Sikander Azam

Transport properties of APdCu(Se2)(Se3) (A = K and Rb): new quaternary copper palladium polyselenides

The electronic structure, effective mass, optical properties and electrical transport coefficients of APdCu(Se2)(Se3) (where A = K and Rb), a new quaternary copper palladium polyselenide, were investigated using a density functional theory calculation within a generalized gradient approximation (GGA) plus the Hubbard term (U) (GGA + U). The electronic band structure shows that the calculated compounds have a direct band gap. From the partial density of states we found that, at an energy of −5.0 eV: (1) the Pd-s state strongly hybridizes with the Se-p state; (2) near the Fermi level the Se-p state hybridizes with the Cu-p state; and (3) at the lower conduction band the Pd-s state forms a strong hybridization with the Cu-s state. The investigation of electronic charge density shows that the Pd–Se and Cu–Se atoms form weak covalent bonds and have strong ionicity, whereas the K/Pd atoms exhibit pure ionic bonding. We also calculated the dielectric function, refractive index, extinction coefficient, absorption coefficient and reflectivity of the compounds. The calculated transport coefficients show the anisotropic nature of the compounds, in agreement with their electronic states. The transport properties reveal stronger carrier transport along the Cu-p/d and Pd-d orbitals, indicating that these orbitals are mainly responsible for the electrical transport. The maximum power factor values of the KPdCu(Se2)(Se3) (RbPdCu(Se2)(Se3)) compounds as a function of relaxation time reach 2.2 (1.8) × 1011, 4.4 (3.5) × 1011 and 1.3 (1.4) × 1011 within Pxx, Pyy and Pzz components, respectively.

Coulomb interaction and spin-orbit coupling calculations of thermoelectric properties of the quaternary chalcogenides Tl2PbXY4 (X = Zr, Hf and Y = S, Se)

The increase in energy demands is leading to growing interest in new thermoelectric inorganic materials, such as the chalcogenides. The recently synthesized quaternary chalcogenide, Tl2PbXY4 (X = Zr, Hf and Y = S, Se), compounds were investigated using the full potential linear augmented plane wave method based on density functional theory. We used the generalized gradient approximation plus the optimized effective Hubbard parameter U to treat the exchange correlation. The existence of heavy metals (Tl, Pb and Hf) required the application of relativistic spin–orbit coupling via a second variational procedure. Tl2PbHfS4, Tl2PbHfSe4, Tl2PbZrS4 and Tl2PbZrSe4 compounds were found to be semiconductors with indirect band gaps of 0.911, 0.659, 0.983 and 0.529 eV, respectively. The types of carriers and electrical transport properties of Tl2PbXY4 (X = Zr, Hf and Y = S, Se) are attributed to the Tl-d and S/Se-s electronic states near the Fermi level. Optical properties were investigated via the calculation of dielectric function and reflectivity. Using Boltzmann theory, we compared the thermoelectric properties and we found that Tl2PbHfS4 could be a good candidate for thermoelectric devices.

Predicted Thermoelectric Properties of the Layered XBi4S7 (X = Mn, Fe) Based Materials: First Principles Calculations

We report a theoretical investigation of electronic structures, optical and thermoelectric properties of two ternary-layered chalcogenides, MnBi4S7 and FeBi4S7 , by combining the first principles density functional calculations and semi-local Boltzmann transport theory. The calculated electronic band structure have demonstrated that both compounds exhibit indirect band gaps. The optical transitions are explored via the dielectric function (real and imaginary parts) along with other related optical constants including refractive index, reflectivity, and energy loss spectrum. These chalcogenides have exhibited interesting thermoelectric properties such as Seebeck’s coefficient, electrical and thermal conductivity, and power factor as function of temperatures.

