KR20200103603A - Graphene-based compound and manufacturing method thereof and composition for graphene-based manufacturing compound and graphene quantum dot - Google Patents

Abstract

The present invention relates to a graphene-based compound which is manufactured from a single-phase composition for manufacturing a graphene-based compound comprising hydrocarbyl amine, a hydroxyl group-containing carbon source and acid, to a manufacturing method thereof, and to graphene quantum dots, wherein the graphene-based compound has excellent luminous efficiency and electrical conductivity.

Description

Graphene-based compound, its manufacturing method, single-phase composition for preparing graphene-based compounds, and graphene quantum dots {GRAPHENE-BASED COMPOUND AND MANUFACTURING METHOD THEREOF AND COMPOSITION FOR GRAPHENE-BASED MANUFACTURING COMPOUND AND GRAPHENE QUANTUM DOT}

The present invention relates to a graphene-based compound, a method for preparing the same, a single-phase composition for preparing a graphene-based compound, and a graphene quantum dot.

Low-dimensional materials composed of carbon atoms include fullerene, carbon nanotubes, graphene, and graphite. When carbon atoms form a hexagonal arrangement and form a ball, they are divided into 0-dimensional structure of fullerene, one-dimensionally rolled carbon nanotubes, two-dimensionally one layer of atoms graphene, and three-dimensionally stacked graphite. can do.

Among them, graphene is not only structurally and chemically very stable, but also exhibits excellent conductivity due to its structural characteristics having a thickness of one atom and relatively few defects in the surface area. In theory, graphene is known as a carbon-based material capable of moving electrons 100 times faster than silicon and flowing current about 100 times more than copper.

On the other hand, quantum dots are semiconducting nano-sized particles having a three-dimensionally limited size so that the size of the particles is smaller than the size of a wavelength, and exhibit excellent optical and electrical properties that semiconducting materials do not have in bulk.

Semiconductor quantum dots are widely used in the bio-imaging field because they can be monitored for a long time and have a constant emission wavelength. However, since most of the semiconductor quantum dots use toxic heavy metal materials as core materials, their use is limited despite their excellent properties. In particular, toxic substances such as cadmium, lead, indium, and selenium are discharged, which may cause environmental and health problems. Because of this potential toxicity, despite the physicochemical excellence of quantum dots, their use for therapeutic purposes is bound to be limited.

As a study to solve this problem, studies of non-organic and non-toxic luminescent materials have been attempted, and as an example, studies of carbon quantum dots having excellent physical properties such as aqueous dispersion characteristics, chemical inertness, and low photobleaching characteristics have been attempted.

Carbon quantum dots have excellent properties such as dispersion characteristics of aqueous solutions, chemical inertness, and low photobleaching characteristics, and synthesis methods include microwave synthesis, electrochemical methods, incomplete oxidation, laser evaporation, plasma treatment, hydrothermal synthesis, and chemical vapor deposition. Has become. Various synthesis methods are known, but they have problems such as low synthesis yield, low economic efficiency due to complex manufacturing processes, low chemical stability, low quantum efficiency, or low-quality luminescence characteristics.

Accordingly, various methods have been studied to improve the luminescence characteristics of carbon quantum dots, including a method of oxidizing the surface using nitric acid, a method of using a strong oxidizing agent such as potassium manganate, and a method of treating a surface protective film. On the other hand, factors affecting the quantum efficiency of carbon quantum dots include ion doping, host material, size and shape of nanoparticles, surface defects, and external environment. In particular, in order to increase the applicability of carbon quantum dots, studies such as improvement of synthesis methods, development of post-treatment processes, and development of composites with heterogeneous elements or heterogeneous materials have been attempted, but there are still problems with low quantum efficiency or low economic efficiency.

Although such carbon quantum dots have high applicability in many fields, there is no known chemical synthesis method that can be uniformly produced in large quantities, and conventional manufacturing methods have low production efficiency, production of uniform particle size, and particle size. There is a problem that it is very difficult to easily control and minimize defects in the surface area.

Accordingly, there is a need for an easy synthesis method capable of improving excellent luminescence characteristics and quantum efficiency by controlling the size and shape of carbon quantum dots.

In order to solve the above problems, an object of the present invention is to provide a graphene-based compound and graphene quantum dots having excellent luminous efficiency and electrical conductivity.

Another object of the present invention is to provide a graphene-based compound and a graphene quantum dot in which the oxidized region of the edge region is regular.

Another object of the present invention is to provide a single-phase composition for producing a graphene-based compound that can control the size and shape and has excellent crystallinity.

Another object of the present invention is to provide a single solution phase preparation method for preparing monodisperse and monocrystalline graphene-based compounds.

Another object of the present invention is to provide a method for producing a graphene-based compound that is simple to synthesize and separate, and has excellent economical efficiency.

As a result of research in order to achieve the above object, the single-phase composition for preparing a graphene-based compound according to the present invention includes a hydrocarbyl amine, a carbon source containing a hydroxyl group, and an acid.

The hydroxyl group-containing carbon source according to an aspect of the present invention may include a saccharide-based compound.

The saccharide-based compound according to an aspect of the present invention may include a monosaccharide compound.

The acid according to an aspect of the present invention may include an organic acid.

The single-phase composition for preparing the graphene-based compound according to an aspect of the present invention may further include a hydrocarbyl alcohol-based solvent.

The hydrocarbyl alcohol-based solvent according to an aspect of the present invention is a substituted or unsubstituted (C4 ~ C20) alkyl alcohol, a substituted or unsubstituted (C4 ~ C20) alkenyl alcohol, or a substituted or unsubstituted (C4 ~ C20) alky It may contain nil alcohol.

The hydrocarbyl amine according to an aspect of the present invention is a substituted or unsubstituted (C3 ~ C20) alkyl amine, a substituted or unsubstituted (C3 ~ C20) alkenyl amine or a substituted or unsubstituted (C3 ~ C20) alkynyl amine. Can include.

The hydroxyl group-containing carbon source and hydrocarbyl amine according to an aspect of the present invention may be included in a molar ratio of 1: 1 to 20.

The hydrocarbyl amine and acid according to an aspect of the present invention may be included in a molar ratio of 1:0.2 to 2.

The pH of the composition according to an aspect of the present invention may have a range of 4 to 7.

The composition according to an aspect of the present invention may be in the form of an aqueous solution.

The graphene-based compound according to an aspect of the present invention may include graphene quantum dots.

