Mécanismes et cinétiques des SAMs
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Annu. Rev. Phys. Chem. 2001. 52:10737Copyright c 2001 by Annual Reviews. All rights reserved
MECHANISMS AND KINETICS OF SELF-ASSEMBLEDMONOLAYER FORMATION
Daniel K SchwartzDepartment of Chemical Engineering, University of Colorado, Boulder, Colorado 80309;
e-mail: [email protected]
Key Words thin film, coatings, SAM, monolayer growth
s Abstract Recent applications of various in situ techniques have dramatically im-proved our understanding of the self-organization process of adsorbed molecular mono-layers on solid surfaces. The process involves several steps, starting with bulk solutiontransport and surface adsorption and continuing with the two-dimensional organiza-tion on the substrate of interest. This later process can involve passage through one ormore intermediate surface phases and can often be described using two-dimensionalnucleation and growth models. A rich picture has emerged that combines elements ofsurfactant adsorption at interfaces and epitaxial growth with the additional complica-
tion of long-chain molecules with many degrees of freedom.
INTRODUCTION
The adsorption of amphiphilic surfactant molecules at interfaces is a well-known
phenomenon that is at the heart of all detergency applications. A single mole-
cular layer (monolayer) of surfactant stabilizes oil droplets and gas bubbles in an
aqueous environment, enhancing the stability of emulsions and foams. In addition
to adsorption at liquid-liquid and liquid-vapor interfaces, amphiphilic moleculesalso adsorb at the solid-liquid interface. Self-assembled monolayers (SAMs) are
distinguished from ordinary surfactant monolayers by the fact that one end of
the molecule (generally the hydrophilic one) is designed to have a favorable and
specific interaction with the solid surface of interest (the substrate). This results in
the formation of a stable monolayer film that remains intact even after the substrate
is removed from solution.
Due to the specific interaction between molecule and substrate, the adsorption
can often be carried out in a variety of solvents, polar and nonpolar, allowing
greater flexibility in molecular design and, therefore, in the types of surface prop-erties that can be modified and controlled. Since the monolayer films are thin and
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108 SCHWARTZ
adhesion, chemical resistance, biocompatibility, sensitization for photon harvest-
ing, molecular recognition for sensor applications, and many others.
Zisman is often credited with originating the SAM concept in his 1946 paper
(1). Work in the early 1980s by Nuzzo & Allara (thiols on gold) (2) and Maoz& Sagiv (trichlorosilanes on silicon oxide) (3) introduced what were to become
the two most popular SAM systems and brought SAMs into the popular scien-
tific consciousness. Interest in these monolayer films has continuously increased
since that time, and the development and application of surface-sensitive exper-
imental techniques (e.g. scanning probe microscopy, vibrational spectroscopy,
and synchrotron X-ray sources) has resulted in an improved understanding of the
film structure and growth process. Poirier recently reviewed scanning tunneling
microscopy (STM) measurements of thiol-based SAMs (4), and a more general
review of SAM structure (with some information about film growth) was previ-ously published by Ulman (5). Ulmans book (6) serves as a useful introduction
to SAMs and thin organic films in general. The current review is more narrowly
focused on the growth process of a variety of SAM systems.
THE BIG PICTURE: General Growth Mechanisms
Bulk Transport and Adsorption
Many processes are involved in SAM growth. A first step is clearly the solution-
phase transport of adsorbate molecules to the solid-liquid interface, which can in-
volve some combination of diffusive and convective transport. This is followed by
adsorption on the substrate with some adsorption rate (related to a sticking prob-
ability). The overall adsorption dynamics may be diffusion-controlled, adsorption-
rate controlled, or in an intermediate mixed-kinetic regime. This part of the self-
assembly process is closely related to the adsorption of surface-active molecules
at the liquid-vapor interface, an area that has been thoroughly studied. Although
the typical quantity of interest at the liquid-vapor interface is surface tension rather
than surface concentration (or coverage), the two quantities are related by the sur-
face equation of state. In fact, most dynamic adsorption models are actually written
in terms of surface concentration and translated into dynamic surface tension pre-
dictions, using an equation of state determined by applying the Gibbs equation (7)
to equilibrium surface tension data. The dynamics of surfactant adsorption were
thoroughly reviewed by Chang & Franses (8), and most of the mathematical devel-
opment presented by them is directly relevant to the initial adsorption stage of SAM
formation. Quantitative aspects of this process are discussed later in this review.
Self-Organization on the Surface
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SELF-ASSEMBLED MONOLAYER FORMATION 109
for example, because their primary function is simply to reduce surface tension.
In SAM formation, therefore, there must be an evolution of the molecular order
as adsorption progresses and the surface coverage increases. For example, the
very early stages of adsorption can be pictured as isolated adsorbed molecules,conformationally disordered and randomly distributed on the substrate. The final
film involves close-packed adsorbate molecules with relatively uniform molecular
orientation and conformation. Although one might imagine a continuous path
from the former structure to the latter, experimental evidence points to a stepwise
process that can be thought of as an isothermal path through a quasiequilibrium
2D-phase diagram like the one schematically illustrated in Figure 1. Possible
states alluded to in this phase diagram include (a) a low-density vapor phase in
which isolated, mobile adsorbate molecules are randomly deposited on the surface,
(b) an intermediate-density phase that could involve conformationally disorderedmolecules or ones lying flat on the surface, and (c) a final, high-density solid
phase in which the molecules are conformationally ordered, close packed, and
Figure 1 Schematic quasi-equilibrium 2D-phase diagram for a generic SAM system. The dotted
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110 SCHWARTZ
standing approximately normal to the surface plane with a possible polar tilt angle
of about 30. As discussed below, other states are, of course, possible.In a hypothetical situation in which the adsorption rate is much slower than any
other process, the monolayer system would follow the equilibrium phase diagram.There are two qualitatively different growth processes suggested by the lines at
temperatures T1 and T2 in Figure 1. If the temperature is lower than the triple
point (e.g. temperature T1), the growth sequence will be similar to the one shown
in Figure 2a. Initially, adsorbed molecules will form a dilute 2D-vapor phase.
At a relatively low surface concentration, the monolayer will enter a coexistence
region between the vapor and the high-density condensed (solid) phase. Domains
(islands) of solid phase will nucleate and grow, surrounded by isolated adsorbate
molecules in the vapor phase. Eventually, these domains will grow to cover the en-
tire substrate. This mechanism is analogous to the three dimensional (3D) processof crystal nucleation and growth from a vapor phase precursor, and the 2D scenario
is typical for epitaxial film growth from the vapor phase (e.g. molecular-beam
epitaxy) (9). At a temperature above the triple point (e.g. T2 in Figure 1), a more
complicated progression will occur as illustrated in Figure 2b. When the vapor
phase reaches a certain surface concentration, islands of an intermediate, low-
density condensed phase will nucleate and grow. This phase may be a disordered
2D-liquid phase or an ordered phase with lower density than the solid phase (e.g.
a lying-down phase where the molecular axis is parallel to the surface plane).
Eventually the vapor phase is completely converted to the low-density condensedphase. As adsorption continues, a second transition occurs involving nucleation,
growth, coalescence, etc, of solid-phase islands surrounded by the low-density
condensed phase. Note that, at any temperature, a snapshot of an incomplete film
during growth will often involve islands of one phase surrounded by another, in
particular, islands of solid phase surrounded by either liquid or vapor phase.
