Quantum dot manufacturing method and quantum dot

Abstract

To provide a method for producing a quantum dot capable of enhancing EQE, and the quantum dot.SOLUTION: The quantum dot of the present invention is a core-shell structure quantum dot. The shell has an intermediate layer and an outermost layer, the intermediate layer is located between the core and the outermost layer, and the intermediate layer contains ZnSeS and a halogen element. Preferably, at least, the external quantum efficiency is 7% or more or the fluorescence quantum yield is 70% or more.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a method for producing cadmium-free core-shell quantum dots, and to the quantum dots. [Background technology]

[0002] Quantum dots emit fluorescence and are also called fluorescent nanoparticles because they are nanometer-sized, semiconductor nanoparticles because their composition is derived from semiconductor materials, or nanocrystals because they have a specific crystalline structure.

[0003] The performance of quantum dots can be expressed by the fluorescence quantum yield (QY) and the external quantum efficiency (EQE).

[0004] When quantum dot displays are used with photoluminescence (PL) as the light-emitting principle, a blue LED is used as the backlight to provide excitation light, which is then converted to green or red light using quantum dots. On the other hand, when electroluminescence (EL) is used as the light-emitting principle, or when quantum dots are used to emit all three primary colors using other methods, blue fluorescent quantum dots are required.

[0005] Cadmium selenide (CdSe) quantum dots, which use cadmium (Cd), are representative of blue quantum dots. However, Cd is internationally regulated, and there have been high barriers to the practical application of materials using CdSe quantum dots.

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

[0007] Furthermore, Non-Patent Document 1 listed below describes in detail a method for directly synthesizing ZnSe using diphenylphosphine selenide, which is thought to be relatively reactive with organic zinc compounds, but is not suitable for producing blue fluorescent quantum dots.

[0008] A method for synthesizing ZnSe in an aqueous system is also reported in the following Non-Patent Document 2. Although the reaction proceeds at low temperatures, the fluorescence half-width is somewhat broad at 30 nm or more, and the fluorescence wavelength is less than 430 nm, making it unsuitable for use as a replacement for conventional blue LEDs to achieve a wide color gamut.

[0009] Additionally, Non-Patent Document 3 below reports a method for synthesizing ZnSe-based quantum dots by forming a precursor such as copper selenide (CuSe) and then cation-exchanging the copper with zinc (Zn). However, because the precursor copper selenide particles are large (15 nm) and the reaction conditions for cation-exchanging copper and zinc are not optimal, copper remains in the ZnSe-based quantum dots after cation exchange. The results of the present study indicate that ZnSe-based quantum dots containing residual copper are unable to emit light. Alternatively, even if they emit light, residual copper results in defect-related emission, resulting in a half-width of the emission spectrum of 30 nm or more. The particle size of the precursor copper selenide also influences this residual copper. Large particles tend to leave residual copper even after cation exchange. Even if ZnSe is confirmed by XRD, the residual copper often prevents light emission. Therefore, Non-Patent Document 3 is cited as an example of residual copper due to the inability to control the particle size of the precursor and optimize the cation-exchange method. Therefore, blue fluorescence is not reported. Although there are many reports of cation exchange methods, 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 Summary of the Invention [Problem to be solved by the invention]

[0012] Incidentally, the external quantum efficiency is calculated by the following (Equation 1). External quantum yield (EQE) = Carrier balance × Light-emitting exciton generation efficiency × Light emission quantum efficiency (fluorescence quantum yield (QY)) × Light extraction efficiency (Equation 1)

[0013] Here, the light extraction efficiency is generally 0.2 to 0.3, so if 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, to obtain a high EQE, quantum dots with a high QY are required.

[0014] Furthermore, if the distance between quantum dots is too close, Förster resonance energy transfer (FRET) occurs, resulting in a decrease in EQE. 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, quantum dots with a high QY that can be covered with a shell of approximately uniform thickness around the entire core have not yet been mass-produced. For example, it has been found that increasing the shell thickness leads to a deterioration in particle shape and a corresponding decrease in QY. Therefore, the present invention has been made in view of the above points, and an object of the present invention is to provide a method for producing quantum dots that can improve EQE, and quantum dots. [Means for solving the problem]

[0016] The quantum dots of the present invention are quantum dots having a core-shell structure, The core and the shell an intermediate layer between the core and the core; and the core contains Zn and Se; the intermediate layer comprises ZnSeS; The shell contains ZnS, and the outermost layer of the shell Contains halogen elements The inner layer of the shell excluding the outermost layer does not contain the halogen, and the halogen element is detected by elemental analysis using energy dispersive X-ray analysis, and the content of the halogen element is 0.01 atom % to 5 atom %. It is characterized by:

[0017] In the present invention, it is preferable that the external quantum efficiency is at least 7% or more, or the fluorescence quantum yield is at least 70%.

[0019] In the present invention, the halogen element is preferably chlorine or bromine. In the present invention, Cu may be contained.

