Quantum dots

Abstract

The objective is to provide quantum dots that can enhance EQE (Energy Optimization). [Solution] The quantum dot of the present invention is a quantum dot having a core-shell structure, wherein an intermediate layer is included between the core and the shell, the core contains Zn and Se, the intermediate layer contains ZnSeS, and the shell contains ZnS and has a substantially constant thickness. In the present invention, it is preferable that the thickness of the shell is 0.5 nm or more and 3 nm or less.

Description

[Technical Field]

[0001] This invention relates to a cadmium-free core-shell quantum dot. [Background technology]

[0002] Quantum dots are also called fluorescent nanoparticles because they emit fluorescence and are on the nanoscale; semiconductor nanoparticles because their composition is derived from semiconductor materials; or nanocrystals because their structure has a specific crystalline structure.

[0003] Quantum dot performance can be described by the fluorescence quantum yield (QY) and external quantum efficiency (EQE).

[0004] When using quantum dots in displays, if photoluminescence (PL) is employed as the light emission principle, a method is used in which a blue LED is used as the excitation light for the backlight, and the quantum dots are used to convert it into green or red light. On the other hand, if, for example, electroluminescence (EL) is employed as the light emission principle, or if all three primary colors are emitted by quantum dots using other methods, blue fluorescent quantum dots are required.

[0005] A representative example of blue quantum dots is cadmium selenide (CdSe) quantum dots, which use cadmium (Cd). However, Cd is internationally regulated, which has presented a significant obstacle to the practical application of materials using CdSe quantum dots.

[0006] On the other hand, the development of quantum dots that do not use Cd is also being considered. For example, the development of chalcopyrite-based quantum dots such as CuInS2 and AgInS2, indium phosphide (InP)-based quantum dots, etc. is progressing (see, for example, Patent Document 1). However, those currently developed generally have a wide fluorescence half-width and are not suitable as blue fluorescent quantum dots.

[0007] In addition, Non-Patent Document 1 below describes in detail a direct method for synthesizing ZnSe using diphenylphosphine selenide, which is considered to be relatively highly reactive with an organozinc compound, but it is not suitable as a blue fluorescent quantum dot.

[0008] In addition, Non-Patent Document 2 below also reports a method for synthesizing ZnSe in an aqueous system. Although the reaction proceeds at a low temperature, the fluorescence half-width is somewhat wide at 30 nm or more, and the fluorescence wavelength is less than 430 nm. Therefore, it is not suitable for achieving a high color gamut by using this as a substitute for a conventional blue LED.

[0009] In addition, Non-Patent Document 3 reports a method for synthesizing ZnSe-based quantum dots by forming a precursor such as copper selenide (CuSe) and then performing cation exchange of copper with zinc (Zn). However, the precursor copper selenide particles are large (15 nm), and the reaction conditions for cation exchange between copper and zinc are not optimal, resulting in residual copper in the ZnSe-based quantum dots after cation exchange. Our investigations have shown that ZnSe-based quantum dots with residual copper cannot emit light. Alternatively, even if light is emitted, residual copper indicates emission originating from defects, resulting in emission with a full width at half maximum (FWHM) of 30 nm or more. The particle size of the precursor copper selenide also influences this copper residue; larger particles are more likely to retain copper after cation exchange, and even if ZnSe can be confirmed by XRD, even a small amount of residual copper can prevent emission. Therefore, Non-Patent Document 3 is cited as an example where copper residue remains due to insufficient control of precursor particle size and optimization of the cation exchange method. Consequently, blue fluorescence has not been reported. While there are many reported cases using the cation exchange method, there are no reports of strong luminescence for the reasons mentioned above. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] International Publication No. 2007 / 060889 Pamphlet [Non-patent literature]

[0011] [Non-Patent Document 1] Organic Electronics 15 (2014) 126-131 [Non-Patent Document 2] Materials Science and Engineering C 64 (2016) 167-172 [Non-Patent Document 3] J. Am. Chem. Soc.(2015)137 29 9315-9323 [Overview of the project] [Problems that the invention aims to solve]

[0012] By the way, the external quantum efficiency is calculated using the following equation (Equation 1). External quantum yield (EQE) = Carrier balance × Efficiency of generating luminescent excitons × Efficiency of emission quantum (fluorescence quantum yield (QY)) × Efficiency of light extraction (Equation 1)

[0013] Here, since the light extraction efficiency is generally 0.2 to 0.3, if we assume that the carrier balance, the generation efficiency of luminescent excitons, and the fluorescence quantum yield are all 1 (100%), the theoretical external quantum yield will be 20 to 30%. Therefore, to obtain a high EQE, quantum dots with high QY are required.

[0014] Furthermore, if quantum dots are too close together, Förster resonance energy transfer (FRET) occurs. As a result, the EQE decreases. Therefore, by using a core-shell structure in which a shell surrounds the core, the distance between the cores can be physically increased, thereby reducing FRET.

[0015] However, conventionally, it has not been possible to mass-produce quantum dots with a shell that has a nearly uniform thickness around the entire circumference of the core and also has high QY. For example, it has been found that increasing the shell thickness deteriorates the particle shape, and consequently, the QY also decreases. Therefore, the present invention has been made in view of these points, and aims to provide a quantum dot capable of enhancing EQE. [Means for solving the problem]

[0016] The quantum dot of the present invention is a quantum dot having a core-shell structure, wherein an intermediate layer is included between the core and the shell, the core comprises Zn and Se, the intermediate layer comprises ZnSeS, and the shell comprises ZnS, and is substantially constant in thickness.

[0017] In the present invention, the thickness of the shell is preferably 0.5 nm or more and 3 nm or less. Furthermore, the thickness of the shell is preferably 2 nm or more and 3 nm or less. The particle shape of the quantum dot is preferably substantially rectangular.

[0018] In the present invention, it is preferable that the material contains a halogen. Furthermore, it is preferable that the halogen is chlorine or bromine. It is also preferable that the material contains Cu. It is preferable that the residual amount of Cu be 100 ppm or less.

[0019] In this invention, it is preferable that the external quantum efficiency is 7% or higher. It is also preferable that the fluorescence quantum yield is 70% or higher.

[0020] In the present invention, it is preferable that the fluorescence wavelength is 410 nm or more and 470 nm or less. It is preferable that the fluorescence wavelength is 430 nm or more and 470 nm or less.

[0021] In this invention, it is preferable that the fluorescence half-width is 20 nm or less. [Effects of the Invention]

[0022] According to the quantum dots of the present invention, quantum dots with good particle shape can be synthesized, improving the QY (Quantum Yield) and ultimately achieving high EQE (Equivalent Efficiency). [Brief explanation of the drawing]

[0023] [Figure 1] Figures 1A and 1B are schematic diagrams of quantum dots in embodiments of the present invention. [Figure 2] This is a schematic diagram of an LED device using quantum dots according to an embodiment of the present invention. [Figure 3] This is a longitudinal cross-sectional view of a display device using an LED device according to an embodiment of the present invention. [Figure 4] This is a flowchart illustrating the manufacturing process of quantum dots in an embodiment of the present invention. [Figure 5]This is the photoluminescence (PL) spectrum of Example 1. [Figure 6] This is the absorption spectrum of Example 1. [Figure 7] This is the X-ray diffraction (XRD) spectrum of Example 1. [Figure 8] This table shows the measurement results for each quantum dot in Examples 1 to 7. [Figure 9] Figure 9A is a photograph of the TEM-EDX analysis results in Comparative Example 1, and Figure 9B is a photograph of the TEM-EDX analysis results in Example 1. [Figure 10] Figure 10A is a schematic partial view of Figure 9A, and Figure 10B is a schematic partial view of Figure 9B. [Modes for carrying out the invention]

[0024] The present invention will be described in detail below in the form of the embodiment described below. However, the present invention is not limited to the embodiment described below, and can be implemented in various ways within the scope of its gist.

