Additive for quantum dot production, mixed solution for shell formation, method for producing core-shell type quantum dots, and dialkylamine-hydrohalide

Abstract

Provided is an additive for quantum dot production that comprises a dialkylamine-hydrohalide and by which defects on the surface of a quantum dot can be repaired without volatilization in a flow method, the reaction time can be shortened due to excellent reactivity, and a reaction solution that can be fed by a pump without containing a solid content at a temperature near room temperature can be obtained. Also provided are: a mixed solution for shell formation, the mixed solution containing the additive; and a method for producing core-shell type quantum dots through the flow method using the additive and mixed solution.

Description

Additive for quantum dot production, mixed solution for shell formation, method for producing core-shell quantum dots, and dialkylamine-hydrohalide salt

[0001] The present invention relates to an additive for producing quantum dots containing a dialkylamine hydrohalide salt as an active ingredient, a mixed solution for forming a shell containing the additive, and a method for producing core-shell quantum dots by a flow method using the additive and the mixed solution.

[0002] In recent years, the development of quantum dots as light-emitting materials has progressed. Cadmium-based quantum dots such as CdSe, CdTe, and CdS are being developed as representative quantum dots due to their excellent light-emitting properties. However, due to the toxicity and environmental impact of cadmium, InP and CuInS ２ , AgInS ２ Cadmium-free quantum dots such as PbS and InAs are also being actively developed.

[0003] To improve the performance of quantum dots, it is necessary to control the individual variations in particle size and surface condition of the reaction product and impart more uniform quality. From this perspective, the synthesis of quantum dots by flow methods using devices such as microreactors and tube reactors has been investigated. For example, Patent Document 1 describes a method in which two or more raw material solutions are joined in a flow channel with an inner diameter of 1 μm to 1 mm and subjected to a first heating reaction to form semiconductor particle precursors, followed by a second heating reaction at a higher temperature than the first heating reaction, thereby obtaining semiconductor particles with uniform particle sizes. Furthermore, Patent Document 2 describes a method in which a multilayer cylindrical mixer is used to configure the confluence where a solution of a compound containing a Group III element and a solution of a compound containing a Group V element are joined, thereby obtaining III-V semiconductor quantum dots that exhibit sharper luminescence characteristics than those obtained by uniform mixing using a T-shaped mixer.

[0004] By performing synthesis using the flow method in this way, it becomes easier to control the reaction temperature and reaction time, and the particle size of the obtained quantum dots becomes uniform, which is expected to improve the full width at half maximum (hereinafter also referred to as FWHM) of the emission peak.

[0005] However, if oxides or defects occur on the surface of quantum dots during their synthesis, excitons are trapped in the impurity levels formed by these defects, resulting in thermal energy deactivation, preventing efficient light emission and raising concerns about a decrease in photoluminescence quantum yield (hereinafter also referred to as PLQY). Therefore, removing oxides and defects from the quantum dot surface and forming a shell layer to protect the easily oxidized surface are important for the efficient and stable synthesis of quantum dots.

[0006] To repair defects on the quantum dot surface, etching with hydrofluoric acid or the like is used to remove oxides and other impurities from the quantum dot surface, resulting in a clean surface. It has also been reported that highly efficient and stable quantum dot synthesis can be achieved by forming a shell after obtaining a clean surface with hydrofluoric acid (see, for example, Non-Patent Document 1).

[0007] However, it is considered difficult to perform surface treatment and shell formation to obtain clean quantum dot surfaces using the flow method. For this reason, for example, in Non-Patent Document 2, core production is performed using the flow method, but the surface treatment process and shell formation process are performed using a batch method using a batch reaction vessel. In addition, Non-Patent Document 3 reports continuous synthesis from core synthesis to shell formation, but the light-emitting performance is significantly inferior to that of the batch method.

[0008] Japanese Patent Application Laid-Open No. 2008-37716 International Publication No. WO2018 / 180456 Pamphlet

[0009] Reinout F. Ubbink, et al., “A Water-Free In Situ HF Treatment for Ultrabright InP Quantum Dots” Chemistry of Materials 2022, 34, 22, 10093-10103Takafumi Moriyama, et al., “Highly luminescent and narrow-band-emitting InP / ZnSe / ZnS quantum dot synthesis by halide modified shell reaction” Appl. Phys. Express 2023, 16, 015504Ajit Vikram, et al., “A Millifluidic Reactor System for Multistep Continuous Synthesis of InP / ZnSeS Nanoparticles” ChemNanoMat 2018, 4, 943-953

[0010] As mentioned above, the improvement in the performance of quantum dots produced by the flow method is limited to core synthesis. There are no examples of the flow method being used for everything from surface treatment to shell formation, resulting in the same quality as that achieved by batch synthesis. Therefore, the advantages of using the flow method to synthesize quantum dots are extremely limited. The reason for this is that the precursor solutions used for surface treatment and shell formation, which follow quantum dot core synthesis, are not optimized. Specifically, depending on the quantum dot synthesis conditions, it is necessary to raise the temperature and flow the reaction solution through a fine channel. However, in this case, components in the reaction solution volatilize and become gases, which remain in the channel, preventing further reactions or making it impossible to control the residence time, resulting in problems such as only obtaining quantum dots with poor PLQY.

[0011] Another problem with the flow method is the long time required for surface treatment and shell formation. While core synthesis typically takes only a few minutes, surface treatment and shell formation require significantly longer processes, ranging from several tens of minutes to several hours. Therefore, achieving a similar residence time using the flow method would require an extremely long flow path, making it impractical. Furthermore, the flow method requires precursor solutions that can be pumped at or near room temperature. Some synthesis methods involve heating a solid-containing solution, such as a powder or slurry, to dissolve the solution at high temperatures and then react using a batch process. However, such solid-containing solutions are difficult to pump effectively, making the flow method unsuitable.

