Quantum dots and their manufacturing method, green phosphor, visualization dye and pigment

Abstract

To provide a quantum dot having a high quantum yield and narrow half-width in a wavelength range of 500 nm or more and 540 nm or less, and a phosphor.SOLUTION: A quantum dot comprises a core 11 and a shell 12 that covers the core. The surface of the core is in contact with the inner surface of the shell. The core comprises single crystal of indium phosphide (InP). The shell comprises single crystal of zinc sulfide (ZnS). The shell has a thickness of 0.2 nm or more and 1.1 nm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to quantum dots and methods for producing the same, green phosphors, visualization dyes and pigments. [Background technology]

[0002] In recent years, quantum dots have been attracting attention as fluorescent materials. When semiconductor crystals with a direct band structure are micronized to crystals smaller than the bulk exciton Bohr radius, the optical absorption and PL (photoluminescence) spectra shift depending on the particle size. This is the result of electrons, holes, and excitons being confined within the microcrystalline particles, resulting in discrete energy states. Such semiconductor crystals are generally called quantum dots. Because quantum dots can modulate the light emission and light absorption spectra by particle size, they are applied to wavelength-tunable phosphors, light-emitting diodes, lasers, light-receiving elements, etc. Recently, quantum dots with a core / shell structure have been developed as phosphor particles in the visible light region, in which a core made of InP or the like with a narrow band gap is covered with a shell made of ZnSe or the like with a wide band gap, as disclosed in Patent Documents 1 to 3, for example.

[0003] Since the three primary colors of light are red, blue, and green, there is a demand for green phosphors and quantum dots. In particular, there is a strong demand for phosphors that emit deep green light (corresponding to an emission wavelength of 520 nm). It is thought that if such phosphors were available, it would be possible to reproduce all the colors that exist in nature, but this has not yet been realized. Furthermore, phosphors and quantum dots are required to have high quantum efficiency (PLQY: Photoluminescence Quantum Yield), PL emission with a narrow half-width, and low toxicity.

[0004] However, current quantum dots do not combine high quantum efficiency and narrow PL half-width in the green wavelength range of 500 nm to 540 nm, and most of them are made of materials that are known to be toxic. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-76827 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-138367 [Patent Document 3] Japanese Patent Application Publication No. 2019-218527 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention aims to provide quantum dots and phosphors that have high quantum yields and narrow half-widths in the wavelength range of 500 nm to 540 nm, as well as methods for producing them. It also aims to provide a highly sensitive and highly visible green visualization dye and a green pigment that develops color under sunlight. [Means for solving the problem]

[0007] The configuration of the present invention to solve the problems is shown below. (Configuration 1) It consists of a core part and a shell part that covers the core part, a surface of the core portion contacting an inner surface of the shell portion; the core portion is made of a single crystal of indium phosphide (InP), The shell portion is made of a single crystal of zinc sulfide (ZnS), The quantum dot has a shell portion having a thickness of 0.2 nm or more and 1.1 nm or less. (Configuration 2) 2. The quantum dot according to configuration 1, wherein the core portion is spherical and has a diameter of 0.58 nm or more and 2.95 nm or less. (Configuration 3) 3. The quantum dot according to claim 1, wherein the core and shell have the same crystal lattice constant. (Configuration 4) 4. The quantum dot of claim 3, wherein the lattice constant is 0.56 nm. (Configuration 5) 5. The quantum dot according to any one of claims 1 to 4, wherein the shell has a thickness of 0.27 nm or more and 1.08 nm or less. (Configuration 6) 6. The quantum dot of claim 5, wherein the shell has a thickness of 0.81 nm. (Configuration 7) It consists of a core part and a shell part that covers the core part, a surface of the core portion contacting an inner surface of the shell portion; the core portion is made of a single crystal of indium phosphide (InP), the shell portion comprises a first shell in contact with the core portion and a second shell in contact with the first shell, the first shell is made of a single crystal of gallium phosphide (GaP); the second shell is made of a single crystal of zinc sulfide (ZnS), the thickness of the first shell portion is 0.27 nm or more and 1.9 nm or less; The quantum dot, wherein the thickness of the second shell portion is 0.27 nm or more and 1.9 nm or less. (Configuration 8) 8. The quantum dot according to any one of configurations 1 to 7, wherein the diameter of the core portion is 2.4 nm or more and 2.95 nm or less. (Configuration 9) mixing, under an inert gas, one or more of indium acetate, indium chloride, indium bromide, and indium iodide, one or more selected from the group consisting of zinc acetate, zinc chloride, zinc bromide, and zinc iodide, one or more selected from the group consisting of palmitic acid, lauric acid, myristic acid, stearic acid, and oleic acid, trioctylphosphine, and one or more selected from the group consisting of 1-octadecene (ODE), dodecane, tetradecane, hexadecane, octadecane, dodecene, tetradecene, and hexadecene to prepare a first mixed solution; subjecting the first mixed liquid to a first heat treatment under vacuum; adding a first solution in which tris(trimethylsilyl) phosphite is dissolved in trioctylphosphine to the first mixed solution in an inert gas environment after the first heat treatment; After the addition of the first solution, subjecting the first mixed solution to a second heat treatment under vacuum; After the second heat treatment, a third heat treatment is performed on the first mixture in an inert gas atmosphere to form an InP core. Mixing the InP core, ODE, dodecanethiol (DDT) or sulfur powder, and zinc oleate (Zn-OA) or zinc stearate (Zn-SA) and maintaining under vacuum to prepare a second mixture; subjecting the second mixture to a fourth heat treatment under an inert gas; a method for producing quantum dots, comprising: after the fourth heat treatment, adding DDT and Zn—OA to the second mixed solution, and performing a fifth heat treatment under an inert gas. (Configuration 10) 10. The method for producing quantum dots according to claim 9, wherein the inert gas is argon gas. (Configuration 11) 11. The method for producing quantum dots according to claim 9, wherein the temperature of the first heat treatment is 90° C. or higher and 140° C. or lower. (Configuration 12) 12. The method for producing quantum dots according to any one of aspects 9 to 11, wherein the temperature of the second heat treatment is 25°C or higher and 60°C or lower. (Configuration 13) 13. The method for producing quantum dots according to any one of aspects 9 to 12, wherein the temperature of the third heat treatment is 210°C or higher and 320°C or lower. (Configuration 14) 14. The method for producing quantum dots according to any one of aspects 9 to 13, wherein the temperature of the fourth heat treatment is 210°C or higher and 320°C or lower. (Configuration 15) 15. The method for producing quantum dots according to any one of aspects 9 to 14, wherein the temperature of the fifth heat treatment is 230°C or higher and 320°C or lower. (Configuration 16) A green phosphor comprising the quantum dot according to any one of configurations 1 to 8. (Configuration 17) A green visualization dye comprising the quantum dot of any one of configurations 1 to 8. (Configuration 18) A green pigment comprising the quantum dots according to any one of configurations 1 to 8. [Effects of the Invention]

