Asymmetry in the Kelvin probe force microscopy images of the tautomerization switching of naphthalocyanine. It is the first time that the charge distribution within a single molecule can be resolved. When a scanning probe tip is placed above a conductive sample, an electric field is generate$ due to the different electrical potentials of the tip and the sample. With KPFM this potential difference can be measured by applying a voltage such that the electric field is compensated. Therefore, KPFM does not measure the electric charge in the molecule directly, but rather the electric field generated by this charge. The field is stronger above areas of the molecule that are charged, leading to a greater KPFM signal. Furthermore, oppositely charged areas yield a different contrast because the direction of the electric field is reversed. This leads to the red and blue areas in the micrograph.
