MOLECULAR SELF-ASSEMBLY OF LONG CHAIN ALCOHOLS,

THIOLS, AND CARBOXYLIC AClDS ON A SINGLE SUBSTRATE

VIA ACID-BASE HYDROLWIC CHEMISTRY

by

Samuel S.Y. Tong

A thesis submitted to the Faculty of Graduate Studies and Research

of McGill University in partial fulfillment of the requirements for the

degree of Master in Science.

August 1996. Department of Chemistry, McGiII University, Montreal, Quebec, Canada. O Samuel S.Y. Tong

. y of Canada du Canada

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This thesis is dedicated to my father. He is my teacher, my mentor and rny inspiration.

ABSTRACT

This thesis reports a novel route to molecular self-assembly based on simple

acid-base hydrolytic chemistry involving the reactions of aminosilanes with

organic species containing acidic protons. From detailed solution chemistry of

trimethyl-, trimethoxy-, and triphenyl-chlorolamino-silanes, two general

processes have been developed for the self-assembly of long alkyl chain

terminated alcohols, thiols, and carboxylic acids on inorganic oxide surfaces Le.,

glass, quartz and single crystal silicon. The first method involves three

successive reactions with the surface moieties where the initially formed

silylchloride layer is converted to a silylamine, which is then reacted further to

give the organic monolayers. The second rnethod involves a single reaction of

the surface with surfactant species formed first by reacting

trimethoxysilylchloride with the appropriate chromophore. The chemistry and

comparative advantages of these two methods are discussed. A discussion on

the complete characterization of newly formed thin films by employing surface

techniques, such as wettability, FTIR-ATR spectroscopy, ellipsometry, and X-ray

reflectivity, is provided. These results indicate that thin films formed using

simple acid-base hydrolytic chemistry are comparable to those obtained from

more established techniques, such as trichlorosilanes on glass, thiols on gold,

and carboxylic acids on silver or alumina. The results presented in this thesis

demonstrate that the acid-base hydrolytic chemistry is a viable and widely

applicable method to molecular self-assembly and can be considered as a

unifying approach to literature methods.

Ésu

Cette thèse présente une nouvelle avenue pour l'auto-assemblage moléculaire

simplement basée sur la chimie hydrolytique acide-base impliquant la réaction

d'aminosilanes avec des espèces organiques comportant des protons acides.

Se basant sur la chimie des triméthyls, trimethoxy et triphénylsilanes, deux

procédés ont été développés pour l'auto-assemblage d'alcools, de thiols et

d'acides carboxyliques à longue chaîne sur des surfaces d'oxydes inorganiques

(quartz, verre et silicium). La première méthode implique trois réactions

successives avec la surface. La seconde méthode implique une seule réaction

de la surface avec un surfactant formé par la réaction du chlorure de

triméthylsilyl avec un chromophore approprié. La chimie et les avantages

comparés de ces deux méthodes sont présentés. La caractérisation complète

du nouveau film mince ainsi formé, par des techniques de surfaces telles des

études de mouillabilité, la spectroscopie FTIR-ATR, I'ellipsornétrie et quelques

réflectivités des rayons-X est également présentée. Ces résultats indiquent que

les films minces préparés par simple réaction hydrolytique acide-base sont

comparables à ceux obtenus par des techniques mieux établies pour la

preparation de trichlorosilanes sur verre, de thiols sur l'or, et d'acides

carboxyliques sur l'argent ou l'alumine. Cette thèse démontre que la chimie

hydrolytique acide-base est une méthode viable et largement applicable à l'auto-

assemblage moléculaire et peut donc être considérée comme une approche

globale pouvant être comparée aux méthodes de la littérature.

ACKNOWLEDGMENTS

I would like to especially thank Prof. Ashok Kakkarfor his guidance throughout

my studies at McGill, and for allowing me to work in his lab.

I would also like to thank al1 my lab-mates: Kevin Bunten, Maria Petrucci, Chi

Ming Yam, and Hongwei Jiang for creating an enjoyable environment to work in.

I would like to express my sincere gratitude to:

Graham Stringer for proof reading the thesis.

Virginie Guillemette for translation of the abstract to French.

Norbert Schuhler and Stephane Brienne for their assistance with the

FTIR-ATR,

Michel Boulayfor his assistance with the infrared spectrometers.

Nahim Saadeh for running the mass spectra.

Financial support from the Department of Chemistry, McGill University in

the form of a teaching assistantship is gratefully acknowledged.

I would also like to thank Renée Charron for her help throughout my stay at

McGill, and al1 my friends in the Otto Maass building.

Finally I would like to thank my parents for their guidance, love and support.

TABLE OF CONTENTS

Abstract ....................................................................................... iii ....................................................................................... Résumé iv

................................................. ....................... Acknowledgments .. v

........................................................................... Table of Contents vi

List of Figures ................................................................................ vii

................................................................................ List of Tables viii

Chapter 1 ............................................................... Introduction

................................................ 1.1 Langmuir-Blodgett Films 3 ................................................ 1.2 Molecular Self-Assembly 4

1.3 Common Molecular Self-Assembly Techniques .................. 6 ................................................................... 1.4 Objectives 7

....................................... 1.5 Acid-Base Hydrolytic Chemistry 9 ......................................................... 1.6 Long Alkyl Chains I O

...................................... 1.7 Methods for film characterization 12

............................................... Chapter 2 Results and Discussion 18

........................................................ 2.1 Solution Chemistry 18 ........................................................ 2.2 Surface Chernistty 28

........................................................... 2.3 Characterization 32 2.4 Corn parison of data with established techniques ................. 45

Chapter 3

Chapter 4

Conclusions and Contribution to Knowledge ................. 47

.............................................................. Experimental 50

................................................................................... References 62

LIST OF FIGURES

Figure 2 .

Figure 3 .

Figure 4 .

Figure 5 . Figure 6 .

Figure 7 .

Figure 8 . Figure 9 . Figure 1 O .

Figure 1 1 .

Figure 12 . Figure 13 . Figure 14 .

Figure 15 .

Figure 16 .

Figure 17 .

Figure 18 .

Schematic diagram of a Langmuir-Blodgett trough for deposition of monolayers ............................................... 3

Formation of a molecularly self-assembled thin film ............. 5

Self-assembly of trichiorosilanes on inorganic oxide ..................................................................... surfaces 7

Molecular self-assembly of alkylthiols on a gold surface ........ 7

Molecular self-assembly of carboxylic acids on silver ........... 8

The reaction of silylamines with a variety of acidic species .... 10

............................................... Contact angle goniometry 13

Attenuated total reflectance ...................................... 14

The schematics of an ellipsometer .................................. 15

Proposed mechanisms for the reaction of silylchlorides with diethylamine .......................................................... 20

.................... FTlR spectra of alkoxytrimethylsilanes in CC4 25

.......... FTlR spectra of alkylmercaptotrimethylsilanes in CC4 26

.................. FTlR spectra of trimethylsilylalkanoates in CC14 27

Routes for molecular self-assembly using acid-base hydrolytic chemistry ....................................................... 28

The reaction of silicon(lV)chloride with glass ...................... 29

The relation of the actual thickness measurements and alkyl chain tilt ............................................................... 35

FTIR-ATR spectra of assemblies formed from alcohols ................................................ using the 3-step method 37

FTIR-ATR spectra of films assembled from thiols using the 3-step process ........................................................ 38

Figure 19 . FTIR-ATR spectra of films assembled from carboxylic acids using the 3-step process ........................................ 39

Figure 20 . FTIR-ATR spectra of films prepared from alcohols using .............................................................. 1 -step method 40

Figure 21 . FTIR-ATR spectra of thiol assemblies prepared using .............................................................. 1 -step method 41

List of Tables

Table 1 . Table 2 .

Table 3 .

.................................................. Contact Angle results 32

Ellipsometry data .......................................................... 34

Prelirninary X-ray reflectivity data ..................................... 44

CHAPTER 1

INTRODUCTION

Optoelectronics and molecular electronics are at the forefront of materials

science due to their potential applications in the communication and information

industries'. These are two distinct areas of study, however they both encounter

similar demands. Fiber optics has fueled the pursuit of efficient nonlinear optics

based signal processing units, such as switches, modulators, and optically

driven parametric amplifiers, while the electronics industry is looking to conserve

space, requit-ing smaller and more efficient circuit elements, insulators and

packaging materials. Such requirements has spawned the development of new

materials,

Many of the currently used systems are near their lirnits of size and

efficiency. Lithium niobate is currently widely used in nonlinear optical (NLO)

devices, however it is relatively inefficient2. In general, the inorganic materials

employed are often unable to fulfill the above-mentioned demands of new

materials industries. Therefore, much interest has been placed on the

development of organic systems to replace these materials3. Organic rnaterials

offer major advantages since they can be tailored to possess larger NLO effects

than lithium niobate and similar inorganic materials2. Similarly, organic systems

are being examined as conducting, insulating and wire materials in

microelectronics since they offer the precision, quality and flexibility4. lntegrated

circuitry already uses organic polymers as insulators and resist rnaterials,

however the stringent requirernents of the microelectronics industry are still

asking for higher quality and unifonity than such polymers can provide.

