Ab initio density functional simulation of structural and electronic properties of MgO ultra-thin adlayers on the (001) Ag surface

To simulate the properties of ultrathin layers of magnesium oxide epitaxially grown on silver (001), we have adopted a periodic slab model, consisting of six layers of Ag covered on both sides with an MgO monolayer. All calculations have been performed with the 98 program. Several DFT functionals were tried and a rich basis set was adopted. The electronic and structural properties of the two bulk materials (Ag and MgO) were quite accurately reproduced. The presence of the metallic substrate was found to have an appreciable influence on the structural and electronic features of the oxide surface. In the most stable configuration (O ions directly above Ag atoms, Mg ions in the hollow sites), the surface is corrugated, and there is a net transfer of electrons from the overlayer to the metal, leading to a substantial reduction of the work function of the metal and to a decrease of the electrostatic field at the surface. The reactivity properties of the supported oxide surface have been investigated by studying the interaction of the composite material with water molecules.     

Ab initio study of electronic,magnetic and optical properties of CuWO4 tungstate

Ab initio calculations based on the density-functional theory have been employed to study the electronic and magnetic properties of copper tungstate CuWO₄, as well as its optical characteristics in the ultraviolet region, up to 40 eV. The electronic structure around the band gap is dominated by the O p- and the Cu d-states and it is quite different from the recent spin-restricted calculations of the same compound. The most stable antiferromagnetic state and the values of magnetic moments at Cu sites and O(3) oxygen atomic sites (closest to Cu²⁺ ions in zigzag antiferromagnetic chains) are in agreement with experiments. The gap is found to be indirect with the acceptable value only after use of the LDA + U rotationally invariant self-consistent full potential linearized augmented plane wave (FP LAPW) approach. The optical spectra are analyzed, compared, and interpreted in terms of calculated band structures. It is shown that absorption process involves significant energy flow from the O ions to the Cu ions.     

Ab initio study of the electronic and optical properties of sillimanite (Al2SiO5) crystal

Ab initio calculations based on the density functional theory (DFT) are used to investigate the electronic and optical properties of sillimanite. The geometrical parameters of the unit cell, which contain 32 atoms, have been fully optimized and are in good agreement with the experimental data. The electronic structure shows that sillimanite has an indirect band gap of 5.18 eV. The complex dielectric function and optical constants, such as extinction coefficient, refractive index, reflectivity and energy-loss spectrum, are calculated. The optical properties of sillimanite are discussed based on the band structure calculations. It is shown that the O-2p states and Al-3s, Si-3s states play the major role in optical transitions as initial and final states, respectively.     

Ab initio molecular dynamics studies of metal clusters

We present results of ourab initio molecular dynamics simulations on the atomic and electronic structure of clusters of divalent metals, aluminum and antimony, which exhibit a range of bonding characteristics e.g. non-metal-metal transition, metallic and covalent respectively. Results of these studies have been used to develop icosahedral AI₁₂X (X = C, Si and Ge) superatoms with 40 valence electrons which correspond to a filled electronic shell. It is found that the doping leads to a large gain in the binding energy as compared to Al₁₃, suggesting this to be a novel way of developing species for cluster assembled materials. Further studies of adsorption of Li, Si and Cl atoms on Al₇ and Al₁₃ clusters show marked variation in the adsorption behaviour of clusters as a function of size and the adsorbate. Silicon reconstructs both the clusters and induces covalency in Al-Al bonds. We discuss the adsorption behaviour in terms of the superatom-atom interactions.     

Structural and electronic properties of AlX (X = P,As, Sb) nanowires: Ab initio study

Present paper discusses the structural stability and electronic properties of AlX (X = P, As and Sb) nanowires in its linear, zigzag, ladder, square and hexagonal type atomic configurations. The structural optimization has been performed in self consistence manner by using generalized gradient approximation with revised Perdew, Burke and Ernzerhof type parameterization. The study observes that in all the three nanowires, the square shaped atomic configuration is the most stable one. The calculated electronic band structures and density of states profile confirms the semiconducting behaviour of linear and zigzag shaped nanowires of AlP, whereas for AlAs and AlSb nanowires are metallic. The ground state properties have also been analysed in terms of bond length, bulk modulus and pressure derivative for all the nanowires along with their bulk counterpart. The lower bulk modulus of all the linear shaped geometries of AlX nanowires in comparison to its bulk counterpart indicates softening of the material at reduced dimension.     