The method for preparing a graphene-based compound according to the present invention includes the steps of: (a) preparing a reaction mixture comprising a hydrocarbyl amine, a hydroxyl group-containing carbon source, and an acid; And (b) heating the reaction mixture.

The reaction mixture according to an aspect of the present invention may further include a hydrocarbyl alcohol-based solvent.

The reaction temperature in step (b) according to an aspect of the present invention may be 60 to 300°C.

It may further include the step of mixing the reaction product obtained after the step (b) according to an aspect of the present invention with an alcohol-based solvent (C1 ~ C3).

It may further include cooling the reaction product obtained after step (b) according to an aspect of the present invention to below the melting point of hydrocarbyl amine.

The reaction mixture in step (a) according to an aspect of the present invention may be a single phase.

The graphene-based compound according to the present invention is prepared by the above-described manufacturing method.

The graphene-based compound according to an aspect of the present invention may include graphene quantum dots.

The graphene quantum dots according to the present invention may have an average particle diameter of 2 nm to 10 μm, and a ratio of I(D)/I(G) may be 1.0 or less.

The graphene quantum dot according to an aspect of the present invention may include a carbonyl group or a -C-O group in an edge region of graphene, and a nitrogen atom of a hydrocarbyl amine may be covalently bonded with a carbon atom in the edge region.

The graphene quantum dots according to an aspect of the present invention may have a relative standard deviation (coefficient of variation) of less than 10% with respect to a particle size distribution.

The graphene quantum dot according to an aspect of the present invention may have a maximum light emission characteristic at 380 to 480 nm in a 300K, methanol solvent.

The graphene quantum dot according to an aspect of the present invention may have single crystal characteristics.

Exciton survival time of a 5mg/mL solution in a 300K, methanol solvent of the graphene quantum dots according to an aspect of the present invention may have 4 ns or less.

The aspect ratio of the long axis length to the thickness of the graphene quantum dot according to an embodiment of the present invention may be 5 or more.

At 300K of the graphene quantum dot according to an embodiment of the present invention, the maximum emission wavelength (λ _em ) and the excitation wavelength (λ _ex ) of PL in a methanol solvent may satisfy the following relationship.

[Relationship 1]

λ _em = A × λ _ex + B

In the above relational expression, A is 0.59 and B is in the range of 208.81 to 238.81.

The graphene quantum dot according to an aspect of the present invention may exhibit a maximum PL at an excitation wavelength of 350 ~ 390nm at 300K.

The graphene quantum dot according to an embodiment of the present invention may have a PL emission spectrum at half width (FWHM) of 100 nm or less at 300K.

The graphene quantum dot according to an embodiment of the present invention may have a maximum emission wavelength in the range of 420 to 480 nm, showing a maximum PL curve at 300K.

In the graphene quantum dot according to an aspect of the present invention, the diameter (Dp) and the carbon/oxygen atomic ratio (C/O ratio) of the graphene quantum dot molecule may satisfy the following relational formula 2.

[Relationship 2]

C/O ratio = -0.221*Dp+C

In the above relational equation 2, the diameter Dp has a unit of nm, and the constant C has a range of 20.48 to 30.48.

The graphene quantum dots according to an aspect of the present invention may contain an oxygen content of 15 atomic% or less in a molecule.

The graphene quantum dots according to an aspect of the present invention may form a hexagonal arrangement.

The graphene-based compound according to the present invention has an advantage in that the average particle diameter is uniform and that it has monodispersity and single crystallinity with high crystallinity.

The graphene-based compound according to the present invention has the advantage of excellent luminescence characteristics and quantum efficiency, including single crystal graphene quantum dots.

The composition for preparing a graphene-based compound according to the present invention is provided in a single phase, and has the advantage of being able to mass-produce with excellent economic efficiency due to simple synthesis and separation.

The method for preparing a graphene-based compound according to the present invention has the advantage of being able to control the size and shape of the graphene-based compound.

1 is a photograph illustrating a single phase of a composition for preparing a graphene-based compound according to an embodiment of the present invention.
Figure 2 is a graphene-based compound according to an embodiment of the present invention a), c) and d) are photographs observed with a transmission electron microscope, b) a graph of the standard deviation of the particle size distribution of the graphene-based compound.
3 is a photograph of a graphene-based compound according to an embodiment of the present invention observed with a transmission electron microscope of the size changed according to the reaction temperature.
4 is a result of XPS analysis of graphene-based compounds according to an embodiment of the present invention. 4A is a result of a graphene compound having an average particle diameter of 5 nm, and FIG. 4B is a result of a graphene compound having an average particle diameter of 70 nm.
5 is a Raman spectroscopy result of a graphene-based compound having an average particle diameter of 5 nm or 70 nm according to an embodiment of the present invention.
6 is a result of PL characteristics of a graphene-based compound according to an embodiment of the present invention. 6A is a result of a graphene compound having an average particle diameter of 5 nm, and FIG. 6B is a result of a graphene compound having an average particle diameter of 70 nm.
7 is a photograph of a graphene-based compound according to an embodiment of the present invention, after UV irradiation, and luminescence characteristics are observed.
8 is a result of measuring time-resolved photoluminescence (TRPL) for each average particle diameter and excitation wavelength of a graphene-based compound according to an embodiment of the present invention.
9 is an FT-IR analysis result according to the average particle diameter of the graphene-based compound according to an embodiment of the present invention.
10 is a result of ultraviolet/visible light transmittance (UV-Vis-Transmittance) of a graphene-based compound according to an embodiment of the present invention.
11 is a photograph of a transparent colorless single phase of a composition for preparing a graphene-based compound according to an embodiment of the present invention observed with the naked eye.

Hereinafter, a graphene-based compound according to the present invention, a method for preparing the same, a single-phase composition for preparing a graphene-based compound, and a graphene quantum dot according to the present invention will be described in detail with reference to the accompanying drawings. The drawings introduced below are provided as an example in order to sufficiently convey the idea according to the present invention to those skilled in the art. Accordingly, the present invention is not limited to the drawings presented below and may be embodied in other forms, and the drawings presented below may be exaggerated to clarify the idea of the present invention. In this case, unless there are other definitions in the technical terms and scientific terms used, they have the meanings commonly understood by those of ordinary skill in the art to which this invention belongs, and in the following description and accompanying drawings, the gist of the present invention A description of known functions and configurations that may unnecessarily obscure will be omitted.

