Low-temperature molten salt enabled synthesis of solid-state emissive carbon dots with high endurance and >99% quantum yields

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Herein, we develop a facile molten salt method to achieve the one-step synthesis of full-color CDs with efficient solid-state emission. Comprehensively, kilogram-scale solid-state CDs with a quantum yield of 90% can be readily synthesized via a salt-assisted approach under mild conditions (100–142°C) within 10 minutes. The spectral characterization and density functional theory calculation confirm that zinc ion coordination can occur in liquated environment, which facilitates the polymerization of precursors at lower temperatures, suppresses the formation of non-radiative recombination channels on their surface, and further enhances luminescence in solid. The machine learning is further used to optimize CDs’ luminous efficiency up to 99.86%, evoking excellent performance CDs-based light-emitting diodes with a maximum luminous efficiency of 156.29 lm W – 1 to drive backlit display with a long-persistent lifetime (T 95 at 100 cd m –2 = 45108.7 h). This work provides a pathway for the design and fabrication of advanced carbon-based solid-state luminescent materials, significantly contributing to the advancement of next-generation lighting and display technologies. Physical sciences/Nanoscience and technology/Nanoscale materials/Quantum dots Physical sciences/Materials science/Nanoscale materials/Nanoparticles Carbon dots Molten salt Solid-state fluorescence Light emitting diodes displays Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Stable phosphors have been fundamental to modern illumination, displays and information exchange, and thus bright fluorescent materials have gained extensive attention all the time. 1 – 4 Due to the potential environmental pollution, high-cost, or low rendering index of classical rare earth-based phosphors, the fluorescent nanomaterials, including semiconductor quantum dots, perovskite nanocrystals, metal clusters, have been widely developed to satisfy the demanding requirements of modern display technology. 5 – 9 Especially in recently years, fluorescent carbon dots (CDs), as an emerging carbon-based nanomaterial, have garnered significant attentions for their tunable emission wavelength, high rendering index, low environment pollution, low toxicity, etc., endowing the promising applications in illumination and displays. 10 – 14 Nevertheless, the employment of the CDs is always limited by the complex preparation process and potential aggregation-caused quenching (ACQ). 15 – 16 To overcome this drawback, diverse strategies, such as confined aggregation, polymer dispersion, self-crosslink, etc., 17 – 21 have been employed to endow the CDs with high solid-state fluorescence (SSF). Since these progressions have gained significant achievements in solid-state fluorescent CDs, there are still numerous challenges: (i) solid-state photoluminescence quantum yields (SS QY) of most SSF CDs are still quite low; (ii) preparation process of SSF CDs always require high-cost precursors and intricate synthesis techniques with high temperature; (iii) scalable synthetic method are difficult to achieve, hindering industrial production and commercial applications (iv) CDs-based solid-state phosphors combining high luminous efficiency and long-term durability have yet to be realized. Therefore, it is still an urgent challenge to develop a facile approach to large-scale synthesis of CDs with high SS QY. Molten salt, as a molten liquid of salts composed of alkali metals or alkaline earth metals, halides, silicates, carbonates, nitrates, and phosphates, etc., have been demonstrated an excellent medium to synthesize the high quality of nanomaterials, such as metal oxides nanoparticles, perovskite nanocrystals, or two-dimensional transition metal dichalcogenides, etc. 22 – 25 With the inherit advantages of low reaction temperature, low viscosity, simplicity, and cost-effectiveness, the molten salts can enable these nanomaterials to spontaneously form and grow in a high quality and large-scale synthesis. 22 , 26 Recently, there have been several researches about the molten salt-assisted synthesis of fluorescent CDs. 27 – 30 Nevertheless, numerous technical challenges still remain in these strategies: (i) the high melting-point of previously used molten salts (> 350°C) may cause the excessive polymerization and carbonization of precursor molecules and thus lead to the reduction in SSF emission (SS QY 1.5 hours) render it difficult to obtain SSF CDs regularly and promptly; (iii) the adverse factors like synthetic repeatability, poor light stability and intricate technological post-treatment result in difficult in the translation from laboratory to industrial production. Therefore, it is still an eager requirement to fully utilize the advantages of molten salts to prepare SSF CDs in high quality and large-scale, ensuring a feasible pathway for the industrial production and application of CDs. In this work, we innovatively employed a low-melting-point molten salts systems, which are composed of sodium chloride (NaCl), potassium chloride (KCl), and zinc chloride (ZnCl₂), to successfully achieve the synthesis of SSF CDs in a high quality and large-scale synthesis. The fluorescent CDs with high solid-state photoluminescence quantum yield of 8.62%-82.51% can be easily prepared under a low temperature (100–142°C) in a short reaction time (5–10 min). With the detailed experiment surveys and theoretical calculations, it reveals that the molten salt can reduce the Gibbs free energy by altering the polymerization pathway of small molecules and meanwhile provide zinc ions for coordination on the surface of CDs to effectively suppress non-radiative transition, thereby reducing the reaction temperature and improve the SSF. By further utilizing the machine learning techniques to optimize the reaction conditions, the SSF CDs with an unprecedented SS QY of ~ 99.86% can be prepared and further employed as phosphors to fabricate high-efficiency CD-based light-emitting diodes (LEDs) with luminous efficiency of 156.29 lm W − 1 , endowing the excellent backlight performance as down-conversion luminescent film with long duration of 45108.7 h. This work provides a novel approach to low-cost and large-scale synthesis of superior SSF CDs, and may further put forward the groundwork of CDs in industrial illumination and display technologies. Results and Discussion Synthesis and optical properties of the full-color SSF CDs As illustrated in Fig. 1 a, the SSF CDs were synthesized with different precursors in a molten salts system which is composed of NaCl, KCl, and ZnCl 2 (Table S1 ). By regulating the precursors and reaction factors, the as-prepared B-, G-, Y-, and NIR-CDs present blue, green, yellow and near-infrared SSF under 365 nm UV lamp (Fig. S1 ). As exhibited in Fig. 1 b-e, these B-, G-, Y-, and NIR-CDs present a SSF peak around 432 nm, 517 nm, 594 nm, and 660 nm with distinct UV-vis absorption. Herein, the B-, G-, Y-, and NIR-CDs present different absorption bands, which are corresponding to the π-π* transition of sp² carbon, n-π* transition of surface heteroatom functional groups. The corresponding SSF spectra reveal a significant color transition from blue to near-infrared region in a Commission Internationale de l’Eclairage (CIE) coordinate (Fig. S2), which is corresponding to the observation in naked-eyes. As shown in Fig. 1 f-i, the excitation-emission mapping of the B-, G-, Y- and NIR-CDs a maximal SSF center at (335 nm, 430 nm), (430 nm, 517 nm), (560 nm, 594 nm) and (640 nm, 660 nm). Herein, the SSF emission of the B-, G-, Y- and NIR-CDs illustrate a decreased excitation-dependence. The PL QYs of the B-, G-, Y- and NIR-CDs are measured with integrating sphere under their optimal excitation wavelength (Fig. S3) and the results are calculated as 62.77%, 82.51%, 63.46%, and 8.62%, which are ever relative high values among the reported SSF CDs (Table S2). The time-resolved PL decay of these SSF CDs can be well fitted with a biexponential curves (Fig. S4) and the results reveal an average lifetime of 6.47 ns, 16.16 ns, 10.18 ns, and 1.07 ns for the B-CDs, G-CDs, Y-CDs, and NIR-CDs, respectively. Structure and components of the full-color SSF CDs To further investigate the excellent optical properties, the SSF CDs were treated with dialysis purification to remove the molten salts. As depicted in Fig. 2 a-d, the transmission electron microscopy (TEM) images reveal that the SSF B-, G-, Y-, and NIR-CDs contain well-dispersed spherical particles, and the high-resolution TEM (HRTEM) image indicate an interplanar lattice spacing of 0.21 nm for all the CDs, which is corresponding to the (100) plane of graphitic carbon. 