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Scanning Tunneling Microscopy Study of L-Cysteine on Au(111) A. S. Dakkouri,† D. M. Kolb,*,† R. Edelstein-Shima,‡ and D. Mandler‡ Department of Electrochemistry, University of Ulm, D-89069 Ulm, Germany, and Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received November 27, 1995. In Final Form: March 15, 1996X The adsorption of L-cysteine on Au(111) from a perchlorate solution was studied by in-situ scanning tunneling microscopy. The potential-induced adsorption of the thiols lifts the reconstruction of Au(111) and at more positive potentials L-cysteine adopts a (x3 × x3)R30° adlayer structure. The potentialinduced desorption of the adlayer does not cause the reconstruction to reappear but instead leaves a roughened surface with a large number of monoatomic high gold islands and monoatomic deep holes.
Introduction The adsorption of thiols on gold surfaces has attracted much interest during the last decade.1-8 This stems from the tendency of long alkanethiols to self-assemble into highly ordered monomolecular layers. Since the thickness as well as the monolayer/electrolyte interface can be easily modified, these self-assembled monolayers (SAMs) have been used for studying electron-transfer,9-14 protein adsorption,15 and specific recognition.16 The structure and properties of SAMs have been examined by spectroscopy,17,18 microscopy,19-28 electrochemistry,10,29-31 and †
University of Ulm. The Hebrew University of Jerusalem. X Abstract published in Advance ACS Abstracts, May 1, 1996. ‡
(1) Ulman, A. An Introduction to Ultra-Thin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (2) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (3) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (4) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (5) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (6) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688. (7) Pertsin, A. J.; Grunze, M. Langmuir 1994, 10, 3668. (8) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1995, 11, 2237. (9) Chidsey, C. E. D. Science 1991, 251, 919. (10) Creager, S. E.; Collard, D. M.; Fox, M. A. Langmuir 1990, 6, 1617. (11) Miller, C.; Cuendet, P.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 877. (12) Miller, C.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 5225. (13) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1992, 96, 2657. (14) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1993, 97, 6233. (15) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164. (16) Mandler, D.; Turyan, I. Electroanalysis, in press and reference therein. (17) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370. (18) Bryant, M. B.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284. (19) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805. (20) Kim, Y.-T.; Bard, A. J. Langmuir 1992, 8, 1096. (21) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222. (22) McCarley, R. L.; Kim, Y.-T.; Bard, A. J. J. Phys. Chem. 1993, 97, 211. (23) Pan, J.; Tao, N.; Lindsay, S. M. Langmuir 1993, 9, 1556. (24) Edinger, K.; Go¨lzha¨user, A.; Demota, K.; Wo¨ll, Ch.; Grunze, M. Langmuir 1993, 9, 4. (25) Liu, G.-Y.; Salmeron, M. B. Langmuir 1994, 10, 367. (26) Han, T.; Beebe, Th.P., Jr. Langmuir 1994, 10, 2705. (27) Delamarche, E.; Michel, B.; Gerber, Ch.; Anselmetti, D.; Gu¨ntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869.
S0743-7463(95)01079-1 CCC: $12.00
other surface-sensitive techniques.32-35 Of particular interest and significance has been the application of scanning microprobe techniques, i.e., STM and AFM. These techniques can be conducted in situ and thus provide information on surface processes such as phase transitions in adlayers on a molecular and atomic level. Due to the fact that long alkanethiols are almost insoluble in aqueous solutions, their adsorption onto metallic surfaces is usually carried out from organic, e.g., ethanolic, solutions at open circuit potential. The resulting adlayers on Au(111) often adopt a hexagonal (x3 × x3)R30° organization in which the alkyl chains are tilted by approximately 30° from the surface normal.36,37 Such organization is driven by the formation of a strong sulfur-gold bond, which tends to lock the molecules into the threefold hollow sites of the substrates, as well as by van der Waals interactions between the chains. On the other hand, the formation of organized layers of short thiols on well-ordered gold surfaces has been scarcely studied by scanning probe microscopies. Porter and co-workers showed STM images19 whereby SAMs of ethanethiolate adsorbed from ethanolic solutions adopted a (x3 × x3)R30° overlayer. L-Cysteine represents an especially interesting case. It is a small and highly polar molecule in which a variety of intermolecular forces, such as hydrogen and electrostatic bonding, can govern its packing on a gold surface. The organization of L-cysteine monolayers on gold was examined by Liedberg and coworkers38,39 using reflection-absorption IR and XPS. They concluded that the amino and carboxyl groups exist in the zwitterionic form. The electrochemistry of L-cysteine on gold electrodes has been the subject of numerous studies.40-49 Fawcett et al.45-48 investigated in detail the adsorption and electrochemistry of L-cysteine on poly(28) Takami, T.; Delamarche, E.; Michel, B.; Gerber, Ch.; Wolf, H.; Ringsdorf, H. Langmuir 1995, 11, 3876. (29) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (30) Sabatini, E.; Rubinstein, I. J. Phys. Chem. 1987, 91, 6663. (31) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510. (32) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (33) Lo´pez, G. P.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1513. (34) Creager, S. E.; Clarke, J. Langmuir 1994, 10, 3675. (35) Offord, D. A.; John, C. M.; Griffin, J. H. Langmuir 1994, 10, 761. (36) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (37) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (38) Uvdal, K.; Bodo¨, P.; Liedberg, B. J. Colloid Interface Sci. 1992, 149, 162. (39) Ihs, A.; Liedberg, B. J. Colloid Interface Sci. 1991, 144, 282. (40) Prada´cˇ, J.; Koryta, J. J. Electroanal. Chem. 1968, 17, 167. (41) Koryta, J.; Prada´cˇ, J. J. Electroanal. Chem. 1968, 17, 177.
