The chemical nature of ion-bombarded carbon: a photoelectron spectroscopic study of 'cleaned’ surfaces of diamond and graphite

Abstract

The effects of ion-bombardment on single-crystal carbon surfaces have been studied by X-ray photoelectron spectroscopy (X. p. s.) partly to establish whether, as previously suggested, the bombardment of diamond leads to surface graphitization. Changes in the x. p. s. of graphite were monitored as a function of exposure to both Ar and 57 Fe ions; and the chemical natures of ion-bombarded diamond and graphite were examined in comparative electron spectroscopic studies of their uptakes of various oxygen species. X. p. s. revealed significant differences between Ar-ion bombarded diamond and graphite both in their C 1s spectra (which also differed significantly from that of cleaved graphite) and in the 2p binding energy of the embedded argon. The C 1s spectra of the 57 Fe implanted graphite specimens (10 15 -10 16 cm -2 , 12.5-85 kV) were identical with those of Ar-ion bombarded graphite, although the Fe was buried beneath the X. p. s. sampling depth. These results show that ion bombardment produces partially disordered but nevertheless structurally distinct surface phases on graphite and diamond crystals. The bombarded surfaces were rather unreactive to O 2 taking up only a small fraction of a monolayer, although exposure to oxygen (or, more effectively, nitric oxide) excited by a microwave discharge resulted in the uptake of several monolayer equivalents of oxygen. Embedded Ar was only very slowly lost during oxidation, confirming the expected low erosion rate under the conditions used. The O 1s X. p. s. from the two substrates showed significant differences although the rates of uptake of oxygen were closely similar. The O 1s peaks were broad, indicating the presence of a minimum of two major oxygen species. Our results, taken in conjunction with others, suggest that >C=O and —>C—O—C<— surface groups were probably present in roughly equal concentrations on the oxygenated carbon surfaces. He ǀ and He ǁ spectra of Ar-ion bombarded diamond obtained before and after oxidation are also intelligible on this basis.