Fullerene and graphene formation from carbon nanotube fragments

We study the long-time annealing behavior of 60-atom fragments of carbon nanotubes using two accelerated molecular dynamics methods. We find that this behavior depends strongly on the geometry of the nanotube fragment. Fragments from (n, n) nanotubes with n ⩽ 5 quickly form closed structures. Whether the terminations of (n, 0) fragments (n ⩽ 9) close depends on the structure of the termination. Those forming a zigzag structure remain open for the duration of our simulations. Fragments from (n, n) nanotubes with n ⩾ 6 exhibit surprising behavior, with (7, 7) fragments unfolding to form thermodynamically unfavorable graphene fragments. These results suggest that small-radius (n, n) fragments will be unreactive with other molecules, large-radius (n, n) fragments will react with other molecules, and the reactivity of (n, 0) fragments will depend on the details of their structure. We include a discussion of the accelerated molecular dynamics methods and some of the implementation details to give the reader a sense of how they can be effectively applied to this kind of system.