High-fluorescence quantum yield and narrow-bandwidth emission carbon dot and preparation method thereof

Abstract

The invention discloses a carbon dot with high fluorescence quantum yield and narrow bandwidth emission, which is prepared by taking citric acid and rhodamine 6G as carbon sources for hydrothermal reaction, and emits orange fluorescence under ultraviolet excitation, the emission wavelength is 556 + / -2nm, the fluorescence quantum yield is not less than 80%, the half-peak width is not more than 50nm, and the narrow bandwidth characteristic of the carbon dot can effectively improve the light color purity. And when being applied to preparation of a high-color-purity photoluminescent diode as photoconversion fluorescent powder, the fluorescent powder shows excellent optical performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of fluorescent nanomaterials and relates to a fluorescent carbon dot that can be used for a photoluminescent diode, in particular to a fluorescent carbon dot with high fluorescence quantum yield and narrow bandwidth emission and a preparation method thereof. Background Art

[0002] As an emerging new type of quasi-zero-dimensional carbon-based material in the field of nanotechnology, carbon dots have shown important application prospects in many fields such as optoelectronic display devices, bioimaging and diagnosis, fluorescence sensing and photocatalysis since they were first reported in 2004, thanks to their wide range of raw materials, simple preparation process, low cytotoxicity and good biocompatibility, as well as unique photophysical properties such as tunable photoluminescence.

[0003] Currently, research on carbon dots mainly focuses on precisely controlling the size distribution of carbon dots, optimizing the chemical modification of surface functional groups to enhance quantum yield, and improving the stability of their luminescence performance through synthesis process optimization and heteroatom doping strategies.

[0004] However, despite significant breakthroughs in the application expansion and performance optimization of carbon dots, their inherently wide spectral emission characteristics (with a half-width at half-maximum (FWHM) generally exceeding 60nm) remain a key scientific challenge hindering breakthroughs in their optoelectronic performance. This wide FWHM (Full-Width at Half-Maximum) spectral emission characteristic not only reduces the color purity of light-emitting devices (CIE color coordinate shift), but also directly affects the human eye's perception of color saturation and image resolution in display technology. This necessitates the introduction of complex structures such as filters or distributed Bragg reflectors in device design to improve the color purity of photoluminescent diodes (LEDs). This inevitably leads to device efficiency losses, reducing the device's external quantum efficiency, and significantly increasing the process complexity and manufacturing cost of LEDs.

[0005] Despite recent breakthroughs in the narrow emission spectrum control of carbon dots, the application of carbon dots in photoluminescent LEDs is still in its infancy. Literature shows that the fluorescence quantum yield (PLQY) of carbon dot-based fluorescent LEDs is generally below 50%.

[0006] Xu et al. (Rational Synthesis of Solid-State Ultraviolet B Emitting CarbonDots via Acetic Acid-Promoted Fractions of sp 3Bonding Strategy. Adv. Mater .2022, 34(17): 2200011) Using acetic acid-promoted sp 3 A hybrid bond formation strategy was used to fabricate solid-state UV-B-band emitting carbon dots, which exhibited excellent photoluminescence properties. However, the PLQY of the prepared carbon dots was only 49%.

[0007] Dong et al. (Ethylenediamine Assisted Synthesis of o-Phenylenediamine-BasedRed Emissive Carbon Quantum Dots: A Strategy to Improve the FluorescenceQuantum Yield. Adv. Opt. Mater . 2025, 13(3): 2402173)Based on o-phenylenediamine with sp 2 Hybrid structures are often used as precursors for the preparation of red-light carbon quantum dots, but the problem of low fluorescence quantum yield can be solved by introducing ethylenediamine to assist in the synthesis of o-phenylenediamine-based red-light carbon quantum dots, thereby improving the fluorescence quantum yield of carbon quantum dots, but it only reaches 33%.

[0008] Despite some efforts, improvements in quantum yield remain limited. This inefficient luminescence leads to severe non-radiative recombination losses within the device, directly restricting improvements in external quantum efficiency (EQE) and brightness, making it difficult to meet the core requirements of commercial display technology for photoelectric conversion efficiency. Therefore, it is extremely necessary to prepare carbon dots with both high PLQY and narrow-bandwidth emission to meet the requirements of high-color-purity photoluminescent LEDs. Summary of the Invention

[0009] The purpose of the present invention is to provide a carbon dot with high fluorescence quantum yield and narrow bandwidth emission and a preparation method thereof, so as to improve the problem of low color purity of traditional carbon dot-based photoluminescent LEDs.

