Principles Total Energy Calculations Research Articles

FP-LAPW investigation of electronic structure of TaN and TaC compounds

We study the structural and electronic properties of tantalum compounds, i.e. TaC and TaN, by means of accurate first principle total energy calculations using the full-potential linearized augmented plane wave plus local orbitals (FP-LAPW + lo) method. The calculations are based on density functional theory and we use the local density approximation (LDA) as well as the generalized gradient approximation (GGA) for the exchange and correlation potential. The calculated electronic structure, total energies, and equilibrium lattice constants are determined for the NaCl structure of TaC and TaN, and these results are discussed in terms of the nature of bonding found in these compounds. In both tantalum compounds studied the Ta 5d-like and N(C) 2p-like states are strongly hybridized. The values of the lattice constants, bulk moduli, and elastic constants have been calculated and are in reasonable agreement with experiment and other theoretical works.

Lithiation of InSb andCu2Sb: A theoretical investigation

In this work the mechanism of Li insertion/intercalation in the anode materials InSb and Cu$_2$Sb is investigated by means of the first principles total energy calculations. The total charge densities for the lithiated products of the two compounds are presented. Based on these results the change in the bonding character on lithiation is discussed. Further, the isomer shift for InSb and Cu$_2$Sb and there various lithiated products is reported. The average insertion/intercalation voltage and volume expansion for transitions from InSb to Li$_2$InSb and Cu$_2$Sb to Li$_2$CuSb are calculated and found to be in good agreement with the experimental values. These findings help to resolve the controversy regarding the lithiation mechanism in InSb.

Defect structure ofGa1−xMnxAs: A cross-sectional scanning tunneling microscopy study

We have studied the atomic scale structure of molecular beam epitaxy grown Ga1-xMnxAs compounds with various Mn concentrations by cross-sectional scanning tunneling microscopy and first principles calculations. Only bright protrusions close to the top layer As atoms are directly correlated with the bulk Mn concentration. Atomically resolved filled and empty states images of this defect are compared to images derived from first principles calculations. We identify the Mn related defect as substitutional Mn in the second layer Ga site. Surprisingly no substitutional Mn is observed in the top-most Ga layer. The experimental results are consistent with the energetics of our first principles total energy calculations.

Trace element effects on precipitation in Al–Cu–Mg–(Ag, Si) alloys: a computational analysis

In order to optimize mechanical properties of age-hardenable Al–Cu–Mg–(Ag, Si) alloys, the nucleation of the strengthening precipitate Ω must be enhanced while competing phases such as S and θ (or θ″/θ ′) must be suppressed. One strategy is to promote the formation of the precursors, i.e., certain solute clusters that aid in Ω nucleation, using trace element additions such as Ag, and/or the elimination of deleterious impurities, e.g., Si. This paper describes the use of computational analysis tools to identify those elements that are either helpful or harmful. Using a multi-component quasi-chemical model, short-range-order parameters were calculated based on CALPHAD databases to indicate cluster formation in Al–Cu–Mg–(Ag, Si) solid solutions. To anticipate the orientation of the plate-like clusters that controls the habit-plane selection of the subsequent precipitates, the anisotropy of the associated strain was considered using the second order elastic tensors obtained from first principle total energy calculations.

Trace element effects on precipitation in Al?Cu?Mg?(Ag, Si) alloys: a computational analysis

In order to optimize mechanical properties of age-hardenable Al–Cu–Mg–(Ag, Si) alloys, the nucleation of the strengthening precipitate Ω must be enhanced while competing phases such as S and θ (or θ″/θ′) must be suppressed. One strategy is to promote the formation of the precursors, i.e., certain solute clusters that aid in Ω nucleation, using trace element additions such as Ag, and/or the elimination of deleterious impurities, e.g., Si. This paper describes the use of computational analysis tools to identify those elements that are either helpful or harmful. Using a multi-component quasi-chemical model, short-range-order parameters were calculated based on CALPHAD databases to indicate cluster formation in Al–Cu–Mg–(Ag, Si) solid solutions. To anticipate the orientation of the plate-like clusters that controls the habit-plane selection of the subsequent precipitates, the anisotropy of the associated strain was considered using the second order elastic tensors obtained from first principle total energy calculations.

Density functional theory investigation of benzenethiol adsorption on Au(111).

