Simulating the thermal stability and phase changes of small carbon clusters and fullerenes

The thermal stability, phases and phase changes of small carbon clusters an d fullerenes are investigated by constant energy Molecular Dynamics simulat ions performed over a wide range of temperatures, i.e., from T = 0 K to abo ve the melting point of graphitic carbon. The covalent bonds between the ca rbon atoms in the clusters are represented by the many-body Tersoff potenti al. The zero temperature structural characteristics of the clusters, i.e., the minimum energy structures as well as the isomer hierarchy can be ration alized in terms of the interplay between the strain energy (due to the surf ace curvature) and the number of dangling bonds in the cluster. Minimizatio n of the strain energy opposes the formation of cage structures whereas min imization of the number of dangling bonds favors it. To obtain a reliable p icture of the processes experienced by carbon clusters as a function of tem perature, both thermal and dynamical characteristics of the clusters are ca refully analyzed. We find that higher excitation temperatures are required for producing structural transformations in the minimum energy structures t han in higher lying isomers;We have also been able to unambiguously identif y some structural changes of the clusters occurring at temperatures well be low the melting-like transition. On the other hand, the melting-like transi tion is interrupted before completion, i.e., the thermal decomposition of t he clusters (evaporation or ejection of C-2 or C-3 units) occurs, from high ly excited configurations, before the clusters have fully developed a liqui d-like phase. Comparison with experiments on the thermal decomposition of C -60 and a discussion of the possible implications of our results on the gro wth mechanisms leading to the formation of different carbon structures are included.