Dynamic force microscopy with quartz tuning forks at high oscillation amplitudes

Dynamic force microscopy ( DFM) with the self- oscillator ( SO) method allows reasonably high scanning rates even with high Q- factors of the resonant force sensor, typical of cantilevers in ultra- high vacuum and of quartz tuning forks. However, due to simpler interpretation of force spectroscopy measurements, small oscillation amplitudes ( sub- nm level) are generally preferred. In applications like 'apertureless' scanning near- field optical microscopy ( SNOM), oscillation amplitudes of the order of 5 - 10 nm are needed to increase optical sensitivity and to apply standard optical artefact suppression methods. This motivates the study of the behaviour of tuning forks driven at such high amplitudes, as compared to usual air- operated cantilevers. Both constant- excitation- amplitude ( CE) and constant- oscillation- amplitude ( CA) modes of SO- DFM are analysed, since the CA mode is more convenient for SNOM applications, denoting remarkable differences. In particular, possible instability effects, previously found in CE mode, are not anticipated for CA mode. It is shown how resonance and approach (' isophase') curves in both modes can be conveniently described in terms of the usual ` normalized frequency shift' gamma and of a ` normalized gain' eta, defined as a measurement of surface dissipation.