Quantum dot

Abstract

To provide a quantum dot in which much Zn can be contained in a surface thereof.SOLUTION: A quantum dot of the present invention includes at least: a core that contains Ag, Ga, and S, or contains Ag, Ga, and Se; and a shell that covers a surface of the core. The shell includes at least Zn, and contains GaS or GaSe between the core and the shell composed of ZnS. An amount of Zn present in the surface is 5% or more in terms of weight ratio with respect to the entirety.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 a particle size of several nm to several tens of nm. Quantum dots emit fluorescence and are also called fluorescent nanoparticles because their size is on the nano-order, semiconductor nanoparticles because their composition is derived from semiconductor materials, or nanocrystals because their structure has a specific crystal structure.

[0003] Quantum dots consist of positively charged metal atoms and negatively charged non-metallic or metalloid atoms that are bonded together by ionic or covalent bonds. The ionic nature of the bond depends on the combination of the properties of the metal and metalloid atoms.

[0004] Quantum dots can change the emission wavelength in various ways depending on the particle size and composition. The performance of quantum dots can be expressed by the fluorescence quantum yield (QY) and the fluorescence 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 to a specific wavelength range. In addition, 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 by using quantum dots as a wavelength conversion material in the visible light region, the important optical properties are the fluorescence quantum yield and the fluorescence half-width.

[0007] Conventionally, the highly efficient quantum dots used mainly contain cadmium (Cd). Quantum dots containing Cd have the advantages of high fluorescence quantum yield and narrow fluorescence half-width. However, due to the toxicity of Cd, there are restrictions on its use in various countries, which has been a major barrier to practical use.

[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] JP 2017-025201 A [Patent Document 2] JP 2018-039971 A [Patent Document 3] JP 2018-044142 A [Patent Document 4] JP 2018-141141 A [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] In addition, even if Zn is added to AgGaS or AgInGaS to eliminate Zn, the cationic species easily diffuses within the grains, and Zn easily diffuses into the core, resulting in defect emission.

[0013] The present invention has been made in view of the above-mentioned circumstances, and has an object to provide quantum dots capable of containing a large amount of Zn on the surface. [Means for solving the problem]

[0014] The quantum dot of the present invention has at least a core containing Ag, Ga, and S or Ag, Ga, and Se, and a shell covering the surface of the core, the shell containing at least Zn, and containing GaS or GaSe between the core and the shell made of ZnS, and the amount of Zn present on the surface has a weight ratio of 5% or more relative to the total. Effect of the Invention

[0015] According to the quantum dot of the present invention, it is possible to incorporate a large amount of Zn into the surface while maintaining the band edge emission. In this way, the surface of the core can be appropriately coated with a shell containing a large amount of Zn, thereby improving the stability of the fluorescent properties and maintaining a high fluorescent quantum yield. [Brief description of the drawings]

[0016] [Figure 1] FIG. 2 is a schematic diagram of a quantum dot according to an embodiment of the present invention. [Diagram 2] 1 is a schematic diagram of an LED device using quantum dots according to an embodiment of the present invention. [Diagram 3] 1 is a vertical 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. [Diagram 5] 1 is an X-ray diffraction (XRD) spectrum in Example 1. [Figure 6] Fluorescence (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] FIG. 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 (a). [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] 3 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 PREFERRED EMBODIMENTS

[0017] 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 can be practiced with various modifications within the scope of the gist. Note that in this specification, the notation "to" means that it includes the lower limit value and the upper limit value.

[0018] Fig. 1 is a schematic diagram of a quantum dot in 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 In.

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

[0021] It is preferable that the shell 5b covering the surface of the core 5a does not contain cadmium (Cd) or indium (In) like the core 5a. In the present embodiment, the shell 5b contains a large amount of zinc (Zn). Specifically, the shell 5b is made of zinc sulfide (ZnS), zinc selenide (ZnSe), zinc gallium selenide (ZnGa 2 Se 4 ), zinc gallium sulfide (ZnGa 2 S 4 Of these, ZnS is preferred. The shell 5b may be in a state of being solid-solved on the surface of the core 5a.

[0022] The shell 5b may also contain copper (Cu) or indium (In). Specifically, the shell 5b may contain copper sulfide (CuS), copper selenide (CuSe), indium sulfide (In 2 S 3 ), indium selenide (In 2 Se 3 ), zinc indium selenide (ZnIn 2 Se 4 ), zinc indium sulfide (ZnIn 2 S 4 ), Copper indium sulfide (CuIn 2 S 4 ), Copper indium selenide (CuIn 2 Se 4 It is preferable that the shell 5b is made of a material other than the core 5a. The shell 5b may be in a state of being solid-solved on the surface of the core 5a.

