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J. Phys. Chem. C 2007, 111, 10904-10914

The Structure of Atomic Sulfur Phases on Au(111) Miao Yu,† H. Ascolani,‡ G. Zampieri,‡ D. P. Woodruff,*,† C. J. Satterley,§ Robert G. Jones,§ and V. R. Dhanak# Physics Department, UniVersity of Warwick, CoVentry CV4 7AL, U.K., CONICET and Centro Ato´ mico Bariloche, 8400 S.C. de Bariloche, Argentina, Department of Physical Chemistry, School of Chemistry, UniVersity of Nottingham, Nottingham NG7 2RD, U.K., and CCLRC Daresbury Laboratory, Warrington WA4 4AD, U.K. ReceiVed: March 15, 2007; In Final Form: May 3, 2007

The structural phases formed by atomic sulfur on Au(111) due to reaction with molecular S2 have been investigated by qualitative low-energy electron diffraction (LEED), scanning tunneling microscopy, and normal incidence X-ray standing wavefield absorption (NIXSW) combined with X-ray photoelectron spectroscopy (XPS). Three phases are identified with increasing coverage, namely, a newly identified (5 × 5) phase, a (x3 × x3)R30° phase, and a “complex” phase. The (5 × 5) phase, with a LEED pattern having the appearance of a “split-spot” (x3 × x3)R30° pattern, is interpreted in terms of local (x3 × x3)R30° ordering within a (5 × 5) ordered domain structure. The S atoms in the (5 × 5) phase occupy fcc hollow sites 1.56 Å above the outermost extended Au(111) bulk atomic scatterer plane. A specific model of the ordering in this phase is proposed that, together with the observed marginal stability of the true, long-range-ordered, (x3 × x3)R30° phase, indicates significant short-range S-S repulsion and probably compressive surface stress. The complex phase, that coexists in a poorly ordered state with the lower coverage atomic chemisorption phases, is interpreted in terms of an incommensurate long-range periodicity, but the NIXSW data shows clear evidence of local commensuration, with the S atoms mainly close to atop sites relative to the underlying Au(111) substrate; these data provide strong support for a previously proposed model based on a sulfide layer of stoichiometry AuS.

1. Introduction While gold is generally regarded as one of the least reactive metals, its resistance to atmospheric corrosion being one reason for its widespread use in jewelry and as an electrical contact, it also has some specific and very valuable surface reactions. Perhaps most remarkable is the fact that very small particles of gold do have strong reactivity and the potential for use in a range of applications in heterogeneous catalysis.1,2 In addition, however, extended gold surfaces, particularly Au (111)-textured thin films, are widely used as the substrate for alkylthiolate selfassembled monolayers, exploiting the particular interaction of Au with the S headgroup atom of these species (e.g., refs 3-5). As such, the interaction of Au(111) with atomic sulfur is also of interest and has attracted significant interest recently. 6-10 The earliest study of the Au(111)/S system was conducted through reaction of the surface with H2S and identified a large surface mesh phase at saturation coverage by qualitative lowenergy electron diffraction (LEED)11 that will hereafter be referred to as the “complex” phase. Through the use of a radioactive tracer technique, this work provided a rather accurate surface coverage for this phase of 0.51 ML. This same phase has been seen more recently by LEED and scanning tunneling microscopy (STM) as a result of reaction with SO2 and has been proposed to be associated with a surface sulfide layer of * Corresponding author. E-mail: [email protected]. † University of Warwick. ‡ CONICET and Centro Ato ´ mico Bariloche. § University of Nottingham. # CCLRC Daresbury Laboratory.

stoichiometry AuS.9,10 This same method of S deposition was reported to lead to a (x3 × x3)R30° phase, identified by the LEED pattern, although STM showed only the (1 × 1) periodicity of the underlying Au(111) substrate.8 By contrast, an Au(111)(x3 × x3)R30°-S phase has been imaged by STM at an electrochemical interface with the S deposited from a solution containing Na2S.7 This electrochemical interface investigation also revealed STM images at higher coverage attributed to the presence of S8 “octomers” (or S8 surface structures12). These images are very similar to those reported for the “complex” phase prepared under ultrahigh vacuum (UHV) conditions and attributed to the AuS layer. One further recent UHV study involved the use of high-resolution core level photoemission, LEED, and thermal desorption from Au(111) subjected to exposure to S2 molecules from a solid-state electrochemical source and was combined with density functional theory (DFT) calculations to determine minimum energy geometries.6 These calculations indicated that at low coverages, atomic S occupies the fcc hollow sites (the 3-fold coordinated sites directly above third layer Au atoms), but for coverages in excess of 0.40 ML, S2 is more stable on the surface than atomic S. These authors also conclude that for coverages in excess of 1 ML, S8 species are present on the surface. Evidently, there are several areas of disagreement in these earlier studies, at least some of which may well be attributable to variations in the extent to which the initial Au(111) surface shows the ideal herringbone reconstruction. It is known that quite low S concentrations lift the reconstruction but almost certainly lead to an inhomogeneous surface. Indeed, coexistence

10.1021/jp072088+ CCC: $37.00 © 2007 American Chemical Society Published on Web 06/29/2007

Structure of Atomic Sulfur Phases on Au(111)

J. Phys. Chem. C, Vol. 111, No. 29, 2007 10905

Figure 1. LEED patterns recorded at an energy of 62 eV from (a) the “split-spot x3” or (5 × 5) phase and (b) from the true (x3 × x3)R30° phase.

of different states of the S-covered surfaces are clearly indicated by the published photoemission data6 and are a key aspect of the conclusions of the present work. Despite these several previous investigations of the Au(111)/S system, however, none of them have included any experimental quantitative structure determination. Here we present the results of an investigation specifically aimed at providing this structural information through the use of the normal incidence X-ray standing wavefield absorption (NIXSW) technique. All experiments were performed under UHV conditions using a solid-state electrochemical S2 source to deposit the sulfur. These measurements are complemented by qualitative LEED and STM measurements, as well as X-ray photoelectron spectroscopy (XPS). On the basis of our results, we provide structural information on three distinct ordered phases of atomic S on Au(111). At the lowest coverages (∼0.25 ML), the LEED observations show a new (5 × 5) phase characterized by a diffraction pattern showing groups of three 1/ order beams symmetrically positioned around the locations 5 expected for the 1/3 order beams of a (x3 × x3)R30° ordering.13 This phase is rather stable and is associated with S atoms chemisorbed in 3-fold coordinated surface sites; a specific model of the ordering, involving a self-organized array of local (x3 × x3)R30° “islands”, is shown to be consistent with all the experimental data. At the highest coverage (∼0.5 ML), data from the “complex” phase, observed in earlier studies, are shown to be consistent with the monolayer (AuS) sulfide layer model recently proposed, in which most of the S atoms occupy nearatop sites relative to the Au atoms in the underlying outermost unreconstructed Au(111) layer. At only a slightly lower coverage than that of the complex phase, we observe the (x3 × x3)R30° LEED pattern but find this ordering to be unstable to heating and conclude, on the basis particularly of the NIXSW data, that a true long-range-ordered (x3 × x3)R30° phase only exists as a minority phase in coexistence with the “complex” phase. 2. Experimental Details The NIXSW experiments were performed at the double crystal monochromator beamline 4.2 of the Synchrotron Radiation Source at the CLRC’s (Central Laboratories for the Research Councils) Daresbury Laboratory. This beamline, described in detail elsewhere,14-16 is fitted with a pair of InSb(111) diffracting crystals and a surface science end-chamber equipped with the usual in situ sample preparation and