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crystalline as well as low-index single-crystal gold electrodes. More recently Johnson and co-workers used the electrochemical quartz crystal microbalance to study the effect of L-cysteine adsorption on gold electrodes in acid and basic media.48,49 We present in the following an STM study on the adsorption/desorption behavior of L-cysteine on Au(111). To the best of our knowledge, L-cysteine on Au(111) has never been studied in situ by STM or AFM. It is shown that an organized monolayer is formed at positive potentials. Lifting of the reconstruction and roughening of the surface also characterize the adsorption of L-cysteine on Au(111). Experimental Section In-situ STM studies were performed in Ulm with a “Topometrix TMX 2010”. The tips used throughout this study were electrochemically etched from a 0.25 mm diameter Pt-Ir (80:20) wire in 3.4 M NaCN and isolated with Apiezon wax down to an active area of the order of 10-8 cm2. The potential of the tip could be controlled independently from that of the working electrode by a bipotentiostat, and it was kept typically between -0.38 and -0.35 V vs SCE. All STM images were recorded in the constantcurrent mode with tunneling currents It between 0.7 and 7.2 nA. Recording a complete scanning tunneling micrograph took about 1 min. The images are represented as “top views”, in which different heights within the image are colored in different shades of gray, with lighter shades corresponding to higher points. The working electrodes for the STM studies were 200 nm thick gold films evaporated onto “Robax” glass (AF 45, Berliner Glas KG) which had a 2 nm thick Cr undercoating for better adhesion of the gold film on the glass. Before each experiment the film electrode with the size of ca. 10 mm × 10 mm was annealed in a hydrogen flame for about 2-3 min at yellow heat, which had been shown to yield high-quality surfaces of (111) orientation.50 Finally, the electrode was allowed to cool down in air for several minutes before it was mounted in the electrochemical cell of the STM. After flame annealing, the gold electrode possesses atomically flat regions of the order of 100 nm in size, separated from each other by monoatomic high steps. For an electrochemical characterization of the L-cysteine adsorption on Au(111) by cyclic voltammetry, a good single-crystal disk of ca. 4 mm diameter and ca. 7 mm thickness was used as working electrode. Prior to each electrochemical experiment, the electrode was annealed several times in a Bunsen burner flame for a few seconds and, after a short cooling period in air, which allowed the red heat to disappear, quenched in Milli-Q water. Cyclic current-potential curves were then obtained in a standard electrochemical cell, for which the dipping method51 could be employed. The electrolytes were prepared from Milli-Q water (Millipore), HClO4 (suprapure, Merck), KClO4 (puriss. p. a., Fluka), and L-cysteine (>99%, Merck). Although the reference electrode was Hg/Hg2SO4/K2SO4 (sat.), all potentials are quoted with respect to the saturated calomel electrode (SCE) (ESCE - EHg/Hg2SO4 ) +0.41 V). (42) Koryta, J.; Prada´cˇ, J. J. Electroanal. Chem. 1968, 17, 185. (43) Prada´cˇ, J.; Ossendorfova, N.; Koryta, J. Croat. Chem. Acta 1973, 45, 97. (44) Ralph, T. R.; Hitchman, M. L.; Millington, J. P.; Walsh, F. C. J. Electroanal. Chem. 1994, 375, 1. (45) Fawcett, W. R.; Fedurco, M.; Kova´cˇova´, Z.; Borkowska, Z. J. Electroanal. Chem. 1994, 368, 265. (46) Fawcett, W. R.; Fedurco, M.; Kova´cˇova´, Z.; Borkowska, Z. J. Electroanal. Chem. 1994, 368, 275. (47) Fawcett, W. R.; Fedurco, M.; Kova´cˇova´, Z.; Borkowska, Z. Langmuir 1994, 10, 912. (48) Tu¨do¨s, A. J.; Vandeberg, P. J.; Johnson, D. C. Anal. Chem. 1995, 67, 552. (49) Tu¨do¨s, A. J.; Johnson, D. C. Anal. Chem. 1995, 67, 557. (50) Will, T.; Dietterle, M.; Kolb, D. M. In Nanoscale Probes of the Solid/Liquid Interface; Gewirth, A. A., Siegenthaler, H., Eds.; NATO ASI Series E; Kluwer: Dordrecht, 1995; Vol. 228, p 137. (51) Dickertmann, D.; Koppitz, F. D.; Schultze, J. W. Electrochim. Acta 1976, 21, 967.
