Evolution of sp2 networks with substrate temperature in amorphous carbon films:: Experiment and theory -: art. no. 014120

The evolution of sp(2) 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 sp(2) site distributions. The experimental results show a transition from tetrahedral a-C (ta-C) to sp(2)-rich structures at similar to 500 K. The sp(2) hybrids are mainly arranged in chains or pairs whereas graphitic structures are only promoted for sp(2) fractions above 80%. The theoretical analysis confirms the preferred pairing of isolated sp(2) sites in ta-C, the coalescence of sp(2) clusters for medium sp(2) fractions, and the pronounced formation of rings for sp(2) fractions > 80%. However, the dominance of sixfold rings is not reproduced theoretically, probably related to the functional form of the interatomic potential used.