Optical excitations of transition-metal dichalcogenides with charged adsorbates

Direct band-gap semiconducting monolayer transition-metal dichalcogenides (TMDs) have been previously shown to demonstrate enhanced light absorption, which has promising applications in photovoltaic devices. This work explores the effects of charged adsorbates in MoS₂, MoTe₂, WS₂ and WTe₂ on their optical properties. Charged impurities on TMDs can give rise to bound states that extend over tens of nanometers due to weak screening in two-dimensional materials, hence it becomes difficult to capture their effects with conventional electronic structure calculations based on density-functional theory. In our approach, we first determine the electronic structure using large-scale tight-binding calculations which include a screened defect potential obtained from first-principles random-phase approximation calculations. Next, the Bethe-Salpeter equation is solved to include excitonic effects and obtain the optical absorption spectrum to compare with photoluminescence experiments. We indicate how trigonal symmetry present at the valleys of the band extrema results in deviation from simple hydrogenic behaviour of the bound states, and show dependence of excitonic resonances on the defect charge.