Adjustment of Bi3+ Luminescence and Thermal Quenching Properties by B'-Site Ion Substitution Strategy in Double Perovskite CaLaMgSb/TaO6:Bi3+ Phosphor.

Thermal quenching has always been one of the most difficult issues in creating high-quality phosphor conversion light-emitting diodes (pc-LED), and a family of strategies are urgently needed to improve the luminescence performance of phosphors at high temperatures. In this contribution, a novel B'-site substitution CaLaMgSbxTa1-xO6:Bi3+ phosphor was constructed using an ion substitution strategy in the matrix with a green activator Bi3+ and a novel double perovskite material. When Sb5+ replaces Ta5+, a surprising increase in luminescence intensity occurs and the thermal quenching properties are greatly improved. The shift of the Raman characteristic peak to a smaller wavenumber and the reduction of the Bi-O bond length confirm that the crystal field environment around Bi3+ changes, which has a substantial effect on the crystal field splitting and nepheline effect of Bi3+ ions, affecting the crystal field splitting energy (Dq). This results in a corresponding increase of the band gap and the thermal quenching activation energy (ΔE) of the activator Bi3+. From the perspective of Dq, the intrinsic relationships among the activator ion band gap, bond length, and Raman characteristic peak changes were analyzed, and a mechanism for regulating luminescence thermal quenching properties was constructed, which provides an effective strategy for improving the promising new materials such as double perovskite.

Thermal Quenching Mechanism of Metal-Metal Charge Transfer State Transition Luminescence Based on Double-Band-Gap Modulation.

Bi3+-related metal-to-metal charge transfer (MMCT) transition phosphors are expected to become a new class of solid-state luminescent materials due to their unique broadband long-wavelength emission; however, the main obstacle to their application is the thermal quenching effect. In this study, one novel thermal quenching mechanism of Bi3+-MMCT transition luminescence is proposed by introducing electron-transfer potential energy (ΔET). Y0.99V1-xPxO4:0.01Bi3+ (YV1-xPxO4:Bi3+) is used as the model; when the band gap of the activator Bi3+ increases from 3.44 to 3.76 eV and the band gap of the host YV1-xPxO4 widens from 2.75 to 3.16 eV, the electron-transfer potential energy (ΔET) decreases and the thermal quenching activation energy (ΔE) increases, which result in the relative emission intensity increasing from 0.06 to 0.64 at 303-523 K. Guided by density functional calculations, the thermal quenching mechanism of the Bi3+-MMCT state transition luminescence is revealed by the double-band-gap modulation model of the activator ion and the matrix. This study improves the thermal quenching theory of different types of Bi3+ transition luminescence and offers one neo-theory guidance for the contriving and researching of high-quality luminescence materials.

Regulating luminescence thermal quenching based on the synergistic effect of energy transfer and energy gap modulation.

Thermal quenching is the core challenge that hinders the application of luminescent materials. Herein, a synergistic mechanism involving energy transfer and energy gap modulation is proposed based on the local crystal field regulation around sensitizers. The substitution of coordination cation V5+/P5+ weakens the crystal field strength of the sensitizer Bi3+, and the weakening of crystal field splitting causes an increase in the 3P1 energy level, thus increasing its energy gap. Compared with the YVO4:Bi3+,Eu3+ phosphor, the thermal stability of the YV0.25P0.75O4:Bi3+,Eu3+ phosphor is significantly improved, and the relative emission intensity of Eu3+ continuously increases with heating and reaches 1.24 times the original intensity at 523 K and does not show a decreasing trend in the studied temperature range. The anti-thermal quenching performance is mainly attributed to the increasing thermal quenching activation energy (ΔE) of the sensitizer by energy gap modulation, which enhances the energy transfer to compensate thermal quenching. Based on the thermal quenching characteristics of the materials, an optical thermometer is designed. The maximum relative sensitivity (Sr) and absolute sensitivity (Sa) are as high as 1.74% K-1 and 0.59 K-1, respectively, and the minimum temperature resolution reaches 0.288 K. The synergistic effect between energy gap modulation of the sensitizer and energy transfer enables the regulation of thermal quenching. Thus, this study provides a new strategy for exploiting high-performance luminescent materials.

Optical thermometry based on europium doped self-activated dual-emitting LiCa3ZnV3O12 phosphor.

In this study, rare-earth-doped self-activated LiCa3ZnV3O12 (LCZV) vanadate phosphors were preparation by a high-temperature solid-state reaction. Their crystal structure, non-contact temperature sensing, and luminescence properties were studied deeply. Excited by ultraviolet light at 340 nm, the emission of [VO4]3- group and the Eu3+ ions were monitored. The highest strength emission peaks at 470 nm and 610 nm for [VO4]3- and Eu3+, respectively, provide favorable signal identification for estimating temperature. Due to thermal quenching behavior and energy transfer, the FIR (Fluorescence Intensity Ratio) from Eu3+ to [VO4]3- exhibits excellent sensitivity performance at 303 K - 523 K. In the meantime, the maximum absolute and relative sensitivities of the obtained phosphors are 0.0068 K-1 and 1.18 % K-1, which are overtopped to those reported previously. Furthermore, for the luminescent color of the CIE diagram with a strong temperature effect, the color coordinate could be verified from (0.2871, 0.3416) to (0.4121, 0.3420), which was matched well with the linear equation. Consequently, the Eu3+ doped LCZV phosphor not only can be used for high-temperature environmental safety signals but also is an extraordinary viable material in the field of optical sensing.

Tuning of the thermal quenching performance of Bi3+-doped scheelite Ca(Mo/W)O4 solid solution phosphors.

The utilization of phosphor materials has always been a significant challenge in terms of improving thermal quenching performance. In this work, the thermal quenching performance tuning mechanism which establishes the band gap and thermal quenching correlation patterns is proposed. The crystal field splitting energy Dq was decreased by changing the surrounding crystal lattice environment of Bi3+ through a solid solution replacement, and the thermal quenching activation energy ΔE of Bi3+ was tuned from 0.117 eV to 0.182 eV accordingly. At 423 K, the luminous intensity increases from 0.101 to 0.396 of the preliminary intensity at 303 K with increasing substitution. In addition, the band gap value of Bi3+ calculated by diffuse reflectance spectroscopy increased from 4.40 eV to 4.72 eV, which corresponds to a linear positive correlation between the band gap and the thermal quenching properties. Furthermore, a monophase white-emitting phosphor with good thermal stability was prepared by constructing a Bi3+-Eu3+ co-doping system. In particular, the relative sensitivity of Sr for temperature measurement applications reached 3.17% K-1 based on the double-luminescence fluorescence intensity ratio. Thus, this modulation scheme can be used as a reference for the design of various phosphor materials with tunable thermal quenching properties in the future.