Adsorption Behavior of 11-Mercapto-1-undecanol on Au(111

Yaw-Chia Yang, Teng-Yuan Chang, and Yuh-Lang Lee*. Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan. J. Phys ...
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J. Phys. Chem. C 2007, 111, 4014-4020

Adsorption Behavior of 11-Mercapto-1-undecanol on Au(111) Electrode in an Electrochemical System Yaw-Chia Yang, Teng-Yuan Chang, and Yuh-Lang Lee* Department of Chemical Engineering, National Cheng Kung UniVersity, Tainan 70101, Taiwan ReceiVed: December 3, 2006; In Final Form: January 15, 2007

In-situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV) were used to study the phase evolution of 11-mercapto-1-undecanol (MUO) adlayer on an Au(111) electrode. The effect of various electrolytes, including HClO4 and H2SO4, on the adsorption behavior was studied. The MUO adsorption was found to initiate mainly at the intersectional corner of herringbone rows of an Au(111) reconstruction structure in both of the electrolytes. The following growth of an adsorbed cluster develops first along the face-centeredcubic (fcc) position of the herringbone structure. In the HClO4 solution, the MUO molecule is first adsorbed in a flat-lying orientation when the dose concentration of MUO is low, growing to an ordered domain of striped structure (β phase) with a molecular arrangement of (12 × x3). When the surface coverage becomes high, the hydrocarbon chains of MUO lift off from the Au(111) plane, forming a more condensed and saturated phase, the φ phase, identified as (x3 × x3)R30°. At a high dose concentration of MUO, however, the striped phase does not appear. Due to the fast adsorption of thiol groups at high dose concentrations, the hydrocarbon chains-gold interaction is inhibited, and therefore, a flat-lying orientation of MUO molecules cannot be obtained. In the H2SO4 solution, the striped phase does not form even at a low dose concentration and, instead, the φ phase appears directly in the low-coverage stage. The distinct phenomena observed for the two electrolytes are attributed to the different interactions of anionic ions with the gold surface. It has been shown that sulfate ions adsorb more strongly than perchlorate ions on a gold surface. The strongly adsorbed sulfate ions in the electrical double layer are supposed to resist the direct contact of hydrocarbon chain with the gold surface which also prevents the formation of a flat-lying orientation.

Introduction During the past 2 decades, self-assembled monolayers (SAMs) of organic molecules on metal surfaces, in particular on Au(111), have attracted significant attention not only in fundamental research but also in industrial applications.1,2 Typically, SAMs are ordered two-dimensional arrays formed by molecules containing thiol groups and alkane chains. The thiol groups anchor the organosulfur compounds on a metal surface via a chemical bond, and the alkane chains provide an interchain van der Waals interaction among the thiol molecules, leading to a regularly packed array structure. In many applications, such as wetting, adhesion, chemical and biological sensing, corrosion, and molecular electronic, SAMs have provided an effective and easy method to control the interfacial property between different materials.3-10 Many studies regarding SAMs were conducted on alkanethiol [HS-(CH2)n-X)] SAMs adsorbed on gold substrates. These systems are considered to be model systems for studying the fundamental phenomena of the self-assembly process.11,12 Various experimental techniques, including X-ray photoelectron spectroscopy,13 infrared reflection absorption,14 electron diffraction, atomic force microscopy (AFM),15 and scanning tunneling microscopy (STM) have been employed to examine the structure and formation behavior of alkanethiol SAMs. It has been found that the growth of the adlayer goes through several stages in the adsorption process. In general, the thiol * Corresponding author. Telephone: 886-6-2757575, ext 62693. Fax: 886-6-2344496. E-mail: [email protected].

