Observation of Deep In-gap States of Line and Point defects in Monolayer WS₂ via Scanning Tunneling Microscopy and Spectroscopy

Atomic defects in crystalline semiconductors strongly affect their electronic properties, such as electron transport and optical response. Defects in two dimensional (2D) semiconductors, such as transition metal dichalcogenides (TMD’s), have a more dramatic impact than bulk counterparts due to less screening and increased substrate interactions. In particular, different atomic vacancies have been reported, but their role in the electronic structure must be established. Deep in-gap states were predicted for different defect structures in 2D TMD’s, but direct experimental observation of defect structure and electronic properties is necessary to tailor TMD’s for device design. Here we report the creation of point and line defects in WS₂ monolayers by vacuum annealing. The electronic properties and structure of these defects was probed using scanning tunneling microscopy and spectroscopy. We find these defects have a rich variety of deep in-gap states, shedding light on the role of defects in TMD electronic properties.