Point defects at surfaces were investigated by using scanning tunnelling microscopy under ultra-high vacuum conditions. The surfaces were bombarded with energy-selected beams of Ar+ and Kr+ ions, with energies of less than 100eV, in order to generate defects at the atomic scale. The bombardment produced mainly C vacancy defects and interstitial defects. The latter were formed by trapping an incident ion beneath the surface C plane. A vacancy defect appeared as a protrusion in scanning tunnelling microscopic images, as did an interstitial defect, but they could be distinguished by measuring the local tunnelling barrier height and tunnelling spectroscopy behavior. They could also be physically separated by heating the defective surface to a temperature which was high enough to evaporate noble-gas interstitials. By these means, it was possible to examine the electronic structure of individual vacancy defects and interstitial defects. The former exhibited a tunnelling barrier height which was much lower than that for an interstitial defect or clean material. Both vacancy defects and interstitial defects increased the local charge density of states near to the Fermi energy. This effect was largest for a vacancy defect, due to its dangling bonds. A v3 x v3 superlattice structure arose from an interstitial defect, but not from a vacancy defect. This disproved the suggestion that the superlattice structure resulted from electron scattering at a vacancy defect site.

Vacancy and Interstitial Defects at Graphite Surfaces: Scanning Tunnelling Microscopic Study of the Structure, Electronic Properties, and Yield for Ion-Induced Defect Creation. J.R.Hahn, H.Kang: Physical Review B, 1999, 60[8], 6007-17