Disordered and ordered C28 solids

Using tight-binding molecular dynamics, we have performed computer experiments to mimic the gas phase growth of a disordered solid composed of C28 fullerenes. The growth has been simulated by repeated low energy collisions of molecules coming from random directions. The resulting solid is composed of undamaged C28 cages, with most fullerenes being three- and four-fold coordinated, similar to C atoms in amorphous materials. The system contains a high percentage of distorted sp2 C sites and only a small proportion of sp3 sites. These results help clarify the structure of disordered films obtained experimentally by small fullerene deposition on surfaces. Furthermore, we have compared the properties of the disordered C28 solid (a-C28) with those of ordered C28 solids. We have found that the energy of a-C28 is close to that of hyperdiamond (0.1 eV/atom higher) and differs by a few meV from that of other ordered structures, such as 2D-hypergraphite, hexagonal and clathrate solids. This indicates that in condensed phases C28 molecules can act as carbon superatoms, while showing more bonding flexibility than C atoms; in particular the capability of acting as six-fold coordinated building blocks of hexagonal solids, which are as stable as a-C28.