[0010] To achieve the above-mentioned purpose of the invention, the present invention provides a high fluorescence quantum yield and narrow bandwidth emission carbon dot. The orange fluorescent carbon dot with an emission wavelength of 556±2nm is prepared by hydrothermal reaction at 140-220°C using citric acid and rhodamine 6G as carbon sources. The fluorescence quantum yield is not less than 80%, and the half-peak width is not greater than 50nm. It can be used as a light conversion phosphor for use in high-color purity photoluminescent diodes.

[0011] The present invention also provides a method for preparing the carbon dots with high fluorescence quantum yield and narrow bandwidth emission, which comprises dispersing citric acid and rhodamine 6G as carbon sources in water, performing a hydrothermal reaction at 140-220° C. to prepare a crude carbon dot product, and then purifying and freeze-drying to obtain an orange solid carbon dot powder.

[0012] Furthermore, in the preparation method of the present invention, the molar ratio of citric acid to rhodamine 6G is preferably 100-800:1.

[0013] Furthermore, in the preparation method of the present invention, the hydrothermal reaction time is preferably 4 to 12 hours.

[0014] Furthermore, the preparation method of the present invention is more preferably a hydrothermal reaction at 200° C. for 8 hours.

[0015] Furthermore, the preparation method of the present invention also includes purification treatment of the hydrothermal reaction product, including conventional centrifugation treatment, microporous membrane filtration treatment and dialysis treatment.

[0016] Furthermore, the dialysis treatment in the preparation method of the present invention is specifically performed using a dialysis bag with a molecular weight cutoff of 500 Da for 48 hours, during which deionized water is replaced every 8 hours.

[0017] Based on the high fluorescence quantum yield and narrow bandwidth emission characteristics of the carbon dots prepared by the present invention, they are suitable for use as light conversion phosphors in the preparation of high color purity photoluminescent diodes.

[0018] In order to explore the effect of the molecular structure of the carbon source on FWHM, the present invention selected 1,5-diaminonaphthalene, 1-pyrenecarboxylic acid, perylene-3,4,9,10-tetracarboxylic dianhydride and other carbon sources with different degrees of conjugation and citric acid for hydrothermal reaction to prepare carbon dots under the same reaction conditions. It was found that the FWHM of carbon dots prepared with carbon sources with planar rigid structures was larger, and as the degree of conjugation of the carbon source molecules increased, the FWHM gradually increased. When rhodamine 6G with a spatially distorted structure was used as the carbon source, the FWHM of the prepared carbon dots was smaller, making it easier to prepare narrow-bandwidth emission carbon dots. Therefore, through comparative analysis of carbon source structures, the introduction of carbon sources with non-planar conjugated structures with spatial distortions can inhibit the emission of carbon dots. S 0 and S 1. The torsional vibration energy difference of this type of small molecule is small, only local rotation and deformation are generated within the molecule, and there is less interaction with the surrounding environment, which helps to achieve narrow bandwidth emission of carbon dots.

[0019] The present invention uses citric acid and rhodamine 6G as reaction raw materials, introduces carbonyl groups to improve the luminescence intensity of carbon dots, and uses a simple hydrothermal reaction to prepare orange carbon dots with high fluorescence quantum yield and narrow-bandwidth emission. This can improve the light conversion efficiency of photo-LEDs and is expected to realize low-cost, high-efficiency lighting and display devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is the fluorescence emission spectrum of the carbon dot aqueous solution prepared in Example 1.

[0021] Figure 2 This is the UV-visible absorption spectrum of the carbon dot aqueous solution prepared in Example 1.

[0022] Figure 3 This is a comparison chart of infrared spectra of carbon dots prepared in Example 1 and Rhodamine 6G.

[0023] Figure 4 This is the fluorescence decay curve of the carbon dots prepared in Example 1 and Rhodamine 6G.

