Oxygen functional group regulated full-color fluorescent graphene quantum dots, preparation method and application

Abstract

The application discloses oxygen-containing functional group regulated full-color fluorescent graphene quantum dots, a preparation method and application, and belongs to the technical field of nano functional materials and optoelectronic devices. The application takes PTCDA (parylene tetracarboxylic dianhydride) as a single carbon source, combines different reaction solvents and chemical additives, and successfully prepares full-color fluorescent graphene quantum dots (GQDs) by using one-step hydrothermal method. The core of the application lies in that the types, contents and C-O and C=O relative proportions of oxygen-containing functional groups on the surface of GQDs can be systematically regulated by changing preparation conditions, the band gap of GQDs is continuously compressed with the increase of the proportion of surface C-O functional groups, and thus the precise and continuous regulation of fluorescent emission wavelength is realized. The method is simple, efficient and low in cost. The prepared high-performance red GQDs are compounded with a polymer matrix PAN to prepare GQDs / PAN composite thin films and electrostatic spinning nanofiber films, and are successfully applied to LED devices, and the LED devices have excellent light emitting performance and good stability, and have great potential in the field of solid-state lighting and display.

Description

Technical Field

[0001] This invention relates to the field of carbon nanotube optoelectronic functional materials technology, and in particular to a method for preparing graphene quantum dots (GQDs) that achieves full-spectrum tunability and narrow half-peak width emission by controlling the types and relative proportions of oxygen-containing functional groups on the surface of graphene quantum dots, as well as the application of this composite luminescent material in solid-state LED lighting, flexible luminescent films and wide color gamut displays. Background Technology

[0002] In recent years, with the evolution of solid-state lighting and display technologies, "full-spectrum LEDs," which can highly simulate natural sunlight, have become a core direction for healthy lighting and next-generation display technologies. Full-spectrum light sources, by supplementing the cyan and deep red light bands missing in traditional LEDs, not only achieve extremely high color rendering indexes (CRI>90) but also significantly reduce blue light hazards, aligning with the human-centered need for healthy circadian rhythms. However, currently commercially available full-spectrum devices heavily rely on complex rare-earth phosphors or semiconductor quantum dots containing heavy metals such as lead and cadmium, facing serious environmental pollution and stringent regulatory restrictions. Graphene quantum dots (GQDs), due to their inherent advantages such as low toxicity, environmental friendliness, and tunable band structure, are widely recognized as ideal candidate materials for constructing green full-spectrum light sources.

[0003] In existing literature and authorized patents, the common techniques for controlling the fluorescence color of GQDs mainly focus on frequently changing the type of carbon source, introducing random co-doping of multiple elements (such as nitrogen, sulfur, and boron), or using strong acids for disordered deep oxidation and trimming. For example, some existing patents (CN 108587615 B) disclose the preparation of single-wavelength GQDs by changing different small organic molecule precursors or using drastically different reaction temperatures and routes. However, obtaining multicolor carbon quantum dots requires fine separation of the upper carbon quantum dot solution using column chromatography, which is overly complex and yields low results.

[0004] Despite the immense potential of GQDs, achieving high-quality, full-spectrum continuous white light remains a significant technological hurdle. Conventional methods for synthesizing GQDs largely limit emission to short-wavelength bands such as blue and green. To obtain the highly efficient long-wavelength (orange and red) emission essential for full-spectrum illumination, complex precursor designs or multi-step, demanding reactions are often required, resulting in extremely low yields and an inability to achieve continuous spectral modulation across the entire visible light spectrum from violet to red. Furthermore, due to the randomness of heteroatom doping, existing GQDs generally suffer from complex surface defect states, leading to excessively wide emission HW widths and low color purity. More critically, when encapsulated as solid-state LED films, the intense π-π stacking between carbon nanoparticles easily induces severe aggregation-induced quenching (ACQ) effects, significantly weakening the optical efficiency and overall color stability of full-spectrum devices.

[0005] To completely overcome the aforementioned limitations, traditional methods for controlling physical dimensions have proven insufficient, necessitating a new approach from the chemical perspective of microscopic bandgap engineering. Systematically adjusting the distribution of electron-withdrawing and electron-donating groups on the surface of GQDs to induce strong charge transfer and significantly and continuously compress the optical bandgap is the fundamental way to solve the problems of long-wavelength emission deficiency and excessively wide full width at half maximum (FWHM). Therefore, exploring a simple method for preparing GQDs that can achieve full-spectrum coverage through the synergistic regulation of electron-withdrawing and electron-donating groups, and developing high-quality, quench-resistant, wide-gamut LED devices based on this method, has become a crucial issue that urgently needs to be addressed in this field. Summary of the Invention

[0006] To address the problems in existing technologies, such as the high dependence of multicolor graphene quantum dot preparation on complex and variable carbon source systems, lack of precise directional control of surface functional groups, difficulty in obtaining efficient long-wavelength light emission, and the tendency for aggregation-induced quenching to occur in solid-state LED packaging, this invention provides a simple and low-cost method for preparing full-color fluorescent GQDs that achieves precise on-demand control of fluorescence color by adjusting the types and proportions of oxygen-containing functional groups on the surface of graphene quantum dots. Furthermore, the prepared high-performance long-wavelength GQDs are applied to LED devices.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] A method for preparing full-color fluorescent graphene quantum dots regulated by oxygen-containing functional groups includes the following steps:

[0009] (1) Preparation of precursor solution: Perylene tetracarboxylic acid dianhydride (PTCDA) was used as a carbon source and dissolved or dispersed in water, ethanol or N,N-dimethylformamide (DMF) to form precursor solution;

[0010] (2) Introduction of additives: Add a regulator to the precursor solution. The regulator is one of dilute sulfuric acid, uric acid, ammonia, sodium hydroxide, 2,6-pyridine acid, or o-phenylenediamine. A specific combination of solvent and regulator corresponds to a specific fluorescent color. By selecting different combinations of solvent and additives, the types, contents and relative proportions of oxygen-containing functional groups on the surface of graphene quantum dots can be systematically changed, thereby achieving continuous control of the fluorescence emission wavelength.

