Thermally-coupled Levels Research Articles

Optical temperature sensing properties of novel RE3+ (RE=Er, Ho) doped BaLa2Ti3O10 phosphors

A novel luminescent material of BaLa2Ti3O10:RE3+ was synthesized by the high-temperature solid phase method, with a typical layered structure belonging to a special crystal system. Under the excitation of 980 nm, BaLa2Ti3O10:4%Er3+ exhibits characteristic emissions centered at 531 nm, 551 nm and 668 nm. It is noteworthy that the color of BaLa2Ti3O10:3%Ho3+ shows a strong green emission at 551 nm and a weak red emission near 664 nm, also represents adjustable yellow-green light with the change of 980nm laser pumped powers. The optical temperature sensing properties were checked by employing different strategies, relating to the thermally-coupled-levels (TCLs) and nonthermally-coupled-levels (NTCLs). The results show that the maximum relative sensitivity of TCLs based on BaLa2Ti3O10:Er3+ is 0.68% K-1(305 K). Similarly, the maximum sensitivity of TCLs is 0.89% K-1(313 K) in BaLa2Ti3O10:Ho3+, which performs well from 313 K to 513 K. It has been found the samples using FIRs of TCLs can produce higher absolute (Sa) and relative (Sr) sensitivities compared with those using Fluorescence Intensity Ratios (FIRs) of NTCLs. The multiple FIRs also achieved superior levels of temperature resolution and repeatability in all cases. Generally, this work provides favorable candidates for the temperature sensors.

Multi-Wavelength Excitable Multicolor Upconversion and Ratiometric Luminescence Thermometry of Yb3+/Er3+ Co-Doped NaYGeO4 Microcrystals.

Excitation wavelength controllable lanthanide upconversion allows for real-time manipulation of luminescent color in a composition-fixed material, which has been proven to be conducive to a variety of applications, such as optical anti-counterfeiting and information security. However, current available materials highly rely on the elaborate core-shell structure in order to ensure efficient excitation-dependent energy transfer routes. Herein, multicolor upconversion luminescence in response to both near-infrared I and near-infrared II (NIR-I and NIR-II) excitations is realized in a novel but simple NaYGeO4:Yb3+/Er3+ phosphor. The remarkably enhanced red emission ratio under 1532 nm excitation, compared with that under 980 nm excitation, could be attributed to the Yb3+-mediated cross-relaxation energy transfers. Moreover, multi-wavelength excitable temperature-dependent (295-823 K) upconversion luminescence realizes a ratiometric thermometry relying on the thermally coupled levels (TCLs) of Er3+. Detailed investigations demonstrate that changing excitation wavelength makes little difference for the performances of TCL-based ratiometric thermometry of NaYGeO4:Yb3+/Er3+. These findings gain more insights to manipulate cross-relaxations for excitation controllable upconversion in single activator doped materials and benefit the cognition of the effect of excitation wavelength on ratiometric luminescence thermometry.

Up-conversion optical temperature sensing characteristics of new KBaGd(MoO4)3:Yb3+,Ho3+ phosphors

To develop new up-conversion luminescent materials for non-contact optical thermometer with high sensitivity and temperature resolution, a battery of KBaGd(MoO4)3:Yb3+,Ho3+ phosphors were fabricated through solid reaction process. The crystal structure, up-conversion luminescence, energy transfer, thermal stability and optical temperature sensing performances were studied in detail. Under 980 nm laser excitation, the KBaGd(MoO4)3:Yb3+,Ho3+ phosphor exhibits distinctive emission bands of Ho3+ at 545, 660, and 755 nm, and excellent illuminant performance. Based on the thermally coupled levels (TCLs) of Ho3+, both the relative sensitivity (Sr) and absolute sensitivity (Sa) display similar change trends, with the highest values of 6.73%/K (@298 K) and 5.69%/K (@298 K), respectively. Furthermore, the highest Sa of 13.90%/K (@623 K) and the ultimate Sr of 0.62%/K (@ 298 K) are achieved based on non-TCLs of Ho3+. Therefore, KBaGd(MoO4)3:Yb3+,Ho3+ phosphor is a promising candidate for self-referenced optical thermometry.

Achieving Ultrahigh Thermal Stability in Cr3+-Activated Garnet Phosphors through Electron Migration between Thermally Coupled Levels.

