Paper NS+NC-ThM9
Non-IPR C₆₀ Solids

Thursday, October 23, 2008, 10:40 am, Room 311

Thin monodisperse films consisting of primarily non-IPR C₆₀ isomers (IPR = Isolated Pentagon Rule) have been generated by depositing vibronically excited C₆₀ ions on HOPG and subsequently sublimating the undesired IPR C₆₀(I_h) isomer from the deposited mixture. The deposition procedure is based on UHV-compatible soft-landing of mass-selected carbon cluster ions on substrates (Low Energy Cluster Beam Deposition, LECBD). Beams of non-IPR C₆₀ cations were produced by electron-impact mediated heating and ionization of various buckminsterfullerenes. The associated excitation processes activate the Stone-Wales rearrangement of carbon atoms in the cage (S-W), which creates non-IPR sites on C₆₀ cages. The relative amount of these S-W isomers, C₆₀ (S-W), deposited from the beam of hot C₆₀ ions scales with the kinetic energy of electrons used for excitation/ionization. Essentially pure C₆₀(S-W) films were obtained simply by heating as deposited (mixed isomer) films up to ~600 K. This results in the thermal desorption of most IPR cages, C₆₀(I_h). The topography of the resulting films, as studied by means of AFM, is governed by the aggregation of the C₆₀(S-W) cages and reflects intercage bonds constituted by non-IPR sites. These covalent intercage bonds are responsible for the higher stability of the C₆₀(S-W) films as mirrored by a sublimation offset at ~1100 K (compared to C₆₀(I_h) which sublimes at ~ 550 K). In contrast to the characteristic doublet structure of the HOMO-derived band in C₆₀ (I_h) films, the valence band of the C₆₀(S-W) films exhibits a triplet with a well distinguishable additional peak at a binding energy of ~2.6 eV. This results from electronic modifications induced by intercage bonds. The C₆₀(S-W) films exhibit a narrower HOMO-LUMO gap than found for C₆₀(I_h) films.