The combination of the Si-C 60 interactions and interactions between C 60 s results predominantly in a hexagonal packing in the first layer. Molecules that impact above a closely packed region in the first monolayer can settle into the hollows between molecules. The fullerenes are stable in these positions as they have at least four near neighbours and are still in the range of the surface- C 60 interactions. In the time simulated here, there is no migration of molecules from these sites to the Si surface. Hence the second layer begins to form prior to the completion of the first monolayer.
Further layers grow in a similar way and produce a hpc crystal structure.
Hexagonal stacking predominates in the first layer, but some cubic stacking can occur. The fullerenes are fairly mobile on the surface and congregate into regularly packed groups. Impact of molecules onto a one monolayer covered region initiates growth of the second layer before the first layer is completed. Due to the nature of the inter-molecular potential, a hcp, rather than fcc crystal is produced. Written by Keith Beardmore , February 1997.