Electronic structures of ReS2, ReSe2 and TcS2 in the real and the hypothetical undistorted structures

The transition-metal dichalcogenides ReX2 (X = Sor Se) and TcS2 with a d(3) electron configuration have distorted; CdCl2 and Cd(OH)(2) structures, respectively, with the Re(Tc) atoms in each layer forming parallelogram-shaped connected clusters (diamond chain). Ab-initio band-structure calculations were performed for ReX2 and TcS2, and the hypothetical undistorted 1T-TcS2 and 3R-ReX2 structures. The calculations show that ReS2, ReS2 and TcS2 are semiconductors with energy gaps of about 1.0 eV, 0.5 eV and 0.7 eV, respectively, while for the undistorted structures the Fermi level is in the partly filled band of d(x2-y2) and d(xy) orbitals of the t(2g) manifold. X-ray photoemission spectra for the core levels and valence band of ReSe2 and ReS2 are presented. The Valence x-ray photoemission spectra showed that ReS2 is a p-type semiconductor with an energy gap of about 1.5 eV, while ReSe2 is an n-type semiconductor. The experimental results are in good agreement with the band-structure calculations.