[0023] The quantum dot 5 of this embodiment has a core 5a of AgGaSe or AgGaS on the surface thereof. 2 S 4 The shell 5b, such as the core 5a, can be appropriately covered. In this embodiment, the diffusion of Ag contained in the core 5a into the shell 5b is suppressed. 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 diffusion of Zn contained in the shell 5b into the inside of the core 5a can be suppressed.

[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, the term "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 entire quantum dots 5. Although the upper limit is not limited, for example, the upper limit is 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 between the quantum dots 5, and to develop the desired optical properties. There is no particular limitation 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 NH 2 , Stearyl (octadecyl)amine: C 18 H 37 NH 2 , dodecyl(lauryl)amine: C 12 H 25 NH 2 , Decylamine: C 10 H21 NH 2 , Octylamine: C 8 H 17 NH 2 (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: C 9 H 19 COOH, octanoic acid: C 7 H 15 COOH (3) Thiols 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: C 8 H 17 SH (4) Phosphine-based Trioctylphosphine: (C 8 H 17 ) 3 P, triphenylphosphine: (C 6 H 5 ) 3 P, tributylphosphine: (C 4 H 9 ) 3 P (5) Phosphine oxides Trioctylphosphine oxide: (C 8 H 17 ) 3 P=O, triphenylphosphine oxide: (C 6H 5 ) 3 P=O, tributylphosphine oxide: (C 4 H 9 ) 3 P=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 makes it possible to suppress surface defects on the quantum dots and to develop higher 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 producing quantum dots will be described. In this embodiment, the objective is to produce a quantum dot that exhibits band edge emission and can stabilize the emission characteristics by increasing the amount of Zn contained in the shell 5b. In this embodiment, band edge emission is possible even with only the core 5a. This point will be described later.

[0032] In a conventional method for producing quantum dots, for example, the surface of AgGaSe is covered 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 method for producing quantum dots of this embodiment has the following features.

[0033] (1) comprising at least a step of producing a core 5a containing Ag, Ga, and S or Ag, Ga, and Se, and a step of coating a surface of the core 5a with a shell 5b; (2) In the step of coating the shell 5b, the surface of the core 5a is coated with GaS 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), for example, GaS or Ga 2 S 3 In addition, when the core surface is coated with GaSe, the Gax Se y (x=1 to 2, y=1 to 6), for example, GaSe or Ga 2 Se 3 "Zn addition" refers to the addition of Zn alone, ZnS, ZnSe, ZnGa 2 Se 4 , ZnGa 2 S 4 This includes the addition of

[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 of the quantum dot 5 of this embodiment. As shown in the left diagram of FIG. 4, after generating an AgGaSe core, as shown in the center diagram of FIG. 4, a GaS shell is coated on the surface of the AgGaSe core. Then, Zn is added later, and as shown in the right diagram of FIG. 4, a ZnS shell is obtained. As shown in the center diagram of FIG. 4, the GaS shell coated on the surface of the AgGaSe core is an important shell for preventing Zn from diffusing to the inside of the core 5a when the next Zn is added. It is to be noted that, after the addition of Zn, Ga is dissolved or removed from the shell to the outside through a washing process, and the amount of Ga remaining in the shell is thought to decrease, but Ga may be contained in the shell. That is, if the shell is ZnGa 2 S 4 Alternatively, a GaS shell may be interposed between the AgGaSe core and the ZnS shell, i.e., the shell may have a two-layer structure of GaS / ZnS.

[0035] Alternatively, in this embodiment, it is preferable that Ga and Zn of GaS are cation-exchanged to coat the ZnS shell. That is, 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 are almost the same as those of AgGaSe / GaS in the right of FIG. 4, as confirmed by the TEM-EDX analysis results. From this, it can be inferred that the ZnS shell coated on the surface of AgGaS is coated by cation exchange. In this way, it is possible to synthesize quantum dots having a core-shell structure with a large amount of Zn on the surface without changing the particle size by cation exchange or anion solid solution.

