First-principles Studies On The Structures And Electronic Properties Of Neutral And Singly Charged Silver Clusters

Clusters are the agglomerates of a few to a few thousand atoms with relative stability and with a radius smaller than 50 nm. Clusters are the borderland between the microscopic single atom and macroscopic solid state, exhibiting extraordinary size-dependent physical and chemical properties. Thus one can design new materials using clusters as the building blocks. Experiments of silver clusters reflect the existence of shell effects similar to those observed in alkali clusters. Due to the supring similarity with the simple alkali clusters, the extensive studies on the silver clusters become the hot topic up to now. Silver clusters also play an important role in some industrial applications, such as photography, catalytic processes, novel electronic materials, surface science, and metal alloy clusters.Structural evolution and electronic propertires of neutral and singly charged Ag_n^v (v=0,±1; n=3-14) as well as neutral Ag_n(n=3-22) clusters have been explored using an extensive, unbiased search based on genetic algorithm and density functional theory (DFT) methods. In the DFT calculations, exchange-correlation interactions are approximated by the generalized gradient approximation (GGA) with PW91 parameterization. A relativistic semi-core pseudopotential (DSPP), and a double numerical basis set plus d-polarization functions (DNP), as implemented in the DMol³ package are employed. We denote this scheme as PW91/DSPP/DNP.Cationic, neutral, and anionic silver clusters have planar shapes for their lowest-energy structures up to n=7, 6, and 6, respectively. Most of the competitive candidates for Ag_n^v (v=0,±1; n=9-14) are found to adopt close-flat structures. The present results obtained at the PW91/DSPP/DNP level are significantly different from those predicted in earlier studies using empirical and semi-empirical potentials, and partly in line with the previous first-principles calculations.It is found that neutral medium-sized Ag_n (n=19, 21, 22) clusters adopt amorphous packing configurations based on a 13-atom icosahedral core. The optimal structure of Ag₂₀, which is a highly symmetric tetrahedron with T_d symmetry, is an fcc-like structure. These results are significantly different from the simple icosahedron-based structural growth pattern predicted in earlier studies using empirical methods. For Ag_n (n=16-18), two competitive candidates for the lowest-energy structures, i.e., hollow-cage structure and icosahedron-based compact structures of flat shape are found. The most stable structures of both Ag₁₆ and Ag₁₈ adopt the hollow cage configuration with C_2v symmetry.The dependences of the most stable structures of Ag_n^v (v=0,±1; w=3-14) and Ag_n(n=3-22)on second difference energy, average binding energies per atom (E_b), highest occupied and lowest unoccupied molecular orbital energy gaps (HOMO-LUMO gap), vertical ionization potentials (IP_v), and adiabatic electron affinities (EA) are studied in detail. The calculated IP_v and EA for the optimized structures are compared with the corresponding and available experimental values. The neutral Ag_n (n=6, 8, 14, 18, 20) clusters are suggested to be "magic" clusters by an analysis of their geometric and electronic properties.The present results provide useful insight into understanding the structural evolution and electronic properties of neutral and singly charged Ag clusters. The silver clusters give some useful information for the experimental further studies.