J. Phys. Chem. 1996, 100, 19917-19926
19917
Formation of Self-Assembled Butanethiol Monolayers on Au Substrates: Spectroscopic Evidence for Highly Ordered Island Formation in Sub-Monolayer Films Kim D. Truong and Paul A. Rowntree* De´ partement de Chimie, UniVersite´ de Sherbrooke, Sherbrooke, Que´ bec, Canada ReceiVed: October 31, 1995; In Final Form: July 18, 1996X
The initial stages of the formation of butanethiol self-assembled monolayers on well-prepared Au/mica substrates have been studied using infrared spectroscopic analysis of the CH2 and CH3 stretching vibrations and real-space STM methods. The projection of the molecular axes of the chemisorbed species onto the surface normal is initially less than found with the fully formed films, indicating that in the initial stages of deposition they adopt configurations more prone to the surface; the evolution to the well-packed near-vertical molecular conformation that is usually associated with monolayer structures is observed even for sub-monolayer films that have been equilibrated in solution for several days. We conclude that film formation under lowconcentration conditions proceeds by island growth and that the evolution of these islands can be monitored by the spectral evolution of the infrared bands. We have also established that the infrared absorption of the symmetric methyl stretch (r+, 2876 cm-1) is linearly related to the overall surface coverage, while the absorption frequency is independent of coverage, thus supporting an island growth model in which the local molecular environment is unaffected by the macroscopic coverages. Our results indicate that thiolate desorption during film formation is not a significant process using methanol solvents and that the quality of the Au/mica substrate is crucial in determining the structural evolution of the growing film.
Introduction Highly organized organic thin-film interfaces have been the subject of enormous scientific interest in recent years due to their close structural similarities to biological membrane systems and because of their potential applications in thin-film sensor devices and optical networks;1-3 their capacity to passivate metal surfaces against charge and mass transfers into solution has been used to study rapid electron transfer processes4 and promises to provide new methods to provide inert interfaces between bodily fluids and metallic surfaces of implanted prosthetic devices. Controlling the structure and composition of the organic thin film is a natural prerequisite for the application of these systems; accordingly, a great deal of fundamental research on organic layers has been dedicated to the study of the surface and “bulk” properties of the fully formed monolayers. The two most commonly employed techniques for the preparation of these films are based on Langmuir-Blodgett methods (i.e., physisorption of preorganized organic films onto planar substrates) and molecular self-assembly, whereby amphiphilic molecules are chemisorbed from solution onto appropriate substrates by reactive terminal groups of the molecular adsorbate and, in the process, develop into high-density films.5,6 Within the latter classification, self-assembled monolayers (SAMs) composed of organic thiols adsorbed onto metal surfaces (notably single-crystal Au(111) and evaporated Au/mica and Au/Si films with Au(111) surface textures) have received the most attention by far, due to the ease with which these SAMs can be prepared using standard “wet-chemistry” methods, the absence of significant gold oxidation under ambient conditions, and the remarkably high degree of ordering that can be present at the Au(111)-S interface,7,8 in the organic bulk phase,9-12 and at the surface;13-17 the recent STM studies of Poirier et al.15,16 indicate that a remarkable degree of surface ordering can be achieved even with short-chain systems in which the interchain interactions are relatively weak. * Author to whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, November 15, 1996.
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The vast majority of the extensive literature on alkanethiol SAMs employ the film preparation methodology described in the original work by Nuzzo et al.18 for the deposition of disulfides onto gold surfaces: metal substrates are immersed for ∼24 h in thiol (or disulfide) solutions that contain ∼102- to 103-fold excess thiol than that required for full monolayer formation. Although the mechanistic details of the chemisorption process are not fully resolved (and indeed, there are outstanding questions regarding the composition19 and the structure7,20 of the Au(111)-S interface), the overall result is the elimination of the terminal sulfur-bonded hydrogen to form a thiolate species chemically bound to the substrate; the 24 h “incubation” period allows the molecular film to assemble into the final crystalline-like solid phase11 and provides the possibility for the desorption of physisorbed and chemisorbed contaminants from the gold surface into solution. In the case of methylterminated alkanes CH3(CH2)n-1SH (Cn) adsorbed onto Au(111) surfaces, it is now accepted that the aVerage intermolecular separation of the alkane chains is 4.99 Å, leading to a model with the S atoms chemisorbed to the Au(111) surface on a (x3×x3)R30° lattice, corresponding to the next nearest neighbor separation of gold atoms (or equivalently, the distance between next nearest 3-fold hollow sites that are believed to provide the strongest bonding for the thiolate20). Optimal packing of the n-alkane component of the SAM is achieved by tilting the molecular axis with respect to the surface normal while maintaining the average 4.99 Å S-S separation. In several cases, c(4×2)14 and “pinstriped”15-17 superlattices have been observed in well-formed films, showing that anisotropic interchain interactions are important aspects of the energy balance in the fully equilibrated film structures. Despite the widespread study of these systems by virtually every surface sensitive technique (e.g., infrared reflection absorption spectroscopy (IR-RAS),10,21 scanning tunneling and atomic force microscopy ([STM, AFM),15-17,22,23 atomic diffraction,13,14,24 X-ray photoelectron spectroscopy, contact angle measurements, ellipsometry,11 electrochemical methods,4 X-ray © 1996 American Chemical Society
