Enhanced Upconversion Luminescence Research Articles

Enhancing up-conversion luminescence in Yb/Er co-doped CsPbCl3 single crystals

The search for and fabrication of materials for the near-infrared range of the spectrum, including telecommunication windows, is an extremely important task in modern technology. This work presents the results of studies on the photoluminescence of CsPbCl3 single crystals with varying contents of ytterbium and erbium impurities, grown using the Bridgman method. The existence of electrically neutral Yb3+–VPb–Er3+ complexes within the crystal structure was confirmed. Resonant absorption of excitation radiation λ = 980 nm by Yb3+ ions reveals a three-step energy transfer channel from Yb3+ to Er3+ ions within a single complex. The estimated quantum yield of up-conversion luminescence at λ = 524 nm (0.02%) associated with erbium ions indicates the presence of electronic transitions to higher energy levels, with the possibility of subsequent emission within the third telecommunication window.

Dual–resonance enhancement and polarization control of upconversion luminescence of NaYF4:Yb,Er nanoparticles on 2D metal gratings

In this work, we report enhancement and polarization control of multi–color (green/red) upconversion luminescence (UCL) of lanthanide–doped (NaYF4:Yb,Er) nanoparticles by dual–resonance modes of surface plasmons (SP) on two–dimensional (2D) metal gratings in orthogonal directions. Investigations are performed for the SP resonance modes to be tuned, by adjusting the grating periods, to match any one or two of the green–UCL, red–UCL and excitation wavelengths. It is shown that, for the upconversion nanoparticles (UCNPs) on the 2D gratings with both of the dual resonance modes matching the green–/red–UCL or excitation wavelength, enhancements of the UCLs are roughly doubled, compared with their one–dimensional (1D) counterparts. And when the dual resonance modes match the green– or red–UCL and the excitation wavelength, enhancements of the UCLs are further greatly improved, in spite that the resonance modes for the excitation and emission light are in orthogonal directions. As the emitted UCL photons are largely coupled with the dual SP resonance modes, the radiated green– and red–UCL light can be distinguished by their linear polarization states in orthogonal directions. It is also indicated that polarization of the excitation light can influence the polarization of the UCL light when coupled with the SP resonance modes.

Enhanced upconversion luminescence of Er/Tm for highly sensitive multi-mode optical nanothermometer

A novel class of Er3+/Tm3+/Yb3+ co-doped Sc4Zr3O12 nanocrystals have been successfully synthesized via a hydrothermal method. The enhanced green and red emissions of Er3+are achieved through the mediation of Tm3+. Decay curves and DFT calculations indicate this enhancement is due to the cross relaxation processes and narrowed bandgap. Based on the temperature-dependent upconversion characteristics, a multi-mode optical thermometer is developed using the fluorescence intensity ratio (FIR) technique of thermally coupled levels (TCLs) and non-thermally coupled levels (NTCLs), as well as the valley to peak ratio (VPR) technique. The absolute sensitivity (Sa) and relative sensitivity (Sr) derived from 1G4(b)/2H11/2 NTCLs reach as high as 11.84% K−1 and 1.89% K−1, respectively. Additionally, the optimal sensitivities occur at 303 K, perfectly aligning with the physiological temperature range. A comparison of different fitting models for NTCLs reveals that the exponential model offers the best performance for this system, with a correlation coefficient of 99.9%. This work provides new insights into upconversion enhancement of multi-emission centers and lays the theoretical and experimental foundation for developing high-sensitivity and high-precision optical nanothermometers.

