High-sensitivity temperature sensing via color change: A study on the thermal coupling and non-thermal coupling energy levels in YVO 4 :Tm 3+ /Er 3+

High-sensitivity temperature sensing via color change: A study on the thermal coupling and non-thermal coupling energy levels in YVO 4 :Tm 3+ /Er 3+ | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article High-sensitivity temperature sensing via color change: A study on the thermal coupling and non-thermal coupling energy levels in YVO 4 :Tm 3+ /Er 3+ Zhensheng Lu, Hongxia Tang, Meilin Song, Changxing Yu, Changwen Wang This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9457832/v1 This work is licensed under a CC BY 4.0 License Status: Under Revision Version 1 posted 10 You are reading this latest preprint version Abstract In this study, a highly sensitive fluorescence temperature sensing strategy accompanied by color change, based on thermal population and non-radiative transition, has been developed. To enhance the temperature sensitivity in high-temperature regions and to observe significant color changes with the naked eye, the fluorescence spectrum based on thermal population and non-radiative transition has been discussed and designed. The co-doped YVO 4 :Tm 3+ /Er 3+ luminescent material was prepared by the high-temperature solid-phase method. Utilizing the strong absorption of ultraviolet (UV) by the V-O charge transfer band, the material was excited with 310 nm UV light. A significant color change from blue to blue-green and finally to green was achieved in the luminescent material within the temperature range of 450 K. The two fluorescence intensity ratios ( FIR ) were analyzed. In the high-temperature region, the FIR from the non-thermal coupling levels 1 G 4 and 2 H 11/2 satisfy the Boltzmann distribution. The relative sensitivity ( S r ) reached its maximum at 583 K, with a maximum value of 1.74% K − 1 . This strategy increased the sensitivity by 8.5 times at 583 K compared to the thermal coupling level temperature measurement strategy. A color transition recognizable by the naked eye was achieved within the 450 K range, demonstrating thermal anti-behavior, providing a reference for enhancing relative sensitivity in high-temperature regions. temperature sensing thermal coupling relative sensitivity colour change Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction Accurate temperature measurement is crucial in fields like aerospace, medical technology, and scientific research. Rare-earth ion-doped materials have gained wide attention in non-contact thermometry [ 1 – 6 ] . These materials offer several excellent features. They provide high relative sensitivity and rapid response times. Moreover, they do not rely on the inherent emissivity of the object. They also do not disrupt the measured temperature field. Currently, thermometry systems use either thermally coupled levels (TCLs) or non-thermally coupled levels (NTCLs) [ 7 , 8 ] . However, both systems rely on expensive spectral testing equipment. In contrast, color-recognition-based thermometry effectively reduces both device fabrication and practical application costs. This greatly broadens the application scenarios of fluorescence thermometry [ 9 ] . Therefore, developing new fluorescent materials based on color recognition is highly important. It has become a major research focus and a key challenge in the current field of non-contact fluorescence thermometry. However, rare-earth ions naturally have many characteristic emission levels. This causes their emission spectra in the visible range to show multiple peaks. To some extent, this limits color recognizability during fluorescence thermometry. For example, Meng et al. developed Pr 3+ -doped sensors that emit light in the red, green, yellow, and blue visible bands [ 10 ] . Singh K prepared Eu 3+ -based materials with emission peaks covering the green, red, and yellow regions [ 11 ] . Ding et al. developed Er 3+ upconversion materials that show dual emissions in both the red and green bands [ 12 ] . We believe that to specifically design color-recognition-based materials, we must systematically analyze the fundamental luminescence of rare-earth ions. Essentially, designing a temperature-dependent, color-recognizable sensor requires focusing on three core aspects: First, we must carefully select two luminescent centers from rare-earth ions. Both must have excellent monochromatic emission in the visible range. Second, focusing on the overall relative sensitivity, we must independently analyze and synergistically control the emission properties of both centers. This helps build a stable, color-recognizable system. Third, to improve color recognition sensitivity, we must build a dual-center system with inverse thermally induced emission behaviors. This means as temperature rises, one color's intensity decreases while the other's increases. This process must consider the emission properties of both ions, with a special focus on their anti-quenching performance. In this study, we developed a color-recognizable fluorescent thermometry material for the visible range. Based on a systematic analysis of emission mechanisms, we selected the characteristic emission levels of Tm 3+ and Er 3+ as the dual luminescent centers. We used yttrium vanadate (YVO 4 ) as the host. The target material was prepared using a high-temperature solid-state method. Through temperature-dependent emission spectra tests, we observed that Er 3+ produces green fluorescence. This green emission shows anti-thermal quenching and its intensity increases as temperature rises. Meanwhile, the blue fluorescence of Tm³⁺ decays rapidly with increasing temperature. Thus, we successfully built a dual-center system with inverse thermally induced emission behaviors. We quantitatively analyzed the fluorescence intensity ratio (FIR) of the dual centers. This provided an FIR-temperature calibration curve for temperature detection. We further analyzed key performance parameters, including absolute sensitivity Sa, relative sensitivity Sr, and temperature uncertainty ΔT . Based on this, we imported the temperature-dependent fluorescence spectral data into a chromaticity diagram for colorimetric analysis. We calculated the theoretical emission color at each temperature. We then compared and verified these calculations with the actual colors photographed in our experiments. We successfully achieved a continuous and reversible color change from blue to blue-green, and finally to green, in the temperature range of 323 K to 773 K. This study provides an important experimental and theoretical basis for the future targeted design of highly sensitive, color-recognizable fluorescent thermometry materials. 