Enhancing upconversion emissions and temperature sensing properties by incorporating Mn2+ for KLu2F7:Yb3+/Er3+ nanocrystals based on thermally and non-thermally coupled levels

Abstract

Upconversion (UC) nanocrystal phosphors have received increasing attention for designing optical temperature sensors. Developing an efficient UC phosphor with superior temperature sensing properties remains a challenging task. Herein, we report dual-emitting and thermochromic Mn²⁺ co-doped KLu₂F₇:Yb³⁺/Er³⁺ nanocrystals, synthesized by a modified hydrothermal method. The marked influence of Mn²⁺ on the UC luminescence and thermal sensing performances of the KLu₂F₇:Yb³⁺/Er³⁺ nanocrystals was shown. The introduction of Mn²⁺ into the KLu₂F₇:Yb³⁺/Er³⁺ nanocrystals resulted in a slight change in crystal structure and morphology. The Mn²⁺ ions were shown to improve the UC emission, which might be attributed to the distortion of the local symmetry. Moreover, the optical temperature sensing properties of the nanocrystals were investigated. Based on the fluorescence intensity ratio of the thermally coupled levels and non-thermally coupled levels corresponding to the Arrhenius equation, the thermal sensing behavior of KLu₂F₇:Yb³⁺/Er³⁺ and KLu₂F₇:Yb³⁺/Er³⁺/Mn²⁺ was studied in the range of 303–543 K. Compared with the previous study, the KLu₂F₇:Yb³⁺/Er³⁺/Mn²⁺ nanocrystals exhibited outstanding temperature sensing properties based on non-thermally coupled levels. This study not only discussed the influence of Mn²⁺ doping on KLu₂F₇:Yb³⁺/Er³⁺ nanocrystals working as nanothermometers, but also provided a general approach to enhance the properties of optical temperature sensors by introducing non-thermally coupled levels.