Mapping contact potential differences with noncontact atomic force microscope using resonance frequency shift versus sample bias voltage curves

We have measured cantilever resonance frequency versus sample bias voltage and generated frequency vs bias (f-V) curves using an ultrahigh-vacuum noncontact atomic force microscope (UHV NC-AFM). Using the f-V data, we calculated the contact potential difference (CPD) between the tip and the sample. These CPD measurements were compared with those that were directly observed with a scanning Kelvin probe force microscope (SKPM) on the same atomically resolved area of the sample using a UHV-AFM. The CPD values obtained by both methods were similar, however, it was difficult to obtain CPD values that agreed precisely on the atomic scale.