Mechanism of Formation of Au Vacancy Islands in Alkanethiol

mechanism by which Au vacancy islands form during assembly of ... large terraces the vacancies nucleate into islands while the adatoms migrate and ads...
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Langmuir 1997, 13, 2019-2026

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Mechanism of Formation of Au Vacancy Islands in Alkanethiol Monolayers on Au(111) G. E. Poirier National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received August 7, 1996. In Final Form: January 6, 1997X Using an ultrahigh-vacuum scanning tunneling microscope we have discovered evidence for a novel mechanism by which Au vacancy islands form during assembly of alkanethiol monolayers on Au(111). Our results suggest a model whereby excess Au atoms are forced out of the surface layer by relaxation of the compressed herringbone reconstruction. This creates adatoms on, and vacancies in, the surface layer. On large terraces the vacancies nucleate into islands while the adatoms migrate and adsorb at ascending step edges. At saturation coverage of alkanethiols the surface exhibits ≈6% of a monolayer of vacancy islands. These results show that complex interactions between the assembling thiols and the herringbone reconstruction influence the mesoscopic aspects of the final monolayer surface.

Introduction In 1957, alkanethiol (HS(CH2)nX, abbreviated CnX)modified metal surfaces were explored as a means to affect dropwise condensation of steam and thereby realize more efficient heat transfer in steam generators.1,2 More recently, these material systems are being investigated for potential applications in biosensing,3 biomimetics,4 and corrosion inhibition.5 Early researchers realized that these amphiphiles attach to the surface via “metallophilic” head groups and that the hydrocarbon tails align with the surface normal, forming a monolayer “brush”.6 Later infrared studies quantified the molecular tilt angle and determined that the monolayers are dense and solid-like.7,8 Diffraction studies established that the molecules adopt a commensurate crystalline packing.9 These structural tools were all spatially-averaging and therefore left unanswered questions about the number, nature, and structure of surface defects. Scanning tunneling microscopy (STM) was used to address these questions. Early STM studies by Haussling10 and others11-15 revealed that alkanethiol monolayers on Au(111) exhibit a distribution of pitlike defects, defects that do not exist on the bare Au surfaces. This raised concerns about the physical and electrochemical blocking ability of alkanethiol monolayers in applications such as biosensing or corrosion control. Three studies attributed the pitlike defects to regions of missing or loosely-packed alkanethiols10-12 while three others suggested they were an electronic artifact of X

Abstract published in Advance ACS Abstracts, March 1, 1997.

(1) Blackman, L. C. F.; Dewar, M. J. S. J. Chem. Soc. 1957, 162. (2) Nagle, W. M.; Drew, T. B. Trans. Am. Inst. Chem. Eng. 1933, 30, 217. (3) Haussling, L.; Knoll, W.; Ringsdorf, H.; Schmitt, F.-J.; Yang, J. Makromol. Chem., Macromol. Symp. 1991, 46, 145. (4) DiMilla, P. A.; et al. J. Am. Chem. Soc. 1994, 116, 2225. (5) Chailapakul, O.; Sun, L.; Xu, C.; Crooks, R. M. J. Am. Chem. Soc. 1993, 115, 12459. (6) Emmons, H. Trans. Am. Inst. Chem. Eng. 1939, 35, 109. (7) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (8) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (9) Camillone, N.; Chidsey, C. E. D.; Liu, G.-y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (10) Haussling, L.; Michel, B.; Ringsdorf, H.; Rohrer, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 569. (11) Sun, L.; Crooks, R. M. J. Electrochem. Soc. 1991, 138, L23. (12) Kim, Y.-T.; Bard, A. J. Langmuir 1992, 8, 1096. (13) Mizutani, W.; Michel, B.; Schierle, R.; Wolf, H.; Rohrer, H. Appl. Phys. Lett. 1993, 63 (2), 147. (14) Durig, U.; Zuger, O.; Michel, B.; Haussling, L.; Ringsdorf, H. Phys. Rev. B 1993, 48, 1711. (15) Anselmetti, D.; et al. Europhys. Lett. 1993, 23 (6), 421.

