Electrostatic force spectroscopy on insulating surfaces: the effect of capacitive interaction

Scanning surface probes delivered from atomic force microscopy (AFM) are expected to investigate local electrostatic properties on insulating surfaces by forces. Electrostatic force spectroscopy is especially suitable to clarify the capacitive interaction. In order to perform it at a well-defined tip-surface separation, we developed a dynamic mode, in which the tip-surface separation is regulated by maintaining the cantilever oscillation amplitude with an active feedback, while the electrostatic force gradient is simultaneously detected with a variable resonant frequency shift. Using the method, it turns out that the quadratic dependence of the electrostatic force gradient on an applied bias observed on an insulating Al2O3(0001) is comparable to those on a metallic Au(111). It results from the potential difference between the tip and the insulator surface being only one order smaller than that between the tip and the metal surface despite the spacing between electrodes for the insulator being 106 times larger than for the metal, because the capacitive interaction is modified primarily between the sharp AFM tip and the surface.