Properties of non-IPR fullerene films versus size of the building blocks

This perspective focuses on the cage size dependent properties of novel solid fullerene nanofilms grown by soft-landing of mass-selected Cn+ (48, 50, 52, 54, 56, 58, 62, 64, 66 and 68) onto room temperature graphite surfaces under ultra-high vacuum conditions. Such non-isolated-pentagon-ring (non-IPR) fullerene materials are not accessible to standard fullerene preparation methods. The component molecular building blocks of non-IPR films were generated by electron impact induced ionization/fragmentation of sublimed IPR-C70(D5h) (→Cn (n = 68, 66, 64, 62)) or IPR-C60(Ih) (→Cn (n = 58, 56, 54, 52, 50)). Non-IPR fullerene films on graphite grow via formation of dendritic Cn aggregates, whereas deposition of IPR fullerenes under analogous conditions (via deposition of unfragmented C60+ and C70+) leads to compact islands. The latter are governed by weak van der Waals cage–cage interactions. In contrast, the former are stabilized by covalent intercage bonds as mediated by the non-IPR sites (primarily adjacent pentagon pairs, AP). A significant fraction of the deposited non-IPR Cn cages can be intactly (re)sublimed by heating. The corresponding mean desorption activation energies, Edes, increase from 2.1 eV for C68 up to 2.6 eV for C50. The densities of states in the valence band regions (DOS), surface ionization potentials (sIP) and HOMO–LUMO gaps (Δ) of semiconducting non-IPR films were measured and found to vary strongly with cage size. Overall, the n-dependencies of these properties can be interpreted in terms of covalently interconnected oligomeric structures comprising the most stable (neutral) Cn isomers—as determined from density functional theory (DFT) calculations. Non-IPR fullerene films are the first known examples of elemental cluster materials in which the cluster building blocks are covalently but reversibly interconnected.