Quantum dot, wave length conversion member, back light unit, image display device, and production method of quantum dot

Abstract

To provide a quantum dot improved in light stability, and a production method of the same.SOLUTION: A core-shell type quantum dot includes a group I-III-VI semiconductor nanocrystal core, and a single or a plurality of semiconductor nanocrystal shells covering the group I-III-VI semiconductor nanocrystal core, wherein an excited electron level of the core shell type quantum dot satisfying a restriction condition estimated by an effective mass approximation method is one between a bottom edge of a conduction band of the group I-III-VI semiconductor nanocrystal core and a bottom edge of a conduction band of the semiconductor nanocrystal shell.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a quantum dot, a wavelength conversion member, a backlight unit, an image display device, and a method for manufacturing quantum dots. [Background technology]

[0002] In single-crystal semiconductor nanoparticles, when the crystal size is less than the Bohr radius of the exciton, a strong quantum confinement effect occurs, resulting in discrete energy levels. The energy levels depend on the crystal size, making it possible to tune the light absorption and emission wavelengths by adjusting the crystal size. Furthermore, the quantum confinement effect makes light emission from single-crystal semiconductor nanoparticles highly efficient. Since the light emission is essentially an emission line, achieving a uniform particle size distribution would enable high-brightness, narrow-band emission. This phenomenon, resulting from the strong quantum confinement effect in nanoparticles, is called the quantum size effect, and semiconductor nanocrystals utilizing this property are being investigated as quantum dots for a wide range of applications. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-152904 [Patent Document 2] International Publication No. 2022 / 138905 [Non-patent literature]

[0004] [Non-Patent Document 1] Nozik et al., Highly efficient band-edge emission from InP quantum dots, Appl. Phys. Lett., 68, 3150(1996) [Non-patent document 2] JP Park, J.-J. Lee, S.-W. Kim, Highly luminescent InP / GaP / ZnS QDs emitting in the entire color range via a heating up process, Sci. Rep. 6: 30094(2016) [Non-patent document 3] Yang Li, Xiaoqi Hou, Xingliang Dai, Zhenlei Yao, Liulin Lv, Yizheng Jin, and Xiaogang Peng, Stoichiometry-controlled InP-based quantum dots: synthesis, photoluminescence, and electroluminescence, J.Am.Chem.Soc., 2019,141,6448-6452 [Non-patent document 4] Watcharaporn Hoisang, Taro Uematsu, Tsukasa Torimoto, and Susumu Kuwabata, Photoluminescence Stability Enhancement of AgInGaS / GaSx Core / Shell Quantum Dots with Thicker Shells by THE Addition of Gallium Diethyldithiocarbamate, Chem. Lett., 2021, 50, 1863-1866 | doi:10.1246 / cl.210363 Summary of the Invention [Problem to be solved by the invention]

[0005] One application of quantum dots is being considered for use as phosphor materials for displays. If narrow-band, highly efficient emission can be achieved, it will be possible to express colors that could not be reproduced with existing technology, and so quantum dots are attracting attention as next-generation display materials. CdSe has been considered as the quantum dot with the best luminescence properties, but its high toxicity limits its use, and it has been necessary to find Cd-free materials.

[0006] Therefore, quantum dots with InP cores have attracted attention. Three years after the MIT group reported on CdSe, visible light emission was confirmed in 1996 (Non-Patent Document 1). Since then, it has become clear that quantum size effects can cover the RGB spectrum (red: λ = 630 nm, 1.97 eV; green: λ = 532 nm; blue: λ = 465 nm), and this has led to intensive research. However, InP has been found to have inferior optical properties compared to CdSe. One of the challenges is improving the quantum efficiency of InP quantum dots. Quantum dots are essentially nanosized semiconductor crystalline particles. The surface is highly active, while the core, with its small band gap, is highly reactive. Therefore, cores such as CdSe or InP alone are prone to defects such as dangling bonds on the crystal surface. Therefore, core-shell semiconductor crystalline particles have been produced, in which the shell is a semiconductor nanocrystal with a larger band gap than the core and a small lattice mismatch. CdSe-based quantum dots have achieved quantum yields approaching 100%. On the other hand, although InP can be similarly improved by covering it with a shell, the quantum yield remains at 60% to 80%, and further improvement in quantum yield is desired. Also, while CdSe-based quantum dots have an emission full width at half maximum (FWHM) of less than 30 nm, achieving the sharp emission characteristics required for display applications, InP has a large FWHM of 35 nm or more, and improvements in quantum yield as well as FWHM are desired.

[0007] The reason for the large FWHM is that the band gap of InP changes more significantly with particle size than that of CdSe, and even if the particle size distribution is similar to that of CdSe, the FWHM becomes wider. This is because, as is clear from equation (1) below, which represents the quantum size effect, the band gap of InP, which has a small effective mass, changes more significantly with particle size than that of CdSe.

[0008] Therefore, there is a demand for materials with a large effective mass that can emit green and red light due to the quantum size effect. Chalcopyrite-type semiconductor nanoparticles are promising quantum dots. The chalcopyrite structure is a tetragonal crystal with two zincblende-type crystal structures, which are found in II-VI and III-V semiconductors such as ZnS and InP, arranged side by side. By creating mixed crystals with various compositions and particle sizes, it is possible to create crystals with a wide range of band gap emission wavelengths, from 0.26 to 3.5 eV. However, until now, quantum dots with a chalcopyrite structure have mainly emitted broad light due to defect emission, and none have exhibited band-edge emission.

[0009] Recently, as described in Patent Document 1, quantum dots with a chalcopyrite structure that exhibit band-edge emission have been obtained by using amorphous GaS as a shell for AgInGaS2 quantum dots. Based on this report, quantum dots exhibiting band-edge emission, such as AgGaSe2 and AgGaInS2, which also exhibit high quantum yields, have been developed, and are promising candidates for Cd- and Pb-free quantum dots, as described in Patent Document 2. However, as described in Non-Patent Document 4, chalcopyrite-type quantum dots with thin shell thicknesses have been shown to be less stable and to gradually decrease in quantum yield in air. Meanwhile, photostability is an important issue to consider when considering quantum dot applications, yet there have been few reports on the photostability of chalcopyrite-type quantum dots. In fact, studies of chalcopyrite-type quantum dots have revealed that they have low photostability, which is a problem.

