Upconversion fluorescence enhancement of NaYF4:Yb/Re nanoparticles by coupling with SiO2 opal photonic crystals

Abstract

Upconversion fluorescent material such as NaYF4:Yb/Re (Re is lanthanide ions) nanoparticles has considerable application prospect, but the fluorescence efficiency is usually too low. In this paper, an effective approach has been validated to enhance the fluorescence intensity of NaYF4:Yb/Re by coupling with SiO2 opal photonic crystals (PCs). NaYF4:Yb/Tm and NaYF4:Yb/Er were prepared by thermal decomposition method and ligand exchange, and then spin-coated on the surface of SiO2 opal PCs with different photonic band gap (PBG). The PBG properties of PCs/UCNPs composite films were measured by transmission spectra. The effect of overlapping extent between PBG and UCNPs’ emission on upconversion emission enhancement was studied through fluorescence spectra. The result shows that when the SiO2 opal PCs with the PBG at 448 nm were composited with NaYF4:Yb/Tm, the upconversion fluorescence at 450 nm was enhanced mostly, which is 34-fold. Similarly, the highest enhancement, 23-fold, was observed for fluorescence of NaYF4:Yb/Er at 541 nm when composited with SiO2 opal PCs having PBG at 544 nm. According to the analysis, the upconversion fluorescence can be enhanced through coupling with PCs by utilizing the light reflection and light localization effect of PCs. At the same time, the highest enhancement is obtained when the PBG of PCs is exactly overlapping with the wavelength of upconversion fluorescence. Our study potentially will contribute to promoting the sooner practical application of upconversion nanoparticles.