[0012] Therefore, an object of the present invention is to provide an additive for quantum dot production that can repair defects on the surface of quantum dots in a flow method without volatilizing, has excellent reactivity so that the reaction time can be shortened, and is capable of producing a reaction liquid that does not contain solids and can be pumped at around room temperature; a mixed solution for shell formation that contains the additive; and a method for producing core-shell quantum dots by a flow method.

[0013] Hydrofluoric acid, which is added as a surface treatment agent to improve the luminescence properties of quantum dots, is highly volatile and cannot be used in the flow method. Therefore, the inventors investigated whether it could be used as a surface treatment agent by reducing its volatility, and found that a salt of hydrogen fluoride and an amine having a specific structure does not volatilize in the flow method and can also repair defects on the quantum dot surface, leading to the completion of the present invention.

[0014] That is, the present invention provides an additive for producing quantum dots, which comprises a dialkylamine hydrohalide salt represented by the following general formula (1):

[0015] (In the formula, each R independently represents an optionally substituted branched alkyl group having 4 to 20 carbon atoms, and X represents a halogen atom.)

[0016] The present invention also provides a mixed solution for forming a shell, which contains at least the additive for producing quantum dots and a shell precursor.

[0017] The present invention also provides a method for producing core-shell quantum dots by a flow method, which includes a step of merging a flow path through which a solution containing a core flows, a flow path through which a solution containing the additive flows, and a flow path through which a shell precursor solution flows to obtain a mixed solution, and a step of supplying the obtained mixed solution to a tubular reactor and causing a reaction while the mixed solution passes through the reactor, and which continuously performs these steps.

[0018] The present invention also provides a method for producing core-shell quantum dots by a flow method, which includes a step of merging a flow path through which a solution containing a core flows and a flow path through which the shell-forming mixed solution flows to obtain a mixed solution, and a step of supplying the obtained mixed solution to a tubular reactor and causing a reaction while the mixed solution passes through the reactor, and which performs these steps continuously.

[0019] The present invention also provides a dialkylamine hydrofluoride salt represented by the following general formula (2):

[0020] (In the formula, each R' independently represents an optionally substituted branched alkyl group having 4 to 20 carbon atoms.)

[0021] According to the present invention, it is possible to provide an additive for quantum dot production that can repair defects on the quantum dot surface without volatilizing in a flow method, a mixed solution for shell formation that contains the additive, and a method for producing core-shell quantum dots by a flow method using the additive and the mixed solution.

[0022] FIG. 1 is a diagram showing a preferred embodiment of the method for producing core-shell quantum dots of the present invention. FIG. 2 is a diagram showing a preferred embodiment of the method for producing core-shell quantum dots of the present invention. FIG. 3 is a diagram showing a preferred embodiment of the method for producing core-shell quantum dots of the present invention. FIG. 4 is a diagram showing an emission spectrum of an octane dispersion of InP / ZnSe / ZnS quantum dots obtained in Example 1 and Comparative Example 1. FIG. 5 is a diagram showing the emission spectrum of an octane dispersion of di(2-ethylhexyl)amine hydrofluoride used in Example 1. １ 1H-NMR spectrum of di(2-ethylhexyl)amine hydrofluoride used in Example 1. １９ F-NMR spectrum.

[0023] The additive for producing quantum dots of the present invention (hereinafter referred to as "the additive of the present invention") comprises a dialkylamine hydrohalide salt represented by the following general formula (1).

[0024] (In the formula, each R independently represents an optionally substituted branched alkyl group having 4 to 20 carbon atoms, and X represents a halogen atom.)

[0025] The dialkylamine hydrohalide salt is difficult to volatilize due to its high boiling point, and is a liquid at room temperature. Therefore, when treating the surface of quantum dots by a flow method, gas does not remain in the flow channel of the reactor, and the liquid can be easily pumped. Therefore, the dialkylamine hydrohalide salt can be used as an additive in the synthesis of quantum dots by a flow method.

[0026] In the general formula (1), R each independently represents an optionally substituted branched-chain alkyl group having 4 to 20 carbon atoms, preferably 6 to 18 carbon atoms. Examples of such branched-chain alkyl groups include sec-butyl, isobutyl, tert-butyl, isopentyl, neopentyl, tert-pentyl, isohexyl, isoheptyl, isooctyl, 2-ethylbutyl, 2-ethyl-1-pentyl, 2-ethylhexyl, 3-methylhexyl, 2,4-dimethyl-3-hexyl, and isodecyl. These branched-chain alkyl groups may have a substituent, and examples of the substituent include a hydroxy group, a halogen atom, a cyano group, and an amino group. Among these, R is preferably an unsubstituted branched-chain alkyl group, and a 2-ethylhexyl group is particularly preferred, from the viewpoint of imparting physical properties such as a high boiling point when forming a salt with hydrogen halide without losing the function of the hydrogen halide as a surface treatment agent. That is, the dialkylamine of the dialkylamine hydrohalide salt represented by the general formula (1) is preferably di(2-ethylhexyl)amine.

[0027] In the general formula (1), X is a halogen element. Examples of the halogen element include fluorine, chlorine, bromine, and iodine. Among these, fluorine is preferable from the viewpoint of effectively repairing defects on the quantum dot surface. That is, the dialkylamine hydrohalide salt represented by the general formula (1) is preferably a dialkylamine hydrofluoride salt represented by the following general formula (2):

[0028] (In the formula, each R' independently represents an optionally substituted branched alkyl group having 4 to 20 carbon atoms.)

[0029] Examples of R' in the general formula (2) include the same groups as those of R in the general formula (1), and preferred groups are also the same.

