Determining hybridization differences for amorphous carbon from the XPS C 1s envelope

Two kinds of amorphous carbon (a-C) thin film samples were synthesized, one by magnetron sputtering, the other by filtered cathodic arc deposition. X-ray photoelectron spectroscopy (XPS) showed that the C 1s envelope for each sample was primarily a convolution of three peaks which are assignable to sp2 hybridized carbon, sp3 hybridized carbon (shifted from the sp2 peak by ∼ 1 eV) and an sp2 satellite peak, possibly due to π-π∗ transition. The satellite typically comprised just under 14% of the total sp2 signal and was shifted from the main sp2 peak by ∼ 2 eV. The relative amounts of each bonding type were determined by comparing the integrated areas of the sp2 and sp3 peaks. Auger electron spectroscopy (AES) was also performed on these samples, along with highly ordered pyrolytic graphite (100% sp2) and natural diamond (100% sp3) for comparison purposes. The binding energy width D between the highest maximum and lowest minimum values of the first derivative C (KLL) spectra were obtained and a linear relationship between D and the amount of sp3 bonding assumed. When comparing sp3 values obtained by AES and deconvoluted XPS spectra an error range within ±1.5% was found, making XPS a satisfactory method for obtaining hybridization information. It was noted that Ar+ ion bombardment has a substantial effect on the surface sp2/sp3 ratio, as observed by XPS. These results are related to other studies of ion bombardment of graphite and diamond.