Total Energy Pseudopotential Calculations Research Articles

2D hexagonal boron nitride for solar energy conversions

The optoelectronic properties of free standing monolayer (ML) hexagonal boron nitride (h-BN) is investigated for potential solar energy conversion applications using the density functional theory (DFT) full potential linearized augmented plane wave (FP-LAPW) method. In addition, the bulk optical properties have also been calculated for the sake of comparison. The dielectric functions, optical conductivities and the optical constants are evaluated using the relaxed structures from electronic total energy pseudopotential calculations. The results reinforce previous research on h-BN DUV optoelectronics and demonstrate the suitability of its use as a component in deep ultraviolet (DUV) and energy conversion devices.

First-principles investigation of monatomic gold wires under tension

Ab initio pseudopotential total-energy calculations on infinite monatomic chains of Au are performed within density functional theory. We use the density functional perturbation theory to study the phonon spectra of these gold wires as a function of strain. Our results show that there does not seem to be a range of strain for which the linear chain is stable, contrary to what was stated by Ribeiro and Cohen [Phys. Rev. B 68 (2003) 035423]. For low strain, the zigzag chain is the stable geometry; while for higher strain, the chains with two or more aligned gold atoms are found to be more stable. At the limit between these two regimes, we predict a transition structure (an asymmetric zigzag chain) to be the most stable.

Plane wave density functional theory studies of the structural and the electronic properties of amino acids attached to graphene oxide via peptide bonding

We studied the electronic and the structural properties of amino acids (alanine, glycine, and histidine) attached to graphene oxide (GO) by peptide bonding, via plane wave pseudopotential total-energy calculations within the local spin density approximation (LSDA). The HOMO-LUMO gap, the Hirshfeld charges, and the equilibrium geometrical structures exhibit keen variations depending on the species of the attached amino acid. The GO-amino acid system appears to be a good candidate as a biosensor.

Two-dimensional Sc2C: A reversible and high-capacity hydrogen storage material predicted by first-principles calculations

Abstract Recently, a new family of two-dimensional (2D) MXene materials was prepared by exfoliating the MAX phases ( ACS Nano 2012, 6 , 1322). Among all possible MXene phases, theoretically 2D Sc 2 C possesses the highest surface area per weight and thus is expected to have the highest gravimetric hydrogen storage capacities. In this work, using first-principles total energy pseudopotential calculations, we systematically investigated the hydrogen storage properties of 2D Sc 2 C phase. Depending on different adsorption sites, the hydrogens are bound by three modes: chemisorption, physisorption and Kubas-type interactions with the binding energies of 4.703, 0.087 and 0.164 eV respectively. The maximum hydrogen storage capacity was calculated to be 9.0 wt.%, which meets the gravimetric storage capacity target (5.5 wt.% by 2015) set by the U.S. DOE. Ab-initio molecular dynamic simulations confirmed that 3.6 wt.% hydrogen molecules storaged by Kubas-type interactions can be adsorbed and released reversibly at ambient conditions.

Structural Stability and Energy of a Pd2Ni Nanofilm: Ab Initio Calculations

Recently, using molecular dynamics simulation in conjunction with an embedded-atom method potential, we have predicted Pd2Ni surface-sandwich ordering at the nanoscale. These findings open up a range of opportunities for the synthesis of new kinds of Pd-Ni nanostructures such as a five-layer Pd2Ni nanofilm from which a Pd2Ni nanotube might be fabricated. In this paper, we report on an ab initio spatial optimization and structural energy calculation of a five-layer Pd2Ni nanofilm, which are performed using plane-wave pseudopotential total energy calculations in the generalized gradient approximation of density functional theory. The results of the ab-initio calculations show that the five-layer Pd2Ni nanofilm is structurally stable and its energy is approximately 0.4 eV higher than the energy of a bulk crystal alloy having the same composition.

