Plasma modification of vanadium oxynitride surfaces: Characterization by in situ XPS experiments and DFT calculations.

Plasma modification of transition metal nitride/oxynitride (MOxNy) surfaces for enhanced surface properties is highly desirable, given the scalability of such methods and limitations of thermal treatments. In situ x-ray excited photoelectron spectroscopy demonstrates that the O2 plasma oxidation of VOxNy films generates non-lattice N1s surface features with binding energies near 396.5 eV, which are associated with the nitrogen reduction reaction activity but not observed upon thermal oxidation. The NH3 plasma generates N1s surface features near 400.5 eV binding energy. The O2+NH3 plasma generates both types of N1s features. Annealing in UHV to <1000 K reverses plasma-induced changes to N1s spectra. Density functional theory (DFT) calculations integrated with the experiments indicate that the plasma-induced N1s features at ∼396.5 eV and 400.5 eV are V≡N: and V-NH2 sites, respectively, with significantly lower thermal stabilities than lattice N sites. These results provide practical insight regarding the plasma modification of MOxNy surfaces for important applications.