Quantum dots

Abstract

To provide a quantum dot that has a narrow fluorescence half-width and a high fluorescence quantum yield, and emits blue fluorescence.SOLUTION: A quantum dot (5) according to the present invention includes at least Zn and Se and does not include Cd, and has a particle diameter of 5 nm or more and 20 nm or less. In addition, the quantum dot (5) according to the present invention includes at least Zn and Se and does not include Cd, and has a fluorescence quantum yield of 5% or more and a fluorescence half-width of 25 nm or less. In the present invention, the fluorescence lifetime can be set to 50 ns or less.SELECTED DRAWING: Figure 1A

Description

Technical Field

[0001] The present invention relates to cadmium - free quantum dots and a method for manufacturing the same.

Background Art

[0002] Quantum dots emit fluorescence. Because their size is on the nano - order, they are also called fluorescent nanoparticles; because their composition is derived from semiconductor materials, they are called semiconductor nanoparticles; or because their structure has a specific crystal structure, they are also called nanocrystals.

[0003] Examples of properties representing the performance of quantum dots include fluorescence quantum yield (QY) and full width at half maximum of fluorescence (FWHM). When quantum dots are used as wavelength - conversion materials in the visible - light region, their most significant feature is the wide range of colors that can be represented, that is, a high color gamut. In order to achieve a high color gamut using quantum dots, it is important to narrow the full width at half maximum of fluorescence.

[0004] As an application of a display using quantum dots, when adopting photoluminescence (PL) as the emission principle, a method is adopted in which a blue LED is used as excitation light for the backlight, and quantum dots are used to convert it into green light or red light. On the other hand, for example, when adopting electroluminescence (EL) as the emission principle, or when all three primary colors are made to emit light using quantum dots by other methods, blue - emitting quantum dots are required. In that case, when achieving a high color gamut, it is necessary not only for green and red, but also for the full width at half maximum of blue light to be narrow. Therefore, when all three RGB colors are made to emit light using quantum dots, it is necessary for the full width at half maximum of the fluorescence of the blue - emitting quantum dots to be narrow.

[0005] As typical examples of blue quantum dots, cadmium selenide (CdSe) - based quantum dots using cadmium (Cd) can be cited. However, Cd is internationally regulated, and there has been a high barrier to the practical application of materials using CdSe quantum dots.

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

[0007] In addition, Non - Patent Document 1 below describes in detail a direct synthesis method of ZnSe using diphenylphosphine selenide, which is considered to have relatively high reactivity with an organic zinc compound, but it is not suitable as a blue - emitting quantum dot.

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

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

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

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

[0012] As described above, although research and development of blue quantum dots is progressing, none of the quantum dots have reached a level where they can be mass-produced, nor have they reached the performance level required to replace Cd-based quantum dots in terms of fluorescence full width at half maximum and fluorescence quantum yield.

[0013] Therefore, the present invention has been made in view of these points, and aims to provide a quantum dot that emits blue fluorescence with a narrow fluorescence half-width and a high fluorescence quantum yield.

[0014] Furthermore, the present invention aims to provide a quantum dot in a core-shell structure that exhibits high luminescence efficiency and emits blue fluorescence with a narrow fluorescence half-width.

[0015] Furthermore, the present invention aims to provide a method for producing quantum dots that can be synthesized safely and in mass production. [Means for solving the problem]

[0016] This invention relates to ZnSe Consists of Core and ZnS Consists of It is a core-shell structure composed of a shell and a particle size of 5 nm to 20 nm, and on the surface of the ZnS, Cl or Br modifies Furthermore, the fluorescence quantum yield is 76% or higher. It is characterized by the following:

[0017] In this invention, it is preferable that the fluorescence half-width is 25 nm or less.

[0018] In this invention, it is preferable that the fluorescence wavelength is in the range of 410 nm to 490 nm.

[0019] In this invention, the particle shape may be polygonal. [Effects of the Invention]

[0021] According to the quantum dots of the present invention, quantum dots with uniform particle shape and size can be synthesized, which allows for a narrower fluorescence half-width and improved wide color gamut.

[0022] Furthermore, the quantum dot manufacturing method of the present invention makes it possible to safely and mass-produce quantum dots with a narrow fluorescence half-width and that do not contain cadmium. [Brief explanation of the drawing]

[0023] [Figure 1A] Figure 1A is a schematic diagram of a quantum dot in an embodiment of the present invention. [Figure 1B] Figure 1B is a schematic diagram of a quantum dot in an embodiment of the present invention. [Figure 2] This is a schematic diagram of an LED device using quantum dots according to an embodiment of the present invention. [Figure 3] This is a longitudinal cross-sectional view of a display device using an LED device according to an embodiment of the present invention. [Figure 4] This is the photoluminescence (PL) spectrum of ZnSe / ZnS in Example 1. [Figure 5] This is the PL spectrum of ZnSe / ZnS in Example 2. [Figure 6] This is the PL spectrum of ZnSe / ZnS in Example 3. [Figure 7A] This is a scanning electron microscope (SEM) image of ZnSe in Example 1. [Figure 7B] This is a schematic diagram of Figure 7A. [Figure 8A] This is an SEM image of ZnSe / ZnS in Example 1. [Figure 8B] This is a schematic diagram of Figure 8A. [Figure 9A] This is an SEM image of ZnSe in Example 2. [Figure 9B] This is a schematic diagram of Figure 9A. [Figure 10A]This is an SEM image of ZnSe / ZnS in Example 2. [Figure 10B] Figure 10A is a schematic diagram. [Figure 11A] This is an SEM image of ZnSe in Example 3. [Figure 11B] Figure 11A is a schematic diagram. [Figure 12A] This is an SEM image of ZnSe / ZnS in Example 3. [Figure 12B] This is a schematic diagram of Figure 12A. [Figure 13] These are the X-ray diffraction (XRD) spectra of ZnSe and ZnSe / ZnS in Example 1. [Figure 14] These are the XRD spectra of ZnSe and ZnSe / ZnS in Example 2. [Figure 15] These are the XRD spectra of ZnSe and ZnSe / ZnS in Example 3. [Figure 16] Figure 15 is a magnified view of the XRD spectrum. [Modes for carrying out the invention]

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

[0025] Figures 1A and 1B are schematic diagrams of quantum dots in this embodiment. The quantum dot 5 shown in Figure 1A is a cadmium (Cd)-free nanocrystal.

[0026] In this embodiment, the quantum dot 5 contains at least zinc (Zn) and selenium (Se), but does not contain cadmium (Cd). Specifically, the quantum dot is preferably a nanocrystal containing Zn and Se, Zn and Se and sulfur (S), or Zn and Se and tellurium (Te).

[0027] Quantum dot 5 exhibits fluorescence properties due to band-edge emission, and its nano-sized particles exhibit quantum size effects.

[0028] Here, "nanocrystal" refers to nanoparticles having a particle size of several nanometers to tens of nanometers. In this embodiment, a large number of quantum dots 5 can be generated with a substantially uniform particle size.

[0029] The main components of quantum dot 5 are Zn and Se, Zn, Se and S, or Zn, Se and Te, but other elements may also be present. However, it is preferable that Cd and phosphorus (P) are not included. Organophosphorus compounds are expensive and easily oxidized in air, which destabilizes their synthesis and can lead to increased costs, unstable fluorescence properties, and complicated manufacturing processes.

[0030] In this embodiment, the quantum dot 5 has a fluorescence half-width of 25 nm or less. "Fluorescence half-width" refers to the full width at half maximum (FWHM), which indicates the spread of the fluorescence wavelength at half the peak intensity of the fluorescence intensity in the fluorescence spectrum. Preferably, the fluorescence half-width is 23 nm or less. More preferably, the fluorescence half-width is 20 nm or less. Even more preferably, the fluorescence half-width is 15 nm or less. In this embodiment, the fluorescence half-width can be narrowed, thereby improving the wide color gamut.

[0031] In this embodiment, as will be described later, the reaction system for synthesizing quantum dots 5 involves first synthesizing a copper chalcogenide as a precursor, and then performing a metal exchange reaction on the precursor. By producing quantum dots 5 based on such an indirect synthesis reaction, the fluorescence full width at half maximum (FWHM) can be narrowed, specifically achieving a FWHM of 25 nm or less.

[0032] It is preferable that a large number of organic ligands 2 are coordinated on the surface of the quantum dots 5. Thereby, aggregation of the quantum dots 5 can be suppressed, and the intended optical properties can be exhibited. Furthermore, by adding an amine or thiol-based ligand, it is possible to greatly improve the stability of the quantum dot luminescence properties. The ligands that can be used in the reaction are not particularly limited, and for example, the following ligands can be cited as representative ones.

