Renee D. Diehl

Structural studies of alkali metal adsorption and coadsorption on metal surfaces

Abstract The study of the adsorption and the coadsorption of alkali metals on single-crystal metal surfaces is a very active sub-field of surface science, partly because of the importance of technological applications (promotion of catalytic reactions, enhanced oxidation, increases in electron emission rates), but also because of the fundamental interest in how these “simple” adsorbates modify the surface. In the past few years there has been a marked increase in the number of structural studies of alkali adsorption and coadsorption systems, motivated in part by developments in theoretical models of adsorption. This article reviews recent studies of alkali adsorption and coadsorption using specifically structural techniques, with the intention of highlighting recent developments, providing a useful reference base to the community, and drawing attention to some unifying concepts.

A LEED structural study of the Pd{110}-(1×1) surface and an alkali-metal-induced (1×2) surface reconstruction

Abstract From a LEED study of the {110} surface of palladium it was concluded that an oscillatory relaxation occurs with respect to the bulk interlayer spacing in the top two layers of a laterally unreconstructed surface. The first and second interlayer spacings were found to be −6±2% and 1±2%, respectively, with respect to the bulk value. These results compare with recent embedded atom theory predictions that a first layer contraction of ∼11% occurs. Low coverages of the alkali metals Cs and Na are shown to induce a (1×2) reconstruction of the Pd{110} surface. We present details of coverage and alkali metal dependence of the (1×1) to (1×2) phase transition. A LEED study has been carried out to examine the feasibility of a wide range of possible structures exhibiting the required lateral periodicity. We favour models in which all selvedge atoms remain at or near bulk-like positions, the “missing row” and “saw-tooth” models being the most satisfactory so far tested.

Pseudomorphic Growth of a Single Element Quasiperiodic Ultrathin Film on a Quasicrystal Substrate

Surface Science Research Centre and Department of Physics,The University of Liverpool, Liverpool L69 3BX, UKWe have synthesised a thin film of copper with a quasi-periodic structure by the adsorption ofcopper atoms on the five-fold surface of the icosahedral quasicrystal Al-Pd-Mn at room temperature.The quasi-periodicity of the thin film is manifested in low energy electronic diffraction (LEED)measurements and in the existence of Fibonacci relationships between rows of copper atoms imagedusing scanning tunneling microscopy (STM). These findings demonstrate the feasibility of single-element quasi-periodic thin film formation using quasicrystals as templates.

Quasicrystal surfaces: structure and potential as templates

We present a review of recent progress in determining the surface structure of quasicrystals, with emphasis on their connections to mathematical tiling models. The review focusses in particular on the five-fold surface of icosahedral Al-Pd-Mn and the ten-fold surface of decagonal Al-Ni-Co. We also assess their potential as templates for the formation of two-dimensional quasicrystalline overlayers with reference to recent investigations of atomic and molecular adsorption.

Tiling of the fivefold surface of Al70Pd21Mn9

The nature of the five-fold surface of Al(70)Pd(21)Mn(9) has been investigated using scanning tunneling microscopy. From high resolution images of the terraces, a tiling of the surface has been constructed using pentagonal prototiles. This tiling matches the bulk model of Boudard et. al. (J. Phys.: Cond. Matter 4, 10149, (1992)), which allows us to elucidate the atomic nature of the surface. Furthermore, it is consistent with a Penrose tiling T^*((P1)r) obtained from the geometric model based on the three-dimensional tiling T^*(2F). The results provide direct confirmation that the five-fold surface of i-Al-Pd-Mn is a termination of the bulk structure.