[0024] Figure 5 The morphology and particle size statistics of the carbon dots prepared in Example 1 were observed under a transmission electron microscope.

[0025] Figure 6 This is the fluorescence emission spectrum of carbon dots prepared in Example 2.

[0026] Figure 7 This is the fluorescence emission spectrum of carbon dots prepared in Example 3.

[0027] Figure 8 This is the fluorescence emission spectrum of carbon dots prepared in Example 4.

[0028] Figure 9 This is the fluorescence emission spectrum of carbon dots prepared in Comparative Example 1.

[0029] Figure 10 This is the fluorescence emission spectrum of carbon dots prepared in Comparative Example 2.

[0030] Figure 11 This is the fluorescence emission spectrum of carbon dots prepared in Comparative Example 3.

[0031] Figure 12 The fluorescence spectrum of the photo-LEDs prepared using carbon dots in Example 1 is shown. Implementation Method

[0032] The following embodiments are only used to more clearly illustrate the technical solutions of the present invention so that those skilled in the art can better understand and utilize the present invention, but are not intended to limit the scope of protection of the present invention.

[0033] Unless otherwise specified, the production processes, experimental methods or detection methods involved in the embodiments of the present invention are all conventional methods in the prior art, and their names and / or abbreviations are conventional names in the field and are very clear and unambiguous in the relevant fields of use. Those skilled in the art can understand the conventional process steps based on the names and apply the corresponding equipment to implement them according to conventional conditions or the conditions recommended by the manufacturer.

[0034] The various instruments, equipment, raw materials or reagents used in the embodiments of the present invention are not particularly limited in their sources and are all conventional products that can be purchased through regular commercial channels or prepared according to conventional methods well known to those skilled in the art.

[0035] The carbon dots with high fluorescence quantum yield and narrow bandwidth emission in the following specific examples of the present invention are prepared according to the following steps:

[0036] S1: Add citric acid and rhodamine 6G to deionized water, sonicate them at room temperature to fully dissolve them, and then transfer them to a stainless steel autoclave lined with polytetrafluoroethylene.

[0037] S2: placing the stainless steel autoclave in an oven for a hydrothermal reaction, cooling the reaction to room temperature, taking out the reaction solution, and centrifuging the crude carbon dot product solution to remove impurities and precipitates to obtain a clarified carbon dot solution;

[0038] S3: The carbon dot solution contains a large amount of unreacted small molecule raw materials and over-carbonized carbon materials. Impurities are removed by filtration and dialysis to obtain a purified orange carbon dot aqueous solution;

[0039] S4: Freeze the purified orange carbon dot aqueous solution into a solid, and freeze-dry to remove deionized water to prepare orange carbon dot powder. Example

[0040] Example 1

[0041] 4.387 g of citric acid and 0.025 g of rhodamine 6G (molar ratio 400:1) were weighed and added to 15 mL of deionized water. The mixture was ultrasonically treated in a 59 kHz high-frequency ultrasonic cleaner at room temperature for 10 min to fully dissolve the mixture. The mixture was then transferred to a 50 mL stainless steel autoclave lined with polytetrafluoroethylene.

[0042] A sealed stainless steel autoclave was placed in an oven and heated to 200°C for a hydrothermal reaction for 8 hours. The reaction solution was cooled to room temperature and centrifuged at 10,000 rpm to remove impurities and precipitate, yielding a clear carbon dot solution.

[0043] The clarified carbon dot solution was first filtered using a 0.22 μm disposable microporous filter membrane, and then dialyzed using a dialysis bag with a molecular weight cutoff of 500D (the water was changed every 8 hours, for a total of 48 hours) to remove the unreacted small molecule raw materials and over-carbonized carbon material impurities contained in the carbon dot solution, obtaining a purified orange carbon dot aqueous solution.

[0044] The orange carbon dot aqueous solution was frozen into a solid at -79°C, and freeze-dried to remove moisture to obtain orange carbon dot powder.

[0045] A small amount of the prepared orange carbon dot powder was dispersed in deionized water for relevant physical and chemical property tests.

[0046] Figure 1 Figure 2 shows the fluorescence spectra of carbon dots at different excitation wavelengths. It can be seen that the emission peak of carbon dots is located at 556.5 nm, and there is no significant change in the emission peak at different excitation wavelengths, with the full width at half maximum (FWHM) being 49.5 nm.

