Fluorescence Intensity Ratio Thermometry Research Articles

Yb,Er,Tm:Sc2O3 single crystal fibers for multi-mode optical thermometry

Fluorescence intensity ratio (FIR) thermometers have obtained great attention in industrial production, medical diagnosis and scientific research. However, the sensitivity has a large decline at high temperatures, which seriously limits the practical applications. Here, the Yb,Er,Tm:Sc2O3 single crystal fibers were grown by laser-heated pedestal growth (LHPG) technique and its multi-mode FIR thermometry based on thermal coupled energy levels (TCLs) and non-thermal coupled energy levels (NTCLs) has been well demonstrated. The temperature measuring range of Yb,Er,Tm:Sc2O3 single crystal fibers covers 298–973 K, with an increase of 2 orders of magnitude in absolute sensitivity. The relative sensitivity is higher than 0.02 % K−1 in the whole T-range of 298∼973 K, with a maximum value of 1.02 % K−1. Especially at the high temperatures of 973 K, it can still maintain 0.31 % K−1. This work proposes an innovative fluorescence intensity ratio detection strategy to achieve complementary disadvantages between TCLs and NTCLs and provides an effective method for improving detection sensitivity.

Optical temperature sensing of Yb,Er:GSAG single crystal in both upconversion and downconversion dual-mode luminescence

Fluorescence intensity ratio thermometry represents a novel non-contact approach for rapid and precise temperature measurements. In this study, a Yb,Er co-doped Gd3Sc2Al3O12 single crystal (Yb,Er:GSAG) was investigated, exploiting the dual-mode luminescence of upconversion and downconversion for optical temperature sensing applications. The crystal exhibited distinct luminescence behaviors when subjected to 266 nm, 355 nm, and 980 nm laser excitations, manifesting a dark blue appearance under 266 nm ultraviolet (UV) excitation and a green hue under 355 nm and 980 nm excitations. Specifically, under 355 nm excitation, the crystal demonstrated a predominantly pure green upconversion, with a green-to-red ratio of 38. For the first time, we propose an investigation into the temperature sensing behavior under dual-mode luminescence in the same crystal, utilizing the fluorescence intensity ratio of the 2H11/2 and 4S3/2 thermally coupled energy levels of Er3+. The obtained maximum relative sensitivities (SR) for the two fitted luminescence modes were found to be 101.80×10−4 K−1 at 300 K (980 nm excitation) and 126.39×10−4 K−1 at 300 K (355 nm excitation), respectively. The impressive SR values substantiate Yb,Er:GSAG crystal as highly promising candidates for optical temperature sensing applications.

Highly efficient cyan-red emission in self-activated Sr9In(VO4)7:xEu3+ phosphors for applications in W-LEDs and optical thermometry

Optical thermometry based on fluorescence intensity ratio (FIR) has garnered much attention as it pushes beyond the limitations of traditional thermometers. Here, we present a new strategy for FIR thermometry using dual emissions from Sr9In(VO4)7:xEu3+ (SIVO:xEu3+) phosphors by a single activation. A series of color tunable SIVO:xEu3+ phosphors was successfully synthesized, and their luminescent properties were investigated. The phosphors emit broadband from VO43− groups as well as sharp emissions from Eu3+ ions upon ultraviolet excitation (320 nm) with high thermal stability and little to no concentration quenching of Eu3+. Meanwhile, the SIVO:0.8Eu3+ phosphor demonstrates a high quantum efficiency of 49%. The emitting color could be modulated from cyan to red via regulating the energy transfer efficiency from VO43− to Eu3+ ions by adjusting Eu3+concentration. A prototype LED device based on SIVO:xEu3+ phosphors produces a bright white light with low color temperatures (2967–3741 K) and high color rendering indices (87–88.4%). Moreover, optical thermometry measurements using fluorescence intensity ratio of VO43− and Eu3+ emissions demonstrated optimum absolute sensitivity and relative sensitivity of 0.0975 K−1 and 1.26% K−1, respectively. Regulating the energy transfer between the host and activator proved to be a feasible strategy to modulate the emission color and achieve optical thermometry in a single-phase phosphor. The reported results also provide insights into designing multifunctional materials for both White-LED (W-LED) and thermometry applications.