C60 adsorption on the quasicrystalline surface of Al70Pd21Mn9

Abstract Room temperature adsorption of C 60 on the flat quasicrystalline surface of Al 70 Pd 21 Mn 9 has been investigated using scanning tunnelling microscopy. A dispersed overlayer is formed at low coverage, with avoidance of step-edges. There is no evidence of island formation or clustering. As the coverage is increased, a higher density layer is formed with no evidence of the formation of hexagonal ordered adsorbate structures seen on other substrates. This is followed by the onset of second layer formation. A range of bonding sites for C 60 molecules is implied from measurements of apparent molecular heights and from thermal effects. Detailed analysis of the surface at a low coverage (∼0.065 ML) provides evidence of adsorbate local order, with Fibonacci ( τ -scaling) relationships between the C 60 molecules. Where this occurs, the preferred adsorption site is tentatively identified as the pentagonal hollow. These local correlations however are not found to extend over larger regions of the surface.

Observation of top-site adsorption for Xe on Cu(111)

Abstract A low-energy electron diffraction study of Cu(111)–(√3×√3)R30°–Xe at 50 K indicates that Xe atoms occupy the top sites. The equilibrium Xe–Cu interlayer spacing is 3.60±0.08 A and the spacings of the three top Cu layers are essentially bulk-like. The Xe–Cu spacing agrees well with estimates based on hard-sphere packing. Top-site adsorption for Xe on Cu(111) was unexpected on the basis of previous experimental and theoretical results for noble gas adsorption on metal surfaces. This result is discussed in the light of earlier studies of physisorbed atoms, anticorrugated potentials in He-atom scattering, and possible links to alkali metal adsorption.

The structure of sodium adsorption phases on Al(111)

Abstract The structures formed by the adsorption of Na and Al(111) have been studied by normal incidence standing X-ray wavefield absorption using both the (111) and (111) Bragg reflections in order to obtain not only the Na-Al layer spacings, but also the adsorption sites. In the case of the ordered (√3 × √3)R30°-Na phase, at a nominal coverage of 0.33 ML, and at lower coverages down to approximately 0.12 ML, Na is found to adsorb in a coverage independent site which involves a substitution of some of the top layer Al atoms; if the substrate layer spacings remain unchanged by this reconstruction, the effective radius of the adsorbed Na species is found to be 1.67 A. The higher coverage (2 × 2)-Na phase is found to involve (at least) two distinct Na adsorption sites, but each of these has a very similar layer spacing relative to the extended (111) substrate scatterer lattice planes. A specific model of this phase, involving two reconstructed layers each of stoichiometry NaAl 2 is proposed, which accounts in a quantitative way for the present X-ray standing wave data and previous X-ray absorption data, and in a qualitative way for published high resolution soft X-ray photoelectron spectroscopy results. As Al and Na are essentially immiscible in the bulk, this two-layer structure appears to be an entirely new kind of surface phase.

Evolution of topological order in Xe films on a quasicrystal surface

We report results of the first computer simulation studies of a physically adsorbed gas on a quasicrystalline surface Xe on decagonal Al-Ni-Co. The grand canonical Monte Carlo method is employed, using a semiempirical gas-surface interaction, based on conventional combining rules, and the usual Lennard-Jones Xe-Xe interaction. The resulting adsorption isotherms and calculated structures are consistent with the results of LEED experimental data. The evolution of the bulk film begins in the second layer, while the low coverage behavior is epitaxial. This transition from epitaxial fivefold to bulklike sixfold ordering is temperature dependent, occurring earlier (at lower coverage) for the higher temperatures.

The adsorption sites of rare gases on metallic surfaces: a review

During the past six years, the adsorption geometries of several rare gases in structures having several different symmetries on a variety of substrates were determined using low-energy electron diffraction (LEED). In most of these studies, a preference is found for the rare gas atoms to adsorb in the low-coordination sites. Only in the case of adsorption on graphite has a clear preference for a high-coordination site for a rare gas atom been found. This unexpected behaviour is not yet completely understood, although recent density functional theory (DFT) calculations for these and similar surfaces suggest that this is a general phenomenon. This paper reviews the early studies that were presages of the discovery of top site adsorption for rare gases, the discovery itself, and the present state of understanding of this curiosity. It also details some of the features of the LEED experiments and analysis that are specific to the case of rare gas adsorption.