Adsorption of Pt and Bimetallic PtAu Clusters on the Partially Reduced Rutile (110) TiO2 Surface: A First-Principles Study

An extensive study of the adsorption of small Ptn (n = 1–8) and bimetallic Pt2Aum (m = 1–5) clusters on the partially reduced rutile (110) TiO2 surface has been performed via total energy pseudopotential calculations based on density functional theory. Structures, energetics, and electronic properties of adsorbed Ptn and Pt2Aum clusters have been determined. The surface oxygen vacancy site has been found to be the nucleation center for the growth of Pt clusters. These small Pt clusters strongly interact with the partially reduced surface and prefer to form planar structures for n = 1–6 since the cluster–substrate interaction governs the cluster growth at low Pt coverage. We found a planar-to-three-dimensional structural transition at n = 7 for the formation of Ptn clusters on the reduced TiO2 surface. GGA+U calculations have also been performed to get a reasonable description of the reduced oxide surface. We observed significant band gap narrowing upon surface–Ptn cluster interaction which leads to the formation of gap localized Pt states. In the case of bimetallic Pt–Au clusters, Aum clusters have been grown on the Pt2–TiO2 surface. The previously adsorbed Pt dimer at the vacancy site of the reduced surface acts as a clustering center for Au atoms. The presence of the Pt dimer remarkably enhances the binding energy and limits the migration of Au atoms on the titania surface. The charge state of both individual atoms and clusters has been obtained from the Bader charge analysis, and it has been found that charge transfer among the Pt atoms of Ptn clusters and the metal oxide surface is stronger compared to that of Au clusters and the Pt2–TiO2 system.

First-principles study of phase stability of stoichiometric vanadium nitrides

First-principles pseudopotential total-energy and phonon-spectrum calculations of various phases of stoichiometric vanadium nitrides with a small number of vanadium and nitrogen vacancies, as well as with small carbon and oxygen admixtures, were carried out. It was found that the stoichiometric hexagonal and tetragonal vanadium nitrides (VN) were dynamically stable. The NaCl-type VN exhibited lattice instability. The total energy for the computed ground-state phases of VN increased in the sequence hexagonal (WC type, $P\overline{6}m2$)--hexagonal (AsNi type, $P{6}_{3}/mmc$)--tetragonal ($P{4}_{2}/mcm$)--cubic (NaCl type, $Fm\overline{3}m$)--cubic (ZnS type, $F\overline{4}3m$)--cubic (CsCl type, $Pm\overline{3}m$). At low temperatures, the vacancies in both sublattices stabilize the triclinic phases of V${}_{x}$N${}_{x}$ for $x<0.94$, which is consistent with tetragonal symmetry. At high temperatures, the stability of the NaCl- and WC-type VN structures was estimated using an approach based on band-energy smearing. The results obtained indicate that the cubic structure of VN with the fully occupied sublattices will be stable only at high temperatures. The small carbon and oxygen admixtures do not stabilize the cubic phase compared to the hexagonal one. The phase stability of the cubic and hexagonal structures of V${}_{x}$N${}_{x}$ was explained on the basis of the peculiarities of their electronic densities of states near the Fermi level. The predicted structural transformations in stoichiometric vanadium nitrides are compatible with those observed experimentally.

Structural stability and electronic properties of M2TaN3,ε-TaN and MTa2N3(M = Ti, Zr, Hf) compounds

Using a Plane-Wave Pseudo-Potential (PWPP) method, total energy and band structure calculations for M2TaN3, e-TaN and MTa2N3 (M = a transition metal, TM) compounds have been performed in order to understand their structural stability and electronic properties. To do that, we first focus on e-TaN compounds. The exchange correlation is treated using the Generalized Gradient Approximation (GGA). The Virtual Crystal Approximation (VCA) is then used to examine the structural stability when substituting Ti, Zr or Hf to a Ta atom either in a cell corner [c(M)(Ta2N3)] or inside the cell [in(M2)(TaN3)]. Actually, substitution of Ta by a Ti, Zr or Hf atom at a corner site does slightly change the corresponding lattice constant. Also we calculate ground-state quantities such as elastic constants, shear moduli, Young’s modulus and bulk modulus as well as Poisson’s ratio. The corresponding results for band structures and densities of states are shown as well. As far as we know our work is a pioneer attempt to determine elastic, mechanic and electronic properties for M2TaN3 and MTa2N3 (M = TM) compounds.

