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

The UV–visible absorption spectra of Agn and Agn−1Cu1 (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 Agn and Agn−1Cu1 (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 Ag2 to Ag3, from Ag4 to Ag13, from Ag20 to Ag32, from Ag1Cu1 to Ag2Cu1, from Ag3Cu1 to Ag4Cu1, from Ag5Cu1 to Ag12Cu1, from Ag19Cu1 to Ag31Cu1, and from Agn to Agn−1Cu1 (n=2–8, 20, 32); redshifts obviously appear as following changes: from Ag3 to Ag4 and from Ag13 to Ag20, from Ag2Cu1 to Ag3Cu1, from Ag4Cu1 to Ag5Cu1, from Ag13Cu1 to Ag19Cu1, and from Ag13 to Ag12Cu1. 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 Ag2 to Ag8, from Ag1Cu1 to Ag7Cu1, then, they (except Ag12Cu1) gradually strengthen with increasing cluster size, however, the intensity of Ag12Cu1 is larger than that of Ag19Cu1. 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.

Graphical abstractThe absorption spectra of silver and Ag-Cu clusters are related with their sizes and compositions. Blueshifts and redshifts are observed as sizes and compositions vary.Figure optionsDownload full-size imageDownload as PowerPoint slide