Heterodyne AC Kelvin Probe Force Microscopy for Nanoscale Surface Potential Imaging in Liquids

Knowledge of electric charge and potential distributions at the nanoscale is of great interest in the fields of material science and biology. The required high measurement accuracy, spatial resolution, and applicability to aqueous environments are not always provided by conventional techniques such as Kelvin probe force microscopy (KPFM) due to averaging artifacts and the use of a dc bias. This article presents the development of an atomic force microscopy measurement mode, enabling quantitative surface potential measurements of nanoscale structures with high measurement accuracy in air and liquid (aqueous) environments. Averaging artifacts caused by the influence of the cantilever cone, cantilever beam and tip-sample distance in dc-bias-free KPFM (AC-KPFM) are eliminated by the implemented heterodyne detection and single-pass operation. The accuracy of the potential measurement as compared to amplitude-modulated KPFM (AM-KPFM) modes is greatly improved while keeping the advantages of closed-loop and dc-bias-free operation. Experiments on a gold-aluminum test sample and collagen fibrils show quantitative surface potential measurements on nanoscale structures and operability in an aqueous environment.