S0743-7463(96)00777-9

Figure 1. Constant-current UHV STM topograph of the bare Au(111) herringbone reconstruction. The inset shows a ball model of a single (x3 × 23) surface unit cell. The surface layer is 4.4% uniaxially compressed. Compression causes surfaceto-subsurface atomic registry to vary from unfaulted (ABC) stacking, to bridging, to faulted (ABA) stacking, to bridging, and back to unfaulted stacking. Bridging rows adopt alternating 60° bends, half of which contain surface-confined dislocation.

the STM contrast mechanism.13-15 Later work by Edinger et al. showed that the pit depth was 2.4 Å, consistent with the Au(111) single-atom step height, suggesting that the pits were defects in the Au layer rather than defects in the alkanethiol layer.16 Specifically, they were assigned to two-dimensional islands of Au vacancies.16 The Au vacancy island model was later confirmed by a number of other STM studies.17-25 (16) Edinger, K.; Golzhauser, A.; Demota, K.; Woll, C.; Grunze, M. Langmuir 1993, 9, 4. (17) Schonenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10 (3), 611. (18) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10 (9), 2853. (19) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10 (10), 3383. (20) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966. (21) Bucher, J.-P.; Santesson, L.; Kern, K. Langmuir 1994, 10 (4), 979.

This article not subject to U.S. Copyright.

Published 1997 by the American Chemical Society

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Figure 2. Constant-current UHV STM topographs of alkanethiol monolayers on Au(111). (A) Octanethiol monolayer on Au(111) 2 days after removal from the incubation bath. (B) Butanethiol monolayer on Au(111) 5 h after removal from the incubation bath. (C) Butanethiol monolayer on Au(111) 3 days after removal from the incubation bath. The right halves of images A-C are highlighted to show pit areas and centroids. (D) Cross-sectional profile corresponding to the line trace in part C showing a 2.4 Å depth of vacancy island features.

Once the correct structural interpretation of the pitlike defects was established, the question then turned to the mechanism by which they form. Edinger et al. proposed that the vacancy islands (VIs) were formed by etching of Au in the alkanethiol solutions, a mechanism that was suggested by atomic absorption spectroscopy measurements showing dissolved Au species in the incubation bath.16 This Au-etching mechanism was called into question by more recent experiments showing VI formation even for assembly by gas-phase transport.5,26 A recent report suggested that the VIs were formed by adsorbateinduced shrinkage of the surface lattice constant;27 however, data existing in the literature indicate that the (22) Delemarche, E.; Michel, B.; Kang, H.; Gerber, C. Langmuir 1994, 10, 4103. (23) Sondag-Huethorst, J. A. M.; Shonenberger, C.; Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6826. (24) Cavalleri, O.; Hirstein, A.; Kern, K. Surf. Sci. 1995, 340, L960. (25) McCarley, R. L.; Dunaway, D. J.; Willicut, R. J. 1993, 9, 2775. (26) Poirier, G. E.; Pylant, E. D. Science 1996, 272 (5265), 1145.

surface lattice constant is actually increased during alkanethiol monolayer assembly.9,18,28 The goal of this paper is to provide evidence for a novel picture of the mechanism by which Au VIs form during assembly of alkanethiol monolayers on Au(111). The studies were accomplished by using gas-phase transport of alkanethiol vapor onto clean Au(111) single crystals in an ultrahigh-vacuum (UHV) scanning tunneling microscope. This deposition protocol results in the same distribution of VI defects as is seen when the monolayers are formed in ethanol solutions.5,26 Our results suggest that the currently-proposed etching mechanism is not entirely accurate. We begin the paper by characterizing the VI fractional area. We follow this by discussing evidence for the existence of mobile Au-adatoms during monolayer assembly. We then discuss VI nucleation and (27) McDermott, C. A.; McDermott, M. T.; Green, J. B.; Porter, M. D. J. Phys. Chem. 1995, 99, 13257. (28) Anselmetti, D.; et al. Europhys. Lett. 1994, 27 (5), 365.