[0011] (3) Hydrothermal reaction: The mixed solution obtained in step (2) is transferred to a high-pressure reactor lined with polytetrafluoroethylene.

[0012] (4) Post-processing: After the reaction is completed, the product is naturally cooled to room temperature, centrifuged to remove large particles, then dialyzed or filtered to remove small molecule impurities, and finally freeze-dried to obtain powdered graphene quantum dots (GQDs).

[0013] Furthermore, in step (1), the concentration of PTCDA in the solvent is 1-3 mg / mL. This concentration setting is crucial for successfully obtaining high-quality quantum dots. If the concentration is too high (>3 mg / mL), excessive cross-linking and severe aggregation of molecules are easily triggered during precursor dispersion and hydrothermal reaction, leading to deep carbonization of the product and the formation of non-fluorescent amorphous carbon blocks or large carbon spheres. If the concentration is too low (<1 mg / mL), the effective collision probability between the carbon source and the regulator molecules is significantly reduced, resulting in extremely low reaction yield. Therefore, 1-3 mg / mL is the optimal range to ensure appropriate molecular fragmentation and nucleation while avoiding carbonization quenching.

[0014] Furthermore, in step (2), when the regulator is dilute sulfuric acid, uric acid, ammonia, sodium hydroxide, or 2,6-pyridinic acid, the mass ratio of the regulator to PTCDA is (400-600):1. Because the aromatic ring structure of PTCDA is highly stable, in order to effectively open and trim the anhydride rings at its edges and precisely control the ratio of CO to C=O on the surface, an extremely high concentration of chemical reaction microenvironment must be provided. This extreme mass ratio (400-600 times excess) can generate a powerful chemical driving force: on the one hand, the excess regulator can instantly coat the edges of the broken carbon skeleton, performing in-situ passivation and effectively preventing secondary disordered cross-linking of reaction intermediates; on the other hand, a large number of polar molecules (such as OH-) can be absorbed by the regulator. - H + (Or amino groups) can deeply participate in nucleophilic substitution or hydrolysis reactions, ensuring that the types of oxygen-containing functional groups (such as the degree of hydroxylation) on the surface of graphene quantum dots reach the expected saturation. If the ratio is lower than 400:1, the ring-opening cleavage is incomplete, the product luminescence is disordered and the yield is low; if it is higher than 600:1, it will destroy the conjugated structure and bring great difficulties to subsequent dialysis purification.

[0015] Furthermore, in step (2), when the regulator is o-phenylenediamine, acetic acid needs to be added to adjust the pH of the reaction system to 5-6, and the mass ratio of o-phenylenediamine to PTCDA is (5-15):1. This specific combination is the core key to achieving precise long-wavelength (red light) emission. Because o-phenylenediamine has high reactivity of diamino groups, it mainly condenses with the anhydride of PTCDA through nucleophilic addition-elimination reactions, deeply anchoring the nitrogen-containing structure (pyridine nitrogen and graphitic nitrogen) at the edge of the quantum dot. The mass ratio is set to (5-15):1 because it does not require a large amount of reagents to drive the reaction, as in simple hydrolysis. This moderate ratio can ensure sufficient heteroatom (nitrogen) doping to significantly compress the optical band gap, while avoiding the polymerization of excess o-phenylenediamine into impurity small molecule fluorophores. The introduction of acetic acid to adjust the pH to a weakly acidic microenvironment of 5-6 plays an irreplaceable role: at pH 5-6, the amino group is in a moderately protonated state, retaining sufficient nucleophilic attack capability, while the weakly acidic environment greatly promotes dehydration condensation and amidation ring-closure reactions. If the system is too acidic (pH < 4), the amino group is completely protonated and loses its nucleophilicity, making doping impossible; if the system is too alkaline, hydrolysis reactions outweigh condensation reactions, the surface is dominated by C–O, leading to a degradation of the emission wavelength towards blue / green light, and failing to obtain the expected high-purity red fluorescence.

[0016] Furthermore, precise control of fluorescence color can be achieved through specific combinations of solvents and regulators. Water, as a highly polar proton solvent, tends to induce hydrolysis of PTCDA. When combined with weak regulators (uric acid, ammonia), the degree of ring-opening is limited, and a large number of electron-withdrawing C=O groups are retained on the surface, resulting in a wide band gap and purple / blue light emission. When combined with a strong base (NaOH), deep hydrolysis and ring-opening occur, leading to a surge in surface C–O (hydroxyl groups), and the electron-donating effect causes the emission to shift from red to green. Moreover, in an aqueous system, even with the addition of a nitrogen-containing regulator (o-phenylenediamine), the hydrolysis reaction still has a competitive advantage, making it difficult to achieve deep nitrogen-doped condensation, thus only achieving green light. DMF, as a polar aprotic solvent, can provide a unique weak reducing and special coordination environment. It greatly inhibits the simple hydrolysis reaction and instead promotes deep dehydration condensation and amidation between the regulator and PTCDA. For example, when both NaOH and DMF are added, the aqueous system produces green light, while the DMF system, due to the formation of special nitrogen-containing defect states, exhibits a red-shift to yellow light. Similarly, when o-phenylenediamine is added, the DMF system perfectly promotes its efficient condensation with the acid anhydride, anchoring large conjugated nitrogen groups (pyridine nitrogen / graphite nitrogen) into the carbon lattice, resulting in strong intramolecular charge transfer (Push-Pull effect). This leads to extreme band gap compression, achieving a remarkably high-purity red fluorescence. Ethanol, as a weakly polar proton solvent, promotes the reaction along the esterification pathway, resulting in mild surface functional group reconstruction, thus filling and enriching the spectral gaps in the mid-to-long wavelength range (yellow and orange light).

