Energy Band Structure Calculations Research Articles

Two-Band Calculations on the Upper Critical Field of Sc2Fe3Si5

The ternary compound Sc2Fe3Si5 has attracted much attention because of the various anomalous physical properties. The specific heat experiment and energy band structure calculation suggest that Sc2Fe3Si5 is a two-gap superconductor. Based on this, we analyze the upper critical field for superconducting Sc2Fe3Si5 crystals using the two-band Ginzburg-Landau theory. A two-parameter variational approach is adopted to obtain the upper critical field in arbitrary direction. The temperature and angular dependences of the upper critical field are plotted. The results reproduce the experimental data in a very broad temperature range and strongly support previous specific heat data and theoretical calculation, pointing to the existence of two energy gaps in Sc2Fe3Si5. The anisotropy of the upper critical field is also studied and is about 2, in accordance with the experimental result. Moreover our calculations indicate that Sc2Fe3Si5 has rather a three-dimension character, in agreement with the energy band calculation.

Hydrogen adsorption on sulphur-doped SiC nanotubes

Hydrogen (H2) is an energy carrier and clean fuel that can be used for a broad range of applications that include fuel cell vehicles. Therefore, development of materials for hydrogen storage is demanded. Nanotubes, in this context, are appropriate materials. Recently, silicon carbide nanotube (SiCNTs) have been predicted as potential nanomaterials for hydrogen storage, and atomic doping into the nanotubes improves the H2 adsorption. Here, we report H2 adsorption properties of sulphur-doped (S-doped) SiCNTs using first-principles calculations based on density functional theory. The H2 adsorption properties are investigated by calculations of energy band structures, density of states (DOS), adsorption energy and Mulliken charge population analysis. Our findings show that, compared to the intrinsic SiCNT, S-doped SiCNT is more sensitive to H2 adsorption. H2 gas adsorption on S-doped C-sites of SiCNT brings about significant modulation of the electronic structure of the nanotube, which results in charge transfer from the nanotube to the gas, and dipole–dipole interactions cause chemisorptions of hydrogen. However, in the case of H2 gas adsorption on S-doped Si-sites of the nanotube, lesser charge transfer from the nanotube to the gas results in physisorptions of the gas. The efficient hydrogen sensing properties of S-doped SiCNTs, studied here, may have potential for its practical realization for hydrogen storage application.

Synthesis, characterization and DFT calculations of electronic and optical properties of CaMoO4

The electronic and optical properties of calcium molybdate (CaMoO4) have been determined by X-ray diffraction, spectroscopic measurements and calculations of energy-band structures, density of states, and optical response functions by density functional theory. The chemical bonding analysis indicates that Mo–O bonds exhibit more covalent character than the Ca–O bond. The linear photon-energy-dependent dielectric functions, conductivity, refractive index, reflectivity and extinction coefficients were investigated and analyzed. The results are in agreement with previous theoretical works and the experimental data. Reflectivity spectra revealed that the CaMoO4 promises as good coating materials in the energy region of 9.3–11.6eV with reflectivity larger than 75%.

Electronic and Thermoelectric Properties of Layered Sn- and Pb-Doped Ge2Sb2Te5 Alloys Using First Principle Calculations

A computational study on stable hexagonal phase of undoped, and Sn- and Pb-doped Ge2Sb2Te5 (GST) phase change materials has been carried out. The electronic structure, lattice dynamics and thermoelectric properties of doped GST have been extensively investigated using ab initio methods with virtual crystal approximation. The hexagonal symmetry of the GST is maintained with the addition of Sn and Pb dopants. The lattice parameters and atomic volume of the Sn-doped GST structure is larger than that of the undoped GST. Electronic band structure calculations show that there is an increase in band gap with the increase in the concentration of Sn (≤4.4 at.%). However, with the addition of a very small amount of Pb, there is a continuous decrease in lattice parameters and band gap values. The calculated energy band structure is then used in combination with the Boltzmann transport equation to calculate the thermoelectric parameters of GST and Sn- and Pb-doped materials. Seebeck coefficient (S), electronic thermal conductivity (κe) and the thermoelectric figure-of-merit (ZT) have been calculated with the help of BoltzTraP code. It was found that the thermoelectric properties of GST are enhanced with the addition of Sn.