[0030] The method for synthesizing the dialkylamine hydrohalide salt represented by general formula (1) is not particularly limited, and examples include a method in which hydrohalic acid, dialkylamine, and a reaction solvent are mixed together, followed by degassing the water. Alternatively, when the dialkylamine is liquid, the hydrohalic acid and dialkylamine may be mixed without using a solvent, followed by degassing the water. Examples of the reaction solvent that can be used include water, acetonitrile, propionitrile, isovaleronitrile, benzonitrile, acetone, methyl ethyl ketone, methyl isobutyl ketone, acetylacetone, acetophenone, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, methanol, ethanol, isopropanol, cyclohexanol, phenol, methyl acetate, ethyl acetate, isopropyl acetate, phenyl acetate, tetrahydrofuran, tetrahydropyran, diethyl ether, t-butyl methyl ether, cyclohexyl methyl ether, anisole, diphenyl ether, hexane, cyclohexane, benzene, toluene, 1-decene, 1-octadecene, and squalane.

[0031] The synthesis reaction between the hydrohalic acid and the dialkylamine may be carried out in the solvent described above, or, if the dialkylamine is liquid at room temperature, a solvent may not be used. If synthesis is carried out without a solvent, the reaction can be directly used in the flow-type production method for core-shell quantum dots described below without the need for a solvent removal process. When a solvent is used, for example, the hydrohalic acid and the solvent may be mixed to obtain a solution, and then the dialkylamine may be added; the dialkylamine and the solvent may be mixed, and then the hydrohalic acid may be added; or the hydrohalic acid and the dialkylamine may be added to the solvent. Among these, from the viewpoint of ease of handling of the hydrohalic acid, it is preferable to mix the hydrohalic acid and the solvent to obtain a solution, and then add the dialkylamine. The content of hydrogen halide in the solution using a solvent-free liquid dialkylamine-hydrohalide salt and a solvent is preferably 0.1% by mass or more and 50% by mass or less, particularly 1% by mass or more and 20% by mass or less. When a high-boiling point solvent such as 1-octadecene or squalane is used, the obtained dialkylamine-hydrohalide solution can be used as is in the flow-type production method for core-shell quantum dots described below without removing the solvent.

[0032] In the case of salts of hydrogen fluoride and amines, it is known that hydrogen fluoride is extremely stable up to about 10 equivalents relative to the amines, and hardly releases hydrogen fluoride even at 60°C. Therefore, the amount of dialkylamine used is preferably 0.1 mol to 5 mol, particularly 0.2 mol to 2 mol, per mol of hydrogen halide. The reaction at this time is completed instantaneously, but because reaction heat is generated, it is preferable to control the temperature to 0°C to 100°C, particularly 0°C to 80°C.

[0033] The dialkylamine hydrohalide salt has a boiling point of 300°C or higher and therefore does not easily volatilize even at high temperatures, and is also liquid at room temperature. Therefore, it is possible to perform a surface treatment that repairs defects on the quantum dot surface, and in the flow method, gas does not remain in a flow path such as a tube, and the liquid can be easily pumped. Therefore, it can be used as an additive in the synthesis of quantum dots by the flow method.

[0034] The method for producing core-shell quantum dots of the present invention is a flow method using the additive of the present invention. More specifically, a core-containing solution, a shell precursor solution, and a solution of the additive of the present invention are supplied to a tubular reactor and reacted while passing through to continuously produce core-shell quantum dots. Examples of the reactor include those shown in Figures 1, 2, and 3.

[0035] By using the additive of the present invention, liquid can be delivered by a liquid delivery means such as a pump without volatilization or solid precipitation in the flow channel, and defects on the quantum dot surface can be repaired by the flow method, allowing quantum dots with excellent luminescence properties to be efficiently obtained. The flow method is a synthesis method in which a reaction is carried out continuously, in which raw materials are continuously flowed into a tubular reactor to carry out the reaction and continuously obtain the product. In particular, the present invention refers to a synthesis method in which a reaction solution is continuously passed through a minute flow channel such as a microreactor or a tubular reactor to carry out the reaction. Below, a preferred embodiment of the quantum dot production method of the present invention is described.

[0036] (Core) In the method for producing core-shell quantum dots of the present invention, a solution in which cores are dispersed is used. Examples of the cores include cadmium-based quantum dots such as CdSe, CdTe, and CdS, and cadmium-free quantum dots such as InP, CuInS, and ZnTeSe. An embodiment in which the core is an InP-based quantum dot will be described in detail below.

[0037] Various phosphorus sources can be used for the core InP-based quantum dots depending on the chemical synthesis method employed, and examples include phosphine derivatives such as silylphosphine compounds and aminophosphine compounds, phosphine gas, etc. From the viewpoints of ease of obtaining quantum dots, ease of availability, and control of the particle size distribution of the obtained quantum dots, a silylphosphine compound represented by the following general formula (a) is preferred. The silylphosphine compound used as the phosphorus source is a tertiary compound, that is, a compound in which three silyl groups are bonded to a phosphorus atom.

[0038] (In the formula, each R independently represents an alkyl group having 1 to 5 carbon atoms or an aryl group having 6 to 10 carbon atoms.)

[0039] The alkyl group having 1 to 5 carbon atoms, represented by R in the general formula (a), is preferably a linear or branched alkyl group, and specific examples thereof include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, a sec-butyl group, a tert-butyl group, an isobutyl group, an n-amyl group, an isoamyl group, and a tert-amyl group.

[0040] Examples of the aryl group having 6 to 10 carbon atoms represented by R in the general formula (a) include a phenyl group, a tolyl group, an ethylphenyl group, a propylphenyl group, an isopropylphenyl group, a butylphenyl group, a sec-butylphenyl group, a tert-butylphenyl group, an isobutylphenyl group, a methylethylphenyl group, and a trimethylphenyl group.

