  Structural defects are widely regarded as detrimental to the optoelectronic
properties of monolayer transition metal dichalcogenides, leading to concerted
efforts to eliminate defects via improved materials growth or post-growth
passivation. Here, using steady-state and ultrafast optical spectroscopy,
supported by ab initio calculations, we demonstrate that sulfur vacancy defects
act as exciton traps. Current chemical treatments do not passivate these sites,
leading to decreased mobility and trap-limited photoluminescence. We present a
generalizable treatment protocol based on the use of passivating agents such as
thiols or sulfides in combination with a Lewis acid to passivate sulfur
vacancies in monolayer MoS$_2$ and WS$_2$, increasing photoluminescence up to
275 fold, while maintaining mobilities. Our findings suggest a route for simple
and rational defect engineering strategies, where the passivating agent varies
the electronic properties, thereby allowing the design of new heterostructures.
