Vacancy-driven reversal of layer-dependent friction in two-dimensional MoSe2

The atomic-layer dependence of friction is a fundamental characteristic of two-dimensional (2D) materials, yet the impact of atomic defects on it remains largely unexplored. Here, we report the reversal of frictional layer-number dependence in chemical vapor deposition grown ${\mathrm{MoSe}}_{2}$ by tuning the selenium vacancy concentration. While stoichiometric triangular ${\mathrm{MoSe}}_{2}$ follows the conventional trend where bilayer friction is lower than that of the monolayer, Se-deficient hexagonal ${\mathrm{MoSe}}_{2}$ exhibits an anomalous behavior with bilayer friction exceeding the monolayer. A similar defect-dependent reversal is further observed in ${\mathrm{WSe}}_{2}$, while triangular ${\mathrm{WSe}}_{2}$, ${\mathrm{MoS}}_{2}$, and ${\mathrm{WS}}_{2}$ exhibit the conventional layer dependence consistent with the proposed mechanism. Density functional theory calculations and experiments reveal that Se vacancies impede interlayer charge redistribution and weaken van der Waals coupling, reducing adhesion energy from 0.538 to 0.498 J/${\mathrm{m}}^{2}$. The consequent reduction in effective film stiffness amplifies the out-of-plane puckering effect, enhancing energy dissipation. These findings demonstrate that defect engineering provides an effective strategy for modulating the nanotribological properties of 2D lubricating interfaces.