Wilayat Khan

Exploring the optoelectronic structure and thermoelectricity of recent photoconductive chalcogenides compounds, CsCdInQ3 (Q = Se, Te)

The photoconductive quaternaries, CsCdInQ3 (Q = Se, Te), have been recently synthesized and have been shown to be potential materials for hard X-ray and γ-ray detection. These materials have relatively high densities and band gaps in the range of 1.5–3 eV, which make them fulfill the requirement of hard detection devices. In the present work, we investigate the metal chalcogenide, CsCdInQ3 as deduced from a full potential linearized augmented plane wave method based on density functional formalism. The direct band gaps are estimated at the level of the EV-GGA functional, as 2.11 and 1.75 eV for CsCdInSe3 and CsCdInTe3, respectively. These values are in good agreement with the experimental measurements (2.40 and 1.78 eV) obtained from solid-state UV-vis optical spectroscopy. Optical parameters, including the dielectric constant, absorption coefficient, energy loss function reflectivity and refractive index, were also reported to investigate the potential role of these metal chalcogenide compounds for solar conversion application. Our calculated optical band gap was compared to the measured experimental values on a Lambda 1050 UV-vis-IR spectrophotometer in the range of 300–1500 nm. The thermoelectric properties discuss the variation of the electrical and thermal conductivity, Seebeck coefficient and power factor with the temperature variation using the Boltzmann transport theory.

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.

Correlation between the electronic structure, effective mass and thermoelectric properties of rare earth tellurides Ba2MYTe5 (M = Ga, In)

Rare earth telluride compounds, namely Ba2MYTe5 (M = Ga, In), are the focus of this work due to their semiconducting nature, optoelectronic and thermoelectric properties. Their band gaps lie in the range of 1.08 to 1.36 eV, providing these compounds with opto- and thermo-electric properties. Here, we have studied the rare earth telluride single crystals of Ba2MYTe5 (M = Ga, In) using the full potential linearized augmented plane wave package WIEN2k. The direct band gaps were calculated using the modified Becke–Johnson approach which is in good agreement with the band gaps obtained from diffuse reflectance spectra. The density of states reveals a strong hybridization between Y 5s/4p, Ga 3d, Te 5p and Y 4d orbitals, indicative of covalent bonding. Besides, the electronic charge density contour discloses a mix of ionic and covalent bonding between the elements. We also report the thermoelectric properties studied through the temperature dependent electronic and thermal conductivities, as well as the Seebeck coefficient and the power factor using the BoltzTraP code.

Theoretical study on optical and thermoelectric properties of the direct band gap α/β-Ca2CdAs2 pnictide semiconductors

This article reports the utilization of the density functional theory within the Perdew–Burke–Ernzerhof generalized gradient functional for the electronic structure, optical and thermoelectric properties of the α-Ca2CdAs2 and β-Ca2CdAs2 single crystals. The theoretical calculations show that both the compounds are direct band gap semiconductors with optical band gaps of 0.96 eV and 0.79 eV at the point for α- and β-phases, respectively, which demonstrate that these compounds are optically active. The total and projected density of states, electronic charge density and optical properties, such as complex dielectric function, energy loss function, absorption coefficient, reflectivity and refractive index of α/β-Ca2CdAs2, were studied using the abovementioned technique. The calculated results for the energy band structures are compared with experimental and other simulated data. Temperature-dependent thermoelectric properties, such as electrical and thermal conductivity, Seebeck coefficient, power factor and figure of merit, were calculated. Our results show that α- and β-Ca2CdAs2 compounds are suitable materials for non-linear optical properties and thermoelectric devices.

Local vs Nonlocal States in FeTiO3 Probed with 1s2pRIXS: Implications for Photochemistry

Metal–metal charge transfer (MMCT) is expected to be the main mechanism that enables the harvesting of solar light by iron–titanium oxides for photocatalysis. We have studied FeTiO3 as a model compound for MMCT with 1s2pRIXS at the Fe K-edge. The high-energy resolution XANES enables distinguishing five pre-edge features. The three first well distinct RIXS features are assigned to electric quadrupole transitions to the localized Fe* 3d states, shifted to lower energy by the 1s core–hole. Crystal field multiplet calculations confirm the speciation of divalent iron. The contribution of electric dipole absorption due to local p-d mixing allowed by the trigonal distortion of the cation site is supported by DFT and CFM calculations. The two other nonlocal features are assigned to electric dipole transitions to excited Fe* 4p states mixed with the neighboring Ti 3d states. The comparison with DFT calculations demonstrates that MMCT in ilmenite is favored by the hybridization between the Fe 4p and delocalized Ti 3d orbitals via the O 2p orbitals.

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.