Effect of relaxation on the energetics and electronic structure of clean Ag3PO4(111) surface**Project supported by the National Natural Science Foundation of China (Nos. 51472081, 51102150, 61106046), the Development Funds of Hubei Collaborative Innovation Center (Nos. HBSKFMS2014003, HBSKFMS2014011), and the Foundation for High-Level Talents (No. GCRC13014).

The effect of relaxation on the energetics and electronic structure of clean Ag3PO4(111) surface has been studied, carried out using first-principles density functional theory (DFT) incorporating the GGA+U formalism. After atomic relaxation of the Ag3PO4(111) surface, it is found that O atoms are exposed to the outermost surface, due to an inward displacement of more than 0.06 nm for the two threefold-coordinated Ag atoms and an outward displacement of about 0.004 nm for three O atoms in the sublayer. The atomic relaxations result in a large transfer of surface charges from the outermost layer to the inner layer, and the surface bonds have a rehybridization, which makes the covalence increase and thus causes the surface bonds to shorten. The calculated energy band structures and density of states of the Ag3PO4(111) surface present that the atomic relaxation narrows the valence band width 0.15 eV and increases the band gap width 0.26 eV. Meantime, the two surface peaks for the unrelaxed structure disappear at the top of the valence band and the bottom of the conduction band after the relaxed structure, which induces the transformation from a metallic to a semi-conducting characteristic.

First-Principles Study of Electronic Structure and Optical Properties of V-Doped CrSi<sub>2</sub>

The electronic structure and optical properties of V-doped CrSi2 have been calculated by using the first-principle peudo-potential plane-wave method based on the density functional theory.The parameters and properties of structure were given and the theory data were offered to research the effect of V doping into CrSi2. The calculations of energy band structure, total density of states, partial density of states of V-doped CrSi2were analysed. Fermi level enters into valence band which makes the V-doped CrSi2to be p-type semiconductor that improves the electrical conductivity of material. Additionally, the optical parameters of V-doped CrSi2were also discussed. It was found that both static dielectric constant and static refractive index increase after doping.

Electronic structures of the ferrimagnetic double-perovskites Sr2XReO6 (X = Cr, Mn, Fe, Ni) with the modified Becke-Johnson potential

We investigate the electronic structures and magnetic properties of the ferrimagnetic double-perovskites Sr2XReO6 (X = Cr, Mn, Fe, Ni) by using Tran and Blaha’s modified Becke and Johnson exchange potential. The calculated results show that Sr2XReO6 (X = Cr, Fe) are half-metals, and Sr2XReO6 (X = Mn, Ni) are insulators, which is in accordance with the experimental results. By using the ionic picture, we explain the trend in the density of states and total magnetic moment with X changing from Cr to Ni. We find that the insulating property is associated with X2 +, while the metallic character is associated with X3 +. The Re t 2g splitting in the minority channel leads to insulativity of Sr2XReO6 (X = Mn, Ni), and the Re t 2g splitting of Sr2MnReO6 is caused by lattice distortion, while Sr2NiReO6’s splitting is due to the heavier atomic mass of Ni. When spin-orbit coupling is included, the spin polarization of Sr2XReO6 (X = Cr, Fe) drifts off 100%, and the related gaps of Sr2XReO6 (X = Mn, Ni) become narrow. The spin-orbit coupling results in a significant increase in the total magnetic moment due to an unquenched Re orbital moment. Our calculated energy band structures show that Sr2MnReO6 is a spin gapless semiconductor, which can realize fully polarized spin-down electrons and spin-up holes.

Electronic and optical properties of the LiCdX (X = N, P, As and Sb) filled-tetrahedral compounds with the Tran–Blaha modified Becke–Johnson density functional

The structural, electronic and optical properties of the LiCdN, LiCdP, LiCdAs and LiCdSb filled-tetrahedral compounds have been explored from first-principles. The calculated structural parameters are consistent with the available experimental results. Since DFT with the common LDA and GGA underestimates the band gap, we use a new developed functional able to accurately describe the electronic structure of semiconductors, namely the Tran–Blaha-modified Becke–Johnson potential. The four investigated compounds demonstrate semiconducting behavior with direct band gap ranging from about 0.32 to 1.65eV. The charge-carrier effective masses are evaluated at the topmost valence band and at the bottommost conduction band. The evolution of the value and nature of the energy band gap under pressure effect is also investigated. The frequency-dependent complex dielectric function and some macroscopic optical constants are estimated. The microscopic origins of the structures in the optical spectra are determined in terms of the calculated energy band structures.