[0036] In the conventional manufacturing method, even if ZnS coating is performed, Zn immediately diffuses to the inside of the core, resulting in defective emission, and it was not possible to perform a large amount of ZnS coating. In contrast, in this embodiment, the crystallinity of ZnS obtained by adding Zn after GaS coating was confirmed. In the conventional shell coating, for example, oleylamine is used, but it is thought that oleylamine becomes a ligand that hinders shell coating. In fact, it is known that if a GaS coating operation that does not shift the XRD is performed, defective emission occurs immediately when Zn is added. In contrast, in this embodiment, for example, DDT (dodecanethiol), which is not an amine system, is used for shell coating. Other than DDT, ODE can also be used. However, oleylamine can also be included as long as it is not the main solvent. It is known that after the use of DDT, the XRD shifts during the GaS coating process, as shown in the experimental results described later. In this way, in this embodiment, by using GaS in a DDT (or ODE) solvent, it is possible to coat Ag without diffusing it to the surface.

[0037] According to the above-described method for producing quantum dots 5 of the present embodiment, it is possible to suppress the diffusion of Ag contained in the core 5a into the shell, and further, in a form in which the core 5a contains Se, it is possible to appropriately separate Se in the core 5a from S contained in the shell 5b. According to the method for producing quantum dots 5 of the present embodiment, it is possible to increase the amount of Zn contained in the shell 5b. 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 ratio. A method for producing quantum dots will now be described in detail. First, in this embodiment, quantum dots are synthesized in one pot by heating 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 to a lower temperature of 280° C. or less.

[0039] In this embodiment, an organic silver compound or an inorganic silver compound is used as a raw material for Ag. Although not particularly limited, for example, silver acetate: AgOAc, silver nitrate: AgNO 3 , as halides, silver chloride: AgCl, silver bromide: AgBr, silver iodide: AgI, as carbamates, silver diethyldithiocarbamate: Ag(SC(=S)N(C 2 H 5 ) 2 ), Silver dimethyldithiocarbamate: Ag(SC(=S)N(CH 3 ) 2 ), etc. can be used.

[0040] In this embodiment, the Ag raw material may be added directly to the reaction solution, but may also be dissolved in an organic solvent in advance to a constant 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 raw material for Ga. Although not particularly limited, for example, gallium acetate: Ga(OAc) 3 , gallium nitrate:GaNO 3 , gallium acetylacetonate: Ga(acac) 3 , as a halide, gallium chloride: GaCl 3 , gallium bromide: GaBr 3 , gallium iodide: Ga 2 I 3 , gallium diethyldithiocarbamate as the carbamate: Ga[(SC(=S)N(C 2 H 5 ) 2 ] 3 , etc. can be used.

[0042] In this embodiment, the Ga source may be added directly to the reaction solution, but may also be dissolved in an organic solvent in advance to give 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 (C 8 H 17 ) 3 P=Se, or tributylphosphine selenide, in which selenium is dissolved in tributylphosphine: (C 4 H 9 ) 3 P=Se or a solution in which selenium is dissolved in a high boiling point solvent that is a long chain hydrocarbon such as octadecene can be used. When synthesizing AgGaSe, the selenium raw material species contributes greatly to the fluorescent properties. In particular, a solution in which Se is dissolved in a mixture of oleylamine and dodecanethiol (Se-OLAm / DDT) shows good luminescence properties. In the early stages of luminescence, two types of luminescence can be confirmed in normal chalcopyrite quantum dots: a PL spectrum considered to be band edge luminescence and a PL spectrum considered to be defect luminescence, and the luminescence intensity ratio of band edge luminescence / defect luminescence is almost always 10 or less. Thereafter, as the reaction proceeds further, the intensity of the defect luminescence gradually decreases, and the intensity of the band edge luminescence also increases accordingly. However, as in this embodiment, when Se-DDT / OLAm is used as the Se source, a single peak is observed from the early stages of luminescence, the band edge luminescence / defect luminescence is 10 or more, and almost no peak considered to be defect luminescence can be confirmed. In addition, the fluorescence half width is 30 nm or less. The fluorescence lifetime is also short, at less than 20 ns until it reaches 1 / e, and only peaks that are not defect emission can be confirmed at the initial stage 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 H37 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: C 8 H 17 In addition, S-ODE raw material in which sulfur is dissolved in octadecene:ODE, S-DDT raw material 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 can be used. There is no particular limitation on the S raw material.

