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SPM Principles
Scanning Capacitance Microscopy
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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_{ac}^{2}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 w_{r}. References |
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