NMR evidence for strained carbon bonding in tetrahedral amorphous carbon Original Research Article

13C NMR is used to study the carbon environment in a dense form of amorphous carbon containing a large fraction of sp3 bonds prepared from a cathodic arc source. The results show a chemical shift of 66 ppm intermediate between graphite and diamond shifts and not previously reported for a pure carbon form. This shift is assigned to the tetrahedral component of the network on the basis of a comparison of various types of NMR techniques and by analogy with the effect of amorphisation on crystalline silicon observed using 29Si NMR. A peak is also observed at 140 ppm which is definitively assigned to sp2 carbon. Cross-polarisation results reveal the presence of approximately 1% of hydrogen which is associated with the tetrahedral network, perhaps in a fashion analogous to the passivation of the dangling bonds in amorphous silicon.