Nanoscale Mapping of the Surface Potential: Multifrequency Modulation Open-Loop Kelvin Probe Force Microscopy

In this letter, a new open-loop amplitude modulation Kelvin probe force microscopy is presented for nanoscale mapping of the surface potential. In this method, measurement of the contact potential difference (CPD) is performed with a normal probe under two electrostatic excitations. One excitation oscillates the probe in its second resonance mode with an amplitude of less than 5 nm and the other nonresonant excitation modulates this oscillation at a frequency much lower than the probe's first resonant frequency. With this approach, the probe-sample CPD can be measured directly from the periodic variations of the modulated amplitude and the phase without feedback control. Nanoscale mapping of the surface potential of the graphene-on-silicon sample shows that the proposed method has comparable performance to the conventional amplitude modulation Kelvin probe force microscopy. Further experimental results demonstrate that the proposed method can accurately map the surface potentials of various conductor, semiconductor and insulator materials.