[0020] The method for producing quantum dots according to the present invention is a method for producing quantum dots having a core-shell structure, comprising the steps of: forming a core containing Zn and Se; Coating the surface of the core S The shell has an intermediate layer and an outermost layer, and the intermediate layer The aforementioned the step of forming the shell is located between the core and the outermost layer and includes a step of coating the core with ZnSeS as the intermediate layer; The outermost layer is coated with ZnS, combining an acidic compound and a zinc halide compound; It is characterized by the following. In the present invention, it is preferable that the step of coating the outermost layer is divided into at least a first half and a second half, and that the first half comprises a step of blending the acidic compound but not blending the zinc halide compound, and the second half comprises a step of blending both the acidic compound and the zinc halide compound.

[0021] In the present invention, it is preferable to use at least one of hydrogen chloride, hydrogen bromide, and trifluoroacetic acid as the acidic compound, and it is preferable to use at least one of zinc chloride and zinc bromide as the zinc halide compound. [Effects of the Invention]

[0022] According to the method for producing quantum dots of the present invention, quantum dots with a good particle shape can be synthesized, the QY can be improved, and a high EQE can be obtained. [Brief explanation of the drawings]

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

[0024] An embodiment of the present invention (hereinafter abbreviated as "embodiment") will be described in detail below. Note that the present invention is not limited to the following embodiment, and various modifications can be made within the scope of the gist of the present invention.

[0025] 1A and 1B are schematic diagrams of quantum dots in this embodiment. The quantum dots 5 shown in FIGS. 1A and 1B are nanocrystals that do not contain cadmium (Cd). "Nanocrystals" refer to nanoparticles having a particle size of approximately several nm to several tens of nm. In this embodiment, a large number of quantum dots 5 can be produced with a substantially uniform particle size.

[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) or sulfur (S). However, it is preferable that the core 5a does not contain cadmium (Cd) or indium (In).

[0027] Similarly to the core 5a, the shell 5b covering the surface of the core 5a preferably does not contain cadmium (Cd) or indium (In). In this embodiment, the shell 5b contains a large amount of zinc (Zn). Specifically, the shell 5b is preferably made of zinc sulfide (ZnS), zinc selenide (ZnSe), or zinc sulfide selenide (ZnSeS). Of these, ZnS is preferred. The shell 5b may be in a state of being solid-solved on the surface of the core 5a. In this embodiment, by adopting a core-shell structure, a further increase in the fluorescence quantum yield (QY) can be expected while maintaining a narrow fluorescence half-width.

[0028] In the quantum dot 5 of this embodiment, the entire surface of the core 5a can be covered with a shell 5b made of ZnS or the like to a predetermined thickness. 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, i.e., the shell 5b may have a structure of two or more layers. As an example, the shell 5b may have a layered structure made of ZnSeS / ZnS.

[0029] The quantum dots 5 may have a circular cross section as shown in FIG. 1A, or a polygonal cross section as shown in FIG. 1B. In the case of a polygonal shape, for example, a substantially rectangular or substantially triangular shape is preferable. In this embodiment, the core 5a of the quantum dot 5 preferably contains at least Zn and Se, which makes it easy for the core 5a constituting the quantum dot 5 to be formed into a polyhedron (for example, a substantially cubic shape) by crystal growth. That is, in this embodiment, the quantum dots 5 can be formed in a good shape with a uniform particle shape, rather than an irregular shape. In this embodiment, the shell 5b can be formed with a substantially uniform thickness around the entire periphery of the core 5a. Although not limited thereto, the thickness of the shell 5b can be set to 0.5 n m~3 n It can be formed in about 1 m, preferably n m or more 2.5 n This is because an acidic compound is blended into the shell raw material, as will be explained later in the manufacturing method. In addition, in this embodiment, a halogenated compound is blended into the shell raw material, which can improve the QY.

[0030] As shown in Figures 1A and 1B, it is preferable that a large number of organic ligands 11 are coordinated to the surface of the quantum dots 5. This makes it possible to suppress aggregation of the quantum dots 5 and to achieve the desired optical properties. Furthermore, the addition of amine or thiol-based ligands can significantly improve the stability of the quantum dot luminescence properties. There are no particular limitations on the ligands that can be used in the reaction, but the following ligands are representative examples: (1) Aliphatic primary amines Oleylamine: 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 (2) Fatty acid type 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: CH 19 COOH, octanoic acid: CH 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: CH 17 SH (4) Phosphine Trioctylphosphine: (C8H 17 )3P, triphenylphosphine: (C6H5)3P, tributylphosphine: (C4H9)3P (5) Phosphine oxides 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] It is also preferable that inorganic ligands are mixed with organic ligands. This can further suppress surface defects on quantum dots, resulting in the development of superior optical properties. The ligands are not particularly limited, but halogens such as F, Cl, Br, and I are typical examples.

