High-sensitivity and wide-temperature-range dual-mode optical thermometry under dual-wavelength excitation in a novel double perovskite tellurate oxide

Optical behaviors of Mn4+-modified cubic type ZnTiO3:Eu3+ nanocrystals: Application in optical thermometers based on fluorescence intensity ratio and lifetime

Cubic type ZnTiO3 nanophosphors were fabricated by using hydrothermal method. The photoluminescence behaviors of Eu3+/Mn4+ co-doped ZnTiO3 crystals and Mn4+-modified ZnTiO3:Eu3+ crystals were investigated. The red and near-infrared emissions assigned to Eu3+ and Mn4+ were observed at the excitation wavelength of 465 nm for Eu3+/Mn4+ co-doped ZnTiO3 crystals and Mn4+ modified ZnTiO3:Eu3+ crystals. Eu3+/Mn4+ co-doped ZnTiO3 crystals revealed the most intense near-infrared emission assigned to Mn4+. For Mn4+-modified ZnTiO3:Eu3+ crystals, the red emissions assigned to Mn4+ revealed an obvious enhancement with the concentration of Mn4+ increasing from 0.01 mol% to 0.1 mol%, and then revealed a concentration quenching behavior at the Mn4+ concentration of 0.5 mol% and 1 mol%. Under 465 nm excitation wavelength, the emission assigned to 2Eg → 4A2g of Mn4+ in ZnTiO3 revealed stronger temperature dependence compared with that assigned to 5D0 → 7F2 transition of Eu3+. Based on the fluorescence intensity ratio between Eu3+ and Mn4+, the maximum relative sensitivity of 3.15 %/K at 305 K for the 0.05 mol% Mn4+ modified ZnTiO3:Eu3+ was achieved, which was higher than that of 2.9 %/K for Eu3+/Mn4+ co-doping ZnTiO3 nanocrystals. Based on the lifetime of the emission of Mn4+, the highest relative sensitivity of 1.4 %/K was obtained in the 0.05 mol% Mn4+ modified ZnTiO3:Eu3+, which was lower than that based on fluorescence intensity ratio. It indicated that the surface modification by transition metals should improve the temperatures sensing performance and had potential applications in optical thermometers.

Designing dual-emission phosphors for temperature warning indication and dual-mode luminescence thermometry

Color tunable phosphors of Mn4+ and Tb3+ co-doped double-perovskite SrGdLiTeO6 (SGLT) were synthesized in this work. The crystal parameters and photoluminescence performances were investigated in detail. By taking advantage of the different thermal quenching strengths between Mn4+ and Tb3+ ions, the emission color of SGLT:0.7%Mn4+/1%Tb3+ changed from red to green, which could be used for high-temperature temperature warning indication. Moreover, according to the luminescence intensity ratio (LIR) technique, wide temperature-range optical thermometry was developed and further, the maximum relative sensitivity (SR1) value of the SGLT:0.7%Mn4+/5%Tb3+ phosphor was determined to be 1.49% K-1 at 560 K. On the other hand, the sensing properties were also analyzed based on the temperature-dependent lifetime method. The most interesting thing is that the maximum SR2 value reached 1.88% K-1 at 573 K. This work proved that the Mn4+ and Tb3+ co-doped double-perovskite SrGdLiTeO6 could be potentially used in temperature warning indication and high sensitivity luminescence thermometry.

