Direct Detection of Highly Localized Metal-Metal Interface Plasmons from Bimetallic Nanoparticles

Tailoring of photon-matter interaction in solid material is critical for surface plasmon resonance-based sensing. This can be achieved from suitable material with interface engineering. The modified plasma oscillations in metal-metal interfaces are highly sought-after phenomena in plasmonics; however, such a localized nature of this oscillation has never been reported. Here we present the first evidence of localized interface plasmons from CoAg bimetallic nanoparticles by employing scanning transmission electron microscopy-electron energy-loss spectroscopy. We found that the localized interface plasmons oscillate with a frequency in between in-plane dipole localized surface plasmon resonance (LSPR) mode and quasiplanar mode. Moreover, we observed that the localized interface plasmon resonance is stronger than in-plane dipole LSPR which was characterized by comparing the quality factor of the energy-loss peaks. Such interface plasmon resonance was not distinctly observed from ensembles of CoAg nanoparticles by optical excitation incident normally; however, a broader in-plane dipole mode was observed compared to similar pure Ag nanoparticles. This direct detection of plasmons confined to the interface region could drive to future engineering of bimetallic interfaces with improved plasmonic activity.