Atomically-Resolved Oxidative Erosion and Ablation of Basal Plane HOPG Graphite Using Supersonic Beams of O2 with Scanning Tunneling Microscopy Visualization

The detailed mechanism and kinetics for the oxidative erosion and ablation of highly oriented pyrolytic graphite (HOPG) with molecular oxygen has been examined by monitoring the spatiotemporal evolution of the reacting interface. This has been accomplished using a new, unique gas-surface scattering instrument that combines a supersonic molecular beam with a scanning tunneling microscope (STM) in ultrahigh vacuum. Using this new instrument, we are able to tightly control the energy, angle, and flux of impinging oxygen along with the surface temperature and examine the reacted surface spanning atomic, nano, and mesocopic length-scales. We observe that different oxidation conditions produce morphologically distinct etching features: anisotropic channels, circular pits, and hexagonal pits faceted along crystallographic directions. These outcomes depend upon independent effects of oxygen energy, incident angle, and surface temperature. Reaction probability increased with beam energy and demonstrated non-Arrhenius behavior with respect to surface temperature, peaking at around 1375 K. At the incident collision energies used, it was found that beam impingement angle had only minor effects on the reaction probability and etch pit morphology. Comparison of the relative reactivity of higher grade versus lower grade HOPG indicates that the formation of etched channels largely depends on the presence of grain boundaries. We have also observed the transition to multilayer etching. The influence of structural inhomogeneities such as defects and grain boundaries can now be assessed by real-time visualization of reacting interfaces. For example, the insertion of intentionally created point defects via ion sputtering leads to marked enhancement in interfacial reactivity. The approach used herein has allowed us to correlate time-evolving surface morphology with atomic-level interfacial kinetics and dynamics, providing new insight into the reactivity of materials in aggressive, energetic environments.