[0008] The present invention provides quantum dots and phosphors having high quantum yields and narrow half-widths in the wavelength range of 500 nm to 540 nm, as well as highly sensitive and highly visible green visualization dyes and green pigments that develop color under sunlight. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view showing the structure of a quantum dot of the present invention. [Figure 2] FIG. 1 is a characteristic diagram showing the bandgaps of InP, GaP, and ZnS side by side. [Figure 3] FIG. 1 is a flow chart showing the steps for producing quantum dots of the present invention. [Figure 4] This is an SEM photograph of an InP core. [Figure 5] FIG. 1 is a characteristic diagram showing powder X-ray diffraction of the prepared sample. [Figure 6] FIG. 1 is a characteristic diagram showing the PL spectrum of the prepared sample. [Figure 7] FIG. 1 is a characteristic diagram showing the ZnS shell thickness dependence of the PL internal yield and the PL peak half-width of the prepared sample. [Figure 8] FIG. 1 is a characteristic diagram showing the influence of the degree of vacuum on the first exciton peak characteristics in the optical absorption spectrum of an InP core. [Figure 9] FIG. 10 is a characteristic diagram showing the influence of a degassing process on the first exciton peak characteristics in the absorption spectrum of an InP core. [Figure 10] This is a TEM image of the crystal lattice of a core / shell particle. [Figure 11] FIG. 1 is a characteristic diagram showing powder X-ray diffraction of the prepared sample. [Figure 12] FIG. 1 is a characteristic diagram showing the PL spectrum of the prepared sample. [Figure 13] FIG. [Figure 14] FIG. 1 is a characteristic diagram showing powder X-ray diffraction of InP core and coherent InP / GaP / ZnS core / shell / shell particles. [Figure 15] FIG. 1 is a characteristic diagram showing the optical absorption and PL spectra of a core / shell / shell particle made of coherent InP / GaP / ZnS. [Figure 16] FIG. 10 is a characteristic diagram showing the influence of gas supply time on the first exciton peak characteristics in the absorption spectrum of an InP core. [Figure 17] This is a TEM image of the crystal lattice of a core / shell particle. [Figure 18] FIG. 1 is a diagram of a process for thickening the ZnS shell. [Figure 19] FIG. 1 shows the optical absorption spectrum of an InP / ZnS core / shell and a method for measuring valley depth from the spectrum. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that the expression AB in the text means A or more and B or less.

[0011] (Embodiment 1) In the first embodiment, quantum dots having a core / shell structure made of InP / ZnS will be described.

[0012] <Structure and effects> Through extensive research, the inventors have discovered that quantum dots 1, which have a core / shell structure consisting of InP cores 11 sized to exhibit a green fluorescence (PL) peak in the 510-540 nm wavelength range and covered with ZnS shells 12 with a thickness of 1.1 nm or less, exhibit a PL peak that can be modulated in the 500-530 nm wavelength range depending on the particle size, have high symmetry near the PL spectrum peak, a PL peak full width at half maximum (PL-FWHM) of 40 nm or less, and a PL quantum yield (PLQY) of 60% or more, as shown in Figure 1. Furthermore, the inventors have discovered that the first exciton peak in the optical absorption spectrum is steep, has a valley depth of 0.51 nm or less, and a full width at half maximum of 50 nm or less, providing excellent properties. As shown in FIG. 19, Valley-depth (VD) is defined by the following formula (1). VD=1-[Absorbance (Minimum) / Absorbance (Maximum)] ···(1)

[0013] As described above, the quantum dot 1 of the first embodiment is a quantum dot with a core / shell structure in which the core 11 is made of InP and the shell 12 is made of ZnS. The size of the InP core 11 is adjusted to form the quantum dot 1 in the green wavelength region of 500-530 nm using the InP core 11. When a semiconductor crystal with a direct transition band structure is miniaturized to a crystal smaller than the bulk exciton Bohr radius, electrons, holes, and excitons are confined within the microcrystalline particles, and their energy states become discrete, causing the optical absorption and PL spectra to shift depending on the particle size. Applying this principle, the emission and optical absorption spectra were modulated by the dot size. The specific diameter of the InP core 11 for this purpose is between 0.27 nm and 1.08 nm.

[0014] Here, a shift of approximately 10 nm is observed between the PL peak calculated from the InP core size and the PL peak of quantum dot 1 consisting of InP / ZnS. If the size of InP core 11 is set so that the PL peak of InP core 11 has a wavelength of 510-540 nm, the PL peak of InP / ZnS quantum dot 1 will have a wavelength of 500-530 nm.

[0015] This 10 nm shift is due to the crystal lattice contraction at the surface of the InP core 11 caused by the ZnS coating as the thin-film shell 12. As will be described in detail later, we have found that when a coherent core-shell material is fabricated using ZnS with a density of three or fewer monomolecules as the shell material, the lattice constant of the InP core 11 is 0.56 nm, which is intermediate between the original lattice constants of InP and ZnS. This causes the InP lattice to contract, increasing the band gap and shifting the PL peak relatively 10 nm toward shorter wavelengths. Therefore, in the first embodiment, it is essential to set the size of the InP core 11 based on the assumption that the PL peak will shift 10 nm.

[0016] It is preferable to increase the monodispersity of the InP core 11. Increasing the monodispersity makes it possible to provide quantum dots with favorable properties, such as a highly symmetric PL spectrum, a narrow half-width of the PL peak, and a steep peak of the first excitation level in the optical absorption spectrum.