Recently, Cr3+-activated near-infrared (NIR) phosphors have received much more attention due to their excellent photoluminescence (PL) properties. However, most of them suffer from poor thermal stability which limits further application. Herein, a novel Lu2CaGa4SnO12:Cr3+ phosphor with broadband NIR emission (λem = 750 nm) is synthesized successfully. Despite the good luminescence property, its PL intensity decreases obviously with temperature (I425K = 79%). To improve the thermal stability, a series of Lu2+xCa1-xGa4+xSn1-xO12:Cr3+ (x = 0-1.0) solid solutions with tunable thermal quenching performance have been designed. It is found that the fluorescence intensity ratio (FIR) of 4T2 → 4A2 to 2E → 4A2 [I(4T2)/I(2E)] transitions (i.e. electron occupation) decreases monotonously with increasing [Lu3+-Ga3+] co-substitution, resulting from a strengthened crystal field strength and increased energy difference between 4T2 and 2E energy levels. Benefiting from the various thermal population and energy difference Δ', the PL thermal quenching behavior of Lu2+xCa1-xGa4+xSn1-xO12:Cr3+ can be adjusted easily, and the corresponding mechanism is explored in detail. Most notably, the emission intensity of Lu2+xCa1-xGa4+xSn1-xO12:Cr3+ at 425 K can reach up to 142% compared with that at 300 K, which may be the best for Cr3+-activated NIR phosphors. This work may provide an alternative path for the development of thermally stable broadband NIR phosphors.

Enhancement of upconversion luminescence and temperature sensing performance in Er3+/Yb3+ doped Lu2O3 and Lu2O2S phosphors by incorporation of lithium ions

A series of spherical Er3+/Yb3+ doped Lu2O3 and Lu2O2S phosphors co-doping with varying concentrations of Li+ (0, 1, 3, 5, and 7 mol%) were synthesized by homogeneous precipitation method followed by direct calcination or sulfurization. The crystal structure and morphology of the as-prepared phosphors were characterized by XRD, FTIR and SEM. Meanwhile, the upconversion (UC) luminescence spectra were recorded under 980 nm laser excitation at temperatures ranging from 303 to 573 K. The incorporation of lithium ions led to an enhancement of UC luminescence intensity by about 15 times at room temperature. The introduction of Li+ could also affect the temperature sensitivity of the samples. Furthermore, optical temperature sensing of the phosphors was achieved by analyzing the fluorescence intensity ratio (FIR) of thermally coupled levels (TCLs) and nonthermally coupled levels (non-TCLs). Impressively, utilizing the non-TCLs approach resulted in maximum Sa values of 8.80 %K−1 and 11.06 %K−1 for the Er3+/Yb3+/5 % Li+ doped Lu2O3 and Lu2O2S phosphors, leading to an approximately 17-fold and 23-fold increase compared to that of TCLs-based approach, respectively. The results of this work demonstrate the great potential of Er3+/Yb3+/Li+ doped Lu2O3 and Lu2O2S phosphors for optical thermometers.

Investigation of Ca2Al2SiO7: Pr3+/Gd3+ UVB upconverting phosphor and its optical thermometric properties

