First-principle studies of the geometries and electronic properties of Cu_m Si_n (2≤m+n≤7)clusters

The equilibrium geometries and electronic properties of Cu_mSi_n (2 \le m+n \le 7) clusters have been studied by using density functional theory at the B3LYP/6-311+G (d) level. Our results indicate that the structure of CuSi_n (n<6) keeps the frame of the corresponding Si_n cluster unchanged, while for Cu_nSi clusters, the rectangular pyramid structure of Cu_4Si is shown to be a building block in many structures of larger Cu_nSi clusters. The growth patterns of Cu_mSi_n clusters become more complicated as the number of Cu atoms increases. Both the binding energies and the fragmentation energies indicate that the Si--Si bond is stronger than the Cu--Si bond, and the latter is stronger than the Cu--Cu bond. Combining the fragmentation energies in the process Cu_mSi_n \to Cu+Cu_m - 1Si_n and the second- rder difference \Delta_2E(m) against the number of Cu atoms of Cu_mSi_n, we conclude that Cu_mSi_n clusters with even number of Cu atoms have higher stabilities than those with odd m. According to frontier orbital analyses, there exists a mixed ionic and covalent bonding picture between Cu and Si atoms, and the Cu d orbitals contribute little to the Cu--Si bonding. For a certain cluster size (m+n = 3, 4, 5, 6, 7), the energy gaps of the most stable Cu_mSi_n clusters show odd--even oscillation with changing m, the clusters with odd m exhibit stronger chemical reactivity than those with even m.