Direct identification of atomic binding sites on a crystal

Abstract

The atomic resolution of the field ion microscope, in conjunction with its ability to remove and identify individual atomic layers, allowed us to map unambiguously the unit cell of the (111) plane of tungsten and to determine directly the location of single tungsten atoms adsorbed on this plane. Adatoms have been observed to occupy two binding sites only. The predominant site corresponds to a normal lattice position. The second site is of similar symmetry, in that the adatom sits between three first layer atoms; however, at this position the adatom is located above an atom in the second rather than the third lattice layer. The former site is favored energetically, but only by ≈ 0.5 eV. All observations have been made at high fields, however, it is shown from studies of migration and other effects that the binding sites identified in the field ion microscope are typical of a normal, field free environment.