Dielectric-dependent screened Hartree-Fock exchange potential and Slater-formula with Coulomb-hole interaction for energy band structure calculations.

We previously reported a screened Hartree-Fock (HF) exchange potential for energy band structure calculations [T. Shimazaki and Y. Asai, J. Chem. Phys. 130, 164702 (2009); T. Shimazaki and Y. Asai, J. Chem. Phys. 132, 224105 (2010)]. In this paper, we discuss the Coulomb-hole (COH) interaction and screened Slater-formula and determine the energy band diagrams of several semiconductors, such as diamond, silicon, AlAs, AlP, GaAs, GaP, and InP, based on the screened HF exchange potential and Slater-formula with COH interaction, to demonstrate the adequacy of those theoretical concepts. The screened HF exchange potential and Slater-formula are derived from a simplified dielectric function and, therefore, include the dielectric constant in their expressions. We also present a self-consistent calculation technique to automatically determine the dielectric constant, which is incorporated into each self-consistent field step.

Study on Electronic Structure and Optical Properties of 21R-SiC Polytype

The density functional theory based on first-principles plane wave ultrasoft pseudopotential method is adopted to study the electronic structure and the optical linear response function of 21R-SiC. The calculated data include the lattice parameter, the energy band structure, the density of states, the electric charge density, the complex dielectric function and the absorption coefficient, theoretically giving the relationship between the electronic structure and the optical properties of 21R-SiC. The dielectric function and the extinction coefficient of 21R-SiC are investigated by using the calculated energy band structure to provide the theoretical basis for the design and application of 21R-SiC photoelectric material. At the same time, the calculating result also provides the possibility for us to accurately monitor and control the growth process of SiC material.

Structural, elastic and electronic structure of LiCu2Si, LiCu2Ge and LiAg2Sn intermetallic compounds

Structural, elastic and electronic properties of the intermetallic compounds, LiCu2Si, LiCu2Ge and LiAg2Sn, are investigated using ab initio technique as implemented in the (FP-(L) APW + lo) scheme based on the Wu–Cohen generalized gradient approximation. The computed equilibrium lattice parameters for the compounds are in good agreement with previous theoretical result and experimental data. The elastic constants reveal that the compounds are elastically stable, ductile and also exhibit large elastic anisotropy. The elastic wave velocities in the [100], [110] and [111] directions and their pressure dependence are presented and discussed. A set of isotropic elastic parameters and their related properties, namely bulk and shear moduli, Young’s modulus, hardness parameter, Poisson’s ratio, compressibility, Lame’s constant, average sound velocity, Debye temperature are predicted in the frame work of the Voigt–Reuss–Hill approximation for the polycrystalline aggregates of the compounds. Energy band structure calculations as well as the site and angular momentum projected density of states show that the compounds are metallic with mainly hybridized Cu or Ag d and Si, Ge or Sn p bands crossing the Fermi level.

First principles calculation on ternary stannide phase narrow band gap semiconductor Na2MgSn

The electronic structures, chemical bonding, elastic and optical properties of the ternary stannide phase Na2MgSn were investigated by using density-functional theory (DFT) within generalized gradient approximation (GGA). The calculated energy band structures show that Na2MgSn is an indirect semiconductor material with a narrow band gap 0.126 eV. The density of state (DOS) and the partial density of state (PDOS) calculations show that the DOS near the Fermi level is mainly from the Na 2p, Mg 3p and Sn 5p states. Population analysis suggests that there are strongly bonded Mg-Sn honeycomb layers in Na2MgSn. Basic physical properties, such as lattice constant, bulk modulus, shear modulus, elastic constants cij were calculated. The elastic modulus E and Poisson ratio v were also predicted. The results show that Na2MgSn is mechanically stable soft material and behaves in a brittle manner. Detailed analysis of all optical functions reveals that Na2MgSn is a better dielectric material, and reflectivity spectra show that Na2MgSn promise as good coating materials in the energy regions 6.24–10.49 eV.