[0033] (1) Aliphatic primary amine-based 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-based Oleic acid: C 17 H 33 COOH, Stearic acid: C 17 H 35 COOH, Palmitic acid: C 15 H 31 COOH, Myristic acid: C 13 H 27 COOH, Lauryl (dodecane) acid: C 11 H 23 COOH, Decanoic acid: C9H 19 COOH, Octanoic acid: C7H 15 COOH (3) Thiol-based Octadecanethiol: C 18 H 37 SH, Hexadecanethiol: C 16 H 33 SH, Tetradecanethiol: C 14 H 29 SH, Dodecanethiol: C 12 H 25 SH, Decanethiol: C 10 H 21 SH, Octanethiol: C8H 17SH (4) Phosphine systems Trioctylphosphine:(C8H 17 )3P, Triphenylphosphine:(C6H5)3P, Tributylphosphine:(C4H9)3P (5) Phosphine oxide systems Trioctylphosphine oxide: (C8H 17 )3P=O, triphenylphosphine oxide: (C6H5)3P=O, tributylphosphine oxide: (C4H9)3P=O

[0034] In this embodiment, the fluorescence quantum yield of quantum dot 5 is 5% or more. Preferably, the fluorescence quantum yield is 20% or more, more preferably 50% or more, and even more preferably 80% or more. Thus, in this embodiment, the fluorescence quantum yield of the quantum dot can be increased.

[0035] In this embodiment, the particle size of the quantum dots 5 can be adjusted to be between 5 nm and 20 nm. Preferably, the particle size of the quantum dots 5 can be reduced to 15 nm or less, and more preferably to 10 nm or less. In this way, in this embodiment, the particle size of the quantum dots 5 can be reduced, and the variation in the particle size of each quantum dot 5 can be reduced, making it possible to obtain quantum dots 5 of uniform size. As a result, in this embodiment, the fluorescence half-width of the quantum dots 5 can be narrowed to 25 nm or less, as described above, and the wide color gamut can be improved.

[0036] Furthermore, in this embodiment, the fluorescence lifetime of quantum dot 5 can be reduced to 50 ns or less. In addition, in this embodiment, the fluorescence lifetime can be adjusted to 40 ns or less, and even to 30 ns or less. Thus, in this embodiment, the fluorescence lifetime can be shortened, but it can also be extended to about 50 ns, and the fluorescence lifetime can be adjusted depending on the application.

[0037] In this embodiment, the fluorescence wavelength can be freely controlled to approximately 410 nm to 490 nm. In this embodiment, the quantum dot 5 uses a chalcogen element other than Zn, specifically a solid solution based on ZnSe. In this embodiment, the fluorescence wavelength can be controlled by adjusting the particle size and composition of the quantum dot 5. In this embodiment, the fluorescence wavelength can preferably be 430 nm or higher, and more preferably 450 nm or higher. Furthermore, with quantum dot 5 based on ZnSeTe, the fluorescence wavelength can be 450 nm to 490 nm. In addition, the fluorescence wavelength can be 480 nm or lower, or 470 nm or lower.

[0038] Thus, in the quantum dot 5 of this embodiment, it is possible to control the fluorescence wavelength to blue.

[0039] The quantum dot 5 shown in Figure 1B has a core-shell structure having a core 5a and a shell 5b covering the surface of the core 5a. As shown in Figure 1B, it is preferable that a large number of organic ligands 2 are coordinated to the surface of the quantum dot 5.

[0040] In this embodiment, the above-described fluorescence full width at half maximum (FWHM) and fluorescence quantum yield can also be obtained in a core-shell structure. Specifically, in the quantum dot 5 of the core-shell structure shown in Figure 1B, the fluorescence FWHM can be 25 nm or less. Furthermore, the fluorescence quantum yield can be 5% or more. The preferred ranges for fluorescence FWHM and fluorescence quantum yield can be the ranges described above.

[0041] Furthermore, the particle size of the quantum dot 5 in the core-shell structure shown in Figure 1B is between 5 nm and 20 nm. Compared to the core-only structure shown in Figure 1A, the particle size is slightly larger when using a core-shell structure, but it is possible to maintain a particle size of 20 nm or less, thus obtaining quantum dots 5 in a core-shell structure with very small particle sizes.

[0042] The core 5a of the quantum dot 5 shown in Figure 1B is the nanocrystal shown in Figure 1A. Therefore, it is preferable that the core 5a be formed from ZnSe, ZnSeS, or ZnSeTe. The shell 5b, like the core 5a, does not contain Cd. The material of the shell 5b is not particularly restricted, but it is formed from, for example, ZnS.

[0043] The shell 5b may be in a state of solid solution on the surface of the core 5a. In Figure 1B, the boundary between the core 5a and the shell 5b is shown with a dotted line, which means that it is acceptable whether or not the boundary between the core 5a and the shell 5b can be confirmed by analysis. In this embodiment, the core 5a alone has the characteristic of emitting fluorescence.

[0044] The quantum dot 5 with a core-shell structure shown in Figure 1B, like Figure 1A, allows for free control of the fluorescence wavelength between approximately 410 nm and 490 nm. In this embodiment, it is possible to control the fluorescence wavelength to blue. Furthermore, while the fluorescence lifetime of the quantum dot 5 with a core-shell structure can be set to 50 ns, the fluorescence lifetime can be shortened by using a core-shell structure compared to a single core 5a with the same composition and particle size.

[0045] Next, the method for producing the quantum dot 5 of this embodiment will be described. First, in this embodiment, a copper chalcogenide precursor is synthesized from an organic copper compound, or an inorganic copper compound and an organic chalcogen compound. Specifically, the copper chalcogenide precursor is preferably Cu2Se, Cu2SeS, Cu2SeTe, Cu2SeTeS, CuSe, CuSeS, CuSeTe, or CuSeTeS.

[0046] In this embodiment, the Cu raw material is not particularly limited, but for example, the following organocopper reagents and inorganic copper reagents can be used. Specifically, as acetate salts, copper(I) acetate:Cu(OAc), copper(II) acetate:Cu(OAc)2, and as fatty acid salts, copper stearate:Cu(OC(=O)C 17 H 35 )2. Copper oleate: Cu(OC(=O)C 17 H33 )2. Copper myristate: Cu(OC(=O)C 13 H 27 )2. Copper dodecanoate: Cu(OC(=O)C 11 H 23 )2, copper acetylacetonate: Cu(acac)2, both monovalent and divalent compounds can be used as halides, such as copper(I) chloride: CuCl, copper(II) chloride: CuCl2, copper(I) bromide: CuBr, copper(II) bromide: CuBr2, copper(I) iodide: CuI, copper(II) iodide: CuI2, etc.

[0047] In this embodiment, the Se raw material used is an organic selenium compound (organo chalcogenide). While the structure of the compound is not particularly limited, for example, trioctylphosphine selenide, obtained by dissolving Se in trioctylphosphine: (C8H 17 )3P=Se, or tributylphosphine selenide ((C4H9)3P=Se) obtained by dissolving Se in tributylphosphine can be used. Alternatively, a solution obtained by dissolving Se at high temperature in a high-boiling point solvent such as a long-chain hydrocarbon like octadecene (Se-ODE), or a solution obtained by dissolving Se in a mixture of oleylamine and dodecanethiol (Se-DDT / OLAm) can be used.

[0048] In this embodiment, Te is used as a raw material in the form of an organic tellurium compound (organo chalcogen compound). While the structure of the compound is not particularly limited, for example, trioctylphosphine telllide (C8H) is obtained by dissolving Te in trioctylphosphine. 17 )3P=Te, or tributylphosphine tellurides such as (C4H9)3P=Te, obtained by dissolving Te in tributylphosphine, can be used. Dialkyl diterlides such as diphenyl diterlide (C6H5)2Te2, or solutions of Te dissolved in a mixture of oleylamine and dodecanethiol (Te-DDT / OLAm) can also be used.

[0049] In this embodiment, an organocopper compound or an inorganic copper compound is mixed with an organochalcogen compound and dissolved. As a solvent, octadecene can be used as a high-boiling point saturated hydrocarbon or unsaturated hydrocarbon. In addition, aromatic high-boiling point solvents such as t-butylbenzene and high-boiling point ester solvents such as butylbutyrate (C4H9COOC4H9) and benzylbutyrate (C6H5CH2COOC4H9) can be used, but aliphatic amine compounds, fatty acid compounds, aliphatic phosphorus compounds, or mixtures thereof can also be used as solvents.

[0050] At this time, the reaction temperature is set to a range of 140°C to 250°C to synthesize the copper chalcogenide precursor. Preferably, the reaction temperature is lower, between 140°C and 220°C, and even more preferably, between 140°C and 200°C.

[0051] Furthermore, although there are no particular limitations on the reaction method in this embodiment, it is important to synthesize Cu2Se, Cu2SeS, Cu2SeTe, and Cu2SeTeS with uniform particle sizes in order to obtain quantum dots with a narrow fluorescence half-width.

[0052] Furthermore, the particle size of copper chalcogenide precursors such as Cu2Se, Cu2SeS, Cu2SeTe, and Cu2SeTeS is preferably 20 nm or less, more preferably 15 nm or less, and even more preferably 10 nm or less. The composition and particle size of these copper chalcogenide precursors enable wavelength control of ZnSe-based quantum dots. Therefore, it is important to perform appropriate particle size control.

