Solid carbon quantum dot

Abstract

The purpose of the present invention is to provide a novel solid carbon quantum dot that exhibits excellent light emission characteristics when irradiated with light. A solid carbon quantum dot that solves the above problem comprises a carbon nitride crystal structure and a graphite crystal structure, wherein: an X-ray diffraction spectrum obtained by X-ray diffraction has a peak PCN that belongs to the carbon nitride crystal structure and that has a diffraction angle 2θ which has a maximum value in the range of larger than 26° but not more than 29° and a peak PGr1 that belongs to the graphite crystal structure and that has a diffraction angle 2θ which has a maximum value in the range of 10-12°; and when a line connecting the point at which the diffraction (scattering) angle is the smallest at a diffraction angle 2θ of 11-15° and the point at which the diffraction (scattering) intensity is the smallest at a diffraction angle ２θ of 34-40° is a baseline, the half width of a diffraction peak PGr2 that belongs to the graphite crystal structure and that is located at a diffraction angle 2θ of 25° is 2-20°.

Description

solid carbon quantum dots

[0001] The present invention relates to solid carbon quantum dots.

[0002] Conventionally, a white LED has been known that combines a blue LED with an inorganic fluorescent material that is excited by light from the blue LED and emits yellow light (with a wavelength of approximately 520 nm to 580 nm). However, inorganic fluorescent materials are expensive, and there is a need to develop alternative materials to inorganic fluorescent materials.

[0003] Carbon quantum dots are stable carbon-based microparticles with particle diameters ranging from several nanometers to several tens of nanometers, and some carbon quantum dots are known to exhibit excellent fluorescence properties. Carbon quantum dots also have the advantage of being relatively inexpensive to manufacture, without the need to include rare elements, as is the case with inorganic fluorescent materials. Therefore, much research has been conducted on carbon quantum dots in recent years. However, many of these quantum dots have insufficient light-emitting properties, and further improvements are needed.

[0004] On the other hand, carbon quantum dots are expected to be used in applications other than luminescent materials. For example, Non-Patent Document 1 describes the use of a mixture of carbon nitride and graphene quantum dots as a humidity sensor material. Furthermore, Non-Patent Document 2 describes the simultaneous measurement of urea and dopamine using a mixture of carbon nitride and graphene quantum dots. Furthermore, Non-Patent Document 3 describes a mixture of sulfur-doped carbon nitride and graphene quantum dots that is useful as an electrode catalyst.

[0005] Morsy et al., Design and implementation of humidity sensor based on carbon nitride modified with graphene quantum dots, Natureportfolio Scientific Reports, 2023, No.23, 2891Xia et al., Facile preparation of metal-free graphitic-like carbon nitride / graphene oxide composite for simultaneous determination of uric acid and dopamine, Microchemical Journal, 2023, No.190, 108726Chenyu et al., Sulfur-doped graphitic carbon nitride decorated with graphene quantum dots for an efficient metal-free electrocatalyst, Journal or Materials Chemistry A, 2015,3, 1841-1846

[0006] However, the mixtures of carbon nitride and graphite (or graphene) quantum dots described in Non-Patent Documents 1, 2, and 3 above are not significantly different from conventional carbon quantum dots in terms of fluorescent properties, and therefore cannot be used as a light-emitting material.

[0007] The present invention has been made in view of the above-mentioned problems, and has an object to provide novel solid carbon quantum dots that exhibit excellent light-emitting properties when irradiated with light.

[0008] The present invention relates to a method for producing a carbon nitride crystalline structure comprising a carbon nitride crystalline structure and a graphite crystalline structure, and in an X-ray diffraction spectrum obtained by an X-ray diffraction method, a peak P attributable to the carbon nitride crystalline structure has a maximum value in a range of a diffraction angle 2θ of more than 26° and not more than 29°. ＣＮand a peak P attributable to the crystalline structure of the graphite, which has a maximum value in the range of a diffraction angle 2θ of 10° or more and 12° or less. Ｇｒ１ and a diffraction peak P attributable to the crystalline structure of the graphite located at a diffraction angle 2θ=25° when a line connecting a point where the diffraction (scattering) angle is minimum when the diffraction angle 2θ is 11° or more and 15° or less and a point where the diffraction (scattering) intensity is minimum when the diffraction angle 2θ is 34° or more and 40° or less is used as a baseline. Ｇｒ２ The present invention provides solid carbon quantum dots having a half-width of 2° or more and 20° or less.

[0009] The present invention provides novel solid carbon quantum dots that exhibit excellent light-emitting properties when irradiated with light.

[0010] FIG. 1 shows the X-ray diffraction spectrum of the solid carbon quantum dots prepared in Example 1, the X-ray diffraction spectrum of a commercially available carbon nitride, and the X-ray diffraction spectrum of commercially available graphene carbon quantum dots. FIG. 2A shows the X-ray diffraction spectrum of the carbon nitride, graphite carbon quantum dots, and a mixture thereof described in Non-Patent Document 1. FIG. 2B shows the X-ray diffraction spectrum of the carbon nitride, graphene carbon quantum dots, and a mixture thereof described in Non-Patent Document 2. FIG. 2C shows the X-ray diffraction spectrum of the sulfur carbon nitride, graphene quantum dots, and a mixture thereof described in Non-Patent Document 3. FIGS. 3A and 3B are both images of the solid carbon quantum dots prepared in Example 1 observed with a transmission electron microscope. FIG. 4 shows the results of infrared spectroscopy analysis of the solid carbon quantum dots prepared in Example 1. FIG. 5A shows the results of X-ray photoelectron spectroscopy analysis of the solid carbon quantum dots prepared in Example 1, and FIG. 5B shows the results of X-ray photoelectron spectroscopy analysis of the commercially available carbon nitride. Figure 6 shows the results of analysis of boron by X-ray photoelectron spectroscopy of the solid carbon quantum dots prepared in Example 1. Figure 7 shows the X-ray diffraction spectra of the unpurified and purified solid carbon quantum dots prepared in Example 1, the X-ray diffraction spectrum of commercially available carbon nitride, and the X-ray diffraction spectrum of commercially available graphene carbon quantum dots.

[0011] In this specification, a numerical range indicated by "to" means a numerical range including the numerical values ​​written before and after "to".

[0012] The solid carbon quantum dots of the present invention are novel quantum dots having a carbon nitride crystal structure and a graphite crystal structure. In this specification, the term "carbon nitride crystal structure" refers to graphitic carbon nitride (g-C ３ N ４ The term "graphite crystal structure" refers to a crystal structure in which six-membered ring structures consisting of carbon and nitrogen atoms are connected in a plane, also referred to as a graphene crystal structure. In this specification, the term "graphite crystal structure" includes not only a structure in which multiple graphenes are stacked, but also a structure in which graphene exists as a single layer. In the solid carbon quantum dots of the present invention, some carbon atoms in the graphite crystal structure may be substituted with other atoms (e.g., nitrogen atoms). Furthermore, other atoms (e.g., boron atoms, alkali metals, alkaline earth metals, etc. contained in the raw materials) may be bonded to the carbon nitride crystal structure or the graphite crystal structure, or functional groups derived from the raw materials may be bonded to the carbon nitride crystal structure or the graphite crystal structure.