[0025] Figures 1A and 1B are schematic diagrams of quantum dots in this embodiment. The quantum dots 5 shown in Figures 1A and 1B are cadmium (Cd)-free nanocrystals. "Nanocrystal" refers to nanoparticles having a particle size of several nanometers to several tens of nanometers. In this embodiment, a large number of quantum dots 5 can be produced with substantially uniform particle sizes.

[0026] 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). The core 5a may also contain tellurium (Te) and sulfur (S). However, it is preferable that the core 5a does not contain cadmium (Cd) or indium (In).

[0027] Furthermore, it is preferable that the shell 5b coated on the surface of the core 5a also does not contain cadmium (Cd) or indium (In), similar to the core 5a. In this embodiment, the shell 5b contains a large amount of zinc (Zn). Specifically, it is preferable that the shell 5b consists of zinc sulfide (ZnS), zinc selenide (ZnSe), or zinc selenide sulfide (ZnSeS). Of these, ZnS is preferred. The shell 5b may also be in a solid solution state on the surface of the core 5a. In this embodiment, by adopting a core-shell structure, it is possible to expect a further increase in fluorescence quantum yield (QY) while maintaining a narrow fluorescence half-width.

[0028] In this embodiment, the quantum dot 5 can be coated with a shell 5b made of ZnS or the like to a predetermined thickness over the entire surface of the core 5a. Alternatively, an intermediate layer may be interposed between the core 5a and the shell 5b. For example, this intermediate layer may be the first layer of the shell, meaning the shell 5b may have a structure of two or more layers. As an example, a shell 5b with a laminated structure made of ZnSeS / ZnS can be presented.

[0029] As shown in Figure 1A, the quantum dot 5 may have a circular cross-section, or as shown in Figure 1B, it may have a polygonal cross-section. In the case of a polygonal shape, for example, a roughly rectangular or roughly triangular shape is preferable. In this embodiment, it is preferable that the core 5a of the quantum dot 5 contains at least Zn and Se, which makes it easier for the core 5a constituting the quantum dot 5 to be formed into a polyhedron (for example, a roughly cubic shape) by crystal growth. That is, in this embodiment, the quantum dot 5 can be formed in a good shape with uniform particle shapes, rather than an amorphous shape. In this embodiment, the shell 5b can be formed around the entire circumference of the core 5a with a roughly constant thickness. Although not limited, the thickness of the shell 5b can be formed to about 0.5 mm to 3 mm, preferably to 1 mm or more and 2.5 mm or less. This is due to the incorporation of an acidic compound into the shell raw material, as will be explained in the manufacturing method described later. In addition, in this embodiment, a zinc halide compound is incorporated into the shell raw material, which can improve QY.

[0030] As shown in FIGS. 1A and 1B, it is preferable that a large number of organic ligands 11 are coordinated on the surface of the quantum dots 5. Thereby, aggregation of the quantum dots 5 can be suppressed, and the target optical properties can be exhibited. Furthermore, by adding an amine or thiol-based ligand, it is possible to greatly improve the stability of the quantum dot light-emitting characteristics. The ligands that can be used in the reaction are not particularly limited, and for example, the following ligands can be cited as representative ones. (1) Aliphatic primary amine-based Oleylamine: C , 11 , 13 , 12 , , 14 , 16 , 17 , 18 , 10 , 19 , 25 , 23 , 29 , 27 , , 33 , 31 , 37 , 15 , 21 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 (2) Fatty acid-based 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 11 H 23 COOH, Decanoic acid: C9H 19 COOH, Octanoic acid: C7H 15 COOH (3) Thiol-based Octadecanethiol: C 18 H 37 SH, Hexadecanethiol: C 16 H 33 SH, Tetradecanethiol: C 14 H 29 SH, Dodecanethiol: C 12 H 25 SH, Decanethiol: C 10 H 21 SH, Octanethiol: C8H 17 SH (4) Phosphine systems Trioctylphosphine:(C8H 17 )3P, Triphenylphosphine:(C6H5)3P, Tributylphosphine:(C4H9)3P (5) Phosphine oxide systems Trioctylphosphine oxide: (C8H 17 )3P=O, triphenylphosphine oxide: (C6H5)3P=O, tributylphosphine oxide: (C4H9)3P=O (6) Alcohol-based Oleyl alcohol: C 18 H 36 O

[0031] Furthermore, it is preferable that inorganic ligands are coordinated in combination with organic ligands. This can further suppress quantum dot surface defects and enable the development of higher optical properties. The ligands are not particularly limited, but halogens such as F, Cl, Br, and I are typical examples.

[0032] In this embodiment, elemental analysis of the quantum dot 5 by energy dispersive X-ray spectroscopy (EDX) reveals the presence of halogen elements in addition to Zn, Se, and S. The halogen element is preferably chlorine (Cl) or bromine (Br).

[0033] While there is no limit to the halogen element content, it is significantly less than Zn, Se, and S, and the halogen element content is approximately 0.01 atom% to 5 atom%. Preferably, the halogen element content is between 0.5 atom% and 2 atom%. "atom%" represents the ratio when the total number of atoms constituting the quantum dot 5 is set to 100. The halogen element content can be measured by EDX analysis.

[0034] 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 increased to 7% or more. Preferably, the EQE can be increased to 9% or more, more preferably to 9.5% or more, even more preferably to 10% or more, and even more preferably to 10.5% or more. The EQE can be evaluated using an LED measuring device and is determined by its maximum value.

[0035] Furthermore, EQE can be improved by increasing QY, as shown in (Equation 1) above. Therefore, it is preferable to increase the QY of quantum dot 5 in order to obtain a high EQE. In this embodiment, QY can be set to 70% or more, preferably 75% or more, more preferably 80% or more, even more preferably 85% or more, even more preferably 90% or more, and most preferably 95% or more.

[0036] In this embodiment, the quantum dot 5 preferably has a fluorescence half-width of 20 nm or less. "Fluorescence half-width" refers to the full width at half maximum, which indicates the spread of the fluorescence wavelength at half the peak intensity of the fluorescence intensity in the fluorescence spectrum. Furthermore, it is more preferable that the fluorescence half-width be 15 nm or less. In this embodiment, the fluorescence half-width can be narrowed, thereby improving the wide color gamut.

[0037] In this embodiment, as described later, the reaction system for synthesizing quantum dot 5 involves first synthesizing a copper chalcogenide as a precursor, and then performing a metal exchange reaction on the precursor. By producing quantum dot 5 based on such an indirect synthesis reaction, the fluorescence full width at half maximum can be narrowed.

[0038] Furthermore, in this embodiment, the fluorescence lifetime of quantum dot 5 can be reduced 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 about 50 ns, allowing for adjustment of the fluorescence lifetime depending on the application.

