MOLECULAR SELF-ASSEMBLY OF ON - Bienvenue au site Web
Transcript of MOLECULAR SELF-ASSEMBLY OF ON - Bienvenue au site Web
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
Acquisitions and Acquisitions et Bibliographic Services services bibliographiques 395 Wellington Street 395, rue Wellington Ottawa ON K I A ON4 Ottawa ON K I A ON4 Canada Canada
The author has granted a non- L'auteur a accordé une licence non exclusive licence allowing the exclusive permettant à la National L i b r w of Canada to Bibliothèque nationale du Canada de reproduce, loan, distribute or sell reproduire, prêter, distribuer ou copies of this thesis in microfom, vendre des copies de cette thèse sous paper or electronic formats. la fome de microfiche/&, de
reproduction sur papier ou sur format électronique.
The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts fiom it Ni la thèse ni des extraits substantiels may be printed or otherwise de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son permission. autorisation.
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.
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)
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.
wavenumbers (l/cm)
Figure 19 KIR-ATR spectra of assemblies formed from carboxylic acids using the 3-step process
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).
REFERENCES
' Swalen J.D.; Allara D.L.; Andrade J.D.; Chandross E.A.; Garoff S.; lsrealachvili
J.; McCarthy T.J.; Murray R.; Pease R.F.; Rabolt J.F.; Wynne K.J.; Yu H.
Langmuir, 1987, 3, 932-950.
* Marks T.J.; Ratner M.A.; Angew. Chem., Int. Ed. Engl., 1995, 34, 155-173.
Ulman A. A n Introduction to Ultrathin Organic films, Academic Press, New
York, 1991.
lnabe T.; Lomax J.F.; Marks T.J.; Tobin J.; Lyding J.W.; Kannawurf C.R.;
Wynne K.J. Macromolecules, 1985, 17, 260.
Andrade J.D.; Hlady V. Adv. Polym. Sci., 1986, 79, 1.
Allum K.G.; Hancock R.D.; Howell I.V.; Lester T.E.; McKenzie S.; Pitkethyl
R.C.; Robinson P.J. J. Organomet. Chem., 1976, 107,393-405.
' Porter M.D.; Bright TB.; Allara D.L.; Chidsey C.E.D. J. Am. Chem. Soc., 1987,
709,3559-3568. 8 Walczak M.M.; Chung C.; Stole S.M.; Widng C.A.; Porter M.D. J. Am. Chem.
SOC., 1991, 1 13, 237002378. 9 Roberts G.G.; Petty MC.; Baker S.; Fowler M.T.; Thomas N.J. Thin Solid
Films, 1985, 132, 113.
' O Zisman W.A. Adv. Chem. Ser., 1964, 43, 1.
' Sagiv J. J. Am. Chem., 1980, 102, 92. 12 Sagiv J. Isr. J. Chem., 1979, 18, 339-345.
l3 Allen D.S.; Kubota F.; Orihashi Y.; Li D.; Marks T.J.; Zhang T.G.; Lin W.P.;
Wong G.K. Polym. Preprints., 1991, 32, 86-87.
l4 Nuzzo R.G.; Allara D.L. J. Am. Chem. Soc., 1983, 105,4481
'' Kumar A.; Abbott N.L.; Kim E.; Biebuyck H.A.; Whitesides G.M. Acc. Chem.
Res., 1 995,28,219-226
l6 Liabinis P.E.; Whitesides G.M. J. Am. Chem. Soc., 1992, 114, 9022-9028.
" Allara D.L.; Nuzzo R.G. Langmuir, 1985, 1, 45
'' Schlotter N.G.; Porter m.D.; Bright T.B.; Allara DL. Chem. Phys. Leu., 1986,
132,93
l9 Lindford M.R.; Fenter P.; Eisenberger P.M.; Chidsey C.E.D. J. Am. Chem.
SOC., 1995, 117,3145-3155
Fessenden R.; Fessenden J.S. Chem. Rev., 1961, 61,361 -388
2' Pike R.M. J. Org. Chem., 1961,25, 232
* Larsson E.; Marin R.E.I. Acta Chem. Scand., 1950, 4, 45 23 Andersen H.H. J. Am. Chem. Sac., 1951, 73,5802
24 Yarn C.M.; Kakkar A.K. J. Chem. Soc. Chem. Commun., 1995,907-909
*' Tao Y.T. J. Am Chem. Soc., 1993, 1 15, 435094358 26 Wassetman SR.; Whitesides G.M.; Tidswell LM.; Ocko B.M.; Pershan P.S.;
Axe J.D. J. Am. Chem. Soc., 1989, 11 1,5852-5861.
27 Tidswell LM.; Ocko B.M.; Pershan P.S.; Wasserman S.R.; Whitesides G.M.;
Axe J.D. Phys. Rev. B., 1990, 41, 1111-1127. 28 Corriu R.J.P.; Guerin C. J. Organomet. Chem., 1980, 189,231-320.
*' Patai S.; Rappoport Z. The Chemistry of Organosilicon compounds, 1989,
John Wiley & Sons, p.839.
30 Abel E.W. J. Org. Chem. 1960, 4406.
3' Folkers J.P.; Gonnan C.B.; Laibinis P.E.; Buchholz S.; Whitesides G.M.;
N u u o R.G. Langmuir, 1995, 11,813-824
32 Netzer L.; lsovici R.; Sajiv J. Thin Solid Films, 1983, 100, 67-76.
33 Parikh A.N.; Allara D.L.; Azouz 1.9.; Rondelez F. J. Phys. Chem., 1994, 98,
7577-7590.
Allara D.L.; Swalen J. J. Phys. Chem., 1982, 86, 4675-4678.
35 Evans, S.D.; Shana R.; Ulman A. langmuir, 1991, 7,156.
" Maoz R.; Sajiv J. J. Colloici and Interface Sci., 1984, 100, 465.
37 Wasserrnan S.R.; Tao Y.T.; Whitesides G.M. Langmuir, 1989, 5, 1074-87.