31 – 33 The statistical data reveal an approximate size distribution around 3.41, 3.18, 2.46, and 2.63 nm for the B-, G-, Y-, and NIR-CDs (Fig. 2 e). The size-independent emission implies that the SSF of the CDs are not just from the quantum size effect. In the X-ray diffraction (XRD) patterns, all these SSF CDs present the peaks of crystalline molten salts and an obvious a characteristic amorphous carbon peak around 25.6°can be observed (Fig. 2 f), indicating the successful carbonization and polymerization of CDs from the precursors. 19 , 34 The chemical competent of the SSF B-, G-, Y-, and NIR-CDs was analyzed with Fourier-transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). After removing the molten salts, all these CDs samples present the C = C vibration peak at 1572 cm – 1 , the C = O vibration peak at 1630 cm – 1 , and OH vibration peak at 1406 cm – 1 , and the NIR-CDs present obvious C–N vibration peak (1113 cm – 1 ) and N–H vibration peak (1494 cm – 1 and 754 cm – 1 ), implying the N-related long-wavelength emission. 35 – 37 The full XPS survey for the pure CDs with dialysis reveal the elements of C (~ 284 eV), N (~ 400 eV), O (~ 531 eV) and Zn (~ 1022 eV) (Table S3 and Table S4), implying the surface metal ions from molten salt. 38 – 39 Meanwhile, the high-resolution XPS C1s spectra reveal the C–C/C = C, C–O/C–N, and C = O (Fig. 2 i-l), and the high-resolution O1s spectra reveal the C = O and C–O (Fig. 2 m-p), further implying the surface ion exchange between the CDs and molten salts. Additionally, these CDs present the pyridine N, pyrrole N and graphite N in the N1s spectra (Fig. S5), suggesting the different light emission principles of B-, G-, Y-, and NIR-CDs. The formation and light emission mechanism of the SSF CDs To investigate the formation and light emission mechanism of the SSF CDs, more surveys are executed. To test the temperature threshold for the formation of SSF CDs, the reaction temperatures were continuously recorded until the sample was fully formed. As illustrated in Fig. 3 a-d, the highest recorded temperature of the B-, G-, Y-, and NIR-CDs are as low as 100°C, 110°C, 120°C, and 142°C, respectively. Such low temperature can enable the chemical reaction of surface passivation or branch linking to further improve the light emission of CDs or develop furtherword applications. After dissolved in DMF, these CDs solutions present obvious different light emission from the SSF CDs powder. As shown in Fig. 3 e-h, the maximal excitation-emission centers of the CDs solutions present slight blueshift to (365 nm, 450 nm), (445 nm, 495 nm), (475 nm, 510 nm), and (450 nm, 520 nm). Meanwhile, the PL QYs of the B-, G-, Y-, and NIR-CDs are 14.93%, 57.72%, 1.28% and 4.11%, which are far lower than their SSF powder. On the condition, the density functional theory (DFT) calculations are further employed to explore the mechanism. On account of the G-CDs from the citric acid and urea in molten salts, the strong p-π conjugation between the hydroxyl and carbonyl oxygen in citric acid delocalizes the π electrons and further increase the polarity of O–H bond, making it easier to combines with –NH 2 to form ammonia gas. Thus, the citric acid and urea can self-link and further form the CDs through polymerization and carbonization. When there are molten salts in the reaction, the salts ions can provide a liquid environment to facilitate the departure of H + and the citric acid and urea are more likely to undergo dehydration condensation between –COOH and –NH 2 to form polymers, enabling the formation of CDs. The corresponding Gibbs free energy change (ΔG) of the CDs with and without molten salts can be calculated. As depicted in Fig. 3 i-j, the ΔG from free citric acid and urea to polymer is 0.52 eV while the ΔG from the citric acid and urea in molten salts is -27.64 eV. With such low ΔG, the reaction of citric acid and urea can spontaneously undergo, enabling the low temperature threshold for the formation of SSF CDs. The corresponding light emission mechanism are further investigated with the same model. As illustrated in Fig. 3 k, the highest occupied molecular orbits (HOMO) and lowest unoccupied molecular orbits (LUMO) energy levels of CDs with and without surface zinc ion are compared with density functional theory (DFT). Herein, the CDs with and without surface zinc ion exhibit slight reduced bandgap, indicating that the zinc ions can trigger the redshifted emission. Meanwhile, the average vibrational frequency of CDs is 1122.05 while the SSF CDs is 1060.36. The lower average vibrational frequency of SSF CDs implies that the surface zinc ion can suppress the vibrational deactivation pathway of the S1 state and thus reduce the nonirradiation, enabling the enhancement of SSF (Fig. 3 l). The hypothesis also can be demonstrated with another survey. As shown in Fig. S6 and Fig. S7, when the SSF CDs are prepared in molten salts without ZnCl 2 or with other zinc salts, the products present different SSF performance, in which the sample prepared without ZnCl 2 exhibits significantly reduced FL intensity and the sample prepared with zinc chloride, zinc sulfate, zinc nitrate, and zinc acetate are exhibit approximate SSF emission as the SSF CDs. Machine learning-assisted improvements of the SSF CDs On account of the SSF enhancement from molten salts, we believe that the reaction factors can influence the SSF performances of these CDs and thereby the SS QY can be further improved. To achieve this hypothesis, the machine learning techniques are employed to optimize the reaction parameters. 40 – 42 Herein, a dataset was constructed by preparing 120 samples in laboratory, and the citric acid mass (CA), urea mass (UA), NaCl mass (NaCl), KCl mass (KCl), ZnCl 2 mass (ZnCl 2 ), reaction time (Time), and reaction temperature (Temp) were recorded as features, and the measured Photoluminescence quantum yield (PLQY) were set as the labels. Before training the model, we preprocessed the dataset and evaluated the mutual information between the features and the labels. As illustrated in Fig. 4 a, the results reveal that ZnCl₂ did not significantly contribute to the labels, speculating that the range from the collected ZnCl₂ data may exceed the threshold of contribution to the PLQY. Therefore, the ZnCl₂ feature was removed from the feature set. The Pearson correlation coefficient matrices present a strong positive correlation between features CA and UA (Fig. 4 b). To avoid multicollinearity, the feature transformation was performed, and the Ratio (UA/CA) and Sum (UA + CA) were added into the feature sets. After feature transformation, all feature pairs showed low linear correlation (Fig. 4 c), approving the effectiveness of the feature selection. During the training phase, the performance of XGBoost (XGB), random forest (RF), support vector regression (SVR), and elastic net (ELN) were evaluated on the dataset with the grid search to optimize the model parameters and evaluate the performances with root mean square error (RMSE) and the coefficient of determination (R 2 ). As depicted in Fig. 4 d and Fig. 4 e, the results revealed that the XGBoost outperformed the lower RMSE and the higher R 2 than other models on this dataset, demonstrating its excellent generalization ability. Thus, the XGBoost were selected to perform the PLQY prediction task. With the optimal feature combination to maximize the PLQY, a high-density feature grid involving 134,198,064 feature combinations were built and input into the XGBoost for PLQY prediction (Table S5). As a result, the XGBoost can output a highest predicted PLQY and the parameter was used to prepare sample. As shown in Fig. 4 f, the as-prepared CDs present a solid-state PLQY as high as 99.86%, which is the highest value in ever reported solid-state CDs (Table S1 ). On the condition, the laboratory factors were further employed in a large-scale synthesis and ~ 1,338.5 g of SSF CDs were obtained in a single reaction (Fig. 4 g), indicating the promising potential of molten salt-assisted SSF CDs in