[0010] As described above, quantum dots using chalcopyrite-type semiconductor nanoparticles as cores have the problem of low photostability. The reason for this low photostability is thought to be that the number of bound levels of excited electrons is large, and the existence of multiple bound states during relaxation after excitation reduces stability. Therefore, a method for controlling the composition and particle size of nanoparticles is important. The present invention has been made in consideration of the above-mentioned problems, and aims to provide quantum dots with improved photostability and a method for producing the same. [Means for solving the problem]

[0011] The present invention has been made to achieve the above-mentioned object, and provides a core-shell quantum dot comprising a Group I-III-VI semiconductor nanocrystal core and one or more semiconductor nanocrystal shells covering the Group I-III-VI semiconductor nanocrystal core, wherein the excited electron level in the core-shell quantum dot, which satisfies constraints estimated by the effective mass approximation method, is located between the lower end of the conduction band of the Group I-III-VI semiconductor nanocrystal core and the lower end of the conduction band of the semiconductor nanocrystal shell.

[0012] Such quantum dots have improved photostability.

[0013] At this time, the I-III-VI group semiconductor nanocrystal core is CuIn x Ga 1-x Se2, CuIn x Ga 1-x S2, AgIn x Ga 1-x Se2, AgIn x Ga 1-x S2 (where 0≦X≦1) or quantum dots that are mixed crystals thereof.

[0014] Such semiconductor nanocrystal cores are preferred because the band gap can be easily adjusted and they are capable of emitting visible light.

[0015] In this case, the semiconductor nanocrystal shell is CuIn having an energy gap larger than the energy gap of the I-III-VI group semiconductor nanocrystal core. Y Ga 1-Y Se2, CuIn Y Ga 1-Y S2, AgIn Y Ga 1-Y Se2, AgIn Y Ga 1-Y The quantum dots may be those containing one or more layers of semiconductor nanocrystals selected from S2, ZnSe, and ZnS (where 0≦y≦1) or mixed crystals thereof.

[0016] Such semiconductor nanocrystal shells are preferred because they allow for easy band gap tuning and are capable of emitting visible light.

[0017] In this case, the wavelength conversion member can be one in which the quantum dots are dispersed.

[0018] This results in improved light stability.

[0019] At this time, a backlight unit including the wavelength conversion member and an excitation light source, The backlight unit may be such that the excited electron level in the core-shell quantum dots is within a range of excitation energy of light from the excitation light source, based on the top of the valence band of the I-III-VI group semiconductor nanocrystal core.

[0020] This results in improved light stability.

[0021] In this case, an image display device can be provided with the backlight unit.

[0022] This results in improved light stability.

[0023] The present invention also provides a method for producing core-shell quantum dots, comprising: a core synthesis step of synthesizing a III-VI semiconductor nanocrystal core in a solution, and then adding a solution in which a Group I precursor is dissolved to the solution in which the III-VI semiconductor nanocrystal core is dispersed, to produce a I-III-VI semiconductor nanocrystal core; and a shell synthesis step of forming a semiconductor nanocrystal shell on the surface of the I-III-VI semiconductor nanocrystal core, wherein in the core synthesis step, the particle size of the I-III-VI semiconductor nanocrystal core is adjusted so that the excited electron level in the core-shell quantum dot, which satisfies constraints estimated by effective mass approximation, is located between the lower end of the conduction band of the I-III-VI semiconductor nanocrystal core and the lower end of the conduction band of the semiconductor nanocrystal shell.

[0024] According to this method for producing quantum dots, quantum dots with improved photostability can be produced. [Effects of the Invention]

[0025] As described above, the quantum dots of the present invention have improved photostability. The quantum dot manufacturing method of the present invention makes it possible to manufacture quantum dots with improved photostability. [Brief explanation of the drawings]

[0026] [Figure 1] 1 shows an example of a quantum dot according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention will be described in detail below, but the present invention is not limited thereto.

[0028] As discussed above, there is a need for quantum dots with improved photostability and methods for producing the same.

[0029] As a result of extensive research into the above-mentioned problems, the present inventors have found that core-shell quantum dots comprising a Group I-III-VI semiconductor nanocrystal core and one or more semiconductor nanocrystal shells covering the Group I-III-VI semiconductor nanocrystal core, wherein the excited electron level in the core-shell quantum dots that satisfy constraints estimated by the effective mass approximation method is between the lower end of the conduction band of the Group I-III-VI semiconductor nanocrystal core and the lower end of the conduction band of the semiconductor nanocrystal shell, can improve photostability, and have completed the present invention.

[0030] The present inventors have also discovered that quantum dots with improved photostability can be produced by a method for producing core-shell quantum dots, the method comprising: a core-synthesis step of synthesizing a Group III-VI semiconductor nanocrystal core in a solution; and then adding a solution in which a Group I precursor is dissolved to the solution in which the Group III-VI semiconductor nanocrystal core is dispersed, to produce a Group I-III-VI semiconductor nanocrystal core; and a shell-synthesis step of forming a semiconductor nanocrystal shell on the surface of the Group I-III-VI semiconductor nanocrystal core, the particle size of the Group I-III-VI semiconductor nanocrystal core being adjusted in the core-synthesis step so that the excited electron level in the core-shell quantum dot, which satisfies a constraint estimated by effective mass approximation, is located between the lower end of the conduction band of the Group I-III-VI semiconductor nanocrystal core and the lower end of the conduction band of the semiconductor nanocrystal shell. This discovery led to the completion of the present invention.

[0031] The following description will be made with reference to the drawings.

[0032] [Quantum dots] First, the core-shell quantum dot according to the present invention will be described. As shown in Fig. 1, the core-shell quantum dot 10 according to the present invention is a core-shell quantum dot comprising a Group I-III-VI semiconductor nanocrystal core 1 and a single or multiple semiconductor nanocrystal shells 2 (2A, 2B) covering the Group I-III-VI semiconductor nanocrystal core. Fig. 1 shows an example of a core-shell quantum dot comprising multiple semiconductor nanocrystal shells.

