S.M. Driver

Scanning tunnelling microscopy study of the interaction of dimethyl disulphide with Cu(111)

Scanning tunnelling microscopy has been used to study the interaction of dimethyl disulphide, (CH3S)2, with Cu(111) at room temperature, which is known to produce adsorbed methanethiolate, CH3S-, by SS bond scission at the surface. The results provide clear evidence for the formation of a pseudo-square surface reconstruction induced by the adsorbate; this reconstruction can be reconciled with the results of previous structural studies of this surface by the techniques of X-ray standing wave and surface extended X-ray absorption fine structure. Two further ordered adsorption phases are observed. One of these, a hexagonal honeycomb phase, is proposed to involve bridge and hollow site adsorption on an unreconstructed Cu surface and is metastable; it is identified as a methanethiolate species previously only studied at low temperature by photoelectron spectroscopy and X-ray standing waves. A third phase of hexagonal symmetry appears to be an alternative endpoint to the pseudo-square phase, but may be due to higher adsorbate coverages; the nature of the structure of the underlying copper surface in this case is unclear.

A structural study of the interaction of SO2 with Cu(111)

Abstract The technique of chemical-shift normal-incidence X-ray standing waves (CS-NIXSW) has been applied to a study of the interaction of SO 2 with Cu(111), yielding quantitative information on the local adsorption geometry of adsorbed SO 2 at low temperature and coadsorbed atomic sulfur with an SO x species, identified on the basis of near-edge X-ray absorption fine structure (NEXAFS) as SO 3 . Atomic sulfur appears to occupy a mixture of face-centred cubic and hexagonal close-packed hollow sites, while the SO 3 species adsorbs with its C 3v axis perpendicular to the surface atop a surface copper atom with the SO bonds out of plane such that the oxygen atoms are closer to the surface; there appears to be some local distortion of the outermost copper layers around this species. While SO 2 is found to adsorb with its molecular plane essentially perpendicular to the surface, and the data are most readily interpreted in terms of a bridging geometry bonding through the oxygen atoms, there are marked inconsistencies between these results and those of an earlier surface extended XAFS (SEXAFS) investigation of this species on Cu(111), and alternative interpretations are discussed.

Scanning tunnelling microscopy investigation of the oxygen-induced faceting and nano-faceting of a vicinal copper surface

Abstract Scanning tunnelling microscopy (STM) has been applied to investigate the well-known tendency of Cu(100) vicinals to facet to Cu(410) in the presence of chemisorbed oxygen. Room-temperature imaging was performed on a nominally (610) face of Cu following different oxygen exposures and annealing treatments. The well-annealed oxygen-exposed surface shows large (410) facets but a rather small tendency to produce comparably sized (100) facets; these only occur in conjunction with (401) facets which arise due to a miscut of the crystal surface by a few degrees out of the [001] zone relative to a pure ( n 10) face. Instead, the additional (100) surface needed to make up the average surface orientation occurs in the form of regions of wider (100) terraces between groups of terraces of (410) width, leading to nanofacet regions with modulated step-edge spacings. Oxygen dosing around room temperature produces small (410) facets elongated in the [001] direction, with pile-up of the kinks resulting from the azimuthal twist leading to 〈100〉 step-edge facets as a precursor to the formation of (100) and (401) two-dimensional facets which form on annealing. Explicit evidence is found for significant adatom diffusion at room temperature, even over the oxygen-induced (410) facets. Atomic-scale images of nanofacets, which can be regarded (4 n 10) regions with n an integer, provide some further information on specific atomic models of oxygen-covered Cu(100) vicinals in this zone.

Nitrogen-induced pseudo-(100) reconstruction of the Cu(111) surface identified by STM

Abstract The N-induced reconstruction of Cu(111) has been studied using scanning tunnelling microscopy (STM). The results fully support a model, previously proposed on the basis of qualitative interpretation of the LEED pattern, in which the outermost Cu atom layer adopts a distorted pseudo-(100) structure with the adsorbed atomic N forming a c(2×2) periodicity on this reconstructed Cu layer. Both the c(2×2) and (1×1) periodicities of this pseudo-(100) surface were imaged under different tip conditions; these show clearly that there is no ‘clock reconstruction’ of the pseudo-(100) layer as reported for C on Ni(111). A periodic corrugation observed in the height of the reconstructed phase has been modelled by a simple hard-sphere simulation of the effects of the variations in local registry between the pseudo-(100) layer and the underlying (111) substrate based on a true (25×7 3 )rect. coincidence surface mesh.

The structure of sulphur adsorption phases on Ni(111) studied by X-ray standing wavefield absorption

Abstract The technique of normal-incidence X-ray standing wavefield (NIXSW) absorption has been applied to an investigation of several structural phases of S on Ni(111), specifically the (2 × 2), (√3 × √3)R30° and (5√3 × 2)rect. phases, with a view in particular to clarifying recent controversy over the structure of the last of these. Absolute adsorbate site determination has been effected through real-space triangulation using both (111) and (111) Bragg reflections. For the (2 × 2) phase the results are consistent with a simple overlayer structure with S atoms occupying the “fcc” hollow sites (directly above third-layer Ni atoms), in agreement with earlier determinations by LEED, ion scattering and SEXAFS. The same local site is found to be occupied in the (√3 × √3)R30° phase, although the NISXW data indicate poorer local order. For the (5√3 × 2)rect. phase, a simple undistorted missing-row model is found to be inconsistent with the data, as is any ideal “coincidence lattice” structure, including a simple undistorted pseudo-(100) surface reconstruction. A modification of this latter model, in which some S atoms penetrate the pseudo-(100) layer to bond to the underlying (111) substrate does, however, contain the main ingredients needed to fit the data. By using not only (111) and (111) NIXSW but also (111) and (111) absorption profiles, recorded from a near single-domain structure formed on a miscut crystal surface, specific information on some of the structural parameters of this modified reconstruction model is obtained and compared with complementary data from other published studies.