industrial production. In addition, after continuously illumination under 365 nm lamp, the as-prepared G-CDs powder present almost the same SSF emission (Fig. 4 h and Fig. 4 i), demonstrating their excellent photostability. Multi-color LED and backlit displays application of the SSF CDs The full-color light emission wavelength and high PL QY inspire us to develop the CDs as phosphors in light-emitting diodes (LEDs). Herein, the SSF CDs powder was first evenly mixed with polydimethylsiloxane and then coated onto a 365 nm UV or 400 nm blue LED chips (Fig. S8). The as-prepared LEDs from B-, G-, Y-, and NIR-CDs are named as B-, G-, Y-, and NIR-LEDs, respectively. Figure S9 shows the actual photographs of these LEDs, and Fig. S10 presents the emission spectra of these LEDs, indicating the light emission conversion from the SSF CDs. The corresponding CIE coordinates also demonstrate the success light conversion (Fig. S11). On the condition, the luminous efficiency of these LEDs devices was measured to evaluate the luminescent performance. As shown in Fig. 5 a-d, the B-, G-, Y-, and NIR-LEDs present a maximum luminous efficiency of 23.46, 156.29 and 15.26 and 0.03 lm W – 1 , respectively. The impressive performance of G-LEDs is almost the highest values in the CDs-based or QDs-based LED devices (Table S6), approving the potential applications of these SSF CDs in light conversion. In recent years, high-brightness mini-LED backlight displays have become increasingly popular for high-end applications and thus the optimized G-CDs were further used as the phosphors in mini-LED. Subsequently, a 450 nm blue mini-LED chip-on-board (COB) were prepared with a green-emitting and red-emitting conversion film of G-CDs and (Sr,Ca)AlSiN 3 (Fig. S12 and Fig. S13). As a result, these two color conversion films were combined and successful to create a mini-LED white backlight (Fig. S14). As shown in Fig. 5 e, the mini-LED backlight was integrated with a commercial TFT-LCD panel to design a complete display device (Fig. S15 and Fig. S16). The EL emission spectrum of the mini-LED white backlight present obvious white color and the CIE coordinate is calculated as (0.31, 0.29) (Figure S17). With an initial luminance of 10599.8 cd cm – 2 , the T 95 (the time that the luminance intensity decreases to 95% initial value) was 10.69 hours (Fig. 5 g). With the formula L 0 n T 95 = constant, the T 95 values were estimated as 731.6 hours at initial 1000 cd cm – 2 and 45108.7 hours at initial 100 cd cm – 2 , respectively. The long lifespan of the mini-LED white backlights demonstrates their durability for long-term display applications. In addition, as a simple demonstration, the display exhibits a clear blue sky, green plants, and red flowers, showcasing the vibrant color saturation and high luminance of the CDs-based device (Fig. 5 h). Conclusion In summary, we developed a facile low-temperature molten salt method for the large-scale synthesis of full-color SSF CDs, successfully overcoming the limitations of traditional preparation methods in terms of temperature, time, and productivity. The as-prepared SSF CDs can possess an unprecedented SS QY of 99.86% after a further machine learning assisted optimization and present excellent performance in high luminous efficiency LEDs and mini-LED backlight displays with high brightness and long-term durability. This work provides a novel perspective on the design of high-performance solid-state fluorescent carbon dots, which contribute to the ongoing progress in next-generation illumination and display technologies. Methods Materials Citric acid (C₆H₈O₇, > 99.5%), urea (CH 4 N 2 O, > 99.0%), phloroglucinol (C 6 H 6 O 3 , > 99.0%), o-phenylenediamine (C 6 H 8 N 2 , > 98.0%), sodium chloride (NaCl, > 99.5%), potassium chloride (KCl, > 99.8%) and zinc chloride (ZnCl 2 , > 98.0%) were purchased by Aladdin Reagent Co., Ltd (Shanghai, P. R. China). All reactants were analytical pure and used without further purification. The deionized water was used throughout this work. The photodiode chips were purchased from Fangpu Optoelectronics Co., Ltd (Shenzhen, P. R. China). The chip-on-boards were purchased from Huoxing Photoelectric Technology Co., LTD (Shenzhen, P. R. China). The TFT-LCD was purchased from Qinglong Technology Co., LTD (Shenzhen, P. R. China). Preparation of B-, G-, Y- and NIR-CDs 2.00 g sodium chloride, 2.55 g potassium chloride and 4.66 g zinc chloride were used as the molten salt system. 1 g citric acid, 2 g urea and molten salts were fully mixed in a beaker, and then the beaker was transferred to a heating table. After heated and stirred at 250℃ for 5 minutes, the products were transferred to a glove box and fully grounded to obtain the B-CDs powder. Similarly, 1 g citric acid, 2 g urea and molten salts were mixed and heated at 250℃ for 8 minutes to obtain the G-CDs powder. 1 g phloroglucinol, 2 g urea and molten salts were mixed and heated at 250℃ for 10 minutes to obtain the Y-CDs powder. 1g o-phenylenediamine and molten salts were mixed and heated at 300℃ for 10 minutes to obtain the NIR-CDs powder. Fabrication of LEDs and light conversion films The 365 nm LED chips were used to manufacture the blue LEDs, and 400 nm chips were used to manufacture the green, yellow, and red LEDs. Actually, 0.35 g B-, G-, Y- and NIR-CDs powder were thoroughly mixed with 0.5 g polydimethylsiloxane, and stirred continuously to form a uniform viscous mixture. Then, part of the mixture was coated in the center of the LED chips, and the products were transferred to a drying oven, and cured at 70°C for 120 minutes to obtain B-, G-, Y- and NIR-LED devices. For the red-emitting film, 0.01 g (Sr,Ca)AlSiN₃ was mixed with 1 g polydimethylsiloxane, and then the mixture was thoroughly stirred and added to a square mold, and cured at 70℃ for 2 hours. For the green-emitting film, 0.8 g G-CDs and 2 g polydimethylsiloxane were mixed, and stirred well and added to the previous square mold. After curing at 70℃ for 4 hours, the light conversion film was obtained. Characterization The morphology and structure were characterized using transmission electron microscopy (TEM, JEOL-2010). The crystalline property was evaluated in a Bruker-D8 Discover X-ray diffractometer with the Cu Kα line (λ = 1.54 Å) as the irradiation source. The X-ray photoelectron spectroscopy (XPS) was measured on a Kratos AXIS HIS 165 spectrometer with a monochromatized Al KR X-ray source (1486.7 eV). Fourier transform infrared (FT-IR) spectroscopy was performed using a Thermo Scientific Nicolet iZ 10 spectrometer in the KBr tablets. The fluorescence spectrum of was measured by a F-7000 spectrofluorometer (Hitachi, Japan). The absorption spectrum was measured by an UV-vis spectrophotometer (Hitachi, UH-4150). Fluorescence attenuation curve and PLQY were measured by Horiba spectrometer. The luminescence spectrum, lumen efficiency of LEDs and aging curve of white backlight devices are measured by FPD optical characteristics test system. Declarations Data availability The raw data that support the plots within this paper and the other findings of this study are available from the corresponding authors upon reasonable request. Conflicts of interest There are no conflicts to declare. Author contributions All authors analyzed and interpreted the data, contributed to the writing of the manuscript, discussed the results and implications, and edited the manuscript at all stages. 