Quartz tuning forks as sensors for attractive-mode force microscopy under ambient conditions

We present investigations of the frequency versus distance behavior of a quartz tuning-fork-based atomic force microscope. We show that if the amplitude of the motion A of the tip is large, then the apparent shape of the tip-surface interaction curve depends on A. For smaller amplitudes of oscillation (A less than or similar to3 nm), we find that the shape of the interaction curve becomes independent of A. In this low amplitude limit, a simple relation between the observed frequency shift and the underlying interaction allows quantitative determination of tip-sample forces. Tuning fork sensors open a window for dynamic-mode force microscopy in a regime where conventional microfabricated sensors are overwhelmed by long range capillary forces. (C) 2001 American Institute of Physics.