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

TL;DR

This paper provides a theoretical analysis of discrepancies in lanthanide-based luminescence thermometry, highlighting the importance of electronic thermalization and experimental conditions for accurate temperature measurements.

Contribution

It introduces a fundamental theoretical framework that explains sources of inconsistencies in calibration parameters and guides the development of more reliable primary thermometers.

Findings

Thermalization of electronic Stark sublevels influences LIR temperature dependence.

Disruptions in Boltzmann thermalization cause significant measurement errors.

Understanding these effects improves the interpretation of luminescence thermometry data.

Abstract

Trivalent lanthanide ion-doped nanoparticles are widely employed as nanoscale thermometers, driving rapid advancements in real-world applications. When the Luminescence Intensity Ratio (LIR) technique is used, these thermometric systems typically require a calibration process to obtain 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 calibration parameters substantially differ from their microscopic counterparts, which is commonly ignored in the literature. This study addresses some sources for 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 consequently in measuring the macroscopic parameters. As a result, attempts to construct primary thermometers without prior calibration can result in temperature measurement errors exceeding 20 K. Additionally, we show that pathways disrupting Boltzmann thermalization, influenced by experimental conditions, 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, also paving the way for developing more accurate and reliable primary thermometers.