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Table 1. Apparent Vacancy Island Fractional Coverages for Various Alkanethiol Monolayers on Au(111) sample

molecule

assembly protocol, storage

VI fraction

1 (Figure 2A) 2 (Figure 2B) 3 (Figure 2C) 4 5 6 7 8 9

HS(CH2)7CH3 HS(CH2)3CH3 HS(CH2)3CH3 HS(CH2)3CH3 HS(CH2)2(CF2)7CF3 HS(CH2)9CH3 HS(CH2)6OH HS(CH2)6OH HS(CH2)6OH

≈12 h, 1 mM EtOH, 2 days UHV 14 h, 1 mM EtOH, 5 h UHV 14 h, 1 mM EtOH, 3 days UHV 14 h, 1 mM EtOH, 10 days UHV 17 h, 1 mM EtOH, 7 days UHV 13 h, 5 µM EtOH, 50 °C, 6 days UHV 1 h 10-6 Torr gas phase, image imed 5 h 10-6 Torr, 1 day UHV 1 day 10-6 Torr, 1 day UHV

0.09 0.06 0.06 0.06 0.09 0.06 0.05 0.06 0.08

island-shaped anomalies. We close the paper by discussing implications of the VIs on the efficacy of alkanethiol monolayer interfaces. Experiment Our measurements were performed in a multichamber UHV surface analysis system with a base pressure of 3 × 10-8 Pa (2 × 10-10 Torr) and equipped with a rapid-entry load-lock. Single crystals of Au(111) were sputter-cleaned and then annealed to 500 or 600 °C for 10 min. Following this preparation, X-ray photoelectron spectroscopy revealed a contamination-free surface and STM topographs showed the herringbone reconstruction characteristic of clean Au(111). These topographs were used to establish the sample’s crystallographic orientation with respect to the STM scan direction. For gas-phase deposition, neat alkanethiol liquids were stored in an ambient-temperature, blackened glass vial that is attached to a variable-aperture leak valve on the UHV chamber. C6OH is a hygroscopic liquid; therefore, freeze-pump-thaw purification cycles were used prior to gas-phase deposition in UHV. In situ quadrupole mass spectrometry was used to confirm the purity of the alkanethiol vapors. All observed mass/charge peaks could be attributed to cracking fragments of the parent thiol. Dosing pressures were typically 1 × 10-5 Pa (1 × 10-7 Torr). For liquid-phase deposition, µmol/L to mmol/L solutions of alkanethiols in ethanol were prepared and the coverage was controlled by varying the incubation time. STM tips were prepared from single-crystal tungsten wire using a dc etch, and STM imaging was done in high-impedance (Rt > 10 GΩ), constant-current mode.

Background Prior to discussing the experimental results, we review what is known about the bare Au(111) surface and the Au surface at the thiol/Au interface. Figure 1 shows an STM topograph of the herringbone reconstructed bare Au(111) surface. The reconstructed surface accommodates one extra Au atom for every 22 or 23 bulk lattice constants, giving rise to a 4.4% uniaxial surface compression.29 This compression induces variations in surface-to-subsurface atomic registry such that the stacking arrangement alternates between normal ABC stacking and faulted ABA stacking with faulted and unfaulted regions delineated by rows of bridging Au atoms.30 These bridging rows are manifest in STM topographs as corrugations with a 0.15 Å amplitude aligned with substrate 〈121〉 directions.31 The surface has an orthorhombic unit cell with dimensions of (x3 × 23) that is comprised of paired ridges and can adopt one of three orientational registries. To further reduce surface energy, the ridges form hyperdomains characterized by alternating 60° bends reminiscent of a herringbone pattern, hence the name.29,31 Certain bend apices, indicated in Figure 1, contain surface-confined dislocations and are more reactive sites.31 The crystal structure of saturation coverage alkanethiols on Au(111), as measured by grazing-incidence X-ray (29) Sandy, A. R.; Mochrie, S. G. J.; Zehner, D. M.; Huang, K. G.; Gibbs, D. Phys. Rev. B 1991, 43 (6), 4667. (30) Woll, C.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Phys. Rev. B 1989, 39, 7988. (31) Chambliss, D. D.; Wilson, R. J.; Chiang, S. J. Vac. Sci. Technol. B 1991, 9 (2), 933.

