Addressing Discrepancies between Theory and Experiments in Boltzmann Luminescence Thermometry with Ln³⁺ Ions

High Resolution Image Download MS PowerPoint Slide Trivalent lanthanide ion-doped nanoparticles are widely employed as nanoscale thermometers, driving rapid advancements in real-world applications. When using the Luminescence Intensity Ratio (LIR) technique, a calibration process is often required to determine macroscopic calibration parameters. However, despite extensive studies from various research groups, significant discrepancies are observed among the reported values, even for identical Ln 3+ –host systems under similar experimental conditions. Also, in many cases, the obtained parameters substantially differ from their microscopic counterparts, which is commonly overlooked in the literature. This study addresses sources of these inconsistencies by providing fundamental theoretical insights into the measurement process. We demonstrate that the thermalization of the electronic population within the Stark sublevels of a given manifold plays a crucial role in the LIR’s temperature dependence and thereby in measuring the macroscopic parameters. This fact has direct implications on developing LIR-based primary thermometers. Additionally, we show that pathways disrupting Boltzmann thermalization, influenced by experimental conditions, such as excitation power and cross-relaxation, also affect the evaluation of the macroscopic quantities. These findings contribute to a more robust theoretical framework for interpreting and understanding ratiometric Boltzmann luminescence thermometry experiments.