Innovative synthesis of Tm³⁺/Er³⁺-doped Yb-YGG single crystals for upconversion-based white light emission

In recent years, there has been a growing interest in developing advanced materials for photonics applications, particularly for efficient white light emission, which is crucial for technologies like solid-state lighting and display devices. One interesting approach for emitting white light is Upconversion (UC) luminescence. This study focuses on synthesis, by a new and alternative method, and characterization of yttrium ytterbium gallium garnet (Yb-YGG) single crystals, specifically Y1.8Yb1.2Ga5O12, doped with various concentrations of Er3+ and Tm3+ ions. These crystals were studied intending to enhance UC luminescence properties for white light generation by modifying their respective emissions in the red, green, and blue (RGB) regions. Unlike traditional methods such as Czochralski, these crystals were produced by controlled nucleation and growth through gradual cooling of a specific glass mixture, leading to Yb-YGG single crystals doped with different concentrations of Yb3+, Er3+, and Tm3+ ions. By optimizing the Yb3+/Tm3+/Er3+ ratio, the study allowed to obtain micrometric single crystals that efficiently emit white light via UC. The crystals were characterized by X-ray diffraction, optical and electronic microscopies, EDS and luminescence spectroscopy.

Enhanced upconversion luminescence of Er3+/Y3+ co-doped tellurite glasses for volumetric three-dimensional display

Large-sized Er3+/Y3+ co-doped tellurite glasses with high image quality were prepared by a conventional melt-quenching method for volumetric three-dimensional display. The effects of the concentration of Er3+ and Y3+ on the two-frequency upconversion luminescence properties and volumetric three-dimensional display were investigated. Under dual-wavelength excitation at 850 nm and 1550 nm, the two-frequency two-step upconversion luminescence intensity at 546 nm in the Er3+/Y3+ co-doped tellurite glass was significantly enhanced by about 44% compared with that in the Er3+ doped tellurite glass. However, the upconversion luminescence mechanism and its dynamic process were further obtained. A range of the high-resolution and high-contrast volumetric three-dimensional images can be achieved in the Er3+/Y3+ co-doped tellurite glass with optimal doping concentration, utilizing a coordinated control system of galvanometer scanner and digital micro-mirror devices. The results indicate that Er3+/Y3+ co-doped tellurite glass has promising potential for widely volumetric 3D displays.

Preparation and significant enhancement of upconversion luminescence of α-Ba2ScAlO5:Yb3+/Er3+ phosphors by Ca2+ ions doping

The intensity of upconversion luminescence (UCL) poses a significant challenge for oxides characterized by high phonon energy. Herein, a significant enhancement of red UCL is achieved through the introduction of an intermediate band (IB) in α-Ba2ScAlO5: Yb3+/Er3+ by Ca2+ ions doping. The absorption spectra verify the existence of the IB. The IB of α-Ba2ScAlO5: Yb3+/Er3+/Ca2+ increases the host's absorption of excited photons, provides more electrons for the excitation of Er3+, and realizes the enhancement of UCL emission. The optimal concentration of Ca2+ in α-Ba2ScAlO5: 0.2Yb3+, 0.03Er3+ is 0.15. Compared with undoped Ca2+ samples, the red and green UCL intensity of α-Ba2ScAlO5: Yb3+, Er3+, 0.15Ca2+ increased by 48.2 and 10.6 times, respectively. The mechanism of Ca2+ enhanced UCL was investigated by analyzing the UCL spectra, absorption spectra and lifetime decay curves of α-Ba2ScAlO5: Yb3+/Er3+/Ca2+. The temperature-dependent UCL properties of α-Ba2ScAlO5: Yb3+/Er3+/Ca2+ were also studied by the 2H11/2 (525 nm) and 4F9/2 (667 nm) energy level radiation transitions. The observed linear correlation between luminescence intensity and temperature, coupled with its high sensitivity, indicates the promising potential for temperature sensing applications. This study not only opens a new avenue for enhancing UCL but also holds significant implications for sparking widespread interest in non-contact temperature sensing applications leveraging UCL.

Enhancement of upconversion luminescence in NaBiF4: Er3+/Yb3+/Lu3+ for temperature detection

Optical thermometer has got lots of attention by virtue of non-contact, fast response speed, high sensitivity, anti-interference ability, which is less affected by the environment. However, most fluorescent thermometers still suffer from low sensitivity. Here, an optical thermometer based on NaBiF4: Er3+/Yb3+/Lu3+ with enhanced upconversion luminescence is reported. The sample emits strong green light of Er3+ under 980 nm laser excitation and with the increase of Lu3+, the whole luminescence intensity of NaBiF4: 2 % Er3+/20 % Yb3+/ 2.5 % Lu3+ is enhanced by about 3 times, which due to local crystal field distortion. Meanwhile, the lifetime of Er3+ is prolonged and the temperature detection performance is also improved with the substitution of Lu3+. The relative sensitivity reaches a maximum of 0.841 % K−1 at 301 K. The results suggest that it is an optical nanothermometer with great potential.

