxy chromophores. the procrss c m be continued indetinitely. in
principle. to form multilayered structures. The potencial of our new acid-base hydrolpic
technique in the fabrication of mul t i iaye~d thin films. was enplored using bifunctional
chromophores. 2.4-hexadiyne- 1.6-di01 (1) and 5.7-dodecadiyne- 1.12-di01 (2). Af'r one
layer of 1 o r 2 was self-assembled on ihe substraie. it was cleaned by sonicating in THF
for 5 min. followed by drying in an oven at 120 OC for 5 min. A sep-by-step reaction
methodology (Scheme 6.2) of reacting the huer monolayer with Si(NEtJ,, followed by
the reaction with molecules 1 or 2 led to thin film assemblics of up to 10-layers.
The evolution of the multilayered structures was monitored by conmct angle
goniornetry. ellipsometry. FT-IR. XPS. etc. The thin fdm assemblies on silicon wafers
show a linear relationship between the film thickncss and thc number of layers (Figures
6.1 and 6.2). The linear regression line through the first five data points shows a dope o f
12.5 h y e r and 21 M a y e r for 1 and 2, respectively. and the latter are comparable to the
monolayer thicknesses- The ellipsornetric thickness of multilayered thin films were found to
be in the range of S A to S A of ihe calculated thickness (Tables 6.3 and 6.4). There
was an increaîe in thicknesdlayer from 1-layer sample (12 A) to the lû-layer samplr ( 15 Â)
for 1. and an increasc in LhicknessAayer from 1-layer sample (20 A) to 10-layer sample (24
A) for 2. This may be largely due to a more effective cleaning process which is employcd
for the monolayers on silicon substrates chan is possible for the multilayer samples." It is
probable that in the process of multiiayer formatic. I with Si(NE&), o r chromophore
~ a c t i o n . a small amount of loosely held material. eithcr by adsorption on top of ùic
monolaycr surfaces or inclusion in the rnonolayer bulk. is introduced." The peak for the
Si-O stretching frequency at -1 1 0 0 cm-' in the FT-IR spectrum was found to increarï: in
intensity upon multilayer deposition. Ir should be noted. however. that 15 and 24 A are
ccrtainly vcry reasonable thickness values for films with surface groups of 1 and 2.
respec tivcl y. Precision of h i c kness measurernents also tends to decfcase wi th incrcasi ng
number of layers. The unccrtainty on the rcadings of film thickness at various positions
across the face of the sample increases from ca. f 2 A Cor 1-layer samples to ca. f6Â for
10-laycr samples of both 1 and 2. These results art: similar to thc multiiayers of
HOC2,Si/Si prepared by continuing the adsorption-reduction sequence for monolayers of
MeO,CC,,Si/Si - -- and HOC,S i/Si."
For the thin fdms prepared from 1, t h e ~ is a gradua1 decrease in the static contact
angle of water from -35' for a monolayer to -20' for a 10 layered film. Similady. the
asymmeuic and syrnmetric stretching frequencies of methylenc groups on the 71h layer
decmased to 2922 and 2852 cm" from thc initial values of 2929 and 2859 cm-' rcspcctively.
in the 1st layer flable 6.3 and Figure 6.3). The contact angles of water and FTiR-ATR
CH, stretching frequencies reflect a gencral tendency to increasing order in the monotayers
with increasing layer number. This may be due to (i) the incrcasing X-lt interactions of the
diacetylene groups and (ii) the formation of an effective Si-O-Si cross-linking network
within each layer that results in better dignrnent of the chrornophorc on the surface.
For the thin films with the surface group of 2. the asymmetric and symmetric
stretching frquencies of methylene groups on the IOm layer also decreased to 2922 and
2852 cm'' from 2926 and 2856 cm-'. respectively. in the 1'' layer (Table 6.4 and Figun:
6.4). The FT-IR data, once again, points to increasing order in the thin tilms with
increasing layer number. T o substantiate these rcsults. we decided to cap the monolayer and
mullilayercd assemblies w i ~ h monolayers of OTS or C,,OH. Thc contact angles of watcr
from thesccapped i h films (Tables 6.3-6.4) were found to tx in thc range 110 - 1 1 5 O .
and the asymmetric and symmetric methylene stretching fqucnc ies wen: tound to bc 29 18
and 2850 cm", respectively. The terminal OTS/C,,OH rnonolayers gave the expected
thickness of 24 - 26A. These results suggest h a t Si(NE&), is highly efficient in cteating a
siloxane network that leads to close-packed multilayered thin films.
O 2 4 6 8 10 Layer number
6.1. Eiiipsomeuic thickness of the 1 to 10 layered thin fiIms of 2.4-hexadiyne- 1.6-
4 6 Lay er num ber
Figure 6.2. Ellipsomenic thichess of the 1 to 10 layered thin films of 5.7- dodecadiyne- 1.12-di01 (2).
Table 6.3. Static contact angles of water (CA,,,).
ATR data for the multilayers prepared Si( 100) substrates
ellipsometric thickncsses (Te) and FTIR- h m 2.4-hexadiyne- 1 -6-di01 (1) on
Num ber of CAtrzo, f1° layers of 1
1 + OTS
2929 (br). 2859 (br). 2 187, 2 134
29 18, 2849
2926 (br), 2864 (br). 2 156. 2 125
2925. 2860, 2176. 2125
2924,2858. 2176, 2 125
2924,2854, 2 177 , 2 135
2923,2854. 2190. 2165
2922, 2852. 2175, 2128
29 18, 2850
2923.2853, 2176 ,2144
2923,2853, 2181, 2133
2923,2854, 2 170 , 2 120
2918. 2850
7 + OTS
8
Y
10 I
10 + OTS
110 36t2
115
20
20
20
115
120 24
L 12+6
1 30E6
148 +6
172 k6
\ I 1 I I 3000 2975 2450 2325 2900 y5 2850 2825 2800
Waveriuaiber cm'
Figure 6.3. ETR-ATR (nonpolarized) specûa for the 1 to 10 layered (top to bottom) thin
films of 2,4-hexadiyne-1,6-di01 (1) on S i ( 1 0 ) substrates in the region 2800- 3000 cm".
Table 6.4. Static contact angles of water (CA,,). ellipsornetric thicknrsses (Tc) and FïJR-
ATR data for the multilayers prepared from 5.7-dodecadiyne- 1.12-di01 (2) on
Si( 100) Substrates
v,(CH2). v, (CH?), v(C=C). cm-'
Y
10
1 0 + OTS
55
60
115
2 12k6
24W6
265+6
2922, 2822, 2 174. 2 135
2922. 2852, 2 168. 2 129
29 18, 2850
saoo 2975 2950 2925 2900 q 5 mo 2825 mo UwerrarbQl cm-
Figure 6.4. =-ATR (nonpolarîzed) spectra for rhe 1 to 10 layered (top to bottom) thin
films of 5.7-dodecadiyne- 1.12-di01 (2) on Si(100) substrates in the region 2800-3000 cm-'-
6.5 Stability of Mono- a n d Multilayers
WC havc mcently r epor t~d '~" dctailed studies of thc stability of monolaycrs prcpared
from rnonofunctional hydroxyl-terminated chromophores using acid-base hydrolytic
chemistry under a variety of conditions. These monolayers were found to be susceptible to
hydrolysis in boiling water and chlorofom. acids and bases. We would like to look into the
possibility that the construction of multiiayered structures via a step-by-step deposition
melhodology might enhance the stability of the ihin films. Hencc. we examined the
behavior of mono- and mullilaycred thin films of 24-hexadiyne- 1.6-di01 (1) and 5.7-
dodecadiyne- 1.12-di01 (2) on Si( 100) subsuates. in hot organic solvents, watcr. acid.
base. and ai high tempemures. As shown in Table 6Sa, monolayers of 1 and 2 an:
hydrolyzed upon treaûnent with boiling chloroform, methanol and waier. Aqueous acids
and bases completely etch the films. The behavior of these S k M s arc: similar to thow
rcported earlier in the literature for long alkane chain assemblies."
The contact angles of water on hydrophobic surfaccs of long hydrocarbon chains and
CH, symmeuic and asymmetric frequencies in the FTIR-ATR spectra c m be used to
distinguish ordered and disordered interfaces. For examplc. a densely packed SAM of
octadccyltrichlorosilane on glas shows a contact angle of - 1 10" with water. and a liquid-
like disordercd film has a contact angle of 90-100". It is difficult to notice such sharp
changes in the thin films of small chromophores (1 and 2) exposing OH groups to surface.
Thus, to compare tllin tilm chancteristics using contact angles of water and FTIR-ATR. thc
mono- and multilayered thin films of ttic dihydroxy chrornophorcs wcre cappcd with a
SAM of octadecyltrichlorosilane (OTS). The results of thesc studies arc summarized in
Table 6.5b-d. The results of stability tests donc on a SAM of octadecanol arc also
included. As discussed earlier.16 upon treating a SAM of octadecanol with boiling water.
chloroform and methano1. the thickness and contact angle of water decrease, and the
asymmetric and symmelnc methylene suetches in the FTiR-ATR specua move to 2923 and
2850 cm" from the initial valucs of 29 18 and 2849 cm" (Table 6.5b). In contrasi, a SAM
of 1 capped with OTS is affected by these solvents to a lesxr extent. The thickness and
contact angle of water decrease only by 5 A and 10" rcspectively, after boiiing solvent
treatmenl and remain unchanged upon heating at 150 OC in an oven for 1 h. These rcsults
are similar to the stabiiity tests on a SAM of OTS on oxidired ~ i l i c o n . ~ These results
suggcst that a SAM of OTS acts as a protective coating for the dihydroxy monolaycr.
