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Abstract:
For the first time to our knowledge, the 13C solid-state magic angle spinning (MAS) NMR spectrum of a 99% 13C enriched tetrahedral amorphous-carbon (ta-C) thin film containing a high concentration of fourfold coordinated carbon species (82%) is reported along with measured NMR spectra for the ta-C film after low temperature annealing (650 degreesC). Differential changes are observed for the 13C MAS NMR chemical shifts and linewidths of both the fourfold (diamondlike) and threefold (graphitelike) coordinated carbon species within the thin films with increasing annealing time; however, there was no change (±2%) in the relative fourfold content. These spectral changes are associated with the large compressive stress reduction (6-8 GPa) in the carbon film. Ab initio calculations of the 13C NMR chemical shift, along with shift variations as a function of atomic volume are reported for amorphous carbon and crystalline diamond. Using the observed spectral variations in the solid-state 13C MAS NMR, along with the ab initio chemical shift calculations, the effect of annealing on the ta-C films is discussed and related to current models of thermal stress relaxation in ta-C thin films.