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Nat Commun 15:4843 Han Y et al (2020) Machine-learning-driven synthesis of carbon dots with enhanced quantum yields. ACS Nano 14:14761–14768 Hong Q et al (2022) Customized carbon dots with predictable optical properties synthesized at room temperature guided by machine learning. Chem Mater 34:998–1009 Additional Declarations There is NO Competing Interest. Supplementary Files SupplementaryInformation.docx supplementary information Cite Share Download PDF Status: Published Journal Publication published 01 Sep, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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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-5969251","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":416827715,"identity":"b4370130-7109-4e28-89ee-d1103c145fe4","order_by":0,"name":"Qing Lou","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA7ElEQVRIie3RsQrCMBCA4RPBLod1TEEcnSOCCAq+SougSwfBSVARhLq0D+PmWAnUJTjHTRcnF7cWHExUHNOOgvmHQEo+EnoAJtMPVlWLC4C2td6f399cPal8ztSdMBnS4kTWp8JvkmLEClvislsg5bwySx8MbMunkO00BHm76/EDOocoEQQZOOGNliKuIcTvEC9IsMqPI0EJA/lCWi4FBQgIvzNxKYNBQTLHmiQQu/IWkkcwmXa9IEb1k51VPEbCr5N9pCFygttTFiwHapT39NFr2Jvh9pxpiEqOg303qJZYD15kmXfGZDKZ/rkng1pPvO4VJsYAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-8852-8453","institution":"Zhengzhou University","correspondingAuthor":true,"prefix":"","firstName":"Qing","middleName":"","lastName":"Lou","suffix":""},{"id":416827716,"identity":"dc5cbf67-f9cf-40b8-a88d-798c6479677e","order_by":1,"name":"Yu Lan","email":"","orcid":"","institution":"Guangxi University","correspondingAuthor":false,"prefix":"","firstName":"Yu","middleName":"","lastName":"Lan","suffix":""},{"id":416827717,"identity":"58897988-0c92-4695-9e51-81dd9da6bdbd","order_by":2,"name":"Guangsong Zheng","email":"","orcid":"","institution":"Zhengzhou University","correspondingAuthor":false,"prefix":"","firstName":"Guangsong","middleName":"","lastName":"Zheng","suffix":""},{"id":416827718,"identity":"78c4ffe0-3fb3-4fed-a66a-a8485911449e","order_by":3,"name":"Run-Wei Song","email":"","orcid":"","institution":"Zhengzhou University","correspondingAuthor":false,"prefix":"","firstName":"Run-Wei","middleName":"","lastName":"Song","suffix":""},{"id":416827719,"identity":"0598a777-d6e9-41a1-b4ae-ef94bb3e713a","order_by":4,"name":"Jing-Nan Hao","email":"","orcid":"","institution":"Zhengzhou University","correspondingAuthor":false,"prefix":"","firstName":"Jing-Nan","middleName":"","lastName":"Hao","suffix":""},{"id":416827720,"identity":"27e4e464-7f1a-4f1d-9eac-ab5887e343ad","order_by":5,"name":"Jia-Lu Liu","email":"","orcid":"","institution":"Zhengzhou University","correspondingAuthor":false,"prefix":"","firstName":"Jia-Lu","middleName":"","lastName":"Liu","suffix":""},{"id":416827721,"identity":"1bc7f675-93a8-441c-b6cb-8786f9623c2b","order_by":6,"name":"Cheng-Long Shen","email":"","orcid":"","institution":"Zhengzhou University","correspondingAuthor":false,"prefix":"","firstName":"Cheng-Long","middleName":"","lastName":"Shen","suffix":""},{"id":416827722,"identity":"1172c476-7ccc-49cb-bc8d-dfbf206908e1","order_by":7,"name":"Jinyang Zhu","email":"","orcid":"","institution":"Zhengzhou University","correspondingAuthor":false,"prefix":"","firstName":"Jinyang","middleName":"","lastName":"Zhu","suffix":""},{"id":416827723,"identity":"1ed0775a-863d-4b31-b687-0cac2b8ea694","order_by":8,"name":"Sheng Cao","email":"","orcid":"https://orcid.org/0000-0002-6203-9088","institution":"Guangxi University","correspondingAuthor":false,"prefix":"","firstName":"Sheng","middleName":"","lastName":"Cao","suffix":""},{"id":416827724,"identity":"6afdc480-a2e0-4a98-8fbd-f8282f100f64","order_by":9,"name":"Jialong Zhao","email":"","orcid":"https://orcid.org/0000-0001-9020-1436","institution":"School of Physical Science and Technology, State Key Laboratory of Featured Metal Materials and Lifecycle Safety for Composite Structures, Guangxi University, Guangxi Key Laboratory of Processing fo","correspondingAuthor":false,"prefix":"","firstName":"Jialong","middleName":"","lastName":"Zhao","suffix":""},{"id":416827725,"identity":"5d75543a-b7cd-4a11-bb10-5706a1a09603","order_by":10,"name":"Chongxin Shan","email":"","orcid":"https://orcid.org/0000-0001-7119-5325","institution":"Zhengzhou University","correspondingAuthor":false,"prefix":"","firstName":"Chongxin","middleName":"","lastName":"Shan","suffix":""}],"badges":[],"createdAt":"2025-02-06 02:10:20","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5969251/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5969251/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-025-63653-2","type":"published","date":"2025-09-01T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":76673804,"identity":"acd9f129-2e65-484f-ba80-063f3d49074d","added_by":"auto","created_at":"2025-02-19 14:07:11","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":649588,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSynthesis and optical properties of the full-color SSF CDs. a \u003c/strong\u003eSchematic illustration of the low-temperature molten salt-assisted synthesis of SSF CDs. \u003cstrong\u003eb-e\u003c/strong\u003e UV-vis absorption spectra and SSF emission spectra of the B-, G-, Y-, and IR-CDs (Insets: the optical photograph of SSF CDs powder under 365 nm lamp. \u003cstrong\u003ef-i\u003c/strong\u003e Excitation-emission mapping of the B-, G-, Y-, and NIR-CDs powder.\u003c/p\u003e","description":"","filename":"figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-5969251/v1/a293ed5f7da56080e9ae7bb4.png"},{"id":76674803,"identity":"84c12282-d9b3-4c76-814e-8b359a526dad","added_by":"auto","created_at":"2025-02-19 14:15:14","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1594958,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStructural characterization of the full-color SSF CDs.\u003c/strong\u003e \u003cstrong\u003ea-d\u003c/strong\u003e TEM images of the B-, G-, Y-, and NIR-CDs (inset: HRTEM images and corresponding size distribution). \u003cstrong\u003ee-h\u003c/strong\u003e The relative average size (e), XRD pattern (f), FT-IR spectra (g), and full XPS survey (h) of the B-, G-, Y-, and NIR-CDs. \u003cstrong\u003ei-l\u003c/strong\u003e High-resolution XPS C1s spectra of the B- (i), G- (j), Y- (k), and NIR-CDs (l). \u003cstrong\u003em-p\u003c/strong\u003e High-resolution XPS O1s spectra of the B- (m), G- (n), Y- (o), and NIR-CDs (p).\u003c/p\u003e","description":"","filename":"figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-5969251/v1/4207ceabc036a49245e31463.png"},{"id":76674807,"identity":"63db13ed-04d2-4803-a9f1-f97f14fba3de","added_by":"auto","created_at":"2025-02-19 14:15:16","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1727492,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe formation and light emission mechanism of the SSF CDs from molten salts.\u003c/strong\u003e \u003cstrong\u003ea-d\u003c/strong\u003e The reaction temperature of B- (a), G- (b), Y- (c), and NIR-CDs (d). \u003cstrong\u003ee-h\u003c/strong\u003e The excitation-emission mapping of B- (e), G- (f), Y- (g), and NIR-CDs (h) in aqueous solution (insets: the optical photographs of these CDs aqueous solution. \u003cstrong\u003ei-j\u003c/strong\u003e Schematic illustration of the formation of CDs from precursors with (i) and without molten (j). \u003cstrong\u003ek\u003c/strong\u003e The calculated HOMO-LUMO energy gap and vibrational frequency (Freq) of CDs in the model. \u003cstrong\u003el\u003c/strong\u003e Schematic illustration of the SSF enhancement mechanism of CDs from the zinc ion coordination.\u003c/p\u003e","description":"","filename":"figure3.png","url":"https://assets-eu.researchsquare.com/files/rs-5969251/v1/7003f9a0555a0c0ddd4d649d.png"},{"id":76673807,"identity":"91dd2885-b359-4188-8b04-4ccc824f4612","added_by":"auto","created_at":"2025-02-19 14:07:12","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":690408,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMachine learning-assisted improvements of the SSF CDs and their kilogram preparation.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003eEstimation of the mutual information between features and labels. \u003cstrong\u003eb-c \u003c/strong\u003ePearson correlation coefficient matrices before feature transformation (b) and after feature transformation (c). \u003cstrong\u003ed\u003c/strong\u003e Model performance evaluation of Random Forest (RF), XGBoost (XGB), Support Vector Regression (SVR), and Elastic Net (ELN). \u003cstrong\u003ee \u003c/strong\u003eEvaluation of the generalization performance of the trained XGBoost model. \u003cstrong\u003ef\u003c/strong\u003e PLQY of the G-CDs prepared with the optimal reaction parameters predicted by XGBoost. \u003cstrong\u003eg\u003c/strong\u003e Photograph of the kilogram-scale G-CDs under sunlight and 365 nm UV lamp. \u003cstrong\u003eh\u003c/strong\u003e SSF emission spectra of the G-CDs under continuous 365 nm illumination. \u003cstrong\u003ei\u003c/strong\u003e Peak strength of SSF emission spectra of the G-CDs under continuous 365 nm illumination.\u003c/p\u003e","description":"","filename":"figure4.png","url":"https://assets-eu.researchsquare.com/files/rs-5969251/v1/19b0d81e13f3e7dcdb5e42b0.png"},{"id":76673805,"identity":"975760a6-e4cd-40a9-b005-dd231412472e","added_by":"auto","created_at":"2025-02-19 14:07:12","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":816255,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSSF CDs for multi-color LED and backlit displays.