[0013] The solid carbon quantum dots of the present invention are solid at 25°C and 1 atmosphere. Furthermore, the solid carbon quantum dots have a peak at a specific location in the XRD spectrum when analyzed by X-ray diffraction (hereinafter also referred to as "XRD") method. Figure 1 shows the XRD spectrum of the solid carbon quantum dots of the present invention prepared in Example 1 described below. As shown in Figure 1, the solid carbon quantum dots of the present invention have a peak P ＣＮ The solid carbon quantum dots of the present invention have a peak P attributable to the crystalline structure of graphite, which has a maximum value in the range of a diffraction angle 2θ of 10° or more and 12° or less. Ｇｒ１ The peak P Ｇｒ１ is a peak attributed to a partially oxidized structure of graphite. Ｇｒ１The solid carbon quantum dots of the present invention further exhibit a diffraction peak P at a diffraction angle 2θ=25° in the XRD spectrum, which is attributed to the crystalline structure of graphite. Ｇｒ２ The line connecting the point where the diffraction (scattering) intensity is minimum when the diffraction angle 2θ is 11° or more and 15° or less and the point where the diffraction (scattering) intensity is minimum when the diffraction angle 2θ is 34° or more and 40° or less is used as a baseline, and the diffraction peak P located at a diffraction angle 2θ = 25° is Ｇｒ２ The half-width of a peak in the XRD spectrum represents the crystallite size of the crystalline structure. The peak P Ｇｒ２ The half-value width of the peak P represents the size of the crystallites in the crystal structure of graphite, and the larger the half-value width, the smaller the crystallite size. Ｇｒ２ When the half-width of the peak P is 20°, the crystallite size of the graphite is approximately the size of two benzene rings. Ｇｒ２ When the half-width of the peak P is 2° or more, it can be said that graphite exists as microcrystals. Ｇｒ２ When the half-width of the peak P is 2° or more and 20° or less, the size of the graphite crystallites becomes sufficiently small, and there are many contact points with the crystalline structure of the carbon nitride, so that the solid carbon quantum dots exhibit very excellent luminescence properties. Ｇｒ２ The half-width of the graphite quantum dots of the present invention is preferably 8° or more and 15° or less. In the XRD spectrum of the solid carbon quantum dots of the present invention, another peak attributable to the crystalline structure of graphite is usually observed in the range of the diffraction angle 2θ of 40° or more and 45° or less.

[0014] Here, the XRD spectrum of the solid carbon quantum dots of the present invention is clearly different from the XRD spectrum of a mixture (the mixture described in the above-mentioned Non-Patent Documents 1 and 2) obtained by synthesizing and mixing carbon nitride and graphite (or graphene) quantum dots. FIG. 2A shows the XRD spectrum described in Non-Patent Document 1 (Figure 7 in Non-Patent Document 1), and FIG. 2B shows the XRD spectrum described in Non-Patent Document 2 (Figure 1D in Non-Patent Document 2). FIG. 2C shows the XRD spectrum described in Non-Patent Document 3 (Figure 3(d) in Non-Patent Document 2). As shown in FIGS. 2A, 2B, and 2C, when carbon nitride and graphite are simply mixed, no peaks of the crystalline structure derived from graphite (graphene) are observed in the diffraction angle 2θ range of 23° to 26° (FIGS. 2A and 2B), or no peaks of the crystalline structure derived from carbon nitride are observed in the diffraction angle 2θ range of more than 26° to 29° (FIG. 2C). In other words, the carbon quantum dots of the present invention are substances with a crystal structure different from that of a mixture of carbon nitride and graphite quantum dots, and this crystal structure is believed to contribute to their excellent light-emitting properties.

[0015] Although the reason why the solid carbon quantum dots of the present invention exhibit excellent luminescence properties is unclear, it is believed that in the solid carbon quantum dots, the carbon nitride crystalline structure and the graphite microcrystalline structure are not independently mixed, but rather covalent bonds exist between them, and they are intricately combined at the molecular level. Furthermore, in the solid carbon quantum dots, the carbon nitride crystalline structure and the graphite are intricately combined at the molecular level, which is thought to result in an expanded conjugated system. It is also possible that this is due to boron atoms and alkali metals and / or alkaline earth metals present in the system during the production of the carbon quantum dots. In any event, the solid carbon quantum dots of the present invention have luminescence properties far superior to conventional quantum dots, and exhibit extremely high internal quantum efficiency when irradiated with excitation light having a wavelength of, for example, about 400 nm.

[0016] The chemical composition of the solid carbon quantum dots of the present invention is not particularly limited, but the amount of carbon atoms is preferably 10% by mass or more and 25% by mass or less, more preferably 12% by mass or more and 18% by mass or less, relative to the total amount of the solid carbon quantum dots. Furthermore, the amount of nitrogen atoms is preferably 15% by mass or more and 35% by mass or less, more preferably 20% by mass or more and 30% by mass or less, relative to the total amount of the solid carbon quantum dots. Furthermore, the mass ratio of the amount of nitrogen atoms to the amount of carbon atoms is preferably 1.0 to 2.0, more preferably 1.2 to 1.8. Furthermore, the molar ratio of the amount of nitrogen atoms to the amount of carbon atoms is preferably 0.8 to 1.7, more preferably 1.0 to 1.5. Furthermore, the amount of boron atoms is preferably 5% by mass or more and 30% by mass or less, more preferably 10% by mass or more and 15% by mass or less, relative to the total amount of the solid carbon quantum dots. The mass ratio of the amount of boron atoms to the amount of carbon atoms is preferably 0.1 to 1.5, more preferably 0.5 to 1.0. The molar ratio of the amount of boron atoms to the amount of carbon atoms is preferably 0.1 to 1.7, more preferably 0.5 to 1.2. Furthermore, the total amount of alkali metals and alkaline earth metals relative to the total amount of the solid carbon quantum dots is preferably 0.1 to 30% by mass, more preferably 3.0 to 8.0% by mass.

[0017] The average particle size of the solid carbon quantum dots measured by observation with an atomic force microscope (AFM) is preferably 1 nm or more and 100 nm or less, more preferably 1 nm or more and 80 nm or less. When the average particle size of the carbon quantum dots is within this range, the properties of quantum dots are easily obtained. The average particle size of the carbon quantum dots is a value calculated by measuring the diameters of three or more carbon quantum dots with an AFM and calculating the average value of these diameters.

[0018] Furthermore, the wavelength of light for exciting the solid carbon quantum dots is not particularly limited, but the solid carbon quantum dots can usually be excited by light with a wavelength of 200 nm or more and 780 nm or less. Furthermore, the maximum emission wavelength of the fluorescence emitted by the solid carbon quantum dots with excitation light of this wavelength is usually 380 nm or more and 780 nm or less, and can be, for example, 440 nm or more and 580 nm or less. When irradiated with excitation light, the light emitted by the solid carbon quantum dots is mainly fluorescence. The maximum emission wavelength can be determined by obtaining the spectral distribution when irradiated with light of a predetermined wavelength using a spectrofluorometer or the like equipped with an integrating sphere unit.

