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How Substitutional Point Defects in Two-Dimensional WS$_2$ Induce Charge Localization, Spin-Orbit Splitting, and Strain

Control of impurity concentrations in semiconducting materials is essential to device technology. Because of their intrinsic confinement, the properties of two-dimensional semiconductors such as transition metal dichalcogenides (TMDs) are more sensitive to defects than traditional bulk materials. The technological adoption of TMDs is dependent on the mitigation of deleterious defects and guided incorporation of functional foreign atoms. The first step towards impurity control is the identification of defects and assessment of their electronic properties. Here, we present a comprehensive study of point defects in monolayer tungsten disulfide (WS$_2$) grown by chemical vapor deposition (CVD) using scanning tunneling microscopy/spectroscopy, CO-tip noncontact atomic force microscopy, Kelvin probe force spectroscopy, density functional theory, and tight-binding calculations. We observe four different substitutional defects: chromium (Cr$_{\text{W}}$) and molybdenum (Mo$_{\text{W}}$) at a tungsten site, oxygen at sulfur sites in both bottom and top layers (O$_{\text{S}}$ top/bottom), as well as two negatively charged defects (CDs). Their electronic fingerprints unambiguously corroborate the defect assignment and reveal the presence or absence of in-gap defect states. The important role of charge localization, spin-orbit coupling, and strain for the formation of deep defect states observed at substitutional defects in WS$_2$ as reported here will guide future efforts of targeted defect engineering and doping of TMDs.