19918 J. Phys. Chem., Vol. 100, No. 51, 1996 crystallography7), it is remarkable that so little is known concerning the process of self-assembly and the kinetic factors that determine the ultimate film definition and quality. Such time-domain characterizations may also be important for the use of SAMs for microcontact pattern formation,25 where the choice of alkanethiol is determined by the balance between the need for fast adsorption and film stabilization (thus favoring highly mobile species) combined with the requisite high resolution of edge structures (thus favoring long-chain species with slow lateral transport rates and negligible airborne transport). The ellipsometric and contact-angle measurements of Bain et al.11 showed that the formation of long-chain C10 and C18 SAMs proceeds rapidly in 10-3 M solutions, with the film properties being fully evolved within 24 h; it was noted that the film characteristics evolved in a non-Langmuirian fashion in 10-6 M (or weaker) solutions. These authors proposed that SAM formation is the result of two distinct processes that operate on significantly different time scales in millimolar solutions. Initial adsorption from millimolar solutions is very rapid and is essentially complete within a few seconds or minutes; this is followed by an autoorganization process (possibly including the lateral diffusion of adsorbates and the elimination of entrapped solvent26 from the film) that can require several hours. Karpovich and Blanchard27 have recently studied in-situ the kinetics of adsorption of C8 and C18 films from n-hexane and cyclohexane solvents onto gold-plated quartz microbalance surfaces. As with previous workers, they found that the adsorption phase in millimolar solutions required ∼10-20 s; they concluded that the deposition phase is an equilibrium process and that the equilibrium constant favors desorption for shorter chain systems, while longer chain systems tend to maintain higher surface coverages for a given thiol concentration. These are among the first quantitative measurements of the adsorption process for the alkanethiol/Au(111) systems. The second, slower stage of film formation has been studied by Ha¨hner et al.12 using NEXAFS; on the basis of the relative intensities of the C-H and C-C NEXAFS peak intensities, they concluded that long-chain (C22) SAMs initially deposit with highly disordered chain conformations and that the second, slower phase consists of the extension of the individual alkyl chains without lateral transport of thiols or gold atoms across the metal surface, and additional adsorption presumably takes place as the molecular disentanglement exposes more of the metal surface to the thiol solution. These authors also concluded that solvent desorption was not a significant aspect of the temporal evolution of the SAM structure and that island-growth mechanisms were not operant in these long-chain systems. Despite the works cited above, there are many unknown aspects of the adsorption and formation of SAMs. These include the role of surface uniformity, the ability to distinguish and resolve the two phases of film formation, the mobility of the adsorbed phases, and the precise correlations between surface structure, surface dynamics, and surface “quality” as measured by the various techniques. Films prepared by vacuum deposition under cryogenic conditions24 tend to form large, open structures, presumably due to the reduced lateral diffusion of the adsorbed alkanethiols. Shimazo26 has shown that the electrochemical properties of organic SAMs are highly sensitive to the film deposition conditions, with more compact layers being formed with extended deposition times; the role of solvent inclusions into the developing monolayer was also evident, and the liberation of these trapped species provides avenues by which water can be incorporated into films during subsequent electrochemical characterizations. In view of the potential applica-
Truong and Rowntree tion of these films in electrochemical environments and the overall requirement for information on the film formation dynamics, it would be useful to better understand the role of preparative methodologies on the organic surface characteristics. Infrared spectroscopy is well-suited to these studies because of its nondestructive nature and its intrinsically high resolution for resolving the band shifts associated with changing molecular conformations. Most studies of SAMs to date have been based on comparisons among closely related systems (e.g., as a function of the alkane chain length10 or chain termination11) or by considering the evolution of the SAM spectra for a single species as a function of the substrate temperature.28,29 The present work will explore a different and unique aspect of monolayer dynamics by considering the spectral evolution of a single adsorbate species, butanethiol (C4), during the process of film adsorption and self-organization. Butanethiol was selected for this study because of the characteristic absence of IR-RAS absorptions due to the methylene groups in the spectrum of the SAM and, as we show herein, their characteristic presence in the case of nonequilibrated films; this provides a clear indication of the structural transformations that occur as the system evolves. In addition, the short butanethiol chain permits the molecule to minimize interactions with neighboring adsorbates at experimentally accessible low surface coverages; the low-coverage limit for the completely independent motion of Cm chains will vary approximately as m-2, such that longer chain systems require fewer molecules per unit area to have equivalent independence among the molecular adsorbates (the m-1 variation of the molecules per unit area for cryogenically deposited films measured by Camillone et al.24 is due to the formation of highly organized planar arrays of molecules). By spectroscopically examining the film formation process, we have been able to follow the motion and average conformations of the adsorbates in disorganized films, as well as the evolution toward full SAM structures. To the best of our knowledge, this is the first IRRAS study of its kind, and the methodology presented herein can be directly applied to other systems in order to provide detailed information on film formation mechanisms. In recent years, the term self-assembled monolayer has been applied to an extremely wide variety of adsorbed phases, such that it often refers more to the mechanism of adsorption (e.g., RS-Au or RSi-O-Si bonding) than to the adsorbed phase’s structural or coverage characteristics. We restrict the use of the term self-assembled monolayer to those cases where highly organized and complete monolayers are demonstrably present; all other adsorbates will be considered simply as films. Films are further classified as (1) nonequilibrated or as equilibrated, according to the degree to which the film structures have achieved limiting characteristics as a function of time (e7 days), and (2) sub-monolayer or monolayer, according to the macroscopic surface coverage. Experimental Section (1) IR-RAS Characterization. Grazing incidence infrared reflection-absorption spectroscopy30 (IR-RAS) was used to characterize the films. All spectra were obtained with ppolarized light incident at 85° with respect to the surface normal using a commercial specular reflectance apparatus (Harrick) installed in a Magna-550 spectrometer (Nicolet) equipped with an MCT-A detector; the sample compartment and optical bench were purged with desiccated and CO2-scrubbed air (Balston 7545). Data were collected with a resolution of 0.5 cm-1, a moving mirror speed of 1.98 cm/s, and Happ-Ganzel apodization and postprocessed to an equivalent of 1 cm-1. The absorbance data are reported as -1000 log(R/R0), where R and R0 are the
Self-Assembled Butanethiol Monolayers on Au Substrates
J. Phys. Chem., Vol. 100, No. 51, 1996 19919
Figure 1. STM images of Au/mica films before (A) and after (B) acid cleaning the surface to eliminate all traces of organic impurities or adsorbates. Brighter regions correspond to higher elevations of the surface. The etch pits observed after the cleaning process are exactly one Au atom deep (2.4 Å) and are unrelated to the pitting observed by several workers following the deposition of the organic thiols onto the gold substrates. Image A is a 100 nm scan, while B is 175 nm across. Imaging conditions are described in the text.
