Low temperature annealing in tetrahedral amorphous carbon thin films observed by ¹³C NMR spectroscopy

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 °C). 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.