molecules adsorb randomly in the initial stage and then form a striped structure at the low-coverage regime. Finally, a densely packed phase is observed when the coverage becomes higher. However, different results regarding the phase revolution and adlattice structure have been reported in the literature. In these studies, the alkanethiol SAMs were fabricated in an ultrahighvacuum system, an electrolyte solution, or ambient condition using thiol molecules of different structures.16-23 Apparently, the adsorption behavior and adlayer structure of the SAMs are closely related to the fabrication method of the adlayer, as well as the molecular structure of the alkanethiols. Most of the studies on the formation behavior and adlayer structure of SAMs are concentrated on methyl-terminated alkanethiol SAMs. It is well-known that the head group of an alkanethiol, as well as its chain length, will affect the structure and surface characteristic of a SAM. For many applications, terminal groups, such as COOH, NH2, and OH, are more important than CH3.24,25 However, very few studies have been conducted on the structure characteristics of alkanethiol with a specific functional head group. In this work, the adsorption behavior of an OH-terminated alkanthiol, 11-mercapto-1undecanol [HS(CH2)11OH, MUO], on Au(111) surface is investigated using an in-situ STM in an electrochemical system. The different characteristics between hydroxy and methyl groups on the adlayer structure are compared in this manuscript. Since a SAM structure is also known to be affected by the electrolyte in which a SAM forms, HClO4 and H2SO4 were used as electrolytes to investigate their effect on the adsorption behavior of the MUO SAM. The voltammetric behavior of a flame-

10.1021/jp068290e CCC: $37.00 © 2007 American Chemical Society Published on Web 02/20/2007

Adsoption Behavior of MUO on Au(111) Electrode

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annealed Au(111) electrode in HClO4 and H2SO4 solutions is examined first. This result is then related to the adsorption behavior of MUO molecules in the two electrolytes. Experimental Section 11-Mercapto-1-undecanol was purchased from Aldrich and used without further purification. Perchloric acid and sulfuric acid supplied by Cica-Merck (ultrapure grade) were used to prepare the supporting electrolytes with a concentration of 0.1 M. Au(111) electrode was prepared according to a procedure described elsewhere.26 For voltammetric measurement, the Au single-crystal bead was cut and mechanically polished with successively finer grades of Al2O3. The final stage of sample preparation involved the electrode under a hydrogen flame, followed by quickly quenching it in hydrogen-saturated Millpore water.27 The Au electrode was then transferred into the STM or electrochemical cell with a drop of water protecting the surface from contamination. The electrochemical cell had a three-electrode configuration, including a reversible hydrogen electrode (RHE) as the reference electrode and a Pt wire as the counter electrode. All potentials in this paper refer to the RHE scale. Cyclic voltammetry was performed with a CHI-703 potentiostat (Austin, TX). The scanning tunneling microscope used in this work is a Nanoscope-E (Digital Instruments, Santa Barbara, CA) with a single tube scanner (high-resolution A-head with a maximum scan area of 500-600 nm). The tip was prepared by electrochemically etching a tungsten wire (0.25 mm in diameter) in 2 M KOH. The tip was painted with a thin layer of nail polish for insulation.28 In this work, the scanning tunneling microscope was operated in the constant-current mode.

Figure 1. Typical cyclic voltammograms at 50 mV/s for bare Au(111) electrodes in 0.1 M H2SO4 (dotted trace) and HClO4 (dasheddotted trace) solutions. The solid line is an MUO-coated Au(111) electrode in 0.1 M H2SO4.