Surface plasmon polariton-enhanced upconversion luminescence for biosensing applications.

Upconversion luminescence (UCL) has great potential for highly sensitive biosensing due to its unique wavelength shift properties. The main limitation of UCL is its low quantum efficiency, which is typically compensated using low-noise detectors and high-intensity excitation. In this work, we demonstrate surface plasmon polariton (SPP)-enhanced UCL for biosensing applications. SPPs are excited by using a gold grating. The gold grating is optimized to match the SPP resonance with the absorption wavelength of upconverting nanoparticles (UCNPs). Functionalized UCNPs conjugated with antibodies are immobilized on the surface of the fabricated gold grating. We achieve an UCL enhancement up to 65 times at low excitation power density. This enhancement results from the increase in the absorption cross section of UCNPs caused by the SPP coupling on the grating surface. Computationally, we investigated a slight quenching effect in the emission process with UCNPs near gold surfaces. The experimental observations were in good agreement with the simulation results. The work enables UCL-based assays with reduced excitation intensity that are needed, for example, in scanning-free imaging.

Highly sensitive optical thermometer based on Li+-doped erbium ytterbium silicate films

This work provides a new material, Li+-doped erbium ytterbium silicate, with reliable and high temperature sensing sensitivity for optical thermometers. The Li+-doped erbium ytterbium silicate films were prepared by RF magnetron sputtering. The film shows strong green visible emission, which is related to the radiation transitions from thermally coupled energy levels 2H11/2, 4S3/2 to 4I15/2. Lithium doping reduces the field symmetry around erbium and improves the radiation transition efficiency, and Yb doping improves the pump light absorption. Lithium or ytterbium is advantageous to enhance up-conversion luminescence and temperature sensing sensitivity of erbium ions. Therefore, the temperature dependence of green up-conversion luminescence in the range of 80–460 K of the Li+-doped erbium ytterbium silicate films have been found to be a promising optical thermometer material with a high relative sensitivity of 16.9 % K−1, which is higher than the optimum value of other Er3+ materials. In addition, we have provided a preliminary temperature measurement scheme based on the Li+-doped erbium ytterbium silicate film.

Defect level induced negative thermal quenching of β-Ba3LuAl2O7.5: Yb3+, Er3+ phosphor

Temperature-dependent behavior of phosphors has recently attracted significant research attention. This is due to the widespread applications of phosphors. Unfortunately, the thermal quenching effect in many phosphors greatly limits their performance at high temperatures. Herein, negative thermal quenching phosphor β-Ba3LuAl2O7.5: 0.3Yb3+, 0.03Er3+ is synthesized using the high-temperature solid-state method. X-ray Rietveld refinement and EPR results indicate the defect state of oxygen vacancy. Temperature-dependent upconversion luminescence spectra demonstrate that the green and red emission intensities increase with temperature to reach maximums at 398 K and 348 K respectively. Diffuse reflectance spectrum and downshifting luminescence spectrum prove that the defect state is near the 4F7/2 state of Er3+ ions. The defect state traps electrons at room temperature and releases them to populate the 4F7/2 state of Er3+ ion at elevated temperatures to cause the observed thermal enhancement of upconversion luminescence. This material's negative thermal quenching effect makes it a suitable candidate for applications in displays and anti-counterfeiting.