2. Experiments and Tests The YVO 4 :2% Tm 3+ 0.5% Er 3+ samples were synthesized using a high-temperature solid-state method. The precursor materials included Er 2 O 3 , Tm 2 O 3 , Y 2 O 3 , and NH 4 VO 3 (99.99% purity, Aladdin). First, stoichiometric amounts of these powders were accurately weighed using an electronic balance. They were then thoroughly ground for 1 hour to ensure uniform mixing. Next, the mixed powder was calcined in a muffle furnace at 1000°C for 6 hours. To evaluate the fluorescence properties, the prepared samples were pressed into pellets. The excitation and emission spectra were measured using a fluorescence spectrometer (Zolix Instruments Co., Ltd.). This spectrometer was equipped with a 450 W xenon lamp as the excitation source. 3. Results and discussion Analyzing the fluorescence behavior of dual-emitting centers requires understanding their excitation processes. Figure 1 (a) illustrates the specific excitation and de-excitation pathways. Under 310 nm ultraviolet (UV) excitation, electrons transition from the ground state to the excited state. This occurs through the charge transfer band (CTB) of the V-O bonds. Figure 1 (b) presents the excitation spectra of YVO 4 :Er 3+ /Tm 3+ , monitored at 474 nm corresponding to the characteristic 1 G 4 emission of Tm 3+ , and at 550 nm corresponding to the 4 S 3/2 emission of Er 3+ .Following this, the absorbed energy is transferred to the excited states of the rare-earth ions (Tm 3+ and Er 3+ ) via energy transfer. Finally, due to the crystal field effect, these excited rare-earth ions return to their ground states. This relaxation happens through either radiative or non-radiative transitions. This complete cycle defines the excitation and de-excitation processes of the material. To verify the thermal and fluorescence behaviors of the co-doped ions, variable-temperature emission spectra were measured. As shown in Fig. 2 , the samples were excited at 310 nm via the strong absorption of the V-O bond. This excitation produced a strong blue emission from the 1 G 4 → 3 H 6 transition of Tm³⁺. It also produced weak green emissions from the 2 H 11/2 → 4 I 15/2 and 4 S 3/2 → 4 I 15/2 transitions of Er³⁺. As the temperature increases, the 1 G 4 → 3 H 6 blue emission quenches rapidly. For Er³⁺, the 2 H 11/2 → 4 I 15/2 green emission initially increases and then decreases due to thermal population effects. Meanwhile, the 4 S 3/2 → 4 I 15/2 emission steadily decreases. We attribute the weakening of the Tm³⁺ blue emission to two combined factors. These are the weakened absorption of the V–O bond and enhanced non-radiative transitions. The Er³⁺ green emission is also affected by these same processes. However, there is an energy gap of approximately 850 cm − 1 between the 2 H 11/2 and 4 S 3/2 levels. Because of this gap, strong thermal population occurs. This thermal population partially offsets the fluorescence quenching effect. As a result, it enables a stronger thermal anti-quenching behavior. Both the strong blue and weak green emissions maintain good monochromaticity. However, a minor red emission is observed near 650 nm. This red emission also originates from the 1 G 4 level of Tm 3+ . Its intensity is low, and it quenches rapidly as the temperature rises. Therefore, it does not significantly affect the overall color transition in the visible range. To further analyze the thermal behavior, we calculated the integrated intensities of the three main emissions. Figure 3 illustrates these changes with temperature. Blue dots ( 1 G 4 → 3 H 6 of Tm 3+ ): This blue emission quenches significantly as the temperature rises. Notably, at 623 K, its intensity drops below the sum of the green emissions. This suggests the overall emission theoretically begins to change from blue to green in this range. At 743 K and 773 K, the blue emission becomes weaker than each individual green emission. This indicates a complete theoretical transition to a green color. Dark green dots ( 2 H 11/2 → 4 I 15/2 of Er 3+ ): This emission strengthens as the temperature increases from 323 K to 573 K. This happens due to the thermal population from the 4 S 3/2 level. From 573 K to 773 K, the intensity decreases. However, it remains stronger than its initial value at 323 K. This overall trend results from the combined effects of thermal population and non-radiative transitions. Light green dots ( 4 S 3/2 → 4 I 15/2 of Er 3+ ): This emission shows a continuous decrease in fluorescence intensity as the temperature increases. As mentioned earlier, the 2 H 11/2 and 4 S 3/2 levels of Er 3+ are a pair of thermally coupled levels (TCLs). Therefore, the intensity ratio of I 2 to I 3 is expected to follow the Boltzmann distribution. To further understand the fluorescence intensity ratio (FIR) of the energy levels, we applied two different strategies across different temperature ranges. As shown in Fig. 4 , we first analyzed the FIR ( I 2 / I 3 ) of the thermally coupled levels (TCLs) in a lower temperature range (323 K to 623 K). Then, we analyzed the FIR ( I 2 / I 1 ) of the non-thermally coupled levels (NTCLs) in a higher temperature range (593 K to 773 K). For TCLs, the particle population ratio between the two levels naturally follows the Boltzmann distribution. Similarly, for NTCLs, we hypothesize that their FIR also follows a Boltzmann-type distribution at high temperatures due to non-radiative transitions. To verify this, we fitted the temperature-dependent FIR data using the following equation [ 13 , 14 ] : $$\:R=B\text{e}\text{x}\text{p}\left(-\frac{\varDelta\:E}{kT}\right)$$ 1 , In Eq. ( 1 ), R represents the fluorescence intensity ratio. B is a constant related to the degeneracy of the energy levels. \(\:\varDelta\:E\) is the effective energy difference. k is the Boltzmann constant (with a value of 0.695 cm − 1 K − 1 , and T is the absolute thermodynamic temperature. As shown in Fig. 4 , the FIR ( I 2 / I 3 ) of the TCLs strictly follows the Boltzmann distribution between 323 K and 623 K. The fitting result yields \(\:\varDelta\:E\) /k = 1225 K − 1 . This value is highly consistent with the actual energy gap between the 2 H 11/2 and 4 S 3/2 levels in singly Er 3+ -doped systems [ 15 ] . Between 593 K and 773 K, the FIR ( I 2 / I 1 ) of the NTCLs also shows an excellent Boltzmann fit. This fitting gives \(\:\varDelta\:E\) /k = 8828 K − 1 . Notably, for NTCLs, this value does not represent a physical energy gap between the two emission levels. Rather, it indicates that the overall luminescence kinetics satisfy a Boltzmann distribution. In this temperature range, the 1 G 4 level of Tm 3+ experiences severe non-radiative transitions. Simultaneously, the upper level of Er 3+ is populated by thermal excitation from its lower level. These combined thermal effects drive the system to follow this distribution. To further evaluate the temperature sensing performance of thermally coupled and non-thermally coupled systems, the relative sensitivity was examined. The relative sensitivity S r is defined as [ 16 , 17 ] : $$\:{S}_{r}=\frac{1}{R}\frac{dR}{dT}\times\:100\%.