[0053] Furthermore, in this embodiment, to obtain quantum dots with a narrower fluorescence half-width, it is important to solid-solve S in the core. For this reason, it is preferable to add a thiol in the synthesis of the precursor Cu2Se or Cu2SeTe. To obtain quantum dots with an even narrower fluorescence half-width, it is more preferable to use Se-DDT / OLAm as the Se starting material. The thiol is not particularly limited, but 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 H 21 SH, Octanethiol:C8H 17 SH, etc.

[0054] Next, organozinc compounds or inorganic zinc compounds are prepared as raw materials for ZnSe, ZnSeS, ZnSeTe, or ZnSeTeS. Organozinc compounds and inorganic zinc compounds are stable in air and easy to handle. While the structure of the organozinc compounds and inorganic zinc compounds is not particularly limited, it is preferable to use zinc compounds with high ionic properties in order to efficiently carry out the metal exchange reaction. For example, the following organozinc compounds and inorganic zinc compounds can be used: namely, zinc acetate: Zn(OAc)2, zinc nitrate: Zn(NO3)2 as acetate salts, and zinc stearate: Zn(OC(=O)C) as fatty acid salts. 17 H 35 )2. Zinc oleate: Zn(OC(=O)C 17 H 33 )2, Zinc palmitate: Zn(OC(=O)C 15 H 31 )2. Zinc myristate: Zn(OC(=O)C 13 H 27 )2. Zinc dodecanoate: Zn(OC(=O)C 11 H 23)2, zinc acetylacetonate: Zn(acac)2, as halides, zinc chloride: ZnCl2, zinc bromide: ZnBr2, zinc iodide: ZnI2, as zinc carbamates, zinc diethyldithiocarbamate: Zn(SC(=S)N(C2H5)2)2, zinc dimethyldithiocarbamate: Zn(SC(=S)N(CH3)2)2, zinc dibutyldithiocarbamate: Zn(SC(=S)N(C4H9)2)2, etc. can be used.

[0055] Next, the above-mentioned organozinc compounds or inorganic zinc compounds are added to the reaction solution in which the copper chalcogenide precursor was synthesized. This causes a metal exchange reaction between the Cu of the copper chalcogenide and Zn. The metal exchange reaction is preferably carried out at a temperature of 150°C to 300°C. Furthermore, it is even more preferable to carry out the metal exchange reaction at a lower temperature, between 150°C and 280°C, and more preferably between 150°C and 250°C.

[0056] In this embodiment, the metal exchange reaction between Cu and Zn proceeds quantitatively, and it is preferable that the nanocrystals do not contain precursor Cu. This is because if precursor Cu remains in the nanocrystals, the Cu acts as a dopant, emitting light through a different luminescence mechanism and broadening the fluorescence full width at half maximum. The amount of residual Cu is preferably 100 ppm or less relative to Zn, more preferably 50 ppm or less, and ideally 10 ppm or less.

[0057] In this embodiment, ZnSe-based quantum dots synthesized by the cation exchange method tend to have a higher Cu content than ZnSe-based quantum dots synthesized by the direct method, but good luminescence properties can be obtained even if the Cu content is around 1 to 10 ppm relative to Zn. Furthermore, it is possible to determine whether a quantum dot was synthesized by the cation exchange method based on the Cu content. In other words, by synthesizing using the cation exchange method, the particle size can be controlled with the copper chalcogenide precursor, making it possible to synthesize quantum dots that are inherently difficult to react. Therefore, the Cu content is advantageous in determining whether the cation exchange method was used.

[0058] Furthermore, in this embodiment, a compound is needed that plays an auxiliary role in releasing the metal of the copper chalcogenide precursor into the reaction solution through coordination or chelation during metal exchange.

[0059] Compounds having the above-mentioned role include ligands that can form complexes with Cu. For example, phosphorus-based ligands, amine-based ligands, and sulfur-based ligands are preferred, and among these, phosphorus-based ligands are even more preferred due to their high efficiency.

[0060] This ensures proper metal exchange between Cu and Zn, enabling the production of quantum dots with a narrow fluorescence half-width based on Zn and Se. In this embodiment, the cation exchange method described above allows for mass production of quantum dots compared to direct synthesis.

[0061] In other words, in direct synthesis methods, organozinc compounds such as diethylzinc (Et2Zn) are used to increase the reactivity of the Zn raw material. However, diethylzinc is highly reactive and ignites in air, so it must be handled under an inert gas stream, making the handling and storage of the raw material difficult. Furthermore, reactions using it involve risks such as heat generation and ignition, making it unsuitable for mass production. Similarly, reactions using selenium hydride (H2Se), for example, to increase the reactivity of the Se raw material are also unsuitable for mass production from a toxicity and safety standpoint.

[0062] Furthermore, in reaction systems using highly reactive Zn or Se raw materials as described above, although ZnSe is produced, particle formation is not controlled, resulting in a broader fluorescence half-width of the resulting ZnSe.

[0063] In contrast, in this embodiment, a copper chalcogenide precursor is synthesized from an organic copper compound or an inorganic copper compound and an organic chalcogen compound, and quantum dots are synthesized by metal exchange using the copper chalcogenide precursor. Thus, in this embodiment, quantum dots are synthesized first by synthesizing a copper chalcogenide precursor, and not directly. This indirect synthesis eliminates the need to use reagents that are too reactive and dangerous to handle, making it possible to safely and stably synthesize ZnSe-based quantum dots with a narrow fluorescence half-width.

[0064] Furthermore, in this embodiment, it is possible to perform a one-pot metal exchange between Cu and Zn without isolating and purifying the copper chalcogenide precursor, thereby obtaining quantum dots with the desired composition and particle size. Alternatively, the copper chalcogenide precursor may be isolated and purified before use.

[0065] Furthermore, in this embodiment, the synthesized quantum dots exhibit fluorescence properties without undergoing various treatments such as washing, isolation and purification, coating, or ligand exchange.

[0066] However, as shown in Figure 1B, the fluorescence quantum yield can be further increased by coating a core 5a made of nanocrystals such as ZnSe, ZnSeS, ZnSeTe, or ZnSeTeS with a shell 5b made of ZnS, ZnSeS, etc. Furthermore, by adopting a core-shell structure, the fluorescence lifetime can be shortened compared to before the shell coating.

[0067] Furthermore, as shown in Figure 1B, by covering the core 5a with the shell 5b, it is possible to shorten or lengthen the wavelength compared to the core 5a. For example, when the particle size of the core 5a is small, covering it with the shell 5b tends to lengthen the wavelength, while when the particle size of the core 5a is large, covering it with the shell 5b tends to shorten the wavelength. The magnitude of the wavelength change also differs depending on the coating conditions of the shell 5b.

[0068] Furthermore, by adding ZnSe, ZnSeS, ZnSeTe, or ZnSeTeS, which are the same raw materials, to core 5a made of nanocrystals such as ZnSe, ZnSeS, ZnSeTe, or ZnSeTeS obtained by the cation exchange method, it is possible to obtain quantum dot 5 core 5a with any particle size while maintaining uniform particle size. Therefore, it is easy to control the wavelength between 410 nm and 490 nm while maintaining the fluorescence half-width of 25 nm or less.

[0069] Furthermore, in this embodiment, the core-shell structure can be synthesized at the copper chalcogenide precursor stage. For example, Cu2Se / Cu2S can be synthesized by continuously adding an S raw material to Cu2Se, which is a copper chalcogenide precursor, and then ZnSe / ZnS can be obtained by subsequently performing a metal exchange between Cu and Zn.

[0070] Furthermore, in this embodiment, the S raw material used in the core-shell structure is not particularly limited, but the following raw materials are typical examples.

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

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

[0073] Furthermore, in this embodiment, the Zn raw material used for the core-shell structure can be the raw material described above.

[0074] Furthermore, the optimal coating thickness can also be calculated from the progression of fluorescence lifetime. As the number of times coating material is added increases, the particle size increases, and the fluorescence lifetime gradually shortens. Simultaneously, the fluorescence quantum yield also increases, so it was found that the fluorescence quantum yield is highest when the fluorescence lifetime is shortest.

[0075] Furthermore, in this embodiment, the less amine-based solvent used when coating the shell 5b, the easier it is to coat the shell 5b and the better the luminescence characteristics can be obtained. Moreover, the luminescence characteristics after coating the shell 5b differ depending on the ratio of amine-based solvent, carboxylic acid-based solvent, or phosphine-based solvent.

[0076] Furthermore, the quantum dots 5 synthesized by the manufacturing method of this embodiment can be aggregated by adding a polar solvent such as methanol, ethanol, or acetone, allowing for the separation and recovery of the quantum dots 5 from the unreacted raw materials. The recovered quantum dots 5 can then be dispersed again by adding toluene, hexane, or the like. By adding a ligand solvent to this redispersed solution, the luminescence properties and their stability can be further improved. The change in luminescence properties due to the addition of this ligand differs significantly depending on whether or not the shell 5b coating operation is performed. In this embodiment, quantum dots 5 coated with shell 5b can have their fluorescence stability particularly improved by adding a thiol-based ligand.