Results and Discussion Cyclic Voltammetry. Linear sweep voltammetry was performed using a freshly prepared Au(111) electrode in 0.1 M H2SO4 (dotted trace) or 0.1 M HClO4 (dashed-dotted trace) solution. The steady-state cyclic voltammograms (CVs) recorded at 50 mV/s show an extensive double-layer charging region between 0.05 and 1.0 V. A pair of broad anodic-cathodic peaks at 0.65 (A1) and 0.58 V (C1) appears in the HClO4 solution, which can be attributed to the anion-induced reconstruction of the Au(111) surface from (x3 × 22) to (1 × 1) structure.23,29-31 In the H2SO4 solution, the anodic peak observed at 0.58 V is also due to surface reconstruction, and the broader peak at 0.77 V (A3) is attributed to the adsorption of sulfate ions.30,31 These feature peaks are typical characteristics of a well-ordered Au(111) surface, and their appearance indicates that a welldefined Au(111) surface was exposed to the solution. It is noteworthy that the double layer capacity due to the structural transition in 0.1 M H2SO4 is larger than the capacity in 0.1 M HClO4 and the reconstruction structure transition occurs earlier in H2SO4. These results indicate that sulfate ions adsorb more strongly than perchlorate ions on the gold surface, a result consistent with the literature.32,33 The CV shown by a solid line in Figure 1 was recorded for a MUO-modified Au(111) electrode. The Au(111) electrode was first held at 0.15 V in a 0.1 M H2SO4 (or HClO4) solution containing 1 mM MUO. The electrolyte solution was drained after 20 min and replaced by a pure 0.1 M H2SO4 (or HClO4) solution. The feature peaks of the clear Au(111) electrode disappear due to the adsorption of MUO. The MUO SAM was found to be stable in H2SO4 or HClO4 solution as examined by repeating the CV scanning between 0.05 and 1.0 V. Therefore,

Figure 2. Time-dependent STM images showing the phase evolution of MUO adsorbed on Au(111) performed at 0.15 V in 0.1 M HClO4. The dose concentration of MUO is 10 µM. The first image (a) reveals the general herringbone feature of (x3 × 22) reconstructed Au(111) structure. The images b-f were acquired at 3, 9, 20, 27, and 30 min, respectively, after the introduction of MUO. The scan areas are 150 × 150 nm2.

the formation and structural study of the MUO SAM was controlled at this potential region. In-Situ STM Imaging of the Phase Evolution. Electrode in HClO4 Solution. Before the introduction of MUO, the Au(111) electrode was held at 0.15 V in an HClO4 solution. The herringbone feature shown in Figure 2a reveals the typical surface characteristics of Au(111) surface, known as the (x3 × 22) reconstruction.23,29-31 After introducing about 10 µM MUO, the evolution of the adsorbed structure within 30 min was recorded and is shown in Figure 2b-f. In the initial

4016 J. Phys. Chem. C, Vol. 111, No. 10, 2007 adsorption stage, only nanometer-scale pits, corresponding to the adsorbed MUO clusters, appear on the terraces (3 min, Figure 2b). The adsorbed clusters formed in the early stage are mainly located on the intersectional corner of herringbone rows, indicating the preferred adsorption of MUO molecules on these sites. As the adsorption time increases, rowlike adsorption domains, located at the furrows of the herringbone structure, appear gradually, and a typical image acquired after 9 min is shown in Figure 2c. The rowlike adsorbed domains are identified as a striped phase, growing along the furrows of the herringbone structure and forming mainly as single-row domains. A closer inspection indicates that the striped structure grows initially on the face-centered-cubic (fcc) stacking region rather than on the hexagonal close-packed (hcp) one. It has been reported that the free energy change due to the adsorption of thiol molecules on fcc sites is 5-6 kcal/mol more than that on hcp sites, which gives a reasonable explanation for the preferred adsorption of MUO on the fcc lattice sites.34,35 By prolonging the adsorption time, the single-row domain develops gradually across the herringbone bridge and finally on the hcp region. The lateral growth of the single-row domains leads to coalescence between domains, forming larger domains with well-aligned multirow morphology. A series of STM images showing the detail evolution from single-row to multirow domains are given in the Supporting Information (Figure s1). The striped structure is referred to the β phase hereafter. The herringbone structure of the clean Au(111) surface disappears gradually due to the increasing adsorbed amount of MUO. An image acquired at about 20 min after MUO introduction is shown in Figure 2d. Although a few hcp regions of the herringbone structure (the broad bright rows) can still be observed in Figure 2d, nearly all the fcc regions are occupied by adsorbed MUO. Three ordered domains of the same structure, oriented 120° between each other, were found, as indicated in Figure 2d (domains I-III). A similar structure was also observed by Fitts et al. for a decanethiol SAM prepared in an ultrahighvacuum system.17 It is impressive to see few local defects amid the main striped structure, as indicated by the dashed circle marked in Figure 2d. The defect structure is similar to the etching pits or gold vacancies which are commonly observed on a thioladsorbed gold substrate,17,36-38 and their appearance indicates the formation of a new phase (the φ phase). The new adsorbed structure develops gradually at the expense of the striped phase and the unoccupied hcp positions. In an STM image acquired 7 min after Figure 2d (Figure 2e), large domains of the φ phase are observed, as well as the initially nucleated defects amid the striped phase. The following phase transformation proceeds more rapidly, and the striped β phase disappears completely within 3 min (Figure 2f). Electrode in H2SO4 Solution. The adsorption of MUO in the H2SO4 solution was performed in the same conditions as that in the HClO4 solution. A series of time-dependent STM images are shown in Figure 3. A notable difference of the phase evolution can be seen for the two electrolytes. After introducing the MUO into the STM cell, several dark pits appear gradually, as indicated by the STM images taken after 3 (Figure 3a) and 7 min (Figure 3b). As in the HClO4 solution, the adsorption initiates mainly at the intersectional corner of herringbone rows and extends along the furrows of the fcc stacking regions. However, the striped structure found in the HClO4 solution does not appear in this system. In an STM image acquired 15 min after the introduction of MUO (Figure 3c), larger adsorbed domains develop amid the herringbone structure. Instead of the