Tablc 6.5a. Results of the stability studies on SAMs of 2,4-hexadiyne- 1 -6-di01 (1) and
5-7-dodecadiyne- 1.12-di01 (2)
S tabili ty Test
1 Before anv treament
Heat. 150 OC, 1 hr
Boiling CHCI,, 1 hr
Boiling MeOH, 1 hr
Boiling Aq. 10% 1M
BoiIing Aq. 10% r- etched
etched
v,, v,. CA1120
cm-' 12"
2925, 2856 (br) 45
2925, 2856 (br) 45
v,. v,.
cm" +2"
2925,2855 (br) 60
2925.2855 (br) 2
T a b l e 6Sb. Results of the stability studies on a SAM of octadecanol and a thin Cdm of
2,4-hexadiyne- 1.6-di01 (1) capped with OTS
1 Heat, 150 OC, 1 hr
Boilin CHCl,, 1 hr + 1 Boiling water, 1 hr 13
etched
34
etched 1 Boiling Aq. 10% 1 M
Boiling Aq, 10%
As multilayers of 1 are constructed. their stabiLity towards boiling solvcnts incrcases
(Table 6 . 5 ~ ) . After the 4th layer, no damage by boiling solvents to the thin films was
observed. Thc thickness and contact angles of water in the multilayers werc not affcctcd
èven d ~ r 1 h immersion in the boiling solvcnts. Howcvcr, the thin lilms were vcry
sensitive to hot base. and even a IO layer assembly was visibly pitted. Similar results werc
obtained with mono- and multilayers of 2 capped with OTS (Table 6.5d).
139
Table 6 . 5 ~ Results of the stability studies on multiiayers of 2.4-hexadiyne- 1.6-di01 (1)
çapped with OTS
Thin films of 1 capped
with OTS on Si(100)
1 1 -1ayer + OTS CA,
2-layen +OTS CA,,
T,
' 3-layers + OTS CA,
T,
4-layers + OTS CA,,
Te
5-layers + OTS CA,
Te
6-layers + OTS CA,,
T,
7-layers+OTS CA,,
T,
8-lriyers + OTS CA,?,
Te
9-layers + OTS CA,,,
T,
IO-layers + OTS CA,,
Te
Boiling
HzS04,
1 hr - 100
32
105
46
Befoce any
Veatrnent
1 ioO
36A
110
50
Boiling
H,O,
1 hr
100
34
105
46
Table 6.5d. Results of the stability studies on multilayers of 5-7-dodecadiyne- 1-12-di01
(2) capped with OTS
2-, 6- and 10-layered
thin films of 2 capped
with OTS on Si(100)
Before anv treatrnent
Heat, 150 O C , 1 hr
4 hr
1 dav
Water, 25 OC. 1 hr
Boiiing Water. 1 hr
6.6 UV-Vis Exposure of Mono- and Multilayeis
For the thin films prepared from 1 on quartz. the intensity of absorption at -3 15 nm
in the UV-Vis absorption spectra increased upon multiiayercd thin film deposition (Figure
6.5). This is consistent with the increase in density of chromophores and a change in the
orientation of the molecules upon multilayer formation.
The thin film assemblies were irradiated with a UV lamp opemting at 365 nm for up
to 120 min undcr a n i v o p n aunosphew. m e intensity of chc h, dccrcascd signitkmtiy
(Figure 6.6). with thc sübsequent formation of a blue film that could be observed by a
naked eye. This suggests that a structural change has taken place with the C=C bonds
becoming panllel to the substrate after UV exposure, as reported prcviously."
SM) 400
Wavelsngth (nm)
Figure 6.5. W-Vis spectra of the 1 to 10 layered thin films of 2.4-hexadiyne- 1,6-di01 (1)
on quartz. The inset shows the UV-Vis specua of the 1 and 2 l a y e ~ d thin
films.
C I .
3 ' Cr: w
200 300 400 500
Wavelengai (nm)
Figure 6.6. UV-Vis spectra of a 10-layered thin film of 2,4-hexadiyne- 1.6-di01 (1) upon
exposure to UV-lamp for a period of 0 . 5 , 15. 30.60 and 120 min (top to
bonom).
However. only a broad absorption maximum with low intensity between 570 and 610 nm.
which might correspond to the formation of the blue polymer after exposure of. for
cxarnple. carboxylic acid-terminaied alkanethi01 diacetylene3' to UV lighr was observed. A
similar formation of a blue f h was observed by a naked eyc. upon U V exposure of the
thin film of 2. with a weak and broad absorption maximum in the 570-6 10 nm wavelength
region. The low intensity of these bands may be due to the fact that the absorption of C=C
bonds, which form upon UV-polymerization, is not strong enough for identification.It is
d s o possible that the topochernical polymerization in these thin films leads to a different
polymer backbone structure.'6b For a monolayer of 2.4-hexadiyne- 1.6-diol. the thickness is
calculated to k 14G A. and upon topochernical polyn?crization. leading to a -=-=-=-=-
type of structure. it decreases to 1 2 s a. The measured thickncss of such a film was found
to be 12+2 A by ellipsornetry. and the lauer decreased to 1 l f 2 A upon UV-Vis exposure.
Both water contact angle and ellipsorneuic thickness for the diacetylene films showed only
a slight dccrease upon UV-Vis exposure, indicating that there has bcen no gross-
deteriontion of the film quality on polyrnerization. A similar behavior has been observed
for the diacetylenic diaikyl disulfidr thin film on $old."
6.7 Conclusion
Self-assembled mono- and multilayers of a variety of organic chromophores
krrninated with OH groups on both ends on SitlOO) (Si/SiO,) surfaces have been
succcssfully preparcd by a simple acid-base hydroiytic chemistry route. Multilaycrs of up to
10 layers were fabricated from the dihydroxy molecules. and there was no incrcasing
disorder upon addition of each successive layer. The stability of the thin films is
significantly enhanced upon layer-by-layer deposition of the multilayered thin films. An
increase in the intensity of the km was observed upon rnultilayeied thin füm construction.
It suggests that there is an incrcasc in density of chromophores and a change in the
molccular aiignment upon multilayer formation. Upon UV exposure. the intensity of the
A, dxreased s ignikant iy with the subsequent formation of a blue film. and is
accompanied by a structural change where the C=C bonds becorne parallel to the substrate
upon topochernical polymerization.
6.8 Experi men ta1 Section
6.8.1 Materials
H ydroquinone (HO-p-C,H,-OH), bip henol (HO-p-C,H,-C,H,-OH) and 2.4-
hcxadiync- l,6-di01 (HOCH,C=CCgCH,OH). were purchased from Aldrich Chernical
Company Inc., and used wi thout further purification. 5,7-Dodecadiyne- 1,12-diol. 1 -4-
bis(6-hydroxyhexynyl)benzene, 4.4'-bis(6-hydroxyhexyny1)biphenyl. 9.1 O-bis(6-
hydroxyhexynyl)anthracene, 1.4-bis@-hydroxyphcnylethyny1)benzene. 4.4' -bis@-
hydroxypheny1ethynyl)biphenyl. were prepared by modification of published
procedu res. lg-'O neir purities were verified b y routine analytical methods including ' H
NMR, mass spectra and FT-IR spectroscopy. Toluene and THF were dried and distiIled
over sodium. Single side polished Si( 100) substntes were purchased from Nova Electronic
Materials.
5,7-Dodecadiyne- 1,12-diol, HO(CH,),C=C-C=C(CH,),OH To a 1 0 ml
Schlenk llask, chu-ged with 10 ml freshly distilled pyndinc. 5-hcxyn- 1-01 ( 1 .O ml. 8.7
mmol) and copper(1) chloridc (0.1 g) werc added. The solution wsis stirrcd rigorously aî
room temperature under oxygen for 4 h, The pyridine was rcmoved in vacuo, and
dicthylether ( 1 O ml) was added to the dark green liquid that turncd pale green. The solvent
was rcmoved in vacuo, affording a white solid. It was ~crystal l ized from diethyl ether-
Yicld: (0.7 g. 82%); Mass Spcc (FAB): 194; IR (KBr) u (OH): 3391. u (CH2): 2950.
293 1. 2901. 2865 cm". u (CC): 2 185, 2 139 cm*'; 'H NMR (270 MHz, CDCI,) 8 ppm
1,4-bis(6-hydroxyhexynyl)benzene, HO(CH,),C=C(p-C,H,)C=C(CH,),O H
To a 100 ml Schlenk flask charged with 10 mi freshly distilled diethylamine, 5-hexyn-1-01
( 1.5 ml, 0.012 mol). bis(trimethylphosphine)palladium(U~ chloride (0.06 g ) and copper(1)
iodide (0.1 g) were added. To the above solution. p-diiodobenzene (2.0 g. 0.006 mol)
dissolved in 10 ml diethylarnine was addcd dropwisc through an addition funnel, Thc
resulting solution was stirred at room tempcrature under a nitrogen atmospherc for 18 h.
The solvent was removed in vacuo. and the precipitate was extracted into dichlorornethanc.
It was washcd 2-3 times with dilute hydrochlotic acid and deionized watcr. The
dichloromethane layer was dried over MgSO, for 4 h. it was fdtered and then the solvent
was removed in vacuo, A pale yellow solid was obtained after reçrystaliization irom
dicthylcdier. Yield: (0.7 g. 43%); Mass Spec (El): 270; IR (KBr) u (OH): 3361 cm*'. u
(C,H,): 3076, 3036. 1507. 1483 cm-'. u (CH,): 2945. 2925. 2895. 2865. 2838 cm-', u
( C S ) : 2231 cm''; 'H NMR (200 MHz. CDCl,) 6 ppm 7.21 (d. 4H. J = 6 Hz. C,H,). 3.7 1
(t. 4H. J = 6 Hz, -CH,-OH), 2.45 (t. 4H. J = 6 Hz. -CH,-C=C-), 1.72 (m. 8H. -CH2-
CH2). 1.42 (S. 2H. -OH).
4,4'-bis(6-hydroxyhexyny l)biphenyl, HO(CH2)4C=C(p-C6H4C6H4)c=c
(CH2)40H T o a IO0 ml Schlcnk flask chargcd with 10 ml h ;h ly distilled diethylaminc.