[0077] The applications of the quantum dot 5 shown in Figures 1A and 1B are not particularly limited, but for example, the quantum dot 5 of this embodiment that emits blue fluorescence can be applied to wavelength conversion members, lighting members, backlight devices, and display devices.

[0078] The quantum dot 5 of this embodiment can be applied to a wavelength conversion member, lighting member, backlight device, and display device, for example, when photoluminescence (PL) is used as the light emission principle, it becomes possible to emit blue fluorescence when irradiated with UV light from a light source. Alternatively, when electroluminescence (EL) is used as the light emission principle, or when all three primary colors are emitted by quantum dots by other means, a light-emitting element that emits blue fluorescence can be created using the quantum dot 5 of this embodiment. In this embodiment, by including the quantum dot 5 of this embodiment that emits blue fluorescence, along with quantum dots that emit green fluorescence and quantum dots that emit red fluorescence, it becomes possible to emit white light.

[0079] Figure 2 is a schematic diagram of an LED device using quantum dots according to this embodiment. As shown in Figure 2, the LED device 1 of this embodiment is composed of a housing case 2 having a bottom surface 2a and side walls 2b surrounding the bottom surface 2a, an LED chip (light-emitting element) 3 placed on the bottom surface 2a of the housing case 2, and a fluorescent layer 4 filled inside the housing case 2 and sealing the upper side of the LED chip 3. Here, the upper side is the direction in which the light emitted from the LED chip 3 is emitted from the housing case 2, and is the direction opposite to the bottom surface 2a with respect to the LED chip 3.

[0080] The LED chip 3 is placed on a base wiring board (not shown), which may constitute the bottom surface of the housing case 2. As the base board, for example, a configuration in which a wiring pattern is formed on a substrate such as glass epoxy resin can be presented.

[0081] LED chip 3 is a semiconductor element that emits light when a voltage is applied in the forward direction, and has a basic configuration in which a P-type semiconductor layer and an N-type semiconductor layer are joined at a PN junction.

[0082] As shown in Figure 2, the fluorescent layer 4 is formed of a resin 6 in which a large number of quantum dots 5 are dispersed.

[0083] Furthermore, the resin composition in which the quantum dots 5 are dispersed in this embodiment may also contain a fluorescent substance other than the quantum dots 5. Examples of fluorescent substances include sialon-based materials and KSF(K2SiF6:Mn 4+ While red phosphors and other materials are available, the material is not particularly limited.

[0084] The resin 6 constituting the fluorescent layer 4 is not particularly limited, but polypropylene (PP), polystyrene (PS), acrylic resin, methacrylate, MS resin, polyvinyl chloride (PVC), polycarbonate (PC), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polymethylpentene, liquid crystal polymer, epoxy resin, silicone resin, or mixtures thereof can be used.

[0085] The LED device using quantum dots in this embodiment can be applied to a display device. Figure 3 is a longitudinal cross-sectional view of a display device using the LED device shown in Figure 2. As shown in Figure 3, the display device 50 is composed of a plurality of LED devices 20 and a display unit 54 such as a liquid crystal display facing each LED device 20. Each LED device 20 is arranged on the back side of the display unit 54. Each LED device 20 has a structure in which an LED chip is sealed with a resin in which a large number of quantum dots 5 are diffused, similar to the LED device 1 shown in Figure 2.

[0086] As shown in Figure 3, multiple LED devices 20 are supported by a support 52. Each LED device 20 is arranged at a predetermined interval. Each LED device 20 and the support 52 constitute a backlight 55 for the display unit 54. The support 52 is not limited in shape or material, and can be a sheet, plate, or case. As shown in Figure 3, a light diffuser plate 53 or the like may be interposed between the backlight 55 and the display unit 54.

[0087] By applying the quantum dot 5 with a narrow fluorescence half-width in this embodiment to the LED device shown in Figure 2, the display device shown in Figure 3, etc., it becomes possible to effectively improve the light emission characteristics of the device.

[0088] Furthermore, the resin composition in which the quantum dots 5 of this embodiment are dispersed in a resin can also be formed into a sheet or a film. Such a sheet or film can be incorporated into, for example, a backlight device.

[0089] Furthermore, in this embodiment, a wavelength conversion member in which multiple quantum dots are dispersed in a resin can be formed as a molded body. For example, the molded body in which quantum dots are dispersed in a resin is housed in a container having a storage space by press-fitting or the like. In this case, it is preferable that the refractive index of the molded body is smaller than the refractive index of the container. As a result, a portion of the light that enters the molded body is totally reflected by the inner wall of the container. Therefore, the beam of light leaking out from the side of the container to the outside can be reduced. In this way, by applying the quantum dots with a narrow fluorescence half-width in this embodiment to wavelength conversion members, lighting members, backlight devices, and display devices, it is possible to effectively improve the luminescence characteristics. [Examples]

[0090] The effects of the present invention will be explained below with reference to examples and comparative examples of the present invention. However, the present invention is not limited in any way by the following examples.

[0091] In this invention, the following raw materials were used to synthesize Cd-free quantum dots. Furthermore, the following measuring instruments were used to evaluate the synthesized quantum dots.

[0092] <Raw materials> Anhydrous copper acetate: Manufactured by Wako Pure Chemical Industries, Ltd. Octadecene: Manufactured by Idemitsu Kosan Co., Ltd. Oleylamine: Manufactured by Kao Corporation (Farmin) Oleic acid: Lunac OV manufactured by Kao Corporation Dodecanethiol (Se-DDT): Manufactured by Kao Corporation, Thiocalcol 20 Trioctylphosphine: Manufactured by Hokko Chemical Co., Ltd. Anhydrous zinc acetate: Manufactured by Kishida Chemical Co., Ltd. Selenium (4N: 99.99%): Manufactured by Shinko Chemical Co., Ltd. Sulfur: Manufactured by Kishida Chemical Co., Ltd. <Measuring equipment> Fluorescence spectrometer: JASCO Corporation F-2700 Ultraviolet-Visible Spectrophotometer: Hitachi V-770 Quantum yield measurement device: QE-1100, manufactured by Otsuka Electronics Co., Ltd. X-ray diffraction device (XRD): Bruker D2 PHASER Scanning-line electron microscope (SEM): Hitachi SU9000 Fluorescence lifetime analyzer: Hamamatsu Photonics C11367

[0093] [Example 1] 2546 mg of anhydrous copper acetate (Cu(OAc)), 28.5 mL of oleylamine (OLAm), and 46.5 mL of octadecene (ODE) were placed in a 300 mL reaction vessel. The mixture was then heated at 150 °C for 20 minutes with stirring under an inert gas (N2) atmosphere to dissolve the starting materials.

[0094] To this solution, 8.4 mL of Se-DDT / OLAm solution (0.285 M) was added and the mixture was heated at 150°C for 10 minutes with stirring. The resulting reaction solution (CuSe) was cooled to room temperature.

[0095] Subsequently, 24.092 g of anhydrous zinc acetate (Zn(OAc)), 60 mL of trioctylphosphine (TOP), and 2.4 mL of oleylamine (OLAm) were added to the Cu2Se reaction solution, and the mixture was heated at 180°C for 30 minutes with stirring under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSe) was cooled to room temperature.

[0096] Ethanol was added to the reaction mixture cooled to room temperature to generate a precipitate, which was then collected by centrifugation. 72 ml of octadecene:ODE was added to the precipitate and dispersed.

[0097] Subsequently, 24.092 g of anhydrous zinc acetate (Zn(OAc)), 30 mL of trioctylphosphine (TOP), 3 mL of oleylamine (OLAm), and 36 mL of oleic acid were added to 72 ml of ZnSe-ODE solution, and the mixture was heated at 280°C for 30 minutes with stirring under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSe) was cooled to room temperature.

[0098] The resulting reaction solution was measured using a fluorescence spectrometer. The results showed optical properties with a fluorescence wavelength of approximately 430.5 nm and a fluorescence full width at half maximum of approximately 15 nm.

[0099] The resulting reaction solution was measured using a quantum efficiency measurement system. The results showed a fluorescence quantum yield of approximately 30%. The fluorescence lifetime was measured to be 48 ns.

[0100] Furthermore, the dispersion solution of the obtained ZnSe particles was measured using a scanning electron microscope (SEM) and an X-ray diffraction (XRD) apparatus. Figure 7A shows the SEM measurement results, Figure 7B is a schematic diagram of Figure 7A, and the dotted line in Figure 13 shows the X-ray diffraction (XRD) measurement results. From the SEM results, the particle size was approximately 5 nm. In addition, from the XRD results, it was found that the crystal was cubic and coincided with the crystal peak position of ZnSe.

[0101] Separately from the above, 47 ml of ZnSe reaction solution was mixed with ethanol to generate a precipitate, which was then collected by centrifugation. 35 ml of octadecene:ODE was added to the precipitate and dispersed.

[0102] 35 mL of dispersed ZnSe-ODE solution was heated at 310°C for 20 minutes with stirring under an inert gas (N2) atmosphere.