$$ 2 Figure 5 presents the relative sensitivity curves derived from the two fluorescence intensity ratio (FIR) fits. For the thermally coupled system, the maximum relative sensitivity occurs at 323 K, reaching 0.83% K − 1 . As is typical for thermally coupled-based thermometers, S r decreases monotonically with increasing temperature. While the sensitivity is relatively high near room temperature, it declines to below 0.2% K − 1 at 623 K, consistent with theoretical predictions. This sensitivity can be partially enhanced by enlarging the energy gap between the thermally coupled levels. For the non-thermally coupled system, the maximum relative sensitivity is attained at 583 K, with a value of 1.74% K − 1 . This result indicates superior performance of NTC thermometry in the high-temperature regime. In the Tm 3+ /Er 3+ co-doped host, the 1 G 4 level of Tm 3+ undergoes strong thermal quenching at elevated temperatures due to enhanced non-radiative relaxation. The 2 H 11/2 level of Er 3+ is subject to the same quenching mechanisms. However, thermal population from the lower-lying 4 S 3/2 level offsets this quenching, sustaining the green emission intensity. The contrast between severe blue quenching and moderate green attenuation drives the observed chromatic transition. The realization of visually discernible color shifts over a limited temperature span is of considerable practical importance. Such behavior requires several conditions as below: distinct thermal quenching rates between the two emitting centers, sufficient spectral separation between their emission bands, and minimal interference from parasitic radiative transitions. In contrast to upconversion processes, where multiphoton excitation populates multiple emitting states in both Tm 3+ and Er 3+ , the down-conversion luminescence of Tm 3+ /Er 3+ in the YVO 4 matrix exhibits excellent monochromaticity. Using the CIE 1931 standard colorimetric system, the emission spectra were converted into chromaticity coordinates [ 18 – 20 ] . Figure 6 shows a pronounced color evolution—from blue to blue-green, and finally to green—is observed across the temperature range of 323– 773 K. The transition becomes particularly evident beyond 593 K, corroborating the relative sensitivity analysis. 4. Conclusions In summary, a sensitive fluorescence thermometry strategy based on thermal population and non-radiative transitions was demonstrated in YVO 4 :Tm 3+ /Er 3+ . The material exhibits a continuous color change from blue (323 K) to blue-green, then to green (773 K), driven by the strong thermal quenching of Tm 3+ blue emission and the anti-thermal behavior of Er 3+ green emission. FIR analysis shows that both thermally coupled ( 2 H 11/2 / 4 S 3/2 ) and non-thermally coupled ( 1 G 4 / 2 H 11/2 ) level pairs follow a Boltzmann-type temperature dependence. The relative sensitivity reaches 0.83% K⁻¹ at 323 K for the thermally coupled system and 1.74% K − 1 at 593 K for the non-thermally coupled system, an 8.5-fold enhancement over conventional thermally coupled thermometry. This dual-center design enables visually discernible color transitions over a 450 K span and provides a viable pathway for high-sensitivity optical thermometry in elevated-temperature regimes. Declarations Disclosures. The authors declare no conflicts of interest. Funding. Supported by the Project of Basic Scientific Research Funds of Heilongjiang Education Department (Grant Nos. YWF10236250214) Author Contribution Zhensheng Lu and Changwen Wang wrote the main manuscript text and Hongxia Tang and Meilin Song prepared figures. All authors reviewed the manuscript. Data availability. Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request. References Zhang B, Guo X, Zhang Z, Fu Z, Zheng H (2022) Luminescence thermometry with rare earth doped nanoparticles: Status and challenges. J Lumin 250:119110. https://doi.org/10.1016/j.jlumin.2022.119110 Maciel GS, Rakov N (2013) Anomalous up-conversion dynamics in rare-earth doped yttrium oxide powders. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9457832","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":638255149,"identity":"7d015940-1e6d-4d28-b87e-1eeedf26789c","order_by":0,"name":"Zhensheng Lu","email":"","orcid":"","institution":"Suihua University","correspondingAuthor":false,"prefix":"","firstName":"Zhensheng","middleName":"","lastName":"Lu","suffix":""},{"id":638255150,"identity":"fc896974-fbb3-4367-91bb-dddd6aabbc52","order_by":1,"name":"Hongxia Tang","email":"","orcid":"","institution":"Suihua University","correspondingAuthor":false,"prefix":"","firstName":"Hongxia","middleName":"","lastName":"Tang","suffix":""},{"id":638255151,"identity":"67ce8e0d-54a6-48af-88c2-9f8e6f5cce6d","order_by":2,"name":"Meilin Song","email":"","orcid":"","institution":"Suihua University","correspondingAuthor":false,"prefix":"","firstName":"Meilin","middleName":"","lastName":"Song","suffix":""},{"id":638255152,"identity":"935de83a-e7f4-4a03-a8b6-83ba31539731","order_by":3,"name":"Changxing Yu","email":"","orcid":"","institution":"Suihua University","correspondingAuthor":false,"prefix":"","firstName":"Changxing","middleName":"","lastName":"Yu","suffix":""},{"id":638255153,"identity":"7161c2da-56cb-4f5d-a554-e9515e93e2a5","order_by":4,"name":"Changwen Wang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABDElEQVRIiWNgGAWjYLACxgYoI6HCRo6fmfnwA+K1PDiTZizZzpZmQLQWxodthxM3nOdRkMCn2uD42cMvfu6wy5OPSD72ILEtzdj4MA+DAUONTTROLWfy0ix7zyQXG95ISzdIOGcjZ3aY98ADhmNpuQ04tJgdyDEzZmxjTtw4O8dMIqEszdjsMF+CAWPDYdxazr8BaakHasn/JpHAdjhxczOPgQReLTdyjB8zAn09XzqHTSIB5H1mAlrsb7wxY+xtO564Qf4Z0GHAQJY4DAzkBDx+kezPMf7ws606cX7P4WeSP0BR2X/48IMPNTY4tQABGzgWDA4giyXgVg4CzB9ApDweQ0fBKBgFo2CEAwCkD2Nm78koPwAAAABJRU5ErkJggg==","orcid":"","institution":"Suihua University","correspondingAuthor":true,"prefix":"","firstName":"Changwen","middleName":"","lastName":"Wang","suffix":""}],"badges":[],"createdAt":"2026-04-18 19:53:32","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9457832/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9457832/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":109090613,"identity":"59a1fa2c-d07b-4815-a2e2-8e850c00e038","added_by":"auto","created_at":"2026-05-12 