Research On The Structure And Electronic Properties And X-ray Absorption Near Edge Structure Of Gold Clusters

During the past few years, gold clusters have attracted the attention of a wide range of researchers .Apart from the understanding of the finite size effect on the physicochemical properties, the interest on gold clusters is primarily due to various applications, ranging from molecular electronic devices, catalysis, etc.As is well known, the catalysis of nanoclusters is dependent on environment, cluster size, and the geometrical and electronic structure. But the present experiment condition was not able to determining the microcluster structure, now the information of clusters'structures can only be derived from theoretical calculations.In this paper, we study the geometry structure and x-ray absorption near edge structure of Aun (n?28) clusters. First, we have carried out the geometry structure optimization for Aun(n?28) clusters. Calculation was performed using the ab initio molecular dynamics simulation method as implemented in VASP. The total energy calculation used projected augmented wave (PAW) method under the spin–polarized generalized gradient wave (GGA). We have chosen the Perdew-Wang(PW91) exchange–correlation functional. The atomic pseudopotentials include the mass-velocity term and Darwin term to account for the scalar-relativistic effects. A simple cubic supercell with theгpoint for the Brillouin zone integration was considered for these calculations. The geometries are considered to be converged when the forces on each ion become 0.05eV/ A? . The cutoff energy for the plane–wave basis set was 280ev.The total-energy convergence was test with respect to the plane-wave basis size and simulation cell size and the total energy was found to be accurate within 0.0001ev. The calculation results are in good agreement with experiment and other theoretical calculations. It shows that the parameters are the best choice for the study of the gold clusters.The ground states geometries of Aun(n?28)clusters are found to favor planar configurations to 13,which is much larger than in copper and silver clusters, owing to the strong relativistic effects and the strong hybridization of the gold atomic 5d and 6s orbital. From n=14 onwards, Aun clusters show significant change in the geometry arrangement, which favor three-dimensional configurations. Aun(n?28)clusters'average bond and average coordination number increase with the atomic number increasing. The stability analysis has been carried out by calculating the average binding energy, the second order difference in energy and energy gap. The binding energy increases with the increase in size for Aun (n>5) and exhibits an odd-even effect,which has a relatively higher binding energy with clusters having even numbers of atoms than neighboring clusters having odd numbers of atoms . The second difference energy exhibits obvious even-odd oscillation on increasing cluster size. There are no obvious odd-even oscillating behaviors in the energy gap. For the calculated electron affinities of gold clusters also show a remarkable characteristic oscillation effect, which implies that the clusters with odd numbers of atoms have higher EA values than the neighboring clusters with even numbers of atoms. Based on the results obtained for Aun , it is found that Aun having even numbers of atoms are more stable than those with odd numbers of atoms.Secondly, we calculated the X-ray absorption near edge structure of Aun clusters. Calculations, based on a self full multiple scattering, self-consistent field ,real-space Green's function approach implemented in the ab initio FEFF8 code .The resulting calculated XANES should be average over all atoms absorption coefficient, because Au atom in each shell has a different absorption coefficient. Our results using FEFF8 for the different Aun clusters show that XANES provide a characteristic signature of clusters shape. The calculated L1,L 2edge spectra of the different size Aun clusters are compared .The peak in the edge was found to increase as function of cluster size .And it exits a more clear peak in the edge and the peak move ahead in the post edge when it presents for 3D structures. We also compared with the L1 edge spectra of atom in the different sites of Aun clusters. The atom spectra of the different sites are difference. The atom spectra exits obvious peak in the edge, when the number its surrounding atom is increased. The results show the Aun XANES provide information on clusters geometry and shape. Finally, we compared the experimental date with of Au metal with the calculated L 3 XANES edge of the Au cluster .The results show that as the cluster size increases, there is an increase in the white line, and closer to the Au metal.So, through the theory research of the Aun cluster XANES, we could afford some important information for the research of the cluster structure in the experiment.