Thermal Stability Improvement of Cr3+‐Activated Broadband Near‐Infrared Phosphors via State Population Optimization

AbstractTechnological progress has accelerated the researches into broadband near‐infrared (NIR) luminescent materials as next‐generation intelligent NIR light sources; however, poor thermal stability restricts the applications of NIR phosphors. Herein, new insights into Cr3+‐activated NIR phosphors with improved thermal stability are provided. The photoluminescence intensity originating from the 4T2→4A2 broadband emission of CaLu2Al4SiO12:Cr3+(CLAS:Cr) with optimal electron occupation (4T2/2E) increases up to 118% at 475 K compared with that at room temperature, which is the best for Cr3+‐activated broadband NIR phosphor. Accordingly, new ideas for the design of thermally stable phosphors are proposed by optimizing the thermal population between4T2and2E states and electron migration from2E to4T2state. Finally, multifunctional applications of CLAS:Cr in NIR fluorescence intensity ratio thermometry, X‐ray detection, NIR phosphor‐converted light‐emitting diodes, and bio‐imaging are demonstrated.

Ratiometric optical thermometry based on upconversion luminescence with different multi-photon processes in CaWO4:Tm3+/Yb3+ phosphor.

Ratiometric optical thermometry based on upconversion (UC) luminescence with different multi-photon processes in CaWO4:Tm3+,Yb3+ phosphor was developed. A new fluorescence intensity ratio (FIR) thermometry, utilizing the ratio of the cube of 3F2,3 emission to the square of 1G4 emission of Tm3+ and retaining the feature of anti-interference of excitation light source fluctuations, is proposed. Under the hypotheses of the UC terms being neglected in the rate equations and the ratio of the cube of 3H4 emission to the square of 1G4 emission of Tm3+ being a constant in a relatively narrow temperature range, the new FIR thermometry is valid. The correctness of all hypotheses was confirmed by testing and analyzing the power-dependent emission spectra at different temperatures and the temperature-dependent emission spectra of CaWO4:Tm3+,Yb3+ phosphor. The results prove that the new ratiometric thermometry based on UC luminescence with different multi-photon processes is feasible through optical signal processing, and maximum relative sensitivity of the thermometry is 6.61% K-1 at 303 K. This study provides guidance in selecting UC luminescence with different multi-photon processes to construct ratiometric optical thermometers with anti-interference of excitation light source fluctuation.

Optical thermometry based on fluorescence intensity ratio of Dy3+-Doped oxysilicate apatite warm white phosphor

The development of non-contact thermometers is one of the important applications of phosphors. In this work, a photoluminescence strategy by introducing Dy3+ for application in fluorescence intensity ratio thermometry (FIR) was investigated in detail. The FIR approach employs the fluorescence intensity ratios of 4I15/2-6H15/2 and 4F9/2-6H13/2, due to the Boltzmann variant distribution between two excited states at specific temperatures. The crystal structure, morphology, and luminescence behaviors of CaLa4(SiO4)3O: Dy3+ were studied. When the excitation wavelength is 348 nm, two strong emission peaks of CaLa4-x(SiO4)3O: xDy3+ at 478 and 573 nm respectively, as well as two small emission peaks located at 456 nm and 661 nm. The optical temperature measurement performance of the phosphors at different temperatures (298 K–548 K) was achieved because of the different thermal quenching transitions of 4F9/2-6H15/2 and 4F9/2-6H13/2. The maximum relative sensitivity and absolute sensitivity of the as-prepared phosphors as optical thermometers could reach as high as 0.1673% K−1 at 298 K and 0.007087% K−1 at 548 K, respectively. Therefore, we hope that our research on the temperature measurement of CaLa4(SiO4)3O: Dy3+ series phosphors can lay a certain research foundation and reference value for the subsequent research on the temperature measurement direction of Dy3+ doped phosphors.

