Quantum dots

Abstract

To provide a quantum dot capable of containing a large amount of Zn on the surface.SOLUTION: The quantum dots according to the present invention each include a core containing at least Ag, Ga, and S or Ag, Ga, and Se and a shell covering the core, wherein the shell contains at least Zn and exhibits fluorescence properties with a fluorescence FWHM of 35nm or less and a fluorescence quantum efficiency of 70% or more, and the quantum dots contain Zn in an amount of 5% by mass or more and 40% by mass or less based on the total mass of the quantum dots.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to quantum dots with a core / shell structure. [Background technology]

[0002] Quantum dots are inorganic nanoparticles composed of thousands to tens of thousands of atoms and with diameters of several to tens of nanometers. Quantum dots emit fluorescence and are also called fluorescent nanoparticles because they are nanometer-order in size, semiconductor nanoparticles because their composition is derived from semiconductor materials, or nanocrystals because they have a specific crystalline structure.

[0003] Quantum dots consist of positively charged metal atoms and negatively charged non-metallic or metalloid atoms, which are bonded together by either ionic or covalent bonds, depending on the combined properties of the metal and metalloid atoms.

[0004] Quantum dots can vary their emission wavelength depending on their particle size and composition. Quantum dot performance can be measured by their fluorescence quantum yield (QY) and full width at half maximum (FWHM).

[0005] One of the properties of quantum dots is photoluminescence. Quantum dots can absorb wavelengths in a specific wavelength range and emit light by converting them into other wavelengths in a specific range. Furthermore, the absorption and emission wavelengths can be controlled by the structure, composition, and size of the quantum dots, and these characteristics can be utilized for a variety of purposes.

[0006] For example, when quantum dots are used as wavelength conversion materials in the visible light region, one of their characteristics is that they can express a wide range of colors, i.e., a wide color gamut. In realizing a wide color gamut using quantum dots in the visible light region, the important optical properties are the fluorescence quantum yield and the fluorescence half-width.

[0007] Previously, highly efficient quantum dots mainly contained cadmium (Cd). Cd-containing quantum dots have the advantages of high fluorescence quantum yield and narrow fluorescence half-width. However, due to the toxicity of Cd, their use is restricted in various countries, which has been a major barrier to their practical application.

[0008] In response to this, many studies are being conducted on the development of quantum dots that do not contain Cd. For example, the following patent documents describe AIS or AIGS quantum dots or AISe or AIGSe quantum dots that contain Ag, In, and S, or Ag, In, Ga, and S, or Ag, In, and Se, or Ag, In, Ga, and Se. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 2017-025201 [Patent Document 2] Japanese Patent Application Publication No. 2018-039971 [Patent Document 3] Japanese Patent Application Laid-Open No. 2018-044142 [Patent Document 4] Japanese Patent Application Laid-Open No. 2018-141141 [Patent Document 5] WO2018 / 159699 [Non-patent literature]

[0010] [Non-Patent Document 1] NPG Asia Materials volume 10. 2018, pp713-726 [Non-patent document 2] ACS Publications 2018,10,49,41844-41855 [Non-patent document 3] ACS Publications Nano Mater. 2020, 3, 3275-3287 [Non-patent document 4] The Journal of Physical Chemistry Letters; Ligand-Induced Luminescence Transformation in AgInS2 Nanoparticles: From Defect Emission to Band-Edge Emission Summary of the Invention [Problem to be solved by the invention]

[0011] Incidentally, when Zn is used in AgGaS or AgInGaS, the difference in valence (Zn is divalent, Ag is monovalent, and Ga or In is trivalent) leads to defect emission, which tends to broaden the fluorescence half-width.

[0012] Furthermore, even if Zn is added to AgGaS or AgInGaS to eliminate Zn, the cationic species easily diffuse into the grains, which allows Zn to easily diffuse into the core, resulting in defect luminescence.

[0013] The present invention has been made in view of the above-mentioned points, and has as its object to provide quantum dots that can have a large amount of Zn contained on the surface. [Means for solving the problem]

[0014] The quantum dots of the present invention have a core containing at least Ag, Ga, and S or Ag, Ga, and Se, and a shell covering the surface of the core, wherein the shell contains at least Zn and exhibits fluorescent properties with a fluorescence half-width of 35 nm or less and a fluorescence quantum yield of 70% or more, and the Zn content of the quantum dots is 5% by mass or more and 40% by mass or less. [Effects of the Invention]

[0015] The quantum dots of the present invention can contain a large amount of Zn on the surface while maintaining band-edge emission. By appropriately coating the surface of the core with a shell containing a large amount of Zn, the stability of the fluorescent properties can be improved and a high fluorescent quantum yield can be maintained. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic diagram of a quantum dot 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] 1 is a conceptual diagram showing a manufacturing process of quantum dots according to an embodiment of the present invention. [Figure 5] 1 shows an X-ray diffraction (XRD) spectrum in Example 1. [Figure 6] 1 shows photoluminescence (PL) spectra in Examples 1 and 2. [Figure 7] FIG. 1(a) is a photograph of the TEM-EDX analysis results (Se+S and Ag+Zn) in Example 1, and FIG. 1(b) is a partial schematic diagram of FIG. [Figure 8] 10(a) is a photograph of the results of high-resolution STEM analysis in Example 2, and FIG. 10(b) is a partial schematic diagram of FIG. [Figure 9]FIG. 1(a) is a photograph of the TEM-EDX analysis results (Ag+Zn) in Example 2, and FIG. 1(b) is a partial schematic diagram of FIG. [Figure 10] FIG. 1(a) is a photograph of the TEM-EDX analysis results (Zn) in Example 1 and Comparative Example, and FIG. 1(b) is a partial schematic diagram of FIG. [Figure 11] 1 shows photoluminescence (PL) spectra in Example 3 and Comparative Example 2. [Figure 12] (a) is a photograph of the TEM-EDX analysis results (Ag+Ga) of the core of Example 3. (b) is a fluorescence (Photoluminescence: PL) spectrum of the core of Example 3. (c) is a partial schematic diagram of (a). DETAILED DESCRIPTION OF THE INVENTION

[0017] An embodiment of the present invention (hereinafter abbreviated as "embodiment") will be described in detail below. The present invention is not limited to the following embodiment, and can be practiced in various modifications within the scope of the gist. In this specification, the notation "to" means that the lower limit and upper limit are included.

