Origin of Photoluminescence of Ag25(SR)18- Nanoparticles: Ligand and Doping Effect

Recent theoretical insights into the origin of photo-luminescence of thiolate-protected gold nanoclusters raise the question of whether the observed luminescence mechanism is valid for related silver nanoparticles. To this end, we perform density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations on the Ag-25(SR)(18)(-) (R = H, PhMe2) nanocluster, which is currently the only thiolate-protected silver cluster that has a matching analogue in gold. The geometric and electronic structural modifications of Ag-25(SH)(18)(-) upon photo excitation are found to be similar but less pronounced than those of Au-25(SH)(18)(-) at the same level of theory. The Stokes shift is calculated to be 0.37 eV and the replacement of R = H model ligands by R = PhMe2 decreases the Stokes shift in contrast to the increase in Stokes shift for aliphatic ligands previously observed in the Au-25 system. The calculated emission energy agrees well with the experimental photoluminescence energy when the typical underestimation of DFT calculations is considered. Similar to Au-25, luminescence of Ag-25 arises from a HOMO-LUMO transition where core-based superatomic P and D orbitals are involved. Heteroatom doping of Au-25 and Ag-25 clusters with Ag/Au does not affect the origin of the photoemission of these systems. However, the radiative lifetimes of the lowest singlet excited states (S-1) and emission energies are slightly affected.