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

The initial growth stage of C60 thin film on graphite substrate has been investigated by scanning tunneling microscopy in ultrahigh vacuum at room temperature. The C60 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 C60 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 C60 layers on graphite (first layer) and on C60 surfaces (second and higher layers) are caused by the restriction of the C60 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.