Direct mapping of the lateral force gradient on Si(111)-7x7

Lateral force gradient of down to 0.01 N/m on Si(111)-7x7 was directly detected by dynamic lateral-force microscopy with an amplitude of 81 pm. Positive and negative torsional resonance frequency shifts of a silicon cantilever caused by the attractive interaction inward and outward tip ditherings were detected on adatom and nonadatom sites, respectively. The lateral force of down to subpiconewton was measurable with direct lateral-force spectroscopy. The converted lateral force predicts a possibility of the stick-slip motion in the noncontact region. The theoretical calculations were in good qualitative agreement with the experiments.