| As a transition metal clusters, Ag clusters have some unexpected properties. These properties have been widely used in physics, chemistry, materials and other fields. The structure of the clusters greatly affects their properties, but the present experimental conditions were not able to determine the micro-cluster structures, now the information of clusters'structure can only be derived from theoretical calculations. To solve these problem, the VASP procedures were used to calculate their structure by us. First of all,we calculate the structure, energy gap and the vertical ionization energy of Agn (n=2-6) through VASP procedures, compared with the theoretical calculation results and experimental data in published literature. We found the calculation results in this article are in good agreement with the literature, which verify the correctness of calculations.Then this article optimized Agn(n = 2-19) clusters structure with VASP in the relativistic core potential approximation ,using density functional theory methods. We calculate energy, energy gap of Agn (n = 2-19) clusters on those ground state structure. By comparison, we found that there was no peak before white line, which is due to K edge is corresponding to the electrons transition from 1 s to 2 s. According to transition selection rule, only P-state can transition to the S state. But the lowest empty state in transition-metal is d state. So transition is not allowed before the white line. However, the L2 and L3 are respectively corresponding to 2P1/2, 2P3/2 transition. In addition to S-state, there is D-state transition, it is allowed transition before the adsorption edge peak, so there is adsorption peak.Furthermore, this article also compares three atoms'XANES in the structure of Ag13, the adsorption spectra of atomic is different. The vibration intensity in XANES of the center atom has relatively large vibration intensity. But vibration intensity of the edge atom is light. Using this method, we can determine the surrounding information of the absorption atom in the clusters.Finally, magnetic moment of Ag clusters is also calculated in this article. According to results, Agn (n =2-12): when n is even, the average atom magnetic moment is 0; when n is odd, the average atom magnetic moment is gradually reduced. Because of fact that there is a s electron in Ag atom, when n is an even number there are even s electrons, spin angle momentum coupling is 0, there is no magnetic moment of clusters. When n is odd, there are odd s electrons, spin angle momentum coupling is not 0, clusters have magnetic moment. But when n =13, the average magnetic moment is the largest. Because the structure is icosahedral, which is high degree of symmetry. When n is between 13 and 19 except 18, the even clusters have magnetic moment. This structure has a central atom, which has the higher coordination number. The orbital overlap between the 4d orbit of the central atom and the outer 4d orbit increase. Therefore, the 4d low-energy electron transfer from the outer layer of atoms to the center. Then uncouple electrons of the outer layer atoms are increased, so magnetic moment increases . But when n> 13, the even-numbered clusters also have magnetic moment. Because we add an atom on the previous structure to get the structures of Agn(n = 13-17). So the even clusters also have magnetic moment. But there is a central atom, or two at this time, and the structure of clusters is not very symmetrical so the magnetic moment of clusters decreases.
|
PDF Full Text Request