Method for manufacturing quantum dots and quantum dots

Abstract

Disclosed is a quantum dot production method and quantum dot that can improve EQE. The quantum dot production method of the present application is characterized by comprising a step of producing a core and a step of coating a shell on the surface of the core. The step of coating the shell is performed by mixing an acidic compound and a zinc halide compound in a shell material. The step of coating the shell is preferably divided into a first half and a second half. The shell is coated in the first half using a shell material mixed with the acidic compound but not mixed with the zinc halide compound, and the shell is coated in the second half using a shell material mixed with both the acidic compound and the zinc halide compound.

Description

TECHNICAL FIELD

[0001] The present application relates to a manufacturing method of a quantum dot of a core-shell structure not containing cadmium, and a quantum dot. BACKGROUND

[0002] A quantum dot emits fluorescence, and is also called a fluorescent nanoparticle because of its size of nanometer order, a semiconductor nanoparticle because of its composition derived from a semiconductor material, or a nanocrystal because of its structure having a specific crystal structure.

[0003] As examples of indicating the performance of a quantum dot, a fluorescence quantum yield (QY) and an external quantum efficiency (EQE) can be listed.

[0004] As a use of a display using a quantum dot, in a case where photoluminescence (PL) is adopted as a light emitting principle, a method of using a blue LED as excitation light in a backlight and using a quantum dot to convert to green light and red light is adopted. On the other hand, for example, in a case where electroluminescence (EL) is adopted as a light emitting principle, or in a case where all of 3 primary colors are made to emit light by a quantum dot by another method, or the like, a quantum dot of blue fluorescence is required.

[0005] As a quantum dot of blue, as a representative example, a cadmium selenide (CdSe)-based quantum dot using cadmium (Cd) can be listed. However, Cd is internationally restricted, and there is a high barrier in the practical use of a material using a quantum dot of CdSe.

[0006] On the other hand, a quantum dot not using Cd is also under study and development. For example, development of a chalcopyrite-based quantum dot such as CuInS2, AgInS2, an indium phosphide (InP)-based quantum dot, and the like is being carried out (for example, refer to Patent Literature 1). However, the quantum dots developed at present generally have a relatively wide fluorescence half value width, and are not suitable as a quantum dot of blue fluorescence.

[0007] Further, a direct synthesis method of ZnSe using diphenylphosphine selenide which is considered to be relatively high in reactivity with an organic zinc compound is described in detail in Non-Patent Literature 1 below, but is not suitable as a quantum dot of blue fluorescence.

[0008] Further, a synthesis method of ZnSe in an aqueous system is also reported in Non-Patent Literature 2 below. Although the reaction is carried out at low temperature, the fluorescence half value width is 30 nm or more, which is slightly wide, and the fluorescence wavelength is insufficient at 430 nm or more, and thus it is not suitable to use it as a substitute for a conventional blue LED to achieve high color gamut.

[0009] Furthermore, Non-Patent Document 3 below reports a method for synthesizing ZnSe-based quantum dots by cation exchange of copper with zinc (Zn) after forming a precursor such as copper selenide (CuSe). However, the copper selenide particles used as the precursor are as large as 15 nm, and the reaction conditions for cation exchange of copper and zinc are not optimal, resulting in copper residues in the ZnSe-based quantum dots after cation exchange. As discussed in this invention, ZnSe-based quantum dots with copper residue cannot emit light. Alternatively, even if light is emitted in the case of copper residue, it is due to defects, resulting in a half-width of the emission spectrum greater than 30 nm. The particle size of the copper selenide precursor also affects this copper residue; larger particles easily lead to copper residue after cation exchange, and even if XRD confirms it as ZnSe, it often fails to emit light due to trace amounts of copper residue. Therefore, Non-Patent Document 3 cites examples of copper residues resulting from the inability to control the particle size of the precursor and optimize the cation exchange method. Therefore, blue fluorescence is not reported. There are many reported examples of using the cation exchange method, but for the reasons mentioned above, there are no reported examples of strong luminescence.

[0010] Existing technical documents

[0011] Patent documents

[0012] Patent Document 1: International Publication No. 2007 / 060889

[0013] Non-patent literature

[0014] Non-patent literature 1: Organic Electronics 15(2014)126-131

[0015] Non-patent literature 2: Materials Science and Engineering C 64(2016)167-172

[0016] Non-patent literature 3: J. Am. Chem. Soc. (2015) 137 29 9315-9323 Summary of the Invention

[0017] The problem that the invention aims to solve

[0018] In addition, the external quantum efficiency is calculated by the following equation (1).

[0019] External quantum efficiency (EQE) = carrier balance × luminescent exciton generation efficiency × luminescence quantum efficiency (fluorescence quantum yield (QY)) × light extraction efficiency (Equation 1)

[0020] Here, since the light extraction efficiency is generally 0.2 to 0.3, when the carrier balance, the generation efficiency of luminescent excitons, and the fluorescence quantum yield are all 1 (100%), the theoretical external quantum yield is 20 to 30%. Therefore, quantum dots with high QY are needed to obtain high EQE.

[0021] Furthermore, when quantum dots are too close to each other, Foster resonance energy transfer (FRET) occurs. (Resonance Energy Transfer). As a result, EQE decreases. Therefore, by employing a core-shell structure that surrounds the nucleus, the distance between the nuclei can be physically separated, thus reducing FRET.

[0022] However, it has not yet been possible to mass-produce quantum dots with high QY that can coat the core with a shell of approximately uniform thickness. For example, as the shell thickness increases, the particle shape deteriorates, and the QY decreases accordingly.

[0023] Therefore, the present invention has been made in view of the aspects involved, and its object is to provide a method for manufacturing quantum dots that can improve EQE, as well as quantum dots.

[0024] Solution for solving the problem

[0025] The method for manufacturing quantum dots according to the present invention is characterized by comprising: a step of generating a core and a step of coating the surface of the core with a shell, wherein in the step of coating the shell, an acidic compound and a zinc halide compound are mixed in the shell raw material.

[0026] Preferably, the present invention involves coating the surface of a core containing at least Zn and Se with ZnS.

[0027] Preferably, the shell coating process is divided into at least a first half and a second half. The shell is coated multiple times by using a shell material mixed with the acidic compound but not the zinc halide compound in the first half, and using a shell material mixed with both the acidic compound and the zinc halide compound in the second half.

[0028] Preferably, the present invention uses at least one of hydrogen chloride, hydrogen bromide, or trifluoroacetic acid as the acidic compound.

[0029] Preferably, the present invention uses at least one of zinc chloride or zinc bromide as the zinc halide compound.

[0030] Preferably, the core of the present invention is composed of ZnSe or ZnSeTe.

[0031] The quantum dot of the present invention has a core and a shell covering the surface of the core, characterized in that it contains a halogen element and has an external quantum efficiency of 7% or more.

[0032] The quantum dot of the present invention has a core and a shell covering the surface of the core, characterized in that it contains a halogen element and has a fluorescence quantum yield of 70% or more.

[0033] The quantum dot of the present invention has a core and a shell covering the surface of the core, characterized in that the shell is formed by mixing an acidic compound and a zinc halide compound in a shell raw material.

[0034] Preferably, in this invention, the core contains at least Zn and Se, and the shell is composed of ZnS.

[0035] Invention Effects

[0036] According to the quantum dot manufacturing method of the present invention, it is possible to synthesize quantum dots with good particle shape, improve QY, and even obtain high EQE. Attached Figure Description

[0037] exist Figure 1 middle, Figure 1 A and Figure 1 B is a schematic diagram of quantum dots in an embodiment of the present invention.

[0038] Figure 2 This is a schematic diagram of an LED device using quantum dots according to an embodiment of the present invention.

[0039] Figure 3 This is a longitudinal sectional view of a display device using the LED device in an embodiment of the present invention.

[0040] Figure 4 This is a flowchart illustrating the manufacturing process of quantum dots in embodiments of the present invention.

[0041] Figure 5 The photoluminescence (PL) spectrum of Example 1 is shown.

[0042] Figure 6 This is the absorption spectrum of Example 1.