[0017] Specific combinations and corresponding fluorescent colors include:

[0018] GQDs emitting blue fluorescence were prepared using water as a solvent and uric acid as a modulator.

[0019] GQDs emitting cyan fluorescence were prepared using water as a solvent and ammonia as a modulator.

[0020] GQDs emitting green fluorescence were prepared using water as a solvent and sodium hydroxide solution as a modulator.

[0021] GQDs emitting blue fluorescence were prepared using water as a solvent and 2,6-pyridinic acid as a modulator.

[0022] Using water as a solvent and o-phenylenediamine as a modulator, GQDs emitting green fluorescence were prepared.

[0023] GQDs emitting green fluorescence were prepared using DMF as solvent and uric acid as modulator.

[0024] GQDs emitting green fluorescence were prepared using DMF as solvent and ammonia as modulator.

[0025] GQDs emitting yellow fluorescence were prepared using DMF as solvent and sodium hydroxide solution as modulator.

[0026] GQDs emitting blue fluorescence were prepared using DMF as solvent and 2,6-pyridinic acid as modulator.

[0027] GQDs emitting red fluorescence were prepared using DMF as a solvent and o-phenylenediamine as a modulator.

[0028] GQDs emitting green fluorescence were prepared using ethanol as a solvent and uric acid as a modulator.

[0029] GQDs emitting green fluorescence were prepared using ethanol as a solvent and ammonia as a modulator.

[0030] GQDs emitting yellow fluorescence were prepared using ethanol as a solvent and sodium hydroxide solution as a modifier.

[0031] GQDs emitting orange fluorescence were prepared using ethanol as solvent and 2,6-pyridinic acid as modulator.

[0032] GQDs emitting yellow fluorescence were prepared using ethanol as a solvent and o-phenylenediamine as a modulator.

[0033] Furthermore, the hydrothermal reaction temperature in step (3) is 140-180℃, and the hydrothermal reaction time is 6-12 hours. Due to the abnormally stable perylene ring skeleton of the precursor PTCDA, if the reaction temperature is below 140℃, the reaction system cannot overcome the activation energy barrier required for hydrolysis ring opening and heteroatom rearrangement doping, resulting in PTCDA being unable to effectively degrade into nanoscale quantum dots, with extremely low yield and almost no luminescence; conversely, if the temperature is above 180℃, the excessive heat energy will trigger excessive carbonization and cross-linking, and the surface oxygen / nitrogen functional groups (C–O, C=O, amide bonds, etc.) that were originally carefully grafted by the regulator will be largely thermally decomposed and stripped, and GQDs will lose the chemical environment for bandgap regulation, not only failing to achieve long-wavelength luminescence such as red and orange, but even causing complete fluorescence quenching, turning into black carbon slag with no luminescence activity. If the reaction time is less than 6 hours, the nucleus growth and edge passivation are insufficient, leaving a large number of unreacted small molecules and intermediate oligomers in the product, resulting in an extremely wide full width at half maximum (FWHM) and low color purity in the emission spectrum. If the time is longer than 12 hours, it will lead to excessive growth of GQDs grain size and irreversible agglomeration and precipitation, significantly reducing their absolute fluorescence quantum yield. Therefore, the synergistic limitation of 140-180℃ and 6-12 hours, while ensuring appropriate carbon framework trimming, maximizes the preservation of surface functional groups that control emission color, and has a decisive influence on the realization of full-color emission.

[0034] The present invention also provides full-color fluorescent graphene quantum dots prepared by the preparation method described above. The fluorescence emission spectrum of the full-color fluorescent graphene quantum dots covers the visible light range of 390 nm to 630 nm. Furthermore, by changing the combination of solvent and additives, the fluorescence of multiple specific wavelengths within the visible light range can be controlled on demand and the full color coverage can be achieved.

[0035] This invention also provides the application of the full-color fluorescent graphene quantum dots in the fabrication of LED devices. Taking a red fluorescent graphene quantum dot LED device as an example, the fabrication method includes the following steps:

[0036] a. Select GQDs powder that emits red fluorescence, mix the selected GQDs powder with the polymer matrix and dissolve it in deionized water, and sonicate to form a uniform composite solution;

[0037] b. Thin-film LED preparation: The composite solution is magnetically stirred, and then the spinning solution is electrospun to prepare a nanofiber membrane with orange or red fluorescence effect. The fiber membrane is then mixed with silicone and coated onto the surface of the LED chip for encapsulation.

[0038] c. Perform photoelectric performance testing on the packaged LED devices.

[0039] Furthermore, in step b, the polymer matrix is ​​one or more of polyacrylonitrile (PAN), polyvinyl alcohol (PVA), polymethyl methacrylate (PMMA), polycarbonate (PC), or epoxy resin. All of these polymer matrices possess excellent optical transparency (high transmittance) and good film-forming / spinning processing properties. More importantly, as spatially isolating matrices, they can effectively anchor GQDs in the inter-chain spaces, fundamentally suppressing the aggregation-induced quenching (ACQ) effect common in carbon nanotube luminescent materials, thus ensuring that the red LED still maintains extremely high quantum yield and color purity in solid-state packaging.

[0040] Furthermore, in step a, the mass ratio of GQDs to the polymer matrix is ​​1:(5-15). This mass ratio range has a significant impact on the luminescence performance of the device. If the polymer matrix ratio is too low (i.e., mass ratio > 1:5), the local concentration of GQDs in the solid film is too high, which can easily trigger a strong self-absorption effect and aggregation quenching, resulting in a sharp drop in luminescence efficiency and broadening of the half-maximum width at half-maximum. If the polymer matrix ratio is too high (i.e., mass ratio < 1:15), the concentration of effective luminescent centers in the composite film is insufficient, and it cannot fully absorb the excitation light from the bottom chip, resulting in poor color conversion capability and severe light leakage. Therefore, a ratio of 1:(5-15) is the optimal range for balancing anti-quenching effect and luminescence intensity.