[0047] Figure 2 The UV-visible absorption spectrum of carbon dots is given. Carbon dots have two obvious absorption peaks, mainly concentrated in the ultraviolet region and the green region. The absorption peak at 348nm comes from the n-π* transition of the C=O bond, and the strong absorption peak at 520nm comes from the large sp 2 It is worth noting that under 520 nm excitation, the carbon dots exhibit the highest fluorescence efficiency, with a PLQY of 83.5%.

[0048] Figure 3 This is a comparison chart of infrared spectra of carbon dots and single-component rhodamine 6G. -1 The absorption peaks at 2929cm-1 are all derived from C=O stretching vibration. Compared with rhodamine 6G, the C=O content of carbon dots has increased to a certain extent, which has a certain impact on the fluorescence properties of carbon dots. -1 The absorption peak at is attributed to the -CH2- stretching vibration, indicating that the carbon dots have a certain degree of flexible short chains that can balance the FWHM fluctuations caused by the introduction of C=O.

[0049] Figure 4 The fluorescence decay curves of carbon dots and single-component rhodamine 6G are shown in Figure 2. When the excitation wavelength is 375nm, the fluorescence decay of carbon dots and rhodamine 6G at 556nm is single exponential decay, with average fluorescence lifetimes of 6.03 and 5.88ns, respectively. The fluorescence lifetime of carbon dots is single exponential decay, indicating that their luminescence is determined by the carbon core. Compared with rhodamine 6G, the fluorescence decay of carbon dots is single exponential decay, which is more obvious than that of rhodamine 6G. τ 1 was increased by 0.15ns, indicating that the introduction of carbonyl groups can increase the fluorescence lifetime of carbon dots, thereby improving their PLQY, which also supports the speculation that carbonyl groups are beneficial to the improvement of PLQY.

[0050] Figure 5 The carbon dots are observed under a transmission electron microscope. It can be seen that the carbon dots are quasi-spherical particles, evenly dispersed and without obvious agglomeration. The particle size of the carbon dots is statistically analyzed, and the particle size is mainly distributed between 2.4 and 3.6 nm, with an average particle size of 3.07 nm. At the same time, the high-resolution transmission electron microscope can see the obvious lattice fringes of the carbon dots, indicating that they have good crystallization properties. The interplanar spacing is 0.22 nm, corresponding to the (100) crystal plane of graphite carbon, indicating that there are sp 2 Hybridized carbon.

[0051] Example 2

[0052] 2.193 g of citric acid and 0.025 g of rhodamine 6G (molar ratio 200:1) were weighed and added to 15 mL of deionized water. The mixture was ultrasonically treated in a 59 kHz high-frequency ultrasonic cleaner at room temperature for 10 min to fully dissolve the mixture. The mixture was then transferred to a 50 mL stainless steel autoclave lined with polytetrafluoroethylene.

[0053] A sealed stainless steel autoclave was placed in an oven and heated to 200°C for a hydrothermal reaction for 8 hours. The reaction solution was cooled to room temperature and centrifuged at 10,000 rpm to remove impurities and precipitate, yielding a clear carbon dot solution.

[0054] The clarified carbon dot solution was first filtered using a 0.22 μm disposable microporous filter membrane, and then dialyzed using a dialysis bag with a molecular weight cutoff of 500D (the water was changed every 8 hours, for a total of 48 hours) to remove the unreacted small molecule raw materials and over-carbonized carbon material impurities contained in the carbon dot solution, obtaining a purified orange carbon dot aqueous solution.

[0055] The orange carbon dot aqueous solution was frozen into a solid at -79°C, and freeze-dried to remove moisture to obtain orange carbon dot powder.

[0056] A small amount of the prepared orange carbon dot powder was dispersed in deionized water for testing. Figure 6 As shown in the fluorescence spectrum, the emission peak of carbon dots is located at 555.5 nm.

[0057] Calculations show that the PLQY of the prepared carbon dots reaches 81.3% and the FWHM is 48.2 nm.