[0039] In this embodiment, the fluorescence wavelength can be freely controlled to approximately 410 nm to 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, the fluorescence wavelength can preferably be 430 nm or higher, and more preferably 440 nm or higher. Thus, in the quantum dot 5 of this embodiment, it is possible to control the fluorescence wavelength to blue.

[0040] Next, the method for manufacturing the quantum dot 5 of this embodiment will be described. The method for manufacturing the quantum dot 5 of this embodiment includes the steps of generating a core and coating the surface of the core with a shell, and the step of coating with a shell is characterized by blending an acidic compound and a zinc halide compound with the shell raw material.

[0041] <Method for synthesizing cores> The method for synthesizing the core will now 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.

[0042] In this embodiment, the Cu raw material is not particularly limited, but for example, the following organocopper reagents and inorganic copper reagents can be used. Specifically, as acetate salts, copper(I) acetate:Cu(OAc), copper(II) acetate:Cu(OAc)2, and as fatty acid salts, copper stearate:Cu(OC(=O)C17 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 acetylacetonate: Cu(acac)2, both monovalent and divalent compounds can be used as halides, such as copper(I) chloride: CuCl, copper(II) chloride: CuCl2, copper(I) bromide: CuBr, copper(II) bromide: CuBr2, copper(I) iodide: CuI, copper(II) iodide: CuI2, etc.

[0043] In this embodiment, the Se raw material used is an organic selenium compound (organo chalcogenide). While the structure of the compound is not particularly limited, for example, trioctylphosphine selenide, obtained by dissolving Se in trioctylphosphine: (C8H 17 )3P=Se, or tributylphosphine selenide ((C4H9)3P=Se) obtained by dissolving Se in tributylphosphine can be used. Alternatively, a solution obtained by dissolving Se at high temperature in a high-boiling point solvent such as a long-chain hydrocarbon like octadecene (Se-ODE), or a solution obtained by dissolving Se in a mixture of oleylamine and dodecanethiol (Se-DDT / OLAm) can be used.

[0044] In this embodiment, Te is used as a raw material in the form of an organic tellurium compound (organo chalcogen compound). While the structure of the compound is not particularly limited, for example, trioctylphosphine telllide (C8H) is obtained by dissolving Te in trioctylphosphine. 17 )3P=Te, or tributylphosphine tellurides such as (C4H9)3P=Te, obtained by dissolving Te in tributylphosphine, can be used. Dialkyl diterlides such as diphenyl diterlide (C6H5)2Te2, or R2Te2, can also be used.

[0045] In this embodiment, an organocopper compound or an inorganic copper compound is mixed with an organochalcogen compound and dissolved. As a solvent, octadecene can be used as a high-boiling point saturated hydrocarbon or unsaturated hydrocarbon. In addition, aromatic high-boiling point solvents such as t-butylbenzene and high-boiling point ester solvents such as butylbutyrate (C4H9COOC4H9) and benzylbutyrate (C6H5CH2COOC4H9) can be used, but aliphatic amine compounds, fatty acid compounds, aliphatic phosphorus compounds, or mixtures thereof can also be used as solvents.

[0046] At this time, the reaction temperature is set to a range of 140°C to 250°C to synthesize the copper chalcogenide precursor. Preferably, the reaction temperature is lower, between 140°C and 220°C, and even more preferably, between 140°C and 200°C.

[0047] Furthermore, although there are no particular limitations on the reaction method in this embodiment, it is important to synthesize Cu2Se, Cu2SeS, Cu2SeTe, and Cu2SeTeS with uniform particle sizes in order to obtain quantum dots with a narrow fluorescence half-width.

[0048] Next, organozinc compounds or inorganic zinc compounds are prepared as raw materials for ZnSe, ZnSeS, ZnSeTe, or ZnSeTeS. Organozinc compounds and inorganic zinc compounds are stable in air and easy to handle. While the structure of the organozinc compounds and inorganic zinc compounds is not particularly limited, it is preferable to use zinc compounds with high ionic properties in order to efficiently carry out the metal exchange reaction. For example, the following organozinc compounds and inorganic zinc compounds can be used: namely, zinc acetate: Zn(OAc)2, zinc nitrate: Zn(NO3)2 as acetate salts, and zinc stearate: Zn(OC(=O)C) as fatty acid salts. 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 acetylacetonate: Zn(acac)2, as halides, zinc chloride: ZnCl2, zinc bromide: ZnBr2, zinc iodide: ZnI2, as zinc carbamates, 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. can be used.

[0049] Next, the above-mentioned organozinc compounds or inorganic zinc compounds are added to the reaction solution in which the copper chalcogenide precursor was synthesized. This causes a metal exchange reaction between the Cu of the copper chalcogenide and Zn. The metal exchange reaction is preferably carried out at a temperature of 150°C to 300°C. Furthermore, it is even more preferable to carry out the metal exchange reaction at a lower temperature, between 150°C and 280°C, and more preferably between 150°C and 250°C.

[0050] In this embodiment, the metal exchange reaction between Cu and Zn proceeds quantitatively, and it is preferable that the nanocrystals do not contain precursor Cu. This is because if precursor Cu remains in the nanocrystals, the Cu acts as a dopant, emitting light through a different luminescence mechanism and broadening the fluorescence full width at half maximum. The amount of residual Cu is preferably 100 ppm or less relative to Zn, more preferably 50 ppm or less, and ideally 10 ppm or less.

[0051] In this embodiment, ZnSe-based quantum dots synthesized by the cation exchange method tend to have a higher Cu content than ZnSe-based quantum dots synthesized by the direct method, but good luminescence properties can be obtained even if the Cu content is around 1 to 10 ppm relative to Zn. Furthermore, it is possible to determine whether a quantum dot was synthesized by the cation exchange method based on the Cu content. In other words, by synthesizing using the cation exchange method, particle size can be controlled with the copper chalcogenide precursor, and a synthesis method that is inherently less reactive becomes possible, so the Cu content is advantageous in determining whether the cation exchange method was used.

[0052] Furthermore, in this embodiment, a compound is needed that plays an auxiliary role in releasing the metal of the copper chalcogenide precursor into the reaction solution through coordination or chelation during metal exchange.

[0053] Compounds having the above-mentioned role include ligands that can form complexes with Cu. For example, phosphorus-based ligands, amine-based ligands, and sulfur-based ligands are preferred, and among these, phosphorus-based ligands are even more preferred due to their high efficiency.

[0054] This ensures proper metal exchange between Cu and Zn, enabling the production of quantum dots with a narrow fluorescence half-width based on Zn and Se. In this embodiment, the cation exchange method described above allows for mass production of quantum dots compared to direct synthesis.

[0055] In other words, in direct synthesis methods, organozinc compounds such as diethylzinc (Et2Zn) are used to increase the reactivity of the Zn raw material. However, diethylzinc is highly reactive and ignites in air, so it must be handled under an inert gas stream, making the handling and storage of the raw material difficult. Furthermore, reactions using it involve risks such as heat generation and ignition, making it unsuitable for mass production. Similarly, reactions using selenium hydride (H2Se), for example, to increase the reactivity of the Se raw material are also unsuitable for mass production from a toxicity and safety standpoint.

[0056] Furthermore, in reaction systems using highly reactive Zn or Se raw materials as described above, although ZnSe is produced, particle formation is not controlled, resulting in a broader fluorescence half-width of the resulting ZnSe.