\u003c/strong\u003e \u003cstrong\u003ea-d\u003c/strong\u003e Luminous efficiency of the LEDs prepared with B- (a), G- (b), Y- (c) and NIR-CDs (d) (insets: the photographs of the corresponding LEDs). \u003cstrong\u003ee\u003c/strong\u003e Schematic illustration of the mini-LED backlight display. \u003cstrong\u003ef\u003c/strong\u003e EL spectrum of the mini-LED backlight. \u003cstrong\u003eg\u003c/strong\u003e Aging curve of the mini-LED backlight. The initial luminance is 10599.8 cd cm\u003csup\u003e–2\u003c/sup\u003e, and T\u003csub\u003e95\u003c/sub\u003e is calculated at 1000 cd cm\u003csup\u003e–2\u003c/sup\u003e and 100 cd cm\u003csup\u003e–2\u003c/sup\u003e. \u003cstrong\u003eh\u003c/strong\u003e Photographs of the mini-LED white backlights and display demo.\u003c/p\u003e","description":"","filename":"figure5.png","url":"https://assets-eu.researchsquare.com/files/rs-5969251/v1/b7eec12720242e7b38c3bbac.png"},{"id":90384550,"identity":"53419af1-a716-4c44-9f4a-83afa7d1b0a4","added_by":"auto","created_at":"2025-09-02 07:09:51","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6910660,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5969251/v1/99e4154c-8c66-4fdc-baf2-a8a99b4bc7a3.pdf"},{"id":76675308,"identity":"c67da7c0-6a6e-4e09-9895-02084f87dbe2","added_by":"auto","created_at":"2025-02-19 14:23:13","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":3322007,"visible":true,"origin":"","legend":"supplementary information","description":"","filename":"SupplementaryInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-5969251/v1/5ad4110305643f5197093f9a.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Low-temperature molten salt enabled synthesis of solid-state emissive carbon dots with high endurance and \u003e99% quantum yields","fulltext":[{"header":"Introduction","content":"\u003cp\u003eStable phosphors have been fundamental to modern illumination, displays and information exchange, and thus bright fluorescent materials have gained extensive attention all the time.\u003csup\u003e\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e Due to the potential environmental pollution, high-cost, or low rendering index of classical rare earth-based phosphors, the fluorescent nanomaterials, including semiconductor quantum dots, perovskite nanocrystals, metal clusters, have been widely developed to satisfy the demanding requirements of modern display technology.\u003csup\u003e\u003cspan additionalcitationids=\"CR6 CR7 CR8\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e Especially in recently years, fluorescent carbon dots (CDs), as an emerging carbon-based nanomaterial, have garnered significant attentions for their tunable emission wavelength, high rendering index, low environment pollution, low toxicity, etc., endowing the promising applications in illumination and displays.\u003csup\u003e\u003cspan additionalcitationids=\"CR11 CR12 CR13\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e Nevertheless, the employment of the CDs is always limited by the complex preparation process and potential aggregation-caused quenching (ACQ).\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e To overcome this drawback, diverse strategies, such as confined aggregation, polymer dispersion, self-crosslink, etc.,\u003csup\u003e\u003cspan additionalcitationids=\"CR18 CR19 CR20\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e have been employed to endow the CDs with high solid-state fluorescence (SSF). Since these progressions have gained significant achievements in solid-state fluorescent CDs, there are still numerous challenges: (i) solid-state photoluminescence quantum yields (SS QY) of most SSF CDs are still quite low; (ii) preparation process of SSF CDs always require high-cost precursors and intricate synthesis techniques with high temperature; (iii) scalable synthetic method are difficult to achieve, hindering industrial production and commercial applications (iv) CDs-based solid-state phosphors combining high luminous efficiency and long-term durability have yet to be realized. Therefore, it is still an urgent challenge to develop a facile approach to large-scale synthesis of CDs with high SS QY.\u003c/p\u003e \u003cp\u003eMolten salt, as a molten liquid of salts composed of alkali metals or alkaline earth metals, halides, silicates, carbonates, nitrates, and phosphates, etc., have been demonstrated an excellent medium to synthesize the high quality of nanomaterials, such as metal oxides nanoparticles, perovskite nanocrystals, or two-dimensional transition metal dichalcogenides, etc.\u003csup\u003e\u003cspan additionalcitationids=\"CR23 CR24\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e With the inherit advantages of low reaction temperature, low viscosity, simplicity, and cost-effectiveness, the molten salts can enable these nanomaterials to spontaneously form and grow in a high quality and large-scale synthesis.\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e Recently, there have been several researches about the molten salt-assisted synthesis of fluorescent CDs.\u003csup\u003e\u003cspan additionalcitationids=\"CR28 CR29\" citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e Nevertheless, numerous technical challenges still remain in these strategies: (i) the high melting-point of previously used molten salts (\u0026gt;\u0026thinsp;350\u0026deg;C) may cause the excessive polymerization and carbonization of precursor molecules and thus lead to the reduction in SSF emission (SS QY\u0026thinsp;\u0026lt;\u0026thinsp;26.4%); (ii) the long reaction time (\u0026gt;\u0026thinsp;1.5 hours) render it difficult to obtain SSF CDs regularly and promptly; (iii) the adverse factors like synthetic repeatability, poor light stability and intricate technological post-treatment result in difficult in the translation from laboratory to industrial production. Therefore, it is still an eager requirement to fully utilize the advantages of molten salts to prepare SSF CDs in high quality and large-scale, ensuring a feasible pathway for the industrial production and application of CDs.\u003c/p\u003e \u003cp\u003eIn this work, we innovatively employed a low-melting-point molten salts systems, which are composed of sodium chloride (NaCl), potassium chloride (KCl), and zinc chloride (ZnCl₂), to successfully achieve the synthesis of SSF CDs in a high quality and large-scale synthesis. The fluorescent CDs with high solid-state photoluminescence quantum yield of 8.62%-82.51% can be easily prepared under a low temperature (100\u0026ndash;142\u0026deg;C) in a short reaction time (5\u0026ndash;10 min). With the detailed experiment surveys and theoretical calculations, it reveals that the molten salt can reduce the Gibbs free energy by altering the polymerization pathway of small molecules and meanwhile provide zinc ions for coordination on the surface of CDs to effectively suppress non-radiative transition, thereby reducing the reaction temperature and improve the SSF. By further utilizing the machine learning techniques to optimize the reaction conditions, the SSF CDs with an unprecedented SS QY of ~\u0026thinsp;99.86% can be prepared and further employed as phosphors to fabricate high-efficiency CD-based light-emitting diodes (LEDs) with luminous efficiency of 156.29 lm W\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, endowing the excellent backlight performance as down-conversion luminescent film with long duration of 45108.7 h. This work provides a novel approach to low-cost and large-scale synthesis of superior SSF CDs, and may further put forward the groundwork of CDs in industrial illumination and display technologies.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSynthesis and optical properties of the full-color SSF CDs\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, the SSF CDs were synthesized with different precursors in a molten salts system which is composed of NaCl, KCl, and ZnCl\u003csub\u003e2\u003c/sub\u003e (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). By regulating the precursors and reaction factors, the as-prepared B-, G-, Y-, and NIR-CDs present blue, green, yellow and near-infrared SSF under 365 nm UV lamp (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). As exhibited in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb-e, these B-, G-, Y-, and NIR-CDs present a SSF peak around 432 nm, 517 nm, 594 nm, and 660 nm with distinct UV-vis absorption. Herein, the B-, G-, Y-, and NIR-CDs present different absorption bands, which are corresponding to the π-π* transition of sp\u0026sup2; carbon, n-π* transition of surface heteroatom functional groups. The corresponding SSF spectra reveal a significant color transition from blue to near-infrared region in a Commission Internationale de l\u0026rsquo;Eclairage (CIE) coordinate (Fig. S2), which is corresponding to the observation in naked-eyes. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef-i, the excitation-emission mapping of the B-, G-, Y- and NIR-CDs a maximal SSF center at (335 nm, 430 nm), (430 nm, 517 nm), (560 nm, 594 nm) and (640 nm, 660 nm). Herein, the SSF emission of the B-, G-, Y- and NIR-CDs illustrate a decreased excitation-dependence. The PL QYs of the B-, G-, Y- and NIR-CDs are measured with integrating sphere under their optimal excitation wavelength (Fig. S3) and the results are calculated as 62.77%, 82.51%, 63.46%, and 8.62%, which are ever relative high values among the reported SSF CDs (Table S2). The time-resolved PL decay of these SSF CDs can be well fitted with a biexponential curves (Fig. S4) and the results reveal an average lifetime of 6.47 ns, 16.16 ns, 10.18 ns, and 1.07 ns for the B-CDs, G-CDs, Y-CDs, and NIR-CDs, respectively.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eStructure and components of the full-color SSF CDs\u003c/h3\u003e\n\u003cp\u003eTo further investigate the excellent optical properties, the SSF CDs were treated with dialysis purification to remove the molten salts. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea-d, the transmission electron microscopy (TEM) images reveal that the SSF B-, G-, Y-, and NIR-CDs contain well-dispersed spherical particles, and the high-resolution TEM (HRTEM) image indicate an interplanar lattice spacing of 0.21 nm for all the CDs, which is corresponding to the (100) plane of graphitic carbon.\u003csup\u003e\u003cspan additionalcitationids=\"CR32\" citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e The statistical data reveal an approximate size distribution around 3.41, 3.18, 2.46, and 2.63 nm for the B-, G-, Y-, and NIR-CDs (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee). The size-independent emission implies that the SSF of the CDs are not just from the quantum size effect. In the X-ray diffraction (XRD) patterns, all these SSF CDs present the peaks of crystalline molten salts and an obvious a characteristic amorphous carbon peak around 25.6\u0026deg;can be observed (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef), indicating the successful carbonization and polymerization of CDs from the precursors.\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e The chemical competent of the SSF B-, G-, Y-, and NIR-CDs was analyzed with Fourier-transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). After removing the molten salts, all these CDs samples present the C\u0026thinsp;=\u0026thinsp;C vibration peak at 1572 cm\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, the C\u0026thinsp;=\u0026thinsp;O vibration peak at 1630 cm\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, and OH vibration peak at 1406 cm\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, and the NIR-CDs present obvious C\u0026ndash;N vibration peak (1113 cm\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e) and N\u0026ndash;H vibration peak (1494 cm\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e and 754 cm\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e), implying the N-related long-wavelength emission.\u003csup\u003e\u003cspan additionalcitationids=\"CR36\" citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e The full XPS survey for the pure CDs with dialysis reveal the elements of C (~\u0026thinsp;284 eV), N (~\u0026thinsp;400 eV), O (~\u0026thinsp;531 eV) and Zn (~\u0026thinsp;1022 eV) (Table S3 and Table S4), implying the surface metal ions from molten salt.\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e Meanwhile, the high-resolution XPS C1s spectra reveal the C\u0026ndash;C/C\u0026thinsp;=\u0026thinsp;C, C\u0026ndash;O/C\u0026ndash;N, and C\u0026thinsp;=\u0026thinsp;O (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ei-l), and the high-resolution O1s spectra reveal the C\u0026thinsp;=\u0026thinsp;O and C\u0026ndash;O (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003em-p), further implying the surface ion exchange between the CDs and molten salts. Additionally, these CDs present the pyridine N, pyrrole N and graphite N in the N1s spectra (Fig. S5), suggesting the different light emission principles of B-, G-, Y-, and NIR-CDs.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eThe formation and light emission mechanism of the SSF CDs\u003c/h3\u003e\n\u003cp\u003e \u003c/p\u003e \u003cp\u003eTo investigate the formation and light emission mechanism of the SSF CDs, more surveys are executed. To test the temperature threshold for the formation of SSF CDs, the reaction temperatures were continuously recorded until the sample was fully formed. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea-d, the highest recorded temperature of the B-, G-, Y-, and NIR-CDs are as low as 100\u0026deg;C, 110\u0026deg;C, 120\u0026deg;C, and 142\u0026deg;C, respectively. Such low temperature can enable the chemical reaction of surface passivation or branch linking to further improve the light emission of CDs or develop furtherword applications. After dissolved in DMF, these CDs solutions present obvious different light emission from the SSF CDs powder. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee-h, the maximal excitation-emission centers of the CDs solutions present slight blueshift to (365 nm, 450 nm), (445 nm, 495 nm), (475 nm, 510 nm), and (450 nm, 520 nm). Meanwhile, the PL QYs of the B-, G-, Y-, and NIR-CDs are 14.93%, 57.72%, 1.28% and 4.11%, which are far lower than their SSF powder. On the condition, the density functional theory (DFT) calculations are further employed to explore the mechanism. On account of the G-CDs from the citric acid and urea in molten salts, the strong p-π conjugation between the hydroxyl and carbonyl oxygen in citric acid delocalizes the π electrons and further increase the polarity of O\u0026ndash;H bond, making it easier to combines with \u0026ndash;NH\u003csub\u003e2\u003c/sub\u003e to form ammonia gas. Thus, the citric acid and urea can self-link and further form the CDs through polymerization and carbonization. When there are molten salts in the reaction, the salts ions can provide a liquid environment to facilitate the departure of H\u003csup\u003e+\u003c/sup\u003e and the citric acid and urea are more likely to undergo dehydration condensation between \u0026ndash;COOH and \u0026ndash;NH\u003csub\u003e2\u003c/sub\u003e to form polymers, enabling the formation of CDs. The corresponding Gibbs free energy change (ΔG) of the CDs with and without molten salts can be calculated. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ei-j, the ΔG from free citric acid and urea to polymer is 0.52 eV while the ΔG from the citric acid and urea in molten salts is -27.64 eV. With such low ΔG, the reaction of citric acid and urea can spontaneously undergo, enabling the low temperature threshold for the formation of SSF CDs. The corresponding light emission mechanism are further investigated with the same model. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ek, the highest occupied molecular orbits (HOMO) and lowest unoccupied molecular orbits (LUMO) energy levels of CDs with and without surface zinc ion are compared with density functional theory (DFT). Herein, the CDs with and without surface zinc ion exhibit slight reduced bandgap, indicating that the zinc ions can trigger the redshifted emission. Meanwhile, the average vibrational frequency of CDs is 1122.05 while the SSF CDs is 1060.36. The lower average vibrational frequency of SSF CDs implies that the surface zinc ion can suppress the vibrational deactivation pathway of the S1 state and thus reduce the nonirradiation, enabling the enhancement of SSF (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003el). The hypothesis also can be demonstrated with another survey. As shown in Fig. S6 and Fig. S7, when the SSF CDs are prepared in molten salts without ZnCl\u003csub\u003e2\u003c/sub\u003e or with other zinc salts, the products present different SSF performance, in which the sample prepared without ZnCl\u003csub\u003e2\u003c/sub\u003e exhibits significantly reduced FL intensity and the sample prepared with zinc chloride, zinc sulfate, zinc nitrate, and zinc acetate are exhibit approximate SSF emission as the SSF CDs.