Band gap in tungsten sulphoselenide single crystals determined by the optical absorption method

Optical absorption in single crystals of tungsten sulphoselenides has been measured at room temperature with the incident light beam normal to the basal plane, i.e. along c-axis of the grown crystal flakes. Results have been analysed on basis of two-dimensional (2D) and three-dimensional models (3D). Absorption near the fundamental edge was found to be due to indirect allowed transition on the basis of (3D) model and due to indirect forbidden transition on the basis of a (2D) model. Both transitions involve two phonons. The energy gaps and phonon energies have been estimated. The indirect and direct band gaps of WSxSe2−x vary smoothly with the S/Se composition x, indicating that the nature of band edges is similar for WSe2, WS2 and compounds of intermediate composition. The dependence of band gaps on sulphur/selenium content in WSxSe2−x has been adequately explained on the basis of band structure calculations reported in the literature.