On The Existence And Stability Of Carbon -based Silicon Fullerenes -a Density Functional Theoretic Study
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The electronic and geometric properties of silicon-carbon fullerene-like nanostructures with two, four, six, twenty and twenty four carbon atoms on the surface of the Si60 cages by substitution, as well as inside the cage at various symmetry orientations have been studied within the generalized gradient approximation to density functional theory (GGA-DFT). The Perdew-Wang 91 (PW91) functional has been used to treat exchange and correlation energies. Full geometry and spin optimizations have been performed using Gaussian 03 suite of programs and the LANL2DZ basis set. Thus for the silicon atom, the Hay-Wadt pseudopotential with the associated basis set are used for the core and valence electrons respectively. For the carbon atom, the Dunning/Huzinaga double zeta basis set is employed. Results on electronic state, binding energy per atom, HOMO-LUMO gap (Band gap), vertical ionization potential (energy required to ionize the system), vertical electron affinity (energy required to add an electron to the system) and dipole moment for all the stable silicon -carbon fullerene like nanostructures are presented and discussed in detail. The average Si-C bond length and average C-C bond length are also presented. The natures of the bondings between the Si-C and C-C are studied in detail using the Natural Bond Orbital program (NBO). It was found that the optimized silicon-carbon fullerene like nanostructures have increased stability compared to the bare Si60 cage. The binding energy per atom for the stable silicon-carbon fullerene like nanostructures increase with the number of carbon atoms, with the structures having carbon atoms on the surface having higher binding energies (3% for Si58C2 to 38% for Si36C24) than the structures with carbon atoms inside the Si60 cage (2.5 % for Si60C2 to 27% for Si60C24). The stability of these nanostructures depends on the orientation of carbon atoms, as well as on the nature of bonding between silicon and carbon atoms (for two and four carbon atoms) and nature of onding between Si-C and C-C bonding (for six, twenty and twenty four carbon atoms).