Linearized equations for the electronic transport properties of thin metal films

Starting from the multidimensional statistical model of grain boundaries a new unique general expression is proposed for the resistivity, the temperature coefficient of resistivity and the Hall coefficient of monocrystalline, polycrstalline and columnar films. The ranges of validity of linearized equations are determined. Experiments of other workers related to columnar films are interpreted satisfactorily in this way.     

Ab initio simulations of the mechanics and electrical transport of Pt nanowires

Based on first principles, theoretical studies of atomic-scale platinum contacts are presented. A short monatomic wire freely suspended between tips is seen to vibrate as its tensile load increases. The main vibration mode is transversal for lower tensions and longitudinal for higher tensions up to the breaking of the nanowire. The computed conductance exhibits oscillation in the sub-picosecond regime that can be well correlated to the mechanical oscillations of the nanowire. Both the values for the maximum tensile load and the average conductance agree well with available experimental measurements.     

Adsorbate and defect effects on electronic and transport properties of gold nanotubes

First-principles calculations have been performed to study the effects of adsorbates (CO molecules and O atoms) and defects on electronic structures and transport properties of Au nanotubes (Au(5, 3) and Au(5, 5)). For CO adsorption, various adsorption sites of CO on the Au tubes were considered. The vibrational frequency of the CO molecule was found to be very different for two nearly degenerate stable adsorption configurations of Au(5, 3), implying the possibility of distinguishing these two configurations via measuring the vibrational frequency of CO in experiments. After CO adsorption, the conductance of Au(5, 3) decreases by 0.9G(0) and the conductance of Au(5, 5) decreases by approximately 0.5G(0). For O-adsorbed Au tubes, O atoms strongly interact with Au tubes, leading to around 2G(0) of drop in conductance for both Au tubes. These results may have implications for Au-tube-based chemical sensing. When a monovacancy defect is present, we found that, for both tubes, the conductance decreases by around 1G(0). Another type of defect arising from the adhesion of one Au atom is also considered. For this case, it is found that, for the Au(5, 3) tube, the defect decreases the conductance by nearly 1G(0), whereas for Au(5, 5), the decrease in conductance is only 0.3G(0).     

Ab initio insights into the visible luminescent properties of ZnO

The luminescence spectrum of ZnO exhibits, besides a UV band-edge recombination line, a broad visible band around 2.2-2.4 eV whose origin has not been satisfactorily established. Recently, analysis of the luminescence of nanostructured materials with high surface-to-volume ratios has led some authors to suggest that the band could be related to surface states. This work presents a novel ab initio study of the most relevant ZnO surfaces and their intrinsic point defects. It reveals the existence of intragap surface states 0.5 eV above the valence band maximum. If additional bulk defect levels are considered, several bulk-to-surface transitions are compatible with the observed visible luminescence.     

Formation and properties of thin films of iron silicides on Si(111) Surface: Ab initio simulation

Density functional theory in the generalized gradient approximation has been used to calculate the total energy and model the atomic and electronic structures of thin FeSi films with CsCl type lattice and γ-FeSi₂ films with CaF₂ fluorite type lattice on a Si(111) surface. It is shown that, upon the adsorption of two monolayers of iron atoms on Si(111), the most energetically favorable process is the growth of a γ-FeSi₂ film with CaF₂ type structure. The electronic structure of a silicide film formed upon the adsorption of one monolayer of iron atoms exhibits features that are characteristic of both FeSi and γ-FeSi₂. The density of states calculated for the γ-FeSi₂ well agrees with the experimental photoemission spectra reported in the literature.     

Ab initio studies of single-height Si(001) steps

We have studied the Si(001) surface with single-height steps by ab initio molecular dynamics simulations. Surface dimers were found to be unstable with respect to buckling for all geometries considered. However, the ground state reconstruction depends on the type of step. For the SA step, the c(2 × 4) geometry is induced by the step edge, while, for the SB step, the p(2 × 2) reconstruction is more stable. The binding sites and diffusion barriers for a single Si adatom were investigated via the adiabatic trajectory method. In agreement with other studies of the flat surface, fast diffusion takes place along the dimer rows. The local changes to buckling induced by the adatom are sizable and lead to changes in the activation barriers for diffusion, in particular for the path perpendicular to the dimer rows. We also investigated the diffusion of the adatom over the rebonded SB step. The calculations show that there is no additional barrier for the arrival of the adatom at the edge from the upper terrace, while a barrier of at least 1 eV exists for the arrival of the adatom from the lower edge. In step flow growth involving the rebonded SB step, most of the adatoms will thus arrive from the upper terrace.     