characterization facilities. A concentric hemispherical analyzer (with the entrance lens at 40° to the incident photon beam in the horizontal plane) was used to measure the energy distribution curves (EDCs) of photoemitted electrons at fixed pass energies. The Au(111) crystal sample was cleaned in situ by the usual combination of cycles of (1.0 keV) argon ion bombardment and flash annealing to 825 K, followed by a longer (20 min) anneal at 800-825 K. This treatment produced a well-ordered surface showing the (22 × x3) rect. LEED pattern characteristic of the herringbone reconstruction, devoid of impurities as assessed by the synchrotron radiation XPS and Auger electron spectroscopy. In the NIXSW technique,17 an X-ray Bragg reflection is established in the underlying crystal through interference of the incident and Bragg-reflected waves. The location of the adsorbate atoms in the X-ray standing wavefield is then determined by measuring the X-ray absorption variation at the adsorbate atom, as the X-ray energy is varied, and the standing wavefield sweeps through the adsorbate atom. This allows one to obtain quantitative information on the location of the adsorbate atoms relative to the underlying substrate. NIXSW measurements require that the X-ray absorption at the S atoms is monitored as a function of X-ray energy as this energy is scanned through the Bragg reflection at normal incidence to the Bragg scatterer planes. NIXSW data using the (111) scatterer planes, parallel to the surface (at a nominal photon energy of 2640 eV), provide a measure of the S adsorbate positions perpendicular to the surface. Additional measurements, using scatterer planes inclined out of the surface, provide a means of determining the lateral position of the S adsorbate atoms by triangulation and thus a determination of the adsorption site; in the present case (1h11) and (200) normal incidence Bragg reflections at nominal energies of 2640 and 3047 eV were used for this purpose. All NIXSW measurements were made at a sample temperature of ∼160 K to reduce the effects of thermal vibrations; the lower temperature also leads to slightly higher energies for the Bragg conditions due to the thermal contraction of the gold lattice. In order to monitor the photon energy dependence of this X-ray absorption at the S atoms, measurements were taken of the S 1s photoemission, in the form of narrow-range (20 eV) energy distribution curves (EDCs), at each photon energy as this was stepped, in 0.2 eV increments, over a 14 eV range covering the standing-wavefield condition. Each of these EDCs was fitted to a peak and a background, and the variation of the peak intensity with photon energy, normalized

10906 J. Phys. Chem. C, Vol. 111, No. 29, 2007 to the average values on either side of the standing wavefield region, provided a measure of the relative photoabsorption at these atoms. Similarly normalized plots of the background intensity versus photon energy provided a measure of the Au substrate NIXSW absorption profile. The STM experiments were conducted at room temperature in a different UHV surface science chamber, located at the University of Warwick, using an Omicron Vakuumphysik “µSTM” mounted on a spring and magnetic eddy current damping system. Electrochemically etched polycrystalline W tips were used, conditioned in situ by Ar+ ion bombardment and annealing to 770 K. These were then subjected to field desorption (-300 V sample bias) against a clean Ag surface and to gap voltage pulses (×10) during scanning, in order to clean the tip or to modify its condition to alter the resolution obtained, as discussed below. All images presented here were collected in the constant tunneling current mode. The sample cleaning method was similar to that used in the chamber at Daresbury Laboratory, but in this case in addition to observing the LEED pattern, STM was used to check that the surface after cleaning revealed a well-ordered herringbone reconstruction. In both chambers the sulfur overlayers were prepared by exposure to the S2 flux provided by a solid-state Pt/Ag/AgI/ Ag2S/Pt electrochemical cell. The cell was operated with an applied potential of 200 mV at a temperature of 200 °C, the output being checked in situ by mass spectrometry. 3. Results 3.1. Phase Characterization: LEED, STM, and XPS. Qualitative LEED studies of the Au(111) surface as a function of increasing S coverage revealed three distinct long-range ordered structures. Two of these, the (x3 × x3)R30° phase, with a nominal expected coverage of 0.33 ML, and the “complex” phase at higher coverage, previously calibrated to be 0.51 ML, have been reported previously.8,11 We note, however, that none of the earlier reports have included photographs of the (x3 × x3)R30° LEED pattern in the publications (despite showing the (1 × 1) LEED pattern associated with a S coverage of ∼0.1 ML8); this may, or may not, be significant in the light of our own results described below. In particular, at lower coverages than those associated with the (x3 × x3)R30° and “complex” LEED patterns, we observed a new ordered phase in which groups of three diffracted beams appear centered on the 1/3 order locations characteristic of the (x3 × x3)R30°. Figure 1 shows a comparison of this “split-spot x3” phase LEED pattern with that from the true (x3 × x3)R30° phase. One important feature of this lower coverage phase is that the locations of the superstructure beams (i.e., the magnitude of the splitting relative to the 1/3 order locations) was always the same in many preparations with somewhat different S coverages, and specifically these beams are located at, or very close to, the exact 1/5 order positions. As such, they are effectively a subset of the diffracted beams expected from a (5 × 5) unit mesh, and for this reason we will hereafter refer to this as a (5 × 5) phase. Somewhat surprisingly, this (5 × 5) phase was significantly more stable than the (x3 × x3) phase. Both phases were prepared by deposition of S2 onto the Au(111) crystal at room temperature but then were quickly cooled to low temperature (∼160 K) for all subsequent measurements. Heating the (x3 × x3) phase to ∼330 K was sufficient to irreversibly destroy the ordering, while annealing to ∼450 K led to the appearance of the “complex” phase LEED pattern. Moreover,

Yu et al.

