Bonding, Thermal and Ambient Stability of Nitrogen-Terminated Diamond (100) Surfaces by Plasma Exposure Studied by Ex-Situ XPS, HREELS, and DFT Modeling

We report on our experimental studies of the physicochemical properties, density functional theory (DFTDFT) modelingDFT modeling and ambient stability of nitrided diamond (100) surface prepared by exposing a hydrogenated diamond (100) surface to high-purity nitrogen plasmas. The nitrogen-plasmaPlasma exposures include microwave (MW) and radio frequency (RF) (at pressures: 3×10–2 (damaging) and 7 ×10–2 Torr (non-damaging)) nitrogen plasmas. The nitrogen bonding configuration, thermal stability and concentration on the nitride surfaces and structural damage are investigated by X-ray photoelectron spectroscopy (XPSXPS) and high resolution electron energy loss spectroscopy (HREELSHREELS). The highest level of structural damage to the upper atomic layers of the diamond is induced by exposure to the damaging RF(N2)RF(N), followed by non-damaging RF(N2) and MW(N2). For ambient exposed MW(N2) processed surface, nitrogen is adsorbed mainly in C‒N/C=NC=N state. Whereas, for ambient exposed non-damaging- and damaging-RF(N2) treated surfaces, nitrogen is bonded in mixed C‒N/C=N and C≡NC≡N states. Adventitious oxygen is adsorbed onto the nitride surfaces in a predominant COx(ads) bonding configuration alongside a small C‒NOx (ads) component depending on the degree of surface defects. The damaging RF(N2) exposed surface exhibits a lower oxygen and nitrogen thermal stability than the other cases. The largest amount of oxygen ambient adsorption occurs on the damaging RF(N2) (O = 2.8 at.%) exposed surface, whereas for MW(N2) (O = 0.8 at.%) and non-damaging RF(N2) (O = 1.3 at.%) exposed surfaces, a lower oxygen concentration is observed. It is concluded that the near-surface damage induced by the plasma interaction promotes adventitious oxygen adsorption (in various bonding configurations, including COx(ads) and C‒NOxC‒NO). From the HREELSHREELS analysis, it was determined that the hydrogen adsorbed on the diamond (100) surface is not removed by exposure to the different nitrogen plasmasPlasma. These measurements show that NH(ads) species are formed on the surface and are desorbed upon vacuum annealing in the 500–700 °C range. The DFTDFT computed vibrational modes of various nitrogen bonding configurations are in qualitative agreement with the HREELS data. This study may be of importance in all ex-situ applications influenced by the near-surface physicochemical and electronic properties of nitrogen-terminated H-Diamond (100) surfaces.