Amplitude curves and operating regimes in dynamic atomic force microscopy

The experimental dependence of the amplitude on the average tip-sample distance has been studied to understand the operation of an atomic force microscope with an amplitude modulation feedback. The amplitude curves can be classified in three major groups according to the existence or not of a local maximum and how the maximum is reached (steplike discontinuities vs. smooth transitions). A model describing the cantilever motion as a forced nonlinear oscillator allows to associate the features observed in the amplitude curves with the tip-sample interaction force. The model also allows to define two elemental tip-sample interaction regimes, attractive and repulsive. The presence of a local maximum in the amplitude curves is related to a transition between the attractive and the repulsive regime.