13:33:06","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":120199,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Schematic diagram of the V-O charge transfer band, energy level diagrams of Tm\u003csup\u003e3+\u003c/sup\u003e and Er\u003csup\u003e3+\u003c/sup\u003e. (b) Excitation spectrum of YVO\u003csub\u003e4\u003c/sub\u003e:Er\u003csup\u003e3+\u003c/sup\u003e/Tm\u003csup\u003e3+\u003c/sup\u003e\u003c/p\u003e","description":"","filename":"image1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-9457832/v1/636f9ae78ab420a74c5f43e0.jpeg"},{"id":109092492,"identity":"2be65508-b617-49ef-a839-15c9ea77038c","added_by":"auto","created_at":"2026-05-12 13:41:27","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":152152,"visible":true,"origin":"","legend":"\u003cp\u003eTemperature-dependent emission spectra of YVO\u003csub\u003e4\u003c/sub\u003e:Tm\u003csup\u003e3+\u003c/sup\u003e/Er\u003csup\u003e3+\u003c/sup\u003e co-doped material excited by 310 nm ultraviolet light, where the color transition from blue to red represents the temperature range from 323 K to 773 K.\u003c/p\u003e","description":"","filename":"image2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-9457832/v1/3f0bee32e9cb53707eafdc71.jpeg"},{"id":109090560,"identity":"cf6abe1e-c2ed-42f3-b110-34e4a25193e8","added_by":"auto","created_at":"2026-05-12 13:32:24","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":112209,"visible":true,"origin":"","legend":"\u003cp\u003eTemperature-dependent integrated emission intensities of \u003cem\u003eI\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e (\u003csup\u003e1\u003c/sup\u003eG\u003csub\u003e4\u003c/sub\u003e-\u003csup\u003e3\u003c/sup\u003eH\u003csub\u003e6\u003c/sub\u003e), \u003cem\u003eI\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e (\u003csup\u003e2\u003c/sup\u003eH\u003csub\u003e11/2\u003c/sub\u003e-\u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e15/2\u003c/sub\u003e), and \u003cem\u003eI\u003c/em\u003e\u003csub\u003e3\u003c/sub\u003e (\u003csup\u003e4\u003c/sup\u003eS\u003csub\u003e3/2\u003c/sub\u003e-\u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e15/2\u003c/sub\u003e).\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-9457832/v1/b2c31a4d64119a564dfa0922.png"},{"id":109090810,"identity":"3d58e8a8-93f0-4468-9eea-cb205260118e","added_by":"auto","created_at":"2026-05-12 13:34:25","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":183623,"visible":true,"origin":"","legend":"\u003cp\u003eFluorescence intensities ratio of (a) thermally coupled energy levels (\u003cem\u003eI\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e/\u003cem\u003eI\u003c/em\u003e\u003csub\u003e3\u003c/sub\u003e) in the lower temperature range (323 K – 623 K), and (b) non-thermally coupled energy levels (\u003cem\u003eI\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e/\u003cem\u003eI\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e) of in the higher temperature range (593K to 773K). Fitted curve based on fluorescence intensity ratio.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-9457832/v1/5a08ec12693150dfa97e0b9f.png"},{"id":109092437,"identity":"51b48f16-f3e7-459b-bf64-712a434cf0e9","added_by":"auto","created_at":"2026-05-12 13:41:10","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":119018,"visible":true,"origin":"","legend":"\u003cp\u003eRelative sensitivity of \u003cem\u003eR\u003c/em\u003e(\u003cem\u003eI\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e/\u003cem\u003eI\u003c/em\u003e\u003csub\u003e3\u003c/sub\u003e) and \u003cem\u003eR\u003c/em\u003e(\u003cem\u003eI\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e/\u003cem\u003eI\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e)\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-9457832/v1/4fe16ac86dd1d62bb0947643.png"},{"id":109092439,"identity":"5bb82dca-9440-4893-a691-f9eaca67f201","added_by":"auto","created_at":"2026-05-12 13:41:11","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":102900,"visible":true,"origin":"","legend":"\u003cp\u003eColor coordinates and color change process of YVO\u003csub\u003e4\u003c/sub\u003e:Tm\u003csup\u003e3+\u003c/sup\u003e/Er\u003csup\u003e3+\u003c/sup\u003e co-doped luminescent materials at different temperatures. The insets are the luminescent colors of the samples photographed by a camera at temperatures of 323 K, 593 K, and 773 K.\u003c/p\u003e","description":"","filename":"image6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-9457832/v1/02a0b10950519ed054025b10.jpeg"},{"id":109094798,"identity":"dadcd2a2-7605-40da-8421-42dd4c058954","added_by":"auto","created_at":"2026-05-12 13:53:33","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1088601,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9457832/v1/987b7c6c-8a83-4977-8260-42e805a2ae99.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"High-sensitivity temperature sensing via color change: A study on the thermal coupling and non-thermal coupling energy levels in YVO 4 :Tm 3+ /Er 3+","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eAccurate temperature measurement is crucial in fields like aerospace, medical technology, and scientific research. Rare-earth ion-doped materials have gained wide attention in non-contact thermometry\u003csup\u003e[\u003cspan additionalcitationids=\"CR2 CR3 CR4 CR5\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e. These materials offer several excellent features. They provide high relative sensitivity and rapid response times. Moreover, they do not rely on the inherent emissivity of the object. They also do not disrupt the measured temperature field. Currently, thermometry systems use either thermally coupled levels (TCLs) or non-thermally coupled levels (NTCLs) \u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e. However, both systems rely on expensive spectral testing equipment. In contrast, color-recognition-based thermometry effectively reduces both device fabrication and practical application costs. This greatly broadens the application scenarios of fluorescence thermometry \u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e. Therefore, developing new fluorescent materials based on color recognition is highly important. It has become a major research focus and a key challenge in the current field of non-contact fluorescence thermometry.