In the present invention, unless otherwise defined,% or ratio is interpreted to mean weight percent or weight ratio.

In the present invention, "hydrocarbyl" refers to a radical having one bonding site derived from hydrocarbon, and specifically, for example, selected from alkyl, cycloalkyl, aryl, alkenyl, alkynyl, etc. It may be a combination of these.

"Substituted" in the present invention is halogen, hydroxyl group, cyano group, carboxyl group, carboxylate, (C1 ~ C20) alkyl, (C1 ~ C20) alkoxy, (C2 ~ C20) alkenyl, (C2 ~ C20 ) It means substituted with one or more substituents selected from alkynyl, (C6~C20)aryl and (C6~C20)heteroaryl.

The present invention relates to a graphene-based compound, a method for preparing the same, a single-phase composition for preparing a graphene-based compound, and a graphene quantum dot.

Hereinafter, a single-phase composition for preparing a graphene-based compound according to the present invention will be described in detail.

The single-phase composition for preparing a graphene-based compound according to the present invention is provided to prepare a graphene-based compound, and includes a hydrocarbyl amine, a hydroxyl group-containing carbon source, and an acid. That is, a homogeneous single-phase composition can be prepared by combining a hydrocarbyl amine, a hydroxyl group-containing carbon source, and an acid.

Through the single-phase composition for preparing a graphene-based compound according to the present invention, the size and shape of the graphene-based compound can be controlled, and a single crystal graphene-based compound having excellent crystallinity can be prepared. As a result, excellent luminous efficiency and electrical characteristics can be expressed that exceeds the limitations of existing graphene-based compounds.

The single phase refers to a homogeneous phase excluding a dispersed phase such as an emulsion even though it is transparent in appearance.

According to an aspect of the present invention, the single-phase composition for preparing the graphene-based compound may have a transmittance of 80% or more in a wavelength range of 450 to 800 nm. Preferably, the transmittance may be 85% or more. It can be seen that the single-phase composition for preparing a graphene-based compound according to the present invention has the same transmittance as shown in FIG. 10, and there is no shoulder peak, so that a colorless, transparent single phase as shown in FIG. 11 is formed. Can be confirmed.

According to one aspect of the present invention, the hydrocarbyl amine is a substituted or unsubstituted (C3 ~ C20) alkyl amine, a substituted or unsubstituted (C3 ~ C20) alkenyl amine or a substituted or unsubstituted (C3 ~ C20) alkynyl amine It may include. Preferably it may include a substituted or unsubstituted (C4 ~ C20) alkyl amine, a substituted or unsubstituted (C4 ~ C20) alkenyl amine or a substituted or unsubstituted (C4 ~ C20) alkynyl amine.

Preferably, the hydrocarbyl amine may be linear and may be a primary amine (R-NH ₂ ). For specific examples, the hydrocarbyl amine is butylamine, hexylamine, dodecylamine, nonylamine, decylamine, undecylamine, tridecylamine, tetradecylamine, pentadecylamine, hexadecylamine, heptadecylamine and It may be any one or a mixture of two or more selected from octadecylamine and the like.

Although such amines are close to hydrophobicity, it is difficult to prepare a single phase composition, but a single phase may be formed by combining the compositions according to the present invention.

In addition, by including the hydrocarbyl amine as described above, a covalent bond can be formed through polar interaction with a carbon source containing a hydroxyl group, and by forming a single phase more stably when forming a single phase composition, through subsequent heat treatment A single crystal graphene-based compound having a high degree of crystallinity can be prepared.

According to an aspect of the present invention, the hydroxyl group-containing carbon source may include a saccharide-based compound. The saccharide-based compound may include, for example, any one or a mixture of two or more selected from monosaccharide compounds, disaccharide compounds, oligosaccharide compounds, and polysaccharide compounds. Preferably, the saccharide-based compound may include a monosaccharide compound.

The monosaccharide compounds may be classified into disaccharides, trisaccharides, tetrasaccharides, pentose sugars, and hexasaccharides according to the number of carbon atoms, and may preferably contain hexasaccharides to provide a single crystal graphene-based compound according to the present invention. . The hexasaccharide may include, for example, any one or a mixture of two or more selected from glucose, fructose, galactose, and mannose.

When the carbon source containing a hydroxyl group as described above is included in the composition, a single phase can be easily formed in the solution, and polar interactions and covalent bonds with hydrocarbyl amine can be induced in the single phase composition. As described above, the hydroxyl group-containing carbon source can form a covalent bond with the hydrocarbyl amine in the single-phase composition, and Amadori rearrangement can be easily induced, and through a subsequent heat treatment process, high A single crystal graphene-based compound having a degree of crystallinity can be prepared. In addition, since a graphene-based compound having a regular oxidized region of the edge region can be prepared, excellent luminescence characteristics can be expressed.

According to an aspect of the present invention, the hydroxyl group-containing carbon source may include 0.01 to 20% by weight based on the total weight of the single-phase composition for preparing a graphene-based compound, but is not limited thereto. As a non-limiting example, it may contain 1 to 18% by weight, preferably 5 to 15% by weight, and when included in the above content, a graphene-based compound can be obtained in a high yield, and a graphene-based compound having a uniform shape Can be obtained.

According to an aspect of the present invention, the hydroxyl group-containing carbon source and the hydrocarbyl amine may be included in a molar ratio of 1:1 to 20 in the single-phase composition for preparing a graphene-based compound. Preferably, it may be included in a molar ratio of 1: 2 to 10, more preferably 1: 4 to 10. In the case of containing a carbon source containing a hydroxyl group and a hydrocarbyl amine in the above content, the polar interaction between the carbon source containing a hydroxyl group and a hydrocarbyl amine can be most effectively induced and stably contained in a single phase, making it easy to use graphene-based compounds. Can be prepared, and the yield of the graphene-based compound can be improved.

According to an aspect of the present invention, the hydroxyl group-containing carbon source may be provided as an aqueous solution. Accordingly, the single-phase composition for preparing the graphene-based compound according to the present invention can be provided in an aqueous solution.

In a conventional method for producing a single crystal graphene-based compound, a liquid phase process is impossible, or an unstable phase may be formed in an aqueous solution to separate the phase, and an emulsified structure such as an emulsion may be formed. However, the present invention forms a stable aqueous solution, through which a graphene-based compound can be stably prepared, and a single crystal graphene-based compound having a higher degree of crystallinity can be provided.

