Scanning Capacitance Force Microscopy
Scanning Capacitance Microscopy is kind of dynamic EFM. Generally  in EFM. the cantilever is biased directly by Vtip=Vdc + Vac sin(wt), where Vac is referred to as the driving voltage. Scanning is executed on some height h above the sample surface in according with the profile defined during the first scanning in Semicontact mode. The capacitive force Fcap(z) between the tip and a sample surface at potential Vs is
Fcap(z) =(1/2) (Vtip - Vs)2(dC/dz)
where C(z) is the tip-surface capacitance dependent on tip geometry, surface topography and tip-surface separation z.
Second harmonic of the capacitive force depends only on (dC/dz) and Vac
Fcap2w(z) =(1/2) (dC/dz)Vac2 sin(2wt)
and can be used for acquisition additional information, e.g distribution of the surface capacity over the sample. For maximization of the second harmonic oscillations the ac frequency w is adjusted to be equal to half of cantilever resonance frequency wr.
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