Metal-Core/Dielectric-Shell/Metal-Cap Composite Nanoparticle for Upconversion Enhancement

We have developed an upconversion composite nanoparticle composed of a metal core, an upconversion shell, and a metal cap. Numerical simulation of the nanocomposite revealed that hybridization of the localized surface plasmon modes of the core and the cap results in the emergence of novel bonding and antibonding modes. The latter mode has wide tunability in the resonance wavelength and strong field confinement at the position of the upconversion shell. For the fabrication of the composite nanoparticle, we developed a process that combines liquid-phase synthesis and vapor deposition processes. The scattering spectra of single composite nanoparticles agreed well with those in the numerical simulation. The comparison of the upconversion intensity between the metal-core/dielectric-shell structure and the metal-core/dielectric-shell/metal-cap structure revealed that the cap formation increases the intensity several folds