Vacancy and interstitial defects at graphite surfaces: Scanning tunneling microscopic study of the structure, electronic property, and yield for ion-induced defect creation

Abstract

Point defects at graphite surfaces are investigated by scanning tunneling microscopy (STM) under ultrahigh vacuum conditions. Graphite surfaces are bombarded with energy-selected beams of ${\mathrm{Ar}}^{+}$ and ${\mathrm{Kr}}^{+}$ ions at low energies (100 eV) to generate the defects on atomic scale. The ion bombardment produces mostly carbon vacancy defects (VD's) and interstitial defects (ID's), the latter formed by trapping an incident ion beneath the surface carbon plane. A VD appears as a protrusion in STM image and so does an ID, but they can be distinguished from each other in the measurements of local tunneling barrier height (\ensuremath{\Phi}) and tunneling spectroscopy $(I\ensuremath{-}V$ curve). They can also be physically separated by heating the defected surface to a temperature high enough to evaporate the noble-gas interstitials. By employing these methodologies, we are able to examine the electronic structure of individual VD's and ID's. A VD exhibits a \ensuremath{\Phi} value substantially lower than an ID or a clean graphite. Both VD and ID increase the local charge density of states near the Fermi energy, but this effect is largest for a VD due to its dangling bonds. A $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ superlattice structure appears from an ID, but not from a VD. This observation disproves the existing theoretical interpretation that the superlattice structure results from electron scattering at a VD site. The absolute yield is measured for production of VD's and ID's at ion impact energies of 40--100 eV. The features of the yield curves, including the dependency on ion energy and the threshold energies for defect creation, provide reasonable explanations for the ion-surface collisional events leading to VD and ID creation in the low-energy regime.