Controlling Surface Plasmon Resonance of Metal Nanocap for Upconversion Enhancement

An upconversion composite nanoparticle (NP) composed of an Er and Yb doped Y₂O₃ (Y₂O₃:Yb,Er) NP and a Au nanocap was developed, and the effect of the coverage on the optical responses was studied by numerical simulations and optical measurements of single composite NPs. The formation of a composite NP and the control of the coverage were confirmed by transmission electron microscope observations. By controlling the coverage, the scattering peak due to the magnetic dipole surface plasmon resonance shifted from around 700 nm to over 900 nm. The upconversion intensity was also strongly modified by the coverage. Clear correlation was observed between the surface plasmon resonance wavelength and the upconversion intensity; the intensity increased as the resonance wavelength approached the excitation wavelength (975 nm). The largest enhancement factors of the upconversion with respect to that of a Y₂O₃:Yb,Er NP without a Au nanocap were 64 and 101-fold for the green (∼560 nm) and red (∼660 nm) emissions, respectively