Regulating Exciton De-Trapping of Te4+ -Doped Zero-Dimensional Scandium-Halide Perovskite for Fluorescence Thermometry with Record High Time-Resolved Thermal Sensitivity

Fluorescence thermometry has been propelled to the forefront of scientific attention due to its high spatial resolution and remote non-invasive detection. However, recent generations of thermometers still suffer from limited thermal sensitivity (Sr ) below 10% change per Kelvin. Herein, this work presents an ideal temperature-responsive fluorescence material through Te4+ -doped 0D Cs2 ScCl5 ·H2 O, in which isolated polyhedrons endow highly localized electronic structures, and the strong electron-phonon coupling facilitates the formation of self-trapped excitons (STEs). With rising temperature, the dramatic asymmetric expansion of the soft lattice induces increased defects, strong exciton-phonon coupling, and low thermal activation energy, which evokes a rapid de-trapping process of STEs, enabling several orders of magnitude changes in the fluorescence lifetime over a narrow temperature range. After regulating the de-trapping process with different Te4+ doping, a record-high Sr (27.36% K-1 ) of fluorescence lifetime-based detection is achieved at 325 K. The robust stability against multiple heating/cooling cycles and long-term measurements enables a low temperature uncertainty of 0.067 K. Further, the developed thermometers are demonstrated for the remote local monitoring of operating temperature on internal electronic components. It is believed that this work constitutes a solid step towards building the next generation of ultrasensitive thermometers based on low-dimensional metal halides.

Halide Double Perovskite Nanocrystals Doped with Rare-Earth Ions for Multifunctional Applications

Most lead-free halide double perovskite materials display low photoluminescence quantum yield (PLQY) due to the indirect bandgap or forbidden transition. Doping is an effective strategy to tailor the optical properties of materials. Herein, efficient blue-emitting Sb³⁺ -doped Cs₂ NaInCl₆ nanocrystals (NCs) are selected as host, rare-earth (RE) ions (Sm³⁺ , Eu³⁺ , Tb³⁺ , and Dy³⁺ ) are incorporated into the host, and excellent PLQY of 80.1% is obtained. Femtosecond transient absorption measurement found that RE ions not only served as the activator ions but also filled the deep vacancy defects. Anti-counterfeiting, optical thermometry, and white-light-emitting diodes (WLEDs) are exhibited using these RE ions-doped halide double perovskite NCs. For the optical thermometry based on Sm³⁺ -doped Cs₂ NaInCl₆ :Sb³⁺ NCs, the maximum relative sensitivity is 0.753% K^-1 , which is higher than those of most temperature-sensing materials. Moreover, the WLED fabricated by Sm³⁺ -doped Cs₂ NaInCl₆ :Sb³⁺ NCs@PMMA displays CIE color coordinates of (0.30, 0.28), a luminous efficiency of 37.5 lm W^-1 , a CCT of 8035 K, and a CRI over 80, which indicate that Sm³⁺ -doped Cs₂ NaInCl₆ :Sb³⁺ NCs are promising single-component white-light-emitting phosphors for next-generation lighting and display technologies.

Fluorescence Lifetime-Based Luminescent Thermometry Material with Lifetime Varying over a Factor of 50

Recently, the growing demand for temperature detection is pushing forward the flourishing development of noncontact optical thermometry. Herein, a new red phosphor Sr₂InTa_1-xO₆:xMn⁴⁺ (SIT:xMn⁴⁺) was first constructed and systematically investigated. Based on the fairly rapid decline of the lifetime from 0.403 to 0.008 ms by increasing the temperature from 25 to 450 K, a noncontact optical thermometer can be made from phosphor SIT:0.003Mn⁴⁺ with S_r = 1.396% K^-1 at 375 K and S_a = 0.0012 K^-1 at 300 K. Because the luminescence is based on the outermost 3d orbits of Mn⁴⁺, the lifetime of SIT:xMn⁴⁺ is quite sensitive to the temperature, leading to a rapid decline of the lifetime with the increase in temperature. Moreover, multiple rounds of variable-temperature studies were performed to demonstrate the stability and reversibility of SIT:0.003Mn⁴⁺. This work suggests that Mn⁴⁺-phosphors are promising candidates for application as optical thermometric material.

