  Transition metal dichalcogenides (TMDs) represent an entire new class of
semiconducting 2D materials with exciting properties. Defects in 2D TMDs can
crucially affect their physical and chemical properties. However,
characterization of the presence and spatial distribution of defects is limited
either in throughput or in resolution. Here, we demonstrate large area mapping
of reactive sulfur-deficient defects in 2D-TMDs coupling single-molecule
localization microscopy with fluorescence labeling using thiol chemistry. Our
method, reminiscent of PAINT strategies, relies on the specific binding by
reversible physisorption of fluorescent probes to sulfur-vacancies via a thiol
group and their intermittent emission to apply localization of the labeled
defects with a precision down to 15 nm. Tuning the distance between the
fluorophore and the docking thiol site allows us to control F\"oster Resonance
Energy Transfer (FRET) process and reveal large structural defects such as
grain boundaries and line defects, due to the local irregular lattice
structure. Our methodology provides a simple and fast alternative for
large-scale mapping of non-radiative defects in 2D materials and paves the way
for in-situ and spatially resolved monitoring of the interaction between
chemical agent and the defects in 2D materials that has general implications
for defect engineering in aqueous condition.