Innovative approach to tailoring upconversion luminescence in Yb3+/Eu3+ doped CaZrO3 perovskite host matrix

This study presents the synthesis and characterization of upconverter nanostructures consisting of a perovskite host matrix, CaZrO3, co-doped with trivalent ytterbium (Yb3+) and europium (Eu3+), denoted as CaZrO3:Yb3+/Eu3+. Single-doped CaZrO3:Eu3+ samples were excited using a 394 nm UV source, while CaZrO3:Yb3+/Eu3+ samples were excited with a 980 nm diode laser source. Yb3+ ions efficiently absorb the 980 nm light and transfer the absorbed energy to the nearby ion Eu3+ ions, leading to the emission of visible light around 590, 614, and 655 nm, respectively. The emission spectra of Eu3+ ions exhibit an initial increase with increasing Yb3+ concentration, peaking at 7 mol%, followed by a decrease due to luminescence quenching. The decay lifetime of Eu3+ ions under 300 nm excitation decreases simultaneously with increasing Yb3+ concentration, indicating energy migration between Yb3+ and Eu3+. Detailed explanations of the energy transfer mechanism between Yb3+ and Eu3+ ions are provided, supported by equations and diagrams. This study provides valuable insights into the optimization of dopant concentrations for enhancing upconversion luminescence in CaZrO3:Yb3+/Eu3+ nanostructures, with 3 mol% Eu3+ concentration identified as optimal based on preliminary experiments. The findings presented in this paper contribute to the understanding of energy transfer processes in luminescence upconversion materials and highlight the potential of CaZrO3:Yb3+/Eu3+ nanostructures for applications in lighting, displays, and bio-imaging. This work represents a significant advancement in the field of upconversion luminescence emission, particularly in the utilization of CaZrO3 codoped with Yb3+ and Eu3+, and lays the foundation for further research in this area.

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.

Co-precipitation synthesis and thermally enhanced upconversion luminescence properties of ScPO4:Yb3+/Er3+ phosphors for temperature sensing

A series of ScPO4:Yb3+/Er3+ upconversion luminescent (UCL) materials with different ytterbium and erbium doping concentrations have been synthesized by a co-precipitation method. Concentration-dependent UCL spectra revealed that the optimal composition was realized for the 15 at%Yb3+ and 1.0 at% Er3+-doping concentration with a strong UCL. Temperature-dependent UCL spectra of ScPO4:Yb3+/Er3+ phosphors indicate that this phosphor possesses thermally enhanced UCL properties. Under the excitation of 980 nm laser, ScPO4:Yb3+/Er3+ can emit intense green and red located at 526/550 nm and 668 nm attributed to the characteristic transitions 2H11/2/4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 of the Er3+ ion, respectively. In addition, the temperature sensing properties of the samples were investigated based on the thermally coupled energy levels (2H11/2 and 4S3/2) of Er3+. In the temperature range of 298∼573 K, the best relative sensitivity (Sr) is 1.28 %/K at 298 K. The above results indicate that ScPO4:Yb3+/Er3+ phosphors have excellent UCL performance and can be applied in luminescence display and temperature sensing.

Dye-sensitized lanthanide-doped upconversion nanoprobe for enhanced sensitive detection of Fe3+ in human serum and tap water

Iron ion (Fe3+) detection is crucial for human health since it plays a crucial role in many physiological activities. In this work, a novel Schiff-base functionalized cyanine derivative (CyPy) was synthesized, which was successfully assembled on the surface of upconversion nanoparticles (UCNPs) through an amphiphilic polymer encapsulation method. In the as-designed nanoprobe, CyPy, a recognizer of Fe3+, is served as energy donor and β-NaYF4:Yb,Er upconversion nanoparticles are adopted as energy acceptor. As a result, a 93-fold enhancement of upconversion luminescence is achieved. The efficient energy transfer from CyPy to β-NaYF4:Yb,Er endows the nanoprobe a high sensitivity for Fe3+ in water with a low detection limit of 0.21 μM. Moreover, the nanoprobe has been successfully applied for Fe3+ determination in human serum and tap water samples with recovery ranges of 95 %-105 % and 97 %-106 %, respectively. Moreover, their relative standard deviations are all below 3.72 %. This work provides a sensitive and efficient methodology for Fe3+ detection in clinical and environmental testing.

