Electronic structure and optical properties of layered dichalcogenides: TiS2 and TiSe2

The electronic-energy-band structure of the layered dichalcogenides TiS2 and TiSe2 has been determined using the Korringa-Kohn-Rostoker (KKR) method. Crystal potentials were constructed from overlapping atomic charge densities in the muffin-tin approximation for both full Slater exchange and Kohn-Sham-Gaspár exchange and several different starting atomic configurations. Detailed results are given here for the potential based on the atomic configuration 3d2 4s2 on the titanium atoms, 3p4 on the sulfur atoms, and 4p4 on the selenium atoms and with full Slater exchange. An energy gap of 2.0-2.7 eV between the occupied 3p sulfur bands and the empty 3d titanium bands is found for all potentials studied thereby confirming the traditional view of TiS2 as a semiconductor. TiSe2 is also found to be a semiconductor with a direct energy-band gap of 1.2 eV. The joint density of states calculated from the energy bands of these ab initio calculations are found to agree quite well with recently measured transmission spectra.