Influence of Mo-vacancy concentration on the structural, electronic and optical properties of monolayer MoS₂: A first-principles study

Effects of Mo-vacancy concentration on the structural, electronic and optical properties of monolayer MoS₂ have been investigated using the first-principles calculations. Results show that Mo-vacancy is prone to form in monolayer MoS₂ under S-rich condition. S atoms around Mo-vacancy exhibit an outward relaxation, whereas Mo atoms around Mo-vacancy show an inward relaxation. At low Mo-vacancy concentration, some localized impurity states are induced in the band gap of monolayer MoS₂, coupled with a band gap increment. As the Mo-vacancy concentration increases, the impurity states become delocalized and mix with the upper valence bands, resulting in the band gap decrease. The covalent character of Mo-S bonding is enhanced upon the introduction of Mo-vacancy, and the enhancement is weakened as the Mo-vacancy concentration increases. Optical properties calculations show that the static dielectric constant increases with the increasing Mo-vacancy concentration. The imaginary part of complex dielectric function exhibits a little blue shift for monolayer MoS₂ with low Mo-vacancy concentration, whereas the imaginary part of complex dielectric function shows distinct red shift for monolayer MoS₂ with high Mo-vacancy concentration.