H. R. Sharma

The Intermetallic Compound ZnPd and its Role in Methanol Steam Reforming

The rich literature about the intermetallic compound ZnPd as well as several ZnPd near-surface intermetallic phases is reviewed. ZnPd is frequently observed in different catalytic reactions triggering this review in order to collect the knowledge about the compound. The review addresses the chemical and physical properties of the compound and relates these comprehensively to the catalytic properties of ZnPd in methanol steam reforming—an interesting reaction to release hydrogen for a future hydrogen-based energy infrastructure from water/methanol mixtures. The broad scope of the review covers experimental work as well as quantum chemical calculations on a variety of Pd-Zn materials, aiming at covering all relevant literature to derive a sound state-of-the-art picture of the understanding gained so far.

Quasicrystal surfaces: structure and growth of atomic overlayers

We review recent developments in surface studies of single-grain quasicrystals under ultra high-vacuum conditions, focusing on two different topics: surface structure and growth of atomic overlayers on surfaces. Quasicrystalline phases are currently used for investigation of the first topic are icosahedral (i) Al-Pd-Mn, i-Al-Cu-Fe, i-Al-Cu-Ru, i-Ag-In-Yb and decagonal (d) Al-Ni-Co, and d-Al-Cu-Co. We report the progress made with all of these phases. The second topic covers the study of single-element overlayer growth by vapor deposition.

Templated quasicrystalline molecular ordering.

Quasicrystals are materials with long-range ordering but no periodicity. We report scanning tunneling microscopy (STM) observations of quasicrystalline molecular layers on 5-fold quasicrystal surfaces. The molecules adopt positions and orientations on the surface consistent with the quasicrystalline ordering of the substrate. Carbon-60 adsorbs atop sufficiently separated Fe atoms on icosahedral Al-Cu-Fe to form a unique quasicrystalline lattice, whereas further C60 molecules decorate remaining surface Fe atoms in a quasi-degenerate fashion. Pentacene (Pn) adsorbs at 10-fold symmetric points around surface-bisected rhombic triacontahedral clusters in icosahedral Ag-In-Yb. These systems constitute the first demonstrations of quasicrystalline molecular ordering on a template.

The surface science of quasicrystals

The surfaces of quasicrystals have been extensively studied since about 1990. In this paper we review work on the structure and morphology of clean surfaces, and their electronic and phonon structure. We also describe progress in adsorption and epitaxy studies. The paper is illustrated throughout with examples from the literature. We offer some reflections on the wider impact of this body of work and anticipate areas for future development.

Templated three-dimensional growth of quasicrystalline lead

Quasicrystals, materials with aperiodic long-range order, have been found in intermetallics, soft materials such as colloids and supermolecules, and also in two-dimensional monolayer films. Here we present the first example of three-dimensional growth of a single-element quasicrystalline film. Using a hitherto unexplored template, the icosahedral Ag-In-Yb quasicrystal, and various experimental techniques combined with theoretical calculations of adsorption energies, we find that lead atoms deposited on the surface occupy the positions of atoms in the rhombic triacontahedral cluster, the building block of the substrate, and thus grow in layers with different heights and adsorption energies. We show that the adlayer-adlayer interaction is crucial for stabilizing this epitaxial quasicrystalline structure. The finding opens an avenue for further investigation of the impact of the aperiodic atomic order over periodic order on the physical and chemical properties of materials.

First UHV surface studies of single-grain icosahedral Ag–In–Yb quasicrystal

We present the first ultrahigh vacuum (UHV) surface study of single-grain icosahedral Ag–In–Yb quasicrystal by using X-ray photoelectron spectroscopy (XPS), reflection high-energy electron diffraction (RHEED), and scanning tunnelling microscopy (STM). This new quasicrystal is found to exhibit surfaces with steps and terraces, upon suitable preparation conditions. The Fourier transform of high-resolution STM images and RHEED patterns reveal that the surface after annealing at appropriate temperatures retains quasicrystalline long-range order. The analysis of XPS data measured from this surface shows a near bulk chemical composition in the surface region.

Structure and reactivity of Bi allotropes on the fivefold icosahedral Al-Pd-Mn quasicrystal surface.

The growth of Bi on a pseudomorphic Bi monolayer on the fivefold surface of the icosahedral Al-Pd-Mn quasicrystal has been investigated using low energy electron diffraction and scanning tunnelling microscopy. Initially randomly oriented pseudocubic islands are formed with a preference for an even number of layers. Subsequently a morphological transformation takes place to hexagonal Bi islands, which align along high symmetry directions of the substrate. The Bi flux is found to have a strong effect on which island structure is preferred. When C(60) is adsorbed on the three different allotropes of Bi present in this system, hexagonal C(60) islands are formed in each case. On the pseudocubic and hexagonal islands, the C(60) islands are aligned with the substrate. We discuss the energetic, kinetic and geometrical factors which influence the morphological transformation referred to above.

Ordering of adsorbed species on quasicrystal surfaces

We present an overview of experimental studies of the structure of adsorbed atomic and molecular species on quasicrystal surfaces with a particular focus on those systems where quasiperiodic ordering of the adsorbate structure occurs. Atomic adsorption is illustrated with examples from studies of Fe and Gd adsorption on the five-fold surface of i-Al–Pd–Mn. Overlayer structure is correlated with several physical parameters to identify trends. It is concluded that the chemical properties of the adsorbate (metallic radius, electronegativity, melting temperature) are important indicators of quasiperiodic structuring of the adsorbate system. These correlations allow predictions of those elements likely to yield pseudomorphic overlayers. Structural effects upon molecular adsorption on clean quasicrystal surfaces have been less successful. A new approach to ordered molecular adsorption is described and illustrated for C60 adsorption on a Pb monolayer film on the ten-fold surface of d-Al–Ni–Co.

Nucleation and growth of pseudomorphic monolayers on quasicrystal surfaces

We present a review of experimental studies of adsorption systems where pseudomorphic growth on quasicrystal surfaces has been observed. From these studies, we correlate information on the known or suggested adsorption and nucleation sites. For fivefold surfaces of icosahedral quasicrystals, two favoured locations emerge: the centres of equatorially truncated pseudo-Mackay clusters, and the centres of pentagonal hollows. The information on adsorption sites on decagonal quasicrystals is less definitive. Nevertheless the correlation of known information leads to a simple picture of pseudomorphic growth, at least for the first monolayer. We also remark on the possibilities for studies using quantitative surface structural techniques.

Faceting of the two-fold decagonal Al71.8Ni14.8Co13.4(0 0 1 1̄ 0) surface studied by He diffraction

The two-fold decagonal-Al71.8Ni14.8Co13.4(0 0 1 1 0) surface is investigated by highly surface sensitive He atom scattering. The diffraction pattern shows that the topmost surface layer preserves a perfect quasi-crystalline ordering expected from the bulk. The surface is found to develop facets with (1 0 0 0 0)-equivalent orientation, inclined by 18° to the original surface. The facets are of sufficient size to observe diffraction from them. The facet planes are identical to those identified in an as grown sample.