One of the major challenges facing new materials based systems is the

incorporation of organics into rnacrostructures, such as thin films, with the

desired packing density, orientation and order. Such characteristics are

important if high precision and efficiency are required. There are two main

techniques which have been developed for the construction of organized and

densely packed thin films on solid substrates: Langmuir-Blodgett and molecular

self-assembly'. Both techniques have been extensively studied for use in a

variety of functions, in addition to nonlinear optics and micro electronics. For

example, the ability to assemble oriented thin films is of interest biologically2,

where protein films can be fomed for chromotographic supports for anitgen

detection, or used as biosensors which are ernbedded in lipid or polymer films

for sensing and gating applications5. One can also use organized assemblies as

an anchor for organometallic catalysts6. This would essentially heterogenize

homogeneous catalysts, allowing for new steric and intermolecular influences,

offering enhanced activity and product selectivity.

The great potential offered by auto-assemblies is largely due to the

unique properties and morphology of the ordered thin films. These properties

are much different than most bulk systems and are not as easily studied. Films

of simple long alkyl chains have been widely investigated to provide insight into

their short range intermolecular interactions, packing ability and organizati~n'~~.

New methods producing Langmuir-Blodgett and molecular self-assembled thin

films are continuously being developed to incorporate specific properties,

allowing for the development of future technologies.

1.1 LANGMUIR-BLODGETT FILMS

The Langmuir-Blodgett technique produces highly organized mono- and

multilayer thin filmsg. The technique involves spreading a monolayer on water

and compressing it such that the molecules are aligned and well packed. The

solid substrate is then dipped through the layer of molecules while the surface

pressure is maintained. This technique yields a film where the molecules have a

head-to-head and tail-to-tail configuration.

moving barder tall -

'O control pressure

+

Figure 1 A schematic diagram of a Langmuir-Blodgett trough for deposition of monolayers.

Langmuir-Blodgett technique has many disadvantages. The films formed

are not necessarily thermodynamically stable, nor are the films strongly bound to

the surface. This method can only work for molecules that have both

hydrophobic and hydrophilic ends which, in tum, limits the variety of these films.

1.2 MOLECULAR SELF ASSEMBLY

Molecular self-assembly is a technique that was developed approximately

thirty years agol*, but not extensively studied until about fifteen years later".

The self-assembled monolayers are fomed spontaneously from immersion of a

substrate into a solution of an active surfactant. This process offers the ability to

use aqueous or non-aqueous solvents for the formation of layers of molecules

which are chemically bonded to a substrate.

The process of molecular self assembly is greatly dependent on the

nature of the surfactant used. Surfactants traditionally used for such purposes

consist of 3 components (Fig. 2). The head-group is responsible for the

chemisorption of the species ont0 the substrate. The strength of the head-group

to the surface bond determines the effectiveness of the self-assembly route.

The spontaneous formation of such bonds releases energy, therefore, "pushing"

the surface species close together, allowing al1 binding sites to be occupied.

The resulting proximity of these surface species allows for intermolecular forces,

such as van der Waals attractions to be dominant.

The body of the surfactant often is composed of alkyl groups. The

spacing between the chains causes interchain van der Waals forces to be the

main interactions in such films. This assists in the packing and order found

within these films. The final component of the surfactant molecule is the

terminal functionality, such as - C Y in simple alkyl chains. These eventually

fonn the surface groups of the films, and govern properties such as

hydrophilicityl hydrophobicity of the films.

surface active head group

new surface

Figure 2 Formation of a molecularly self-assembled thin film.

Along with the mechanisrn of formation of the head-group to surface bond,

factors that affect the resultant assembly structure of the film include the lattice

spacing of the substrate, head-to-head spacing, and the natural chain-to-chain

spacing. These competing factors affect the uniformity and crystallinity of the

film. Disorder of the chains can cause gauche conformations, which do not pack

as closely.

Chemisorption is the most important factor governing molecular self-

assembly. This spontaneous process foms monolayers under equilibrium

conditions, giving films that are generally more uniforni, chemically and

therrnodynamically stable, and more robust in comparison with similar Langmuir-

Blodgett films. Molecular self-assembly can accommodate much flexibility of the

structure of the surfactant, enabling ease of structural tailoring of the films. The

areas of molecular engineering and self-organization may be the key to future

technologies.

1.3 COMMON MOLECULAR SELF-ASSEMBLY TECHNIQUES

A number of techniques for molecular self-assembly have been

developed, and much of the work being performed to date has been focused on

uncovering the fundamental understanding of self-as~embly~~~. Some of the first

examples of surfactants employed for molecular self-assembly are the

alkyltrichlorosilanes'2. These chromophores self-assemble on hydroxylated

surfaces, such as SiO2, or single crystal silicon and glass.

Figure 3 Self-assembly of trichlorosilanes on inorganic oxide surfaces.

The alkyltrichlorsilanes react with the surface silanol (-Si-OH) groups along with

the moisture at the surface, forming polysiloxane bound to the surface. These

reactions occur almost instantaneously to result in a very stable/durable thin film.

This method, using different organic chains, is often employed for the

preparation of stationary phases for chromatography columns, and has led to

the formation of muiltilayers and films with nonlinear optical propertied3.

Much of the recent literature dealing with molecular self-assembly has

been based on the films made from alkanethiols on gold surfaces14.

Figure 4 Molecular self-assembly of alkylthiols on a gold surface.

Chemisorption results in an electron transfer from Au0 to the sulfur of the thiol,

weakening the S-H bond. This leads to a covalent, though slightly polar, bound

alkylthiolate film. These films have been widely studied to determine the

mechanism of assembly on the surface, and have been explored for a number of

technological applications. Other metal substrates, such as Pt, Ag, Cu have

15,16 been reported to promote molecular self-assembly of thiols .

Carboxylic acids have been assembled on surfaces, such as AI2O3 l7 and

Ag20 18. These long chain n-alkanoic acids assemble on the surfaces through

acid-base reactions, yielding an ionic Ag' -0OC-R bond.

Figure 5 Molecular self-assernbly of carboxylic acids on silver ".

Other methods for preparing similar self-assembled thin films are

currently under investigation, including the use of alkenes on hydrogen

terminated si~icon'~. Each of these methods have been shown to give well

organized thin films in a single, relatively efficient, high yielding step.

However, each of these processes has limitations. Often, problems arise

while attempting to tailor organic films, for example, change in the body of the

surfactant can alter the electronic environment of the head group, and therefore

may not allow chemisorption to occur. Further, synthesis of compounds

terminated with specific head groups may prove to be difficult. Many of the

substrates used for self-assembly are very expensive. In contrast, glass and

single crystal silicon are relatively inexpensive substrates, and chemistry on

them can easily be utilized on semiconductors and cerarnics. Therefore, it would

be beneficial to develop a single method for self assembly on glass and single

crystal silicon that would allow the build-up of thin films for a wide variety of

surfactants.

1.4 OBJECTIVES

As rnentioned earlier, the established methods for molecular self-

assembly have al1 been end group substrate dependent. A versatile route to

molecular self-assernbly which can incorporate long chain alcohols, thiols and

carboxylic acids will contribute significantly to the fundamental understanding of

this process. One of the possible routes is the acid-base hydrolysis of

silylamines with terminal organic acids 20. Acid-base hydrolytic chemistry

would allow for the use of a variety of acidic species as possible surfactants,

including alcohols, thiols, and carboxylic acids. The benefits from using such a

flexible method seem limitless. The formation of such films would allow us to

compare the effects of the terminal group on the mode of self-assembly, and

also to compare the effects of the procedure of both the assembly and the

substrate on the consequent films.

1.5 ACID-BASE HYDROLYTIC CHEMISTRY

The chemistry of silylamines has been well do~urnented*~. They have

weak Si-N bonds and are therefore susceptible to hydrolysis by any acidic

species (Fig. 6). The reactions of silylamines with alcohols and silanols have

been shown both to be affected by steric hindrance around the Si-N bond, or

around the carbon containing the hydroxyl groups, but is not affected by the

acidity of the alcoho12'.

H-A = thiols H-S-R

alcohols H-O-R O

16-19

carboxylic acids H-O-ILR 4-5

Figure 6 The reaction of silylamines with a variety of acidic species.

Thiols react in a similar manner, however, they tend to be less reactive.

Only few reports of silylthiolç have been reportedZ2. Carboxylic acids, being

much more acidic, are able to react very quickly with silylamines readily cleaving

the Si-N bond. These reactions can be performed under relatively mild

conditions, though are dependent on the acidity of the carboxylic acid.

Silyl-amines are foned from halosilanes by their reaction with excess

amines. The halide released during the reaction is precipitated as the amine

s a ~ t ~ ~ .

The chemistry shown above offers potential to assemble organic thin films on

silicon surfaces. We have explored this chemistry on inorganic oxide surfaces,

and it has been used to self-assemble long alkyl chah alcohols, thiols, and

carboxylic acids. Sirnilar chemistry is possible using t in-arnine~~~. The Sn-

amine bond is much more basic than the silicon counterpart, therefore is more

sensitive to attack by acidic species, such as terminal alkynes.

1.6 LONG-ALKYL CHAINS

The use of long straight alkyl chains in self-assembly will aid in

understanding the fundamental interfacial properties of the resulting films.