Figure 3. Height histogram from a constant-current STM topograph, a subset of which is shown in Figure 2C. Peaks centered at 0 and -2.4 Å are comprised of terrace-level and vacancy-island-level pixels, respectively. Normalized histogram integrals for the unprocessed data (solid line) and for data processed to correct for tip artifacts (dashed line) are shown at ×1 and ×10. Tip “convolution” artifacts decrease the apparent vacancy island fraction.

diffraction (GIXD) and STM, is a commensurate c(4 × 2) superlattice of a (x3×x3)R30 arrangement.9,18,28 The lattice constant measured by GIXD is 5.01 ( 0.02 Å,9,32 and that measured by STM is 5.1 ( 0.5 Å.18 Both measurements are consistent with the bulk-terminated Au(111) next-nearest-neighbor distance of 4.988 Å. There is no indication of incommensuration or discommensuration in either the GIXD or the STM. In contrast to the previous literature suggestion that alkanethiol monolayer assembly induces a contraction of the surface Au layer,27 these data indicate that the surface undergoes a 4.4% uniaxial in-plane expansion to the bulk lattice constant. Results Concomitant with monolayer assembly and surface relaxation, a distribution of VIs appears on the surface.26 Figure 2 shows sections of STM topographs of an octanethiol monolayer surface, and two different butanethiol monolayer surfaces. The variation in VI size distribution that is apparent in these topographs results from Ostwald ripening.20,24 The fractional area represented by the VIs can be quantified by calculating height histograms of the STM topographs. Plotted in Figure 3 is a height histogram of Figure 2C. Figure 2C was chosen to illustrate the histogram analysis because it has a coarse vacancy island distribution with a low perimeter-to-area ratio, thus minimizing the number of VI edge pixels. The histogram is comprised of a peak centered at 0 Å (terracelevel pixels) and a peak at -2.5 Å (VI-level pixels). Integrating the histogram yields a VI fractional area of (32) Camillone, N.; Chidsey, C. E. D.; Liu, G.-y.; Scoles, G. J. Chem. Phys. 1993, 98, 4234.

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Figure 4. (A) Constant-current UHV STM topograph of Au(111) exposed to 100 Langmuirs of mercaptohexanol. Herringbone features are due to bare, reconstructed Au(111), striped features are commensurate surface-aligned mercaptohexanol rows on relaxed Au(111), dark features are nascent vacancy islands, and isolated protrusions (circled and enumerated) are attributed to Au adatoms. Certain vacancy islands (pointing fingers) are elongated in 〈101〉 directions. The region indicated by rectangle B is expanded and rendered in the adjacent frame. (B) Height-mapped surface rendering from the region indicated in part A showing a Au adatom adjacent to a surface-aligned thiol island. (C) Horizontal cross-sectional profiles from the corresponding adatom features in part A. Profiles show an apparent adatom width of 13 Å and a height of 1.5 Å. Profile 3 shows an apparent height of 1 Å for a surface-aligned thiol island.