Enlarging Sensitivity of Fluorescence Intensity Ratio-Type Thermometers by the Interruption of the Energy Transfer from a Sensitizer to an Activator

The occurrence of energy transfer (ET) would enhance the luminescence of the activator but sacrifice that of the sensitizer. However, the novel Sm³⁺-doped Ca₂TbSn₂Al₃O₁₂ (CTSAO) phosphor reported here seems to be an exception. In the series of CTSAO:xSm³⁺ phosphors investigated, something unexpected occurs; the activator, Sm³⁺, did not gain any energy compensation from the sensitizer, Tb³⁺, when temperature increases. Instead, when the loss of Sm³⁺ luminescence accelerates, simultaneously, the loss of Tb³⁺ luminescence accordingly alleviates. By careful calculations on the ET efficiency of the CTSAO:0.06Sm³⁺ phosphor at different temperatures, it is surprisingly found that the efficiency keeps decreasing as temperature increases. It means that the Tb³⁺-Sm³⁺ energy transfer is capable of being interrupted by an increasing temperature. By simulation, it is found that the occurrence of thermal interruption of energy transfer benefits the achievement of a higher temperature sensing sensitivity. In this sense, making use of the thermal interruption of energy transfer could become a novel route for further design of the fluorescence intensity ratio-type luminescence thermometers.

Designing bifunctional platforms for LED devices and luminescence lifetime thermometers: a case of non-rare-earth Mn4+ doped tantalate phosphors

Non-rare-earth Mn⁴⁺ doped tantalate (Sr₂GdTaO₆) phosphors exhibiting deep-red emission were synthesized. Afterward, the phase structure, morphology, and optical properties (e.g., emission spectra, concentration quenching, decay curves, thermal stability, quantum yields, etc.) were systematically investigated. Under the optimal conditions, the Sr₂GdTaO₆:0.005Mn⁴⁺ phosphor showed an excellent color purity of 96.41% while the chromaticity coordinates were (0.721, 0.279). Besides, the optimal sample exhibited good thermal stability, and, hence, it can be packaged into light-emitting diode (LED) devices. Red-emitting LED devices could show strong far-red emission and could be suggested for plant cultivation lighting. On the other hand, white-emitting LED devices could find use in indoor illumination. Moreover, with the aid of temperature-dependent lifetime (TDL), a good relative sensing sensitivity (1.73% K^-1 at 453 K) of the luminescent thermometer was established. Herein, all the above findings suggested that Sr₂GdTaO₆:Mn⁴⁺ phosphors are a potential candidate for bifunctional platforms of solid-state lighting and luminescence lifetime thermometers.

Highly Sensitive Dual-Mode Optical Thermometry of Er3+/Yb3+ Codoped Lead-Free Double Perovskite Microcrystal

In this Letter, erbium (Er³⁺) and ytterbium (Yb³⁺) codoped perovskite Cs₂Ag_0.6Na_0.4In_0.9Bi_0.1Cl₆ microcrystal (MC) is synthesized and demonstrated systematically to the most prospective optical temperature sensing materials. A dual-mode thermometry based on fluorescence intensity ratio and fluorescence lifetime provides a self-reference and highly sensitive temperature measurement under dual wavelength excitation at a temperature from 300 to 470 K. Combined with the white-light emission derived from self-trapped excitons (STEs), the characteristic emission peak of Er³⁺ ions can be observed under 405 nm laser excitation. The fluorescence intensity ratio (FIR) between perovskite and Er³⁺ is used as temperature-dependent probe signal, of which maximum value for relative and absolute sensitivities reaches to 1.40% K^-1 and 8.20 × 10^-2 K^-1. Moreover, Er³⁺ luminescence becomes stronger with the feeding Yb³⁺ increasing under 980 nm laser excitation. The energy transfer of Er³⁺ and Yb³⁺ is revealed by power-dependent photoluminescence (PL) spectroscopy, and the involved upconversion mechanism pertains to the two-photon excitation process. The results reveal that the Er³⁺/Yb³⁺ codoped lead-free double perovskite MC is a good candidate for a thermometric material for the novel dual-mode design.