Thermal stability and rehybridization of carbon bonding in tetrahedral amorphous carbon

We perform a quantitative investigation of the energetics of thermally induced sp3→sp2 conversion of carbon-carbon bonds in tetrahedral amorphous carbon (ta-C) films by using near-edge x-ray absorption fine structure (NEXAFS) and Raman spectroscopy. We investigate the evolution of the bonding configuration in ta-C thin films subjected to high temperature annealing in flowing Argon gas using a rapid thermal annealing furnace over the range of 200–1000 °C. We observe no substantial change in bonding structure below 600 °C. Changes in the NEXAFS and Raman spectra start to appear above 600 °C, and by 1000 °C a significant increase in the sp2 bonding in the film is observed. No oxygen bonding is detected in the NEXAFS spectra, but we do observe an isosbestic point, demonstrating that the thermally driven sp3→sp2 conversion reaction occurs without passing through an intermediate transition state. This allows us to use NEXAFS spectra of thermally annealed ta-C films to quantitatively determine that the activation energy for directly converting the sp3-bonded carbon to the sp2-bonded configuration is 3.5±0.9 eV.