Optoelectronic Structure and Related Transport Properties of Ag2Sb2O6 and Cd2Sb2O7

Using the full-potential linearized augmented-plane wave method, the electronic structure and thermoelectric properties of Ag2Sb2O6 and Cd2Sb2O7 compounds have been explored. The modified Becke–Johnson potential was applied to treat the exchange–correlation energy term. The electronic band structures reveal that the valence-band maximum and conduction-band minimum occur at Γ point, indicating that Ag2Sb2O6 and Cd2Sb2O7 are direct energy bandgap semiconductors. Strong hybridization appeared between Ag (Cd)-s/p and O-s/p states. The optical properties, i.e., complex dielectric function, reflectivity, refractive index, and energy loss function, reveal high reflectivity in the ultraviolet energy range, indicating usefulness of these materials in shields from high-energy radiation. Combining transport theory and the outputs from the full-potential linearized augmented-plane wave calculations, the thermoelectric properties were analyzed as functions of temperature. Due to their high thermopower and narrow bandgap, Ag2Sb2O6 and Cd2Sb2O7 are suitable materials for application in optoelectronic and thermoelectric devices.

Electronic and Thermoelectric Properties of Ternary Chalcohalide Semiconductors: First Principles Study

Ternary chalcohalides have been widely utilized for different device applications. The thermoelectric properties of SbSI, SbSeI and SbSBr have been investigated by theoretical simulations, and the findings have been performed using BoltzTraP code, based on semi-classical Boltzmann transport theory. In this study, we simulated the electronic structures using the Englo-Vosko generalized gradient approximation employed in the WIEN2k program. From the electronic band structures, we found a combination of light and heavy bands around the Fermi level in the valence band, which strongly affect the effective masses of the carriers. The entire thermoelectric parameters, like the electrical, the electronic part of the thermal conductivities, the Seebeck coefficient and the power factor have been analysed as functions of temperature and chemical potential. The correlation between the effective masses and the thermoelectric properties is also included in the discussion because the effective mass reveals the mobility of the carriers which in turn affect the thermoelectric properties. The substitution of sulfur reveals high electrical conductivity and a smaller Seebeck coefficient based on effective mass leads to the increase in the power factor.

Tailoring the electronic structure and optical properties of cadmium-doped zinc oxides nanosheet

Abstract Cd-doped ZnO nanosheet (ZnO NS) were investigated using a full-potential linearized augmented plane wave method within the generalized gradient approximation (GGA) to calculate the electronic structure and its optical response. The calculated band structures have shown that the Cd-doped ZnO NS is a direct band gap semiconductor at Γ with 1.50 eV band gap. The contribution of each atom/orbital were commented in light of total and partial densities of states. We also derived the optical constants (mainly the dielectric constants ε1(0) and ε2(0)), the absorption coefficient I(ω), refractive index n(ω), extinction coefficient k(ω), and energy-loss function L(ω). The spectrum of absorption coefficient has revealed to increase rapidly for photon energies higher than 2.5 eV. The absorption spectrum was found to be limited in energy region due to different contributions electronic transitions that occurred within ZnO NS and effect of Cd doping. Reducing the band gap of ZnO NS to low values is suitable process for light-emitting devices and solar cells applications.

Optoelectronic and Magnetic Properties of Eu2Si5N8: An Ab-initio Study

Abstract Eu2Si5N8 is considered the most important compound in the development of inorganic materials with high potential and performance. Therefore, the electronic, magnetic and optical properties of Eu2Si5N8 are investigated here using density functional theory. The electronic interactions are described within the generalised gradient approximation, GGA+U (where U is the Hubbard Coulomb energy term). The calculated energy gap was 3.5 eV for the investigated compound, resulting in a direct band gap semiconductor. The optical constants, including the dielectric function, refractive index, absorption coefficient, reflectivity, and energy loss function were calculated for radiation up to 14 eV. The optical properties demonstrate that the main differences in absorption, reflectivity, energy-loss function and refractive index occur in the infrared and visible regions for the spin-up and spin-down states, which makes this material an excellent candidate for optical memory devices.