Second-Order Nonlinear Optical Properties Of Au₂₀ Clusters By Coinage And Alkali Metal Polyatomic Doping

Nonlinear optical materials are the newly materials which developed in recent years, and have application in both technology and research fields. Finding and designing the nonlinear optical materials with predominant properties had becoming the main issue in nonlinear optical area. Gold clusters which have potential nonlinear properties had been wide applied in optical devices and electronic equipments et al. The electronic structures, adsorption, and optical properties of gold clusters have been investigated theoretically and experimentally. In this paper, coinage and alkali metal atoms are designed to dope in pure Au20clusters to further improve its nonlinear optical properties, then the first-principle calculations which based on density functional theory are used to obtain the doped structures, electronic structures and second-order nonlinear optical properties, the properties of pure Au20clusters are also listed for comparison. Based on the difference of dopping positions, we designed two dopping modes, vertex dopping mode and tetrahedral doping mode. From dopping positions and dopping atomic kinds, this paper discussed the influences of the dopping atoms on the clusters’ nonlinear optical properties. In general, the main work and conclusions of this thesis are arranged as following parts:(1) The influences of one kind of doping atoms and two doping modes on nonlinear optical properties. Coinage and alkali metal atoms dopped in different positions of the tetrahedral Au20cluster in two dopping modes, and the basic geometry structures of the dopped clusters are calculated on PBEPBE/LanL2DZ level, then the average binding energy, energy gap and Mulliken charge of the clusters are calculated. In order to test the influence of density functional method on the optical properties of clusters, five different methods are selected, and finally PBEPBE/LanL2DZ and CAM-B3LYP/LanL2DZ are used to calculate the first-order hyperpolarizabilities of all doped clusters.The results show that dopping could influence the geometry structures of the clusters. Clusters with vertex doping mode have smaller deformation than tetrahedral doping clusters, especially for double-and triple-atomic doping clusters. From the changes in energy gap and average binding energy, we believe that copper doped clusters are more stable than other clusters. These two doping modes, vertex and tetrahedral doping modes all enhanced the nonlinear optical response of the clusters, compare with pure Au20clusters. Especially for the alkali doped clusters, which have large enhancement over the pure gold cluster. Mulliken charge analysis indicated that, the charge transited from vertex and surface positions to edged positions are contributed to the nonlinear optical response. Considering the stability and experimental probability, we recommend vertex doping mode. Based on PBEPBE/LanL2DZ level, the first order hyper-polarizability of the tetrahedral doping clusters can reach a maximum β30=149257atomic unit, a.u.for short.While that of vertex doping clusters on CAM-B3LYP level also got a maximum/β0=56289a.u.. In short, different alkali atomic number and types doped on tetrahedral Au20clusters can largely enhanced the nonlinear optical response.(2) The influence of Multi-element doping mode on nonlinear optical properties. Based on upper work, and for further optimized the nonlinear optical properties of doped tetrahedral gold clusters, two different alkali atomic associations are chosen to replace the four vertex positions of Au20clusters, then geometry optimization are calculated on PBEPBE/LanL2DZ level and got the ground structures of the doped clusters.the average binding energy, energy gap and first order hyper-polarizability of the clusters are calculated and analysis.Therefore, alkali multi-atomic doping have little influence on the geometry structures, but the first order hyperpolarizability are elevated. The charge in the clusters transited from vertex and surface positions to the edged positions. We also found that, the multi-dopped atoms with large electronegativity could efficient improve the static dipole moment, and this could make contribution to enhanced the nonlinear response.