Scanning Capacitance Microscopy

Scanning Capacitance Microscopy is kind of dynamic EFM. Generally [1] in EFM. the cantilever is biased directly by V_tip=V_dc + V_ac sin(wt), where V_ac 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 F_cap(z) between the tip and a sample surface at potential V_s is

F_cap(z) =(1/2) (V_tip - V_s)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 V_ac

F_cap2w(z) =(1/2)(dC/dz) V_ac²sin(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 w_r.
 

    References

1. Nanotechnology 12, 485 (2001). 
2. Appl. Phys. Lett. 52, 1103 (1988). 
3. J. Appl. Phys. 61, 4723 (1987).

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