[0019] Furthermore, the solid carbon quantum dots can have an internal quantum efficiency of 50% or more, or even 55% or more, when irradiated with light having a wavelength of 200 to 780 nm. Furthermore, the internal quantum efficiency when irradiated with light having a wavelength of 400 nm, for example, is preferably 50% or more, more preferably 55% or more, and even more preferably 60% or more. The internal quantum efficiency can also be determined by obtaining the spectral distribution when irradiated with light having a wavelength of 400 nm using a spectrofluorometer or the like equipped with an integrating sphere unit.

[0020] (Method for Producing Solid Carbon Quantum Dots) The method for producing the above-mentioned solid carbon quantum dots is not particularly limited, but they can be produced by, for example, the following two production methods. Each method will be described in detail below.

[0021] (1) First Production Method The first production method includes a step of preparing a homogeneous mixture containing at least carbon atoms, nitrogen atoms, boron atoms, and an alkali metal and / or an alkaline earth metal (mixture preparation step), a step of heating the mixture substantially in the absence of a solvent (thermal synthesis step), and a step of purifying the obtained composition (purification step). Each step will be described below.

[0022] Mixture Preparation Step In the mixture preparation step, a homogeneous mixture containing at least carbon atoms, nitrogen atoms, boron atoms, and alkali metals and / or alkaline earth metals is prepared. Specific raw materials preferably include a combination of one or more organic compounds (excluding those containing boron atoms) having reactive groups and one or more boron compounds containing boron atoms. Furthermore, it is preferable that either one or both of the organic compound and the boron compound contain an alkali metal and / or alkaline earth metal. In this specification, a compound containing a boron atom is referred to as a "boron compound," and even if the compound has a reactive group in its molecule, it is still considered a boron compound if it contains a boron atom.

[0023] The type of alkali metal or alkaline earth metal that can be contained in the organic compound or boron compound is not particularly limited, and examples include lithium, sodium, potassium, magnesium, calcium, barium, etc. However, from the viewpoints of reactivity, safety, material availability, etc., sodium, potassium, or calcium is preferred.

[0024] The total amount of alkali metals and alkaline earth metals in the mixture is preferably 0.1 mol % to 30 mol %, more preferably 0.5 mol % to 20 mol %, and even more preferably 1 mol % to 10 mol %, based on the sum of the moles of carbon atoms and nitrogen atoms contained in the organic compound and boron compound. It is preferable that the alkali metals and alkaline earth metals are mainly derived from the organic compound and boron compound, but a portion of them may be derived from other compounds.

[0025] The organic compound may be any compound having a reactive group. In this specification, the term "reactive group" refers to a group that is bonded to a carbon atom in the organic compound and that causes a polycondensation reaction between organic compounds in the heating step described below, and contributes to the formation of the main skeleton of the solid carbon quantum dots. Specific examples of the reactive group include a carboxy group, a hydroxy group, an epoxy group, an amide group, a sulfo group, an amino group, and groups in which an alkali metal or an alkaline earth metal is bonded or coordinated to these groups.

[0026] The organic compound may be composed of only one type of compound or two or more types of compounds, provided that at least one of the organic compounds contains nitrogen atoms. The amount of nitrogen atoms relative to the total amount of the organic compounds is preferably 30% by mass or more and 50% by mass or less, more preferably 33% by mass or more and 50% by mass or less, and even more preferably 36% by mass or more and 48% by mass or less. When the amount of nitrogen atoms in the organic compound is 30% by mass or more and 50% by mass or less, the above-mentioned carbon nitride crystal structure and graphite crystal structure are easily formed.

[0027] The organic compound may be composed solely of a compound that contains a nitrogen atom and has a reactive group (hereinafter also referred to as a "nitrogen-containing organic compound"), or may be composed of a compound that does not contain a nitrogen atom and has a reactive group (hereinafter also referred to as a "nitrogen-free organic compound") and a nitrogen-containing organic compound.

[0028] Examples of nitrogen-containing organic compounds include amine compounds, nitrogen-containing sugars, imidazoles, triazines, triazoles, triazenes, guanidines, and oximes, as well as salts of these with alkali metals and / or alkaline earth metals. As the organic compound, only one of these compounds may be used, or two or more of these compounds may be used. Furthermore, these compounds may be solid or liquid at room temperature.

[0029] Examples of the amine compound include 1,2-phenylenediamine, 1,3-phenylenediamine, 1,4-phenylenediamine, 2,6-diaminopyridine, urea, thiourea, ammonium thiocyanate, ethanolamine, 1-amino-2-propanol, melamine, cyanuric acid, barbituric acid, folic acid, ethylenediamine, diethylenetriamine, triethylenetetramine, polyethyleneimine, dicyandiamide, guanidine, aminoguanidine, formamide, glutamic acid, and the like. Examples of amino acids include citric acid, aspartic acid, cysteine, arginine, histidine, lysine, glutathione, RNA, DNA, cysteamine, methionine, homocysteine, taurine, thiamine, N-[3-(trimethoxysilyl)propyl]ethylenediamine, 4,5-difluoro-1,2-phenylenediamine, sulfanilic acid, o-phosphoserine, adenosine 5'-triphosphate, guanidine phosphate, guanylurea phosphate, 3-aminopropyltriethoxysilane, and the like.

[0030] Examples of nitrogen-containing sugars include glucosamine, chitin, chitosan, etc. Examples of imidazoles include 1-(trimethylsilyl)imidazole, etc. Examples of triazines include 1,2,4-triazine, and examples of triazoles include 1,3,5-triazine, 1,2,3-triazole, and 1,2,4-triazole. Examples of triazenes include 1,3-diphenyltriazene and 1-methyl-3-p-tolyltriazene, examples of guanidines include guanidine and arginine, and examples of oximes include benzamide oxime and p-benzoquinone dioxime. Examples of alkali metal and alkaline earth metal salts of the above compounds include sodium glutamate and sodium imidazole-4-acetate, etc.

[0031] Of the above nitrogen-containing organic compounds, amine compounds are preferred from the viewpoints of availability and reactivity in the heating step described below.

[0032] On the other hand, examples of nitrogen-free organic compounds include carboxylic acids, alcohols, phenols, sugars, and salts thereof with alkali metals and / or alkaline earth metals. The organic compound may contain only one of these, or may contain two or more of these. Furthermore, these compounds may be solid or liquid at room temperature.

[0033] The carboxylic acid may be a compound having one or more carboxy groups in the molecule (excluding those corresponding to nitrogen-containing organic compounds, phenols, or sugars). Examples of the carboxylic acid include monocarboxylic acids such as formic acid, acetic acid, 3-mercaptopropionic acid, and α-lipoic acid; divalent or higher polyvalent carboxylic acids such as oxalic acid, malonic acid, succinic acid, maleic acid, fumaric acid, itaconic acid, polyacrylic acid, (ethylenedithio)diacetic acid, thiomalic acid, tetrafluoroterephthalic acid, and 2-phosphonobutane-1,2,4-tricarboxylic acid; and hydroxy acids such as citric acid, glycolic acid, lactic acid, tartaric acid, malic acid, and 5-sulfosalicylic acid.