\u003c/p\u003e \u003cp\u003eHowever, rare-earth ions naturally have many characteristic emission levels. This causes their emission spectra in the visible range to show multiple peaks. To some extent, this limits color recognizability during fluorescence thermometry. For example, Meng et al. developed Pr\u003csup\u003e3+\u003c/sup\u003e-doped sensors that emit light in the red, green, yellow, and blue visible bands \u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e. Singh K prepared Eu\u003csup\u003e3+\u003c/sup\u003e-based materials with emission peaks covering the green, red, and yellow regions \u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e. Ding et al. developed Er\u003csup\u003e3+\u003c/sup\u003e upconversion materials that show dual emissions in both the red and green bands \u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e. We believe that to specifically design color-recognition-based materials, we must systematically analyze the fundamental luminescence of rare-earth ions. Essentially, designing a temperature-dependent, color-recognizable sensor requires focusing on three core aspects: First, we must carefully select two luminescent centers from rare-earth ions. Both must have excellent monochromatic emission in the visible range. Second, focusing on the overall relative sensitivity, we must independently analyze and synergistically control the emission properties of both centers. This helps build a stable, color-recognizable system. Third, to improve color recognition sensitivity, we must build a dual-center system with inverse thermally induced emission behaviors. This means as temperature rises, one color's intensity decreases while the other's increases. This process must consider the emission properties of both ions, with a special focus on their anti-quenching performance.\u003c/p\u003e \u003cp\u003eIn this study, we developed a color-recognizable fluorescent thermometry material for the visible range. Based on a systematic analysis of emission mechanisms, we selected the characteristic emission levels of Tm\u003csup\u003e3+\u003c/sup\u003e and Er\u003csup\u003e3+\u003c/sup\u003e as the dual luminescent centers. We used yttrium vanadate (YVO\u003csub\u003e4\u003c/sub\u003e) as the host. The target material was prepared using a high-temperature solid-state method. Through temperature-dependent emission spectra tests, we observed that Er\u003csup\u003e3+\u003c/sup\u003e produces green fluorescence. This green emission shows anti-thermal quenching and its intensity increases as temperature rises. Meanwhile, the blue fluorescence of Tm\u0026sup3;⁺ decays rapidly with increasing temperature. Thus, we successfully built a dual-center system with inverse thermally induced emission behaviors.\u003c/p\u003e \u003cp\u003eWe quantitatively analyzed the fluorescence intensity ratio (FIR) of the dual centers. This provided an FIR-temperature calibration curve for temperature detection. We further analyzed key performance parameters, including absolute sensitivity Sa, relative sensitivity Sr, and temperature uncertainty \u003cem\u003eΔT\u003c/em\u003e. Based on this, we imported the temperature-dependent fluorescence spectral data into a chromaticity diagram for colorimetric analysis. We calculated the theoretical emission color at each temperature. We then compared and verified these calculations with the actual colors photographed in our experiments. We successfully achieved a continuous and reversible color change from blue to blue-green, and finally to green, in the temperature range of 323 K to 773 K. This study provides an important experimental and theoretical basis for the future targeted design of highly sensitive, color-recognizable fluorescent thermometry materials.\u003c/p\u003e"},{"header":"2. Experiments and Tests","content":"\u003cp\u003eThe YVO\u003csub\u003e4\u003c/sub\u003e:2% Tm\u003csup\u003e3+\u003c/sup\u003e 0.5% Er\u003csup\u003e3+\u003c/sup\u003e samples were synthesized using a high-temperature solid-state method. The precursor materials included Er\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, Tm\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, Y\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, and NH\u003csub\u003e4\u003c/sub\u003eVO\u003csub\u003e3\u003c/sub\u003e (99.99% purity, Aladdin). First, stoichiometric amounts of these powders were accurately weighed using an electronic balance. They were then thoroughly ground for 1 hour to ensure uniform mixing. Next, the mixed powder was calcined in a muffle furnace at 1000\u0026deg;C for 6 hours. To evaluate the fluorescence properties, the prepared samples were pressed into pellets. The excitation and emission spectra were measured using a fluorescence spectrometer (Zolix Instruments Co., Ltd.). This spectrometer was equipped with a 450 W xenon lamp as the excitation source.\u003c/p\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003eAnalyzing the fluorescence behavior of dual-emitting centers requires understanding their excitation processes. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a) illustrates the specific excitation and de-excitation pathways. Under 310 nm ultraviolet (UV) excitation, electrons transition from the ground state to the excited state. This occurs through the charge transfer band (CTB) of the V-O bonds. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b) presents the excitation spectra of YVO\u003csub\u003e4\u003c/sub\u003e:Er\u003csup\u003e3+\u003c/sup\u003e/Tm\u003csup\u003e3+\u003c/sup\u003e, monitored at 474 nm corresponding to the characteristic \u003csup\u003e1\u003c/sup\u003eG\u003csub\u003e4\u003c/sub\u003e emission of Tm\u003csup\u003e3+\u003c/sup\u003e, and at 550 nm corresponding to the \u003csup\u003e4\u003c/sup\u003eS\u003csub\u003e3/2\u003c/sub\u003e emission of Er\u003csup\u003e3+\u003c/sup\u003e.Following this, the absorbed energy is transferred to the excited states of the rare-earth ions (Tm\u003csup\u003e3+\u003c/sup\u003e and Er\u003csup\u003e3+\u003c/sup\u003e) via energy transfer. Finally, due to the crystal field effect, these excited rare-earth ions return to their ground states. This relaxation happens through either radiative or non-radiative transitions. This complete cycle defines the excitation and de-excitation processes of the material.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo verify the thermal and fluorescence behaviors of the co-doped ions, variable-temperature emission spectra were measured. