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

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 (Au-25(SH)(18)(-) and Ag-25(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+I and between the HOMO and HOMO-I 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.