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Abstract: Gas sensor devices have traditionally comprised thin films of metal oxides, with tin oxide, zinc oxide and indium oxide being some of the most common materials employed. With the recent discovery of novel metal oxide nanostructures, sensors comprising nano-arrays or single nanostructures have shown improved performance over the thin films. The improved response of the nanostructures to different gases has been primarily attributed to the highly single crystalline surfaces as well as large surface area of the nanostructures. In this paper the properties of clean and defected quasi one-dimensional ZnO nanostructures, including hexagonal and triangular nanowires, nanotubes and facetted nanotubes are reviewed. The adsorption of atoms and molecules on the ZnO nanostructures are also reviewed and the findings are compared to studies examining similar reactions on nanostructured metal oxide surfaces for sensing purposes. While both experimental and theoretical approaches have been employed to examine gas sensor reactions, this review focuses on studies that employ electronic structure calculations, which primarily concentrate on using density functional theory. Computational studies have been useful in elucidating the reaction mechanism, binding strength, charge transfer as well as other electronic and structural properties of the nanomaterials and the gas-sensor interaction. Despite these studies there are still significant areas of research that need to be pursued that will assist in the link between theoretical and experimental findings, as well as advancing the current chemical and physical understanding of these novel materials. A summary and outlook for future directions of this exciting area of research is also provided. [Copyright &y& Elsevier]

Abstract: Density functional theory calculations were used to model the structure and properties of ZnO and surfaces. Our calculations of the clean surface indicate that the Zn atom moves towards the surface, while the O atom moves away from the surface slightly, resulting in a tilting and shortening of the Zn–O bond. On the clean surface, the movements are similar but smaller in magnitude. Surface energy calculations indicate that the surface is more stable than the surface. The clean surface models were used to investigate the adsorption of N2O on these surfaces. More than one minimum energy structure was found for each surface, all showing relatively small binding energy values that reflect physisorbed surface species. Even though the geometry of N2O was little changed after adsorption, small surface relaxations and adsorbate-induced reconstructions were observed, resulting in a further tilting of the Zn–O surface bond so that the O atoms sit even higher on the surface. On both surfaces N2O prefers to adsorb to a surface Zn atom, via the O atom on the surface and via the N atom on the surface. Charge density differences, electron localisation function plots, Bader charges and vibrational frequency values were also calculated, with all these properties reflecting the weak interaction between the adsorbate and surface. Adsorption mainly leads to charge polarisation within the adsorbate molecule and the surface, resulting in a small transfer of at most 0.02e from the surface to the adsorbate. [Copyright &y& Elsevier]

Abstract: The adsorption of H2S on Fe(100) is examined using ab initio molecular dynamics at 298 and 1808K. Dissociation of H2S occurs at both temperatures simulated, to leave adsorbed S and two H atoms. The dissociation occurs via a two step process and the mechanism is found to be different depending on the temperature of the reaction. At 1808K, diffusion of the dissociated H atoms into the subsurface region is also observed. [Copyright &y& Elsevier]

Abstract: Density functional theory was employed to examine the interactions of atomic sulfur with the Fe(100) and Fe(110) surfaces. Vibrational frequency calculations were performed to determine the nature of stationary points at the high symmetry atop, bridge and hollow adsorption sites and to indicate the direction the adsorbate would move across the surface. The values were also used in the determination of the rate constant for hopping of S from one energy minimum site to another. Ab initio molecular dynamics (MD) simulations were then performed to monitor the mobility of the S atom on the (100) surface at different temperatures up to the melting point of Fe (1808K) and were compared to our previously obtained ab initio MD results for S/Fe(110) [N. Todorova, M.J.S. Spencer, I. Yarovsky, Australian Institute of Physics 16th Biennial Congress, 2005, Canberra, Australia, ISBN 0-9598064-8-2]. [Copyright &y& Elsevier]

Abstract: The effect of S contamination on the properties of Fe(100) is examined using density functional theory (DFT) calculations. S is adsorbed at 1/2 monolayer coverage in atop, bridge and hollow sites in a c(2×2) arrangement. The effect of S on the clean surface properties is first examined for the three adsorption sites and compared with experimental and other theoretical data. S is found to adsorb preferentially in hollow sites on the isolated surface in agreement with experiment. The adhesion energy at different interfacial separations is then calculated and the effect of S on the interfacial properties of Fe(100) is characterised quantitatively and qualitatively. S is found to enhance adhesion at larger separations though at the equilibrium interfacial separation the maximum interfacial strength is reduced. [Copyright &y& Elsevier]

Abstract: A geometry optimisation of the perovskite and hexagonal phases of BaTiO3 has been conducted using Density Functional Theory (DFT) within the Local Density Approximation (LDA) and Generalised Gradient Approximation (GGA) schemes. The LDA was found to give lattice parameters closer to experiment than the GGA. A study of oxygen vacancies in the hexagonal phase has been performed and the results suggest an O(1) type (face sharing) vacancy is more stable than an O(2) type (corner sharing) vacancy in the octahedral structure. In addition, the effect of different Ru doping concentrations on the structure and stability of the hexagonal phase has been investigated. [Copyright &y& Elsevier]

We have used density functional theory to examine the effects of avalanche in adhesion between Fe(100) surfaces, in registry and out of registry. When the central layers of the two surfaces are constrained the surface layers are attracted towards each other, forming a strained crystal region at intermediate interfacial separations. When the constraints in the z-direction are lifted, the surfaces avalanche together. In addition, when the surfaces are allowed to move sideways, we find that an interface initially out of registry will tend to avalanche towards an interface that is in registry. [ABSTRACT FROM AUTHOR]