Formation and evolution of sp² clusters in amorphous carbon networks as predicted by molecular dynamics annealing simulations

A comparative study of clustering of sp² -bonded atoms in the as-deposited and annealed diamond-like amorphous carbon (DLC) films is presented. The as-deposited DLC with a grown-in compressive stress of ~10 GPa is modelled using amorphous networks generated by ion-beam film deposition simulations with a modified hydrocarbon potential of Brenner. The DLC networks were annealed in the temperature range of 600-2000 K, using molecular-dynamics with the same interatomic potential. The size and type of the sp² clusters were analysed as a function of the annealing temperature and simulation parameters. An essential finding of this study is that at the density less than 3.0 g/cm³ the structure of DLC can become unstable with respect to formation of large sp² clusters. In particular, molecular-dynamics simulations show how the small sp² clusters coalesce, forming the large ones. The influence of the sp² clustering on the mechanical properties of DLC is also discussed.