[0057] In contrast, in this embodiment, a copper chalcogenide precursor is synthesized from an organic copper compound or an inorganic copper compound and an organic chalcogen compound, and quantum dots are synthesized by metal exchange using the copper chalcogenide precursor. Thus, in this embodiment, quantum dots are synthesized first by synthesizing a copper chalcogenide precursor, and not directly. This indirect synthesis eliminates the need to use reagents that are too reactive and dangerous to handle, making it possible to safely and stably synthesize ZnSe-based quantum dots with a narrow fluorescence half-width.

[0058] Furthermore, in this embodiment, it is possible to perform a one-pot metal exchange between Cu and Zn without isolating and purifying the copper chalcogenide precursor, thereby obtaining quantum dots with the desired composition and particle size. Alternatively, the copper chalcogenide precursor may be isolated and purified before use. Furthermore, in this embodiment, the synthesized quantum dots exhibit fluorescence properties without undergoing various treatments such as washing, isolation and purification, coating, or ligand exchange.

[0059] <How to compose a shell> The method for synthesizing the shell will be explained using the flowchart shown in Figure 4. In this embodiment, for example, after synthesizing a ZnSe core, the surface of the ZnSe core is coated with, for example, ZnSeS. To coat with ZnSeS, for example, a mixture of Se-TOP solution, S-TOP solution, and zinc oleate is added to a solution in which the ZnSe core is dispersed, and the mixture is heated at a predetermined temperature while stirring. By repeating this operation multiple times, the surface of the ZnSe can be coated with ZnSeS.

[0060] In this embodiment, after washing ZnSe / ZnSeS, it is dispersed in, for example, octadecene (ODE), and then trioctylphosphine (TOP) and oleic acid are added, and the mixture is heated and stirred under predetermined heat treatment conditions (for example, 320°C for 10 minutes).

[0061] Next, in this embodiment, the ZnS shell is coated. In this embodiment, it is preferable to divide the process of coating the ZnS shell into at least two parts: the first half and the second half. First, in the first half of the process of coating the ZnS shell, 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, dodecanethiol (DDT), and TOP are added, and then an acidic oxide is added. In this embodiment, this shell source mixture containing the acidic oxide is added and heated while stirring under predetermined heating conditions. The predetermined heating conditions are, for example, a heating temperature of 320°C and a heating time of 10 minutes. In this embodiment, the operation of adding the shell source mixture and heating is repeated multiple times. Figure 4 shows 10 repetitions, but "10 times" is just an example and does not limit the number of repetitions. However, it is preferable to specify the number of repetitions within the range of 5 to 15 times. After that, it is cooled to room temperature.

[0062] In this embodiment, an acidic compound is added to the shell source mixture in the first shell coating step, but the zinc halide compound that is added in the second shell coating step is not added. It is known that adding a zinc halide compound to the shell source mixture in the first shell coating step reduces QY. Therefore, a zinc halide compound is not added to the shell source mixture in the first shell coating step.

[0063] Next, in this embodiment, the latter half of the shell coating process is performed. In the latter half of the shell coating process, a shell source mixture containing an acidic compound and a zinc halide compound is added to a solution in which ZnSe / ZnSeS / ZnS are dispersed. To this shell source mixture, for example, zinc oleate (Zn(OLAc)2) solution, dodecanethiol (DDT), and TOP are added along with the zinc halide compound and the acidic compound. Thus, in the latter half of the shell coating process, the shell source mixture containing the acidic compound and the zinc halide compound is added and heated while stirring under predetermined heating conditions. The predetermined heating conditions are, for example, a heating temperature of 320°C and a heating time of 10 minutes. In this embodiment, the operation of adding the shell source mixture and heating is repeated multiple times. Figure 4 shows 10 repetitions, but "10 times" is just an example and does not limit the number of repetitions. However, it is preferable to specify the number of repetitions within the range of 5 to 15 times.

[0064] The mixture is then cooled to room temperature, washed, and dispersed by adding ODE. This process, from adding the shell source mixture to dispersing with ODE, is repeated until the desired shell thickness is achieved. Thus, the latter part of the shell coating process is characterized by the addition of a shell source mixture containing acidic compounds and zinc halide compounds.

[0065] In this embodiment, the aim is to improve EQE, which requires improving QY and further optimizing the particle shape. If QY can be increased, EQE can be improved as shown in (Equation 1).

[0066] The optimization of particle shape is explained as follows: When the cores of quantum dots are close together, Förster resonance energy (FRET) is generated, leading to a decrease in EQE. Therefore, by using a core-shell structure in which a shell surrounds the core, the cores can be physically separated, and it is thought that FRET can be reduced. However, increasing the shell thickness deteriorates the particle shape, and consequently, QY also decreases. Furthermore, conventional methods have had problems such as defects occurring because the shell could not be covered with the predetermined thickness across the entire surface of the core, or the shell thickness becoming locally thick, which deteriorates the particle shape. As a result, it was not possible to appropriately reduce FRET, and thus EQE could not be effectively reduced.

[0067] Therefore, in this embodiment, the QY can be improved by adding a small amount of zinc halide compound to the core. In particular, by not adding the zinc halide compound in the first half of the shell coating process, but only in the second half, the QY can be effectively improved. Furthermore, since the particle shape deteriorates if the shell source mixture is continuously added, adding an acidic compound to the shell source mixture etches the areas of the shell that are locally thicker, thereby straightening the shape and allowing for the creation of good particle shapes with polygonal cross-sections.

[0068] In this embodiment, it is preferable to add the zinc halide compound to zinc oleate in an amount of approximately 0.5 mol% to 3 mol%, and more preferably to an amount of approximately 1 mol% to 2 mol%.

[0069] In this embodiment, at least one can be selected as the acidic compound from hydrogen chloride (HCl), hydrogen bromide (HBr), hydrogen iodide (HI), trifluoroacetic acid (TFA), trifluoromethanesulfonic acid (TfOH), acetic acid (AA), sulfuric acid (H2SO4), phosphoric acid (H3PO4), etc. Of these, it is preferable to use at least one of hydrogen chloride (HCl), hydrogen bromide (HBr), and trifluoroacetic acid (TFA). This allows for a high QY and good particle shape of quantum dots. In this embodiment, for example, a hydrogen oxide-ethyl acetate solution can be added to the shell source mixture.

[0070] In this embodiment, it is preferable to use zinc chloride (ZnCl2), or at least one of zinc bromide (ZnBr2), zinc fluoride (ZnF2), or zinc iodide (ZnI2) 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. Furthermore, in this embodiment, the S raw material used in the core-shell structure is not particularly limited, but the following raw materials are typical examples.

[0071] In other words, as a thiol, octadecanethiol:C 18 H 37 SH, Hexanedecanethiol:C 16 H 33 SH, Tetradecanethiol:C 14 H 29 SH, Dodecanethiol:C 12 H 25 SH, Decanethiol:C 10 H 21 SH, Octanethiol:C8H 17Solutions can be used in which sulfur is dissolved in a high-boiling solvent that is a long-chain phosphine hydrocarbon such as SH, benzenethiol (C6H5SH), or trioctylphosphine (S-TOP); further, solutions can be used in which sulfur is dissolved in a high-boiling solvent that is a long-chain hydrocarbon such as octadecene (S-ODE); or solutions can be used in which sulfur is dissolved in a mixture of oleylamine and dodecanethiol (S-DDT / OLAm).