Order, orientation and alignment of the alkyl chains affects the collective

properties of the films, therefore, these characteristics must be studied and

understood before tailoring of the surface species can be achieved.

The effect of variation of the length of the alkyl chain will be investigated

so that any change in the order and packing of the film can be determined. In

these studies, only even numbers of carbons in the alkyl chains are used to

avoid any discrepancies due to the odd-even e f f e ~ t ~ ~ . It has been shown that

the odd-even effect alters the characteristics of the film.

1.7 METHODS FOR FILM CHARACTERIZATION.

Characterization of thin films is very important. Many traditional

techniques for chernical analysis are not able to determine many of the surface

properties of such films. The commonly employed surface characterization

techniques are discussed below:

A) Contact Angle Goniometry.

Contact angles are used to estimate the quality of stable thin films'. The

contact angle is the angle at the contact point of a liquid drop on a surface. It is

measured from the liquid-solid interface to the liquid-vapour interface. Therefore

if the liquid spreads easily over the surface, contact angles are around oO, while,

if the drop sits perfectly spherically on the surface, contact angles are greater

than 90'. These contact angles depend on the surface tension of the solid

surface.

Figure 7 Contact angle goniometry.

Most commonly, water and hexadecane are used as the wetting liquids.

When water is used, hydrophilicity of the surface will determine the contact

angle. A hydrophilic surface would yield contact angles near 0'. However, using

hexadecane on a similar surface, relatively high contact angles would be

obtained.

This method is very useful for the detemination of the quality of

assembled films of long alkyl chain species. The methyl terminated end groups

yield a surface of hydrophobic species. The more closely packed the rnethyl

groups are, the larger the contact angle with water is, and lesser with

hexadecane. Contact angle goniometry enables us to qualitatively determine

the packing and uniformity of the assernbled thin films.

B) lnfrared Spectroscopy.

lnfrared spectroscopy is a very valuable tool. It shows both chernical

bonding as well as rnolecular orientation. This technique would therefore be

ideal for analyzing the process of self-assembly. It would help determine the

presence of a chemical bond, and the organization of the alkyl chains. However,

there are problems when it cornes to self-assembled thin films. The monolayers

are very thin, and need a set-up that would allow for the required sensitivity. The

intensity of the signal will also be dependent on the coverage, thickness, and

density of the film.

In order to overcome such limitations, Attenuated Total Reflectance (ATR)

spectroscopy is used. This method employs a prism that allows the incoming

beam to bounce off the intemal surfaces a nurnber of times before exiting. Such

multiple reflections intensify the signal to a reasonably detectable level. The

prisms norrnally employed are made of silicon, germanium, KRS-5, or ZnSe.

Figure 8 Attenuated total reflectance.

The sample with monolayer can be pressed against the crystal, allowing the IR

beam to penetrate the film approximately a micrometer before reflecting.

Due to the solid state nature of the thin films, ATR is a useful tool for the

determination of the packing and the crystallinity of the film. The detection of the

IR bands depends on the orientation of the band itself. Vibrations that are

parallel to the surface of the crystals are most readily detected due to the

polarization of the IR beam. Those perpendicular to the surface are difficult to

detect. For example, alkyl groups that have a trans molecular orientation and

which are perpendicular to the surface, will have their methylene C-H vibrations

parallel to the surface, while disordered, cis oriented alkyl groups, MI1 have

diff iculty in detecting the C-H vibrations, which are perpendicular to the surface.

C) Ellipsometry

Ellpsometry is used to determine the optical properties and the physical

structure of thin films, and it employs the change in polarization of an incident

plane-polarked monochromatic beam once it is reflected from a surface

material, as elliptically polarized light.

Figure 9 The schematics of an ellipsometer.

The ellipsornetric angles (v and A) are therefore determined. y is the ratio of the

change in amplitude for s and p polarizations of the reflected light, and A is the

difference in the phase ~h i f t s~ l *~ . These values are determined from the Fresnal

Reflection coefficients of the p and s polarizations, r, (parallel to the plane of

incidence) and r, (perpendicular to plane of incidence). p. the ratio of r,, and r,

are measured by ellipsometry and are related to v :

A= measured analyzer angle P= polarizer angle

v=A A= 2P+ d2

p= rdr, = tan y exp(iA)

These can therefore be related to the thickness and the refractive index of a film.

Ellipsometry is very useful in obtaining information in a non-destructive,

and a non-perturbing rnanner, with a thickness resolution of 1 to 2 A.

D) X-ray Ref lectivity

X-ray reflectivity is a useful method for analyzing film thickness and

~niforrnit f~~~'. Thickness is determined from the analysis of the X-rays reflected

from the sample. Synchrotron radiation is used as the source of X-rays. X-ray

reflectivity rneasures the intensity of these reflected X-rays (R) as a function of

the angle (0) between the incoming X-ray and the sample. The intensity varies

due to the difference in the phases from the air to monolayer and monolayer to

substrate interfaces.

sutstrate : film air l I

The intensity (R) can be related to the derivative of the electron density along the

normal z axis by:

where qz is the change in momentum of the X-ray during reflection, p, is the

electron density of the bulk substrate, and RF is the Fresnal Reflectivity, the

intensity of the X-rays reflected from bare substrate. This equation describes the

pattern of interference from the reflection of the X-rays. The pattern is

dependent on the distance separating the two interfaces. Therefore, this is a

direct measurernent of the thickness of the monolayer (unlike ellipsornetry). This

method does suffer from some drawbacks, foremost being the X-ray darnage to

the sample. Exposure of organic monolayers to synchrotron radiation have been

shown to degrade them, thus resulting in a change of the surface properties.

CHAPTER 2

RESULTS AND DISCUSSION

2.1 SOLUTION CHEMISTRY

The simple acid-base hydrolytic chemistry of silylamines described in the

introduction offers potential for the molecular self-assembly of a variety of

chromophores on solid surfaces. Sirnilar to the chemistry of trichlorosilanes, it is

expected that a single layer of crosslinked chlorosiloxanes would be obtained

once we react silicon(lV)chloride with clean glass or silicon surfaces, which can

then be converted into silylamine terminated surfaces, and subsequently to the

chromophoric monolayers.

To help us understand the chemistry of silylamines, we initially focused on the

solution chemistry.

Much of the chemistry of silylamines has been reported ex tens ive~f~~~~.

We have used a number of solution reactions to help provide insight into this

chemistry. Determining an ideal model compound proved challenging,

particularly when required to mimic the steric and electronic environment of the

surface bound species.

Commonly, silylamines are formed from their corresponding silylhalides.

Some of the readily available silylchlorides include trimethylsilylchloride,

triphenylsilylchloride, and trimethoxysilylchloride. Triphenylsilylchloride was

chosen because the effect of increased steric bulk on the reactions could be

studied, however, the electronic influences of the phenyl groups on Si must also

be borne in minci. Trirnethoxysilylchlorides can be used to model the electronic

environment of the surface bound silylchlorides. The methoxy groups, however,

are themselves susceptible to condensation reactions producing oligomers or

polymers. Trimethylsilylchloride was used as a model because it is not able to

condense with other silylchlorides, and its methyl groups have more comparable

electronic properties than phenyl groups.

The formation of the silylamine from diethylamine and the corresponding

silylchloride was investigated initially:

R3SiCI + 2 NEt2H + R3SiNEt2 + NEt2HHCI R=Me, MeO, Ph

Trimethysilylchloride and trimethoxysilylchloride react instantaneously under

ambient conditions with diethylamine producing the desired silylamine, and the

salt NEt2HH-CI as a precipitate. Upon examination of the 'H NMR, the intensity

of the methoxy peaks decreases slightly upon reaction, possibly indicating some

condensation of the methoxy groups. For the reaction to occur with

diethylamine, triphenylsilylchloride requires heating of the reaction mixture to

80°C for 48h, however it is still not complete. Constant removal of this salt

during the reaction may be required to obtain high yields in this reaction.

There are numerous possible mechanistic routes that would lead to the

formation of the silylamines from trimethylsilylchloride and diethylamine. One

possibility is for the amine to approach the silylchloride bond at a 90' angle.

While the other proposed mechanism is for the amine to attack from the

backside, leading to a 5 coordinate intermediate, resulting in a similar

28,29 product .

Mechanisrn 1 /

Figure 10 Proposed mechanisms for the reaction of silylchlorides with diethylamine

The proposed mechanisms would allow for sterics and electronics to influence

the reaction, both possibly explaining the observations of the above reactions.

It has been noted that nucleophilic displacement of silylchlorides results in the

inversion of stereochemistry at chiral silicon ~ e n t e r s ~ ~ ' " and suggests that th2

reaction proceeds through mechanisrn 2.

Reactions of silylamines with long alkyl chain alcohols, thiols and

carboxylic acids were examined. Trimethyl- and trimethoxy-diethylaminosilanes

were reacted with alcohols at room temperature over a period of up to 12h.

Long chain carboxylic acids were found to react instantaneously to give the

corresponding silylesters. Thiols do not react as quickly with these silylamines.

They require heating to 80°C for 48h before they are found to react completely.

