Asymmetric Light Absorption and Radiation of Ag–Cu Hybrid Nanoparticles

In this article, we study the plasmonic properties of phase-separated Ag–Cu nanoparticles. The particles are fabricated using simple thin film evaporation followed by particle formation by vacuum annealing. The formed particles feature a two-faced Janus structure. Characterization is carried out at the single particle level utilizing transmission electron microscopy in combination with electron energy loss spectroscopy and cathodoluminescence, and modeled by finite element method simulations. We find that these particle sustain two kinds of resonances: resonances localized to the Ag half of the particle and resonances involving the entire particle. This is due to the difference in onset energy for interband transitions for the two metals. As the resonances are excited in Ag, large enhancements of energy absorption can be achieved in the Cu half of the particle. We also find a slight asymmetry to the emission of the particles in cathodoluminescence, with a preferential emission toward the silver side of the particle. Enhanced energy absorption into Cu means an increased number of generated hot electrons. This together with the ease of fabrication of the particles makes such structures interesting candidates for plasmon enhanced photocatalysis. Furthermore, because of the inherent phase separation of the materials, stability even at elevated temperatures is enhanced. Not being limited to the Ag–Cu system, a similar approach should work equally well for other phase-separated systems.