[0072] The reactivity differs depending on the S raw material used, and as a result, the coating thickness of shell 5b (e.g., ZnS) can be varied. The reactivity of thiol-based materials is proportional to their decomposition rate, while the reactivity of S-TOP or S-ODE changes in proportion to their stability. Therefore, by selecting the appropriate S raw material, it is possible to control the coating thickness of shell 5b and thus the final fluorescence quantum yield.

[0073] Furthermore, in this embodiment, the less amine-based solvent used when coating the shell 5b, the easier it is to coat the shell 5b and the better the luminescence characteristics can be obtained. Moreover, the luminescence characteristics after coating the shell 5b differ depending on the ratio of amine-based solvent, carboxylic acid-based solvent, or phosphine-based solvent.

[0074] Furthermore, the quantum dots 5 synthesized by the manufacturing method of this embodiment can be aggregated by adding a polar solvent such as methanol, ethanol, or acetone, allowing for the separation and recovery of the quantum dots 5 from the unreacted raw materials. The recovered quantum dots 5 can then be dispersed again by adding toluene, hexane, or the like. By adding a ligand solvent to this redispersed solution, the luminescence properties and their stability can be further improved. The change in luminescence properties due to the addition of this ligand differs significantly depending on whether or not the shell 5b coating operation is performed. In this embodiment, quantum dots 5 coated with shell 5b can have their fluorescence stability particularly improved by adding a thiol-based ligand.

[0075] The applications of the quantum dot 5 shown in Figures 1A and 1B are not particularly limited, but for example, the quantum dot 5 of this embodiment that emits blue fluorescence can be applied to wavelength conversion members, lighting members, backlight devices, and display devices.

[0076] The quantum dot 5 of this embodiment can be applied to a wavelength conversion member, lighting member, backlight device, and display device, for example, when photoluminescence (PL) is used as the light emission principle, it becomes possible to emit blue fluorescence when irradiated with UV light from a light source. Alternatively, when electroluminescence (EL) is used as the light emission principle, or when all three primary colors are emitted by quantum dots by other means, a light-emitting element that emits blue fluorescence can be created using the quantum dot 5 of this embodiment. In this embodiment, by including 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, it becomes possible to emit white light.

[0077] Figure 2 is a schematic diagram of an LED device using quantum dots according to this embodiment. As shown in Figure 2, the LED device 1 of this embodiment is composed of a housing case 2 having a bottom surface 2a and side walls 2b surrounding the bottom surface 2a, an LED chip (light-emitting element) 3 placed on the bottom surface 2a of the housing case 2, and a fluorescent layer 4 filled inside the housing case 2 and sealing the upper side of the LED chip 3. Here, the upper side is the direction in which the light emitted from the LED chip 3 is emitted from the housing case 2, and is the direction opposite to the bottom surface 2a with respect to the LED chip 3.

[0078] The LED chip 3 is placed on a base wiring board (not shown), which may constitute the bottom surface of the housing case 2. As the base board, for example, a configuration in which a wiring pattern is formed on a substrate such as glass epoxy resin can be presented. LED chip 3 is a semiconductor element that emits light when a voltage is applied in the forward direction, and has a basic configuration in which a P-type semiconductor layer and an N-type semiconductor layer are joined at a PN junction. As shown in Figure 2, the fluorescent layer 4 is formed of a resin 6 in which a large number of quantum dots 5 are dispersed.

[0079] Furthermore, the resin composition in which the quantum dots 5 are dispersed in this embodiment may also contain a fluorescent substance other than the quantum dots 5. Examples of fluorescent substances include sialon-based materials and KSF(K2SiF6:Mn 4+ While red phosphors and other materials are available, the material is not particularly limited.

[0080] The resin 6 constituting the fluorescent layer 4 is not particularly limited, but 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 can be used.

[0081] The LED device using quantum dots in this embodiment can be applied to a display device. Figure 3 is a longitudinal cross-sectional view of a display device using the LED device shown in Figure 2. As shown in Figure 3, the display device 50 is composed of a plurality of LED devices 20 and a display unit 54 such as a liquid crystal display facing each LED device 20. Each LED device 20 is arranged on the back side of the display unit 54. Each LED device 20 has a structure in which an LED chip is sealed with a resin in which a large number of quantum dots 5 are diffused, similar to the LED device 1 shown in Figure 2.

[0082] As shown in Figure 3, multiple LED devices 20 are supported by a support 52. Each LED device 20 is arranged at a predetermined interval. Each LED device 20 and the support 52 constitute a backlight 55 for the display unit 54. The support 52 is not limited in shape or material, and can be a sheet, plate, or case. As shown in Figure 3, a light diffuser plate 53 or the like may be interposed between the backlight 55 and the display unit 54.

[0083] By applying the quantum dot 5 in this embodiment to the LED device shown in Figure 2, the display device shown in Figure 3, etc., it is possible to effectively improve the light emission characteristics of the device. In particular, the EQE can be improved when the quantum dot of this embodiment is applied to a QLED element. 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.

[0084] Furthermore, the resin composition in which the quantum dots 5 of this embodiment are dispersed in a resin can also be formed into a sheet or a film. Such a sheet or film can be incorporated into, for example, a backlight device.

[0085] Furthermore, in this embodiment, a wavelength conversion member in which multiple quantum dots are dispersed in a resin can be formed as a molded body. For example, a molded body in which quantum dots are dispersed in a resin can be housed in a container having a storage space by press-fitting or the like. In this case, it is preferable that the refractive index of the molded body is smaller than the refractive index of the container. As a result, some of the light that enters the molded body is totally reflected by the inner wall of the container. Therefore, the beam of light leaking out from the side of the container to the outside can be reduced. In this way, by applying the quantum dots in this embodiment to wavelength conversion members, lighting members, backlight devices, and display devices, it is possible to effectively improve the luminescence characteristics. [Examples]

[0086] The effects of the present invention will be explained below with reference to examples and comparative examples of the present invention. However, the present invention is not limited in any way by the following examples.

[0087] In this invention, the following raw materials were used to synthesize Cd-free blue fluorescent quantum dots. Furthermore, the following measuring instruments were used to evaluate the synthesized quantum dots. <Raw materials> Anhydrous copper acetate: Manufactured by Wako Pure Chemical Industries, Ltd. Octadecene: Manufactured by Idemitsu Kosan Co., Ltd. Oleylamine: Manufactured by Kao Corporation (Farmin) Oleic acid: Lunac OV manufactured by Kao Corporation Dodecanethiol (DDT): Manufactured by Kao Corporation, Thiocalcol 20 Trioctylphosphine (TOP): Manufactured by Hokko Chemical Co., Ltd. Anhydrous zinc acetate: Manufactured by Kishida Chemical Co., Ltd. Selenium (4N: 99.99%): Manufactured by Shinko Chemical Co., Ltd. Sulfur: Manufactured by Kishida Chemical Co., Ltd. Hydrogen chloride: Manufactured by Kokusan Chemical Co., Ltd. Zinc chloride: Manufactured by Kanto Chemical Co., Ltd. Hydrogen bromide: Manufactured by Tokyo Chemical Industry Co., Ltd. Zinc bromide: Manufactured by Kishida Chemical Co., Ltd. <Measuring equipment> Fluorescence spectrometer: JASCO Corporation F-2700 Ultraviolet-Visible Spectrophotometer: Hitachi V-770 Fluorescence quantum yield analyzer: QE-1100, manufactured by Otsuka Electronics Co., Ltd. X-ray diffraction device (XRD): Bruker D2 PHASER Scanning-line electron microscope (SEM): Hitachi SU9000 Fluorescence lifetime analyzer: Hamamatsu Photonics C11367 LED measuring device: Manufactured by SpectraCorp Transmission electron microscope (TEM): JEOL Ltd. JEM-ARM200-CF XEDS detector: JEOL Ltd. JED2300T

[0088] [Example 1] [Synthesis method of ZnSe core] Into a 300 mL reaction vessel, 728 mg of copper acetate anhydrous: Cu(OAc)2, 19.2 mL of oleylamine: OLAm, and 31 mL of octadecene: ODE were added. Then, under an inert gas (N2) atmosphere, it was heated with stirring at 165 °C for 20 minutes to dissolve the raw materials.

