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Scanning Tunneling Microscopy Characterization of Organoselenium Monolayers on Au(111) Mark H. Dishner, John C. Hemminger,* and Frank J. Feher* Department of Chemistry, University of California, Irvine, CA 92697 Received April 17, 1997. In Final Form: July 8, 1997X Self-assembled monolayers were formed by immersing Au(111) surfaces into solutions of benzeneselenol and diphenyl diselenide. Chemisorption of benzeneselenol produced numerous gold islands (20-200 Å in diameter) on the surface and a selenium-containing monolayer with a fundamental repeat unit of (3x3 × 2x3)R30°. Upon annealing at 60 °C in air, these gold islands coalesce to form larger, hexagonal-shaped islands. Chemisorption of diphenyl diselenide is qualitatively similar and upon annealing appears to produce the same surface structures observed with benzeneselenol. These organoselenium-modified surfaces were investigated by scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES).
Introduction For over a decade it has been known that organosulfur reagents will chemisorb to gold surfaces and spontaneously form ordered monolayers.1-5 These reagents can be used to anchor virtually any functional group to a gold surface in order to construct practical devices.6,7 For example, the exceptional tolerance of organosulfur monolayers to a variety of functional groups has allowed for the development of chemically specific sensors based on surface acoustic wave and quartz crystal microbalance technology.8 In contrast to the many reports of monolayers derived from organosulfur reagents containing functional groups in the ω-position, there has been relatively little work done to build similarly ordered organic monolayers on metals without using sulfur as the anchor group.9 The ability to change the anchor can potentially make selfassembled monolayer (SAM) technology useful for environments where an organosulfur monolayer would be unsuitable. For example, it is known that thiol monolayers are quantitatively oxidized to the corresponding sulfonate in the presence on UV light and oxygen.10-12 In addition, the stability of alkanethiol monolayers is strongly dependent on temperature; most alkanethiol monolayers melt and anneal below 100 °C.13-16 The identification of adsorbates that offer the advantages of organosulfur X
Abstract published in Advance ACS Abstracts, August 1, 1997.
(1) Ulman, A. In Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (2) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365-385. (3) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 44814483. (4) Nuzzo, R. G.; Zergarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733-740. (5) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (6) Zhou, Y.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M.; Wells, M. J. Am. Chem. Soc. 1996, 118, 3773-3774. (7) Carey, R. I.; Folkers, J. P.; Whitesides, G. M. Langmuir 1994, 10, 2228-2234. (8) Renken, J.; Dahint, R.; Grunze, M. Anal. Chem. 1996, 68, 176182. (9) Samant, M. G.; Brown, C. A.; Gordon, J. G. I. Langmuir 1992, 8, 1615-1618. (10) Dishner, M. H.; Feher, F. J.; Hemminger, J. C. Chem. Commun. 1996, 1971-1972. (11) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626-628. (12) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305-5306. (13) Dishner, M. H.; Hemminger, J. C.; Feher, F. J. Lagmuir 1996, 12, 6176-6178. (14) Cavalleri, O.; Hirstein, A.; Kern, K. Surf. Sci. Lett. 1995, 340, L960-L964. (15) Bucher, J.-P.; Santesson, L.; Kern, K. Langmuir 1994, 10, 979983.
S0743-7463(97)00397-1 CCC: $14.00
reagents (i.e., self-assembly, ease of preparation, and tolerance to other functional groups), as well as enhanced mechanical stability and resistance to photochemical degradation, would be advantageous. In this paper we report scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES) characterization of ordered monolayers formed by the deposition of benzeneselenol and diphenyl diselenide onto Au(111). To the best of our knowledge, this is only the second example of an ordered organoselenium monolayer on Au(111),9 and the first to be studied by STM. Experimental Section AV-STM grade scratch-free green mica was purchased from Asheville-Schoomaker, and 99.9999% pure gold was obtained from Aldrich and Alfa-Aesar. Sylvania 5 mm alumina crucibles were used for resistive evaporation of gold. An Edwards E306A evaporator was used for gold deposition, and the preparation of films has been described in detail elsewhere.17 Benzeneselenol and diphenyl diselenide were purchased from Aldrich. Benzeneselenol was vacuum distilled (25 °C, 1 × 10-6 Torr) to remove the diselenide and stored in a sealed ampule in a nitrogen glovebox. As an added precaution against adventitious impurities, the benzeneselenol solutions used to prepare monolayers were pretreated with enough gold powder (1.5-3 µm diameter) to adsorb 10% of the benzeneselenol. Diphenyl diselenide was used as received. Monolayers of benzeneselenol were formed by immersing a Au(111) film in benzeneselenol (4.0 mM in diethyl ether) for 2 h in a nitrogen glovebox. Upon removal of the newly formed monolayers from solution, the samples were rinsed successively with diethyl ether and ethanol before being dried in a stream of nitrogen. Benzeneselenol-derived monolayers were annealed for 2.5 h in air at 60 °C. Chemisorption of diphenyl diselenide was performed by immersing a Au(111) film in diphenyl diselenide (4.0 mM in diethyl ether) for 10 min, rinsing as described above, and then annealing in air for 45 min at 60 °C. STM images were recorded in constant current mode with bias and current settings that produced the highest signal to noise ratio. Typical settings were 300 mV and 1.0 nA. All images were recorded on a Besocke microscope, which has a beetle geometry. Auger electron spectroscopy was performed on a Perkin-Elmer 32-150 spectrometer with an incident beam of 2000 eV electrons at 1 mA in ultrahigh vacuum (UHV).