On basis of fluorescence intensity ratio (FIR) over certain narrow coupled levels of rare-earth ions, up-conversion of RE-doped materials perform excellent temperature-sensing abilities for thermometry applications. A series of Pr3+/Gd3+ co-doped Ca2Al2SiO7 phosphors (Ca2Al2SiO7:Pr3+/Gd3+) were prepared by a conventional high-temperature solid-state reaction. Under excitation of blue ∼445 nm, the Ca2Al2SiO7:Pr3+/Gd3+ phosphor yields intense photoluminescence (PL) bands around 489, 528, 537, 580, 604, 613, and 646 nm due to electronic transition of 3P0 → 3H4, 3P0 → 3H5, 3P1 → 3H5, 3P1 → 3H6, 1D2 → 3H4, 3P0 → 3H6, and 3P0 → 3F2 of Pr3+, respectively. Their relative intensities are sensitively determined by Pr3+ contents, which, from 0.1 % to 5 %, tune the emission color from pink to warm white under excitation of 445 nm. It is of great interest that intense irradiation using a blue ∼445 laser does make efficient ultraviolet B (UVB) up-conversion at about 297, 306, and 313 nm, just stemming from the 6P3/2 → 8S7/2, 6P5/2 → 8S7/2, and 6P7/2 → 8S7/2 of Gd3+, respectively. A typical two-photon absorption process was proved to account for the UVB up-conversion, i.e. successive absorption of two blue ∼445 nm photons populating Pr3+ 4f5d excited state, which further efficiently transfer its energy to the 6IJ/6PJ states of Gd3+ neighbor for UVB emission. Moreover, the thermally coupled levels (TCLs) of Gd3+: 6P3/2-6P7/2, and Gd3+: 6P5/2-6P7/2 were thoroughly investigated for temperature sensing in 298–598 K. Maximum relative sensitivity of the 6P3/2-6P7/2 TCLs, i.e. Sr(I306/I313), is proved to be 0.96 % K−1 at 298 K, and that of the 6P5/2-6P7/2 TCLs, i.e. Sr(I297/I313), is 1 % K−1 at 373 K. The relevant thermal stabilities of the Ca2Al2SiO7:Pr3+/Gd3+ thermometric candidate was well validated with 10 cycles of heating and cooling processes in 298–598 K. Such continuous explorations of Pr3+/Gd3+ co-doping system will significantly promote the development of robust UVB up-converting phosphors as well as their practical applications in the field of high-resolution optical thermometry.

Multifunctional Optical Sensing with Lanthanide-Doped Upconverting Nanomaterials: Improving Detection Performance of Temperature and Pressure in the Visible and NIR Ranges.

Temperature and pressure are fundamental physical parameters in the field of materials science, making their monitoring of utmost significance for scientists and engineers. Here, the NaSrY(MoO4)3:0.02Er3+/0.01Tm3+/0.15Yb3+ nanophosphor is developed as an optical sensor material. Under 975 nm laser excitation, the upconversion characteristics and optical detection performance of the multifunctional sensing platform of temperature and pressure (vacuum) are investigated. We have successfully developed a novel detection platform that enables optical detection of pressure (vacuum) and temperature. This platform utilizes thermally coupled levels (TCLs) and non-TCLs of Er3+ and Tm3+ to achieve ratiometric detection. The multimodal optical temperature and pressure detection based on TCLs and non-TCLs is successfully realized by using different emission bands of double emission centers, which makes it possible for self-referencing optical temperature and pressure measurement modes. These results indicate that the developed nanophosphor is a promising candidate for optical sensors, and our findings suggest potential strategies for modulating the sensor properties of luminescent materials doped with rare-earth ions.

Multi-mode excited upconversion and downshifting of Ho3+/Yb3+ doped lead-free double perovskite Cs2NaBiCl6 for anti-counterfeiting and thermometry

Rear earth(RE)-doped halide perovskite has generated as an amazing optical material for multicolor luminescence. Free-lead halide double perovskites have attracted interesting because of their non-toxicity and higher instability. Here, RE3+ doped chloride-based double perovskite phosphors are reported. A series of heavy concentration Ho3+/Yb3+ doped Cs2NaBiCl6(CNBC) phosphors are prepared with a solvothermal route. The band structure of CNBC is calculated through density functional theory (DFT). The luminescence properties of CNBC:Ho3+ and CNBC:Ho3+/Yb3+ are investigated through upconversion(UC) and downshifting(DS) processes. From CNBC:Ho3+, the brightness orange, deep red and pale green lights are achieved under excitation at 365, 452 and 980 nm. The multicolor emissions of CNBC:Ho3+ are used to multimode fluorescent anti-counterfeiting. Comparing with CNBC:Ho3+ samples, the UC emission intensity of CNBC:Ho3+/Yb3+ are dramatically enhanced. The brightness green light can be seen by naked eyes under super lower power pumping(∼10 mW). The temperature sensing properties of CNBC:Ho3+/Yb3+ phosphor are investigated systematically based on the luminescence intensity ratio(LIR) of thermally coupled levels(TCLs)(5F1/5G6,5F2,3/3K8) and non-TCLs(5F4/5S2,5F5). The maximum relative sensitivity(SR) reaches 5.089% K−1 at 293 K according to the LIR of non-TCLs. The work will broaden the application field of RE3+-doped free-lead perovskite such as anti-counterfeiting and thermometry.

