Molecular dynamics simulations of nanoscale and sub-nanoscale friction behavior between graphene and a silicon tip: analysis of tip apex motion.

A sliding object on a crystal surface with a nanoscale contact will always experience stick-slip movement. However, investigation of the slip motion itself is rarely performed due to the short slip duration. In this study, we performed molecular dynamics simulation and frictional force microscopy experiments for the precise observation of slip motion between a graphene layer and a crystalline silicon tip. The simulation results revealed a hierarchical structure of stick and slip motion. Nanoscale stick and slip motion is composed of sub-nanoscale stick and slip motion. Sub-nanoscale stick and slip motion occurred on a timescale of a few ps and a force scale of 10(-1) nN. The relationship between the trajectories of the silicon tip and stick-slip peak revealed that in-plane and vertical motions of the tip provide information about stick and slip motion in the sub-nanoscale and nanoscale ranges, respectively. Parametric studies including tip size, scan angle, layer thickness, and flexibility of the substrate were also carried out to compare the simulation results with findings on lateral force microscopy.