Local coordination of carbon atoms in amorphous carbon

Amorphous carbon has been studied by ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ NMR with the techniques of static single-pulse excitation, magic-angle spinning, and cross polarization with magic-angle spinning. A spin-lattice relaxation time of 0.7 s was obtained for ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ by progressive saturation. Two different components are seen in the NMR spectrum of ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ in the static sample. These were resolved using a tensor fitting routine with Gaussian broadening functions. The downfield component corresponds to ‘‘${\mathit{sp}}^2$-like’’ carbon atoms, which comprise 93.6% of the total signal. This component was fit to an axially symmetric shift tensor with ${\mathrm{\sigma }}_{\mathrm{?}}$=-28 ppm, ${\mathrm{\sigma }}_{\mathrm{\perp }}$=209 ppm, and σ¯=130 ppm. The remaining upfield component is assigned to ‘‘${\mathit{sp}}^3$-like’’ carbon atoms. These were fit to a symmetric shift tensor with σ¯=62 ppm. The concentration of the dangling bonds, as inferred from spin counting by ESR, is about 2×${10}^{20}$ ${\mathrm{cm}}^{\mathrm{-}3}$. About two thirds of the carbon atoms are not seen in the static NMR measurement because of the high concentration of the unpaired electrons, which leads to severe inhomogeneous line broadening. These carbon atoms are detected under magic-angle spinning by the sidebands, which spread over a range of 2000 ppm. The first moment is located at 130 (±5) ppm. The upper limit of the fraction of hydrogenated carbon is estimated to be 1.5%.