According to one aspect of the present invention, the acid may include an organic acid. The organic acid may be a (C1 ~ C8) organic acid, and one or more acid groups may be included in one molecule. Preferably, it may be an organic acid (C1 to C3), and for a specific example, it may be any one or a mixture of two or more selected from acetic acid, formic acid, propionic acid, etc., but is not limited thereto. Preferably, it may be acetic acid in order to effectively form a single phase, suppress side reactions, and obtain monocrystalline and monodisperse graphene-based compounds.

When the above organic acid is included, a stable and uniform single-phase composition can be provided, and a monodisperse graphene-based compound can be obtained in high yield without phase separation during the subsequent heat treatment of the composition, and the light emission characteristic with a narrow half width is excellent. A single crystal graphene-based compound can be prepared.

The reactive composition according to the conventional method for preparing a graphene-based compound is difficult to provide in a single phase, has low production efficiency, and is difficult to artificially control the size and surface condition of particles. On the contrary, the present invention provides a stable and uniform single-phase composition, thereby exhibiting high crystallinity, providing a single crystal graphene-based compound, and implementing excellent light-emitting characteristics and electrical characteristics.

According to an aspect of the present invention, the hydrocarbyl amine and acid may be included in a molar ratio of 1:0.2 to 2 in a single-phase composition for preparing a graphene-based compound. Preferably, it may be included in a molar ratio of 1: 0.3 to 2. When the hydrocarbyl amine and the acid are included in the above content, the acid may promote the reaction of the hydrocarbyl amine with the carbon source containing the hydroxyl group, as well as induce a single crystal growth.

According to one aspect of the present invention, the single-phase composition for preparing the graphene-based compound may further include a hydrocarbyl alcohol-based solvent. The hydrocarbyl alcohol-based solvent may preferably be a (C4 ~ C20) hydrocarbyl alcohol-based solvent.

The hydrocarbyl alcohol-based solvent is, for example, a substituted or unsubstituted (C4 ~ C20) alkyl alcohol, a substituted or unsubstituted (C4 ~ C20) alkenyl alcohol, or a substituted or unsubstituted (C4 ~ C20) alkynyl alcohol. Can include. Preferably, a substituted or unsubstituted (C4 ~ C10) alkyl alcohol, a substituted or unsubstituted (C4 ~ C10) alkenyl alcohol, or a substituted or unsubstituted (C4 ~ C10) alkynyl alcohol may be included. More preferably, a substituted or unsubstituted (C4 ~ C7) alkyl alcohol, a substituted or unsubstituted (C4 ~ C7) alkenyl alcohol, or a substituted or unsubstituted (C4 ~ C7) alkynyl alcohol may be included. Specific examples include n-butanol, isobutanol, sec-butanol, tert-butanol, n-pentanol, isopentanol, sec-pentanol, tert-pentanol, hexanol, heptanol, octanol, nonanol , Decanol, undecanol, dodecanol and pentadecanol may be any one or two or more mixed solvents selected from. In order to provide a more homogeneous single-phase composition, preferably in n-butanol, isobutanol, sec-butanol, tert-butanol, n-pentanol, isopentanol, sec-pentanol, tert-pentanol and hexanol. It may be any one or a mixed solvent of two or more selected. When the hydrocarbyl alcohol-based solvent as described above is included, the acidity can be adjusted, and thus the reaction of the hydrocarbyl amine, the carbon source containing a hydroxyl group, and the acid can be activated to provide a highly crystalline graphene-based compound.

According to one aspect of the present invention, the pH of the single-phase composition for preparing the graphene-based compound may have a range of 4 to 7. Preferably, the pH may have a range of 4.5 to 6.5. When it has the above pH, it is possible to suppress side reactions when preparing a graphene-based compound.

Hereinafter, a method for preparing a graphene-based compound according to the present invention will be described in detail.

The method for preparing a graphene-based compound according to the present invention includes the steps of: (a) preparing a reaction mixture comprising a hydrocarbyl amine, a hydroxyl group-containing carbon source, and an acid; And (b) heating the reaction mixture.

According to an aspect of the present invention, the method for preparing the graphene-based compound may be carried out at atmospheric pressure. Unlike the slow crystal growth rate at the time of hydrothermal reaction at high pressure, by performing at normal pressure, a graphene-based compound having a fast crystal growth rate and uniform single crystal can be prepared.

According to an aspect of the present invention, the reaction mixture in step (a) may be prepared in a stable and uniform single phase by mixing a hydrocarbyl amine, a carbon source containing a hydroxyl group, and an acid.

According to an aspect of the present invention, the reaction mixture of step (a) may be further heated. The heating temperature may be 60 to 100 ℃. Preferably the heating temperature may be 60 to 90 ℃. Through heating as described above, some non-uniform reaction mixtures are also induced into a uniform single phase, so that single crystal and monodisperse graphene-based compounds can be prepared.

The reaction mixture according to an aspect of the present invention may further include a hydrocarbyl alcohol-based solvent. As described above, by further including a hydrocarbyl alcohol-based solvent, a monocrystalline graphene-based compound can be prepared by activating the reaction of a hydrocarbyl amine, a carbon source containing a hydroxyl group, and an acid. In addition, it is possible to control the size of the graphene-based compound, and can be provided with monodispersity.

The hydrocarbyl alcohol-based solvent may preferably be a (C4 ~ C20) hydrocarbyl alcohol-based solvent, for example, a substituted or unsubstituted (C4 ~ C20) alkyl alcohol, a substituted or unsubstituted (C4 ~ C20) Alkenyl alcohol, or substituted or unsubstituted (C4 ~ C20) alkynyl alcohol may be included. Preferably, a substituted or unsubstituted (C4 ~ C10) alkyl alcohol, a substituted or unsubstituted (C4 ~ C10) alkenyl alcohol, or a substituted or unsubstituted (C4 ~ C10) alkynyl alcohol may be included. More preferably, a substituted or unsubstituted (C4 ~ C7) alkyl alcohol, a substituted or unsubstituted (C4 ~ C7) alkenyl alcohol, or a substituted or unsubstituted (C4 ~ C7) alkynyl alcohol may be included. Specific examples include n-butanol, isobutanol, sec-butanol, tert-butanol, n-pentanol, isopentanol, sec-pentanol, tert-pentanol, hexanol, heptanol, octanol, nonanol , Decanol, undecanol, dodecanol and pentadecanol may be any one or two or more mixed solvents selected from. In order to provide a graphene-based compound having a higher degree of crystallinity, preferably n-butanol, isobutanol, sec-butanol, tert-butanol, n-pentanol, isopentanol, sec-pentanol, tert-pentanol and hexane It may be any one or two or more mixed solvents selected from ol and the like.

