Morphology of C_n thin films (50 ≤ n < 60) on graphite: Inference of energy dissipation during hyperthermal deposition

摘要

Low energy cluster beam deposition, LECBD, under UHV conditions has been used to generate thin films comprising monodispersed non-IPR fullerenes, C_n, 50 ≤ n < 60, on pyrolithic graphite surfaces (HOPG). The morphology of the resulting C_n deposits has subsequently been studied by ex situ atomic force microscopy. Deposition experiments were carried out under nominally normal incidence and at hyperthermal incident kinetic energies, E_o, varied between 1 and 40 eV. Surface temperatures during deposition, T_s, were varied from 300 K to the desorption onset of ～700 K. Initial sticking of C_n cages is governed by the lateral density of step edges, which act as pinning and nucleation centres for migrating cages. Consequently in the early deposition stages, the surface exhibits large areas of empty terraces, while the step edges themselves are well-decorated. The terraces in turn become decorated by dendritic C_n islands in later deposition stages. Both, the mean size of these 2D islands and the mean distance between nearest islands, δ, scale with the size of the terraces. When increasing the primary kinetic energy, the fractal-like islands become smaller and less dendritic in shape. The mean initial sticking coefficient decays exponentially with increasing E_o. The island topography has also been found to depend sensitively on the deposition temperature. Instead of the dendritic/fractal islands generated at room temperature, densely packed islands terminated by smooth rims are observed upon deposition at elevated temperatures. We rationalize our findings in terms of a three step deposition process involving: (ⅰ) conversion of perpendicular E_o into hyperthermal surface parallel gliding/sliding motion, (ⅱ) friction-dissipation of this surface-parallel kinetic energy within an (unexpectedly large) mean free path A followed by (ⅲ) thermal diffusion. A, is observed to scale with E_o and T_s.