Host-Dependent Structural and Optical Properties and Upconversion Ratiometric Thermometry of Yb 3+ /Er 3+ -doped NaYF 4 , NaGdF 4 , NaScF 4 Nanophosphors.

This work presents a systematic study of the structural, morphological, and optical properties, along with the upconversion ratiometric thermometry of NaGd 0.78 Yb 0.2 Er 0.02 F 4 , NaY 0.88 Yb 0.1 Er 0.02 F 4 , NaY 0.78 Yb 0.2 Er 0.02 F 4 , and NaSc 0.78 Yb 0.2 Er 0.02 F 4 nanophosphors. Structural analysis confirms the hexagonal β-phase for all the nanophosphors except NaSc 0.78 Yb 0.2 Er 0.02 F 4 .Upconversion emission studies show emission bands corresponding to the 2 H 11/2 → 4 I 15/2 , 4 S 3/2 → 4 I 15/2 , and 4 F 9/2 → 4 I 15/2 transitions of Er 3+ ions. Upconversion emission intensities and greento-red emission intensity ratios strongly depend on Yb 3+ concentration and phonon energy of the hosts. The temperature-dependent population distribution between the thermally coupled levels, 2 H 11/2 and 4 S 3/2 , was used for upconversion-based thermometry. The plot of temperature sensitivity as a function of absolute temperature shows a linear variation for all these nanophosphors. The NaSc 0.78 Yb 0.2 Er 0.02 F 4 shows the lowest temperature sensitivity, possibly because of the higher phonon energy of this host. Thus, the findings of this work provide valuable insights into the role of sensitizer and host matrix in designing efficient upconversion nanothermometers.