Manipulating Upconversion Emission and Thermochromic Properties of Ho3+/Yb3+-Codoped Al2Mo3O12 Microparticles by Negative Lattice Expansion for Multimode Visual Optical Thermometry

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.