[0017] In the InP core / ZnS shell structure, the VBM (Valence Band Maximum)-CBM (Condition Band Minimum) energy band is such that the ZnS band completely sandwiches the InP band (band gap), as shown in Figure 2. Because the energy bands are completely sandwiched, carriers remain in the core without spilling out into the shell. This makes it easier to obtain a narrow full width at half maximum (PL-FWHM) and a high quantum yield (PLQY). If they were not sandwiched, carriers would seep into the shell, and radiative recombination of carriers would occur not only within the core but also at the core / shell interface and even in the shell, resulting in a wider full width at half maximum (PL-FWHM) and a lower quantum yield (PLQY).

[0018] Generally, when an InP core 11 is coated with a ZnS shell 12, the difference in lattice constant between the original InP and ZnS materials is large, at 7.7%, resulting in low lattice matching at the core / shell interface. Therefore, to reduce the lattice strain energy at the core / shell interface, lattice defects such as dangling bonds and dislocations are generated at the interface. These defects act as non-radiative deactivation channels, reducing the probability of radiative recombination of carriers. As a result, the quantum yield (PLQY) is low, at less than 20%, for example.

[0019] A core / shell / shell structure has been proposed to improve the lattice matching at the core / shell interface. A typical semiconductor combination is an InP core, ZnSe inner shell, and ZnS outer shell. The lattice constant difference between InP and ZnSe is 3.3%, and the lattice constant difference between ZnSe and ZnS is 4.4%, so the lattice mismatch at the heterojunction interface is reduced compared to the case of InP / ZnS. This double-shell structure is not a problem when used as a phosphor emitting at wavelengths longer than 600 nm. However, at wavelengths shorter than 530 nm, the bandgap of InP increases to the point where it is no longer possible to sandwich the InP bandgap with the ZnSe bandgap. As mentioned above, if the InP bandgap is no longer able to sandwich the InP bandgap, carriers will seep into the shell, and radiative recombination of carriers will occur not only within the core but also at the core / shell interface and even in the shell. This results in an increase in the full width at half maximum (PL-FWHM) and a decrease in the quantum yield (PLQY). Furthermore, this double-shell structure uses selenium and selenium compounds, which have toxicity issues and have been selected as substances to be investigated by the Joint Industry Guideline (JIG).

[0020] As described above, the inventors have found that when InP / ZnS coherent core-shell quantum dots 1 are fabricated using ZnS with a molecular weight of three or less as the material for shell 12, the lattice constant of the surface of InP core 11 is modulated from the original 0.587 nm to 0.56 nm, and the lattice constant of ZnS shell 12 is also modulated from the original 0.542 nm to 0.56 nm. Because the core and shell have the same lattice constant, the lattice matching at the core / shell interface is good, even though different semiconductors are bonded together. Furthermore, as mentioned above, the ZnS shell 12 completely sandwiches the band gap of the core 11 in terms of energy, so carriers are tightly confined within the core crystal (preventing them from escaping to the shell), increasing the radiative recombination probability. As a result, a high quantum yield (PLQY) of over 60% can be achieved.

[0021] The thickness of the ZnS shell 12 is preferably 1 to 3 monomolecules, more preferably 3 monomolecules. In terms of nm, the thickness of the ZnS shell 12 is preferably 0.27 to 1.08 nm, more preferably 1.08 nm. This allows for a high PLQY and a narrow PL-FWHM.

[0022] As described above, the quantum dot 1 of the first embodiment is a quantum dot having highly monodisperse core particles, a highly symmetric PL spectrum, a narrow half-width of the PL peak, a steep peak at the first excitation level in the optical absorption spectrum, and a high quantum yield PLQY.

[0023] The quantum dot 1 of the first embodiment and materials containing it can be used as a green phosphor that has a high quantum yield, a narrow PL half-width, and does not contain any materials that are known to be toxic. Specifically, when used in displays such as liquid crystal displays and organic electroluminescence displays, it can be used as a low-power, high-brightness green phosphor. Furthermore, the quantum dot 1 of the first embodiment and materials containing it can also be used as a green visualization dye that has a high quantum yield, a narrow PL half-width, and does not contain any materials that have been identified as toxic. Specifically, it can be used as a green visualization dye for use in surgery. When illuminated, it becomes a highly bright and distinct marker, making cancer surgery, for example, easier. Furthermore, the quantum dot 1 of the first embodiment and materials containing it can also be used as a green pigment or green cosmetics. This makes it possible to provide a pigment and cosmetics with the unique characteristic that they do not emit a noticeable color in the dark when no light is shining, but efficiently emit a color near the green apex of the chromaticity diagram under light such as sunlight.

[0024] <Manufacturing method> The method for producing the quantum dots 1 will be described with reference to the production flow chart of FIG. First, indium acetate, indium chloride, indium bromide, indium iodide and zinc acetate, zinc chloride, zinc bromide, zinc iodide are mixed under an inert gas atmosphere with at least one selected from the group consisting of palmitic acid, lauric acid, myristic acid, stearic acid, and oleic acid, trioctylphosphine, and at least one selected from the group consisting of 1-octadecene (ODE), dodecane, tetradecane, hexadecane, octadecane, dodecene, tetradecene, and hexadecene to produce a first mixture (step S11). Examples of the inert gas include rare gases such as argon gas, krypton gas, and neon gas, and nitrogen gas. The temperature during mixing is not particularly limited, but room temperature (e.g., 20°C to 25°C) is preferred for ease of handling. Palmitic acid is particularly preferred as an acid, as it provides a high yield.

[0025] Next, the first mixed liquid is subjected to a first heat treatment under vacuum (step S12). Here, the degree of vacuum is preferably 60 Pa or less. By setting the degree of vacuum at 60 Pa or less, a narrow first exciton peak in the optical absorption spectrum can be obtained. There is no particular lower limit for the degree of vacuum, but considering ease of handling and cost, it is preferable to set it at 15 Pa or more. Furthermore, since evaporation of ODE begins at 15 Pa or less, it is necessary to lower the heat treatment temperature. The temperature of the first heat treatment is preferably 90° C. or higher and 140° C. or lower, and the time of the first heat treatment is preferably 2 hours or higher and 12 hours or lower, since these are temperatures and times sufficient to remove impurities.

