This paper presents a novel feedback based Scanning Probe Microscopy method which enables quantitative surface potential measurements without the need of the DC bias of Kelvin Probe Force Microscopy. In addition to the sinusoidal excitation signal at frequency ω, a sinusoidal signal with the frequency 2ω is applied to the conductive cantilever. By modulating the amplitude of the signal at 2ω, the resulting electric force component at the frequency ω can be nullified by a feedback controller. When the force and, hence, the cantilever oscillation is zero, the required amplitude represents the quantitative surface potential. Recording this amplitude while scanning over the sample allows to acquire a two dimensional map of the surface potential. The AC-KPFM method, shown analytically and with experimental results, keeps the compensation based principle of classical KPFM, resulting in quantitative measurements but without the need of a DC bias.
Kohl, D., Mesquida, P., & Schitter, G. (2017). Quantitative AC - Kelvin Probe Force Microscopy. Microelectronic Engineering, 176, 28–32. https://doi.org/10.1016/j.mee.2017.01.005