Yang et al.

Figure 3. Time-dependent STM images showing the phase evolution of MUO adsorbed on Au(111) performed at 0.15 V in 0.1 M H2SO4. The dose concentration of MUO is 10 µM. These images were acquired at 3 (a), 7 (b), 15 (c), 20 (d), 28 (e), and 35 min after MUO introduction. The scan sizes are 300 × 300 nm2. The bias voltage and set point current are 245 mV and 3 nA, respectively.

appearance of the striped pattern, only pinhole defects corresponding to the φ phase are observed in these domains. With increasing adsorption time, the MUO domains grow at the expense of the herringbone structure. The evolution is shown in Figure 3d-f. An image corresponding to the complete coverage of the MUO was taken at about 35 min after the introduction of MUO. The STM results obtained in the HClO4 solution indicate that the φ phase is a thermodynamically preferred structure, and the striped phase is a kinetically preferred structure in the evolution of the MUO adsorption. The appearance of striped phase indicates that, even though the hydrocarbon chain-gold interaction is much weaker than thiol-gold interaction, the hydrocarbon chains of MUO molecules still have a chance to contact the gold surface when the coverage ratio is small. However, the phase transition from striped (β) to φ phase in the later stage suggests that the φ phase has a lower surface energy and is the final structure of MUO adsorbed on the Au(111) surface. In the H2SO4 solution, the disappearance of the β phase may imply that this phase does not form or its life time is too short to be observed by STM.39 Apparently, there is something in the H2SO4 solution which prevents the formation of the metastable structure found in the HClO4 solution. Molecular Arrangements of the“Striped”Structure. The highresolution STM image shown in Figure 4a reveals the detailed molecular arrangement of the striped structure. It is clear that

Adsoption Behavior of MUO on Au(111) Electrode

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Figure 4. High-resolution STM image (a) and the corresponding ball model proposed (b) for the striped phase of the MUO adlayer on the Au(111) surface. The adlayer was prepared at 0.15 V, and the imaging conditions were 240 mV and 3 nA.

