Nature of tip-sample interaction in dynamic mode atomic force microscopy

The dependence curve of the resonance frequency shift of a dynamic mode atomic force microscope (AFM) cantilever on the distance between the tip and the sample is examined. For a system with clean semiconductor sample and a metal-coated tip, the obtained curve exhibited a larger frequency shift compared to one with a uncoated Si tip, and an increasing deviation from van der Waals characteristics as the separation decreased. This is due to an additional attractive force which becomes dominant at a small separation. This force is considered to play a crucial role in high-resolution imaging of semiconductor surfaces with a dynamic mode AFM.