ABSTRACT
CONTEXT AND RESULTS: In this work the first-principles calculations of the structural, electronic and thermoelectric properties of monolayer TiSe2 are presented. The optimized lattice parameter of monolayer TiSe2 shows excellent agreement with the experimental value. The computed band structure and density of states calculations predict metallic nature of monolayer TiSe2 with overlapping of 0.44 eV between the lowest conduction band and top valance band at high symmetry point M. The position of pseudogap formed by Ti-3d orbitals near the Fermi level confirms the mechanical stability of monolayer TiSe2. Due to the influence of positive strain (tensile strain), the Ti-Se bond length increases and the layer height decreases. The applied tensile strain changes the metallic nature of TiSe2 into a semiconductor with opening of bandgap. It has also been observed that the positions of conduction band minima and valance band maxima change with strain. The charge analysis shows that charge transfer from Ti to Se atom increases when tensile strain is applied, while an opposite trend is observed with compression. The computed thermoelectric coefficients i.e. Seeback coefficient, power factor and figure of merit are in good agreement with the experimental data. The temperature dependence of these coefficients is also reported. COMPUTATIONAL METHOD: The density functional theory based calculations are reported employing the PBE-GGA ansatz using the plane wave-pseudopotential method embodied in the Quantum ESPRESSO package. The self-consistent field calculations are performed over a dense Monkhorst-Pack net of 12 × 12 × 1 k-points. The energy convergence criteria for the self-consistent field calculation were set to 10-6 Ry/atom with a cutoff energy of 90 Ry. The thermoelectric properties are computed by combining the band structure calculations with the Boltzmann transport equation using Boltztrap2 peckage.
ABSTRACT
CONTEXT AND RESULTS: The elastic, electronic, and vibrational properties of the ground state of the rocksalt SnTe and PbTe are investigated. The deduced elastic constants, namely, shear modulus, Young's modulus, and Poisson's ratio are in very good agreement with the experimental and other theoretical data. The electronic band structure and density of states are obtained with and without considering the spin-orbit coupling. The bandgaps of SnTe and PbTe with (without) spin-orbit coupling are 0.11 (0.05) eV and 0.01 (0.78) eV, respectively. The bandgaps with spin-orbit interactions are nearer to experimental data. The hybrid functionals give higher values of bandgaps for both the SnTe and PbTe. In both compounds, the bandgap increases with volume. The valence bandwidths, however, decrease with increasing volume. The vibrational frequencies are found in reasonable agreement with the experiment. The frequencies increase with pressure. COMPUTATIONAL METHOD: In this work, the ab initio calculations of SnTe and PbTe crystals are carried out applying plane wave pseudopotential method using the QUANTUM ESPRESSO package. The PBE exchange and correlation functional based on GGA is considered. The fully relativistic norm-conserving pseudopotentials for Sn, Pb, and Te are used. The self-consistent field calculations are performed over a dense MP net of 18 × 18 × 18 k-points. The energy cut-off of 70 Ryd was found sufficient to achieve convergence of 10-6 Ryd in total energy of the crystals.
