Nd 3+ -Mediated Phonon Engineering in Fluoride Hosts: Toward Zero-Thermal Quenching NIR-II Luminescence and High-Sensitivity Thermometry

Temperature manipulates the radiative process of near-infrared (NIR) emitting materials, which is essential for constructing luminescence thermometry. However, an elevated temperature usually causes thermal quenching of luminescence, restricting the sensitivity of thermometers. Herein, an approach to populating the excited state of lanthanides at elevated temperatures is introduced, leading to zero-thermal quenching of the NIR-II emission (∼1530 nm) of Er³⁺ in β-NaLuF₄ via Nd³⁺-mediated phonon-assisted energy transfer under 808 nm excitation. Notably, the phonon engineering depends on the distance between the interacting ions, enabling unique NIR-II emission originating from the ⁴I_11/2 → ⁴I_15/2 transition (∼984 nm) of Er³⁺. Additionally, the enormously suppressed emissions of Nd³⁺ and unusually enhanced emissions of Er³⁺ are exploited to construct ratiometric NIR-II thermometers, which achieves exceptional thermal sensitivity and resolution (S_r = 1.23% K^-1, δT = 0.23 at 313 K). By leveraging this anomalous optical response to temperature, we further demonstrated the broad applicability of the moderate mismatch energy level strategy across micro/nano-fluoride hosts. These findings not only offer valuable insights into the design of zero-thermal quenching of NIR-II luminescence materials, but also open up promising avenues for developing high-performance NIR-II ratiometric thermometers in advanced photonics applications.