Optical Properties of Pt and Ag-Pt Nanoclusters from TDDFT Calculations: Plasmon Suppression by Pt Poisoning

The optical properties of alloyed Ag-Pt nanoclusters are theoretically investigated as a function of composition and chemical ordering via a time-dependent density-functional-theory (TDDFT) approach. Clusters with icosahedral structure ranging in size between 55 and 146 atoms are considered, large enough to start observing strong adsorption peaks related to surface plasmon resonances (SPR) in pure Ag systems. Strikingly it is found that even the modest Pt content here considered, ranging between 14% and 24%, is sufficient to substantially damp the optical response of these clusters. The effect is most disruptive when Pt atoms are scattered at the cluster surface, where the Ag SPR is mostly located, especially at the cluster apex, while the most intense residual peaks occur as Pt 5d -> Ag 5p transitions at a Pt(core)/Ag(shell) interface and are strongly blue-shifted by 0.7-1.0 eV with respect to the analogous Ag peaks. Smaller Pt-13 and Pt-38 clusters are also studied for comparison, finding a nonplasmonic behavior but a strong involvement of Pt 5d orbitals in the optical response.