[0103] To this solution, 1.1 mL of a mixture of 2.2 mL of S-TOP solution (2.2 M) and 11 mL of zinc oleate:Zn(OLAc)2 solution (0.8 M) was added, and the mixture was heated at 310°C for 20 minutes with stirring. This procedure was repeated 12 times. The resulting reaction solution (ZnSe / ZnS) was cooled to room temperature.

[0104] Ethanol was added to the resulting reaction solution to generate a precipitate, which was then collected by centrifugation. Hexane was then added to the precipitate to disperse it.

[0105] The hexane-dispersed ZnSe / ZnS was measured using a fluorescence spectrometer. As a result, as shown in Figure 4, optical properties were obtained with a fluorescence wavelength of approximately 423 nm and a fluorescence full width at half maximum of approximately 15 nm.

[0106] A similar solution was measured using a quantum efficiency measurement system. The results showed a fluorescence quantum yield of approximately 60%. The fluorescence lifetime was measured to be 44 ns.

[0107] Furthermore, the obtained dispersion solution of ZnSe / ZnS particles was measured using a scanning electron microscope (SEM) and an X-ray diffraction (XRD) apparatus. Figure 8A shows the SEM measurement results, Figure 8B is a schematic diagram of Figure 8A, and the solid line in Figure 13 shows the X-ray diffraction (XRD) measurement results. From the SEM results, the particle size was approximately 12 nm. In addition, from the XRD results, it was found that the crystal was cubic, and the maximum peak intensity was shifted 1.1° higher than the crystal peak position of ZnSe.

[0108] [Example 2] 291 mg of copper acetate (Cu(OAc)), 4.8 mL of oleylamine (OLAm), and 7.75 mL of octadecene (ODE) were placed in a 100 mL reaction vessel. The mixture was then heated at 150 °C for 5 minutes with stirring under an inert gas (N2) atmosphere to dissolve the starting materials.

[0109] To this solution, 1.4 mL of Se-DDT / OLAm solution (0.285 M) was added and the mixture was heated at 150°C for 30 minutes with stirring. The resulting reaction solution (Cu2Se) was cooled to room temperature.

[0110] Subsequently, 2682 mg of anhydrous zinc acetate (Zn(OAc)), 10 mL of trioctylphosphine (TOP), and 0.4 mL of oleylamine (OLAm) were added to the Cu2Se reaction solution, and the mixture was heated at 180°C for 10 minutes with stirring under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSe) was then cooled to room temperature.

[0111] Ethanol was added to the reaction mixture cooled to room temperature to generate a precipitate, which was then collected by centrifugation. 12 ml of octadecene:ODE was added to the precipitate and dispersed.

[0112] Subsequently, 2682 mg of anhydrous zinc acetate (Zn(OAc)), 5 mL of trioctylphosphine (TOP), 0.5 mL of oleylamine (OLAm), and 6 mL of oleic acid (OLAc) were added to 12 ml of ZnSe-ODE solution, and the mixture was heated at 280°C for 30 minutes with stirring under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSe) was cooled to room temperature.

[0113] The resulting reaction solution was measured using a fluorescence spectrometer. The results showed optical properties with a fluorescence wavelength of approximately 437 nm and a fluorescence full width at half maximum of approximately 15 nm.

[0114] Ethanol was added to the resulting reaction solution (a few ml) to generate a precipitate, which was then collected by centrifugation. Hexane was then added to the precipitate to disperse it.

[0115] ZnSe dispersed in hexane was measured using a quantum efficiency measurement system. The results showed a fluorescence quantum yield of approximately 37%. The fluorescence lifetime was measured to be 13 ns.

[0116] Measurements were also performed using a scanning electron microscope (SEM) and an X-ray diffraction (XRD) apparatus. Figure 9A shows the SEM measurement results, Figure 9B is a schematic diagram of Figure 9A, and the dotted line in Figure 14 shows the X-ray diffraction (XRD) measurement results. From the SEM results, the grain size was approximately 6.0 nm. Furthermore, from the XRD results, it was found that the crystal was cubic and its crystal peak position coincided with that of ZnSe.

[0117] Separately from the above, 23 ml of ZnSe reaction solution was prepared by adding ethanol to generate a precipitate. The precipitate was collected by centrifugation, and 17.5 ml of octadecene:ODE was added to the precipitate to disperse it.

[0118] To 17.5 mL of dispersed ZnSe-ODE solution, 1 mL of oleic acid (OLAc) and 2 mL of trioctylphosphine (TOP) were added, and the mixture was heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0119] To this solution, 0.5 mL of a mixture of 1 mL of S-TOP solution (1 M) and 5 mL of zinc oleate:Zn(OLAc)2 solution (0.4 M) was added, and the mixture was heated at 320°C for 10 minutes while stirring. This procedure was repeated 8 times.

[0120] Subsequently, 2 ml of oleic acid (OLAc) was added and the mixture was reacted at 320°C for 10 minutes. Then, 2 ml of trioctylphosphine (TOP) was added and the mixture was heated with stirring at 320°C for 10 minutes. The resulting reaction solution (ZnSe / ZnS) was cooled to room temperature.

[0121] The resulting reaction solution was measured using a fluorescence spectrometer. As a result, as shown in Figure 5, optical properties were obtained with a fluorescence wavelength of approximately 435 nm and a fluorescence half-width of approximately 16 nm.

[0122] Ethanol was added to the resulting reaction solution to generate a precipitate, which was then collected by centrifugation. Hexane was then added to the precipitate to disperse it.

[0123] The hexane-dispersed ZnSe / ZnS was measured using a quantum efficiency measurement system. The results showed a fluorescence quantum yield of approximately 81%. The fluorescence lifetime was measured to be 12 ns.

[0124] Measurements were also performed using a scanning electron microscope (SEM) and an X-ray diffraction (XRD) apparatus. Figure 10A shows the SEM measurement results, Figure 10B is a schematic diagram of Figure 10A, and the solid line in Figure 14 shows the X-ray diffraction (XRD) measurement results. From the SEM results, the grain size was approximately 8.5 nm. From the XRD results, it was found that the crystal was cubic, and the maximum peak intensity was shifted 0.4° higher than the crystal peak position of ZnSe.

[0125] [Example 3] 291 mg of anhydrous copper acetate (Cu(OAc)), 4.8 mL of oleylamine (OLAm), and 7.75 mL of octadecene (ODE) were placed in a 100 mL reaction vessel. The mixture was then heated at 170 °C for 5 minutes with stirring under an inert gas (N2) atmosphere to dissolve the starting materials.

[0126] To this solution, 1.4 mL of Se-DDT / OLAm solution (0.285 M) was added and the mixture was heated at 170°C for 30 minutes with stirring. The resulting reaction solution (CuSe) was cooled to room temperature.

[0127] Subsequently, 2922 mg of anhydrous zinc acetate (Zn(OAc)), 10 mL of trioctylphosphine (TOP), and 0.4 mL of oleylamine (OLAm) were added to the Cu2Se reaction solution, and the mixture was heated at 180°C for 30 minutes with stirring under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSe) was then cooled to room temperature.

[0128] Ethanol was added to the reaction mixture cooled to room temperature to generate a precipitate, which was then collected by centrifugation. 12 ml of octadecene:ODE was added to the precipitate and dispersed.

[0129] Subsequently, 2922 mg of anhydrous zinc acetate (Zn(OAc)), 5 mL of trioctylphosphine (TOP), 0.5 mL of oleylamine (OLAm), and 3 mL of oleic acid (OLAc) were added to 12 ml of ZnSe-ODE solution, and the mixture was heated at 280°C for 20 minutes with stirring under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSe) was cooled to room temperature.

[0130] The resulting reaction solution was measured using a fluorescence spectrometer. The results showed optical properties with a fluorescence wavelength of approximately 448 nm and a fluorescence full width at half maximum of approximately 15 nm.

[0131] Ethanol was added to the resulting reaction solution to generate a precipitate, which was then collected by centrifugation. Hexane was then added to the precipitate to disperse it.

[0132] ZnSe dispersed in hexane was measured using a quantum efficiency measurement system. The results showed a fluorescence quantum yield of approximately 6%. The fluorescence lifetime was measured to be 25 ns.

[0133] Measurements were also performed using a scanning electron microscope (SEM) and an X-ray diffraction (XRD) apparatus. Figure 11A shows the SEM measurement results, Figure 11B is a schematic diagram of Figure 11A, and the dotted line in Figure 15 shows the X-ray diffraction (XRD) measurement results. From the SEM results, the grain size was approximately 8.2 nm. Furthermore, from the XRD results, it was found that the crystal was cubic and its crystal peak position coincided with that of ZnSe.

[0134] Separately from the above, 20 ml of ZnSe reaction solution was prepared by adding ethanol to generate a precipitate. The precipitate was collected by centrifugation, and 17.5 ml of octadecene:ODE was added to the precipitate to disperse it.

[0135] To 17.5 mL of dispersed ZnSe-ODE solution, 1 mL of oleic acid (OLAc) and 2 mL of trioctylphosphine (TOP) were added, and the mixture was heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0136] To this solution, 0.5 mL of a mixture containing 0.2 mL of DDT, 0.8 mL of trioctylphosphine (TOP), and 5 mL of zinc oleate (Zn(OLAc)2) solution (0.4 M) was added, and the mixture was heated at 320°C for 10 minutes with stirring. This procedure was repeated 8 times.