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

[0033] The content of the halogen element is not limited, but is sufficiently smaller than that of Zn, Se, and S, and is about 0.01 atom % to 5 atom %. The content of the halogen element is preferably about 0.5 atom % or more and 2 atom % or less. "Atom %" is the ratio when the number of all atoms constituting the quantum dots 5 is taken as 100. The amount of the halogen element can be measured by EDX analysis.

[0034] In a quantum dot light-emitting diode (QLED) using the quantum dots 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, the EQE can be 9.5% or more, even more preferably, the EQE can be 10% or more, and even more preferably, the EQE can be 10.5% or more. The EQE can be evaluated using an LED measuring device and is found as the maximum value.

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

[0036] The quantum dots 5 of this embodiment preferably have a fluorescence half-width of 20 nm or less. The "fluorescence half-width" refers to the full width at half maximum, which indicates the spread of the fluorescence wavelength at half the intensity of the peak fluorescence intensity in the fluorescence spectrum. Furthermore, the fluorescence half-width is more preferably 15 nm or less. In this way, this embodiment can narrow the fluorescence half-width, thereby achieving a wider color gamut.

[0037] In this embodiment, as will be described later, the quantum dots 5 are synthesized using a copper chalcogenide precursor as a reaction system, and then the precursor is subjected to a metal exchange reaction. By producing the quantum dots 5 based on such an indirect synthesis reaction, the fluorescence half-width can be narrowed.

[0038] Furthermore, in this embodiment, the fluorescence lifetime of the quantum dots 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. In this way, in this embodiment, the fluorescence lifetime can be shortened, but can also be extended to about 50 ns, making it possible to adjust the fluorescence lifetime depending on the intended use.

[0039] In this embodiment, the fluorescence wavelength can be freely controlled to approximately 410 nm or more and 470 nm or less. Specifically, the quantum dots 5 in this embodiment are ZnSe-based solid solutions. In this embodiment, the fluorescence wavelength can be controlled by adjusting the particle size and composition of the quantum dots 5. In this embodiment, the fluorescence wavelength can be preferably set to 430 nm or more, and more preferably set to 440 nm or more. In this way, the quantum dots 5 of this embodiment can control the fluorescence wavelength to blue.

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

[0041] <Core synthesis method> A method for synthesizing the core will now be described. First, in this embodiment, a copper chalcogenide precursor is synthesized from an organic copper compound or an inorganic copper compound and an organic chalcogen compound. Specifically, the copper chalcogenide precursor is preferably CuSe, CuSeS, CuSeTe, or CuSeTeS.

[0042] In this embodiment, the Cu raw material is not particularly limited, but the following organic copper reagents or inorganic copper reagents can be used. That is, as acetates, copper acetate (I): Cu(OAc) and copper acetate (II): Cu(OAc)2, 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 acetylacetonate: Cu(acac)2, and both monovalent and divalent halides can be used, such as copper chloride (I): CuCl, copper chloride (II): CuCl2, copper bromide (I): CuBr, copper bromide (II): CuBr2, copper iodide (I): CuI, and copper iodide (II): CuI2.

[0043] In this embodiment, an organic selenium compound (organic chalcogenide) is used as the Se raw material. The structure of the compound is not particularly limited, but for example, trioctylphosphine selenide (C8H2O5) obtained by dissolving Se in trioctylphosphine is used. 17Alternatively, a solution of Se dissolved in a high-boiling solvent such as a long-chain hydrocarbon, such as octadecene, at high temperature (Se-ODE), or a solution of Se dissolved in a mixture of oleylamine and dodecanethiol (Se-DDT / OLAm) can be used.

[0044] In this embodiment, Te is obtained by using an organic tellurium compound (organic chalcogen compound) as a raw material. The structure of the compound is not particularly limited, but for example, trioctylphosphine telluride (C8H2O3) obtained by dissolving Te in trioctylphosphine is used. 17 )3P=Te, or tributylphosphine telluride (C4H9)3P=Te in which Te is dissolved in tributylphosphine, etc. Dialkyl ditellurides (R2Te2) such as diphenyl ditelluride (C6H5)2Te2 can also be used.

[0045] In this embodiment, an organic copper compound or an inorganic copper compound is mixed with and dissolved in an organic chalcogen compound. Octadecene can be used as a high-boiling saturated or unsaturated hydrocarbon solvent. Other examples of high-boiling aromatic solvents include t-butylbenzene and high-boiling ester solvents such as butyl butyrate (C4H9COOC4H9) and benzyl butyrate (C6H5CH2COOC4H9). Alternatively, aliphatic amines, fatty acid compounds, aliphatic phosphorus compounds, or mixtures thereof can also be used as solvents.

[0046] At this time, the copper chalcogenide precursor is synthesized at a reaction temperature set in the range of 140° C. to 250° C. The reaction temperature is preferably lower, 140° C. to 220° C., and more preferably still lower, 140° C. to 200° C.