[0041] Through numerous creative experiments, this invention has discovered that GQDs covering the entire visible spectrum can be prepared by using PTCDA as a precursor and systematically combining three solvents (water, ethanol, and DMF) and five additives (uric acid, ammonia, sodium hydroxide, 2,6-pyridinic acid, and o-phenylenediamine).

[0042] The mechanism of this invention lies in the fact that by selecting different combinations of solvents and additives, the types, contents, and relative proportions of oxygen-containing functional groups on the surface of graphene quantum dots (GQDs) can be systematically varied, thereby achieving continuous control over the fluorescence emission wavelength. The X-ray photoelectron spectroscopy (XPS) O 1s fine spectra of the seven different colored GQDs prepared exhibit a regular evolution: as the emission wavelength shifts from violet to red to red, the O / C atomic ratio on the GQDs surface first increases and then slightly decreases. The O / C ratio is approximately 0.20 for the blue sample, reaches a maximum of approximately 0.38 for the green sample, and is between 0.28 and 0.30 for the red sample. Simultaneously, the peak area ratio of C–O bonds (corresponding to hydroxyl and epoxy groups) and C=O bonds (corresponding to carbonyl and carboxyl groups) undergoes a systematic reversal. The surface of short-wavelength emission GQDs is dominated by C=O and O=C–O functional groups, with a C–O / C=O ratio of less than 1. The C–O / C=O ratio of medium-wavelength (green and yellow) GQDs increases significantly to over 1.5, indicating a substantial increase in the content of surface hydroxyl groups. The C=O ratio of long-wavelength (orange and red) GQDs rises again, but at this time the C=O mainly comes from the nitrogen-oxygen amide structure introduced by the DMF system, rather than simple carboxyl groups.

[0043] The changes in the composition of the aforementioned oxygen-containing functional groups directly affect the positions of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of graphene quantum dots: when the surface oxygen-containing groups are predominantly C–O (hydroxyl groups), the electron-donating effect is enhanced, the HOMO energy level rises, the band gap narrows, and fluorescence red-shifts; when the surface is predominantly C=O (carbonyl / carboxyl groups), the electron-withdrawing effect is enhanced, the band gap increases, and fluorescence blue-shifts. In this invention, this systematic evolution of the relative abundance of C–O and C=O functional groups and the O / C ratio is achieved by precisely combining specific solvents (water, ethanol, DMF) and regulators (uric acid, ammonia, sodium hydroxide, 2,6-pyridinic acid, o-phenylenediamine) to guide the precursor to undergo a directional chemical reaction. For short-wavelength emission (violet and blue light): using water as a solvent, combined with weak acid / weak base regulators such as uric acid or ammonia, the reaction conditions are mild. The modulator primarily induces limited hydrolysis and cleavage of the anhydride ring in PTCDA, resulting in a product surface dominated by C=O and O=C–O (carbonyl / carboxyl) groups, with a low O / C atomic ratio (approximately 0.20) and a C–O / C=O ratio less than 1. At this stage, a strong electron-withdrawing effect dominates, leading to a large band gap and exhibiting short-wavelength violet / blue emission. For medium-wavelength emission (cyan, green, and yellow): Introducing strong alkaline modulators such as sodium hydroxide, or combining with a water / ethanol system, induces deep ring-opening of the anhydride ring and the introduction of a large number of hydroxyl groups. This intensifies nucleophilic substitution and hydrolysis within the reaction system, causing a surge in the C–O (hydroxyl / epoxy) content on the GQDs surface, significantly increasing the C–O / C=O ratio to above 1.5, and reaching a peak O / C ratio (approximately 0.38). Due to the strong electron-donating effect of the hydroxyl groups, the HOMO energy level is significantly raised, the band gap narrows significantly, and the fluorescence shifts towards the green and yellow wavelengths. For long-wavelength emission (orange and red light): DMF or ethanol is used as the solvent, along with a modulator containing an amino / aromatic ring (such as o-phenylenediamine or 2,6-pyridinic acid), to conduct a hydrothermal reaction at a specific pH. In this reaction, not only ring-opening reactions occur, but also deep amidation and condensation reactions. The synergistic effect of the nitrogen-containing solvent and the nitrogen-containing modulator anchors the large-volume amide / imide nitrogen-oxygen structures (specific C=O) and polarization defects at the edge of the GQDs. In this state, the O / C ratio is moderate (approximately 0.28-0.30), but the synergistic doping of N / O atoms further compresses the optical band gap.

[0044] Compared with the prior art, the present invention has the following significant advantages:

[0045] 1. Band modulation dominated by the types and proportions of oxygen-containing functional groups:

[0046] This invention, through systematic characterization, reveals a definite correspondence between the fluorescence emission wavelengths of the prepared series of GQDs and the types and relative contents of their surface oxygen-containing functional groups. Specifically, GQDs with a higher proportion of C–O functional groups (hydroxyl, epoxy groups) exhibit long-wavelength emission (green to red), while GQDs with a higher proportion of C=O functional groups (carbonyl, carboxyl groups) exhibit short-wavelength emission (violet to blue). This systematic evolution of the oxygen-containing functional group composition drives the elevation of the highest occupied molecular orbital energy level of GQDs and the continuous compression of the optical band gap, thereby achieving precise and continuous tuning of the fluorescence emission wavelength from 390 nm to 630 nm.

[0047] 2. Size Coordination and Surface State Defect Construction:

[0048] With the directional grafting and introduction of specific oxygen- and nitrogen-containing functional groups, the nucleation and grain growth kinetics of the reaction system are significantly altered. Systematic adjustment of the relative proportions and modification levels of different types of surface functional groups (such as C–O, C=O, and amide structures) leads to the breaking of local symmetry in the carbon framework, resulting in an increase in the proportion of sp³ hybrid defects and structural disorder within the GQDs system, which in turn causes a synchronous increase in its lateral crystal size. This micro-size effect, closely synergistic with specific surface oxygen / nitrogen-containing defect states, effectively reshapes the micro-energy level distribution of the system, thereby breaking the high-energy transition limitations of conventional wide bandgap light and providing abundant low-energy radiation recombination centers for long-wavelength (orange and red) emission.

