Dual-mode up/down-conversion optical thermometry of Pr<sup>3+</sup>-regulated Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub>:Er<sup>3+</sup> phosphors

Abstract

Photothermal sensing is crucial in developing smart wearable devices. However, designing and synthesizing luminescent materials with suitable multi-wavelength emission and constructing multiple sets of probes in a single material system is a huge challenge for constructing sensitive temperature sensors with a wide temperature range. In this paper, Pr<sup>3+</sup>, Er<sup>3+</sup> single-doped and double-doped Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub> phosphors are successfully prepared by high temperature solid phase method, and their structures, morphologies, excitation wavelengths and temperature-dependent fluorescence properties are characterized by XRD, SEM, fluorescence spectrometer and self-made heating device. Firstly, the photoluminescences of the synthesized series of samples are investigated. The results show that comparing with the single-doped Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub>: Er<sup>3+</sup> sample, the up/down-conversion spectra of Pr<sup>3+</sup>, Er<sup>3+</sup> co-doped phosphors under 808 nm/380 nm excitation show that the green fluorescence emission of Er<sup>3+</sup> is enhanced. In addition, under 980 nm excitation, Pr<sup>3+</sup> can effectively regulate the fluorescence energy level population pathway, so that the electrons are more effectively arranged in the <sup>2</sup>H<sub>11/2</sub> and <sup>4</sup>S<sub>3/2</sub> energy levels in the excitation process. The red emission is weakened and the green emission is enhanced, which improves the signal resolution of the fluorescent material and has a significant influence on the optical temperature measurement. Secondly, the up-conversion fluorescence property of Er<sup>3+</sup> under 808 nm/980 nm laser excitation in Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub>:Er<sup>3+</sup> and Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub>:Pr<sup>3+</sup>,Er<sup>3+</sup> phosphors are investigated. The results show that the red and green fluorescence emissions of Er<sup>3+</sup> are two-photon processes. Finally, the up/down-conversion dual-mode temperature sensing properties of Er<sup>3+</sup> in Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub>:Er<sup>3+</sup> and Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub>:Pr<sup>3+</sup>, Er<sup>3+</sup> phosphors are investigated. It is found that both materials have good optical temperature measurement performances. The Pr<sup>3+</sup> doping optimizes the dual-mode optical temperature measurement performances of Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub>:Er<sup>3+</sup> phosphors derived from the thermal coupling energy level of Er<sup>3+</sup> ions. In addition, the up/down-conversion fluorescence mechanism of Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub>:Er<sup>3+</sup> and Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub>:Er<sup>3+</sup>, Pr<sup>3+</sup> phosphors are proposed, and the enhanced green fluorescence by Pr<sup>3+</sup> co-doping is attributed to the energy transfer from Pr<sup>3+</sup> ions to Er<sup>3+</sup> ions, leading to the increase of green fluorescence level population and the decrease of red fluorescence level population of the Er<sup>3+</sup> ions. This new dual-mode optical temperature measurement material provides a material basis and optical temperature measurement technology for exploring other temperature measurement materials.