[0033] Furthermore, the core-shell quantum dot 10 is characterized in that the excited electron level that satisfies the constraint conditions estimated by the effective mass approximation method is one between the lower end of the conduction band of the Group I-III-VI semiconductor nanocrystal core 1 and the lower end of the conduction band of the semiconductor nanocrystal shell 2.

[0034] As mentioned above, we have conducted extensive research into the challenge of improving the photostability of quantum dots containing core semiconductor nanocrystals made of chalcopyrite-type semiconductors, group I-III-VI elements. As a result, we have discovered that if the structure is such that there is only one bound level that satisfies the boundary conditions estimated by the effective mass approximation (EMA) method, it is possible to effectively suppress exciton leakage, improving photostability and, as a result, providing quantum dots that do not contain highly toxic metals such as Cd and Pb.

[0035] First, we will explain the estimation of the constraint conditions using the effective mass approximation. In the effective mass approximation, electrons and holes are confined in a spherical quantum dot in the confinement potential of the shell material, and the effective mass m e , m h This method involves predicting the behavior of excitons, which are quasi-particles with a specific molecular weight, using the Schrödinger equation.

[0036] The three-dimensional Schrödinger equation for a particle of mass m in a potential V(r) can be expressed as a time-independent equation as follows:

number

[0037] Here, the confining potential of V(r) is core 0 inside the quantum dot and V in the shell shell Assuming this, it is expressed as follows: V(r) = 0 (r < r core ) V(r) = V shell (r > r core )

[0038] Under this boundary condition, the above equation (1) can be converted to spherically symmetric polar coordinates as follows:

number

number

[0039] where k core , k shell is the effective mass of the electron and hole m * Using this, we get the following:

number

[0040] The above equations (2) and (3) are the spherical Bessel function of the first kind J0 and the Hankel function of the first kind or the second kind h0 (1) Since it has a solution of the form

number

number

[0041] In addition, the following boundary conditions are satisfied to ensure that the joints are continuous and smoothly connected.

number

number

[0042] From equations (4), (5), (6), and (7), the following equation (8) is derived.

number

[0043] Numerical analysis of the above equation (8) can sometimes yield multiple solutions. Focusing on excited electrons, if the excited electron level in the core-shell quantum dot is between the lower end of the conduction band of the Group I-III-VI semiconductor nanocrystal core and the lower end of the conduction band of the semiconductor nanocrystal shell, the excited electrons will be in only one bound state before relaxing and emitting light, enabling efficient light emission and improving photostability. On the other hand, if multiple bound states are possible (i.e., multiple excited electron levels in the core-shell quantum dot between the lower end of the conduction band of the Group I-III-VI semiconductor nanocrystal core and the lower end of the conduction band of the semiconductor nanocrystal shell), the electron distribution will increase as the energy increases from the lowest bound energy level, resulting in a wide range of leakage and ineffective electron binding. Therefore, if multiple bound states are possible, photostability will be reduced. The conditions for such multiple bound states are often a relatively large effective mass and a large core particle size.

[0044] (semiconductor nanocrystal core) The semiconductor nanocrystal core made of I-III-VI elements has a larger effective mass than InP, and tends to have a relatively large band gap, so the particle size is also larger and it is easy to take on multiple bound states. However, the large effective mass also leads to a large electron density and a large absorption coefficient, making it useful as a light conversion material. X Ga 1-X Se2, CuIn X Ga 1-X S2, AgIn X Ga 1-X Se2, AgIn X Ga 1-X S2 (where 0≦X≦1) or a mixed crystal thereof, it is useful and preferable because the band gap can be easily adjusted by changing the particle size and visible light can be emitted. Hereinafter, the "semiconductor nanocrystal core made of Group I-III-VI elements" may be simply referred to as the "core."

[0045] (semiconductor nanocrystal shell) The material of the single or multiple semiconductor nanocrystal shells that coat the I-III-VI semiconductor nanocrystal core is not particularly limited as long as it satisfies the conditions for the excited electron level in the core-shell quantum dot described above. Y Ga 1-Y Se2, CuIn Y Ga 1-Y S2, AgIn Y Ga 1-Y Se2, AgIn Y Ga 1-Y It is preferable to use a semiconductor nanocrystal shell material containing one or more layers of semiconductor nanocrystals selected from S2, ZnSe, and ZnS (where 0≦y≦1) or mixed crystals thereof. Such semiconductor nanocrystal shells are also preferable because the band gap can be easily adjusted and they are capable of emitting visible light. Hereinafter, the "semiconductor nanocrystal shell" may be simply referred to as the "shell."

[0046] In addition, CuIn as a semiconductor nanocrystal core X Ga 1-X Se2, CuIn X Ga 1-X S2, AgIn X Ga 1-X Se2, AgIn X Ga 1-X S2 (where 0≦X≦1) or a mixed crystal thereof, CuIn having a band gap larger than that of the semiconductor nanocrystal core. Y Ga 1-Y Se2, CuIn Y Ga 1-Y S2, AgIn Y Ga 1-Y Se2, AgIn Y Ga 1-YBy growing crystals of S2 (where 0≦Y≦1) or mixed crystals thereof, the crystal systems between the core and shell become chalcopyrite type, which is preferable because lattice mismatch is unlikely to occur and the lattice mismatch is small. Note that, since chalcopyrite easily reacts with water, oxygen, etc. in the air when it becomes nanoparticles, it is preferable to coat the outermost surface with ZnSe or ZnS, which are stable in the air.

[0047] The formation of the core and shell layers can be confirmed by measuring particle images obtained with a transmission electron microscope (TEM), measuring the increase in particle size, performing elemental analysis using energy dispersive X-ray spectrometry (EDX), and calculating the proportions of elements after the synthesis of the core and shell layers.

[0048] Furthermore, the core-shell quantum dots of the present invention preferably have organic ligands coordinated to their surfaces in order to impart dispersibility and reduce surface defects.