[0043] Figure 7 The X-ray diffraction (XRD) spectrum is shown in Example 1.

[0044] Figure 8 This is a table showing the measurement results of each quantum dot in Examples 1 to 7.

[0045] exist Figure 9 middle,Figure 9 A is a photograph of the TEM-EDX analysis results from Comparative Example 1. Figure 9 B is a photograph of the TEM-EDX analysis results from Example 1.

[0046] exist Figure 10 middle, Figure 10 A is Figure 9 A partial schematic diagram. Figure 10 B is Figure 9 Partial diagram of B. Detailed Implementation

[0047] Hereinafter, one embodiment of the present invention (hereinafter referred to as "Embodiment") will be described in detail. Furthermore, the present invention is not limited to the following embodiment, and various modifications can be made within its scope.

[0048] Figure 1 A, Figure 1 B is a schematic diagram of the quantum dot in this embodiment. Figure 1 A and Figure 1 Quantum dot 5 shown in B is a cadmium (Cd)-free nanocrystal. "Nanocrystal" refers to nanoparticles with a particle size of approximately a few nm to tens of nm. In this embodiment, multiple quantum dots 5 can be generated with a substantially uniform particle size.

[0049] In this embodiment, the quantum dot 5 has a core-shell structure consisting of a core 5a and a shell 5b covering the surface of the core 5a. The core 5a is preferably a nanocrystal containing at least zinc (Zn) and selenium (Se). In addition, the core 5a may also contain tellurium (Te) and sulfur (S). However, the core 5a is preferably free of cadmium (Cd) and indium (In).

[0050] Furthermore, the shell 5b covering the surface of the core 5a is also preferably free of cadmium (Cd) and indium (In), similar to the core 5a. In this embodiment, the shell 5b contains a large amount of zinc (Zn). Specifically, the shell 5b is preferably composed of zinc sulfide (ZnS), zinc selenide (ZnSe), or zinc selenide sulfide (ZnSeS). ZnS is preferred. Alternatively, the shell 5b may be in a state of solid solution dissolved on the surface of the core 5a. In this embodiment, by employing a core-shell structure, it is expected that the fluorescence quantum yield (QY) will be further improved while maintaining a narrow fluorescence half-width.

[0051] In this embodiment, the quantum dot 5 can completely cover the surface of the core 5a with a shell 5b of ZnS or the like, with a predetermined thickness. Furthermore, an intermediate layer can be sandwiched between the core 5a and the shell 5b. For example, this intermediate layer can be the first layer of the shell, meaning the shell 5b can have a structure of two or more layers. As an example, a shell 5b with a laminated structure composed of ZnSeS / ZnS can be shown.

[0052] Quantum dot 5 can be like Figure 1 As shown in Figure A, the cross-section is circular, or it can be like... Figure 1 As shown in B, the cross-section is polygonal. In the case of polygons, for example, approximately rectangular or approximately triangular shapes are suitable. Although in this embodiment, the core 5a of the quantum dot 5 preferably contains at least Zn and Se, the core 5a constituting the quantum dot 5 is prone to form a polyhedron (e.g., approximately cubic) due to crystal growth. That is, in this embodiment, the quantum dot 5 can be formed into a good shape with consistent particle shape, rather than an amorphous shape. In this embodiment, the shell 5b can be formed on the outer periphery of the core 5a with an approximately fixed thickness. Although not limited, the thickness of the shell 5b can be formed to about 0.5 mm to 3 mm, preferably 1 mm to 2.5 mm. This is because, as explained in the manufacturing method described later, an acidic compound is mixed into the shell material. Furthermore, although in this embodiment, a zinc halide compound is mixed into the shell material, this can improve the QY (quantum hysteresis).

[0053] Preferably, such as Figure 1 A and Figure 1 As shown in Figure B, multiple organic ligands 11 are coordinated on the surface of quantum dot 5. This suppresses the aggregation of quantum dots 5 and allows them to exhibit the desired optical properties. Furthermore, the stability of the quantum dot luminescence properties can be significantly improved by adding amine or thiol-based ligands. There are no particular limitations on the ligands that can be used in the reaction, but the following ligands are listed as representative examples.

[0054] (1) Aliphatic primary amine series

[0055] Oleamine: C 18 H 35 NH2, stearyl (octadecyl)amine: C 18 H 37 NH2, dodecyl (lauryl)amine: C 12 H 25 NH2, decylamine: C 10 H 21 NH2, Octylamine: C8H 17 NH2

[0056] (2) Fatty acid system

[0057] Oleic acid: C 17 H 33 COOH, stearic acid: C 17 H 35 COOH, palmitic acid: C 15 H 31 COOH, myristic acid: C 13 H 27 COOH, Lauric acid: C 11H 23 COOH, Decanoic acid: C9H 19 COOH, Caprylic acid: C7H 15 COOH

[0058] (3) Thiol series

[0059] Octadecyl mercaptan: C 18 H 37 SH, hexadecyl mercaptan: C 16 H 33 SH, tetradecyl mercaptan: C 14 H 29 SH, dodecanethiol: C 12 H 25 SH, Decathiol: C 10 H 21 SH, Octethiol: C8H 17 SH

[0060] (4) Phosphine series

[0061] Trioctylphosphine: (C8H) 17 3P, triphenylphosphine: (C6H5)3P, tributylphosphine: (C4H9)3P

[0062] (5) Phosphine oxide

[0063] Trioctylphosphine oxide: (C8H) 17 3P=O, Triphenylphosphine oxide: (C6H5)3P=O, Tributylphosphine oxide: (C4H9)3P=O

[0064] (6) Alcohol series

[0065] Oil alcohol: C 18 H 36 O

[0066] Preferably, inorganic and organic ligands are mixed for coordination. This allows for better suppression of quantum dot surface defects and the expression of higher optical properties. The ligands are not particularly limited, but halogens such as F, Cl, Br, and I are representative examples.

[0067] In this embodiment, elemental analysis of the quantum dot 5 using energy dispersive X-ray spectroscopy (EDX) revealed the presence of halogens in addition to Zn, Se, and S. The preferred halogens are chlorine (Cl) or bromine (Br).

[0068] While the content of halogens is not limited, it is sufficiently low compared to Zn, Se, and S, with a halogen content of approximately 0.01 atom% to 5 atom%. Preferably, the halogen content is between 0.5 atom% and 2 atom%. "Atom%" refers to the percentage when the total number of atoms constituting quantum dot 5 is set to 100. The amount of halogens can be determined by EDX analysis.

[0069] In the quantum dot light-emitting diode (QLED) using the quantum dot 5 of this embodiment, the external quantum efficiency (EQE) can be effectively improved. In this embodiment, the EQE can be 7% or more. Preferably, the EQE can be 9% or more, more preferably 9.5% or more, further preferably 10% or more, and even more preferably 10.5% or more. The EQE can be evaluated using an LED measuring device and obtained as the maximum value.

[0070] Furthermore, as shown in Equation 1 above, EQE can be improved by increasing QY. Therefore, in order to obtain a high EQE, it is preferable to increase the QY of quantum dot 5. In this embodiment, QY can be 70% or more, preferably 75% or more, more preferably 80% or more, further preferably 85% or more, even more preferably 90% or more, and most preferably 95% or more.

[0071] In this embodiment, the quantum dot 5 is preferably characterized by a fluorescence half-width of 20 nm or less. "Fluorescence half-width" refers to the full width at half maximum (WWHM) of the fluorescence wavelength at half the peak intensity of the fluorescence spectrum. Furthermore, a fluorescence half-width of 15 nm or less is more preferable. This narrowing of the fluorescence half-width in this embodiment enables an improvement in color gamut.

[0072] In this embodiment, as described later, the reaction system for synthesizing quantum dots 5 involves a metal-exchange reaction of the precursor after synthesis using a copper chalcogenide as a precursor. By fabricating quantum dots 5 based on this indirect synthesis reaction, the fluorescence half-width can be narrowed.

[0073] Furthermore, in this embodiment, the fluorescence lifetime of quantum dot 5 can be set to 50 ns or less. Alternatively, in this embodiment, the fluorescence lifetime can be adjusted to 40 ns or less, 30 ns or less, or even 20 ns or less. Thus, in this embodiment, the fluorescence lifetime can be shortened, but it can also be extended to around 50 ns, and the fluorescence lifetime can be adjusted according to the intended use.

