Deep-UV Plasmonics with Stable In2Au Alloy (Blue Gold) Nanoparticles.

Surface plasmon resonances in nanoparticles are intensely studied in fundamental and applied research as they permit investigating the interplay between atomic and electronic structures and generate strongly enhanced local electric fields. For the commonly used noble metals it is not possible to extend this resonance below ∼400 nm, into the deep ultraviolet, even though such an extension is highly desirable for numerous applications. We report the successful fabrication and characterization of surfactant-free In2Au nanoparticles in the size range of 5-10 nm diameter which, if properly protected against oxidation, are entirely metallic, highly crystallized in the blue gold structure and exhibit plasmon resonances down to 280 nm. We characterized the crystalline and the chemical structure of these particles as well as their plasmonic response on both the ensemble-averaged and the single particle level. Machine-learning assisted data treatment of the single-particle experiments allowed identifying and separating chemical phases as well as the unequivocal separation of the surface plasmon resonance from the background signal. The possibility of unambiguously measuring plasmonic signals in individual particles in the size range <10 nm using STEM-EELS constitutes a major advance in nanoalloy nano-optics. These results introduce the In2Au nanoalloy as a material for the tailored fabrication of alloy-based UV-plasmonic nanostructures and their implementation in applications such as photocatalysis, biosensing and high-energy hot carrier generation.