[0018] 1A is a schematic diagram of a quantum dot according to this embodiment. The quantum dot 5 shown in FIG. 1A is a nanocrystal that does not contain Cd.

[0019] 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 silver (Ag), gallium (Ga), and sulfur (S), or Ag, Ga, and selenium (Se). However, it is preferable that the core 5a does not contain cadmium (Cd) or indium (In).

[0020] The core 5a may also contain copper (Cu) or indium (In). The Ga / In ratio is preferably 5 or more, more preferably 10 or more, and even more preferably 30 or more.

[0021] Like 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), zinc gallium selenide (ZnGa2Se4), or zinc gallium sulfide (ZnGa2S4). Of these, ZnS is preferred. The shell 5b may be in a state of being solid-solution on the surface of the core 5a.

[0022] The shell 5b may also contain copper (Cu) or indium (In). Specifically, the shell 5b is preferably made of copper sulfide (CuS), copper selenide (CuSe), indium sulfide (In2S3), indium selenide (In2Se3), zinc indium selenide (ZnIn2Se4), zinc indium sulfide (ZnIn2S4), copper indium sulfide (CuIn2S4), or copper indium selenide (CuIn2Se4). The shell 5b may be in a solid solution state on the surface of the core 5a.

[0023] The quantum dot 5 of this embodiment can appropriately coat the surface of the core 5a made of AgGaSe or AgGaS with a shell 5b made of ZnS or ZnGa2S4 or the like. In this embodiment, the Ag contained in the core 5a is prevented from diffusing into the shell 5b. Furthermore, when the core 5a contains Se, the Se contained in the core 5a and the S contained in the shell 5b can be appropriately separated. Furthermore, the Zn contained in the shell 5b can be prevented from diffusing into the core 5a.

[0024] In this embodiment, the shell 5b may further contain gallium (Ga), or GaS or GaSe may be interposed between the core 5a and the shell 5b as the first layer of the shell.

[0025] Here, "nanocrystal" refers to nanoparticles having a particle size of about 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 amount of Zn present on the surface of the quantum dots 5 can be increased. Specifically, the amount of Zn has a weight ratio of 5% or more, preferably 10% or more, and more preferably 20% or more, relative to the total weight of the quantum dots 5. There is no upper limit to this amount, but the upper limit is, for example, about 40%.

[0027] The Ga / In ratio of the entire quantum dots 5 is preferably 10 or more, more preferably 30 or more, and even more preferably 50 or more. Furthermore, the Ga / Zn ratio is preferably 1 or less, and even more preferably 0.5 or less.

[0028] As shown in Fig. 1, 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. There are no particular limitations on the ligands that can be used in the reaction, but the following ligands are representative examples.

[0029] (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 H31 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

[0030] It is also preferable that inorganic ligands are coordinated together 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.

[0031] Next, a method for manufacturing quantum dots will be described. In this embodiment, the object is to manufacture quantum dots that exhibit band-edge emission and are capable of stabilizing the emission characteristics by increasing the amount of Zn contained in the shell 5b. In this embodiment, band-edge emission is possible even with the core 5a alone. This point will be described later.

[0032] In a conventional method for producing quantum dots, for example, the surface of AgGaSe is coated with a shell containing Se and then Zn is added, but according to this embodiment, the amount of Zn on the core surface can be increased by the following method. That is, the quantum dot production method of this embodiment has the following features.

[0033] (1) The method includes a step of forming a core 5a containing at least Ag, Ga, and S or Ag, Ga, and Se, and a step of coating the surface of the core 5a with a shell 5b. (2) In the step of coating the shell 5b, GaS is coated on the surface of the core 5a, and then Zn is added. Here, the valence of GaS is not taken into consideration. x S y (x=1 to 2, y=1 to 6), and can be expressed as GaS or Ga2S3. Also, when GaSe is coated on the core surface, Ga x Se y (x=1 to 2, y=1 to 6), and is represented by, for example, GaSe or Ga2Se3. "Addition of Zn" includes addition of simple Zn, as well as addition of ZnS, ZnSe, ZnGa2Se4, and ZnGa2S4.

[0034] In this embodiment, it is preferable to coat the surface of the core 5a with GaS and then with ZnS. FIG. 4 is a conceptual diagram of the manufacturing process for the quantum dot 5 of this embodiment. After generating an AgGaSe core as shown in the left diagram of FIG. 4, a GaS shell is coated on the surface of the AgGaSe core as shown in the center diagram of FIG. 4. Zn is then added, resulting in a ZnS shell as shown in the right diagram of FIG. 4. As shown in the center diagram of FIG. 4, the GaS shell coated on the surface of the AgGaSe core is important for preventing Zn from diffusing into the core 5a during the next Zn addition. Although the amount of Ga remaining in the shell after Zn addition is expected to decrease due to dissolution or cleaning processes, Ga may still be contained in the shell. That is, the shell may be ZnGa2S4. Alternatively, a GaS shell may be interposed between the AgGaSe core and the ZnS shell. That is, the shell may have a two-layer structure of GaS / ZnS.

[0035] Alternatively, in this embodiment, it is preferable that Ga and Zn in GaS are cation-exchanged to form a ZnS shell. Specifically, the particle size of AgGaSe / GaS in the center of FIG. 4 and the particle size of AgGaS / ZnS in the right of FIG. 4 were almost identical, as confirmed by TEM-EDX analysis. From this, it can be inferred that the ZnS shell coating the surface of AgGaS is formed by cation exchange. Thus, quantum dots with a core-shell structure and a high Zn content on the surface can be synthesized by cation exchange or anion solid solution without changing the particle size.