[0034] The alcohol is preferably a monohydric alcohol having one hydroxy group and six or more carbon atoms, or a polyhydric alcohol having two or more hydroxy groups (excluding those corresponding to nitrogen-containing organic compounds, carboxylic acids, phenols, or sugars). Examples of monohydric alcohols having six or more carbon atoms include higher alcohols such as hexanol and octanol. On the other hand, examples of polyhydric alcohols include ethylene glycol, glycerol, erythritol, pentaerythritol, ascorbic acid, polyethylene glycol, sorbitol, etc.

[0035] The phenol may be any compound having a structure in which a hydroxy group is bonded to a benzene ring (excluding nitrogen-containing organic compounds). Examples of phenols include phenol, catechol, resorcinol, hydroquinone, phloroglucinol, pyrogallol, 1,2,4-trihydroxybenzene, gallic acid, tannin, lignin, catechin, anthocyanin, rutin, chlorogenic acid, lignan, curcumin, etc.

[0036] Examples of sugars, which are non-nitrogen-containing organic compounds, include glucose, sucrose, cellulose, etc. Examples of alkali metal salts and alkaline earth metal salts of the above compounds include trisodium citrate dihydrate, tripotassium citrate monohydrate, sodium ascorbate, calcium acetate, etc.

[0037] The nitrogen-free organic compound preferably has a reactive group that efficiently undergoes a condensation reaction with the nitrogen-containing organic compound, and is preferably a carboxylic acid, an alcohol, a phenol, an alkali metal salt thereof, or an alkaline earth metal salt thereof.

[0038] Furthermore, the total amount of the organic compounds (nitrogen-containing organic compounds and nitrogen-free organic compounds) in the mixture prepared in the mixture preparation step is preferably 20% by mass or more and 60% by mass or less, and more preferably 35% by mass or more and 55% by mass or less, relative to the total amount of the organic compounds and the boron compound. When the amount of the organic compounds relative to the total amount of the organic compounds and the boron compound is within the above range, solid carbon quantum dots having the above-mentioned carbon nitride crystal structure and graphite crystal structure are easily obtained.

[0039] On the other hand, the boron compound used in preparing the mixture may be any compound containing a boron atom, for example, boron alone or a compound containing boron. The boron compound may be composed of one type of compound or two or more types of compounds.

[0040] Specific examples of boron compounds include boron, boric acid, sodium tetraborate, boron oxide, trimethyl borate, triethyl borate, trioctadecyl borate, triphenyl borate, 2-ethoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, triethanolamine borate, 2,4,6-trimethoxyboroxine, tris(trimethylsilyl)borate, tris(2-cyanoethyl)borate, 3-aminophenyl diboronic acid, 2-anthraceneboronic acid, 9-anthraceneboronic acid, phenylboronic acid, 3,5-bis(trifluoromethyl)phenylboronic acid, 4,4'-biphenyldiboronic acid, 2-bromophenylboronic acid, 4-bromo-1-naphthaleneboronic acid, 3-bromo-2-fluorophenylboronic acid, 4-carboxyphenylboronic acid, 3-cyanophenylboronic acid, 4-cyano-3-fluorophenylboronic acid, 3, 5-Difluorophenylboronic acid, 4-(diphenylamino)phenylboronic acid, 3-fluorophenylboronic acid, 3-hydroxyphenylboronic acid, 4-mercaptophenylboronic acid, 1-naphthaleneboronic acid, 9-phenanthreneboronic acid, 1,4-phenylenediboronic acid, 1-pyreneboronic acid, 2-aminopyrimidine-5-boronic acid, 2-bromopyridine-3-boronic acid, 2-fluoropyridine-3-boronic acid, 4 -pyridylboronic acid, quinoline-8-boronic acid, 4-aminophenyl pinacol boronate, 3-hydroxyphenyl pinacol boronate, 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyridine, diboronic acid, sodium borohydride, sodium tetrafluoroborate, 1-ethyl-3-methylimidazolium tetrafluoroborate, boron trifluoride, boron tribromide, and the like.

[0041] The amount of boron atoms in the boron compound is preferably 3 mass % or more, more preferably 5 mass % or more, based on the total amount of the boron compound, which allows the boron atoms to be arranged on the surface of the carbon quantum dots.

[0042] The total amount of the boron compound is preferably 40% by mass or more and 80% by mass or less, more preferably 42% by mass or more and 70% by mass or less, and even more preferably 45% by mass or more and 65% by mass or less, based on the total amount of the organic compound and the boron compound. When the amount of the boron compound relative to the total amount of the organic compound and the boron compound is within the above range, solid carbon quantum dots having the above-mentioned carbon nitride crystal structure and graphite crystal structure can be easily prepared.

[0043] In this process, compounds other than the organic compound and the boron compound may be further mixed within a range that does not impair the objectives and effects of the present invention. However, the amount of the organic compound and the boron compound in the mixture is preferably 50% by mass or more, more preferably 70% by mass or more, relative to the total amount of the mixture. When the total amount of the organic compound and the boron compound is 50% by mass or more, the above-mentioned solid carbon quantum dots can be efficiently prepared. In addition, the compound other than the organic compound and the boron compound may be a compound containing an alkali metal or an alkaline earth metal.

[0044] Examples of compounds other than organic compounds and boron compounds include compounds containing a phosphorus atom, a sulfur atom, a silicon atom, a fluorine atom, or the like, but not having the above-mentioned reactive groups (hereinafter also referred to as "other compounds"). Mixing other compounds in this process can produce solid carbon quantum dots containing heteroatoms other than nitrogen and boron. The mixture may contain only one type of other compound, or two or more types.

[0045] Examples of compounds containing a phosphorus atom include elemental phosphorus, phosphoric acid, phosphorus oxide, 1-hydroxyethane-1,1-diphosphonic acid, phytic acid, ammonium phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, o-phosphorylethanolamine, phosphorus chloride, phosphorus bromide, triethyl phosphonoacetate, tetrakis(hydroxymethyl)phosphonium chloride, methyl phosphate, triethyl phosphite, nitrilotris(methylenephosphonic acid), N,N,N',N'-ethylenediaminetetrakis(methylenephosphonic acid), and the like.

[0046] Examples of compounds containing a sulfur atom include sulfur, sodium thiosulfate, sodium sulfide, sodium sulfate, sulfuric acid, methanesulfonic acid, p-toluenesulfonic acid, and sodium hydrosulfide.

[0047] Examples of compounds containing silicon atoms include tetrachlorosilane, tetraethoxysilane, and the like.

[0048] Examples of the compound containing a fluorine atom include 2,2,3,3,4,4-hexafluoro-1,5-pentanediol diglycidyl ether, sodium fluoride, and the like.

[0049] The amount of other compounds in the mixture is appropriately selected according to the desired amount of heteroatoms, but is typically preferably 0% to 20% by mass, and more preferably 3% to 10% by mass, relative to the total amount of the mixture. When the amount of other compounds is 3% by mass or more, the effect of adding the other compounds is easily obtained. On the other hand, when the amount of other compounds is 10% by mass or less, the amount of organic compounds and boron compounds becomes sufficiently large relative to each other, making it possible to efficiently prepare solid carbon quantum dots.