We have studied the adsorption of benzenethiol molecules on the Au(111) surface by using first principles total energy calculations. A single thiolate molecule is adsorbed at the bridge site slightly shifted toward the fcc-hollow site, and is tilted by 61 degrees from the surface normal. As for the self-assembled monolayer (SAM) structures, the (2 square root of 3 x square root of 3)R30 degrees herringbone structure is stabilized against the (square root 3 x square root 3)R30 degrees structure by large steric relaxation. In the most stable (2 square root 3 x square root 3)R30 degrees SAM structure, the molecule is adsorbed at the bridge site with the tilting angle of 21 degrees, which is much smaller compared with the single molecule adsorption. The van der Waals interaction plays an important role in forming the SAM structure. The adsorption of benzenethiolates induces the repulsive interaction between surface Au atoms, which facilitates the formation of surface Au vacancy.

First principles study of oxygen vacancy migration in tantalum pentoxide

First principles total energy calculations were performed in order to determine oxygen vacancy migration energies in Ta2O5. A simplified version of the crystalline orthorhombic phase of Ta2O5 was used in this study. O vacancies in the chosen model of Ta2O5 can be broadly classified into ‘cap’ and ‘in-plane’ sites based on their location in the lattice. The cap type of vacancies display the largest barriers both for migration to a neighboring cap site or to a neighboring in-plane site, thus behaving as oxygen vacancy ‘sinks’. A lowering of the barriers to migration is generally seen when the vacancies are doubly positively charged. All calculations were performed within the local density approximation of density functional theory, and the elastic band method was used in the estimation of migration barriers.

Ab initio total energy calculations of copper nitride: the effect of lattice parameters and Cu content in the electronic properties

We have studied the structural and electronic properties of bulk copper nitride by performing first principles total energy calculations using the full-potential linearized augmented plane wave (FP-LAPW) method. In our study we have considered two types of cells: the ideal cubic anti-ReO 3 structure corresponding to Cu 3N, and a unit cell with an extra Cu atom at the center of the cube. In the first case, our calculated lattice parameter a=3.82 Å is in excellent agreement with the experimental value a=3.807 Å. The structure is semiconductor with a small indirect band-gap. The increasing of the lattice parameter results in larger band-gaps. An addition of an extra Cu atom at the center of the cell results in a slightly larger lattice parameter a=3.88 Å, and the structure becomes fully metallic. Our calculated value is similar to the experimental lattice parameter corresponding to a metallic copper nitride film.

On compressibility of osmium metal

On the basis of the high-pressure diamond anvil cell experiments on Os metal, Cynn et al. [Phys. Rev. Lett. 88, 135701-1 (2002)] have reported that this metal has lower compressibility than diamond. In the present work we have reanalysed the experimental data of Cynn et al. We find that the bulk moduli of Os and diamond are close to each other, implying that Os metal is as incompressible as diamond, but not more so. Our first principles total energy calculations using the full potential linearised augmented plane wave method on Os and diamond also suggest the same results.

Yttrium nitride thin films grown by reactive laser ablation

Yttrium nitride thin films were grown on silicon substrates by laser ablating an yttrium target in molecular nitrogen environments. The composition and chemical state were determined with Auger electron, X-Ray photoelectron, and energy loss spectroscopies. The reaction between yttrium and nitrogen is very effective using this method. Ellipsometry measurements indicate that the films are metallic. We attribute this behavior to a small oxygen contamination. Each oxygen atom introduces two additional electrons to the unit cell, resulting in a complex semiconductor–ionic–metallic system. These results are corroborated by first principles total energy calculations of clean and oxygen doped YN.

First principles study of oxygen vacancy defects in tantalum pentoxide

First principles total energy calculations were performed to characterize oxygen vacancy defects in tantalum pentoxide (Ta2O5). A simplified version of the crystalline orthorhombic phase of Ta2O5 was used in this study. Results indicate that O vacancies in Ta2O5 can be broadly classified based on their location in the lattice. One type of vacancy that occupies the “in-plane” sites displays deep or midgap occupied states and shallow unoccupied states, while a second type occupying “cap” sites results in shallow occupied states. For a wide range of Fermi levels or chemical potentials, the neutral and +2 charged states of the in-plane type vacancy and the +2 charge state of the cap type vacancy are found to be most stable.

