Inequivalent atoms and imaging mechanisms in ac-mode atomic-force microscopy of Si(111)7x7

ac-mode atomic-force microscopy (AFM) has been used to image the Si(111)7X7 reconstruction. The comer holes and adatoms in the 7X7 unit cell as well as isolated atomic defects are clearly resolved. In addition, we observe a contrast between inequivalent adatoms, the center adatoms appearing 0.13 Angstrom higher than the corner adatoms. We show that AFM does not image the true atom positions nor the charge density in the dangling bonds. Rather, our data suggest that the contrast is due to a variation in the chemical reactivity of the adatoms or to a tip-induced atomic-relaxation effect reflecting the stiffness of the surface lattice.