[0036] In conventional manufacturing methods, even after ZnS coating, Zn quickly diffuses into the core interior, resulting in defect emission, making it impossible to coat a large amount of ZnS. In contrast, in this embodiment, the crystallinity of ZnS obtained by post-adding Zn after GaS coating was confirmed. Conventionally, oleylamine, for example, is used for shell coating. However, oleylamine is thought to act as a ligand that interferes with shell coating. In fact, it has been found that when GaS coating without XRD shift is performed, defect emission immediately occurs upon Zn addition. In contrast, in this embodiment, for example, DDT (dodecanethiol), which is not an amine, is used for shell coating. Instead of DDT, ODE can also be used. However, oleylamine can also be included as long as it is not the main solvent. Note that, as shown in the experimental results described below, XRD shifts occur during the GaS coating process after the use of DDT. Thus, in this embodiment, using GaS in a DDT (or ODE) solvent allows for coating without Ag diffusing to the surface.

[0037] According to the method for producing quantum dots 5 of this embodiment, the diffusion of Ag contained in the cores 5a into the shells can be suppressed, and further, in a configuration in which the cores 5a contain Se, the Se in the cores 5a and the S contained in the shells 5b can be appropriately separated. Furthermore, according to the method for producing quantum dots 5 of this embodiment, the amount of Zn contained in the shells 5b can be increased. Although not particularly limited, the amount of Zn in the quantum dots 5 can be adjusted to 5% or more, preferably 10% or more, and more preferably 20% or more, by weight. A method for producing quantum dots will now be described in detail. First, in this embodiment, quantum dots are synthesized by heating in one pot from an organic silver compound, an organic gallium compound, and selenium.

[0038] At this time, AgGaSe is synthesized at a reaction temperature set in the range of 100° C. to 320° C. Note that the reaction temperature is preferably set at a lower temperature of 280° C. or less.

[0039] In this embodiment, an organic silver compound or an inorganic silver compound is used as the Ag raw material, and although not particularly limited, examples thereof include silver acetate (AgOAc), silver nitrate (AgNO), halides such as silver chloride (AgCl), silver bromide (AgBr), and silver iodide (AgI), and carbamates such as silver diethyldithiocarbamate (Ag(SC(=S)N(C2H5)2) and silver dimethyldithiocarbamate (Ag(SC(=S)N(CH3)2).

[0040] In this embodiment, the Ag raw material may be added directly to the reaction solution, or may be dissolved in an organic solvent in advance to a certain concentration and used as the Ag raw material solution.

[0041] In this embodiment, an organic gallium compound or an inorganic gallium compound is used as a Ga source material. Although not particularly limited, examples include gallium acetate: Ga(OAc)3, gallium nitrate: GaNO3, and gallium acetylacetonate: Ga(acac)3; halides include gallium chloride: GaCl3, gallium bromide: GaBr3, and gallium iodide: Ga2I3; and carbamates include gallium diethyldithiocarbamate: Ga[(SC(=S)N(C2H5)2]3.

[0042] In this embodiment, the Ga source material may be added directly to the reaction solution, but may also be dissolved in an organic solvent in advance to prepare a solution of a certain concentration, which may then be used as the Ga source solution.

[0043] In this embodiment, an organic selenium compound (organic chalcogen compound) can be used as a raw material for Se. For example, trioctylphosphine selenide (C8H2O5), in which selenium is dissolved in trioctylphosphine, can be used. 17)3P=Se, or tributylphosphine selenide (C4H9)3P=Se, in which selenium is dissolved in tributylphosphine, or a solution of selenium dissolved in a high-boiling solvent such as a long-chain hydrocarbon, such as octadecene, can be used. When synthesizing AgGaSe, the selenium source species significantly contributes to the fluorescent properties. In particular, a solution of Se dissolved in a mixture of oleylamine and dodecanethiol (Se-OLAm / DDT) exhibits excellent luminescence properties. Typical chalcopyrite-based quantum dots exhibit two types of PL spectrum at the initial stage of emission: band-edge PL and defect PL. The intensity ratio of band-edge / defect PL is usually less than 10. Subsequently, as the reaction proceeds, the intensity of defect PL gradually decreases, and the intensity of band-edge PL often increases. However, when Se-DDT / OLAm is used as the Se source as in this embodiment, a single peak is observed from the beginning of emission, the band edge emission / defect emission ratio is 10 or more, and peaks that are considered to be defect emission are hardly observed. Furthermore, the fluorescence half-width is 30 nm or less. The fluorescence lifetime is also short, at 20 ns or less until it reaches 1 / e, and only peaks that are not defect emission can be observed in the early stages of emission.

[0044] Next, in this embodiment, the surface of the nanocrystal core 5a is coated with a shell 5b, thereby further increasing the fluorescence quantum yield. As described above, in this embodiment, the surface of the core is first coated with GaS, and then Zn is added. Here, the Ga source is as described above.

[0045] In this embodiment, an organic sulfur compound such as thiol can be used as a raw material for S. For example, 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 H21 SH, Octanethiol: CH 17 Other examples include S-ODE raw materials in which sulfur is dissolved in octadecene (ODE), S-DDT raw materials in which sulfur is dissolved in dodecanethiol, disulfide-based and thiuram-based S raw materials, and S-OLAm / DDT in which S is dissolved in oleylamine and dodecanethiol. The S raw materials are not particularly limited.

[0046] In addition, organic zinc compounds and inorganic zinc compounds are used as Zn sources. Organic zinc compounds and inorganic zinc compounds are raw materials that are stable in air and easy to handle. These raw materials can also be used as ligands. There are no particular limitations on the structure of the organic zinc compounds and inorganic zinc compounds, but for example, the organic zinc compounds and inorganic zinc compounds shown below can be used. As acetate salts, 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. Furthermore, in this embodiment, quantum dots can be obtained in one pot without isolating and purifying the precursor.

[0047] After the core-shell structure is formed, the product is purified with a specific solvent. For example, trioctylphosphine (TOP) can be used, but a high fluorescence quantum yield can be obtained even without TOP. TOP can also be included as a ligand. In this embodiment, the synthesized reaction solution may be centrifuged.

[0048] In the quantum dot manufacturing method described above, an AgGaSe core is produced, but it is also possible to produce a core containing at least Ag, Ga, and S. The Ag raw material, Ga raw material, and S raw material are as described above. The core can also contain Cu or In. Regarding the shell, if ZnSe or ZnGa2Se4 is ultimately desired, it is considered preferable that the first shell layer be GaSe. However, it is also possible to coat the GaSe layer with ZnS or ZnGaS. Furthermore, if the first shell layer is GaS, it is considered possible to ultimately obtain a shell containing ZnS, ZnGa2S4, or both. However, it is also possible to coat the GaS layer with ZnSe or ZnGaSe. In this embodiment, after coating GaS, cation exchange between Ga and Zn allows coating with a ZnS shell.