[0058] Example 3

[0059] 1.096 g of citric acid and 0.025 g of rhodamine 6G (molar ratio 100:1) were weighed and added to 15 mL of deionized water. The mixture was ultrasonically treated in a 59 kHz high-frequency ultrasonic cleaner at room temperature for 10 min to fully dissolve the mixture. The mixture was then transferred to a 50 mL stainless steel autoclave lined with polytetrafluoroethylene.

[0060] A sealed stainless steel autoclave was placed in an oven and heated to 180°C for a hydrothermal reaction for 8 hours. The reaction solution was cooled to room temperature and centrifuged at 10,000 rpm to remove impurities and precipitate, yielding a clear carbon dot solution.

[0061] The clarified carbon dot solution was first filtered using a 0.22 μm disposable microporous filter membrane, and then dialyzed using a dialysis bag with a molecular weight cutoff of 500D (the water was changed every 8 hours, for a total of 48 hours) to remove the unreacted small molecule raw materials and over-carbonized carbon material impurities contained in the carbon dot solution, obtaining a purified orange carbon dot aqueous solution.

[0062] The orange carbon dot aqueous solution was frozen into a solid at -79°C, and freeze-dried to remove moisture to obtain orange carbon dot powder.

[0063] A small amount of the prepared orange carbon dot powder was dispersed in deionized water for testing. Figure 7 As shown in the fluorescence spectrum, the emission peak of carbon dots is located at 556.1 nm.

[0064] Calculations show that the PLQY of the prepared carbon dots reaches 82.7% and the FWHM is 49.8 nm.

[0065] Example 4

[0066] 4.387 g of citric acid and 0.025 g of rhodamine 6G (molar ratio 400:1) were weighed and added to 15 mL of deionized water. The mixture was ultrasonically treated in a 59 kHz high-frequency ultrasonic cleaner at room temperature for 10 min to fully dissolve the mixture. The mixture was then transferred to a 50 mL stainless steel autoclave lined with polytetrafluoroethylene.

[0067] A sealed stainless steel autoclave was placed in an oven and heated to 220°C for a hydrothermal reaction for 10 hours. The reaction solution was cooled to room temperature and centrifuged at 10,000 rpm to remove impurities and precipitate, yielding a clear carbon dot solution.

[0068] The clarified carbon dot solution was first filtered using a 0.22 μm disposable microporous filter membrane, and then dialyzed using a dialysis bag with a molecular weight cutoff of 500D (the water was changed every 8 hours, for a total of 48 hours) to remove the unreacted small molecule raw materials and over-carbonized carbon material impurities contained in the carbon dot solution, obtaining a purified orange carbon dot aqueous solution.

[0069] The orange carbon dot aqueous solution was frozen into a solid at -79°C, and freeze-dried to remove moisture to obtain orange carbon dot powder.

[0070] A small amount of the prepared orange carbon dot powder was dispersed in deionized water for testing. Figure 8 As shown in the fluorescence spectrum, the emission peak of carbon dots is located at 555.4 nm.

[0071] Calculations show that the PLQY of the prepared carbon dots reaches 82.1% and the FWHM is 49.2 nm.

[0072] Comparative Example 1

[0073] 4.387 g of citric acid and 0.009 g of 1,5-diaminonaphthalene (molar ratio 400:1) were weighed and subjected to hydrothermal reaction in full accordance with the method in Example 1 to prepare carbon dots, which were finally freeze-dried to obtain light blue carbon dot powder.

[0074] Take a small amount of light blue carbon dot powder and disperse it in deionized water for testing. Figure 9 The fluorescence spectrum of the carbon dots shows an emission peak at 406.3 nm, demonstrating excitation-independent properties. The emission peak intensity is highest at an excitation wavelength of 300 nm, with a PLQY of 15.4% and a full width at half maximum (FWHM) of 58.3 nm. This confirms the key role of the spatially distorted structure in the narrow bandwidth, while a planar, rigid carbon source results in an increased FWHM.

[0075] Comparative Example 2

[0076] 4.387 g of citric acid and 0.014 g of 1-pyrenecarboxylic acid (molar ratio 400:1) were weighed and subjected to hydrothermal reaction in full accordance with the method in Example 1 to prepare carbon dots, which were finally freeze-dried to obtain dark blue carbon dot powder.

