Open loop Kelvin probe force microscopy with single and multi-frequency excitation

Conventional Kelvin probe force microscopy (KPFM) relies on
closed loop (CL) bias feedback for the determination of surface
potential (SP). However, SP measured by CL-KPFM has been shown to be
strongly influenced by the choice of measurement parameters due to
non-electrostatic contributions to the input signal of the bias feedback
loop. This often leads to systematic errors of several hundred mV and
can also result in topographical crosstalk. Here, open loop (OL)-KPFM
modes are investigated as a means of obtaining a quantitative, crosstalk
free measurement of the SP of graphene grown on Cu foil, and are
directly contrasted with CL-KPFM. OL-KPFM operation is demonstrated in
both single and multi-frequency excitation regimes, yielding
quantitative SP measurements. The SP difference between single and
multilayer graphene structures using OL-KPFM was found to be 63 ± 11 mV,
consistent with values previously reported by CL-KPFM. Furthermore, the
same relative potential difference between Al2O3-coated graphene and Al2O3-coated
Cu was observed using both CL and OL techniques. We observed an offset
of 55 mV between absolute SP values obtained by OL and CL techniques,
which is attributed to the influence of non-electrostatic contributions
to the input of the bias feedback used in CL-KPFM.