[0049] In the quantum dot manufacturing method of this embodiment, after the core is formed, a predetermined element is added to synthesize the shell. In this case, In may be included in the initial stage of the core generation reaction, but it is preferable that In is not included. This allows for good luminescence characteristics to be obtained.

[0050] Furthermore, in this embodiment, when Zn is incorporated into the quantum dots, it is preferable to add Zn while taking the following points into consideration. First, Zn is not added during the initial reaction but is added in the final step. This is because if Zn is contained inside the particles, defect luminescence may predominate, or only defect luminescence may be observed. Therefore, by adding Zn in the final step, the purpose is to cause a reaction only on the particle surface. Second, Zn is added at a low temperature. Here, low temperature means approximately 150 to 250°C. If the temperature when Zn is added is high, Zn will react into the particle interior, which is likely to result in defect luminescence. Therefore, it is preferable to cause the reaction only on the particle surface at a low temperature in order to limit the reaction to the particle surface.

[0051] In addition, in this embodiment, when synthesizing AgGaSe, it is preferable to use gallium acetylacetonate: Ga(acac)3 rather than gallium chloride as the Ga raw material, since good light emitting properties can be obtained. The Se raw material is preferably Se-OLAm / DDT, which can effectively suppress defect emission. Furthermore, the fluorescence quantum yield can be further improved by adding TOP to the separated quantum dots, although the addition of TOP is not essential.

[0052] As described above, according to the quantum dot manufacturing method of this embodiment, the surface of a core containing at least Ag, Ga, and S or Ag, Ga, and Se can be appropriately coated with a shell containing a large amount of Zn. This allows for the accurate production of quantum dots exhibiting band-edge emission with a fluorescence half-width of 35 nm or less, making mass production possible. In this embodiment, the core can be appropriately coated with a shell made of ZnS, thereby improving the stability of the fluorescence properties and achieving a high fluorescence quantum yield, specifically a fluorescence quantum yield of 70% or more.

[0053] Band-edge emission will now be described. In this embodiment, band-edge emission is possible not only with a core-shell structure but also with the core alone. Here, in the inventions described in patent documents, band-edge emission has not been confirmed, particularly for quantum dots of Groups 11-13-16, and the core alone does not emit band-edge emission. In each patent document, a specific shell manipulation is performed to obtain band-edge emission.

[0054] Furthermore, the methods defined as shell coating in each patent document and each non-patent document are not shell coating, but rather surface treatment of the core by shell coating, which is assumed to cause the core to emit band-edge light. The quantum dots of this embodiment achieve this band-edge emission without surface treatment. Additionally, the quantum dots realized in this embodiment have the same characteristics as conventional core-shell structures.

[0055] The core-shell structure of conventional quantum dots refers to quantum dots in which the core and shell compositions are clearly separated by different elements and the entire particle is crystallized. Another characteristic is that the shell coating results in an XRD peak position that differs from that of the core. The characteristics of this core-shell structure cannot be proven by the methods used to define shell coating in patent documents and non-patent documents. On the other hand, the quantum dots of this embodiment are crystallized quantum dots in which Ag, Ga, or Se are clearly separated in the core and Zn or S in the shell. Even after shell coating, the XRD peak position changes, just like conventional quantum dots. Therefore, this embodiment is the first quantum dot to achieve a clear shell coating.

[0056] Furthermore, in this embodiment, the emission intensity is high even for the core alone, and the fluorescence half-width can be narrowed even without a core-shell structure, and band edge emission has been confirmed.

[0057] Furthermore, in this embodiment, the fluorescent wavelength can be controlled from green to red by appropriately adjusting the particle size and composition of the quantum dots 5. Therefore, the fluorescent wavelength for green emission is preferably 500 nm or more and 560 nm or less, more preferably 510 nm or more and 550 nm or less, and even more preferably 520 nm or more and 540 nm or less. For red emission, the fluorescent wavelength is preferably 600 nm or more and 660 nm or less, more preferably 610 nm or more and 650 nm or less, and even more preferably 620 nm or more and 640 nm or less. In this embodiment, as described above, it is also possible to adjust the fluorescence wavelength within the range of 500 nm to 700 nm. The quantum dots 5 of this embodiment exhibit fluorescent properties with a fluorescence half-width of 35 nm or less and a fluorescence quantum yield of 70% or more.

[0058] Here, "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. The fluorescence half-width is preferably 35 nm or less. The fluorescence half-width is more preferably 30 nm or less. In this way, the fluorescence half-width can be narrowed, thereby improving the color gamut.

[0059] The fluorescence quantum yield of the quantum dots 5 of this embodiment is preferably 70% or more, more preferably 80% or more, and even more preferably 90% or more. In this way, this embodiment can increase the fluorescence quantum yield of the quantum dots.

[0060] Chalcopyrite is generally a material that emits defect light with a fluorescence half-width of 70 to 100 nm. In contrast, the quantum dots 5 of this embodiment have a narrow fluorescence half-width, a high fluorescence quantum yield, and a fluorescence lifetime that is much shorter than that of defect light. From these characteristics, it is presumed that the quantum dots 5 of this embodiment emit light at the band edge. In the method for producing quantum dots according to the present embodiment, Cu can be added when the AgGaSe core or AgGaS core is produced.

[0061] The Cu source may be an organic or inorganic copper compound, such as, but not limited to, copper acetate (Cu(OAc)2), copper nitrate (Cu(NO3)2), halides such as copper chloride (CuCl2), copper bromide (CuBr2), and silver iodide (CuI2), and carbamates such as copper diethyldithiocarbamate (Cu(SC(=S)N(C2H5)2)2 and copper dimethyldithiocarbamate (Cu(SC(=S)N(CH3)2)2).

[0062] In this embodiment, the Cu raw material may be added directly to the reaction solution, or may be dissolved in an organic solvent in advance to a certain concentration and used as the Cu raw material solution.

