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≲3 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.