The structure of the surface phase: a new normal-incidence X-ray standing wave study

Abstract The local geometry of the adsorbed sulphur atoms in the Cu(111)( 7 × 7 )R19°-S surface phase has been investigated with normal-incidence X-ray standing wavefield (NIXSW) absorption using the (111), (111) and (200) Bragg reflections. The results are compared with the predictions for previously proposed models based on earlier investigations using NIXSW [but only the (111) reflection], surface extended X-ray absorption fine structure, quantitative low energy electron diffraction, surface X-ray diffraction (SXRD) and scanning tunneling microscopy. The best agreement is found for the model deduced from SXRD based on Cu 4 S adclusters, but for the specific overlayer substrate registry in which these clusters are centred atop an outermost substrate layer Cu atom. All of the other models show significant inconsistencies with these new data.

The local adsorption structure of SO2 on Ni(111) : a normal incidence X-ray standing wavefield determination

Abstract A normal incidence X-ray standing wavefield study of the structure of molecular SO 2 on Ni(111) has been conducted, using photoabsorption at both the O and S atoms and real-space site triangulation using {111} scatterer planes both parallel to, and at 70° to, the surface plane. Both O and S atoms are found to be in the vicinity of atop sites, although the S atoms are displaced significantly further from these high symmetry sites. S K-edge NEXAFS confirms an earlier finding that the molecule lies with its molecular plane parallel to the surface. The detailed sites can only be reconciled with a model in which the SO 2 molecules are centred close to hollow sites (with equal occupation of both types of hollow) and the internal conformation of the molecule, especially the OSO bond angle, is significantly different from that of the gas-phase molecule. Specifically, the OSO bond angle is estimated to be no more than 100°, while the data indicate an SO bond-length expansion of 5% or more. This change is attributed to the unusual π-bonding (for which there appears to be no analogue in coordination compounds) and thus partial occupation of the 3b 1 π∗ LUMO of the molecule.

Missing rows on oxygen-covered Cu(100) vicinal surfaces: a scanning tunnelling microscopy investigation

Scanning tunnelling microscopy images of oxygen-covered Cu(410) facets, of the structure, and of the faceting of Cu(610) in the presence of adsorbed oxygen have been used to investigate the basic elements of the atomic-scale structure of these surfaces. The existence of stable (100) nanofacets of widths corresponding to those of (4n 1 0) faces is shown to be consistent with a common structural model for all of these surfaces based on the well known structural unit of three adjacent [001] Cu atom rows and one missing row which comprises the surface. This model implies that on the (410) surface the missing row lies at the bottom of each atomic step on this surface. All these structures have a common local geometry for the adsorbed O atoms coordinated to four Cu atoms.

Photoelectron diffraction investigation of the local adsorption site of N on Cu(111)

Scanned-energy mode photoelectron diffraction has been used to investigate the local structure of N adsorbed on Cu(111). The results favour an N-induced pseudo-(100) reconstruction as proposed previously, but indicate that this comprises two reconstructed Cu layers. The N atoms occupy hollow sites approximately coplanar with the outermost pseudo-(100) layer at a height of about 2 A above a Cu atom from the underlying pseudo-(100) layer; the N atoms thus have fivefold co-ordination.

Alkali-promoted oxidation of Al(111): RbO and KO coadsorption and the role of surface structure

Abstract Synchrotron radiation core-level photoemission from the Al 2p, K 3p and Rb 4p states has been used to characterise the role of preadsorbed Rb and K on the interaction of oxygen with an Al(111) surface. Specific precoverages have been used corresponding to two different (√3 × √3)R30° surface structures, a metastable low temperature phase involving alkali atoms in atop sites, and a stable higher temperature phase with substitutional alkali atoms. In all cases a significant promotion of both dissociation and oxidation is seen relative to the activity of the clean Al(111) surface. In comparison with earlier results for Na O coadsorption Rb is found to promote oxidation most strongly and Na least strongly with K being intermediate; the Rb room temperature substitutional phase, in particular, shows oxidation at the lowest oxygen exposures and no indication for the RbO chemisorption precursor comparable with the NaO one identified on the Na-covered surface. By contrast the Rb-atop and K-atop geometry surfaces do show evidence of some discrete chemisorption states in the Al 2p spectra of the type seen on alkali-free Al(111), but only in the presence of other spectral structure assigned to mixed-coordination geometries. At low temperatures the effect of both Rb and K on oxidation, but not on initial oxygen adsorption, is generally suppressed, an effect ascribed to the role of bulk diffusion. Measurements of normal incidence X-ray standing wavefield absorption for the Rb O coadsorption structures at very low oxygen exposure also indicate that no simple single sites are occupied, particularly in the case of the more reactive Rb-substitutional phase at room temperature, although there appears to be a relatively well-defined OAl layer spacing attributed to small but laterally incommensurate oxide islands. Measured work function changes at low oxygen exposure in the Rb O and Na O systems can be reconciled with oxygen penetration of the alkali layer except for the Na-substitutional phase, for which the data are qualitatively consistent with the previously reported atop geometry.