Microwave N2 plasma nitridation of H-diamond (111) surface studied by ex situ XPS, HREELS, UPS, TPD, LEED and DFT

Abstract

We report on an experimental investigation and density functional theory (DFT) modeling of the physico-chemical properties of nitrided diamond (111) surface prepared by exposing a hydrogenated diamond (111) surface to high purity nitrogen microwave plasma (MW(N2)). Ex-situ X-ray photoelectron spectroscopic analysis showed that the maximum nitrogen coverage is ∼1/4 monolayer alongside coadsorbed hydrogen, as corroborated by high-resolution electron energy loss spectroscopy (HREELS). The observed broad peak at ∼630 °C in the N2 temperature programmed desorption spectrum suggests that nitrogen desorption substantially occurs below 700 °C. As nitride surface exhibits an increase in electron affinity/work function compared to the hydrogenated surface. Low energy electron diffraction of the nitride surface exhibits a 1×1 pattern, which confirms that the MW(N2) exposure results in low-level damage to the diamond (111) surface; these results complement well with the presence of 1st order optical phonon peak (∼300 meV), the characteristic signature of a highly ordered diamond surface, on the HREEL spectrum. DFT simulations reveal that it is facile for nitrogen to insert into the CH bond on H-diamond (111), forming NH(ad) species adsorbed over C(111), which dimerizes into NH-NH(ad) at increasing coverages. The computed modes of vibration are in qualitative agreement with the HREELS data.