According to an aspect of the present invention, the reaction temperature during heating in step (b) is not particularly limited, but the reaction temperature may be 60 to 300°C. Preferably, the reaction temperature may be 60 to 200 ℃.

The reaction time can be performed for 30 minutes to 10 hours, but is not limited. Preferably, it can be performed for 30 minutes to 5 hours, and the average particle diameter of the graphene-based compound can be adjusted according to the reaction temperature. Specifically, particles having a small average particle diameter can be produced at high temperatures, and particles having a large average particle diameter can be produced at low temperatures.

It may further include the step of mixing the reaction product obtained after the step (b) according to an aspect of the present invention with an alcohol-based solvent (C1 ~ C3). The (C1 ~ C3) alcohol-based solvent may be any one or a mixture of two or more selected from methanol, ethanol, propanol and isopropanol, for a specific example. The reaction can be stopped by mixing the reaction product with the above (C1-C3) alcohol-based solvent.

According to an aspect of the present invention, it may further include cooling the reaction product obtained after step (b) to a melting point of hydrocarbyl amine or lower. While the reaction product obtained by hydrothermal reaction carried out at high temperature and high pressure has a low yield according to a complicated purification process such as dialysis, the reaction product according to the present invention is cooled to precipitate hydrocarbyl amine in a single-phase reaction product. By removing and extracting with unreacted amines, the yield can be further improved.

The graphene-based compound prepared by the above-described manufacturing method according to the present invention may include graphene quantum dots having excellent luminescence properties.

The graphene quantum dots according to the present invention may have an average particle diameter of 2 nm to 10 μm. Preferably, it may be 2 nm to 2 μm, and more preferably 2 nm to 500 nm. Graphene quantum dots according to the present invention may have a wide range of average particle diameter.

The graphene quantum dots according to the present invention are prepared from a single-phase composition containing a hydrocarbyl amine, a hydroxyl group-containing carbon source and an acid, and in more detail, a covalent bond with a hydrocarbyl amine and a hydroxyl group-containing carbon source, amadori ash Through the arrangement and subsequent heat treatment processes, graphene quantum dots having excellent luminescence properties can be manufactured.

The types and contents of the hydrocarbyl amine, the hydroxyl group-containing carbon source and the acid are the same as described above, and thus are omitted.

According to an aspect of the present invention, the aspect ratio (L/T) of the long axis length (L) to the thickness (T) of the graphene quantum dot may be 5 or more. The aspect ratio may be specifically 5 to 1,000, preferably 10 to 1,000. In the case of having the above aspect ratio, a luminous effect can be maximized, a single crystal can be obtained, and excellent luminous efficiency and electrical characteristics can be exhibited.

Graphene quantum dots according to the present invention may have a ratio of I(D)/I(G) of 1.0 or less. In this case, the lower limit of I(D)/I(G) is also not particularly limited, but may be preferably 0.3 or more.

Specifically, as shown in FIG. 5, the I(D) indicating the degree of defects in the surface area in the Raman spectrum is the maximum peak intensity (au) of the D band appearing at 1300-1400 cm ^-1 , and I(G) is 1550. It is the maximum peak intensity (au) of the G band at ~1650 cm ^-1 .

Having a ratio of I(D)/I(G) as described above means that graphene quantum dots are manufactured that are substantially free of defects. Conventional quantum dots have a lot of crystal defects because the distribution of atoms located on the surface is very large compared to the bulk, and there is a problem that electrons are easily lost because of the high energy state. In contrast, the graphene quantum dots according to the present invention are substantially free of defects, and thus luminous efficiency can be remarkably improved.

In this case, "substantially free from defects" includes not only the absence of defects in the surface area, but also most functional groups due to oxidation reactions are present in the edge area, and there is little or no functional group in the surface area. Includes that.

For example, as shown in FIG. 5, even if the graphene quantum dots are manufactured by varying the size of the graphene quantum dots, it can be confirmed that the graphene quantum dots according to the present invention are substantially free of defects.

According to an aspect of the present invention, the graphene quantum dots may include a carbonyl group or a -C-O group in the edge region of graphene, and the nitrogen atom of the hydrocarbyl amine may be covalently bonded with the carbon atom of the edge region.

As described above, graphene quantum dots can be grown by covalently bonding the carbonyl group or -C-O group present in the edge region of the graphene quantum dot and the amine group of the hydrocarbyl amine.

According to an aspect of the present invention, the graphene quantum dots may have a relative standard deviation (coefficient of variation) of less than 10% with respect to the particle size distribution. Preferably, the particle size distribution may have a relative standard deviation (coefficient of variation) of less than 8%. By having a low relative standard deviation as described above, it means that the graphene quantum dots according to the present invention have monodispersity. By exhibiting monodispersity as described above, it is possible to realize stable and excellent light-emitting characteristics and electrical characteristics.

In this case, the relative standard deviation (coefficient of variation) refers to a ratio of the standard deviation to the average value or a coefficient indicating the reproducibility of the device, and usually refers to a relative display of scatter.

According to an aspect of the present invention, the graphene quantum dots may have maximum light emission characteristics at 380 to 480 nm in a 300K, methanol solvent. Specifically, the graphene quantum dots may have a maximum emission wavelength in the range of 420 to 480 nm showing a maximum PL curve at 300K. The graphene quantum dots according to the present invention may have the same emission wavelength region even when a wide range of excitation wavelengths is applied. In general graphene quantum dots, the PL emission wavelength deviates from the emission region of a color other than blue emission according to the wavelength range of the excitation wavelength due to many defects in graphene. In contrast, the graphene quantum dots according to the present invention stably have maximum light emission characteristics within the above range even with a change in excitation wavelength, so that the graphene quantum dots may preferably exhibit blue light emission characteristics. In order to clearly exhibit the blue light emission characteristics as described above, there must be few defects in the graphene quantum dots, and the graphene quantum dots according to the present invention are substantially free of defects, so that excellent blue light emission characteristics can be expressed.