Again, from 'H nmr, the reactions using the trimethoxysilylamine tended to give

products which were partially oligomerized especially if the reaction mixture was

heated,

The reactions using triphenylsilyldiethylamine do not occur as readily

compared to the other systems studied. Long chained alcohols and thiols do not

react, even on heating for several days. In contrast, carboxylic acids reacted at

80°C, after 24h.

The reaction of alcohols, thiols, or carboxylic acids with the silylamines

occur via acid-base hydrolysis. These reactions with thiols have been reported,

however vety few of such compounds have been made 20*2'130. The difference in

reactivity observed with the triphenylsilydiethylamine could be due to the more

bulky phenyl rings. Phenyl rings are also electron withdrawing. This results in a

silicon-nitrogen bond which is less susceptible to cleavage by acidic species.

Such an environment is not expected to be encountered by the silicon species

bound at the surface.

The same silicon bound species can be obtained by reacting the

silylchloride directly with the acidic species in the presence of a base (such as

triethylamine). This way, the base is able to pull away the resulting H-CI by-

product, thereby driving the reaction to completion.

These reactions tend to work more faster than those with the silylamine. This is

probably due to the weaker silicon-chloride bond. Even the reactions using

triphenylsilylchloride proceed to completion using acids, thiols and alcohols, and

heating was needed only with the thiol. Trimethyl and trimethoxysilylchlorides

both react quickly at room temperature with alcohols and carboxylic acids, while

thiols require 24h before completion. Formation of the NEt3HCI salt seems to

limit the rate of reaction, and removal of this salt tends to help push the reaction

dong.

The stability of the trimethylsilyl compounds in the presence of both water

and methanol were examined. To the trimethylsilyl compounds made from

alcohols, thiols and carboxylic acids, one molar equivalent of water or methanol

was added. These reactions were monitored using 'H nmr, and the samples

were heated if no initial reaction was observed. Alkoxytrimethylsilanes (1) are al1

stable in the presence of both water and methanol, while hydrolysis of the

silicon-sulfur bond was observed after heating the alkylmercaptotrimethylsilanes

(2) over a period of 2 to 3 days. As expected, the trimethylsilylalkanoates (3) are

very susceptible to hydrolysis by both water and methanol. The reactivity of

these species correspond directly to the acidity of alcohols, thiols, and carboxylic

acids.

lnfrared spectroscopy is used to distinguish any changes in the

environment of alkyl groups. Typically, the most distinguishable bands are at

2960-2970 cm-' from the CHû asymmetric stretches, 2915-2930 cm" from the

CH2 asymmetric, and the 2840-2850 cm" band from the CH2 symmetric

stretching frequency. This region is able to distinguish between alkyl groups

samples that are crystalline and those in solution.

The solution infrared spectra of the resulting trimethylsilane terminated

species have been obtained (see Figures 11-13), and have been found to be

very similar to the corresponding alcohols, thiols, and carboxylic acids. This

general agreement of the peak positions tells us that once bound to the silicon,

the structural integrity of the alkyl chains is not significantly perturbed.

Therefore, once on the surface, it is not expected that the alkyl groups would

possess stretching frequencies much different than those from the solution

models.

From the infrared spectra of the solution models, we can see a few trends

in the peak positions and intensities as the chain length is increased in al1 the

alcohols, thiols and carboxylic acids. There is a relative decrease in the intensity

of the methyl peaks as the chain length increases, especially the CH3 symmetric

band, at about 2872 cm" due to the increase in methylene groups. Though not

very significant, the peak positions for the CH3 asymmetric, CH2 asymmetric,

and CH2 symmetric bands al1 seern to decrease as the chain length is increased.

lnfrared spectroscopy is therefore very useful since it is sensitive to

minute changes in the structure of the alkyl groups. It would be very beneficial if

such characterization methods were sensitive Po thin films.

The use of the solution models has been useful to establish the potential

for reactions on the surface, and can be used to model the results obtained at

the surfaces. The chemistry in solution establishes that the use of acid-base

hydrolytic chemistry can be used as a foundation for molecular self-assembly.

Wavenumbers (llcm)

Figure 11 R I R spectra of alkoxytrimethylsilanes in CCls.

Figure 12 FTlR spectra of alkylrnercaptotrimethylsilanes in CC14.

Q> U c 0 e E: rr a

m m

-

C

-

C

C

- O <O

m P QD V ) * (D

Q) Q) OD CO O Cu CU (U CU CU a0

CU

wave numbers (llcrn)

Absorbance

2.2 SURFACE CHEMISTRY

Acid base hydrolysis, as described previously, offers a potential route to

the formation of molecularly self-assembled thin films on glass and silicon

surfaces. Two methods of self-assembly have been developed:

3-STEP METHOD CI

1

1 -STEP METHOD

R-H CI 1

3= -O-(CH2)n-CH3 -S-(CH2)n-CH3 tol. -OOC-(CH2)n-CH3

' AJ Figure 14 Routes for molecular self-assembly using acid-base hydrolytic chemistry

The first method, a 3atep process, involves a sequence of 3 reactions with the

substrate. Glass or silicon, cleaned to expose maximum number of surface

hydroxyl groups, is reacted with silicon(lV)chloride, producing a bound

silylchloride film. This is then converted to the silylamine using excess

diethylamine. Finally the desired surfactant molecules can then be added,

yielding self-assembled thin films.

Figure 15 The reaction of silicon(lV)chloride with glass likely occurs through 3 stages although they may occur in any order. (i) adsorption ont0 the glass surface; (ii) hydrolysis; (iii) polymerization.

The bonding of the molecules to the surface, as shown above, represent

a general pattern which has been widely used in literature3. The reactions of

chlorosilanes (e.g. SiC14) with the hydroxyl groups on the inorganic oxide

surfaces in the presence of molecular amounts of water, generally lead to a

Si-O-Si polymer network, and the represented Si-CI moieties perpendicular to

the surface.

The reaction conditions were first optimized in order to obtain the

maximum coverage. All the reactions were performed under a nitrogen

atmosphere to prevent the surface from being contaminated. Because of the

susceptibility of the silicon-chloride bond to hydrolysis, it was necessary to limit

the amount of water present in the system. However, the silicon dioxide

surfaces were not, and did not have to be completely devoid of moisture. Water

is required to forrn siloxane linkages between the resulting silylchlorides (step (ii)

fig.15). The silicon(lV)chloride reaction with glass is facile at room temperature.

The substrate was immersed for a brief period in a solution of ammonia before

drying. Ammonia helps bind the silylchloride to the surface silanol. This step

has been found to increase the surface c~verage*~. The samples are left in the

solution of silicon(lV)chloride for -14h in order to completely cover the surface.

After the formation of the layer of silylchlorides, the surface is reacted with

diethylamine. It was found that a temperature between 70-75OC optimized the

reaction. Below this temperature the resulting NEt2HHCI salt is insoluble in

toluene, often causing the samples to be coated with a film of NEt2HHCI, while

above 90°C, coverage is less due to thermal decomposition.

The amount of time in the diethylamine solution was also investigated.

The best results were obtained for the reaction with diethylamine at 75OC for

48h. However, it is believed that, even at this temperature, a film of the salt is

still able to form at the surface. This could be minimised by washing the

substrate with warrn toluene, then replacing the sample in the solution of

diethylamine and allowing the surface to react further. It was found that the best

results were obtained after 2 such washings each after 24h of subsequent

immersions in the diethylamine solution.

The amount of time in the solution of the surfactant is also very important,

and each type of surfactant has been found to require slightly different

conditions. The long alkylchain alcohols require heating at 70°C for 24 hours,

while carboxylic acids need to be heated for 60°C for 24 h. Alkylthiols require

48h of heating to 80°C before obtaining maximum coverage.

We also tried reacting the chloride surface directly with the alcohols in the

presence of triethylamine. These reactions were perfoned under a nitrogen

atmosphere at 80'~ and always produced films of poor quality. This is probably

due to the formation of the NEt3HCI salt at the surface.

The second method, the 1-step method, uses the trimethoxysilyl bound

alcohols, thiols and carboxylic acids, as described in the previous chapter. The

trimethoxysilyl bound surfactants were prepared by reacting

trimethoxysilylchloride with the acidic chromophores in the presence of

triethylamine. The NEt3HCI salt was then filtered off, and the clean glass or

silicon substrates were then immersed in the filtrate. Maximum coverage for al1

types of acidic species investigated was obtained after reacting the slides for

24h at 60°C. These reactions produce methanol as the condensation by-

product. We were not able to prepare films of carboxylic acids using this one-

step process probably due to the hydrolysis of the silylester bond by methanol.

The stability of the resulting films was also studied. Water and methanol

both destroy the films created from carboxylic acids. The thiolate films were

slowly hydrolyzed by water and methanol, however the films made from long

chain alcohols were not affected, a similar observation to that of the solution

models.

A. Contact Angles

Table 1 summarizes the static contact angles of deionized water on the

thin films produced. Using contact angles, we are able to examine the quality of

Table 1 Contact angles obtained from films of a) alcohols; b) thiols; c) carboxylic acids using the 3 step process, and d) alcohols; and e) thiols using the 1-step process

3-STEP PROCESS 1-STEP PROCESS

the film, and compare them to the analogous structures prepared using different

methods. Nomally, for alkyl groups, close packed methyl terminated films

would have water contact angles of 105-1 15')'. The data was reproducible to

within 2' on a given sample.

