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) Sc₂Mo₃O₁₂:Er³⁺/Ho³⁺ nanocrystals is reported. Mechanistic investigations affirm that Ho³⁺ ion can work as an energy reservoir and back-transfer to Er³⁺ ion with the assistance of lattice phonon at elevated temperatures. Moreover, the Ho³⁺-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 Er³⁺ and Ho³⁺ 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.