Interactions and chemical transformations of coronene inside and outside carbon nanotubes.

By exposing flat and curved carbon surfaces to coronene, a variety of van der Waals hybrid heterostructures are prepared, including coronene encapsulated in carbon nanotubes, and coronene and dicoronylene adsorbed on nanotubes or graphite via π-π interactions. The structure of the final product is determined by the temperature of the experiment and the curvature of the carbon surface. While at temperatures below and close to the sublimation point of coronene, nanotubes with suitable diameters are filled with single coronene molecules, at higher temperatures additional dimerization and oligomerization of coronene occurs on the surface of carbon nanotubes. The fact that dicoronylene and possible higher oligomers are formed at lower temperatures than expected for vapor-phase polymerization indicates the active role of the carbon surface used primarily as template. Removal of adsorbed species from the nanotube surface is of utmost importance for reliable characterization of encapsulated molecules: it is demonstrated that the green fluorescence attributed previously to encapsulated coronene is instead caused by dicoronylene adsorbed on the surface which can be solubilized and removed using surfactants. After removing most of the adsorbed layer, a combination of Raman spectroscopy and transmission electron microscopy was employed to follow the transformation dynamics of coronene molecules inside nanotubes.

Jahn-Teller distortion of ionized and excited carbon nanotubes.

Due to their rotational (C(n)) symmetry, neutral zigzag and armchair type nanotubes possess doubly degenerate orbitals. As the energies of highest occupied molecular orbital and lowest unoccupied molecular orbital are usually different, neutral nanotubes exhibit a nondegenerate ground state. Ionized or excited forms of these tubes, however, may undergo Jahn-Teller distortion if degenerate orbitals show up in the vicinity of the Fermi level. Theoretical calculations in a simple one-eletron-per-site model show that the Jahn-Teller distortion energies are in order-of-magnitude of 0.1 eV for short pieces of tubes, and are associated with a significant geometry change. Distortions tend to diminish quickly with increasing tube length, except if multiple ionizations are considered.