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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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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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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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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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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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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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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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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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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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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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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126 SCHWARTZ

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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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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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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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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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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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

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