[0041] These alkyl and aryl groups may have one or more substituents, and examples of the substituent for the alkyl group include a hydroxy group, a halogen atom, a cyano group, an amino group, etc., and examples of the substituent for the aryl group include an alkyl group having from 1 to 5 carbon atoms, an alkoxy group having from 1 to 5 carbon atoms, a hydroxy group, a halogen atom, a cyano group, an amino group, etc. When the aryl group is substituted with an alkyl group or an alkoxy group, the number of carbon atoms of the aryl group includes the number of carbon atoms of the alkyl group or alkoxy group.

[0042] In the general formula (a), the plurality of Rs may be the same or different. In addition, the three silyl groups (—SiR ３ As the silylphosphine compound represented by the general formula (a), those in which R is an alkyl group having from 1 to 4 carbon atoms or a phenyl group that is unsubstituted or substituted with an alkyl group having from 1 to 4 carbon atoms are preferred from the viewpoint of excellent reactivity with other molecules such as an indium source as a phosphorus source during the synthesis reaction, and a trimethylsilyl group is particularly preferred.

[0043] Various indium sources can be used for the core InP-based quantum dots depending on the chemical synthesis method employed. From the viewpoints of ease of obtaining quantum dots, ease of availability, and control of the particle size distribution of the obtained quantum dots, organic indium carboxylates are preferred, such as saturated aliphatic indium carboxylates such as indium acetate, indium formate, indium propionate, indium butyrate, indium valerate, indium caproate, indium enanthate, indium caprylate, indium pelargonate, indium caprate, indium laurate, indium myristate, indium palmitate, indium margarate, indium stearate, indium oleate, and indium 2-ethylhexanoate; and unsaturated indium carboxylates such as indium oleate and indium linoleate. In particular, from the viewpoints of availability and control of particle size distribution, it is preferable to use at least one selected from the group consisting of indium acetate, indium laurate, indium myristate, indium palmitate, indium stearate, and indium oleate, and it is most preferable to use an indium salt of a higher carboxylic acid having from 12 to 18 carbon atoms.

[0044] Examples of chemical synthesis methods for the InP-based quantum dots include known methods such as the sol-gel method (colloid method), hot soap method, reverse micelle method, solvothermal method, molecular precursor method, hydrothermal synthesis method, and flux method. Furthermore, batch or flow reaction may be used. The method for producing InP-based quantum dots in the present invention preferably involves mixing a phosphorus source and an indium source, reacting them at a temperature of 20°C or higher and 150°C or lower to obtain an InP quantum dot precursor, and then reacting the mixture at a temperature of 200°C or higher and 350°C or lower to obtain the InP-based quantum dots.

[0045] The reaction between the phosphorus source and the indium source is preferably carried out in an organic solvent from the viewpoints of reactivity and stability. Examples of organic solvents include nonpolar solvents from the viewpoint of the stability of the phosphorus source, the indium source, and the like. Examples of solvents that are preferred from the viewpoints of reactivity and stability include aliphatic hydrocarbons, unsaturated aliphatic hydrocarbons, aromatic hydrocarbons, trialkylphosphines, and trialkylphosphine oxides. Examples of aliphatic hydrocarbons include n-hexane, n-heptane, n-octane, n-nonane, n-decane, n-dodecane, n-hexadecane, and n-octadecane. Examples of unsaturated aliphatic hydrocarbons include 1-undecene, 1-dodecene, 1-tetradecene, 1-hexadecene, and 1-octadecene. Examples of aromatic hydrocarbons include benzene, toluene, xylene, and styrene. Examples of trialkylphosphines include triethylphosphine, tributylphosphine, tridecylphosphine, trihexylphosphine, trioctylphosphine, and tridodecylphosphine. Examples of the trialkylphosphine oxide include triethylphosphine oxide, tributylphosphine oxide, tridecylphosphine oxide, trihexylphosphine oxide, trioctylphosphine oxide, and tridodecylphosphine oxide.

[0046] The core InP-based quantum dots may be quantum dots (also referred to as composite quantum dots of In, P, and M) made of a composite compound containing, in addition to In and P, an element M other than phosphorus and indium. From the viewpoint of improving quantum yield, the element M is preferably at least one selected from the group consisting of Be, Mg, Ca, Mn, Cu, Zn, Cd, B, Al, Ga, N, As, Sb, and Bi. Representative examples of InP-based quantum dots containing the element M include InGaP, InZnP, InAlP, InGaAlP, InNP, InAsP, InPSb, and InPBi. To obtain InP-based quantum dots containing the element M, a solution containing a compound containing the element M may be added to the reaction solution when heating a solution containing an InP quantum dot precursor, or a solution containing an InP quantum dot precursor may be added to the reaction solution when heating a solution containing a compound containing the element M. The compound containing element M means, when element M is Be, Mg, Ca, Mn, Cu, Zn, Cd, B, Al, or Ga, a chloride, bromide, or iodide of element M, or a higher carboxylate salt having from 12 to 18 carbon atoms, and when the higher carboxylate salt is used, the higher carboxylate may be the same as or different from the carboxylic acid of the indium carboxylate used in the reaction. When element M is N, As, Sb, or Bi, a compound having three silyl groups or amino groups bonded to element M can be suitably used.

[0047] (Shell Precursor) The shell precursor used in the method for producing core-shell quantum dots of the present invention is a raw material solution composed of a compound containing a shell-forming element. The shell precursor solution preferably contains an element selected from Zn, S, Se, Te, Ga, P, N, Cd, Mg, and Al as the shell-forming element, and preferably contains at least Zn. Suitable coating compounds constituting the formed shell include ZnS, ZnSe, ZnSeS, ZnTe, ZnSeTe, ZnTeS, ZnO, ZnOS, ZnSeO, ZnTeO, etc.

