Influence of Imaging Parameters on AFM Surface Potential Measurements in Aqueous Solutions

This paper presents surface potential measurements by closed-loop AC-KPFM in aqueous solutions. In contrast to conventional KPFM, the proposed method omits the use of a dc-bias, therefore preventing electrochemical & electrokinetic effects, enabling operation in water. The ability for reproducible surface potential measurements in aqueous solutions is demonstrated and the influence of key imaging parameters on its performance are investigated, in particular the excitation frequency of the drive signal. It is found that proper operation of AC-KPFM in water is only possible in a regime, where movement of the ions in the solution is suppressed. In the case of highly deionized water, an excitation frequency of >30 kHz is necessary to achieve a comparable performance as conventional KPFM operation in air.