\u003c/p\u003e\n\u003ch3\u003eMachine learning-assisted improvements of the SSF CDs\u003c/h3\u003e\n\u003cp\u003e \u003c/p\u003e \u003cp\u003eOn account of the SSF enhancement from molten salts, we believe that the reaction factors can influence the SSF performances of these CDs and thereby the SS QY can be further improved. To achieve this hypothesis, the machine learning techniques are employed to optimize the reaction parameters.\u003csup\u003e\u003cspan additionalcitationids=\"CR41\" citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e Herein, a dataset was constructed by preparing 120 samples in laboratory, and the citric acid mass (CA), urea mass (UA), NaCl mass (NaCl), KCl mass (KCl), ZnCl\u003csub\u003e2\u003c/sub\u003e mass (ZnCl\u003csub\u003e2\u003c/sub\u003e), reaction time (Time), and reaction temperature (Temp) were recorded as features, and the measured Photoluminescence quantum yield (PLQY) were set as the labels. Before training the model, we preprocessed the dataset and evaluated the mutual information between the features and the labels. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, the results reveal that ZnCl₂ did not significantly contribute to the labels, speculating that the range from the collected ZnCl₂ data may exceed the threshold of contribution to the PLQY. Therefore, the ZnCl₂ feature was removed from the feature set. The Pearson correlation coefficient matrices present a strong positive correlation between features CA and UA (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). To avoid multicollinearity, the feature transformation was performed, and the Ratio (UA/CA) and Sum (UA\u0026thinsp;+\u0026thinsp;CA) were added into the feature sets. After feature transformation, all feature pairs showed low linear correlation (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec), approving the effectiveness of the feature selection. During the training phase, the performance of XGBoost (XGB), random forest (RF), support vector regression (SVR), and elastic net (ELN) were evaluated on the dataset with the grid search to optimize the model parameters and evaluate the performances with root mean square error (RMSE) and the coefficient of determination (R\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e). As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee, the results revealed that the XGBoost outperformed the lower RMSE and the higher R\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e than other models on this dataset, demonstrating its excellent generalization ability. Thus, the XGBoost were selected to perform the PLQY prediction task. With the optimal feature combination to maximize the PLQY, a high-density feature grid involving 134,198,064 feature combinations were built and input into the XGBoost for PLQY prediction (Table S5). As a result, the XGBoost can output a highest predicted PLQY and the parameter was used to prepare sample. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef, the as-prepared CDs present a solid-state PLQY as high as 99.86%, which is the highest value in ever reported solid-state CDs (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). On the condition, the laboratory factors were further employed in a large-scale synthesis and ~\u0026thinsp;1,338.5 g of SSF CDs were obtained in a single reaction (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg), indicating the promising potential of molten salt-assisted SSF CDs in industrial production. In addition, after continuously illumination under 365 nm lamp, the as-prepared G-CDs powder present almost the same SSF emission (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eh and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ei), demonstrating their excellent photostability.\u003c/p\u003e\n\u003ch3\u003eMulti-color LED and backlit displays application of the SSF CDs\u003c/h3\u003e\n\u003cp\u003e \u003c/p\u003e \u003cp\u003eThe full-color light emission wavelength and high PL QY inspire us to develop the CDs as phosphors in light-emitting diodes (LEDs). Herein, the SSF CDs powder was first evenly mixed with polydimethylsiloxane and then coated onto a 365 nm UV or 400 nm blue LED chips (Fig. S8). The as-prepared LEDs from B-, G-, Y-, and NIR-CDs are named as B-, G-, Y-, and NIR-LEDs, respectively. Figure S9 shows the actual photographs of these LEDs, and Fig. S10 presents the emission spectra of these LEDs, indicating the light emission conversion from the SSF CDs. The corresponding CIE coordinates also demonstrate the success light conversion (Fig. S11). On the condition, the luminous efficiency of these LEDs devices was measured to evaluate the luminescent performance. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea-d, the B-, G-, Y-, and NIR-LEDs present a maximum luminous efficiency of 23.46, 156.29 and 15.26 and 0.03 lm W\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, respectively. The impressive performance of G-LEDs is almost the highest values in the CDs-based or QDs-based LED devices (Table S6), approving the potential applications of these SSF CDs in light conversion. In recent years, high-brightness mini-LED backlight displays have become increasingly popular for high-end applications and thus the optimized G-CDs were further used as the phosphors in mini-LED. Subsequently, a 450 nm blue mini-LED chip-on-board (COB) were prepared with a green-emitting and red-emitting conversion film of G-CDs and (Sr,Ca)AlSiN\u003csub\u003e3\u003c/sub\u003e (Fig. S12 and Fig. S13). As a result, these two color conversion films were combined and successful to create a mini-LED white backlight (Fig. S14). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee, the mini-LED backlight was integrated with a commercial TFT-LCD panel to design a complete display device (Fig. S15 and Fig. S16). The EL emission spectrum of the mini-LED white backlight present obvious white color and the CIE coordinate is calculated as (0.31, 0.29) (Figure S17). With an initial luminance of 10599.8 cd cm\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e, the T\u003csub\u003e95\u003c/sub\u003e (the time that the luminance intensity decreases to 95% initial value) was 10.69 hours (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eg). With the formula L\u003csub\u003e0\u003c/sub\u003e\u003csup\u003en\u003c/sup\u003eT\u003csub\u003e95\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;constant, the T\u003csub\u003e95\u003c/sub\u003e values were estimated as 731.6 hours at initial 1000 cd cm\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e and 45108.7 hours at initial 100 cd cm\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e, respectively. The long lifespan of the mini-LED white backlights demonstrates their durability for long-term display applications. In addition, as a simple demonstration, the display exhibits a clear blue sky, green plants, and red flowers, showcasing the vibrant color saturation and high luminance of the CDs-based device (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eh).\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn summary, we developed a facile low-temperature molten salt method for the large-scale synthesis of full-color SSF CDs, successfully overcoming the limitations of traditional preparation methods in terms of temperature, time, and productivity. The as-prepared SSF CDs can possess an unprecedented SS QY of 99.86% after a further machine learning assisted optimization and present excellent performance in high luminous efficiency LEDs and mini-LED backlight displays with high brightness and long-term durability. This work provides a novel perspective on the design of high-performance solid-state fluorescent carbon dots, which contribute to the ongoing progress in next-generation illumination and display technologies.