LaOF:Yb³⁺, Er³⁺ Upconversion Nanophosphors Operating at Low Laser Powers for Nanothermometry Applications

Luminescence nanothermometers based on LaOF:Yb3+, Er3+ upconversion nanoparticles (UCNPs) were investigated in this study. The fluorescence intensity ratio (FIR) of the UCNPs was recorded in the visible (red) and near-infrared (NIR) regions at different temperatures. The impact of laser heating on the UCNPs was prevented by identifying the optimal operating conditions of the laser that could be used to record the FIRs without compromising the integrity of the UCNPs. The behavior of the UCNPs against the laser exposure time and power was analyzed to recognize the critical point of the laser power, below which laser heating of the UCNPs was negligible. This approach can be perceived as a benchmark for testing luminescent materials using nanothermometry