Spatially anisotropic etching of graphite by hyperthermal atomic oxygen

Area of Science:

• Surface science
• Materials science
• Chemical kinetics

Background:

• Highly ordered pyrolytic graphite (HOPG) is a model system for studying surface reactions.
• Hyperthermal atomic oxygen is relevant to space environments and plasma processing.

Purpose of the Study:

• To investigate the surface morphologies resulting from the reaction of HOPG with hyperthermal atomic oxygen.
• To understand the kinetic factors governing the formation of unique surface structures.

Main Methods:

• Exposure of HOPG to a beam of atomic oxygen (O((3)P)) and molecular oxygen at hyperthermal energies (~8 km/s).
• Characterization of surface morphology using atomic force microscopy (AFM) and scanning tunneling microscopy (STM).
• Analysis of reaction kinetics at moderate temperatures (298-423 K).

Main Results:

• Formation of numerous multilayer circular pits with diameters from nanometers to micrometers.
• Observed convex curvature of pit bottoms, with the highest point at the center.
• Correlation of pit formation with spatially anisotropic kinetics and oxygen atom fluence.

Conclusions:

• The unique pit morphology arises from the interplay of pit nucleation kinetics, anisotropic HOPG reactivity (lateral and downward), and atomic oxygen fluence.
• The high reactivity of translationally hot oxygen atoms significantly influences the overall surface evolution.
• Spatially anisotropic kinetics are crucial in determining the resulting surface structures.