[0046] In addition, an organic zinc compound or an inorganic zinc compound is used as the Zn source. The organic zinc compound or the inorganic zinc compound is a raw material that is stable in air and easy to handle. This raw material can also be used as a ligand. The structure of the organic zinc compound or the inorganic zinc compound is not particularly limited, but for example, the organic zinc compound and the inorganic zinc compound shown below can be used. Zinc acetate as an acetate salt: Zn(OAc) 2 , Zinc nitrate: Zn(NO 3 ) 2 , as fatty acid salt, 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 a halide, zinc chloride: ZnCl2 , Zinc bromide: ZnBr 2 , zinc iodide: ZnI 2 , Zinc diethyldithiocarbamate as zinc carbamate: Zn(SC(=S)N(C 2 H 5 ) 2 ) 2 , zinc dimethyldithiocarbamate: Zn(SC(=S)N(CH 3 ) 2 ) 2 , zinc dibutyldithiocarbamate: Zn(SC(=S)N(C 4 H 9 ) 2 ) 2 etc. can be used. Moreover, in this embodiment, it is possible to obtain quantum dots 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 without TOP. TOP can also be included as a ligand. In this embodiment, the synthesized reaction solution may be centrifuged.

[0048] In the above quantum dot manufacturing method, 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. As for the shell, it is also possible to finally produce a ZnSe or ZnGa 2 Se 4 When it is desired to obtain the above, it is considered that the first shell is preferably GaSe. However, after covering with GaSe, it is also possible to cover with ZnS or ZnGaS. Also, when the first shell is GaS, it is preferable to cover with ZnS or ZnGa 2 S 4 Alternatively, it is believed that a shell containing both can be obtained, although it is also possible to coat GaS followed by ZnSe or ZnGaSe. In this embodiment, after coating GaS, a ZnS shell can be coated by cation exchange between Ga and Zn.

[0049] In the method for producing quantum dots according to the present embodiment, after the core is formed, a specific element is added to synthesize the shell. In this case, although In may be included in the initial stage of the core generation reaction, it is preferable that In is not included. This allows for good light emission characteristics to be obtained.

[0050] In addition, in this embodiment, when Zn is included in the quantum dots, it is preferable to add Zn while paying attention to the following points. First, Zn is not added during the initial reaction but is added in the final step. This is because if Zn is included inside the particles, there is a risk that defect emission will be dominant or only defect emission will be confirmed. Therefore, the purpose of adding Zn in the final step is to react only on the particle surface. Second, Zn is added at a low temperature. Here, low temperature means about 150 to 250°C. If the temperature when Zn is added is high, Zn will react to the inside of the particles, which is likely to result in defect emission. Therefore, in order to stop the reaction to the particle surface, it is preferable to react only on the particle surface at a low temperature.

[0051] In this embodiment, when synthesizing AgGaSe, the Ga source is gallium acetylacetonate (Ga(acac)) rather than gallium chloride. 3 By using the above, good light emitting properties can be obtained, which is preferable. The Se raw material is preferably Se-OLAm / DDT, which can effectively suppress defect emission. In addition, 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 method for producing quantum dots of this embodiment, the surface of a core containing at least Ag, Ga, and S or Ag, Ga, and Se can be appropriately covered with a shell containing a large amount of Zn. This allows quantum dots having band edge emission with a fluorescence half width of 35 nm or less to be accurately produced and mass-produced. In this embodiment, the core can be appropriately covered with a shell made of ZnS, which improves the stability of the fluorescence characteristics and allows a high fluorescence quantum yield, specifically a fluorescence quantum yield of 70% or more to be obtained.

[0053] The band edge emission will be explained. In this embodiment, band edge emission is possible not only in the core-shell structure but also in the core alone. Here, in the inventions described in the patent documents, it is stated that 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 predetermined shell operation is performed to obtain band edge emission.

[0054] In addition, the method defined as shell coating in each patent document and each non-patent document is not shell coating, but rather, it is assumed that the core emits band edge light by performing a surface treatment of the core through a shell coating operation. The quantum dots of this embodiment realize this band edge emitting core without performing a surface treatment. In addition, the quantum dots realized in this embodiment have the same characteristics as those of a conventional core-shell structure.

[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 whole particle is crystallized. Another feature is that the shell coating shows a different XRD peak position from the core. The features of this core-shell structure cannot be proven by the methods in which each patent document and each non-patent document defines shell coating. 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. After shell coating, the XRD peak position changes in the same way as conventional quantum dots. Therefore, this embodiment is the first quantum dot in which a clear shell coating has been realized.

