Bridging Boltzmann and Non‐Boltzmann Optical Thermometry Enabled by Multimodal Perovskite Nanocrystals

ABSTRACT Multimodal luminescent nanomaterials that operate across different excitation regimes and biological optical windows open new opportunities for contactless temperature sensing in complex environments. Here, we present a multifunctional optical thermometry platform based on Na 0.5 Gd 0.5 TiO 3 perovskite nanocrystals co‐doped with Yb 3+ , Er 3+ , and Cr 3+ ions, integrating upconversion, downshifting, and near‐infrared persistent luminescence within a single nanocrystalline system. The developed materials exhibit multimodal emission spanning the UV–vis–NIR spectral range under excitation at 375, 975, and 1532 nm. Optical thermometry was demonstrated over a broad temperature range using both Boltzmann and non‐Boltzmann luminescence intensity‐ratio approaches, with particular emphasis on excitation and emission pathways within the biological windows. The incorporation of Cr 3+ ions plays a key role in enhancing near‐infrared emission and enabling persistent luminescence, thereby expanding the number of available thermometric channels. Notably, non‐Boltzmann thermometry under 1532 nm excitation— located within the third biological window—yields relative sensitivities up to 8.1% K −1 , exceeding previously reported values. Finally, temperature readout is demonstrated in biologically relevant media, including blood‐derived environments, confirming reliable operation under strong absorption and scattering conditions. Overall, this work establishes multimodal perovskite nanocrystals as a versatile nanophotonic platform that bridges Boltzmann and non‐Boltzmann optical thermometry across multiple excitation and emission regimes.