The contact potential difference (CPD) between carbon contamination (CC) layers and the several substrates on which they were deposited has been measured as a function of the film thickness by means of Kelvin probe force microscopy (KPFM). The observed CPD trends may be divided into three categories: (i) an increase, or decrease, in CPD with thickness up to a saturation value with sign inversion with respect to the substrates (Al and Si); (ii) an oscillation with no sign inversion (substrates, gold and platinum); (iii) an oscillation through sign inversion (palladium substrate). Effects (ii) and (iii) seem to be typical of CC, since they have not been observed for other materials, including evaporated carbon. Several possible causes of the above two effects are examined, but a satisfactory interpretation has not been found yet. The sensitivity of KPFM is such that CC layers 10 nm thick are easily visible, whereas they are hardly detectable by topography.
