Density Functional Theory In Local Approximation Research Articles

Structural, electronic and optical studies of Sr2NiTeO6 double perovskite by first-principle DFT–LDA + U calculation

The structural, electronic and optical properties of monoclinic Sr2NiTeO6 double perovskite have been studied using corrected density functional theory–local density approximation with applying of Hubbard U corrected energy (DFT–LDA + U) method in plane-wave pseudopotential basis. The influence of on-site Coulomb interaction on structural, electronic and optical properties of Sr2NiTeO6 double perovskite compound, which consists of strongly localized Ni 3d electrons, were investigated. The highly Coulomb repulsion between electrons was corrected using Hubbard U parameter, varying from 0 to 8 eV for Ni 3d orbitals. The calculated results demonstrated that Sr2NiTeO6 double perovskite was sensitive to the change in U values. The optimized structural properties give a good agreement with experiment and other calculation data with lattice parameters a = 5.673 Å, b = 5.565 Å, c = 7.847 Å, α = 89.999°, β = 90.257° and γ = 90.000°. The DFT–LDA + U predicted the calculated electronic band structure of Sr2NiTeO6 was showed metallic behaviour (0 and 2 eV) and insulator behaviour (4, 6 and 8 eV). The density of state (DOS) shows that there was a significant effect on hybridization of Ni 3d and O 2p states at the conduction and valence band, respectively. Moreover, the results of optical studies such as dielectric function, absorption and reflectivity were found significant to the variation of U values applied indicates that the U values give a better description on the electronic localization of Ni 3d states.

Electron—Phonon Interaction, Phonon and Electronic Structures of Layered Electride Ca2N

The phonon and electronic properties, the Eliashberg function and the temperature dependence of resistance of electride Ca2N are investigated by the DFT-LDA (density functional theory in local density approximation) plane-wave method. The phonon dispersion, the partial phonon density of states and the atomic eigenvectors of zero-center phonons are studied. The electronic band dispersion and partial density of states conclude that Ca2N is a metal and the Ca 3p, 4s and N 2p orbitals are hybridized. For the analysis of an electron-phonon interaction and its contribution of the Eliashberg function to resistance was calculated and a temperature dependence of resistance due to electron-phonon interaction was found.

ZnO/ZnS (hetero)structures: ab initio investigations of polytypic behavior of mixed ZnO and ZnS compounds

The range of feasible ZnO/ZnS polytypes has been explored, predicting alternative structural arrangements compared with previously suggested or observed structural forms of ZnO/ZnS compounds, including bulk crystal structures, various nanostructures, heterostructures and heterojunctions. All calculations were performed ab initio using density functional theory–local density approximation and hybrid Heyd–Scuseria–Ernzerhof functionals. Specifically, pure ZnO and ZnS compounds and mixed ZnO1–x S x compounds (x = 0.20, 0.25, 0.33, 0.50, 0.60, 0.66 and 0.75) are investigated and a multitude of possible stable polytypes for ZnO/ZnS compounds creating new possibilities for synthesis of new materials with improved physical and chemical properties are identified.

Simulation of the phase transition of graphite to the diamond-like LA3 phase

The phase transition of graphite to a diamond-like LA3 phase is simulated by the methods of the density functional theory (DFT). The calculations are performed in the local density approximation (LDA) and the generalized gradient approximation (GGA). It is found that the structural transformation must occur at a pressure of 60 or 74 GPa according to calculations based on the DFT–LDA and DFT–GGA, respectively. The height of the potential barrier separating the structural state corresponding to the LA3 phase from the state corresponding to graphite exceeds 0.13 eV/atom. This indicates the possibility of stable existence of the diamond-like LA3 phase under standard conditions.

