Load dependence of two-dimensional atomic-scale friction

We investigated the load dependence of two-dimensional atomic-scale friction between a ${\mathrm{MoS}}_2$ surface and a ${\mathrm{Si}}_3$ ${\mathrm{N}}_4$ tip. We found that the amplitude of the frictional force hysteresis is determined by the effective adhesive radius itself. Besides, we observed nearly zero kinetic frictional force at the normal load of 2.3×${10}^{\mathrm{-}8}$ N. The load dependence of the effective adhesive radius suggests that the appearance of the negative frictional force correlates clearly with the load dependence.