[0074] In this embodiment, the fluorescence wavelength can be freely controlled between approximately 410 nm and 470 nm. Specifically, the quantum dot 5 in this embodiment is a solid solution based on ZnSe. In this embodiment, the fluorescence wavelength can be controlled by adjusting the particle size and composition of the quantum dot 5. In this embodiment, a fluorescence wavelength of 430 nm or higher is preferred, and 440 nm or higher is more preferable.

[0075] Thus, in the quantum dot 5 of this embodiment, the fluorescence wavelength can be controlled to be blue.

[0076] Next, the manufacturing method of the quantum dot 5 according to this embodiment will be described. The manufacturing method of the quantum dot 5 in this embodiment is characterized by including: a process of generating a core and a process of coating a shell on the surface of the core, wherein the shell coating process involves mixing an acidic compound and a zinc halide compound in the shell raw material.

[0077] <Methods for Nuclear Synthesis>

[0078] The method for synthesizing the nucleus will be described. First, in this embodiment, a copper chalcogenide precursor is synthesized from an organocopper compound or an inorganic copper compound and an organochalcogen compound. Specifically, the copper chalcogenide precursor is preferably Cu2Se, Cu2SeS, Cu2SeTe, or Cu2SeTeS.

[0079] In this embodiment, the Cu raw material is not particularly limited, but for example, the following organic copper reagents and inorganic copper reagents can be used. That is, as acetates, copper acetate (I): Cu(OAc) and copper acetate (II): Cu(OAc)2 can be used; as fatty acid salts, copper stearate: Cu(OC(=O)C 17 H 35 2. Copper oleate: Cu(OC(=O)C 17 H 33 2. Copper myristate: Cu(OC(=O)C 13 H 27 2. Copper dodecanoate: Cu(OC(=O)C 11 H 23 2. Copper acetylacetone: Cu(acac)2. As a halide, it can be used as a monovalent or divalent compound, such as copper chloride (I): CuCl, copper chloride (II): CuCl2, copper bromide (I): CuBr, copper bromide (II): CuBr2, copper iodide (I): CuI, copper iodide (II): CuI2, etc.

[0080] In this embodiment, an organoselenium compound (organochalcogenide) is used as the Se raw material. The structure of the compound is not particularly limited, but for example, trioctylphosphine selenide obtained by dissolving Se in trioctylphosphine can be used: (C8H... 17 )3P=Se or tributylphosphine selenide obtained by dissolving Se in tributylphosphine: (C4H9)3P=Se, etc. Alternatively, a solution obtained by dissolving Se in a high-boiling-point solvent of a long-chain hydrocarbon such as octadecene at high temperature (Se-ODE), or a solution obtained by dissolving Se in a mixture of oleylamine and dodecyl mercaptan (Se-DDT / OLAm), etc., can be used.

[0081] In this embodiment, Te uses an organotelluric compound (organochalcogenide) as a raw material. The structure of the compound is not particularly limited, but for example, trioctylphosphine telluride, obtained by dissolving Te in trioctylphosphine, can be used: (C8H 17 Tributylphosphine telluride (C4H9)3P=Te, or tributylphosphine telluride obtained by dissolving Te in tributylphosphine, etc. In addition, dialkyl ditelluride (C6H5)2Te2 and other dialkyl ditelluride (R2Te2) can also be used.

[0082] In this embodiment, an organocopper compound or an inorganic copper compound is mixed with and dissolved in an organochalcogenide compound. Octadene can be used as a solvent as a high-boiling-point saturated hydrocarbon or unsaturated hydrocarbon. In addition, tert-butylbenzene can be used as an aromatic high-boiling-point solvent, butyl butyrate (C4H9COOC4H9) and benzyl butyrate (C6H5CH2COOC4H9) can be used as high-boiling-point ester solvents, but aliphatic amine compounds or fatty acid compounds, aliphatic phosphorus compounds, or mixtures thereof can also be used as solvents.

[0083] At this point, the reaction temperature is set in the range of 140°C to 250°C to synthesize the copper chalcogenide precursor. Furthermore, the reaction temperature is preferably lower, between 140°C and 220°C, and more preferably even lower, between 140°C and 200°C.

[0084] Furthermore, in this embodiment, the reaction method is not particularly limited, but in order to obtain quantum dots with narrow fluorescence half-values, it is important to synthesize Cu2Se, Cu2SeS, Cu2SeTe, and Cu2SeTeS with uniform particle size.

[0085] Next, organozinc compounds and inorganic zinc compounds are prepared as raw materials for ZnSe, ZnSeS, ZnSeTe, or ZnSeTeS. These organozinc compounds and inorganic zinc compounds are stable in air and easy to handle. The structures of the organozinc compounds and inorganic zinc compounds are not particularly limited, but zinc compounds with high ionicity are preferred for more efficient metal exchange reactions. For example, the following organozinc compounds and inorganic zinc compounds can be used: Zinc acetate: Zn(OAc)2, Zinc nitrate: Zn(NO3)2; Zinc stearate: Zn(OC(=O)C)2, as an acetate salt. 17 H 35 2. Zinc oleate: Zn(OC(=O)C 17 H 33 2. Zinc palmitate: Zn(OC(=O)C 15 H 31 2. Zinc myristate: Zn(OC(=O)C 13 H 27 2. Zinc dodecanoate: Zn(OC(=O)C 11 H 23 2. Zinc acetylacetone: Zn(acac)2. As a halide, zinc chloride: ZnCl2, zinc bromide: ZnBr2, zinc iodide: ZnI2 can be used. As zinc carbamate, zinc diethyldithiocarbamate: Zn(SC(=S)N(C2H5)2)2, zinc dimethyldithiocarbamate: Zn(SC(=S)N(CH3)2)2, zinc dibutyldithiocarbamate: Zn(SC(=S)N(C4H9)2)2, etc.

[0086] Next, the aforementioned organozinc compound and inorganic zinc compound are added to the reaction solution from which the copper chalcogenide precursor was synthesized. This results in a metal-exchange reaction between Cu and Zn, producing a copper chalcogenide. The metal-exchange reaction is preferably carried out at a temperature between 150°C and 300°C. More preferably, the metal-exchange reaction is carried out at a lower temperature, between 150°C and 280°C, and even more preferably, between 150°C and 250°C.

[0087] In this embodiment, the metal exchange reaction between Cu and Zn is preferably carried out quantitatively, and the nanocrystals are free of precursor Cu. This is because when precursor Cu remains in the nanocrystals, Cu acts as a dopant and increases the fluorescence half-width by emitting light through another luminescence mechanism. The residual amount of Cu relative to Zn is preferably 100 ppm or less, more preferably 50 ppm or less, and ideally 10 ppm or less.

[0088] In this embodiment, ZnSe-based quantum dots synthesized using the cation exchange method tend to have a higher Cu residual content compared to ZnSe-based quantum dots synthesized using the direct method. However, even with a Cu content of approximately 1 to 10 ppm relative to Zn, good luminescence properties can still be obtained. Furthermore, the Cu residual content can be used to determine whether quantum dots were synthesized using the cation exchange method. That is, by using the cation exchange method for synthesis, the particle size can be controlled by the copper chalcogenide precursor, making a synthesis method that was originally difficult to react possible. Therefore, the Cu residual content has an advantage in determining whether the cation exchange method was used.

[0089] Furthermore, in this embodiment, when performing metal exchange, a compound is required that has an auxiliary function of freeing the metal of the copper chalcogenide precursor in the reaction solution through coordination or chelation.

[0090] Examples of compounds that can form complexes with Cu include ligands that can perform the aforementioned functions. For example, phosphorus-based ligands, amine-based ligands, and thio-based ligands are preferred, with phosphorus-based ligands being more preferred for their high efficiency.

[0091] Therefore, by appropriately performing metal exchange between Cu and Zn, quantum dots with narrow fluorescence half-widths based on Zn and Se can be manufactured. In this embodiment, the cation exchange method described above enables mass production of quantum dots compared to direct synthesis.