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

[0048] Next, an organic zinc compound or an inorganic zinc compound is prepared as a raw material for ZnSe, ZnSeS, ZnSeTe, or ZnSeTeS. The organic zinc compound or inorganic zinc compound is a raw material that is stable in air and easy to handle. There are no particular limitations on the structure of the organic zinc compound or inorganic zinc compound, but in order to efficiently carry out the metal exchange reaction, it is preferable to use a zinc compound with high ionicity. For example, the organic zinc compound or inorganic zinc compound shown below can be used. That is, as acetates, zinc acetate: Zn(OAc)2, zinc nitrate: Zn(NO3)2, and as fatty acid salts, zinc stearate: Zn(OC(=O)C 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 organic zinc compound or inorganic zinc compound is added to the reaction solution in which the copper chalcogenide precursor has been synthesized. This causes a metal exchange reaction between Cu in the copper chalcogenide and Zn. The metal exchange reaction is preferably carried out at a temperature of 150°C or higher and 300°C or lower. It is also more preferable to carry out the metal exchange reaction at a lower temperature, 150°C or higher and 280°C or lower, and even more preferably 150°C or higher and 250°C or lower.

[0050] In this embodiment, the Cu-Zn transmetallation reaction proceeds quantitatively, and it is preferable that the nanocrystals do not contain any Cu precursor. If Cu precursor remains in the nanocrystals, it acts as a dopant, emitting light via a different emission mechanism, broadening the fluorescence half-width. The amount of remaining Cu relative to Zn is preferably 100 ppm or less, 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 residual content than ZnSe-based quantum dots synthesized by the direct method. However, good luminescence characteristics can be obtained even if the Cu content is about 1 to 10 ppm relative to Zn. It is possible to determine whether quantum dots were synthesized by the cation exchange method based on the Cu residual content. That is, synthesis by the cation exchange method allows particle size control using a copper chalcogenide precursor, which makes it possible to use a synthesis method that is inherently less reactive. Therefore, the Cu residual content is advantageous in determining whether the cation exchange method was used.

[0052] Furthermore, in this embodiment, when metal exchange is carried out, a compound is needed that plays an auxiliary role in liberating the metal of the copper chalcogenide precursor into the reaction solution by coordination or chelation.

[0053] Compounds having the above-mentioned role include ligands capable of forming a complex with Cu. For example, phosphorus-based ligands, amine-based ligands, and sulfur-based ligands are preferred, and among these, phosphorus-based ligands are more preferred due to their high efficiency.

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

[0055] In other words, in the direct synthesis method, an organozinc compound such as diethylzinc (EtZn) is used to increase the reactivity of the Zn raw material. However, diethylzinc is highly reactive and flammable in air, so it must be handled under an inert gas flow, making it difficult to handle and store as a raw material. Furthermore, reactions using it involve the risk of heat generation and fire, making it unsuitable for mass production. Similarly, reactions using selenium hydride (HSe), for example, to increase the reactivity of the Se raw material are also unsuitable for mass production due to their toxicity and safety.

[0056] Furthermore, in reaction systems using highly reactive Zn and Se raw materials as described above, ZnSe is produced, but particle generation is not controlled, resulting in a broad 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 by first synthesizing the copper chalcogenide precursor, rather than directly. This indirect synthesis eliminates the need for reagents that are too reactive and dangerous to handle, and allows for the safe and stable synthesis of ZnSe-based quantum dots with narrow fluorescence half-widths.

[0058] In this embodiment, the metal exchange between Cu and Zn can be carried out in one pot without isolating and purifying the copper chalcogenide precursor, thereby obtaining quantum dots with the desired composition and particle size. On the other hand, the copper chalcogenide precursor may be isolated and purified before use. Furthermore, in this embodiment, the synthesized quantum dots exhibit fluorescent properties without undergoing various treatments such as washing, isolation and purification, coating treatment, and ligand exchange.

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

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

[0061] Next, in this embodiment, a ZnS shell is coated. In this embodiment, it is preferable to divide the ZnS shell coating process into at least two parts. First, in the first part 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, a zinc oleate (Zn(OLAc)2) solution, dodecanethiol (DDT), and TOP are added, and then an acidic oxide is added. In this embodiment, the shell source mixture containing the acidic oxide is added and heated with stirring 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 of the shell source mixture and heating are repeated multiple times. Although FIG. 4 shows the number of repeated operations as 10, "10 times" is an example and is not intended to limit the number of times. However, it is preferable to specify the number of repetitions to be within the range of approximately 5 to 15 times. The mixture is then cooled to room temperature.

[0062] In this embodiment, an acidic compound is added to the shell source mixed solution in the first half of the shell coating process, but a zinc halide compound, which is blended in the second half of the shell coating process, is not added. It has been found that adding a zinc halide compound to the shell source mixed solution in the first half of the shell coating process reduces the QY. Therefore, a zinc halide compound is not added to the shell source mixed solution in the first half of the shell coating process.