Advanced temperature sensing with Er3+/Yb3+ co-doped Ba2GdV3O11 phosphors through upconversion luminescence.

Optical thermometry is a non-contact temperature sensing technique with widespread applications. It offers precise measurements without physical contact, making it ideal for situations where contact-based methods are impractical. However, improving the accuracy of optical thermometry remains an ongoing challenge. Herein, enhancing the thermometric properties of luminescent thermometers through novel materials or strategies is crucial for developing more precise sensors. Hence, the present study focuses on the application of four-mode luminescence thermometric techniques in sol-gel synthesized Er3+/Yb3+ co-doped Ba2GdV3O11 phosphors for optical temperature sensing in the temperature range of 298-573 K. The upconversion (UC) luminescence is achieved under excitations of 980 nm or 1550 nm, resulting in bright yellow-green emission in the visible spectral range. Temperature sensing is realized by exploiting the UC emissions of 4S3/2, 2H11/2 and 4F7/2 bands, which represent intensity ratios of thermally coupled levels (TCELs) and non-thermally coupled levels (NTCELs) of Er3+/Yb3+, along with the emission lifetimes at 4S3/2. The relative sensitivity (Sr) values for TCELs exhibit a gradual decrease with rising temperature, reaching a maximum of 1.1% K-1 for 980 nm excitation and 0.86% K-1 for 1550 nm excitation at 298 K. Conversely, for NTCELs, the highest Sr value observed is 0.9% K-1 at 298 K for 1550 nm excitation. Moreover, the emission lifetimes at 4S3/2 yield notably high Sr values of up to 5.0% μs K-1 (at 425 K). Furthermore, the studied phosphors have a sub-degree thermal resolution, making them excellent materials for accurate temperature sensing. Overall, this study provides a promising new direction for the development of more precise and reliable optical thermometry techniques, which could have important implications for a range of scientific and industrial optical temperature sensing applications.

Multi-dimensional tailoring of the thermometric behaviors of Er3+/Yb3+-codoped Y2W3O12 polychromatic upconverting microparticles for multi-mode visual optical thermometry

To combat the thermal quenching and inadequate sensitivities of luminous materials, the Y1.98–2xW3O12:0.02Er3+/2xYb3+ (YWO:Er3+/2xYb3+; 0.01 ≤ x ≤ 0.11) microparticles with splendid negative thermal expansion (NTE) effect were designed. Excited at 980 nm, the produced samples emit polychromatic upconversion (UC) emissions triggered by the energy back transfer from Er3+ to Yb3+ and it has been systematically discussed via steady-state rate theory. The thermally enhanced UC emissions are accomplished in YWO:Er3+/2xYb3+ microparticles due to the distinctive NTE properties. Moreover, according to the temperature-associated UC emissions of Er3+ arising from its thermally coupled levels (TCLs) and non-thermally coupled levels (non-TCLs), the thermometric properties of synthesized microparticles are investigated. When non-TCLs are adopted, the absolute and relative sensitivities of resultant microparticles are 0.0129% and 1.7875% K−1, respectively. Furthermore, through manipulating doping content and spatial mode, the temperature sensing capacities of YWO:Er3+/2xYb3+ microparticles are multi-dimensionally regulated. In addition, contactless temperature recognition can also be realized through analyzing the thermochromic features of final products and its relative sensitivity is 2.1604% K−1. Ultimately, visual optical temperature detection is obtained in designed microparticles via its temperature-related emitting colour. Our findings may provide an important insight into the design of luminescent materials with high sensitivities for multi-mode visual optical thermometry.

Luminescent thermometry with YVO4:Er/Nd: Achieving high sensitivities within the 1st and 2nd biological windows