It is important to recognize that the picture painted in the previous paragraph
is somewhat oversimplified. For example, the adsorption rate will not always be
much slower than other surface processes, and, therefore, partial monolayers may
be quite far from equilibrium. If the nucleation and growth of condensed-phasedomains do not keep up with the deposition rate, the less condensed phase will
become super concentrated (i.e. it will have a density greater than the equilibrium
coexistence concentration), and, thus, its density may vary considerably during the
growth of the more condensed phase. This behavior is well known in vapor phase
thin-film deposition, where the surface concentration of free adsorbate atoms is
understood to vary during island nucleation and growth, and is likely to occur
during SAM growth as well. However, the surface concentration in the vapor
phase will always be small and amount to a negligible fraction of the molecules
on the surface. In the case of a 2D-liquid phase, however, the surface densityis not negligible, and, in fact, the film thickness is directly related to the surface
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SELF-ASSEMBLED MONOLAYER FORMATION 111
Figure2
Cartoonsd
epictingtypicalsequencesofaself-assembledmonolayerstructureduringgrowth
below(A)andabove(B)atriplepointlikethatsho
wninFigure1.
(A)Belowth
etriplepoint,growth
proceedsfroma2D-vaporphase,
throughasolid-vap
orcoexistenceregion,
tothesolidphase.
(B)Above
thetriplepoint,theSA
M
mustpassthroughthreeph
asesandtwocoexistenceregions.Theintermediate
low-densityphasemaybeadisordered(liquid)phase,
alying-downphase,
etc.
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112 SCHWARTZ
The molecules used to create SAMs have numerous degrees of freedom, and,
therefore, it is quite possible that the equilibrium phase diagram could be more
complicated and involve a greater number of condensed phases than implied in
Figure 1. It could include a lying-down phase and a disordered-liquid phase, forexample. However, there are numerous other possibilities. Langmuir monolayers
of long-chain fatty acids, for example, are known to display a variety of liquid
crystalline and crystalline phases (10) that differ in the polar tilt angle, the azi-
muthal direction of molecular tilt (i.e. nearest-neighbor vs next-nearest-neighbor
direction), and rotational freedom (herringbone vs rotator). To date there is no
firm experimental evidence for liquid-crystalline phases, transient or equilibrium,
in SAMs. Given their ubiquity in Langmuir monolayers and Langmuir-Blodgett
films, however, one would not be surprised if they were observed in SAMs with
the appropriate experimental studies.Therefore, although it is certainly overly simplistic, the phase diagram of
Figure 1 will be used as a conceptual framework to describe the experimentally
observed growth mechanisms of various SAM systems discussed in the following
sections. Two general experimental strategies have been used to study monolayer
growth: (a) in situ studies under actual deposition conditions in real time and
(b) studies on quenched partial monolayers removed from solution and possibly
rinsed to remove loosely attached adsorbate molecules. Although in situ experi-
ments have become increasingly important in recent years, many publications in
the literature report experiments that used quenched films. The clear advantageof in situ experiments is that one avoids the issue of whether the film structure
is altered by the quenching process. This is not a trivial matter, since there is
clear evidence that quenching can alter the film coverage and morphology in some
molecular systems. On the other hand, working with quenched films permits the
use of certain techniques not applicable in situ, such as contact angle and X-ray
photoelectron spectroscopy. Furthermore, one can work over a longer range of
time scales (i.e. concentrations). Although experiments on quenched films often
report reliable and useful information (particularly on qualitative issues), one must
view subtle quantitative conclusions based on quenched films with appropriateskepticism until they are confirmed by more direct experiments.
Vapor Phase-Deposited Thiol Films
Although not strictly considered SAMs, films created by vapor phase (molecular
beam) deposition of alkylthiols on gold share many structural characteristics with
solution-deposited films. Furthermore, studies on these films have the advantages
of ultra-high-vacuum substrate cleanliness and the availability of traditional in situ
surface characterization techniques. Although it is clear that solvent interactionsare potentially important for SAMs (perhaps even more so during the growth
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SELF-ASSEMBLED MONOLAYER FORMATION 113
Poirier & Pylants (11) STM observations of vapor-deposited thiols on single-
crystal Au(111) were the first report of the general mechanism of thiol mono-
layer growth. Studying the formation process of C6C10 alkylthiols, both methyl
and hydroxy terminated, Poirier & Pylant reported a two-step process startingwith the nucleation and growth of islands of striped phases from a lower-
density lattice-gas phase. Based on the observed periodicity of these striped phases,
it was proposed that they consisted of molecules lying flat on the gold surface,
in either a head-to-head or a head-to-tail arrangement. The growth of the stripe
phase islands was accompanied by the appearance of gold atom vacancies (pits).
After the surface was covered by the stripe phase, continued deposition resul-
ted in islands of molecules arranged in a way consistent with an epitaxial
overlayer (
3
3)R30 on the Au(111) surface. The lateral density neces-
sary to form this structure implied thiol molecules with an orientation nearlyperpendicular to the substrate. This sequence is similar to the one suggested in
Figure 2b.
Schreiber and coworkers (12) presented a multitechnique study (X-ray diffrac-
tion, atom diffraction, and X-ray photoelectron spectroscopy) of the vapor phase
growth process of C10 thiol on Au(111). Their results were qualitatively consistent
with Poirier & Pylants observations (11) at low temperatures with a few added
details. The atom diffraction suggested that the striped phase became disordered
prior to nucleation of the upright [(
3
3)R30] phase. Also, experiments at
temperatures above 15C found an additional intermediate 2D-liquid phase be-tween the striped phase and the standing-up solid phase. To incorporate this into
a phase diagram like Figure 1, one would have to add another low-density con-
densed phase. It is interesting that Schreiber and coworkers found that the size of
correlated [(
3
3)R30] domains that grew from the liquid phase were sig-nificantly larger than those that nucleated from the striped phase. This suggested
that the defect structure of the final film may be intimately related to deposition
conditions and mechanisms. In two subsequent papers (13, 14), the growth pro-
cess of the [(
3
3)R30] phase was studied in greater detail. It was found
that at >15C, the growth rate of the [(33)R30] phase was approximatelyproportional to the adsorbate pressure in the gas phase. However, at lower temper-
atures, the growth rate was proportional to the square of the pressure, suggesting
that a bimolecular process may be rate limiting.
Thiol on Gold Self-Assembled Monolayers
Contact angle and ellipsometry experiments on quenched, incomplete alkanethiol
SAMs on gold by Bain et al (15) revealed at least two time scales in the growth
process. For a typical solution of 1 mM in ethanol, the contact angle and filmthickness were observed to reach90% of their final values within the first minute
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114 SCHWARTZ
with time scales differing by 2 orders of magnitude. They also noted that themonolayer properties of longer-chain n-alkyl thiols (n> 8) were consistent but that
shorter-chain thiol monolayers were qualitatively different in a way that suggested
greater disorder. Sun & Crooks (16) decorated defects in quenched partial SAMsby underpotential electrochemical deposition of Cu. They monitored the decrease
in SAM defect density as a function of exposure time using STM and found that
the number of defects disappeared on a time scale of several hours, consistent
with Bains (15) results. This study also provided evidence, albeit indirect, that an
islanding mechanism was involved in alkylthiol SAM growth. They also observed
pits 0.5 nm deep that were characteristic of the SAM-covered gold surface.These pits did not appear to be holes in the SAM layer, however, because Cu islands
did not nucleate in these locations. These intriguing results inspired a multitude
of measurements designed to confirm and later explain the existence of multipletime scales.