[0089] To this solution, 4.56 mL of Se-DDT / OLAm solution (0.7 M) was added, and it was heated with stirring at 165 °C for 30 minutes. The obtained reaction solution (CuSe) was cooled to room temperature.

[0090] Thereafter, 7376 mg of zinc acetate anhydrous: Zn(OAc)2, 40 mL of trioctylphosphine: TOP, and 1.6 mL of oleylamine: OLAm were added to the CuSe reaction solution, and it was heated with stirring at 200 °C for 1 hour under an inert gas (N2) atmosphere. The obtained reaction solution (ZnSe) was cooled to room temperature.

[0091] Ethanol was added to the reaction solution cooled to room temperature to cause precipitation, and centrifugation was performed to collect the precipitate. 96 ml of octadecene: ODE was added to the precipitate and dispersed.

[0092] Thereafter, 7376 mg of zinc acetate anhydrous: 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, and it was heated with stirring at 290 °C for 30 minutes under an inert gas (N2) atmosphere. The obtained reaction solution (ZnSe) was cooled to room temperature. The obtained reaction solution was measured with a fluorescence spectrometer. As a result, optical properties with a fluorescence wavelength of about 446.5 nm and a fluorescence half-width of about 14 nm were obtained.

[0093] [Method for coating a shell on the ZnSe core] Ethanol was added to 40 ml of ZnSe reaction solution to generate a precipitate, which was then collected by centrifugation. 35 ml of octadecene:ODE was added to the precipitate and dispersed.

[0094] 35 mL of dispersed ZnSe-ODE solution was mixed with 2 mL of oleic acid (OLAc) and 4 mL of trioctylphosphine (TOP), and heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0095] To this solution, 0.9 mL of a mixture consisting of 0.5 mL of Se-TOP solution (1M), 0.5 mL of S-TOP solution (1M), and 5 mL of zinc oleate:Zn(OLAc)2 solution (0.4M) was added, and the mixture was heated at 320°C for 10 minutes while stirring. This procedure was repeated four times.

[0096] Subsequently, ethanol was added to the resulting reaction mixture to generate a precipitate, which was then collected by centrifugation. 35 ml of octadecene (ODE) was added to the precipitate to disperse it. Then, as before, 2 ml of oleic acid (OLAc) and 4 ml of trioctylphosphine (TOP) were added, and the mixture was heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0097] To this solution, 0.9 mL of a mixture consisting of 0.4 mL of DDT, 1.6 mL of trioctylphosphine (TOP), 0.12 mL of ethyl chloride solution (4M), and 10 mL of zinc oleate (Zn(OLAc)2) solution (0.4M) was added, and the mixture was heated at 320°C for 10 minutes with stirring. This procedure was repeated 10 times.

[0098] Subsequently, ethanol was added to the resulting reaction mixture to generate a precipitate, which was then collected by centrifugation. 35 ml of octadecene (ODE) was added to the precipitate to disperse it. Then, as before, 2 ml of oleic acid (OLAc) and 4 ml of trioctylphosphine (TOP) were added, and the mixture was heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0099] To this solution, 0.9 mL of a mixture consisting of 0.4 mL of DDT, 1.6 mL of trioctylphosphine (TOP), 0.12 mL of ethyl hydrogen chloride solution (4M), 0.1 mL of zinc chloride-TOP oleic acid solution (0.8M), and 10 mL of zinc oleate (Zn(OLAc)2) solution (0.4M) was added, and the mixture was heated at 320°C for 10 minutes with stirring. This procedure was repeated 10 times.

[0100] Subsequently, ethanol was added to the resulting reaction mixture to generate a precipitate, which was then collected by centrifugation. 35 ml of octadecene (ODE) was added to the precipitate to disperse it. Then, as before, 2 ml of oleic acid (OLAc) and 4 ml of trioctylphosphine (TOP) were added, and the mixture was heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0101] To this solution, 0.9 mL of a mixture consisting of 0.4 mL of DDT, 1.6 mL of trioctylphosphine (TOP), 0.2 mL of ethyl hydroxide solution (4M), 0.1 mL of zinc chloride-TOP oleic acid solution (0.8M), and 10 mL of zinc oleate (Zn(OLAc)2) solution (0.4M) was added, and the mixture was heated at 320°C for 10 minutes with stirring. This procedure was repeated 10 times. The resulting reaction solution was measured using a fluorescence spectrometer. As a result, as shown in Figure 5, optical properties were obtained with a fluorescence wavelength of approximately 442 nm and a fluorescence full width at half maximum of approximately 15 nm. Ethanol was added to the resulting reaction solution to generate a precipitate, which was then collected by centrifugation. Hexane was added to the precipitate to disperse it. The resulting dispersion was measured using a UV-Vis spectrometer. As a result, the UV-Vis absorption spectrum shown in Figure 6 was obtained. Figure 7 shows the X-ray diffraction (XRD) spectrum of Example 1. From the results in Figure 7, crystal peaks of a cubic crystal composed of Zn, Se, and S were confirmed.

[0102] <Measurement results> ZnSe / ZnSeS / ZnS dispersed in hexane was measured using a quantum efficiency measurement system. The fluorescence quantum yield was approximately 96%. The fluorescence lifetime was measured to be 16 ns. Elemental analysis (EDX) revealed Zn: 42 atom%, Se: 11 atom%, S: 41 atom%, and Cl: 1 atom%. Analysis of images obtained by TEM showed a shell thickness of 2.0 nm. Furthermore, by applying the quantum dots obtained in Example 1, a light-emitting element having the following stacked structure was manufactured. ITO / PEDOT:PSS / PVK / QD layer / LiZnO / Al When this element was evaluated using an LED measuring device, the maximum value of its external quantum efficiency (EQE) was found to be 18.6%.

[0103] [Example 2] The synthesis was carried out under the same conditions as in Example 1, except that the zinc chloride-TOP·oleic acid solution used in Example 1 was replaced with a zinc bromide-TOP·oleic acid solution. [Example 3] The synthesis was carried out under the same conditions as in Example 2, except that the hydrogen chloride-ethyl acetate solution (4M) used in Example 2 (see description in Example 1) was replaced with a hydrogen bromide-acetic acid solution. [Example 4] The synthesis was carried out under the same conditions as in Example 1, except that the hydrogen chloride-ethyl acetate solution (4M) used in Example 1 was replaced with trifluoroacetic acid. [Example 5] The synthesis was carried out under the same conditions as in Example 2, except that the hydrogen chloride-ethyl acetate solution (4M) used in Example 2 (see description in Example 1) was replaced with trifluoroacetic acid.