The contact angles from the 3-step process are lower than the ideal

values for close packed systems, indicating pooriy ordered films. The values for

the films made from the carboxylic acids and thiols rnay not be entirely

representative of the initial film quality since water would at least partially

hydrolyze the silylester or silyl thiolate bonds. The values for the thiols are even

lower than the acids or alcohols. Again this may be because the silicon-sulfur

bond is hard to form, especially with very sterically hindered silicon species.

The results obtained from the 1 -step process show that these films are of

much higher quality. Comparatively higher contact angles are indicative of very

low surface free energy due to highly packed methyl groups3'. This better

packing rnay be due to the methoxy groups polymerizing by condensation in

solution first before binding to the surface, similar to that observed from the

alkyltrichlorosilanes. This rnay bypass rnany of the problems that rnay be

encountered during absorption ont0 the surface.

There is a slight trend in contact angles with increasing chain length. This

rnay infer that the shorter alkyl chains are Iess organized. However, this rnay

also be due to the sensitivity of the probe liquid, water, to the underlying glass

substrate. The silicon-oxygen bonds in the glass rnay have dipole-dipole

interactions with the water droplet, and would cause the contact angles to

decrease more with thinner films.

B. Ellipsometry

Optical ellipsornetry uses the changes in polarized light reflected from the

sample to indirectly measure film thicknesses. This technique was applied as a

convenient and precise means of determining the average monolayer thickness

of the films, and to follow the progress of film growth. The thicknesses obtained

are from samples on single crystal silicon 100, (res.: 3-7 ohm.cm, thickness: 13-

17 mils, grade: SSP). The oxide layers on the silicon samples may Vary, and

were first measured by the ellipsometer. Four to five measurements were taken

from each sample, and an average of these values is reported. Across the

surface, film thickness values would typically Vary 2-3A.

Table 2 Ellipsometry thickness measurements

3-STEP PROCESS

n A theoret 3 14 11 7 15 16 9 15 18.5

11 19 21 13 16 23.5 15 20 26 17 23 29

1-STEP PROCESS

theoret 11 16

18.5 2 1 26 29

theoret 13

15.5 18 21 26 29

In order to relate these data to a simple structural model, thicknesses were

calculated for an ideal film consisting of n-alkyl groups fully extended in an all-

trans conformation perpendicular to the surface. Assuming al1 tram

conformation, the projection of the carbon-carbon bond ont0 the surface normal

(assuming the bond angles at the C atoms are 109.5') is 1.26A, Si-O 1.33A, C-O

1.35A, C-S 1.72A, CO-O 1.22A, and the terminal methyl group -CHû is 1.92A.

Upon cornparison of the data with the theoretical values, it can be

concluded that the thickness corresponds to the formation of films of one

molecular layer. In general, we can Say there is a linear increase in the

ellipsometric thickness with chain length. This indicates that there is consistency

in both the conformational and spatial orientation of the alkyl chains for al1 chain

lengths.

theoretical thickness

actual thickness

7- Figure 16 The relation of the actual thickness measurements and alkyl chah tilt

The data shows that the alkyl groups in rnolecularly self-assembled thin

films are at about an average of a 35' tilt. A tilt angle of about 30' is normally

expected due to the packing arrangement of the alkyl chainsZ5. Our films have a

slightly larger tilt, and it may indicate that our films are not as well ordered.

Disordered segments of alkyl chains would result from chains which are

conformationally or thermally disordered as a result of gauche kinks andlor weak

cohesive interchain interactions.

C. FTlR-ATR

lnfrared spectroscopy measurements via Attenuated Total Reflectance

were made in order to examine the contributions of chain conformation,

orientation, and packing due to the changes in chain length. The ATR

attachment uses multiple refiections, allowing for increased sensitivity to such

thin films. The FTIR-ATR spectra were al1 taken from samples on single crystal

silicon, using a KRSd crystal, at a 45' angle.

Figure 17 FTIR-ATR spectra of assemblies formed frorn alcohols using the 3-step method

Absorbance

wavenumbers (l/cm)

Figure 19 KIR-ATR spectra of assemblies formed from carboxylic acids using the 3-step process

Wavenumbers (llcm)

Figure 20 KIR-ATR spectra of films prepared from alcohols using 1-step method

Wawnumbers (l/cm)

Figure 21 FTIR-ATR spectra of thiol assemblies prepared using 1 -step method.

The spectra produced (Fig. 16-20) are normally characterized by the presence of

three distinct absorbances in the ranges of 2855-2866cm", 29 1 %2930crn-', and

2956-2962 cm". These bands can be assigned straightforwardly as primary

contributions from the methyl C-H antisymmetric stretching, and methylene C-H

antisymmmetric and symmetric stretching modes, re~pectively~~.

The general agreement of the band positions of the thin film spectra with

those for the bulk long chain surfactants demonstrates that the structural

integrity of the alkyl chains is not significantly peiturbed due to the formation of

the assembly. The band frequencies are able to provide insight into the

intermolecular environment of the alkyl chains in these assemblies. Bulk

crystalline alkyl groups would have va (CH*) close to 291 8cm-', while in solution

1 7 3 4 va(CHn) is shifted to 2929cm- .

The films formed using the 3-step process al1 have well defined

absorbances. The band intensities, in general, reflect the proportion of methyl to

methylene groups, thus shoiter alkyl chains have relatively larger methyl

absorbances. Frequency shifts in the spectra of the thin film, while small, are

significant and very informative. The position of v,(CH2) also tends to decrease

as the chain length increases. This indicates that the average local environment

of an individual chain with more methylene groups are closer to their neighboring

chains, minimizing free volume.

The films forrned using the 1-step process with alcohols have ATR

spectra with the most prominent trend in v,(CH2) shifts, spanning from a fluid-like

2930 cm-' for the butanol derivative, to a more crystalline 2922cma1 for the

octadecanol. The thiols have films which al1 have slightly lower frequencies,

indicating that they are more crystalline-like. This is somewhat surprising since

the contact angles from such samples showed that these were not very well

packed. The carboxylic acids did not have much shift in absorbance position at

all. They also showed the least crystalline character.

The films forrned using the one step process do not have as well defined

absorbances in the IR. Trends in the band positions of these samples are not

well defined. The alcohols show spectra of the short chained moieties to have

broader bands. With thiols, al1 the absorbances due to the alkyl groups are very

broad, and there is no apparent trend with the band frequencies.

Upon closer examination and comparison of the one-step to the three-

step processes, the films obtained using the one-step process tend to have

broader absorbances in the IR. This may again indicate that the films from one-

step process are less uniform and less ordered. This is contrary to the data

obtained using contact angles and ellipsornetry. This could indicate that the

packing of the methyl groups may be independent of the packing of the inner

methylene groups, and is governed by the mechanistics of the molecular self-

assembly process.

D. X-ray Reflectivity

X-ray reflectivity measures the intensity of the X-ray reflected frorn the

sam ple to directly measure the film thickness. Some prelirninary X-ray

reflectivity data has been obtained, however, due to limitations of the beamtime,

only a few samples have been run so far. The data obtained show thicknesses

consistent with the values obtained using ellipsometry. Further studies are in

progress at Brookhaven National Laboratories.

Table 3 Preliminary X-ray reflectivity data

I 3-STEP PROCESS

theoret ri

1-STEP PROCESS

2.4 COMPARISON WlTH ESTABLISHED TECHNIQUES

The advantage of using acid-base hydrolytic chemistry for molecular self-

assembly is the ability to use a variety of surfactant species on a single

substrate. This allows one to examine the roles of the substrate and surfactants

in molecular self-assembly. Upon comparison of films prepared using literature

methods e.g., alkyltrichlorosilanes on glass, alkylthiols on Au, and carboxylic

acids on Ag or Al, with those prepared using acid-base hydrolysis, it is apparent

that the acid-base hydrolytic route is capable of producing films with similar

quality and microstructures.

For example, the static contact angles of water obtained from thin films

35 25 prepared by deposition of thiols on Au , carboxylic acids on Ag , and

36,37 trichlorosilanes on glass , range from 95-10oO for short alkyl chains, to -105-

11 5' for alkyl chains longer than 12 carbons. In comparison, the thin films

prepared by using the newly developed 1-step method gave contact angles in

the range of 114-122' for long chain alcohols, and 102-1 18' for thiols. The

contact angles of films produced using the 3-step method were lower than these,

which might indicate lower packing density in these films. It should also be

noted that Si-S and Si-OOC bonds are susceptible to hydrolysis, and their lower

contact angles may not be entirely representative of the film characteristics.

The ellipsometry data also are, in general, consistent with those from the

established m e t h ~ d s ~ ' ~ ~ ' ~ ~ . As discussed earlier, the data, on average, points to

a 35' tilt from the surface normal for the alkyl thin films. This is slightly larger

than the 30' tilt angle expected for the ideal monolayer structures. However, the

data from both thiols on gold and carboxylic acids on silver, show that these

monolayers have their alkyl groups at just a 25' tilt. These discrepancies

indicate that chemisorption may induce changes in optical response of the

substrates relative to the bare substrate, or may also be due to changes in the

refractive index due to the changes in density of alkyl chain packing.