[0092] In direct synthesis, organozinc compounds such as diethylzinc (Et₂Zn) are used to improve the reactivity of Zn feedstock. However, due to the high reactivity of diethylzinc, it is ignitable in air, requiring handling under an inert gas stream, making feedstock handling and storage difficult. Furthermore, the reaction with diethylzinc involves risks such as heat generation and ignition, making it unsuitable for mass production. Similarly, from a toxicity and safety perspective, reactions using selenium hydride (H₂Se) to improve the reactivity of Se feedstock are also unsuitable for mass production.

[0093] Furthermore, although ZnSe was generated in a reaction system using highly reactive Zn and Se raw materials as described above, particle generation was not controlled, resulting in a broadened fluorescence half-width of the generated ZnSe.

[0094] In contrast, in this embodiment, a copper chalcogenide precursor is synthesized from an organocopper compound or an inorganic copper compound and an organochalcogenide compound, and then metal exchange is performed using the copper chalcogenide precursor to synthesize quantum dots. Thus, in this embodiment, quantum dots are first synthesized via the synthesis of a copper chalcogenide precursor, rather than through direct synthesis. This indirect synthesis allows for the safe and stable synthesis of ZnSe-based quantum dots with narrow fluorescence half-values ​​without the need for highly reactive and hazardous reagents.

[0095] Furthermore, in this embodiment, the copper chalcogenide precursor is not separated or purified; instead, a metal exchange between Cu and Zn is performed in a single pot, thereby obtaining quantum dots with the desired composition and particle size. Alternatively, the copper chalcogenide precursor can be used after a single separation and purification.

[0096] Furthermore, in this embodiment, the synthesized quantum dots do not undergo any treatments such as washing, separation and purification, coating, or ligand exchange, and still express fluorescence properties.

[0097] <Methods for synthesizing shells>

[0098] use Figure 4 The flowchart shown illustrates the method for synthesizing the shell. In this embodiment, for example, after synthesizing the ZnSe core, ZnSeS is coated onto the surface of the ZnSe core. The ZnSeS coating is achieved, for example, by adding a mixture of Se-TOP solution, S-TOP solution, and zinc oleate to a solution in which the ZnSe core is dispersed, and then stirring and heating at a predetermined temperature. This operation is repeated multiple times, thereby enabling the coating of ZnSeS onto the surface of ZnSe.

[0099] In this embodiment, after washing ZnSe / ZnSeS, it is dispersed, for example, in octadecene (ODE), and then trioctylphosphine (TOP) and oleic acid are added. The mixture is stirred and heated under specified heat treatment conditions (e.g., 320°C for 10 minutes).

[0100] Next, in this embodiment, ZnS shells are coated. In this embodiment, it is preferable to divide the ZnS shell coating process into at least two parts: a first half and a second half. First, in the first half of the ZnS shell coating process, a shell-source mixture (shell raw material) containing an acidic compound is added to a solution in which ZnSe / ZnSeS is dispersed. Specifically, zinc oleate (Zn(OLAc)2) solution, dodecyl mercaptan (DDT), and TOP are added, followed by an acidic acid oxide. In this embodiment, the shell-source mixture containing the acidic acid oxide is added, and the mixture is stirred and heated under specified heating conditions. The specified heating conditions are, for example, a heating temperature of 320°C and a heating time of 10 minutes. In this embodiment, the addition and heating of the shell-source mixture are repeated multiple times. Figure 4 The number of repetitions is recorded as 10, but "10 times" is a single example and the number of repetitions is not limited. However, it is preferable to specify the number of repetitions in the range of about 5 to 15 times. Then, cool to room temperature.

[0101] In this embodiment, an acidic compound is added to the shell source mixture in the first half of the shell coating process, but the zinc halide compound mixed in the second half of the shell coating process is not added. It was found that QY decreased when the zinc halide compound was added to the shell source mixture in the first half of the shell coating process. Therefore, the zinc halide compound is not added to the shell source mixture in the first half of the shell coating process.

[0102] Next, the shell coating process in the latter half of this embodiment is performed. In this latter shell coating process, a shell source mixture containing an acidic compound and a zinc halide compound is added to a solution dispersing ZnSE / ZnSeS / ZnS. The zinc halide compound and the acidic compound are added to this shell source mixture, for example, along with a zinc oleate (Zn(OLAc)2) solution, dodecanethiol (DDT), and TOP. Thus, in the latter shell coating process, the shell source mixture containing the acidic compound and the zinc halide compound is added, and the mixture is stirred and heated under specified heating conditions. Specified heating conditions include, for example, a heating temperature of 320°C and a heating time of 10 minutes. In this embodiment, the addition and heating of the shell source mixture are repeated multiple times. Figure 4 In this context, the number of repetitions is recorded as 10, but "10 times" is just one example and the number of repetitions is not limited. However, it is preferable to specify the number of repetitions within the range of approximately 5 to 15 times.

[0103] Then, cool to room temperature, wash, and disperse by adding ODE. Repeat the process of adding the shell source mixture to disperse the ODE until the specified shell thickness is achieved.

[0104] Thus, the latter half of the shell coating process is characterized by the addition of a shell source mixture containing acidic compounds and zinc halide compounds.

[0105] Although this implementation aims to improve EQE, this requires improving QY and also achieving appropriate particle shape. If QY can be improved, as shown in Equation 1, EQE can be improved.

[0106] Regarding the appropriateness of particle shape, the following explanation is provided. Specifically, when the nuclei of quantum dots are close to each other, the Foster resonance energy (FRET) is generated, leading to a decrease in EQE. Therefore, it is believed that by employing a core-shell structure with a shell surrounding the nuclei, the nuclei can be physically separated, thereby reducing FRET. However, as the shell thickness increases, the particle shape deteriorates, and QY also decreases. Furthermore, there has always been the problem of not being able to cover the entire surface of the nuclei with a shell of a specified thickness, resulting in defects or localized thickening of the shell, and deterioration of the particle shape. Consequently, it is impossible to adequately reduce FRET and effectively reduce EQE.

[0107] Therefore, in this embodiment, by adding zinc halide compounds little by little to the core, the QY (Quality of Y) can be improved. In particular, the zinc halide compounds are added only in the latter half of the shell coating process and not in the former half, thereby effectively improving the QY. Furthermore, when the shell source mixture is continuously added, the particle shape deteriorates. Therefore, by adding acidic compounds to the shell source mixture, the shape of the locally thick shell areas can be adjusted by etching, resulting in a particle shape consistent with a polygonal cross-section.

[0108] In this embodiment, it is appropriate to add about 0.5 mol% to 3 mol% of zinc halide compound relative to zinc oleate, and more preferably about 1 mol% to 2 mol%.

[0109] In this embodiment, at least one acidic compound can be selected from hydrogen chloride (HCl), hydrogen bromide (HBr), hydrogen iodide (HI), trifluoroacetic acid (TFA), trifluoromethanesulfonic acid (TfOH), acetic acid (AA), sulfuric acid (H2SO4), and phosphoric acid (H3PO4). Preferably, at least one of hydrogen chloride (HCl), hydrogen bromide (HBr), and trifluoroacetic acid (TFA) is used. This yields high QY and well-formed quantum dot particle shapes. In this embodiment, for example, a hydrogen chloride-ethyl acetate solution can be added to the shell-source mixture.

[0110] In this embodiment, zinc chloride (ZnCl2) or at least one of zinc bromide (ZnBr2), zinc fluoride (ZnF2), and zinc iodide (ZnI2) is preferably used as the zinc halide compound. In this embodiment, for example, a zinc chloride-TOP·oleic acid solution can be added to the shell-source mixture.

[0111] Furthermore, in this embodiment, the S material used for the core-shell structure is not particularly limited, but the following materials can be listed as representative materials.

