Dual-mode up/down-conversion optical thermometry of Pr³⁺-regulated Li_0.9K_0.1NbO₃:Er³⁺ phosphors

Photothermal sensing is crucial for the development of 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, Pr³⁺, Er³⁺ single-doped and double-doped Li_0.9K_0.1NbO₃ phosphors were successfully prepared by high temperature solid phase method, and their structure, morphology, excitation wavelength and temperature-dependent fluorescence properties were characterized by XRD, SEM, fluorescence spectrometer and self-made heating device. Firstly, the photoluminescence of the synthesized series of samples was investigated. The results show that compared with the single-doped Li_0.9K_0.1NbO₃: Er³⁺sample, the up/down-conversion spectra of Pr³⁺, Er³⁺ co-doped phosphors under 808/380 nm excitation show that the green fluorescence emission of Er³⁺ is enhanced. In addition, under 980 nm excitation, Pr³⁺ can effectively regulate the fluorescence energy level population pathway, so that the electrons are more effectively arranged in the ²H_11/2 and ⁴S_3/2 energy levels during 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 impact on the optical temperature measurement. Secondly, the up-conversion fluorescence property of Er³⁺ under 808/980 nm laser excitation in Li_0.9K_0.1NbO₃:Er³⁺and Li_0.9K_0.1NbO₃:Pr³⁺,Er³⁺phosphors were investigated. The results show that the red and green fluorescence emissions of Er³⁺ are two-photon processes. Finally, the up/down-conversion dual-mode temperature sensing properties of Er³⁺ in Li_0.9K_0.1NbO₃:Er³⁺ and Li_0.9K_0.1NbO₃:Pr³⁺,Er³⁺ phosphors were investigated. It was found that both materials have good optical temperature measurement performance. Pr³⁺ doping optimizes the dual-mode optical temperature measurement performance of Li_0.9K_0.1NbO₃:Er³⁺ phosphors derived from the thermal coupling energy level of Er³⁺ions. In addition, the up/down-conversion fluorescence mechanism of Li_0.9K_0.1NbO₃:Er³⁺ and Li_0.9K_0.1NbO₃:Er³⁺,Pr³⁺ phosphors is proposed, and the enhanced green fluorescence by Pr³⁺ co-doping is attributed to the energy transfer from Pr³⁺ to Er³⁺ ions, leading to the increase of green fluorescence level population and the decrease of red fluorescence level population of the Er³⁺ ions. This new dual-mode optical temperature measurement material provides a material basis and optical temperature measurement technology for exploring other temperature measurement materials.