[0077] Take a small amount of dark blue carbon dot powder and disperse it in deionized water for testing. Figure 10 The fluorescence spectrum of the carbon dots shows that the emission peak is located at 402.1nm, which has excitation-independent characteristics. The emission peak intensity is the largest when the excitation wavelength is 280nm, the PLQY is 18.1%, and the FWHM is 61.6nm.

[0078] Comparative Example 3

[0079] 4.387 g of citric acid and 0.0224 g of perylene-3,4,9,10-tetracarboxylic dianhydride (molar ratio 400:1) were weighed and hydrothermally reacted according to the method in Example 1 to prepare carbon dots, which were finally freeze-dried to obtain blue carbon dot powder.

[0080] Take a small amount of blue carbon dot powder and disperse it in deionized water for testing. Figure 11 The fluorescence spectrum of the carbon dots shows that the emission peak is located at 459.8 nm, which has excitation-independent characteristics. The emission peak intensity is the largest when the excitation wavelength is 280 nm, the PLQY is 9.2%, and the FWHM is 86.3 nm.

[0081] Application Example 1

[0082] The orange carbon dot powder prepared in Example 1 was dispersed in anhydrous ethanol to prepare a carbon dot ethanol solution with a concentration of 8 mg / mL.

[0083] Weigh 250 mg of polyvinyl pyrrolidone (Mn = 220,000) and add it to 1 mL of anhydrous ethanol. Stir magnetically at 60°C in a closed state until it is completely dissolved. After the solution becomes clear, add 1 mL of the prepared carbon dot ethanol solution and stir until the solution has a uniform color.

[0084] 0.5 mL of the solution was evenly dropped onto the sapphire glass surface and baked in a vacuum drying oven at 50°C for 5 h. After the anhydrous ethanol was completely evaporated, a carbon dot / polyvinyl pyrrolidone fluorescent film was obtained.

[0085] The fluorescent film is assembled with 450nm blue light LEDs to obtain a high color purity photoinduced LED device.

[0086] Figure 12 The fluorescence spectra of the photoinduced LEDs are given. The device assembled with carbon dot / polyvinyl pyrrolidone fluorescent film and 450nm blue LEDs can emit bright orange light with an emission peak at 584.3nm and a full width half maximum (FWHM) of 49.8nm.

[0087] When the operating voltage is 6V, the device has the highest luminous brightness, with a maximum value of 2375cd m -2 .

[0088] The above embodiments of the present invention do not describe all details in detail, nor do they limit the present invention to the above embodiments. Various changes, modifications, substitutions, and variations made by those skilled in the art without departing from the principles and purpose of the present invention are intended to be included within the scope of protection of the present invention.

Claims

1. A carbon dot with high fluorescence quantum yield and narrow bandwidth emission, prepared by hydrothermal reaction of citric acid and rhodamine 6G at 140-220°C. The orange fluorescent carbon dot has an emission wavelength of 556±2nm, a fluorescence quantum yield of not less than 80%, and a half-peak width of not more than 50nm. The carbon dot is used in high-color-purity photoluminescent diodes.

2. The method for preparing carbon dots with high fluorescence quantum yield and narrow bandwidth emission according to claim 1 is to disperse citric acid and rhodamine 6G as carbon sources in water, perform a hydrothermal reaction at 140-220°C to prepare a crude carbon dot product, and prepare an orange solid carbon dot powder through purification and freeze-drying.

3. The preparation method according to claim 2, wherein The molar ratio of the citric acid to the rhodamine 6G is 100-800:

1.

4. The preparation method according to claim 2 or 3, wherein The hydrothermal reaction time is 4 to 12 hours.

5. The preparation method according to claim 2, wherein The hydrothermal reaction temperature is 200° C., and the reaction time is 8 h.

6. The preparation method according to claim 2, wherein The purification includes dialysis treatment of the reaction solution.

7. The preparation method according to claim 6, wherein Dialysis was performed using a dialysis bag with a molecular weight cut-off of 500 Da for 48 h, during which deionized water was replaced every 8 h.

8. Use of the carbon dots with high fluorescence quantum yield and narrow emission bandwidth as claimed in claim 1 as light conversion phosphors in the preparation of high color purity photoluminescent diodes.

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