Extraordinary growth of C₆₀ on a GaAs(001) As-rich 2 × 4 surface

We have systematically investigated, by using scanning tunneling microscopy, the adsorption and film growth of C₆₀ on the various GaAs(001) surface phases prepared by molecular-beam epitaxy. For most phases, the C₆₀ overlayer exhibits the usual close-packed fcc(111) configuration with its lattice constant close to that of the bulk C₆₀ crystal. However, in the case of C₆₀ on the As-rich 2 × 4 substrate, the epitaxial growth is found to be quite different and unique; C₆₀ film takes its (110) crystalline axis; the C₆₀ overlayer is highly strained with a lattice expansion of ∼13%, and this structure is very stable at least up to 10 ML. We will address the underlying formation mechanism of this new structure in terms of a charge transfer from the As-dangling bonds to C₆₀S and a site-specific C₆₀-substrate interaction, as confirmed by molecular dynamic simulations. The present system provides a unique opportunity to study fullerene and/or noble-gas related two-dimensional phenomena, and demonstrates a potential for fabrication of novel fullerene-based devices, such as strained superlattice structures.