5.7%. The STM topographs shown in Figure 2A and B have higher VI dispersions; therefore, a greater fraction of the image pixels lie on VI edges. This leads to overlapping peaks in the height histograms. Nevertheless, the histograms of these topographs suggest VI fractional areas that are also close to 5%. In fact, the data in Table 1 suggest that the VIs comprise 5% to 9% of the surface area regardless of alkane chain length, end group, or deposition protocol. Time dependent studies show that the VI fraction smoothly approaches 5% to 9% and levels off as the monolayer approaches saturation. This fixed VI fraction suggests that the VIs may be associated with relaxation of the herringbone reconstruction. One caveat of the VI fractional-area estimate is that surface features are “convolved” or, more precisely, dilated by the finite-radius tip. By this effect, STM topographs underestimate the width of surface depressions. Therefore, VI fractional areas determined from unprocessed topographs such as those reported in Table 1 should be viewed as lower bounds. In order to estimate the error caused by these morphological limitations of STM we employ an erosion algorithm.33 An outer bound on the tip profile was ascertained from atomic step features in the topographs. As seen in Figure 3, this algorithm has the effect of increasing the apparent VI fractional area of Figure 2C from 5.7% to 7.3%. Applying this algorithm to Figure 2A and B results in similar increases. Because (33) Villarrubia, J. S. Surf. Sci. 1994, 321, 287.

the tip profile used in the erosion is an upper bound, these higher values should be viewed as upper bounds on the VI fractional area.33 Thus, whereas the experimental uncertainty is tip dependent, we can report that the VI fractional area associated with Figure 2C is somewhere between 0.057 and 0.073. Observation of Au Adatoms Relaxation of the compressed surface lattice would give rise to 4.4% of a monolayer (ML) of excess Au atoms in the surface layer. This excess density should create inplane compressive stress that could result in ejection of the excess Au atoms. Evidence for ejected Au atoms exists in STM topographs acquired at intermediate stages of gas-phase alkanethiol monolayer assembly. Figure 4A shows an STM topograph comprised of herringbone ridges, striped domains of mercaptohexanol,26 nascent VIs, and isolated protrusions. Figure 4B shows a height-mapped gray scale rendering of the protrusion indicated by the rectangle in Figure 4A. Figure 4C shows horizontal crosssectional profiles through the corresponding enumerated protrusions in Figure 4A. The protrusions exhibit a uniform apparent height of ≈1.5 Å and a uniform apparent width of ≈13 Å. While their apparent height is less than the Au(111) single-atom-step height of 2.35 Å, we can attribute this to a difference in electronic structure for an isolated adatom compared to a close-packed adatom island (comparable heights are observed for Pt adatoms on Pt-

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Figure 5. Sequential constant-current UHV STM topographs of a partial decanethiol monolayer on Au(111). (A) Surface region exhibiting six adatom features (circled and enumerated). (B) Same surface region after a several minute time delay. Circles are placed in the same relative positions as in frame A. Features 4-7 have migrated from their previous positions. Vacancy islands are not observed on this surface because nucleation occurs at higher alkanethiol coverage for methyl-terminated homologues than for hydroxy-terminated homologues. (C) Horizontal cross-sectional profiles from corresponding adatom features in A. Profiles show an apparent adatom width of 20 Å and a height of 1.5 Å. Asymmetric adatom profiles are attributed to “convolution” with an asymmetric tip geometry.

(111)34). While their apparent width is larger than the Au metal atom diameter of 2.88 Å, we can attribute this to “convolution” by the finite-radius tip. The crosssectional profile (Figure 4C, no. 3) traverses both an isolated protrusion and a striped-phase thiol island and indicates that these two features exhibit distinct and distinguishable height signatures in addition to differing shape signatures. The isolated protrusions are not observed on bare Au(111) surfaces in UHV or on Au(111) exposed to saturation doses of gas-phase alkanethiols, suggesting that they are not due to adsorption of residual gases. The thiol purity was confirmed by in situ mass spectrometry prior to gas-phase deposition; therefore, thiol impurities are an unlikely source of the features. We attribute these protrusions to Au adatoms. Figure 5A and B shows sequential STM topographs acquired at an intermediate coverage of decanethiol. Figure 5C shows cross-sectional profiles from the enumerated adatom features in Figure 5A. The adatom features exhibit a uniform apparent height of ≈1.5 Å and a uniform apparent width of ≈20 Å. The two frames were separated in time by several minutes, during which adatom features 4-7 migrated from their prior positions. While this motion may be partially tip-induced, a high intrinsic mobility is consistent with the low, 3-fold coordination number for adatoms on fcc(111). We attribute elongated adatom features (such as Figure 4A, no. 1) to adatom motion whereas the round features (such as Figure 4A, nos. 2 and 3) are attributed to an adatom trapped at a herringbone elbow site and in a thiol island, respectively. The intermediate-coverage STM topographs (Figures 4 and 5) provide evidence for Au adatoms; however, the saturation coverage topographs (Figure 2) give no indication of adatom islands. The fate of the adatoms thus remains unresolved. Using time-lapse STM movies, we can observe adatom motion; however, rapid adatom diffusion makes full trajectory determination problematic. (34) Zeppenfeld, P.; Lutz, C. P.; Eigler, D. M. Ultramicroscopy 1992, 42-44, 128.

