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

Abstract

This paper is an attempt to clarify the question of the damping signal in non-contactatomic force microscopy. For more than ten years now, the non-contact atomic force hasbeen used as a powerful tool for investigating topography and/or mechanical properties ofsurfaces at the nanometre scale. In non-contact mode the cantilever is inside a closed loopand the frequency of the loop depends on the tip–surface interaction. Variations of thefrequency shift as a function of the tip–surface distance are now well understood, andtheoretical 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 theautomatic gain control used to keep the amplitude of the oscillation constant. This signalgives information about the dissipative forces, and true atomic resolution is alsoobtained with the damping signal. Various theoretical models have been proposedfor explaining the dissipation, but as far as the atomic scale is concerned, mostof them predict much smaller dissipation than the ones often observed. In anattempt to clarify the situation, a new fast virtual machine has been built using acoarse graining method. This new machine represents real progress compared topreceding virtual machines because it improves the computing time by more than twoorders of magnitude. This machine allows us to extract the contribution of theinstrument to the damping signal and thus to propose a method for experimentalistsfor removing any artefact measurements. On the other hand, it is shown thatthe damping signal can be strongly enhanced if the gains of the automatic gaincontrol and the scan speed of the tip are chosen close to some critical values.Finally, theoretical results are successfully compared to experimental results.