Theoretical Study On Structural Evolution And Photoluminescence Of Ligand-Protected Silver Clusters

Ligand-protected metal nanoclusters have found a wide range of applications in catalysis,materials,and biomedicine due to their unique physicochemical properties.Researchers have conducted more systematic experimental and theoretical studies on the structural evolution,electronic properties,and thermodynamic stability of ligand-protected gold nanoclusters,but silver nanoclusters have been studied mainly in terms of structural resolution and properties,with less research on their structural evolution evolution and the conformational relationships between their structures and properties.In this paper,the structural evolution law of face-centered cubic stacked silver clusters protected by thiol and phosphine ligands and the photoluminescence mechanism of thiolate-protected silver clusters have been intensively investigated using the density functional theory（DFT）and time-dependent density functional theory（TDDFT）.The paper is divided into the following two parts:Part Ⅰ:By resolving the structure of six experimentally reported face-centered cubic（FCC）silver nanoclusters,these silver clusters could be classified into two categories according to their silver core and surface Ag-S ligands.A generic Ag-S network model is proposed to describe the Ag-S skeleton structure of these two types of clusters.Based on the Ag-S network model,a series of structural models of silver clusters（n×m×l）with different sizes,length/diameter ratios,and thickness/width ratios are constructed in one-dimensional,two-dimensional,and three-dimensional directions by the method of crystal plane growth.The electronic structure and thermodynamic stability of these clusters were theoretically investigated,and general expressions for the number of Ag and S atoms（marked by n and m）in two types of FCC conformations Ag_n（SR）_m（PR′）₈clusters were proposed.Where n=1 2（abc+1）,m=1 2（ab+ac+bc-3）in the first type of FCC silver cluster and n=1 2 abc+27-2a-2b-2c,m=1 2（ab+ac+bc-2a-2b-2c+3）in the second type of FCC silver cluster.Finally,by calculating the electron affinity energy and ionization energy of the reported 11 clusters,the relationship between the charges carried by the clusters and their electron electron affinity energy and ionization energy is summarized,and the charges for building the FCC cluster structure are predicted.Part 2:The photoluminescence mechanism of classical Ag₂₅（SR）₁₈^-cluster and the effect of doping with metallic elements（M=Au,Pt）on the optical properties（absorption and emission spectra）of this cluster are investigated.We study the key parameters which affected the luminescence properties of the cluster,and the relationship between the structure and luminescence properties of the MAg₂₅（SR）₁₈^-is summarized.Then the type of emitted light of the cluster is derived,and the type of excited state electron transition is determined.Theoretical calculations show that the doping of Au and Pt atoms in the center of Ag₂₅（SR）₁₈^-cluster can increase the HOMO-LUMO energy gap,shift the maximum absorption peak to the short wavelength,and significantly increase the contribution of the D orbitals of the central metal atoms to the lowest empty orbitals（LUMO）.During the transition from first single excited state to the ground state（S₁→S₀）,the doped Au and Pt atoms could increase the emited energy of the MAg₂₄（SR）₁₈^z-cluster due to the large HOMO-LUMO gaps in the ground state.The increasing of fluorescence radiation rate is ascribed to the large transition dipole moment and oscillator intensity in the S₁→S₀process.During the transition from first triplet excited state to ground state（T₁→S₀）,the large phosphorescence radiation rates of the Au Ag₂₄（SR）₁₈^-and Pt Ag₂₄（SR）₁₈^2-clusters are mainly from the increasing of electronic coupling matrix element.The non-radiative rate of the clusters is much larger than the radiative rate,and the inter-system acrossing rate is larger than the reverse inter-system acrossing rate,which leads to a phosphorescence emission type of the MAg₂₄（SR）₁₈^z-clusters.The transition of excited states is characterized as localized excitation with a small amount of charge transfer.