Numerous studies involving measurements of film mass and average thickness
were conducted to explore the overall coverage kinetics of thiol SAMs on gold.
The results of many of these were qualitatively consistent with the observations
of Bain et al (15), finding fast and slow time scales. Shimazu and coworkers (17)
performed in situ quartz crystal microbalance (QCM) experiments on ferrocene-
substituted thiols in hexane solution. They observed a fast adsorption step (a few
seconds in 0.5 mM solution) followed by a process with a time scale
2 orders of
magnitude slower. Their results were consistent with a single molecular layer after800 s. QCM and STM studies on quenched monolayers by Kim et al (18) detected
a slow build-up of multilayers (over a period of days) during C18 thiol SAM growth
from ethanol solution. Schneider & Buttrys in situ QCM experiments (19) also
considered multilayer formation. However, their results suggested gradual conver-
sion of physisorbed multilayers to a chemisorbed monolayer. Schneider & Buttry
also observed a significant solvent effect. In dimethylformamide solution, adsorp-
tion was rapid; however, a complete monolayer was never formed. In acetonitrile
solution, on the other hand, adsorption was slower, but the physisorbed film was
slowly converted to a densely packed monolayer. Schneider & Buttry suggestedthat the final monolayer quality had an inverse relationship with the solubility of
the thiol in the solvent. In situ QCM experiments by Frubose & Doblhofer (20)
revealed two distinct time scales in adsorption from 0.1 mM thiol solutioninitial
adsorption in2 min followed by a much slower process taking >1 h. Their mea-surements of gradually decreasing electrochemical impedance during that latter
process suggested that the slow time scale corresponded to healing of the SAM.
In situ surface plasmon resonance (SPR) experiments by DeBono and coworkers
(21) found two adsorption time scales differing by a factor of
100. For C12 or C16
thiols from ethanolic solution, the initial fast step resulted in 80% of monolayercoverage. The C16 rate constant was faster than the C12 for both steps. For the C6
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SELF-ASSEMBLED MONOLAYER FORMATION 115
Figure 3 Chain length dependence of formation kinetics for C8 (circles), C12 (squares),
C16 (triangles), and C18 (diamonds) thiols from 1.0 mM ethanolic solutions. The film
thicknesses were calculated from in situ surface plasmon resonance measurements. Up
to three distinct kinetic regimes were observed, depending on chain length. (a) Details
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SELF-ASSEMBLED MONOLAYER FORMATION 117
formation. The relative band intensities suggested that the alkyl chains were, on
average, lying close to the surface at first and closer to the surface normal after 15
min of immersion. The invariance of the band frequencies suggested that the local
molecular environments were insensitive to coverage. These data were consistentwith a picture in which islands of vertically oriented molecules form and grow to
cover the surface. Terrill and coworkers (31) obtained IR spectra from quenched
C16 thiol SAMs immersed in ethanolic solutions (106102 M) for periods of
11 days. Using the position of the antisymmetric methylene stretch as a signa-ture of chain disorder, they found that long times (from several hours to several
days depending on concentration) were necessary to reach the most conformation-
ally ordered state. They also observed that the ordering was faster on smoother
substrates. On the other hand, Bensebaa et al (32) reported that this same peak
position reached its ultimate value, representative of well-ordered alkyl chains,after only a 45-s immersion in 5 M ethanol solution (for quenched films of a C22thiol).
Himmelhaus et al (33) performed sum frequency generation spectroscopy stud-
ies on quenched C22 thiol SAMs adsorbed from 3 M ethanolic solution, monitor-
ing the various C-H stretch bands over 2 days of immersion time. They found three
distinct regimes of growth. The first stage (initial 5 min of immersion) involved
formation of Au-S bonds. The coverage reached 80%90% after this stage. The
second stage (515 min of immersion) was characterized by a transition of the
hydrocarbon chains from a highly kinked to an all-trans conformation. The finalstage (20 min to 2 h of immersion) involved reorientation of the terminal methyl
groups from a state in which methyl groups were disordered relative to one another
to one in which they were aligned. The authors pointed out that this sequence im-
plies that the ordering process can be viewed as consecutive steps originating at the
gold interface and moving toward the film surface. Humbert and coworkers (34)
performed SFG studies on quenched para-nitroanilino C12 thiol SAMs deposited
from 2 M ethanolic solution. They observed a marked change in molecular ori-
entation over the first 30 min, followed by a slower change over the next 90 min,
after which their observations ended.In recent years, several scanning-probe-microscopy experiments have shed
light on the thiol growth process. In a sequence of two papers (35, 36), Yamada
& Uosaki performed in situ STM experiments monitoring alkylthiol growth on
Au(111) from micromolar heptane solutions. They observed three basic steps.
Initially, patches of adsorbed molecules were observed, but no periodic struc-
tures were detected on molecular-length scales. The authors suggested that these
patches might correspond to a disordered phase. In this stage of growth, pits
(or vacancy islands) were formed in the gold. The second step involved the
appearance of patches in which striped patterns were observed. Althoughseveral periodic length scales were found, all were greater than the molecu-
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118 SCHWARTZ
of thiol molecules lying down on the surface in various ordered epitaxial ar-
rangements. In the third and final stage of growth, islands of apparently greater
film thickness formed and grew to cover the surface. A hexagonal pattern was
observed on these islands, consistent with the (3 3)R30 epitaxial ar-rangement of thiol molecules well-known from STM and scattering experiments
(as discussed above). This growth sequence is reminiscent of the path dis-
cussed in an earlier section of this review through a quasi-equilibrium phase
diagram at temperatures above the triple point (Figure 2b). Xu and cowork-
ers (37) performed detailed and quantitative in situ atomic-force microscopy
(AFM) experiments to follow the growth of C18 and C22 thiol molecules from
2-butanol solution (see Figure 4). Their observations were consistent with those
of Yamada & Uosaki but provided direct height information. They first observed
the formation of patches that were 0.5 nm high, consistent with molecules lyingon the surface. At longer exposure times, islands that were 1.8 nm higher than the
lying-down phase were observed to nucleate and grow, consistent with a structure
in which molecules were approximately vertically oriented. The transition from
lying down to standing up was faster for the C22 than the C18 thiol film. For 0.2 mM
C18 thiol solution, the time elapsed between the initial appearance of lying-down
Figure 4 In situ topographic atomic-force microscopy images of Au(111) obtained at various
times after injection of a solution of C18 thiol (0.2 mM in 2-butanol). The area of each frame is
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SELF-ASSEMBLED MONOLAYER FORMATION 119
patches, and essentially complete coverage of the standing-up phase was 1015
min. Tamada et al (38) also observed island growth with AFM on quenched partial
monolayers.
Silane Self-Assembled Monolayers
The growth of trichlorosilane (similarly trimethoxy- or triethoxysilane)-based
SAMs is unique among SAM systems in that it involves an irreversible covalent
cross-linking step. This is critical to the desirable properties of this class of SAMs,
including their chemical and mechanical robustness on a variety of substrates.
There is also the potential for hydrolytic bond formation to OH surface groupsthat would immobilize adsorbate molecules. Again, this is critical to the stability of
the final monolayer. The kinetics of this step relative to the self-assembly processcan clearly have dramatic implications on the growth mechanisms and final film
structure. This adds a number of complications, because the rate of hydrolysis
is sensitive to water content, pH, and temperature. It is interesting that, since
the molecular packing is ultimately determined by the covalent siloxane network,
one does not find long-range molecular order in these SAMs as one does in thiol
SAMs, where the epitaxial arrangement on the Au lattice dictates the molecular
arrangement.
