CaGdF₅ based heterogeneous core@shell upconversion nanoparticles for sensitive temperature measurement

Lanthanide-doped upconversion nanoparticles (UCNPs) have attracted great attention in temperature sensing because of their widespread thermal quenching effect (TQE), a phenomenon in which luminescence intensity decreases as the temperature increases. However, enhancing the TQE of activated ions without changing the dopants or the host is still challenging. Herein, Yb³⁺ and Er³⁺ codoped UCNPs in a cubic CaGdF₅ host were synthesized by a coprecipitation method for optical temperature sensing. Compared with the homogeneous shell (CaGdF₅), those heterogeneous (CaF₂) shelled UCNPs exhibited stronger upconversion luminescence (UCL) due to the significantly reduced multiphonon nonradiative relaxation. Further, we investigated the effects of homogeneous and heterogeneous shells on TQE. The relationship between the intensity ratio of the green emission bands of Er³⁺ ions (²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2) and temperature are obtained for these two core@shell UCNPs. The results demonstrated that the UCNPs with CaF₂ shells are more sensitive to temperature in the 200-300 K. The maximum thermal sensitivity of CaGdF₅:Yb,Er@CaF₂ could reach 2.2% K^-1 at 200 K. These results indicate that the heterogeneous core@shell UCNPs are promising for use as optical temperature sensors.