Figure 1. Cyclic current-potential curves for Au(111) in 0.1 M KClO4 + 1 mM HClO4 before (- - -) and after (s) addition of 3.3 × 10-6 M L-cysteine. Scan rate: 20 mV/s.
Results and Discussion Figure 1 shows a cyclic current-potential curve for Au(111) in the presence of L-cysteine, which is in very good agreement with cyclic voltammograms already reported in the literature.47-49 The current peaks at ca. -0.40 and 0.30 V vs SCE are ascribed to the adsorption of L-cysteine. Since the potential scan is limited on the negative end to about -0.50 V vs SCE due to the beginning of hydrogen evolution, it is difficult to determine whether at that potential the Au(111) surface is free of adsorbed L-cysteine or not. By carrying out adsorption measurements of polycrystalline gold electrodes in an alkaline medium, where the hydrogen evolution reaction is shifted to much more negative potentials, Tu¨do¨s et al.49 were able to observe an adsorption peak at about -1.15 V vs SCE. It thus appears that at least for polycrystalline gold some L-cysteine is already on the surface at the most negative potentials accessible in this study. In Figure 2 a sequence of STM images is shown taken during the adsorption and desorption of L-cysteine. Figure 2a depicts the Au(111) surface at the initial potential of -0.42 V vs SCE, at which the freshly prepared gold electrode was immersed. The double rows of the (22 × x3) reconstruction can be clearly seen. From the high-quality image of the reconstructed surface we conclude that only little if any L-cysteine is adsorbed on the Au(111) surface at that potential, which is in accordance with the cyclic current-potential curve (Figure 1). When the potential is raised to more positive values, L-cysteine begins to adsorb on the Au(111) surface (Figure 2b). In this STM image, which represents the same area as in Figure 2a, the reconstruction rows are still visible, indicating that the (22 × x3) reconstruction is not yet lifted. However, there is a clear loss of image quality accompanying the change of the potential in anodic direction which can be attributed to the presence of highly mobile admolecules on the surface. After having scanned the potential up to 0.41 V vs SCEsa potential where maximum coverage of L-cysteine has been reached but which is still negative enough to avoid its oxidationsthe reconstruction seems to be gradually lifted (Figure 2c), as is indicated by the appearance of a few monoatomic high gold islands. Because of the more densely packed (22 × x3) structure ca. 4.4% of the unreconstructed Au(111) surface ought to be covered with monoatomic high gold islands after complete lifting of the
L-Cysteine
on Au(111)
Figure 2. Sequence of STM images (53 nm × 53 nm) for a Au(111) electrode in 0.1 M KClO4 + 1 mM HClO4 + 3.3 × 10-6 M L-cysteine, showing the adsorption and desorption of Lcysteine. It ) 2.7 nA (a and b), 7.2 nA (c), 0.7 nA (d-f).
Figure 3. High-resolution STM image (13 nm × 13 nm) displaying the hexagonal arrangement of L-cysteine molecules on Au(111). ESCE ) 0.41 V; 0.1 M KClO4 + 1 mM HClO4 + 3.3 × 10-6 M L-cysteine. It ) 0.7 nA.
reconstruction.52,53 However, the surface area taken up by the gold islands in Figure 2c is much less than the expected value. The reason for this could lie in a densely packed adlayer that hinders either lifting of the reconstruction or surface diffusion of the excess gold atoms to form islands. At 0.41 V vs SCE an ordered structure of L-cysteine on Au(111) can indeed be observed by STM. This structure is shown in Figure 3, from which a hexagonal overlayer with a nearest-neighbor distance of 0.49 ( 0.02 nm can be determined. This is in good agreement with a (52) Tao, N. J.; Lindsay, S. M. Surf. Sci. Lett. 1992, 274, L546. (53) Hagebo¨ck, J. Diploma Thesis, Ludwig-Maximilian-Universita¨t Mu¨nchen, 1992.