[0137] Subsequently, 2 ml of oleic acid (OLAc) was added and the mixture was reacted at 320°C for 10 minutes. Then, 2 ml of trioctylphosphine (TOP) was added and the mixture was heated with stirring at 320°C for 10 minutes. The resulting reaction solution (ZnSe / ZnS) was cooled to room temperature.

[0138] The resulting reaction solution was measured using a fluorescence spectrometer. As a result, as shown in Figure 6, optical properties were obtained with a fluorescence wavelength of approximately 447 nm and a fluorescence full width at half maximum of approximately 14 nm.

[0139] Ethanol was added to the resulting reaction solution to generate a precipitate, which was then collected by centrifugation. Hexane was then added to the precipitate to disperse it.

[0140] The hexane-dispersed ZnSe / ZnS was measured using a quantum efficiency measurement system. The results showed a fluorescence quantum yield of approximately 62%. The fluorescence lifetime was measured to be 16 ns.

[0141] Measurements were also performed using a scanning electron microscope (SEM) and an X-ray diffraction (XRD) apparatus. Figure 12A shows the SEM measurement results, Figure 12B is a schematic diagram of Figure 12A, and the solid line in Figure 15 shows the X-ray diffraction (XRD) measurement results. From the SEM results, the grain size was approximately 9.8 nm. From the XRD results, the crystal was cubic. Figure 16 shows a magnified view of the maximum peak intensity in Figure 15. From this, it was found that the maximum peak intensity was shifted 0.1° higher in angle than the crystal peak position of ZnSe.

[0142] [Example 4] 291 mg of copper acetate (Cu(OAc)), 4.8 mL of oleylamine (OLAm), and 7.75 mL of octadecene (ODE) were placed in a 100 mL reaction vessel. The mixture was then heated at 150 °C for 5 minutes with stirring under an inert gas (N2) atmosphere to dissolve the starting materials.

[0143] To this solution, 1.4 mL of Se-DDT / OLAm solution (0.285 M) was added, and the mixture was heated at 150°C for 30 minutes while stirring.

[0144] To this solution, a mixture of 0.1 mL of 0.5 M Te TOP solution and 2.5 mL of 0.1 M Se ODE solution was added dropwise over 1 minute. The mixture was then heated at 150°C for 30 minutes with stirring. The resulting reaction solution (CuSeTe) was cooled to room temperature.

[0145] Subsequently, 2682 mg of anhydrous zinc acetate (Zn(OAc)), 10 mL of trioctylphosphine (TOP), and 0.4 mL of oleylamine (OLAm) were added to the CuSeTe reaction solution, and the mixture was heated at 180°C for 20 minutes with stirring under an inert gas (N2) atmosphere.

[0146] Ethanol was added to the reaction mixture cooled to room temperature to generate a precipitate, which was then collected by centrifugation. 12 ml of octadecene:ODE was added to the precipitate and dispersed.

[0147] Subsequently, 2922 mg of anhydrous zinc acetate (Zn(OAc)), 5 mL of trioctylphosphine (TOP), and 0.5 mL of oleylamine (OLAm) were added to 12 ml of ZnSeTe-ODE solution, and the mixture was heated at 180°C for 20 minutes with stirring under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSeTe) was cooled to room temperature.

[0148] Ethanol was added to 8.7 ml of the obtained ZnSeTe reaction solution to generate a precipitate, which was then collected by centrifugation. 8.75 ml of octadecene:ODE was added to the precipitate and dispersed.

[0149] 8.75 mL of dispersed ZnSeTe-ODE solution was mixed with 0.5 mL of oleic acid (OLAc) and 1 mL of trioctylphosphine (TOP), and heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0150] To this solution, 0.25 mL of a mixture of 0.25 mL of Se-TOP solution (1 M) and 2.5 mL of zinc oleate:Zn(OLAc)2 solution (0.4 M) was added, and the mixture was heated at 320°C for 10 minutes with stirring. This procedure was repeated six times, and then the mixture was cooled to room temperature.

[0151] The resulting ZnSeTe / ZnSe reaction solution was measured using a fluorescence spectrometer. As a result, optical properties were obtained with a fluorescence wavelength of approximately 461 nm and a fluorescence full width at half maximum of approximately 24 nm.

[0152] [Example 5] 291 mg of copper acetate (Cu(OAc)), 4.8 mL of oleylamine (OLAm), and 7.75 mL of octadecene (ODE) were placed in a 100 mL reaction vessel. The mixture was then heated at 180 °C for 5 minutes with stirring under an inert gas (N2) atmosphere to dissolve the starting materials.

[0153] To this solution, 1.4 mL of Se-DDT / OLAm solution (0.285 M) was added and the mixture was heated at 180°C for 30 minutes while stirring.

[0154] To this solution, a mixture of 0.1 mL of 0.5 M Te TOP solution and 1.35 mL of 0.1 M Se ODE solution was added dropwise over 1 minute. The mixture was then heated at 180°C for 20 minutes with stirring. The resulting reaction solution (CuSeTe) was cooled to room temperature.

[0155] Subsequently, 2682 mg of anhydrous zinc acetate (Zn(OAc)), 7.5 mL of trioctylphosphine (TOP), and 0.25 mL of oleylamine (OLAm) were added to the CuSeTe reaction solution, and the mixture was heated at 180°C for 20 minutes with stirring under an inert gas (N2) atmosphere.

[0156] Ethanol was added to the reaction mixture cooled to room temperature to generate a precipitate, which was then collected by centrifugation. 12 ml of octadecene:ODE was added to the precipitate and dispersed.

[0157] Subsequently, 2682 mg of anhydrous zinc acetate (Zn(OAc)), 7.5 mL of trioctylphosphine (TOP), and 0.25 mL of oleylamine (OLAm) were added to 12 ml of ZnSeTe-ODE solution, and the mixture was heated at 180°C for 20 minutes with stirring under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSeTe) was cooled to room temperature.

[0158] Ethanol was added to 10 ml of the obtained ZnSeTe reaction solution to generate a precipitate, which was then collected by centrifugation. 8.75 ml of octadecene:ODE was added to the precipitate and dispersed.

[0159] 8.75 mL of dispersed ZnSeTe-ODE solution was mixed with 0.5 mL of oleic acid (OLAc) and 1 mL of trioctylphosphine (TOP), and heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0160] To this solution, 2.5 mL of a mixture of 0.25 mL of Se-TOP solution (1 M) and 5 mL of zinc oleate:Zn(OLAc)2 solution (0.4 M) was added, and the mixture was heated at 320°C for 10 minutes with stirring. This procedure was repeated four times, and then the mixture was cooled to room temperature.

[0161] The resulting ZnSeTe / ZnSe reaction solution was measured using a fluorescence spectrometer. As a result, optical properties were obtained with a fluorescence wavelength of approximately 479 nm and a fluorescence half-width of approximately 23 nm.

[0162] Examples 1 to 5 are summarized in Table 1 below. By changing the synthesis conditions of CuSe or CuSeTe, it was possible to control the fluorescence half-width to 25 nm or less and the fluorescence wavelength within the range of 410 nm to 490 nm (preferably 420 nm to 480 nm).

[0163] [Table 1]

[0164] Examples 1-3 demonstrated that increasing the reaction temperature and time allowed for the enlargement of Cu2Se particles and control of the fluorescence wavelength. However, achieving fluorescence wavelengths above 450 nm with a ZnSe core proved difficult. Therefore, we synthesized CuSeTe first, then synthesized ZnSeTe via cation exchange, which enabled the creation of fluorescent particles with fluorescence wavelengths in the 460 nm and 470 nm ranges.

[0165] The distinguishing features of the ZnSeTe quantum dots in Examples 4 and 5 are that the fluorescence half-width is 25 nm or less, and the fluorescence wavelength can be controlled within the range of 420 nm to 480 nm.

[0166] ZnSeTe quantum dots obtained by directly reacting Zn, Se, and Te are already known and have a fluorescence half-width of 25 nm or more.

[0167] In Example 3 (and similarly in Examples 7 and 8 described later), the fluorescence wavelength was 447 nm, securing a wavelength around 450 nm, and a high fluorescence quantum yield was obtained. When ZnSe is produced by direct synthesis, the particles become larger, making them prone to internal defects and reducing the fluorescence quantum yield. In contrast, Example 3 was able to secure a high fluorescence quantum yield.

[0168] The results of Examples 3-5 are presumed to be unique to cation exchange.

[0169] [Example 6] In a 100 mL reaction vessel, 2182 mg of anhydrous copper acetate (Cu(OAc)), 4.8 mL of oleylamine (OLAm), and 7.75 mL of octadecene (ODE) were added. The mixture was then heated at 165 °C for 5 minutes with stirring under an inert gas (N2) atmosphere to dissolve the starting materials.