Figure 2. Comparison of the LEED pattern from the “complex” phase, recorded at an electron energy of 85 eV, with a simulation of this pattern using the LEEDpat2 computer program.18 In the upper half of the figure, the real space mesh and the associated reciprocal mesh of this single rotational domain are shown. These simulated patterns show only the relative positions of diffracted beams and not their relative intensities. The larger spots identify the (1 × 1) beams of the substrate.

even at ∼160 K the order in this phase, as judged by the LEED pattern, deteriorated progressively under the incident electron beam, while at room-temperature this type of deterioration was seen even in the absence of the electron beam. By contrast, the (5 × 5) phase was stable in the electron beam at low temperature and was only irreversibly destroyed by heating to ∼670 K. The quality of the order of the (5 × 5) phase did deteriorate somewhat with time at room temperature, and this effect was accelerated by modest heating (to temperatures below ∼670 K), but the good order was restored by recooling the sample to ∼160 K. Recooling a disordered (x3 × x3) phase did not restore the good order of the original preparation. By contrast, the “complex” phase LEED pattern was made much sharper by annealing the sample to ∼450 K and was then extremely stable at room temperature over periods of days. This behavior clearly indicates that there is some energetic barrier to the formation of this phase, consistent with the surface reconstruction that would be required to form a surface sulfide phase as has been proposed.9,10 The LEED pattern from the “complex” phase is shown in Figure 2, together with a simulation of the pattern based on a unit mesh essentially identical to that suggested in the original study that reported this phase.11 In the matrix notation, the unit 0.33 -2.67 . As shown in Figure mesh shown in Figure 2 is 3.33 2.33 2, the simulated LEED pattern for this mesh reproduces almost all (but not all) of the beams seen in the experimental diffraction pattern. Strictly, with each component of this matrix being a rational fraction, this would imply a true commensurate mesh 1 -8 . This much of dimensions (3 × 3) times larger, i.e., 10 7 larger mesh would, of course, lead to a greatly increased number of diffracted beams, but many of them may have a low intensity. In particular, if the long-range periodicity is actually a consequence of a complex ordering of a smaller structural unit, due

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Structure of Atomic Sulfur Phases on Au(111)

J. Phys. Chem. C, Vol. 111, No. 29, 2007 10907 TABLE 1: XPS Chemical Shifts Observed for Different States of S on Au(111) in the Present Investigation and in the Earlier Study by Rodriguez et al.6,a Rodriguez et al.

this work

S 1s KE BE shift S 2p3/2 BE BE shift (eV) (eV) name/phase (eV) (eV) name/phase 160.8 161.6 163.4

0.8 2.6

S hollow S Sn

529 528 526

1 3

(5 × 5) complex multilayer

a For the present work the photoelectron kinetic energies (as shown in Figure 3) are given at a nominal photon energy of 3000 eV. No absolute calibration of the conversion to binding energy was undertaken, but the relative scale should still yield meaningful chemical shifts.

Figure 3. S 1s X-ray photoelectron energy spectra recorded at a nominal photon energy of 3 keV from several different surface preparations of Au(111)/S phases.

to a regular pattern of antiphase domain boundaries, only a small subset of the extra beams may have significant intensity. We will show below that this type of behavior can account for the observed LEED pattern of the (5 × 5) phase, but building a more detailed model of the “complex” phase is far more challenging. On the basis of the LEED pattern (and the STM data) alone, it is not really clear whether or not the “complex” phase is commensurate but, as will be discussed below, the NIXSW data from this phase does appear to indicate commensuration. Some further characterization of these phases was provided by S 1s XPS recorded using 3 keV synchrotron radiation during the NIXSW experiments. A representative set of these spectra, normalized to the incident photon flux, is shown in Figure 3. Two aspects are of note, namely, the relative intensities and photoelectron binding energies. If we accept the previous calibration of the S coverage of the “complex” phase as 0.51 ML, then the coverage in the (5 × 5) phase at approximately half this value is ∼0.25 ML. Notice, though, that the coverage corresponding to the (x3 × x3) LEED pattern is only slightly lower than that of the “complex” phase; indeed, one preparation of the (x3 × x3) phase shown in Figure 3 appears to have a slightly higher coverage than the “complex” phase. By contrast, we would expect a perfect (x3 × x3) phase to have a coverage of 0.33 ML. We will return to this apparent discrepancy later. Also shown in Figure 3 are some spectra recorded following much higher S2 exposures to the sample with intensities that clearly imply coverages of more than a single atomic or molecular layer. The objective of including these high-coverage measurements was to provide a connection to the earlier high-resolution S 2p XPS investigation of Rodriguez et al. .6 These authors identified three distinct chemically shifted states of S on the surface (Table 1). At low coverages, the dominant peak had a S 2p3/2 photoelectron binding energy of 160.8 eV, while a second peak at a binding energy of ∼161.6 eV became significant at slightly higher coverages, although both peaks were found to be present at all coverages. A third component with a binding energy of ∼163.4 eV dominated at high (approaching 1 ML) coverages. The first two components were both attributed to atomic S on the surface, the lowest binding energy state being labeled “S hollow” by these authors who attributed this peak to S atoms adsorbed in the 3-fold coordinated hollow sites favored by their

Figure 4. Representative STM images of the clean Au(111) surface (top) and the “complex” S phase (bottom) on this surface. Imaging conditions (sample voltage and tunneling current): (a) -1.32 V, 1.10 nA; (b) -0.43 V, 1.10 nA; (c) -1.71 V, 1.06 nA; (d) -1.28 V, 1.34 nA. Representative unit meshes are superimposed on the images of the complex phase. The images are not corrected for any effects of thermal drift.

DFT calculations of the chemisorption. No specific attribution other than “S” was given to the second component. The third peak was believed to be associated with molecular Sn and labeled accordingly. Note that this earlier work reported coverages of up to 1 ML, with the significant presence of the Sn component, even at 300 K. Although these authors show three-(doublet)-component fits to their S 2p photoemission spectra, the energies of the three components do not appear to have been constrained to constant values, and the binding energies shown in Table 1 and mentioned in the text correspond to the values they quote for “small doses of S2”. While our S 1s spectra of Figure 3 are certainly not high resolution, clear binding energy shifts are seen with increasing coverage, indicative of multiple components. Relative to the dominant peak at the lowest coverages, the chemical shifts of the second and third components in the study of Rodriguez et al. are 0.8 and 2.6 eV, and the data of Figure 3 indicate a shift of ∼1 eV in going from the (5 × 5) phase to the “complex” and (x3 × x3) phases and ∼3 eV to the multilayer (Table 1). No attempt to fit these low-resolution data to three component peaks was made, so these shifts are indicative rather than exact. On the basis of these values, however, it seems that the first and third peaks seen by Rodriguez et al. can clearly be assigned

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Figure 5. Large-area STM images of the Au(111) surface after the formation of the “complex” surface phase, showing the appearance of single atomic layer islands of vacancies and adatoms. Imaging conditions (sample voltage and tunneling current): (a) -1.45 V, 1.06 nA; (b) -1.40 V, 1.06 nA.