Interaction of BrPDI, BrGly, and BrAsp with the Rutile TiO2(110) Surface for Photovoltaic and Photocatalytic Applications: A First-Principles Study

The adsorption of perylenediimide (PDI)-based dye compounds (BrPDI, BrGly, and BrAsp) on the defect-free unreconstructed (UR) rutile TiO2(110) surface has been studied using total energy pseudopotential calculations based on density functional theory. All dye molecules form moderate chemical bonds with the defect-free UR rutile (110) surface in the most stable adsorption configurations. Electronic structure analysis reveals that HOMO and LUMO levels of the adsorbed dye molecules appear within the band gap and conduction band region of the UR surface, respectively. The effect of model slab thickness on interaction strength between the dye and the UR surface has also been examined. Unlike on four-layer slabs, BrGly and BrAsp molecules are dissociatively adsorbed on the three-layer slabs. Interaction between the BrPDI and partially reduced UR rutile (110) as well as the platinized UR surface has also been considered to figure out the effects of O vacancy and previously adsorbed Pt clusters on the binding, electronic, and structural properties of the dye–surface system. The BrPDI molecule prefers to bind to the O vacancy site of the partially reduced UR surface. The existence of the small Ptn (n = 1, 3, and 5) clusters on the reduced UR surface does not significantly alter the binding strength between the surface and BrPDI.

Structural, electronic and optical properties of smallest (2,2) carbon nanotube: A plane-wave pseudopotential total energy calculation

Based on plane-wave pseudopotential total energy calculations, we investigate the structural, electronic and optical properties of the smallest (2,2) carbon nanotube. Our calculated results indicate that severe structural changes take place due to the large curvature in the smallest (2,2) carbon nanotube. The energy gap obtained from ultrasoft pseduopotential and norm-conserving pseudopotential is 0.132 eV and 0.166 eV, respectively. Distributions of charge density indicate that the electron localization in the bonds perpendicular to the axis of (2,2) carbon nanotube optimized by using norm-conserving pseudopotential has a more distinct trend than that optimized by using ultrasoft pseudopotential, which leads to a relative smaller energy gap in the latter. For the electric field parallel to the nanotube axis, the ε″ spectrum of the dielectric function in the low-energy region is dominated by a single peak, which can be compared with the recent Raman spectra measures. In high-energy region, the optical anisotropy is found to be remarkable.

An ab initio study of the electronic structure and elastic properties of the newly discovered ternary carbide [formula omitted

The electronic structure and elastic properties of the newly discovered ternary layered carbide Ti 4GaC 3 were investigated by means of the first-principle plane-wave pseudopotential total energy calculation method based on density functional theory. The computed results, including lattice constants and internal coordinates, are in good agreement with experimental values. The elastic moduli of ideal polycrystalline Ti 4GaC 3 were predicted from the individual elastic constants by Voigt approximation. The band structure shows that the electrical conductivity is metallic and anisotropic, with a high density of states at the Fermi energy. The elastic properties are anisotropic, related to the Ti–Ga bonds being relatively weaker than the Ti–C bonds.

First-Principles Study of Equilibrium Properties and Electronic Structures of GeTe–Sb2Te3 Pseudobinary Crystalline Films

On the basis of first-principles pseudopotential total energy calculations, Ge1Sb2Te4, Ge2Sb2Te5, and Ge1Sb4Te7 of GeTe–Sb2Te3 pseudobinary systems were studied. First, the most possible models for Ge1Sb2Te4 were compared and investigated in detail to identify the most favorable one by examining the lattice configurations, total energy, cohesive energy, and X-ray diffraction powder curves. Then, we discussed the density of states and band structures of selected models of the GeTe–Sb2Te3 pseudobinary system and compared the corresponding values with each other to find similar electronic structures that are due to the similar structures with repeating units of Sb2Te3 and GeTe layers. Furthermore, we pointed out why Ge2Sb2Te5 has the most stable structure among the three compounds by referring to a “Te–Te layer” model.