Ultra-High-Sensitive Temperature Sensing Based on Er3+ and Yb3+ Co-Doped Lead-Free Double Perovskite Microcrystals

Fluorescence intensity ratio (FIR) thermometry, a new contactless temperature measurement, can achieve accurate measurements in a harsh environment. In this work, all-inorganic lead-free Cs2AgInCl6: Er-Yb and Cs2AgBiCl6: Er-Yb microcrystals emit bright green up-conversion emission, which are synthesized by precipitation at a low temperature (80 °C). In up-conversion emission, FIR of the 2H11/2 → 4I15/2 band to the 4S3/2 → 4I15/2 band exhibits temperature dependence, which can be used as the temperature measurement parameter, so-called FIR thermometry. Moreover, the theoretically accurate measurement range is from 100 to 600 K, achieving maximum absolute sensitivities from 0.0130 to 0.0113 K-1, respectively. The principle of up-conversion and high sensitivity is well explained by calculating the partial density of states. Compared to the reported thermometry materials based on the FIR method, the prepared all-inorganic lead-free Cs2AgInCl6: Er-Yb and Cs2AgBiCl6: Er-Yb microcrystals show outstanding temperature measurement width and sensitivity, becoming a potential candidate for high-sensitivity optical temperature sensors.

Compensation effect of electron traps for enhanced fluorescence intensity ratio thermometry performance

A significant enhancement effect of electron traps on the optical thermometry performance in ZnGa2O4:Mn is demonstrated by experimental and calculation results.

High performance FIR thermometry on the basis of the redshift of CTB by dual-wavelength alternative excitation in Eu3+:YVO4.

Compared with the forbidden 4f transition of rare earth ions, the strong absorption of the charge transfer band (CTB) enabled fluorescence thermometry to have high luminescence efficiency. Based on the temperature induced redshift of CTB, a high performance fluorescence intensity ratio (FIR) thermometry performed by dual-wavelength alternative excitation was studied. By way of the rising and falling edges of CTB in Eu3+ doped YVO4, monochrome sensitivity as a function of excitation wavelength was studied in the range of 303-783 K. The excitation wavelength with the highest positive monochrome sensitivity was determined, as well as that with the negative one. The optimum FIR temperature sensing strategy is proposed, and the theoretical highest relative sensitivity (Sr) is calculated to be 1.86% K-1, with the lowest uncertainty (ΔT) of 0.1 K at 783 K.

The design of dual-switch fluorescence intensity ratio thermometry with high sensitivity and thermochromism based on a combination strategy of intervalence charge transfer and up-conversion fluorescence thermal enhancement.

Currently, the temperature sensing performances of inorganic photoluminescence materials based on fluorescence intensity ratio technology have become a research hotspot in the optical thermometry field due to their non-contact sensing, fast response and high stability. However, several problems have obstructed the development of optical temperature sensing materials, including low sensitivity and narrow temperature measurement ranges. In view of the above dilemma, a new optical thermometer La2Mo3O12:Yb3+,Pr3+ designed based on the combination strategy of intervalence charge transfer and up-conversion fluorescence thermal enhancement was developed. Under excitation at 450 nm, the thermometer can work in a range from 298 to 648 K and the relative sensitivity reaches as high as 2.000% K-1 at 648 K. Under excitation at 980 nm, the thermometer can sense temperature with a wide range from 298 to 748 K and the relative sensitivity reaches as high as 4.325% K-1 at 598 K. A dual-switch optical temperature sensing material with high-sensitivity and a wide temperature measurement range has been successfully developed. Our research design strategies will give inspiration to the research on multi-switch temperature sensing materials with high sensitivity and a wide temperature measurement range.

Temperature Dependent Up-Conversion Luminescence Properties of Er<sup>3+</sup> Doped KNN Ultrafine Powders Prepared by Pulsed Laser Ablation in Liquid

Er3+ doped potassium sodium niobate (KNN: Er) ultrafine powders have been prepared by pulsed laser ablation in water. X-ray diffraction (XRD) pattern of the sample demonstrated that the as-synthesized powders were crystalized in orthorhombic phase. Scanning electron microscopy (SEM) and transmittance electron microscopy (TEM) images exhibited that the morphology of ultrafine powders are cube-like. Under the excitation of 980 nm laser, the sample exhibits green emission, which is originated from the transition of thermal coupled energy levels (2H11/2, 4S3/2) to ground state level 4I15/2. Temperature dependent up-conversion emission intensity associated with thermal quenching of total green emission band and the fluorescence intensity ratio (FIR) between two sub-emission bands related to population of thermal coupled energy levels are investigated for temperature sensing in the temperature range of 300 K to 480 K. The temperature sensing performances related to different technique were discussed. A maximum relative sensitivity reaches 1.01% K-1 at 464 K for emission intensity thermometry and that is 0.84% K-1 at 374 K for FIR thermometry technique. All these results show that KNN: Er ultrafine phosphors prepared via pulsed laser ablation in water have prospect for non-contact temperature sensing.