reflected infrared intensities for the sample and the reference surfaces, respectively. All spectra have been baseline corrected using two points selected outside of the spectral range presented in the figures. (2) STM Characterization. STM images were obtained using a homemade in-air STM system using a standard piezotube scanner and Viton suspension/support for vibration isolation. Lateral dimensions were calibrated by the atomically resolved images on graphite surfaces, and vertical heights were calibrated by the 2.36 Å step heights on freshly prepared Au(111) films. Tips were electrochemically etched tungsten (0.5 M NaOH, 13 Vac) or electrochemically etched 0.9/0.1 PtIr wires (2 N KOH, 6 N NaCN, 7-13 Vac). The STM environment was mildly desiccated to minimize humidity and to reduce parasitic currents to below 2 pA; typical tunneling parameters were Vsample ) +0.25 V and I ≈ 30 pA; tunneling currents were measured at the tip, which was maintained at virtual ground. Gold surfaces could be imaged under most conditions, but acceptable images of the C4 SAMs were only observed using positive sample biases (i.e., electrons tunneling from the tip into the SAM) and low tunnel currents. Under no circumstances were adsorbate-like structures observed on clean gold surfaces. (3) Substrate Preparation. Evaporated gold on mica films were prepared using methods similar to those described by DeRose et al.31 Mica sheets (Proscience/TechniGlass, ASTM V2) were cleaved to expose a clean crystalline surface and immediately loaded into an all-metal evaporation chamber pumped by a turbomolecular pump (Leybold). Following chamber evacuation to ∼10-7 Torr, the mica substrates were heated to ∼300-350 °C for 4-12 h in order to degas the crystal surfaces and the vacuum system; ∼1000 Å of gold (JohnsonMatthey, 99.9999%) was then evaporated onto the surface at an rate of ∼1 Å/s while maintaining the substrate heating. The gold/mica film was heated for another ∼3 h after evaporation in order to produce uniform and atomically flat surfaces. STM images of these freshly evaporated films show the anticipated atomically resolved terrace structure, with individual plateau areas of ∼104-106 Å2 (Figure 1A) and frequently display the
long-range 23 × x3 reconstruction of the gold surface that is typical of high-quality surfaces that are strongly Au(111) textured; close examination of the data of Figure 1A revealed that all terrace levels were decorated by this reconstruction. Samples were stored in sealed polycarbonate dishes prior to use. Immediately before use, the substrates were rinsed in absolute methanol and quickly washed in sulfochromic acid (Chromage/ H2SO4) to oxidize any residual hydrocarbon impurities adventitiously adsorbed onto the gold surface. This highly reproducible cleaning procedure eliminates surface contaminants while preserving the atomically defined surface structure of the thin film; a secondary effect32 is the formation of topographical structures on the gold surface such as one-atom-deep etch pits and one-atom-high island structures (Figure 1B). These structures are not related to the depressions often observed in freshly prepared SAM surfaces.33 In surfaces exposed to relatively concentrated thiol solutions, we find that these nanostructures are largely eliminated in the final surface topography; exposure to dilute solutions containing less than one full monolayer leads to only a partial elimination of the structures. IR-RAS spectra taken of freshly cleaned films have absorptions in the C-H stretching region (2800-3000 cm-1) below ∼0.1 milli-absorbance units (mAU), showing them to be free of organic impurities. The interferogram for each freshly cleaned substrate was stored for use as a reference (R0) in all subsequent IRRAS measurements using this substrate. This process was absolutely essential in obtaining highly consistent results for the sub-monolayer films. (4) Film Deposition. The (x3×x3)R30° SAM structure described previously implies a full monolayer coverage of 4.64 × 1014 molecules/cm2, or 7.70 × 10-10 mol/cm2 on Au(111) surfaces. We have used this estimate in all considerations of full and sub-monolayer films. “Concentrated” solutions of butanethiol in freshly distilled methanol were prepared with ∼10-102-fold excess thiol (with respect to 1.0 ML coverage) present in solutions, while extremely dilute solutions (10-810-7 M) were prepared by sequential dilutions of the pure thiol
19920 J. Phys. Chem., Vol. 100, No. 51, 1996
Truong and Rowntree
Figure 2. Comparison spectra of butanethiol (C4) and dodecanethiol (C12) SAMs on well-prepared Au/mica substrates. Band assignments and designations are provided in Table 1 and explained in the text.
reagent in the same solvent. All solutions were freshly prepared prior to use; nitrogen-degassed solvents showed no significant improvements with respect to non-degassed solvents if both were freshly prepared. Deposition was performed at room temperature in 50 mL polycarbonate sample vials, with the immersion time specified for each experiment. Assuming a diffusion coefficient of ∼10-5 cm2/s for the butanethiol in methanol, the mean square displacement of the thiols in ∼24 h is on the order of 1-2 cm; this is approximately the maximum thiol-surface separation in the deposition cell. Accordingly, deposition times for the preparation of fully equilibrated sub-monolayer films from extremely dilute solutions were always in excess of 24 h. Spectroscopic Background A structural model of a single butanethiol molecules adsorbed onto a gold surface is schematically shown in Figure 2; the configuration shown is for the all-trans conformation tilted ∼30° from the surface normal, with no twist of the carbon backbone with respect to the surface normal (i.e., the surface normal can be found in the plane of the C-C-C skeleton). The C-SAu bonding angle in this diagram corresponds to the sp3 hybridization of the sulfur atom that has been assumed in most structural studies to date.6 Sellers et al.20 have used ab initio methods to predict that a linear sp configuration of an isolated thiol bound into the 3-fold hollow site of the gold surface will have virtually identical binding energy as the sp3 model; in the absence of experimental verification of the sp model, we have also assumed the conventional sp3 form. The IR-RAS results presented in this work are based on the frequencies and absolute intensities of the absorption bands in the “high-frequency” C-H stretching region (2800-3000 cm-1). Figure 3 shows a comparison between the IR-RAS spectra of C4 and C12 SAMs; the C12 spectrum was obtained using the conventional methods used for full SAM formation and is representative of intermediate and long-chain alkanethiol films containing an even number of carbon atoms per molecule. Assignments of the various bands34 are shown for each band and are summarized in Table 1, along with the vibrational frequencies of the liquid and solid phases from Pemberton’s Raman data35 and the C4 SAM/Au frequencies from this study. To a first approximation the optical absorption associated with a given vibration mode, I(ν), will be given by I(ν) ) |µ(ν)| cos2(Ψ), where µ(ν) is the frequency-dependent transition dipole moment (TDM) of the vibrational mode and Ψ is the angle between µ and the polarization vector of the incident radiation;30 in the present case of p-polarized radiation incident at 85°, Ψ is very close to the angle of the TDM from the surface normal,
Figure 3. Schematic diagram showing the definitions of the tilt and twist angles of butanethiol molecules adsorbed onto the Au surface; in this case, the tilt angle is ∼30°, and the twist angle is 0°. The geometry shown here corresponds to the all-trans conformation bonded to the surface with a sp3 hybridization of the sulfur center.