[0170] To this solution, 1.14 mL of Se-DDT / OLAm solution (0.7 M) was added and the mixture was heated at 165°C for 30 minutes with stirring. The resulting reaction solution (Cu2Se) was cooled to room temperature.

[0171] Subsequently, 21844 mg of anhydrous zinc acetate (Zn(OAc)), 10 mL of trioctylphosphine (TOP), and 0.4 mL of oleylamine (OLAm) were added to the Cu2Se reaction solution, and the mixture was heated at 180°C for 45 minutes with stirring under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSe) was then cooled to room temperature.

[0172] Ethanol was added to the reaction mixture cooled to room temperature to generate a precipitate, which was then collected by centrifugation. 12 ml of octadecene:ODE was added to the precipitate and dispersed.

[0173] Subsequently, 21844 mg of anhydrous zinc acetate (Zn(OAc)), 10 mL of trioctylphosphine (TOP), 1 mL of oleylamine (OLAm), and 6 mL of oleic acid (OLAc) were added to 12 ml of ZnSe-ODE solution, and the mixture was heated at 280°C for 20 minutes with stirring under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSe) was then cooled to room temperature.

[0174] The resulting reaction solution was measured using a fluorescence spectrometer. The results showed optical properties with a fluorescence wavelength of approximately 447.5 nm and a fluorescence half-width of approximately 14 nm.

[0175] Ethanol was added to 20 ml of the obtained ZnSe reaction solution to generate a precipitate, which was then collected by centrifugation. 17.5 ml of octadecene:ODE was added to the precipitate and dispersed.

[0176] To 17.5 mL of dispersed ZnSe-ODE solution, 1 mL of oleic acid (OLAc) and 2 mL of trioctylphosphine (TOP) were added, and the mixture was heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0177] To this solution, 0.5 mL of a mixture consisting of 0.5 mL of Se-TOP solution (1 M), 0.125 mL of DDT, 0.375 mL of trioctylphosphine:TOP, and 5 mL of zinc oleate:Zn(OLAc)2 solution (0.4 M) was added, and the mixture was heated at 320°C for 10 minutes with stirring. This procedure was repeated four times.

[0178] Subsequently, ethanol was added to the resulting reaction mixture to generate a precipitate, which was then collected by centrifugation. 17.5 ml of octadecene (ODE) was added to the precipitate to disperse it, and 1 ml of oleic acid (OLAc) and 2 ml of trioctylphosphine (TOP) were added as before. The mixture was then heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0179] To this solution, 0.5 mL of a mixture containing 0.5 mL of DDT, 1.5 mL of trioctylphosphine (TOP), and 10 mL of zinc oleate (Zn(OLAc)2) solution (0.4 M) was added, and the mixture was heated at 320°C for 10 minutes with stirring. This procedure was repeated 10 times.

[0180] (A) Subsequently, ethanol was added to the resulting reaction mixture to generate a precipitate, which was then collected by centrifugation. 17.5 ml of octadecene:ODE was added to the precipitate to disperse it, and 1 ml of oleic acid:OLAc and 2 ml of trioctylphosphine:TOP were added as before. The mixture was then heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0181] (B) To this solution, 0.5 mL of a mixture of 0.5 mL of DDT, 1.5 mL of trioctylphosphine (TOP), and 10 mL of zinc oleate (Zn(OLAc)2) solution (0.4 M) was added, and the mixture was heated at 320°C for 10 minutes with stirring. This procedure was repeated 6 times. After that, the mixture was heated at 320°C for 30 minutes with stirring.

[0182] Subsequently, the reaction solution was subjected to the washing and coating procedures described in (A) and (B) above, repeating them three times until the final target reaction solution (ZnSe / ZnS) was obtained, and then cooled to room temperature.

[0183] The resulting reaction solution was measured using a fluorescence spectrometer. The results showed optical properties with a fluorescence wavelength of approximately 444 nm and a fluorescence full width at half maximum of approximately 15 nm.

[0184] Ethanol was added to the resulting reaction solution to generate a precipitate, which was then collected by centrifugation. Hexane was then added to the precipitate to disperse it.

[0185] The hexane-dispersed ZnSe / ZnS was measured using a quantum efficiency measurement system. The results showed a fluorescence quantum yield of approximately 62%. The fluorescence lifetime was measured to be 15 ns.

[0186] [Example 7] In a 100 mL reaction vessel, 2182 mg of anhydrous copper acetate (Cu(OAc)), 4.8 mL of oleylamine (OLAm), and 7.75 mL of octadecene (ODE) were added. The mixture was then heated at 165 °C for 5 minutes with stirring under an inert gas (N2) atmosphere to dissolve the starting materials.

[0187] To this solution, 1.14 mL of Se-DDT / OLAm solution (0.7 M) was added and the mixture was heated at 165°C for 30 minutes with stirring. The resulting reaction solution (Cu2Se) was cooled to room temperature.

[0188] Subsequently, 21844 mg of anhydrous zinc acetate (Zn(OAc)), 10 mL of trioctylphosphine (TOP), and 0.4 mL of oleylamine (OLAm) were added to the Cu2Se reaction solution, and the mixture was heated at 180°C for 45 minutes with stirring under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSe) was then cooled to room temperature.

[0189] Ethanol was added to the reaction mixture cooled to room temperature to generate a precipitate, which was then collected by centrifugation. 12 ml of octadecene:ODE was added to the precipitate and dispersed.

[0190] Subsequently, 21844 mg of anhydrous zinc acetate (Zn(OAc)), 10 mL of trioctylphosphine (TOP), 1 mL of oleylamine (OLAm), and 6 mL of oleic acid (OLAc) were added to 12 ml of ZnSe-ODE solution, and the mixture was heated at 280°C for 20 minutes with stirring under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSe) was then cooled to room temperature.

[0191] The resulting reaction solution was measured using a fluorescence spectrometer. The results showed optical properties with a fluorescence wavelength of approximately 447.5 nm and a fluorescence half-width of approximately 14 nm.

[0192] Ethanol was added to 20 ml of the obtained ZnSe reaction solution to generate a precipitate, which was then collected by centrifugation. 17.5 ml of octadecene:ODE was added to the precipitate and dispersed.

[0193] To 17.5 mL of dispersed ZnSe-ODE solution, 1 mL of oleic acid (OLAc) and 2 mL of trioctylphosphine (TOP) were added, and the mixture was heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0194] To this solution, 0.5 mL of a mixture consisting of 0.5 mL of Se-TOP solution (1 M), 0.125 mL of DDT, 0.375 mL of trioctylphosphine:TOP, and 5 mL of zinc oleate:Zn(OLAc)2 solution (0.4 M) was added, and the mixture was heated at 320°C for 10 minutes with stirring. This procedure was repeated four times.

[0195] Subsequently, ethanol was added to the resulting reaction mixture to generate a precipitate, which was then collected by centrifugation. 17.5 ml of octadecene (ODE) was added to the precipitate to disperse it, and 1 ml of oleic acid (OLAc) and 2 ml of trioctylphosphine (TOP) were added as before. The mixture was then heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0196] To this solution, 0.5 mL of a mixture containing 0.5 mL of DDT, 1.5 mL of trioctylphosphine (TOP), and 10 mL of zinc oleate (Zn(OLAc)2) solution (0.4 M) was added, and the mixture was heated at 320°C for 10 minutes with stirring. This procedure was repeated 10 times.

[0197] (C) Subsequently, ethanol was added to the resulting reaction mixture to generate a precipitate, which was then collected by centrifugation. 17.5 ml of octadecene:ODE was added to the precipitate to disperse it, and 1 ml of oleic acid:OLAc and 2 ml of trioctylphosphine:TOP were added as before. The mixture was then heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0198] (D) To this solution, 0.5 mL of a mixture of 0.5 mL of DDT, 1.5 mL of trioctylphosphine (TOP), and 10 mL of zinc oleate (Zn(OLAc)2) solution (0.4 M) was added, and the mixture was heated at 320°C for 10 minutes with stirring. This procedure was repeated 6 times. After that, the mixture was heated at 320°C for 30 minutes with stirring.

[0199] Subsequently, the washing and coating procedures described in (C) and (D) above were repeated twice with this reaction solution.

[0200] Subsequently, ethanol was added to the resulting reaction mixture to generate a precipitate, which was then collected by centrifugation. 17.5 ml of octadecene (ODE) was added to the precipitate to disperse it, and 1 ml of oleic acid (OLAc) and 2 ml of trioctylphosphine (TOP) were added as before. The mixture was then heated at 320°C for 10 minutes with stirring under an inert gas (N2) atmosphere.

[0201] To this solution, 0.5 mL of a mixture containing 0.5 mL of DDT, 1.5 mL of trioctylphosphine (TOP), and 10 mL of zinc oleate (Zn(OLAc)2) solution (0.4 M) was added, and the mixture was heated at 320°C for 10 minutes with stirring. This procedure was repeated 6 times. After that, the mixture was heated at 320°C for 30 minutes with stirring.

[0202] (E) To this solution, 1.0 mL of a mixture of 0.25 mL of DDT, 0.75 mL of TOP, and 5 mL of 0.4 M ZnBr2 oleic acid-TOP solution was added and heated at 320°C for 10 minutes. The final reaction solution (ZnSe / ZnS) was obtained and cooled to room temperature.

