Erbium as an energy trap center for manipulating NIR-II luminescence of Ho³⁺ in fluoride towards phonon-based ratiometric thermometry

Thermal quenching has been known to entangle with luminescence naturally, which is primarily driven by a multi-phonon relaxation (MPR) process. Considering that MPR and the phonon-assisted energy transfer (PAET) process may interact cooperatively plays a critical role in conducting the thermal response of luminescence thermometry. Herein, an energy mismatch system of Yb 3+ / Ho 3+ / Er 3+ co-doped β -NaLuF 4 hollow microtubes was delicately proposed to combat thermal quenching of near-infrared (NIR)-II luminescence of Ho 3+ via premeditated Er 3+ -mediated PAET processes under 980 nm excitation. Meanwhile, the mechanism of anti-thermal quenching is attributed to the Er 3+ as an energy trap center to facilitate the PAET process, thereby enabling a considerable energy transfer efficiency of over 80% between Er 3+ and Ho 3+ without Yb 3+ ions as sensitizers. Leveraging the accelerated PAET process at increased temperature and superior emission, the phonon-tuned NIR-II ratiometric thermometers were achieved based on fluoride beyond the reported oxide host, enabling excellent relative sensitivity and resolution ( S r =0.57 % K −1 , δ T =0.77 K). This work extends the significant effect of PAET on overcoming the notorious thermal quenching, and offers a unique physical insight for constructing phonon-tuned ratiometric luminescence thermometry.