Brillouin zone spin filtering mechanism of enhanced tunneling magnetoresistance and correlation effects in aCo(0001)/h -BN/Co(0001)magnetic tunnel junction

The 'Brillouin zone spin filtering' mechanism of enhanced tunneling magnetoresistance (TMR) is described for magnetic tunnel junctions (MTJ) and studied on an example of the MTJ with hcp Co electrodes and hexagonal BN (h-BN) spacer. Our calculations based on local density approximation of density functional theory (LDA-DFT) for Co(0001)/h-BN/Co(0001) MTJ predict high TMR in this device due to Brillouin zone filtering mechanism. Owning to the specific complex band structure of the h-BN the spin-dependent tunneling conductance of the system is ultra-sensitive to small variations of the Fermi energy position inside the BN band gap. Doping of the BN and, consequentially, changing the Fermi energy position could lead to variation of the TMR by several orders of magnitude. We show also that taking into account correlation effects on beyond DFT level is required to accurately describe position of the Fermi level and thus transport propertied of the system. Our study suggests that new MTJ based on hcp Co-Pt or Co-Pd disordered alloy electrodes and p-doped hexagonal BN spacer is a promising candidate for the spin-transfer torque magnetoresistive random-access memory (STT-MRAM).

Ab-initio DFT FP-LAPW GGA and LDA TB-mBJ and SO theoretical study of structural and elastic properties of Zinc-Blende crystal phase GaAs1−xBix alloys

We present an ab-initio theoretical study of structural and elastic properties of GaAs1−xBix alloys in the Zinc-Blende (ZB) phase. We use a recent version of Wien2k package code based on Density Functional Theory (DFT) Full Potential and Linearized Augmented Plane Waves (FP-LAPW) method including recent Tran-Blaha modified Becke–Johnson correction of the exchange potential (TB-mBJ) and the spin–orbit interaction (SO). The calculations are performed within the Local Density Approximation (LDA) as well as the Generalized Gradient Approximation (GGA). We study first the structural properties of GaAs1−xBix alloys by solving Murnaghan equation of state. Our results show that the ZB phase is the lowest equilibrium crystal structure of GaAs1−xBix in the whole alloy composition range, in agreement with previous theoretical predictions. The variations versus Bi contents of the ZB GaAs1−xBix lattice constant a0, bulk modulus B0 and its pressure derivative B0′ are also found very close to other theoretical and experimental data, but with much smaller bowing effects indicating a better resolution thanks to TB-mBJ correction. The variations of B0 versus the reverse equilibrium volume of the unit cell (1/V0) are found to be described by the simple linear empirical expression B0 = −0.21068 + 0.16695/V0 which is close to the theoretical prediction for III-V semiconductors with, however, somewhat lower linear coefficients values, suggesting a more metallic behavior. In a second part of this work, we use Birch–Murnaghan approach to study the elastic properties of GaAs1-xBix alloys. The elastic stiffness coefficients, C11, C12 and C44, and their variations versus alloy composition were determined for ZB GaAs1−xBix alloy. Their values in GaAs and GaBi binary compounds are found in very good agreement with available experimental and/or theoretical data. Their variations in GaAs1−xBix alloy show a monotonic decrease with increasing Bi contents, indicating a softening behavior as is typically the case for metallic alloys. The values of the bulk modulus derived with the help of C11, C12 and C44 elastic stiffness coefficients are found very close to B0 values derived directly from Murnaghan equation of state, indicating the good accuracy of Cij coefficients deduced from Birch–Murnaghan procedure.

Quasiparticle theoretical characterization of electronic and optical properties of the photocatalytic material Ti3−δO4N

Abstract

Calculation of the spectrum of quasiparticle electron excitations in organic molecular semiconductors

A quasiparticle electronic spectrum belongs to the characteristics of nanoobjects that are most important for applications. The following methods of calculating the electronic spectrum are analyzed: the Kohn-Sham equations of the density functional theory (DFT), the hybrid functional method, the GW approximation, and the Lehmann approximation used in the spectral representation of one-electron Green’s function. The results of these approaches are compared with the data of photoemission measurements of benzene, PTCDA, and phthalocyanine (CuPc, H2Pc, FePc, PtPc) molecules, which are typical representatives of organic molecular semiconductors (OMS). This comparison demonstrates that the Kohn-Sham equations of DFT incorrectly reproduce the electronic spectrum of OMS. The hybrid functional method correctly describes the spectrum of the valence and conduction bands; however, the HOMO-LUMO gap width is significantly underestimated. The correct gap width is obtained in both the GW approximation and the Lehmann approach, and the total energy in this approach can be calculated in the local density approximation of DFT.

