Manipulating Upconversion Emission and Thermochromic Properties of Ho³⁺/Yb³⁺-Codoped Al₂Mo₃O₁₂ Microparticles by...

To ameliorate the inherent thermal quenching behaviors of upconverting materials, a series of Ho³⁺/Yb³⁺-codoped Al₂Mo₃O₁₂ (i.e., Al₂Mo₃O₁₂:Ho³⁺/2xYb³⁺) microparticles were developed. Upon excitation at 980 nm, intense upconversion (i.e., UC) emissions arising from Ho³⁺ are observed, and their optimal states occur at x = 0.09. Besides, the UC mechanisms of these generated emissions from ⁵F₄/⁵S₂ and ⁵F₅ levels all pertain to a two-photon absorption process. Furthermore, modified thermal quenching performances are realized in the resultant microparticles, in which the intensities of the UC emissions arising from ⁵F₄/⁵S₂ levels decrease as the temperature increases, while that of the UC emission from the ⁵F₅ level increases and then decreases with the increase of temperature. The coexistence of nonradiative transition promoted crossrelaxation, and energy transfer routes can be responsible for the above phenomenon. By studying the diverse UC emission characteristics at high temperatures, we revealed the thermometric properties of Al₂Mo₃O₁₂:Ho³⁺/2xYb³⁺ microparticles, where their sensitivities can be regulated by selecting the spectral mode and dopant contents. According to the intensity ratio of the UC emissions originating from ⁵F₅ → ⁵I₈ to (⁵F₄,⁵S₂) → ⁵I₇ transitions at different temperatures, one obtains that the relative and absolute sensitivities of the developed compounds reach up to 0.464% and 0.1739 K^-1, respectively. Additionally, by the analysis of the thermochromic performances of final products, their thermometric characteristics were also investigated. Note that the environmental temperature is able to be facilely read out by distinguishing the emitting color. These results verify that the Al₂Mo₃O₁₂:Ho³⁺/2xYb³⁺ microparticles are promising luminescent materials for multimode visual optical thermometry.