[0063] It is believed that Cu acts as a catalyst during core formation. In other words, the results of TEM-EDX analysis show that it is difficult to quantify Cu, and XRD also gives similar results regardless of whether Cu is added or not. For this reason, it is speculated that Cu acts as a catalyst rather than being dissolved in the elements that make up the core.

[0064] When cores are generated, the addition of Cu can increase the quantum yield (QY) to a level equivalent to or greater than that achieved without Cu addition, and reduce the fluorescence half-width to a level equivalent to or less than that achieved without Cu addition. Depending on the application of quantum dots, the addition of Cu can produce better results. The uses of quantum dots 5 shown in Figure 1 are not particularly limited, but some specific examples are listed below.

[0065] 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.

[0066] 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.

[0067] The LED chip 3 is a semiconductor element that emits light when a forward voltage is applied, and has a basic structure in which a P-type semiconductor layer and an N-type semiconductor layer are PN-junctioned. As shown in Figure 2, the fluorescent layer 4 is formed from a resin 6 in which a large number of quantum dots 5 are dispersed.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] By applying the quantum dots 5 having a narrow fluorescence half-width in this embodiment to the LED device shown in Figure 2 or the display device shown in Figure 3, it is possible to effectively improve the light-emitting characteristics of the device.

[0073] 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. [Example]

[0074] 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.

[0075] <Raw materials> In the experiment, the following raw materials were used to synthesize quantum dots with a core / shell structure of AgGaSe / ZnS. (solvent) Oleylamine: Kao Corporation Dodecanethiol: manufactured by Kao Corporation (Silver raw material) Silver acetate: Aldrich (Gallium raw material) Gallium acetylacetonate: manufactured by Tokyo Chemical Industry Co., Ltd. (selenium) Selenium: Shinko Chemical Industry Co., Ltd. (zinc) Zinc acetate: Kishida Chemical Co., Ltd. Zinc bromide: Kishida Chemical Co., Ltd. (sulfur raw material) Sulfur: Kishida Chemical Co., Ltd. <Measuring equipment> Fluorescence spectrometer: JASCO F-2700 Quantum yield measurement device: Otsuka Electronics QE-1100 Scanning electron microscope (SEM): Hitachi SU9000 X-ray diffraction device (XRD): Bruker D2 PHASER

[0076] [Example 1] A 100 mL reaction vessel was charged with 0.5 mL of a 0.2 M solution of silver acetate (Ag(OAc)) in oleylamine (OLAm), 336.7 mg of gallium acetylacetonate (Ga(acac)), 20.0 mL of oleylamine (OLAm), and 2.0 mL of dodecanethiol (DDT). The mixture was heated under an inert gas (N2) atmosphere with stirring to dissolve the raw materials.

[0077] This solution was dissolved at 150°C for 10 minutes, and 0.3 ml of a 0.7 M solution of selenium (Se) dissolved in a mixed solvent of dodecanethiol (DDT) and oleylamine (OLAm) in a volume ratio of 5:2 was added thereto. The temperature was then raised from 150°C to 320°C and stirred for a total of 10 minutes. The resulting reaction solution was then cooled to room temperature.

[0078] The resulting reaction solution was centrifuged at 5500 rpm for 3 minutes to obtain a precipitate. The precipitate was redispersed in toluene, ethanol was added, and the mixture was centrifuged at 7500 rpm for 3 minutes to obtain a precipitate. The precipitate was then redispersed in 10.0 ml of dodecanethiol (DDT). The precipitate obtained here was also analyzed using an X-ray diffraction (XRD) analyzer. The results are shown in Figure 5A.

[0079] This dispersion was placed in a 100 mL reaction vessel and heated at 270°C for 10 minutes under an inert gas (N2) atmosphere, and 0.15 mL of a 0.4 M solution of sulfur (S) in dodecanethiol (DDT) and 0.2 mL of a 0.1 M solution of gallium acetylacetonate (Ga(acac)3) in oleyl alcohol (OLOH) were added dropwise five times every 10 minutes. The resulting reaction solution was then cooled to room temperature.

[0080] Toluene and ethanol were added to the resulting reaction solution, which was then centrifuged at 7500 rpm for 3 minutes to obtain a precipitate. The precipitate was then redispersed in 10.0 ml of dodecanethiol (DDT). The precipitate was then analyzed using an X-ray diffraction (XRD) analyzer. The results are shown in Figure 5B.

[0081] Put this dispersion into a 100 mL reaction vessel, heat it at 180 °C for 10 minutes under an inert gas (N2) atmosphere, and alternately add 0.1 ml of a 0.2 M solution obtained by dissolving zinc bromide: ZnBr2 in dodecanethiol: DDT and 0.1 ml of a 0.4 M solution obtained by dissolving sulfur: S in dodecanethiol: DDT 5 times each at intervals of 10 minutes. Then, cool the resulting reaction solution to room temperature.

[0082] <Experiment on Fluorescence Half-Width and Quantum Yield> The QD dispersion solution was measured with a fluorescence spectrometer and a quantum efficiency measurement system. As a result, optical properties with a fluorescence wavelength of 637.5 nm, a fluorescence half-width of 32.11 nm, and a quantum yield of 76% were obtained as shown in Fig. 6. In addition, the precipitate obtained here was measured using an X-ray diffraction (XRD) apparatus. The result is shown as C in Fig. 5.

[0083] <Analysis Results of X-ray Diffraction (XRD) Apparatus> Fig. 5 shows the maximum peaks of the analysis results of samples A, B, and C obtained in each synthesis process using an XRD apparatus. From this result, it was confirmed that the XRD peak shifted to the high-angle side from A to B. In this synthesis process, the sample obtained by adding Ga and S to AgGaSe2 (sample A) obtained as the core is B. Also, the sample obtained by adding Zn and S to the sample B obtained here is C. In this synthesis process as well, it was confirmed that the XRD peak further shifted to the high-angle side from B to C. Considering such a peak shift, it can be inferred that Ag contained in the core does not diffuse into the shell and the shell containing a large amount of Zn can be coated.

