NMR EVIDENCE FOR STRAINED CARBON BONDING IN TETRAHEDRAL AMORPHOUS-CARBON

C-13 NMR is used to study the carbon environment in a dense form of amorphous carbon containing a large fraction of sp(3) bonds prepared from a cathodic are 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 Si-29 NMR. A peak is also observed at 140 ppm which is definitively assigned to sp(2) 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.