Effect of noble metal nanoparticles on the upconversion of NaYF4:Er,Yb

Although enhancement of up-conversion (UC) phosphors by metal nanoparticles (NPs) has been regularly reported, it is not yet easy to use the results to generalize the specific conditions leading to enhancement and there is a lack of systematic studies assessing a wide range of parameters. Therefore a prototyping mixing protocol was implemented to assess the effect of the size of gold (Au) and silver (Ag) NPs on the performance of hexagonal NaYF 4 :Er,Yb UC phosphor prepared by high-temperature co-precipitation, producing particles of size 22 nm in solution. Separately, Au and Ag metal NPs of 17 – 45 nm and 12 – 32 nm, respectively, were prepared using reduction methods. Solution mixing and freeze drying was used to prepare powder samples, initially with metal-to-phosphor particle ratio ∼1:50. Transmission electron microscopy was used to confirm the mixing, while X-ray diffraction patterns indicated no change in the structure of the phosphor resulting from mixing. Despite the range of metal NP sizes used, neither Au or Ag mixing resulted in enhancement. Using Au NPs of intermediate size (30 nm), lower mixing ratios to ∼1:600 were assessed, but this only resulted in quenching, which became stronger as more Au NPs were added. These results indicate that plasmonic enhancement may require quite specific conditions to achieve or that the mixing process did not facilitate the interaction conditions leading to plasmonic enhancement. Despite not achieving plasmonic enhancement, the results of this study are important to consider for the broad systematic conditions assessed as well as the novel mixing protocol used.