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

Abstract

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 ∼500K. 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. © 2005 The American Physical Society.