Effect of valence band splitting on the absorption spectra of monolayer MoS2 in presence of sulphur vacancies

Single layer Molybdenum Disulfide (MO$S$2), a direct band gap transitional metal dichalcogenide (TMDC) has attracted a lot of research and study due to its excellent electro-optical integrity. Optical absorption within this monolayer Mos2is extremely influenced by the presence of sulphur vacancies. At reduced dimensions the interaction between these vacancy created trap centres and charge carriers becomes more prominent leading to the formation of bound excitons. Here we demonstrate the absorption spectra of a single layer Mos2through many body perturbation theory in the presence of sulphur vacancy sites. We use a fully relativistic approach within the GW approximation containing the non collinear core correction with full spinor wave functions. The absorption spectra calculation is achieved through the Bathe-Salpeter equation to include the excitonic excitations at room temperature. Our computations exhibit a Gaussian absorption spectra observed with double excitonic peaks A and B, unlike the step function profile without the incorporation of excited states. This double peak corresponds to valence band splitting at the K point of the brillouin zone which is most prominent at the top of the valence band and is a result of spin orbit coupling of the excitons. The absorption edge demonstrates a red shift when investigated in the presence of sulphur vacancies which can be attributed to inter excitionic interactions and the reduction in bandgap in the presence of sulphur vacancies. A change in the value of absorption coefficient is observed as a result of localization of excitons in the traps. Our outcomes clarify the vacancy and exciton material science of Mos2offering another course towards fitting its physical properties by defect engineering.