[0026] Thereafter, in an inert gas environment, a first solution in which tris(trimethylsilyl) phosphite is dissolved in trioctylphosphine is added to the first mixed solution (step S13).

[0027] Thereafter, the first mixed liquid is subjected to a second heat treatment under vacuum (step S14). Here, the degree of vacuum is preferably 60 Pa or less. By setting the degree of vacuum to 60 Pa or less, it is possible to obtain a first exciton peak with a narrow half-width of the optical absorption spectrum and a large valley depth. There is no particular lower limit to the degree of vacuum, but in consideration of ease of handling and cost, it is preferably 15 Pa or more. The temperature of the second heat treatment is preferably 20° C. or higher and 60° C. or lower, and the time of the second heat treatment is preferably 5 minutes or higher and 20 minutes or lower, since these are sufficient temperatures and times to remove impurities.

[0028] Thereafter, the first mixture is subjected to a third heat treatment under an inert gas to produce an InP core (step S15). Examples of the inert gas include rare gases such as argon gas, nitrogen gas, and mixtures thereof. There are no particular restrictions on the pressure of the inert gas, but atmospheric pressure is preferred for ease of handling. Furthermore, the temperature of the third heat treatment is preferably 210° C. or more and 320° C. or less, and the time of the third heat treatment is preferably 2 minutes or more and 30 minutes or less in order to suppress Ostwald ripening and obtain a uniform particle size.

[0029] Next, the prepared InP core, ODE, dodecanethiol (DDT) or sulfur powder, and zinc oleate (Zn-OA) or zinc stearate (Zn-SA) are mixed and held under vacuum to prepare a second mixture (step S16). Here, the degree of vacuum is preferably 60 Pa or less. By setting the degree of vacuum to 60 Pa or less, a first exciton peak with a narrow half-width of the optical absorption spectrum can be obtained. There is no particular lower limit to the degree of vacuum, but in consideration of ease of handling and cost, it is preferably 15 Pa or more. The heat treatment temperature is preferably 90° C. or higher and 140° C. or lower, and the heat treatment time is preferably 20 minutes or higher and 30 minutes or lower, as these are appropriate synthesis temperatures and times for a green phosphor having a narrow PL half-width.

[0030] Thereafter, the second mixed liquid is subjected to a fourth heat treatment under an inert gas (step S17). Examples of the inert gas include rare gases such as argon gas, nitrogen gas, and mixtures of these gases. There are no particular restrictions on the pressure of the inert gas, but atmospheric pressure is preferred for ease of handling. Furthermore, the temperature of the fourth heat treatment is preferably 210°C or higher and 320°C or lower, and the time of the fourth heat treatment is preferably 20 minutes or higher and 30 minutes or lower, as these are appropriate temperatures and times for uniformly growing a ZnS shell on the InP core that has been fabricated.

[0031] Thereafter, DDT and Zn—OA are added to the second mixture (step S18), and a fifth heat treatment is carried out under an inert gas (step S19), thereby producing quantum dots. Examples of the inert gas include rare gases such as argon gas, nitrogen gas, and mixtures of these gases. There are no particular restrictions on the pressure of the inert gas, but atmospheric pressure is preferred for ease of handling. The temperature of the fifth heat treatment depends on the desired ZnS shell thickness and is controlled within a range of 230°C to 320°C. The duration of the fifth heat treatment is preferably 20 minutes to 30 minutes at any reaction temperature, as this is the appropriate temperature and time for growing a ZnS shell monolayer by monolayer on the InP core / ZnS shell. The heat treatment process diagram shown in Figure 18 shows the relationship between the amount of DDT and Zn-OA additive solution, the heat treatment temperature, and the shell layer.

[0032] Through the above steps, it is possible to provide quantum dots 1 having highly monodisperse core particles, highly symmetric PL spectra, narrow half-width PL peaks, steep peaks in the first excitation level of the optical absorption spectrum, and high quantum yields (PLQY).

[0033] (Embodiment 2) In the second embodiment, quantum dots having a core / shell / shell structure made of InP / GaP / ZnS will be described.

[0034] The InP core conforms to the first embodiment. The GaP shell preferably has a thickness of 0.27 nm or more and 1.9 nm or less, and in terms of the number of molecules, 1 monomolecular or more and 7 monomolecular or less. The ZnS shell preferably has a thickness of 0.27 nm or more and 1.9 nm or less, and in terms of the number of molecules, 1 monomolecular or more and 7 monomolecular or less. It is more preferable that the GaP shell be 1 monomolecular or less, and the ZnS shell be 3 monomolecular or less. This improves the lattice matching of InP / GaP / ZnS. That is, although the lattice constants of the InP, GaP, and ZnS materials themselves are 0.587 nm, 0.545 nm, and 0.542 nm, respectively, when the InP core conforms to the first embodiment and the shell has the above thickness, the lattice constants of the InP core surface, GaP shell, and ZnS shell are all modulated to 0.56 nm. Furthermore, as shown in Figure 2, the band gap of the GaP shell almost sandwiches that of the InP core, and the band gap of the ZnS shell completely sandwiches that of the GaP shell, so charge carriers are tightly confined within the core crystal (they do not escape to the shell), increasing the radiative recombination probability.

[0035] Therefore, the quantum dots of the second embodiment, like those of the first embodiment, have highly monodisperse core particles, highly symmetric PL spectra, narrow full width at half maximum of the PL peak, a steep peak at the first excitation level in the optical absorption spectrum, and a high quantum yield PLQY.

[0036] The quantum dots and materials containing them of the second embodiment can be used as green phosphors that have high quantum yields, narrow PL half-widths, and do not contain any materials that are known to be toxic. Specifically, when used in quantum dot displays, they can be used as low-power, high-color-rendering, and high-brightness green phosphors. Furthermore, the quantum dots of the second embodiment and materials containing them can also be used as green visualization dyes that have high quantum yields, narrow PL half-widths, and do not contain any known toxic materials. Specifically, they can be used as green visualization dyes for surgery. When illuminated, they become bright, highly visible markers, making cancer surgery, for example, easier. The quantum dots of the second embodiment and materials containing them can also be used as green pigments and green cosmetics. This makes it possible to provide pigments and cosmetics with the unique characteristic of not producing a noticeable color in the dark, but efficiently producing a color near the green apex of the chromaticity diagram under light such as sunlight.