[0112] That is, as a thiol, it can be used to dissolve sulfur in octadecyl mercaptan: C 18 H 37 SH, hexadecyl mercaptan: C 16 H 33 SH, tetradecyl mercaptan: C 14 H 29 SH, dodecanethiol: C 12 H 25 SH, Decathiol: C 10 H 21 SH, Octethiol: C8H 17 SH, benzenethiol: solutions made in high-boiling-point solvents such as C6H5SH or trioctylphosphine (S-TOP), solutions made by dissolving sulfur in high-boiling-point solvents such as octadecene (S-ODE), or solutions made by dissolving in a mixture of oleylamine and dodecyl mercaptan (S-DDT / OLAm), etc.

[0113] Depending on the sulfur (S) feedstock used, the reactivity varies, resulting in varying shell 5b coating thicknesses (e.g., ZnS). The reactivity of thiols is proportional to their decomposition rate, while the reactivity of S-TOP or S-ODE is proportional to their stability. Therefore, by differentiating the S feedstock used, the shell 5b coating thickness can be controlled, and consequently, the final fluorescence quantum yield can also be controlled.

[0114] Furthermore, in this embodiment, the solvent used for shell 5b coating is such that the less amine-based solvent used, the easier the shell 5b coating is, and the better the luminescence properties can be obtained. Moreover, the luminescence properties after shell 5b coating vary depending on the ratio of amine-based, carboxylic acid-based, or phosphine-based solvents.

[0115] Furthermore, the quantum dots 5 synthesized by the manufacturing method of this embodiment can be condensed by adding a polar solvent such as methanol, ethanol, or acetone, thereby separating and recovering the quantum dots 5 from unreacted raw materials. The recovered quantum dots 5 are then redispersed by adding toluene or hexane. Adding a solvent that acts as a ligand to the redispersed solution further enhances the luminescence properties and improves their stability. The change in luminescence properties resulting from the addition of this ligand varies significantly depending on whether a shell 5b coating is present. In this embodiment, the quantum dots 5 with a shell 5b coating exhibit particularly improved fluorescence stability by adding a thiol-based ligand.

[0116] right Figure 1 A and Figure 1The application of quantum dot 5 shown in B is not particularly limited, but for example, the quantum dot 5 of this embodiment that emits blue fluorescence can be applied to wavelength conversion components, lighting components, backlight devices, and display devices.

[0117] The quantum dot 5 of this embodiment can be applied to a part of a wavelength conversion component, an illumination component, a backlight device, and a display device. For example, when photoluminescence (PL) is used as the light-emitting principle, blue fluorescence can be emitted by UV irradiation from a light source. Alternatively, when electroluminescence (EL) is used as the light-emitting principle, or when all three primary colors are emitted by quantum dots through other methods, a light-emitting element that emits blue fluorescence using the quantum dot 5 of this embodiment can be used. In this embodiment, white light can be emitted by a light-emitting element (full-color LED) containing the quantum dot 5 of this embodiment that emits blue fluorescence, along with quantum dots that emit green fluorescence and quantum dots that emit red fluorescence.

[0118] Figure 2 This is a schematic diagram of an LED device using quantum dots according to this embodiment. The LED device 1 of this embodiment is configured to have, as shown in the diagram... Figure 2 As shown, the housing 2 has a bottom surface 2a and side walls 2b surrounding the bottom surface 2a; an LED chip (light-emitting element) 3 disposed on the bottom surface 2a of the housing 2; and a phosphor layer 4 filled inside the housing 2 and sealing the upper surface side of the LED chip 3. Here, "upper surface side" refers to the direction in which light emitted from the LED chip 3 is released from the housing 2, that is, the direction opposite to the bottom surface 2a relative to the LED chip 3.

[0119] The LED chip 3 is disposed on a substrate wiring board (not shown), which may also form the bottom surface of the housing 2. For example, a structure in which a wiring pattern is formed on a substrate such as glass epoxy resin can be shown as the substrate.

[0120] LED chip 3 is a semiconductor element that emits light when a forward voltage is applied, and has a basic structure consisting of a P-type semiconductor layer and an N-type semiconductor layer connected by a PN junction.

[0121] like Figure 2 As shown, the fluorescent layer 4 is formed of resin 6 with multiple quantum dots 5 dispersed therein.

[0122] The resin composition, in which quantum dots 5 are also dispersed, may contain quantum dots 5 and a fluorescent substance different from quantum dots 5. As fluorescent substances, there are silane-based fluorescent substances, KSF(K2SiF6:Mn) 4+ Red fluorescent materials, etc., are allowed, but there are no specific restrictions on the materials used.

[0123] The resin 6 constituting the fluorescent layer 4 is not particularly limited, but can be polypropylene (PP), polystyrene (PS), acrylic resin, methacrylate, MS resin, polyvinyl chloride (PVC), polycarbonate (PC), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polymethylpentene, liquid crystal polymer, epoxy resin, silicone resin, or mixtures thereof.

[0124] The quantum dot LED device using this embodiment can be applied to display devices. Figure 3 For use Figure 2 A longitudinal sectional view of the LED device shown. Figure 3 As shown, the display device 50 is configured as a display section 54, such as having a plurality of LED devices 20 and a liquid crystal display opposite each LED device 20. Each LED device 20 is disposed on the rear side of the display section 54. Each LED device 20 and... Figure 2 Similarly, the LED device 1 shown also has a structure in which the LED chip is sealed with resin by diffusing multiple quantum dots 5.

[0125] like Figure 3 As shown, multiple LED devices 20 are supported by a support body 52. ​​The LED devices 20 are arranged at predetermined intervals. Each LED device 20 and the support body 52 constitute a backlight 55 relative to the display unit 54. The support body 52 can be sheet-like, plate-like, or shell-like, and its shape and material are not particularly limited. Figure 3 As shown, a light diffusion plate 53 or similar material can also be sandwiched between the backlight 55 and the display unit 54.

[0126] By applying the quantum dot 5 in this embodiment to Figure 2 The LED device shown Figure 3 The quantum dots used in this embodiment can effectively improve the light-emitting characteristics of display devices, etc. In particular, they can improve the EQE when applied to QLED elements. In this embodiment, an EQE of 7% or more can be obtained, preferably 9% or more, more preferably 10% or more, and even more preferably 10.5% or more.

[0127] Furthermore, the resin composition obtained by dispersing the quantum dots 5 of this embodiment in a resin can also be formed into a sheet or film. Such sheets or films can be assembled, for example, into a backlight device.

[0128] Furthermore, in this embodiment, a wavelength conversion component in which multiple quantum dots are dispersed in resin can be formed into a molded body. For example, the molded body formed by dispersing quantum dots in resin is housed in a container with a storage space by pressing or other methods. In this case, the refractive index of the molded body is preferably less than the refractive index of the container. As a result, a portion of the light entering the molded body undergoes total internal reflection at the inner wall of the container. Therefore, light beam leakage from the side of the container to the outside can be reduced. Thus, by applying the quantum dots in this embodiment to wavelength conversion components, illumination components, backlight devices, and display devices, the light emission characteristics can be effectively improved.

[0129] Example

[0130] The effects of the present invention will be described below through embodiments and comparative examples. Furthermore, the present invention is not limited to any of the following embodiments.

[0131] In this invention, the following raw materials are used to synthesize Cd-free blue fluorescent quantum dots. Furthermore, the following measuring instruments are used to evaluate the synthesized quantum dots.

[0132] <Raw Materials>

[0133] Anhydrous copper acetate: manufactured by Wako Pure Chemical Industries, Ltd.

[0134] Octadene: Manufactured by Idemitsu Kosan Co., Ltd.

[0135] Oleylamine: Phamine manufactured by Kao Co., Ltd.

[0136] Oleic acid: LUNAC OV manufactured by Kao Corporation

[0137] Dodecanethiol (DDT): Thiocarcol 20 manufactured by Kao Corporation

[0138] Trioctylphosphine (TOP): Manufactured by Hokko Chemical Co., Ltd.

[0139] Anhydrous zinc acetate: Manufactured by Kishida Chemical Co., Ltd.

[0140] Selenium (4N: 99.99%): Manufactured by Shinshin Chemical Co., Ltd.

[0141] Sulfur: Manufactured by Kishida Chemical Co., Ltd.

[0142] Hydrogen chloride: Manufactured by a domestic chemical company.

[0143] Zinc chloride: Manufactured by Kanto Chemical Co., Ltd.

