Sharp Interface and Highly Efficient Upconversion Luminescence of CaF₂@NaYbF₄:Er@CaF₂ Sandwich-Structured Nanoparticles

Sandwich nanostructures doped with lanthanides, where sensitizers and activators are confined to the intermediate layer between the inner and outer inert NaYF 4 layers, have been shown to be the most efficient upconversion nanoparticles (UCNPs). However, in order to achieve a high upconversion luminescence (UCL) quantum yield, a thick outer inert NaYF 4 layer and a large-size nanostructure are required, which limits their applications in biological fields such as bioimaging and biosensing. In this work, we have synthesized CaF 2 @NaYbF 4:Er@CaF 2 sandwich nanostructures and found that their UC emission efficiency remarkably increases with increasing thickness of the intermediate active layer, even though the outer CaF 2 layer is thin and incomplete. In contrast to the diffuse NaYF 4 @NaYbF 4:Er interfaces in NaYF 4 @NaYbF 4:Er@NaYF 4 sandwich structures, the sharp CaF 2 @NaYbF 4:Er interfaces not only effectively suppress energy migration to defects and surface quenchers but also guarantee a shorter Yb–Er distance on average. This leads to a reduction in energy loss and a highly efficient upconversion of energy transfer. As a result, the 20 nm-CaF 2 @NaYbF 4:2%Er@CaF 2 nanoparticles emit ultrabright UCL, which is twice as intense as the UCL emitted from the larger 37 nm-α-NaYF 4 @NaYbF 4:2%Er@NaYF 4 nanoparticles. These findings will enable the versatile design of bright upconversion nanoparticles with relatively small sizes, meeting the requirements for biological applications.