No time for surface charge: how bulk conductivity hides charge patterns from KPFM in contact-electrified surfaces

Abstract

Kelvin probe force microscopy (KPFM) is a powerful tool for studying contact electrification, using an tiny tip to image voltages caused by transferred charge. It has been used in stationary studies focused on finding patterns (e.g. heterogeneity) in transferred charge, but also in dynamic studies aimed at understanding how charge escapes. Here, we show that the charge dynamics in all but the very best insulators are too fast for patterns found in stationary studies to be meaningful. Using a custom-built system, we are able to quickly (~30 s) transfer samples from our contact-charging apparatus to the atomic force microscopy (AFM). For materials at the lower end of `good insulators', we see potential decay that is shorter than the timescale of a typical KPFM scan (~10 minutes). We develop a minimal model to explain this decay based on bulk conductivity, and show that the measured timescale increases with the prediction from this model. To rule out surface conductivity, we perform additional experiments with the macroscopic method of scanning Kelvin probe microscopy. Our results highlight an important but widely overlooked consideration for dynamic vs. stationary studies in contact-electrified surfaces, and suggest that some patterns observed in the latter, e.g. charge heterogeneity, are not as widespread as previously thought.