to the (5 × 5) and multilayer phases, whereas the intermediate peak appears to be associated with both the “complex” and (x3 × x3) phases (or some mixture of them). On the basis of the NIXSW results reported below, we will actually assign this intermediate component specifically to the “complex” phase. We note that Rodriguez et al. actually associated the lowest binding energy “S hollow” state with the (x3 × x3) phase, as identified by the presence of this ordered LEED pattern, but they also show that the coverage associated with this LEED pattern corresponds to a substantial component of the second “S” state being present. This is consistent with our own results (Figure 3), but we will show that although we can, indeed, attribute the lowest binding energy peak to S atoms chemisorbed in hollow sites, the presence of the (x3 × x3) LEED pattern does not mean that such simple chemisorption is the dominant form of S on the surface. Note that our preparations of the “complex” phase, the highest coverage phase investigated in detail by us, always involved annealing to a temperature above that (∼400 K) at which Rodriguez et al. report desorption of S2, so our investigations of this phase clearly do not correspond to the molecular phase seen at high coverage by these authors. All STM measurements were made at room temperature. The results of these measurements are closely similar to the earlier UHV STM study8,9 in which S deposition was achieved by reaction of the Au surface with SO2. Specifically, the results showed loss of the herringbone reconstruction of the clean surface at low S coverages but no images showing anything other than (1 × 1) periodicity prior to the formation of the complex phase. Figure 4 shows images of the clean surface at two different magnifications, clearly showing the herringbone reconstruction, together with two images of the “complex” phase under different tip conditions. The periodicity displayed in these images of the “complex” phase is consistent with the 8.2 Å × 8.6 Å unit mesh used in Figure 2 to simulated the LEED pattern from this surface. The failure to achieve STM images showing the (x3 × x3) periodicity was tentatively attributed by Min et al.8 to high mobility of the S atoms in this phase at room temperature. Our failure to find evidence in the STM images for the (5 × 5) phase seems likely to have the same origins. This interpretation is reinforced by our observation that the sharpness of the LEED patterns of both of these phases deteriorated at room temperature, although for the (5 × 5) phase this process was reversible on recooling. One further feature of the STM data is the appearance of single atomic layer holes and islands that accompany the formation of the “complex” phase, a clear indication that there is a change in Au atomic density within one or more of the outermost layers. Figure 5 shows two such images. Figure 5a shows a region with large holes or vacancy islands, very similar

Figure 6. Representative experimental NIXSW absorption profiles recorded for both the S adsorbate atoms (9) and the Au substrate atoms (/) together with the theoretical best fits (full and dashed lines). The associated structural parameter values are given in Table 2.

to images presented previously by Biener et al.,9 while Figure 5b shows a somewhat different region showing similar large vacancy islands but with adatom islands superimposed both on the upper terrace and in the bottom of the vacancy islands. In both cases, the height difference between the various levels corresponds closely to the interplanar spacing of the Au(111) substrate. This behavior is clearly consistent with a major reconstruction of the surface to produce a layer having a muchreduced density of Au atoms relative to that of a bulk substrate layer, the rejected Au atoms giving rise to the islands. These are precisely the properties of the previously proposed singlelayer AuS structure10 3.2. NIXSW Local Structure Determination. Figure 6 shows representative NIXSW absorption profiles recorded from the (5 × 5), (x3 × x3) and “complex” phases. These experimental NIXSW profiles were analyzed using the XSWfit automated fitting procedure19 to yield two structural parameters: 17 the coherent position d (where H specifies the Miller indices H of the scatterer planes) and the coherent fraction fco. Note that dH is defined in units of the bulk interlayer spacing of the scatterer planes, DH. In the simplest case of an absorber occupying a single well-defined site, dH is equal to the perpendicular distance of this site from the scattering planes, while fco is a measure of the degree of local order. In particular, fco can only take values between 0 and 1; low values arise from dynamic (thermal vibrations) or static local disorder or from

Structure of Atomic Sulfur Phases on Au(111)

J. Phys. Chem. C, Vol. 111, No. 29, 2007 10909

TABLE 2: Summary of NIXSW Structural Parameter Values for the Three Phases of Atomic S on Au(111) as Identified by Their LEED Patterns (x3 × x3)

(5 × 5) (111) (1h11) (200)

“complex”

dH

fco

dH

fco

dH

fco

0.73 ( 0.03 0.88 ( 0.03 0.74 ( 0.03

0.50 ( 0.10 0.58 ( 0.05 0.80 ( 0.05

0.04 ( 0.03 0.31 ( 0.03 0.71 ( 0.03

0.43 ( 0.07 0.35 ( 0.07 0.55 ( 0.08

0.05 ( 0.03 0.31 ( 0.03 0.70 ( 0.03

0.88 ( 0.05 0.57 ( 0.05 0.41 ( 0.08

multiple site occupation. The shape of the profiles is also influenced by two nonstructural parameters, the Gaussian instrumental broadening ∆E (mainly due to the finite resolution of the monochromator) and the absolute energy of the Bragg reflection EB. These parameters can be determined by fitting the substrate standing wave profile and are then fixed for the analysis of the adsorbate absorption profiles, which are then fitted by only adjusting the structural parameters. The coherent position and coherent fraction values used to achieve the bestfit theoretical curves of Figure 6 are summarized in Table 2. Perhaps the most striking feature of the parameter values of Table 2 is the fact that the coherent positions of the (x3 × x3) phase are essentially identical to those of the “complex” phase, and quite different from those of the (5 × 5) phase. A (x3 × x3) phase is expected to be associated with just one adsorbed S atom per surface unit mesh in identical local sites, most probably of high symmetry (such as the hollow sites favored by DFT calculation6), whereas the large unit mesh of the complex phase must involve several distinct local adsorption geometries relative to the surface, and may even be incommensurate. It is therefore very difficult to understand how the NIXSW structural parameters of these two phases can be the same. There are, however, several observations that indicate that the observation of the (x3 × x3) LEED pattern may not indicate that the associated surface is dominated by a true (x3 × x3) ordered surface phase. First, we have already noted that the (x3 × x3) phase is not truly stable at, or slightly above, room temperature. Second, careful inspection of the associated (x3 × x3) LEED pattern shows a diffuse ring of higher intensity through the 1/3 order diffracted beams, a region of the diffraction pattern in which the “complex” phase displays many diffracted beams, and annealing such a surface to ∼450 K led to the evolution of a sharp “complex” phase LEED pattern. In addition, we have already noted that surface preparations showing the (x3 × x3) LEED pattern actually have a S coverage and a S 1s photoelectron binding energy