[0049] 3. Edge structure modification and sealing to suppress luminescence quenching:

[0050] During the fabrication of graphene quantum dots, numerous broken chemical bonds inevitably form at the edges of the carbon framework. These edge defects convert energy intended for luminescence into ineffective heat, resulting in dim luminescence. To achieve high-brightness luminescence, this invention achieves precise modification and repair of these edge defects through a synergistic reaction of specific solvents and modifiers. The introduced amine groups and other specific nitrogen / oxygen-containing functional groups not only participate in changing the luminescence color but also firmly bind to the broken chemical bonds at the quantum dot edges. This effective edge structure protection mechanism ensures that the vast majority of absorbed energy is converted into light, fundamentally overcoming the technical bottleneck of dim luminescence in long-wavelength (especially red) quantum dots, thus significantly improving their luminescence efficiency and brightness (i.e., quantum yield). Attached Figure Description

[0051] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0052] Figure 1 These are digital photographs of six different colored GQDs prepared in Example 1 of this invention under natural light (top row) and 365 nm ultraviolet light (bottom row).

[0053] Figure 2 The images show the fluorescence emission spectra of six different colored GQDs prepared in Example 1 of this invention, demonstrating continuous spectral coverage from purple to red.

[0054] Figure 3 These are XPS images of six different colored GQDs prepared in Example 1 of this invention.

[0055] Figure 4 Emission spectrum diagrams and physical images of LEDs of different colors. Detailed Implementation

[0056] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to represent selected embodiments of the invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0057] Example 1

[0058] This embodiment describes in detail a method for preparing panchromatic fluorescent GQDs using a single carbon source (PTCDA) combined with different solvents and modulators.

[0059] (1) Preparation of precursor solution:

[0060] Weigh out perylene tetracarboxylic acid dianhydride (PTCDA) powder and ultrasonically disperse it in deionized water, anhydrous ethanol, or N,N-dimethylformamide (DMF), respectively, for 30 minutes until uniformly dispersed, maintaining its concentration at 2 mg / mL.

[0061] (2) Introduction and combination of regulators:

[0062] According to the desired target emission color, specific modifiers are added to the above precursor solutions respectively. Figure 1 This embodiment prepares six different colors of GQDs:

[0063] Blue GQDs (b-GQDs): The solvent is deionized water, and the regulator is uric acid (the mass ratio of uric acid to PTCDA is 500:1).

[0064] Cyan GQDs (c-GQDs): The solvent is deionized water, and the regulator is ammonia (the mass ratio of ammonia to PTCDA is 500:1).

[0065] Green GQDs (g-GQDs): The solvent is deionized water, and the regulator is a 0.1M sodium hydroxide solution (NaOH to PTCDA mass ratio is 500:1).

[0066] Yellow GQDs (y-GQDs): The solvent is DMF, and the modifier is a 0.1M sodium hydroxide solution (NaOH to PTCDA mass ratio is 500:1).

[0067] Orange GQDs (o-GQDs): The solvent is anhydrous ethanol, and the regulator is 2,6-pyridinedicarboxylic acid (2,6-pyridinedicarboxylic acid to PTCDA mass ratio 500:1).

[0068] Red GQDs (r-GQDs): The solvent is DMF, and the regulator is o-phenylenediamine, wherein the mass ratio of o-phenylenediamine to PTCDA is 10:1. Glacial acetic acid is added dropwise to adjust the pH of the reaction system to 5.5.

[0069] (3) Hydrothermal reaction:

[0070] The above six mixed solutions were transferred to 50 mL stainless steel high-pressure reactors lined with polytetrafluoroethylene, sealed, and placed in an oven. The temperature was raised to 160°C and maintained for 10 hours.

[0071] (4) Separation and purification:

[0072] After the reaction was complete, the mixture was allowed to cool naturally to room temperature. First, it was centrifuged at 10,000 rpm for 15 minutes, and the supernatant was collected to remove unreacted large carbon source particles. Then, the supernatant was placed in a dialysis bag with a molecular weight cutoff of 1000 Da and dialyzed in deionized water for 48 hours (changing the water every 8 hours) to remove unreacted small molecule impurities. Finally, the dialyzed solution was freeze-dried to obtain solid GQDs powders of various luminescent colors.

[0073] Characterization and evolution analysis of oxygen-containing functional groups: The fine O 1s spectra of the six GQDs obtained in Example 1 were fitted using X-ray photoelectron spectroscopy (XPS). The results showed that the O / C atomic ratio of the GQDs changed systematically as the emission wavelength shifted from blue to red: 0.21 for the blue sample, increasing to 0.38 for the green sample, and 0.29 for the red sample. Simultaneously, the peak area ratio of C–O (approximately 533.2 eV) to C=O (approximately 531.8 eV) underwent a systematic reversal: C=O dominated in the blue sample, with a C–O / C=O ratio of 0.6; the C–OH content significantly increased in the green sample, reaching a C–O / C=O ratio of 1.8; and the C=O proportion increased again in the red sample, but at this point, the C=O mainly belonged to nitrogen- and oxygen-containing amide or imide structures. The changes in the types and relative contents of the aforementioned oxygen-containing functional groups are highly correlated with the redshift of the fluorescence emission wavelength, indicating that precise control of oxygen-containing surface states is the key chemical basis for achieving panchromatic emission.

[0074] Figure 2 The photoluminescence (PL) behavior of the full-spectrum GQDs was shown to exhibit a significant and continuous spectral evolution, with the optimal emission peak position gradually redshifting from 440 nm in b-GQDs to 640 nm in r-GQDs, covering almost the entire visible light region.