[0104] Figure 8 is a table summarizing the measurement results for Examples 1 to 5. TEM images of each quantum dot obtained in Examples 1 to 5 are also included. As shown in Figure 8, in all of Examples 1 to 5, the EQE was increased to 7% or higher. In particular, in Example 1, the EQE was improved to 18.6%. Also, in any of the examples, the QY could be made 70% or more. In particular, in Example 2, the QY could be improved to 98%.

[0105] Also, in each example, the fluorescence half-width could be made 20 nm or less. Furthermore, in any of the examples, the fluorescence wavelength could be confined within the range of 410 nm to 470 nm, showing blue fluorescence. Also, the shell thickness of each example was in the range of about 2 nm to 2.5 nm. Note that the shell thickness can be estimated from the photograph of the analysis results of TEM-EDX.

[0106] As shown in the SEM photographs of each example in Fig. 8, it was found that the particle shape of the quantum dots was substantially rectangular (substantially cubic) and good. That is, it is considered that the ZnSe core was crystallized into a substantially rectangular shape, and a shell with a predetermined thickness was coated over the entire circumference thereof, thereby maintaining the substantially rectangular particle shape. This is presumably because the acidic compound was incorporated into the shell source mixture, and the effect of etching the portions where the particle shape deteriorated was exerted.

[0107] [Example 6] Among the synthesis processes used in Example 1, the <Synthesis method of ZnSe core> was the same, and a part of the <Method of coating the shell on the ZnSe core> was changed to synthesize quantum dots. Hereinafter, the <Method of coating the shell on the ZnSe core> of Example 6 will be described.

[0108] <Method of coating the shell on the ZnSe core> Ethanol was added to 40 ml of the ZnSe reaction solution to generate precipitation, and the precipitation was collected by centrifugation. 35 ml of octadecene: ODE was added to the precipitation and dispersed.

[0109] 2 mL of oleic acid: OLAc and 4 mL of trioctylphosphine: TOP were added to 35 mL of the dispersed ZnSe-ODE solution, and it was heated with stirring at 320 °C for 10 minutes under an inert gas (N2) atmosphere.

[0110] To this solution, 0.9 mL of a mixture consisting of 0.5 mL of Se-TOP solution (1M), 0.5 mL of S-TOP solution (1M), and 5 mL of zinc oleate:Zn(OLAc)2 solution (0.4M) was added, and the mixture was heated at 320°C for 10 minutes while stirring. This procedure was repeated four times.

[0111] Subsequently, ethanol was added to the resulting reaction mixture to generate a precipitate, which was then collected by centrifugation. 35 ml of octadecene (ODE) was added to the precipitate to disperse it. Then, as before, 2 ml of oleic acid (OLAc) and 4 ml of trioctylphosphine (TOP) were added, and the mixture was heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0112] To this solution, 0.9 mL of a mixture consisting of 0.6 mL of DDT, 1.4 mL of trioctylphosphine (TOP), 0.24 mL of ethyl chloride solution (4M), and 10 mL of zinc oleate (Zn(OLAc)2) solution (0.48M) was added, and the mixture was heated at 320°C for 10 minutes with stirring. This procedure was repeated 10 times.

[0113] Subsequently, ethanol was added to the resulting reaction mixture to generate a precipitate, which was then collected by centrifugation. 35 ml of octadecene (ODE) was added to the precipitate to disperse it. Then, as before, 2 ml of oleic acid (OLAc) and 4 ml of trioctylphosphine (TOP) were added, and the mixture was heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0114] To this solution, 0.9 mL of a mixture consisting of 0.6 mL of DDT, 1.4 mL of trioctylphosphine (TOP), 0.24 mL of ethyl hydroxide solution (4M), 0.1 mL of zinc chloride-TOP oleic acid solution (0.8M), and 10 mL of zinc oleate (Zn(OLAc)2) solution (0.48M) was added, and the mixture was heated at 320°C for 10 minutes with stirring. This procedure was repeated 10 times.

[0115] <Measurement results for Example 6> Hexane-dispersed ZnSe / ZnSeS / ZnS was measured using a quantum efficiency measurement system. As a result, the fluorescence quantum yield was approximately 90%. Also, as a result of measuring the fluorescence lifetime, it was 20 ns. As a result of analyzing the image obtained by TEM, the shell thickness was 2.7 nm.

[0116] [Example 7] The <synthesis method of ZnSe core> and <coating method of shell on ZnSe core> of Example 1 were used as they were. Finally, 2.0 mL of a zinc chloride-TOP zinc chloride-TOP·oleic acid solution (0.8 M) was added, and it was heated while stirring for 20 minutes. [Measurement Results of Example 7] Hexane-dispersed ZnSe / ZnSeS / ZnS was measured using a quantum efficiency measurement system. As a result, the fluorescence quantum yield was approximately 84%. Also, as a result of measuring the fluorescence lifetime, it was 25 ns. As a result of elemental analysis (EDX), it was Zn: 32 atom%, Se: 12 atom%, S: 50 atom%, Cl: 6 atom%. As a result of analyzing the image obtained by TEM, the shell thickness was 2.0 nm.

[0117] In Example 6, the shell was made thicker than in Example 1 in order to more effectively prevent Förster resonance energy transfer (FRET). Specifically, while the shell thickness in Example 1 was 2 nm, in Example 6, the shell thickness was increased to 2.7 nm. Also, in Example 6, it was possible to suppress the decrease in fluorescence quantum yield (QY) as much as possible compared to Example 1.

[0118] In Example 7, the chlorine content was increased for the purpose of reducing Zn without ligands on the surface of the quantum dots. That is, while the Cl content in Example 1 was 1 atom%, the Cl content in Example 7 was 6 atom%. [Comparative Example 1] Comparative Example 1 is an example in which a shell was coated without mixing an acidic compound and a zinc halide compound in the shell source mixture. Specifically, the shell was coated by the following steps.

[0119] In a 100 mL reaction vessel, 2182 mg of anhydrous copper acetate (Cu(OAc)), 4.8 mL of oleylamine (OLAm), and 7.75 mL of octadecene (ODE) were added. The mixture was then heated at 165 °C for 5 minutes with stirring under an inert gas (N2) atmosphere to dissolve the starting materials.

[0120] To this solution, 1.14 mL of Se-DDT / OLAm solution (0.7 M) was added and the mixture was heated at 165°C for 30 minutes with stirring. The resulting reaction solution (CuSe) was cooled to room temperature.

[0121] Subsequently, 21844 mg of anhydrous zinc acetate (Zn(OAc)), 10 mL of trioctylphosphine (TOP), and 0.4 mL of oleylamine (OLAm) were added to the Cu2Se reaction solution, and the mixture was heated at 180°C for 45 minutes with stirring under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSe) was then cooled to room temperature.

[0122] Ethanol was added to the reaction mixture cooled to room temperature to generate a precipitate, which was then collected by centrifugation. 12 ml of octadecene:ODE was added to the precipitate and dispersed.

[0123] Subsequently, 21844 mg of anhydrous zinc acetate (Zn(OAc)), 10 mL of trioctylphosphine (TOP), 1 mL of oleylamine (OLAm), and 6 mL of oleic acid (OLAc) were added to 12 ml of ZnSe-ODE solution, and the mixture was heated at 280°C for 20 minutes with stirring under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSe) was then cooled to room temperature.

[0124] The resulting reaction solution was measured using a fluorescence spectrometer. The results showed optical properties with a fluorescence wavelength of approximately 447.5 nm and a fluorescence half-width of approximately 14 nm.