closely similar to that of the “complex” phase. Finally, we note that LEED, like all crystal diffraction techniques, shows selectively the diffraction pattern of those regions of a surface having the best long-range order. It is well-established that even if only a few percent of a surface has good long-range order, the LEED pattern will be dominated by this component of the surface. Such a situation leads to some enhancement of the diffuse background, but the amplitude of this background is much lower than that of the diffracted beams. We therefore conclude that surface preparations displaying the (x3 × x3) LEED pattern are actually dominated by poorly ordered regions of the “complex” phase; LEED picks out the locally ordered (x3 × x3) phase regions, but NIXSW simply averages over the whole surface, so the NIXSW structural parameters are dominated by the majority “complex” phase. We now consider the NIXSW data from the (5 × 5) phase. Notice that the large unit mesh of this phase might lead us to expect that it involves multiple adsorption sites, but as we shall show in the following section, the LEED pattern can be reconciled with a model involving partial occupation of identical sites. However, we first consider the NIXSW data in isolation. A general property of this technique is that, in situations in which the coherent positions can be directly related to single adsorbate-substrate interlayer spacings, the measurements at different scatterer planes can be used to triangulate the adsorption site. The three highest symmetry (3-fold rotational) adsorption sites on fcc (111) surfaces (atop, hcp hollow, fcc hollow) correspond to positions atop first, second, and third layer substrate atoms. If the adsorbate-outermost substrate layer spacing is z111, one can therefore regard these three sites as corresponding to atop sites with effective layer spacings of z111, z111 + D111, and z111 + 2D111. The (1h11) planes are inclined at an angle of 70.5° to the (111) surface planes, so these sites than are spaced above the (1h11) planes by z111/3, (z111 + D111)/3, and (z111 + 2D111)/3 where the factor 1/3 is simply cos 70.5°. Figure 7 provides a simple schematic view of this (1h11) triangulation for these three high-symmetry adsorption sites. For the (200) planes the tilt angle is 54.7°, so the multiplying factor turns out to be 1/x3. Bearing in mind that the dH values are the ratio of the actual distance and DH, if the (111) coherent position is 0.73 (as in Table 2 for the (5 × 5) phase), one can readily calculate the expected (1h11) and (200) coherent position values for the three high-symmetry adsorption sites. These are shown in Table 3; the experimental values are clearly consistent with fcc hollow site occupation. TABLE 3: Comparison of the Experimental NIXSW Coherent Position Values for the (5 × 5) Phase with Theoretical Triangulation for the Three High-Symmetry Adsorption Sites, Assuming the Experimental Value of d(111) (Shown in Italic)

Figure 7. Schematic diagram of a side-view of the Au(111) surface with S atoms in the three high-symmetry adsorption sites showing how these are distinguished by NIXSW measurements at both (111) and scatterer planes. Note that S atoms in the atop, hcp, and fcc sites are atop Au atoms in the first, second, and third substrate layers, respectively.

dH

experiment

theory atop

theory hcp hollow

theory fcc hollow

(111) (1h11) (200)

0.73 ( 0.03 0.88 ( 0.03 0.74 ( 0.03

0.73 0.24 0.48

0.73 0.58 0.15

0.73 0.91 0.82

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This simple analysis, however, neglects the fact that the coherent fraction values for the surface characterized by the (5 × 5) LEED pattern, even for the (111) reflection, are too low to be consistent with a single adsorption site. To understand the consequences of multiple adsorption sites we note that the coherent fraction and coherent position can be related to the spatial distribution of the absorber atoms relative to the nearest scatterer plane f(z), defined by the spacing coordinate z, and the following equation.17

fco exp(2πidH) )

∫0d f(z) exp(2πiz/DH)dz H

(1)

The parameters fco and dH thus define the amplitude and phase of one Fourier component of the absorber site distribution projected along one direction (perpendicular to the relevant Bragg scatterer planes). The left-hand side of this equation can be represented as a vector in an Argand diagram with length fco and direction determined by the phase angle 2πdH relative to the positive real axis.20 The right-hand side of the equation is then a summation (integral) over component vectors of length f(z) and phase angle 2πz/dH. This interpretation is particularly useful in summing over discrete sites (when f(z) comprises a series of essentially discrete values). Notice that one consequence of this is that if one has two distinct sites which differ in their z values by DH /2, thus corresponding to opposing vectors in the Argand diagram, they may cancel one another. If they are equally occupied, the cancellation is perfect and the resulting coherent fraction (the length of the resultant vector) is zero. If the occupation is unequal, the resultant vector has the direction (and thus the coherent position value) of the majority state but its length (the coherent fraction) is greatly reduced. With this result in mind, we note that both the (111) and (1h11) NIXSW coherent fractions measured from the surface characterized by a (5 × 5) LEED pattern are rather low, and that the coherent positions for these two conditions differ from those of the “complex” phase by 0.68 and 0.57, respectively. By contrast, for the (200) NIXSW, the coherent fraction for this surface is quite high but the coherent position in this case is almost identical to that for the “complex” phase. This means that if the preparation of the (5 × 5) phase, like that of the nominal (x3 × x3) phase, also leads to some fraction of the surface having poorly ordered regions of the “complex” phase, and the presence of this second phase could account for the experimental coherent fractions values. Such an admixture would cause no reduction of the (200) coherent fraction of the (5 × 5) phase, would lower the (1h11) coherent fraction with very little change in the coherent position, and would similarly lower the (111) coherent fraction but produce a modest change in the associated coherent position, because the two contributing Argand vectors are less exactly opposed. To develop this model in a more quantitative fashion, we assume that in the nominal (5 × 5) phase preparations, a fraction x if the surface is locally in the complex phase, while the remaining fraction (1 - x) consists of patches of “pure” (5 × 5) phase. The measured NIXSW parameters in this case, fco and dH, can be expressed, through minor modification of eq 1, in the form

fco exp(2πidH) ) {x fcomplex exp(2πidH(complex)) + (1 - x)f(5×5) exp(2πidH(5×5))} (2) which can be represented in an Argand diagram as a sum of two vectors, in each case the direction being determined by the phase factor (and thus the phase-specific coherent position) and

Figure 8. Argand diagram representation of the (111) NIXSW from the nominal (5 × 5) phase showing the influence of 30% co-occupation of the “complex” phase on the true structural parameters for the (5 × 5) phase.