[0144] Hydrogen bromide: Manufactured by Tokyo Chemical Industry Co., Ltd.

[0145] Zinc bromide: manufactured by Kishida Chemical Co., Ltd.

[0146] <Measuring Equipment>

[0147] Fluorescence spectrophotometer: F-2700 manufactured by Japan Spectrophotometer Co., Ltd.

[0148] UV-Vis spectrophotometer: Hitachi, Ltd. V-770

[0149] Fluorescence quantum yield measurement device: Otsuka Electron Co., Ltd. QE-1100

[0150] X-ray diffraction (XRD) apparatus: Bruker D2 PHASER

[0151] Scanning electron microscope (SEM): Hitachi SU9000

[0152] Fluorescence lifetime measurement device: Hamamatsu Photonics C11367

[0153] LED measuring device: manufactured by SPECTRA CO-OP

[0154] Transmission electron microscope (TEM): JEM-ARM200-CF manufactured by Nippon Electron Ltd.

[0155] XEDS detector: JED2300T manufactured by Nippon Electronics Co., Ltd.

[0156] [Example 1]

[0157] <Synthetic methods of ZnSe cores>

[0158] Add 728 mg of anhydrous copper acetate (Cu(OAc)2), 19.2 mL of oleylamine (OLAm1), and 31 mL of octadecene (ODE) to a 300 mL reaction vessel. Then, under an inert gas (N2) atmosphere, heat at 165 °C with stirring for 20 minutes to dissolve the raw materials.

[0159] Add 4.56 mL of Se-DDT / OLAm solution (0.7 M) to the solution, stir and heat at 165 °C for 30 minutes. Cool the resulting reaction solution (CuSe) to room temperature.

[0160] Then, 7376 mg of anhydrous zinc acetate (Zn(OAc)2), 40 mL of trioctylphosphine (TOP), and 1.6 mL of oleylamine (OLAm) were added to the CuSe reaction solution, and the mixture was stirred and heated at 200 °C for 1 hour under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSe) was then cooled to room temperature.

[0161] Ethanol was added to the reaction solution cooled to room temperature to produce a precipitate. The precipitate was recovered by centrifugation. 96 ml of octadecene:ODE was added and dispersed in the precipitate.

[0162] Then, 7376 mg of anhydrous zinc acetate (Zn(OAc)2), 40 mL of trioctylphosphine (TOP), 4 mL of oleylamine (OLAm), and 24 mL of oleic acid (OLAc) were added to 96 mL of the ZnSe-ODE solution. The mixture was stirred and heated at 290 °C for 30 minutes under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSe) was then cooled to room temperature.

[0163] The obtained reaction solution was measured using a fluorescence spectrophotometer. The results showed that the optical properties were approximately 446.5 nm in fluorescence wavelength and approximately 14 nm in fluorescence half-width.

[0164] <Coating method for ZnSe core coated in shell>

[0165] Ethanol was added to 40 ml of ZnSe reaction solution to produce a precipitate. The precipitate was recovered by centrifugation, and 35 ml of octadecene:ODE was added and dispersed in the precipitate.

[0166] Add 2 mL of oleic acid: OLAc and 4 mL of trioctylphosphine: TOP to 35 mL of dispersed ZnSe-ODE solution, and stir and heat at 320 °C for 10 minutes under an inert gas (N2) atmosphere.

[0167] Add 0.9 mL of a mixture of 0.5 mL of Se-TOP solution (1 M), 0.5 mL of S-TOP solution (1 M), and 5 mL of zinc oleate:Zn(OLAc)2 solution (0.4 M) to the solution, stir and heat at 320 °C for 10 minutes. Repeat this operation 4 times.

[0168] Then, ethanol was added to the obtained reaction solution to produce a precipitate, which was recovered by centrifugation. 35 mL of octadecene:ODE was added and dispersed in the precipitate. Then, 2 mL of oleic acid:OLAc and 4 mL of trioctylphosphine:TOP were added as before, and the mixture was stirred and heated at 320 °C for 10 minutes under an inert gas (N2) atmosphere.

[0169] Add 0.9 mL of a mixture of 0.4 mL DDT, 1.6 mL trioctylphosphine:TOP, 0.12 mL hydrogen chloride-ethyl acetate solution (4 M), and 10 mL zinc oleate:Zn(OLAc)2 solution (0.4 M) to the solution, stir and heat at 320 °C for 10 minutes. Repeat this operation 10 times.

[0170] Then, ethanol was added to the obtained reaction solution to produce a precipitate, which was recovered by centrifugation. 35 mL of octadecene:ODE was added and dispersed in the precipitate. Then, 2 mL of oleic acid:OLAc and 4 mL of trioctylphosphine:TOP were added as before, and the mixture was stirred and heated at 320 °C for 10 minutes under an inert gas (N2) atmosphere.

[0171] Add 0.9 mL of a mixture of 0.4 mL DDT, 1.6 mL trioctylphosphine:TOP, 0.12 mL hydrogen chloride-ethyl acetate solution (4 M), 0.1 mL zinc chloride-TOP·oleic acid solution (0.8 M), and 10 mL zinc oleate:Zn(OLAc)2 solution (0.4 M) to the solution, stir and heat at 320 °C for 10 minutes. Repeat this operation 10 times.

[0172] Then, ethanol was added to the obtained reaction solution to produce a precipitate, which was recovered by centrifugation. 35 mL of octadecene:ODE was added and dispersed in the precipitate. Then, 2 mL of oleic acid:OLAc and 4 mL of trioctylphosphine:TOP were added as before, and the mixture was stirred and heated at 320 °C for 10 minutes under an inert gas (N2) atmosphere.

[0173] Add 0.9 mL of a mixture of 0.4 mL DDT, 1.6 mL trioctylphosphine:TOP, 0.2 mL hydrogen chloride-ethyl acetate solution (4 M), 0.1 mL zinc chloride-TOP·oleic acid solution (0.8 M), and 10 mL zinc oleate:Zn(OLAc)2 solution (0.4 M) to the solution, stir and heat at 320 °C for 10 minutes. Repeat this operation 10 times.

[0174] The obtained reaction solution was measured using a fluorescence spectrometer. The results showed that, for example... Figure 5 As shown, the optical properties include a fluorescence wavelength of approximately 442 nm and a fluorescence half-width of approximately 15 nm.

[0175] Ethanol was added to the obtained reaction solution to produce a precipitate, which was recovered by centrifugation. Hexane was then added and dispersed in the precipitate. The resulting dispersion was analyzed using a UV-Vis spectrophotometer. The results showed that... Figure 6 The ultraviolet-visible absorption spectrum.Figure 7 This is the X-ray diffraction (XRD) spectrum of Example 1. According to... Figure 7 The results confirmed the crystallization peaks of the cubic crystal composed of Zn, Se, and S.

[0176] <Measurement Results>

[0177] The fluorescence quantum yield of ZnSe / ZnSeS / ZnS dispersed in hexane was determined using a quantum efficiency measurement system. The results showed a fluorescence quantum yield of approximately 96%. Furthermore, the fluorescence lifetime was measured to be 16 ns. Elemental analysis (EDX) revealed the following: Zn: 42 atom%, Se: 11 atom%, S: 41 atom%, Cl: 1 atom%. The TEM image showed a shell thickness of 2.0 nm.

[0178] Furthermore, the quantum dots obtained from Example 1 were used to fabricate a light-emitting element having the following layered structure.

[0179] ITO / PEDOT: PSS / PVK / QD layer / LiZnO / Al

[0180] The results of evaluating the element using an LED measuring device show that the maximum external quantum efficiency (EQE) is 18.6%.

[0181] [Example 2]

[0182] Except for replacing the zinc chloride-TOP·oleic acid solution used in Example 1 with the zinc bromide-TOP·oleic acid solution, the synthesis was carried out under the same conditions as in Example 1.

[0183] [Example 3]

[0184] Except that the hydrogen chloride-ethyl acetate solution (4M) used in Example 2 (as described in Example 1) was replaced with a hydrogen bromide-acetic acid solution, the synthesis was carried out under the same conditions as in Example 2.