[0075] Figure 3 The sample was shown to consist mainly of two characteristic peaks: C1s (approximately 285 eV) and O1s (approximately 532 eV), indicating that carbon and oxygen are the main constituent elements of the material. Further analysis of the high-resolution C1s and N1s energy dispersive spectroscopy (EDS) provides a more direct view of the changes in the internal chemical structure of the material. With the adjustment of reaction conditions, different samples exhibited significant differences in the type and proportion of functional groups, indicating that significant structural reconstruction occurred within the system. In the r-GQDs sample, a relatively obvious pyridinic nitrogen (Pyridinic-N) characteristic peak appeared at approximately 398 eV.

[0076] Table 1. Content and proportion of each element in full-spectrum GQDs in XPS

[0077]

[0078] Table 1 shows that the oxygen-to-carbon ratio (O / C) of the samples increased significantly from 0.25 for b-GQDs to 1.13 for r-GQDs, indicating that more oxygen-containing functional groups were gradually introduced onto the material surface during the reaction.

[0079] Table 2. C=O and CO content and ratio of full-spectrum GQDs in XPS

[0080]

[0081] Table 2 shows that the atomic ratio of C=O to CO increased from 1.17 to 3.35, indicating that strong electron-withdrawing groups such as carboxyl groups gradually became dominant in the system.

[0082] Example 2

[0083] To enhance and expand the application scenarios and operability of the present invention, this embodiment extends the heating means in the technical solution by an equivalent means.

[0084] Extension of reaction heating method: After preparing the precursor and adding the regulator, the conventional hydrothermal heating in step (3) of Example 1 can be replaced by microwave-assisted heating. A mixture of water or DMF is placed in a microwave synthesizer, the microwave power is set to 600-800 W, and the reaction temperature is controlled at 150-170℃. Utilizing the internal heating effect of microwaves on polar molecules, the reaction time, which originally required 6-12 hours, can be significantly shortened to 30-60 minutes. The GQDs prepared by this extended method not only have an improved yield but also exhibit fluorescence emission peak positions consistent with the conventional hydrothermal method, effectively improving the efficiency of industrial production.

[0085] Application Example 1

[0086] This application example details how to formulate freeze-dried GQDs solid powder into a high-purity, highly dispersible luminescent concentrate, providing a standardized precursor for subsequent processing of optoelectronic thin films and LED devices. The specific preparation steps are as follows:

[0087] (1) Accurately weigh the GQDs solid powder of a specific color obtained by the above preparation method and freeze-dried. Based on the polarity characteristics of the polymer matrix used in the subsequent preparation of the composite luminescent film or electrospinning, select a suitable solvent: if polyvinyl alcohol (PVA) is used as the polymer matrix, deionized water is used as the matching solvent; if polyacrylonitrile (PAN) or epoxy resin is used as the polymer matrix, N,N-dimethylformamide (DMF) is used as the matching solvent. Add the weighed GQDs powder to the selected matching solvent, controlling the mass-volume concentration of GQDs in the solvent to be between 10 and 50 mg / mL.

[0088] (2) Seal the container containing the above mixture and place it in an ultrasonic cleaner or cell disruptor for continuous ultrasonic treatment under ice-water bath conditions for 15-30 minutes. Utilizing the cavitation effect of ultrasound, the nanoscale agglomerations formed by intermolecular forces during the freeze-drying process of GQDs powder are completely broken down, allowing them to completely deagglomerate in a specific solvent and achieve a monodisperse state. The ice-water bath environment effectively prevents solvent evaporation or attenuation of the optical properties of quantum dots caused by ultrasonic heat generation.

[0089] (3) Use a polytetrafluoroethylene (PTFE) organic microporous membrane or an aqueous microporous membrane with a pore size of 0.22 μm to perform injection filtration on the above-mentioned ultrasonically treated mixture. This step removes a very small number of undispersed dead particles or large-sized impurities. Collect the filtrate to obtain a GQDs luminescent concentrate with no visible particles and optical-grade transparency. Finally, seal the concentrate and store it in a 4°C refrigerator away from light for later use.

[0090] Application Example 2

[0091] This paper focuses on optimizing the process parameters of "red-emitting GQDs," which currently present the greatest technical challenge, and details their standard application in high-performance solid-state LED lighting.

[0092] (1) Optimal synthesis of red-light GQDs: to obtain the highest quantum yield and maximum sp 2 For the conjugated red-light GQDs, the reaction conditions were optimized as follows: high-purity DMF was used as the solvent, and the concentration of the carbon source PTCDA was set at 3 mg / mL. o-phenylenediamine was used as the modulator, with a mass ratio optimized to 15:1, and acetic acid was added to adjust the pH to 5.0. The hydrothermal temperature was increased to 180℃, and the reaction time was extended to 12 hours to promote efficient and deep nitrogen doping, significantly reducing the material's band gap.

[0093] (2) Preparation of anti-quenching GQDs / PAN composite luminescent film and LED: Accurately weigh the red GQDs powder and polyacrylonitrile (PAN, molecular weight 150,000) powder obtained above. Dissolve both in DMF at a mass ratio of 1:10 (GQDs:PAN) and magnetically stir at 60℃ for 12 hours to form a uniform viscous spinning precursor solution. Use an electrospinning device to prepare GQDs / PAN nanofiber membranes under the conditions of 15 kV voltage, 15 cm receiving distance, and 1.0 mL / h feed rate. This porous network structure can effectively isolate GQDs particles and fundamentally suppress the aggregation-induced quenching (ACQ) effect caused by π-π stacking. A GQDs / PAN red fluorescent nanofiber film of appropriate size is cut and uniformly mixed with a high-transmittance silicone base. This silicone base acts not only as a binder but also as an index-matching layer to reduce total internal reflection loss of blue light when it enters the air from the high-refractive-index semiconductor. The cut GQDs composite film is then precisely attached to the uncured silicone base. Optical silicone with a hemispherical or flat-top lens structure is then dripped onto the attached GQDs film for overall encapsulation. The entire LED device is then placed in an oven and cured in a stepped temperature range of 100℃-150℃ for 1-2 hours to allow the silicone to fully cross-link and solidify. Due to the synergistic doping of specific solvents and additives in this invention, the product has an extremely narrow half-width at half-maximum (FWHM < 40 nm). The resulting white LED device not only has a color rendering index (CRI) of over 90 but also significantly enhanced color gamut coverage under the NTSC standard, perfectly meeting the application requirements of high-end wide color gamut displays.

