Cn films (n=50, 52, 54, 56, and 58) on graphite:: Cage size dependent electronic properties -: art. no. 054705

Novel semiconducting materials have been prepared under ultrahigh-vacuum conditions by soft-landing mass-selected C-n(+) (50 <= n < 60; even n) on highly oriented pyrolytic graphite surfaces at mean kinetic energies of 6 eV. In all cases, C-n films grow according to the Volmer-Weber mechanism: the surface is initially decorated by two-dimensional fractal islands, which in later deposition stages become three-dimensional dendritic mounds. We infer that C-n aggregation is governed by reactive sites comprising adjacent pentagons (or heptagons) on individual cages. The resulting covalent cage-cage bonds are responsible for the unusually high thermal stability of the films compared to solid C-60. The apparent activation energies for intact C-n sublimation range from 2.2 eV for C-58 to 2.6 eV for C-50 as derived from thermal desorption spectra. All C-n films exhibit a common valence-band ultraviolet photoelectron spectroscopy spectral feature located around the center of a broad highest occupied molecular-orbital (HOMO)-derived band (E-B similar to 2.5 eV). This feature has been assigned to C-n units covalently linked to each other in polymeric structures. To within experimental accuracy, the same work function (4.8 eV) was determined for thick films of all C-n studied. In contrast, HOMO ionization potentials were cage size dependent and significantly lower than that obtained for C-60. C-58 exhibited the lowest HOMO (6.5 eV). Band gaps of C-n films have been determined by depositing small amounts of Cs atoms onto the topmost film layer. HOMO-lowest unoccupied molecular-orbital-derived band gaps between 0.8 eV (C-52) and 1.8 eV (C-50) were observed, compared to 1.5 eV for solid C-60. (c) 2006 American Institute of Physics.