[0049] The ligand preferably contains an aliphatic hydrocarbon from the viewpoint of improving dispersibility in a non-polar solvent. Examples of such ligands include oleic acid, stearic acid, palmitic acid, myristic acid, lauric acid, decanoic acid, octanoic acid, oleylamine, stearyl (octadecyl)amine, dodecyl (lauryl)amine, decylamine, octylamine, octadecanethiol, hexadecanethiol, tetradecanethiol, dodecanethiol, decanethiol, octanethiol, trioctylphosphine, trioctylphosphine oxide, triphenylphosphine, triphenylphosphine oxide, tributylphosphine, and tributylphosphine oxide. These may be used alone or in combination.

[0050] [Wavelength conversion material] Furthermore, there is provided a wavelength converting material in which the quantum dots according to the present invention are dispersed. Examples of wavelength converting materials include, but are not limited to, wavelength conversion films and color filters. It is possible to obtain a wavelength converting material having a target emission wavelength, good color reproducibility, good luminous efficiency, and improved photostability.

[0051] The method for producing the wavelength converting material according to the present invention is not particularly limited and can be appropriately selected depending on the purpose. When producing a wavelength conversion film, the quantum dots according to the present invention can be dispersed in a resin by mixing them with the resin. In this step, the quantum dots dispersed in a solvent can be added to the resin and mixed to disperse them in the resin. Alternatively, the quantum dots can be dispersed in a resin by removing the solvent and adding the powdered quantum dots to the resin and kneading them. Alternatively, there is a method in which monomers or oligomers, which are components of the resin, are polymerized in the presence of the quantum dots. The method for dispersing the quantum dots in a resin is not particularly limited and can be appropriately selected depending on the purpose.

[0052] The solvent for dispersing the quantum dots is not particularly limited as long as it is compatible with the resin used. The resin material is also not particularly limited, and silicone resin, acrylic resin, epoxy resin, urethane resin, etc. can be selected appropriately depending on the desired properties. These resins preferably have high transmittance to increase their efficiency as wavelength conversion materials, and a transmittance of 80% or more is particularly desirable.

[0053] Furthermore, the quantum dots may contain substances other than quantum dots, such as fine particles of silica, zirconia, alumina, titania, etc. as light scatterers, or may contain inorganic or organic phosphors. Examples of inorganic phosphors include YAG, LSN, LYSN, CASN, SCASN, KSF, CSO, β-SIALON, GYAG, LuAG, and SBCA, and examples of organic phosphors include perylene derivatives, anthraquinone derivatives, anthracene derivatives, phthalocyanine derivatives, cyanine derivatives, dioxazine derivatives, benzoxazinone derivatives, coumarin derivatives, quinophthalone derivatives, benzoxazole derivatives, and pyrarizone derivatives.

[0054] Alternatively, a wavelength converting material can be obtained by applying a resin composition in which quantum dots are dispersed to a transparent film such as PET or polyimide, curing the resin to form a resin layer, and laminating the resulting layer. The transparent film can be coated with a spraying method such as spraying or inkjet printing, spin coating, bar coating, doctor blade printing, gravure printing, or offset printing. The thicknesses of the resin layer and the transparent film are not particularly limited and can be selected appropriately depending on the application.

[0055] [Backlight unit] Furthermore, there is provided a backlight unit including the wavelength conversion member according to the present invention and an excitation light source. Such a backlight unit has improved light stability. In this case, it is preferable that the excited electron level of the core-shell quantum dot according to the present invention is within a range of excitation energy of light from the excitation light source, based on the top of the valence band of the I-III-VI group semiconductor nanocrystal core.

[0056] More specifically, it may be a backlight unit in which a wavelength converting material, such as a wavelength converting film, is placed on a light guide panel surface to which an excitation light source, such as a blue LED, is coupled.

[0057] [Image display device] Furthermore, there is provided an image display device including the above backlight unit. Such an image display device has improved light stability.

[0058] [Quantum dot manufacturing method] The method for producing quantum dots will be described in detail below. As described above, the method for producing core-shell quantum dots according to the present invention includes a core synthesis step in which a Group III-VI semiconductor nanocrystal core is synthesized in a solution, and then a solution in which a Group I precursor is dissolved is added to the solution in which the Group III-VI semiconductor nanocrystal core is dispersed to produce a Group I-III-VI semiconductor nanocrystal core; and a shell synthesis step in which a semiconductor nanocrystal shell is formed on the surface of the Group I-III-VI semiconductor nanocrystal core. In the core synthesis step, the particle size of the Group I-III-VI semiconductor nanocrystal core is adjusted so that the excited electron level in the core-shell quantum dot that satisfies the constraint condition estimated by the effective mass approximation method is located between the lower end of the conduction band of the Group I-III-VI semiconductor nanocrystal core and the lower end of the conduction band of the semiconductor nanocrystal shell. This allows the production of quantum dots with improved photostability. The method for estimating the constraint condition using the effective mass approximation method is as described above.

[0059] As mentioned above, CuIn X Ga 1-X Se2, CuIn X Ga 1-X S2, AgIn X Ga 1-X Se2, AgIn X Ga 1-X S2 (where 0≦X≦1) or a mixed crystal thereof, and a CuIn coating the core. Y Ga 1-Y Se2, CuIn Y Ga 1-Y S2, AgIn Y Ga 1-Y Se2, AgIn Y Ga 1-Y Core-shell quantum dots are preferably formed with a shell containing one or more layers of semiconductor nanocrystals selected from S2, ZnSe, and ZnS (where 0≦y≦1) or mixed crystals thereof. The method for producing core-shell quantum dots according to the present invention will be described in more detail below.

[0060] (Core synthesis process) In the core synthesis step, a Group III-VI semiconductor nanocrystal core is synthesized in a solution, and then a solution in which a Group I precursor is dissolved is added to the solution in which the Group III-VI semiconductor nanocrystal core is dispersed to produce a Group I-III-VI semiconductor nanocrystal core. Then, as described above, the particle size of the Group I-III-VI semiconductor nanocrystal core is adjusted so that the excited electron level in the core-shell quantum dot that satisfies the constraints estimated by the effective mass approximation method is located between the lower end of the conduction band of the Group I-III-VI semiconductor nanocrystal core and the lower end of the conduction band of the semiconductor nanocrystal shell.