YVO4:Er3+, Nd3+ powders were prepared by the modified sol gel synthesis route using glucose as chelating agent and characterized by X-ray powder diffraction (XRD) and scanning electron microscope (SEM). Under 660 nm excitation, four infrared emission bands located between 790 and 845 nm, 870 and 930 nm, 950 and 1030 nm, and between 1050 and 1100 nm were observed, and were assigned the transitions 4F5/2 – 4I9/2 (Nd) and 4I9/2 – 4I15/2 (Er), 4F3/2 – 4I9/2 (Nd), 4I11/2 – 4I15/2 (Er), and 4F3/2 – 4I11/2 (Nd), respectively. The performance of these powders as optical temperature sensors using the luminescence intensity ratio (LIR) technique between thermally coupled levels (TCLs) and non-thermally coupled levels (non-TCLs) was studied within the temperature range of 293–373 K. Using different combinations of the emission intensities at 809, 880, 914 and 1003 nm, three distinct (LIR) temperature sensors schemes were built, with both excitation and emission within the first and second biological window. These LIRs relative sensitivity (Sr) and temperature uncertainties (δT) were investigated for samples with different Er and Nd concentrations. Sr values up to 2.08 ± 0.03%. K−1 and δT smaller than 1 K, were obtained. Repeatability and reproducibility tests demonstrated the reliability of the proposed LIRs.

Self-calibrated multi-mode optical thermometry in up-converting phosphor Cs2NaErCl6: Yb3+

With the development of temperature measurement technology, people put forward higher requirements for the accuracy and precision of temperature measurement. However, most of the current luminescent thermometers use a single optical signal for temperature detection, which has a certain range of temperature insensitivity, resulting in inaccurate temperature measurement. Here, an all-inorganic rare-earth halide double perovskite Cs2NaErCl6: Yb3+ (CNEC: Yb3+) phosphor was synthesized, which not only expressed triple-mode thermometric properties based on thermally coupled levels (TCLs), non-thermally coupled levels (N-TCLs) and lifetime (FL), but also capable of providing self-referencing and high-sensitivity temperature measurements under excitation at 980 nm. Benefiting from the low phonon energy of the matrix, high doping concentration and the excellent internal up-conversion quantum efficiency (UCQY) about 6.52% of optimal composition CNEC: 40%Yb3+, all modes exhibit superior sensitivities, reversibility and thermal resolution under 980 nm irradiation. In addition, a flexible thin film thermometer composed of CNEC: 40%Yb3+ was prepared to realized multi-mode thermal monitoring of local hot spots of electronic components. The excellent three-mode temperature measurement results indicated that CNEC: Yb3+ phosphors have great potential in temperature sensing.

Effect of Doping Concentration and Excitation Power on Upconversion and Temperature Sensitivity of Gd3Ga5O12:Yb3+/Er3+ Phosphors

Yb/Er codoped Gd3Ga5O12 nanocrystalline upconverting phosphors were produced by the sol-gel pechini method at 1000 °C annealing temperature. The phosphor structure, morphological features, and luminescent properties of the fabricated material were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), and photoluminescence measurements (PL). Upconversion luminescence characteristics were investigated in the range of 450-850 nm by a 975 nm laser source. Emission, optical, and theoretical thermal behaviors were analyzed with respect to Er3+ ion content and the increasing excitation power. Temperature sensitivity calculations based on the fluorescence intensity ratio were performed by employing the thermally-coupled levels of Er3+. The maximum sensitivity was calculated with the optimal value of 0.83x10-2 K-1 for Gd3Ga5O12:2%Yb3+,0.5%Er3+ nanophosphor. The results pointed out that Yb/Er codoped Gd3Ga5O12 may be a potential candidate for optical temperature sensors and lighting.

Microwave-assisted hydrothermal synthesis of LaOF:Yb3+, Ho3+ nanorods with high thermoresponsive upconversion luminescence for thermometry

Non-contact thermometry based on upconversion luminescence from lanthanide-doped nanoparticles is the best choice when temperature probes with higher precision, accuracy, sensitivity, and spatial resolution are required. Here, LaOF:Yb3+,Ho3+ upconversion nanoparticles (UCNPs) were demonstrated to be thermometers with a high sensitivity and thermal stability within the temperature range of 303–673 K. Microwave-assisted hydrothermal synthesis was used to obtain these rod-shaped UCNPs with a trigonal crystal structure. LaOF:Yb3+,Ho3+ UCNPs displayed green emissions due to the thermally-coupled levels (5F4, 5S2) and red emissions from the Stark-split sublevels of the 5F5 energy level belonging to Ho3+ under 980 nm continuous-wave laser irradiation. The upconversion emission originating from these transitions were remarkably sensitive to the temperature variation. Their fluorescence intensity ratios were explored to evaluate their relative (SR) and absolute (SA) sensitivities. Optical heating induced by continuous laser irradiation was also examined to assess the degradation of the luminescence caused by laser exposure.

