Influence of cobalt-doping on the optical properties of zinc tristhiourea sulfate (ZTS) single crystal

Zinc tris-thiourea sulfate (ZTS) single crystals, pure and doped with different concentrations of cobalt, were synthesized and grown by the slow evaporation technique of supersaturated aqueous solutions at 315 K. Co2+ concentration in the solution was 2, 4 and 6 mol%. Optical transmittance was measured for the obtained crystals as a function of wavelength in the range of 190-900 nm. The present investigation shows that doping ZTS crystal with Co2+ modifies its optical properties. The absorption coefficient of ZTS crystal exhibits an exponential dependence on photon energy following Urbach relation. Doping ZTS crystal with Co2+ changes the values of optical transmittance, cut off wavelength, absorption coefficient, optical energy gap, Urbach tail energy and steepness parameter. The changes are systematic i.e. the mentioned optical parameters increase or decrease continuously with increasing Co2+ concentration in the growth solution. The electrical conduction of ZTS crystal is increased with cobalt doping. The Urbach tail energy changes reversely with the optical energy gap.