5-hcxyn- 1-01 ( 1.5 ml. 0.0 12 mol), bis(trimethylphosphinc)palladium(ii) chloride (0.06 g)
and copper(1) iodide (0.1 g) were added. To the above pale yellow solution, p-
diiodobiphenyl (2.4 g, 0.006 mol) dissolved in 10 ml diethylamine was added dropwise
using an addition funnel. The resulting solution was stirred at room temperature under an
atmosphcrc of nitrogen for 18 h. The solveni was rcmovcd in vacuo and ttic bright yellow
pmipitate was extracted into dichloromethane. It was washcd 2-3 t i rna with di lue
hydrwhloric acid and deionized water. The dichlorornethanc layer was dried over MgSO,
for 4 h. it was filtercd and the solvent was removed in vacuo. A pale yellow precipitate was
obtained afrer k i n g recrysdl izd from diethylether. Yield: (0.6 g. 306); Mass Spec
(FAB): 346; IR (KBr) u (OH): 3378 cm". u (C',Hg): 3089. 3033. 1493 cm". u (CH,):
2945, 2924, 2 9 0 . 2859. 2841 cm-'. u ( C S ) : 2230, 2210 cm"; 'H NMR (200 MHz.
CDCI,) 8 ppm 7.46 (m. 8H. C,,H,). 3.73 (t. 4H. J = 6 Hz. -=?-OH). 2.49 (L 4H. J = 6
Hz, -=,-CS-), 1.72 (m. 8H. -CH,-CH,). 1-20 (S. 2H, -OH).
9, IO-bis(6-hydroxyhexynyl)anthracene, HO(CH,),C=C(p-C ,,H,)C=C(CH,),
OH To a 100 ml Schlenk flask. c h q e d with 15 ml frcshly distilled diethylaminc, 5-
hexyn- 1-01 ( 1.5 ml. 0.0 12 mol). bis(trimethylphosphine)palladiumO chloridc (0.06 g)
and copper(1) iodide (0- 1 g) were added in that crder. To the above solution. a solution of
p-dibromoanthracenc (2.02 g. 0.006 mol) dissolved in 10 ml diethylamine was addcd
dropwise through an addition funnel- The resulting solution mixture was refluxed for 1 day
under an aunosphere of nitrogen. The solvent was removed in vacuo and the green
precipitate was extracted into dichloromethane. It was washed 2-3 times with dilute
hydrochloric acid and deionized water, The dichloromethane layer was dried over MgSOj
for 4 h, it was filtered and the solvent was removed in vacuo. A yellow solid was cbtaincd
aftcr k i n g recrys!allized from diethylether. Yield: (0.72 g. 50%); Mass Spec (EI): 370. IR
iKBr) u (OH,: 3300 cm-'. u (C,,H,): 3059. 1619 cm-'. u (CH,): 2944.2864. 2825 c d . u
( C g ) : 2207 cm*'; 'H NMR ( 2 0 MHz. CDCI,) 6 pprn 8.54. 7.55 (d. 8H. J = 1 Hz.
C,,H,). 3.79 (t. 4H, J = 4 Hz. -=,-OH). 2.82 (t, 2H. J = 4 Hz. -CH,-C=C-), 1.92 (m.
8H. -CH2-CH2). 1.35 (S. 2H. -OH).
1,4-bis(p-hydroxyphenylethynyl)benzene, HO(p-C,H,)C=C(p-C6HI)C-C@-
C,H,)OH To a LOO ml Schlenk flask. charged with 10 ml freshly distilled diethylarnine. p-
iodophcnol ( l . l g, 0.005 mol). bis(trimethylphosphine)pailadium(lI) chloride (0.02 g) and
copper(1) iodide (0.03 gf were added. To the above solution. p-diethynylbenzene (0.25 g.
0.002 mol) dissolved in 10 ml diethylamine was added dropwise through an additional
hnnel. The resulting solution mixture was s t imd ovemight at room temperature under an
atrnosphcre of niuogen. The diethylamine was rhen removed in vacuo and dichloromethane
was added. The yeiiow precipitate obtained which was insoluble in dichloromethane was
filtered, washed 2-3 times with dilute hydrochlonc acid and deionized water. and
rccrystailized from methanol. Yield: (0.36 g. 50%); Mass Spec (ET): 3 10. IR (KBr) u
(OH): 3329 cm". u (C,H,): 3065. 3041. 1519 cm". u ( C S ) : 2216 cm-'; ' H NMR (200
MHz. CD;OD) G ppm 7.44. 7.35. 6.79 (d. 12H. J = 8 Hz. C,H,). 3.35 (S. 2H. -OH).
4,4'-bis@-hydroxyphenylethynyl)biphenyl, HO(p-CsH4)C=C(p-CsHC
CsH4)C=C(p-CsH4)OH To a 1 0 0 ml Schlenk flask. charged with 15 ml frcshly
distilled diethylamine, p-iodop henol (0.82 g - 0.003 mol),
bis(irimethylphosphine)palladium(U) chloride (0.01 g) and copper(0 iodide (0.02 g) wen:
added in that order. To the above solution, we added dropwise the solution of p-
diethynylbiphenyl (0.3 g. 0.00 l mol) dissolved in 10 ml diethylamine through additional
funnel. The rcsulting solution mixture was stirred overnight at room temperature under an
atmospherc of nitïogen. The diethylamine was then removed in vacuo and dichloromethane
was added. The yellow precipiiate which was insoluble in dichloromethane was filtered.
washed 2-3 timcs with dilutc hydrochlonc acid and deionizcd water. and recrystallized from
methanol. Yicld: (0.3 g. 50%); Mass Spcc (EI): 386. IR (KBr) u (OH): 3384. 3277 cm-'. u
(C,H,): 3065. 3038. 1512. 1488 cm". u ( C C ) : 2215 cm": 'H NMR (200 MHz. d-
DMSO) 6 ppm 7.78. 7.6 l. 7.42. 6.83 (d. 16H. J = 8 Hz. C,H,), l. I J ( S . 2H, -OH).
6.8.2 Su bstrate Preparation
The glass. quartz or single crystai Si watèrs werc first cleaned i) by soakinp in soap
solution and sonocating for 1 h; ii) repeated washings with dcionizcd water: iii) m u n e n t
wilh a solution mixture containing 70% conc. H,SO, and 30% H,O, - - (piranha solution) at
1 0 0 OC for 1 h. Caution: Piranha solulion is highly explosive. and care should be taken
while using this mixture; iv) repeated washing with deionizcd water, and v) Fmally heating
in oven at 150 OC for 5 min and vacuum drying for 5 min to remove physisorbcd water.
6.8.3 Preparation of SAMs
The surface functionalization invoived a IWO-step process of i) rcaction with 0.5 %
solution by volume of Si(NE&),. which can be conveniently prepmdl%y the =action of
SiCl, and excess dry NEt,H. in dry toluene at room temperature for 8 h; toilowed by ii)
treaunent with O. 1 96 by weight of the organic chromophore in dry THF at 40 OC for 1 day
after sonicating in dry toluene for 5 min to remove excess Si(NE5), physisorbed on thc
surface. After thorough washing with anhydrous THF, the subsuates werc dned at 120 "C
for 5 min.
6.8.4 Preparation of Multilayers
The multilayered thin fih assemblies of 2.4-hexadiyne-1,6-di01 (1) or 5.7-
dodccadiync- 1,12-di01 (2) were prepared by immersion of the monolaycr subsuates into
0.5 % by volume of anhydrous toluene solution of Si(NEb), at room temperature for 8 hr,
followed hy treatment with O. 1 % by weight of anhydrous THF solution of 1 or 2 at 40 OC
for up to 24 h. aher sonicating in10 anhydrous loluene for 5 min. The two-laycr suhsuatcs
thus obtained wcrc thoroughly washcd with anhydrous THF and rhen dried at 120 OC for 5
min. The proccdum was repcated to obtain multilaycred thin films.
6.8.5 Contact-Angle (CA,,,) Measurements
The static and advancing contact angles were measured with a Rame-Hart NRL 100
goniometcr. On average. 6 drops of water were measured on different areas of the polished
side of a silicon w d e r for each sample. and the values repoiled are the mean values with a
maximum range of SO. The advancing contact angles of captive drops were found to be
roughly 5 O above the static values of sessile (free-stimding) drops. If the drop was allowed
to M l from the needle of the syringe to the surface. smailer contact angles were usually
obtained k a u s e of the mechanical vibration^.'^
6.8.6 Fourier Transform Infrared Spectroscopy in the Attenuated Total Reflection Mode (FTIR-ATR)
The organic thin fihs were grown on the < l m surfaces of the single side polished
silicon wafers. A KRS crystal was sandwiched between the reflective faces of two silicon
wafers (1 -2 cm X 4.0 cm). and the angle of non-polarized light was set at an angle of 45O.
AU spectra were run for 4ûûû s a n s at a resolution of 4 cm-'. using a Bruker IFS-48
specuometer. A specuum of two clean silicon wafers with a sandwiched KRS crystal was
measured as a background correction.
6.8.7 Ellipsometry
A Gaertner Scientïîïc ellipsorneter openting at 633 nm &-Ne laser (h = 6328 A) was
employed. The angle of incidence was 70.0°. and the compensator was set at 45.0'. AU
reported values are the average of at least six measurements taken at differcnt locations on
the sampie. The error bar of the thickness increases from +t A for 1 <O 4-layer. to fi A for
8 to 10-iaycr. il.: thickness was calculated by cor,iparing data from the samc substmte
bcfore and after functionalization and using a value of 1.46 for thc nrfractive index, This
value is based on the assumption that the monolayer is similar to bulk paraffins with a
refractive index of 1-45? If the monolayer is more crystalline-like. similar to polyethylene.
the refractive index thus should be within 1.49- 1 .55.'6 It was found that an increase of O. 1
in the refractive index from 1.45 to 1.55 resulted in a decrease in the measured thickness by
-2 a.
6.8.8 X-ray Photoelectron Spectroscopy (XPS)
monochrornatized MgK, X-ray source to produce the photoemission of electrons from the
coir: levels of the surface atoms. About 50 A of depth was probed for a d e e t o r
perpendicular to the surface. The analyzed surface was 2 x 3 mm. AU peak positions were
corrected for carbon at 285.0 eV in binding energy to adjust for charging effects. The
power of the source was 3 0 watts and a pressure of 10' mbar.
6.8.9 UV-Polymerization
Topochemicai polymerization was camied out by placing the substrates into a
container. and irradiating them under a nitrogen purge for up to 120 min with a medium
pressure Hg vapor lamp (Ace-Hanovia photochernical lamp, mode1 7830,230-430 nm with
the main spike at 365 nm), with the sample-to-source distance of approx. 3 cm.