[0050] In this process, a layered clay mineral may be further added. Adding a layered clay mineral facilitates the preparation of solid carbon quantum dots of uniform size by using the interlayer spaces of the layered clay mineral as a template. Examples of layered clay minerals include smectite, layered double hydroxides, kaolinite, and mica. Among these, smectite or layered double hydroxide is preferred because it has an average interlayer spacing suitable for forming solid carbon quantum dots. Note that while layered clay minerals may contain alkali metals or alkaline earth metals, these alkali metals or alkaline earth metals do not substantially affect the composition of the carbon quantum dots. Therefore, in this specification, the amount of alkali metals or alkaline earth metals contained in the layered clay mineral is not included in the amount of alkali metals or alkaline earth metals in the mixture.

[0051] Smectite is a clay mineral that swells with water, etc., and examples thereof include saponite, montmorillonite, hectorite, beidellite, nontronite, sauconite, stevensite, and the like.

[0052] On the other hand, layered double hydroxides are double hydroxides in which trivalent metal ions are dissolved in a divalent metal oxide, and examples thereof include hydrotalcite, hydrocalumite, hydromagnesite, and pyroaurite.

[0053] In addition, in order to adjust the average layer spacing of the layered clay mineral, the layered clay mineral may be swollen with a solvent. Examples of solvents include water, methanol, ethanol, hexane, toluene, chloroform, dimethylformamide, and dimethyl sulfoxide. However, in the heating step described below, heating is performed substantially without a solvent. Therefore, it is preferable to use a small amount of solvent, and further, it is preferable that the boiling point of the solvent is lower than the temperature at which organic compounds and the like are carbonized in the heating step (hereinafter also referred to as the "heating temperature"). In other words, in the heating step described below, it is preferable to adjust the amount and type of solvent so that all of the solvent volatilizes before the temperature of the mixture reaches the heating temperature.

[0054] The amount of layered clay mineral in the mixture is preferably 0% by mass to 20% by mass, more preferably 0% by mass to 10% by mass, relative to the total amount of the mixture, and even more preferably substantially free of layered clay mineral. "Substantially free" means 5% by mass or less relative to the total amount of the mixture. The amount of layered clay mineral is more preferably 3% by mass or less, even more preferably 1% by mass or less, relative to the total amount of the mixture, and particularly preferably completely free. When the amount of layered clay mineral is 20% by mass or less, the amounts of organic compounds and boron compounds are sufficiently large relative to each other, allowing for efficient preparation of solid carbon quantum dots.

[0055] The method for mixing the mixture is not particularly limited as long as it allows the organic compound, the boron compound, and, if necessary, other compounds to be mixed uniformly. For example, the mixture may be mixed by grinding in a mortar or by pulverizing in a ball mill or the like. When the mixture is in a solid state, it is preferable to mix the mixture so that the particle size is approximately uniform.

[0056] When the mixture is solid, the particle size of the mixture can be evaluated by measuring the major axis of each particle using an optical microscope. The average particle size of the mixture is preferably 0.1 μm or more and 300 μm or less, more preferably 0.1 μm or more and 100 μm or less. Furthermore, 99% of the particle sizes of the particles contained in the mixture are preferably 0.1 μm or more and 300 μm or less, more preferably 0.1 μm or more and 100 μm or less. When the particle size of the mixture is within the above range, the mixture is easily mixed uniformly, and solid carbon quantum dots can be efficiently prepared while suppressing the residue of unreacted components.

[0057] Furthermore, when the organic compound, boron compound, or other compound is liquid, the solid component may be dissolved, mixed, or dispersed in the liquid component to be mixed. Alternatively, each material may be dissolved, mixed, or dispersed in a small amount of solvent to be mixed. In this case, the amount and type of solvent are adjusted so that heating can be performed substantially without solvent in the heating step described below. Specifically, it is preferable to adjust the amount and type of solvent so that all of the solvent volatilizes before the temperature of the mixture reaches the heating temperature. In this specification, the term "solvent" refers to a compound that is liquid at 25°C under 1 atmosphere and does not correspond to the organic compound or boron compound.

[0058] Heating Step In the heating step, the mixture prepared in the mixture preparation step is heated substantially without solvent. In this specification, "substantially solvent-free" means that the amount of solvent in the mixture is 5% by mass or less relative to the total amount of the mixture when the temperature (heating temperature) at which the organic compound or the like is carbonized is reached. The amount of solvent in the mixture at the heating temperature is more preferably 2% by mass or less, and even more preferably 0% by mass. Therefore, as described above, the mixture may contain a solvent at the start of heating as long as it is sufficiently volatilizable by the heating temperature. Note that the compounds used as raw materials for solid carbon quantum dots, i.e., organic compounds, boron compounds, etc., may be liquid at the heating temperature.

[0059] The method for heating the mixture may be any method that can carbonize the organic compound or the like to produce solid carbon quantum dots, and examples thereof include heating with a heater and irradiating with electromagnetic waves.

[0060] When the mixture is heated using a heater or the like, the heating temperature is preferably 70°C or higher and 700°C or lower, more preferably 100°C or higher and 500°C or lower, and even more preferably 100°C or higher and 300°C or lower. The holding time at the heating temperature is preferably 0.01 hours or higher and 45 hours or lower, more preferably 0.1 hours or higher and 30 hours or lower, and even more preferably 0.5 hours or higher and 10 hours or lower. The particle size and thus the emission wavelength of the resulting solid carbon quantum dots can be adjusted by the heating time. Heating may also be performed in a non-oxidizing atmosphere while circulating an inert gas such as nitrogen.

[0061] When irradiating with electromagnetic waves (e.g., microwaves), the wattage is preferably 1 W or more and 1500 W or less, more preferably 1 W or more and 1000 W or less. Furthermore, the heating time using electromagnetic waves (e.g., microwaves) is preferably 0.01 hours or more and 10 hours or less, more preferably 0.01 hours or more and 5 hours or less, and even more preferably 0.01 hours or more and 1 hour or less. The particle size of the resulting solid carbon quantum dots, and therefore the emission wavelength, can be adjusted by adjusting the irradiation time of the electromagnetic waves.

[0062] The electromagnetic wave irradiation can be carried out using, for example, a semiconductor-type electromagnetic wave irradiation device. The electromagnetic wave irradiation is preferably carried out while checking the temperature of the mixture. The electromagnetic wave irradiation is preferably carried out while adjusting the heating temperature to be 70°C or higher and 700°C or lower.

[0063] In the purification step, the composition obtained in the heating step is purified to remove unreacted components and by-reaction components. The purification method is not particularly limited, but washing with an organic solvent is preferred.

[0064] The organic solvent used for washing may be composed of one kind of compound or two or more kinds of compounds. The organic solvent may be a polar solvent, a non-polar solvent, or a mixed solvent thereof, and more preferably contains at least one kind of polar solvent in order to remove unreacted components and by-reaction components derived from the compound having a reactive group used in the mixture preparation step.