TABLE 1. Assignments and Frequencies of the C-H Stretching Modes of Butanethiola mode
designation
liquid (cm-1)
solid (cm-1)
νs(CH2), sym str νs(CH2), sym str νs(CH3), sym str νs(CH2, FR), sym str νa(CH2), asym str νa(CH2, FR), asym str νs(CH3, FR), sym str νa(CH3)op, asym str, op νa(CH3)ip, asym str, ip
d+ dβ+ r+ dFR+ dβdRrFR+ rbra-
2848 2864 2875 2905 2914 2926 2936 2964 2964
2842 2857 2873 2900 2925 2937 2956 2968
SAM/Au (cm-1) 2854 2876 2898 2915 2929 2936 2956 2965
a Symbols used: ip ) in plane; op ) out of plane; sym ) symmetric; asym ) antisymmetric; FR ) Fermi resonance; R ) CH2 group adjacent to terminal S atom; β ) CH2 group adjacent to R. Mode assignments and designations taken from Parikh et al.,34 the liquid and solid phase frequencies are from BryantPemberton et al.,35 while the SAM/Au frequencies are from this work.
as determined by the tilt and twist angles. Bands associated with vibrations of the terminal CH3 groups are herein denoted by r, while those associated with the vibrations of the CH2 units of the backbone chain are denoted d. Superscript + and identify the symmetric and antisymmetric vibrational modes, respectively. In addition, several bands (e.g., rFR+, 2936 cm-1, dFR+, 2898 cm-1) are associated with inter- and intrachain Fermi resonances. Figure 4 shows the value of cos2(Ψ) for the r( and d( TDMs (i.e., the quantity that is approximately proportional to the IRRAS intensities) as a function of the tilt and twist angles of the molecular backbone for the all-trans configuration of an evenchain-length n-alkane such as butanethiol. In the case of n-alkanethiol SAM structures, the combined results of ellipsometry, IR-RAS, X-ray crystallography, atomic diffraction, and molecular simulations show that the average chain tilt of the n-alkanethiols adsorbed on Au(111) is ∼26-35° from the surface normal; recent evidence has also shown that the idealized structural unit cell of the SAMs contains two distinct molecular orientations with approximately equal but opposite twists of the all-trans carbon skeletal plane about the molecular axes by (∼50°, although it is not yet certain which chain lengths support this conformation, and under which film-processing conditions.
Self-Assembled Butanethiol Monolayers on Au Substrates
Figure 4. Calculated values of cos2(Ψ) as a function of the tilt and twist angles for the r( and d( vibrational bands, where Ψ is the angle of the molecular axis with respect to the polarization vector of the incident radiation. The behavior of the r( bands corrresponds to an n-alkanethiol with an even number of carbon atoms.
Several trends are shown in Figure 4 that will be central to this work. First, it is seen that the cos2(Ψ) behavior of the r+ TDM is relatively insensitive to minor excursions of the tilt and twist angles away from those specified above for the wellformed SAM structures, since the r+ curves have relatively flat profiles in this region. Secondly, although both the d+ and dbands have exactly zero intensity for molecular arrangements parallel to the surface normal (tilt ) 0°), they have opposing tendencies as a function of the twist angle of the C-C-C skeleton for nonzero tilt angles. We stress that the curves plotted in Figure 4 are simply the result of the molecular geometries and do not include effects such as the variation of the film’s refractive index with coverage or the presence of defects in the molecular conformations; this information is not available a priori for the sub-monolayer films that are the object of this work. Nonetheless, the trends presented in Figure 4 capture the essential behavior of the various vibrational modes as the molecular geometry changes; each of the band intensity trends predicted by full-spectral simulations34 (as a function of the tilt/ twist angles) is reproduced by this simple projection model. Results and Discussion This study employs two parallel techniques to monitor the formation of the SAMs. Section A will present the IR-RAS spectra for C4 and C12 SAMs adsorbed on the Au/mica substrates. Section B will present and analyze the ex-situ IRRAS spectra obtained by controlled exposures of Au/mica surfaces to a 5 µM butanethiol solution. This will provide the first spectroscopic information on the alkanethiol/Au film deposition and autoorganization processes. In section C, the spectroscopic investigation of fully equilibrated sub-monolayer films will be used to identify the intermediate molecular conformations during film growth. Finally, these two approaches will be merged in section D to study the evolution of the IR-RAS spectra of sub-monolayer films during the process of equilibration. Section E will present STM images of the butanethiol/Au/mica film structures that show the real-space structures of the growing crystalline domains and the disordered “liquid-like” regions lacking molecular resolution.
J. Phys. Chem., Vol. 100, No. 51, 1996 19921 (A) IR-RAS Spectra of C4 and C12 SAMs. The most apparent difference between the IR spectra of the C4 and C12 SAMs (Figure 3) is the marked absence of the d( bands in the C4 SAM results, while they dominate the spectra of the C12 SAM (and all other long-chain SAMs) to the point of obscuring adjacent CH3-derived bands (e.g., rFR+, 2936 cm-1). The only clear evidence of the CH2 absorption bands in the C4 SAM spectrum is the weak d- shoulder observed at 2922 cm-1, with an intensity of ∼0.3 milli-absorbance units (mAU); in the case of the C12 SAM spectrum, the absorption of the d- band (2920 cm-1, 2.5 mAU) exceeds the r band intensities by at least a factor of 2. Our C4 SAM spectrum is in qualitative agreement with the results of Porter et al.10 for SAMs adsorbed onto Au/ Cr/Si substrates, with the exception that (1) our r( bands are systematically 1-2 cm-1 lower in frequency, (2) we resolve the rb- mode (2955 cm-1), and (3) the intensity of our ra- band (2964 cm-1) is ∼40% lower than that measured in the previous study; both data sets show that the d( bands are extremely weak. Interestingly, the r+ (2877 cm-1) band in our C12 SAM spectrum is also 2 cm-1 lower in frequency than that tabulated by Porter et al. As discussed above, the relatively weak ra- band in our data may reflect a greater degree of ordering of the methyl termination than previously observed with Au/Cr/Si substrates. The intensity of the d( bands is of course related to the number of methylene groups in the complete film; our studies have shown that the d( band intensities are observable only for C4 and longer chains and that they vary approximately linearly with the n-alkane chain length. Porter’s analysis10 indicated that the d+ intensities per CH2 are lower for C6-C12 SAMs than those for longer chain systems; along with the ellipsometric measurements of the film thicknesses and the absorption frequencies of the d( bands (relative to the bulk liquid and solid phase data of C8 and C22, respectively), this indicated that the structural order of their short-chain SAMs was significantly less than that of the longer chain systems. Although it has been argued that short-chain alkanethiol systems must be less organized than their longer chain counterparts due to their weaker interchain interactions, this is not necessarily true; Chang et al.36 have shown that even short-chain systems on silver substrates can be highly ordered and that short-chain “tail groups” attached to biphenyl thiolates on gold are just as well structured as their longer chain counterparts. We have found that the IR-RAS spectra of C4 SAMs are sensitive to the topography of the substrate gold;37 SAMs formed on Au/mica surfaces that were prepared with less substrate heating display systematic variations in d( intensities, indicating that the angles of the molecular axis with respect to the local surface normal are greater on “rough” substrates than on the well-prepared surfaces. It should be noted that even these “rough” substrates displayed atomically separated terraces, but with more defects and grain boundaries than found with the higher quality substrates.31 The observation that the r+ peak positions reported herein for the C4 and C12 SAMs adsorbed onto Au/mica are systematically lower in frequency than those reported using Au/Cr/Si substrates suggests that the details of the substrate topology play an important role in the final SAM structure; by inference, we believe that the adsorption dynamics will also be influenced by the substrate topology, especially for the short-chain alkane thiols. (B) IR-RAS Spectra during Molecular Adsorption and Film Formation. Figure 5 shows the evolution of the C-H stretch spectra of butanethiol films as a function of the time of exposure to the 5 µM solution. The first spectrum, taken after 1.0 min exposure, shows a weak r+ absorption band (∼2880 cm-1), a broad series of unresolved absorption bands in the 2885-2950 cm-1 region that overlaps the barely discernible
19922 J. Phys. Chem., Vol. 100, No. 51, 1996
Figure 5. Temporal evolution of a C4 film formed in a 5 µM solution of butanethiol/methanol. Deposition times are indicated for each spectrum.
