Effect of an atomic scale protrusion on a tip surface on molecular stick-slip motion and friction anisotropy in friction force microscopy

A molecular dynamics (MD) method is used to simulate the molecular stick-sl ip motion and the friction anisotropy observed experimentally between an at omic force microscope (AFM) tip and an ordered monolayer of n-alkane chains which tilt in one of six equivalent stable directions. A slider with a sin gle atomic scale protrusion, connected to an external force control unit vi a three orthogonal springs, is used to model the AFM tip apex with cantilev er springs under feedback regulation of the applied normal force. Although there is almost no interfacial commensurability between the tip atomic latt ice and the sample molecular lattice, molecular lattice-resolved images are observed due to molecular scale stick-slip motion when the size of the pro trusion is comparable to the molecular lattice constant. The present MD sim ulation can provide an explanation of why we can see a molecular lattice in contact AFM.