First principles total energy calculations of the adsorption of germane and digermane on Si(0 0 1)-c(2 × 4)

First principles total energy studies are performed to investigate the energetics, and the atomic structure of the adsorption of germane (GeH 4), and digermane (Ge 2H 6) on the Si(0 0 1)-c(2 × 4) surface. It has been observed experimentally that adsorption of Ge 2H 6 is a dissociative process, which first yields GeH 3 and then GeH 2 fragments as products. We first study the adsorption of GeH 2 considering two different models; the intra-row and the on-dimer geometries. Our results show that the on-dimer site is more stable than the intra-row geometry by 0.44 eV. This is not a surprise since in the absence of H atoms, adsorption in the on-dimer site leaves no dangling bonds. In contrast, when the GeH 2 fragment is considered together with two H atoms, the intra-row geometry is favored energetically as compared with the on-dimer site, in good agreement with experiment. Similar results have been previously obtained for the adsorption of SiH 2 on Si(0 0 1). Digermane adsorption is explored according to two different geometries. In the first one, we have considered the adsorption as two GeH 3 fragments, while in the second, we have considered the adsorption as two GeH 2 fragments plus 2 H fragments. In good agreement with experiments, it is found that the latter geometry is energetically more favorable.

Ab initio study of the structural and electronic properties of the complex structures of RuO 2

We have studied structural and electronic properties of RuO 2 in the two complex structures (rutile and orthorhombic) by means of accurate first principle total energy calculations using the full potential linearized augmented plane wave (FP-LAPW). The calculations are presented within the local-density approximation and the generalized gradient approximation. The orthorhombic (CaCl 2-type Pnnm) structure is just a deformation of the rutile (P4 2/mnm). Until now, considerable work has been performed on the rutile structure; however, no work has been performed on the orthorhombic structure. The structural properties are in good agreement with experiment. Our calculations of the electronic properties have been compared with experiment and previous calculations and a good agreement is found.

First principles total energy calculations of the adsorption of single Cl2 and Br2 molecules on the Si(001)‐c(2 × 4) surface

AbstractWe have performed first principles total energy calculations to investigate the adsorption of Cl2 and Br2 molecules on the Si(001)‐c(2 × 4) surface. First of all, our results indicate that the adsorption of Cl2 and Br2 as molecules is not energetically favorable. Instead, Cl2 (Br2) dissociates into two Cl (Br) subunits. The most stable geometry was achieved when each Cl (Br) subunit is bonded to a Si atom of the same Si dimer along the dangling bonds. Three other geometries were considered in our study: the first one corresponds to adsorption of the two subunits on Si dimer sites in adjacent silicon dimer rows. The second corresponds to adsorption of the two Cl (Br) subunits along dangling bonds of two consecutive dimers of the same Si dimer row, with the two Cl(Br) atoms in the same side of the Si dimer row. The third one is similar to the second, but the ad‐atoms are now in a staggered arrangement, occupying sites in opposite sides of the Si dimer row. These are adsorption configurations that have been observed experimentally for Cl2 on Si(001). Our results indicate that energetics and atomic geometries are similar for Cl2 and Br2. We have also calculated energy barriers for the ad‐atom to move along and across the Si dimer rows. It is found that the barriers for Br are smaller than the barriers for Cl. This result can explain the differences observed in the adsorption of Cl2 and Br2 molecules.

First Principles Calculations of the Electronic Properties of Bulk Cu2O, Clean and Doped with Ag, Ni, and Zn.

Abstract Using first principles total energy calculations we have studied the electronic properties of bulk Cu2O, clean and doped with Ag, Ni and Zn. The calculated ground state structure of clean Cu2O is cubic. Its lattice constant a=4.30 A, and bulk modulus B0=108 GPa are in excellent agreement with experimental values a=4.27 A, and B0=112 GPa. In its ground state, Cu2O is a semiconductor material with a small direct band gap at the Γ point. Our calculated value of ∼0.5 eV is smaller than the experimental value of ∼2.0 eV, reflecting the problems of local density functional theory calculating the excited states. Doping with Ag, results in a reduction of the band gap. This result is important, since it implies that the band gap of Cu2O can be engineered by varying the amount of Ag in the crystal. Doping with Ni gives rise to a p-type semiconductor with the impurity levels above the valence band maximum, while doping with Zn results in a n-type semiconductor, with impurity levels above the conduction band minimum.

