Geometric, energetic, and bonding properties of neutral and charged copper-doped silicon clusters

The geometric, energetic, and bonding properties of CuSiₙ ($n=4,$ 6, 8, 10, and 12) clusters in neutral and charged states are studied systematically using a hybrid density functional method (B3LYP). The Siₙ frameworks in most isomers of CuSiₙ are found to adopt the geometries of the ground-state or low-lying isomers of Siₙ or Siₙ₊₁, with Cu at various substitutional or adsorption sites. Several cagelike structures with Cu at the center site are found for CuSi₁₀ and CuSi₁₂. A hexagonal double-chair structure with Cu at the center, which bears a similarity to the structure of a regular hexagonal prism recently reported for WSi₁₂⁺ [H. Hiura et al., Phys. Rev. Lett. 86, 1733 (2001)], is identified as the best candidate for the ground state of CuSi₁₂. The Cu-Si bond in CuSiₙ is strong for the substitutional and the center-site structures, but weak for the adsorption structures where charge transfer and resulting ionic interaction is found to play a more important role. The Cu atom reveals a similar bonding character to the replaced Si atom in the substitutional structures except for CuSi₁₂, where the Cu atom both in the substitutional and in the center-site structures is found to form multicenter bonds with as many as nine (substitutional) to 12 (center-site) Si atoms. Various energetic properties, including binding and dissociation energies, ionization potentials, electron affinities, and vertical detachment energies are reported for CuSiₙ.