The FTlR-ATR spectra of the samples prepared using the 3-step method

are very representative of those obtained using the conventional methodç. Al1

these show a trend of va(CH2) band positions ranging from about 2930cm" for

short alkyl chain films down to near 291 8cm-'.

Direct relationships between al1 these methods cannot be deduced due to

the differences in the substrate material. The substrate will govern the

maximum coverage, and thus, the packing density of the films. Upon

com parison of al1 the self-assem bly techniques, the acid-base hydrolysis

approach is able to produce films of similar quality. Thus, we have been able to

unify the conventional processes of molecular self-assembly using this novel

approach.

CHAPTER 3

CONCLUSIONS AND CONTRIBUTIONS TO KNOWLEDGE

Acid-base hydrolysis of silylamines with appropriate chromophores

containing acidic protons is a convenient, general and versatile route to

molecular self-assembly. Using this new approach, we have prepared

monolayers of long alkyl chain alcohols, thiols and carboxylic acids on a single

subçtrate Le., inorganic oxide surfaces of glass, quartz and single crystal silicon.

The flexibility of the chemistry involved offered two novel methods:

A) complete acid-base hydrolytic process on the sutface in a succession of

three steps: i. reaction of surface hydroxyl groups with SiC14; ii. Treatment of the

monolayer of silylchloride with excess diethylamine in toluene at 80°C; iii. Finally

the reaction of surface bound silylarnines with the acidic chromophores.

B) Acid-base hydrolysis of trimethoxysilylchloride in solution with chromophores

in the presence of amines, and then reaction of the trimethoxysilyl terminated

alkyl chains with the surface hydroxyl groups.

The former is referred to as the 3-step method, and the latter as the 1-step

method. The surface chemistry was preceded by the preparation and

characterization of solution models, and optimization of reaction conditions for

surface chemistry.

The surface bound species were characterized using well established

techniques such as wettability (contact angle goniornetry), FTlR spectroscopy in

the ATR mode, ellipsometry, and in some cases by X-ray reflectivity using

synchrotron radiation. The results from these studies indicate that the acid-base

hydrolytic approach developed in Our laboratory is capable of producing

organized surfaces of a variety of chrornophores on a single substrate.

Therefore, this approach could be considered as a unifying molecular self-

assembly route to silanes on glass, thiols on gold and carboxylic acids on

alumina or silver. Molecular self-assembly of long alkyl chain thiols and

carboxylic acids on inorganic oxide surfaces are the first examples of such

surfaces. We are now beginning to understand the role substrate plays in

rnolecular self-assembly. Once the X-ray reflectivity studies are cornpleted on

our newly formed monolayers, some useful correlations will be forthcoming.

The results of the above study can also help in distinguishing the abilities

of the 3-step and 1-step methods in generating well-organized monolayers.

From the contact angle measurements, we conclude that the 1-step method

produces thin films that are more organized and densely packed than those from

the 3-step method. However, the FTIR-ATR spectra showed that the long alkyl

chain monolayers foned by the 1-step method have much fluid-like character,

more so than the 3-step method. This rnay indicate some disorder in the films.

These results are contradictory, but it may also indicate that the factors affecting

the packing of the terminal methyls group may be independent of the

organization of the bulk of the methylene structure.

This study also helps us understand some fundamental properties of

newly formed thin films. As expected, the chain length plays a significant role in

the organization of molecules by van der Waals attractions. As the chain length

increases, the thin films become more well packed and ordered. The results of

the hydrolytic stability tests indicated that the Si-O bound alkanes are stable to

hydrolysis, while the Si-S and Si-OOC bound alkanes are susceptible to

decomposition by hydrolysis, the former being more stable.

In conclusion, acid-base hydrolytic chemistry on inorganic oxide surfaces

offers potential for the build up of thin films by molecular self-assernbly. The

results dernonstrate that this is a unified approach to known methods to the

preparation of thin films. It is a highly important area to study to understand

fundamental characteristics of molecular self-assembly before we begin to

develop thin film technologies for industrial applications.

CHAPTER 4

EXPERIMENTAL

The reagents were purchased from Aldrich, and used as received. The

solvents used were purified using conventional methods, for example,

diethylamine was dried by refluxing over potassium hydroxide for 1 hour. All

solution model preparations were perforrned under nitrogen atmosphere using

standard chemical glassware and Schlenk line techniques.

'H NMR spectra were recorded on a JEOL-270 spectrometer, operating

at 270 MHz. The chemical shifts are reported relative to tetrarnethylsilame.

Mass spectra were obtained by CI or El on a KRATOS-MS25RFA instrument.

Transmission lnfrared spectra were recorded in solid state and in solution (CC14)

on a BRUKER IFS-48 spectrometer.

Trimethylsilyldiethyiamine (1).

Trimethylsilyldiethylamine can be bought through Aldrich Chemicals, or

made using the following process:

To a 50mL flask containing 15mL of freshly distilled toluene, trimethylsilylchloride

(O.lg, 0.92mmol) and 1 mL of diethyl amine (excess) were added. The reaction

mixture was left to stir under an atmosphere of N2(g1 for 4h at ambient

temperature. The resulting salt was then filtered off, and the solvent from the

filtrate was removed in vacuo, affording a clear liquid. Yield: 70%.* 'H NMR

(270 MHz, C6D6) ppm 2.7 (q, 4H, JH-H= 7 HZ), 1.1 (t, 6H, &+=~Hz), 0.1 (s, 9H).

* Mass spectroscopy was not obtained due to unavailabiliîy of the instrument.

Trimethoxysilyldiethylamine (2).

In a 50mL Schlenk flask charged with 15mL of toluene, an excess (1 ml)

of diethylamine was added to 100pL (1 16mg, 0.73mmol) trimethoxysilylchloride.

The mixture was allowed to react under an atmosphere of N2(g) for 12h at

ambient temperature. The resulting salt was then filtered off, and the solvent

from the filtrate was then rernoved in vacuo, affording a clear liquid. Yield:

72%.* 'H NMR (270 MHz, C&) ppm 3.53 (s, 9H), 2.4 (q, 4H, JHSH= 7 Hz), 1 .O

(t, 6H, JH-H=~Hz).

Triphenylsilyldiethylamine (3).

To 15mL of toluene in a Schlenk flask, 1 mL (large excess) diethylamine

was added along with (IOOmg, 0.34mmol) triphenylsilylchloride. This was left to

react under an atmosphere of Nqg) for 24h at 75OC. The resulting salt was then

filtered off, and the solvent from the filtrate was then removed in vacuo, affording

a white solid. Yield: 65%.* 'H NMR (270 MHz, CsDs) ppm 7.8 (m, 6H), 7.2 (m,

9H), 3.0 (q, 4H, JH-~= 7 HZ), 0.94 (t, 6H, JH.~=7Hz).

Al koxytrimethylsilanes (4).

To a 50mL Schlenk flask charged with 15mL of freshly distilled toluene,

trimethylsilyldiethylamine (66m9, 0.45mmol) and the alcohol (0.45mmol) were

added. The solutions were stirred for 10-12 h at 60°C under Na. The solvent

was then removed in vacuo, affording a clear oil from 4 and 6 carbon alcohols,

and white powder from higher chah alcohols.

a. Butoxytrimethylsilane. Yield: 78%. MS (CI): 148. IR vc-H: 2960 2933

-1 1 2871 cm . H NMR (270 MHz, C6D6) 6 pprn 3.50 (t, 2H, JH-~ = 6 HZ), 1.49 (m,

2H, JH-H = 7 HZ), 1.35 (m, ZH, JH-~ = 7 Hz), 0.86 (t, 3H, JH+ = 7 HZ), 0.1 0 (s, 9H).

b. Trimethylsilyloctylether. Yield: 77%. MS (El): 204. IR VC-H: 2957 2929

2858 cm". 'H NMR (270 MHz, C6D6) 6 pprn 3.54 (t, 2H, JH-~ = 6 HZ), 1.55 (m,

2H, JH-H = 7 Hz), 1.2-1.4 (m, IOH), 0.89 (t, 3H, JH-~ = 6 HZ), 0.12 (s, 9H).

c. Trimethylsilyldecylether. Yield: 82%. MS (CI): 230. IR vc-H: 2956 2928

2856 cm-'. 'H NMR (270 MHz, C6Ds) 6 pprn 3.53 (1, 2Hl JH-~ = 6 HZ), 1 5 4 (m,

2H, JH-H = 7 HZ), 1.2-1.4 (ml 14H), 0.89 (t, 3H, JH-~ = 6 HZ), 0.1 0 (s, 9H).

d. Trimethylsilyldodecylether. Yield: 80%. MS (CI): 258. IR vc-H: 2956

2927 2855 cm-'. 'H NMR (270 MHz, C6D6) 6 pprn 3.53 (t, 2H, JH-~ = 6 HZ)! 1.54

(m, 2H, A-H = 6 Hz), 1.2-1.45 (m, 18H), 0.89 (t, 3H, JH-~ = 6 HZ), 0.1 1 (s, 9H).

e. Trimethylsilyltetradecylether. Yield: 79%. MS (CI): 286. IR vc.~: 2956

2927 2854 cm". 'H NMR (270 MHz, C6D6) 6 pprn 3.53 (t, 2H, JHH = 7 HZ), 1.54

(m, 2H, JH-H = 6 Hz), 1.2-1.45 (m, 22H), 0.89 (t, 3H, JH-~ = 6 Hz), 0.10 (s, 9H).

f. Trimethylsilylhexadecylether. Yield: 84%. MS (CI): 31 6. IR vc.~: 2956

2927 2854 cm-'. 'H NMR (270 MHz, C6Ds) 6 pprn 3.55 (t, 2H, JHmH = 6 Hz), 1 5 6

(m, ZH, JH-H = 6 Hz), 1.2-1.4 (m, 26H), 0.91 (t, 3H, JHsH = 6 HZ), 0.13 (s, 9H).

g. Trimethylsil yloctadecylether. Y ield: 84%. MS (CI): 343. I R vc.~: 2956

2927 2854 cm-'. 'H NMR (270 MHz, CsD6) 6 ppm 3.56 (t, 2H, JHmH = 6 HZ), 1.56

(m, 2H, JH.H = 7 Hz), 1.2-1.45 (m, 18H), 0.92 (t, 3H, = 6 Hz), 0.13 (s, 9H).

