Yb3+/Tm3+ Boltzmann thermometers at room temperature – Origin and limits of their thermometric activity

The Yb³⁺/Tm³⁺ pair is a promising near-infrared luminescent thermometer, especially close to the physiological temperature, yet the mechanisms governing its thermometric behavior remain poorly understood. Here, we investigate the excited-state dynamics of (Gd,Y)₂O₃:Yb³⁺(15%),Tm³⁺(0.5–3%) phosphors synthesized by a modified Pechini method. Temperature-dependent emission measurements under 980 nm excitation reveal thermally coupled ³F_2,3 and ³H₄ levels, enabling Boltzmann-based temperature sensing with sensitivities of approximately 1% K⁼¹ near physiological temperatures. Despite the large energy gap (≈1890 cm^-1), the onset of thermometric activity (T_on) occurs around 300 K owing to nonradiative coupling mediated by forced electric dipole. However, the formally forbidden ³F_2,3→³H₆ emissions result in weak luminescence of the ³F_2.3 levels, thereby increasing temperature uncertainty and highlighting an intrinsic trade-off between sensitivity and precision. These findings provide design guidelines for improving the performance of near-infrared Yb³⁺/Tm³⁺ luminescent thermometers.