We report 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, that is, for tip heights similar to the size of the molecule, the nonspherical distribution of π- and σ-electrons often conceals the effect of the partial charges within the molecule. Only for large distances does the LCPD map converge toward the simple field of a dipole for a polar molecule.

This work was carried out in the group of Prof. Silvio Decurtins under the guidance of Dr. Shi-Xia Liu in collaboration with IBM Research - Zurich.

References:

• B. Schuler, S.-X. Liu, Y. Geng, S. Decurtins, G. Meyer, L. Gross;
  "Contrast formation in Kelvin probe force microscopy of single π-conjugated molecules"
  Nano Lett., 14, 3342-3346, (2014); doi:10.1021/nl500805x.