Sharp Interface and Highly Efficient UpconversionLuminescence 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.