6.9 References
( 1 ) (a) Jackman, R. Je; Wilbur, J. L.; Whitesides. G , M. Science 1995, 269, 664. (b)
Chidsey, C.E.D. Science 1991, 251, 9 19.
(2) Sagiv, J. J. Am. Chem. Soc. 1980. 102, 92.
(3) Bain. C. D.; Troughton, E. B.: Tao. Y.; Whitesides, G. M.; Nuzzo. R. G. J. Anr
Chem. Soc. 1989, 11 1, 32 1.
(4) Porter, M, D.; Bright. T. B.; Allara, D. L.; Chidsey, C . E. D. J. Arn. Chem. Soc.
1987,109, 3559.
( 5 ) Walczak, M. M.; Chung. C.; Stoie. S. M.; Widrig, C. A.; Porter, M. D. J. Am-
Chem. Soc. 1991, 113, 2370.
(6) Laibinis, P. E.; Whitesidcs. G. M.; Allan, D. L.; Tao, Y.; Parikh. A. N.; Nuzzo. R.
G. J. Am, Chem. Soc. 1991.113, 7152.
(7) Troughton, E. B.; Bain, C. D.; Whitesides. Ci. M.; Nuzzo, R. G.; Allam. D. L.;
Porter, M, D. Langmuir, 1988.4. 365.
(8) Nuzzo. R. G.; Fusco, F. A.; Aliara. D. L. J, A m Chem. Soc. 1987, 109, 2358-
(9) Tao, Y. J. A m Chem. Soc. 1993,115, 4350.
( IO) Linford, M. R-; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am Chem.
Soc. 1995, 11 7, 3 145.
( 1 1) (a) Ulman, A. Adv. Mater. 1990.2.573. ( b ) Swalen, J. D.; Allara, D. L.; Andrade.
J. D.; Chandross, E. A.; Garoff. S.; Israelachvili, J.: McCarthy. T. JI; Murray, R.;
Pease. R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987. 3, 932.
(12) (a) Finklea, H. O.; Robinson, L. R.: Blackburn, A.: Richter, B.; Allara. D.; Bright,
T. Langmuir 1986. 2, 239. (b) Maoz, R.; Sagiv. J. Langmuir 1987.3. 1045.
( 13) Tilman, N; Ulman. A.; Penner, T. L. Langmuir 1989.5, 101.
(14) Maoz, R; Sagiv. J. Langmuir 1987.3. 1045.
( 15) Kato. S; Pac, C. Langmuir 1998, 14. 2372.
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C . M.; Dickie, A.; Malkhasian, A.; Kakkar. A. K.; Whitehead, M. A. Cm. J. Chem.
1998. 76. 1766. (c) Yarn C. M.; Kakkar A. K, J. Chem. Soc., Chem. Commun.
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(19) Hay. A. J- Polyrn- Sc i Pt. A-1. 1%9. 7. 1625
(20) (a) Nguyen, P.; Yuan, 2.; Agocs, L.; Lesley. G.; Mardcr. T. Inorg. Chim. Acta
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(2 1) Wasserman S. R., Tao Y.; Whitesides, G. M- Langmuir 1989.5, 1074.
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Chapter Seven
Molecular Self-Assembly of Alkynyl Terminated Chromophores
via Acid-Base Hydrolytic Chemistry on
Inorganic Oxide Surfaces: Monolayer and Step-by-Step
Multilayered Thin Film Construction
7.1 Introduction
In rccent years. organizcd moleçuIar systems havc attmcted growing attention because
they offer significant potential in technologyl which includes. for example. proicctivc and
patternable materials, surface preparation and modification. chemically rnodified elccuodes.
and biological thin films of proteins. The common techniques' for thc construction of such
systems comprise Langmuir-Blodgen (LB) and molecular scl f-asscrnbly (SA). resulting in
ordered monolayers. Some of the commonly used rnethodologies for molecular self-
assembly are organosilicon derivatives on silica surfaces.' alkanethiols on gold.'" s i l v ~ r . ~
and copper.' dialkyl sulfides on gold."ialkyl disuifides on g ~ l d . ~ alkanoic acids"' on
silvcr. copper and aluminum oxide. 1-alkenes on hydrogcn tcminated Si( l 1 1)". alkûnols!'
and alkynes13 on Si/SiO,. They have hcen shown to fom unifonn and ordcrcd structures.
Thin films with useful aromatic and acctylenic groups havc growing prominence in indusuy
bccausc of their optical and electronic u s . l J
in ordcr to transform thin films into practical devices, rnultilayers of appropriate
thickness need to be reproducibly constructed with minimum rcduction in order." In
gcnerai. multïiayercd thin films on silica bûsed surfaces are fabricated by rnodifying the
monolayer surface to a hydroxylated one. by the reduction of surfacc ester grouplS. thc
hydrobontion-oxidation of the tcrminal vinyl group.16 the photolysis of the nitrate-bearing
group." and the hydrolysis of the boronate-protecting group."
WC have discussed the construction of monolayers and multiiayers of aicohols on
SVSiO, via acid-base hydrolytic chemistry in Chapters 5 and 6. An elaboration of this
approach using arninostannanes can be used to constnict multilayers of aikynes on SVSiO,.
The reactivity of aminostannanes with molecules containing terminal acidic groups has k e n
weU studied.19 Owing to the high basicity of nitmgen in aminosmnanes. the Sn-NE5 bond
c m be easily cleaved by a variety of protic species including acetylenes. Using the acid-
base hydrolytic chemisiry approach. silica surtàces functionalized with Sn-NE- groups can
be easily modified with a number of terminal alkyne molecules with varied backbones.13 In
this chaptcr, we discuss a two-step thin film construction process involving the reaction of
Sn(NEt,), with hydroxyl groups of the inorganic oxide surfaces. followed by the reaction
with a variety of alkyne molecules incorporating alkyl and aromatic moieties in the
backbone. By repetitive reactions with Sn(NE&), followed by H-Cs-R-Cs-H,
multilayeccd rhin füm assemblies were fabricated on glass. quartz. and single-crystal Si.
The mono- and multilayered thin films were characterized using surfrice wet~bil i ty
measutzmcnts, ellipsometry. XPS, FTIR-ATR. and U'd-vis spectroscopy. Our rt3sults
indicate that thin films of appropriate thickness using dialkynyl chromophores could be
easily adsorbed using acid-base hydrolytic chemistry- Surface covcnge of rigid-rod
alkynes. H - C S - R - C S - H (R = p-C6H,, p-C,H,-C,H,, p-C,,H,, -C-=C-p-C,H,-CS-),
was estimated to be 2-7 molecules/100 a' which is comparable to Ihe surface hydroxyl
dcnsity for silica. degassed at 1 0 - 1 5 0 OC. WC have also investigated topochemiçal
polperization of diacetylene moiety in a SAM of H - C C - C C - p - C , H , - C S - C C - H . and
upon UV-Vis exposure, the formation of a blue film was observed. The adsorption of
Co,(CO), in the mono- and multilayered aikyne assemblies was found to be f m i b l e under
room temperature conditions. Thest: alkynyl surfaces may h o m e potential candidates for
tethering transition metai fragments leading to organometaüic surfaces for heterogenizd
homogeneous catiiIysis, one-dimensional conductors, and chemically modified elwuodes
and sens or^,'-'^
7.2 Acid-Base Hydrolysis
When chforostannanes react with LiNR (R = Me, Et), the corresponding
aminostamanes (Me,Sn-NR?. R = Me. Et) are produccd in very good yields.Ig
Aminostamanes then undergo acid-basc hydrolysis wilh molecules containing tcrmuial
acctylcne groups to yield stannyl-alkynes.'g Since the surfaces of inoganic oxides succh as
silica, g l a s and quar tz contain hydroxyl groups, the acid-base hydrolytic chemistry can be
easily applied to these suriaces to give a versatile chemisorption method for a variety of
organic microstnictures. The hydroxyl groups on the surface of silica were firsr treated with
Sn(NEL.,), to produce the surface-anchored Sn-NEt, groups, which furthcr react with
terminal alkyne molecules with varied backbones, leading to ordered thin films based on
simple acid-base hydrolytic chemistry (Scheme 7.1 ). ' B y repeating this IWO-step process
and using bifunctional chromophores (H-Cs-R-CS-H). multilayers of alkynes with
desircd backbones were constmcted (Scheme 7.2).
7.3 Monolayers of Alkynes
The SAh.;s of alkynes with varied chain-lengths wcrt: characterizd by surface
wettability rneasurements. eliipsometry. XPS and FTIR-ATR. and thc data are prcscntcd in
Tables 7.1 and 7.2.
Table 7.1. Static contact mgles of water and HD, thcoretical (TL) and ellipsornetric thicknesses (T,), and XPS data for SAMs of alkyncs on Si(100) substrates
XPS Data (eV) C( 1s). O( 1 s),
Si-Si(2p). Si-O(2p). W 3 4 . 3 4 )
Ta b 1 e 7.2. FïR- ATR data for S AMs of alkyncs on Si( 1 OO) substratcs
"CK-EI* "cd
cm"
vK,H,)
cm-'
29 18 (br), 2848 (br)
Contact angle goniometry has been routinely u x d 10 detcnnine the wetung bchavior
of thin Tùm assemblies.' Upon comparing the water contact angle of a SAM of octadecyne
( 1 L O O ) with that from SAMs of OTS on silica'' and ocÿidecanethiol on gold.' it is apparent
that the new acid-base hydrolytic approach is capable of producing thin films of comparable
qualit. and order. However. the contact angle with hexadecane (CA,,) was round to be
about 10" lower for the occadecyne SAM (25") on Si(100) than those reported for the OTS
SAMs on silica-based surfaces (35-40"). This may be due to (i) the higher hydrophilicity of
[Si]-O-Sn-GC-R in our SAM than [Si]-CH,-R in OTS, and (ii) the Si(100) surface
cmployed in our studies. which have been discussed in Chapter 5.