[0037] Quantum dots with a core / shell / shell structure made of InP / GaP / ZnS can be produced by the following process. The InP core is fabricated in accordance with the first embodiment. The GaP / ZnS shell is formed as follows: A sample is mixed with an InP core, ODE, and GaCl3, and kept at room temperature and a vacuum of 60 Pa or less. When the sample reaches 200°C under atmospheric pressure of inert gas, a GaP shell is first formed, and DDT and Zn-OA are added at the same temperature. The temperature is then raised to 230°C, DDT and Zn-OA are added, and the ZnS shell is then formed by heat treatment at 240°C.

[0038] Through the above process, it is possible to provide quantum dots with highly monodisperse core particles, highly symmetric PL spectra, narrow half-width PL peaks, steep peaks at the first excitation level in the optical absorption spectrum, and high quantum yields (PLQY). [Example]

[0039] Example 1 In Example 1, Sample A made of an InP core was fabricated and its characteristics were evaluated. The fabrication process is shown below. First, indium acetate (0.15 mmol), zinc acetate (0.075 mmol), palmitic acid (0.6 mmol), and 1-octadecene (ODE, 6.3 mL) were mixed and purged with argon gas. Inexpensive nitrogen gas could also be used instead of argon gas. Thereafter, the temperature was raised to 140° C. under a vacuum of 60 Pa or less, and after maintaining the temperature at that temperature for 12 hours, the container was cooled to room temperature and maintained in an argon atmospheric pressure environment. Next, trioctylphosphine (1 mL) containing 0.12 mmol of tris(trimethylsilyl) phosphite was added, and the mixture was heated to 40°C under a vacuum of 60 Pa or less and held at that temperature for 10 minutes. After returning to room temperature, the pressure was adjusted to argon, and the mixture was heated to 300°C and held at that temperature for 10 minutes to obtain an InP core.

[0040] The diameter of the InP core was measured using a scanning transmission electron microscope (SEM) to be 2.42 nm (Fig. 4), and the monodispersity was ±0.3 nm. The lattice constant was calculated to be 0.58 nm from powder X-ray diffraction (XRD) shown in sample A in Figure 5. This value is consistent with the theoretical value (0.58 nm) of InP crystal. As shown in Figure 6 for sample A, the PL spectrum has a low symmetry with two peaks at 510 nm and 660 nm. The full width at half maximum (FWHM) of the PL peak located at 510 nm was 54 nm (Figure 7, open circle). The PL internal quantum yield (PLQY) was 1% (Figure 7, filled circle). As shown in Figure 8, the first exciton peak at 460 nm in the optical absorption spectrum was steep, with a full width at half maximum (FWHM) of 48 nm (Figure 9). The quantum yield PLQY of the InP core alone is sufficiently small.

[0041] Example 2 In Example 2, Sample B, which is a quantum dot with a coherent core / shell structure, was fabricated and its properties were evaluated. First, Sample A, which was an InP core fabricated by the above process, was prepared. The ZnS shell was prepared by the following process. First, InP core (sample A) (2 mL), ODE (2 mL), dodecanethiol (DDT, 0.52 mmol, 124 μL), and zinc oleate (Zn-OA, 0.4 mol / L, 1 mL) were mixed and kept at room temperature under a vacuum of 60 Pa or less for 30 minutes. The pressure was then returned to argon atmospheric pressure, the temperature was raised to 230°C, and the temperature was maintained at 230°C for 20 minutes to obtain InP / ZnS core / shell particles. This was designated Sample B.

[0042] 5, the diffraction peak of sample B is shifted to the lower angle side. The lattice constant calculated from the diffraction peak position was 0.56 nm. The interplanar spacing of the (111) crystals estimated from HRTEM (High Resolution Transmission Electron Microscope) measurements was 0.32 nm (Fig. 10). From this interplanar spacing, the lattice constant was estimated to be 0.56 nm, which was consistent with the value calculated from XRD. From HRTEM observations, the thickness of the ZnS shell was estimated to be 0.54 nm, which corresponds to a shell with a thickness of two monolayers. As shown in Figure 6, the PL spectrum had a peak only at 500 nm and no other peaks, and was highly symmetric. As shown in Figure 7 (the second point from the left), the quantum yield (PLQY) was 37%, and the PL FWHM (full width at half maximum) of the PL peak was 35 nm. The FWHM of the first xenon peak in the optical absorption spectrum was 43 nm.

[0043] Example 3 In Example 3, Sample C, which is a quantum dot with a coherent core / shell structure that gives the maximum PL value, was fabricated and its characteristics were evaluated. First, sample A was prepared as an InP core. The ZnS shell was prepared by the following steps. InP core (2 mL), ODE (2 mL), DDT (124 μL), and Zn-OA (1 mL) were mixed and held at room temperature for 30 minutes at a vacuum of 60 Pa or less. The mixture was then returned to argon atmospheric pressure, heated to 230 °C, and held at that temperature for 20 minutes. DDT (124 μL) and Zn-OA (1 mL) were then added, and the mixture was heated to 240 °C and held at that temperature for 20 minutes to produce quantum dots (sample C) consisting of three monomolecular shells.

[0044] The lattice constant calculated from XRD was 0.56 nm. From HRTEM observation, the thickness of the ZnS shell was estimated to be 0.81 nm. The interplanar spacing of the (111) plane measured by HRTEM was 0.32 nm, and from this interplanar spacing the lattice constant was estimated to be 0.56 nm, which agreed with the value calculated from XRD. As shown in Figure 7 (the third point from the left), the PL internal quantum yield (PLQY) was 60%. The PL spectrum was symmetric, with a peak at 500 nm. The full width at half maximum (FWHM) of the PL peak was 35 nm, and the first exciton peak in the optical absorption spectrum had a FWHM of 44 nm and a valley depth of 0.51 (Figure 19). From the above, the InP / ZnS quantum dots with a coherent core / shell structure consisting of three monomolecular shells exhibited a high PL quantum yield (PLQY) and a narrow PL FWHM.

