Colloquium: Investigation of Rare-Earth Ions Activated NaLa(MoO4)2 Phosphors for LEDs and Temperature Sensing Application

The standard of living of humanity can be sustained and improved by a noteworthy improvement in energy efficiency because of the scarcity of fossil fuels and expected climate change. Owing to the growth in population, pollution, and climate change, energy consumption is increasing day by day and it is expected that it would be double by the end of 2050 globally.¹ One part of energy is utilized for the lighting and operation of display devices. Thus, the improvement of economical light sources for example white-light-emitting diodes (WLEDs) and display devices with high efficiency is in urgent need. However, pc-WLEDs based on YAG:Ce³⁺ phosphor suffer low color rendering index (CRI) and high correlated color temperature (CCT) values.² Solar cells may fulfil a little bit of the demand for energy by utilizing solar energy, however its efficiency also needs to be enhanced significantly. One of the main challenges for the production of solar cells at commercial level is its cost and this challenge could be overcome by enhancing the conversion efficiency. Therefore, to address this problem, luminescent materials are essential due to their wide-ranging applications, including lighting, solar cells, sensors, and biomedical imaging.³ In my thesis we explore the synthesis of rare-earth (RE) ions-activated NaLa(MoO₄)₂ phosphors for multifunctional applications, including solid-state lighting, luminescent solar concentrators (LSC), and luminescent thermometers. These materials leverage various luminescent phenomena such as downconversion (DC), quantum cutting (QC), and upconversion (UC) to tackle the aforementioned issues.⁴ NaLa(MoO₄)₂ phosphors doped with RE ions have attracted significant interest due to their high luminescence efficiency, ease of synthesis, and strong thermal and chemical stability. Co-doping with Li⁺, Ca²⁺, and Bi³⁺ ions further enhance emission intensity by increasing asymmetry in the crystal field around the RE ions.^5,6 Inorganic phosphor materials also play a crucial role in luminescence thermometry, addressing the limitations of conventional contact-based thermometers, such as electromagnetic interference, slow response time, low accuracy, and limited sensitivity. Specifically, NaLa(MoO₄)₂ doped with Tm³⁺/Ho³⁺/Yb³⁺phosphors is utilized for dual-mode light emission in color-tunable lighting applications and luminescence intensity ratio (LIR) based on non-thermally coupled energy levels (NTCLs) for optical temperature sensing.⁷ Moreover, the dual-mode light emission through stokes and antistokes emission processes from Er³⁺ doped NaLa(MoO₄)₂ have been studied for optical thermometers in a broad temperature range from cryogenic to above room temperature based on thermally coupled levels (TCLs) and non-thermally coupled energy levels (NTCLs).⁸ Additionally, multimodal light emissions from Pr³⁺/Yb³⁺-doped NaLa(MoO₄)₂ phosphors are explored for their potential in multifunctional applications.⁹