Figure 5. High-resolution STM images (a, c) and the FTIR filtered pattern (b) for the φ phase of the MUO adlayer on Au(111) surface. The adlayer was prepared at 0.15 V in 0.1 M H2SO4 and the imaging conditions were 300 mV and 250 pA. The corresponding model depicting the arrangement of MUO molecules on the Au(111) surface is shown in d.

the MUO molecules form a well-ordered adlayer on Au(111). The striped rows run parallel to the 〈121〉 direction of the Au(111) surface. The distance between two striped rows is measured to be ca. 34.6 Å, which is about twice the molecular length of MUO. Between two very bright lines, slightly bright lines of short length, aligned regularly along the 〈110〉 direction, can be seen. The information from the STM images suggests that the short bright lines are the hydrocarbon chains of MUO, lying directly on the Au(111) surface. The S atoms of the MUO molecules take a head-to-head configuration, forming the striped rows with strong brightness. Another striped row of low brightness, centered between the stripes of sulfur atoms, belongs

to the position of hydroxyl groups of MUO molecules, which also take a head-to-head configuration. The unit cell of the striped structure is superimposed as a rectangular lattice in Figure 4a. The lengths of the two vectors in the unit cell are measured to be 34.6 ( 0.08 and 5 ( 0.05 Å, respectively, corresponding to 12 and x3 times the Au lattice constant. Therefore, the ordered lattice is determined to be (12 × x3), as proposed in the model in Figure 4b. This structure is similar to the model proposed in the literature for a decanethiol SAM prepared under ultrahigh vacuum or in a solution.22,40 Molecular Arrangements of the φ Phase. High-resolution STM was also performed on the φ phase, and the same structure

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Figure 6. Cyclic voltammogram for MUO-modified Au(111) electrodes recorded at 50 mV/s in 1 M KOH. The MUO SAMs were prepared in 0.1 M HClO4 (solid trace) and 0.1 M H2SO4 (dasheddotted trace) solutions, respectively.

was obtained for both HClO4 and H2SO4 solutions. Regular adsorbed domains, as well as the etched pits, can be clearly observed in a 20 × 20 nm2 scanning image (Figure 5a). The structure in Figure 5a was examined using the Fourier transform pattern (as shown in Figure 5b) and a hexagonal arrangement with an intermolecular spacing of 5 ( 0.2 Å was determined. With the assistance of a molecular resolution image (Figure 5c), the ordered phase was identified to be (x3 × x3)R30°. A corresponding model illustrating the arrangement of MUO molecules on the Au(111) surface is given in Figure 5d. The sulfur atoms of MUO molecules are positioned at the 3-fold sites of the gold atoms with the molecular axis lifting off the gold surface.22,23,28 Electrochemical ReductiVe Desorption of the MUO-Modified Au(111) Surface. The stripping behavior of the MUO-modified

Yang et al. Au(111) electrode was investigated using cyclic voltammetry in a 1 M KOH solution. A full MUO monolayer was first prepared at a potential of 0.15 V for about 30 min. After replacing the MUO solution with a KOH solution, the potential was negatively swept from -0.2 to -1.3 V at a sweep rate of 50 mV/s. As shown in Figure 6, a reductive peak appears at -0.78 V for MUO monolayers prepared in both HClO4 and H2SO4 solutions. The amount of charge estimated for the reductive peak is 85 ( 5 µC/cm2, corresponding to a coverage ratio of 0.33. This value is consistent with the coverage ratio calculated for the structure of the φ phase, (x3 × x3)R30°.12,41,42 Proposed Model Illustrating the EVolution of the Adsorbed Phase. According to the examination of in-situ STM, the growth of the MUO adlayer goes through several stages depending on the MUO coverage and on the electrolyte used. In the initial adsorption stage, where the coverage is very low, the MUO molecules adsorb randomly with weak interaction between molecules. Therefore, no ordered structure can be inspected in this stage. This phase is therefore a two-dimensional gas phase (R phase), as shown in Figure 7. In this stage, the orientations of the adsorbed molecules in HClO4 and H2SO4 solutions are considered to be different. In HClO4, the MUO molecules are expected to lie on the surface with the hydrocarbon chains parallel to the Au(111) surface. In the H2SO4 solution, however, the hydrocarbon chains do not lie directly on the Au(111) surface. It was shown in the previous section that sulfate ions have a stronger interaction with the Au(111) surface than the ClO4- ions. In the HClO4 solution, the hydrocarbon chains are

Figure 7. Schematic model illustrating the phase evolution of MUO molecules adsorbed on the Au(111) surface.