[0084] <Analysis Results of TEM-EDX> The results (observation images) of analyzing the quantum dots of Example 1 above by TEM-EDX are shown in FIG. 7. FIG. 7(a) is a photograph of the analysis results (Se+S and Ag+Zn) of TEM-EDX, and FIG. 7(b) is a partial schematic diagram of FIG. 7(a). The dotted line in FIG. 7(b) indicates the vicinity of the boundary between the core and the shell (it does not show a clear boundary). The same applies to FIGS. 9 and later. The observation image on the left side of FIG. 7 is the analysis result of Se+S, and the observation image on the right side of FIG. 7 is the analysis result of Ag+Zn. From this experimental result, it was found that Se and S exist separately, and specifically, S exists around Se. Also, it was found that Ag and Zn exist separately, and specifically, Zn exists around Ag. Se and Ag are mainly contained in the core, and S and Zn are mainly contained in the shell. However, it was found that the diffusion of the components contained in the core into the shell can be suppressed, and similarly, the diffusion of the components contained in the shell into the core can be suppressed.

[0085] [Example 2] The same manufacturing operations as in Example 1 were followed, but the same operations were carried out up to the GaS coating, and the operation of post-adding Zn was carried out 9 times on the obtained particles. The quantum dot dispersion solution was measured with a fluorescence spectrometer and a quantum efficiency measurement system. As a result, as shown in FIG. 6, optical properties with a fluorescence wavelength of 630 nm, a fluorescence half-width of 34 nm, and a maximum quantum yield of 94% were obtained.

[0086] [High-resolution STEM results] The results of analyzing the quantum dots of Example 2 above by high-resolution STEM are shown in FIG. 8. FIG. 8(a) is a photograph of the analysis results of high-resolution STEM in Example 2, and FIG. 8(b) is a partial schematic diagram of FIG. 8(a). When the particles in FIG. 8 were observed, a crystal lattice could be confirmed from the whole particle. From this result, it can be inferred that the shell is crystallized, and it is considered that the diffusion of the amount of Ag contained in the core and Zn contained in the shell is suppressed.

[0087] [Analysis results of TEM-EDX] The results (observation images) of analyzing the quantum dots of the above Experimental Example 2 by TEM-EDX are shown in Fig. 9. Fig. 9(a) is a photograph of the analysis result (Ag + Zn) of TEM-EDX in Example 2, and Fig. 9(b) is a partial schematic diagram of Fig. 9(a). The observation image in Fig. 9 is the analysis result of Ag + Zn. From this observation image, it was found that Zn exists around Ag, and it is possible to suppress the diffusion of the components contained in the core to the shell, and similarly, it is possible to suppress the diffusion of the components contained in the shell to the core.

[0088] <Measurement results of Zn content> Fig. 10 shows the results of analyzing the Zn content by TEM-EDX. Fig. 10(a) is a photograph of the analysis result (Zn) of TEM-EDX in Example 1 and the Comparative Example, and Fig. 10(b) is a partial schematic diagram of Fig. 10(a). The observation image on the left side of Fig. 10 is the Comparative Example 1, and the observation image on the right side of Fig. 10 is Example 1. Different from the above experimental examples, in the process of the central figure of Fig. 4, the first layer of shell was coated with oleylamine solvent in the Comparative Example 1, and Zn was added later in the process of the right figure of Fig. 4. On the other hand, the Example is the above Experimental Example 1, that is, quantum dots were generated through the manufacturing process shown in Fig. 4.

[0089] From the experimental results shown in Fig. 10, it was confirmed that Example 1 contained more Zn on the surface of the core than Comparative Example 1, and GaS and ZnS could be appropriately coated on the surface of AgGaSe.

[0090] When measuring the Zn content in the quantum dots of the above Comparative Example 1, Example 1 and Example 2, it was about 1% in the Comparative Example 1, about 5% in Example 1, and about 10% in Example 2. It was found that the Zn content in Examples 1 and 2 increased by several times to about 10 times compared with the Comparative Example 1. Also, the Zn content in Example 2 was about twice that of Example 1. This is because the number of times of the Zn post-addition process was increased in Example 2.

[0091] [Example 3] A 100 mL reaction vessel was charged with 1.8 mL of a 0.2 M solution of silver acetate (Ag(OAc)) in oleylamine (OLAm), 0.2 mL of a 0.2 M solution of copper acetate (Cu(OAc)2) in oleylamine (OLAm), 0.2644 mg of gallium acetylacetonate (Ga(acac)3), 20.0 mL of oleylamine (OLAm), and 4.0 mL of dodecanethiol (DDT). The mixture was heated under an inert gas (N2) atmosphere with stirring to dissolve the raw materials.

[0092] This solution was dissolved at 150°C for 10 minutes, and 1.43 ml of a 0.7 M solution of selenium (Se) dissolved in a mixed solvent of dodecanethiol (DDT) and oleylamine (OLAm) in a volume ratio of 5:2 was added thereto. The temperature was then increased from 150°C to 320°C and stirred for a total of 20 minutes. The resulting reaction solution was then cooled to room temperature.

[0093] The resulting reaction solution was centrifuged at 5,500 rpm for 3 minutes to obtain a precipitate. The precipitate was redispersed in toluene, ethanol was added, and the mixture was centrifuged at 7,500 rpm for 3 minutes to obtain a precipitate. The precipitate was then redispersed in 10.0 ml of dodecanethiol:DDT.

[0094] This dispersion was placed in a 100 mL reaction vessel and heated at 270 °C for 5 minutes under an inert gas (N2) atmosphere. Then, 0.25 mL of a 0.8 M solution of sulfur (S) in dodecanethiol (DDT) and 1.3 mL of a 0.1 M solution of gallium acetylacetonate (Ga(acac)3) in oleyl alcohol (OLOH) were mixed. The mixture was heated for a total of 60 minutes. The resulting reaction solution was then cooled to room temperature.

[0095] Toluene and ethanol were added to the resulting reaction solution, which was then centrifuged at 7500 rpm for 3 minutes to obtain a precipitate, which was then redispersed in 10.0 ml of dodecanethiol:DDT.

[0096] This dispersion was placed in a 100 mL reaction vessel and heated at 200°C for 5 minutes under an inert gas (N2) atmosphere. 2.0 mL of a 0.8 M solution of zinc bromide (ZnBr2) dissolved in dodecanethiol (DDT) and 2.0 mL of a 0.8 M solution of sulfur (S) dissolved in dodecanethiol (DDT) were mixed and added dropwise in 0.4 mL increments every 10 minutes. The mixture was heated for a total of 120 minutes. The resulting reaction solution was then cooled to room temperature.

