Stability and electronic properties of Si-doped carbon fullerenes

The electronic structure and stability of several Si-doped ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ fullerenes (endohedral, exohedral, and substitutional) are investigated within the framework of both semiempirical modified-neglect-of-diatomic-overlap (to determine the most stable geometrical arrangements) and ab initio density-functional theory (calculation of the electronic structure and total energies). For endohedral species, the equilibrium configurations show a tendency of the Si atoms to move from the center of the molecule toward the cage, producing sizable expansions of the carbon bonds closer to the encapsulated atoms. Exohedrally doped structures exhibit similar geometrical deformations, being localized (also around the impurity) and relatively large (expansions in the C—C bond lengths as large as $12\%),$ while minor changes in the C—C distances of the cage are obtained in fullerenes with substitutional Si. In agreement with relative abundance spectra, the endohedral silicon-doped carbon fullerenes are found to be less stable when compared to the hollow structures $({\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}),$ with configurations in which the Si atoms are attached to the outside of the cage, and also with arrangements in which carbon atoms of the fullerene are replaced by silicon atoms. The small energy difference between the highest occupied molecular orbital and lowest unoccupied molecular orbital obtained with the inclusion of Si atoms into the fullerene cage as well as the weak Si—C bonds formed in these kind of arrangements seems to be at the origin of this behavior. On the other hand, substitutionally doped molecules are the ground-state configurations since Si atoms can be viewed as carbonlike atoms in the fullerene network, also due to its four valence electrons, binding thus more strongly to the carbon units of the structure. For some particular cases we analyze the influence of oxygen chemisorption on the electronic spectrum and stability of our Si-doped carbon molecules, and finally we give a qualitative estimation of the energy-barrier height for ${\mathrm{Si}}^{+}$ penetration into a ${\mathrm{C}}_{60}$ fullerene.