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The most useful of them is Non-Contact EFM mode based on the two-pass technique. During the second pass the cantilever is piezodrived at resonant frequency and cantilever is grounded or biased by dc voltage V.
Capacitive tip-sample electric force (or rather its derivative) leads to resonance frequency shift. Accordingly amplitude of cantilever oscillation decreases and phase of oscillation changes . Both amplitude and (or) phase of oscillation deviations can be measured and electric potential distribution over the sample surface can be imaged.
This mode of operation has some advantages comparatively to Scanning Kelvin Microscopy (SKM). Registered amplitude or phase deviations images are determined by capacitive tip-sample electric force derivative, e.g. second derivative of tip-sample capacity. As a result Non-Contact EFM leads to higher resolution because the ratio of the parasitic capacitance of the tip side and of the flat part of the cantilever to the useful capacitance of the tip apex should is minimized [2,3].
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