[0045] Example 4 In Example 4, Sample D, which is a quantum dot having a coherent core / shell structure with varying core sizes, was fabricated by the following steps, and its properties were evaluated. The InP core was fabricated by the following process. First, indium acetate (0.15 mmol), zinc acetate (0.075 mmol), palmitic acid (0.6 mmol), and ODE (6.3 mL) were mixed and purged with argon gas. The mixture was then heated to 140 °C under a vacuum of 60 Pa or less, and 0.12 mmol of tris(trimethylsilyl)phosphite dissolved in trioctylphosphine (1 mL) was added. The mixture was then heated to 40 °C under a vacuum of 60 Pa or less and held at that temperature for 10 min. After returning to room temperature, the mixture was returned to atmospheric argon pressure and heated to 300 °C, where it was held for 15 min to produce InP cores. The PL peak of this InP core was observed at a wavelength of 535 nm. A ZnS shell was formed on this InP core under the same fabrication conditions as for sample C, and quantum dots with a coherent core / shell structure (sample D) were fabricated.

[0046] As a result, after the shell formation, the diffraction line shifted to the wide-angle side (FIG. 11), similar to Example 3. The lattice constant calculated from the XRD pattern was 0.56 nm. The quantum yield (PLQY) was 62%, the full width at half maximum (PL-FWHM) was 40 nm, and the PL spectrum was symmetrical, with a peak at 525 nm (Figure 12). The wavelength of 525 nm is located near the apex of the green region of the chromaticity diagram shown in Figure 13. Therefore, the use of these quantum dots as phosphors, dyes, or pigments can expand the range of colors they can cover. Furthermore, these quantum dots were shown to have a high quantum yield (PLQY) of 62% and a narrow full width at half maximum (PL-FWHM).

[0047] Example 5 In Example 5, quantum dots with a coherent InP / GaP / ZnS (core / shell / shell) structure were fabricated by the following steps, and their properties were evaluated. The InP core was obtained in the same manner as in Example 1. The GaP / ZnS shell was formed by mixing an InP core (2 mL), ODE (2 mL), and GaCl (3 mg) and maintaining the mixture at room temperature under a vacuum of 60 Pa for 30 minutes. The mixture was then heated to argon atmosphere and heated to 200 °C. At this point, DDT (124 μL) and Zn-OA (1 mL) were added. The mixture was then heated to 230 °C and maintained for 20 minutes. Then, DDT (124 μL) and Zn-OA (1 mL) were added. The mixture was then heated to 240 °C and maintained for 20 minutes. The resulting quantum dots are triple-shelled, consisting of one GaP shell and two ZnS shells. The GaP shell is 0.27 nm thick, and the ZnS shell is 0.54 nm thick.

[0048] The XRD pattern is shown in FIG. Compared with the XRD of the InP core, the diffraction lines were shifted to the wide-angle side by the GaP / ZnS shell. The lattice constant was calculated to be 0.56 nm from the diffraction lines, indicating a coherent core / shell / shell structure. Since the lattice constant of GaP crystals, 0.545 nm, is almost equal to the lattice constant of ZnS crystals, 0.542 nm, it is thought that the lattice constant was 0.56 nm, the same as in the case of a coherent core / shell structure consisting of InP / ZnS. The InP / GaP / ZnS core / shell / shell particles exhibited a high PLQY of 82%, a narrow PL FWHM of 38 nm, a symmetric PL spectrum with a peak at 513 nm, and a first exciton peak FWHM of 44 nm in the optical absorption spectrum (Fig. 15).

[0049] The PL peak wavelength of this quantum dot, 513 nm, is located near the apex of the green region of the chromaticity diagram shown in Figure 13. Therefore, using this quantum dot (coherent InP / GaP / ZnS (core / shell / shell)) as a phosphor, dye, or pigment can expand the range of colors that can be covered. Furthermore, it was shown that this quantum dot has an extremely high quantum yield (PLQY) and a narrow full width at half maximum (PL-FWHM).

[0050] (Comparative Example 1) In Comparative Example 1, the following sample was produced to examine the effect of the degree of vacuum during synthesis of the InP core. The InP core was fabricated by the following process. First, indium acetate (0.15 mmol), zinc acetate (0.075 mmol), palmitic acid (0.6 mmol), and ODE (6.3 mL) were mixed and purged with argon gas. The mixture was then heated to 140 °C under a vacuum of 120-180 Pa and held at that temperature for 12 h. It was then cooled to room temperature and argon atmosphere was released. Subsequently, 0.12 mmol of tris(trimethylsilyl)phosphite dissolved in trioctylphosphine (1 mL) was added, and the mixture was heated to 40 °C under a vacuum of 120-180 Pa and held at that temperature for 10 min to degas. The mixture was then returned to room temperature, argon atmosphere was released, and the mixture was heated to 300 °C and held at that temperature for 10 min to produce InP cores. As shown in Fig. 8, the steepness of the first exciton peak in the optical absorption spectrum of the InP core was lower than that of sample A, and the full width at half maximum (FWHM) of the peak was only 55 nm. A vacuum of 120-180 Pa is insufficient, and it is preferable to keep it at 60 Pa or less, as shown in the examples.

[0051] (Comparative Example 2) In Comparative Example 2, the following sample was fabricated to examine the effect of the degassing time during synthesis of the InP core. The InP core was fabricated by the following process. The InP core was prepared by mixing indium acetate (0.15 mmol), zinc acetate (0.075 mmol), palmitic acid (0.5 mmol), and ODE (6.3 mL) and purging with argon gas. The mixture was then heated to 140°C under a vacuum of 120-180 Pa, held at that temperature for 2 hours, and then cooled to room temperature and argon atmospheric pressure. Subsequently, trioctylphosphine (1 mL) containing 0.12 mmol of tris(trimethylsilyl)phosphite was added, and the mixture was heated to 40°C under a vacuum of 120-180 Pa and held for 10 minutes to degas. After returning to room temperature, the mixture was argon atmospheric pressure, heated to 300°C, and held at that temperature for 10 minutes to obtain an InP core. Therefore, Comparative Example 2 is identical to Comparative Example 1, except that the heat treatment time at 140°C after mixing the materials was shortened from 12 hours to 2 hours. As a result, the first exciton peak FWHM of the optical absorption spectrum of Comparative Example 2 was broadened to 67 nm (FIG. 16). It was confirmed that the characteristics deteriorated when the degassing time was short.