Trimethylsilylalkylmercaptans (5)

Under an atmosphere of NP, the appropriate alkylthiol (-0.45 mmol) and

trimethylsilyldiethylamine (66 mg, 0.45 mmol) are added to approxirnately 15mL

of toluene in a 50mL Schlenk flask. The solution was then stirred at 80°C for

48h. The solvent was then removed in vacuo, affording slightly yellow oils.

a. Trimethylsilylhexylmercaptan. Yield: 62%. MS (CI): 190. IR VGH: 2958

2929 2858 cm-'. 'H NMR (270 MHz, C6D6) 6 ppm 2.39 (t, 2H, JHmH = 7 HZ), 1.55

(m, 2H, JH-~+ = 6 HZ), 1.1-1.4 (m, 6H), 0.84 (t, 3H, JHmH = 7 Hz), 0.22 (s, 9H).

b. Trimethylsilyloctylmercaptan. Yield: 71%. MS (CI): 218. IR vc-H: 2954

2928 2856 cm". 'H NMR (270 MHz, C6D6) O ppm 2.40 (t, 2H, JH-~ = 7 HZ), 1.56

(m, 2H, JH-~ = 7 HZ), 1.1-1.4 (m, 10H), 0.87 (t, 3H, JH.~ = 7 Hz), 0.22 (s, 9H).

c. Trimethylsilyldecylmercaptan. Yield: 66%. MS (CI): 246. IR vc-H: 2855

2927 2856 cm". 'H NMR (270 MHz, C6D6) O ppm 2.41 (t, 2H, JHmH = 7 HZ), 1.58

(m, 2H, JH-H = 7 HZ), 1.1-1.4 (m, 14H), 0.89 (t, 3H, JKH = 6 HZ), 0.22 (s, 9H).

d. Trimethylsilyldodecylmercaptan. Yield: 70%. MS (CI): 275. IR vc -~ :

2955 2928 2856 cm-'. 'H NMR (270 MHz, C6Ds) 6 ppm 2.42 (t, 2H, JH-~ = 7 Hz),

1.59 (m, 2H, JH-H = 7 HZ), 1.1-1.4 (m, 18H), 0.90 (t, 3H, JH-H = 7 Hz), 0.23 (s,

9H).

e. Trimethylsilylhexadecylmercaptan. Yield: 78%. MS (CI): 330. IR v c ~ :

2954 2927 2856 cm-'. ' H NMR (270 MHz, C6D6) 8 ppm 2.41 (t, 2H, JH-H = 7 HZ),

1.57 (m, 2H, JHmH = 6 Hz), 1.1-1.4 (m, 26H), 0.90 (t, 3H, JH-H = 6 HZ), 0.22 (s,

9H).

f. Trirnethylsilyloctadecylmercaptan. Yield: 76%. MS (El): 358. IR v c ~ :

2954 2927 2855 cm". 'H NMR (270 MHz, C6Ds) 6 ppm 2.46 (t,2H, J H - ~ = 7 Hz),

1.61 (m, 2H, JH-H = 6 Hz), 1.1 -1.4 (m, 30H), 0.92 (1, 3H, JH-H = 6 Hz), 0.23 (s,

9H).

Trimethylsilylalkanoates (6).

The carboxylic acid (0.45 mmol) and trimethylsilyldiethylamine (66m9,

0.45mmol) were added to a 50mL Schlenk flask charged with 15mL of toluene.

This solution was stirred at room temperature for 6h under Na The solvent is

removed in vacuo, affording white solids.

a. Trimethylsilylhexanoate. Yield: 75%. MS (CI): 202. IR VCH: 2958 2931

2872 2861 cm-'. 'H NMR (270 MHz, C6D6) 6 ppm 2.41 (t, 2H, = 8 HZ), 1.74

(m, 2H, JH-H = 7 Hz), 1.2-1.4 (m, 6H), 0.85 (t, 3H, = 7 Hz), 0.27 (s, 9H).

b. Trimethylsilyloctanoate. Yield: 80%. MS (CI): 21 7. IR vc-H: 2957 2929

2872 2858 cm-'. 'H NMR (270 MHz, CsD6) 6 ppm 2.36 (t, 2H, J H - ~ = 8 Hz), 1 . ï O

(m, 2H, JH-H = 6 Hz), 1.2-1.4 (rn, IOH), 0.86 (t, 3H, JH-H = 6 HZ), 0.26 (s, 9H).

cm Trimethylsilyldecanoate. Yield: 77%. MS (CI): 245. IR vc-H: 2957 2928

2856 cm". 'H NMR (270 MHz, C6Ds) 6 ppm 2.16 (1, 2H, J H - ~ = 7 HZ), 1.54 (rn,

2H, JH-H = 7 HZ), 1.1-1.4 (m, 14H), 0.87 (t, 3H, JH.H = 7 Hz), 0.24 (s, 9H).

d. Trimethylsilyldodecanoate. Yield: 82%. MS (CI): 273. IR V ~ H : 2957

2927 2855 cm*'. 'H NMR (270 MHz, CsD6) 6 ppm 2.1 5 (t, 2H, J H - ~ = 7 HZ), 1.53

(m, 2H, JH+ = 7 HZ), 1.1-1.4 (rn, 14H), 0.88 (t, 3H, JHSH = 7 Hz), 0.23 (s, 9H).

e. Trimethylsilylhexadecanoate. Yield: 79%. MS (CI): 329. IR VGH: 2956

2927 2856 cm-'. 'H NMR (270 MHz, CsDs) 6 ppm 2.18 (t, 2H, JH-H = 7 Hz), 1.57

(m, 2H, JH-H = 7 Hz), 1.1-1.4 (m, 18H), 0.89 (t, 3H, JHeH =7 Hz), 0.25 (s, 9H).

f. Trimethylsilyloctadecanoate. Yield: 77%. MS (CI): 356. IR VGH: 2955

2926 2854 cm". 'H NMR (270 MHz, C6D6) 6 ppm 2.19 (t, 2H, JHmH = 7 HZ) , 1.59

(m, 2H, JH-H = 7 Hz), 1.1-1.4 (m, 14H), 0.91 (t, 3H, JH.H = 7 Hz), 0.27 (s, 9H).

Stability of trimethylsilyl adducts.

A study of the stability of the solution models was performed by the

addition of water or methanol, and followed spectroscopically. The samples

were heated to 80°C if no change was observed after 2 days at RT.

Alkoxytrimethoxysilanes (7).

To 20mL of dried toluene, trimethoxysilylchloride (1 16mg, 0.73mmol) and

the appropriate alcohol (0.73mmol) were added. Triethylamine was then added

drop-wise to the solution. This was let to react for 24h at room temperature

under an atmosphere of Na. The resulting mixture was then filtered, and the

solvent in the filtrate was removed in vacuo. This afforded white crystals. IR

spectra of these compounds were not obtained due to their sensitivity to

moisture, but are expected to be similar to those of the trimethylsilyl derivatives.

a. Butoxytrimethoxysilane. Yield: 70%. MS (CI): 195. 'H NMR (270 MHz,

C&) O ppm 3.82 (t, 2H, JH.H = 6 HZ), 3.50 (s, 9H), 1.54 (m, 2H, JHmH = 7 Hz),

1.33 (m, 2H, JH-H = 7 Hz), 0.84 (t, 3H, JH-H = 7 Hz).

b. Octoxytrimethoxysilane. Yield: 73%. MS (CI): 251. 'H NMR (270 MHz,

%De) O pprn 3.85 (t, 2H, JH-H = 6 Hz), 3.52 (s, 9H), 1.59 (m, 2H, JH-~ = 7 Hz),

1.2-1.45 (rn, 1 OH), 0.89 (t, 3H, JHmH = 7 HZ).

c. Decoxytrimethoxysilane. Yield: 68%. MS (CI): 279. 'H NMR (270

MHz, Cs&,) 6 ppm 3.85 (t, 2H, k-H = 7 Hz), 3.51 (s, 9H), 1.60 (m, 2H, JH-~ = 7

Hz), 1.15-1.45 (m, 14H), 0.90 (t, 3H, Jn.H = 7 Hz).

d. Dodecoxytrimethoxysilane. Yield: 72%. MS (El): 307. 'H NMR (270

MHz, Cs&) 6 ppm 3.86 (t, 2H, JH-H = 6 HZ), 3.51 (s, 9H), 1.61 (m, 2H, JHmH = 6

Hz), 1.2-1.45 (m, 18H), 0.91 (t, 3H, JH-~ = 7 HZ).

e. Hexadecoxytrimethoxysilane. Yield: 76%. MS (CI): 363. 'H NMR (270

MHz, C6D6) 6 ppm 3.87 (1, 2H, JH-H = 6 Hz), 3.52 (s, 9H), 1.59 (m, 2H, JHFH = 7

Hz), 1.2-1.45 (m, 1 OH), 0.92 (t, 3H, JH-~ = 7 HZ).