First principles total energy calculations of the structural and electronic properties of ScxGa1–xN

AbstractIn this issue's Editor's Choice [1], results of structural and electronic properties calculations of ScxGa1–xN in the full range of scandium concentrations are presented. Investigations on the relative stability of various configurations have revealed that for x < 0.65, the favoured structure is wurtzite (right figure on the cover) whereas for x > 0.65 it is the rocksalt‐like structure (left figure).The first author, Maria Guadalupe Moreno‐Armenta, received her Ph.D. in Materials Physics from the Centro de Investigación Científica y de Educación Superior de Ensenada in 2000. She is currently assistant professor at the Centro de Ciencias de la Materia Condensada‐UNAM, México. Her research interests are in the area of solid state science, focused on first principles calculations of electronic structure from bulk semiconductors, mainly nitrides, with the main concern to propose materials for potential technological applications. She has recently started studies about the interaction mechanisms of molecules with the solid surface.

Influence of ad-dimers on the incorporation of carbon in the Si(100) surface

We present first principle total energy calculations of carbon incorporation in the Si(100) surface in the presence of silicon ad-dimers either parallel or perpendicular to the dimer rows of the clean surface. By modifying the local stress, the ad-dimers are found to make the carbon penetration in the subsurface layers easier. At low coverages (θ≤0.125), carbon atoms are preferentially adsorbed in the third layer directly below an ad-dimer, and the barrier at the crossing of the second layer on the clean surface is strongly reduced or completely vanishes. At higher coverage (0.125≤θ≤0.25), configurations with part of the carbons at the surface and part in the third layer are favored. These tendencies are enhanced with increasing ad-dimer density.

Oxygen vacancy defects in tantalum pentoxide: a density functional study

First principles total energy calculations were performed in order to characterize O vacancy defects in Ta 2O 5. A simplified version of the crystalline orthorhombic phase of Ta 2O 5 was used in this study. Results indicate that O vacancies in Ta 2O 5 can be broadly classified based on their location in the lattice. One type of vacancy (occupying the ‘in-plane’ sites) displays deep or mid gap occupied states, and shallow unoccupied states, while a second type (occupying ‘cap’ sites) results in shallow occupied states. For a wide range of local Fermi level or chemical potential, the neutral and +2 charged states of the in-plane type vacancy and the +2 charge state of the cap type vacancy are found to be most stable.

First principles total energy calculations of the structural and electronic properties of ScxGa1–xN

AbstractUsing first principles total energy calculations within the the full‐potential linearized augmented plane wave (FP‐LAPW) method, we have investigated the structural and electronic properties of ScxGa1–xN, with Sc concentrations varying from 0% up to 100%. In particular we have studied the relative stability of several configurations of ScxGa1–xN in wurtzite‐like structures (the ground state configuration of GaN), or in rocksalt‐like structures (the ground state configuration of ScN). It is found that for Sc concentrations less than ≈65%, the favored structure is a wurtzite‐like one, while for Sc concentrations greater than ≈65%, the favored structure is a NaCl‐like structure. It is also found that for the wurtzite‐like crystals, the fundamental gap is large and direct. For the rocksalt crystals the fundamental gap is small and indirect, but with an additional larger direct gap. In agreement with the experiments of Little and Kordesch [Appl. Phys. Lett. 78, 2891 (2001)] we found a decrease of the band gap with the increase of the Sc concentration.

Growth morphology, structure, and magnetism of ultrathin Co films on W(111)

The growth morphology, structure, and magnetism of ultrathin Co films on the W(111) surface are studied with conventional and spin polarized low energy electron microscopy and diffraction and first principles total energy calculations. Pseudo-layer-by-layer growth of thick pseudomorphic (ps) Co films is observed at 380 K, while Stranski-Krastanov growth and transformation to a (6\ifmmode\times\else\texttimes\fi{}6) closed-packed structure are observed at higher temperatures. On the basis of the calculations, only one ps Co monolayer is expected to be thermodynamically stable against three-dimensional island formation. Calculations also indicate that the magnetic moment on Co atoms at the interface is reduced due to strong hybridization with the substrate, but remains non-zero even for the single ps monolayer. Metastable ps Co/W(111) is found experimentally to be ferromagnetically ordered at 380 K when film thickness exceeds 7.6 ps ML. Although only in-plane magnetization is observed, substrate atomic steps have a strong influence on the magnetization easy axis and magnetic domain structure, which suggests that the in-plane magnetocrystalline anisotropy is weak.