the length by the prefactor. To constrain the solution, we assume that the true coherent fraction and coherent position for the “complex” phase are as measured and reported in Table 2; i.e., we assume that the preparations of the “complex” phase are not significantly contaminated by any other phase. Bearing in mind that the complex phase is, according to our characterization, the most stable one, this assumption appears reasonable. We further assume that the true coherent fraction for the pure (5 × 5) phase, f (5×5) lies between 0.8 and 1.0, the approximate range expected for a well-ordered phase containing only adsorbate atoms in or near high-symmetry adsorption sites. Figure 8 illustrates the results of this procedure for the (111) NIXSW parameter components. Labeled as “resultant” is a vector in the Argand diagram corresponding to the experimentally measured NIXSW structural parameter values for the (5 × 5) phase of the left-hand side of eq 2). The two component vectors of Figure 8 correspond to the two terms on the righthand side of eq 2, one being based on the experimental NIXSW parameters for the “complex” phase (with only x as an adjustable variable), while the other represents the unknown quantities, the true NIXSW parameters for the pure (5 × 5) phase. The solution shown in Figure 8 then corresponds to a surface coverage of 70% pure (5 × 5) and 30% “complex”. Notice that the true (5 × 5) d(111) value obtained in this way is 0.66, somewhat smaller than the value (0.73) obtained in the experiment from what we now identify as a mixed phase. This value is found to be very insensitive to other possible solutions involving small changes in x and f(5×5), with variations of only 0.01-0.02. A similar analysis of the (1h11) and (200) NIXSW parameter components is found to be consistent with this same solution. We may therefore infer that the NIXSW data from the (5 × 5) phase is consistent with occupation of only fcc hollow sites by the S atoms but that even for this phase some co-occupation of the “complex” phase occurs. Further support for the interpretation in terms of mixed phases comes from the high-resolution S 2p photoemission data of Rodriguez et al.6 if we assign the two distinct atomic S states they identify as being associated with the hollow site adsorption and the “complex” phase, respectively (as in Table 1). For the (5 × 5) phase, the NIXSW data thus leads us to a clear conclusion: that the S atoms occupy fcc hollow sites at a S-Au interlayer spacing of 1.56 Å, deduced from the “true” d(111) value of 0.66. Strictly, this layer spacing is relative to the nearest extended bulk scatterer plane, and not to the outermost Au layer, but on a fcc

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Figure 9. Simulation of (5 × 5) LEED pattern for two S atoms per unit mesh in different relative positions. The open circles represent possible equivalent sites relative to the underlying Au(111) surface. The dark-shaded circles show the locations of the first S atom per (5 × 5) unit mesh. The light-shaded circles show one example of (all equivalent) b-type second S atoms in the (5 × 5) unit mesh. All simulated patterns are averaged over the three rotationally equivalent domains. The size of the spots in the simulated patterns reflects their relative intensities.

metal (111) surface any net relaxation of the outermost AuAu layer spacings can be expected to be very small (∼0.010.02 Å or less), so the value of 1.56 Å should be very close to the true nearest-neighbor S-Au interlayer spacing. Why these S atoms adsorbed in high-symmetry hollow sites should lead to a (5 × 5) ordering will be discussed in the following section. The further implication of this evaluation of the NIXSW data is that no useful structural information is available for the (x3 × x3) phase, because the surface preparation showing this LEED pattern actually comprises mainly the (poorly ordered) “complex” phase; this superficially surprising conclusion is a timely reminder of the ability of a LEED pattern to be dominated by a minority phase if this is the only one with good long-range order. The corollary is that the NIXSW data from the “complex” phase are meaningful. One striking feature of these data is the relatively high coherent fractions. The very high value of the coherent fraction for the (111) NIXSW clearly indicates that all S atoms in this phase are at almost exactly the same height above the underlying substrate. Perhaps even more surprising is that the (1h11) and (200) coherent fractions appear to be very significantly greater than zero; for a planar S layer that is incommensurate, these values should both be identically zero. The implication would appear to be that locally most of the S atoms are in similar sites relative to the substrate, although the true long-range periodicity may be incommensurate. The STM images, of course, clearly show some relatively small motif that is repeated in a complex fashion, so we may infer that within this motif there is a relatively simple registry. Simple triangulation of the NIXSW coherent positions for this phase, ignoring the fact that the (1h11) and (200) coherent fractions are too low for a single high-symmetry adsorption site, would imply the S atoms are close to atop sites; a height above the substrate surface of 1.05D(111) ) 2.48 Å would lead to (1h11) and (200) coherent position values of 0.35 and 0.70, to be compared with the experimental values of 0.31 and 0.70. The reduced coherent fraction values, however, clearly indicate that the true structure must be more complex, a conclusion clearly consistent with the long-range periodicity seen in both LEED and STM. 4. Discussion and Structural Models 4.1. The (5 × 5) and (x3 × x3) Phases. So far we have made no attempt to discuss the origin of the fact that while the LEED pattern of the (5 × 5) phase shows 1/5 order beams in addition to those expected from the underlying (1 × 1) substrate,

only those extra beams that lie close to the 1/3 order positions have significant intensity. The appearance of this “split-spot” (x3 × x3) LEED pattern clearly suggests there may be some relationship to a formal (x3 × x3) phase. In general, spot splitting of this type can be attributed to one of two processes, namely, a so-called compression structure or a structure involving periodic antiphase boundaries. This type of LEED pattern is well-known, for example in some CO adsorption structures, and these two alternative interpretations have been discussed extensively (e.g., refs 21 and 22). True compression structures are typically expected when the adsorbate-adsorbate (repulsive) interaction is much stronger than the amplitude of the adsorbatesubstrate bonding corrugation. They are expected to occur at high coverages and should lead to a continuous range of splitting with changing coverage, many of the associated overlayer structures being incommensurate. In the present case all preparations led to the same splitting, with the extra beams always at (or very close to) 1/5 order positions, so this explanation is inconsistent with the present data. The alternative explanation is periodic antiphase domain boundaries; within the domains, the local ordering is (x3 × x3), but the domain sizes in the present system must always correspond to a (5 × 5) mesh. Interestingly, a rather similar LEED pattern has been reported for CO on Pt(111)23 at a coverage of ∼0.17 ML, but while higher coverage phases in this adsorption system have been interpreted in terms of antiphase domain boundaries, this low-coverage structure seems never to have been determined. To evaluate this model, we have simulated the relative intensities of the “extra” LEED beams using a simple kinematical theory for a single layer of S adatoms, treating each atom as an isotropic scatterer. While it is well-known that multiple scattering (or so-called dynamical theory) is essential to describe the energy dependence of LEED beam intensities, it is also known that the average relative intensities of the beams (i.e., the appearance of the LEED pattern) can be quite effectively simulated by a single scattering description. We start with a model involving a single S atom per (5 × 5) unit mesh (leading to a LEED pattern in which all the 1/5 order beams have similar intensity) and consider the consequences of adding just one additional S atom per (5 × 5) unit mesh to a second site with identical substrate registry to the first S atom. The results are shown in Figure 9. The circles represent sites with identical substrate registry (the substrate is not included in the calculation and not shown in the figure) and within one (5 × 5) unit mesh,

10912 J. Phys. Chem. C, Vol. 111, No. 29, 2007

Yu et al.