The lower contact angle of water (75-82') and hexadecanc ( 12- 15") on the acetylene-
or phenyl-tcrminated surfaces than the correspondhg long alkyl chains (paraftins, ca. 1 LOO
(water) and 35-40' (hexadecane)) may be the result of the stronger interactions between -
C S - H (sp hybridization) or -C,H, (sp' hybridization) groups and watcr chan CH, (sp')
groups.13 as weli as the introduction of disorder into the asemblies by surface
f u n ~ t i o n a l i t ~ . ' ~ The wetting nature of alkynyl surfaces tcrminated with acetylenc or phenyl
groups is comparable to that obtained from thio-alkynyl thin films on goid.''
As the chain-length of the alkyne decreases from -CS-(CH-) , -CS-H to - C S -
(CH2)Z-C=-H. CA,, and CA,, drop from 80' to 75". and 15" to 12'. rcspcctivcly (Figure
7.1). IL rnay be due to the sensitivity of the probe liquid to the undcrlying substratc.'" as
well as the structures becoming increasing di~ordcred.'.~" For shorter chains. the Iilm is
less ordered and dense owing to the lack of cohesive interactions. As the chah lcngth
increases, the cohesive forces becorne strong enough to puil the molecular chains into
'normal' orientation for optimal interaction energ y .5 S tudies of alkanethiols adsorbed on
gold and silver surfaces:-7 and alkynyol on Si/Si0213 showed similar results.
Figure
Hexadi yne
7.1 Static contact mgles of water for monolayers of hexadiyne, octadiyne,
nonadiyne and decadiyne on S i ( l 0 ) subsuate.
The thicknesses of the newly se l f -mmbled thin films wcre mcasured using
ellipsometry. The refractive index for the alkynyl films was assumed CO be 1.46"" for
calculations. This is comparable to the Literature values for the refractive index of
hydrocarbon thin film on Si/SiO, of 1.45- 1-50." The film thickness was calculaied on the
basis of data comparison from the same substrate before and after functionalization. A
detailed discussion of the ellipsometric technique for the characterization of thin f h s has
been reported elsewhere."'" Table 7.1 shows the eU.ipsomeuic thicknesses obtained for
the alkynyl films. Theoretical thicknesses were calculated based on typical values" of bond
Icngths between elements projectcd on thc surface. For example. for a trans-extended
chain. the projections of the Si-O. Sn-O, Sn-Cg. C S , CS-CH,, CH,-CH, and C S - H
bonds onto the surface normal (z -a i s ) are 1.33. 1.58. 2.18. 1.18. 1-46. 1.26 and 1.06 A.
respectively. Hence. a rnonolayer prepared from 19-dccadiyne is expected CO have a
thickness of 18 A. As shown in Figure 7.2. the ellipsornevic thicknesses arc in good
agreement with theoretical value, and suggest the formation of relatively close-packed
monolayers with a slight tilt to normal.
X-ray photoelectron spectroscopy provides a qualitative evidencc of the surfacc
composition of thin films. According to Table 7.1, various alkynyl thin films on Si(100)
were found to consist of silicon: 2p. 99 eV, 2p, 103 eV; carbon: 1s. 285 eV; oxygen: 1s.
532-533 eV; and t h : 3d,, 495-496 eV, 3d,, 487 eV, confirrning the surface composition of
the thin films. The molwules containing conjugatcd brickbones showed a pedc at 291 r=V
[or C , , corresponding to, for example, aromatic carbon.
Theoretical 0 Ellipsometric
1 i I I 1
Hexadiyne Octadiyne Nonadiyne Decadiyne me
Figure 7.2 EUipsometric thickness for monolayers of hexadiyne. octadiyne. nonadiyne and decadiyne on Si(100) subsuare.
Fourier uansfonn infrared specuoscopy iri the atienuatcd toral reflection mode (FIIR-
AIR) has b e n extcnsivdy used for the idenliriication of the newly developed thin films.'
For the alcyne SAMs containhg varied alkyl chain-lengths in the backbone (-(CH,),-. n =
2, 4. 5. 6 ) . the v,(CH2) and v,(CH,) stretching frequencies were observed at -2920 and
-2850 cm". respectively. with the peaks becoming broader and less intense as the chain-
length decreased (Figure 7.3). These observations are consistent with the resulis on
alkyltrichlorosilanes24 and alkynyoll" on glass and thiols5 on gold. demonsirating that the
stmcture of thesc SAMs with shorter chain-length is less ordered and more liquid-like. The
positions for asymmeuic and symrnetric methylene s ~ t c h e s at -2920 and -2850 cm" for
longer alkyl chains indicate that x-a interactions of C S groups may complement the order
of the systems. A typical specual pattern for an ordered hydrocarbon assembly gives the
peak positions for asyrnmetric and symrnetric methylene smtching frequencies at 29 18 and
2849 cm-'. respectively.5-" The v,(CH,) and v,(CH,) smtc hing frequencies of oc t ad~yne
SAM on the Si( 100) surfaces were observed at 29 18 and 2850 cm-'. These m consistent
with prcvious published results for OTS on silica based surfaces" and octadecanethiol on
gold.5 suggesting that b e y are highly ordered.
correspond to the acetylene and phenyI groups of the aikync SAMs. As cornparcd to the
spcctra from a KBr pellet (or nujol) of the pure compound, thc frequencies of the acetylenc
and phenyl groups on surface are s h i k d slightly bccause of the orientation efkcts in the
SAMS.'~ as rçponcd similarly for thio-alkynyl tïlms on gold'O and alkynyol on S ~ / S ~ O ? . ' ~
Figure 7.3 FTIR-ATR (nonpolarized) specua for monolayers of hexadiyne (A), octadiyne (B), nonadiyne (C) and decadiyne (D) on Si(100) substrates in the region 2800-3000 cm-'.
7.4 Estimation of Surface Coverage of Rigid-Rod Alkyne Chromophores
The UV-Vis data for the rigid-rod alkyne chromophores, includinp p-
diethynylbenzne. p-diethynylbiphenyl. pbis(butadiyny1)benzene and p-
diethynylanrhncene. on quartz shown in Table 7.3- A typical UV-Vis spectrum for a
thin film of pdiethynylanthraçene on quartz is shown in Figure 7.4. and the spectral
absorption at 398 nm is comparable to the solution absorption at 401 nm. The peaks
became broad on the surface. and in the case of p-diethynylbenzene. h , shified to -30 nm
lower than that of the solution speçtra, Simiiar observations have b e n reported eariier for
othcr chromophores self-assembled on surface^.'^ Using the Beer-Lambert law and
assuming the value of extinction coefficient on the surfsce as that for solution. an estimation
of the surface coverage of thc chromophores can be obtained. The spectra wcre collected
from a quartz slide functionalized on both sides. Thcrcfore. for calculating surfacc
coverage. absorbance was divided by 2 to obtain the value of -ch individud monolayer.
The surface coverûgc in the rhin films of -2-7 rnolecules/100 a' is reasonable. since the
hydroxyl group density for silica degassed at 100-150 OC has k e n reportcd to bc -3-8
OWlOO A'." This value is also close to the reported surface coverage for other
chromophores. for example. pyridineT3 p ~ r p h y r i n ' ~ and NLO-active based SAMS."
suggesting that the SAMs of the rigid-rod diakynyl chromophores on Si/SiO, are relatively
close-packed.
As a rcsult, a combination of contact angles. cllipsometry. XPS, FT-IR and UV-Vis
data suggcsts that the SAMs of diakynyl chromophores on Si/SiO, arc rclativcly ordercd
and denscly packed, Thc wakr contact angles of the dialkynyl monoiayers with vancd aUyi
chain-Icngth wcre found to be lower (75-80") than the corrcsponding long alkyl chains
(-1 10°) but wcn: comparable to the SAMs ofalkynyol on SilSiO. mportcd previously.'"
Tatlc 7.3. &, (nm), absorbante (A), absorption Coefficient (E). and surface coverage
(8) of rigid-rd alkyne thin films on quartz
Alme Film on Quartz
cm' mol-'
3.28
0.45
0.4
0.35
0.2
A
2 0.25 Y
G.2
O.? 5
0.1
0.05
O 203 250 300 350 400 450 506
Wavelength (nrn)
Figure 7.4 UV-Vis specfrurn of a monolayer of p-diethynyhthracene on quartz.
7.5 UV-Vis Exposure of a SAM of p-Bis(butadiyny1)benzene
The thin film assembly of pbis(butadiyny1)bcnzcnc was exposcd to UV light
opcrating at 365 nm for up to 60 min under a nitrogen atmosphere (Figure 7.5). After a 5-
min irradiation. the intcnsity of hm, at 296 nm in the monolayer decreased significantly
and a blue tiim was observed by a naked eye. Exposure of the film to UV ligh for times
k y o n d 60 min revealed no further change in intensity at 296 nm. The d e c m in intensity
may bc attnbuted to a structural change as the C S bonds becorne parallel to the substrate
upon UV-exposure, as reported previously." A broad peak with weak intensity at 580-620
nm. corresponding to the formation of the blue polymcr." was obscrved. IL may bc due to
the fact that (i) the intensity of Cs bonds are too weak for identilicati~n."~ and (ii) the
topochernical merization in these thin f h s rnay lead to a ditTercnt polymer backbone
s t n ~ c t u r e . ~ ~ ' The water contact angle and cllipsomcuic lhickness (Table 7.4) for thc
diacctyiene film were found to decrease slightly on exposure to UV for 30 min. indicating
that there has k n no great deterioration d t!!e film quality on polymerization. as reported
prcviously." T o p t h e r with a demeau: in v,,. from 2165 io 21 15 cm-'. due to extensive
elcctronic delocalization in the backbone of the polymerixd diacetylenc groups." t.hcsr
results support the UV-potymerization of rhe diacetylenic chromophorc. similar to thosc
repowd for UV-polymerization of SAMs of carboxytic acid-tcnninated alkanethi01
diacctyleneE and dialkyl disulfide diacetylene.j3
Table 7.4. Static contact angle of water (CAwo), eUipsomeÛic thickness (Te) and FïR-
ATR data of a SAM of p-bis(butadiyny1)benzene on a Si(100) substrate upon
exposure to UV-lamp for 30 min
10 20 30 40 50 60 70 W-polymerization t h e (min)
--
H œ - \ - - - - - O - - Film on Si(100)
Figure 7.5 UV-Vis absorption (A- = 296 nm) of a rnonolayer of p-bis(butadiyny1)-
benzene upon exposure to UV-lamp for a period of 0,5. 15,30 and 60 min.