Excellent Sensitive Temperature Sensing Performance Based on Fluorescence Intensity Ratio Technique of Yb3+/Er3+ Codoped Gd2(WO4)3

The efficient upconversion (UC) particles based on fluorescence intensity ratio (FIR) technique are developed for application in novel temperature sensors. Herein, Yb3+/Er3+‐codoped Gd2(WO4)3 particles are prepared by coprecipitation method and subsequent calcination. The phase structure and surface morphology of the particles are systematically analyzed. The UC emission spectrum of Gd2(WO4)3:Yb3+/Er3+ particles are mainly composed of two strong emission bands centered at 530 and 551 nm, respectively. Moreover, the optimum doping concentration of Yb3+ and Er3+ is fixed at 10% and 2%, respectively. UC emission mechanism based on two‐photon process is investigated by studying the pump power excitation dependence and fluorescence decay behavior of Gd2(WO4)3:10%Yb3+/2%Er3+ particles. The excellent temperature sensing capability of Gd2(WO4)3:10%Yb3+/2%Er3+ particles is implemented from thermally coupled levels (TCLs) of Er3+. Notably, the maximum absolute sensitivity (S a) and relative sensitivity (S r) based on TCLs by 2H11/2/4S3/2 levels are 1.39 %K−1 (at 518 K) and 1.18 %K−1 (at 293 K), respectively. As a result, Gd2(WO4)3:10%Yb3+/2%Er3+ particles have a great potential for application in noncontact temperature sensors.

Manipulating the thermometric behaviors of Er3+/Yb3+/Ho3+-tridoped La2Mo3O12 polychromatic upconverting microparticles via adjusting spatial mode and a sensing strategy.

To meet the needs of contactless optical thermometry, Er3+/Yb3+/Ho3+-tridoped La2Mo3O12 (LMO) microparticles were designed and synthesized. Upon exciting with 980 nm light, the synthesized compounds emit glaring upconversion (UC) emissions and their emission colors can be tuned from green to yellow by altering the Ho3+ content. It is found that the optimal doping contents for Yb3+ and Ho3+ in LMO are 9 and 1 mol%, respectively, and the UC emission mechanism involved is a two-photon harvest process. Using the fluorescence intensity ratio (FIR) technique to analyze the temperature responses of the UC emissions arising from thermally coupled levels (TCLs) and non-thermally coupled levels (non-TCLs), the temperature sensing abilities of the synthesized samples were investigated. When the TCLs of Er3+ (2H11/2, 4S3/2) are used, the synthesized microparticles present the highest absolute and relative sensitivities of 0.0085 and 1.0236% K-1, respectively. Moreover, when the non-TCLs of Er3+ (2H11/2) and Ho3+ (5F5) are used, the maximum absolute and relative sensitivities of the synthesized compounds are 0.0296 and 0.6287% K-1, respectively. Clearly, the thermometric characteristics of the final products can be regulated via using different sensing strategies (i.e., TCLs and non-TCLs) and emission combinations (i.e., spatial mode). However, the change of the Ho3+ content has little impact on the temperature sensing capacity of the synthesized products. These results indicate that Er3+/Yb3+/Ho3+-tridoped LMO microparticles are promising candidates for optical thermometers and our findings also provide possible strategies for regulating the thermometric properties of rare-earth ion doped luminescent materials.

Energy Gap Linear Superposition of Thermally Coupled Levels toward Enhanced Relative Sensitivity of Ratiometric Thermometry.

Ratiometric luminescence thermometry (RLT) has attracted considerable attention for its non-invasive, fast response, and strong electromagnetic interference resistance; however, improving relative sensitivity (SR) is of great significance. Herein, we propose a design principle to promote SR by linearly superposing the energy gaps of thermally coupled levels (TCLs) subordinated to luminescence centers. A new fluorescence intensity ratio (FIR') is derived from multiplying the previous FIRs of multi-pair TCLs. Then, a new SR' is significantly enhanced and proves to be the sum of the original SR values. The feasibility of this approach is proclaimed by applying to several materials [Na0.5La0.5TiO3:Yb/Nd, Y2O3:Yb/Er, and (LiMg)2Mo3O12:Yb/Er] with improved SR for RLT. Finally, a flexible film is fabricated for temperature measurement of actual scenes and manifests the superiority of the energy gap linear superposition method as ratiometric thermometry.