rFR+ band (∼2936 cm-1), and the rb- and ra- bands (2956 and 2967 cm-1, respectively); the large unresolved band(s) are found at the frequencies of the dFR+ and d- bands previously identified for the longer n-alkane SAMs, as shown in Figure 3 for the C12 SAMs. The d+ band (2850 cm-1) is clearly resolved as a distinct absorption in the spectra taken after short deposition times. Continued exposure of the surface to the same thiol solution causes a dramatic decrease in the d( bands and a monotonic increase in the r+ and rFR+ bands. This evolution of the film spectra continues until the total time in solution reaches ∼5-15 min, at which point the intensities of all major bands appear to be constant; exposures beyond ∼45 min have no further effect on the IR-RAS spectra, and we therefore presume that the time scales for SAM formation are on the order of 5-15 min under the conditions employed in these works. This deposition sequence has been repeated several times, and the spectral evolution reported above has been observed in each case with only minor differences; as expected, higher thiol concentrations accelerate the deposition/SAM formation process. What is the environment of the adsorbates following the first few seconds of exposure to the solution? In the case of molecules adsorbed randomly onto the gold surface with large interchain separations, the tilt of the molecular axes should be ill-defined, because the conformation and orientation of the molecule would be unconstrained by the presence of adjacent molecular species; it would be assumed a priori that the isolated adsorbates would adopt a conformation and orientation that would increase the van der Waals interactions with the polarizable substrate, i.e., lying as close to prone to the surface as the S-Au bonding structure20 and the internal molecular degrees of freedom would permit. Under these conditions of large tilt angles, the planes of the methylene groups would be close to perpendicular to the surface, and thus we would anticipate, on the basis of the results of Figure 4, that (1) one (or both, depending on the twist angles) of the d absorption bands would be strong and (2) the r+ band would be rather weak for the low-coverage films. These band intensities should also show nonlinear dependencies on the total surface coverage, since both the quantity and the orientation of the molecules would evolve with coverage. This appears to be a good description of the IR-RAS spectra taken after short exposures to the thiol solutions.
Truong and Rowntree
Figure 6. Temporal evolution of the intensities (a) and frequencies (b) of the r+ and ra- bands as a function of the exposure time in 5 µM solutions of butanethiol/methanol. These data were extracted from the spectra of Figure 5.
Consider first the unusually strong d( absorptions in these data at short exposure times; their observation in the Td ) 1 min data of Figure 5 shows that their systematic absence in the C4 SAM spectrum (Figure 3a) is not due to intrinsically weak TDMs of the short-chain thiols, but rather, it must be due to the molecular conformations (i.e., the tilt and twist of the C-C-C chains, as well as any gauche defects) that determine the projection of the TDMs onto the polarization vector of the incident radiation. We therefore conclude that the butanethiol molecules are tilted farther from the surface normal at Td ) 1 min than in the full C4 SAM. The loss of the d+ absorptions within 2 min exposure to the thiol solution, and the subsequent loss of the d- bands within 5-15 min, shows that the molecular axes are adopting conformations more parallel to the surface normal than in the short-time “as-deposited” configuration(s). A detailed analysis of the evolution of the d( bands is made difficult by their poorly resolved structure and changing linewidths; similar problems are encountered with the rFR+ peak positions and intensities due to the interference by the CH2 bands in the 2925 cm-1 region in the short-time spectra. Instead, we have monitored the evolution of the more clearly resolved r+ and ra- bands during deposition, as shown in Figure 6. The peak intensity of these bands (Figure 6a) shows a systematic increase as a function of the exposure to the solution, with asymptotic intensities obtained after ∼10 min of immersion in the 5 µM solution; there is no perceptible induction period before the intensities increase. The increase of the ra- band is much less significant than that of the r+ band; if the ra,b- band intensities are largely determined by the presence of gauche defects in the film adjacent to the methyl termination,34 this may indicate that a large fraction of the film defects are associated with substrate defects such as step edges and grain boundaries; the number of gauche defects in this model would saturate once these substrate defects are decorated by the adsorption of thiols (we will show below that the initially deposited thiols are highly mobile on the Au(111) terraces, such that lateral diffusion could lead to adsorption at higher binding energy sites such as defects). If this description is essentially correct for the short-chain systems, the defect density determined by the IR-RAS r- bands does not represent the overall film
Self-Assembled Butanethiol Monolayers on Au Substrates quality in a statistical sense, but rather, it is a measure of the number of localized substrate defects. This reinforces the importance of highly uniform adsorption surfaces in the study of short-chain adsorption processes. The peak positions of the vibrational modes as a function of coverage/exposure also provide useful information on the deposition/assembly process. Figure 6b shows the evolution of the r+ and ra- band frequencies for the series of spectra shown in Figure 5. These bands systematically shift to lower frequencies as additional material is adsorbed onto the surface and the SAM forms. Red-shifts of ∼4 cm-1 are observed for these methyl stretching vibrational frequencies. The rFR+ band appears to red-shift by ∼2 cm-1 during this time. As with the band intensities, the frequency shifts begin immediately upon immersion of the gold substrate into the thiol solution; the time scale for the evolution of the band intensities (Figure 