[0097] The QD dispersion solution was measured using a fluorescence spectrometer, and the optical properties were observed to be a fluorescence wavelength of 623.5 nm, a fluorescence half-width of 34.70 nm, and a quantum yield of 76%.

[0098] [Example 4] A 300 mL reaction vessel was charged with 5.4 mL of a 0.2 M solution of silver acetate (Ag(OAc)) in oleylamine (OLAm), 0.88092 g of gallium acetylacetonate (Ga(acac)), 120.0 mL of oleylamine (OLAm), and 11 mL of dodecanethiol (DDT). The mixture was heated under an inert gas (N2) atmosphere with stirring to dissolve the raw materials.

[0099] This solution was dissolved at 150°C for 15 minutes, and 4.29 ml of a 0.7 M solution of selenium (Se) dissolved in a mixed solvent of dodecanethiol (DDT) and oleylamine (OLAm) in a volume ratio of 5:2 was added thereto. The temperature was then increased from 150°C to 320°C and stirred for a total of 25 minutes. The resulting reaction solution was then cooled to room temperature.

[0100] The resulting reaction solution was centrifuged at 5500 rpm for 3 minutes to obtain a precipitate. The precipitate was redispersed in toluene, ethanol was added, and the mixture was centrifuged at 7500 rpm for 3 minutes to obtain a precipitate. The precipitate was again redispersed in toluene, ethanol was added, and the mixture was centrifuged at 7500 rpm for 3 minutes to obtain a precipitate. The precipitate was then redispersed in 60.0 ml of dodecanethiol:DDT.

[0101] This dispersion was placed in a 300 mL reaction vessel and heated at 270 °C for 5 minutes under an inert gas (N2) atmosphere. Then, 0.1 mL of a 0.8 M solution of sulfur (S) in dodecanethiol (DDT) was added and heated for 20 minutes. Then, 1.2 mL of a 0.8 M solution of sulfur (S) in dodecanethiol (DDT) and 6.0 L of a 0.1 M solution of gallium acetylacetonate (Ga(acac)3) in oleyl alcohol (OLOH) were added dropwise in 1.44 mL portions five times every 10 minutes. The mixture was heated for a total of 50 minutes. The resulting reaction solution was then cooled to room temperature.

[0102] Toluene, ethanol, and methanol were added to the resulting reaction solution, which was then centrifuged at 7500 rpm for 3 minutes to obtain a precipitate, which was then redispersed in 87.5 ml of octadecene:ODE and 10 ml of trioctylphosphine:TOP.

[0103] This dispersion was placed in a 300 mL reaction vessel and heated at 240 °C for 5 minutes under an inert gas (N2) atmosphere. Then, 10 mL of a 0.8 M solution of zinc acetate (Zn(OAc)2) dissolved in a 1:1 volume ratio of trioctylphosphine (TOP) and oleic acid (OLAc) was mixed with 0.4 mL of a 4 N solution of 6.7 mL of octadecene (ODE), 2.3 mL of trioctylphosphine (TOP), 1.0 mL of dodecanethiol (DDT), and hydrogen chloride (HCl) dissolved in ethyl acetate (C4H8O2). 2.25 mL of the solution was added dropwise eight times every 10 minutes. The mixture was heated for a total of 80 minutes. The resulting reaction solution was then cooled to room temperature.

[0104] The QD dispersion solution was measured using a fluorescence spectrometer, and the optical properties were observed to be a fluorescence wavelength of 622.5 nm, a fluorescence half-width of 36.1 nm, and a quantum yield of 91%.

[0105] [Example 5] A 100 mL reaction vessel was charged with 0.45 mL of a 0.2 M solution of silver acetate (Ag(OAc)) dissolved in oleylamine (OLAm), 0.0734 g of gallium acetylacetonate (Ga(acac)), 10.0 mL of oleylamine (OLAm), 0.917 mL of dodecanethiol (DDT), and 0.0643 mL of a 0.7 M solution of selenium (Se) dissolved in a 1:1 volumetric ratio mixture of dodecanethiol (DDT) and oleylamine (OLAm). The mixture was heated under an inert gas (N2) atmosphere with stirring to dissolve the raw materials.

[0106] This solution was dissolved at 320°C for 1.25 minutes, and then 1 ml of a 0.8 M solution of sulfur (S) in dodecanethiol (DDT) was added and heated for 60 minutes. The resulting reaction solution was then cooled to room temperature.

[0107] Toluene and ethanol were added to the resulting reaction solution, followed by centrifugation at 7500 rpm for 3 minutes to obtain a precipitate. The precipitate was redispersed in toluene, ethanol was added, and the mixture was centrifuged at 7500 rpm for 3 minutes to obtain a precipitate. The precipitate was then redispersed in 5.0 ml of dodecanethiol:DDT. The mixture was centrifuged at 5500 rpm for 3 minutes to obtain a precipitate.

[0108] This dispersion was placed in a 100 mL reaction vessel and heated at 240°C for 3 minutes under an inert gas (N2) atmosphere. 2 mL of a 0.2 M solution of zinc bromide (ZnBr2) in dodecanethiol (DDT) was mixed with 0.53 mL of trioctylphosphine (TOP), and 0.506 mL of this solution was added dropwise three times every 10 minutes. The mixture was heated for a total of 30 minutes. The resulting reaction solution was then cooled to room temperature.

[0109] The QD dispersion solution was measured using a fluorescence spectrometer, and the optical properties were observed to be a fluorescence wavelength of 540.5 nm and a fluorescence half-width of 39.0 nm.

[0110] Comparative Example 2 A 100 mL reaction vessel was charged with 1.8 mL of a 0.2 M solution of silver acetate (Ag(OAc)) in oleylamine (OLAm), 0.2 mL of a 0.2 M solution of copper acetate (Cu(OAc)2) in oleylamine (OLAm), 0.2644 mg of gallium acetylacetonate (Ga(acac)3), 20.0 mL of oleylamine (OLAm), and 4.0 mL of dodecanethiol (DDT). The mixture was heated under an inert gas (N2) atmosphere with stirring to dissolve the raw materials.

