Evolution of sp² networks with substrate temperature in amorphous carbon films: Experiment and theory

The evolution of sp² hybrids in amorphous carbon (a-C) films deposited at different substrate temperatures was studied experimentally and theoretically. The bonding structure of a-C films prepared by filtered cathodic vacuum arc was assessed by the combination of visible Raman spectroscopy, x-ray absorption, and spectroscopic ellipsometry, while a-C structures were generated by molecular-dynamics deposition simulations with the Brenner interatomic potential to determine theoretical sp² site distributions. The experimental results show a transition from tetrahedral a-C (ta-C) to sp² -rich structures at ~500 K. The sp² hybrids are mainly arranged in chains or pairs whereas graphitic structures are only promoted for sp² fractions above 80%. The theoretical analysis confirms the preferred pairing of isolated sp² sites in ta-C, the coalescence of sp² clusters for medium sp² fractions, and the pronounced formation of rings for sp² fractions >80%. However, the dominance of sixfold rings is not reproduced theoretically, probably related to the functional form of the interatomic potential used.