According to an aspect of the present invention, specifically, the graphene quantum dots may exhibit a maximum photoluminescence (PL) at an excitation wavelength of 350 to 390 nm at 300K. Preferably, the maximum PL may be exhibited at an excitation wavelength of 355 to 380 nm at 300K. When light is received from the excitation wavelength and reaches an energy-excited state, energy according to an energy band gap may be emitted to exhibit a maximum PL.

According to an aspect of the present invention, at 300K of the graphene quantum dot, the maximum emission wavelength (λ _em ) and the excitation wavelength (λ _ex ) of PL in a methanol solvent may satisfy the following relationship.

[Relationship 1]

λ _em = A × λ _ex + B

In the above relational expression, A is 0.59 and B is in the range of 208.81 to 238.81. Preferably, B is in the range of 213.81 to 235.81. The graphene quantum dots according to the present invention exhibit single crystallinity and monodispersity, thereby satisfying the above-described relational formula 1.

According to an aspect of the present invention, the graphene quantum dot may have an excitation wavelength of 340 to 380 nm, and a half width (FWHM) of a PL emission spectrum at 300 K may be 100 nm or less. Preferably, the half value width (FWHM) of the PL emission spectrum at 350 to 370 nm excitation wavelength and 300 K may be 100 nm or less. By having a narrow half-width as described above, graphene quantum dots can exhibit light emission characteristics having high color purity.

According to an aspect of the present invention, the graphene quantum dots may have single crystal properties by being prepared in a single phase composition. By having a single crystal characteristic as described above, light emission characteristics and electrical characteristics can be remarkably improved through rapid electron transfer.

According to an aspect of the present invention, exciton survival time using excitation light having a wavelength of 280 nm and 350 nm of a 5 mg/mL solution in a 300K, methanol solvent of the graphene quantum dot may have 4 ns or less. By having a short exciton survival time as described above, it is possible to have excellent light emission quantum efficiency.

In the graphene quantum dot according to an aspect of the present invention, the diameter (Dp) and the carbon/oxygen atomic ratio (C/O ratio) of the graphene quantum dot molecule may satisfy the following relational formula 2.

[Relationship 2]

C/O ratio = -0.221*Dp+C

In the above relational equation 2, the diameter Dp has a unit of nm, and the constant C has a range of 20.48 to 30.48. Preferably, the constant C has a range of 22.48 to 28.48.

Since the graphene quantum dot according to the present invention is substantially free of defects, the above relational expression 2 may be satisfied. Accordingly, it is possible to have excellent luminous efficiency.

The graphene quantum dots according to an aspect of the present invention may contain an oxygen content of 15 atomic% or less in a molecule. Preferably it may contain an oxygen content of 10 atomic% or less. If the oxygen content is high, it means that a large amount of oxidized regions exist in the surface region as well. Unlike conventional graphene quantum dots, whose oxygen content does not exceed 40-50 atomic%, the graphene quantum dots according to the present invention have a small oxygen content, meaning that there is substantially no oxidation region in the surface region other than the oxidation region in the edge region. And, thereby, it is possible to implement a graphene quantum dot having high crystallinity. Moreover, despite the low oxygen content of the graphene quantum dots according to the present invention, the high water dispersion characteristics and quantum efficiency means that there are substantially no defects in the surface area of the graphene quantum dots, and oxidized functional groups exist. Means existing in the edge area.

In addition, the graphene quantum dot according to an aspect of the present invention is generated by oxidation of the surface area of a typical graphene quantum dot, and the epoxide appears in the range of 840 to 850 cm ^-1 in the FT-IR spectrum including the epoxide oxidation region. Unlike the epoxide stretching peak, the peak does not occur. Moreover, when the average particle diameter of the graphene quantum dots is small, no peak exists, and when the average particle diameter increases, some of the epoxide peaks are generated, but this is only a degree that is substantially free of defects.

According to an aspect of the present invention, the graphene quantum dot may be one that satisfies the following relational equation 3 in the FT-IR spectrum.

[Relationship 3]

I _CC /I _OH > 1.6

In the above relational formula 3, the I _CC is the maximum strength absorption peak of the aromatic ring C=C stretch that appears in the range of 1560 to 1760 cm ^-1 , and the I _OH is the -OH maximum strength that appears in the range of 3200 to 3600 cm ^-1 Increase the absorption peak

Preferably, the relational expression 3 may satisfy 1.8 or more, and more preferably 1.9 or more. When the relational equation 3 is satisfied as described above, defects in the graphene quantum dot are substantially absent, and further excellent luminous efficiency may be obtained. According to an aspect of the present invention, the graphene quantum dot is 1560 in the FT-IR spectrum. ~ 1760㎝ has full width at half maximum (FWHM) of ^-1 C = C ring stretch maximum absorption peak appearing in the range of ^-1 or less 100㎝. Preferably, the half value width (FWHM) of the maximum strength absorption peak of the aromatic ring C=C stretch appearing in the range of 1560 to 1760 cm ^-1 in the FT-IR spectrum may be 70 cm ^-1 or less. By having a narrow half width as described above, it is possible to provide a graphene quantum dot having a very high degree of crystallinity.

According to an aspect of the present invention, the graphene quantum dots may have amphiphilic properties that are dissolved in both non-polar and polar solvents. More specifically, when dissolved in a concentration of 10 mg/ml at 300K, it can have amphiphilic properties that are dissolved in a non-polar solvent, a polar solvent, or a mixed solvent thereof.

The non-polar solvent may be, for example, any one or a mixture of two or more selected from (C6 to C20) alkanes, amides, (C6 to C20) aromatic solvents and halogenated solvents. For a specific example, it may be any one or two or more mixed solvents selected from hexane, heptane, octane, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, tetrahydrofuran, dichlorobenzene, chloroform, and chlorobenzene. .

The polar solvent is, for example, any one or two or more selected from (C4 ~ C20) alkyl alcohol, substituted or unsubstituted (C4 ~ C20) alkenyl alcohol, or substituted or unsubstituted (C4 ~ C20) alkynyl alcohol. It may be a mixed solvent. For a specific example, it may be any one or two or more mixed solvents selected from methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol, undecanol, dodecanol, and the like. However, water is excluded among polar solvents.