[0063] Next, in this embodiment, the latter shell coating process is performed. In this latter shell coating process, a shell source mixed solution containing an acidic compound and a zinc halide compound is added to the solution in which ZnSe / ZnSeS / ZnS is dispersed. To this shell source mixed solution, for example, a zinc oleate (Zn(OLAc)2) solution, dodecanethiol (DDT), and TOP are added along with a zinc halide compound and an acidic compound. Thus, in the latter shell coating process, a shell source mixed solution containing an acidic compound and a zinc halide compound is added, and the mixture is heated with 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 mixed solution and heating is repeated multiple times. While FIG. 4 indicates that the number of repeated operations is 10, this "10 times" is merely an example and is not intended to limit the number of times. However, it is preferable to limit the number of repetitions to a range of approximately 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 the ODE is repeated until the desired shell thickness is achieved. As described above, the latter shell coating step is characterized by adding a shell source mixed solution containing an acidic compound and a zinc halide compound.

[0065] In this embodiment, the EQE is improved, but to achieve this, it is necessary to improve the QY and further optimize the particle shape. If the QY can be increased, the EQE can be improved as shown in (Equation 1).

[0066] The optimization of particle shape can be explained as follows. That is, when the distance between quantum dot cores is short, Förster resonance energy (FRET) occurs, leading to a decrease in EQE. For this reason, it is thought that by using a core-shell structure in which the cores are coated with a shell, the cores can be physically separated from each other and FRET can be reduced. However, increasing the shell thickness deteriorates the particle shape, and the QY also decreases accordingly. Furthermore, in the past, there were problems such as the shell not being able to coat the entire surface of the core to the specified thickness, resulting in defects, or the shell becoming locally thick, deteriorating the particle shape. As a result, it was not possible to appropriately reduce FRET and effectively reduce EQE.

[0067] Therefore, in this embodiment, the QY can be improved by gradually adding a zinc halide compound to the core. In particular, the QY can be effectively improved by not adding the zinc halide compound to the first half of the shell coating process but adding it only to the second half of the shell coating process. Furthermore, since the particle shape deteriorates when the shell source mixture is continuously added, adding an acidic compound to the shell source mixture etches the locally thick shell portions, thereby regulating the shape and achieving a good particle shape with a polygonal cross section.

[0068] In this embodiment, the zinc halide compound is preferably added in an amount of about 0.5 mol % to 3 mol %, more preferably about 1 mol % to 2 mol %, based on the zinc oleate.

[0069] In this embodiment, the acidic compound can be at least one selected from hydrogen chloride (HCl), hydrogen bromide (HBr), hydrogen iodide (HI), trifluoroacetic acid (TFA), trifluoromethanesulfonic acid (TfOH), acetic acid (AA), sulfuric acid (HSO), phosphoric acid (HPO), etc. Among 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 a good particle shape of the 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, the zinc halide compound is preferably at least one of zinc chloride (ZnCl), zinc bromide (ZnBr), zinc fluoride (ZnF), and zinc iodide (ZnI). In this embodiment, for example, a zinc chloride-TOP-oleic acid solution can be added to the shell source mixture. In this embodiment, the S raw material used for the core-shell structure is not particularly limited, but the following raw materials are typical examples.

[0071] That is, 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: CH 17Examples of sulfur-containing solvents that can be used include solutions of sulfur dissolved in high-boiling solvents such as long-chain phosphine hydrocarbons (e.g., trioctylphosphine, benzenethiol, CHSH, or trioctylphosphine) (S-TOP), solutions of sulfur dissolved in high-boiling solvents such as long-chain hydrocarbons (e.g., octadecene) (S-ODE), and solutions of sulfur dissolved in a mixture of oleylamine and dodecanethiol (S-DDT / OLAm).

[0072] The reactivity varies 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 compounds is proportional to their decomposition rate, while the reactivity of S-TOP or S-ODE varies 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 the final fluorescence quantum yield.

[0073] In this embodiment, the less the amine-based solvent used in coating the shell 5b, the easier it is to coat the shell 5b and the better the luminescence characteristics can be obtained. Furthermore, the luminescence characteristics after coating the shell 5b differ depending on the ratio of the amine-based solvent to the carboxylic acid-based 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, and the quantum dots 5 can be separated and recovered from unreacted raw materials. The recovered quantum dots 5 can be re-dispersed by adding toluene, hexane, or the like. Adding a solvent that acts as a ligand to this re-dispersed solution can further improve the luminescence characteristics and stability of the luminescence characteristics. The change in luminescence characteristics caused by adding this ligand varies greatly depending on whether or not the shell 5b is coated. In this embodiment, adding a thiol-based ligand to the quantum dots 5 coated with the shell 5b can particularly improve the fluorescence stability.

[0075] The applications of the quantum dots 5 shown in FIGS. 1A and 1B are not particularly limited. For example, the quantum dots 5 of this embodiment that emit blue fluorescence can be applied to wavelength conversion members, lighting members, backlight devices, display devices, and the like.

