Highly sensitive optical thermometers based on unconventional thermometric coupled levels of Tm3+ following a Boltzmann-type distribution in oxyfluoride glass ceramics

Abstract

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 (³F_2,3 → 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 ³F_2,3/³H₄ following a Boltzmann distribution and two unconventional coupled levels of ³F_2,3/¹D₂ and ³F_2,3/¹G₄ governed by a Boltzmann-type distribution. Impressively, the relative sensitivity (S_r) of ³F_2,3/¹D₂ states (I_700nm/I_362nm), ³F_2,3/¹G₄ states (I_700nm/I_478nm), and ³F_2,3/³H₄ states (I_700nm/I_800nm) 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 ³F_2,3/³H₄, the two unconventional coupled levels of ³F_2,3/¹D₂ and ³F_2,3/¹G₄ 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.