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

Polymer-like and diamond-like hydrogenated amorphous carbon films, characte rized by high spin concentrations of 10(20) cm(-3), have been studied by mu ltiple frequency electron paramagnetic resonance (EPR) spectroscopy at 9, 3 5, 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 paramagneti c centers with anisotropic g tensors of g(parallel to)(1) = 2.005, g(perpen dicular to)(1) approximate to 2.003, and g(parallel to)(2) = 2.010, g(perpe ndicular to)(2) approximate to 2.003. We attribute both to localized pi sta tes on nanosized sp(2) graphitic clusters. The g(parallel to) values of the g tensors, which are correlated to the size of the graphite like carbon cl usters, indicate two different cluster sizes with values below 50 Angstrom. (C) 2001 American Institute of Physics.