Comparative study of structural,electronic, optical and thermoelectric properties of GaS bulk and monolayer

The structural, electronic, optical properties of GaS in bulk and monolayer forms have been studied by means of full-potential linearised augmented plane wave calculations within framework of the density functional theory. Generalised gradient approximation and Tran–Blaha modified Becke–Johnson exchange potential (mBJ) were employed for the treatment of exchange-correlation effect in calculations. Our calculated lattice parameters are in good agreement with previous theoretical results and available experimental data. The negative formation enthalpy and cohesive energy indicate that both bulk and monolayer GaS can be synthesised and stabilised experimentally. Our electronic results show that the band gap of GaS monolayer is higher than that of bulk counterpart and strong hybridisation between electronic states of constituent atoms is observed in both cases. The optical properties such as reflectivity, absorption coefficient, refractive index and optical conductivity were derived from calculated complex dielectric function for wide energy range up to 35?eV. Finally, the thermoelectric properties of GaS bulk and monolayer also were calculated using semi-classical Boltzmann theory within the constant relaxation time approximation for investigating their applicability in thermoelectric devices.