Results and Discussion Selenium-containing monolayers can be easily prepared by immersing Au(111) films in diethyl ether solutions of benzeneselenol (4.0 mM, 2.5 h), rinsing with ether, and (16) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 1096610970. (17) Dishner, M. H.; Ivey, M. M.; Grorer, S.; Penner, R. M.; Hemminger, J. C.; Feher, F. J. Submitted for publication in J. Vac. Sci. Technol.
© 1997 American Chemical Society
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Figure 2. The (3x3 × 2x3)R30° fundamental repeat unit of the benzeneselenol-derived domain.
Figure 1. (a) Differentiated 2240 Å × 2240 Å STM image of benzeneselenol on Au(111) recorded at 300 mV and 1.0 nA. In contrast to benzenethiol monolayers, the benzeneselenol monolayer is well ordered and contains no gold vacancy islands. (b) Differentiated STM image of benzeneselenol on Au(111) recorded at 300 mV and 1.0 nA. The image is 280 Å × 280 Å, and benzeneselenol domains exist in the three rotationally equivalent orientations labeled A, B, and C. For clarity, unit cells have been superimposed on domians A and B.
then annealing in air (60 °C, 2.5 h). The presence of adsorbed organoselenium species in these monolayers was confirmed by the appearance of a peak at 1315 eV in the Auger spectrum. This peak, which is not observed in spectra of clean gold films, is characteristic of selenium.18 Colorless solutions of benzeneselenol rapidly oxidize in air to produce yellow solutions of diphenyl diselenide.19 However, monolayers obtained from benzeneselenol both in the presence of air and in an oxygen-free atmosphere are essentially identical. A typical STM image of a monolayer obtained by immersing Au(111) in benzeneselenol is shown in Figure 1a. The most striking features in the image are the many small islands of gold ranging from approximately 20 Å to more than 200 Å. These (18) Davis, L. E.; MacDonald, N. C.; Palmberg, P. W.; Riach, G. E.; Weber, R. E. Handbook of Auger Electron Spectoscopy, 2nd ed.; Physical Electronics Industries, Inc.: Eden Prairie, 1976; pp 113-115. (19) Patai, S.; Rappoport, Z. The Chemistry of Organic Selenium and Tellurium Compounds; John Wiley & Sons Inc.: New York, 19861987; Vol. 2.
islands are not present on a clean Au(111) surface prepared by our methods,17 so they must originate during the chemisorption process. A higher magnification image, shown in Figure 1b, reveals that the monolayer formed by chemisorption of benzeneselenol is highly ordered, with molecular domains in three rotationally equivalent orientations. These domains, which are observed both between the gold islands and on the plateau of each island, intersect straight step edges at angles of 30° and 90°, strongly suggesting that the monolayer unit cell is rotated 30° with respect to the underlying Au(111) lattice. Assuming that the Se atom is located in a gold threefold hollow site and that each bright spot in the STM image represents one Se atom from benzeneselenol, the unit cell most consistent with our data is (3x3 × 2x3)R30°. This unit cell, which is illustrated in Figure 2, derives from a fundamental repeat unit that is 10.1 Å × 15.8 Å. It is quite likely that there is more than one Se atom per unit cell because internal structure is observed within the unit cell, but the details of this internal structure have not yet been identified. Upon further annealing, the gold islands undergo Ostwald ripening and eventually merge with step edges (Figure 3). During this process the geometry of the gold step edges and islands changes to favor large hexagonal islands and steps with 120° turns. These changes are quite surprising because clean surfaces of Au(111) normally exhibit triangular-shaped terraces and steps which strongly favor 60° turns. (Hexagonal facets on a [111] surface have two types of step edges, which are energetically different.20 Triangular facets are formed from the intersection of the most energetically favorable steps.) The observation that benzeneselenol-derived monolayers produce large hexagonal terraces and steps with 120° turns suggests that the organoselenium adsorbate is stabilizing hexagonal facets. Unfortunately, the nature of this interaction is not known because we have not yet been able to obtain molecular-level resolution STM images of annealed benzeneselenol-derived monolayers. The facile air-oxidation of benzeneselenol to diphenyl diselenide greatly complicates any effort to study the chemistry of benzeneselenol. However, the presence of diphenyl diselenide does not appear to affect the course of benzeneselenol chemisorption on Au(111). Regardless of whether chemisorption is performed in air using solutions of benzeneselenol which are noticeably contaminated by yellow diphenyl diselenide or solutions of (20) Li, Y.; DePristo, A. E. Surf. Sci. 1996, 351, 189-199.
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Figure 3. 2240 Å × 2240 Å differentiated STM image recorded at 300 mV and 1.0 nA showing the effect of annealing on the benzeneselenol monolayer and the propensity for these monolayers to stabilize hexagonally shaped gold islands.
Figure 4. Differentiated STM image (2240 Å × 2240 Å) of Au(111) after chemisorption of diphenyl diselenide and annealing. Large, hexagonal-shaped gold islands are clearly visible in the top half of the image.
carefully purified benzeneselenol prepared in a glovebox (
