Forces From Periodic Charging Of Adsorbed Molecules

In a recent publication [N. Kocic et al., Nano Lett. 15, 4406 (2015)], it was shown that gating of molecular levels in the field of an oscillating tip of an atomic force microscope can enable a periodic charging of individual molecules synchronized to the tip's oscillatory motion. Here we discuss further implications of such measurements, namely, how the force difference associated with the singleelectron charging manifests itself in atomic force microscopy images and how it can be detected as a function of tip-sample distance. Moreover, we discuss how the critical voltage for the chargestate transition depends on distance and how that relates to the local contact potential difference. These measurements allow also for an estimate of the absolute tip-sample distance.