[0061] For example, Group III-VI semiconductor nanocrystals can be synthesized by adding a Group VI precursor solution to a Group III precursor solution at a high temperature of 150°C to 350°C, and the particle size of the Group III-VI semiconductor nanocrystals can be adjusted by adding the Group III precursor solution and the Group VI precursor solution to a solution containing a high-boiling organic solvent and a ligand such as an organic acid, amine, phosphine, or thiol added to suppress aggregation. Adding a Group I precursor solution to the above-mentioned Group III-VI semiconductor nanocrystals at a high temperature of 150°C to 350°C allows the Group I precursor to diffuse by a cation exchange reaction using the Group III-VI semiconductor nanocrystals as a template, resulting in the synthesis of Group I-III-VI semiconductor nanocrystals (particles).

[0062] Examples of Group III precursors include indium chloride, indium bromide, indium iodide, indium oxide, indium nitrate, indium sulfate, indium acetate, gallium chloride, gallium acetylacetonate, gallium oxide, gallium nitrate, and gallium sulfate.

[0063] Among these, the raw material can be selected according to the reactivity of the Group VI precursor to be reacted. For example, even when using the less reactive Se=TOP, it is advisable to use a halide, etc. Furthermore, it is known that uniformly Group VI-doped III-VI semiconductor nanocrystal cores can be synthesized by treating a Group VI precursor such as an S=TOP solution with lithium borohydride (Super-Hydride) to improve its nucleophilicity, by using a Group VI precursor in which Se and S are dissolved in diphenylphosphine instead of TOP to adjust the reactivity, or by using a Group VI precursor in which Se and S are dissolved in dodecanethiol and then reacting with an organic acid such as gallium acetylacetonate.

[0064] The method for dissolving the Group III precursor in the solvent is not particularly limited, and a preferred method is, for example, dissolving the precursor by heating to a temperature of 100° C. to 180° C. In particular, reducing the pressure during this process is preferred because it allows dissolved oxygen, moisture, and the like to be removed from the dissolved solution.

[0065] The solvent is not particularly limited and may be appropriately selected depending on the synthesis temperature and the solubility of the precursor. For example, aliphatic unsaturated hydrocarbons such as 1-octadecene, 1-hexadecene, and 1-dodecene; aliphatic saturated hydrocarbons such as n-octadecane, n-hexadecane, and n-dodecane; alkyl phosphines such as trioctylphosphine; phosphines or amines having a long-chain alkyl group such as oleylamine, dodecylamine, and hexadecylamine; and the like can be suitably used.

[0066] The method for dissolving the solid Group VI precursor in the solvent is not particularly limited, and a preferred method is, for example, dissolving by heating at a temperature of 100° C. to 180° C. In particular, reducing the pressure at this time is preferred because dissolved oxygen, moisture, etc. can be removed from the dissolved solution.

[0067] The solvent for dissolving the Group VI precursor may be appropriately selected from the viewpoint of controlling the reactivity so as to obtain the desired particle size and particle size distribution. For example, the solvent may be selected from Group VI precursors obtained by dissolving Se or S in a solution of an aliphatic unsaturated hydrocarbon such as 1-octadecene, 1-hexadecene, or 1-dodecene, an aliphatic saturated hydrocarbon such as n-octadecane, n-hexadecane, or n-dodecane, a phosphine such as trioctylphosphine or diphenylphosphine, or an amine having a long-chain alkyl group such as oleylamine, dodecylamine, or hexadecylamine; an alkylthiol such as dodecanethiol; trialkylphosphine sulfide; bistrialkylsilyl sulfide; trialkylphosphine selenium; trialkenylphosphine selenium; bistrialkylsilyl selenium; trialkylphosphine tellurium; trialkenylphosphine tellurium; bistrialkylsilyl tellurium; and the like.

[0068] Examples of Group I precursors include silver chloride, silver bromide, silver iodide, silver oxide, silver nitrate, silver sulfate, silver acetate, copper chloride, copper bromide, copper iodide, copper oxide, copper nitrate, copper sulfate, copper acetate, etc. Among these, a raw material can be selected appropriately to achieve the desired particle size and composition in accordance with the reactivity of the Group III-VI nanoparticles to be reacted.

[0069] The method for dissolving the Group I precursor in the solvent is not particularly limited, and a preferred method is, for example, dissolving by heating at a temperature of 100° C. to 180° C. In particular, reducing the pressure at this time is preferred because dissolved oxygen, moisture, etc. can be removed from the dissolved solution.

[0070] The solvent is not particularly limited and may be appropriately selected depending on the synthesis temperature and the solubility of the precursor. For example, aliphatic unsaturated hydrocarbons such as 1-octadecene, 1-hexadecene, and 1-dodecene; aliphatic saturated hydrocarbons such as n-octadecane, n-hexadecane, and n-dodecane; alkyl phosphines such as trioctylphosphine; phosphines or amines having a long-chain alkyl group such as oleylamine, dodecylamine, and hexadecylamine; and the like can be suitably used.

[0071] Similarly, the synthesis temperature and retention time are not particularly limited, as they can be appropriately adjusted to obtain the desired particle size and particle size distribution. The solvent is not particularly limited and may be appropriately selected depending on the synthesis temperature and the solubility of the precursor. For example, aliphatic unsaturated hydrocarbons such as 1-octadecene, 1-hexadecene, and 1-dodecene, aliphatic saturated hydrocarbons such as n-octadecane, n-hexadecane, and n-dodecane, alkylphosphines such as trioctylphosphine, and phosphines or amines having a long-chain alkyl group such as oleylamine, dodecylamine, and hexadecylamine can be suitably used.