Nitrogen concentration influence on N-NaTaO3 from first principles

First-principles density functional theory electronic structure calculations were carried out for N-doped NaTaO3 to evaluate the effects of the nitrogen doping concentration on the electronic structures and the photocatalytic activity of NaTaO3. When nitrogen doping at the lower levels (x ≦̸ 0.0417), the isolated N 2p states are formed above the O 2p states while the bottom of conduction band (CB) shifts little in N-doped NaTaO3. With increasing nitrogen doping levels (x ⩾ 0.0625), the N 2p states begin to mix with the O 2p states and thus move the top of valence band (VB) upwards in contrast to the corresponding NaTaO3. Based on the calculated energy band structures and Mulliken electronegativity theory, the schematic illustrations of N-doped NaTaO3 with different concentrations are tentatively proposed. The present study will provide a theoretical basis for developing the nitrogen-doped multimetal oxide photocatalysts with visible-light response.

Electronic and optical properties of the SiB2O4 (B=Mg, Zn, and Cd) spinel oxides: An ab initio study with the Tran–Blaha-modified Becke–Johnson density functional

We report ab initio density functional theory calculations of the structural, electronic and optical properties of the spinel oxides SiMg2O4, SiZng2O4, and SiCd2O4 using the full-potential linearized augmented plane-wave method. The structural parameters calculated using both the local density and generalized gradient approximations to the exchange-correlation potential are consistent with the literature data. To calculate the electronic properties, the exchange-correlation potential is treated with various functionals, and we find that the newly developed Tran–Blaha-modified Becke–Johnson functional significantly improves the band gap. We predict a direct band gap in all of the considered SiB2O4 compounds, and the band gaps continuously decrease as the atomic size of the B element increases. The decrease in the fundamental direct band gap (Γ–Γ) from SiMg2O4 to SiZn2O4 to SiCd2O4 can be attributed to p–d mixing in the upper valence bands of SiZn2O4 and SiCd2O4. The lowest conduction band is well dispersive, similar to that found for transparent conducting oxides such as ZnO. This band is mainly defined by the s and p electrons of the Si and B (B=Mg, Zn, Cd) atoms. The topmost valence band is considerably less dispersive and is defined by O-2p and B–d electrons. The charge-carrier effective masses are evaluated at the topmost valence band and at the bottommost conduction band that were calculated. The frequency-dependent complex dielectric function, absorption coefficient, refractive index, extinction coefficient, reflectivity and electron energy loss function were estimated. We find that the value of the zero-frequency limit of the dielectric function ε(0) increases as the band gap decreases. The origins of the peaks and structures in the optical spectra are determined in terms of the calculated energy band structures.

First principles calculation on electronic structure, chemical bonding, elastic and optical properties of novel tungsten triboride

The electronic structures, chemical bonding, elastic and optical properties of the novel hP24 phase WB3 were investigated by using density-functional theory (DFT) within generalized gradient approximation (GGA). The calculated energy band structures show that the hP24 phase WB3 is metallic material. The density of state (DOS) and the partial density of state (PDOS) calculations show that the DOS near the Fermi level is mainly from the W 5d and B 2p states. Population analysis suggests that the chemical bonding in hP24-WB3 has predominantly covalent characteristics with mixed covalent-ionic characteristics. Basic physical properties, such as lattice constant, bulk modulus, shear modulus and elastic constants Cij were calculated. The elastic modulus E and Poisson ratio ν were also predicted. The results show that hP24-WB3 phase is mechanically stable and behaves in a brittle manner. Detailed analysis of all optical functions reveals that WB3 is a better dielectric material, and reflectivity spectra show that WB3 can be promised as good coating material in the energy regions of 8.5–11.4 eV and 14.5–15.5 eV.

Phase stability, electronic structure and optical properties of BiInO3 under strain

We use density functional calculations to elucidate the effects of compressive and tensile strain on the structural, ferroelectric, electronic, and optical properties of BiInO3. Compared to tensile strain, compressive strain results in larger changes to the related physical properties of BiInO3. At a compressive strain about 6.3%, BiInO3 undergoes a first-order phase transition from ferroelectric state to paraelectric state. In the case of tensile strain, BiInO3 can retain its ferroelectricity, and while in its ferroelectric state, become unstable. The ferroelectric polarization and band gap of BiInO3 can be tuned by strain. The optical dielectric function under strain was also investigated. Based on the calculated energy band structures, the transitions peaks of dielectric function are discussed in detail. The change of energy band structures also indicated that the first-order phase transition in BiInO3 occurs under compressive strain. Our results suggest that BiInO3 should be one of the most promising candidates for replacing the widely used ferroelectric and piezoelectric material PbTiO3, and solve the environmental issues associated with the toxicity of lead.

