Controllable and tuneable growth of NaYbF4:Tm(0.5%)Fe(5%)@Na(Yb/Y)F4-core@shell structures and the effect of their geometry on upconversion luminescence

Lanthanide doped upconversion nanoparticles (UCNPs) are a class of nanomaterials with excellent luminescence properties. The practical use of UCNPs, however, has been hindered by their relatively low upconversion (UC) quantum yields. Enhancing their efficiency, and therefore their brightness, is a critical goal for these emerging materials. In this study, a range of novel core@shell structures were synthesized using NaYbF4:Tm(0.5%)Fe(5%) as the core template to regulate the shell growth of Na(Yb/Y)F-4. Here, we observed that the size and the morphology of the UCNPs can be fine-tuned by controlling the ratio of the Y3+/Yb3+ ions within the shell material, resulting in small rod-like structures that form when using a high concentration of Y3+ and larger hexagonal plate-like structures when using a high concentration of Yb3+. In terms of the optical properties, the UC luminescence and lifetime of the core-only and core@shell structures were measured. Overall, the emission intensity and lifetime of the UCNPs increased with the nanoparticle size. We observed that, under the same experimental conditions, larger core@shell nanostructures with a hexagonal shape showed brighter UC emission, compared to those with a small nanorod shape and core-only. Based on our findings, we propose a variety of energy transfer pathways arise through geometric alteration of the nanostructures upon varying the Y3+/Yb3+ ion concentration ratios within the shell material. Our results could have important significance for understanding the relationship between the geometry of UCNPs and their UC optical properties.