One final point is that passivation was observed for N2O-graphite at 1800 K (Figs. ), whereas for O2 etching, full passivation was not observed above 1600 K. As discussed previously, 2 for NPs to remain passivated under oxidizing conditions, the creation rate for defective sites capable of supporting N2O (or O2) adsorption must be slower than the rate at which surface defects are "healed". Surface defect creation for a fully passivated NP largely results from sublimation removing carbon from the surface layer, but once a defect is present, etching would tend to enlarge the defect, increasing reactivity. Defect healing can occur by isomerization within the fullerene-like surface layer, but as more and more carbon is lost by sublimation, carbon transport from sub-surface layers is required to fill in surface layer defects. Thus, at steady state, passivation depends on the balance between the rates for sublimation, etching, surface layer isomerization, and inter-layer carbon transport. For O2, passivation was not seen above ~1600 K, presumably because at higher TNP the defect creation rate by sublimation, combined with the relatively high reactivity of O2, creates reactive sites faster than they can heal. For N2O, passivation was observed at 1800 K, despite the much higher sublimation rate compared to 1600 K. On the other hand, the surface layer isomerization and inter-layer carbon transport rates presumably are also substantially faster at 1800 K -evidently enough so to maintain passivation.