[0048] As the metal source such as Zn in the shell precursor solution, it is preferable to use a halide or organic carboxylate of the metal. Examples of metal halides include zinc fluoride, zinc chloride, zinc bromide, and zinc iodide as the zinc source. As the organic carboxylate of the metal, it is particularly preferable to use a long-chain fatty acid salt having 12 to 18 carbon atoms in terms of particle size control, particle size distribution control, and improved quantum yield.

[0049] Preferred examples of sulfur sources include linear or branched long-chain alkanethiols having from 8 to 18 carbon atoms, such as dodecanethiol, and trialkylphosphine sulfide compounds having from 4 to 12 carbon atoms, such as trioctylphosphine sulfide. Preferred examples of sulfur itself include sulfur solutions prepared by dissolving powdered sulfur in high-boiling solvents having a diene structure, such as octadecene or oleylamine. Preferred examples of selenium sources include trialkylphosphine selenide compounds having from 4 to 12 carbon atoms, such as trioctylphosphine selenide. Preferred examples of tellurium sources include trialkylphosphine telluride compounds having from 4 to 12 carbon atoms, such as trioctylphosphine telluride.

[0050] (Method for producing core-shell quantum dots) The quantum dots obtained by the method for producing core-shell quantum dots of the present invention have a core-shell structure in which the InP-based quantum dots are used as a core and the core is covered with a shell (coating compound). When covering the core with the shell, defects on the core surface can be repaired by treating the core surface with the additive of the present invention, so that quantum dots with excellent luminescence properties can be efficiently obtained.

[0051] In the method for producing core-shell quantum dots of the present invention, a mixed solution is first obtained by mixing a solution containing the cores as raw materials, a shell precursor solution, and a solution containing the additive of the present invention. For example, the mixed solution can be obtained by merging a flow path through which the core-containing solution flows, a flow path through which the solution containing the additive of the present invention flows, and a flow path through which the shell precursor solution flows.

[0052] The order in which the raw material solutions are mixed is not particularly limited, as long as a mixed solution is obtained before supplying it to the reactor. Alternatively, a mixed solution for forming a shell containing the additive of the present invention and a shell precursor may be prepared in advance, and the mixed solution may be obtained by merging the flow path through which the mixed solution for forming the shell flows with the flow path through which the solution containing the core flows.

[0053] The respective solutions can be delivered using general-purpose delivery pumps such as syringe pumps, plunger pumps, diaphragm pumps, etc. Furthermore, it is preferable to provide a static mixer in the flow path after the joining of the solutions, since this allows for a more uniform mixed solution to be obtained.

[0054] The material constituting the flow path is not particularly limited, and examples thereof include stainless steel, Hastelloy, fluororesin, and glass, and may be selected from the viewpoints of heat resistance, flexibility, chemical resistance, etc. Tubes made of these materials may be connected or combined for use. The inner diameter of the flow path is preferably 0.1 to 50 mm, and particularly preferably 1 to 10 mm.

[0055] The resulting mixed solution is then supplied to a tubular reactor, and the reaction is carried out while the solution passes through. Examples of the tubular reactor that can be used include a microreactor in which the reaction is carried out in a microchannel having a diameter of several tens to several hundreds of micrometers engraved on a glass substrate or the like, and a tube reactor in which the reaction is carried out in a tube having a diameter of several millimeters to several tens of millimeters. Examples of materials for the reactor include those similar to those constituting the channel, and a material that allows the reaction to proceed appropriately can be selected from the viewpoints of heat resistance, chemical resistance, and the like. The material used for the channel leading up to obtaining the mixed solution may be the same as or different from the material for the reactor.

[0056] The flow rate of the mixed solution varies depending on the capacity of the reactor used, but is preferably, for example, 0.005 to 5000 mL / min, particularly 0.05 to 200 mL / min.

[0057] The reactor is heated for the surface treatment of the core and the formation of the shell. The heating temperature for the reaction varies depending on the material used to form the desired shell, but is preferably, for example, 80 to 360°C, particularly 100 to 340°C. Furthermore, from the viewpoint of producing quantum dots free of oxides and defects, this heating may be carried out in multiple stages by directly connecting multiple reactors. For example, it is preferable to heat the first reactor at 50 to 250°C, particularly 120 to 230°C, and then continuously heat the second reactor at 230 to 350°C, particularly 240 to 320°C. This allows for the efficient production of core-shell quantum dots having a shell free of oxides and defects.

[0058] The pressure in the reactor can be adjusted by placing a back pressure control device on the outlet side of the reactor, and is preferably set to, for example, 0.1 to 15 MPa, more preferably 0.2 to 5 MPa.

[0059] In the method for producing core-shell quantum dots of the present invention, the amount of the shell precursor used is preferably 0.5 mol to 100 mol, more preferably 4 mol to 50 mol, per mol of indium in the solution containing InP-based quantum dots, when a metal such as zinc is used. It is also preferable to use the sulfur source and selenium source in amounts corresponding to the amounts of the metals.

[0060] The amount of the additive of the present invention used is preferably 0.1 mol or more and 100 mol or less, and more preferably 0.5 mol or more and 50 mol or less, per mol of indium in the solution containing InP-based quantum dots.

[0061] In the method for producing core-shell quantum dots of the present invention, in order to improve the stability of the resulting quantum dots, a shell can be further formed on the surface of the core-shell quantum dots obtained as described above to produce multi-shell quantum dots. In this case, a flow path through which a solution containing the core-shell quantum dots flows can be joined with a flow path through which a solution containing the additive of the present invention flows and a flow path through which a shell precursor solution flows, or a flow path through which a shell-forming mixed solution flows, and the solutions can be mixed, and the surface treatment and shell formation reaction can be carried out under the same conditions as described above. To further improve stability and form a thick shell, the shell raw materials can be injected in multiple stages into a single reactor, as shown in FIG. 3. This allows for the efficient production of quantum dots with multi-shells that are free of oxides and defects.

