KLaP₄O₁₂:Tb³⁺ Nanocrystals for Luminescent Thermometry in a Single-Band-Ratiometric Approach

One of the challenges currently facing luminescent thermometry is providing an in-depth analysis of thermally dependent processes occurring in the studied type of nanothermometer. By understanding all possible thermal phenomena, the properties of a given kind of nanosensor can be intentionally improved. Verification of the existing theories on a nanometric scale is particularly important. In this work, a comprehensive characterization of the structural and optical properties of nanocrystalline KLaP4O12 doped with different concentrations of Tb3+ ions was performed. It was shown that Tb3+ ions excited using two distinct wavelengths matched to the ground- and excited-state absorption show luminescences of opposite thermal dependencies. This enabled the single-band-ratiometric (SBR) thermometric approach to be implemented. The experimentally obtained results turned out to be consistent with the proposed form of state equations describing the dynamics of populating Tb3+ energy levels as a function of the temperature. As reported here, the first SBR nanothermometer based on Tb3+ ion emission stands out because of its nanometric size and high relative sensitivity, reaching 5%/°C at 0 °C. Finally, on the basis of modeling changes in the individual parameters in the state equations, their impact on the results obtained for SBR thermometers based on Tb3+ ions was widely discussed.