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 NaYF4 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 NaYF4 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 CaF2@NaYbF4:Er@CaF2 sandwich nanostructures and found that their UC emission efficiency remarkably increases with increasing thickness of the intermediate active layer, even though the outer CaF2 layer is thin and incomplete. In contrast to the diffuse NaYF4@NaYbF4:Er interfaces in NaYF4@NaYbF4:Er@NaYF4 sandwich structures, the sharp CaF2@NaYbF4: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-CaF2@NaYbF4:2%Er@CaF2 nanoparticles emit ultrabright UCL, which is twice as intense as the UCL emitted from the larger 37 nm-α-NaYF4@NaYbF4:2%Er@NaYF4 nanoparticles. These findings will enable the versatile design of bright upconversion nanoparticles with relatively small sizes, meeting the requirements for biological applications.