[0072] (Shell layer synthesis process) In the shell synthesis step, a semiconductor nanocrystal shell is formed on the surface of the Group I-III-VI semiconductor nanocrystal core. The structure and composition of the semiconductor nanocrystal shell are not limited as long as the excited electron level in the core-shell quantum dot, which satisfies the constraints estimated by the effective mass approximation method, is between the lower end of the conduction band of the Group I-III-VI semiconductor nanocrystal core and the lower end of the conduction band of the semiconductor nanocrystal shell. The shell layer structure is CuIn Y Ga 1-Y Se2, CuIn Y Ga 1-Y S2, AgIn Y Ga 1-Y Se2, AgIn Y Ga 1-Y It is preferable to use a material containing one or more layers of semiconductor nanocrystals or mixed crystals thereof selected from S2, ZnSe, and ZnS (where 0≦y≦1), and although there are no particular limitations, it is more preferable to use ZnS for the outermost surface from the standpoint of stability.

[0073] Similarly to the core synthesis process, it is desirable to add and dissolve the Group I, III, II, and VI precursors in a solution containing dissolved ligands to prevent aggregation after shell layer synthesis, and similarly to the core synthesis process, the reaction is carried out by adding the Group I precursor solution to the III-VI semiconductor nanocrystals at a high temperature of 150°C or higher and 350°C or lower, whereby the Group I precursor diffuses via a cation exchange reaction using the III-VI semiconductor nanocrystals as a template, thereby forming a Group I-III-VI semiconductor nanocrystal shell. At this time, a Group II precursor solution is added to the reaction solution to carry out a shell layer formation reaction, thereby synthesizing a shell layer in which the Group II precursor is doped into the Group I-III-VI semiconductor nanocrystal shell.

[0074] Examples of Group II precursors include zinc fluoride, zinc chloride, zinc bromide, zinc iodide, zinc acetate, zinc acetylacetonate, zinc oxide, zinc carbonate, zinc carboxylate, dimethylzinc, diethylzinc, zinc nitrate, and zinc sulfate. High reactivity is not required for shell layer synthesis, so zinc carboxylate, zinc acetate, and zinc halide are suitable for use due to their ease of handling and compatibility with the solvent. The method for dissolving the solid Group II precursor material in the solvent is not particularly limited; for example, a method in which the material is dissolved by heating to a temperature of 100°C to 180°C is preferred. In particular, reducing the pressure during this process is preferred because it allows dissolved oxygen and moisture to be removed from the solution.

[0075] Examples of Group VI precursors include sulfur, alkylthiol, trialkylphosphine sulfide, bistrialkylsilyl sulfide, selenium, trialkylphosphine selenium, trialkenylphosphine selenium, and bistrialkylsilyl selenium. Among these, alkylthiols having a long-chain alkyl group, such as dodecanethiol, are preferred as sulfur sources, from the viewpoint of the dispersion stability of the resulting core-shell particles. The method for dissolving the solid Group VI precursor material in a solvent is not particularly limited, and a preferred method is, for example, heating to a temperature of 100°C to 180°C to dissolve the material.

[0076] Furthermore, since the quantum yield is further improved by forming a multi-stage shell structure, a mixed crystal shell layer of ZnSe and ZnS may be synthesized, and then a ZnS layer may be formed last. [Example]

[0077] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples.

[0078] [Example 1] (Core synthesis process) A 100 mL flask was charged with 0.8 mmol of gallium acetylacetonate, 16 mL of oleylamine, and 4 mL of dodecanethiol, and heated to 150 °C to dissolve the gallium precursor. 1.5 mL of a 0.8 M solution of Se in dodecanethiol was added, and the solution turned orange, confirming the synthesis of GaSe nanoparticles. Next, 2 mL of a 0.2 M solution of copper chloride in oleylamine was added, and the temperature was raised to 200 °C and stirred for 20 minutes. The solution turned reddish-brown, confirming the formation of core particles.

[0079] (Shell layer synthesis process) After core synthesis, the solution was cooled to room temperature, and ethanol was added. The solution was then centrifuged at 8000 rpm for 3 minutes to precipitate the quantum dots. The precipitated quantum dots were redispersed in 20 mL of octadecene, then placed in a 100 mL flask and degassed for 20 minutes at room temperature. The flask was then purged with nitrogen and heated to 200 °C.

[0080] Next, 0.4 mL each of a 0.8 M solution of zinc chloride dissolved in trioctylphosphine and a 0.8 M solution of S dissolved in dodecanethiol was added and stirred for 20 minutes. This process was repeated six times, and then the mixture was cooled to room temperature. Ethanol was added to the reaction solution obtained after cooling, and the mixture was separated using a centrifuge to precipitate the quantum dots. The precipitated quantum dots were then redispersed in toluene.

[0081] [Comparative Example 1] (Core synthesis process) A 100 mL flask was charged with 0.8 mmol of gallium acetylacetonate and 2 mL of a 0.2 M solution of copper chloride dissolved in oleylamine, followed by the addition of 16 mL of oleylamine and 4 mL of dodecanethiol. The flask was heated to 150°C to dissolve the gallium precursor. 1.5 mL of a 0.8 M solution of Se dissolved in dodecanethiol was added, and the temperature was raised to 200°C. After stirring for 20 minutes, the solution turned reddish-brown, confirming the formation of core particles.

[0082] (Shell layer synthesis process) Synthesis was carried out in the same manner as in Example 1 to form a shell.

[0083] [Example 2] (Core synthesis process) A 100 mL flask was charged with 0.68 mmol of gallium acetylacetonate, 0.12 mmol of indium acetylacetonate, 16 mL of oleylamine, and 4 mL of dodecanethiol, and heated to 150 °C to dissolve the gallium precursor. 1.5 mL of a 0.8 M solution of Se in dodecanethiol was added, and the solution turned orange, confirming the synthesis of GaInSe nanoparticles. Next, 2 mL of a 0.2 M solution of copper chloride in oleylamine was added, and the temperature was raised to 200 °C and stirred for 20 minutes. The solution turned reddish-brown, confirming the formation of core particles.

