Ho3+-Mediated NIR-II Ratio Thermometer Based on Phonon-Assisted Energy Transfer

Abstract

The development of high performance near-infrared II (NIR-II, 1000-1700 nm) luminescent thermometers presents significant challenges in balancing thermal quenching effects while achieving superior temperature sensitivity for advanced applications in bioimaging and optical sensing. Herein, by engineering dual energy transfer channels (Yb³⁺→Er³⁺→Ho³⁺ and Yb³⁺→Ho³⁺→Tm³⁺), we achieve Ho³⁺-mediated NIR-II ratio thermometers in the higher phonon energy of KLu(MoO₄)₂ based on phonon-assisted energy transfer (PAET), exploiting the strong temperature-dependence downshifting luminescence (Er³⁺: ⁴I_13/2 → ⁴I_15/2 transition, ~1550 nm; Ho³⁺: ⁵I₆ → ⁵I₈ transition, ~1200 nm; Tm³⁺: ³F₄ → ³H₆ transition, ~1617 nm). The rate equations reveal that the phonon energy of the host (~870 cm⁻¹) critically enhances the non-radiative (NR) transitions, allowing efficient population redistribution in the excited state of the lanthanides through PAET processes. Finally, the optimized KLu(MoO₄)₂: 12Yb³⁺, 0.2Ho³⁺, 0.4Tm³⁺ system exhibits exceptional ratiometric thermometric performance with a maximum relative sensitivity (S_r) of 0.45% K^-1 at 313 K. This study highlights the critical role of the multiple phonon mode of the host material in both boosting the downshifting luminescence and providing valuable insights for the design of NIR-II ratiometric luminescence thermometers.