Optical, Raman and vibrational properties of closed shell Ag–Cu clusters from density functional theory: The influence of the atomic structure, exchange-correlation approximations and pseudopotentials

Abstract

The optical, Raman and vibrational properties of 8-atom closed shell Ag–Cu clusters are systemically investigated within the framework of density functional theory with eight kinds of exchange-correction functionals: generalized gradient approximation (GGA) and local-density approximation (LDA), as well as DFT-based semi-core pseudopotential (Dspp) and all-electron (AE). Results reveal that the optical spectra of pure Ag8 and Cu8 clusters are in excellent agreement with the experiments; moreover, both tetracapped tetrahedron (TcTd) and monocapped pentagonal bipyramid (McPb) contribute to the experimental absorption spectra. For the optical, Raman and vibrational spectra of Ag–Cu clusters, the differences between TcTd and McPb, that are intensity and position energies of the strongest peaks as well as the appearance of additional peaks, are visible obviously. Exchange-correction functionals have little effect on the optical, Raman and vibrational spectra of Ag–Cu clusters. On comparing the spectra using GGA with those using LDA, the position energies of the corresponding peaks are lower, which shows obviously in the rich Cu atoms’ clusters. However, pseudopotentials have some effects on them. The intensities of absorption peaks for Ag–Cu clusters using Dspp are weaker than those using AE; in particular, this effect of rich Ag atom clusters is shown obviously. The intensities of Raman peaks for Ag–Cu clusters using Dspp are larger than those using AE. An additional peak appears in the vibrational spectra of TcTd using Dspp but not in the ones using AE for AgnCu8−n (n=2–8) clusters. The vibrational and Raman spectroscopy may be helpful in determining cluster size and structure.