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(x3 × x3)R30° superstructure, each of the bright spots of the hexagonal superlattice corresponding to one L-cysteine molecule. The formation of a commensurate (x3 × x3) structure is expected because of the strong chemical interaction between sulfur and the gold surface in the form of a thiolate, forcing the molecules to pack in registry with the underlying gold lattice. The bright patches in Figure 3 which coexist with the ordered regions of L-cysteine are presumably due to randomly adsorbed L-cysteine. A large number of thiols with different chain lengths are known to form a (x3 × x3)R30° structure on Au(111), as is derived from diffraction and microscopy investigations.4,19,21,23,36,54-67 In addition Gao et al.68 found that sulfide adsorption on Au(111) also yields an ordered (x3 × x3)R30° layer. Therefore, it is suggested that the imaged “molecules” correspond to the sulfur atoms in the thiols bonding to the gold substrate.60 In many STM and AFM studies the formation of holes upon the adsorption of thiols on Au(111)20,24,54,55,69-73 has been observed, which seems very typical for this system. However, although L-cysteine is adsorbed at 0.41 V vs SCE on the Au(111) surface, no signs of hole formation in the gold surface are seen in Figure 2c. When the potential is scanned back to negative values, L-cysteine begins to desorb. Figure 2d shows the substrate at 0.21 V vs SCE. The most striking change in the surface topography is the formation of numerous monoatomic high gold islands (0.25 ( 0.02 nm). These islands are obviously formed out of the excess gold atoms which are expelled from the surface during lifting of reconstruction. Because of the partial desorption of L-cysteine at this potential, surface diffusion of gold atoms becomes feasible and hence island formation is now facilitated. The area now covered with islands corresponds to ca. 12% of the scanned region, which is apparently in disagreement with the expected value of 4.4% (} excess of gold atoms in the reconstructed phase) but can be explained by the fact that the imaged size of the islands is generally too large due to the finite size of the imaging tip.74 In Figure 2d small patches of adsorbed species on the Au(111) surface can be recognized between the gold (54) Sondag-Huethorst, J. A. M.; Scho¨nenberger, C.; Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6826. (55) Scho¨nenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611. (56) Camillone, N., III; Chidsey, C. E. D.; Eisenberger, P.; Fenter, P.; Li, J.; Liang, K. S.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 99, 744. (57) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (58) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447. (59) Alves, C. A.; Smith, E. L.; Widrig, C. A.; Porter, M. D. Proc. SPIEsInt. Soc. Opt. Eng. 1992, 1636, 125. (60) Chidsey, C. E. D.; Liu, G.-Y.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989, 91, 4421. (61) Samant, M. G.; Brown, C. A.; Gordon, J. G., II. Langmuir 1991, 7, 437. (62) Liu, G.-Y.; Salmeron, M. B. Langmuir 1994, 10, 367. (63) Kim, Y.-T.; McCarley, R. L.; Bard, A. J. J. Phys. Chem. 1992, 96, 7416. (64) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (65) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 4234. (66) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (67) Bucher, J.-P.; Santesson, L.; Kern, K. Langmuir 1994, 10, 979. (68) Gao, X.; Zhang, Y.; Weaver, M. J. J. Phys. Chem. 1992, 96, 4156. (69) Kim, Y.-T.; McCarley, R. L.; Bard, A. J. Langmuir 1993, 9, 1941. (70) Ross, C. B.; Sun, L.; Crooks, R. M. Langmuir 1993, 9, 632. (71) Sun, L.; Crooks, R. M. J. Electrochem. Soc. 1991, 138, L23. (72) Ha¨ussling, L.; Michel, B.; Ringsdorf, H.; Rohrer, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 569. (73) Du¨rig, U.; Zu¨ger, O.; Michel, B.; Ha¨ussling, L.; Ringsdorf, H. Phys. Rev. B 1993, 48, 1711. (74) Magnussen, O. M.; Hotlos, J.; Behm, R. J.; Batina, N.; Kolb, D. M. Surf. Sci. 1993, 296, 310.
