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 C-n(+) (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-C-70(D-5h) (-> C-n (n = 68, 66, 64, 62)) or IPR-C-60(I-h) (-> C-n (n = 58, 56, 54, 52, 50)). Non-IPR fullerene films on graphite grow via formation of dendritic C-n aggregates, whereas deposition of IPR fullerenes under analogous conditions (via deposition of unfragmented C-60(+) and C-70(+)) 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 C-n cages can be intactly (re) sublimed by heating. The corresponding mean desorption activation energies, E-des, increase from 2.1 eV for C-68 up to 2.6 eV for C-50. The densities of states in the valence band regions (DOS), surface ionization potentials (sIP) and HOMO-LUMO gaps (D) 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) C-n 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.