Ultraviolet-visible absorption, Raman, vibration spectra of pure silver and Ag-Cu clusters: A density functional theory study

摘要

The UV-visible absorption spectra of Agn and Ag_(n-1)Cu_1 (n=2-8, 13, 20, 32) clusters were investigated in the framework of TD-DFT using LC-ωPBE (ω=0.40) and the Raman and vibration spectra of Ag_n and Ag_(n-1)Cu_1 (n=2-8, 13, 20, 32, 55) clusters were calculated by the DFT/GGA/PBE. The UV-visible absorption, Raman, vibration spectra of pure silver and Ag-Cu clusters are related with their sizes, compositions and structures. Blueshifts are obviously observed as following: from Ag_2 to Ag_3, from Ag_4 to Ag_(13), from Ag_(20) to Ag_(32), from Ag_1Cu_1 to Ag_2Cu_1, from Ag_3Cu_1 to Ag_4Cu_1, from Ag_5Cu_1 to Ag_(12)Cu_1, from Ag_(19)Cu_1 to Ag_(31)Cu_1, and from Ag_n to Ag_(n-1)Cu_1 (n=2-8, 20, 32); redshifts obviously appear as following changes: from Ag_3 to Ag_4 and from Ag_(13) to Ag_(20), from Ag_2Cu_1 to Ag_3Cu_1, from Ag_4Cu_1 to Ag_5Cu_1, from Ag_(13)Cu_1 to Ag_(19)Cu_1, and from Ag_(13) to Ag_(12)Cu_1. On the whole, the wavelengths of the maximum Raman peaks of pure Ag and Ag-Cu clusters lower with increasing cluster size; the intensities of the maximum Raman peaks of pure Ag and Ag-Cu clusters are weakly wavy as clusters change from Ag_2 to Ag_8, from Ag_1Cu_1 to Ag_7Cu_1, then, they (except Ag_(12)Cu_1) gradually strengthen with increasing cluster size, however, the intensity of Ag12Cu1 is larger than that of Ag_(19)Cu_1. The wavenumbers and intensities of the maximum vibration peaks of pure Ag and Ag-Cu clusters fluctuate with increasing cluster size. The calculated vibrational and Raman spectroscopy of pure Ag and Ag-Cu clusters may be helpful in determining sizes and structures of experimental clusters.