[0052] (Comparative Example 3) In Comparative Example 3, the following sample was prepared to examine the influence of evacuation when adding a phosphorus source during synthesis of an InP core. The InP core fabrication process was the same as in Comparative Example 1, except that the degree of vacuum after mixing the materials and purging with argon gas was changed from 120-180 Pa to 60 Pa as shown in the examples, and that degassing treatment was not performed after adding trioctylphosphine (1 mL). That is, the InP core of Comparative Example 3 was fabricated by the following process. First, indium acetate (0.15 mmol), zinc acetate (0.075 mmol), palmitic acid (0.6 mmol), ODE (6.3 mL), and 1-octadecene (ODE, 6.3 mL) were mixed and purged with argon gas. Next, the mixture was heated to 140°C under a vacuum of 60 Pa or less, maintained at that temperature for 12 hours, and then cooled to room temperature and argon atmospheric pressure was established. Next, trioctylphosphine (1 mL) in which 0.12 mmol of tris(trimethylsilyl) phosphite had been dissolved was added, and the mixture was heated to 300°C without degassing and maintained for 10 minutes to obtain an InP core. As a result, the first exciton peak FWHM of the optical absorption spectrum of Comparative Example 3 was 55 nm (FIG. 9).

[0053] <Conclusions from Example 2, Comparative Example 1, Comparative Example 2, and Comparative Example 3> Degassing is necessary in all processes when new chemicals are added. Furthermore, by performing degassing at a vacuum level of 60 Pa or less, a narrow first exciton peak in the optical absorption spectrum can be obtained. In addition, when degassing is performed at 140° C. for 12 hours or more, the first exciton peak in the optical absorption spectrum becomes narrower. As described above, by sufficiently removing impurities including acetic acid through degassing, it is possible to reduce the FWHM of the first exciton peak in the optical absorption spectrum.

[0054] Comparative Example 4 In Comparative Example 4, the following sample was prepared to examine the effect of the molecular chain length of the linear saturated fatty acid. The InP core was prepared by adding trioctylphosphine (1 mL) containing 0.12 mmol of tris(trimethylsilyl)phosphite to indium acetate (0.15 mmol), zinc acetate (0.075 mmol), lauric acid (0.6 mmol), and 1-octadecene (6.3 mL), and heating the mixture at 300°C for 10 minutes in an argon gas atmosphere. The ZnS shell was formed in the same manner as in Example 3.

[0055] The lattice constant calculated from XRD was 0.56 nm. The quantum yield (PLQY) was 50%, and the PL spectrum was symmetric, with a peak at 510 nm. The full width at half maximum (PL-FWHM) was 39 nm. Although coherent core / shell structures could be fabricated by varying the hydrocarbon chain length in the range of C12-C16, palmitic acid provided the best optical properties.

[0056] (Comparative Example 5) In Comparative Example 5, the following sample E was produced to examine the effect of increasing the thickness of the shell layer to four layers in an InP / ZnS structure. The InP core was fabricated in the same manner as in Example 1. The ZnS shell was formed by the following method. First, InP cores (2 mL), ODE (2 mL), DDT (124 μL), and Zn-OA (1 mL) were mixed and held at 230 °C for 20 min in an argon gas atmosphere. Next, DDT (124 μL) and Zn-OA (1 mL) were added, and the mixture was heated to 240 °C and held at that temperature for 20 min. Next, DDT (124 μL) and Zn-OA (1 mL) were added, and the mixture was heated to 250 °C and held at that temperature for 20 min to form a ZnS shell.

[0057] The lattice constant calculated from XRD was 0.55 nm, and the thickness of the ZnS shell measured by high-resolution transmission electron microscopy (HRTEM) was 1.08 nm. The PL quantum yield (PLQY) was 30%, and the PL peak was observed at 510 nm. The full width at half maximum (FWHM) of the PL was 42 nm, and the FWHM of the first exciton peak in the optical absorption spectrum was 48 nm.

[0058] (Comparative Example 6) In Comparative Example 6, Sample F below was produced, and the influence of increasing the thickness of the shell layer by one layer to five layers in an InP / ZnS structure compared to Comparative Example 5 was investigated. The InP core was fabricated in the same manner as in Example 1. The ZnS shell was formed by the following method. First, InP core (2 mL), ODE (2 mL), DDT (124 μL), and Zn-OA (1 mL) were mixed and maintained at 230 °C for 20 min in an argon gas atmosphere. Then, DDT (124 μL) and Zn-OA (1 mL) were added, and the mixture was heated to 240 °C and maintained at that temperature for 20 min. Then, DDT (124 μL) and Zn-OA (1 mL) were added, and the mixture was heated to 250 °C and maintained at that temperature for 20 min. Then, DDT (124 μL) and Zn-OA (1 mL) were added, and the mixture was heated to 260 °C and maintained at that temperature for 20 min. Then, DDT (124 μL) and Zn-OA (1 mL) were added, and the mixture was heated to 270 °C and maintained at that temperature for 20 min. Subsequently, DDT (124 μL) and Zn-OA (1 mL) were added, and the temperature was raised to 280° C. and maintained at the same temperature for 20 minutes to form a ZnS shell.

[0059] The lattice constant calculated from XRD was 0.54 nm, and the thickness of the ZnS shell measured by HRTEM was 1.35 nm. The PL quantum yield (PLQY) was 25%, and the PL peak was observed at 510 nm. The full width at half maximum (FWHM) of the PL peak was 45 nm, and the FWHM of the first ZnS peak in the optical absorption spectrum was 50 nm.