Figure 10. Structural model for the (5 × 5) phase at the highest coverage (0.28 ML) of S atoms in next-nearest neighbor sites leading to local (x3 × x3) domains and the resulting simulated LEED pattern. The S atoms are shown in equivalent hollow sites on the underlying Au(111) surface. The large circles superimposed on the structure highlight the seven-atom “rosette” character of the ordering.

four distinct types of sites are identified; sites labeled “a” are nearest neighbors to the occupied sites at the corner of the marked unit mesh, “b” sites are next-nearest (second-nearest) neighbors, “c” sites are third-nearest neighbors, and “d” sites are fourth-nearest neighbors. The LEED simulations (summed over three rotational domains rendered equivalent by the 3-fold rotational symmetry of the substrate) show that with a second S atom in any one of the “b” sites, the simulated LEED pattern already has the 1/5 order beams centered on 1/3 order locations dominant in intensity. Adding further S atoms into “b” sites further enhances these diffracted beam intensities at the expense of the other “extra” 1/5 order beams. Notice, of course, that the next-nearest neighbor site occupation is a characteristic of the local ordering in a (x3 × x3) structure. Figure 10 shows the maximum coverage that can be achieved in this way, with seven S atoms per (5 × 5) unit mesh, corresponding to a coverage of 0.28 ML. The simulated LEED pattern is in excellent agreement with the experimental results, as is the coverage. We should note that our simulations of the LEED pattern show that some fractional occupation of sites other than those labeled “b” does not substantially change the LEED pattern, so it is certainly possible that the ordering is less perfect than implied by Figure 10. Nevertheless, this maximum coverage structure within this mode, which leads to characteristic “rosettes” of seven S atoms (highlighted by the superimposed circles), provides a rationale for the preferred (5 × 5) ordering. In particular, the marginal stability of the true (x3 × x3) structure and the preference for the structure of Figure 10 strongly suggests that S-S atom interactions in next-nearest neighbor sites on the Au(111) surface are probably slightly repulsive and that a true (x3 × x3) phase has a high degree of compressive surface stress. Indeed, on this basis, we might surmise that the S atoms at the edge of the rosettes may be located slightly off the fully symmetric hollow sites due to this repulsion; the NIXSW structural parameter values are not very sensitive to small lateral displacements of ∼0.1 Å or so. This model, of course, is also consistent with the NISXW analysis of section 3.2 that concluded that the S atoms in the (5 × 5) phase may all be in (essentially) identical fcc hollow sites at a distance above the nearest extended bulk Au(111) scattering plane of 0.66D(111) or 1.56 Å. If we assume that there is no net relaxation of the outermost Au(111) surface planes, this geometry implies a nearest-neighbor S-Au distance of 2.28 ( 0.04 Å. We may anticipate that the local adsorption

site in the (x3 × x3) phase is also the fcc hollow, although in view of the marginal stability, it seems likely that the adsorption energy at this higher coverage will be lower, and hence the bond length may be longer. However, the dominance of the “complex” phase coverages means that we have no information on this structure from the NIXSW measurements. These structural conclusions may be compared with the results of the DFT calculations of Rodriguez et al.6 Somewhat surprisingly, despite these authors observing the (x3 × x3) phase experimentally, their DFT calculations were for two different hypothetical ordered chemisorption phases, namely, a lower coverage (2 × 2) (0.25 ML) and a higher coverage (2 × 1) (0.50 ML). Interestingly, though, these calculations showed that while the fcc hollow site was favored at both coverages, the adsorption energy in the higher coverage phase was some 0.8 eV lower per S atom. The calculated S-Au bond lengths in these two phases were 2.39 and 2.45 Å, respectively. Our inferred experimental bond length is slightly (but significantly) shorter than these calculated values, although, like these theoretical values, it is larger than the value of 2.17 Å in bulk Au2S24 indicating a distinct difference in bonding from that in the bulk compound. 4.2. The “Complex” Phase. As discussed earlier, the “complex” phase is a major challenge for structure determination. The overall periodicity is very long-range and it is not even clear that it is truly commensurate. On the other hand, the STM images show there is a repeated internal structural motif of a size commensurate with a relatively small number of S atoms. Specifically, the near-rectangular 8.2 Å × 8.6 Å mesh proposed in section 3 would, for a coverage of 0.51 ML, contain four or five S atoms. The high NIXSW (111) coherent fraction indicates that the great majority of these S atoms must have closely similar heights above the outermost extended bulk Au(111) scatterer plane and a distance of 1.05D(111) or 2.48 Å. The particularly surprising result from the NIXSW results for this phase is that the (1h11) and (200) coherent fractions are both ∼0.5 or more, clearly implying that the S atoms do have a distinct local registry with the substrate, albeit probably in several different local sites. In view of the fact that the coherent positions triangulate almost perfectly to atop sites, and an interlayer spacing of 2.48 Å for atop sites implies a S-Au bond length of this value, within the range of expected bond lengths, it is certainly tempting to infer that most of the S atoms in the complex do occupy near-atop sites with respect to the underlying

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Au(111) substrate. Of course, we should note that NIXSW provides information on the location of the absorber atoms (in this case the S atoms) relative to the extended underlying bulk crystal but provides no information on the actual location of the outermost Au atoms in the surface. Thus, our conclusion that S atoms occupy near-atop sites relative to the underlying substrate does not imply that the surface comprises only a layer of atomic S atoms in such sites, such a structural model seems highly improbable. A specific model of the “complex” phase in terms of an AuS monolayer has been proposed by Quek et al.10 on the basis of DFT slab calculations and simulations of the previously reported STM images from this phase.9 On the basis of the isolated sulfide layer alone, they show some correspondence with aspects of the STM images but they also consider a model that includes the influence of the substrate, necessarily choosing a commensurate structure to allow the calculations to be performed. 3 0 mesh that they use has essentially the same The 1 3 dimensions as that implied by the LEED pattern and STM images but is not, in itself, consistent with the observed LEED pattern. The key difference from the mesh we propose in section 3.1 is a rotation of ∼4° relative to the underlying substrate. However, an interesting feature of this model is that the S atoms within the AuS layer are all close to atop sites. Our NIXSW data thus provide rather strong support for this model, although the fact that the structure is actually incommensurate (or has a significantly larger commensurate mesh) must imply that the sulfide layer is distorted in some way, perhaps by a regular array of antiphase domain boundaries. The implication is that the optimum lateral periodicity of the resulting sulfide layer is incommensurate with the substrate, but periodic distortions ensure that locally the registry is similar within motifs of ∼8.2 Å × 8.6 Å.