[0111] This solution was dissolved at 150°C for 10 minutes, and 1.43 ml of a 0.7 M solution of selenium (Se) dissolved in a mixed solvent of dodecanethiol (DDT) and oleylamine (OLAm) in a volume ratio of 5:2 was added thereto. The temperature was then increased from 150°C to 320°C and stirred for a total of 20 minutes. The resulting reaction solution was then cooled to room temperature.

[0112] The resulting reaction solution was centrifuged at 5,500 rpm for 3 minutes to obtain a precipitate. The precipitate was redispersed in toluene, ethanol was added, and the mixture was centrifuged at 7,500 rpm for 3 minutes to obtain a precipitate. The precipitate was then redispersed in 10.0 ml of dodecanethiol:DDT.

[0113] This dispersion was placed in a 100 mL reaction vessel and heated at 270°C for 5 minutes under an inert gas (N2) atmosphere, followed by mixing with 0.25 mL of a 0.8 M solution of sulfur (S) in dodecanethiol (DDT). After heating for 20 minutes, the resulting reaction solution was cooled to room temperature.

[0114] Hexane and ethanol were added to the resulting reaction solution, which was then centrifuged at 7500 rpm for 3 minutes to obtain a precipitate, which was then redispersed in 10.0 ml of dodecanethiol:DDT.

[0115] This dispersion was placed in a 100 mL reaction vessel and heated at 200°C for 5 minutes under an inert gas (N2) atmosphere. 2.0 mL of a 0.8 M solution of zinc bromide (ZnBr2) dissolved in dodecanethiol (DDT) and 2.0 mL of a 0.8 M solution of sulfur (S) dissolved in dodecanethiol (DDT) were mixed and added dropwise in 0.4 mL increments every 10 minutes. The mixture was heated for a total of 120 minutes. The resulting reaction solution was then cooled to room temperature.

[0116] The QD dispersion solution was measured using a fluorescence spectrometer, and the optical properties were as follows: fluorescence wavelength: 631.0 nm, fluorescence half-width: 36.24 nm, and quantum yield: 96%.

[0117] <Quantum dots with shells formed by cation exchange> Example 3 is an experimental example in which GaS coating was performed, and Comparative Example 2 is an experimental example in which GaS coating was not performed. Figures 11(a) and (b) show photoluminescence (PL) spectra in Example 3 and Comparative Example 2. Figure 11(b) is an enlarged view of a portion of Figure 11(a).

[0118] 11(b), in Comparative Example 2, an emission spectrum was observed in the vicinity of a shorter wavelength of 500 to 550 nm compared to Example 3. This is presumably because, since GaS was not coated in Comparative Example 2, when the ZnS raw material as the shell was added, Zn underwent cation exchange with Ag, Cu, Ga, etc., which are the cation species of the core.

[0119] In contrast to this, it was found that in Example 3, the ZnS raw material was added after the GaS coating, and thus the short wavelength light emission could be suppressed compared to Comparative Example 2. Therefore, it is presumed that the GaS coating prevents the Zn added later from diffusing into the core.

[0120] From the above experimental results, it is considered that Ga and Zn after GsS coating are changed to ZnS by cation exchange. Also, since the particle sizes of AgGaSe / GaS particles and AgGaSe / ZnS particles have hardly changed, it is presumed that Ga and Zn are involved in cation exchange or anion solid solution.

[0121] <Regarding the band emission of the core> EDX analysis was performed on the core of Example 3. The experimental results are shown in Fig. 12(a). Fig. 12(c) is a partial schematic diagram of (a). As shown in Fig. 12(a), it was found that in the core, Ga (blue) and Ag (purple) exist in almost the same distribution. From this, it is considered that the core-shell structure is not formed. Fig. 12(b) is the fluorescence (Photoluminescence: PL) spectrum of the core of Example 3. As shown in Fig. 12(b), it was found that the fluorescence half-width of the emission spectrum is narrow. In this example, it was found that the core alone emits band emission.

[0122] <Regarding the catalytic action of Cu> In Example 3, Cu was added during the formation of the core. However, in the results of TEM-EDX analysis, accurate quantification of Cu was difficult. Also, in XRD, equivalent results were obtained regardless of the presence or absence of Cu addition. Therefore, it is presumed that Cu acts as a catalyst rather than being dissolved in the elements constituting the core.

Industrial applicability

[0123] According to the present invention, for example, quantum dots showing high-intensity green fluorescence or red fluorescence can be stably obtained. By applying the quantum dots of the present invention to LEDs, backlight devices, display devices, etc., excellent emission characteristics can be obtained in each device.

[0124] This application is based on Japanese Patent Application No. 2020-217158 filed on December 25, 2020, and Japanese Patent Application No. 2021-53070 filed on March 26, 2021. All of this content is incorporated herein.

Claims

1. A quantum dot having a core containing at least Ag, Ga, and S or Ag, Ga, and Se, and a shell covering the surface of the core, the shell contains at least Zn; exhibiting fluorescent properties with a fluorescence half-width of 35 nm or less and a fluorescence quantum yield of 70% or more; Zn is contained in an amount of 5% by mass or more and 40% by mass or less with respect to the entire quantum dots. Quantum dots characterized by:

2. further containing In, and the Ga / In ratio in the entire quantum dot is 10 or more; Quantum dots according to claim 1 , characterized in that the Ga / Zn ratio is 1 or less.

3. 3. The quantum dot according to claim 1, wherein the fluorescence wavelength is in the range of 400 nm to 700 nm.

4. 3. The quantum dot according to claim 1, wherein the core alone exhibits band edge emission.

5. 3. The quantum dot according to claim 1, which exhibits fluorescent properties in which the band edge emission / defect emission ratio is 10 or more and the fluorescence lifetime until the fluorescence reaches 1 / e is 20 ns or less.

6. The quantum dot according to claim 1 or 2, further comprising Cu.

7. The quantum dots have at least inorganic ligands on their surfaces, The quantum dot according to claim 1 or 2, wherein the inorganic ligand contains a halogen.

8. The quantum dot according to claim 1 , wherein the core and the shell do not contain Cd and In.

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