Understanding the Optical Properties of Au@Ag Bimetallic Nanoclusters through Time-Resolved and Nonlinear Spectroscopy

Bimetallic Au@Ag nanoclusters have been shown to possess unique properties compared to their monometallic Au or Ag nanoclusters. In the present work, the optical properties of three bimetallic nanoclusters, namely, Au-15@Ag, Au-18@Ag, and Au-25@Ag have been investigated by steady-state and time-resolved absorption and emission and two-photon absorption measurements. The nanoclusters are composed of a gold core and an alternating Au-Ag-ligand surface shell structure. We observed that the absorption and emission of the bimetallic Au@Ag nanoclusters are enhanced compared to the pure Au nanoclusters; the emission is, additionally, blue shifted. All of the mixed clusters showed impressive two-photon absorption coefficients with Au-25@Ag showing a largely enhanced effect. The excited state dynamics of the Au@Ag nanoclusters was studied by time-resolved transient absorption and emission, which show that the bimetallic nanoclusters have longer excited state lifetimes than the Au nanoclusters. Specifically, the surface-related excited states showed a larger increase in lifetime than the core-related states, which indicates Ag binding on the staple motifs of the Au nanoclusters. This spectroscopic evidence shows that the bimetallic nanoclusters have gained significant Ag character, and that the Ag atoms bind on the staple motifs.