[0062] The solution containing the core-shell quantum dots after the reaction is collected in a collection container. From the viewpoints of workability and safety, the content of the core-shell quantum dots in this case is preferably 0.1 to 50% by mass, and more preferably 1 to 30% by mass.

[0063] The obtained solution containing core-shell quantum dots may be used as is, depending on the intended use, or may be subjected to a purification treatment in order to purify by-products and unreacted impurities in the solution. This purification treatment can be performed by adding an organic solvent such as ethanol, methanol, 2-propanol, acetone, or acetonitrile to the solution containing core-shell quantum dots to precipitate the core-shell quantum dots, separating the solution containing impurities from the core-shell quantum dots, and dispersing the separated core-shell quantum dots in an organic solvent such as toluene or hexane. The solution containing impurities can be separated from the core-shell quantum dots by procedures such as centrifugation, decantation, and suction filtration.

[0064] The core-shell quantum dots obtained by the above method can be suitably used in single-electron transistors, security inks, quantum teleportation, lasers, solar cells, quantum computers, biomarkers, light-emitting diodes, display backlights, color filters, etc.

[0065] The present invention will be described in more detail below with reference to examples, but the present invention is not limited thereto. The properties in the examples were measured by the following methods. (1) Emission spectrum, maximum fluorescence wavelength, FWHM, and PLQY An absolute fluorescence quantum yield measurement device (Quantaurus-QY, manufactured by Hamamatsu Photonics K.K.) was used to measure the hexane dispersion of the obtained quantum dots under measurement conditions of an excitation wavelength of 400 nm and a measurement wavelength of 200 to 1100 nm. (2) NMR spectrum measurement The di(2-ethylhexyl)amine hydrofluoride salt synthesized as an additive was identified by dissolving the sample in CDCl ３ After dissolving in CDCl ３ The measurement was carried out using an NMR measuring device (manufactured by JEOL) under the following conditions. １ H-NMR measurement conditions: Observation frequency: 500 MHz, Number of accumulations: 16, Standard substance: tetramethylsilane (internal standard) １９ F-NMR measurement conditions: Observation frequency: 470 MHz, Number of accumulations: 16, Standard substance: trifluoromethylbenzene (external standard)

[0066] [Synthesis Example 1] <Synthesis of Yellow-Emitting InP Core> 12.0 g of indium myristate was added to 35.6 g of 1-octadecene, and the mixture was heated to 120°C under reduced pressure with stirring and degassed for 1.5 hours. After degassing, the mixture was returned to atmospheric pressure with nitrogen gas and cooled to 40°C, yielding a 1-octadecene solution of indium myristate. The resulting 1-octadecene solution of indium myristate was heated to 40°C under a nitrogen atmosphere, and 25.1 g of a trioctylphosphine solution containing 10% by mass of tris(trimethylsilyl)phosphine was added. The mixture was then maintained at 40°C and allowed to cool naturally to 20°C. This yielded a yellow solution containing an InP quantum dot precursor. Subsequently, the InP quantum dot precursor solution was heated to 250°C, yielding a solution containing a core for producing core-shell quantum dots with a yellow emission color. This solution was used in the examples and comparative examples by adding 108.3 g of trioctylphosphine and 106.9 g of 1-octadecene to give a concentration suitable for the flow method.

[0067] [Synthesis Example 2] <Synthesis of InP Core for Orange Light Emission> 14.3 g of indium myristate was added to 35.6 g of 1-octadecene, and the mixture was heated to 120°C under reduced pressure with stirring and degassed for 1.5 hours. After degassing, the mixture was returned to atmospheric pressure with nitrogen gas and cooled to 40°C, yielding a 1-octadecene solution of indium myristate. The resulting 1-octadecene solution of indium myristate was heated to 40°C under a nitrogen atmosphere, and 25.1 g of a trioctylphosphine solution containing 10% by mass of tris(trimethylsilyl)phosphine was added. The mixture was then maintained at 40°C and allowed to cool naturally to 20°C. This yielded a yellow solution containing InP quantum dot precursors. Subsequently, the InP quantum dot precursor solution was heated to 280°C, yielding a solution containing cores for producing core-shell quantum dots with an orange emission color. This solution was used in the examples by adding 108.3 g of trioctylphosphine and 106.9 g of 1-octadecene to it so that the concentration was suitable for the flow method.

[0068] [Synthesis Example 3] <Synthesis of Green-Emitting InP Core> 12.0 g of indium myristate, 1.6 g of zinc myristate, and 0.6 g of zinc acetate were added to 142.5 g of 1-octadecene, and the mixture was heated to 120°C under reduced pressure with stirring and degassed for 1.5 hours. After degassing, the mixture was returned to atmospheric pressure with nitrogen gas and cooled to 40°C, yielding a 1-octadecene solution of indium zinc myristate. The obtained 1-octadecene solution of indium zinc myristate was heated to 40°C under a nitrogen atmosphere, and 25.1 g of a trioctylphosphine solution containing 10% by mass of tris(trimethylsilyl)phosphine was added. The mixture was then maintained at 40°C and allowed to cool naturally to 20°C. This yielded a yellow solution containing InP quantum dot precursors. Subsequently, the InP quantum dot precursor solution was heated to 250°C, yielding a solution containing cores for producing core-shell quantum dots with a green emission color. This solution was used in the examples by adding 108.3 g of trioctylphosphine to it so that the concentration was suitable for the flow method.

