2318
Langmuir 1997, 13, 2318-2322
Direct Observation of Substrate Influence on Chemisorption of Methanethiol Adsorbed from the Gas Phase onto the Reconstructed Au(111) Surface Mark H. Dishner, John C. Hemminger,* and Frank J. Feher* Department of Chemistry, University of California, Irvine, California 92967 Received November 7, 1996. In Final Form: February 10, 1997X Methanethiol adsorption onto Au(111) was studied in situ by scanning tunneling microscopy in air at room temperature. By maintaining registry on the surface during adsorption, it was demonstrated that gold vacancy islands (i.e., “etch pits”), which are created by the chemisorption of methanethiol, exist in rows that exactly follow the (22×x3) gold reconstruction. Ostwald ripening produces larger gold vacancy islands that are no longer coincident with the original Au(111) reconstruction. Further annealing produces well-ordered domains of methanethiol that have the same surface structures observed for other alkanethiolbased self-assembled monolayers on Au(111). The results demonstrate for the first time that at high rates of dosing, the structure of a fully-formed thiol monolayer on Au(111) is influenced by the original (22×x3) reconstruction of the underlying gold surface. These data strongly suggest that the gold vacancy islands observed in methanethiol, thiophene, and other alkanethiol-based monolayers are not the result of chemical etching.
Sulfur containing monolayers on Au(111) have been studied by a wide variety of surface-analytical techniques under many different conditions.1-3 Interest in these systems has been driven by their technological importance.4-11 While thiols on gold have been extensively studied, many aspects of the chemisorption process, as well as the nature of the chemisorbed species and the details of the self-assembly process, are still subject to controversy.2,12-18 The Au(111) surface is known to undergo a surface reconstruction which results in the top layer of gold atoms adopting a (22×x3)19,20 unit cell (vide infra). It has been generally assumed that this reconstruction is lifted upon X
Abstract published in Advance ACS Abstracts, April 1, 1997.
(1) Ulman, A. In Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (2) Dubois, L. H.; Nuzzo, R. G. Annu. Revs. Phys. Chem. 1992, 437463. (3) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 44814483. (4) Zak, J.; Yuan, H.; Ho, M.; Woo, L. K.; Porter, M. D. Langmuir 1993, 9, 2772-2774. (5) Rubenstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426-429. (6) Kumar, A.; Biobuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498-1511. (7) Kumar, A.; Abbott, N. L.; Kim, E.; Biebuyck, H. A.; Whitesides, G. M. Acc. Chem. Res. 1995, 28, 219-226. (8) Mrksich, M.; Whitesides, G. M. Trends Biotechnol. 1995, 13, 228235. (9) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305-5306. (10) Behm, J. M.; Lykke, K. R.; Pellin, M. J.; Hemminger, J. C. Langmuir 1996, 12, 2121-2124. (11) Mirkin, C. A. Nature 1996, 382, 607. (12) Scho¨nenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611-614. (13) Sondag-Huethorst, J. A. M.; Scho¨nenberger, C.; Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6826-6834. (14) Dishner, M. H.; Feher, F. J.; Hemminger, J. C. Chem. Commun. 1996, 1971-1972. (15) Dishner, M. H.; Hemminger, J. C.; Feher, F. J. Lagmuir 1996, 12, 6176-6178. (16) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 17661770. (17) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825-1831. (18) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216-1218. (19) Chambliss, D. D.; Wilson, R. J. J. Vac. Sci. Technol., B 1991, 9, 928-932. (20) Chambliss, D. D.; Wilson, R. J.; Chiang, S. J. Vac. Sci. Tech. B 1991, 9, 933-937.
S0743-7463(96)01086-4 CCC: $14.00
Figure 1. A differentiated 2240 Å × 2240 Å image of Au(111) exhibiting the characteristic (22×x3) reconstruction. This image was recorded with tunneling parameters of 0.40 V and 1.0 nA.
chemisorption of the thiols and that the original reconstructed surface structure has no effect on the resulting monolayer. For example, Poirier and Pylant recently observed that slow dosing of mercaptohexanol in ultrahigh vacuum (UHV) produced saturated monolayers on Au(111) after exposures of 2500 L.21 These SAMs (selfassembled monolayers) exhibited well-ordered molecular domains with randomly distributed vacancy islands, but there was no obvious relationship between the features in these monolayers, specifically vacancy islands, and the original (22×x3) gold reconstruction. However, observations that the Au(111) reconstruction can have profound effects on both the nucleation of some metals20,22,23 at submonolayer coverages and the nucleation of small (21) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145-1148. (22) Wollschla¨ger, J.; Amer, N. M. Surf. Sci. 1992, 277, 1-7. (23) Chambliss, D. D.; Wilson, R. J.; Chiang, S. Phys. Rev. Lett. 1991, 66, 1721-1724.
