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

Abstract

To ameliorate the inherent thermal quenching behaviors of upconverting materials, a series of Ho 3+ /Yb 3+ -codoped Al 2 Mo 3 O 12 (i.e., Al 2 Mo 3 O 12 :Ho 3+ /2xYb 3+ ) microparticles were developed. Upon excitation at 980 nm, intense upconversion (i.e., UC) emissions arising from Ho 3+ are observed, and their optimal states occur at x = 0.09. Besides, the UC mechanisms of these generated emissions from 5 F 4 / 5 S 2 and 5 F 5 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 5 F 4 / 5 S 2 levels decrease as the temperature increases, while that of the UC emission from the 5 F 5 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 Al 2 Mo 3 O 12 :Ho 3+ /2xYb 3+ 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 5 F 5 → 5 I 8 to ( 5 F 4 , 5 S 2 ) → 5 I 7 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 Al 2 Mo 3 O 12 :Ho 3+ /2xYb 3+ microparticles are promising luminescent materials for multimode visual optical thermometry.

Thermochromic Upconversion Emission in Tm3+/Yb3+-Codoped La2Mo3O12 Microparticles via Negative Thermal Expansion Engineering for Ultrahigh Sensitivity Optical Thermometry

“…It is presented in Figure d that all of the UC emission intensities are enhanced as pump power increases. As everyone knows, the following formula is able to be adopted to clarify the relation between laser pump power ( P ) and UC fluorescence intensity ( I ): , I ∝ P n 0.25em or 0.25em log ( I ) ∝ n 0.25em log ( P ) where n denotes the number of low-energy photons (i.e., NIR light) that are needed to promote the electrons to populate the desired excited levels. According to the measured UC emission spectra as well as eq , the double logarithmic relations between I and P are displayed in Figure e.…”

“…Significantly, although several heating–cooling processes have been carried out, the FIR values are scarcely changed, implying the high stability of developed microparticles. Moreover, the repeatability ( R ) can be calculated by utilizing the following formula: ,, R = [ 1 − max false| Q normala normalv normalg − Q i false| Q a v g ] × 100 % where Q i is the measured FIR value, and Q avg is associated with the mean FIR value. Thereby, the I 702 / I 475 and I 702 / I 651 combinations exhibit high R values of 99.1 and 98.4%, respectively, suggesting that the developed optical thermometers possess admirable reversibility.…”

“…Thereby, the I 702 / I 475 and I 702 / I 651 combinations exhibit high R values of 99.1 and 98.4%, respectively, suggesting that the developed optical thermometers possess admirable reversibility. Additionally, the temperature uncertainty or resolution (Δ T ) is also inspected, since it can reveal the thermal sensing accuracy of the thermometer, which is defined as follows: ,, normalΔ T = 1 S normalr δ FIR FIR δ FIR FIR = true( δ I 1 I 1 true) 2 + true( δ I 2 I 2 true) 2 where I 1 and I 2 are related to fluorescence intensities, δ I 1 and δ I 2 correspond to the errors of I 1 and I 2 , respectively. As a result, the Δ T value of the I 702 / I 475 combination is found to be changed in the range of 0.008–0.036 K with adjusting the temperature ranging from 303 to 583 K, whereas that of the I 702 / I 651 combination ranges from 0.015 to 0.068 K when the operating temperature is controlled in the range of 303–583 K, implying the high temperature resolution of resultant microparticles.…”

“…Upconverting compounds, where the rare-earth ions are employed as an activator and a sensitizer, have been widely concerned, since they have the capacity of changing the long-wavelength irradiation light into diverse short-wavelength emissions. Especially, Yb 3+ is used as a sensitizer owing to its intense absorption in near-infrared (NIR) light, while Nd 3+ , Ho 3+ , Er 3+ , Tm 3+ , etc., are selected as activators. − To develop novel upconverting materials, different types of hosts including fluoride, oxide, molybdate, silicate, etc., − have been employed. Unfortunately, the majority of designed rare-earth-ion-doped upconverting materials suffered from quenched upconversion (UC) emission intensities at high temperatures triggered by the thermal quenching effect, which astricts their feasibility in high-temperature monitoring.…”

Enhanced Sensitivity for Tm3+/Er3+‐Based Thermometer Through Dual‐Wavelength Excitation

“…[ 22–24 ] In contrast, relative sensitivity based on nonthermally coupled levels (NTCL) is independent of energy gap between them, because their temperature‐sensing behaviors are dominated by thermal quenching instead of the Boltzmann law. [ 25–27 ] As a result, temperature‐sensing behaviors based on NTCL are expected to achieve higher sensitivity.…”