[0065] Specific examples of polar solvents include methanol, ethanol, butanol, 1-propanol, dichloromethane, acetone, ethyl acetate, and acetonitrile. Among these, it is preferable to use a mixed organic solvent in which methanol and dichloromethane are mixed at a ratio of 1:1.

[0066] (2) Second Production Method The second production method includes the steps of preparing a homogeneous mixture containing at least carbon, nitrogen, and boron atoms (mixture preparation step), mixing the mixture with an aqueous solution containing an alkali metal and / or alkaline earth metal (aqueous solution mixing step), heating the resulting mixture substantially without solvent (thermal synthesis step), and purifying the resulting composition (purification step). In the aqueous solution mixing step, the amount of aqueous solution mixed with the mixture is preferably 200% by mass or less relative to the amount of the mixture. As described above, the term "substantially solvent-free" in this specification refers to the amount of solvent in the mixture being 5% by mass or less relative to the total amount of the mixture when the mixture reaches a temperature (heating temperature) at which the organic compound or the like is carbonized. In the second production method, an aqueous solution containing an alkali metal and / or alkaline earth metal is added in the aqueous solution mixing step. However, because the amount of the aqueous solution is sufficiently small, the water in the aqueous solution evaporates by the time the temperature reaches the heating temperature at which the organic compound or the like is carbonized. Therefore, the heating step can be performed substantially without solvent. Each step is described below.

[0067] In the mixture preparation step, a homogeneous mixture containing at least carbon atoms, nitrogen atoms, and boron atoms is prepared. More specifically, the mixture is prepared by mixing an organic compound having a reactive group but not containing boron atoms with a boron compound. The organic compound and boron compound used in this step may or may not contain an alkali metal or alkaline earth metal.

[0068] The organic compound may be composed of only one type of compound or two or more types of compounds, provided that at least one of the organic compounds contains nitrogen atoms. The amount of nitrogen atoms relative to the total amount of the organic compounds is preferably 20% by mass or more and 50% by mass or less, more preferably 33% by mass or more and 50% by mass or less, and even more preferably 35% by mass or more and 48% by mass or less. When the amount of nitrogen atoms in the organic compound is 20% by mass or more and 50% by mass or less, the above-mentioned carbon nitride crystalline structure and graphite crystalline structure are easily formed.

[0069] The organic compound may be composed solely of a compound that contains a nitrogen atom and has a reactive group (hereinafter also referred to as a "nitrogen-containing organic compound"), or may be composed of a compound that does not contain a nitrogen atom and has a reactive group (hereinafter also referred to as a "nitrogen-free organic compound") and a nitrogen-containing organic compound. The nitrogen-containing organic compound and the nitrogen-free organic compound are the same as those described in the first production method above.

[0070] On the other hand, the boron compound may be any compound containing a boron atom, for example, boron alone or a compound containing boron. The boron compound may be composed of one type of compound or two or more types of compounds. The boron compound is the same as that described in the first production method.

[0071] The total amount of the boron compound may be 40% by mass or more and 80% by mass or less, preferably 42% by mass or more and 70% by mass or less, and more preferably 45% by mass or more and 65% by mass or less, based on the total amount of the organic compound and the boron compound. When the amount of the boron compound relative to the total amount of the organic compound and the boron compound is within the above range, solid carbon quantum dots having the above-mentioned carbon nitride crystal structure and graphite crystal structure are easily formed.

[0072] In this step, compounds other than the organic compound and the boron compound may be further mixed within a range that does not impair the objectives and effects of the present invention. However, the total amount of the organic compound and the boron compound in the mixture is preferably 50% by mass or more, and more preferably 70% by mass or more, relative to the total amount of the mixture. When the total amount of the organic compound and the boron compound is 50% by mass or more, solid carbon quantum dots can be efficiently prepared. The compounds other than the organic compound and the boron compound are the same as those described in the first manufacturing method.

[0073] The method for mixing the mixture is not particularly limited as long as it can uniformly mix the organic compound, the boron compound, and, if necessary, other compounds. For example, they may be mixed by grinding in a mortar or by pulverizing using a ball mill or the like. When the mixture is solid, it is preferable to mix the mixture so that the particle size is approximately uniform. Furthermore, when the organic compound, the boron compound, or other compounds are liquid, the solid components may be dissolved, mixed, or dispersed in the liquid components. Alternatively, each material may be dissolved, mixed, or dispersed in a small amount of solvent and mixed. In this case, the amount and type of solvent are adjusted so that heating can be performed substantially solvent-free in the heating step described below. Specifically, it is preferable to adjust the amount and type of solvent so that all of the solvent volatilizes before the temperature of the mixture reaches the desired heating temperature.

[0074] When the mixture is solid, the particle size of the mixture can be evaluated by measuring the major axis of each particle using an optical microscope. The average particle size of the mixture is preferably 0.1 μm or more and 300 μm or less, more preferably 0.1 μm or more and 100 μm or less. Furthermore, 99% of the particle sizes of the particles contained in the mixture are preferably 0.1 μm or more and 300 μm or less, more preferably 0.1 μm or more and 100 μm or less. When the particle size of the mixture is within the above range, the mixture is easily mixed uniformly, and solid carbon quantum dots can be efficiently prepared while suppressing the residue of unreacted components.

[0075] Aqueous Solution Mixing Step: The amount of the aqueous solution used in this step is 200% by mass or less, preferably 150% by mass or less, based on the mass of the mixture. When the amount of the aqueous solution is within this range, all of the water evaporates before the temperature of the mixture reaches the desired heating temperature. Therefore, carbon quantum dots can be prepared essentially without using a solvent.

[0076] Here, the type of alkali metal and alkaline earth metal contained in the aqueous solution is not particularly limited, and examples include lithium, sodium, potassium, magnesium, calcium, barium, etc. The aqueous solution may contain only one alkali metal and alkaline earth metal, or two or more alkali metals. From the viewpoints of reactivity, versatility, etc., sodium, potassium, or calcium is preferred as the alkali metal and alkaline earth metal. The aqueous solution can usually be an aqueous solution of an alkali metal salt and / or an alkaline earth metal salt. The counter ions of the alkali metal and alkaline earth metal in the salt are not particularly limited, and can be, for example, hydroxy ions, halogen ions, etc. Specific examples of aqueous solutions include sodium hydroxide aqueous solution, potassium hydroxide aqueous solution, sodium fluoride aqueous solution, sodium chloride aqueous solution, calcium chloride aqueous solution, etc.

[0077] The concentration of the alkali metal salt or alkaline earth metal salt may be within a range in which the amount of alkali metal or alkaline earth metal in the heating step described below falls within a desired range.

[0078] Heating Step In the heating step, a composition obtained by mixing the above-mentioned mixture with an aqueous solution containing an alkali metal and / or alkaline earth metal is heated substantially without a solvent to prepare the above-mentioned solid carbon quantum dots.

