No time for surface charge: how bulk conductivity can hide charge patterns from KPFM

Kelvin Probe Force Microscopy (KPFM) is a powerful tool for studying contact electrification (CE), using electrical signals from a nanoscale AFM tip to spatially map a voltage above a surface that is caused by the presence of charge. Among the most influential results obtained with KPFM are observations of surface charge heterogeneity, i.e. mosaic-like patterns of +/- polarity after CE. Such experiments are often done with PDMS, due to its ability to create conformal contacts. In trying to reproduce such results, we instead observe signatures of spatially uniform surface potential that displays prominent temporal decay over a few minutes. We propose that this is due to the material’s bulk conductivity, which lowers the KPFM potential via the movement of non-CE charge carriers in the electric field gradient between the surface and back electrode. With a simple capacitor model, where the only adjustable parameter is the bulk conductivity, we obtain a value that is consistent with electrical resistivity measurements of PDMS. As further support, we observe the same temporal decay on PMMA and SiO₂ surfaces, but with time constants that scale in agreement with their significantly lower conductivities. Going further, we consider more sophisticated models beyond the simple capacitor to account for non-linearities in the conductive response. Our results call into question the presence/stability of surface charge patterns on certain materials, and highlight the role of bulk conductivity during and after CE.