Highly sensitive optical thermometers based on unconventional thermometric coupled levels of Tm3+ following a Boltzmann-type distribution in oxyfluoride glass ceramics†
Oxyfluoride glass ceramics (GCs) containing β-PbF2 nanocrystals (NCs) doped with Tm3+ and Yb3+ were synthesized by a conventional melt-quenching method and a subsequent glass crystallization route. In β-PbF2:Tm3+/Yb3+ GCs, Tm3+/Yb3+ is surrounded by fluoride with a low phonon energy, ensuring its intense emissions at 362 (1D2 → 3H6), 450 (1D2 → 3F4), 478 (1G4 → 3H6), 648 (1G4 → 3F4), 700 (3F2,3 → 3H6) and 800 (3H4 → 3H6) nm excited by 976 nm laser. Furthermore, temperature-dependent upconversion luminescence (UCL) behaviors of β-PbF2:Tm3+/Yb3+ GCs were systematically investigated to explore the thermometric performance with the assistance of the fluorescence intensity ratio (FIR) method by employing conventional TCELs of 3F2,3/3H4 following a Boltzmann distribution and two unconventional coupled levels of 3F2,3/1D2 and 3F2,3/1G4 governed by a Boltzmann-type distribution. Impressively, the relative sensitivity (Sr) of 3F2,3/1D2 states (I700nm/I362nm), 3F2,3/1G4 states (I700nm/I478nm), and 3F2,3/3H4 states (I700nm/I800nm) was obtained in this work as values of 1.30% K−1, 1.38% K−1, and 0.87% K−1, respectively. Compared with the conventional TCELs of 3F2,3/3H4, the two unconventional coupled levels of 3F2,3/1D2 and 3F2,3/1G4 present preferable properties as temperature probes enabling optical ratiometric thermometers. The research in this work provides an important advance in exploring other innovative FIR methods for optical thermometers to enable an intense and precise probing signal and detect temperature accurately.