CAw,.k2O
1
2165 ,
21 15
Tc. +2 A
CC
before UV-exposure
I after CTV-exposure
VCC, cm-'
80
78
17
16
7 .6 Multilayer Thin Film Assembiies of 1,9-Decadiyne and p-
Diethynylbenzene
As discussed earlier. SAMs of dialkynyl chromophores exposing terminal C g - H
groups to surface can react further with Sn( NE%),. Following the reaction with additional
dialkynyl moieties. 2-layered thin fdm can be fabricated. Repetition of this profess can lead
ta multilayered assem blies. Using this approac h. we have successfully prepared
multilayered thin films of up to 5 layers of 19-decadiyne (1) and p-diethynylbenzene (2)
(Scheme 7.2). Before the deposition of each successive layer. the original layer was
cleaned by sonicating in toluene for 5 min. followed by drying in an oven at - 100 'T for 5
min. A temperature of 70 OC was r e q u k d for the reaction of terminal dkynes with surface-
anchored aminostannane (and vice versa) resulting in a closely packed thin films-
The multilayered thin films were characterized by contact mgle goniomerry.
etlipsomeuy, FT-IR, and UV-Vis spcctroscopy. As shown in Figures 7.6 and 7.7. the
film thickness increases as the number of layers incrases from 1 to 5 for both 1 and 2. The
measured thicknesses match the theoretical values from 1- to 3-lüyered films for both 1 and
2. However. they become less close to each other beyond 3-layered films: from 18 h y e r
in 3-layer sample to 22 h y e r in 5-layer sample for 1 (theoreticai monolayer thickness =
18 A) and from 13 bayer in f l a y e r sarnple to 18 bayer in 5-layer sample for 2
(theoretical monolayer thickness = 13 A). This may k duc to the fact b a t (i) the
monolayers can be cleancd more cffectively than the muldlayen. similar [O the multilayers
of dihydroxy crminated molecules on aminosilanc-anchorcd surface"' and those of
HOC,,Si/Si prepared from MeOCC,,Si/Si;" and (ii) therc is an i n c m in thickncss of
tin-oxide layer from monolayer to multiiayer due to polyrnerization of aminostannane ai
highcr temperature (70 OC) dunng multilayer deposition.
0 Ellipsometric thickness Theoretical thickness
Figure 7.6 Ellipsomeuic thickness of the 1 to 5 Iayered thin films of 1 -9-decadiyne.
c Ellipsometric thickness Theoretical thickness
O 1 2 3 4 5 6 La y er number
Figure 7.7 Eliipsomeuic thickness of the 1 io 5 layered thin tilms of p-diethynylbenzcne.
Funhermorr. the precision of rhickness rneasurrments decreases as the number of
layrrs increases. for example. from c a 9 A for 1-layer samples to ca. f5-6 A for 5-layer
samples of both 1 and 2. This may also be explained by the s m d amount of loosely held
material k i n g adsorbed on top of the mulnlayer surfaces or accumulating in the muitilayer
bulk. during the process of multilayer formation.!'
From the UV-Vis data (Figure 7.8) for thin tilms of 1 from 1-layer ro 5-layer. it was
shown that the absorption a t -220 nm increased with the incrcase in the number of layers.
This is consistent with an increase in chromophore density upon mululayer formation.
La yer Num ber
Figure 7.8 UV-Vis absorption of the 1 to 5 layered thin films of 1.9-decadiyne.
The static contact angle of water for the thin film prcparcd from 1 incrcascs from 81P
for a monolaycr to W for a 5-laycr film. Similarly. thc static contact angle of hexadecane
(HD) increases from 12" to 15" from a monolayer to a 5-layer film (Table 7.5). The daia
suggest ihat the thin fdms become densely packed in the multilayer. It is probably due to
effective Sn-O-Sn cross-linking network within eac h layer which resuits in better alignment
of the chromophores on the surface. increasing n-ic interaction of the acetylene groups. A
sirnilar behavior has been observed for the thin film prepared [rom 2 (Table 7.6).
T a b lc 7.5. Static contact angles of water (CA,,,) and HD (CA,,). ellipsometric
thicknesses (T,). and FTIR-ATR data for the multilayers of 1 ,9-decadiyne o n
Si( 100) substrates
In the FTIR-ATR spcctrum of a thin film of 1. thcn: an: two peaks corrcsponding to
Decadiyne film
on S i ( 1 0 ) I
1 -1ayer
the strctching frequencies. v,,. one for the terminal C S - H (-2 100 cm-') and the othcr for
the bonded to -Sn (-2 130 cm"). The peaks becarne more intense on multilayered thin
CA,,, (CA,)
k 2 O ---II
80 ( 12)
film construction. Similarly, FTIR-ATR spectrum of a thin film of 2 (Table 7.6) showed
two peaks (-2 1 0 0 and -2130 cm") which are comparable in position to chat in the powder
I
Te. A
16iS
I v,,, cm-
pl
2 130, 2097
of HCC-C,H,-CKH (2103 cm-') and (CH,),Sn-Cg-C,H,-CC-Sn(CH,), (2 130 cm-').
These peaks d s o show an increase in intensity with rn ultilayer formation-
Table 7.6. Static contact angles of water (CA,J and HD (CA,), eliipsometnc
thicknesses (TJ, and FTïR-ATR data for the multilayers of p-diethynylben7xne
film on Si(100) 1
7.7 Cobalt Carbonyl Adsorption on Monolayers and Multilayers of 1,Y-
Decadiyne and p-Diethynyl benzene
Cobalt carbonyl is cf considerable interest because of its caialytic activity toward such
reactions as rearrangement of epoxides. reduction df aldehydes and oletln ca rb~x~ la t i on . ' ~
In contrast to most other metais, cobalt carbonyl forms vcry stable complexes with
alkynïs.s6 The selective coordination of cobalt carbonyl with the acetylenc groups makes it
a useful acetylene protecting group." The mction of acetylene with cobdt carbonyl in
solution at room temperature is well-known. and has been snidied by many i n~es t i~a ton . "
We were interesed in finding out the possibility of the xisorption of cobalt carbonyt on thin
films of 1 and 2 undcr similar conditions. Adsorption of cobalt carbonyl on the dialkynyl
thin Tilms was found to prwcrd feasib!y undcr room temperature. After 24 h deposition.
the cobalt carbonyl was successfully adsorbed onto the dialkynyl thin films (1 and 2).
supponed by the ellipsomeiry, contact angles. FI?-IR and UV-Vis data,
The ellipsomcmc data (Table 7.7) for the thin füm assemblies (monolayers o r
multilaycrs) of both 1 and 2 show an increase in thickness after adsorption of cobalt
carbonyl. This extra thickness may be the consequence of the adsorbed cobalt carbonyl on
top of the fùms, reacling with the terminal alkyne group (resultant bond length of Co-C=O
is -3 A''). The adsorption of cobalt carbonyl on alkyne film surfaces of 1 and 2 is further
supported by a drcreasr in the siatic contact angles of water and HD to 65-70' and 510".
rcspectively. These results art: similar to hi01 monolayers with tmninal nitrile. estcr.
aldehyde and ketone groups adsorbed on goldS4
From the FT-JR data for the thin films containing adsorbcd cobalt carbonyl on 1 (Co-
DY) and 2 (Co-DEB). the peak positions for the acetylene and carbonyl groups (Table 7.7)
werc observed at ca. 2105 and 2060 (2025) cm-', respcctivcly. The peak positions for thc
carbonyl groups in the films are only slightly shifted from the reference spectmm obtained
from a KBr peuet of CoZ(CO), (v,. terminal = 2054. 2024). 1t should be noted that the
peak at 1849 cm-' in pure Co,(CO),. attributable to its bridging carbonyl group. is absent in
both films of Co-DY and Co-DEB. similar to that reported for the acètylenic dicobalt
hcxacarbonyl c o m p o ~ n d s . ~ ~ Cobalt carbonyl adsorption on alkyne films is further
corroboraicd by the observation OS A,, at 277 nm (Figure 7.9). which is comparable to
that obtain~ d from Co2(CO), in methylene chloride (km at 276 nrn). Thesc results suggcsi
that cobalt carbonyl have been adsorbed onto the diaikynyl mono- and multilaycred
assemblies via coordination through the acetylene groups with no deteriontion of the film
quali ty.
Ta b lc 7.7. Static contact angles of water (CA,,,,). ellipsometric thickncsscs !T,). and
FTIR-ATR data for the thin films of pdiethynyl benzene (Co-DEB) and 1.9-
dccadiync (Co-DY) on Si(100) substrates dtcr rcaction with cobalt carbonyl
7.8 Conclusion
SeK-assem bled mono- and multilayers of a variety of dialkyne m o l ~ u l e s terminated
with acetylene groups on Si(100) (Si/SiOJ surfaces containing anchored aminostannanes
have b e n successfully prepared by a simple acid-base hydrolytic chemistry route. The
surface covenge of chromophores was found to be 2-7 moleculesllOO A'. comparable to
the density of surface hydroxyl groups on siiica degassed at 1 0 - 1 5 0 OC. Upon UV
exposurc OS a SAM of pbis(butadiyny1)benzene. the intensity of the )i, decreased
significantly with the formation of a blue film, suggesting a structural change. accompanied
by C S bonds becoming more parailel to the sub:..r-alc upon topochcmicd polymerization.
Multilaycrs of 1.9-decadiyne and p-dicthynylknixncr of up to 5 layers wcre tàbricated
using a step-by-step thin film construction methodology. Thc results prescntcd hcrc have
shown that simple =id-base hydrolytic chemistry of aminostannanes with dialkyne
chromophores can be successfully ernployed for the construction of mono- and multilayer
thin t l m assernblies. Finally. cobalt c d n y l was found to adsorb in the thin film
asscmblies by reacting with the acetylenc groups.
