Thermal Enhancement of Er³⁺ NIR‐II Luminescence by Ho³⁺‐Mediated Energy‐Trapping in Negative Thermal Expansion Nanocrystals

Thermal quenching of luminescence materials poses a major obstacle to the technological application of luminescence thermometry. It still remains challenging to attain thermally enhanced light emissions, especially in the second near‐infrared window (NIR‐II). Herein, an anomalous thermal dependence of NIR‐II luminescence in the negative thermal expansion (NTE) Sc 2 Mo 3 O 12 :Er 3+ /Ho 3+ nanocrystals is reported. Mechanistic investigations affirm that Ho 3+ ion can work as an energy reservoir and back‐transfer to Er 3+ ion with the assistance of lattice phonon at elevated temperatures. Moreover, the Ho 3+ ‐mediated energy feedback is strengthened by the thermal contraction between dopant ions, thereby enabling a remarkable thermal enhancement of NIR‐II emission over 11‐fold. The opposite thermal response of Er 3+ and Ho 3+ emissions is harnessed for NIR‐II ratiometric thermometry, registering exceptional performance in a high‐temperature regime ( S r = 1.71% K −1 , δT = 0.2 K at 513 K). These findings may inspire new insights for addressing the thermal quenching of NIR‐II luminescence, which also raises exciting opportunities for flexible thermometry in complex settings.