[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] In this embodiment, the particle size and composition of the quantum dots 5 are appropriately adjusted to control the fluorescence wavelength from green to red. Therefore, the fluorescence wavelength is preferably 500 nm or more and 560 nm or less for green emission, 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. Also, the fluorescence 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 for red emission. In this embodiment, as described above, it is also possible to adjust the fluorescence wavelength within the range of 500 nm or more and 700 nm or less. The quantum dots 5 of this embodiment exhibit fluorescent properties in which the fluorescence half-width is 35 nm or less and the fluorescence quantum yield is 70% or more.

[0058] Here, the term "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 value of the 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 defective light with a fluorescence half-width of 70 to 100 nm. In contrast, the quantum dots 5 of the present embodiment have a narrow fluorescence half-width, a high fluorescence quantum yield, and a fluorescence lifetime that is much shorter than that of defective light emission. From these characteristics, it is presumed that the quantum dots 5 of the present embodiment emit light at the band edge. In the method for producing quantum dots according to the present embodiment described above, Cu can be added when the AgGaSe core or the AgGaS core is produced.

[0061] As a raw material for Cu, an organic copper compound or an inorganic copper compound is used. Although not particularly limited, for example, copper acetate: Cu(OAc) 2 , copper nitrate:Cu(NO 3 ) 2 , as a halide, copper chloride: CuCl 2 , copper bromide:CuBr 2 , Silver iodide: CuI 2 , copper diethyldithiocarbamate as carbamate: Cu(SC(=S)N(C 2 H 5 ) 2 ) 2 , copper dimethyldithiocarbamate: Cu(SC(=S)N(CH 3 ) 2 ) 2, etc. can be used.

[0062] In this embodiment, the above-mentioned Cu source material may be added directly to the reaction solution, but it may also be dissolved in an organic solvent in advance to a constant concentration and used as the Cu source 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 the core is generated, the addition of Cu can increase the quantum yield (QY) to the same level or more and reduce the fluorescence half-width to the same level or less, compared to when Cu is not added. Depending on the application to which the quantum dots are applied, the addition of Cu can provide better results. The uses of the quantum dots 5 shown in FIG. 1 are not particularly limited, but some specific examples are given 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 is configured to include a storage case 2 having a bottom surface 2a and a side wall 2b surrounding the bottom surface 2a, an LED chip (light-emitting element) 3 arranged 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 by the LED chip 3 is emitted from the storage case 2, and indicates the opposite direction to the bottom surface 2a with respect to the LED chip 3.

[0066] The LED chip 3 is disposed on a base wiring board (not shown), which may constitute the bottom surface of the storage case 2. The base board may be, for example, a base material such as glass epoxy resin having a wiring pattern formed thereon.

[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 Fig. 2, the fluorescent layer 4 is formed of a resin 6 in which a large number of quantum dots 5 are dispersed.

[0068] The resin composition in this embodiment in which the quantum dots 5 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 a KSF (K 2 SiF 6 :Mn 4+ ) red phosphor, but the material is not particularly limited.

[0069] The resin 6 constituting the fluorescent layer 4 is not particularly limited, but may be 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, or a mixture thereof.

[0070] The LED device using the quantum dots of this embodiment can be applied to a display device. Fig. 3 is a vertical 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 have 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, the 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 constitute 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 dot 5 having a narrow fluorescence half-width in this embodiment to the LED device shown in FIG. 2 or the display device shown in FIG. 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. EXAMPLES

[0074] The effects of the present invention will be described 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 AgGaSe / ZnS core / shell structure. (solvent) Oleylamine: Kao Corporation Dodecanethiol: 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 Co., Ltd. QE-1100 Scanning electron microscope (SEM): Hitachi SU9000 X-ray diffraction device (XRD): Bruker D2 PHASER

[0076] [Example 1] In a 100 mL reaction vessel, add 0.5 mL of a 0.2 M solution of silver acetate (Ag(OAc)) in oleylamine (OLAm) and 0.5 mL of gallium acetylacetonate (Ga(acac)). 3 36.7 mg, oleylamine (OLAm) 20.0 mL, and dodecanethiol (DDT) 2.0 mL were added. Then, an inert gas (N 2 The raw materials were dissolved by heating under stirring in a 100% ethanol atmosphere.

