Electrostatic force spectroscopy on insulating surfaces: the effect of capacitiveinteraction

Abstract

Scanning surface probes delivered from atomic force microscopy (AFM) are expected toinvestigate local electrostatic properties on insulating surfaces by forces. Electrostatic forcespectroscopy is especially suitable to clarify the capacitive interaction. In order to performit at a well-defined tip–surface separation, we developed a dynamic mode, in which thetip–surface separation is regulated by maintaining the cantilever oscillation amplitude withan active feedback, while the electrostatic force gradient is simultaneously detected with avariable resonant frequency shift. Using the method, it turns out that the quadraticdependence of the electrostatic force gradient on an applied bias observed on an insulatingAl2O3(0001) is comparable to those on a metallic Au(111). It results from the potential differencebetween the tip and the insulator surface being only one order smaller than that betweenthe tip and the metal surface despite the spacing between electrodes for the insulator being106 times larger than for the metal, because the capacitive interaction is modified primarilybetween the sharp AFM tip and the surface.