[0185] [Example 4]

[0186] Except for replacing the hydrogen chloride-ethyl acetate solution (4M) used in Example 1 with trifluoroacetic acid, the synthesis was carried out under the same conditions as in Example 1.

[0187] [Example 5]

[0188] Except for replacing the hydrogen chloride-ethyl acetate solution (4M) used in Example 2 (as described in Example 1) with trifluoroacetic acid, the synthesis was carried out under the same conditions as in Example 2.

[0189] Figure 8This is a table summarizing the measurement results of Examples 1 to 5. TEM images of each quantum dot obtained from Examples 1 to 5 are also published.

[0190] like Figure 8 As shown, in Examples 1 to 5, the EQE is above 7%. In particular, Example 1 can increase the EQE to 18.6%.

[0191] Furthermore, in any embodiment, QY is above 70%. In particular, embodiment 2 can increase QY to 98%.

[0192] Furthermore, in each embodiment, the fluorescence half-width can be below 20 nm. Moreover, any embodiment can control the fluorescence wavelength within the range of 410 nm to 470 nm, exhibiting blue fluorescence.

[0193] Furthermore, the shell thickness of each embodiment ranges from approximately 2 nm to 2.5 nm. The shell thickness can also be estimated from images obtained through TEM-EDX analysis.

[0194] like Figure 8 SEM images of various embodiments show that the quantum dots have a generally rectangular (approximately cubic) particle shape and are well-formed. That is, it can be considered that the approximately rectangular particle shape is maintained by crystallizing the ZnSe core and then encapsulating it with a shell of a specified thickness. This can be attributed to the effect of etching away any areas where the particle shape deteriorates by mixing an acidic compound into the shell-source mixture.

[0195] [Example 6]

[0196] In the synthesis process used in Example 1, the <synthesis method of ZnSe core> was carried out in the same manner, and quantum dots were synthesized by changing a part of the <coating method of shell-coated ZnSe core>. The <coating method of shell-coated ZnSe core> in Example 6 is described below.

[0197] <Coating method for ZnSe core coated in shell>

[0198] Ethanol was added to 40 ml of ZnSe reaction solution to produce a precipitate. The precipitate was recovered by centrifugation, and 35 ml of octadecene:ODE was added and dispersed in the precipitate.

[0199] Add 2 mL of oleic acid: OLAc and 4 mL of trioctylphosphine: TOP to 35 mL of dispersed ZnSe-ODE solution, and stir and heat at 320 °C for 10 minutes under an inert gas (N2) atmosphere.

[0200] Add 0.9 mL of a mixture of 0.5 mL of Se-TOP solution (1 M), 0.5 mL of S-TOP solution (1 M), and 5 mL of zinc oleate:Zn(OLAc)2 solution (0.4 M) to the solution, stir and heat at 320 °C for 10 minutes. Repeat this operation 4 times.

[0201] Then, ethanol was added to the obtained reaction solution to produce a precipitate, which was recovered by centrifugation. 35 mL of octadecene:ODE was added and dispersed in the precipitate. Then, 2 mL of oleic acid:OLAc and 4 mL of trioctylphosphine:TOP were added as before, and the mixture was stirred and heated at 320 °C for 10 minutes under an inert gas (N2) atmosphere.

[0202] Add 0.9 mL of a mixture of 0.6 mL DDT, 1.4 mL trioctylphosphine:TOP, 0.24 mL hydrogen chloride-ethyl acetate solution (4 M), and 10 mL zinc oleate:Zn(OLAc)2 solution (0.48 M) to the solution, stir and heat at 320 °C for 10 minutes. Repeat this operation 10 times.

[0203] Then, ethanol was added to the obtained reaction solution to produce a precipitate, which was recovered by centrifugation. 35 mL of octadecene:ODE was added and dispersed in the precipitate. Then, as before, 2 mL of oleic acid:OLAc and 4 mL of trioctylphosphine:TOP were added, and the mixture was stirred and heated at 320 °C for 10 minutes under an inert gas (N2) atmosphere.

[0204] Add 0.9 mL of a mixture of 0.6 mL DDT, 1.4 mL trioctylphosphine:TOP, 0.24 mL hydrogen chloride-ethyl acetate solution (4 M), 0.1 mL zinc chloride-TOP·oleic acid solution (0.8 M), and 10 mL zinc oleate:Zn(OLAc)2 solution (0.48 M) to the solution, stir and heat at 320 °C for 10 minutes. Repeat this operation 10 times.

[0205] <Measurement results of Example 6>

[0206] The fluorescence quantum yield of ZnSe / ZnSeS / ZnS dispersed in hexane was measured using a quantum efficiency measurement system. The results showed a fluorescence quantum yield of approximately 90%. Furthermore, the fluorescence lifetime was measured to be 20 ns. Analysis of the TEM image revealed a shell thickness of 2.7 nm.

[0207] [Example 7]

[0208] The synthesis method of ZnSe core and the coating method of shell-coated ZnSe core in Example 1 were used directly, but 2.0 mL of zinc chloride-TOP zinc chloride-TOP oleic acid solution (0.8 M) was added at the end, and the mixture was stirred and heated for 20 minutes.

[0209] <Results of the measurement in Example 7>

[0210] The fluorescence quantum yield of ZnSe / ZnSeS / ZnS dispersed in hexane was determined using a quantum efficiency measurement system. The results showed a fluorescence quantum yield of approximately 84%. Furthermore, the fluorescence lifetime was measured to be 25 ns. Elemental analysis (EDX) revealed the following: Zn: 32 atom%, Se: 12 atom%, S: 50 atom%, Cl: 6 atom%. Analysis of the TEM image showed a shell thickness of 2.0 nm.

[0211] In Example 6, to more effectively prevent Foster resonance energy transfer (FRET), the shell was thickened compared to Example 1. Specifically, the shell thickness in Example 6 reached 2.7 nm, compared to 2 nm in Example 1. Furthermore, in Example 6, the decrease in fluorescence quantum yield (QY) was significantly suppressed compared to Example 1.

[0212] In Example 7, the objective was to reduce the amount of Zn without ligands on the surface of the quantum dots while increasing the chlorine content. Specifically, the chlorine content was 6% in Example 7, compared to 1% in Example 1.

[0213] [Comparative Example 1]

[0214] Comparative Example 1 is an example of coating the shell without mixing the acidic compound and zinc halide compound into the shell source mixture. Specifically, the shell is coated through the following steps.

[0215] Add 182 mg of anhydrous copper acetate (Cu(OAc)2), 4.8 mL of oleylamine (OLAm), and 7.75 mL of octadecene (ODE) to a 100 mL reaction vessel. Then, under an inert gas (N2) atmosphere, heat at 165 °C with stirring for 5 minutes to dissolve the starting materials.

[0216] Add 1.14 mL of Se-DDT / OLAm solution (0.7 M) to the solution, stir and heat at 165 °C for 30 minutes. Cool the resulting reaction solution (CuSe) to room temperature.

[0217] Then, 1844 mg of anhydrous zinc acetate (Zn(OAc)2), 10 mL of trioctylphosphine (TOP), and 0.4 mL of oleylamine (OLAm) were added to the Cu2Se reaction solution. The mixture was stirred and heated at 180 °C for 45 minutes under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSe) was then cooled to room temperature.

[0218] Ethanol was added to the reaction solution cooled to room temperature to produce a precipitate. The precipitate was recovered by centrifugation, and 12 ml of octadecene:ODE was added and dispersed in the precipitate.

[0219] Then, 1844 mg of anhydrous zinc acetate (Zn(OAc)2), 10 mL of trioctylphosphine (TOP), 1 mL of oleylamine (OLAm), and 6 mL of oleic acid (OLAc) were added to 12 mL of the ZnSe-ODE solution. The mixture was stirred and heated at 280 °C for 20 minutes under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSe) was then cooled to room temperature.

[0220] The obtained reaction solution was measured using a fluorescence spectrophotometer. The results showed that the optical properties were approximately 447.5 nm in fluorescence wavelength and approximately 14 nm in fluorescence half-width.