Global space-group optimization problem: Finding the stablest crystal structure without constraints

Finding the most stable structure of a solid is one of the central problems in condensed matter physics. This entails finding both the lattice type (e.g., fcc, bcc, and orthorhombic) and (for compounds) the decoration of the lattice sites by atoms of types $A$, $B$, etc. (``configuration''). Most approaches to this problem either assumed that both lattice type and configuration are known, optimizing instead the cell volume and performing local relaxation. Other approaches assumed that the lattice type is known, searching for the minimum-energy decoration. We present here an approach to the global space-group optimization (GSGO) problem, i.e., the problem of predicting both the lattice structure and the atomic configuration of a crystalline solid. This search method is based on an evolutionary algorithm within which a population of crystal structures is evolved through mating and mutation operations, improving the population by substituting the highest total-energy structures with new ones. The crystal structures are not represented by bit strings as in conventional genetic algorithms. Instead, the evolutionary search is performed directly on the atomic positions and the unit-cell vectors after a similarity transformation is applied to bring structures of different unit-cell shapes to a common basis. Following this transformation, we can define a crossover operation that treats, on the same footing, structures with different unit-cell shapes. Once a new structure has been generated by mating or mutation, it is fully relaxed to the closest local total-energy minimum. We applied our procedure for the GSGO in the context of pseudopotential total-energy calculations to the semiconductor systems Si, SiC, and GaAs and to the metallic alloy AuPd with composition ${\mathrm{Au}}_{8}{\mathrm{Pd}}_{4}$. Starting from random unit-cell vectors and random atomic positions, the present search procedure found for all semiconductor systems studied the correct lattice structure and configuration. In the case of ${\mathrm{Au}}_{8}{\mathrm{Pd}}_{4}$, the search retrieved the correct underlying fcc lattice, but energetically closely spaced $(\ensuremath{\sim}2\phantom{\rule{0.3em}{0ex}}\mathrm{meV}∕\mathrm{at.})$ alloy configurations were not resolved. This approach to GSGO opens the way to predicting unsuspected structures by direct optimization using, in the cases noted above, an order of $100$ total-energy ab initio calculations.

Investigation of surface structure related features in the multiple-scattering simulations of photoelectron diffraction of 3C–SiC(0 0 1)- [formula omitted

In this paper we have conducted a systematic study of the photoelectron diffraction (PED) effects from the 3C–SiC(0 0 1)- c ( 4 × 2 ) surface by means of multiple-scattering calculations. Two models have been proposed to describe this surface: alternately up and down dimers (AUDD) [P. Soukiassian, F. Semond, L. Douillard, A. Mayne, G. Djuradin, L. Pizzagalli, C. Joachim, Phys. Rev. Lett. 78 (1997) 907], based on the scanning tunneling microscopy data, and missing-row asymmetric-dimer (MRAD) [W. Lu, P. Krüger, J. Pollmann, Phys. Rev. Lett. 81 (1998) 2292], based on the total energy pseudopotential calculations. By calculating PED patterns from different emitters in these two models, we show that the surface structure induced features are visible even in total emission diffraction patterns. For the overall diffractogram a scatterer properties of the first layer under the surface become crucial due to the backscattering at low kinetic energies. This layer is Si-layer in the MRAD model, but C-layer in the AUDD model. While this causes clear differences between the diffraction patterns of the AUDD and the MRAD models, it is shown to diminish the differences between the PED patterns of the AUDD and the other models which consist of only one monolayer of Si on top.

Interaction of Pt Clusters with the Anatase TiO2(101) Surface: A First Principles Study

The adsorption of Pt(n)() (n = 1-3) clusters on the defect-free anatase TiO(2)(101) surface has been studied using total energy pseudopotential calculations based on density functional theory. The defect-free anatase TiO(2)(101) surface has a stepped structure with a step width of two O-Ti bond distances in the (100) plane along the [10] direction and the edge of the step is formed by 2-fold-coordinated oxygen atoms along the [010] direction. For a single Pt adatom, three adsorption sites were found to be stable. Energetically, the Pt adatom prefers the bridge site formed by 2 2-fold-coordinated oxygen atoms with an adsorption energy of 2.84 eV. Electronic structure analysis showed that the Pt-O bonds formed upon Pt adsorption are covalent. Among six stable Pt(2) adsorption configurations examined, Pt(2) was found to energetically favor the O-O bridge sites on the step edge along [010] with the Pt-Pt bond axis perpendicular to [010]. In these configurations, one of the Pt atoms occupies the same O-O bridge site as for a single Pt adatom and the other one either binds a different 2-fold-coordinated oxygen atom on the upper step or a 5-fold-coordinated Ti atom on the lower terrace. Three triangular and three open Pt(3) structures were determined as minima for Pt(3) adsorption on the surface. Platinum trimers adsorbed in triangular structures are more stable than in open structures. In the most stable configuration, Pt(3) occupies the edge O-O site with the Pt(3) plane being upright and almost perpendicular to the [001] terrace. The preference of Pt(n)() to the coordinately unsaturated 2-fold-coordinated oxygen sites indicates that these sites may serve as nucleation centers for the growth of metal clusters on the oxide surface. The increase in clustering energy with increasing size of the adsorbed Pt clusters indicates that the growth of Pt on this surface will lead to the formation of three-dimensional particles.