Ab initio calculation of the structural and dynamical properties of layered semiconductors

We present a theoretical first-principles investigation of the structure and lattice dynamics of several layered semiconductors. The equilibrium structure as obtained by minimization of the total energy of the bulk materials is in good agreement with experiment. Furthermore, we have investigated the surface of these materials in order to obtain information on the van der Waals epitaxial growth. We found that the relaxed atomic positions at the surface deviate from the ideal ones in the bulk by less than 1%, which is obviously a consequence of the weak interlayer forces. Additionally, bulk phonon-dispersion curves have been calculated along several high symmetry directions within the density-functional perturbation theory (DFPT). The weak interlayer interaction makes the vibrational properties of the bulk very similar to those of the surface. In fact, our ab initio calculations for the bulk reproduce well both the experimental bulk phonon frequencies obtained by inelastic neutron scattering and the experimental surface phonon dispersion measured with inelastic He-atom scattering (HAS).     

Electrical transport properties of iron (II) molybdate

Electrical conductivity, thermoelectric power and static dielectric constant of iron (II) molybdate have been measured in the temperature range 300 to 1000 K on pressed pellets of polycrystalline sample. It has been found that FeMoO₄ is a p-type semiconductor with energy gap 4.1 eV. Different conduction mechanisms have been found below and above 700 K. Below 700 K conduction is due to a small polaron hopping mechanism and above 700 K conduction is due to large polarons as well as normal band conduction mechanism. Activation energy W, ₀ (T) and charge carrier mobility have been estimated in the two temperature ranges 300 to 700 K and 700 to 1000 K. Dielectric constant increases slowly with temperature up to 700 K and above 700 K, it increases exponentially with temperature.     

Ab initio study of the structural, elastic, electronic and optical properties of the antiperovskite SbNMg3

A theoretical study of structural, elastic, electronic and optical properties of the cubic antiperovskite SbNMg₃ is presented using the pseudo-potential plane wave method (PP-PW) within the generalized gradient approximation (GGA). Results are given for lattice constant, elastic constants and their pressure dependence. Band structure, density of states and pressure coefficients of energy gaps are also given. Furthermore, the optical reflectivity, refractive index, extinction coefficient, dielectric function and electron energy loss are calculated for radiation up to 30 eV. The results are compared with the available theoretical and experimental data.     

Ab initio simulation of Si-doped GaAs(110) cross-sectional surfaces

We study different configurations of the (110) cross-sectional surface of Si-doped GaAs, from the isolated Si donor up to an entire donor–acceptor Si bilayer embedded along the (001) growth direction. Electronic potentials, density of electronic states, cross-sectional scanning tunneling microscopy (XSTM) images are calculated using first-principles numerical simulations. Doping configurations with compensating Si impurities in cationic and anionic sites, such as the donor–acceptor bilayer, are characterized by XSTM images with bright signal at negative bias, strongly attenuated when the bias is reversed. These features are characteristic of real samples above the onset of self-compensation. The comparison of the experimental images with the numerical simulations allows to shed light on the microscopic picture of self compensation hitherto associated to a variety of mechanisms – including the formation of complexes of Si with native defects – and to uniquely attribute the observed experimental features to Si donor–acceptor configurations.     

Ab initio study of the adsorption of antimony and arsenic on the Si(110) surface

We have performed first principles total energy calculations to investigate the adsorption of Sb and As adatoms on the Si(110) surface using a (2 × 3) supercell. The energetics and atomic structures have been investigated in four atomic configurations. One structure is obtained by placing 1/3 of a monolayer (ML) of Sb (As) atoms on the Si(110) surface. The other three geometries are obtained by depositing 1 ML of Sb (As) atoms on the surface. In the first case the structure is formed by four trimers, in the second case the geometry is formed by zigzag atomic chains and in the third case the structure contains “microfacets”. The energetics results of the Sb adsorption show that for low coverage the tetrahedrons formed by the adsorption of 1/3 ML is the most stable configuration, while in the monolayer region the zigzag atomic chain is the most stable structure. However, the total energies of the trimer and microfacet structures are slightly higher, indicating that under some conditions, they may be formed. In an experimental report it has been suggested that the adsorption of 1/3 and 1 ML of Sb corresponds to the low and high coverage in the experiments of Zotov et al. [A. V. Zotov, V. G. Lifshifts, and A. N. Demidchik, Surf. Sci. 274, L583 (1992)]. On the other hand, our results of the As adsorption show that for low coverage, the tetrahedrons in the adsorption of 1/3 ML also give the most stable configuration. However, at the 1 ML coverage, a structure formed by microfacets is the most stable structure, in agreement with previous results.