Host-dependent structural and optical properties and upconversion ratiometric thermometry of Yb3+/Er3+-doped NaYF4, NaGdF4, NaScF4nanophosphors.

This work presents a systematic study of the structural, morphological, and optical properties, along with the upconversion ratiometric thermometry of NaGd0.78Yb0.2Er0.02F4, NaY0.88Yb0.1Er0.02F4, NaY0.78Yb0.2Er0.02F4, and NaSc0.78Yb0.2Er0.02F4nanophosphors. Structural analysis confirms the hexagonalβ-phase for all the nanophosphors except NaSc0.78Yb0.2Er0.02F4. Upconversion emission studies show emission bands corresponding to the2H11/2→4I15/2,4S3/2→4I15/2, and4F9/2→4I15/2transitions of Er3+ions. Upconversion emission intensities and green-to-red emission intensity ratios strongly depend on Yb3+concentration and phonon energy of the hosts. The temperature-dependent population distribution between the thermally coupled levels,2H11/2and4S3/2, 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 NaSc0.78Yb0.2Er0.02F4shows 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 the sensitizer and host matrix in designing efficient upconversion nanothermometers.