[0077] This solution was dissolved at 150°C for 10 minutes, and 0.3 ml of a 0.7 M solution obtained by dissolving selenium (Se) in a mixed solvent of dodecanethiol (DDT) and oleylamine (OLAm) with a volume ratio of 5:2 was added thereto. The temperature was then increased from 150°C to 320°C, and the mixture was 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 measured using an X-ray diffraction (XRD) device. The results are shown as A in Figure 5.

[0079] This dispersion was placed in a 100 mL reaction vessel and gassed with inert gas (N 2)Heated at 270 °C for 10 minutes under an atmosphere, 0.15 ml of a 0.4 M solution obtained by dissolving sulfur:S in dodecanethiol:DDT and gallium acetylacetonate:Ga(acac) 3 were dissolved in oleyl alcohol:OLOH, and 0.2 ml of a 0.1 M solution obtained was added dropwise 5 times every 10 minutes. Then, the resulting reaction solution was cooled to room temperature.

[0080] After adding toluene and ethanol to the resulting reaction solution, it was centrifuged at 7500 rpm for 3 minutes to obtain a precipitate. Then, the precipitate was redispersed in 10.0 ml of dodecanethiol:DDT. Also, the precipitate obtained here was measured using an X-ray diffraction (XRD) apparatus. The results are shown as B in Figure 5.

[0081] This dispersion was placed in a 100 mL reaction vessel, and under an inert gas (N 2 ) atmosphere, it was heated at 180 °C for 10 minutes, and 0.1 ml of a 0.2 M solution obtained by dissolving zinc bromide:ZnBr 2 in dodecanethiol:DDT and 0.1 ml of a 0.4 M solution obtained by dissolving sulfur:S in dodecanethiol:DDT were added dropwise alternately 5 times every 10 minutes. Then, the resulting reaction solution was cooled to room temperature.

[0082] <Experiment on fluorescence half-width and quantum yield> The QD dispersion solution was measured using 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 Figure 6. Also, the precipitate obtained here was measured using an X-ray diffraction (XRD) apparatus. The results are shown as C in Figure 5.

[0083] <Analysis results of X-ray diffraction (XRD) apparatus> Figure 5 shows the maximum peaks of the results of analyzing samples A, B, and C obtained in each synthesis process using an XRD apparatus. From these results, it was confirmed that the XRD peak shifted to the high-angle side from A to B. This synthesis process was AgGaSe obtained as the core 2For the sample of (Sample of A), the sample obtained by adding Ga and S is B. Also, for the sample of B obtained here, the sample obtained by adding Zn and S is C. In this synthesis process, it was also confirmed that the XRD peak further shifted to the higher 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 can coat the shell containing a large amount of Zn.

[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, specifically, S exists around Se. Also, Ag and Zn exist separately, specifically, Zn exists around Ag. Se and Ag are mainly contained in the core, and S and Zn are mainly contained in the shell. Thus, it was found that the components contained in the core can be suppressed from diffusing into the shell, and similarly, the components contained in the shell can be suppressed from diffusing into the core.

[0085] [Example 2] The manufacturing operation is in accordance with that of Example 1, but the same operation is carried out until GaS coating, and the operation of post-adding Zn is 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 examined, a crystal lattice could be confirmed throughout the particles. 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 the amount of Zn contained in the shell is suppressed.

[0087] <Analysis Results of TEM-EDX> The results (observation images) of analyzing the quantum dots of Experimental Example 2 above by TEM-EDX are shown in Fig. 9. Fig. 9(a) is a photograph of the analysis results (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 Amount> Fig. 10 shows the results of analyzing the amount of Zn by TEM-EDX. Fig. 10(a) is a photograph of the analysis results (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 Comparative Example 1, and the observation image on the right side of Fig. 10 is Example 1. Different from the above experimental examples, in Comparative Example 1, in the process of the middle figure in Fig. 4, the first layer of shell was coated with oleylamine solvent, and Zn was added later in the process of the right figure in Fig. 4. On the other hand, the example is Experimental Example 1 above, 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 the amount of Zn contained in the quantum dots of the above Comparative Example 1, Example 1, and Example 2 was measured, it was about 1% in Comparative Example 1, about 5% in Example 1, and about 10% in Example 2. It was found that the amount of Zn in Examples 1 and 2 was several times to about 10 times greater than that of Comparative Example 1. Moreover, the amount of Zn 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] In a 100 mL reaction vessel, add 1.8 mL of 0.2 M solution of silver acetate: Ag(OAc) in oleylamine: OLAm, and 1.8 mL of copper acetate: Cu(OAc). 2 0.2 ml of 0.2 M solution of 1000 mg ... 3 0.2644 mg, oleylamine (OLAm) 20.0 mL, and dodecanethiol (DDT) 4.0 mL were added. Then, an inert gas (N 2 The raw materials were dissolved by heating under stirring in a 100% ethanol atmosphere.