Size influence on optical thermometry of Er3+/Yb3+ Co-doped Y2O3 microspheres: From TCLs and Non-TCLs

Realization and optimization of enhanced/tunable optical luminescence properties is highly demanded for reliable and accurate temperature sensing. In this work, uniform monodisperse spherical Er3+/Yb3+ co-doped Y2O3 microcrystals of four different sizes have been synthesized via homogeneous precipitation by changing the reaction time, and the tuning of luminescence intensity/color was achieved by the controllable particles size. On this basis, by studying the particle size dependence of temperature sensing performance, we found that the behavior of absolute sensitivity (Sa) and relative sensitivity (Sr) have highly size dependence. Interestingly, when different FIR strategies were used (thermally coupled levels (TCLs) and nonthermally coupled levels (non-TCLs)), the sensitivity exhibits the opposite behavior. Importantly, when using the non-TCLs strategy, the maximum Sa value for the smallest phosphors was 1.084 K-1, leading to ∼246 times improvement compared to TCLs-based Sa. The ultrahigh Sa indicated that the Er3+/Yb3+ co-doped Y2O3 phosphors have potential in temperature detection, and the results are of guiding significance to studying the sensitivity of Y2O3 particle size-dependent temperature measurement.

Effects of Yb3+ concentration on up-conversion luminescence and temperature sensing characteristics of Tm3+/Yb3+:Y2O3 phosphors

Y2O3:0.5 mol%Tm3+/x mol%Yb3+(x = 3, 5, 7, 10) luminescent materials have been synthesized by the CO2 laser zone melting method. With 980 nm excitation, the up-conversion luminescence (UCL) emission bands corresponding to 1G4→3H6, 1G4→3F4, 3F2,3→3H6, and 3H4→3H6 transitions of Tm3+ ions were obtained in visible and near-infrared spectral regions, among which the luminescence emitted by 1G4→3H6, 3H4→3H6 transitions shows obvious Stark splitting. Besides, the optimal doping concentration of Yb3+ is 5 mol%. The up-conversion luminescence spectra of Y2O3:0.5 mol%Tm3+/5 mol%Yb3+ were recorded from 224 K to 873 K. The temperature sensing properties of 3F2,3 and 3H4, 1G4(a) and 1G4(b) energy levels of Tm3+ ions were investigated by the luminescence intensity ratio (LIR) method in a wide temperature range, and the possibility of using 3H4(a), 3H4(b) thermally coupled levels (TCLs) of Tm3+ ions for temperature sensing was developed. The results show that 1G4(a) and 1G4(b), 3F2,3 and 3H4, 3H4(a) and 3H4(b) of Tm3+ ions are TCLs, and the Stark sub-levels own relatively high absolute sensitivity at low temperature, whereas the 3F2,3 and 3H4 TCLs of Tm3+ ions are more suitable for high temperature measurement. The results show that the Tm3+/Yb3+:Y2O3 luminescent material can be a promising candidate for temperature sensing.

Investigation on the determinant of upconversion luminescence efficiency and temperature measurement sensitivity based on AWO4 (Ba, Sr, Ca, and Mg): Er3+/Yb3+ microcrystals

In this work, a comparative study of the upconversion (UC) luminescence properties of E r 3 + − Y b 3 + -co-doped A W O 4 ( A = B a , Sr, and Ca) scheelite phosphors and E r 3 + − Y b 3 + -co-doped M g W O 4 wolframite phosphors is reported. Under 980 nm excitation, all the samples emit green and red emissions. The E r 3 + − Y b 3 + -doped M g W O 4 phosphor has a higher UC emission intensity. Furthermore, the optical temperature sensing properties in E r 3 + − Y b 3 + -co-doped A W O 4 ( A = B a , Sr, Ca, and Mg) phosphor samples are investigated. The relative sensitivity ( S r ) based on thermally coupled levels (TCLs) increases gradually with an increase in matrix phonon energy. The absolute sensitivity ( S a ) values of the samples are accurately predicted by the chemical bonding parameter ( f c ). The results of the correlation between the UC luminescence properties and the microscopic crystal structure, and the relationship of the temperature sensitivity and the host parameters, provide us with a guiding insight for the selection of a host material with ideal temperature measurement capability.