Theoretical Study On Structure, Stability And Electronic Properties Of Al-Si Clusters

Genetic algorithm?GA?combined with the semi-empirical Hamitonian?AM1/PM3?is used to search the low energy isomers of Al_n Si_m?n=3,5,m?3 and n=4,m?4?and the charged clusters having 20 or 28 valence electrons in this work.The candidates were optimized by the density functional theory PBE0 and B3LYP models to decide the most stable structures,and the electronic properties of them were investigated subsequently.We have not only got the most stable isomers in the literature,but also some new structures by the GA+AM1/PM3 model.There are some isomers with close energies in Al_nSi_m clusters,which indicated that the potential energy surface of Al_nSi_m clusters is relatively gradual.The analyse of geometric structures shows that the geometric structures of Al-Si clusters take transition from planar to nonplanar at n+m=6;an octahedral structure is the building block in the structures of the clusters containing 6 or 7 atoms;the abnormal oblate structures occur at n+m=8,in which the energy is lower than that of the previous;meanwhile,the statistics upon bond lengths suggests that the bond lengths of Si-Si arrange between 2.35².50?,shorter than that of Si-Al and Al-Al.The present results indicate that the stability of Al_nSi_m clusters will be enhanced with increasing Si atoms.It means that the doping Silicon into Aluminum clusters enhances the stability considerably.The magic clusters Al₄Si₂,Al₃Si₃⁺and Al₅Si^-having 20 valence electrons as well as the non-magic clusters Al₄Si₄ and Al₅Si₃^-having 28 valence electrons exhibit pronounced stabilities and large energy gaps.The charge populationindicates that aluminum transfer electrons to silicon,and the charge on Si atoms depends strongly on the number of the connected aluminum atoms.Analysis on the density of states,valence molecular orbital and electron density distributions shows that the valence electrons of Al_nSi_m clusters are shared by the entire cluster,forming shell structures like metal clusters,and the valence orbitals accord with the types predicted by the jellium model.The enhanced stabilities of Al₄Si₂,Al₃Si₃⁺and Al₅Si^-clusters result from their magic valence electrons 20,forming closed shell 1S²1P⁶2S²1D¹⁰;and the outstanding stabilities of Al₄Si₄ and Al₅Si₃^-clusters with28 valence electrons are due to their oblate structures leading to the P,D,F levels spilt considerably.The doped Al_nSi_m clusters by non-metallic impurity Si atom behaves as metal cluster.