DFT Studies on a Series of Nitramines Containing Pyridine Ring

In this article, a series of nitramines containing pyridine ring were studied by density functional theory (DFT). The gas-phase heats of formation were predicted based on the isodesmic reactions and the condensed-phase heats of formation and heats of sublimation were estimated with the Politzer's approach. The detonation velocity and pressure were calculated using the empirical Kamlet-Jacobs equation. Many title compounds have better performance than RDX (hexahydro-1,3,5-trinitro-1,3,5-trizine) and HMX (1,3,5,7-tetranitro-1,3,5,7- tetraazacyclooctane). The impact sensitivity was evaluated with the characteristic height (h50). It is found that most of the studied compounds have lower sensitivities than CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12- hexaazaisowurtzitane). The crystal structures were predicted with the molecular mechanics method and optimized by the CA-PZ local density approximation of DFT. Analysis of the crystal energy gap indicates I-13, II-1, III-1, and IV-1 are nearly conductors and other compounds are semiconductors. For I-1∼I-8 and I-11, the largest contribution to the valence bands is mainly from the p states of the C and N atoms in the pyridine and fused ring and for the other compounds, from the p states of the C and N atoms in the amino group and pyridine.

G0W0 band structure of CdWO4

The full quasiparticle band structure of CdWO4 is calculated within the single-shot GW (G0W0) approximation using maximally localized Wannier functions, which allows one to assess the validity of the commonly used scissor operator. Calculations are performed using the Godby–Needs plasmon pole model and the accurate contour deformation technique. It is shown that while the two methods yield identical band gap energies, the low-lying states are given inaccurately by the plasmon pole model. We report a band gap energy of 4.94 eV, including spin–orbit interaction at the DFT–LDA (density functional theory–local density approximation) level. Quasiparticle renormalization in CdWO4 is shown to be correlated with localization distance. Electron and hole effective masses are calculated at the DFT and G0W0 levels.

Designating a logical electrochemical mechanism through a combined electrochemical and computational method

Graphene shows fascinating applications in electrochemical methods and bionanotechnology, including drug assay, drug delivery, and DNA sensing. In this regard, we model the adsorption of organic compound on a reduced graphene oxide (RGO), using a first principles density functional theory–local density approximation method. The presence of functional groups and the exchange of nitrogen atoms with carbon atoms can significantly alter the overall magnitude of π–π interactions between the adsorbed molecules and RGO by giving rise to strong medium-range interactions involving π-orbitals of the substituents. In the next step, the data obtained from the modeling of the adsorbate on RGO, are coupled with electrochemical data obtained from the reduction of adsorbate, resulting in a logical electrochemical mechanism for the reduction of adsorbate on the electrode surface.

Simulations of friction force microscopy on the KBr(001) surface based onab initiocalculated tip-sample forces

We report on ab initio-based simulations of friction-force microscopy on the KBr(001) surface at zero and nonzero temperature. To simulate sliding friction, we employ an extended three-dimensional (3D) Prandtl-Tomlinson model. The microscopic part of the tip is modeled by ${\mathrm{K}}^{+}$- or Br${}^{\ensuremath{-}}$-terminated tips. We use a tip-surface interaction potential, which is calculated within local-density approximation of density-functional theory and supplemented by a long-range van der Waals interaction resulting from the macroscopic part of the tip. Thermal fluctuations are included via random white noise. The loading force acting on the tip enters the Langevin equation of motion separately from all other forces so that it can be changed at will. We analyze friction as a function of loading force, temperature, and mass of the tip and identify regions of these parameters where distinct stick-slip behavior or ultra-low friction occurs. A comparison of our 3D ab initioresults with those obtained using sinusoidal tip-surface forces (1D model) is very revealing. By and large, both approaches yield results in good agreement at $T$ = 0 K. At higher temperatures, however, distinct differences occur. For example, at $T$ = 295 K, the 1D model calculations overestimate the friction hysteresis and energy dissipation, and for positive loading forces they even can yield a different periodicity in the friction-force profile.