[0094] Application Example 3

[0095] Fabrication of Blue-Emitting GQDs and Their Application in High-Performance Solid-State LED Lighting

[0096] (1) Synthesis of blue light GQDs: Deionized water was used as solvent and the concentration of PTCDA was set to 2 mg / mL. Uric acid was selected as the regulator and the mass ratio of regulator to carbon source PTCDA was 500:1. The mixed solution was placed in a reaction vessel and hydrothermally reacted at 160 °C for 10 hours. After purification and freeze-drying, blue light GQDs powder was obtained.

[0097] (2) LED device fabrication: High-transmittance polyvinyl alcohol (PVA) was selected as the water-soluble matrix. The above-mentioned blue light-emitting GQDs and PVA were mixed in deionized water at a mass ratio of 1:10, and dried on a flat substrate using a blade coating method. The composite light-emitting film was cut, attached to and encapsulated on the surface of a commercially available ultraviolet LED chip using optical silicone. Tests showed that the device emitted bright and pure blue fluorescence.

[0098] Application Example 4

[0099] Fabrication of cyan GQDs and their application in high-performance solid-state LED lighting

[0100] (1) Synthesis of GQDs: Deionized water was used as the solvent and the concentration of PTCDA was set to 2 mg / mL. Ammonia was selected as the regulator, and the mass ratio of regulator to carbon source PTCDA was 500:1. The hydrothermal reaction was carried out at 160 °C for 10 hours to promote the appropriate ring-opening and amino modification of the carbon source. After purification and freeze-drying, GQDs powder was obtained.

[0101] (2) LED device fabrication: Following a similar process to that described in Application Example 3, cyan GQDs powder was mixed with PVA (mass ratio 1:10) aqueous solution to prepare a transparent fluorescent film. This film was then coated onto the surface of an ultraviolet LED chip, cured, and encapsulated. After being powered on, a cyan solid-state light source with high color purity was obtained.

[0102] Application Example 5

[0103] Fabrication of Green GQDs and Their Application in High-Performance Solid-State LED Lighting

[0104] (1) Synthesis of green GQDs: Deionized water was used as the solvent, and the concentration of PTCDA was set at 2 mg / mL. 0.1 M sodium hydroxide solution was selected as a strong nucleophilic regulator, and the mass ratio of regulator to carbon source PTCDA was 500:1. The hydrothermal reaction was carried out at 160 °C for 10 hours to promote deep ring-opening and hydroxylation of the acid anhydride. After purification and freeze-drying, green GQD powder with high quantum yield was obtained.

[0105] (2) LED device fabrication: The green GQDs powder and polyacrylonitrile (PAN) powder (mass ratio 1:10) were dissolved together in DMF, and a green GQDs / PAN composite nanofiber membrane was prepared by electrospinning. The membrane was cut and encapsulated on the surface of a commercial blue LED chip (excitation wavelength 450 nm) using transparent silicone to obtain a high-efficiency green LED device.

[0106] Application Example 6

[0107] Fabrication of yellow-light GQDs and their application in high-performance solid-state LED lighting

[0108] (1) Synthesis of yellow GQDs: High-purity DMF was used as the solvent, and the concentration of PTCDA was set at 2.5 mg / mL. Similarly, 0.1 M sodium hydroxide solution was selected as the regulator, and the mass ratio of regulator to carbon source PTCDA was 500:1. Under the special polar microenvironment of DMF, the reaction was carried out at 160 °C for 10 hours to induce the generation of nitrogen-oxygen-containing special defect states on the surface, thus obtaining yellow GQDs powder.

[0109] (2) LED device fabrication: Yellow GQDs powder and polymethyl methacrylate (PMMA) powder (mass ratio 1:8) were dissolved in a suitable solvent and mixed evenly. A yellow light-emitting anti-quenching film was prepared by spin coating. The film was then combined with a blue LED chip and encapsulated with silicone to obtain a yellow LED device with stable luminous efficacy.

[0110] Application Example 7

[0111] Fabrication of Orange Light GQDs and Their Application in High-Performance Solid-State LED Lighting

[0112] (1) Synthesis of orange-colored GQDs: Anhydrous ethanol was used as a weakly polar protic solvent, and the concentration of PTCDA was 2 mg / mL. 2,6-pyridinedicarboxylic acid was selected as a regulator, and the mass ratio of regulator to carbon source PTCDA was 500:1. The reaction was carried out at 160 °C for 10 hours to guide the reaction along a mild esterification and heteroatom rearrangement route, and orange-colored GQDs powder was obtained.

[0113] (2) LED device fabrication: Using the method in Application Example 1, orange GQDs powder was prepared into a DMF concentrate with a concentration of 20 mg / mL. After being mixed with PAN spinning solution, it was electrospun into shape. It was then attached to a commercial blue LED chip and encapsulated with silicone to obtain a high color rendering and photodegradation resistant orange LED solid-state lighting device.

[0114] Figure 4 The emission spectra and physical images of silicon-based fluorescent LEDs (GQDs) prepared by encapsulating different fluorescent GQDs with silicon substrates are shown. These include (a) b-GQDs, (b) c-GQDs, (c) g-GQDs, (d) y-GQDs, (e) o-GQDs, and (f) r-GQDs. The LEDs exhibit a continuous color change from deep blue to saturated red under different component ratios, demonstrating stable and uniform luminescence.

[0115] Application Example 8

[0116] To enhance and expand the application scenarios and operability of the present invention, this application example extends the polymer matrix and application device in the technical solution by equivalent means.