Figure 8. Time-dependent STM images showing the phase evolution of MUO adsorbed on Au(111) at a higher dose concentration of MUO (50 µM). The adsorption was performed at 0.15 V in 0.1 M HClO4. These images were acquired at 3 (a), 6 (b), and 10 min (c), after MUO introduction. The scan areas are 150 × 150 nm2. The imaging conditions were 235 mV and 3 nA.

Adsoption Behavior of MUO on Au(111) Electrode able to repel the ClO4- ions near the gold surface and contact it directly. This orientation leads to the striped phase in the later stage. In the H2SO4 solution, the strong interaction of sulfate ions with the gold surface gives a higher resistance to the direct contact of the MUO molecular axis and the gold surface. Because only thiol groups, which own the specific adhesion to the gold, can anchor on the gold surface, the striped phase does not appear in this system. It is also inferred that, in the adsorption process, the thiol group of an MUO molecule will first contact the gold surface. For an adsorbed molecule to get a flat-lying orientation, a longer time and a higher space are required. That is, the adsorption rate of the thiol molecules should be slow enough that the hydrocarbon chain of an adsorbed molecule has a chance to interact with the gold surface before the adsorption of other thiol molecules. For the MUO concentration studied here (about 10 µM), the adsorption rate of MUO is slow enough to form a flat-lying orientation. However, according to the inference, the situation should be different for a higher adsorption rate. This inference was examined by increasing the MUO concentration up to about 50 µM. The phase revolution in the HClO4 solution is shown in Figure 8. It was found that the adsorption of MUO occurs more quickly due to the increase of MUO concentration and reaches the saturated coverage within 10 min. Furthermore, the φ phase forms directly, without the appearance of the striped phase. This result not only sustains the previous inference but also implies that the adlayer structure is also determined by the adsorption rate. With increasing surface coverage, the interactions between sulfur-sulfur atoms and between alkyl chains of adsorbed MUO molecules lead to the formation of an ordered striped structure in the HClO4 solution (the β phase shown in Figure 7a). However, this phase does not exhibit in the H2SO4 solution or in a high dose concentration of MUO, in which the gas phase will transfer to the φ phase directly, as shown in Figure 7b. Since the coverage ratio of MUO molecules in the striped phase is not the saturated state, a phase transition from β to φ phase occurs to increase the adsorption amount and decrease the surface free energy when the surface coverage becomes higher. In the phase transition process, the thiol groups of MUO molecules repel the hydrocarbon chains from the gold surface and substitute them in the original sites. Once the φ phase is formed, each pit can be observed simultaneously in the STM image. The appearance of the each pit in the φ phase, but not in the β phase, indicates that the thiol groups have higher adhesion to the gold surface than the hydrocarbon chains. Compared with the adsorption behavior of methyl-terminated alkanthiols reported in the literature, MUO molecules undergo a similar phase revolution in an electrochemical system as part of previous studies performed under ultrahigh-vacuum or solution environment did.17,22,40 However, it is worth noting that the adsorption of a methyl-terminated thiol (dodecanthiol, HS(CH2)11CH3) on the Au(111) surface was also performed in the same conditions as that for MUO, but an ordered adlattice was not observed in this electrochemical system. This result indicates the importance of the terminal group of a thiol molecule, as well as the assembly method and environment, on the adsorption behavior and structure of a SAM. In an electrochemical system, a regular adlayer of alkanethiol can be prepared easily for an OH-terminated thiol but not for a methylterminated one. Apparently, the hydroxyl groups can enhance the adsorption of a thiol molecule onto the gold surface in the present system. However, the real reason is presently not clear.

