Tip-Sample Interactions in Kelvin Probe Force Microscopy: Quantitative Measurement of the Local Surface Potential

We study the influence of different experimental parameters on the interaction between the probe and the sample in Kelvin probe force microscopy (KPFM) measurements. We provide a precise and reproducible determination of the local surface potential (SP) of clean macroscopic highly oriented pyrolytic graphite (HOPG) samples and of organic semiconducting nanostructures of an alkyl-substituted perylene-bis(dicarboximide) (PDI) self-assembled at surfaces. We distinguish two different terms in the measured SP, intrinsic and extrinsic, containing the electrical properties of the studied object and the experimental artifacts, respectively. We investigate the effect of the most relevant experimental parameters including tip-sample distance, relative humidity (RH), and potential applied to the tip, which govern the extrinsic term of the measured SP. Moreover, we devise a theoretical description of the tip-sample interaction taking into account the extra modulation of the probe due to the applied ac potential during the KPFM scan. A deep understanding of all the terms which contribute to the measured SP in air environment made it possible to devise a new protocol to quantify the electrical properties of nano-objects leading to an improvement of the achieved lateral resolution, as demonstrated by the good agreement between the proposed model and the experimental results.