In two X-ray reflectivity studies, Wasserman et al (39) and Tidswell et al (40)
determined the electron density profiles of quenched partial and complete mono-layers formed from C10C18 trichlorosilanes on silicon oxide substrates. They
found that the structure of partial monolayers was inconsistent with molecular
islands. An AFM study by Schwartz and coworkers (41) of quenched C18 silane
SAMs on mica, on the other hand, explicitly observed 2-nm-high islands that
grew to cover the surface with increasing immersion time. They found that the
islands were fractal in shape and that the scaling exponent (fractal dimension)
of1.7 in the early stages of growth was consistent with 2D-diffusionlimitedaggregation. This suggested an island growth mechanism involving collisions
between adsorbate molecules moving randomly on the surface and immobileislands. The assumption of irreversible attachment to islands led to the fractal
shape. This was essentially a view of the sequence shown in Figure 2a from a
kinetic (rather than a thermodynamic) perspective and was clearly inconsistent
with the conclusions of the prior X-ray studies. However, the mica surfaces were
known to have only isolated-exposed OH sites appropriate for anchoring themonolayer, whereas such sites were ubiquitous on silica substrates. A later AFM
study by Bierbaum & Grunze (42) on quenched C18 (and longer) silane SAMs on
silicon oxide observed similarly shaped islands. They did not observe islands on
partial C3 silane monolayers. Interestingly, second harmonic generation experi-ments by Zhao & Kopelman (43) showed that only a small minority of the surface
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120 SCHWARTZ
Kropman and coworkers (44) observed dendritic C18 silane islands in quenched
partial monolayers prepared even on SrTiO3 substrates.
As mentioned above, the competition between various time scales in silane
SAM growth makes the process quite sensitive to variations in preparation condi-tions. For example, contact angle (45) and IR spectroscopy (46) studies showed
that quenched C18 silane SAMs prepared on silica substrates at a temperature below
30C contained well-ordered alkyl chains, while those prepared at higher tem-peratures contained increasing chain disorder. This led to a quasi-equilibrium
picture of silane SAM growth similar to those steps illustrated in Figure 1
and Figure 2 above. However, the nomenclature used was borrowed from the
Langmuir monolayer literature; therefore the liquid phase was labeled LE and the
solid phase LC.
Carraro et al (47) obtained AFM images of quenched partial C18 silane SAMson silicon oxide over a range of temperatures (see Figure 5). At a low temper-
ature (10C), dendritic islands were observed to grow and coalesce to cover thesurface, while at a high temperature (40C) only a homogeneous uniform film was
Figure 5 Atomic-force microscopic images of partial octadecyltrichlorosilane self-assembledmonolayers on silicon oxide removed from 2 mM solution (using hexadecane-carbon tetrachloride
as a solvent) after 30 s of immersion at 10C,25C, and 40C (left to right). The height distribution
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SELF-ASSEMBLED MONOLAYER FORMATION 121
observed. At an intermediate temperature (25C), some dendritic islands wereobserved to nucleate and grow. However, before they could coalesce and cover the
entire surface, the continuous phase between the islands gradually increased its
thickness to that of the islands, ending the film formation. A similar AFM study byGoldmann and coworkers (48) observed quenched partial C18 silane SAMs on sili-
con oxide prepared at 12, 21.5, 26.5, 35, and 43C. They observed regions of threeheights that they considered to be vapor, liquid, and solid, respectively (although
their nomenclature was G, LE, and LC). At 26.5C and below, they observedsequential transitions from vapor to liquid to solid involving domain nucleation
and growth. The vapor-liquid coexistence region was characterized by a foamlike
morphology. At 35C, only the evolution from vapor to liquid was observed.The results of both papers can be interpreted via a phase diagram like Figure 1,
where the triple point is between 10 and 12C if one assumes that the growth is notunder quasi-equilibrium conditions. Below the triple point, the growth process is
typical of 2D vapor-solid coexistence. Above the triple point (but below 30C), thevapor-liquidtransition is observed followed by the liquid-solid transition. Although
solid-phase islands nucleate from the liquid phase, they do not grow quickly enough
to maintain quasi-equilibrium conditions, and the surrounding liquid phase be-
comes more and more super concentrated, therefore thicker. Above 30C, it ispossible that concentrations necessary to nucleate the solid phase are not reached
before the SAM growth is quenched by cross-linking or that the surface density
necessary for nucleation of the solid phase is not accessible via spontaneous ad-sorption from solution. A recent experiment by Sung et al (49) on a quenched C18silane SAM explicitly demonstrated a phase transition consistent with the results
of these temperature-dependent adsorption studies. A partial film was prepared in
the vapor-solid coexistence region at 10C, removed from solution, and heated to30 or 60C. The sample heated to 30C had a lower area fraction of solid-phaseislands than the unheated film and the islands had disappeared completely on the
film heated to 60C. If the film was heated to 60C and then cooled to 30C, islandswere observed to form; if cooled to 10C, the islands were larger and covered more
of the surface. This study explicitly verified the phase diagram paradigm as well asthe mobility of the molecules even after quenching. The mobile state was found to
last for several minutes, after which cross-linking and grafting apparently froze
the film morphology.
In a series of several papers (5052), the Hoffmann group explored the effect
of deposition conditions (water content and solution age) and substrate on the
structure of quenched alkylsiloxane SAMs formed at room temperature. Their
AFM images suggested simultaneous growth by island formation and a contin-
uous disordered-liquid phase. The relative contributions of the two mechanisms
were sensitive to growth conditions. Island growth was favored in deposition solu-tions with higher water content (in toluene solution) or solutions that had been aged
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increasing oxide coating thickness up to about six layers. In situ IR spectroscopy
experiments found that the silane molecules adsorbed initially in a disordered
conformation and gradually aligned and stood up as the coverage increased. Fur-
thermore, they observed an enhancement in the adsorption kinetics with increasingwater content of the deposition solution. In a later in situ AFM study (53) conducted
at room temperature, the same group observed only islands 2.5 nm high duringC18 silane growth. The discrepancy between these observations and the AFM im-
ages of quenched films (which showed both islands and continuous phase) casts
some doubt on the relevance of the quenched-film studies.
Richter and coworkers (54, 55) performed X-ray reflectivity studies of C18 silane
film growth on silicon oxide at room temperature from micromolar-concentration
heptane solution. In in situ experiments, they found density profiles that suggested
that the maximum film thickness did not change during growth but that the averagedensity gradually evolved to that of a complete monolayer. This was consistent
with island growth of approximately vertically oriented molecules. Richter and
coworkers compared the structure of partially formed monolayers during these in
situ experiments with quenched partial monolayers and found systematic differ-
ences. The quenching process apparently introduced free area into the film that was
not restored by reintroducing the quenched film into solvent. This suggested that
some adsorbate molecules were removed during quenching. Although both the
average film thickness and density were affected by the quenching process, the
density decreased more dramatically, which suggested that regions of relativelydensely packed molecules were not significantly affected by quenching, while
other, less dense regions lost most of their molecules, with the remaining molecules
tilting over dramatically. This experiment again sounds a warning regarding over-
interpretation of experiments based on quenched partial monolayers.
