Theoretical Investigation of Molecular and Electronic Structures of Buckminsterfullerene-Silicon Quantum Dot Systems

Density functional theory (DFT) and density functional tight binding (DFTB) molecular dynamics (DFTB/MD) simulations of embedding and relaxation of buckminsterfullerene C₆₀ molecules chemisorbed on (001) and (111) surfaces and inside bulk silicon lattice were performed. DFT calculations of chemisorbed fullerenes on both surfaces show that the C₆₀ molecule deformation was very small and the C₆₀ binding energies were roughly ∼4 eV. The charge analysis shows that the C₆₀ molecule charges on (001) and (111) surfaces were between -2 and -3.5 electrons, respectively, that correlates well with the number of C-Si bonds linking the fullerene molecule and the silicon surface. DFT calculations of the C₆₀ molecule inside bulk silicon confirm that the C₆₀ molecule remains stable with the deformation energy values of between 11 and 15 eV for geometries with different C₆₀ configurations. The formation of some C-Si bonds causes local silicon amorphization and corresponding electronic charge uptake on the embedded fullerene cages. Charge analysis confirms that a single C₆₀ molecule can accept up to 20 excessive electrons that can be used in practice, wherein the main charge contribution is located on the fullerene's carbon atoms bonded to silicon atoms. These DFT calculations correlate well with DFTB/MD simulations of the embedding process. In this process, the C₆₀ molecule was placed on the top of the Si(111) surface, and it was further exposed by a stream of silicon dimers, resulting in subsequent overgrowth by silicon.