Accurate prediction of theSi/SiO2interface band offset using the self-consistentab initioDFT/LDA-1/2 method

We use the density functional theory/local-density approximation (DFT/LDA)-1/2 method [L. G. Ferreira et al., Phys. Rev. B 78, 125116 (2008)], which attempts to fix the electron self-energy deficiency of DFT/LDA by half-ionizing the whole Bloch band of the crystal, to calculate the band offsets of two $\text{Si}/{\text{SiO}}_{2}$ interface models. Our results are similar to those obtained with a ``state-of-the-art'' $GW$ approach [R. Shaltaf et al., Phys. Rev. Lett. 100, 186401 (2008)], with the advantage of being as computationally inexpensive as the usual DFT/LDA. Our band gap and band offset predictions are in excellent agreement with experiments.

Extensive theoretical studies of a new energetic material: Tetrazino‐tetrazine‐tetraoxide (TTTO)

[1,2,3,4]Tetrazino-[5,6-e]-[1,2,3,4]tetrazine-[1,3,5,7]tetraoxide (TTTO, C2N8O4) was suggested to be a possible candidate of high energy density materials (HEDM). The most stable conformation of TTTO is a planar structure with C2h symmetry. Using various high-level ab initio methods including Gaussian-n, Complete Basis Set, Full Coupled Cluster, and W1U models of chemistry together with density functional theory based models, the enthalpies of formation of TTTO at 0 K and standard state were calculated precisely. Moreover, the rovibrational and nuclear magnetic resonance properties were predicted as well. The solid state TTTO was studied using the crystal packing models with the Dreiding force-filed and the plane-wave periodic local-density approximation density functional theory. Three stable polymorphous cells of TTTO have been found with either P212121 or P21/C symmetry. The high heat of formation (>200 kcal/mol), the high density (>2.0 g/cm3), the planar nonpolar electronic structure, and the perfect oxygen balance lead TTTO be a very promising HEDM with exceptional performance. This work provides the first theoretical support for further experimental synthesis and testing.

Ab InitioGWMany-Body Effects in Graphene

We present an ab initio numerical many-body GW calculation of the band plot in freestanding graphene. We consider the full ionic and electronic structure introducing e-e interaction and correlation effects via a self-energy containing non-Hermitian and dynamical terms. With respect to the density-functional theory local-density approximation, the Fermi velocity is renormalized with an increase of 17%, in better agreement with the experiment. Close to the Dirac point the linear dispersion is modified by the presence of a kink, as observed by angle-resolved photoemission spectroscopy. We demonstrate that the kink is due to low-energy pi-->pi* single-particle excitations and to the pi plasmon. The GW self-energy does not open the band gap.

UHV STM I(V) and XPS Studies of Aryl Diazonium Molecules Assembled on Si(111)

Molecular layers formed from 4-trifluoromethylbenzenediazonium tetrafluoroborate and 4-Methylbenzenediazonium tetrafluoroborate have been assembled on H-passivated Si(111) and studied by UHV STM and XPS. STM imaging shows well-developed Si(111) step edges and terraces both on Si(111):H and Si(111) substrates covered with a molecular layer. STM I(V) data acquired at different tip-substrate separations reveals a factor of approximately 10 enhancement in current for positive bias voltage when current flows through the 4-trifluoromethyl molecule when compared to the 4-methyl variant. The observed current enhancement in I(V) can be understood by comparing the projected density of states of the two molecule-Si systems calculated using a density functional theory local density approximation after geometry optimization was performed via the conjugate gradient method. XPS data independently confirm that H-passivated Si(111) remains oxygen free for short exposures to ambient conditions and provide evidence that the molecules chemically react with the silicon surface.

Electronic structure of ScN and YN: density-functional theory LDA and GW approximation calculations

The desirable physical properties of hardness, high temperature stability, and conductivity make the early transition metal nitrides important materials for various technological applications. To learn more about the nature of these materials, the local-density approximation(LDA) and GW approximation i.e. combination of the Green function G and the screened Coulomb interaction W, have been performed. This paper investigates the bulk electronic and physical properties of early transition metal mononitrides, ScN and YN in the rocksalt structure. In this paper, the semicore electrons are regarded as valance electrons. ScN appears to be a semimetal, and YN is semiconductor with band gap of 0.142 eV within the LDA, but are in fact semiconductors with indirect band gaps of 1.244 and 0.544 eV respectively, as revealed by calculations performed using GW approximation.

