EFFECTS OF ION MASS AND ENERGY ON THE DAMAGE INDUCED BY AN ION-BEAM ON GRAPHITE SURFACES - A SCANNING TUNNELING MICROSCOPY STUDY

Scanning tunneling microscopy has been used to study the effects of ionic bombardment of a graphite surface. The low doses used allow for the characterization of single ion impacts for ions of increasing masses, as N-14+, S-32+, Ar-40+ and Xe-131+, and for various energies (from 15 to 40 keV). We show that each impact creates a small hillock on the surface whose size increases as a function of surface damage energy. These hillocks mainly result from the stresses induced in the near-surface region by the damage created during the first collisions of the cascade. Images on the atomic scale reveal local modifications of the structure such as crystallites or five-membered rings. Their presence indicates that nonlinear phenomena may occur when only a few atoms surrounding the impact area are considered. In addition, square-root 3 x square-root 3 R30-degrees superstructures, localized next to some hillocks, are attributed to the effects of point defects in the electronic structure of graphite.