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eMaterials\u003c/h2\u003e \u003cp\u003eCitric acid (C₆H₈O₇, \u0026gt;\u0026thinsp;99.5%), urea (CH\u003csub\u003e4\u003c/sub\u003eN\u003csub\u003e2\u003c/sub\u003eO, \u0026gt;\u0026thinsp;99.0%), phloroglucinol (C\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, \u0026gt;\u0026thinsp;99.0%), o-phenylenediamine (C\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003eN\u003csub\u003e2\u003c/sub\u003e, \u0026gt;\u0026thinsp;98.0%), sodium chloride (NaCl, \u0026gt;\u0026thinsp;99.5%), potassium chloride (KCl, \u0026gt;\u0026thinsp;99.8%) and zinc chloride (ZnCl\u003csub\u003e2\u003c/sub\u003e, \u0026gt; 98.0%) were purchased by Aladdin Reagent Co., Ltd (Shanghai, P. R. China). All reactants were analytical pure and used without further purification. The deionized water was used throughout this work. The photodiode chips were purchased from Fangpu Optoelectronics Co., Ltd (Shenzhen, P. R. China). The chip-on-boards were purchased from Huoxing Photoelectric Technology Co., LTD (Shenzhen, P. R. China). The TFT-LCD was purchased from Qinglong Technology Co., LTD (Shenzhen, P. R. China).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003ePreparation of B-, G-, Y- and NIR-CDs\u003c/h2\u003e \u003cp\u003e2.00 g sodium chloride, 2.55 g potassium chloride and 4.66 g zinc chloride were used as the molten salt system. 1 g citric acid, 2 g urea and molten salts were fully mixed in a beaker, and then the beaker was transferred to a heating table. After heated and stirred at 250℃ for 5 minutes, the products were transferred to a glove box and fully grounded to obtain the B-CDs powder. Similarly, 1 g citric acid, 2 g urea and molten salts were mixed and heated at 250℃ for 8 minutes to obtain the G-CDs powder. 1 g phloroglucinol, 2 g urea and molten salts were mixed and heated at 250℃ for 10 minutes to obtain the Y-CDs powder. 1g o-phenylenediamine and molten salts were mixed and heated at 300℃ for 10 minutes to obtain the NIR-CDs powder.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eFabrication of LEDs and light conversion films\u003c/h2\u003e \u003cp\u003eThe 365 nm LED chips were used to manufacture the blue LEDs, and 400 nm chips were used to manufacture the green, yellow, and red LEDs. Actually, 0.35 g B-, G-, Y- and NIR-CDs powder were thoroughly mixed with 0.5 g polydimethylsiloxane, and stirred continuously to form a uniform viscous mixture. Then, part of the mixture was coated in the center of the LED chips, and the products were transferred to a drying oven, and cured at 70\u0026deg;C for 120 minutes to obtain B-, G-, Y- and NIR-LED devices. For the red-emitting film, 0.01 g (Sr,Ca)AlSiN₃ was mixed with 1 g polydimethylsiloxane, and then the mixture was thoroughly stirred and added to a square mold, and cured at 70℃ for 2 hours. For the green-emitting film, 0.8 g G-CDs and 2 g polydimethylsiloxane were mixed, and stirred well and added to the previous square mold. After curing at 70℃ for 4 hours, the light conversion film was obtained.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eCharacterization\u003c/h2\u003e \u003cp\u003eThe morphology and structure were characterized using transmission electron microscopy (TEM, JEOL-2010). The crystalline property was evaluated in a Bruker-D8 Discover X-ray diffractometer with the Cu Kα line (λ\u0026thinsp;=\u0026thinsp;1.54 \u0026Aring;) as the irradiation source. The X-ray photoelectron spectroscopy (XPS) was measured on a Kratos AXIS HIS 165 spectrometer with a monochromatized Al KR X-ray source (1486.7 eV). Fourier transform infrared (FT-IR) spectroscopy was performed using a Thermo Scientific Nicolet iZ 10 spectrometer in the KBr tablets. The fluorescence spectrum of was measured by a F-7000 spectrofluorometer (Hitachi, Japan). The absorption spectrum was measured by an UV-vis spectrophotometer (Hitachi, UH-4150). Fluorescence attenuation curve and PLQY were measured by Horiba spectrometer. The luminescence spectrum, lumen efficiency of LEDs and aging curve of white backlight devices are measured by FPD optical characteristics test system.\u003c/p\u003e \u003c/div\u003e "},{"header":"Declarations","content":"\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eData availability\u003c/h2\u003e \u003cp\u003eThe raw data that support the plots within this paper and the other findings of this study are available from the corresponding authors upon reasonable request.\u003c/p\u003e \u003c/div\u003e\n\u003ch2\u003eConflicts of interest\u003c/h2\u003e \u003cp\u003eThere are no conflicts to declare.\u003c/p\u003e \u003ch2\u003eAuthor contributions\u003c/h2\u003e \u003cp\u003eAll authors analyzed and interpreted the data, contributed to the writing of the manuscript, discussed the results and implications, and edited the manuscript at all stages.\u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e \u003cp\u003eWe gratefully acknowledge the support of the Natural Science Foundation of China (12261141661, 62027816, and 62204223), the China Postdoctoral Science Foundation (2022TQ0307), and the Natural Science Foundation of Henan Province (242300421179).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eFang M-H, Bao Z, Huang W-T, Liu R-S (2022) Evolutionary generation of phosphor materials and their progress in future applications for light-emitting diodes. 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Chem Mater 34:998\u0026ndash;1009\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Carbon dots, Molten salt, Solid-state fluorescence, Light emitting diodes, displays","lastPublishedDoi":"10.21203/rs.3.rs-5969251/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5969251/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eFluorescent carbon dots (CDs) have garnered significant attention for their unique optoelectronic properties and applications, but their practical employment is hampered by the excessive synthesis temperature, tedious post-processing and limited solid-state luminescence efficiency. Herein, we develop a facile molten salt method to achieve the one-step synthesis of full-color CDs with efficient solid-state emission. Comprehensively, kilogram-scale solid-state CDs with a quantum yield of 90% can be readily synthesized via a salt-assisted approach under mild conditions (100\u0026ndash;142\u0026deg;C) within 10 minutes. The spectral characterization and density functional theory calculation confirm that zinc ion coordination can occur in liquated environment, which facilitates the polymerization of precursors at lower temperatures, suppresses the formation of non-radiative recombination channels on their surface, and further enhances luminescence in solid. The machine learning is further used to optimize CDs\u0026rsquo; luminous efficiency up to 99.86%, evoking excellent performance CDs-based light-emitting diodes with a maximum luminous efficiency of 156.29 lm W\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e to drive backlit display with a long-persistent lifetime (T\u003csub\u003e95\u003c/sub\u003e at 100 cd m\u003csup\u003e\u0026ndash;2\u003c/sup\u003e = 45108.7 h). This work provides a pathway for the design and fabrication of advanced carbon-based solid-state luminescent materials, significantly contributing to the advancement of next-generation lighting and display technologies.\u003c/p\u003e","manuscriptTitle":"Low-temperature molten salt enabled synthesis of solid-state emissive carbon dots with high endurance and \u0026gt;99% quantum yields","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-02-19 14:07:05","doi":"10.21203/rs.3.rs-5969251/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"bea09e8e-a3b8-4512-b49e-3c0cede91385","owner":[],"postedDate":"February 19th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":44438578,"name":"Physical sciences/Nanoscience and technology/Nanoscale materials/Quantum dots"},{"id":44438579,"name":"Physical sciences/Materials science/Nanoscale materials/Nanoparticles"}],"tags":[],"updatedAt":"2025-09-02T07:09:40+00:00","versionOfRecord":{"articleIdentity":"rs-5969251","link":"https://doi.org/10.1038/s41467-025-63653-2","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2025-09-01 04:00:00","publishedOnDateReadable":"September 1st, 2025"},"versionCreatedAt":"2025-02-19 14:07:05","video":"","vorDoi":"10.1038/s41467-025-63653-2","vorDoiUrl":"https://doi.org/10.1038/s41467-025-63653-2","workflowStages":[]},"version":"v1","identity":"rs-5969251","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5969251","identity":"rs-5969251","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}