The topographs in Figures 4A and 5A and B suggest that the instantaneous surface adatom density is low. Island nucleation is a critical phenomenon that only occurs above the threshold monomer areal density.35 We propose that during monolayer assembly the rapidly-diffusing adatoms adsorb at ascending step edges thereby preventing the terrace adatom density from exceeding that required for critical island nucleation. It is unfortunate that the transient nature of the ejected adatom features renders them uncountable. There appear to be certain systems, however, for which the ejected adatoms can be frozen-out and visualized. Figure 6 shows an STM topograph, acquired in vacuum, of Au(111) incubated for 1 h in 0.6 mmol/L thiophenol in ethanol. This exposure results in a saturated thiophenol monolayer. The surface exhibits a distribution of features, ≈50 Å in diameter and 2.5 Å in height. The height and texture of the features are indistinguishable from those of Au singleatom step features. The integrated, normalized height histogram (Figure 6C) suggests that the fractional coverage of these islands is ≈5%, consistent with the 4.4% compression of the reconstructed surface. They are not observed if the thiophenol is deposited by gas-phase deposition, nor are they observed for monolayers of saturated linear hydrocarbons deposited by gas-phase, ethanol solution, or benzene solution (data not shown). The features appear to be unique to monolayers of shortchain, thiol-derivatized, cyclic aromatics, assembled on Au(111) from the solution phase. We attribute these features to Au adatom islands. We speculate that solution phase cyclic aromatics inhibit the migration of surface Au adatoms released during monolayer assembly and thereby allow buildup of adatom density to a level exceeding that for critical island nucleation. Similar island features were observed in prior STM studies of cyclic aromatic monolayer assembly36-38 and also in a prior STM study that used (35) Zinke-Allmang, M.; Feldman, L. C.; Grabow, M. H. Surf. Sci. Rep. 1992, 16 (8), 378. (36) Hara, M.; Sasabe, H.; Knoll, W. Thin Solid Films 1996, 273 (1-2), 66.

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Figure 6. (A) Constant-current UHV STM topograph of a saturation monolayer formed by incubating Au(111) in 0.6 mM thiophenol for 1 h. Two single-atom steps ascend in the upper right. (B) Cross-sectional profile from the solid line in part A showing the 2.5 Å height of the bright features. (C) Height histogram of A. To avoid contribution of ascending terraces, we only count pixels on the left side of the dashed line in part A. The value of the histogram integral at 1.25 Å above the terrace level suggests that bright features comprise ≈5% of a monolayer. Bright features are attributed to Au islands.

electrochemical means to lift the Au(111)-herringbone reconstruction.39 Alkanethiol assembly on the Au(001)(5 × 20) reconstruction, which is a 26% compression, also results in ejection of excess atoms onto the surface layer and subsequent creation of an interconnected Au island network.40 Nucleation of Vacancy Islands Data in the literature indicate that the surface relaxes during alkanethiol monolayer assembly, and the data presented here indicate that this results in creation of mobile adatoms. It is interesting, then, that the final monolayer surface exhibits ≈5% ML of vacancy islands. (37) Boland, T.; Ratner, B. D. Langmuir 1994, 10, 3845. (38) Dhirani, A.-A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. J. Am. Chem. Soc. 1996, 118, 3319. (39) Tao, N. J.; Lindsay, S. M. J. Appl. Phys. 1991, 70 (9), 5141. (40) Poirier, G. E. J. Vac. Sci. Technol. B 1996, 14 (2), 1453.