[0079] Here, the total amount of alkali metals and alkaline earth metals in the composition (composition obtained by mixing the mixture and the aqueous solution) when the heating step is performed may be 0.1 mol % or more and 30 mol % or less, preferably 0.5 mol % or more and 20 mol % or less, and more preferably 1 mol % or more and 10 mol % or less, relative to the sum of the moles of carbon atoms and nitrogen atoms contained in the organic compound and the boron compound. Note that the alkali metals and alkaline earth metals in the composition are mainly derived from the aqueous solution, but as mentioned above, a portion may also be derived from the mixture (e.g., derived from the organic compound, the boron compound, or other compounds).

[0080] In this process, the mixture is mixed with an aqueous solution and then heated. The heating method may be any method capable of carbonizing the organic compound or the like to produce solid carbon quantum dots, and examples thereof include heating with a heater and irradiating with electromagnetic waves. The heating temperature, heating time, and irradiating method and time of electromagnetic waves are the same as those in the first manufacturing method.

[0081] In the purification step, the composition obtained in the heating step is purified to remove unreacted components and by-reaction components. The purification method is not particularly limited, but washing with an organic solvent is preferred.

[0082] The organic solvent used for washing may be composed of one kind of compound or two or more kinds of compounds. The organic solvent may be a polar solvent, a non-polar solvent, or a mixed solvent thereof, and more preferably contains at least one kind of polar solvent in order to remove unreacted components and by-reaction components derived from the compound having a reactive group used in the mixture preparation step.

[0083] Specific examples of polar solvents include methanol, ethanol, butanol, 1-propanol, dichloromethane, acetone, ethyl acetate, and acetonitrile. Among these, it is preferable to use a mixed organic solvent in which methanol and dichloromethane are mixed at a ratio of 1:1.

[0084] (Applications of solid carbon quantum dots) The above-mentioned solid carbon quantum dots have good luminescence properties. Therefore, the solid carbon quantum dots can be used for various applications. The applications of the solid carbon quantum dots are not particularly limited, and can be used in solar cells, displays, security inks, quantum dot lasers, biomarkers, lighting materials, thermoelectric materials, photocatalysts, and separation agents for specific substances, depending on the performance of the carbon quantum dots.

[0085] The solid carbon quantum dots described above are solid at 25° C. and 1 atmosphere, but they may be dispersed in a solvent or the like to form a solution for use in various applications.

[0086] The carbon quantum dots obtained by the above-described manufacturing method mainly emit fluorescence with a wavelength of 520 nm to 580 nm when irradiated with excitation light in the blue wavelength range. Furthermore, the emission wavelength peak is sharp, with high emission intensity in the wavelength range of 520 nm to 580 nm and low emission intensity in other wavelength ranges. Therefore, the carbon quantum dots are very useful as a phosphor material for white LEDs.

[0087] When the solid carbon quantum dots (phosphor material for white LEDs) are used in a white LED, a white LED element with excellent luminous efficiency can be obtained by combining the carbon quantum dots with a blue LED having a wavelength of 420 nm or more and 480 nm or less.

[0088] Specific examples of the present invention will be described below together with comparative examples, but the present invention is not limited to these.

[0089] [Example 1] (1) Preparation of raw material mixture 0.040 g of trisodium citrate dihydrate, 0.070 g of dicyandiamide, and 0.100 g of boric acid were mixed in a mortar while being ground to prepare a mixture. At this time, the granules of each reagent were thoroughly crushed to obtain powders with a substantially uniform particle size, and then the powders were mixed uniformly.

[0090] (2) Heating synthesis step of solid phosphor: The powder mixture and a stirrer were placed in a 15 ml screw-top test tube, which was then sealed with a rubber gasket. While nitrogen was circulating in the test tube, the mixture was heated at 170° C. for 1.5 hours while rotating and stirring the stirrer by applying magnetic force from a stirrer, to synthesize a solid phosphor composition.

[0091] (3) Solvent Cleaning and Purification Step: The entire solid phosphor composition was ground in a mortar to obtain a powder. This powder was dispersed in 10 ml of a 1:1 methanol:dichloromethane mixed solvent to obtain a dispersion. The dispersion was then centrifuged (15,000 rpm, 5 minutes) to recover the insoluble solid precipitate, and the solvent and soluble components were removed. This solvent cleaning was repeated three times. The insoluble solid precipitate was then dried under reduced pressure at 60°C for 6 hours to remove the residual solvent, yielding solid carbon quantum dots.

[0092] (4) Confirmation of Emission State The obtained solid carbon quantum dots were sandwiched between KBr plates and pressed to prepare a measurement sample. Furthermore, a measurement sample was similarly prepared for the sample before the above-mentioned cleaning and purification process. The emission wavelength, external quantum efficiency, and internal quantum efficiency were determined when these were irradiated with excitation light. The wavelength of the excitation light was determined as the wavelength at which the internal quantum efficiency of the sample was maximized (maximum excitation wavelength), and the emission wavelength at this time was determined as the maximum emission wavelength. The above measurement was performed using a spectrofluorometer FP-8500 (manufactured by JASCO Corporation) attached to an integrating sphere unit ILF-835. The solid carbon quantum dots emitted yellow fluorescence when irradiated with light at a wavelength of 400 nm, which immediately ceased to emit when the excitation light irradiation was stopped. The results are shown in Table 1.

[0093] For comparison, the emission states of commercially available carbon nitride (Tokyo Chemical Industry Co., Ltd., product code: G0539) and commercially available graphene carbon quantum dots (Sigma-Aldrich, catalog number 900726 50MG) were also measured. Furthermore, a sample in which carbon nitride and graphene carbon quantum dots were uniformly mixed in a mortar at a mass ratio of 8:2 was also measured in the same manner. The results are shown in Table 1.

[0094] (5) X-ray Diffraction The obtained solid carbon quantum dots were analyzed by X-ray diffractometry to obtain their X-ray diffraction spectrum. X-ray diffractometry was performed using the following equipment and conditions. Equipment: PANalytical X'Pert PRO diffractometer, PIXcel detector; Radiation source: Cu tube; Excitation voltage: 40 kV, 40 mA; Measurement method: θ / 2θ method; Slit: DS=½°; Mask: 10 mm; Scanning range: 5° to 90°; Scanning step: 0.013°; Scanning rate: 4° / min. The X-ray diffraction spectrum obtained for the solid carbon quantum dots of Example 1 is shown in FIG. 1. In the X-ray diffraction spectrum, the carbon nitride (g-C ３ N ４ ) The peak P ＣＮ In addition, a peak P attributable to the crystalline structure of graphite, which has a maximum value at a diffraction angle 2θ of approximately 11°, was observed. Ｇｒ１ Furthermore, a peak P , which is located at a diffraction angle 2θ of approximately 25° and is attributed to the crystalline structure of graphite, was also confirmed. Ｇｒ２ The peak P attributable to the crystalline structure of the graphite was determined by using a line connecting the point where the diffraction (scattering) intensity is minimum when the diffraction angle 2θ is 11° or more and 15° or less and the point where the diffraction (scattering) intensity is minimum when the diffraction angle 2θ is 34° or more and 40° or less as a baseline. Ｇｒ２ The half-width was 12°. The crystallite size of the graphite crystal structure calculated from the half-width was 0.7 nm. The crystallite size was determined using the following Scherrer equation: L = Kλ / (β cos θ) (L is the crystallite size (crystallite diameter), K is the Scherrer constant (0.9 in this case), β is the half-width, and θ is the diffraction angle).