[0084] (Shell layer synthesis process) Next, after core synthesis, the solution was cooled to room temperature and 4 mL of a 0.2 M solution of gallium acetylacetonate in oleylamine was added. Then, 2 mL of a 0.2 M solution of copper chloride in oleylamine was added, and the mixture was heated to 150 °C to dissolve the precursor. Next, 1.5 mL of a 0.8 M solution of S in dodecanethiol was added, and the mixture was heated to 200 °C and stirred for 1 hour. After cooling to room temperature, ethanol was added to the resulting reaction solution, which was then centrifuged at 8000 rpm for 3 minutes to precipitate the quantum dots. The precipitated quantum dots were redispersed in 20 mL of octadecene, added to a 100 mL flask, and degassed for 20 minutes at room temperature. After purging with nitrogen, the mixture was heated to 200 °C. Next, 0.4 mL each of a 0.8 M solution of zinc chloride dissolved in trioctylphosphine and a 0.8 M solution of S dissolved in dodecanethiol was added and stirred for 20 minutes. This process was repeated six times, and then the mixture was cooled to room temperature. Ethanol was added to the reaction solution obtained after cooling, and the mixture was separated using a centrifuge to precipitate the quantum dots. The precipitated quantum dots were then redispersed in toluene.

[0085] [Example 3] (Core synthesis process) A 100 mL flask was charged with 0.8 mmol of gallium acetylacetonate, 16 mL of oleylamine, and 4 mL of dodecanethiol, and heated to 150 °C to dissolve the gallium precursor. 1.5 mL of a 0.8 M solution of Se in dodecanethiol was added, and the solution turned orange, confirming the synthesis of GaSe nanoparticles. The reaction solution was heated to 320 °C, and then 2 mL of a 0.2 M solution of silver acetate in oleylamine was added. After heating and stirring for 20 minutes, the solution turned reddish-brown, confirming the formation of core particles.

[0086] (Shell layer synthesis process) Next, after core synthesis, the solution was cooled to room temperature and 4 mL of a 0.2 M solution of gallium acetylacetonate in oleylamine was added. Then, 2 mL of a 0.2 M solution of silver acetate in oleylamine was added, and the mixture was heated to 150 °C to dissolve the precursor. Next, 1.5 mL of a 0.8 M solution of S in dodecanethiol was added, and the mixture was heated to 320 °C and stirred for 1 hour. After cooling to room temperature, ethanol was added to the resulting reaction solution, which was then centrifuged at 8000 rpm for 3 minutes to precipitate the quantum dots. The precipitated quantum dots were redispersed in 20 mL of octadecene, added to a 100 mL flask, and degassed for 20 minutes at room temperature. After purging with nitrogen, the mixture was heated to 200 °C.

[0087] Next, 0.4 mL each of a 0.8 M solution of zinc chloride dissolved in trioctylphosphine and a 0.8 M solution of S dissolved in dodecanethiol was added and stirred for 20 minutes. This process was repeated six times, and then the mixture was cooled to room temperature. Ethanol was added to the reaction solution obtained after cooling, and the mixture was separated using a centrifuge to precipitate the quantum dots. The precipitated quantum dots were then redispersed in toluene.

[0088] Comparative Example 2 (Core synthesis process) A 100 mL flask was charged with 0.8 mmol of gallium acetylacetonate and 2 mL of a 0.2 M solution of silver acetate dissolved in oleylamine, followed by the addition of 16 mL of oleylamine and 4 mL of dodecanethiol. The mixture was heated to 150°C to dissolve the gallium precursor and silver precursor. 1.5 mL of a 0.8 M solution of Se dissolved in dodecanethiol was added, and the temperature was raised to 200°C. After stirring and heating for 20 minutes, the solution turned reddish-brown, confirming the formation of core particles.

[0089] (Shell layer synthesis process) Synthesis was carried out in the same manner as in Example 3 to form a shell.

[0090] [Example 4] (Core synthesis process) A 100 mL flask was charged with 0.64 mmol of gallium acetylacetonate, 0.16 mmol of indium acetylacetonate, 16 mL of oleylamine, and 4 mL of dodecanethiol, and heated to 150 °C to dissolve the gallium precursor. 1.5 mL of a 0.8 M solution of Se in dodecanethiol was added, and the solution turned orange, confirming the synthesis of GaInSe nanoparticles. The temperature was then raised to 320 °C, and 2 mL of a 0.2 M solution of silver acetate in oleylamine was added. After heating and stirring for 20 minutes, the solution turned reddish-brown, confirming the formation of core particles.

[0091] (Shell layer synthesis process) Synthesis was carried out in the same manner as in Example 3 to form a shell.

[0092] [Average particle size measurement] The average particle size was measured by directly observing at least 20 particles using a transmission electron microscope (TEM), calculating the diameter of a circle having the same area as the projected area of ​​the particle, and using the average value of these values ​​as the average particle size. Based on the core particle size calculated in this way, the possible bound states were numerically determined using the effective mass approximation method.

[0093] [Emission wavelength, emission half-width, and emission efficiency measurements] In the examples and comparative examples, the fluorescence emission properties of the quantum dots were evaluated using a quantum efficiency measurement system (QE-2100) manufactured by Otsuka Electronics Co., Ltd., to measure the emission wavelength, fluorescence emission half-width, and fluorescence emission efficiency (internal quantum efficiency) of the quantum dots at an excitation wavelength of 450 nm.

[0094] [Photostability evaluation] In the examples and comparative examples, the photostability of quantum dots was evaluated by adding a solution redispersed in toluene after synthesis to a nitrogen-purged vial, placing it under a blue excitation light source (TH2-51X51BL: manufactured by CCS Co., Ltd.), and adjusting the output with an LED lighting power supply (PSB3-30024: manufactured by CCS Co., Ltd.) to 1000 cd / m 2After irradiating the sample with excitation light for 2 hours, the fluorescence emission efficiency (internal quantum efficiency) was measured again to determine the rate of decrease.

[0095] The measurement results of Examples 1 to 4 and Comparative Examples 1 and 2 are summarized in Table 1 below.

[0096] [Table 1]

[0097] As described above, the primary particle diameter of the core in Examples 1, 2, 3, and 4 was controlled to be smaller than that in the Comparative Example. By preparing Group III-VI nanoparticles and then reacting them with a Group I precursor, the particle diameter could be controlled (suppressed from increasing). On the other hand, in the Comparative Example, the Group VI precursor was reacted in the presence of Group I and III elements, resulting in the highly reactive Group I element increasing the particle diameter. Furthermore, in Examples 1, 2, 3, and 4, the decrease in quantum yield after 2 hours of irradiation with blue excitation light was improved compared to the Comparative Example, demonstrating improved photostability and improved light resistance. Furthermore, when comparing Example 1, Comparative Example 1, Example 3, and Comparative Example 2, each with a similar composition, the quantum yield was improved in the Examples, demonstrating that excitons could be effectively trapped, resulting in improved quantum yield.