Other Self-Assembled-Monolayer Systems:Organic Acids and Ions
Although thiol- and silane-based systems represent the bulk of the SAM literature,there are a number of reports of monolayers based on organic acids or ions. For
example, alkyl carboxylic, sulfonic, and phosphonic acids have been demonstrated
to form organized monolayers on several metal or metal oxide surfaces. Also,
organic ions, such as quaternary ammonium salt detergents, form stable-monolayer
films on substrates like mica that have a nonzero net charge at accessible pH values.
Aside from the practical significance of expanding the range of substrates that
may be coated with SAMs, these systems offer the opportunity to explore how the
adsorbate-substrate interaction affects the assembly process, because the type of
interactions (acid-base or ionic) are in stark contrast with those in thiol or silaneSAMs.
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124 SCHWARTZ
Figure 6 Comparison of the cosine of the contact angle of water to the fractional coverage
of partially formed films of octadecyltrimethylammonium bromide (OTAB) on mica (open
circles) andoctadecylphosphonicacid (OPA) on mica (filled circles). The different behavior
at low coverage suggests that solid-phase islands on partial OPA self-assembled monolayers(SAMs) are surrounded by a two-dimensional (2D) vapor, while those on OTAB SAMs are
surrounded by a 2D-liquid phase. (Figure adapted with permission from Reference 61.)
Applying the Cassie equation (62) to the case of solid-phase islands in coex-
istence with a dilute phase yields the following predictions for the cosine of the
contact angle :
cos
=cos dilute
+island(cos island
cos dilute)
where island is the fractional surface coverage of the island phase and dilute and
island are the contact angles on a surface composed purely of the respective
phase. This equation predicts that the extrapolated value of cos at zero island
coverage will be equal to the cosine of the contact angle on a surface composed
purely of the dilute phase, which surrounds the islands. As shown in Figure 6,
the extrapolated value for OPA is close to unity, implying that water would wet
the dilute phase. This is consistent with the dilute phase being bare mica or mica
with a very low coverage of adsorbed surfactant moleculesa 2D-vapor phase of
OPA molecules. On the other hand, for OTAB, cos extrapolates to 0.4 at zerocoverage, implying that the dilute phase is fairly hydrophobic, with a contact angle
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SELF-ASSEMBLED MONOLAYER FORMATION 125
Figure 7 Transmission IR spectra of quenched (a) octadecylphosphonic acid (OPA) and
(b) octadecyltrimethylammonium bromide (OTAB) self-assembled monolayers on mica
taken after increasing immersion times in 0.2 mM and 0.1 mM solution, respectively.
(Figure adapted with permission from References 58 and 61.)
of OPA (Figure 7a) and OTAB (Figure 7b) after increasing immersion times (top
to bottom in each figure). For OPA, although the peaks were observed to grow asimmersion time and island coverage increased, the peak positions remained at the
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the molecules on the surface were disordered. On the other hand, the peak positions
for OTAB gradually shifted to lower wave numbers with increasing immersion time
and island coverage. The antisymmetric peak position moved from 2924 cm1,
consistent with disordered alkyl chains, to 2919 cm1. For samples partiallycovered by islands, this peak is presumably the convolution of two peaks, one for
the molecules within islands and one for the molecules in the dilute phase between
islands. The relative weighting of the two peaks changes with the island coverage,
resulting in the apparent peak position shift, which is consistent with a significant
coverage of disordered molecules in the region between islands. For both systems,
the contact angles and IR spectra tell the same storythe dilute phase surrounding
the islands may be either a 2D vapor (as for OPA) or a 2D liquid (as for OTAB),
depending on the system chemistry and thermodynamic conditions.
QUANTITATIVE ASPECTS OF GROWTH PROCESSES
Rate Constants/Time Scales
Many reported rate constants or time scales for SAM growth have been collected
in Figures 8a and 8b, cast in terms of time constants. For cases in which rates con-
stants were reported, the time constants were calculated simply as the inverse of the
rate constants. Although rate and time constants clearly contain the same informa-
tion, time constants are presented in the hope that the plotted values will have moreintuitive value to the reader. Figure 8a displays data for thiol SAMs, and Figure 8b
includes data for silanes and acid-based monolayers. The large scatter in the thiol
data is particularly noticeable. For any concentration in the range 106103 M,the spread in the measured values of time constants is typically 2 orders of
magnitude. Data have been included for a range of chain lengths (C12C22) and a
variety of solvents (mostly alcohols and alkanes). Also, several different theoretical
models were used to extract rate or time constants. However, these variables typi-
cally introduce variations in rates only of order unity. Data included in Figure 8a
were determined using a variety of techniques, both in situ and on quenched partialfilms; however, there is no real pattern or consistency even when considering only
individual techniques or methods. Thus, one is left with the impression that there
may be real differences in the growth kinetics of thiol SAMs in different labora-
tories. It is unclear which parameters are not controlled; one possibility that has
been suggested is the substrate roughness or microcrystallinity. In contrast, the
data in Figure 8b is surprisingly consistent even though time scales are included
for silane SAMs on a variety of substrates (open symbols) in addition to carboxylic
and phosphonic acid SAMs (filled symbols). Again, the data represent a variety of
techniques, some in situ and some on quenched partial films. A casual inspectionalso reveals that these SAMs grew consistently more slowly than the thiol SAMs.
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SELF-ASSEMBLED MONOLAYER FORMATION 127
Figure 8 Time scales of self-assembled monolayer (SAM) growth versus solution concentration
summarized from a variety of reports. (a) All symbols represent Alkanethiol SAMs. (b) Silane
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128 SCHWARTZ
rate on concentration) that one would expect for any model involving adsorption-
limited growth. There appears to be a reasonable agreement for silanes and acids
(Figure 8b) above 105 M. Some of the individual thiol data sets (15, 21, 27) in
Figure 8a are approximately consistent with the inverse dependence of time con-stant and concentration. However, others, such as those from the Blanchard group
(25, 26) and the Georgiadis group (22), report time constants that decrease much
more slowly. It has been suggested (25, 26) that this can be explained by a sig-
nificant desorption rate, which is not concentration dependent. The discrepancy
regarding the concentration dependence of growth kinetics between different labo-
ratories remains unresolved.
Functional Form of Coverage KineticsIn most cases, the coverage kinetics are compared to the simple Langmuir or
Avrami kinetic model, which assumes that the rate of deposition is proportional
to the free space on the surface, that is, d/dt = k(1 ), where k is a rateconstant. This leads to the integrated expression = 1 exp(kt), which hasfrequently been used to fit kinetic data for SAM growth, sometimes because it
agreed quantitatively with the data and on other occasions simply because it was
considered the simplest model to use when the low precision of the data did not
justify using a more complicated model. The time constants reported in the pre-
vious section are simply the inverse of the rate constant, that is, time constant =1/k. In situ nonlinear optical experiments on thiol SAM growth (24, 27, 33, 64)
have typically found that Langmuir kinetics [or a variant involving multiple ad-
sorption sites (27)] described the early stages of adsorption reasonably well,
as did several in situ SPR (21, 65), QCM (25, 26), and AFM (37) studies. In a
careful SPR study that divided the growth process into three regimes, Peterlinz &
Georgiadis (22), however, found that the early stages were equally well described
by second-order Langmuir kinetics or a diffusion-limited form. A second step
was found to obey zero-order kinetics. In electrochemical studies of azobenzene-
containing thiol monolayers, Shao et al (23) found that kinetics based on a Frumkinisotherm approach (which includes adsorbate-adsorbate interactions) described the
data better than Langmuir kinetics. In situ IR (51, 66) and X-ray reflectivity (55)
studies of alkylsilane SAM growth also found reasonable agreement with Lang-
muir kinetics, except for early times (55, 66) or for solutions with high water
content (51). IR spectra of quenched partial monolayers of alkyl carboxylic acids
on aluminum oxide (67) were consistent with Langmuir kinetics, as were in situ
AFM studies (59) of the early stages of phosphonic acid monolayer growth on
mica.
