Heavy Element Approach to Realize Efficient Upconversion Luminescence of Er‐Rich Nanoparticles

Allowing high‐level doping is one typical advantage of the spatial confinement effect of rare earth‐relevant upconversion nanosystems. It also triggers many imaginations in constructing super‐bright upconversion nanomaterials. The latter, unfortunately, has not achieved remarkable progress. Among other factors, the interaction between the rare earth ions and the surrounding hosts also plays an important role. Herein, we demonstrate that heavy element introduction can significantly enhance the upconversion efficiency of the Er3+ highly doped nanosystems. Briefly, cubic alkaline‐earth matrices are employed as model hosts of ultrasmall M1‐xErF2+x (M = Ca, Sr, Ba) nanoparticles, in which the interaction between Er and M is different from the well‐established NaErF4 system. As a result, the optimized SrErF5@SrYF5 nanoparticles with a 5 nm inert shell exhibit superior upconversion emission, which is 8.5 times higher than that of the yet most effective hexagonal phase‐NaErF4@NaYF4 reference nanoparticles. On top of that, the upconversion luminescence of SrErF5@SrYF5 exhibits strong temperature dependence‐a two‐order‐of‐magnitude increase of green emission is observed when the nanoparticles are cooled down from room temperature to 80 K, which highlights its potential in high‐sensitivity temperature sensing. All these results provide a new possibility to further improve the upconversion luminescence of highly doped materials.