[0221] Ethanol was added to 20 ml of the obtained ZnSe reaction solution to produce a precipitate. The precipitate was recovered by centrifugation, and 17.5 ml of octadecene:ODE was added and dispersed in the precipitate.

[0222] Add 1 mL of oleic acid: OLAc and 2 mL of trioctylphosphine: TOP to 17.5 mL of dispersed ZnSe-ODE solution, and stir and heat at 320 °C for 10 minutes under an inert gas (N2) atmosphere.

[0223] Add 0.5 mL of Se-TOP solution (1 M), 0.125 mL of DDT, 0.375 mL of trioctylphosphine:TOP, and 0.5 mL of a mixture of 5 mL of zinc oleate:Zn(OLAc)2 solution (0.4 M) to the solution, stir and heat at 320 °C for 10 minutes. Repeat this operation 4 times.

[0224] Then, ethanol was added to the obtained reaction solution to produce a precipitate, which was recovered by centrifugation. 17.5 mL of octadecene:ODE was added and dispersed in the precipitate. 1 mL of oleic acid:OLAc and 2 mL of trioctylphosphine:TOP were added as before, and the mixture was stirred and heated at 320 °C for 10 minutes under an inert gas (N2) atmosphere.

[0225] Add 0.5 mL of a mixture of 0.5 mL DDT, 1.5 mL trioctylphosphine:TOP, and 0.5 mL zinc oleate:Zn(OLAc)2 solution (0.4 M) to the solution, stir and heat at 320 °C for 10 minutes. Repeat this operation 10 times.

[0226] Then, ethanol is added to the obtained reaction solution to produce a precipitate, the precipitate is recovered by centrifugation, and 17.5 ml of octadecene:ODE is added and dispersed in the precipitate (washing step).

[0227] Next, as before, add 1 mL of oleic acid:OLAc and 2 mL of trioctylphosphine:TOP, and stir and heat at 320°C for 10 minutes under an inert gas (N2) atmosphere. Add 0.5 mL of a mixture of 0.5 mL DDT, 1.5 mL trioctylphosphine:TOP, and 0.5 mL of a 0.4 M zinc oleate:Zn(OLAc)2 solution to this solution, and stir and heat at 320°C for 10 minutes. Repeat this operation 6 times. Then, stir and heat at 320°C for 30 minutes (shell coating process).

[0228] The reaction solution was then subjected to the above-described (cleaning process) and (shell coating process) three times to obtain the reaction solution (ZnSe / ZnS) as the target material, and then cooled to room temperature.

[0229] The obtained reaction solution was measured using a fluorescence spectrophotometer. The results showed that the optical properties were approximately 443 nm in fluorescence wavelength and approximately 15 nm in fluorescence half-width.

[0230] Ethanol was added to the obtained reaction solution to produce a precipitate, which was then recovered by centrifugation, and hexane was added and dispersed in the precipitate.

[0231] The fluorescence quantum yield of ZnSe / ZnSeS / ZnS dispersed in hexane was determined using a quantum efficiency measurement system. The results showed a fluorescence quantum yield of approximately 60%. Furthermore, the fluorescence lifetime was measured to be 14 ns.

[0232] Furthermore, the quantum dots obtained from Comparative Example 1 were used to fabricate a light-emitting element having the following layered structure.

[0233] ITO / PEDOT: PSS / PVK / QD layer / ZnO / Al

[0234] The results of evaluating the element using an LED measuring device show that the maximum external quantum efficiency (EQE) is 4.0%.

[0235] The following compares Example 1 and Comparative Example 1. Table 1 shows the measurement results of Example 1 and Comparative Example 1.

[0236] [Table 1]

[0237] Example 1 Comparative Example 1 Fluorescence wavelength (nm) 441.5 443.0 Half-value width (nm) 15.0 15.0 PLQY (solution) (%) 96 60 Fluorescence lifetime 1 / e (solution) (ns) 16 14 Shell thickness (nm) 2.0 2.0 EQE (%) 18.6 4.0

[0238] It can be seen that the EQE of Comparative Example 1 is lower than that of Example 1. Figure 9 A is a photograph of the TEM-EDX analysis results from Comparative Example 1. Figure 9 B is a photograph of the TEM-EDX analysis results from Example 1. Figure 10 A is Figure 9 A partial schematic diagram. Figure 10 B is Figure 9 Partial diagram of B.

[0239] like Figure 9 A, Figure 9 As shown in Figure B, the TEM-EDX analysis results are represented by three colors (red, blue, and green). However, the central area is mainly mixed with red and blue, appearing roughly purple, while the outer area is mainly mixed with red and green, appearing roughly yellow. Since red represents Zn, blue represents Se, and green represents S, Zn and Se are mainly present in the central area, while Zn and S are mainly present on the outer side. Therefore, from... Figure 9 A, Figure 9 The TEM-EDX analysis image shown in Figure B indicates that the core is ZnSe and the shell is ZnS. Then, the thickness of the approximately yellow portion is determined from the TEM-EDX analysis image, thus allowing the shell thickness to be estimated.

[0240] Depend on Figure 9 A and Figure 10 As can be seen from Comparative Example 1, the shell surrounding the nucleus is not of approximately a fixed thickness, but rather is interrupted in various places or has localized growth. Therefore, the particle shape of Comparative Example 1 deteriorates, making it more susceptible to Foster resonance energy transfer (FRET), and the EQE decreases. Furthermore, Comparative Example 1 did not achieve the same high EQY as Example 1.

[0241] In contrast, in Example 1, as Figure 9 B and Figure 10 As shown in B, the shell completely covers the periphery of the core, the shell has a roughly fixed thickness, and the quantum dot particle shape is roughly rectangular. Thus, the particle shape of Example 1 is superior to that of Comparative Example 1, and a sufficiently high QY can also be obtained compared to Comparative Example 1. Therefore, Example 1 achieves a sufficiently high EQE compared to Comparative Example 1.

[0242] Industrial availability

[0243] According to the present invention, quantum dots emitting blue fluorescence can be stably obtained. Furthermore, by applying the quantum dots of the present invention to LEDs, backlight devices, display devices, etc., excellent light-emitting characteristics can be obtained in each device.

[0244] This application is based on Japanese Special Petition 2021-030560 filed on February 26, 2021. Its entire contents are contained herein.

Claims

1. A method for manufacturing quantum dots, characterized in that, include: The process of generating the nucleus; as well as In the process of coating the surface of the core with a shell, In the process of covering the shell, Acidic compounds and zinc halide compounds are mixed in the shell raw material. ZnS is coated on the surface of a core containing at least Zn and Se, separated by ZnSeS.

2. The method for manufacturing quantum dots according to claim 1, characterized in that, The process of covering the shell is divided into at least a first half and a second half. The shell is coated multiple times by using a shell material containing the acidic compound but not the zinc halide compound in the first half, and a shell material containing both the acidic compound and the zinc halide compound in the second half.

3. The method for manufacturing quantum dots according to claim 1 or 2, characterized in that, The acidic compound is described using at least one of hydrogen chloride, hydrogen bromide, or trifluoroacetic acid.

4. The method for manufacturing quantum dots according to any one of claims 1 or 2, characterized in that, The zinc halide compound is described using at least one of zinc chloride or zinc bromide.

5. A quantum dot having a core and a shell covering the surface of said core, characterized in that, Contains halogen elements, The external quantum efficiency is above 7%. The quantum dot comprises: a core containing at least Zn and Se, ZnS coating the surface of the core, and ZnSeS between the core and the ZnS.

6. A quantum dot having a core and a shell covering the surface of said core, characterized in that, Contains halogen elements, The fluorescence quantum yield is over 70%. The quantum dot comprises: a core containing at least Zn and Se, ZnS coating the surface of the core, and ZnSeS between the core and the ZnS.

7. The quantum dot according to claim 5 or 6, characterized in that, The halogen elements were detected by elemental analysis using an energy-dispersive X-ray spectrometer, and the content of the halogen elements was 0.01 atom% to 5 atom%.

8. The quantum dot according to claim 5 or 6, characterized in that, The halogen element is chlorine or bromine.

9. The quantum dot according to claim 5 or 6, characterized in that, It contains Cu.

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