(x3 × x3) LEED pattern actually correspond to a surface covered mainly by this “complex” phase, with a coverage consistent with this interpretation. For the lower coverage (5 × 5) phase, the NIXSW data clearly identify the S adsorption site as the fcc hollow, directly above a third layer Au atom, with a height above the outermost extended substrate scatter plane of 1.56 Å. This preferred local site is consistent with the results of DFT calculations by Rodriguez et al. for a similar coverage but a different (hypothetical) ordering. The Au-S bond length inferred from our measurements, assuming no net relaxation of the outermost Au(111) surface layers, is 2.28 Å, some 0.10 Å shorter than that suggested by the DFT calculations. While the complex phase is apparently incommensurate, and so may be expected to have no distinct local registry, the NIXSW clearly indicates moderately good local registry. These data can be reconciled with the suggested structure of an AuS monolayer proposed by Quek et al.10 on the basis of DFT calculations and the previously published STM images, although some local distortion must occur to allow local commensuration of a fundamentally incommensurate structure. A particular feature of the AuS layer model consistent with the NIXSW data is a strong preference for the S atoms within the layer to occupy near-atop sites relative to the underlying outermost unreconstructed substrate layer. It is interesting to compare these results of UHV experiments (using different molecules, S2, SO2, H2S, to achieve the coverage of atomic S) with those performed at an electrochemical interface.7 In this latter case, in situ STM imaging revealed a well-ordered (x3 × x3) phase under certain conditions of potential control. Evidently the thermodynamics in the two environments differ sufficiently for this phase to be far more stable.

5. Conclusions

Acknowledgment. The authors acknowledge the allocation of SRS beamtime by the Council for the Central Laboratories for the Research Councils (CCLRC) and the assistance of George Miller at Daresbury Laboratory. M.Y. acknowledges the support of a Warwick Postgraduate Research Fellowship and an ORS award, and C.J.S. acknowledges the support of the EPSRC for a studentship. H.A. and G.Z. acknowledge financial support of ANPCyT (Grant PICT 03-11799).

( )

This combined STM, NIXSW, and qualitative LEED study of the interaction of S2 with the Au(111) provides the first quantitative structural information on the resulting phases and further clarifies the nature of the phases produced. At low coverage, ∼0.25 ML, we identify a new ordered LEED pattern, with the appearance of a split (x3 × x3) pattern, which we identify formally as a (5 × 5) phase. We have shown that this pattern can be reconciled with local (x3 × x3) ordering within a (5 × 5) antiphase domain structure, that can be reconciled in terms of “rosettes” of seven S atoms. This ordering must be attributed to the slightly repulsive S-S interactions for the S atom in next-nearest neighbor sites on the surface. This in turn provides a rationale for the marginal stability of the true (x3 × x3) phase and provides an explanation for the apparent inability to obtain STM images of a (x3 × x3) phase at room temperature as reported previously.8 Our LEED and STM observations also provide a formal description of the periodicity of the higher coverage “complex” phase, following the interpretation implicit in the very much earlier investigation of Kosterlitz et al.11 The NIXSW data also provide further information on the qualitative as well as quantitative character of these phases. In particular, these data show rather clearly that regions of poorly ordered “complex” phase must coexist with the lower coverage chemisorption phases, a conclusion consistent with the highresolution XPS data of Rodriguez et al.,6 if we assign their second “atomic S” component to the “complex” phase. In particular, the conditions that lead to the observation of a

References and Notes (1) Haruta, M. Catal. Today 1997, 36, 153. (2) Woodruff, D. P., Ed. The Chemical Physics of Solid Surfaces, Atomic Clusters: From Gas Phase to Deposited Elsevier: Amsterdam, The Netherlands, 2007, Vol. 12. (3) Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43, 437. (4) Ulman, A. Chem. ReV. 1996, 96, 1533. (5) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (6) Rodriguez, J. A.; Dvorak, J.; Jirsak, T.; Liu, G.; Hrbek, J.; Aray, Y.; Gonza´lez, C. J. Am. Chem. Soc. 2003, 125, 276. (7) Vericat, C.; Vela, M. E.; Andraesen, G. A.; Salvaressa, R. C.; Borgatti, F.; Felici, R.; Lee, T.-L.; Renner, F.; Zegenhagen, J.; MartinGago, J. A. Phys. ReV. Lett. 2003, 90, 075506. (8) Min, B. K.; Alemozafar, A. R.; Biener, M. M.; Biener, J.; Friend, C. M. Top. Catal. 2005, 36, 77. (9) Biener, M. M.; Biener, J.; Friend, C. M. Langmuir 2005, 21, 1668. (10) Quek, S. Y.; Biener, M. M.; Biener, J.; Bhattacharjee, J.; Friend, C. M.; Waghmare, U. V.; Kaxiras, E. J. Phys. Chem. B 2006, 110, 15663. (11) Kosterlitz, M.; Domange, J. L.; Oudar, J. Surf. Sci. 1973, 34, 431. (12) Vericat, C.; Andreasen, G.; Vela, M. E.; Salvarezza, R. C. J. Phys. Chem. B 2000, 104, 302. (13) Shortly after submission of this manuscript, an independent report of the observation of this “split-x3” LEED pattern was published in Biener, M. M.; Biener, J.; Friend, C. M. Surf. Sci. 2007, 601, 1659.

10914 J. Phys. Chem. C, Vol. 111, No. 29, 2007 (14) Dhanak, V. R.; Robinson, A. W.; van der Laan, G.; Thornton, G. ReV. Sci. Instrum. 1992, 63, 1342. (15) Robinson, A. W.; D’Addato, S.; Dhanak, V. R.; Finetti, P.; Thornton, G. ReV. Sci. Instrum. 1995, 66, 1762. (16) Further information on the NIXSW station 4.2 at SRS Daresbury can be found at http://www.srs.ac.uk/srs/stations/station4.2.htm. (17) Woodruff, D. P. Prog. Surf. Sci. 1998, 57, 1. Woodruff, D. P. Rep. Prog. Phys. 2005, 68, 743. (18) Hermann, K.; Van Hove, M. A. http://w3.rz-berlin.mpg.de/∼hermann/LEEDpat/. (19) XSWfit is a procedure, written as an Igor-Pro macro, which automatically fits XSW data. It is based on the formalism originally

Yu et al. developed by D. P. Woodruff in Fortran for calculating the XSW profile for a given set of parameters. Copies can be obtained from Rob Jones, email: [email protected]. (20) Woodruff, D. P.; Cowie, B. C. C.; Ettema, A. R. H. F. J. Phys.: Condens. Matter 1994, 6, 10633. (21) Biberian, J. P.; Van Hove, M. A. Surf. Sci. 1984, 138, 361. (22) Persson, B. N. J.; Tu¨shaus, M.; Bradshaw, A. M. J. Chem. Phys. 1990, 92, 5034. (23) Steininger, H.; Lehwald, S.; Ibach, H. Surf. Sci. 1982, 123, 264. (24) Ishikawa, K.; Isonaga, T.; Wakita, S.; Suzuki, Y. Solid State Ionics 1995, 79, 60.