Yttrium, Gallium-doped Silicon Clusters Density Functional Theory Study

The atomic cluster is one of the elementary cell which forms nanometer structure, and the properties of materials can be designed by exploring the enormous variability in the size, shape, and composition of constituent clusters, so the theoretical research for ground-state structures, stability and electronic properties of its is one of the important subject in the design of micro structure of new material, and has great significant in "custom-made" new material with specific performance. In this paper, the geometries, stabilities and electronic properties of the neutral and charged YSi_n(n=1-16) and GaSi_n(n=1-6) clusters are investigated by using the density functional theory.The geometric structures, stabilities and electronic properties of the neutral charged YSi_n(n=1-16) clusters are successively investigated by using relativistic density functional method with generalized gradient approximation. The most stable structures and the corresponding evolutional rule of the YSi_n clusters are obtained. The results indicate that the most stable structure of YSi_n(n=1-14) clusters keeps the similar framework as the most stable structure of Si_n+1 clusters, and especially in the size range n=11-14, they are prolate in shape and form samdwich structures. When the cluster size goes up to 15, the Y atom abruptly drops into the silicon cage, together with the alteration in the direction of charge-transfer revealed by the Hirsheld charges analysis. The calculated atomic averaged binding energies and fragmentation energies manifest that the YSi_n(n=2, 5, 8, 14)clusters have remarkably enhanced stabilities. Moreover, the gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of YSi_n clusters are universally narrow, compared with those of pure silicon clusters. The calculated HOMO and LUMO energies for Y-encapsulated YSi_n(n=15, 16) are evidently lower than those of the small-sized YSi_n(n=1-14) clusters. Compared with the neutral YSi_n clusters, most of the YSi_n⁺ and YSi_n^- structures basically keep similar frameworks of the neutrals, and the serious deformations in geometries are found for YSi_n(n=4, 5, 7, 10, 13) clusters. At the same time, the atomic averaged binding energies and the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps of most charged YSi_n clusters universally increase, which indicates that the stabilities of charged YSi_n clusters have been improved. The Hirsheld charges of the Y atom in the neutral and charged YSi_n clusters changes from positive to negative at n=15, and the quantity of charge transfer between Y atom and Si atom increases in YSi_n⁺ clusters, but decreases in YSi_n^- clusters. All the results manifest that the effects of the positive charge on YSi_n cluster are larger for electronic levels than geometric structures.GaSi_n(n=1-6) clusters are investigated at the B3LYP/6-311+G(d) level available in Gaussian03 program. A serial stable GaSi_n(n=1-6) clusters are obtained, and the Mulliken and Natural populations, HOMO-LUMO gaps, as well as the binding and fragmentation energies of the most stable structures are calculated. The most stable structure of GaSi_n(n=1-6) clusters basically keeps the analogous frameworks as the Si_n+1 clusters, with Ga atom being adsorbed at a surface site, and the fragmentation energies suggest that GaSi₃ and GaSi₅ have enhanced stability. Charge transfer in GaSi_n(n=1-6) clusters is found proceeding from Ga atom to the Si_n framework in terms of the population analysis; except for GaSi₄ and GaSi₅, the HOMO-LUMO gaps of GaSi_n are universally larger than those of Si_n+1 clusters, which means the chemical stabilities of Si_n clusters are enhanced by the dope of Ga atom.