7.9 Experimental Section
7.9.1 Materials
1.8-Nonadiyne and Sn(NEtJ, were purchased from Aldrich. 1 -5-Hexadiyne. 1.7-
octadiyne and 19-decadiyne were purchased from ChemSarnp. Cobait carbonyl was
purchased from Strem Chemicals and used as received. Other rigid-rod alkynes, including
p-dieth ynylbenzene, p-dieth yny 1 biphenyl, pdiethynylanthracene and p-
bis(butadiyny1)benzene. employed in this study were conveniently pxxpared using literature
proccd~res . '~ Toluene was dried and distilied over sodium.
7.9.2 Substrate Preparation
The glass. quartz o r single crystal S i wafers wen: first cleaned (i) by soaking in soap
solution and sonocating for 1 h. (ii) repeated washing with dsioniired water, (üi) m u n c n t
with a solution mixture c o n t a i ~ n p 70% conc. H2SO4 and 30% H 2 0 2 (piranha solution) at
100 OC for 1 h. Caution: Piranha solution is highly explosive. and care should be taken
while using this mixture. (iv) repeated washing with deionized water. and (v) tinaily
heating in a oven at - 150 OC for 5 min and vacuum drying for 5 min to rcmove physisorbed
water.
7.9.3 freparation of SAMs
The suifice functionalization invvlvcd a tw~-s tcp proccss: the c l a n substrates wcre
trcated with (i) 0.5 56 solution by volume of S ~ I ( N E ~ ~ ) ~ in dry toiucnc for 8 h at room
temperature. followed by (ii) O. 1 56 by weight of the alkyne chromophorc in dry tolucnc
70 OC for 24 h, after sonicating in dry toluene for 5 min to remove cxcess Sn(NEt2)4
physisorbed on the surface. After thorough washing with dry toluene, the subsintes were
dried at - 120 OC for 5 min.
7.9.4 Preparation of Multilayers
The multilayered thin fiim assernblies of 1 -9-decadiyne (1) or p-diethynyibenzene (2)
wcrc prepared by immersion of the monolayer substrates into 0.5 % by volume of dry
toluene solution of Sn(NEtz)4 at 70 OC for 8 h. followed by ucatment with O. 1 % by weight
of dry toluene solution of 1 or 2 at 70 OC for 24 h. after sonicating into dry toluene for 5
min. The two-layer substrates thus obtained were thoroughly washed with dry toluene and
then d r i d at - 120 93 for 5 min. The procedure was repeated to obtain multïiayered thin
films.
7.9.5 Cobalt Carbonyl Adsorption
The mono- and muitiiayered thin film assernblies of (1) and (2) wcre immersed into
0.1 % by weight of anhydrous toluene solution of Co2(CO), at room temperature for 24 h.
followed by thorough washing w i h anhydrous toluene and drying at - 120 O C for 5 min.
7.9.6 Contact-Angle (CA) Measurements
The static and advancing contact angles were rneasured with a Rame-Hm NRL LOO
goniorneter. On average. 6 drops of water ar.d hexadecane (HD! werc measurcd on
different areas of the polished side of a silicon wafer for each sample, and mean values with
a maximum range of e0 are reportd. The advancing contact angles of captive drops were
found to be roughly above the static values of sessile (frcc-standing) drops. Cf the drop
was allowed to fa11 from the nwdle of the syringe 10 the suxl'acc, smaller contact angles
wcre usually obtained due to mechanical vibrations.'
7.9 .7 Fourier Transform Infrared Spectroscopy in the Attenuated Total
Reflection Mode (FTIR-ATR)
The organic thin films were grown on the surfaces of the single side polished
silicon wafcrs. A KRS crystal was sandwichcd bctwcen the niflcctivc: races of two silicon
wafers ( 1.2 cm X 3.0 cm). and the angle of non-polarized light was set at 45'. AU spectra
wcre run for 4000 scans at a resolution of 4 c d . using a Bruker IFS-48 spectrometer. A
spectmm of two clan silicon wafers with a sandwiched KRS crystal was measured as a
bac kground correction.
7.9.8 Ellipsometry
A Gaertner Scientific eliipsometer operathg at 633 nm He-Ne Iaser (A = 6328 A) was
cmpfoyed. The angle of incidence was 70.0". and the compensator was set at 45.0". Ail
reporred values with a maximum range ot32 A an the average of at least six measurcments
taken at different locations on the sample. The thickness was caiculated by comparing daia
from the same substratc before and after functionali7ation and using a value of 1.46 for the
refnctive index. This value is based on the assumption that the monolayer is similar to buik
pamffins with a refractive index of 1.45." If the monolayer is more crystalline-likc. sirnilar
to polyethylene. the refractive index thus should be within 1.49- 1.55.' It was found that an
increase of O- f in the refnctive index from 1.45 to 1.55 resukd in a dcçrease in the
mrasured thickncss by -2 A.
7.9.9 X-ray Photoelectron Spectroscopy (XI'S)
The XPS spectn were obtaincd by using a VG Escalab M W specuomcter with
monochromatizcd Mg& X-ray source to producc: the photoernission of clectrons from the
corc lcvcls of the surface atoms. About 50 Â of depth was probcd for a dctcctor
perpendicular to the surface. The analyzed surface w a 2 x 3 mm. ALI peak positions werc
cotrected for carbon at 285.0 eV in binding energy to adjust for charging effects. The
power of the source was 300 watts and a pressure of lu9 mbar.
7.9.10 UV-Polymerization
Topochemicd polymerizauon was canied out by placing the substntes into a
container and inadiating hem under a niuogen purge I'or up to 60 min with a medium
pressure Hg vapor lamp (Ace-Hanovia photochernical lamp. mode1 7830.230-430 nm with
the main spike at 365 nm), with the sample-sourcc distance of approximatety 3 cm.
7.10 References
(1 Swalen, J- D,; Allara, D. L,; Andrade, J. D.; Chandross. E. A,: Garoff. S-:
Israelachvili. J.; McCarthy. T. J.; Murray. R.; Peasc. R. F.; Rabolt. J. F.; Wynnc, K.
J.; Yu, H. Langmliir 1987.3. 432.
(2) Ulman, A. An i'ntroduction to Ultrathin Organic Filntsfrom Langmuir Blodgett tu Self-
Assembly; Academic Press: Boston, 199 1.
(3) Sagiv. J. J. Am. Chem. Soc. 1980. 102. 92.
(4) Bain, C- D.; Troughton, E. B.; Tao, Y-T.; Evall. J.; Whitesides. G. M.; Nuzzo. R. G .
J. Am. Chem. Soc. 1989, 111, 321.
(5) Porter. M. D.; Bright, T. B.; Allara, D. L.: Chidsey, C. E. D. J. Am. Chem. Soc.
1987,109. 3559.
(6) Walczak, M. M.; Chung, C.; Stolc. S. M.; Widrig. C, A.: Porter. M. D. J. Am.
Chem. Soc. 1991. 11.3. 2370.
(7) Laibins, P. E.; Whitesides, G. M-; Allara. D. L.; Tao. Y-T.; Parikh, A. N.: Nuzzo, R.
Ci. J. Am Chem, Soc. 1991,113. 7152.
(8) Troughton. E. B.; Bain, C- D.: Whitesides. G . M.; Nuzzo. R. G.; Allara. D. L.;
Porter, M. D. Langmuir 1988. 4. 365.
(9) Nuzzo, R, G.; Fusco, F. A,; Allara. D. L. J. Am, Chern, Soc. 1987. 109. 2358.
(10) Tao, Y. T. J- Am Chem. Soc. 1993, 115, 4350-
( 1 1) Linford, M. R.; Fcnter, P.; Eiscnbcrger. P. M.; Chidsey, C . E. D. J. Am. Chem.
Soc. 1995.11 7 , 3 145.
185
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Khan, M, S.; Kakkat, A. K-; Long, N. J.; Lewis.J.; Raithby, P.: Nguyen, P.;
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Tillman, N.; Ulman, A.; Penner, T. L. hngmrrir 1989.5, 101.
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Collins. R. J.; Bae. 1. T.; Sxherson. D. A.: Sukenik. C. N. Langmuir 1996, 12.
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Dhirani. A. A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest. P.; Sita, L. R. J.
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Wasserrnan. S. R.; Whitesides, G- M.; Tidswcll, 1. M.; Ocko, B. M.; Pershan, P.
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Dean. J. A. Lange's Handboûk of Chernisr-; McGraw-Hill: New York. 1992.
Tillman, N.; Ulman. A.; Schildkraut, j - S.; Penner. T. L. J. Am. Chem. Soc. 1988,
Tour. J. M,; Jones. L. II; Pearson, D. L.; Lamba, J. J. S-; Burgin. T. P.;
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(a) Liang, Y.; Schmehl. R. H. J. Chem Soc-. Chem. Commun. 1995. 1007- (b)
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(a) Morrow. B. A.; McFarlan. A. J. Langmuir 1991, 7. 1695- (b) Iler. R. K. The
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Chapter Eight
Conclusions, Contributions to Original Knowledge
and Suggestions for Future Work
8.1 Conclusions
In conclusion. we have developed a new. simple and versatile approach to molecular
self-assembly. Treatrnent of surface hydroxyl groups on inorganic oxides, such as glass,
quartz and singlecrystal silicon. with cornmerciaiiy available or easily synthesized
reagents, such as Si(NEtJ, and Sn(NEtJ,, yields surface-anchored-N% moieties. which
can react with several organic molecules containhg teminal acidic protons. such as
alcohols, thiols, carùoxylic acids, cyclopentadiene. indene. phosphines and alkynes. via
acid-base hydrolysis, leading to molecularly self-assembled monolayers. After optimizing
the deposition conditions, ordered and densely packed mono- and multilayers have been
produced. By means of this simple acid-base hydrolytic chemistry approach. self-
assemblcd monolayers of a variety of long chain alcohols containing teminal alkyl. phenyl
and acetylene groups on inorganic oxide surfaces have b e n successfully prepared.