Given what is known about the details of the growth processes for the variousSAM systems discussed above, it is somewhat surprising that the simple Lang-
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SELF-ASSEMBLED MONOLAYER FORMATION 129
free parameters. Also, although it often describes the coverage kinetics well over
a large range of the growth, there are clear limitations to the Langmuir model at
very early times and for late times. At early times, there is often a discrepancy
between observed coverage and the prediction from the Langmuir form. Some-times an induction period is observed before growth starts. Also, the last 10%
20% of monolayer growth is generally found to evolve with a slower time scale
than the earlier regimes. However, the simple Langmuir model is remarkably
robust, and thus it is worthwhile to consider what this tells us about the growth
process.
For systems in which SAM growth involves a 2D-vapor-to-solid transition, the
signal observed by any of the typical techniques is dominated by the molecules
in the solid islands. Thus Langmuir kinetics are consistent with a growth rate that
is proportional to the area uncovered by islands. If adsorption occurs primarilyon these uncovered regions and the overall SAM growth kinetics is adsorption-
limited, this would explain why Langmuir kinetics work well in such cases. For
cases in which a low-density phase (2D-liquid or lying-down phase) forms be-
fore the solid phase, however, it is somewhat more difficult to justify a simple
Langmuir model. With thiols, for example, one wonders which part of the growth
process is being observed in SPR, QCM, or spectroscopic measurements. Is it the
formation of the lying-down phase, the conversion to the standing-up phase, or
some combination of the two? It is certainly reasonable that the formation of the
original lying-down phase would follow Langmuir kinetics. However, this wouldaccount for only 20% of the final film coverage, whereas many experimentsassert that Langmuir kinetics describes the initial 50%80% of film growth. In the
only experiment that could directly separate the two processes, an in situ AFM
study (37), growth of the two phases were observed to occur sequentially but with
approximately the same time constant. It is not clear, however, that the kinetics
of each of the processes have the same concentration dependence. Another pos-
sibility is that some of the techniques used might not be particularly sensitive
to molecules in the lying-down phase, and/or the signal caused by this phase is
part of the experimental baseline. In such a situation, the experiment would reportessentially the growth of the solid-phase islands, and Langmuir kinetics would
imply that the kinetics are limited by adsorption in the regions covered by the
lying-down phase. These assumptions are not particularly satisfying, however,
and the connection between the detailed growth mechanism and the macroscopi-
cally averaged coverage kinetics remains an open question for thiol SAM growth.
Particularly useful in resolving these issues will be in situ techniques that are ca-
pable of discriminating between surface-bound molecules that are in different 2D
phases.
Solvent Effects
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130 SCHWARTZ
Georgiadis (22) found that the initial stage of thiol SAM growth from heptane solu-
tion was 35% faster than that from ethanolic solution. Karpovich &Blanchard (25) did not observe significant differences using hexane or cyclohexane
as solvents. Dannenberger et al (27) found that solvents affected the thiol SAMgrowth kinetics in the order (fastest to slowest) hexane > ethanol > dodecane >
hexadecane. Although this order coincides with the solvent viscosity (which would
affect the molecular diffusivity in solution), there is ample evidence to suggest that
SAM growth is typically not limited by bulk diffusion at micromolar concentra-
tions or above. It was suggested that a limiting step might involve the displace-
ment of solvent molecules by adsorbate molecules at the surface, so that solvents
with stronger surface interactions would result in slower adsorption and SAM
growth.
Chain Length Effects
The literature is full of dramatically conflicting reports regarding the effects of
chain length on thiol SAM growth kinetics. Regarding the initial fast stage of
growth, Bain et al (15) found that C18 grew faster than C10 from ethanolic solu-
tion, Xu and coworkers (37) reported that C22 formed more quickly than C18from 2-butanol solution, and Jung & Campbell (65) performed a systematic SPR
study and found that the growth rate increased with chain lengths in the range
C2C18 from ethanolic solution. Thus, these studies consistently found that ad-sorption rate increased with chain length. Other studies reported exactly the
opposite trend, however. Peterlinz & Georgiadis (22) reported growth rates for
the initial step in the order C8>C12>C16>C18 from ethanolic solution, and
Dannenberger et al (27) found that growth rates obeyed the trend C4>C12>C22for both ethanolic and hexane solution. Complicating the matter even further,
two additional reports were inconsistent with all of these results. DeBono and
coworkers (21) found that the initial stages of growth for C16 thiol occurred at
about the same rate as C6 and that both were faster than C12 from ethanolic solu-
tion. Karpovich & Blanchard (25) found that the early stages of growth forC8 and C18 thiols (from hexane solution) were approximately equal in overall
rate. Analyzing the concentration dependence of the growth kinetics, they repor-
ted that the adsorption rate for C18 was greater than that for C8, but that the
desorption rates had the opposite behavior. There is also some confusion regard-
ing the chain length dependence of the later slow-growth regime. Peterlinz &
Georgiadis (22) reported that the rate of this process increased with chain length
from C12 to C16 to C18. DeBono et al (21) also found that C16 was faster than C12,
but they observed that the trend was reversed for C6, which was equally as fast
as C16.There is, unfortunately, little basis on which to critically analyze these re-
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SELF-ASSEMBLED MONOLAYER FORMATION 131
a hypothetical activated process for adsorption, one might think that the enhanced
interactions between a longer chain and the surface would lower the energy bar-
rier and increase the adsorption rate (65). On the other hand, if mobility is an
issue, longer chains might move more slowly. It is clear that none of the resultssummarized above are dominated by bulk solution-phase molecular diffusion be-
cause of the absolute rates, the details of the time dependence, and the concentra-
tion dependence of the rate constants. However, one cannot rule out the impor-
tance of molecular mobility in moving through a hypothetical physisorbed layer
(22, 27), etc.
Adsorption Energetics
Several approaches have been used to get at the energetics of thiol SAMs. Bainet al (15) determined desorption rates of alkanethiol SAMs into hexadecane at
83C. Assuming an Arrhenius-type expression, they found that the activation en-ergy for desorption increased by 0.2 kcal/mol for each methylene group. Theirestimate for the absolute activation energy for C22 thiol was 28 kcal/mol. Jung
& Campbell (65) determined the sticking probabilities of various-chain-length
thiols by analyzing the observed SAM growth kinetics with a model incorporating
molecular diffusion in solution and adsorption from the subsurface layer. Again
assuming an activated energy process for adsorption, they reported that the acti-
vation free energy for adsorption decreased by 0.16 kcal/mol per methylenegroup and that the absolute activation extrapolated to 11 kcal/mol for zero chainlength. The Blanchard group (25, 26) determined the free energy of adsorption,
Gads, of C8 and C18 thiols by analyzing the concentration dependence of the ob-
served growth rate constant. They found that Gads=5.5 kcal/mol for C18 and4.4 kcal/mol for C8 thiol SAMs. By measuring the temperature dependence ofGads for C18, they found the molar enthalpy of adsorption, Hads = 48 kcal/mol,and the entropy of adsorption, Sads=48 cal mol1 K1.