[0095] For comparison, X-ray diffraction was also performed on commercially available carbon nitride (Tokyo Chemical Industry Co., Ltd., product code: G0539) and commercially available graphene carbon quantum dots (Sigma-Aldrich, catalog number 900726 50MG). These results are also shown in Figure 1. As is clear from Figure 1, the solid carbon quantum dots prepared above clearly have the crystalline structure of carbon nitride and the crystalline structure of graphite. Furthermore, the half-width of the peaks clearly shows that graphite is present as microcrystals.

[0096] (6) Observation by TEM (Transmission Electron Microscope) Images of the solid carbon quantum dots observed with a transmission electron microscope are shown in Figures 3A and 3B. As shown in Figure 3B in particular, lattice fringes of d = 0.32 nm are observed in the solid carbon quantum dots. These lattice fringes correspond to the lattice spacing of the (002) plane of carbon nitride, confirming the presence of a carbon nitride crystal structure. Furthermore, lattice fringes of d = 0.21 nm are also observed in the solid carbon quantum dots. These lattice fringes correspond to the lattice spacing of the (100) plane of graphite, confirming the presence of a graphite crystal structure.

[0097] (7) Analysis by IR (Infrared Spectroscopy) The solid carbon quantum dots were also analyzed by infrared spectroscopy (IR). The IR spectrum is shown in FIG. 4. As shown in FIG. 4, －１ The peak at 808 cm represents aromatic C—H and suggests the presence of a graphite crystal structure. －１ The peak indicates the presence of a triazine ring and suggests the presence of a carbon nitride structure.

[0098] (8) Analysis by XPS (X-ray Photoelectron Spectroscopy) The solid carbon quantum dots were also analyzed by XPS (X-ray Photoelectron Spectroscopy). The XPS spectrum is shown in Figure 5A. For comparison, a commercially available carbon nitride (manufactured by Tokyo Chemical Industry Co., Ltd., product code: G0539) was also analyzed by XPS (X-ray Photoelectron Spectroscopy). The results are shown in Figure 5B.

[0099] As shown in Figure 5A, the solid carbon quantum dots contain a large amount of graphene-like nitrogen and pyrrolic nitrogen. This indicates the presence of a nitrogen-doped graphene (graphite) structure. On the other hand, as shown in Figure 5B, the carbon nitride contains graphene-like nitrogen and pyridine-like nitrogen in a ratio of 1:9, which is clearly different from the solid carbon quantum dots of the present invention.

[0100] Furthermore, the solid carbon quantum dots were analyzed by XPS (X-ray photoelectron spectroscopy) to determine the amount of boron. As shown in Figure 6, the solid carbon quantum dots had B-C bonds.

[0101] Example 2 A mixture was prepared by mixing 0.051 g of trisodium citrate dihydrate, 0.088 g of dicyandiamide, and 0.120 g of boric acid in the same manner as in Example 1. The mixture was subjected to a heating synthesis process and a washing and purification process in the same manner as in Example 1 to prepare solid carbon quantum dots. The luminescence properties of the obtained solid carbon quantum dots were evaluated in the same manner as in Example 1. XRD analysis was also performed. The results are shown in Table 1.

[0102] [Example 3] A mixture was prepared by mixing 0.051 g of trisodium citrate dihydrate, 0.096 g of dicyandiamide, and 0.150 g of boric acid in the same manner as in Example 1. The mixture was subjected to a heating synthesis process and a washing and purification process in the same manner as in Example 1 to prepare solid carbon quantum dots. The luminescence properties of the obtained solid carbon quantum dots were evaluated in the same manner as in Example 1. XRD analysis was also performed. The results are shown in Table 1.

[0103]

[0104] As shown in Table 1 above, solid carbon quantum dots having a carbon nitride crystal structure and a graphite crystal structure, where the graphite is microcrystalline (with a half-width of 2° or more), exhibited an extremely high internal quantum efficiency of 63% or more with excitation light of 400 nm wavelength (Examples 1 to 3). Furthermore, all of these solid carbon quantum dots had a maximum emission wavelength of 530 nm, and all emitted yellow light. The unpurified solid carbon quantum dots of each example were also measured by X-ray diffraction. The graph obtained is shown in FIG. 7. As shown in FIG. 7, after purification, the peak P , which is attributed to the crystalline structure of graphite and has a maximum value at a diffraction angle 2θ of approximately 11°, Ｇｒ１ was confirmed, whereas in the case of the unpurified product, peak P Ｇｒ１ The peak P was not observed. Unpurified solid carbon quantum dots contain unreacted components and side reaction components, which are thought to prevent the layers of the graphene structure from approaching each other, preventing the formation of a graphite crystal structure. In contrast, by carrying out the purification process, the components are removed, making it easier for the graphene structures to approach each other and crystallize, and the peak P Ｇｒ１ It is believed that the formation of such a graphite crystal structure resulted in improved external and internal quantum efficiencies.

[0105] On the other hand, when carbon nitride and graphene quantum dots were excited independently, the internal quantum efficiency was 4% or less, and efficient light emission was not obtained. Furthermore, when carbon nitride and graphene quantum dots were simply physically mixed, the internal quantum efficiency was 2%, and it was not possible to obtain a high light emission efficiency like that of the solid carbon quantum dots.

[0106] This application claims priority based on Japanese Patent Application No. 2023-152277, filed September 20, 2023. The contents of the specification and drawings of that application are incorporated herein by reference in their entirety.

[0107] The solid carbon quantum dots of the present invention are novel quantum dots that exhibit excellent fluorescence emission when irradiated with light. The solid carbon quantum dots can be used, for example, as a substitute for yellow phosphors in white LEDs. They can also be used in a variety of other products, such as various lighting materials and thermoelectric materials.

Claims

1. A carbon nitride crystal structure and a graphite crystal structure are included, and in an X-ray diffraction spectrum obtained by an X-ray diffraction method, a peak P belonging to the carbon nitride crystal structure has a maximum value in the range of a diffraction angle 2θ of more than 26° and not more than 29°. ＣＮ and a peak P attributable to the crystal structure of the graphite, the peak P having a maximum value in the range of a diffraction angle 2θ of 10° or more and 12° or less. Ｇｒ１ and a diffraction peak P attributable to the crystal structure of the graphite located at a diffraction angle 2θ=25° when a line connecting a point at which the diffraction (scattering) angle is minimum when the diffraction angle 2θ is 11° or more and 15° or less and a point at which the diffraction (scattering) intensity is minimum when the diffraction angle 2θ is 34° or more and 40° or less is used as a baseline. Ｇｒ２ A solid carbon quantum dot, wherein the half width of the .lambda.-ray diffraction grating is 2° or more and 20° or less.

2. The peak P Ｇｒ２ The solid carbon quantum dot according to claim 1 , wherein the half width of the peak is 8° or more and 15° or less.