Strained Si, Ge, andSi1−xGexalloys modeled with a first-principles-optimized full-zonek∙pmethod

The electronic energy band structure of strained and unstrained Si, Ge, and $\mathrm{SiGe}$ alloys is examined in this work using a 30-level $\mathbit{k}∙\mathbit{p}$ analysis. The energy bands are at first obtained with ab initio calculations based on the local-density approximation of density-functional theory, including a $GW$ correction and relativistic effects. The so-calculated band structure is then used to extract the unknown $\mathbit{k}∙\mathbit{p}$ fitting parameters with a conjugate-gradient optimization procedure. In a similar manner, the results of ab initio calculations for strained materials are used to fit the unknown deformation potentials that are included in the present $\mathbit{k}∙\mathbit{p}$ Hamiltonian following the Pikus-Bir correction scheme. We show that the present $\mathbit{k}∙\mathbit{p}$ model is an efficient numerical method, as far as computational time is concerned, which reproduces accurately the overall band structure, as well as the bulk effective density of states and the carrier effective masses, for both strained and unstrained materials. As an application, the present 30-level $\mathbit{k}∙\mathbit{p}$ model is used to describe the band offsets and the variations of the carrier effective masses in the strained ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}∕{\mathrm{Si}}_{1\ensuremath{-}y}{\mathrm{Ge}}_{y}$ system.

Size-dependent properties of ZnmSn clusters: A density-functional tight-binding study

We present the results of our theoretical calculations on structural and electronic properties of ligand-free Zn(n)S(n) [with n ranging from 4 to 104 (0.8-2.0-nm diameter)] clusters as a function of size of the clusters. We have optimized the structure whereby our initial structures are spherical parts of either zinc-blende or wurtzite structure. We have also considered some hollow bubblelike structures. The calculations are performed by using a parametrized linear combination of atomic orbitals-density-functional theory-local-density approximation-tight-binding method. We have focused on the variation of radial distribution function, Mulliken populations, electronic energy levels, band gap, and stability as a function of size for both zinc-blende and wurtzite-derived ZnS clusters. We have also reported the results of some nonstoichiometric Zn(m)S(n) (with m+n=47, 99, 177) clusters of zinc-blende modification.

Electronic and optical properties of Fe, Zn and Pb sulfides

Ab initio quantum-chemical calculations of the spatial and electronic structures of sphalerite (ZnS), pyrite (FeS2) and galena (PbS), using the density functional theory (DFT) local density approximation (LDA) and generalized gradient approximation (GGA), the Hartree–Fock (HF) method and the hybrid functional B3LYP, have been carried out. For galena, the DFT LDA and GGA functionals provided the best estimate of the band gap, from within −0.1 eV to +0.4 eV of the measured value. B3LYP and RHF gave rise to errors of +1.3 and +5.4 eV, respectively. The unit cell parameter error varied from between −1.1% and +2.3% for all the functionals examined. For sphalerite the B3LYP functional provided the best estimate of the band gap (error +0.3 eV). The unit cell parameter error varied between −2.1% and +2.0% for the various DFT functionals and B3LYP. RHF gave rise to an error of +3.8%. For FeS2, the DFT-GGA approach provides the best results for both the unit cell and the band gap. This may be due to mutual cancellation of the crystal field splitting and band separation force, which are of equal but opposite magnitudes. The calculated density of states (DOS) for the conduction band is used to interpret the experimental features of the S 1s XANES (X-ray absorption near-edge structure) spectra obtained using synchrotron radiation. Because of the Δl = ±1 selection rule for electron excitation, the S K-edge XANES spectra represent a transition of the S 1s electron to conduction band S p-like orbitals. The near-edge region, up to 15 eV past the edge is approximated well by the DOS. Individual peaks in the DOS correlate with peaks in the XANES spectra. In addition, the imaginary part of the dielectric function, which reflects the transitions from occupied to unoccupied levels, is used to model the near-edge region of the XANES, using the DFT-GGA formalism. Individual peaks in the XANES spectrum are moderately well resolved using the dielectric function, especially for ZnS and FeS2, while the DOS for the conduction band is more successful in predicting the shape of the XANES spectra for all three minerals.