Structural, electronic and optical properties of TcX2 (X = S, Se, Te) from first principles calculations

The structural, electronic and optical properties of layered TcS2, TcSe2, and TcTe2 in the P (1) over bar structure have been investigated using different density functional (DFT) approximations with and without van der Waals corrections. The van der Waals correction terms are important for the accurate prediction of inter-layer spacing in these layered structures. Elastic constants were calculated from finite size distortions of the unit cell and phonon dispersions of the structures were investigated using density functional perturbation theory. The equilibrium elastic constants satisfy all the stability criteria for triclinic structures, showing stability against small distortions. No negative phonon frequencies were found, confirming dynamically stability. We performed DFT and post-DFT many-body perturbation theory Green's Function (GW) calculations at the so-called G(0)W(0) level of approximation to get reasonable estimates of the fundamental band gap of the compounds. TcS2, TcSe2, and TcTe2 were found to be indirect band gap semiconductors with fundamental band gaps in the infra-red region of the electro-magnetic spectrum. The minimum direct band gaps were found to be in a similar range. Optical absorption spectra calculated at the Bethe-Salpeter-Tamm-Dancoff level of approximation show that the structures are good absorbers of light in the upper infra-red and visible region. (C) 2016 Elsevier B.V. All rights reserved.