An XPS/UPS study of the surface / near-surface bonding in nuclear grade graphites: A comparison of monatomic and cluster depth-profiling techniques

Samples of highly-orientated pyrolytic graphite (HOPG) and nuclear graphite grades, Gilsocarbon and Pile Grade-A (PGA), were examined using x-ray photoelectron spectroscopy (XPS), ultra-violet photoelectron spectroscopy (UPS) and Raman spectroscopy. The photoelectron spectra was used to characterise the surface and sub-surface, particularly with regards to the sp2 and sp3 carbon bonding content. A peak-fitting methodology was applied and the results were in good agreement with those obtained through analysis of the C KLL spectra. Depth-profiling was performed using both monatomic Ar+ ions and cluster Arn+ ions with the former found to cause unwanted damage to the graphite structure with a dramatic increase in sp3 content from ~ 11 % to ~ 88 % in both nuclear grades in the ion bombarded region. Monatomic Ar+ etching was also found to result in ion implantation, leading to a broadening of the C 1s line and an increase in high energy component around the C-O region at ~ 286.0 eV. These effects were not observed when etching with cluster Arn+ ions. Raman spectroscopy also confirmed the difference in induced damage between Ar+ and Arn+ with measured ID/IG ration, within the damaged region only (R0), values of 1.04 and 0.3 respectively.