Adhesion and friction patterns of CVD-grown WS2 monolayer flakes induced by vacancy-rich defect domains

A comprehensive understanding of the relationship between defects concentration, optical properties and mechanical behavior of two-dimensional transition metal dichalcogenides (TMDs) is crucial for their integration as active components in micro- and nanomechanical devices. In this study we characterize the nanoscale contact adhesion and friction of WS2 flakes grown via chemical vapor deposition. We identify two domains named α and β with distinct mechanical properties, which are not apparent in morphological differences but mirror spatial variations of the optoelectronic properties. The α-domains exhibit high photoluminescence (PL) emission, strong Raman response, higher contact adhesion and lower friction, closely resembling the response of pristine WS2 flakes prepared by mechanical exfoliation. Conversely, the β-domains display very low PL emission, weak Raman response with blueshifted fingerprint peaks, lower adhesion and up to six-fold higher friction. Based on experimental evidence and general arguments, we attribute the mechanical heterogeneity between the α- and β-domains to the differentiated densities of sulfur and tungsten atomic vacancies, which are known to selectively populate the two domains. Our results indicate that the tungsten vacancies in the β-domains not only mediate non-radiative recombination processes but also drive a prominent friction enhancement, either by increasing the amplitude and disorder of the WS2 potential energy surface or by impacting the stress distribution within the growing flakes. These findings help identify the type of defects and mechanisms that most significantly affect the properties of TMD monolayer flakes prepared by scalable production routes.