[0092] This solution was dissolved at 150°C for 10 minutes, and 1.43 ml of a 0.7 M solution obtained by dissolving selenium (Se) in a mixed solvent of dodecanethiol (DDT) and oleylamine (OLAm) with a volume ratio of 5:2 was added thereto. The temperature was then increased from 150°C to 320°C, and the mixture was 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 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.

[0094] This dispersion was placed in a 100 mL reaction vessel and gassed with inert gas (N 2) atmosphere at 270 °C for 5 minutes, and 0.25 ml of a 0.8 M solution of sulfur (S) in dodecanethiol (DDT) and gallium acetylacetonate (Ga(acac) 3 The reaction mixture was mixed with 1.3 ml of a 0.1 M solution obtained by dissolving the above in oleyl alcohol (OLOH). The mixture was heated for a total of 60 minutes. The reaction mixture 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 gassed with inert gas (N 2 ) atmosphere at 200°C for 5 minutes, and zinc bromide: ZnBr 2 2.0 ml of a 0.8 M solution obtained by dissolving in dodecanethiol:DDT and 2.0 ml of a 0.8 M solution obtained by dissolving sulfur:S in dodecanethiol:DDT were mixed, and 0.4 ml of each solution was added dropwise every 10 minutes. The mixture was heated for a total of 120 minutes. The resulting reaction solution was cooled to room temperature.

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

[0098] [Example 4] In a 300 mL reaction vessel, add 5.4 mL of a 0.2 M solution of silver acetate (Ag(OAc)) in oleylamine (OLAm) and gallium acetylacetonate (Ga(acac) 3 0.88092g, 120.0mL of oleylamine (OLAm), and 11mL of dodecanethiol (DDT) were added. Then, an inert gas (N 2 The raw materials were dissolved by heating under stirring in a 100% ethanol atmosphere.

[0099] This solution was dissolved at 150°C for 15 minutes, and 4.29 ml of a 0.7 M solution obtained by dissolving selenium (Se) in a mixed solvent of dodecanethiol (DDT) and oleylamine (OLAm) with a volume ratio of 5:2 was added thereto. The temperature was then increased from 150°C to 320°C, and the mixture was 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 gassed with inert gas (N 2 ) atmosphere at 270 °C for 5 minutes, mixed with 0.1 ml of a 0.8 M solution obtained by dissolving sulfur: S in dodecanethiol: DDT, and heated for 20 minutes. Then, 1.2 ml of a 0.8 M solution obtained by dissolving sulfur: S in dodecanethiol: DDT and gallium acetylacetonate: Ga(acac) 3 The mixture was mixed with 6.0 L of a 0.1 M solution obtained by dissolving in oleyl alcohol (OLOH), and 1.44 mL of each solution was added dropwise five times every 10 minutes. The mixture was heated for a total of 50 minutes. The 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 gassed with inert gas (N 2 ) atmosphere at 240°C for 5 minutes to obtain zinc acetate: Zn(OAc) 210 ml of 0.8 M solution obtained by dissolving in trioctylphosphine:TOP and oleic acid:OLAc in a volume ratio of 1:1, 6.7 ml of octadecene:ODE, 2.3 ml of trioctylphosphine:TOP, 1.0 ml of dodecanethiol:DDT, and hydrogen chloride:HCl were dissolved in ethyl acetate:C 4 H 8 O 2 The mixture was mixed with 0.4 ml of a 4N solution of 1,000 ml ...

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

[0105] [Example 5] In a 100 mL reaction vessel, add 0.45 mL of a 0.2 M solution of silver acetate (Ag(OAc)) in oleylamine (OLAm) and 0.5 mL of gallium acetylacetonate (Ga(acac)). 3 0.0734g, 10.0mL of oleylamine:OLAm, 0.917mL of dodecanethiol:DDT, and 0.0643mL of a 0.7M solution obtained by dissolving selenium:Se in a mixed solvent of dodecanethiol:DDT and oleylamine:OLAm in a volume ratio of 1:1 were added. Then, an inert gas (N 2 The raw materials were dissolved by heating under stirring in a 100% ethanol atmosphere.