[0076] The quantum dots 5 of this embodiment can be applied to wavelength conversion members, lighting members, backlight devices, display devices, and the like. 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 quantum dots are used to emit all three primary colors by other methods, the quantum dots 5 of this embodiment can be used to create a light-emitting element that emits blue fluorescence. In this embodiment, a light-emitting element (full-color LED) can be formed that includes quantum dots 5 of this embodiment that emit blue fluorescence, as well as quantum dots that emit green fluorescence and quantum dots that emit red fluorescence, thereby enabling white light emission.

[0077] Fig. 2 is a schematic diagram of an LED device using quantum dots according to this embodiment. As shown in Fig. 2, the LED device 1 according to this embodiment comprises a storage case 2 having a bottom surface 2a and a sidewall 2b surrounding the periphery of the bottom surface 2a, an LED chip (light-emitting element) 3 disposed on the bottom surface 2a of the storage case 2, and a fluorescent layer 4 filled in the storage case 2 and sealing the upper surface side of the LED chip 3. Here, the upper surface side refers to the direction in which light emitted from the LED chip 3 is emitted from the storage case 2, and indicates the direction opposite the bottom surface 2a of the LED chip 3.

[0078] The LED chip 3 is disposed on a base wiring substrate (not shown), which may form the bottom surface of the storage case 2. The base substrate may be, for example, a substrate made of glass epoxy resin or the like, on which a wiring pattern is formed. The LED chip 3 is a semiconductor element that emits light when a forward voltage is applied, and has a basic configuration in which a P-type semiconductor layer and an N-type semiconductor layer are PN junctioned. As shown in FIG. 2, the fluorescent layer 4 is formed from 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 of this embodiment are dispersed may contain the quantum dots 5 and a fluorescent material other than the quantum dots 5. The fluorescent material may be a sialon-based material or KSF (K2SiF6:Mn 4+ ) red phosphor, etc., but the material is not particularly limited.

[0080] The resin 6 constituting the fluorescent layer 4 is not particularly limited, but examples thereof include polypropylene (PP), polystyrene (PS), acrylic resin, methacrylic resin, MS resin, polyvinyl chloride (PVC), polycarbonate (PC), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polymethylpentene, liquid crystal polymer, epoxy resin, silicone resin, and mixtures thereof.

[0081] The LED device using quantum dots of this embodiment can be applied to display devices. Fig. 3 is a longitudinal cross-sectional view of a display device using the LED device shown in Fig. 2. As shown in Fig. 3, a display device 50 is configured to include 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 disposed 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 Fig. 2.

[0082] As shown in Fig. 3, a plurality of LED devices 20 are supported by a support 52. The LED devices 20 are arranged at predetermined intervals. The LED devices 20 and the support 52 together form a backlight 55 for a display unit 54. The support 52 is not particularly limited in shape or material, and may be in the form of a sheet, plate, or case. As shown in Fig. 3, a light diffusion plate 53 or the like may be interposed between the backlight 55 and the display unit 54.

[0083] By applying the quantum dots 5 of this embodiment to the LED device shown in FIG. 2 or the display device shown in FIG. 3, the light-emitting characteristics of the device can be effectively improved. In particular, when the quantum dots of this embodiment are applied to a QLED element, the EQE can be improved. 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, a resin composition in which the quantum dots 5 of this embodiment are dispersed in a resin can be formed into a sheet or 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 a plurality of quantum dots are dispersed in a resin can be formed using a molded body. For example, a molded body in which quantum dots are dispersed in a resin is stored in a container having a storage space by, for example, press-fitting. 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, a portion of the light entering the molded body is totally reflected by the inner wall of the container. Therefore, it is possible to reduce the amount of light leaking to the outside from the side of the container. In this way, by applying the quantum dots of this embodiment to wavelength conversion members, lighting members, backlight devices, display devices, etc., it is possible to effectively improve the light-emitting characteristics. [Example]

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

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

[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 resulting reaction solution (CuSe) was cooled to room temperature.

[0090] Subsequently, 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 resulting 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] Subsequently, 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 resulting reaction solution (ZnSe) was cooled to room temperature. The resulting 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 the ZnSe reaction solution to generate a precipitate, which was then centrifuged to recover the precipitate, and 35 ml of octadecene:ODE was added to the precipitate to disperse it.

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

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

[0096] Ethanol was then added to the resulting reaction mixture to generate a precipitate, which was then centrifuged to recover the precipitate, and 35 ml of octadecene (ODE) was added to disperse the precipitate. Then, 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 under an inert gas (N2) atmosphere with stirring.

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

[0098] Ethanol was then added to the resulting reaction mixture to generate a precipitate, which was then centrifuged to recover the precipitate, and 35 ml of octadecene (ODE) was added to disperse the precipitate. Then, 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 under an inert gas (N2) atmosphere with stirring.

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

[0100] Ethanol was then added to the resulting reaction mixture to generate a precipitate, which was then centrifuged to recover the precipitate, and 35 ml of octadecene (ODE) was added to disperse the precipitate. Then, 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 under an inert gas (N2) atmosphere with stirring.

