Spins and microstructure of hydrogenated amorphous carbon: A multiple frequency electron paramagnetic resonance study

Polymer-like and diamond-like hydrogenated amorphous carbon films, characterized by high spin concentrations of 1020 cm−3, have been studied by multiple frequency electron paramagnetic resonance (EPR) spectroscopy at 9, 35, and 94 GHz. Whereas the low-frequency measurements show only one single symmetric EPR line, the high-frequency measurements reveal the anisotropic nature of the g tensor. They show the presence of two different paramagnetic centers with anisotropic g tensors of g∥(1)=2.005, g⊥(1)≈2.003, and g∥(2)=2.010, g⊥(2)≈2.003. We attribute both to localized π states on nanosized sp2 graphitic clusters. The g∥ values of the g tensors, which are correlated to the size of the graphite like carbon clusters, indicate two different cluster sizes with values below 50 Å.