Solid state magnetic resonance investigation of the thermally-induced structural evolution of silicon oxide-doped hydrogenated amorphous carbon

Due to their increased stability in extreme environments, relative to amorphous hydrogenated carbons (a-C:H), amorphous thin film silicon oxide-doped hydrogenated amorphous carbons (a-C:H:Si:O) are being commercially developed as solid lubricants and protective coatings. Although various properties of a-C:H:Si:O have been investigated, no definitive structure of a-C:H:Si:O has ever been proposed, nor has its thermally-induced structural evolution been thoroughly studied. The aim of this work is to better understand the structure of a-C:H:Si:O through solid-state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopies. Deeper insights into the thermally-driven structural evolution are obtained by annealing a-C:H:Si:O between 50°C and 300°C under anaerobic conditions and taking NMR/EPR measurements after each step. EPR results show that the number of paramagnetic defects decreases by 70% with annealing at 300°C. ¹H NMR shows the hydrogen concentration decreases with annealing temperature from 2 × 10²² g^-1, and then levels off at approximately 0.7 × 10²² g^-1 for anneals between 200°C and 300°C. The carbon-silicon-oxygen network exhibits some structural reorganization, seen directly as a slight increase in the sp²/sp³ ratio in the ¹³C NMR with annealing. These results combined with relaxation data are interpreted according to a two-component structure largely defined by differences in hydrogen and defect contents.