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, the samples were excited at 310 nm via the strong absorption of the V-O bond. This excitation produced a strong blue emission from the \u003csup\u003e1\u003c/sup\u003eG\u003csub\u003e4\u003c/sub\u003e\u0026rarr;\u003csup\u003e3\u003c/sup\u003eH\u003csub\u003e6\u003c/sub\u003e transition of Tm\u0026sup3;⁺. It also produced weak green emissions from the \u003csup\u003e2\u003c/sup\u003eH\u003csub\u003e11/2\u003c/sub\u003e\u0026rarr;\u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e15/2\u003c/sub\u003e and \u003csup\u003e4\u003c/sup\u003eS\u003csub\u003e3/2\u003c/sub\u003e\u0026rarr;\u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e15/2\u003c/sub\u003e transitions of Er\u0026sup3;⁺. As the temperature increases, the \u003csup\u003e1\u003c/sup\u003eG\u003csub\u003e4\u003c/sub\u003e\u0026rarr;\u003csup\u003e3\u003c/sup\u003eH\u003csub\u003e6\u003c/sub\u003e blue emission quenches rapidly. For Er\u0026sup3;⁺, the \u003csup\u003e2\u003c/sup\u003eH\u003csub\u003e11/2\u003c/sub\u003e\u0026rarr;\u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e15/2\u003c/sub\u003e green emission initially increases and then decreases due to thermal population effects. Meanwhile, the \u003csup\u003e4\u003c/sup\u003eS\u003csub\u003e3/2\u003c/sub\u003e\u0026rarr;\u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e15/2\u003c/sub\u003e emission steadily decreases. We attribute the weakening of the Tm\u0026sup3;⁺ blue emission to two combined factors. These are the weakened absorption of the V\u0026ndash;O bond and enhanced non-radiative transitions. The Er\u0026sup3;⁺ green emission is also affected by these same processes. However, there is an energy gap of approximately 850 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e between the \u003csup\u003e2\u003c/sup\u003eH\u003csub\u003e11/2\u003c/sub\u003e and \u003csup\u003e4\u003c/sup\u003eS\u003csub\u003e3/2\u003c/sub\u003e levels. Because of this gap, strong thermal population occurs. This thermal population partially offsets the fluorescence quenching effect. As a result, it enables a stronger thermal anti-quenching behavior. Both the strong blue and weak green emissions maintain good monochromaticity. However, a minor red emission is observed near 650 nm. This red emission also originates from the \u003csup\u003e1\u003c/sup\u003eG\u003csub\u003e4\u003c/sub\u003e level of Tm\u003csup\u003e3+\u003c/sup\u003e. Its intensity is low, and it quenches rapidly as the temperature rises. Therefore, it does not significantly affect the overall color transition in the visible range.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo further analyze the thermal behavior, we calculated the integrated intensities of the three main emissions. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e illustrates these changes with temperature. Blue dots (\u003csup\u003e1\u003c/sup\u003eG\u003csub\u003e4\u003c/sub\u003e\u0026rarr;\u003csup\u003e3\u003c/sup\u003eH\u003csub\u003e6\u003c/sub\u003e of Tm\u003csup\u003e3+\u003c/sup\u003e): This blue emission quenches significantly as the temperature rises. Notably, at 623 K, its intensity drops below the sum of the green emissions. This suggests the overall emission theoretically begins to change from blue to green in this range. At 743 K and 773 K, the blue emission becomes weaker than each individual green emission. This indicates a complete theoretical transition to a green color. Dark green dots (\u003csup\u003e2\u003c/sup\u003eH\u003csub\u003e11/2\u003c/sub\u003e\u0026rarr;\u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e15/2\u003c/sub\u003e of Er\u003csup\u003e3+\u003c/sup\u003e): This emission strengthens as the temperature increases from 323 K to 573 K. This happens due to the thermal population from the \u003csup\u003e4\u003c/sup\u003eS\u003csub\u003e3/2\u003c/sub\u003e level. From 573 K to 773 K, the intensity decreases. However, it remains stronger than its initial value at 323 K. This overall trend results from the combined effects of thermal population and non-radiative transitions. Light green dots (\u003csup\u003e4\u003c/sup\u003eS\u003csub\u003e3/2\u003c/sub\u003e\u0026rarr;\u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e15/2\u003c/sub\u003e of Er\u003csup\u003e3+\u003c/sup\u003e): This emission shows a continuous decrease in fluorescence intensity as the temperature increases. As mentioned earlier, the \u003csup\u003e2\u003c/sup\u003eH\u003csub\u003e11/2\u003c/sub\u003e and \u003csup\u003e4\u003c/sup\u003eS\u003csub\u003e3/2\u003c/sub\u003e levels of Er\u003csup\u003e3+\u003c/sup\u003e are a pair of thermally coupled levels (TCLs). Therefore, the intensity ratio of \u003cem\u003eI\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e to \u003cem\u003eI\u003c/em\u003e\u003csub\u003e3\u003c/sub\u003e is expected to follow the Boltzmann distribution.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo further understand the fluorescence intensity ratio (FIR) of the energy levels, we applied two different strategies across different temperature ranges. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, we first analyzed the FIR (\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e/\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e) of the thermally coupled levels (TCLs) in a lower temperature range (323 K to 623 K). Then, we analyzed the FIR (\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e/\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e) of the non-thermally coupled levels (NTCLs) in a higher temperature range (593 K to 773 K). For TCLs, the particle population ratio between the two levels naturally follows the Boltzmann distribution. Similarly, for NTCLs, we hypothesize that their FIR also follows a Boltzmann-type distribution at high temperatures due to non-radiative transitions. To verify this, we fitted the temperature-dependent FIR data using the following equation \u003csup\u003e[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:R=B\\text{e}\\text{x}\\text{p}\\left(-\\frac{\\varDelta\\:E}{kT}\\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e,\u003c/p\u003e \u003cp\u003eIn Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), \u003cem\u003eR\u003c/em\u003e represents the fluorescence intensity ratio. \u003cem\u003eB\u003c/em\u003e is a constant related to the degeneracy of the energy levels. \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:E\\)\u003c/span\u003e\u003c/span\u003e is the effective energy difference. \u003cem\u003ek\u003c/em\u003e is the Boltzmann constant (with a value of 0.695 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and \u003cem\u003eT\u003c/em\u003e is the absolute thermodynamic temperature.