It should be noted that these measurements are not completely consistent. For
example, one would expect that Gads should be approximately the differencebetween the activation energies for desorption and adsorption. Using the values
from the above references, this would give approximately Gads 827=19 kcal/mol for C18 thiol compared with the5.5 kcal/mol quoted by Karpovichet al (25). However, these absolute free energies involve an approximate value of
the pre-exponential frequency factor in the Arrhenius expression and are, there-
fore, somewhat arbitrary. Considering the change with chain length, one finds that
the activation energy measurements predict that longer chains will be stabilized by
approximately 0.2
+0.16
=0.36 kcal/mol per methylene group. This predicts
that Gads for C18 should be 3.5 kcal/mol lower than that for C8, whereas the valuequoted is only 1.1 kcal/mol lower. Of course, there were numerous simplifications
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132 SCHWARTZ
Submonolayer Island Nucleation, Growth,and Size Distributions
Doudevski et al (59) analyzed their in situ AFM images of OPA SAM growthon mica to determine the time dependence of the island number density as well
as the growth rates of individual islands. The island density (and more gene-
rally the full island size distribution) is frequently used in the literature of vapor-
phase epitaxial growth to characterize the submonolayer film morphology (9, 68
72). Figure 9 shows the island number density per site, (a site is calculated as
the approximate cross-sectional molecular area) N, as a function of time. Three
regimes of growth were observed. For short times (growth regime;
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SELF-ASSEMBLED MONOLAYER FORMATION 133
islands gradually grew. At later times (coalescence regime; >3000 s), the island
density decreased rapidly due to the merging of individual islands. At short times,
the number density was found to have a power law dependence on deposition time
with an exponent of 0.31 0.05, consistent with the point island model predictionof one-third (68, 69). This time dependence also implied that the critical nucleus
consisted of two molecules. The growth kinetics of individual islands also had
a power law form, with an exponent of 0.70 0.08, again consistent with thepoint island model prediction of two-thirds (68, 69). By comparing the rates of
island nucleation and growth, Doudevski et al (59) inferred a value of the surface
diffusivity for adsorbate molecules ofD = 1.1( 0.1) 106 sites/s = 2.9(0.3) 109 cm2/s.
In other work, Doudevski & Schwartz (60) measured the island size distribution
in the aggregation regime (in which the island number density was approximatelyconstant) of OPA SAM growth. As expected qualitatively, with increasing cover-
age the peak position of the distribution gradually moved to larger island size, and
the distribution broadened considerably. They used these distributions to test one of
the fundamental assumptions of cluster growth, the dynamic-scaling assumption.
The essential concept of the dynamic-scaling assumption is that, at a given stage of
growth, there is only a single characteristic length scale. This length can be taken
tobe S, the average island size, which is a function of the fractional island coverage
. If this assumption is correct, then the island size distribution function can be
written asNs () = S2f(s/S ) (68), whereNs() is the number density of islandscontaining s molecules at coverage . That is, the island size distribution can be
factored into two partsone that contains all dependence on coverage and length
scale and another that is a scale-invariant fundamental distribution function, f.
Upon applying this scaling form, the island size distributions obtained at various
stages of the aggregation regime were found to collapse onto a single function
f(s/S) = S21 Ns (), consistent with the dynamic-scaling-assumption predic-tion. The shape of this fundamental size distribution was different than expected
from kinetic Monte Carlo simulations of epitaxial growth (68, 72), in that the peak
was shifted to smaller island sizes and the distribution did not extrapolate to zerofor small island sizes. This suggested the importance of additional processes not
included in these simulations, such as desorption from island edges or long-range
interactions. The shape of the distribution did rule out the possible influence of
Ostwald ripening, however (73).
MANIPULATING GROWTH WITHEXPERIMENTAL PROBES
Owing to the current interest in nanotechnology, there has been recent interest in
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SELF-ASSEMBLED MONOLAYER FORMATION 135
of the SAM structure, coverage, and morphology. This reduces our confidence
in results obtained by techniques that cannot be used in situ, under actual growth
conditions.
The paradigm that is emerging for SAM growth is a fascinating one, combin-ing aspects of surfactant science, epitaxial growth, and nonequilibrium thermo-
dynamics in two dimensions. Qualitative pictures of the growth process are now
reasonably well established for several important systems. However, few of the
mechanisms are understood at a quantitative level. The temperature dependence
of a proposed 2D-phase diagram has been explored for alkylsilane SAMs. The
details of island nucleation and growth have been touched on for alkylphosphonic
acid SAMs. The basic issues of solvent and chain length effects, etc, have been ad-
dressed in numerous studies for thiol SAMs; however, the discrepancies in the liter-
ature are dramatic on these issues, both quantitative and qualitative, and essentiallyunexplainable. A few studies have concerned themselves with the energetics of
adsorption and desorption, but, as yet, no consistency has emerged. These are com-
plicated and rich systems with a variety of interactions of the same order in strength,
that is, adsorbate-solvent, adsorbate-adsorbate, adsorbate-substrate, etc. Although
variable solvent and chain length studies have the ability to address the first and
second types of interaction (at least in a limited way), there has been little effort to
probe the ways in which the adsorbate-substrate interaction affects the growth pro-
cess or the effects that qualitatively different types of intermolecular interactions
might have. Because SAMs are being proposed as a route for surface modificationin increasing numbers of applications, involving a greater variety of substrates and
adsorbate chemical functionality, these issues will become increasingly important.
ACKNOWLEDGMENTS
Thanks go to Gang-Yu Liu, Rosina Georgiadis, and Roya Maboudian (and their
coworkers) for contributing figures to this manuscript and to Chad Taylor for his
careful and critical reading of this manuscript. The author is grateful for support
from the National Science Foundation (award 9980250).
Visit the Annual Reviews home page at www.AnnualReviews.org
LITERATURE CITED
1. Bigelow WC, Pickett DL, Zisman WA.
1946. J. Colloid Interface Sci. 1:513
2. Nuzzo RG, Allara DL. 1983. J. Am. Chem.
Soc. 105:1053. Maoz R, Sagiv J. 1984. J. Colloid Interface
S i 100:465
5. Ulman A. 1996. Chem. Rev. 96:153354
6. Ulman A. 1991.An Introductionto Ultrathin
Organic Films. Boston, MA: Academic
7. Adamson AW, Gast AP. 1997. PhysicalChemistry of Surfaces. New York: Wiley
Interscience Wiley & Sons
yU
9
p
y
-
8/4/2019 Mcanismes et cintiques des SAMs
30/33
136 SCHWARTZ
9. Zhang ZY, Lagally MG. 1997. Science
276:37783
10. Kaganer V, Mohwald H, Dutta P. 1999.Rev.
Mod. Phys. 71(3):77981911. Poirier GE, Pylant ED. 1996. Science
272:114548
12. Schreiber F, Eberhardt A, Schwartz P, Wet-
terer SM, Lavrich DJ, et al. 1998. Phys.
