Chemical and Electronic Repair Mechanism of Sulfur Defects in MoS${}_2$ Monolayers

Using {\em ab initio} density functional theory calculations, we characterize changes in the electronic structure of MoS${}_2$ monolayers introduced by missing or additional adsorbed sulfur atoms. We furthermore identify the chemical and electronic function of substances that have been reported to reduce the adverse effect of sulfur vacancies in quenching photoluminescence and reducing electronic conductance. We find that thiol-group containing molecules adsorbed at vacancy sites may re-insert missing sulfur atoms. In presence of additional adsorbed sulfur atoms, thiols may form disulfides on the MoS${}_2$ surface to mitigate the adverse effect of defects.