[0117] Equivalent substitution of polymer matrix: Besides polyacrylonitrile (PAN), other polymers used as the dispersion matrix can be equivalently replaced by polyvinyl alcohol (PVA), polymethyl methacrylate (PMMA), polycarbonate (PC), or epoxy resin. For example, when preparing a flexible transparent luminescent film, a PVA aqueous solution (10 wt%) is mixed with red GQDs (mass ratio 1:8), and the film is directly formed on a flexible PET substrate using a blade coating method. This composite film not only possesses high flexibility and light transmittance but can also be directly used as a "color conversion layer" in LCD backlight display modules to expand the wide color gamut of the display.

[0118] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing full-color fluorescent graphene quantum dots regulated by oxygen-containing functional groups, characterized in that, Includes the following steps: (1) Preparation of precursor solution: Perylene tetracarboxylic acid dianhydride (PTCDA) was used as a carbon source and dissolved or dispersed in water, ethanol or N,N-dimethylformamide (DMF) to form precursor solution; (2) Introduction of additives: Add a regulator to the precursor solution. The regulator is one of dilute sulfuric acid, uric acid, ammonia, sodium hydroxide, 2,6-pyridine acid, or o-phenylenediamine. A specific combination of solvent and regulator corresponds to a specific fluorescent color. By selecting different combinations of solvent and additives, the types, contents and relative proportions of oxygen-containing functional groups on the surface of graphene quantum dots can be systematically changed, thereby achieving continuous control of the fluorescence emission wavelength. (3) Hydrothermal reaction: The mixed solution obtained in step (2) is transferred to a high-pressure reactor lined with polytetrafluoroethylene, and then subjected to hydrothermal reaction. (4) Post-processing: After the reaction is completed, the product is naturally cooled to room temperature, centrifuged to remove large particles, then dialyzed or filtered to remove small molecule impurities, and finally freeze-dried to obtain powdered graphene quantum dots (GQDs).

2. The preparation method according to claim 1, characterized in that, In step (1), the concentration of PTCDA in the solvent is 1-3 mg / mL.

3. The preparation method according to claim 1, characterized in that, In step (2), when the regulator is dilute sulfuric acid, uric acid, ammonia, sodium hydroxide or 2,6-pyridine acid, the mass ratio of the amount of regulator added to PTCDA is (400-600):

1.

4. The preparation method according to claim 1, characterized in that, In step (2), when the regulator is o-phenylenediamine, acetic acid needs to be added to adjust the pH of the reaction system to 5-6, and the mass ratio of o-phenylenediamine to PTCDA is (5-15):

1.

5. The preparation method according to claim 1, characterized in that, Precise control of fluorescence color is achieved through specific combinations of solvents and modulators. Specific combinations and corresponding fluorescence colors include: GQDs emitting blue fluorescence were prepared using water as a solvent and uric acid as a modulator. GQDs emitting cyan fluorescence were prepared using water as a solvent and ammonia as a modulator. GQDs emitting green fluorescence were prepared using water as a solvent and sodium hydroxide solution as a modulator. GQDs emitting blue fluorescence were prepared using water as a solvent and 2,6-pyridinic acid as a modulator. Using water as a solvent and o-phenylenediamine as a modulator, GQDs emitting green fluorescence were prepared. GQDs emitting green fluorescence were prepared using DMF as solvent and uric acid as modulator. GQDs emitting green fluorescence were prepared using DMF as solvent and ammonia as modulator. GQDs emitting yellow fluorescence were prepared using DMF as solvent and sodium hydroxide solution as modulator. GQDs emitting blue fluorescence were prepared using DMF as solvent and 2,6-pyridinic acid as modulator. GQDs emitting red fluorescence were prepared using DMF as a solvent and o-phenylenediamine as a modulator. GQDs emitting green fluorescence were prepared using ethanol as a solvent and uric acid as a modulator. GQDs emitting green fluorescence were prepared using ethanol as a solvent and ammonia as a modulator. GQDs emitting yellow fluorescence were prepared using ethanol as a solvent and sodium hydroxide solution as a modifier. GQDs emitting orange fluorescence were prepared using ethanol as solvent and 2,6-pyridinic acid as modulator. GQDs emitting yellow fluorescence were prepared using ethanol as a solvent and o-phenylenediamine as a modulator.

6. The preparation method according to claim 1, characterized in that, The temperature of the hydrothermal reaction in step (3) is 140-180℃, and the hydrothermal reaction time is 6-12 hours.

7. A full-color fluorescent graphene quantum dot prepared by the preparation method according to any one of claims 1-6, characterized in that, The fluorescence emission spectrum of the full-color fluorescent graphene quantum dots covers the visible light range from 390 nm to 630 nm, and by changing the combination of solvents and additives, it is possible to achieve on-demand control of fluorescence at multiple specific wavelengths within the visible light range and full color coverage.

8. An application of the full-color fluorescent graphene quantum dots described in claim 7 in the fabrication of LED devices, characterized in that, Taking red fluorescent graphene quantum dot LED devices as an example, their fabrication method includes the following steps: a. Select GQDs powder that emits red fluorescence, mix the selected GQDs powder with the polymer matrix and dissolve it in deionized water, and sonicate to form a uniform composite solution; b. Thin-film LED preparation: The composite solution is magnetically stirred, and then the spinning solution is electrospun to prepare a nanofiber membrane with orange or red fluorescence effect. The fiber membrane is then mixed with silicone and coated onto the surface of the LED chip for encapsulation. c. Perform photoelectric performance testing on the packaged LED devices.

9. The application according to claim 8, characterized in that, In step b, the polymer matrix is ​​one or more of polyacrylonitrile (PAN), polyvinyl alcohol (PVA), polymethyl methacrylate (PMMA), polycarbonate (PC), or epoxy resin.

10. The application according to claim 8, characterized in that, In step a, the mass ratio of GQDs to polymer matrix is ​​1:(5-15).

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