Damping and instability in non-contact atomic force microscopy:: the contribution of the instrument

This paper is an attempt to clarify the question of the damping signal in non-contact atomic force microscopy. For more than ten years now, the non-contact atomic force has been used as a powerful tool for investigating topography and/or mechanical properties of surfaces at the nanometre scale. In non-contact mode the cantilever is inside a closed loop and the frequency of the loop depends on the tip-surface interaction. Variations of the frequency shift as a function of the tip-surface distance are now well understood, and theoretical models are able to explain experimental results. Besides the frequency shift, there is another signal available, the damping signal, which is the error signal of the automatic gain control used to keep the amplitude of the oscillation constant. This signal gives information about the dissipative forces, and true atomic resolution is also obtained with the damping signal. Various theoretical models have been proposed for explaining the dissipation, but as far as the atomic scale is concerned, most of them predict much smaller dissipation than the ones often observed. In an attempt to clarify the situation, a new fast virtual machine has been built using a coarse graining method. This new machine represents real progress compared to preceding virtual machines because it improves the computing time by more than two orders of magnitude. This machine allows us to extract the contribution of the instrument to the damping signal and thus to propose a method for experimentalists for removing any artefact measurements. On the other hand, it is shown that the damping signal can be strongly enhanced if the gains of the automatic gain control and the scan speed of the tip are chosen close to some critical values. Finally, theoretical results are successfully compared to experimental results.