The two-step process involving the reaction of surface hydroxyl groups first with
S ~ ( N E L ) ~ followed by ROH. is more efficient than the ihree-step process involving thc
rcaction of SiC14, NEt2H and ROH in sequence. and produces closeiy-packed and weU-
ordered thin films. The two-step route is able to produce monolayers of sirnilar quality as
the traditional self-assembly routes such as deposition of thiols on gold and
alkyltrichlorosilanes on inorganic oxides. When placed in hot water and organic solvents.
thw films are more susceptible to hydrolysis than the comesponding OTS monolayers on
silica. as expected, but show comparable stabilities at ambient and high temperatures. and
upon treatment with acid and base.
In addition, acid-base hydrolytic chemise of aminosilanes with dihydroxy
terrninated molecules containing rigid-rod type and alkyldiacetylene backbones, has k e n
used to constnict self-assembled mono- and multilayers on inorganic o d e surfaces.
Multilayers of up to 10 layers with good quality and relatively close-packing were
fabricated from the dihydroxy molecules, and there was no increasing disorder upon
addition of each successive layer. The stabiiity of the thùi films is significantly enhanced
upon layer-by-layer deposition of the rnultilaycred thin films- An increase in the intensity of
thc hm,, was observed upon multilayered thin filni construction. Upon W exposure. the
ùitensity of the A,,, decreased significantly with the subsequent formation of a blue film,
and is accompanied by a structural change where the Cc bonds becorne parallel to the
substrate upon topochernical polymerization.
The hydrolysis of surface bound basic tin-amide moieties with acidic protons of
akynyl chromophores l a d s to molecuiar self-assembly of a variety rigid-rod dicynes on
inorganic oxide surfaces. The z-x interactions in the molecules lead to ordered and densely
packcd thin film structures. 'lhe surface coverage of chromophores in these thin films on
quartz was found to be 2-7 molecules/100 A'. comparable to the density of surfacc
hydroxyl groi'ps on silica degassed at 100- 150 'C. Upon W - V i s exposure of SAMs of
W n y l chromophores containincg rigid-rod type diacetylenr: backbones, thc inicnsity of the
A,, decmased significantly with the formation of a blue tïlm. suggesting a structurai
change accompanied by C-=C bonds becoming more parallel to the substrate upon
topochernical polymerization.
Furthemore, self-assembled mono- and multilayers of a variety of organic
chromophores tenninated with acetylenc groups on both ends. on inorganic oxide surfaces.
have k e n successfull y prepared by thc acid-base h ydrolytic c hemistry route. Multilayers of
dialkyncs of up to 5 layers were fabricated and thcre was no incrcasing disorder upon
addition of each successive layer. It is inferred chat simple acid-base hydrolytic chemistry of
arninostannanes with dialkyne chromophores c m be successfully employed for the
construction of relatively close-packed mono- and multilayer thin film assemblies.
Finally, cobalt carbonyl was found to adsorb in the thin film assemblies by reacting
with the acetylene groups. These cobalt-adsorkd awne films may become potential
candidam as heterogenized homogeneous catalystc. one-dimensional conductors and
chcmically modified electrodes and sensors.
8.2 Contributions to Original Knowledge
Molecular self-assernbly has recently become an imporkmt area of research due to its
potential applications in thin füm technology. We have dcveloped a new. genenl and
convenient approach to molecular self-assembly based on acid-base hydrolysis. The
advantage of using acid-base hydrolytic chemistry is the ability to incorponte a vluiety of
func tionalities on inorganic oxide surfaces. These functionalitics are organic compounds
with terminal acidic protons having p k I 25 including acids. thiols. alcohols. phosphinas.
cyclopentadiene. indene and alkynes. The hydroxylated silica based sudaces such as glass.
quartz and single-crystal silicon cm be easily modified by the rcaction of surface-anchored
aminosilanes or aminostannanes with these protic specics.
WC havé constructed SAMs of a variety of alcohols containing tcnninal alkyl. plicnyl
and acetylenc groups on Si(1ûû) (Si/SiO,) surfaces for ihe îirst time via the acid-base
hydrolytic chemistry route. Thc two-step process involvcs the rcaction of surface hydroxyl
groups first with Si(NEî,,), followed by ROH. It is mon: el'ficient than the three-step
process involving the m u o n of SiCl,. NEsH and ROH in sequence. The new simple
acid-base hydrolytic chemistry two-step route is able to produce relatively close-packeâ and
well ordered thin films of similar quality as akyltrichlorosilanes on sihca and alkanethiols
on gold exccpt that these ill.ms are more susceptible to hydrolysis than the correspondhg
OTS monohyers on silica as expected.
A layer-by-layer construction of multilayers of dihydroxy teminated compounds
containing rigid-rod type and akyldiacetylene backbones on Si(100) (SiISiOJ Ma =id-base
hydrolytic chemistq route has been successfully achieved. The di01 teminated
chromophores f o m good quality and ~lat ively close-packed thin films on silicon (silica)
surfaces, and there is no increasing disorder in the thin f i s wirh increasing numbcr of
layers. Capping thin films of di01 with OTS helps to increaw the stability of the rrlms under
varied conditions, suggesting that a SAM of OTS acts as a protective coating for the di01
fdms. Multiiayer formation enhances the stabiiity of the thin films under varied conditions
too. The thin film asscmblies can be subjected to topochernicd polymerization to produce a
blue fidm upon UV-Vis exposure.
Molccular self-assembly of rigid-rod alkynes with extended x-conjugation on Si(100)
(Si/SiO,) via acid-base hydroiysis of surface bound basic tin-amide moieties has been
reported. The x-n interactions in the molecules can lead to rclatively highly ordered and
densely packed thin film assemblies with a comparatively high surface coverage. In
addition. the thin film assembly with diacetylene backbone can be subjected to topochemical
polymerization with the formation of a blue film upon UV-Vis exposure.
A layer-by-layer constniction methodology using Sn(NE&), and dialkyne teminated
chromophores containing alkyl or aromatic type backbone can lead to multiiayered
supramolccular structures with good quality and relatively close-packing on Si(100)
(SUSiOJ; there is no increasing disorder in the thin film asscmblies with increasing numbcr
or layers. Cobalt carbonyl can successfuily adsorb on these thin films under room
tcmperature conditions. These cobalt-adsorbed alkyne f i h s may h o m e potential
candidates as heterogenizcd homogeneous catalysts, one-dimensional conductors and
chemically modified electrodes and sensors.
8.3 Suggestions for Future Work
in order to consuuct good quality, close-packed and highly stable monolayers and
rnuItilayers of alcohol and a w y l chromophores on amino-silane and -siannane
functionalized substrates. a complete mechanism of the two-step process involving the
reaction of surface hydroxyl gmups first with Si(NEt,) JSn(NEs), followed by
alcohoValkyne chromophores should be known. Therefnre. a detaded kinetic study of the
two-step process should be investigated.
A detaiied undcrstanding of thc interactions betwcen atkynyl molecules and the role of
the substrate in molecular self-assembly is essential in building denxly packed thin film
materials. Aikynyl moieties in supramolecular structures c m bc polymcrized topochemicdly
to give one-dimensional conductors and third-order nonlinear optical materials.' An
important requirement for surface polymerization is the orientation of the aikynyl
chromophores on the surface. Therefore, molecular rnodeling and semiempuical
calculations are useful in achieving an understanding of molecular self-assembly of rigid-
rod akynes on solid substrates.
Since closely packed and ordered monolayers of alcohols or alkynes cm be fomed
on glass, quartz and Si(100) (Si/SiOJ. a detailed study of these Lhin f h assernblics on
othcr inorganic oxides such as mica. alurninum oxide and germanium oxide is important to
gct a complete understanding of the molecular self-assernbly of alcohols and alkynes on
inorganic oxide surfaces,
Although eiiipsometry and contact angle rneasuremcnts providr information on thc
surfacc properties and packing of monolayers and multilayers of alcohol and alkync
chromophores on Si(100) (Si/SiO,). ihis information only indirectly reflccts the possible
orientation of aikyl chahs and of substituents contained in the bulk of the film.' To examine
the order in these thin film assemblies. the use of polarized ATR measurements is required.
Furthemore. hy means of X-ray reflectivity, the determination of the electron distribution
in films ostensibly having variations in electron density along the z axis would provide one
direct measure of order in these systems.'
I t was reportedLS that the conjugated, rigid-rod chromophores with a phenylene
ethynylene/diacetylene backbone have significant third-order nonlinear optical properties,
and may act as one-dimensional conductors after subjecting to topochemical polymerization;
we can further study these properties for these alcohol and alkyne films possessing rigid-
rod phenylene ethynylenediacetylene backbones on Si( LOO) (Sif SiO,). In addition. those
alcohol and alkyne chromophores with longer phenylene ethynyienddiacetylene backbones
processing higher third-order nonlinear optical properties may be constructed for
characterization.
Besides cobalt. other transition metals may also adsorb on the alkyne films4 Further
study of adsorption of othcr transistion metals on the alkyne assemblies may be necessary
to explore their potential uses as chemically modified electrodes and sensors.
Since the reactivity6 of Ge(NEt.J, towards protic species and their stability to
hydrolysis are in between those of Si(NEt,), and Sn(NEtJ,. it is useful to study the acid-
base hydrolytic chemistry approach to molecular self-assembly by the treatment of surface
hydroxyl groups on inorganic oxides with Ge(NE&), in order to yield the corresponding
surface-anchored-NEt, moieties followed by reacting with a variety of protic species.
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1 hereby give copyright clcarance for the inclusion of the foilowing papers, of which 1 am
CO-author, in the thesis ofChi Ming Yam.
Simple Acid-Base Hydrolytic Chemistry Approach to Molecular Seif-Assembly: Thin F i s
of Long Chain Alcohols Tenninated with Alkyl, Phenyl, and Acetylene Groups on
Inorganic Oxide Surfaces.
Langmuir 1998, 14, 6941-6947.
Molecular Self-Assembly of Dihydroxy Terminated Molecules via Acid-Base Hydrolytic
Chcmistry on Siiica Surfaces: Step-by-Step Multilayered Thin Film Construction. Langmuir 1999. in press.
Molecular Self-Assembly of Dialkynyl Terminated Chromophores via Acid-Base
Hydrolytic Chemistry on Inorganic Oxide Surfaces: Step-by-Step Multilayercd Thin Film
Construction.
Lnngmuir 1999, submitted.
Q-, Dr. Ashok K. Kakkar