Conservative and dissipative forces measured by self-oscillator atomic force microscopy at constant-drive amplitude

The fundamental-mode oscillation of an atomic force microscope cantilever, operated in the self-oscillator (SO) mode, is analyzed to interpret resonant frequency (f(r)) and oscillation amplitude (A) in terms of probe-sample conservative and dissipative forces. Measurements of f(r) and A versus probe-surface gap, for a H-terminated silicon probe and surface in air, have been carried out in the constant-excitation (CE), SO mode for a variety of SO phases. We provide the full z dependence of A and Deltaf due to both conservative and dissipative forces, evidencing a minimum value of tip-sample closest approach distance expected from theory. The existence of such a minimum preserves tips from destructive interaction in CE mode.