Comparison of Three Ratiometric Temperature Readings from the Er³⁺ Upconversion Emission

The emission of Er³⁺ provides three combinations of emission bands suitable for ratiometric luminescence thermometry. Two combinations utilize ratios of visible emissions (²H_11/2→⁴I_15/2 at 523 nm/ ⁴S_3/2→⁴I_15/2 at 542 nm and ⁴F_7/2→⁴I_15/2 at 485 nm/ ⁴S_3/2→⁴I_15/2 at 545 nm), while emissions from the third combination are located in near-infrared, e.g., in the first biological window (²H_11/2→⁴I_13/2 at 793 nm/ ⁴S_3/2→⁴I_13/2 at 840 nm). Herein, we aimed to compare thermometric performances of these three different ratiometric readouts on account of their relative sensitivities, resolutions, and repeatability of measurements. For this aim, we prepared Yb³⁺,Er³⁺:YF₃ nanopowders by oxide fluorination. The structure of the materials was confirmed by X-ray diffraction analysis and particle morphology was evaluated from FE-SEM measurements. Upconversion emission spectra were measured over the 293-473 K range upon excitation by 980 nm radiation. The obtained relative sensitivities on temperature for 523/542, 485/542, and 793/840 emission intensity ratios were 1.06 ± 0.02, 2.03 ± 0.23, and 0.98 ± 0.10%K^-1 with temperature resolutions of 0.3, 0.7, and 1.8 K, respectively. The study showed that the higher relative temperature sensitivity does not necessarily lead to the more precise temperature measurement and better resolution, since it may be compromised by a larger uncertainty in measurement of low-intensity emission bands.