[0060] (Comparative Example 7) In Comparative Example 7, Sample G below was produced, and the influence of increasing the shell layer thickness by three layers to eight layers in an InP / ZnS structure compared to Comparative Example 6 was investigated. The InP core was fabricated in the same manner as in Example 1. The ZnS shell was formed by the following method: Figure 18 shows the Zn-OA addition process. First, InP core (2 mL), ODE (2 mL), DDT (124 μL), and Zn-OA (1 mL) were mixed and held at 230 °C for 20 minutes in an argon gas atmosphere. Subsequently, DDT (124 μL) and Zn-OA (1 mL) were added, the temperature was raised to 240 °C, and it was held at this temperature for 20 minutes. Subsequently, DDT (124 μL) and Zn-OA (1 mL) were added, the temperature was raised to 250 °C, and it was held at this temperature for 20 minutes. Subsequently, DDT (124 μL) and Zn-OA (1 mL) were added, the temperature was raised to 260 °C, and it was held at this temperature for 20 minutes. Subsequently, DDT (124 μL) and Zn-OA (1 mL) were added, the temperature was raised to 270 °C, and it was held at this temperature for 20 minutes. Subsequently, DDT (124 μL) and Zn-OA (1 mL) were added, the temperature was raised to 280 °C, and it was held at this temperature for 20 minutes. Subsequently, DDT (124 μL) and Zn-OA (1 mL) were added, the temperature was raised to 290 °C, and it was held at this temperature for 20 minutes. Subsequently, DDT (124 μL) and Zn-OA (1 mL) were added, the temperature was raised to 300 °C, and it was held at this temperature for 20 minutes to form a ZnS shell.

[0061] The lattice constant calculated from XRD was 0.54 nm, and the film thickness of the ZnS shell measured by HRTEM was 2.16 nm. The interplanar spacing of the (111) plane measured by HRTEM was 0.31 nm, which was consistent with the interplanar spacing of the ZnS (111) plane (Figure 17), and the lattice constant calculated from this value was consistent with the value calculated from XRD. The quantum yield PLQY was 19%, and the PL peak was observed at 510 nm. The full width at half maximum PL-FWHM was 50 nm, and the FWHM of the first exciton peak of the optical absorption spectrum was 58 nm.

[0062] <ZnS shell thickness dependence> The relationship between the thickness of the ZnS shell and the quantum yield and PL full width at half maximum is shown in Figure 7. This figure arranges the data from samples A to G in order from left to right. From this figure, it can be seen that a high quantum yield and a narrow PL full width at half maximum can be obtained when the thickness of ZnS is 1.08 nm or less, which corresponds to 3 monolayers, and particularly the highest quantum yield and the narrowest PL full width at half maximum can be obtained when the thickness of ZnS is 1.08 nm.

Industrial applicability

[0063] The present invention provides quantum dots and phosphors that have high quantum yields and narrow half-widths in the wavelength range of 500 nm to 540 nm. The wavelength of this phosphor is at the green apex of the chromaticity diagram, making it highly valuable as a green phosphor for color displays. Furthermore, the quantum dots of the present invention can be used in highly sensitive and highly visible green visualization dyes suitable for detecting cancer and other conditions, as well as in green pigment cosmetics that develop color under sunlight. For this reason, we believe that this will greatly contribute to the development of the industry. [Explanation of symbols]

[0064] 1: Quantum dots 11: Core (InP) 12: Shell (ZnS)

Claims

1. It consists of a core part and a shell part that covers the core part, a surface of the core portion contacting an inner surface of the shell portion; the core portion is made of a single crystal of indium phosphide (InP), the shell portion is made of a single crystal of zinc sulfide (ZnS), A quantum dot wherein the shell portion has a thickness of three monolayers.

2. The quantum dot according to claim 1 , wherein the core portion is spherical and has a diameter of 0.58 nm or more and 2.95 nm or less.

3. 3. The quantum dot according to claim 1, wherein the core and shell have the same crystal lattice constant.

4. The quantum dot of claim 3 , wherein the lattice constant is 0.56 nm.

5. The quantum dot according to claim 1 , wherein the shell has a thickness of 1.08 nm.

6. preparing a first mixed solution by mixing, under an inert gas, one of indium acetate, indium chloride, indium bromide, and indium iodide, one or more selected from the group consisting of zinc acetate, zinc chloride, zinc bromide, and zinc iodide, one or more selected from the group consisting of palmitic acid, lauric acid, myristic acid, stearic acid, and oleic acid, trioctylphosphine, and one or more selected from the group consisting of 1-octadecene (ODE), dodecane, tetradecane, hexadecane, octadecane, dodecene, tetradecene, and hexadecene; subjecting the first mixed liquid to a first heat treatment under vacuum; adding a first solution in which tris(trimethylsilyl) phosphite is dissolved in trioctylphosphine to the first mixed solution in an inert gas environment after the first heat treatment; After the addition of the first solution, subjecting the first mixed solution to a second heat treatment under vacuum; After the second heat treatment, a third heat treatment is performed on the first mixture in an inert gas atmosphere to produce an InP core; mixing the InP core, ODE, dodecanethiol (DDT) or sulfur powder, and zinc oleate (Zn-OA) or zinc stearate (Zn-SA) and holding under vacuum to prepare a second mixture; subjecting the second mixture to a fourth heat treatment under an inert gas; after the fourth heat treatment, adding DDT and Zn—OA to the second mixture, and performing a fifth heat treatment under an inert gas.

7. The method for producing quantum dots according to claim 6 , wherein the inert gas is argon gas.

8. 8. The method for producing quantum dots according to claim 6, wherein the temperature of the first heat treatment is 90° C. or more and 140° C. or less.

9. 9. The method for producing quantum dots according to claim 6, wherein the temperature of the second heat treatment is 20°C or more and 60°C or less.

10. 10. The method for producing quantum dots according to claim 6, wherein the temperature of the third heat treatment is 210°C or more and 320°C or less.

11. 11. The method for producing quantum dots according to claim 6, wherein the temperature of the fourth heat treatment is 210°C or more and 320°C or less.

12. 12. The method for producing quantum dots according to claim 6, wherein the temperature of the fifth heat treatment is 230°C or higher and 320°C or lower.

13. A green phosphor comprising the quantum dots according to any one of claims 1 to 5.

14. A green visualization dye comprising the quantum dots of any one of claims 1 to 5.

15. A green pigment comprising the quantum dots according to any one of claims 1 to 5.

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