Improving green Yb3+/Er3+ upconversion luminescence by co-doping metal ions into an oxyfluoride matrix

The most efficient upconversion (UC) nanophosphors are based on fluoride hosts with low phonon energies, which reduce the amount of nonradiative transitions. In particular, NaYF4 doped with lanthanide ions (Yb3+/Er3+) is among the most efficient upconversion phosphors. However, its low thermal stability limits its use for certain applications. Oxide hosts, on the other hand, typically have much better thermal stability, despite the fact that these matrices have higher phonon energies and are thus prone to lower UC efficiencies. As a result, developing host nanomaterials that combine the robustness of oxides with the high upconversion efficiencies of fluorides remains an intriguing prospect.[1, 2] Rare earth oxyfluoride (YOF) matrixes have been developed to combine the benefits of low phonon energy of fluorides with the benefits of oxides for active optical ions.[3] The ongoing challenge is to improve the luminescence properties of lanthanide doped rare earth oxyfluoride materials in order to boost their future applications, such as for example for temperature sensing. In this presentation, we demonstrate the synthesis of water dispersible core-shell SiO2@NaYF4: Yb3+/Er3+ beads, which were then converted into NaYF4: Yb3+/Er3+ hollow capsules after the silica template was removed. Further, a simple high-temperature annealing step induced an in-situ ion-exchange reaction between O2 and NaYF4: Yb3+/Er3+, resulting in pure YOF: Yb3+/Er3+. By introducing different metal ions, such as Gd3+, Li+ and Mn2+, at different co-doping concentration into the YOF host matrix, green UC luminescence of Er3+(2H11/2 and 4S3/2 states) issuccessfully strengthened when excited by a 980 nm laser. Strong green emission is especially important for luminescence thermometry applications.