[0101] To this solution, 0.4 mL of DDT, 1.6 mL of trioctylphosphine:TOP, 0.2 mL of hydrogen chloride-ethyl acetate solution (4 M), 0.1 mL of zinc chloride-TOP-oleic acid solution (0.8 M), and 0.9 mL of a mixture of zinc oleate:Zn(OLAc)2 solution (0.4 M) were added and heated at 320 °C for 10 min with stirring. This procedure was repeated 10 times. The resulting reaction solution was measured using a fluorescence spectrometer, and the optical characteristics were as shown in Figure 5, with a fluorescence wavelength of approximately 442 nm and a fluorescence half-width of approximately 15 nm. Ethanol was added to the resulting reaction solution to generate a precipitate, which was then centrifuged to recover the precipitate, and hexane was added to the precipitate to disperse it. The resulting dispersion was measured using a UV-visible spectrometer. As a result, the UV-visible 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 cubic crystals consisting 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 and found to be 16 ns. Elemental analysis (EDX) showed the following: Zn: 42 atom%, Se: 11 atom%, S: 41 atom%, Cl: 1 atom%. Analysis of the TEM image revealed that the shell thickness was 2.0 nm. Furthermore, the quantum dots obtained in Example 1 were used to manufacture a light-emitting device having the following layered structure. ITO / PEDOT:PSS / PVK / QD layer / LiZnO / Al When this device was evaluated using an LED measurement device, the maximum external quantum efficiency (EQE) was 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 changed to 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 (4 M) used in Example 2 (see the description in Example 1) was changed to 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 (4 M) used in Example 1 was changed to 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 (4 M) (see the description in Example 1) used in Example 2 was changed to trifluoroacetic acid.

[0104] 8 is a table summarizing the measurement results of Examples 1 to 5. TEM photographs of the quantum dots obtained in Examples 1 to 5 are also shown. 8, the EQE was able to be 7% or more in all of Examples 1 to 5. In particular, in Example 1, the EQE was able to be 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 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, thereby maintaining the substantially rectangular particle shape. This is presumably because an 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 steps used in Example 1, the <synthesis method of ZnSe core> was the same, and a part of the <coating method of shell on ZnSe core> was changed to synthesize quantum dots. Hereinafter, the <coating method of shell on ZnSe core> of Example 6 will be described.

[0108] <Coating method of shell on ZnSe core> Ethanol was added to 40 ml of the ZnSe reaction solution to generate precipitation, and centrifugation was performed to recover the precipitation. 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 in an inert gas (N2) atmosphere.

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

[0111] Ethanol was then added to the resulting reaction mixture to generate a precipitate, which was then centrifuged to recover the precipitate, and 35 ml of octadecene (ODE) was added to disperse the precipitate. Then, 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 under an inert gas (N2) atmosphere with stirring.

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

[0113] Ethanol was then added to the resulting reaction mixture to generate a precipitate, which was then centrifuged to recover the precipitate, and 35 ml of octadecene (ODE) was added to disperse the precipitate. Then, 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 under an inert gas (N2) atmosphere with stirring.

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

[0115] <Measurement Results of 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 <method of coating the shell on the 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 a ligand 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 the 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] A 100 mL reaction vessel was charged with 182 mg of anhydrous copper acetate (Cu(OAc)2), 4.8 mL of oleylamine (OLAm), and 7.75 mL of octadecene (ODE). The mixture was heated to 165°C for 5 minutes under an inert gas (N2) atmosphere with stirring to dissolve the raw materials.

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

[0121] Then, 1844 mg of anhydrous zinc acetate (Zn(OAc)), 10 mL of trioctylphosphine (TOP), and 0.4 mL of oleylamine (OLAm) were added to the CuSe reaction solution, and the mixture was heated under an inert gas (N) atmosphere at 180°C for 45 minutes with stirring. The resulting reaction solution (ZnSe) was cooled to room temperature.

[0122] Ethanol was added to the reaction solution cooled to room temperature to generate a precipitate, which was then centrifuged to recover the precipitate, and 12 ml of octadecene:ODE was added to the precipitate to disperse it.

[0123] Then, 12 ml of the ZnSe-ODE solution was mixed with 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), and the mixture was heated under an inert gas (N2) atmosphere at 280°C for 20 minutes with stirring. The resulting reaction solution (ZnSe) was cooled to room temperature.

[0124] The resulting reaction solution was measured using a fluorescence spectrometer, and the optical characteristics were 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 resulting ZnSe reaction solution to generate a precipitate, which was then centrifuged to recover the precipitate, and 17.5 ml of octadecene:ODE was added to the precipitate to disperse it.

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

[0127] To this solution, 0.5 mL of a mixture 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] Ethanol was then added to the resulting reaction solution to generate a precipitate, which was then centrifuged to recover the precipitate. 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 in the same manner as above, and the mixture was 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 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 10 times.

[0130] Thereafter, ethanol was added to the resulting reaction solution to generate a precipitate, which was then centrifuged to recover the precipitate, and 17.5 ml of octadecene:ODE was added to the precipitate to disperse it (washing step).

