Structural And Nonlinear Optical Properties Of Au_n-m X_m (X=Ag,Cu) Clusters

Because of their physical and chemical behavior,gold clusters have been widely applied in electronic equipment,functional nano material,optical material and catalyst fields.Large numbers of experimental and theoretical investigations have been processed,including structural characteristics,electronic properties,catalytic properties,adsorption effects and nonlinear optical properties researches.Gold clusters can be served as material of optical devices for their great nonlinear optical properties.In this thesis,first-principle calculations are used to obtain the nonlinear optical parameters of gold-based clusters.The effect on structural characteristics,electronic properties and nonlinear optical properties of doping Ag or Cu atoms into pure gold clusters are discussed.The influence of doping position, impurity consistence,size effect and dissimilarity of doping Ag or Cu atoms are discussed.This thesis is arranged as following two parts:1.Structural characteristics and nonlinear optical properties of planar gold-based clustersPure Au_n(n=10,16) and their alloyed clusters Au_n-mX_m(X=Ag,Cu;m=1,2) are theoretically investigated for the structural characteristics,electronic properties, and nonlinear optical properties using density-functional theory(DFT) at B3LYP/LanL2DZ level.Doping exhibits little effect on geometric structural characteristics and energy gaps but enormous augmenting of the second-order nonlinear optical(NLO) coefficients.The primary causation of the enhancement ofβis the lack of centro-symmetry feature in these alloyed clusters.Time-dependent density-functional theory(TD-DFT) analysis at the same basis set show doping increases the ground to excited state transition electric dipole moments and transition dipoles,which make important roles in enhancing the second-order NLO coefficients. Increasing number of X atoms increases the NLO coefficients of clusters because the X atoms act as electron donors in alloyed clusters.Increasing the length of clusters can also augment theβvalues in our investigation.The reported effect can be utilized to design effective gold-based alloyed clusters for NLO applications. 2.Structural characteristics and nonlinear optical properties of tetrahedral Au₂₀ cluster and its alloyed clustersThe PBE method is used to calculate the ground structures,energy gaps and binding energies of tetrahedral Au₂₀ cluster and its alloyed clusters at LanL2DZ level. Three different Au atoms of Au₂₀ cluster are replaced by Ag or Cu atoms respectively, which produce six stable alloyed clusters:Ag_vertex,Ag_edged,mg_center,Cu_vertex,Cu_edged and Cu_center clusters.The first-order hyperpolarizabilities of pure and alloyed clusters are obtained and analyzed.Doping exhibits little effect on energy gaps and average binding energies but great changing of the first-order hyperpolarizabilities of Au₂₀ cluster.Vertex-doped clusters have largerβvalues than Au₂₀ cluster,well other alloyed clusters have smallerβvalues than Au₂₀ cluster.The reason is the charge transferring from the edged atoms to the vertex one makes dominant contributions to the nonlinear optical effects.By doping electropositive Ag atom on vertex of Au₂₀ cluster,the process of charge transfer can be enhanced,which enlarge theβvalue of vertex-doped clusters. In non-vertex-doped clusters the charge transfer from the edged atoms to the vertex ones are blocked.So the edged-doped and face-center-doped clusters have smallβvalues.The Cu-doped clusters exhibit similar to Ag-doped clusters.Cu atom is less electropositive in comparison with Ag atom,so the changing ofβvalues of Cu-doped clusters is less than Ag-doped clusters.