J. Phys. Chem. C, Vol. 111, No. 10, 2007 4019 Conclusions The present results indicate that the adsorption of MUO molecules on a gold surface is initiated on the intersectional corner of herringbone rows of an Au(111) reconstruction structure and develops along the face-centered-cubic (fcc) furrow. However, the phase revolution and adlayer structure are controlled by the electrolyte used and by the dose concentration of MUO. In an HClO4 solution, the MUO molecules can adsorb in a flat-lying orientation at low MUO concentrations, leading to a striped phase with a molecular arrangement of (12 × x3). When the MUO coverage becomes high, the molecular axis of MUO will lift off from the Au(111) plane, forming a more condensed and saturated phase (φ phase), identified as (x3 × x3)R30°. However, at a high dose concentration of MUO, the hydrocarbon chain-gold interaction is inhibited by the fast adsorption of thiol groups of MUO molecules and therefore, the flat-lying orientation cannot be obtained. In such a case, the striped phase does not form and the φ phase will appear directly at a low surface coverage. When H2SO4 is used as the electrolyte, the striped phase is inhibited even at low dose concentrations of MUO, being attributable to the strong interaction of sulfate ions to the gold surface which prevents the direct contact of hydrocarbon chain and the gold surface. Acknowledgment. We would like to thank Professor ShuehLin Yau (National Central University, Taiwan) for his helpful discussion in connection with the writing of this manuscript. This work was supported by the National Science Council of Taiwan through Grant Nos. NSC 95-ET-7-006-003-ET and NSC 94-2214-E-006-022. Both are gratefully acknowledged. Supporting Information Available: A series of STM images showing the detailed evolution from single-row to multirow domains. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43, 437. (2) Ulman, A. An Introduction to Ultrathin Films: From LangmuirBlodgett to Self-Assembly; Academic Press: Boston, 1991. (3) Ulman, A. Chem. ReV. 1996, 96, 1533. (4) Crooks, R. M.; Ricco, A. J. Acc. Chem. Res. 1998, 31, 219. (5) Finklea, H. O.; Hanshew, D. O. J. Am. Chem. Soc. 1992, 114, 3173. (6) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5444. (7) Chidsey, C. E. D.; Bertozzi, C. R.; Putrinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (8) Schneeweiss, M. A.; Kolb, D. M. Phys. Status SolIdi B 1999, 173, 51. (9) Poirier, G. E. Langmuir 1997, 13, 2019. (10) McDermott, C. A.; McDermott, M. A.; Green, J. B.; Porter, M. D. J. Phys. Chem. 1995, 99, 13257. (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) Esplandiu´, M. J.; Hagenstro¨m, H.; Kolb, D. M. Langmuir 2001, 17, 828. (13) Fenter, P.; Eberhardt, A.; Liang, K. S.; Eisenberger, P. J. Chem. Phys. 1997, 106, 1600. (14) Hayashi, T.; Kodama, C.; Nozoye, H. Appl. Surf. Sci. 2001, 169170, 100. (15) Yamada, R.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1558. (16) Yamada, R.; Uosaki, K. Langmuir 1997, 13, 5218. (17) (a) Poirier, G. E. Langmuir 1999, 15, 1167. (b) Fitts, W. P.; White, J. M.; Poirier, G. E. Langmuir 2002, 18, 1561. (c) Fitts, W. P.; White, J. M. Langmuir 2002, 18, 2096. (18) Toerker, M.; Staub, R.; Fritz, T.; Schmitz-Hu¨bsch, T.; Sellam, F.; Leo, K. Surf. Sci. 2000, 445, 100. (19) Hagenstro¨m, H.; Schneeweiss, M. A.; Kolb, D. M. Langmuir 1999, 15, 2435.

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