Quantitative Surface Potential Measurements by AC Electrostatic Force Microscopy

Abstract

Electric charge distributions and the associated surface potentials at the nanoscale play a key role in many areas from material sciences to biology. The increasing field of sample analysis in liquid environments of biologically relevant ionic concentration demands for invasive-free measurements, hardly achievable with current techniques. This paper presents the development of a novel Atomic force Microscopy measurement mode, termed AC-EFM, enabling quantitative surface potential measurements at the nanoscale. It circumvents the use of a dc-bias, which leads to parasitic electrochemical effects in liquids in conventional methods, by the application of an amplitude modulated high frequency voltage. The surface potential is measured through the shift in cantilever resonance frequency, which itself is detected by a phase-locked loop. Measurements with externally applied sample potentials validate the derived model and experiments on a standard KPFM sample show improved spatial resolution, when compared to conventional methods. The capability of AC-EFM to quantitatively measure surface potentials at the nanoscale without the use of a dc-bias is demonstrated.