© 1997 American Chemical Society
Chemisorption of Methanethiol
Langmuir, Vol. 13, No. 8, 1997 2319
Figure 2. (a) Schematic representation of the gold (22×x3) reconstruction as seen from above; the unit cell is shown by the red rectangle, which has dimensions of 63.36 Å × 4.99 Å. The reconstruction stripes that are experimentally observed are the result of stacking these unit cells in the direction of the short axis (i.e., the next-nearest-neighbor direction). (b) The same reconstruction but viewed from within the gold surface (i.e., end on) (note: the vertical displacement of gold atoms creates a region that is filled by atoms in hcp sites (the vertical displacement of gold atoms has been exaggerated in this figure)).
islands of 4-mercaptopyridine24 suggest that there may still be circumstances where chemisorption of a thiol is influenced (or dictated) by this reconstruction. In this paper, we report the results from an in situ scanning tunneling microscopy (STM) study of the chemisorption of methanethiol on reconstructed Au(111). This study, which was performed in air with very rapid dosing of methanethiol, shows well-ordered arrays of vacancy islands on Au(111) which are aligned with the hexagonal close-packed (hcp) sites of the (22×x3) reconstruction. Our results clearly demonstrate that the gold reconstruction can affect the structure of a newly formed selfassembled thiol monolayer and they strongly suggest that the vacancy islands or “etch pits” observed in Au(111)supported alkanethiol SAMs are not the result of traditional chemical etching. Results and Discussion The STM image of a typical Au(111) sample used in our work is shown in Figure 1. This region of the sample was chosen because it has several screw dislocations (one such screw dislocation is indicated by the arrow) along with other uniquely identifiable features; furthermore, the image is well-reconstructed and the (22×x3) reconstruction is easily seen as straight stripes that are 63.36 Å wide (assuming a width of 22 gold atoms) and occasionally exhibit 120° turns. The reconstructed surface has also been described as having a herringbone pattern.25,26 A model of the rectangular unit cell for the straight stripes of the reconstruction is depicted in Figure 2a. The short edge of the unit cell is 4.99 Å long (x3 times the gold-gold distance of 2.88 Å) and lies along the next-nearest-neighbor (24) Hara, M.; Sasabe, H.; Knoll, W. Thin Solid Films 1996, 273, 66-69. (25) Meyer, J. A.; Baikie, I. D.; Kopatzki, E.; Behm, R. J. Surf. Sci. 1996, 365, L647-L651. (26) Barth, J. V.; Brune, H.; Ertl, G.; Behm, R. J. Phys. Rev. B 1990, 42, 9307-9318.
direction of gold atoms in a (111) plane. The long edge is 63.36 Å long and is aligned along the nearest-neighbor direction of gold atoms in a (111) plane. As one travels along the long edge of the unit cell, the environment of the surface gold atoms changes from face-centered cubic (fcc), at one edge, through a transitional period of the so-called “bridging sites”19,20,23 to hcp in the center, and then the gold surface atom environment returns to fcc at the other edge by again going through the bridging transition. The gold atoms in bridging sites have been shown to be vertically displaced by 0.20 Å. The gold atoms in hcp sites are also vertically displaced but only by 0.12 Å;26 an exaggerated model of the vertical displacement for the reconstruction is depicted in Figure 2b. In Figure 1, the reconstruction appears as alternating light and dark stripes; the dark stripes are gold atoms in fcc sites and the light stripes are the vertically displaced gold atoms in bridging and hcp sites. While it is possible to resolve the subtle vertical displacement of the gold atoms in bridging and hcp sites, it is not easily visible in large area images such as the one shown in Figure 1. The long axis of the unit cell shown in Figure 2a spans from the center of one of the dark stripes in Figure 1 to the center of the next dark stripe. The unique features in Figure 1 (e.g., screw dislocations) allow for surface registry from one image to the next during our experiments. It is therefore possible to examine the same surface region before exposure to methanethiol (Figure 1), after exposure to methanethiol (Figure 3), and after the monolayer has been annealed (Figure 4). Shortly after adsorption of methanethiol (
