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KPFM
Kelvin
Probe Force Microscopy
In the Kelvin method (or vibrating capacitor
method) two conductors are arranged as a parallel
plate capacitor with a small spacing. In a simple
model the contact potential difference (CPD)
between the two materials is VCPD = 1/e(Ô2-Ô1),
where Ô1 and Ô2 are the work
functions of the conductors including changes due
to adsorption layers on the surface. A periodic vibration
of the distance between the two plates at w results
in current i(t) given by
i(t)=VCPDwDCcoswt
where
D C is the change in capacitance. For the actual measurements
of VCPD , an additional bucking voltage
is applied between two plates until the space in between
is field free and the current I(t) goes to zero.
High lateral resolution CPD
measurements can be achieved by using a modified version
of the ac SFM. The
principle is similar to the Kelvin method exept that
forces are measured instead of currents. An additional
voltage is applied between tip and sample until the
electric field in between vanishes, as determined
by measuring the force caused by the applied voltage.
In [1] the ac voltage between tip and sample was applied
at the resonance frequency while the piezo-induced
vibration of the cantilever was driven slightly above
resonance at fres + D f. The signal change
at the piezo-induced vibration amplitude was used
to control the dastance between rip and sample. The
second feedback loop was used to measure the CPD
by minimazing the electric field between tip and sample.
1. Appl. Phys. Lett. 58, 2921 (1991).
KPM
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