6a) agrees closely with the time scale for the evolution of the band frequencies (Figure 6b). These trends are also in qualitative agreement with the temporal evolution of the wetting contact angles and ellipsometric film thicknesses measured by Bain et al.11 for the formation of C18 SAMs in 10-5-10-6 M solutions. They are also consistent with the reorientation of the C22 SAM observed by Ha¨hner et al.12 The Raman data35 (Table 1) show that the frequencies of the C-H stretching modes of butanethiol in the liquid phase are systematically ∼1-5 cm-1 higher than those observed in the solid phase; the only exceptions to this trend are the rFR+ band (νs-νl ) 1 cm-1) and the ra,b- bands, where the ra- and rbcontributions are not resolved in the liquid. The spectral differences between the two condensed phases are the result of the increased ordering and higher density found in the solid, crystalline phase. We therefore believe that the trends that we have observed in the band intensities and frequencies for the adsorbed film are the direct consequences of changes in the conformation (and perhaps surface coverage) that occur during film organization. We have already shown that the changes in the d( band intensities are due to the decreased angle between the surface normal and the molecular tilt axis; this is accompanied by the increased ordering and closer packing that occur as the film assembles, as witnessed by the growth and spectral shifts of the r( bands. The data of Figure 5 spectroscopically “capture” the evolution of the system from a disordered C4 film to a structured C4 SAM. At the present time we cannot explain the slight red-shift in the rFR+ band during film deposition in the absence of similar trends in the bulk phase butanethiol data, but it may be due to the interaction of the methyl vibrations with the lower lying bending structure via the Fermi resonance, which makes such modes exceptionally sensitive to the local environment of the growing film.38 (C) IR-RAS Spectra of Sub-Monolayer Fully Equilibrated Films. It is not possible to directly isolate the processes of adsorption and organization using the data presented in section B; both occur for C4/Au(111) in relatively short times (i.e., less than ∼15 min in 5 µM solutions. We have prepared and studied sub-monolayer films in order to isolate intermediate structural state(s) that may exist after the first exposure to the thiol solution, yet prior to SAM completion. Figure 7 shows the IR spectra for films with surface coverages of 0.28-0.8 ML, along with the spectrum of a full C4 SAM, after allowing sufficient time for complete equilibration (usually 2-4 days) of the submonolayer film; exposure of these samples to their respective thiol solutions for an additional 2-4 days did not change any of the spectra from that shown in Figure 7, showing that these are in fact stable structural states. The spectrum of the lowest coverage film (Φ ≈ 0.28 ML) includes a weak but well-defined
J. Phys. Chem., Vol. 100, No. 51, 1996 19923
Figure 7. IR-RAS spectra for fully equilibrated C4 films adsorbed on Au/mica surfaces for 0.28, 0.47, 0.80, and 1.0 ML surface coverages. The data corresponding to full monolayer coverages were obtained using a SAM formed in a 20-fold excess thiol/methanol solution; all other films were prepared using known quantities of thiol to prepare submonolayer films, assuming a full monolayer thiol coverage of 7.7 × 10-10 mol/cm2.
r+ absorption at 2876 cm-1 and a stronger and broader rFR+ band, with broad absorption extending across the 2950-2970 cm-1 regime for the ra- and rb- bands. Although weak, there is evidence of the d- band (∼2925 cm-1), but there is no trace of the d+ band (∼2850 cm-1) as seen in the C12 data of Figure 3 or the short-time exposure C4 films of Figure 5. With increasing total coverage, the r+ band intensity monotonically increases, while the rFR+ band is seen to sharpen slightly, followed by increasing absorption at higher coverages. The bandwidth of the r+ peak changes very little for coverages above 0.3 ML (fwhm ) 8 cm-1 for Φ ) 0.47 ML, 7 cm-1 for Φ ) 1.0 ML). The ra- bandwidth monotonically decreases and its intensity increases with increasing surface coverage. It is interesting to note that the ra- band intensity increases by only ∼60% as the coverage is increased by 360%; this could be due to an overall greater level of organization in the higher coverage films, or to the “decoration” of defects model outlined above. One of the most striking aspects of the data in Figure 7 is that despite slight differences in band shape and intensities, the spectra of the sub-monolayer films are remarkably similar to those of the fully formed C4 SAM. For example, the position of the r+ peak (2876 ( 0.5 cm-1) remains precisely that found for the full monolayer spectrum as the coverage is adjusted from 0.28 to 1.0 ML. The r+ band position is a sensitive measure of the environment of the methyl termination in the film, and it is even affected by the chain length of the n-alkane thiol.10 The constant r+ band position shown in Figure 7 is therefore a spectroscopic demonstration that the methyl groups for the submonolayer fully equilibrated films are found in very similar structural states and that this environment is locally very similar to that of the full SAM. Another important parallel to the full C4 SAM spectrum is the absence of the significant methylene d( band intensity in the fully equilibrated sub-monolayer IRRAS spectra, eVen at Very low coVerages; this is in sharp contrast to the data presented in section B, where intense d( bands were observed for low coverages in the initially disordered stages of film formation. With precisely the same reasoning as was used above, the low d( signals are caused by the small
19924 J. Phys. Chem., Vol. 100, No. 51, 1996
Truong and Rowntree
Figure 8. IR-RAS Intensity of the r+ vibrational mode as a function of the average surface coverage of the film. The data corresponding to full monolayer coverages were obtained using a SAM formed in a 20fold excess thiol/methanol solution; all other films were prepared using known quantities of thiol to prepared sub-monolayer films, assuming a full monolayer thiol coverage of 7.7 × 10-10 mol/cm2.