The graphene quantum dots are grown as single crystals to exhibit nonpolarity due to substantially no defects in the surface area, and polarity may be indicated by forming -OH or -COOH in the edge area. Conventional graphene quantum dots are only dissolved in polar solvents and are difficult to provide for various solution processes. In contrast, graphene quantum dots having amphiphilic properties that are soluble in both non-polar and polar solvents can be easily applied to a solution process using various solvents, and through this, it can be confirmed that graphene quantum dots have very high crystallinity.

According to an aspect of the present invention, the graphene quantum dot may have a hexagonal structure. By having a hexagonal structure as described above, when graphene quantum dots are applied on a substrate, a uniform hexagonal array can be achieved. By forming a hexagonal arrangement as described above, the graphene quantum dots are packed with high density, so that it can have more excellent luminous efficiency.

The graphene-based compound according to the present invention, a method for preparing the same, a single-phase composition for producing a graphene-based compound, and a graphene quantum dot according to the present invention will be described in more detail below. However, the following examples are only one reference for describing the present invention in detail, and the present invention is not limited thereto, and may be implemented in various forms.

Further, unless otherwise defined, all technical and scientific terms have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. The terms used in the description herein are merely to effectively describe specific embodiments and are not intended to limit the invention.

In addition, the unit of the additive not specifically described in the specification may be a weight %.

[Example 1]

10 ml of 1-hexanol was bubbled with nitrogen for 1 hour at 80 ml/min for 1 hour. Thereafter, 1 ml of an aqueous solution in which D-glucose was dissolved in an amount of 10% by weight was added thereto, and nitrogen supply was cut off, followed by stirring at 80°C for 1 hour. After stirring, 800 mg of 1-hexadecylamine and 200 mg of acetic acid were added thereto. Nitrogen was again supplied at 80 ml/min, and the mixture was stirred at 150° C. for 1 hour. The stirred composition was washed with methanol and sonicated. After cooling at -25 ℃ for 6 hours, filtered through a PTFE 0.22㎛ membrane to obtain a supernatant to obtain a graphene-based compound having an average particle diameter of 5 nm.

[Experimental Example 1] Confirmation of the dispersion phase of the composition.

When the composition was prepared by changing the types of hydrocarbyl amine, hydrocarbyl alcohol-based solvent and acid as shown in Table 1, it was confirmed whether it had a single phase.

Sample No. mountain menstruum Amine One nitric acid Hexanol Hexadecylamine 2 Oxalic acid Hexanol Hexadecylamine 3 Formic acid Hexanol Hexadecylamine 4 Citric acid Hexanol Hexadecylamine 5 Acetic acid Butanol Hexadecylamine 6 Acetic acid Heptanol Hexadecylamine 7 Acetic acid Hexanol Butylamine 8 Acetic acid Hexanol Hexylamine 9 Acetic acid Hexanol Dodecylamine

As shown in FIG. 1, when using formic acid instead of acetic acid, a transparent single-phase composition was provided, whereas when oxalic acid, nitric acid and citric acid were used, an opaquely dispersed composition was provided. Through this, it was confirmed that the composition for preparing a graphene-based compound according to the present invention can provide a single-crystal graphene-based compound having a higher crystallinity by implementing a single phase when it contains (C1~C3) an organic acid. In addition, when the hydrocarbyl alcohol-based solvent contained heptanol having 7 or more carbon atoms, it was confirmed that a slightly heterogeneous composition was provided. When a graphene-based compound was prepared with a composition in which the composition was not prepared in a single phase as described above, the crystallinity of the graphene-based compound was reduced, so that it did not have single crystallinity, and a graphene-based compound having a monodispersed constant size and shape was not prepared. .

In addition, in the case of hydrocarbyl amine, a stable and uniform single-phase composition was prepared even when various amines from butylamine to dodecylamine were used, and it was confirmed that the graphene-based compound had excellent single crystallinity and monodispersity. there was.

That is, the composition for preparing a graphene-based compound according to the present invention can provide a single-phase composition by inducing polar interactions and covalent bonds between a hydrocarbyl amine including an acid and a carbon source containing a hydroxyl group, thereby having a high crystallinity. It was confirmed that a single crystal graphene-based compound could be prepared.

[Experimental Example 2] Observation of the shape of a graphene-based compound.

As shown in FIG. 2, it was confirmed that the graphene-based compound prepared in Example 1 had monodispersity having a uniform size and shape. As shown in FIG. 2, it was confirmed that the graphene-based compound prepared in Example 1 had a hexagonal structure, and formed a hexagonal arrangement according to the hexagonal structure as shown in FIG. 2A. In addition, it was confirmed through b) of FIG. 2 that the particle diameter distribution had a standard deviation of 6.07%, which is less than 7%.

After changing the reaction temperature of 150 ℃ in Example 1 to 140, 135, 130 and 120 ℃, as shown in Figure 3, the graphene-based compound prepared from the example was used in a transmission electron microscope (TEM, JEM-3011 HR, JEOL), it was confirmed that it was manufactured in a certain size and shape. When the average particle diameter is 10 nm, the reaction temperature is 140°C, when the average particle diameter is 30 nm, the reaction temperature is 135°C, and when the average particle diameter is 50 nm, the reaction temperature is 130°C and the average particle diameter is 70 At nm, the reaction temperature was 120°C. As described above, it was confirmed that not only the size of the graphene-based compound can be adjusted according to the reaction temperature, but also a graphene-based compound having a uniform shape can be provided even if the reaction temperature is changed.

[Experimental Example 3] Confirmation of the structure of a graphene-based compound.

As shown in FIG. 4A, when the graphene compound of Example 1 was analyzed by XPS, it contained 91.91 element% of carbon, 4.32 element% of nitrogen, and 3.77 element% of oxygen. As a result, it was confirmed that a graphene-based compound having substantially no defects was prepared. In addition, as shown in b of FIG. 4, when the graphene-based compound having an average particle diameter of 70 nm prepared by changing the reaction temperature of 150° C. to 170° C. in Example 1 was also subjected to XPS analysis, carbon content was 87.45 element% and nitrogen nitrogen. It was confirmed that a graphene-based compound having a very low oxygen content and a substantially defect-free graphene compound was prepared even though the reaction temperature was varied and the size was changed through the composition of 3.86 element% and 8.69 element% of oxygen. Through this, it was confirmed that the oxygen content of the conventional graphene-based compound exhibited a single crystal characteristic by having a remarkably low oxygen content compared to the high oxygen content of 40 to 50 element%.

