Phonon-assisted upconversion luminescence thermal enhancement of NaYS₂:Yb³⁺,Nd³⁺ for optical temperature sensing

Upconversion luminescence presents obvious advantages in optical temperature sensing, nevertheless the application in complex scenarios is limited by luminescence thermal quenching. Herein, NaYS₂:Yb³⁺,Nd³⁺ phosphors are synthesized by the solid-gas reaction method and used for temperature sensing. A series of emissions for Nd³⁺ from visible to near-infrared region are achieved based on energy transfer from Yb³⁺ to Nd³⁺ under 980 nm excitation. With elevating the temperature, significant thermal enhancement effect of nearly three orders of magnitude is detected in near-infrared emission of ⁴F_7/2 → ⁴I_9/2, which is attributed to phonon-assisted energy transfer of Yb³⁺→Nd³⁺ and the corresponding thermal population effect. The thermal behaviors of thermally coupled energy levels for ⁴G_7/2/²G_9/2 and ⁴F_7/2/⁴F_5/2 in NaYS₂:Yb³⁺,Nd³⁺ are evaluated by luminescence intensity ratio technique. Remarkably, the optical thermometer based on the enhanced emission of ⁴F_7/2/⁴F_5/2 shows excellent temperature measurement performance, which is expected to be applied in wide-temperature-range and highly-sensitive temperature sensing. These results not only provide a pathway to realize high performance temperature sensing, but also heighten the understanding of UC emission thermal enhancement.