Theoretical investigation on the adsorption of lithium atom on the Si _n cluster (n=2-7)

Ab initio calculations are carried out to study the adsorption of Lithium atom on the Si_n cluster with n ranging from 2 to 7. At the MP2/6-31G(d) level, the structures of the neutral Si_n clusters and the Si_nLi clusters (n=2-7) are optimized. The single-point energy at QCISD/6-311+G(d,p) level for the optimized isomers are further performed. Harmonic vibrational frequency analysis at the MP2/6-31G(d) level is also undertaken to confirm that the optimize geometries are stable. Based on our results, the most favorable sites for Li adsorption on the Si_2-7 clusters are the bridge sites. In addition, the vertical ionization energies of the Si_nLi clusters and the electron affinities of the Si_n clusters are also calculated. The clear parallelism between the vertical ionization energies of Si_nLi and the electron affinities of Si _n is found. This is consistent with the fact that the framework of the Si_n in the Si_nLi cluster is similar to the structure of the corresponding negative ion Si_n^-.