Energetics for the new forms of carbon-clusters

The growth of closedC _n -structures like fullerenes, shelled fullerenes, tubules and capped tubules controlled by the interplay of surface- and bending-energy is studied. Tubules are less stable than corresponding fullerenessC _n , and these are forn>n _c less stable than shelled fullerenes. Growth of fullerenes from graphite sheets requires bond breaking and bond rearrangement to form pentagons and finite temperatures to overcome energy barriers. Thermodynamical arguments are used to discuss the temperature- and size-dependence of the formation of the new forms of carbon. We argue that trapping of foreign atoms or molecules inside the closed structure may be achieved most efficiently by mixing these with carbon clusters before caging occurs.