projection of the d( TDMs onto the polarization vector of the incident radiation, and not by intrinsically weak TDMs. The butanethiol molecules in the fully equilibrated sub-monolayer films are clearly not lying parallel to the surface (as found for the as-deposited molecules in section B), and therefore the structural states of these fully equilibrated films do not have the molecules arranged as-deposited on the gold substrate. Further information is provided by considering the absorption intensities. If the adsorbates were arranged randomly across the surface, we would anticipate that the various absorption bands would show complex trends as the total surface coverage is changed. The intensity of the r+ band (2876 cm-1) as a function of the average surface coverage is shown in Figure 8; the nearly constant line shape and lack of interfering absorption bands allow the peak intensity to be used as a measure of the total absorbance. The error bars shown in Figure 8 reflect the uncertainties in precisely isolating background and peak structures in the sub-monolayer spectra. These data show that the r+ absorption for the fully equilibrated films is directly proportional to the total surface coverage. This linearity may be partly due to the relative insensitivity of the cos2(Ψ) of the r+ TDM on the tilt and twist angles in the vicinity of the optimized SAM structure, as shown in Figure 4, such that minor angular excursions will not cause large fluctuations in the observed intensities. Large variations in tilt/twist angles will cause the projections of the r+ TDM onto the surface normal to change significantly, however, and thus make a linear relationship between coverage and signal levels difficult to achieve. This does not appear to be the case. We therefore conclude that the molecules are not statistically distributed on the surface at low and intermediate coverages, but rather, are localized into island-like domains on the surface, such that the molecular configuration and local environment closely resemble those of the full SAM, regardless of the true macroscopic surface coverage. This island model is consistent with (1) the insensitivity of the r+ band position to the surface coverage, (2) the linear relationship of the r+ intensity and the surface coverage, and (3) the absence of the d( bands in all of the spectra of sub-monolayer fully equilibrated C4 films. It is interesting to note that the sample for Φ ) 1.0 ML intensity measurement is a complete SAM surface formed from a solution containing approximately 20-fold excess of the number of molecules required to form one monolayer; higher concentrations prepared surfaces with equivalent IR-RAS spectra. The close agreement between the calculated submonolayer coverages and the full SAM results suggests that the desorption of thiolates from the surface while in solution is not an important process; if an adsorption/desorption equilibrium was a significant effect, the low-coverage data would show
Figure 9. Temporal evolution of the IR-RAS spectra for a C4 film with a fully equilibrated coverage of 0.60 ML. The deposition time for each spectrum is indicated.
optical absorptions significantly below the linear trend of Figure 8 (i.e. at equilibrium, there would be non-negligible quantities of thiol remaining in solution). In addition, if we consider the equilibrium constant measured by Karpovich et al.27 for C8 adsorption from n-hexane solvents (Keq ) 1930 M-1), we would predict that under our experimental conditions and with solutions containing exactly one monolayer of thiol, less than 3 × 10-4 ML would be adsorbed onto the surface at equilibrium. This is not reconcilable with our results that show significant molecular deposition even when using significantly more dilute solutions, and we therefore believe that desorption is not an important phenomena in these studies using methanol solvents. (D) IR-RAS Spectra of Sub-Monolayer Nonequilibrated Films. The previous sections have characterized the initial stages of the deposition process and the formation of islandlike structures of adsorbates at low total surface coverages; in this section, we link the two approaches with data that show that the precursors to island formation are in fact disordered and isolated adsorbates. Figure 9 shows the IR spectra of a Au/mica surface following 200 and 6000 min of exposure to the thiol solution; the total thiol content in solution is that required to produce an average coverage of 0.60 ML. At relatively short times (200 min), the d+ band (2860 cm-1) is strong and well-resolved; the d- band (∼2925 cm-1) is also clearly seen in the data. Once again, this confirms that the absence of the d( bands in the spectra of fully equilibrated submonolayer films shown in Figure 7 is the consequence of molecular conformations in the developing film. Thus far, the spectrum is remarkably similar to that found for the short exposures to the 5 µM solution in Figure 5. The d( bands reduce in intensity with increased exposure to the thiol solution; during this period, the quantity of adsorbed material may increase slightly, but more importantly, the adsorbed species can reorganize to approach the fully equilibrated structures described in section B. Clearly, the adsorption of more butanethiol onto the surface without structural rearrangements must entail an increased peak intensities; the reduced absorbances of the d( bands with time are therefore evidence that the film structure is adopting the fully equilibrated structure with smaller tilt angles from the surface normal. The observation of significant and transient d( intensity only at short times is spectroscopic proof that the molecular orientations have evolved in time to form the island-like structures prior to monolayer completion. Even more telling is the observation that the intensity of both d( bands approaches zero as the film approaches equilibrium; as shown in Figure 4, this cannot be
Self-Assembled Butanethiol Monolayers on Au Substrates
J. Phys. Chem., Vol. 100, No. 51, 1996 19925
Figure 10. STM images of butanethiol SAMs/Au/mica showing large and well-structured “p×x3” domain structures occupying a single terrace of the gold surface (A, 75 nm scan area) and a higher magnification view of several “pinstripe” domains coexisting with a central disordered region (B). Imaging conditions are described in the text.
the result of changes in the average molecular twist, since this would serve to increase the intensity of one d band while decreasing the other. Both d( bands can approach zero intensity only by a decrease in the average tilt angle of the adsorbates. The behavior of the d( bands of Figure 9 can only be understood therefore in terms of a gradual evolution of molecular axes toward the surface normal; we have repeatedly observed this behavior with coverages from 0.28 to 0.8 ML. In all cases, the nonequilibrated films show strong d( bands and weak r+ bands; upon equilibration, we observe strong r+ bands and negligible d( bands, as observed for the complete SAM. This provides additional support for our model of island-like growth mechanisms in the sub-monolayer regime. Further confirmation is obtained by considering the spectral shifts of the r+ band of the 0.6 ML data shown in Figure 9 as the deposition time is extended from 200 to 6000 min. There is a small but reproducible ∼1 cm-1 red-shift of the r+ band during this time; the shift to lower vibrational frequencies is also observed in the condensed phase n-alkanes during the liquid-to-solid transition,35 i.e., as the system forms a crystalline solid. This is physically reasonable, since the IR-RAS data also show that the average molecular orientation has evolved into a more perpendicular conformation during the time between these two spectra; it is not surprising that the average environment of the alkane chain has also evolved from a liquid-like (lower density, poorly organized) state into a more organized and uniform solid-like state. As noted above, the frequency of the r+ vibration is insensitive to the surface coverage if the film is fully equilibrated, and we thus conclude that the slight redshift is caused by the adsorbates collectively adopting an orientation directed away from the plane of the surface. It should be noted that at the present time we cannot quantify the absolute quantity of adsorbed thiol in the nonequilibrated spectra, since the linear calibration based on the r+ band intensity (Figure 8) is applicable only to fully equilibrated films with similar film structures. (E) STM Images of Disordered and Ordered Regions of C4 SAMs. The unique ability of scanning probe techniques to probe the real-space configuration of surface films has permitted
