Structural properties of amorphous hydrogenated carbon. III. NMR investigations

Our NMR studies give experimental evidence of bonding heterogeneity in samples of a-C:H on the nanometer scale. Two classes of protons were identified with distinctly different spin-lattice-relaxation behavior. The difference in the spin-lattice relaxation provides a means of spectral editing of cross-polarization magic-angle spinning, combined rotation and multiple-pulse spectroscopy, dipolar dephasing spectra, and multiple quantum NMR experiments. This combination of the various NMR techniques allows for a detailed structural investigation of a-C:H, e.g., the sp²:sp³ ratio, the relative amount of hydrogenated and nonhydrogenated carbons, etc. A model incorporating the heterogeneity is established and discussed. The NMR results are compared with neutron spectroscopy and diffraction data.