Attractive and repulsive tip-sample interaction regimes in tapping-mode atomic force microscopy

Attractive and repulsive tip-sample interaction regimes of a force microscope operated with an amplitude modulation feedback were investigated as a function of tip-sample separation, free amplitude, and sample properties. In the attractive regime, a net attractive force dominates the amplitude reduction while in the repulsive regime the amplitude reduction is dominated by a net repulsive force. The transition between both regimes may be smooth or steplike, depending on free amplitude and sample properties. A steplike discontinuity is always a consequence of the existence of two oscillation states for the same conditions. Stiff materials and small free amplitudes give rise to steplike transitions while the use of large free amplitudes produce smooth transitions. Simulations performed on compliant samples showed cases where the cantilever dynamics is fully controlled by a net attractive force. Phase-shift measurements provide a practical method to determine the operating regime. Finally, we discuss the influence of those regimes in data acquisition and image interpretation.