Quantify point defects in monolayer tungsten diselenide

Point defects may significantly influence the electrical and optoelectronic properties of two-dimensional (2D) tungsten diselenide (WSe₂), while precise information about point defects distribution (e.g. species and concentration) in monolayer (ML-) WSe₂are hard to obtain. In this letter, we tried to partly fill this knowledge gap via performing quantitative and statistical analysis of intrinsic point defects in WSe₂monolayers prepared by three so-called main-stream approaches i.e. mechanical exfoliation (ME), chemical vapor deposition (CVD), and molecular beam epitaxy (MBE), which are promising for providing high-quality samples. Via a conjunction of statistic atomic-resolution annular dark-field scanning transmission electron microscopy imaging, software-based automated defect identification and counting, together with image simulations, defect species and concentrations were quantitatively determined. Seven types of intrinsic point defects were identified in ML-WSe₂and the most dominant one is selenium mono-vacancy (V_Se) (corresponding to one Se atom missing), irrespective of the synthetic route and growth conditions. Exact contents and diversity of point defects depend on the specific preparation method: CVD grown ML-WSe₂is the most defective (for example, the density of V_Sereaches 1.48% in atomic ratio), followed by ME (∼0.85 at% for V_Se) and MBE grown samples (∼0.49 at% for V_Se). Our results, though still with limited sampling, provide preliminary quantitative information of point defects in ML-WSe₂, which can serve as a reference to achieve the precisely controlled large-scale sample growth and establish the structure-property relationship of 2D transition-metal dichalcogenides materials.