Stably and highly sensitive FIR thermometry over a wide temperature range of 303-753 K based on the GdVO4:Eu3+ and Al2O3:Cr3+ hybrid particles.

Fluorescence intensity ratio (FIR) temperature sensors provide an effective method to control or study fine variations in physical and biological research because of their high sensitivity, accuracy, and spatial resolution. However, it is difficult to maintain high sensitivity over a wide temperature range using FIR temperature sensors because of the limits of the Boltzmann distribution law. In this study, sensitivity amplification for a wide temperature range in FIR thermometry based on GdVO4:Eu3+ and Al2O3:Cr3+ hybrid particles is achieved. The mechanism of the non-monotonic temperature dependence of the relative sensitivity (Sr) is studied. The results demonstrate that the Sr stably keeps ∼2.4% per K over a wide temperature range of 303-753 K, thus providing a basis for the extensive application of FIR temperature sensors.

Thermometric analysis of the near-infrared emission of Nd3+ in Y2SiO5 ceramic powder prepared by combustion synthesis

Nd3+:Y2SiO5 crystalline ceramic powder prepared by combustion synthesis exhibited down-conversion fluorescence when the sample was exposed to CW laser radiation at 532 nm. Three emission bands centered at 879.36 nm, 804.86 nm and 753.21 nm, corresponding to transitions (4F7/2,4S3/2) → 4I9/2, (4F5/2,2H9/2) → 4I9/2 and 4F3/2 → 4I9/2, were observed. A noticeable change on the relative intensities of the transitions in the temperature range 298–673 K occured due to thermal coupling among the excited electronic states. By means of the fluorescence intensity ratio (FIR) technique, we obtained a maximum absolute sensitivity of 2.5 × 10−3 K−1 at 650 K and a relative sensitivity of 2.11% K−1 at 300 K for this material. The Judd-Ofelt standard intensity parameters ratio Ω4/Ω6, a spectroscopy quality factor for laser applications, was estimated from the FIR thermometry data and a value of 1.75 was obtained. The potential use of this material in thermal sensing at the first biological window (700–950 nm) and in lasing applications is discussed.

Highly sensitive fluorescence intensity ratio thermometry by breaking the thermal correlation of two emission centers

Fluorescence intensity ratio thermometry based on hybrid Eu3+-doped GdVO4 and Cr3+-doped Al2O3 particles has been studied. We aim at overcoming the limitation that the relative sensitivity (Sr) decreases with the square of temperature owing to the thermal relation between the two emitting levels in one matrix in terms of the Boltzmann distribution law. The design of hybrid Eu3+-doped GdVO4 and Cr3+-doped Al2O3 particles constructs two emission bands with opposite temperature dependences. Furthermore, the Sr makes a breakthrough by obtaining greater sensitivity in a high-temperature range; the maximum value obtained is 1.4%K-1 at 633K.

Investigation on the Fluorescence Intensity Ratio Sensing Thermometry Based on Nonthermally Coupled Levels.

Fluorescence intensity ratio (FIR) of rare earth ions has been widely used in real-time and accurate temperature sensing because of its superiority of rapid response, self-reference, and noncontact in recent years. However, the energy gap (ΔE) restriction of thermally coupled levels (TCLs) has hindered the sensitivity and practical use of such detectors. Herein, we investigate the FIR thermometry based on nonthermally coupled levels (NTCLs) of rare earth ions for fabricating a sensitive, precise temperature detector. Compared with the traditional FIR thermometry based on TCLs (TCL-FIR), the designed NTCL-FIR sensing thermometry exhibits a series of excellent performance including extremely low temperature uncertainty (∼0.27 K), an ultrahigh temperature sensitivity (>10% K-1), and satisfactory signal recognition ability. The rise of sensitivity and recognition is attributed to breaking the ΔE restriction of TCLs by using an Arrhenius equation. The proposed ideas and methods of NTCL-FIR sensing thermometry can not only improve the performance of temperature sensing devices but also more importantly contribute to the practical development of rare earth ions.

