Site discrimination of adatoms in Si(111)-7x7 by noncontact atomic force microscopy

Site discrimination of adatoms in Si(111)-7 x 7 by dynamic mode noncontact atomic force microscopy (NC-AFM) in ultrahigh vacuum has been demonstrated. At a fixed frequency shift, NC-AFM images were acquired at various amplitudes. At amplitudes less than 30 nm(p-p), no atomic image was observed. At 33 nm(p-p), clearly resolved atomic images showed 6 bright atoms in one half of each 7 x 7 unit cell and 6 less bright atoms in the other half of each unit cell. Furthermore, the corner adatoms were observed to be higher. At 36 nm(p-p), such differences were not observed. The observed site differences are not derived by the differences in the actual positions of adatoms, but by the differences in the interactions between the atom at the AFM tip apex and the adatoms on the Si surface. (C) 1997 Elsevier Science BV.