[0106] This solution was dissolved at 320°C for 1.25 minutes, and 1 ml of a 0.8 M solution obtained by dissolving sulfur (S) in dodecanethiol (DDT) was mixed thereon 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, and the mixture was centrifuged 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 gassed with inert gas (N 2 ) atmosphere at 240°C for 3 minutes, and zinc bromide: ZnBr 2 2 ml of a 0.2 M solution obtained by dissolving in dodecanethiol: DDT and 0.53 ml of trioctylphosphine: TOP were mixed, and 0.506 ml was added dropwise three times every 10 minutes. The mixture was heated for a total of 30 minutes. The resulting reaction solution was cooled to room temperature.

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

[0110] [Comparative Example 2] In a 100 mL reaction vessel, add 1.8 mL of 0.2 M solution of silver acetate: Ag(OAc) in oleylamine: OLAm, and 1.8 mL of copper acetate: Cu(OAc). 2 0.2 ml of 0.2 M solution of 1000 mg ... 3 0.2644 mg, oleylamine (OLAm) 20.0 mL, and dodecanethiol (DDT) 4.0 mL were added. Then, an inert gas (N 2 The raw materials were dissolved by heating under stirring in a 100% ethanol atmosphere.

[0111] This solution was dissolved at 150°C for 10 minutes, and 1.43 ml of a 0.7 M solution obtained by dissolving selenium (Se) in a mixed solvent of dodecanethiol (DDT) and oleylamine (OLAm) with a volume ratio of 5:2 was added thereto. The temperature was then increased from 150°C to 320°C, and the mixture was 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 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.

[0113] This dispersion was placed in a 100 mL reaction vessel and gassed with inert gas (N 2 ) atmosphere at 270°C for 5 minutes, and 0.25 ml of a 0.8 M solution obtained by dissolving sulfur (S) in dodecanethiol (DDT) was mixed in. 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 gassed with inert gas (N 2 ) atmosphere at 200°C for 5 minutes, and zinc bromide: ZnBr 2 2.0 ml of a 0.8 M solution obtained by dissolving in dodecanethiol:DDT and 2.0 ml of a 0.8 M solution obtained by dissolving sulfur:S in dodecanethiol:DDT were mixed, and 0.4 ml of each solution was added dropwise every 10 minutes. The mixture was heated for a total of 120 minutes. The resulting reaction solution was cooled to room temperature.

[0116] The QD dispersion solution was measured using a fluorescence spectrometer, and the optical properties obtained were a fluorescence wavelength of 631.0 nm, a fluorescence half-width of 36.24 nm, and a quantum yield of 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] As shown in Fig. 11(b), in Comparative Example 2, compared with Example 3, an emission spectrum was confirmed in the vicinity of short wavelengths of 500 to 550 nm. This is presumably because in Comparative Example 2, since GaS was not coated, when the ZnS raw material was added as a shell, Zn underwent cation exchange with cations such as Ag, Cu, and Ga, which are the cation species of the core.

[0119] In contrast, in Example 3, it was found that by adding the ZnS raw material after coating with GaS, the emission of short wavelengths could be suppressed compared to Comparative Example 2. For this reason, it is presumed that the Zn added later does not diffuse to the inside of the core due to the coated GaS.

[0120] From the above experimental results, it is considered that Ga and Zn after GaS coating have 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 undergoing 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 a 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 also emits band emission.

[0122] <Regarding the catalytic action of Cu> In Example 3, Cu was added during the formation of the core, but the results of the TEM-EDX analysis showed that it was difficult to accurately quantify Cu. In addition, the same results were obtained in the XRD regardless of whether Cu was added or not. For this reason, it is presumed that Cu acts as a catalyst rather than being dissolved in the elements that make up the core. [Industrial Applicability]

[0123] According to the present invention, for example, quantum dots that exhibit high luminance green or red 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.

[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, the contents of which are incorporated herein in their entirety.

Claims

1. At least a core containing Ag, Ga, and S or Ag, Ga, and Se, and a shell covering the surface of the core, The shell comprises at least Zn; GaS or GaSe is included between the core and the shell made of ZnS, A quantum dot, characterized in that the amount of Zn present on the surface has a weight ratio of 5% or more with respect to the whole.

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

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

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

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