Fractal structures in fullerene layers: Simulation of the growth process

The molecular-level processes responsible for fractal-dendritic growth of second-layer C-60 islands on large and compact first-layer C-60 islands deposited on graphite substrate are investigated by a combination of scanning tunneling microscopy (STM) and kinetic Monte Carlo (kMC) simulations. Molecular dynamics (MD) simulations are performed to determine the activation barriers and jump rates for diffusion of C-60 molecules on a C-60 layer. The rates of the thermally activated processes, determined in MD simulations, are used in kMC simulations performed to explore the connections between the elementary growth mechanisms and the shapes of the growing islands. The geometry of the underlying substrate is found to result in the formation of two characteristic molecular arrangements along the edges of the growing islands (A-step and B-step on a close-packed surface). The difference in the molecular mobility along the A-step and B-step configurations, along with the difference in the probability of a C-60 molecule to diffuse from a corner of an island to A-step or B-step edges of the island, are identified as key factors responsible for the formation of characteristic triangular shapes of the fractal islands and their orientation with respect to the underlying surface structure. The fractal shapes of the second-layer C-60 islands predicted in kMC simulations are in a good agreement with STM images taken under comparable deposition conditions.