[0203] The resulting reaction solution was measured using a fluorescence spectrometer. The results showed optical properties with a fluorescence wavelength of approximately 444 nm and a fluorescence full width at half maximum of approximately 15 nm.

[0204] Ethanol was added to the resulting reaction solution to generate a precipitate, which was then collected by centrifugation. Hexane was then added to the precipitate to disperse it.

[0205] The hexane-dispersed ZnSe / ZnS was measured using a quantum efficiency measurement system. The fluorescence quantum yield was approximately 76%. The fluorescence lifetime was measured to be 17 ns. Elemental analysis (EDX) revealed the following: Zn: 53 atom%, Se: 16 atom%, S: 23 atom%, Br: 1 atom%.

[0206] [Example 8] The fluorescence quantum yield of ZnSe / ZnS obtained by performing the same procedure as in (E) of [Example 7], except that ZnBr2 was replaced with ZnCl2, was 80%.

[0207] [Example 9] The fluorescence quantum yield of ZnSe / ZnS obtained by performing the same procedure as in (E) of [Example 7], except that ZnBr2 was replaced with ZnI2, was 69%.

[0208] Examples 7 to 9 showed that by coating a 445 nm ZnSe core with ZnS and then adding a solution containing zinc halide, the fluorescence quantum yield improved by approximately 7% to 18% compared to Example 6 (see Table 2 below). Furthermore, compositional analysis of the QDs obtained in Example 7 after washing revealed the presence of bromine (Br), suggesting that the surface was modified with halogen, and that this effect contributed to the improved fluorescence quantum yield.

[0209] [Table 2]

[0210] [Comparative Example 1] In a 100 mL reaction vessel, 0.833 mL of zinc oleate:Zn(OLAc)2-ODE solution (0.4 M) and 10 mL of Se-ODE solution (0.1 M) were added, and the mixture was heated at 280 °C for 35 minutes with stirring under an inert gas (N2) atmosphere.

[0211] The resulting reaction solution was measured using a fluorescence spectrometer. The results showed optical properties with a fluorescence wavelength of approximately 455.0 nm and a fluorescence full width at half maximum of approximately 45.2 nm.

[0212] [Comparative Example 2] 2182 mg of anhydrous copper acetate (Cu(OAc)), 7.75 mL of oleylamine (OLAm), and 4.8 mL of octadecene (ODE) were placed in a 300 mL reaction vessel. The mixture was then heated at 150 °C for 10 minutes with stirring under an inert gas (N2) atmosphere to dissolve the starting materials.

[0213] To this solution, 2.8 mL of Se-DDT / OLAm solution (0.285 M) was added and the mixture was heated at 150°C for 30 minutes with stirring. The resulting reaction solution (CuSe) was cooled to room temperature.

[0214] Subsequently, 20.682 g of anhydrous zinc acetate (Zn(OAc)), 10 mL of trioctylphosphine (TOP), and 0.4 mL of oleylamine (OLAm) were added to the CuSe reaction solution, and the mixture was heated at 180°C for 30 minutes with stirring under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSe) was cooled to room temperature.

[0215] Ethanol was added to the reaction mixture cooled to room temperature to generate a precipitate, which was then collected by centrifugation. 24 ml of octadecene:ODE was added to the precipitate and dispersed.

[0216] Subsequently, 24 ml of ZnSe-ODE solution was mixed with 20.682 g of anhydrous zinc acetate (Zn(OAc)), 10 ml of trioctylphosphine (TOP), and 1 ml of oleylamine (OLAm). The mixture was heated at 180°C for 30 minutes with stirring under an inert gas (N2) atmosphere. The resulting reaction solution (ZnSe) was then cooled to room temperature.

[0217] The resulting reaction solution was measured using a fluorescence spectrometer. The results showed optical properties with a fluorescence wavelength of approximately 438.5 nm and a fluorescence half-width of approximately 13 nm.

[0218] The resulting reaction solution was measured using a quantum efficiency measurement system. The results showed a fluorescence quantum yield of approximately 11%. The fluorescence lifetime was measured to be 43 ns.

[0219] Ethanol was added to 20 ml of the above ZnSe reaction solution to generate a precipitate, which was then collected by centrifugation. 17.5 ml of octadecene:ODE was added to the precipitate and dispersed.

[0220] 17.5 mL of dispersed ZnSe-ODE solution was heated at 310°C for 20 minutes with stirring under an inert gas (N2) atmosphere.

[0221] To this solution, 0.545 mL of a mixture of 0.73 mL of S-TOP solution (2.2 M) and 3.63 mL of zinc oleate:Zn(OLAc)2 solution (0.8 M) was added, and the mixture was heated at 310°C for 10 minutes with stirring. This procedure was repeated 8 times. The resulting reaction solution (ZnSe / ZnS) was cooled to room temperature.

[0222] Ethanol was added to the resulting reaction solution to generate a precipitate, which was then collected by centrifugation. Hexane was then added to the precipitate to disperse it.

[0223] The hexane-dispersed ZnSe / ZnS was measured using a fluorescence spectrometer. As a result, as shown in Figure 4, optical properties were obtained with a fluorescence wavelength of approximately 437.5 nm and a fluorescence full width at half maximum of approximately 13.8 nm.

[0224] The same solution was measured with a quantum efficiency measurement system. As a result, the fluorescence quantum yield was about 33%. Also, as a result of measuring the fluorescence lifetime, it was 44 ns.

[0225] An experiment was also conducted to assume the optimal number of coating additions in Example 1 from the number of coating additions and the change in fluorescence lifetime. The results are shown in Table 3 below.

[0226]

Table 3

[0227] From this, it can be predicted that the optimal number of coating additions is 6 to 8 times, where the fluorescence quantum yield is high and the fluorescence lifetime is short. Although this varies depending on the coating method, when confirmed from the particle size, the same tendency can be seen in any material. Thus, in all of these examples, consideration has been given to maximizing the coating thickness (particle size) when the fluorescence quantum efficiency is highest and the fluorescence lifetime is shortest.

[0228] In addition, in Example 1 and Comparative Example 2 in the above experiment, when coating the shell (ZnS), S-TOP was used as the sulfur source. At this time, it was effective for the ZnSe in Example 1 with a small particle size (short fluorescence wavelength), but it was not suitable for Comparative Example 2 with a fluorescence wavelength of about 440 nm for ZnSe, and it was found that the particle shape deteriorated and the fluorescence quantum yield after coating also decreased.

[0229] In contrast, in Example 6, the sulfur source was changed to DDT. As a result, it was found that a good fluorescence yield and particle shape could be maintained with a fluorescence wavelength of 440 nm or more, and ZnS could be coated. The above content is summarized in Table 4 below.

[0230]

Table 4

[0231] From the above experimental results, it was found that in each embodiment, the particle size of the quantum dots can be adjusted within the range of 5 nm or more and 20 nm or less. Also, in each embodiment, it was found that the fluorescence quantum yield is 5% or more and the fluorescence half-width is 25 nm or less. Further, the fluorescence lifetime could be made 50 ns or less. Also, it was found that the fluorescence wavelength can be adjusted within the range of 410 nm or more and 490 nm or less. In the core-shell structure composed of Zn, Se, and S, it was found that the position of the maximum intensity peak in XRD is on the high-angle side 0.05° to 1.2° higher than the crystal peak of the ZnSe core alone.

[0232] From this peak shift to the high-angle side, it can be seen that the lattice constant has changed by coating ZnS on the ZnSe core. Furthermore, it has also been found from this result that the amount of this peak shift is proportional to the amount of ZnS coating. Also, in the current result, although the position of XRD is infinitely close to the peak position of ZnS, since it emits blue light (430 - 455 nm), it is considered that the core is ZnSe and ZnS is coated thereon.

[0233] That is, from the peak shift from the ZnSe core and the blue light emission, it becomes possible to infer that it is a core-shell structure composed of Zn, Se, and S.

Industrial Applicability

[0234] According to the present invention, quantum dots that emit blue fluorescence can be stably obtained. By applying the quantum dots of the present invention to an LED, a backlight device, a display device, etc., excellent light emission characteristics can be obtained in each device.

[0235] This application is based on Japanese Patent Application No. 2019-170996 filed on September 20, 2019, and Japanese Patent Application No. 2020-024688 filed on February 17, 2020. The entire contents are incorporated herein.

Claims

1. It is a core-shell structure consisting of a core made of ZnSe and a shell made of ZnS. The particle size is between 5 nm and 20 nm. The surface of the ZnS is modified with Cl or Br. The fluorescence quantum yield is 76% or higher. A quantum dot characterized by the following features.

2. The quantum dot according to claim 1, characterized in that the fluorescence half-width is 25 nm or less.

3. The quantum dot according to claim 1 or 2, characterized in that the fluorescence wavelength is in the range of 410 nm to 490 nm.

4. A quantum dot according to any one of claims 1 to 3, characterized in that the particle shape is polygonal.

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