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the FIR (\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e/\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e) of the TCLs strictly follows the Boltzmann distribution between 323 K and 623 K. The fitting result yields \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:E\\)\u003c/span\u003e\u003c/span\u003e/k\u0026thinsp;=\u0026thinsp;1225 K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. This value is highly consistent with the actual energy gap between the \u003csup\u003e2\u003c/sup\u003eH\u003csub\u003e11/2\u003c/sub\u003e and \u003csup\u003e4\u003c/sup\u003eS\u003csub\u003e3/2\u003c/sub\u003e levels in singly Er\u003csup\u003e3+\u003c/sup\u003e-doped systems \u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e. Between 593 K and 773 K, the FIR (\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e/\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e) of the NTCLs also shows an excellent Boltzmann fit. This fitting gives \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:E\\)\u003c/span\u003e\u003c/span\u003e/k\u0026thinsp;=\u0026thinsp;8828 K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Notably, for NTCLs, this value does not represent a physical energy gap between the two emission levels. Rather, it indicates that the overall luminescence kinetics satisfy a Boltzmann distribution. In this temperature range, the\u003csup\u003e1\u003c/sup\u003eG\u003csub\u003e4\u003c/sub\u003e level of Tm\u003csup\u003e3+\u003c/sup\u003e experiences severe non-radiative transitions. Simultaneously, the upper level of Er\u003csup\u003e3+\u003c/sup\u003e is populated by thermal excitation from its lower level. These combined thermal effects drive the system to follow this distribution.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo further evaluate the temperature sensing performance of thermally coupled and non-thermally coupled systems, the relative sensitivity was examined. The relative sensitivity \u003cem\u003eS\u003c/em\u003er is defined as \u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e:\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:{S}_{r}=\\frac{1}{R}\\frac{dR}{dT}\\times\\:100\\%.$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e presents the relative sensitivity curves derived from the two fluorescence intensity ratio (FIR) fits. For the thermally coupled system, the maximum relative sensitivity occurs at 323 K, reaching 0.83% K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. As is typical for thermally coupled-based thermometers, \u003cem\u003eS\u003c/em\u003er decreases monotonically with increasing temperature. While the sensitivity is relatively high near room temperature, it declines to below 0.2% K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 623 K, consistent with theoretical predictions. This sensitivity can be partially enhanced by enlarging the energy gap between the thermally coupled levels.\u003c/p\u003e \u003cp\u003eFor the non-thermally coupled system, the maximum relative sensitivity is attained at 583 K, with a value of 1.74% K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. This result indicates superior performance of NTC thermometry in the high-temperature regime. In the Tm\u003csup\u003e3+\u003c/sup\u003e/Er\u003csup\u003e3+\u003c/sup\u003e co-doped host, the \u003csup\u003e1\u003c/sup\u003eG\u003csub\u003e4\u003c/sub\u003e level of Tm\u003csup\u003e3+\u003c/sup\u003e undergoes strong thermal quenching at elevated temperatures due to enhanced non-radiative relaxation. The \u003csup\u003e2\u003c/sup\u003eH\u003csub\u003e11/2\u003c/sub\u003e level of Er\u003csup\u003e3+\u003c/sup\u003e is subject to the same quenching mechanisms. However, thermal population from the lower-lying \u003csup\u003e4\u003c/sup\u003eS\u003csub\u003e3/2\u003c/sub\u003e level offsets this quenching, sustaining the green emission intensity. The contrast between severe blue quenching and moderate green attenuation drives the observed chromatic transition.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe realization of visually discernible color shifts over a limited temperature span is of considerable practical importance. Such behavior requires several conditions as below: distinct thermal quenching rates between the two emitting centers, sufficient spectral separation between their emission bands, and minimal interference from parasitic radiative transitions. In contrast to upconversion processes, where multiphoton excitation populates multiple emitting states in both Tm\u003csup\u003e3+\u003c/sup\u003e and Er\u003csup\u003e3+\u003c/sup\u003e, the down-conversion luminescence of Tm\u003csup\u003e3+\u003c/sup\u003e/Er\u003csup\u003e3+\u003c/sup\u003e in the YVO\u003csub\u003e4\u003c/sub\u003e matrix exhibits excellent monochromaticity. Using the CIE 1931 standard colorimetric system, the emission spectra were converted into chromaticity coordinates \u003csup\u003e[\u003cspan additionalcitationids=\"CR19\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows a pronounced color evolution\u0026mdash;from blue to blue-green, and finally to green\u0026mdash;is observed across the temperature range of 323\u0026ndash; 773 K. The transition becomes particularly evident beyond 593 K, corroborating the relative sensitivity analysis.\u003c/p\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eIn summary, a sensitive fluorescence thermometry strategy based on thermal population and non-radiative transitions was demonstrated in YVO\u003csub\u003e4\u003c/sub\u003e:Tm\u003csup\u003e3+\u003c/sup\u003e/Er\u003csup\u003e3+\u003c/sup\u003e. The material exhibits a continuous color change from blue (323 K) to blue-green, then to green (773 K), driven by the strong thermal quenching of Tm\u003csup\u003e3+\u003c/sup\u003e blue emission and the anti-thermal behavior of Er\u003csup\u003e3+\u003c/sup\u003e green emission. FIR analysis shows that both thermally coupled (\u003csup\u003e2\u003c/sup\u003eH\u003csub\u003e11/2\u003c/sub\u003e/\u003csup\u003e4\u003c/sup\u003eS\u003csub\u003e3/2\u003c/sub\u003e) and non-thermally coupled (\u003csup\u003e1\u003c/sup\u003eG\u003csub\u003e4\u003c/sub\u003e/\u003csup\u003e2\u003c/sup\u003eH\u003csub\u003e11/2\u003c/sub\u003e) level pairs follow a Boltzmann-type temperature dependence. The relative sensitivity reaches 0.83% K⁻\u0026sup1; at 323 K for the thermally coupled system and 1.74% K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 593 K for the non-thermally coupled system, an 8.5-fold enhancement over conventional thermally coupled thermometry. This dual-center design enables visually discernible color transitions over a 450 K span and provides a viable pathway for high-sensitivity optical thermometry in elevated-temperature regimes.