[0069] [Synthesis Example 4] <Synthesis of Red-Emitting InP Core> 6.4 g of indium myristate was added to 14.3 g of 1-octadecene, and the mixture was heated to 120°C under reduced pressure with stirring and degassed for 1.5 hours. After degassing, the mixture was returned to atmospheric pressure with nitrogen gas and cooled to 40°C, yielding a 1-octadecene solution of indium myristate. The resulting 1-octadecene solution of indium myristate was heated to 40°C under a nitrogen atmosphere, and 10.0 g of a trioctylphosphine solution containing 10% by mass of tris(trimethylsilyl)phosphine was added. The mixture was then maintained at 40°C and allowed to cool naturally to 20°C. This yielded a yellow solution containing InP quantum dot precursors. Subsequently, the InP quantum dot precursor solution was heated to 280°C to yield a base InP quantum dot reaction solution. In a separate reaction vessel, 9.5 g of indium myristate was added to 21.4 g of 1-octadecene, and the mixture was heated to 120 ° C. under reduced pressure with stirring and degassed for 1.5 hours. After degassing, the mixture was returned to atmospheric pressure with nitrogen gas and cooled to 40 ° C., yielding a 1-octadecene solution of indium myristate. The resulting 1-octadecene solution of indium myristate was heated to 40 ° C. under a nitrogen atmosphere, and 15.0 g of a trioctylphosphine solution containing 10% by mass of tris(trimethylsilyl)phosphine was added. The mixture was then maintained at 40 ° C. for 10 minutes and allowed to cool naturally to 20 ° C. This yielded a yellow solution containing InP quantum dot precursors. The previously prepared base InP quantum dot reaction solution was reheated to 260 ° C., and a newly prepared InP quantum dot precursor solution was added over 60 minutes, yielding a solution containing cores for producing core-shell quantum dots with a red emission color. This solution was used in the examples by adding 108.3 g of trioctylphosphine and 106.9 g of 1-octadecene to it so that the concentration was suitable for the flow method.

[0070] [Example 1] <Additive for quantum dot production> 20.84 g of a 48% by mass aqueous solution of hydrogen fluoride was added to 40.0 g of di(2-ethylhexyl)amine, and the mixture was heated to 100°C under reduced pressure with stirring and degassed for 3 hours. After degassing, the pressure was returned to atmospheric pressure with nitrogen gas and cooled to room temperature, yielding liquid di(2-ethylhexyl)amine hydrofluoride (hereinafter also referred to as HF additive). The obtained liquid contained 10% by mass of hydrogen fluoride and was colorless and clear at room temperature. In addition, １ The results of measuring the H-NMR spectrum are shown in FIG. １９ The results of measuring the F-NMR spectrum are shown in FIG.

[0071] <ZnSe Shell Precursor> 33.03 g of zinc acetate and 101.69 g of oleic acid were added to 439.02 g of oleylamine, and the mixture was heated to 120 ° C. under reduced pressure while stirring and degassed for 3 hours. After degassing, the mixture was returned to atmospheric pressure with nitrogen gas and cooled to 60 ° C., and 98.1 g of a separately prepared ethanol solution containing 50% by mass of zinc chloride was added. The mixture was heated to 80 ° C. under reduced pressure while stirring and degassed for 1 hour to distill off the ethanol. After degassing, the mixture was returned to atmospheric pressure with nitrogen gas and cooled to 60 ° C. 78.56 g of trioctylphosphine selenide was added, and the mixture was heated to 80 ° C. under reduced pressure while stirring and degassed for 1 hour, and then allowed to cool naturally to produce a yellowish ZnSe shell precursor solution. The resulting solution solidified and lost fluidity at around 20 ° C., so it was heated to 40 ° C. when used in the subsequent flow reaction.

[0072] <ZnS Shell Precursor> 33.03 g of zinc acetate and 101.69 g of oleic acid were added to 80.51 g of trioctylphosphine, and the mixture was heated to 120°C while stirring under reduced pressure and degassed for 3 hours. After degassing, the mixture was returned to atmospheric pressure with nitrogen gas and cooled to 60°C. 75.12 g of trioctylphosphine sulfide was added, and the mixture was heated to 80°C while stirring under reduced pressure and degassed for 1 hour, and then allowed to cool naturally to produce a ZnS shell precursor solution. The resulting solution was clear and colorless at room temperature.

[0073] <Shell Formation by Flow Method> The solution containing the core for producing yellow-emitting core-shell quantum dots obtained in Synthesis Example 1, the HF additive, the ZnSe shell precursor solution, and the ZnS shell precursor solution were each held in a screw-cap bottle and connected to a liquid-transfer pump via a PTFE tube. A stainless steel tube with an outer diameter of 3.2 mm was connected to the outlet side of the pump, and quantum dot surface treatment and shell formation were carried out using a coiled tube reactor, as shown in Figure 1, in which the coiled tube was heated by a heater. The lengths of three reactors 1, 2, and 3 were adjusted to a capacity of 10 ml each, and each was connected in series. The InP core quantum dot solution and the ZnSe shell precursor solution were delivered at 0.7 ml / min and 0.7 ml / min, respectively, using the respective pumps. After mixing them before reactor 1, the HF additive delivered at 0.1 ml / min was added, and the mixture was sent to reactor 1 and heated to 200 °C, thereby removing the surface oxide of the InP quantum dots and capping the surface dangling bonds with zinc chloride. The reaction solution was sent directly to reactor 2, which was set at 280°C, where quantum dots having an InP / ZnSe core-shell structure were synthesized. Next, the reaction solution was mixed with an HF additive and a ZnS shell precursor solution, and sent to reactor 3, which was set at 280°C, where quantum dots having an InP / ZnSe / ZnS multi-shell structure were synthesized. The reaction solution was cooled by flowing through a tube connected to a water tank, then passed through a back pressure control device set at 0.3 MPa, and collected in a screw-cap bottle installed at the tube outlet.

