Local force gradients on Si(111) during simultaneous scanning tunneling/atomic force microscopy

The authors report simultaneous scanning tunneling and force imaging of Si(111) 7×7 with sub-angstrom oscillation amplitudes. Both constant height and constant current scans with tungsten tips/levers always showed larger attractive stiffness over corner holes than over adatoms, the opposite of theoretical expectations. Constant height scans show that this cannot be explained by interaction of tip motion with long range forces. Silicon levers, however, sometimes exhibited inversions of force contrast following local tip changes. The authors suggest that there may be charge variations between atomic sites on the surface, which produce electrostatic tip forces additional to the covalent forces usually regarded as dominant.