Quantitative NEXAFS and solid-state NMR studies of sp3/(sp2 + sp3) ratio in the hydrogenated DLC films

Abstract

The classification of diamond-like carbon (DLC) films is imperative in guiding large-scale industrial productions. However, it is not easy to classify this issue based on the structural composition and obtained properties, especially, when the film contains a certain amount of hydrogen. In the present study, two types of hydrogenated DLC films were prepared by electron-cyclotron-resonance chemical-vapor-deposition method with pure C2H2 and Ar gasses with applied negative bias voltages of 0 and 0.5kV. The structural and surface analyses of these films have shown significant changes of physical and mechanical properties with high reproducibility and low oxygen contents. The comparison of H/C atomic ratio extracted from the combination of Rutherford backscattering and elastic recoil detection analysis and the solid-state nuclear magnetic resonance (NMR) illustrated that the NMR analysis of the present work provides a significant contribution. Therefore, we compared quantitatively the sp3/(sp2+sp3) ratios in the hydrogenated DLC films using the near-edge X-ray absorption fine-structure (NEXAFS) and solid-state NMR methods. The signals of hydrocarbon sp3 (CH, CH2 and CH3) and a network of sp3 hybridized carbon (C-sp3) can be separated by the solid-state 13C NMR measurement, from which it is concluded that the former do not enable a detailed interpretation for the internal structural changes of the hydrogenated DLC films with the increase of applied bias voltage.