A comprehensive study of thermoelectric and transport properties of β-silicon carbide nanowires

The temperature dependence of the Seebeck coefficient, the electrical and thermal conductivities of individual β-silicon carbide nanowires produced by combustion in a calorimetric bomb were studied using a suspended micro-resistance thermometry device that allows four-point probe measurements to be conducted on each nanowire. Additionally, crystal structure and growth direction for each measured nanowire was directly obtained by transmission electron microscopy analysis. The Fermi level, the carrier concentration, and mobility of each nanostructure were determined using a combination of Seebeck coefficient and electrical conductivity measurements, energy band structure and transport theory calculations. The temperature dependence of the thermal and electrical conductivities of the nanowires was explained in terms of contributions from boundary, impurity, and defect scattering.

Tuning photocatalytic performance of the near-infrared-driven photocatalyst Cu2(OH)PO4 based on effective mass and dipole moment

Recently, Cu2(OH)PO4 was found as the first photocatalyst active in the near-infrared(NIR) region of the solar spectrum (Angew. Chem., Int. Ed., 2013, 52, 4810; Chem. Eng. News, 2013, 91, 36), motivating us to explore systemically its photocatalytic mechanism under near-infrared light and how to improve and tune its photocatalytic performance. Herein, electronic structures, and effective masses of electron and hole at energy band edges are theoretically investigated by employing spin-polarized density functional theory calculations. The calculated energy band structure supports the absorption spectra of Cu2(OH)PO4 in the NIR region corresponding to the electron excitation from the valence band to the unoccupied bands in the gap. Our charge density analysis indicates that the O atoms in the hydroxyl serves as the effective bridge for the favoring separation of the photogenerated electron-hole pairs. Furthermore, the effective masses of electron and hole analysis demonstrate that the separation and transfer of photogenerated carriers along the [011] direction may be more effective than other possible directions. A qualitative comparison of carrier transfer ability along all the directions in the specific planes is displayed by the three-dimensional band structure. Interestingly, the calculated net dipole moment for the two basic units of Cu2(OH)PO4, octahedron and trigonal bipyramid, indicate that the macroscopic dipole moment for Cu2(OH)PO4 is zero, however, the distorted octahedron unit has a net dipole moment, which enables us to tune the macroscopic dipole moment by doping. The present work provides theoretical insight leading to a better understanding of the photocatalytic performance of Cu2(OH)PO4 and it may be beneficial to prepare more efficient Cu2(OH)PO4 for NIR light photocatalysis, which will also be helpful to design and prepare novel photocatalysts.

First principles calculation of electronic structure, chemical bonding and elastic properties of ultra-incompressible Re2P

The electronic structures, chemical bonding and elastic properties of the Co2P-type structure phase ultra-incompressible Re2P (orthorhombic phase) were investigated by density-functional theory (DFT) within generalized gradient approximation (GGA). The calculated energy band structures show that the orthorhombic structure phase Re2P is metallic material. The density of state (DOS) and the partial density of state (PDOS) calculations show that the DOS near the Fermi level is mainly from the Re-5d state. Population analysis suggests that the chemical bonding in Re2P has predominantly covalent character with mixed covalent-ionic character. Basic physical properties, such as lattice constant, bulk modulus, shear modulus, and elastic constants Cij, were calculated. The elastic modulus and Poisson ratio were also predicted. The results show that the Co2P-type structure phase Re2P is mechanically stable and behaves in a brittle manner.

First-Principles Study of the Electronic Structure and Thermoelectric Properties of Al-Doped ZnO

ZnO materials doped with elements such as Al, Ga, etc. are of great interest for high-temperature thermoelectric applications. In this work, the effects of Al doping on the electronic structure and thermoelectric properties of the ZnO system are presented. The energy band structure and density of states of Al-doped ZnO were investigated using the projector-augmented plane wave pseudopotential method within the local density approximation. The calculated energy band structure was then used in combination with the Boltzmann transport equation to calculate the thermoelectric parameters of Al-doped ZnO. The electronic structure calculation showed that the position of the Fermi level of the doped sample was shifted to a higher energy level compared with the undoped material. The conduction band near the Fermi energy was a combination of hybridized Zn sp-orbitals and Al s-orbital. The calculated thermoelectric properties were compared with the experimental results, showing some agreement. For the Al-doped ZnO system, the Seebeck coefficient was shown to be negative and its absolute value increased with temperature. The electrical conductivity and electronic thermal conductivity followed the trend of the experimental results.