Papers by Keyword: Electronic Structure

Abstract: We report on theoretical and experimental studies of the electronic structure of triclinic copper tungstate, CuWO₄. The present data carried out within a Density Functional theory (DFT) framework reveal the importance of spin-resolved calculations for this tungstate. The spin-resolved DFT calculations indicate that that Cu 3d electronic states dominate the top of the valence band, whereas the main input of W 5d states occur at its bottom and lower part. However, the principal contributions to the valence band of CuWO₄ come from O 2p electronic states giving the main input to the whole valence-band region except of its top. The chemical bonding in CuWO₄ is characterized by essential contribution of the covalent component in addition to the ionic component. The present theoretical data are found to be in excellent agreement with the experimental XPS and XES measurements of polycrystalline copper tungstate.

Abstract: The effect of biaxial and uniaxial strains on the electronic structure of anatase is studied using Density Functional Theory (DFT) calculation with ultrasoft pseudopotential and a generalized gradient approximation (GGA) Perdew-Burke Ernzerhof (PBE) exchange-correlation. The lattice constant is optimized using the Birch-Murnaghan equation of states (BM-EOS) to get an optimized geometric structure of anatase TiO₂. We apply biaxial and uniaxial strains to this optimized structure up to 16% and find that the applied strains change the band gap energy compared to a pure anatase with a different band gap energy up to 1.61 eV for biaxial strain and 0.35 eV for uniaxial strain. The biaxial strains increase gap energies except at +16% tensile strain, decreasing the gap energy to 0.04 eV. Uniaxial strains tend to increase as the strains increase except at-12 and-16%; their gap energy differences are 0.08 and 0.20 eV, respectively, smaller than that of the zero strain. The results also show that the applied 16% tensile strain significantly lengthens the atomic bonds; thus, we conclude that the maximum strain applied to anatase TiO₂ is 16%.

Abstract: Here we provide a description of arsenic (As) adsorption on the cellulose biopolymer using first-principles density functional theory. In all studied configurations, the process of As adsorption on the cellulose is an exothermic process indicated by the negative binding energy. The cellulose's hydroxyl and hydroxymethyl groups significantly interact with As atom, characterized by the binding energy. In all optimized configurations, the interactions are mainly described as chemical bonding. This claim is supported by the overlap of the electron localization function (ELF) in the interface of As and cellulose in all studied adsorption sites. The adsorption of As on the cellulose introduces new states in the vicinity of the Fermi energy, leading to the lower bandgap of the cellulose-As systems. Overall, these results imply that the As atom can be trapped and detected using cellulose-based material. These findings offer an explanation of earlier research works on cellulose-As systems. This work will also serve as a reference for fabricating cellulose-based material for sensing and removing As.

Abstract: Theoretical ab initio calculations of the electronic structure were performed for the non-stoichiometric Mn_1.75Co_1.25Al Heusler alloy and compared with the electronic structure of the stoichiometric Mn₂CoAl full Heusler alloy. Both compounds are assumed to have the L2₁-type crystal structure in the calculations, the non-stoichiometry is taken into account as a substitution of a Mn atom in a supercell. The calculation for the non-stoichiometric composition of Mn_1.75Co_1.25Al showed that taking non-stoichiometry into account leads to a decrease of the total magnetic moment. In comparison with the inverse type of Mn₂CoAl, in both Mn₂CoAl and Mn_1.75Co_1.25Al, the metallic type of the total density of states at the Fermi level was obtained in our calculations. In Mn_1.75Co_1.25Al, the total density of electronic states is found to be close to the one of the stoichiometric Mn₂CoAl alloy in the majority spin projection, and in the minority spin projection spin polarization leads to the formation of the more intense peaks due to the appearance of an additional non-stoichiometric cobalt with a significant magnetic moment, as well as an increase in the magnetic moments of the other magnetic ions.

Abstract: InSb-based self-assembled quantum dots (SAQDs) are very promising for the mid-infrared (3-5 μm) optical range. We have analysed the electronic structure and optical properties of InAs_xSb_1-x/InAs dots. In this paper, we present the results of the modelling of electronic structure and optical properties from photoluminescence (PL) measurement for InAs_xSb_1-x/InAs SAQDs, focusing on the effects of SAQD morphology and composition. In particular, we analyse the electronic structure of InAs_xSb_1-x/InAs SAQD of various shapes, aspect ratios and compositions. We also suggest a method of assessing the geometry and composition of InAs_xSb_1-x/InAs quantum dots using their optical spectra and limited microscopy information. The calculated transition energies agree well with the experimental results. The results show that the geometry of the dot can be estimated from the optical spectra if the composition is known, and vice versa.