Rev. B 57:1247681
13. Eberhardt A, Fenter P, Eisenberger P. 1998.
Surf. Sci. 397:L28590
14. Schwartz P, Schreiber F, Eisenberger P,
Scoles G. 1999. Surf. Sci. 423:20824
15. Bain CD, Troughton EB, Tao Y, Evall J,Whitesides GM, Nuzzo RG. 1989. J. Am.
Chem. Soc. 111:32125
16. Sun L, Crooks RM. 1991. J. Electrochem.
Soc. 138:L2325
17. Shimazu K, Yagi I, Sato Y, Uosaki K. 1992.
Langmuir8:138587
18. Kim Y-T, McCarley RL, Bard AJ. 1993.
Langmuir8:194144
19. Schneider TW, Buttry DA. 1993. J. Am.
Chem. Soc. 115:123919720. Frubose C, Doblhofer K. 1995. J. Chem.
Soc. Faraday Trans. 91:194953
21. DeBono RF, Loucks GD, Dellamanna D,
Krull UJ. 1996. Can. J. Chem. 74:67788
22. Peterlinz KA, Georgiadis R. 1996. Lang-
muir12:473140
23. Shao HB, Yu HZ, Cheng GJ, Zhang HL,
Liu ZF. 1998.Ber. Bunsenges. Phys. Chem.
102:11117
24. Buck M, Grunze M, Eisert F, Fischer J,
Trager F. 1992. J. Vac. Sci. Technol. A
10:92629
25. Karpovich DS, Blanchard GJ. 1994. Lang-
muir10:331522
26. Schessler HM, Karpovich DS, Blanchard
GJ. 1996. J. Am. Chem. Soc. 118:9645
51
27. Dannenberger O, Buck M, Grunze M.
1999. J. Phys. Chem. B 103:22021328. Kawasaki M, Sato T, Tanaka T, Takao K.
2000. Langmuir 16:171928
30. Truong KD, Rowntree PA. 1996. J. Phys.
Chem. 100:1991726
31. Terrill RH, Tanzer TA, Bohn PW. 1998.
Langmuir14:8455432. Bensebaa F, Voicu R, Huron L, Ellis TH,
Kruus E. 1997. Langmuir 13:533540
33. Himmelhaus M, Eisert F, Buck M, Grunze
M. 2000. J. Phys. Chem. B 104:576
84
34. Humbert C, Buck M, Calderone A, Vi-
gneron JP, Meunier V, et al. 1999. Phys.
Status Solidi A. 175:12936
35. Yamada R, Uosaki K. 1997. Langmuir
13:52182136. Yamada R, Uosaki K. 1998. Langmuir
14:85561
37. Xu S, Cruchon-Dupeyrat SJN, Garno JC,
Liu GY, Jennings GK, et al. 1998.J. Chem.
Phys. 108:500212
38. Tamada K, Hara M, Sasabe H, Knoll W.
1997. Langmuir 13:155866
39. Wasserman SR, Whitesides GM, Tidswell
IM, Ocko BM, Pershan PS, Axe JD. 1989.
J. Am. Chem. Soc. 111:58526140. Tidswell IM, Ocko BM, Pershan PS,
Wasserman SR, Whitesides GM, Axe JD.
1990. Phys. Rev. B 41:111128
41. Schwartz DK, Steinberg S, Israelachvili J,
Zasadzinski JAN. 1992. Phys. Rev. Lett.
69:335457
42. Bierbaum K, Grunze M, Baski AA, Chi
LF, Schrepp W, Fuchs H. 1995. Langmuir
11:214350
43. Zhao XL, Kopelman R. 1996. J. Phys.
Chem. 100:1101418
44. Kropman BL, Blank DHA, Rogalla H.
1998. Thin Solid Films 329:18590
45. Brzoska JB, Shahidzadeh N, Rondelez F.
1992. Nature 360:71921
46. Parikh AN, AllaraDL, Azouz IB,Rondelez
F. 1994. J. Phys. Chem. 98:75777590
47. Carraro C, Yauw OW, Sung MM,
Maboudian R. 1998. J. Phys. Chem. B102:444145
48. Goldmann M, Davidovits JV, Silberzan P.
yU
9
p
y
-
8/4/2019 Mcanismes et cintiques des SAMs
31/33
SELF-ASSEMBLED MONOLAYER FORMATION 137
Y, Maboudian R. 2000. J. Phys. Chem. B
104:155659
50. Vallant T, Brunner H, Mayer U, Hoffmann
H, Leitner T, et al. 1998. J. Phys. Chem. B102:719097
51. Vallant T, Kattner J, Brunner H, Mayer U,
Hoffmann H. 1999. Langmuir15:533946
52. Brunner H, Vallant T, Mayer U, Hoff-
mann H, Basnar B, et al. 1999. Langmuir
15:1899901
53. Resch R, Grasserbauer M, Friedbacher G,
Vallant T, Brunner H, et al. 1999. Appl.
Surf. Sci. 140:16875
54. Richter AG, Durbin MK, Yu C-J, Dutta P.1998. Langmuir 14:598083
55. Richter AG, Yu CJ, Datta A, Kmetko J,
Dutta P. 2000. Phys. Rev. E61:60715
56. Woodward JT, Ulman A, Schwartz DK.
1996. Langmuir 12:362629
57. Woodward JT, Schwartz DK. 1996. J. Am.
Chem. Soc. 118:786162
58. Woodward JT, Doudevski I, Sikes HD,
Schwartz DK. 1997. J. Phys. Chem. B
101:75354159. Doudevski I, Hayes WA, Schwartz DK.
1998. Phys. Rev. Lett. 81:492730
60. Doudevski I, Schwartz DK. 1999. Phys.
Rev. B 60:1417
61. Hayes WA, Schwartz DK. 1998.Langmuir
14:591317
62. Cassie AB. 1952. Discuss. Faraday Soc.
75:5041
63. Porter MD, Bright TB, Allara DL, Chidsey
CED. 1987. J. Am. Chem. Soc. 109:3559
68
64. Dannenberger O, Wolff JJ, Buck M. 1998.
Langmuir 14:46798265. Jung LS, Campbell CT. 2000. Phys. Rev.
Lett. 84:516467
66. Cheng SS, Scherson DA, Sukenik CN.
1992. J. Am. Chem. Soc. 1114:54365437
67. Chen SH, Frank CW. 1989. Langmuir
5:97887
68. Amar JG, Family F, Lam PM. 1994. Phys.
Rev. B 50:878197
69. Tang LH. 1993. J. Phys. I3:93550
70. Evans JW, Bartz JA, Sanders DE. 1986.Phys. Rev. A 34:143448
71. Amar JG, Family F. 1996. Thin Solid Films
272:20822
72. Amar JG, Family F. 1995. Phys. Rev. Lett.
74:206669
73. Zinke-Allmang M, Feldman LC, Grabow
MH. 1992. Surf. Sci. Rep. 16:377
463
74. Xu S, Liu GY. 1997. Langmuir 13:12729
75. Xu S, Miller S, Laibinis PE, Liu GY. 1999.Langmuir 15:724451
76. Xu S, Laibinis PE, Liu GY. 1998. J. Am.
Chem. Soc. 120:935661
77. Piner RD, Zhu J, XuF, Hong S, Mirkin CA.
1999. Science 283:66163
78. Hong SH, Zhu J, Mirkin CA. 1999. Lang-
muir15:7897900
79. Amro NA, Xu S, Liu GY. 2000. Langmuir
16:30069
yU
9
p
y
-
8/4/2019 Mcanismes et cintiques des SAMs
32/33
-
8/4/2019 Mcanismes et cintiques des SAMs
33/33
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