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eDisclosures.\u003c/h2\u003e \u003cp\u003eThe authors declare no conflicts of interest.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding.\u003c/h2\u003e \u003cp\u003eSupported by the Project of Basic Scientific Research Funds of Heilongjiang Education Department (Grant Nos. YWF10236250214)\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eZhensheng Lu and Changwen Wang wrote the main manuscript text and Hongxia Tang and Meilin Song prepared figures. All authors reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eData availability.\u003c/h2\u003e \u003cp\u003eData underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eZhang B, Guo X, Zhang Z, Fu Z, Zheng H (2022) Luminescence thermometry with rare earth doped nanoparticles: Status and challenges. J Lumin 250:119110. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jlumin.2022.119110\u003c/span\u003e\u003cspan address=\"10.1016/j.jlumin.2022.119110\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMaciel GS, Rakov N (2013) Anomalous up-conversion dynamics in rare-earth doped yttrium oxide powders. 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J Photochem Photobiol A 459:116044. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jphotochem.2024.116044\u003c/span\u003e\u003cspan address=\"10.1016/j.jphotochem.2024.116044\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"journal-of-fluorescence","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jofl","sideBox":"Learn more about [Journal of Fluorescence](https://www.springer.com/journal/10895)","snPcode":"10895","submissionUrl":"https://submission.nature.com/new-submission/10895/3","title":"Journal of Fluorescence","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"temperature sensing, thermal coupling, relative sensitivity, colour change","lastPublishedDoi":"10.21203/rs.3.rs-9457832/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9457832/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn this study, a highly sensitive fluorescence temperature sensing strategy accompanied by color change, based on thermal population and non-radiative transition, has been developed. To enhance the temperature sensitivity in high-temperature regions and to observe significant color changes with the naked eye, the fluorescence spectrum based on thermal population and non-radiative transition has been discussed and designed. The co-doped YVO\u003csub\u003e4\u003c/sub\u003e:Tm\u003csup\u003e3+\u003c/sup\u003e/Er\u003csup\u003e3+\u003c/sup\u003e luminescent material was prepared by the high-temperature solid-phase method. Utilizing the strong absorption of ultraviolet (UV) by the V-O charge transfer band, the material was excited with 310 nm UV light. A significant color change from blue to blue-green and finally to green was achieved in the luminescent material within the temperature range of 450 K. The two fluorescence intensity ratios (\u003cem\u003eFIR\u003c/em\u003e) were analyzed. In the high-temperature region, the \u003cem\u003eFIR\u003c/em\u003e from the non-thermal coupling levels \u003csup\u003e1\u003c/sup\u003eG\u003csub\u003e4\u003c/sub\u003e and \u003csup\u003e2\u003c/sup\u003eH\u003csub\u003e11/2\u003c/sub\u003e satisfy the Boltzmann distribution. The relative sensitivity (\u003cem\u003eS\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e) reached its maximum at 583 K, with a maximum value of 1.74% K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. This strategy increased the sensitivity by 8.5 times at 583 K compared to the thermal coupling level temperature measurement strategy. A color transition recognizable by the naked eye was achieved within the 450 K range, demonstrating thermal anti-behavior, providing a reference for enhancing relative sensitivity in high-temperature regions.\u003c/p\u003e","manuscriptTitle":"High-sensitivity temperature sensing via color change: A study on the thermal coupling and non-thermal coupling energy levels in YVO 4 :Tm 3+ /Er 3+","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-05-12 13:04:15","doi":"10.21203/rs.3.rs-9457832/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-05-11T12:27:16+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-10T05:20:17+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"192333465483329782913814120012155130307","date":"2026-05-09T16:58:50+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-08T04:53:19+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"78595261570529588288005764931089368141","date":"2026-05-05T03:46:30+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"328024862877023231492352727845158682382","date":"2026-05-05T01:20:29+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-05-04T16:57:17+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-04-24T01:03:20+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-04-24T01:02:28+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Fluorescence","date":"2026-04-18T19:49:15+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-fluorescence","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jofl","sideBox":"Learn more about [Journal of Fluorescence](https://www.springer.com/journal/10895)","snPcode":"10895","submissionUrl":"https://submission.nature.com/new-submission/10895/3","title":"Journal of Fluorescence","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"2648f725-ba39-450e-b70a-44ed4122dadc","owner":[],"postedDate":"May 12th, 2026","published":true,"recentEditorialEvents":[{"type":"decision","content":"Revision requested","date":"2026-05-11T12:27:16+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-10T05:20:17+00:00","index":21,"fulltext":""},{"type":"reviewerAgreed","content":"192333465483329782913814120012155130307","date":"2026-05-09T16:58:50+00:00","index":20,"fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-08T04:53:19+00:00","index":19,"fulltext":""},{"type":"reviewerAgreed","content":"78595261570529588288005764931089368141","date":"2026-05-05T03:46:30+00:00","index":17,"fulltext":""},{"type":"reviewerAgreed","content":"328024862877023231492352727845158682382","date":"2026-05-05T01:20:29+00:00","index":16,"fulltext":""},{"type":"reviewersInvited","content":"7","date":"2026-05-04T16:57:17+00:00","index":"","fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[],"tags":[],"updatedAt":"2026-05-12T13:04:16+00:00","versionOfRecord":[],"versionCreatedAt":"2026-05-12 13:04:15","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9457832","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9457832","identity":"rs-9457832","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}