Efficiency enhancement of photovoltaic cells under infrared light irradiation by synergistic upconversion luminescence of NaYF4:Yb3+/Er3+/Tm3+@TiO2-CQDs

Rare-earth doped upconversion luminescence (UCL) is of great significance in improving the utilization of infrared (IR) light from solar radiation, especially in the efficiency enhancement of photovoltaic (PV) cells. However, the low level of luminescence efficiency remains a great challenge. Here, we designed NaYF4:Yb3+/Er3+/Tm3+@TiO2-CQDs upconversion luminescent materials. The construction of the core–shell structure and the loading of carbon quantum dots (CQDs) resulted in a 9-fold enhancement of upconversion luminescence and a 3.02% enhancement of the efficiency of the photovoltaic cell. These data indicate that the construction of the core–shell structure can effectively inhibit the surface quenching of the luminescent particles, and that the energy transfer between the carbon quantum dots and the rare earth ions has a synergistic effect on the efficiency enhancement of the photovoltaic cell. Meanwhile, the TiO2 shell layer also enables the prepared materials to have better properties of self-cleaning and degradation of organic pollutants, which will further improve the visible light utilization of PV cells. NaYF4:Yb3+/Er3+/Tm3+@TiO2-CQDs provides new ideas for the practical application of PV cell efficiency enhancement and the design of new materials.

Towards core–shell engineering for efficient luminescence and temperature sensing

Research on the core–shell design of rare earth-doped nanoparticles has recently gained significant attention, particularly in exploring the synergistic effects of combining active and inert shell layers. In this study, we successfully synthesized 8 types of spherical core–shell Na-based nanoparticles to enhance the efficiency of core–shell design in upconversion luminescence and temperature sensing through the strategic arrangement of inert and active layers. The most effective upconversion luminescence was observed under 980 nm and 808 nm laser excitation using NaYF4 inert shell NaYF4:Yb3+, Er3+@ NaYF4 and NaYF4@ NaYF4:Yb3+, Nd3+ core–shell nanostructures. Moreover, the incorporation of the NaYbF4 active shell structure led to a significant increase in relative sensitivity in ratio luminescence thermometry. Notably, the NaYF4:Yb3+, Nd3+, Er3+@ NaYbF4 core–shell structure demonstrated the highest relative sensitivity of 1.12 %K−1. This research underscores the crucial role of inert shell layers in enhancing upconversion luminescence in core–shell structure design, while active layers play a key role in achieving high-sensitivity temperature detection capabilities.

Significantly enhanced upconversion luminescence intensity and tailorable chromaticity of Sn4+-doped NaYF4:Yb3+/Er3+

The internal modification with rare earth ion doping proved to be a very effective strategy for improving the luminescent properties of NaYF4-based upconversion materials. However, greatly enhancing the luminescence efficiency of NaYF4:Yb3+/Er3+ remains a major challenge. Herein, the effects of Sn4+ doping on the structure and luminescent performance of such material were explored. The hydrothermal molten-salt method was applied to synthesize the Sn4+-doped NaYF4:Yb3+/Er3+ upconversion materials, and their crystal structures, morphologies, surface chemical composition and element states, and luminescence performance were characterized. It was found that Sn4+ doping can significantly enhance the luminescence intensity and tailor the chromaticity of NaYF4:Yb3+/Er3+. In particular, the green (G) and red (R) luminescence intensity levels of the 30 mol% Sn4+ doped material were increased by factors of 26.97 and 38.91, respectively. The R/G ratio was incremented from 0.34 for the undoped material to 0.83 for the 40 mol% Sn4+ doped counterpart. The Sn4+ doping led to the change of lattice distortion and crystal growth pattern of NaYF4:Yb3+/Er3+. The mechanism for Sn4+ doping to affect the luminescence properties of the prepared upconversion material was also explored. The changes in luminescence intensity levels and R/G ratios could be attributed to the highly asymmetric distorted lattice and crystal field resulting from Sn4+ doping.