Each (1 × 23) primitive unit cell of the reconstructed surface contains one extra Au atom. If one Au atom was ejected from each primitive unit cell and these adatoms all migrated to step edges, then the saturation Au/thiol interface would be atomically flat and defect free. Alternately, we propose that two atoms per (1 × 23) unit cell are ejected. Thereby the fully relaxed surface will contain ≈4.4% ML of vacancies, in close agreement with the observed VI fraction. The excess vacancies released during monolayer assembly nucleate stable vacancy islands, in contrast to the adatoms which appear to migrate and join ascending step edges. We attribute this disparity in nucleation behavior to a disparity in particle diffusion constants. Adatoms diffuse by hopping between adjacent 3-fold hollow sites by way of a 2-fold transition state.41,42 In contrast, vacancy (41) Liu, C.-L.; Adams, J. B. Surf. Sci. 1992, 265, 262.

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Figure 7. Sequential constant-current UHV STM topographs of a partial monolayer of mercaptohexanol on Au(111). (A) 180 Langmuirs of mercaptohexanol results in a single vacancy island (C) nucleated at a herringbone elbow. (B) Same surface region as in part A after exposure to an additional 120 Langmuirs of mercaptohexanol. The growth of mercaptohexanol monolayer islands induces migration of herringbone elbow defects along [101]. Additional vacancy islands nucleate at herringbone elbows (arrows). Opposing ‘Y’-type defects (D) and (E) counterpropagate, annihilating five herringbone elbow defects and merging. Vacancy islands do not migrate.

Figure 8. Schematic of vacancy island formation. (A) The bare, reconstructed surface layer has a relative density of 1.044. (B) Alkanethiol assembly lifts the reconstruction thereby creating adatoms on, and vacancies in, the surface layer. (C) As the monolayer approaches saturation, adatoms adsorb at ascending step edges and vacancies nucleate stable islands. At saturation, the Au surface has a relative in-plane density of 1.

diffusion occurs when an atom on the vacancy perimeter hops into the empty site. The perimeter atoms are eightcoordinate and migrate via a crowded transition state.43 Molecular dynamics studies on fcc(111) find that adatoms have a significantly lower site-hopping activation barrier than do vacancies.44 Likewise, embedded atom method (EAM) calculations for Ni(111) find an activation barrier for adatom diffusion of 0.056 eV while that for vacancies is 1.41 eV.41 Because of the exponential dependence, this (42) Feibelman, P. J.; Nelson, J. S.; Kellogg, G. L. Phys. Rev. B 1994, 49 (15), 10548. (43) Voter, A. F. (Private communication). (44) DeLorenzi, G.; Jacucci, G.; Pontikis, V. Surf. Sci. 1982, 116, 391. (45) Poirier, G. E.; Pylant, E. D.; White, J. M. J. Chem. Phys. 1996, 105, 2089.