Instability of graphite structure induced by excited holes

We are proposing a brand new method to synthesize diamond via electronically excited states. An ab initio pseudopotential total-energy calculation has been carried out in order to investigate the stability of the graphite structure under hole dopings (substitutional B, Be or intercalated F). As a result, the graphite is unstabilized by doping holes. The activation energy for the conversion from the graphite to the diamond dramatically decreases, which will result in drastic decrease of the transition pressures and temperatures. On the other hand, the local lattice relaxation around the acceptor atom may adversely affect the whole lattice changes in the diamond structure.

Ab initiocalculations of the electronic structure and bonding characteristics ofLaB6

Lanthanum hexaboride (${\mathrm{LaB}}_{6}$, NIST SRM--660a) is widely used as a standard reference material for calibrating the line position and line shape parameters of powder diffraction instruments. The accuracy of this calibration technique is highly dependent on how completely the reference material is characterized. Critical to x-ray diffraction, this understanding must include the valence of the La atomic position, which in turn will influence the x-ray form factor $(f)$ and hence the diffracted intensities. The electronic structure and bonding properties of ${\mathrm{LaB}}_{6}$ have been investigated using ab initio plane-wave pseudopotential total energy calculations. The electronic properties and atomic bonding characteristics were analyzed by estimating the energy band structure and the density of states around the Fermi energy level. The calculated energy band structure is consistent with previously reported experimental findings; de Haas--van Alphen and two-dimensional angular correlation of electron-positron annihilation radiation. In addition, the bond strengths and types of atomic bonds in the ${\mathrm{LaB}}_{6}$ compound were estimated by analyzing the Mulliken charge density population. The calculated result revealed the coexistence of covalent, ionic, and metallic bonding in the ${\mathrm{LaB}}_{6}$ system and partially explains its high efficiency as a thermionic emitter.

Structural and magnetic properties of Mn-doped GaAs(1 1 0) surface

We have investigated STM images of the (1 1 0) cross-sectional surface of Mn-doped GaAs using first principles total-energy pseudopotential calculations. We focus on configurations with Mn interstitial in the uppermost surface layers. In particular, we have found that Mn impurities, surrounded by Ga or As atoms, introduce in both cases strong local distortions in the GaAs(1 1 0) surface, with bond length variations up to 8% on surface and non-negligible relaxations effects propagating up to the third subsurface layer. In both cases interstitial Mn induces a spin-polarization on its nearest neighbors, giving rise to a ferromagnetic Mn–As and to antiferromagnetic Mn–Ga configuration.

First-Principles Study of Hydrogen Impurity in HgO

We investigate the structural and electronic properties of hydrogen impurity in HgO through first-principles pseudo-potential total-energy calculations. The most stable position of the interstitial hydrogen is identified to be at the location between two O atoms, each of which is in two adjacent chains, forming a H-O bonding. Our electronic-structure calculations indicate that the interstitial hydrogen is a shallow-donor-type impurity. We also examined the formation enthalpy of the interstitial hydrogen. This indicates that HgO can be easily contaminated by H.

Microscopic Study of Defects in Perovskite Oxide

Through first-principles pseudopotential total-energy calculations, we investigate the formation energetics and the electronic structures and structural properties of the native defects available in LiNbO3. We found that the formation enthalpies of Li-vacancy and Nb-antisite defects where lower compared to other defects. We examined the detailed electronic structures and structural properties of Nb-antisite. We identified a large Jahn-Teller distortion of Nb-antisite that can be induced by light illumination, by which the electronic structure is also remarkably changed. We discuss the role of the bistability of Nb-antisite in the photorefractivity.