Abstract: The intermediate band semiconductor of AgGa_1-xCr_xS₂ is investigated by the first principles calculations and further confirmed by the experimental results. The band structures of pure and Cr-doped crystals were calculated and it is shown that the crystal with a direct energy band gap of about 0.95 eV for AgGaS₂. Because of Cr dopant, a metallic intermediate band (IB) is successfully formed in the host. From the partial density of states (PDOS) of Cr-doped AgGaS₂, the IB mainly comes from the hybridization of the Cr-3d and S-3p states. Based on the theoretical predications, the Cr-doped AgGaS₂ is synthesized by the high-temperature solid state reaction. Two extra absorption responses are detected in the absorption spectra. The optical absorption coefficients are enhanced in the visible radiation range due to the formation of metallic and isolated IB. Therefore, Cr-doped AgGaS₂ with an intermediate band is suggested as a potential material to enhance the efficiency of solar cells.

Abstract: We report on experimental and theoretical studies of the electronic structure of ternary Tl₄CdI₆ alloy. Our XPS results indicate low hygroscopicity of its surface. The first-principle calculations indicate that the valence-band region of Tl₄CdI₆ is dominated by contributions of I 5p states (mainly at the top and the central portion), while its bottom is prevailed by contributions of Tl 6s states. The theoretical data indicate that the Tl₄CdI₆ compound is a direct gap semiconductor with the band gap value of Eg = 2.03 eV. The calculations reveal that the significant covalent component (in addition to ionic component) is characteristic for the chemical Tl–I and Cd–I bonds of Tl₄CdI₆.

Abstract: The current study investigated the electrical properties of Cadmium Telluride (CdTe)by using the first principle of density functional theory (DFT). The nanocrystals suggested being varied constantly over the network systematically so that the lowest value for energy is obtained, through which stability is obtained and through this exceptionality, the measurements of the properties are in their exact state. The conduction and the valence bandwidths were also studied. The investigations targeted the “highest occupied molecular orbital” (HOMO) [Ionization Potential], and the “lowest unoccupied molecular orbital” (LUMO) [Electron Affinity]. Total and cohesive energies, the atomic iconicity, electron affinity, energy gap (Eg), and the density of states (DOS) for 8, 16, 54, and 64 atoms. The results showed that the shape of the conduction and valence affect the crystal groups significantly, and the energy gap exhibited very close results to their practical counterparts that were previously conducted. When the lattice constant decreases the modulus of bulk and the waves of sound speed increase with the increase of the core atoms number. Subsequently, the applied pressure increases the Plasmon energy and bulk modulus. The key of study is to inspect if using materials in their nanoscale state gives special physical, electronic and optical properties through which devices are manufactured with high efficiency in the solar cell industry. Where the compound becomes a point of a sleeve, and the fluorescent peak shifts across the visible field to the UV field. This was obtained by controlling the size of the compound in 54 and 64, at which the energy gap showed an increase, which would make it more preferred to stimulate the electron from the valence band to the conduction band.

Abstract: The first-principles calculations by CASTEP program based on the density functional theory is applied to calculate the cohesive energy, enthalpy of formation, elastic constant, density of states and Mulliken population of Ag₃Sn、AgZn₃ and Ag₅Zn₈. Furthermore, the elastic properties, bonding characteristics, and intrinsic connections of different phases are investigated. The results show that Ag₃Sn、AgZn₃ and Ag₅Zn₈ have stability structural, plasticity characteristics and different degrees of elastic anisotropy; Ag₃Sn is the most stable structural, has the strongest alloying ability and the best plasticity. AgZn₃ is the most unstable structure, has the worst plasticity; The strength of Ag₅Zn₈ is strongest, AgZn₃ has the weakest strength, the largest shear resistance, and the highest hardness. Ag₅Zn₈ has the maximum Anisotropy index and Ag₃Sn has the minimum Anisotropy index. Ag₃Sn、AgZn₃ and Ag₅Zn₈ are all have covalent bonds and ionic bonds, the ionic bonds decrease in the order Ag₃Sn>Ag₅Zn₈>AgZn₃ and covalent bonds decreases in the order Ag₅Zn₈>Ag₃Sn>AgZn₃.

Abstract: As a potential functional material in the perovskite family, the KCaF₃ on electronic structure, elasticity, Debye temperature and anisotropy are studied based on density functional theory (DFT). Above all, the structural parameters of KCaF₃ crystal are optimized. Then the elastic constants and Debye temperature are calculated. The results show that: (1) KCaF₃ is composed of covalent bonds, in which the Ca-F bond is stronger than K-F. (2) Ca atom mainly contributes for the electronic properties of KCaF₃. (3) The structural parameters of KCaF₃ is in fair agreement with the experimental data. (4) The anisotropy of KCaF₃ was analyzed from the pure and quasi waves, of which the longitudinal wave velocity in the direction of [100] is the larger than the others two directions ([110] and [111]). Finally, The homogenized elastic moduli (bulk modulus B, shear modulus G, Young's modulus E), Pugh and Poisson ratio, are obtained. This research is meaningful and thus to provides a good theoretical guidance for the design the new ABX₃-type material with better performance.