Trimethoxysilylalkylmercaptans (8).

Trimethoxylsilylchloride (1 1 6mg, 0.73mmol) and the appropriate alkylthiol

(0.73mmol) were added to approximately 20mL of dry toluene. ImL of

triethylarnine was then added drop-wise to the solution. This was let to react at

60°C for 24h under an atmosphere of Na. The resulting mixture waç then

filtered, and the solvent was removed frorn the filtrate in vacuo, affording pale

yellow oils and crystals. IR spectra of these cornpounds were not obtained due

to their sensitivity to moisture.

a. Trimethoxysilylhexylmercaptan. Yield: 67%. MS (CI): 238. 'H NMR

(270 MHz, C6D6) 8 ppm 3.51 (s, 9H), 2.53 (t, 2H, JH-H = 7Hz), 1.61 (m, 2H, J H - ~

= 7 Hz), 1.2-1.4 (m, 6H), 0.88 (t, 3H, J H - ~ = 7 HZ).

b. Trimethoxysilyloctylmercaptan. Yield: 61 %. MS (CI): 267. 'H NMR

(270 MHz, C6D6) 8 ppm 3.52 (s, 9H), 2.64 (t, 2H, JH.ti = 7 Hz), 1.64 (rn, 2H, J H - ~

= 7 Hz), 1.1-1.45 (m, IOH), 0.91 (t, 3H, J H - ~ = 7 Hz).

c. Trimethoxysilyldecylmercaptan. Yield: 70%. MS (CI): 296. 'H NMR

(270 MHz, C6D6) 8 ppm 3.51 (s, 9H), 2.64 (1, 2H, JH-H = 7 HZ), 1.65 (m, 2H, JHqH

= 7 Hz), 1.1-1.45 (m, 14H), 0.92 (t, 3H, J H - ~ = 7 HZ).

d. Trimethoxysilyldodecylmercaptan. Yield: 78%. MS (El): 323. 'H NMR

(270 MHz, 8 ppm 3.51 (s, 9H), 2.64 (t, ZH, J H - ~ = 7 Hz), 1.65 (m, 2H, JHWH

= 7 Hz), 1.1 -1.45 (m, 18H), 0.92 (t, 3H, JHH = 7 HZ).

e. Trimethoxysilylhexadecylmercaptan. Yield: 74%. MS (CI): 380. 'H

NMR (270 MHz, C6D6) 6 ppm 3.52 (s, 9H), 2.64 (t, 2H, J H . ~ = 8 HZ), 1.65 (in, 2H,

JHmH = 7 Hz), 1.1 -1.45 (m, 26H), 0.91 (t, 3H, JHaH = 7 HZ).

f. Trimethoxysilyloctadecylrnercaptan. Yield: 71%. MS (CI): 407. 'H

NMR (270 MHz, GDs) 6 ppm 3.54 (s, 9H), 2.62 (t, 2H, J H - ~ = 7 HZ), 1.65 (m, 2H,

J H - ~ = 7 HZ), 1.1-1.45 (m, 30H), 0.91 (t, 3H, = 7 HZ).

Hexadecoxytriphenylsilane (9).

To 15mL of diethylether, triphenylsilylchloride (200mg, 0.68mmoI) and

hexadecanol (1 64mg, 0.68mmol) are added under a nitrogen atmosphere. A

slight excess of triethylamine (100pL) was then added dropwise. The reaction

mixture was left to stir for 24h at room temperature before rernoval of the salt by

filtration. The solvent of the filtrate was then removed in vacuo, affording a white

solid. Yield: 71 %.* 'H NMR (270 MHz, CsDs) 6 ppm 7.75 (m, 6H), 7.20 (rn, 9H),

3.81 (t, 2H, JH-H = 7 Hz), 1.60 (m, 2H, J H - ~ = 6 HZ), 1.1-1.45 (m, 26H). 0.91 (t, 3H,

JHmH= 7 HZ).

Triphenylsilyldodecylmercaptan (1 0).

In 15mL of toluene, triphenylsilylchloride (30mg, 0.lOmmol) and 24 pL of

dodecanethiol (21mg, 0.lOmrnol) were reacted in the presence of 20pl of

triethylamine. This reacted at room temperature for 72h. The salt is removed by

filtration, and the solvent of the filtrate is removed in vacuo, resulting in a white

solid. Yield: 55%.* 'H NMR (270 MHz, C&) 6 ppm 7.85 (m, 6H), 7.21 (m, 9H),

2.46 (q, 2H, JHmn = 7 Hz), 1.0-1.60 (m, 20H).

Triphenylsilyldodecanoate (1 1).

a) In toluene under an atmosphere of N2(g), triphenylsilylchloride (30mg,

0.lOmmol) and lauric acid (23rng, 0.lOrnmol) are reacted in the presence of

100p.L of triethylamine for 24h at ambient temperature. The salt is removed by

filtration and the solvent from the filtrate is removed in vacuo, affording a white

solid. Yield: 75%.

b) To triphenylsilydiethylamine (50mg , 0.15mmol) in 15 ml of toluene, lauric

acid (30mg, O.15mmol) is added. The reaction mixture is heated to 80°C for

24h under an atmosphere of Nz(~). The solvent was then removed in vaccuo

affording a white solid. Yield: 80%.

* 'H NMR (270 MHzl &De) 6 ppm 7.87 (in, 6H), 7.23 (ml 9H), 2.24 (q, 2H, JH-" =

7 Hz), 1.50 (m, 2H, JH+= 7 HZ), 1 . O 4 -4 (rn, 1 6H), 0.94 (t,3H, JH+= 7 Hz).

Self Assembly on Glass and Silicon using 3-step Method

Substrate preparation. Glass and single crystal silicon substrates were

prepared first by cleaning using a soap solution with ultrasonocation at 60°C for

1 h. The substrates were then washed using distilled water, then put in piranha

solution (30% H24, 70% concentrated sulfuric acid), and heated to 1 Oo°C for 1 -

2h. Once washed with distilled water, the substrates were placed in a solution of

arnmonia for 5 min. The substrates were then dried under a stream of N2(g)

Film preparation. The clean substrates were first placed in a solution of silicon

tetrachloride in dry toluene, and let to react under an atmosphere of N2 for 24h

at room temperature. The surfaces were then washed with toluene 3 times, and

placed in a solution of diethylamine in toluene. These samples reacted at 80°C

for 24h, then washed again with toluene. The alcohols, alkylthiols, and

carboxylic acids were dissolved in dry toluene at a concentration of -0.03M. The

washed samples were then placed in the solution, and left to react at 70-80°C

for 24-48h, depending on the resulting films. The samples are then removed

from solution, and washed and cleaned using toluene, then characterized.

Self Assembly on Glass and Silicon using latep Method

Substrate preparation. Glass and single crystal silicon substrates were

prepared first by cleaning using a soap solution with sonocation at 60°C for 1 h.

The substrates were then washed using distilled water, then put in piranha

solution (30% H202, 70% concenfrated sulfuric acid), and heated to 100~C for 1-

2h. Once washed with distilled water, the substrates were then dried under a

stream of N2(g).

Film preparation. The trimethoxylsilyl adducts were dissolved in dry toluene at

a concentration of -0.02M. The clean substrate was added to the solution, and

was heated to 60°C for at least 24h. These samples were then washed and

cleaned before characterization.

Contact Angle Measurement.

Static contact angles were measured with a Rame-Hart NRL Model 100

goniorneter using deionized water and hexadecane. At least 3 drops were used

for the reported contact angle readings, and the readings were reproducible to

within 2'.

Ellipsometry Measurement.

A Gaertner ellipsometer was used for thickness measurements. The He-

Ne laser (632.8nm) light incidented at 70' on the sample and reflected into the

analyzer. A real refractive index of 1.45 was assumed for the thickness

ca~culations~~.

Attenuated Total Reflectance Spectroscopy Analysis.

A Graseby Specac Variable Angle ATR in conjunction with a Bruker IFS-

48 infrared spectrometer was used for attenuated total reflectance spectroscopy.

The incident angle of 45' was used on a KRS-5 crystal. One thousand scans

were collected at 4cm" resolution.

X-Ray Reflectivity Measurements.

Prelirninary measurements were performed using the National

Synchotron Light Source (NSLS) at Brookhaven National Laboratory

(wavelength/energy range = 5-8 keV).

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