Unveiling surface and subsurface atomic vacancies in MoS₂ with lateral force microscopy

Atomic-scale defects govern materials properties, with their influence amplified in low-dimensional systems. Understanding and controlling such defects is therefore essential for both fundamental science and technological applications. However, precise defect identification and localization remain major challenges. Lateral force microscopy (LFM), an atomic force microscopy mode that measures friction forces through the lateral torsion of the cantilever, is an appealing technique for probing surface properties. Nevertheless, it has conventionally been regarded as insensitive to structural features beneath the topmost atomic layer. In this study, we overturn this paradigm by demonstrating that LFM can not only detect but also classify atomic vacancies in MoS₂ according to their depth. By combining experiments with molecular dynamics and Prandtl-Tomlinson (PT) modeling, we identify unique frictional fingerprints associated with each defect type: surface vacancies produce a characteristic 'drop-and-rise' signature, whereas subsurface vacancies generate a pronounced exit barrier without an entry drop. While molecular dynamics validates these specific MoS₂ signatures, the PT model reveals a generic, geometric lattice-disruption mechanism, extending the validity of our findings beyond MoS₂. Furthermore, we apply these findings to compare the defect density and defect types in MoS₂ flakes obtained via chemical vapor deposition (CVD) and mechanical exfoliation.