[0098] The present specification includes the following aspects. [1]: A core-shell quantum dot comprising a Group I-III-VI semiconductor nanocrystal core and one or more semiconductor nanocrystal shells covering the Group I-III-VI semiconductor nanocrystal core, wherein the excited electron level in the core-shell quantum dot that satisfies the constraints estimated by the effective mass approximation method is one between the lower end of the conduction band of the Group I-III-VI semiconductor nanocrystal core and the lower end of the conduction band of the semiconductor nanocrystal shell. [2]: The I-III-VI group semiconductor nanocrystal core is CuIn x Ga 1-x Se2, CuIn x Ga 1-x S2, AgIn x Ga1-x Se2, AgIn x Ga 1-x The quantum dot of the above [1], which is a semiconductor nanocrystal or a mixed crystal thereof selected from S2 (where 0≦X≦1). [3]: The semiconductor nanocrystal shell has an energy gap larger than that of the I-III-VI group semiconductor nanocrystal core. Y Ga 1-Y Se2, CuIn Y Ga 1-Y S2, AgIn Y Ga 1-Y Se2, AgIn Y Ga 1-Y The quantum dot according to [1] or [2] above, which comprises one or more layers of semiconductor nanocrystals selected from S2, ZnSe, and ZnS (where 0≦y≦1) or mixed crystals thereof. [4]: A wavelength conversion member in which the quantum dots according to [1], [2] or [3] are dispersed. [5]: A backlight unit comprising the wavelength conversion member of [4] above and an excitation light source, wherein the excited electron level in the core-shell quantum dot is one within a range of excitation energy of light from the excitation light source, based on the top of the valence band of the I-III-VI group semiconductor nanocrystal core. [6]: An image display device comprising the backlight unit of [5] above. [7]: A method for producing core-shell quantum dots, comprising: a core synthesis step of synthesizing a III-VI semiconductor nanocrystal core in a solution; and then adding a solution in which a Group I precursor is dissolved to the solution in which the III-VI semiconductor nanocrystal core is dispersed to produce a I-III-VI semiconductor nanocrystal core; and a shell synthesis step of forming a semiconductor nanocrystal shell on the surface of the I-III-VI semiconductor nanocrystal core, wherein in the core synthesis step, the particle size of the I-III-VI semiconductor nanocrystal core is adjusted so that the excited electron level in the core-shell quantum dot, which satisfies a constraint condition estimated by effective mass approximation, is located between the lower end of the conduction band of the I-III-VI semiconductor nanocrystal core and the lower end of the conduction band of the semiconductor nanocrystal shell.

[0099] The present invention is not limited to the above-described embodiments. The above-described embodiments are merely examples, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and that exhibits similar effects is included within the technical scope of the present invention. [Explanation of symbols]

[0100] 1... Group I-III-VI semiconductor nanocrystal core, 2, 2A, 2B...semiconductor nanocrystal shell, 10...quantum dot.

Claims

1. A core-shell quantum dot comprising a Group I-III-VI semiconductor nanocrystal core and one or more semiconductor nanocrystal shells covering the Group I-III-VI semiconductor nanocrystal core, A quantum dot characterized in that the core-shell quantum dot, which satisfies the constraint conditions estimated by the effective mass approximation method, has one excited electron level between the lower end of the conduction band of the I-III-VI group semiconductor nanocrystal core and the lower end of the conduction band of the semiconductor nanocrystal shell.

2. The I-III-VI group semiconductor nanocrystal core is CuIn ｘ Ga １－ｘ Se ２ , CuIn ｘ Ga １－ｘ S ２ , AgIn ｘ Ga １－ｘ Se ２ , AgIn ｘ Ga １－ｘ S ２ 2. The quantum dot according to claim 1, wherein the quantum dot is a semiconductor nanocrystal or a mixed crystal thereof selected from the group consisting of: (where 0≦X≦1).

3. The semiconductor nanocrystal shell is CuIn having an energy gap larger than the energy gap of the I-III-VI semiconductor nanocrystal core. Ｙ Ga １－Ｙ Se ２ , CuIn Ｙ Ga １－Ｙ S ２ , AgIn Ｙ Ga １－Ｙ Se ２ , AgIn Ｙ Ga １－Ｙ S ２ 2. The quantum dot according to claim 1, which comprises one or more layers of semiconductor nanocrystals selected from the group consisting of ZnSe and ZnS (where 0≦y≦1) or mixed crystals thereof.

4. A wavelength conversion member comprising the quantum dots according to any one of claims 1 to 3 dispersed therein.

5. A backlight unit comprising the wavelength conversion member according to claim 4 and an excitation light source, A backlight unit characterized in that the excited electron level in the core-shell quantum dots is one within a range of excitation energy of light from the excitation light source, based on the top of the valence band of the I-III-VI group semiconductor nanocrystal core.

6. An image display device comprising the backlight unit according to claim 5.

7. A method for producing core-shell quantum dots, comprising: a core synthesis step of synthesizing a Group III-VI semiconductor nanocrystal core in a solution, and then adding a solution in which a Group I precursor is dissolved to the solution in which the Group III-VI semiconductor nanocrystal core is dispersed to produce a Group I-III-VI semiconductor nanocrystal core; a shell synthesis step of forming a semiconductor nanocrystal shell on the surface of the Group I-III-VI semiconductor nanocrystal core; a core-shell quantum dot synthesis step for synthesizing quantum dots, the core-shell quantum dots being characterized by adjusting the particle size of the I-III-VI group semiconductor nanocrystal core so that an excited electron level in the core-shell quantum dots that satisfies constraints estimated by effective mass approximation is located between the lower end of the conduction band of the I-III-VI group semiconductor nanocrystal core and the lower end of the conduction band of the semiconductor nanocrystal shell.

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