Atomic-force microscopy was used to image the (111)-7x7 reconstruction. Corner holes and adatoms in the 7x7 unit cell, as well as isolated atomic defects, were clearly resolved. In addition, a contrast was detected between inequivalent adatoms; in that the center adatoms appeared to be 0.013nm higher than the corner adatoms. It was shown that atomic-force microscopy did not image the true atomic positions nor the charge density in the dangling bonds. The present data instead suggested that the contrast was due to a variation in the chemical reactivity of the adatoms or to a tip-induced atomic relaxation effect that reflected the surface stiffness of the lattice.

R.Erlandsson, L.Olsson, P.Mårtensson: Physical Review B, 1996, 54[12], R8309-12