We investigate the stability of Cu-exposed ZnO(0001) surface structures in an oxygen environment using density functional theory and the method of ab initio atomistic thermodynamics. A two-dimensional phase diagram is constructed which identifies stable surface structures as a function of the copper and oxygen chemical potentials. Two structures with a ( √ 3 ×√3)R30◦ unit cell are found to be prominently stable at intermediate oxygen and copper chemical potentials. These phases are characterized by a single adlayer of Cu4O3 and Cu12O13 stoichiometry on ZnO(0001). We rationalize recent experimental observations in the literature in terms of our results.

The polar Zn-ZnO(0001) surface is involved in the catalysis of methanol synthesis and the water–gas-shift reaction. We use density functional theory calculations to explore the favorable binding geometries and energies of adsorption of several molecular species relevant to these reactions, namely carbon monoxide (CO), carbon dioxide (CO2), water (H2O) and methanol (CH3OH). We also consider several proposed reaction intermediates, including hydroxymethyl (CH2OH), methoxyl (CH3), formaldehyde (CH2O), methyl (CH3), methylene (CH2), formic acid (HCOOH), formate (HCOO), formyl (HCO), hydroxyl (OH), oxygen (O) and hydrogen (H). For each, we identify the preferred binding geometry at a coverage of 1/4 monolayers (ML), and report calculated vibrational frequencies that could aid in the identification of these species in experiment. We further explore the effects on the binding energy when the adsorbate coverage is lowered to 1/9 and 1/16 ML.

Physics — The Cu/ZnO(0001) surface is widely used as a cata-lyst for the production of H 2 -gas from methanol and is thus of considerable relevance to the emergent hydrogen economy. A key to the further development of this cata-lyst system is a detailed atomic-scale understanding of the relation between surface structure and function versus environmental conditions such as copper content and state of surface oxidation. Towards this goal, we use density functional theory within the framework of ab initio atomistic thermodynamics to conduct a detailed survey of conceivable surface structures under variety of Cu exposures. This produces a surface phase diagram that reveals several distinct regimes of surface reconstruction under oxygen-rich and poor conditions. We correlate our findings with experimental studies, including recent scanning tunneling microscopy results by Dulub et al [1].

We present a first-principles theoretical study of the atomic geometry and electronics states of the InP(1 1 1)A surface under In- and P-rich conditions. The In-rich surface, characterised by an In vacancy per unit (2 x 2) cell, obeys the electron counting rule (ECR) and is semiconducting. Under P-rich conditions we have considered two surface reconstructions: (2 x 2) with 3/4 monolayer (ML) P coverage and (root 3 x root 3) with 1 ML coverage. In complete agreement with a recent experimental work by Li et al., it is found that the (root 3 x root 3) reconstruction is more stable than the (2 x 2) reconstruction. However, the (root 3 x root 3) reconstruction has a metallic band structure and thus does not satisfy the ECR. The stability of this reconstruction is explained to arise from a competition between the ECR and a significant elastic deformation in the surface region. We confirm the suggestion by,Li et al. that this surface can be passivated both chemically as well as electronically with 1/4 ML coverage of hydrogen. (c) 2006 Elsevier B.V. All rights reserved.

We have employed the pseudopotential method and the density functional scheme to study the atomic geometry and electronic states of the GaSb(001) surface such as (1×3), c(2×6) and (4×3) reconstructions. It is found that both of (1×3) and c(2×6) reconstructions are characterised by metallic band structures, and thus violate the so-called electron counting rule, one of the main building principles of the stability of compound semiconductor surfaces. We establish that the stability of these reconstructions results from significant elastic deformation in the top atomic layers of the surface, a process which overcomes the penality incurred by the violation of the electron counting rule. The atomic geometry and electronic states for the two reconstructions are compared and contrasted with each other. The α and β phases of the (4×3) reconstruction also show large atomic relaxations but are semiconducting and obey the electron counting rule.

From ab initio studies employing the pseudopotential method and the density functional scheme, we report on progressive changes in geometry, electronic states, and atomic orbitals on Si(001) by adsorption of different amounts of Bi coverage. For the 1/4ML coverage, uncovered Si dimers retain the characteristic asymmetric (tilted) geometry of the clean Si(001) surface and the Si dimers underneath the Bi dimer have become symmetric (untilted) and elongated. For this geometry, occupied as well as unoccupied surface states are found to lie in the silicon band gap, both sets originating mainly from the uncovered and tilted silicon dimers. For the 1/2ML coverage, there are still both occupied and unoccupied surface states in the band gap. The highest occupied state originates from an elaborate mixture of the pz orbital at the Si and Bi dimer atoms, and the lowest unoccupied state has a ppσ* antibonding character derived from the Bi dimer atoms. For 1ML coverage, there are no surface states in the fundamental bulk band gap. The highest occupied and the lowest unoccupied states, lying close to band edges, show a linear combination of the pz orbitals and ppσ* antibonding orbital characters, respectively, derived from the Bi dimer atoms.

We present a study of ad-dimers during the initial stages of the adsorption of Bi and Si on the Si(001) surface by employing an ab initio planewave pseudopotential method and the density functional scheme. For Bi adsorption, the B site with the ad-dimer lying along the substrate Si dimer row is decisively more energetically favorable (by 0.20 eV) than the A site with the ad-dimer lying perpendicular to the substrate Si dimer row. For Si adsorption, the B site is only marginally favorable (at most by 0.07 eV) to the A site. Results of energy barrier, atomic geometry, and electronic states during the rotation process between the A and B sites are presented and compared with recent scanning tunneling microscopy (STM) results. Our results suggest that the interconversion between the two equilibrium sites is intrinsically less frequent than estimated from STM investigations and previous theoretical works (for Si ad-dimer).

In this work we have performed a theoretical study of the atomic geometry and scanning tunelling microscopy simulation of the GaAs(111)A–Se(23×23) surface. The calculated geometry with Se trimers on H3 sites agrees well with results reported recently from STM and RHEED experiments. Simulated STM images, corresponding to orbital localisation near the fundamental gap, support the experimental observations of bright spots on Se trimers.

In this work we have performed a theoretical study of the atomic geometry and scanning tunelling microscopy simulation of the GaAs(1 1 1)A–Se( 2 3 ×2 3 ) surface. The calculated geometry with Se trimers on H 3 sites agrees well with results reported recently from STM and RHEED experiments. Simulated STM images, corresponding to orbital localisation near the fundamental gap, support the experimental observations of bright spots on Se trimers. Keywords Density functional calculations Surface electronic phenomena (work function, surface potential, surface states, etc.) Chemisorption Gallium arsenide Chalcogens