Optical thermometry using efficient upconversion luminescence in Er3+ self-sensitized NaYS2 under multi-wavelength excitation

Abstract

Herein, a series of Er³⁺ self-sensitized NaYS₂ phosphors are synthesized by the solid-gas reaction method for a novel upconversion luminescence thermometer. Er³⁺ possesses abundant excited state energy levels in the near-infrared region, enabling efficient upconversion luminescence by absorbing different near-infrared wavelength light. Compared to 980 nm excitation, the emission intensity is enhanced by nearly an order of magnitude under 1532 nm excitation, which can be attributed to the larger absorption cross-section of ⁴I_13/2 and stronger absorption efficiency for Er³⁺. Based on the luminescence intensity ratio technique, the optical thermometry behaviors of NaYS₂:Er³⁺ under different wavelength excitation are evaluated by employing the thermally coupled energy levels of ²H_11/2/⁴S_3/2. It can be deduced that the excitation wavelength has no significant effect on the temperature sensing parameters. Compared to other typical upconversion luminescence thermometers, NaYS₂:Er³⁺ thermometer exhibits not only excellent sensitivity performance but also high upconversion luminescence efficiency, which is expected to be applied in wide-temperature-range and highly-sensitive temperature sensing.