Geometric artefact suppressed surface potential measurements

Due to the finite scanning probe microscopy (SPM) tip radius and the resulting geometric convolution between the tip and the sample surface, nano-resolution surface potential (SP) or electric force measurement (EFM) cannot be free from topographic artefacts. For conventional Kelvin probe microscopy (KPM), only the first harmonic component of the tip oscillation signal (either oscillation amplitude for amplitude-modulated AM-KPM or frequency shift for frequency-modulated FM-KPM) induced by the applied ac voltage is typically used. However, the first harmonic signal depends not only on tip-sample potential difference, but also on the capacitance gradient (AM-KPM) or the second-order gradient (FM-KPM), the main cause of topographical artefacts. Since the second-order harmonic component is proportional only to the capacitance gradient or second-order gradient, we are able to extract true potential difference signals, free of geometric artefacts, by dividing the first- and second-order harmonics. Surface potential mapping on an equipotential surface verifies that this alternative method significantly reduces the magnitude of topological artefacts. In addition, adoption of the proposed imaging method reduces the dependence of the measured potential on the tip-sample separation by more than an order of magnitude.