Kelvin Probe Force Microscopy with Atomic Resolution

Abstract

The surface potential distribution measured using Kelvin probe force microscopy (KPFM) is influenced by the contact potential difference (CPD) between the tip and surface, the stray capacitance of the cantilever, and fixed monopole charges on the surface and tip. The interpretation of atomic-scale KPFM contrast studies has been controversial. Here, we investigate the contrast mechanism in KPFM with atomic resolution. First, the effect of stray capacitance on potential measurements is explored in the FM-, AM-, and heterodyne AM-KPFM modes. The distance dependence of the modulated electrostatic force in AM-KPFM is much weaker than that in FM- and heterodyne AM-KPFM, and the stray capacitance of the cantilever, which strongly affects potential measurements in AM-KPFM, is almost completely eliminated in FM- and heterodyne AM-KPFM. The very small local contact potential difference (LCPD) corrugation in AM-KPFM is attributed to an artefact induced by the topographic feedback. Next, an investigation of the LCPD on a TiO₂ (110)-1 × 1 surface and atom-dependent bias-distance spectroscopic mapping are performed. The LCPD of TiO₂ (110) is dominated not only by the permanent surface dipole between the tip apex atom and the surface, but also by the dipoles induced by the chemical interaction between the tip and sample. Finally, we propose a new multiple-image method for obtaining the frequency shift, tunneling current, and LCPD images. For the first time, we obtain three images on a TiO₂(110) surface with atomic resolution at 78 K. The LCPD has a higher value on a defect site than on the nearby O rows because excess electrons caused by surface defects are delocalized on the nearby Ti rows. The multiple-image method can be used to investigate the charge transfer phenomena between nanoparticles and surface sites and to elucidate the mechanisms of catalytic reactions.