Enhanced optical temperature sensing and upconversion emissions based on the Mn2+ codoped NaGdF4:Yb3+,Ho3+ nanophosphor

Abstract

In this study, to explore new phosphors for temperature sensing with high detection sensitivity, Yb³⁺/Ho³⁺/Mn²⁺ doped hexagonal NaGdF₄ nanoparticles were designed. These nanoparticles were synthesized by a hydrothermal method while using oleic acid as both a stabilizing and a chelating agent. The morphology and the crystal size of the upconversion nanoparticles (UCNPs) were effectively controlled through the addition of Mn²⁺ dopant contents into the NaGdF₄:Yb/Ho system. Moreover, an enhancement in the overall upconversion luminescence (UCL) spectra of Mn²⁺ doped UCNPs for the NaGdF₄ host compared to Mn²⁺ free UCNPs was observed, which resulted from the close back-energy transfer between Ho³⁺ and Mn²⁺ ions (⁵F₃ and ⁵F₄,⁵S₂ (Ho³⁺) → ⁴T₁ (Mn²⁺) → ⁵F₅ (Ho³⁺)). The calculated color coordinates displayed that with the increase of pump power densities, the tendency of orange-red output turned toward the green region, revealing that the UC emission can be fine-tuned in a wide range of pump power densities. Importantly, the temperature sensitivity of NaGdF₄:Yb/Ho doped with Mn²⁺ based on thermally coupled levels (⁵F₄ and ⁵S₂) and non-thermally coupled levels (⁵F₅ and ⁵S₂) of Ho³⁺ was significantly enhanced compared to other jobs. The maximum sensitivity reached 0.0051 K⁻¹ at 303 K and 0.199 K⁻¹ at 523 K, respectively.