activation barrier difference implies a room-temperature adatom-to-vacancy diffusion constant ratio of 1023. It is known that the EAM tends to underestimate the diffusion barrier of adatoms on fcc(111);42 however, even allowing an adatom activation barrier of 0.56 eV, the diffusion constant ratio is still 1015. This relatively slow surface vacancy diffusion allows the vacancy density to build to a level exceeding that for critical island nucleation. Proximate to step edges, which act as vacancy sinks, the vacancy density, and therefore the VI nucleation probability, should remain low (Au terraces with VI-free perimeters have been reported in the literature: ref 14, Figure 3; ref 5, Figure 3; ref 17, Figure 1; and ref 45, Figure 4). Ejection of excess surface Au atoms creates vacancies that migrate in the surface layer and eventually nucleate islands. Figures 4A and 7A and B show that, for mercaptohexanol monolayers, the preferred nucleation site is the herringbone elbow, a site that contains a surfaceconfined dislocation and that acts as the locus of nucleation in Ni, Co, and Fe heteroepitaxy.31,46-48 Following nucleation, the nascent VIs expand by capturing diffusing vacancies. A fraction of the nascent VIs exhibit anomalous geometries characterized by elongation in 〈101〉 directions (Figure 4B, pointing fingers). We speculate that these anomalous VI geometries may result from a combination of two factors: a thermodynamic preference for 〈101〉 aligned steps, which are {111} microfacets, over 〈121〉 aligned steps, which are {110} microfacets, and a sequential mechanism of VI nucleation, lateral displacement of the herringbone elbow defect,26 and repeated VI nucleation, as suggested in Figure 7. When the monolayer reaches saturation and thermalizes, the VIs adopt geometries that minimize the perimeter-to-area ratio (see Figure 2). (46) Voigtlander, B.; Meyer, G.; Amer, N. M. Surf. Sci. Lett. 1991, 255, L529. (47) Voigtlander, B.; Meyer, G.; Amer, N. M. Phys. Rev. B 1991, 44, 10354. (48) Stroscio, J. A.; Pierce, D. T.; Dragoset, R. A.; First, P. N. J. Vac. Sci. Technol. A 1992, 10 (4), 1981.

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The proposed relaxation-mediated, in-plane Au redistribution model is illustrated schematically in Figure 8. The bare, reconstructed surface layer is 4.4% compressed relative to the bulk. During alkanethiol monolayer assembly, the reconstruction is lifted and two atoms per (1 × 23) primitive unit cell are ejected onto the terrace level, thereby creating one net vacancy per (1 × 23) unit cell. The adatoms diffuse rapidly and adsorb at ascending step edges while the vacancies nucleate islands in the terraces. It was reported by Edinger et al. that the alkanethiol incubation bath contains dissolved Au.16 We note in Table 1 that the two highest measured VI fractional areas were obtained from solution-phase deposition, whereas the lowest fractional area was obtained by gas-phase deposition. We speculate, therefore, that a distribution of VIs comprising 4.4% of a monolayer arises concomitant with monolayer formation by the relaxation-mediated, in-plane Au redistribution mechanism presented here and that any dissolved Au in the incubation bath arises from etching of these VI features or other step-edge surface defects in concentrated thiol solutions. Conclusion We have presented UHV STM evidence for a novel mechanism of formation of vacancy island defects in alkanethiol monolayers assembled on Au(111). Our data

Poirier

suggest that Au atoms are ejected from the surface layer in an amount that is related to the herringbone compression. Relaxation of the herringbone reconstruction appears to be the driving force for Au atom ejection. The driving force for ejection of extra Au adatoms remains unresolved but may be addressed using theoretic modeling. While these vacancy island defects are ubiquitous and largely unavoidable, they do not diminish the desirable properties of alkanethiol monolayers, namely, the physical and electron transfer blocking abilities. The vacancy islands account for only ≈5% of a monolayer and are uniformly covered by alkanethiols, as evidenced by STM19 and by electrochemical measurements.8 They are structurally equivalent to step defects, defects that are unavoidable on real surfaces. Effectively, the VIs merely increase the substrate step density. Because the VI fractional area is fixed, any adverse effects of the VI edges will be mitigated by forming a coarse distribution of VIs. This can be achieved by decreasing the growth rate by using µmol/L solutions rather than mmol/L solutions or by depositing from the gas phase. Acknowledgment. G.E.P. gratefully acknowledges the expert assistance of and stimulating discussions with E. D. Pylant, J. S. Villarrubia, A. F. Voter, M. H. Dishner, and J. G. Hagedorn. LA960777Z