[0074] <Washing Step> 10 g of hexane was added to 10 g of the obtained dispersion, followed by the addition of 20 g of ethanol and stirring. The InP / ZnSe / ZnS quantum dots were recovered as a precipitate by centrifugation. The recovered InP / ZnSe / ZnS quantum dots were suspended in 10 g of hexane to obtain a hexane dispersion of InP / ZnSe / ZnS quantum dots. Another 10 g of ethanol was added to this dispersion, followed by stirring and centrifuging to obtain a precipitate of InP / ZnSe / ZnS quantum dots. The recovered InP / ZnSe / ZnS quantum dots were suspended in 2 g of octane to obtain an octane dispersion of purified InP / ZnSe / ZnS quantum dots.

[0075] Examples 2 to 13 In Example 1, InP / ZnSe / ZnS quantum dots were synthesized by a flow method under the same conditions as in Example 1, except that the liquid supply rates of the solution containing the core for producing core-shell quantum dots corresponding to the desired emission color (core solution), the ZnSe shell precursor solution (ZnSe precursor), the ZnS shell precursor solution (ZnS precursor), and the HF additive (additive) were set to the conditions shown in Table 1, and the set temperatures of reactors 1, 2, and 3 were changed as shown in Table 1. The remaining conditions were the same as in Example 1, and the InP / ZnSe / ZnS quantum dots were evaluated after undergoing a cleaning process.

[0076] Examples 14 and 15 An octane dispersion of InP / ZnSe / ZnS quantum dots was obtained by the same procedure as in Example 1, except that oleylamine was used instead of trioctylphosphine when preparing the ZnS precursor solution in Example 1 and the liquid delivery conditions were set as shown in Table 1.

[0077] Examples 16-17 An octane dispersion of InP / ZnSe / ZnS quantum dots was obtained by the same procedure as in Example 1, except that dodecanethiol was used as the sulfur source instead of the ZnS precursor solution in Example 1 and the liquid delivery conditions were set as shown in Table 1.

[0078] Comparative Example 1 InP / ZnSe / ZnS quantum dots were continuously synthesized by carrying out the reaction under the same conditions as in Example 1, except that no HF additive was added, and the resultant was evaluated after a cleaning process.

[0079] Comparative Examples 2 to 8 The additives for quantum dot production in Example 1 were synthesized using oleylamine, 3-(undecyloxy)propylamine, dioctylamine, dibenzylamine, dicyclohexylamine, trioctylamine, and tri(2-ethylhexyl)amine instead of di(2-ethylhexyl)amine. However, the obtained amine hydrofluorides were all solids and could not be pumped, and therefore could not be used for shell formation by the flow method.

[0080] Comparative Example 9 A commercially available reagent triethylamine-hydrofluoride salt was diluted with triethylamine to a hydrogen fluoride content of 10% by weight and used instead of the HF additive in Example 1. However, the gasification of triethylamine in reactor 1 caused a rapid expansion of the volume, making it impossible to control the residence time in the reactor, and therefore, a shell could not be formed thereafter.

[0081] Comparative Example 10 A commercially available reagent pyridine hydrofluoride salt was diluted with pyridine to a hydrogen fluoride content of 10% by weight and used instead of the HF additive in Example 1. Gasification occurred in reactor 1, and the volume increased significantly, making it impossible to control the residence time in the reactor, and subsequent shell formation was not possible.

[0082] [Comparative Example 11] Hydrofluoric acid was diluted with acetone to a hydrogen fluoride content of 10 wt %, and used instead of the HF additive in Example 1. Gasification occurred in reactor 1, and the volume increased significantly, making it impossible to control the residence time in the reactor, and subsequent shell formation was not possible.

[0083]

[0084] The results in Table 1 show that by using the additive of the present invention, core-shell quantum dots with high quantum yields can be produced by the flow method. Furthermore, comparing Example 1, which uses the additive of the present invention, with Comparative Example 1, which does not, as shown in Figure 4, a shoulder-shaped emission spectrum is observed in the shorter wavelength range of around 480 to 530 nm in the emission spectrum of Comparative Example 1, which does not use the additive of the present invention. This indicates that some quantum dots do not have a shell that can be formed properly because the surface of the InP core is not clean.

Claims

1. An additive for quantum dot production comprising a dialkylamine hydrohalide salt represented by the following general formula (1): (In the formula, each R independently represents an optionally substituted branched alkyl group having 4 to 20 carbon atoms, and X represents a halogen atom.) 2. The additive for producing quantum dots according to claim 1, wherein X is fluorine, chlorine or bromine.

3. The additive for producing quantum dots according to claim 1, which is used for surface treatment of quantum dots.

4. A mixed solution for forming a shell, comprising at least the additive for producing quantum dots according to any one of claims 1 to 3 and a shell precursor.

5. The shell-forming mixed solution according to claim 4, wherein the shell precursor contains at least one element selected from the group consisting of Zn, S, Se, Te, Ga, P, N, Cd, Mg and Al as a shell-forming element.

6. A method for producing core-shell quantum dots by a flow method, comprising the steps of: obtaining a mixed solution by merging a flow path through which a solution containing a core flows, a flow path through which a solution containing the additive according to any one of claims 1 to 3 flows, and a flow path through which a shell precursor solution flows; and supplying the obtained mixed solution to a tubular reactor and carrying out a reaction while passing the mixed solution through the reactor, wherein these steps are carried out continuously.

7. A method for producing core-shell quantum dots by a flow method, comprising the steps of: obtaining a mixed solution by merging a flow path through which a solution containing a core flows and a flow path through which the mixed solution for forming a shell described in claim 4 flows; and supplying the obtained mixed solution to a tubular reactor and carrying out a reaction while passing the mixed solution through the reactor; and carrying out these steps continuously.

8. Dialkylamine hydrofluoride represented by the following general formula (2): (In the formula, each R' independently represents an optionally substituted branched alkyl group having 4 to 20 carbon atoms.)

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