Kelvin probe force microscopy in ultra high vacuum using amplitude modulation detection of the electrostatic forces

We present a detailed study of contact potential difference measurements in ultra high vacuum using Kelvin probe force microscopy. A dependence of the contact potential difference on the tip-sample distance was measured and is explained by an inhomogeneous work function of the highly doped silicon cantilever. On semiconducting samples the measured contact potential difference additionally depends on the AC sample voltage. By investigating this effect for different AC voltages and tip-sample distances, we can conclude that bias-induced bandbending has to be considered in a quantitative analysis of the contact potential difference on semiconductors. Using amplitude modulation detection of the electrostatic forces at the second resonance frequency, a high sensitivity can be achieved at low AC voltages. Spatially resolved contact potential difference measurements on submonolayer fullerene islands grown on graphite were used to determine the lateral resolution of approximately 20 nm. (C) 2000 Elsevier Science B.V. All rights reserved.