Upconversion Luminescence of Er and Yb Codoped NaYF₄ Nanoparticles with Metal Shells

Upconversion photoluminescence (PL) of a composite nanoparticle consisting of an Er and Yb codoped NaYF₄ core and a Au shell is studied theoretically and experimentally. We first investigate the effects of a Au shell on the radiative and nonradiative emission rates of a dipole placed in a core, the absorption and scattering cross sections of a composite nanoparticle, and the electric field within a core at the excitation wavelength. We then synthesize the composite nanoparticle and study the PL properties. From the analyses of the PL data in combination with the data obtained by theoretical calculations, the mechanism of the enhancement and quenching of upconversion PL by the formation of a Au shell is studied