First-principles Study On The Properties Of Group Ⅲ Metal And Ga_mN_n Clusters

Low-dimension systems such as clusters, quantum dots, quantum wires and ultrathin films are different from bulk materials in properties. To understand the low-dimension effect, the investigation of clusters is a start point. The electronic structure s2p1 make the groupⅢmetal clusters show some unique properties, so they have been intensively studied recently.We calculate the geometries structure and binding energy of Al, Ga and In clusters by Density-functional theory. Their electronic structures and bond properties are analyzed and we give reason to explain spin-polarized of smaller cluster. We calculate the geometries structure and binding energy of GamNn clusters too and analyse their stability. The main findings are as follows:(1) The ground state of Al2 and Al4 clusters are spin-polarized triplet state. Al6 have two stable structures and their energy is almost equal: a singlet octahedral structure and a triplet prism structure. The ground state of Al8 cluster is singlet. Al3, Al5, Al7 are all doublet and spin-polarized, none of then form a highly spin-polarized state. But the doublet state and the quarrtet state of Al3 cluster are small difference in energy.(2) The ground state of Ga2 and Ga4 cluster are spin-polarized triplet state too, their energy of triplet state are a lot lower than the singlet state. The ground state geometry of Ga6 cluster are three-prism structure which have C2v symmetry, Although it is spin polarized, its singlet and triplet energy only have small difference. Ga3, Ga5, Ga7 clusters are spin-polarized doublet state too, Ga7 cluster have the most stable shell structure, its quartet state energy is higher than doublet state by 1.04eV.(3) The ground state of In2, In4, In6 cluster are spin-polarized triplet state too. In2 and In4 clusters have strong spin-polarized ability. The triplet energy of In6 cluster is lower than the singlet by 0.1eV. As the difference to Al3 and Ga3 clusters, In3 cluster form quartet state, but the doublet energy is only 0.024eV lower than the quartet one. Agreement with Ga7 and Al7 cluster, In7 cluster have stable electron shell. The ground state of In8 cluster is none spin-polarized singlet state.(4) Comparison of three clusters,there are laws about spin polarization for same size clusters too. Two-atom cluster's spin polarization is weaker and weaker with bond length increasing. For three-atom cluster, their spin polarization is stronger and stronger with atom number increasing. We find that Tl3 cluster is spin-polarized quartet state too. Gan cluster have the most weakly spin-polarized in Aln, Gan and Inn (n=5-7) cluster. We also find that the trend of Ga clusters spin-polarized ability decreasing with cluster size increasing is larger than In clusters when n=3-8, the ability of spin-polarized of In clusters is larger than Ga cluster.(5) For GamN1 cluster, N atom is generally located in the center of cluster. N atom generally has three or four Ga coordination. The binding energy of Ga3N1 cluster is lowest in GamN1 cluster. For m<5, the ionicity of Ga-N bond is weak, when m>5, it become strong and their binding energy per Ga atoms invariable. For GamN2 cluster, when m<5, the two N atoms usually occur with N2 polymer. Ga-N bond in those clusters is always weakly. When m≥5,the two N atoms are usually separated. N-N bond is disconnected and form more Ga-N bond. In these cluster, the ionicity of Ga-N bond is stronger than cluster with m<5. Our result show that the clusters are less stable when m<5, but relatively stable when m≥5.Our most results are consistent with previous studies and some are good agreement with the experimental dates. So our calculating method is feasible. Although some of our results still need to verify by further experimental and theoretical calculation, we believe that they are helpful on further understanding the ground state structure and electronic properties of larger size of group-Ⅲmetal cluster and GamNn cluster.