Origin of Discrete Optical Absorption Spectra of M₂₅(SH)₁₈⁻ Nanoparticles (M = Au, Ag)

Abstract

Time-dependent density functional theory calculations at the SAOP/TZP level of theory are employed to study the optical absorption spectra of pure metal nanoparticles (Au25(SH)18− and Ag25(SH)18−) and mixed metal “core-shell” systems (Au13Ag12(SH)18− and Ag13Au12(SH)18−). All four systems exhibit discrete absorption spectra. The splitting between the LUMO and LUMO+1 and between the HOMO and HOMO−1 varies markedly with the metal used in the core and in the oligomeric thiolate ligands. The optical absorption spectra are not separable into core and ligand contributions; geometric and electronic interactions between the two fragments are responsible for complex absorption spectra.