C 60 thin film growth on graphite: Coexistence of spherical and fractal-dendritic islands

Abstract

The initial growth stage of ${\mathrm{C}}_{60}$ thin film on graphite substrate has been investigated by scanning tunneling microscopy in ultrahigh vacuum at room temperature. The ${\mathrm{C}}_{60}$ layer grows in a quasi-layer-by-layer mode and forms round, monolayer high islands on the graphite surface. The islands are confined by terraces on the graphite surface and the mobility of ${\mathrm{C}}_{60}$ fullerenes across steps is low in all layers. The second and all subsequent layers adopt a fractal-dendritic shape, which was confirmed by calculating the fractal dimension ($D=1.74$ prior to island coalescence) and is in agreement with a diffusion limited aggregation. The profound differences between the growth of ${\mathrm{C}}_{60}$ layers on graphite (first layer) and on ${\mathrm{C}}_{60}$ surfaces (second and higher layers) are caused by the restriction of the ${\mathrm{C}}_{60}$ mobility on the highly corrugated fullerene surfaces. The orientation of the fractal islands follows the hexagonal symmetry of the densely packed (111) surface of the fullerene lattice, which introduces a bias in the direction of molecule movement. The differences in surface topography on the nanoscale determine the mode of film growth in this van der Waals bonded system.