Investigating heterogeneous defects in single-crystalline WS₂ via tip-enhanced Raman spectroscopy

Nanoscale defects in two-dimensional (2D) transition metal dichalcogenides (TMDs) alter their intrinsic optical and electronic properties, and such defects require investigation. Atomic-resolution techniques such as transmission electron microscopy detect nanoscale defects accurately but are limited in terms of clarifying precise chemical and optical characteristics. In this study, we investigated nanoscale heterogeneous defects in a single-crystalline hexagonal WS₂ monolayer using tip-enhanced Raman spectroscopy (TERS). We observed the Raman properties of heterogeneous defects, which are indicated by the shifted A₁′(Γ) modes appearing on the W- and S-edge domains, respectively, with defect-induced Raman (D) mode. In the edge region, various Raman features occur with nanoscale defects. In addition, the TERS signals from single-crystalline WS₂ indicate the existence of two majority defects in each domain, which imply S- and W-dominated vacancies. Quantum mechanical computations were performed for each majority defect and demonstrated the defect-induced variation in the vibrational phonon modes. TERS imaging promises to be a powerful technique for determining assorted nanoscale heterogeneous defects as well as for investigating the properties of other nanomaterials.