[0125] Ethanol was added to 20 ml of the obtained ZnSe reaction solution to generate a precipitate, which was then collected by centrifugation. 17.5 ml of octadecene:ODE was added to the precipitate and dispersed.

[0126] To 17.5 mL of dispersed ZnSe-ODE solution, 1 mL of oleic acid (OLAc) and 2 mL of trioctylphosphine (TOP) were added, and the mixture was heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0127] To this solution, 0.5 mL of a mixture consisting of 0.5 mL of Se-TOP solution (1 M), 0.125 mL of DDT, 0.375 mL of trioctylphosphine:TOP, and 5 mL of zinc oleate:Zn(OLAc)2 solution (0.4 M) was added, and the mixture was heated at 320°C for 10 minutes with stirring. This procedure was repeated four times.

[0128] Subsequently, ethanol was added to the resulting reaction mixture to generate a precipitate, which was then collected by centrifugation. 17.5 ml of octadecene (ODE) was added to the precipitate to disperse it, and 1 ml of oleic acid (OLAc) and 2 ml of trioctylphosphine (TOP) were added as before. The mixture was then heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0129] To this solution, 0.5 mL of a mixture containing 0.5 mL of DDT, 1.5 mL of trioctylphosphine (TOP), and 10 mL of zinc oleate (Zn(OLAc)2) solution (0.4 M) was added, and the mixture was heated at 320°C for 10 minutes with stirring. This procedure was repeated 10 times.

[0130] Subsequently, ethanol was added to the resulting reaction solution to generate a precipitate, which was then collected by centrifugation. 17.5 ml of octadecene:ODE was added to the precipitate and dispersed (washing step).

[0131] Next, as before, 1 mL of oleic acid (OLAc) and 2 mL of trioctylphosphine (TOP) were added, and the mixture was heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere. To this solution, 0.5 mL of a mixture of 0.5 mL of DDT, 1.5 mL of trioctylphosphine (TOP), and 10 mL of zinc oleate (Zn(OLAc)2) solution (0.4 M) was added, and the mixture was heated at 320°C for 10 minutes with stirring. This procedure was repeated six times. After that, the mixture was heated at 320°C for 30 minutes with stirring (shell coating process).

[0132] Subsequently, the reaction solution was subjected to the above-mentioned (washing step) and (shell coating step) operations three times to finally obtain the target reaction solution (ZnSe / ZnS), which was then cooled to room temperature.

[0133] The resulting reaction solution was measured using a fluorescence spectrometer. The results showed optical properties with a fluorescence wavelength of approximately 443 nm and a fluorescence full width at half maximum of approximately 15 nm. Ethanol was added to the resulting reaction solution to generate a precipitate, which was then collected by centrifugation. Hexane was then added to the precipitate to disperse it.

[0134] The hexane-dispersed ZnSe / ZnSeS / ZnS was measured using a quantum efficiency measurement system. The results showed a fluorescence quantum yield of approximately 60%. The fluorescence lifetime was measured to be 14 ns. Furthermore, by applying the quantum dots obtained in Comparative Example 1, a light-emitting device having the following stacked structure was manufactured. ITO / PEDOT:PSS / PVK / QD layer / ZnO / Al When this element was evaluated using an LED measuring device, the maximum value of its external quantum efficiency (EQE) was found to be 4.0%. Below, we compare Example 1 and Comparative Example 1. Table 1 shows the measurement results for Example 1 and Comparative Example 1.

[0135] [Table 1]

[0136] Comparative Example 1 was found to have a lower EQE compared to Example 1. Figure 9A is a photograph of the TEM-EDX analysis results in Comparative Example 1, and Figure 9B is a photograph of the TEM-EDX analysis results in Example 1. Figure 10A is a schematic diagram of a portion of Figure 9A, and Figure 10B is a schematic diagram of a portion of Figure 9B.

[0137] As shown in Figures 9A and 9B, the TEM-EDX analysis results are shown in three colors (red, blue, and green). The central part is mainly a mixture of red and blue, appearing roughly purple, while the outer part is mainly a mixture of red and green, appearing roughly yellow. Since red represents Zn, blue represents Se, and green represents S, it can be concluded that the central part mainly contains Zn and Se, while the outer part mainly contains Zn and S. Therefore, from the TEM-EDX analysis results shown in Figures 9A and 9B, it can be inferred that the core is ZnSe and the shell is ZnS. Furthermore, the shell thickness can be estimated by measuring the thickness of the roughly yellow area in the TEM-EDX analysis results.

[0138] Figures 9A and 10A show that in Comparative Example 1, the shell covering the core was not of a nearly constant thickness, but was interrupted in places, and there were areas where the shell grew locally. Therefore, the particle shape of Comparative Example 1 was deteriorated, making it easier for Förster resonance energy transfer (FRET) to occur, and resulting in a decrease in EQE. In addition, Comparative Example 1 did not achieve the same high QY as Example 1.

[0139] In contrast, in Example 1, as shown in Figures 9B and 10B, the shell neatly covered the entire circumference of the core, the shell had a substantially constant thickness, and the particle shape of the quantum dots was substantially rectangular. Thus, the particle shape of Example 1 was better than that of Comparative Example 1, and a sufficiently higher QY was obtained compared to Comparative Example 1. As a result, a sufficiently higher EQE was obtained in Example 1 compared to Comparative Example 1. [Industrial applicability]

[0140] According to the present invention, quantum dots that emit blue fluorescence can be stably obtained. Furthermore, by applying the quantum dots of the present invention to LEDs, backlight devices, display devices, etc., excellent luminescence characteristics can be obtained in each device. [Explanation of Symbols]

[0141] 1:LED device 2: Storage Case 3: LED chip 4: Fluorescent layer 5: Quantum dots 5a: Core 5b: Shell 11:Organic ligand 50:Display device 52:Support 53: Light Diffuser 54:Display section 55: Backlight

Claims

1. A quantum dot with a core-shell structure, The core includes an intermediate layer between it and the shell. The aforementioned core includes Zn and Se, The aforementioned intermediate layer includes ZnSeS, The aforementioned shell contains ZnS and has a substantially constant thickness. A quantum dot characterized by the following features.

2. The quantum dot according to claim 1, characterized in that the thickness of the shell is 0.5 nm or more and 3 nm or less.

3. The quantum dot according to claim 2, characterized in that the thickness of the shell is 2 nm or more and 3 nm or less.

4. The quantum dot according to claim 1 or 3, characterized in that the particle shape of the quantum dot is substantially rectangular.

5. A quantum dot according to claim 1 or 3, characterized in that it contains a halogen.

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

7. A quantum dot according to claim 1 or 3, characterized by containing Cu.

8. The quantum dot according to claim 7, characterized in that the remaining amount of Cu is 100 ppm or less.

9. The quantum dot according to claim 1 or 3, characterized in that its external quantum efficiency is 7% or more.

10. The quantum dot according to claim 1 or 3, characterized in that the fluorescence quantum yield is 70% or more.

11. A quantum dot according to claim 1 or 3, characterized in that its fluorescence wavelength is 410 nm or more and 470 nm or less.

12. The quantum dot according to claim 11, characterized in that the fluorescence wavelength is 430 nm or more and 470 nm or less.

13. The quantum dot according to claim 1 or claim 3, characterized in that its fluorescence half-width is 20 nm or less.

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