As shown in Figure 9, when FT-IR analysis of the graphene-based compound having an average particle diameter of 5 nm and 70 nm according to the present invention, the epoxide stretching peak appearing at 846.1138 cm ^-1 is a graph having a small average particle diameter of 5 nm. It was confirmed that it did not occur in the fin-based compound. Through this, it can be confirmed that there is no oxidation region in the surface region of the graphene-based compound according to the present invention, that is, there is substantially no defect in the surface region. In addition, in the graphene-based compound having a relatively large average particle diameter of 70 nm, the epoxide stretching peak at 846.1138 cm ^-1 was slightly generated, but this is interpreted to the extent that there are no defects in the surface area.

In addition, as shown in Figure 9, the aromatic ring C = C stretch absorption peak height (I _CC ) shown at 1660 cm ^-1 by FT-IR analysis of the graphene-based compound having an average particle diameter of 5 nm and 70 nm according to the present invention The ratio (I _CC /I _OH ) of the -OH absorption peak height (I _OH ) appearing at 3270 cm ^-1 was calculated. At this time, it was confirmed that the graphene-based compound having an average particle diameter of 5 nm satisfies I _CC /I _OH of 1.944, and the graphene-based compound having an average particle diameter of 5 nm satisfies I _CC /I _OH of 2.567. Through this, it was confirmed that the graphene-based compound according to the present invention has excellent quality substantially without defects.

[Experimental Example 4] Luminous Characteristics of Graphene Quantum Dots

When the PL characteristics of the graphene quantum dots of Example 1 were observed as shown in FIG. 6, it was confirmed that the maximum emission peak was expressed at an excitation wavelength of 350 to 390 nm. At this time, it was confirmed that the maximum emission peak was expressed at 420 to 450 nm, and it was confirmed that the half width (FWHM) of the emission peak was very narrow as 100 nm or less. As such, it was confirmed that the maximum emission peak at an excitation wavelength of 50 to 390 nm was expressed at 420 to 450 nm, and the graphene quantum dots according to the present invention exhibited blue emission characteristics and excellent color purity as shown in FIG. 7.

Exciton survival time through time-resolved photoluminescence (TRPL, Time-Resolved Photoluminescence) in excitation light having wavelengths of 280nm and 350nm in a 300K, methanol solvent according to the size of the graphene quantum dots according to the present invention Was measured. As shown in FIG. 8, even if the size of the graphene quantum dots is changed, the exciton survival time was 4 ns or less, which was confirmed to have a very short exciton survival time. The graphene quantum dots according to the present invention have very good light emission quantum efficiency as they have a short exciton survival time.

In addition, it was confirmed that graphene quantum dots were prepared as a solution in which micelles were formed by adding a surfactant to an aqueous solution in which D-glucose was dissolved in an additional 10% by weight, but it was difficult to control the size and the graphene quantum dots with poor crystallinity were produced. I did.

In addition, graphene oxide prepared including a carbon source and an acid was first prepared, and then thermally decomposed with hydrocarbyl amine to prepare graphene quantum dots. However, it was confirmed that defects occurred in the pyrolysis process, and an additional reducing agent was required to remove them, and graphene quantum dots having poor crystallinity were produced because the defects were not completely removed.

It was confirmed that the graphene quantum dots according to the present invention have single crystallinity and monodispersity with high crystallinity, size control, and excellent luminescence properties. As such graphene quantum dots have excellent luminous efficiency, when applied in an organic light-emitting device such as an organic light-emitting diode, remarkably improved characteristics can be expressed.

As described above, in the present invention, it has been described by specific matters and limited embodiments and drawings, but this is provided only to help a more general understanding according to the present invention, and the present invention is not limited to the above embodiments, and the present invention Those of ordinary skill in the field to which this belongs can make various modifications and variations from this description.

Therefore, the idea according to the present invention is limited to the described embodiments and should not be determined, and all those having equivalent or equivalent modifications to the claims as well as the claims to be described later belong to the scope of the inventive idea. will be.

Claims (14)

Graphene quantum dots with an average particle diameter of 2 nm to 10 μm and a ratio of I(D)/I(G) of 1.0 or less. The method of claim 1,
The graphene quantum dots include a carbonyl group or -CO group in an edge region of graphene, and a nitrogen atom of a hydrocarbyl amine is covalently bonded to a carbon atom in the edge region. The method of claim 1,
The graphene quantum dots have a relative standard deviation (coefficient of variation) of less than 10% with respect to the particle size distribution. The method of claim 1,
The graphene quantum dots are 300K, graphene quantum dots having maximum light emission characteristics at 380 to 480 nm in a methanol solvent. The method of claim 1,
The graphene quantum dots are graphene quantum dots having single crystal properties. The method of claim 1,
The graphene quantum dots having an exciton survival time of 4 ns or less in 300K of the graphene quantum dots, 5 mg/mL solution in methanol solvent. The method of claim 1,
Graphene quantum dots having an aspect ratio of the long axis length to the thickness of the graphene quantum dots is 5 or more. The method of claim 1,
At 300K of the graphene quantum dots, the maximum emission wavelength (λ _em ) and excitation wavelength (λ _ex ) of PL in a methanol solvent are graphene quantum dots satisfying the following relational equation 1.
[Relationship 1]
λ _em = A × λ _ex + B
In the above relational expression, A is 0.59 and B is in the range of 208.81 to 238.81. The method of claim 1,
The graphene quantum dots are graphene quantum dots showing the maximum PL at an excitation wavelength of 350 ~ 390nm at 300K. The method of claim 1,
The graphene quantum dots are graphene quantum dots having a half-width (FWHM) of 100 nm or less of a PL emission spectrum at 300K. The method of claim 1,
The graphene quantum dots are graphene quantum dots with a maximum emission wavelength in the range of 420 ~ 480nm showing the maximum PL curve at 300K. The method of claim 1,
The graphene quantum dots are graphene quantum dots in which the diameter (Dp) and carbon/oxygen atomic ratio (C/O ratio) of the graphene quantum dot molecule satisfy the following relational formula 2.
[Relationship 2]
C/O ratio = -0.221*Dp+C
In the above relational equation 2, the diameter Dp has a unit of nm, and the constant C has a range of 20.48 to 30.48. The method of claim 1,
The graphene quantum dots are graphene quantum dots containing an oxygen content of 15 atomic% or less in a molecule. The method of claim 1,
The graphene quantum dots are graphene quantum dots forming a hexagonal arrangement.

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