Enhancement of Upconversion Luminescence through Three-Dimensional Design of Plasma Ti3O5-Coupled Structural Domain-Limiting Effects.

Upconversion luminescence plays a crucial role in various technological applications, and among the various valence states of lanthanide elements, Ln3+ has the highest stability. The 4f orbitals of these elements are in a fully empty, semifull, or full state. This special 4f electron configuration allows them to exhibit rich discrete energy levels. However, the 4f-4f transition of Ln3+ rare earth ions itself is prohibited, resulting in a lower luminescence efficiency. This limitation greatly hinders the practical application of upconversion luminescence. In this study, we report nanostructured luminescence-enhanced substrate platforms with both semiconductive local surface plasmons and spatially confined domain effects on a single defect semiconductor substrate. By coupling NaYF4:Yb-Er nanoparticle emitters to the surface of Ti3O5 NC-arrays plasmonic nanostructures, an ultrabright luminescence with a 32-fold increase in green emission and a 40-fold increase in red emission was achieved. Furthermore, the fluorescence resonance energy transfer characteristics observed in the R6G/NaYF4/Ti3O5 NC-array composite film enable accurate detection of fluorescent molecules. The results provide an innovative and intelligent approach to enhance the upconversion luminescence intensity of rare-doped nanoparticles and develop highly sensitive molecular detection systems based on the above luminescence enhancement.

Synthesis and upconversion luminescence properties of Ho3+-Yb3+ co-doped glass ceramics containing LiGd(WO4)2

Ho3+-Yb3+ co-doped glass ceramics containing LiGd(WO4)2 were prepared by melt crystallization method. By using the methods of differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmittance, and scanning electron microscopy (SEM), the optimal heat treatment condition for the samples was identified to be 580 °C/140 min. In these conditions, the transmittance of glass ceramic is about 75 % in the 380–780 nm range. In the upconversion emission spectra, strong green emission (543 nm) and red emission (650 nm) due to 5F4/5S2, 5F5→5I8 transitions are observed at 980 nm excitation. Additionally, the optimum doping concentrations of Ho3+ and Yb3+ were 0.12 % and 0.4 %. The crystal field changes around dopant ions were analyzed using J-O theory and Eu3+ probe to explain the up-conversion luminescence enhancement of glass ceramics containing LiGd(WO4)2. The chrominance coordinates of 0.12 % Ho3+-0.4 % Yb3+ co-doped glass ceramics are located in the green light region, and the color purity is 95.6 %. In summary, Ho3+-Yb3+ co-doped transparent glass ceramics containing LiGd(WO4)2 have potential applications in infrared detection and green solid-state lasers.

Large enhancement of red upconversion luminescence in beta Ba2Sc0.67Yb0.3Er0.03AlO5 phosphor via Mn2+ ions doping for thermometry

Here, this study reports single-band red upconversion emission in β-Ba2ScAlO5: Yb3+/Er3+ phosphor by doping Mn2+. The optimum concentration of Mn2+ ions in β-Ba2ScAlO5: Yb3+/Er3+ phosphor was 0.20. The intensity of red and green emissions is increased by 27.4 and 19.3 times, respectively. Compared with the samples without Mn2+ ions, the red-green integral strength ratio of β-Ba2ScAlO5: Yb3+/Er3+/Mn2+ sample was significantly increased by 28.4 times, reaching 110.9. The UCL mechanism was explored by analyzing the down-conversion luminescence spectra, absorption spectra, UCL spectra, and upconversion fluorescence lifetime decay curves of Yb3+/Er3+/Mn2+ co-doped β-Ba2ScAlO5. The enhancement of upconversion red light is achieved through energy transfer between defect bands and Er3+ ions, as well as energy transfer between Mn2+ ions and Er3+ ions. In addition, the Mn2+ doped β-Ba2ScAlO5: Yb3+/Er3+ red UCL phosphors have great potential for ambient temperature sensing in the 298–523 K temperature range. The maximum sensitivity of β-Ba2ScAlO5: Yb3+/Er3+/Mn2+ phosphor as a temperature sensor at 523 K is 0.0247 K−1.