Surface-plasmon properties of noble metals with exotic phases

Noble-metal nanoparticles have been the industry standard for plasmonic applications due to their highly populated plasmon generations. Despite their remarkable plasmonic performance, their widespread use in plasmonic applications is commonly hindered due to limitations on the available laser sources and relatively low operating temperatures needed to retain mechanical strength in these materials. Motivated by recent experimental works, in which exotic hexagonal-closed-packed (HCP) phases have been identified in gold (Au), silver (Ag) and copper (Cu), we present the plasmonic performance of two HCP polytypes in these materials using high-accuracy first-principles simulations. The isolated HCP phases commonly reach thermal and mechanical stability at high temperatures due to monotonically decreasing Gibbs free energy differences compared to the face-centered cubic (FCC) phases. We find that several of these polytypes are harder and produce bulk plasmons at lower energies with comparable lifetimes than their conventional FCC counterparts. It also leads to the localized surface-plasmon resonance (LSPR) in perfectly spherical HCP-phased nanoparticles, embedded onto dielectric matrices, at substantially lower energies with comparable lifetimes to their FCC counterparts. LSPR peak locations and lifetimes can be tuned by controlling the operational temperature, the dielectric permittivity of hosting matrix and the grain size. Our work suggests that noble-metal nanoparticles can be tailored to develop exotic HCP phases to obtain novel plasmonic properties.