Investigation on Two Forms of Temperature-Sensing Parameters for Fluorescence Intensity Ratio Thermometry Based on Thermal Coupled Theory.

Absolute temperature sensitivity (Sa) reflects the precision of sensors that belong to the same mechanism, whereas relative temperature sensitivity (Sr) is used to compare sensors from different mechanisms. For the fluorescence intensity ratio (FIR) thermometry based on two thermally coupled energy levels of one rare earth (RE) ion, we define a new ratio as the temperature-sensing parameter that can vary greatly with temperature in some circumstances, which can obtain higher Sa without changing Sr. Further discussion is made on the conditions under which these two forms of temperature-sensing parameters can be used to achieve higher Sa for biomedical temperature sensing. Based on the new ratio as the temperature-sensing parameter, the Sa and Sr of the BaTiO3: 0.01%Pr3+, 8%Yb3+ nanoparticles at 313 K reach as high as 0.1380 K-1 and 1.23% K-1, respectively. Similarly, the Sa and Sr of the BaTiO3: 1%Er3+, 3%Yb3+ nanoparticles at 313 K are as high as 0.0413 K-1 and 1.05% K-1, respectively. By flexibly choosing the two ratios as the temperature-sensing parameter, higher Sa can be obtained at the target temperature, which means higher precision for the FIR thermometers.

Relative sensitivity variation law in the field of fluorescence intensity ratio thermometry.

We study the variation law of relative sensitivity in the field of fluorescence intensity ratio thermometry. It is theoretically demonstrated that there must be only one maximum value of relative sensitivity in the case in which there is a positive offset in fitting function. Moreover, the method to obtain this maximum is proposed. Experimental results, taking the D15/D50 levels of Eu3+ as examples, are in excellent accordance with the conclusion. The mechanism behind is then investigated, and other populating processes imposed on the D15 level, which exert negative outcome on thermal sensitivity, are found to play a key role in determination of this unique variation law.

Modified calculation method of relative sensitivity for fluorescence intensity ratio thermometry.

The calculation method of relative sensitivity (Sr) for fluorescence intensity ratio (FIR) thermometry is discussed, taking the F33-H63 and H43-H63 transitions of Tm3+ as examples. The value of Sr is calculated using its original definition, and is found to largely deviate from the result obtained using the conventional method that is widely used at present. This deviation is found to stem from the neglect of an offset. A modified expression of Sr is proposed, which shows the true performance of FIR technology and makes it possible to precisely compare the Sr values obtained using various methods.

Eliminating the “decoupling” effect of fluorescence intensity ratio thermometry for an upconversion pumped system

To eliminate the “decoupling” effect, which causes the fluorescence intensity ratios (FIR) to deviate from the pure Boltzmann distribution law, a correcting method for an upvonversion pumped system was theoretical derived and experimentally verified. Theoretical results indicated that the double exponential decay of the lower level was entirely involved in the decay of the upper level, which could be used to determine the thermal population degree (η). Taking Tm3+ ions as examples, by analyzing the temperature dependence of the fluorescent dynamic curves originating from the thermally coupled pair, the η values at different temperatures were obtained and the corresponding correcting curve is given. The corrected FIR abides by the Boltzmann law, even in the lower temperature range.

Modified FIR thermometry for surface temperature sensing by using high power laser.

The FIR (fluorescence intensity ratio) technique for optical thermometry has attracted considerable attention over recent years due to its high sensitivity and high spatial resolution. However, it is thought that a heating effect induced by incident light may lead to temperature overestimations, which in turn impedes the reliability of this technique for applications which require high levels of accuracy. To further improve the FIR technique, this paper presents a modified calibration expression, which is suitable for surface temperature sensing, based on the temperature distribution (calculated through COMSOL software). In addition, this modified method is verified by the experimental data.