We report on the contrast formation in the local contact potential difference (LCPD) measured by Kelvin probe force microscopy (KPFM) on single charge-transfer complexes (CTCs)on a NaCl bilayer on Cu(111). At different tip heights, we found quantitatively different LCPD contrasts that characterize different properties of the molecule. In the small distance regime, the tip penetrates the electron density of the molecule, and the contrast is related to the size and topography of the electron shell of the molecule. For larger distances, the LCPD contrast corresponds to the electrostatic field above the molecule. However, in the medium-distance regime, i.e., for tip heights similar to the size of the molecule, the non-spherical distribution of pi-and sigma-electrons often conceals the effect of the partial charges within the molecule. Only for large distances does the LCPD map converge towards the simple field of a dipole for a polar molecule.
By: Bruno Schuler, Shi-Xia Liu, Yan Geng, Silvio Decurtins, Gerhard Meyer, Leo Gross
Published in: Nano Letters, volume 14, (no ), pages 3342–3346 in 2014
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