[0131] Next, as before, 1 mL of oleic acid (OLAc) and 2 mL of trioctylphosphine (TOP) were added and heated with stirring at 320°C for 10 minutes under an inert gas (N2) atmosphere. 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 to this solution, and the mixture was heated with stirring at 320°C for 10 minutes. This procedure was repeated six times. The mixture was then heated with stirring at 320°C for 30 minutes (shell coating process).

[0132] Thereafter, the reaction solution was subjected to the above (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, and the optical characteristics were a fluorescence wavelength of approximately 443 nm and a fluorescence half-width of approximately 15 nm. Ethanol was added to the resulting reaction solution to generate a precipitate, which was then centrifuged to recover the precipitate, and hexane was added to disperse the precipitate.

[0134] ZnSe / ZnSeS / ZnS dispersed in hexane was measured using a quantum efficiency measurement system. The fluorescence quantum yield was approximately 60%. The fluorescence lifetime was measured to be 14 ns. Furthermore, the quantum dots obtained in Comparative Example 1 were used to manufacture a light-emitting device having the following layered structure. ITO / PEDOT:PSS / PVK / QD layer / ZnO / Al When this device was evaluated using an LED measurement device, the maximum external quantum efficiency (EQE) was 4.0%. Below, Example 1 is compared with Comparative Example 1. Table 1 shows the measurement results of Example 1 and Comparative Example 1.

[0135] [Table 1]

[0136] It was found that Comparative Example 1 had a lower EQE than Example 1. Fig. 9A is a photograph of the TEM-EDX analysis results for Comparative Example 1, and Fig. 9B is a photograph of the TEM-EDX analysis results for Example 1. Fig. 10A is a partial schematic view of Fig. 9A, and Fig. 10B is a partial schematic view of Fig. 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 portion is primarily a mixture of red and blue, appearing almost purple, while the outer portion is primarily a mixture of red and green, appearing almost yellow. Since red represents Zn, blue represents Se, and green represents S, it was determined that the central portion is primarily composed of Zn and Se, while the outer portion is primarily composed of 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 approximately yellow portion of the TEM-EDX analysis results.

[0138] 9A and 10A, in Comparative Example 1, the shell covering the core was not of a uniform thickness, and there were some breaks and localized areas where the shell had grown. Therefore, the particle shape of Comparative Example 1 was poor, Förster resonance energy transfer (FRET) was likely to occur, and the EQE was reduced. Furthermore, Comparative Example 1 did not achieve as high a QY as Example 1.

[0139] In contrast, in Example 1, as shown in Figures 9B and 10B, the shell neatly covered the entire periphery of the core, the shell had a substantially uniform thickness, and the quantum dot particle shape was substantially rectangular. Thus, the particle shape of Example 1 was better than that of Comparative Example 1, and a sufficiently higher QY than that of Comparative Example 1 could be obtained. As a result, Example 1 was able to obtain a sufficiently higher EQE than that of Comparative Example 1. [Industrial Applicability]

[0140] According to the present invention, quantum dots that emit blue fluorescence can be stably obtained. By applying the quantum dots of the present invention to LEDs, backlight devices, display devices, etc., excellent light-emitting properties 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 core-shell quantum dot, an intermediate layer between the core and the shell; the core comprises Zn and Se; the intermediate layer contains ZnSeS; the shell comprises ZnS; a halogen element is contained in the outermost layer of the shell, and the halogen is not contained in the inner layer of the shell other than the outermost layer, The halogen element is detected by elemental analysis using energy dispersive X-ray analysis, and the content of the halogen element is 0.01 atom % to 5 atom %. Quantum dots characterized by:

2. At least an external quantum efficiency of 7% or more, or a fluorescence quantum yield of 70% or more, The quantum dot according to claim 1 .

3. The halogen element is chlorine or bromine.

3. The quantum dot according to claim 1 or claim 2.

4. Contains Cu, The quantum dot according to any one of claims 1 to 3.

5. A method for producing quantum dots with a core-shell structure, comprising: forming a core comprising Zn and Se; the shell covering the surface of the core has an intermediate layer and an outermost layer, the intermediate layer is located between the core and the outermost layer, The step of generating the shell comprises: coating the core with ZnSeS as the intermediate layer; The method includes a step of coating ZnS as the outermost layer and blending an acidic compound and a zinc halide compound. The method for producing quantum dots is characterized by the above.

6. The step of coating the outermost layer is divided into at least a first half and a second half, In the first half, the acidic compound is blended without blending the zinc halide compound; In the latter half, a step of blending both an acidic compound and a zinc halide compound; The method for producing quantum dots according to claim 5, further comprising:

7. 7. The method for producing quantum dots according to claim 5, wherein the acidic compound is at least one of hydrogen chloride, hydrogen bromide, and trifluoroacetic acid.

8. 8. The method for producing quantum dots according to claim 5, wherein at least one of zinc chloride and zinc bromide is used as the zinc halide compound.

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