Effects of nitrogen substitution in amorphous carbon films on electronic structure and surface reactivity studied with x-ray and ultra-violet photoelectrori spectroscopies

摘要

We investigated the effects of incorporating a low percentage of nitrogen on the local and the electronic structures of amophous carbon (a-C) using X-ray photoelectron spectroscopy and ultra-violet photoełectron spectroscopy (UPS). Nitrogen-doped amoφhous carbon films (a-CN_x) with varying nitrogen contents were prepared by a thermal decomposition method using a mixture of CH_4 + NH_3 under atmosphere. A slight shift of the C Is core-level spectrum toward the higher binding energy side was detected in a-CN_x as a function of nitrogen content, whereas a shift of the Fermi level (E_F) cannot be confirmed from the UPS results. This was interpreted as a chemical shift between carbon and nitrogen atoms rather than as a shift of the E_F. The C 1 s peak shifts can be explained by the presence of two kinds of C-N local structures and the charge transferred bulk C-C components by nitrogen atoms. The two kinds of deconvoluted C 1s components could be well correlated with the two N 1s components. Two localized states were detected below the E_F in UPS spectra of a-CN_x-, which could be assigned to defect bands. These defects played a significant role in the surface reactivity, and were stabilized in a-CN-_x. The adsorption and reaction of NO were carried out on a-CN_x as well as a-C films. It was found that both defect sites and O~(2-) species were responsible on a-C, while O~(2-) species were selectively active for NO adsorption on a-CN_x. We concluded that nitrogen doping reduces defect density to stabilize the surface of a-C, while at the same time inducing the selective adsoφtion capability of NO.