Three-energy-level-cascaded strategy for a more sensitive luminescence ratiometric thermometry

Luminescence ratiometric thermometry has gained increasing attention in recent years because of its several merits, such as non-invasive operating mode, fast response time, excellent spatial resolution, and so forth. It can be realized by using the thermally coupled energy levels (TCELs) of rare earth ions. The maximum thermal sensitivity for the TCELs-based luminescence ratiometric thermometry has been found to be ∼2000/kT². Here we show that using a three-energy-level-cascaded strategy can surpass this maximum value. In scheelite host material, the ⁴G_11/2-⁶H_15/2 transition of Dy³⁺ ion was observed successfully upon UV excitation. Moreover, the Dy³⁺ ion's ⁴G_11/2/⁴I_15/2/⁴F_9/2 excited states were confirmed to be thermally linked with each other by studying on the photoluminescence of the as-prepared Dy³⁺-embedded phosphors at different temperatures. By utilizing the ⁴G_11/2/⁴F_9/2 states, rather than the commonly investigated ⁴I_15/2/⁴F_9/2 ones in the previous literatures, the relative sensitivity was found to as high as 2414/kT² over the experimental temperature range. It surpasses the theoretical maximum value 2000/kT². In the end, we study the relationship between the relative sensitivity and temperature resolution, and find that a higher relative sensitivity cannot always ensure a better temperature resolution. In order to achieve a better temperature resolution in practice, both the relative sensitivity and the relative uncertainty of the luminescence intensity ratio that is used for indicating temperature should be considered simultaneously.