Noncontact optical thermometers based on the luminescence intensity ratio of two thermally coupled energy levels, exhibiting high sensitivity, excellent accuracy, fast response, and low environment dependence, have attracted great interests in scientific research, life activities, and industrial manufacturing processes. However, the use of optical thermometers in extreme atmospheres (below 150 K) is usually limited by the required large temperature activation because of the relatively big energy difference (200 cm-1 ≤ ΔE ≤ 2000 cm-1). Here, we propose a strategy to alleviate the ultralow temperature-sensing problem by exploiting and utilizing the near-infrared (NIR) thermally coupled Stark sublevels of Tm3+ (3H4|0 → 3H6/3H4|1 → 3H6, ΔE ≈ 300 cm-1) that is much sensitive to minimal temperature variation, especially at ultralow temperatures because of the tiny energy difference. The integration of ultralow temperature-sensitive Tm3+ ions and room-temperature-sensitive Er3+ ions in an ultrasmall α-NaYbF4:Tm3+@CaF2@NaYF4:Yb3+/Er3+@CaF2 core/multishell nanoparticle (∼15 nm) as a dual-mode upconversion luminescent nanoprobe enables the broad-range temperature detection from 10 to 295 K. This structure induces ∼14 times NIR emission and ∼sixfold green upconversion luminescence output in comparison with the α-NaYbF4:Tm3+ core and α-NaYbF4:Tm3+@CaF2@NaYF4:Yb3+/Er3+ core/shell/shell nanoparticles. The maximum absolute and relative sensitivities of this dual-mode temperature sensor reach 0.67% and 3.06% K-1, respectively, showing the advantage of the concurrent utilization of the Tm3+ NIR 801/820 nm band ratio and the typical Er3+ visible 521/538 nm band ratio for a wide-range temperature-sensing purpose. This work provides a promising strategy to develop accurate and effective, contactless broad-range/ultralow temperature sensors.
Keywords: broad-range upconverting thermometry; dual-mode nanoprobe; luminescence intensity ratio; ultralow temperature.