Langmuir 2000, 16, 6693-6700
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Adsorption Structures of 1-Octanethiol on Cu(111) Studied by Scanning Tunneling Microscopy S. M. Driver and D. P. Woodruff* Department of Physics, University of Warwick, Coventry CV4 7AL, U.K. Received March 23, 2000. In Final Form: May 30, 2000 The interaction of 1-octanethiol with Cu(111) at room temperature in ultrahigh vacuum has been studied using scanning tunneling microscopy (STM). Two structural phases are observed, a “honeycomb” phase and a pseudo-(100) reconstructed surface phase, which are essentially equivalent to methanethiolate structures observed on Cu(111) in a recent study using STM. The pseudo-(100) structure can also be reconciled with the results of a prior study of this phase using normal-incidence X-ray standing wavefield absorption, which indicated the presence of adsorbate-induced reconstruction but was unable to establish further details of the local structure. We discuss the similarity of the behavior of octanethiol and methanethiol on Cu(111), contrast this with alkanethiol behavior on Au(111), and discuss the role of S-substrate interactions in the structures of these self-organized monolayers.
1. Introduction The interaction of alkanethiols (R-SH) with metal surfaces, especially gold, has attracted considerable interest because of the tendency of the thiolate species (R-S-), obtained by deprotonation of the thiol, to form self-assembled monolayers (SAMs), prepared either in solution or in ultrahigh vacuum (UHV).1 The bulk of recent work of this kind concerns alkanethiol adsorption on Au(111). For saturation coverage, the “standard model”, as it has been termed, is that thiolate forms a (x3 × x3)R30° phase on the Au(111) surface on which the “herringbone” reconstruction of the clean surface is lifted. In this phase the alkane chains are oriented at around 30° to the surface normal.2-4 The S atoms have been assumed to occupy 3-fold coordinated hollow sites on the metal surface, although this appears to be a matter of conjecture rather than being based on any quantitative structural study. The assumption is that the self-assembly is governed primarily by interaction between the adjacent alkane chains, which dictate, for example, the packing density and, in a related fashion, the chain tilt angle. Of course, the fact that this phase is commensurate with the substrate and has a small unit mesh size clearly implies that the lateral corrugation of the adsorbate-substrate interaction potential is relevant to the ordering, but the S-S atom spacing in this model is ∼5 Å, and the S atoms are thus assumed to have no significant interaction. This view has been challenged in the past few years by experimental evidence that there are two distinct adsorption sites for the S atoms.5 This has been interpreted in terms of thiolate dimerization, with adsorption being through a disulfide with an S-S spacing of 2.2 Å, a value characteristic of S-S bonding.6,7 This is proposed to account for observations of a c(4 × 2) superlattice of the * Corresponding author. E-mail D.
[email protected]. Tel: +44 24 76523378. Fax: +44 24 76692016. (1) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (2) Dubois, L. H.; Nuzzo, R. G.; Annu. Rev. Phys. Chem. 1992, 43, 437. (3) Sellars, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (4) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (5) Yeganeh, M. S.; Dougal, S. M.; Polizzotti, R. S.; Rabinowitz, P. Phys. Rev. Lett. 1995, 74, 1811. (6) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216.
