Observation of the graphite surface by reflection electron microscopy during STM operation

The basal plane of highly oriented pyrolytic graphite (HOPG) has been studied by reflection electron microscopy (REM) before, during, and after tunnelling with a scanning tunnelling microscope (STM). The STM was designed to replace the standard specimen holder of a commercially available transmission electron microscope (TEM). Three types of contrast are found to be causally linked to tunnelling: (1) A dark feature which follows the tip during scanning; we attribute this to reversible elastic shear strain due to repulsion between the surface and tip. This extends over tens of nanometers. (2) A permanent bright region which forms under the tunnelling tip with repeated scanning; we attribute this to the increase in RHEED reflectivity which results from cleaning of the surface by abrasion and/or oxidation, possibly catalysed by the W tip. WO₃ particles from the tip may play a role in the oxidation reaction at the surface, as observed in independent HREM studies. (3) A permanent bright band of contrast extending beyond the tunnelling tip; further study of this defect is required.