R. McGrath

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.

Surface relaxation of SrTiO3(001)

Surface X-ray diffraction has been used to examine the 300 K structure of SrTiO3(001)1×1 with a termination of 78% TiO and 22% SrO. The data indicate that a lateral ferroelectric distortion is absent on both terminations, consistent with a recent theoretical calculation. Although the negligible Ti atom relaxation found on the TiO termination is consistent with medium-energy ion scattering data, it differs from recent density functional theory calculations by 0.13 and 0.16 A. In contrast, the Sr displacement towards the SrO surface of 0.22±0.07 A is in good agreement with theory.

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.

Pseudomorphic Starfish: Nucleation of Extrinsic Metal Atoms on a Quasicrystalline Substrate

Abstract Direct observation of extrinsic Al atoms on the fivefold surface of icosahedral Al–Cu–Fe indicates that they form pseudomorphic islands resembling starfish. The starfish occupy specific types of sites on the laterally-bulk-terminated quasicrystalline surface. We postulate that the nucleation event consists of a diffusing Al atom dropping into an empty site at the centre of a substrate pentagon. Growth consists of the addition of five Al atoms (nearly) in lattice sites. These 6-atom starfish do not grow laterally as coverage increases, leading to islands of uniform size, and to early roughening.

STM and SPA-LEED studies of O-induced structures on Rh(100) surfaces

Abstract Oxygen adsorption on the Rh(100) surface has been studied by spot profile analysis low energy electron diffraction (SPA-LEED) and scanning tunnelling microscopy (STM). Our results show that oxygen can be reliably dosed onto the Rh(100) surface to produce distinct p(2 × 2) and c(2 × 2) phases. Additionally, the symmetry of the phase formed at saturation coverage is identified as (2 × 2)p4g rather than (2 × 2)p2gg as had been proposed previously. STM images of both the c(2 × 2) and (2 × 2)p4g phases show that the O atoms sit in alternate four-fold hollow sites. It is proposed that the difference between these two structures is a result of overall oxygen coverage. In the c(2 × 2) case we speculate that surface strain is relieved by the formation of islands, but that for the saturation coverage (2 × 2)p4g structure surface strain is reduced by the O atom burrowing into the surface, causing the characteristic clock reconstruction of the substrate Rh atoms.

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.

Self-assembly, structure, and electronic properties of a quasiperiodic lead monolayer

A quasiperiodic Pb monolayer has been formed on the fivefold surface of the Al-Pd-Mn quasicrystal. Growth of the monolayer proceeds via self-assembly of an interconnected network of pentagonal Pb stars, which are shown to be

Oxygen-induced ordering of potassium in a coadsorbate phase: a SPA-LEED and STM study of K-O/Ni(100)

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