(x3 × x3)R30° site,8,9 which has been found in He atom diffraction, X-ray diffraction and scanning tunneling microscopy (STM) studies. By contrast, however, a recent STM study of the adsorption of an asymmetric disulfide on Au(111) shows clear evidence for the scission of the S-S bond (as had been previously supposed), the two different resulting thiolate species forming separate ordered phases.10 In addition to these saturation structures, a variety of so-called “pinstripe” structures have been observed at subsaturation coverages, formed either by subsaturation dosing or by annealing of the saturated surface. In these structures, the alkane chains are thought to lie parallel with the surface, anchored by the headgroups in either hollow sites or a mixture of hollow and atop sites.11 An STM experiment in UHV demonstrated the evolution of these structures from a lattice-gas phase, through a pinstripe phase, and finally to the saturation structure.12 While there are clearly some remaining contradictions between the structural conclusions of some of the studies of these alkanethiolate SAMs on Au(111), an implicit assumption of all of this work seems to be that the Au(111) surface is not reconstructed by the adsorption (strictly, it is assumed to be unreconstructed from the actual clean surface structure to an ideally terminated bulk structure). By contrast, a series of structural studies on methanethiolate (CH3S-) on Cu(111), first by surfaceextended X-ray absorption fine structure (SEXAFS) and rather basic NIXSW (normal incidence X-ray standing wavefield absorption)13 and subsequently by high-resolution S 2p photoelectron spectroscopy14 and chemical-shift NIXSW,15 have shown that at room temperature this adsorbate does produce a significant reconstruction of the (7) Fenter, P.; Schreiber, F.; Berman, L.; Scoles, G.; Eisenberger, P.; Bedzyk, M. J. Surf. Sci. 1998, 412/413, 213. (8) Camillone, N.; III.; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 4234. (9) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (10) Noh, J.; Hara, M. Langmuir 2000, 16, 2045. (11) Staub, R.; Toerker, M.; Fritz, T.; Schmitz-Hu¨bsch, T.; Sellam, F.; Leo, K. Langmuir 1998, 14, 6693. (12) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (13) Prince, N. P.; Seymour, D. L.; Woodruff, D. P.; Jones, R. G.; Walter, W. Surf. Sci. 1989, 215, 566. (14) Kariapper, M. S.; Grom, G. F.; Jackson, G. J.; McConville, C. F.; Woodruff, D. P.; J. Phys.: Condens. Matter 1998, 10, 8661. (15) Jackson, G. J.; Woodruff, D. P.; Jones, R. G.; Singh, N. K.; Chan, A. S. Y.; Cowie, B. C. C.; Formoso, V. Phys. Rev. Lett. 2000, 84, 119.
10.1021/la000447l CCC: $19.00 © 2000 American Chemical Society Published on Web 07/13/2000
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outermost Cu atom layer. Moreover, recent studies of SAMs formed by the C8 1-octanethiol (CH3(CH2)7SH) adsorption on Cu(111) and Ag(111) using (NIXSW) and near-edge X-ray absorption fine structure (NEXAFS)16,17 and of the C6 and C12 alkanethiols by SEXAFS and NEXAFS18 showed that these systems also involve significant adsorbate-induced reconstruction of the outermost metal atom layer. The nature of these reconstructions is unknown, however, but has been assumed to involve some density-lowering reconstruction of the outermost Cu atom layer, which retains the 3-fold symmetry of the substrate. The SEXAFS study of the C6 and C12 thiolates on Cu(111) did, however, provide clear evidence that the adsorbed species in this case are not disulfides, as has been proposed on Au(111). Further evidence for the lack of disulfides on Cu(111) comes from an earlier XAFS study of the adsorption of dimethyl disulfide on this surface, showing a clear distinction between the intact adsorbed disulfide and the resulting thiolate phase.19 The present STM study of the interaction of 1-octanethiol with Cu(111) was undertaken to clarify the structure of the reconstructed phase, and indeed to identify any other ordered adsorption structures. Of particular relevance to this study is the previous work on methanethiolate on Cu(111). Valence20 and core level14,15 photoelectron spectroscopy (as well as the SEXAFS and NIXSW structural work13,15) all indicate that the same methanethiolate adsorbate is formed on Cu(111) by exposure to methanethiol (CH3SH) via deprotonation or to dimethyl disulfide (DMS; (CH3S)2) via S-S bond scission. NIXSW data from this surface show a S-outermost Cu layer spacing that is well-defined but smaller than can be reconciled with the S atoms of the thiolate molecule occupying simple chemisorption sites in a (111) Cu layer, while there is near-zero coherence of the S atom positions parallel to the surface. This is interpreted in terms of a reconstruction involving enlarged hollow sites, which is either incommensurate or involves a large coincidence mesh with several locally inequivalent S adsorption sites. The NIXSW study of the 1-octanethiolate on Cu(111) yielded essentially identical results. The more detailed spectroscopic characterization of the methanethiolate on Cu(111)14,15 showed that there are two distinct thiolate states on this surface. One of these was found to be dominant at low temperatures (