the direct elucidation of unit cell structures and organizational dynamics in SAM systems; the UHV-STM work of Poirier et al.15,16 has elucidated the film structures of full monolayers of short-chain n-alkanethiols as well as the structural reformations associated with disorder-order transformations. Our STM images confirm that the film rearranges in a complex manner while adopting the SAM structure. This is shown in Figure 10A for the C4 SAM film showing large ordered regions covering terraces of the Au(111) film (75 nm); in these images, the stripe separation is 10.17 Å. Ordered regions occupying different terrace levels of the substrate rarely show orientational correlations. This structure is a variant on the “p×x3” structure that has been observed with helium atom diffraction and STM measurements for C4-12 on single-crystal substrates;14,15,24 higher resolution images show that the rows that are visible in Figure 10A are the CH3 terminations of the C4 adsorbates, arranged with ∼5 Å separations.39 Figure 10B shows the coexistence of ordered and intersecting “crystalline-like” domains with disordered regions. The presence of molecular adsorbates within the featureless region of Figure 10B has been confirmed by the lack of significant height variations between the ordered and disordered domains. The abnormally bright regions of these organic surfaces represent elevations of approximately 0.5 Å above the surface of the film; they are stable with time and do not appear to be caused by contamination or impurities. Interestingly, they are frequently observed at the intersection of the pinstripe structures of two (or more) crystalline domains, and therefore they may represent the “puckering” of molecular conformations that results from the collision of two growing domains with different internal molecular orientations. It is also interesting to note that the small etch pits produced in the bare gold surfaces (Figure 1B) are not observed in the STM images of films that have been in deposition solutions for extended periods; this restructuring of the gold substrate/organic film has also been reported for etch pits produced during thiol adsorption.15,16
19926 J. Phys. Chem., Vol. 100, No. 51, 1996 Conclusions This study has used IR-RAS to examine the adsorption of butanethiol from dilute butanethiol/methanol solutions onto highquality gold surfaces; these measurements are the first spectroscopic characterizations of controlled sub-monolayer depositions of n-alkanethiol/Au systems, and it is seen that this methodology has significant potential in elucidating the temporal evolution of the adsorbates from sub-monolayer nonequilibrated films to fully equilibrated SAMs. Specifically, we have shown the following. (1) The evolution of the molecular chain tilt toward the surface normal was monitored by the decreased intensity of the CH2 bands (d( modes) as the adsorbed film was permitted to equilibrate and to autoassemble. The invariance of the band frequencies as a function of the total surface coverage strongly suggests that the local environment of the adsorbed molecules is highly uniform and insensitive to coverage. The adsorbed species in sub-monolayer films tend to aggregate into islandlike formations such that the molecular axes are collectively oriented away from the surface; the local structure and density of these islands are essentially identical to those of the complete C4 SAM. (2) In the case of fully equilibrated C4 films on high-quality gold surfaces, the optical absorbance of the symmetric methyl stretch (2876 cm-1) depends linearly on the total surface coverage, thus providing a convenient method to calibrate surface coverages for less controlled deposition conditions. (3) The adsorption of butanethiol onto the gold surface is found to proceed quantitatively in the 0.3-1.0 ML coverage regime, with no evidence of significant adsorption/desorption equilibrium for alkanethiol films in the methanol solvent; it is proposed that previously reported desorption phenomena for longer chain alkanethiols may be associated with microscopically rough gold substrates or with variations in solubilities for the specific thiol/solvent combinations used. (4) The spectral red-shift of all significant IR-RAS absorption bands pertaining to the C-H stretches shows the evolution of the film environment during the deposition/assembly process. Acknowledgment. This research was supported by the Natural Sciences and Engineering Research Council (Canada). We thank J. Kang for her expert assistance with the STM measurements. References and Notes (1) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Isreaelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne K. J.; Yu, H. Langmuir 1987, 3, 932. (2) Al-Mohamad, A.; Smith, C. W.; Al-Saffar, I. S.; Slifkin, M. A. Thin Solid Films 1990, 189, 175, and references therein.
Truong and Rowntree (3) Narayamurty, V. Science 1987, 235, 1023. (4) Chidsey, C. E. D. Science 1991, 251, 919. (5) Ulman, A. Introduction to Ultrathin Organic Films from LangmuirBlodgett to Self-Assembly; Academic Press: San Diego, CA , 1991. (6) Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43, 437. (7) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (8) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (9) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (10) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (11) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (12) Ha¨hner, G.; Wo¨ll, Ch.; Buck, M.; Grunze, M. Langmuir 1993, 9, 1955. (13) Chidsey, C. E. D.; Liu, G.-Y. ; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989, 91, 4421. (14) Camillone, N., III; Eisenberger, P.; Leung, T. Y. B.; Schwartz, P.; Scoles G.; Poirier G. E.; Tarlov, M. J. J. Chem. Phys. 1994, 101, 11031. (15) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383. (16) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966. (17) Sho¨nenberger, C.; Jorritsma, J.; Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. J. Phys. Chem. 1995, 99, 3259. (18) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (19) Li, Y.; Huang, J.; McIver, R. T., Jr.; Hemminger, J. J. Am. Chem. Soc. 1992, 114, 2428. (20) Sellers, H.; Ulman, A.; Snidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (21) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (22) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805. (23) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222. (24) Camillone, N., III; Leung, T. Y. B.; Schwartz, P.; Eisenberger, P.; Scoles, G. Langmuir 1996, 12, 2737. (25) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 4581. (26) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1994, 117, 372. (27) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 3315. (28) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, 93, 767. (29) Benebaa, F.; Ellis, T. H.; Badia, A.; Lennox, R. B. J. Vac. Sci. Technol. A 1995, 13, 1331. (30) Chabal, Y. J. Surf. Sci. Rep. 1988, 8, 211. (31) DeRose, J. A.; Thundat, T.; Nagahara, L. A.; Lindsay, S. M. Surf. Sci. 1991, 256, 102. (32) Kang, J.; Rowntree, P. Submitted for publication. (33) McDermott, C. A.; McDermott, M. T.; Green, J.-B.; Porter, M. D. J. Phys. Chem. 1995, 99, 13257. (34) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927. (35) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284. (36) Chang, S. C.; Chao, I.; Tao, Y. T. J. Am. Chem. Soc. 1994, 116, 6792. (37) Truong, K. D.; Rowntree, P. Submitted for publication. (38) Kodati, V. R.; El-Jastimi, R.; Lafleur, M. J. Phys. Chem. 1994, 98, 12191. (39) Kang, J.; Rowntree, P. Langmuir 1996, 12, 2813.
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