First-principles Studies Of The Electric Dipole Properties Of Metal Clusters

Increasing interests are focused on the nanomaterial, being their fundamental scientific values and potential applications. With the repaid development of numerical methods such as the density functional theory (DFT), the physicochemical properties of these materials can be quantitatively described, and several unexpected novel phenomena are predicted. Nanoclusters are the basic building units to assemble large functional material, and thus, understand of their fundamental characters are crucial to guide the following researches. The valuable investigations show that the properties of clusters are largely determined by their ground state structures, up to now, however, the structures can not be directly measured experimentally. Alternately, previous works indicated that the electric dipole moments (EDM) and polarizabilities (EDP) are sensitive on the geometrical structures of cluster, which would reflect the structure indirectly. Therefore, we studied the EDM and EDP of Bi and PbSn clusters, with the aim to interpret the recently experimental measurement and to confirm the ground state structures of these clusters by comparison with the experimental and theoretical values.1. The electric dipole properties of Bin (n=2～24,40,80) clusters.The electric dipole moments and polarizabilities of small Bin clusters with n=2～24,40, and 80 are investigated by the first-principle within density functional theory (DFT) and the finite field method. It pays more attention to the mutual influence between cluster structures and electric dipole properties. The results show that the electric dipole moments exist in most of the Bin clusters and present strong odd-even oscillation. Additionally, the electric dipole moment is closely related to the symmetry of structures, high-symmetry cluster configuration often leads to the "quenching" for electric dipole moment. The oscillation increase in polarizabilities of Bin clusters is caused by the chain structure evolution, this phenomenon has been informed significantly different on the polarization characteristics of the metal and semiconductor clusters. On this basis of above discussions, we design a simple model that can well explain the continuous increase in polarizability and anisotropy. These results will help to determine the ground state structures of Bin(n=2～24,40,80) from many isomers in future experiments.2. The structural stability and electric dipole properties of PbnSn (n=1～19) alloy clustersThe stability and electric dipole properties of PbnSn (n=1～19) clusters are investigated by using the finite field method within the DFT. The calculated results show that the stabilities of PbnSn clusters are enhanced by doping a single Sn atom into Pbn clusters, and the experimental fragmentation behavior has been well explained in our theoretical investigation. Most importantly, the transformation from compact structure to loose samdwich occurs at the cluster size with total atom number of N=14. The HOMO-LUMO energy gaps increase firstly, with the following decreasing behavior, which indicates the transformation of characters from metallicity to semiconductor. It is showed that the high-symmetry of structure prohibit the generation of electric dipole moment. In addition, the EDPs are found to be closely related to the electronic structures, which can be illustrated that large HOMO-LUMO gaps are usually relate to a small electric dipole moment in small sizes (N<14). However, the conclusion is not strict application for large clusters, where the dipole moment and HOMO-LUMO gap present the same growth pattern. Except the cluster size N=7, our obtained polarizabilities diverged from the experimental values, which may be due to the mixed state or excite state presented in the experiment.