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islands. The height of these features (printed in light gray) is 0.13 ( 0.01 nm, which is far too low to be due to the gold substrate. We therefore ascribe them to adsorbed L-cysteine. At even more negative potentials (-0.19 V vs SCE, Figure 2e) the number of gold islands has apparently decreased. This process is most likely a result of an electrochemical annealing effect75 which takes place at this potential. The ongoing desorption of L-cysteine causes formation of holes (see below) on the one hand and an increased mobility of gold atoms on the other hand. The latter in turn causes gold atoms of the gold islands to fill the depressions in the substrate. A comparison of Figure 2d (0.21 V vs SCE) and Figure 2e (-0.19 V vs SCE) illustrates this annealing effect. At -0.19 V vs SCE (Figure 2e) monoatomic deep channels can be observed for the first time. This indicates that an etching process takes place simultaneously with the L-cysteine desorption. A similar process which results in hole or channel formation has been reported by several research groups.24,54,55,67 Such types of depressions are found for thiols of all chain lengths but are not observed for the bare gold surface (except after applying oxidationreduction cycles to the Au(111) electrode, see below). Some groups have demonstrated by using the quartz crystal microbalance22,49 and atomic absorption spectroscopy24,55 that a certain percentage of the Au(111) surface is dissolved in thiol-containing solutions. Therefore, it can be concluded that the holes are created during desorption of the thiol. However, we believe that the channel structure is the result of a redistribution process rather than genuine dissolution, caused by a weakening of the gold-gold surface bond due to a strong gold-sulfur bond. At potentials cathodic of the first adsorption/desorption peak in the cyclic current-potential curve (Figure 1), the channel structure has developed further. Figure 2f displays an STM image of the apparently bare substrate at -0.42 V vs SCE. The measured depth of the channels is 0.28 ( 0.03 nm, which corresponds to about the height of a monoatomic high step on Au(111) and thus indicates that it is not L-cysteine which forms the structure but it is the gold substrate itself which is eroded. Because the image does not appear fuzzy like for a surface with a mobile adlayer, it can be assumed that the L-cysteine has been desorbed to a great extent at this potential. The holes and channels in Figure 2f amount to about 25% of the total surface area. It is conceivable that the number of the holes or the size of the channels caused by (75) Stickney, J. L.; Villegas, I.; Ehlers, C. B. J. Am. Chem. Soc. 1989, 111, 6473.
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desorption was originally even larger but was already partly reduced by the depressions being filled with gold atoms from the gold islands. It may be noteworthy that a similar channel structure has been observed on Au(111) surfaces after applying several oxidation-reduction cycles (ORCs):76,77 Starting with a freshly prepared Au(111) surface, the first ORC created monoatomic deep holes, which were more or less uniformly distributed over the terraces. These holes increased considerably in size with each of the following ORCs, merged, and formed channels while the number of newly created monoatomic deep holes remained small. In Figure 2f no indications for a potential-induced reconstruction are seen.77 Obviously the flat areas are too small to sustain the (22 × x3) structure. Indeed, on wider terraces which are sometimes observed, the reconstruction double rows appeared again (not shown). The depth of the channels and the percentage of the bright gray area (ca. 75%) in Figure 2f rule out that L-cysteine is the cause of this observed structure. Conclusions At potentials cathodic of -0.40 V vs SCE little or no L-cysteine is adsorbed on the Au(111) surface and the (22 × x3) reconstruction can be clearly seen in the STM images. Positive of the first adsorption peak in the cyclic voltammogram the Au(111) reconstruction is still present, but its image is obscured by mobile admolecules. At more positive potentials (e.g., positive of the second adsorption peak) the reconstruction is lifted and large areas of hexagonally packed admolecules with a nearest-neighbor spacing of 0.49 ( 0.02 nm are observed, which is consistent with a (x3 × x3)R30° superstructure on Au(111). Hence, the maximum coverage of L-cysteine is 1/3. The surface diffusion of gold atoms is suppressed in the presence of the ordered L-cysteine layer, and therefore only a few islands are formed during lifting of the reconstruction. During desorption of L-cysteine not only the mobility of the gold atoms on the surface is enhanced and monoatomic high islands are formed but also a channel structure is created by the corrosive interaction of the admolecules with the gold surface. Acknowledgment. This work was supported by the Volkswagen Foundation (Grant No. I/68223). LA9510792 (76) Vitus, C. M.; Davenport, A. J. J. Electrochem. Soc. 1994, 141, 1291. (77) Kolb, D. M.; Dakkouri, A. S.; Batina, N. In Nanoscale Probes of the Solid/Liquid Interface; Gewirth, A. A., Siegenthaler, H., Eds.; NATO ASI Series E; Kluwer: Dordrecht, 1995; Vol. 288, p 263.
