Probing tribological evolution in atomically thin MoS₂ at different scales

Atomic-scale stick-slip motion and the nanoscale strengthening effect are foundational phenomena in the nanotribology of atomically thin two-dimensional (2D) materials, yet their sub-nanoscale origins and load-dependent evolution remain insufficiently characterized. Herein, we systematically investigate the sub-nanoscale friction behaviors of atomically thin molybdenum disulfide (MoS₂) under controlled loads using a calibrated atomic force microscope (AFM), with a focus on quantifying the strengthening effect and sub-nanoscale stick-slip motion. Our results reveal that the nanoscale strengthening effect intensifies with increasing applied load but weakens as the number of MoS₂ layers increases, attributed to the reduced out-of-plane flexibility in thicker films. Critically, we identify the slip distance during the slip phase as a reliable metric for sub-nanoscale stick-slip motion. The slip distance increases with the frequency of sub-nanoscale stick-slip events and exhibits a load-dependent transition, remaining nearly constant under low loads before increasing with higher loads, and ultimately decreasing at ultrahigh loads. This transition arises from two competing mechanisms: under low loads, the evolution of interfacial contact quality dominates the strengthening effect and suppresses sub-nanoscale stick-slip motion. Under moderate-to-high loads, the puckering effect becomes dominant, enhancing sub-nanoscale stick-slip events and increasing slip distance. At ultrahigh loads, the nanoscale strengthening effect transitions to static friction, which quenches sub-nanoscale stick-slip motion and reduces slip distance. Furthermore, the load-dependent sub-nanoscale stick-slip motion is closely correlated with changes in tip-MoS₂ contact area and contact geometry, both modulated by load and MoS₂ layer thickness. These findings advance 2D material tribology from the nanoscale to sub-nanoscale, providing critical insights for designing low-friction coatings and high-performance micro/nanoelectromechanical systems (MEMS/NEMS).