Quantum dot, wavelength conversion member, backlight unit, image display device, and method for producing quantum dot

Abstract

The present invention provides a core-shell quantum dot comprising a Group I-III-VI semiconductor nanocrystal core and a single or multiple semiconductor nanocrystal shells covering the I-III-VI semiconductor nanocrystal core, wherein there is one excited electron level in the core-shell quantum dot that satisfies a binding condition estimated by an effective-mass approximation method between the bottom of the conduction band of the I-III-VI semiconductor nanocrystal core and the bottom of the conduction band of the semiconductor nanocrystal shell. Consequently, a quantum dot having improved light stability and a method for producing same are provided.

Description

Quantum dots, wavelength conversion member, backlight unit, image display device, and method for manufacturing quantum dots

[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.

[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 emitted 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.

[0003] JP 2020-152904 A International Publication No. 2022 / 138905

[0004] Nozik et al. , Highly efficient band-edge emission from InP quantum dots, Appl. Phys. Lett. , 68, 3150 (1996) J. P. 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) 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 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

[0005] One application of quantum dots is being considered for use as phosphor materials for displays. Realizing narrow-band, highly efficient light emission would enable the expression of 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 light-emitting properties, but its high toxicity limits its use, making it necessary to explore Cd-free materials.

[0006] Therefore, quantum dots with an InP core have attracted attention. Three years after the MIT group reported on CdSe, visible light emission was confirmed in 1996 (Non-Patent Document 1). Subsequently, it became clear that quantum size effects could cover the RGB spectrum (red: λ = 630 nm, 1.97 eV; green: λ = 532 nm; blue: λ = 465 nm), and intensive research has been conducted on this material. However, InP has been found to have inferior optical properties compared to CdSe. One of the issues is improving the quantum efficiency of InP quantum dots. Quantum dots are essentially nano-sized semiconductor crystalline particles, and their surfaces are highly active, while their cores, with their small band gaps, are 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, its 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 FWHM as well as quantum yield are desired.

[0007] The reason for the large FWHM is that the band gap of InP changes more significantly with respect to 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 the equation (1) below that expresses the quantum size effect, InP, which has a small effective mass, changes its band gap more significantly with respect to particle size than 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 a promising candidate for such 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 crystals with various particle sizes, it is possible to create crystals with a wide range of emission wavelengths, from 0.26 to 3.5 eV, in terms of band gap. 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, AgInGaS ２ By using amorphous GaS as a shell in quantum dots, quantum dots with a chalcopyrite structure that exhibit band-edge emission have been obtained. Based on this report, as described in Patent Document 2, AgGaSe, which exhibits band-edge emission with high quantum yield, has been developed. ２ and AgGaInS ２ Quantum dots exhibiting band-edge emission such as these have been developed and are promising candidates for Cd- and Pb-free quantum dots. However, as described in Non-Patent Document 4, it has been shown that chalcopyrite-type quantum dots with thin shell thicknesses have low stability and gradually decrease quantum yield in the atmosphere. On the other hand, photostability is an important issue to consider when considering the application of quantum dots, but there have been few reports on the photostability of chalcopyrite-type quantum dots. In fact, when chalcopyrite-type quantum dots are examined, it has been found 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.

[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 core-shell quantum dot satisfies constraints estimated by the effective mass approximation method and has a single excited electron level 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 Se ２ , CuIn ｘ Ga １－ｘ S ２ , AgIn ｘ Ga １－ｘ Se ２ , AgIn ｘ Ga １－ｘ S ２ (where 0≦X≦1) or a quantum dot that is a mixed crystal 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 Se ２ , CuIn Ｙ Ga １－Ｙ S ２ , AgIn Ｙ Ga １－Ｙ Se ２ , AgIn Ｙ Ga １－Ｙ S ２, ZnSe, and ZnS (where 0≦y≦1) or a mixed crystal 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] In this case, the backlight unit may include the wavelength conversion member and an excitation light source, and the excited electron level in the core-shell quantum dots may be 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.

[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 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, wherein the particle size of the Group I-III-VI semiconductor nanocrystal core is 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.

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

[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.

[0026] 1 shows an example of a quantum dot according to the present 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 core-shell quantum dots satisfying constraints estimated by the effective mass approximation method have a single excited electron level 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, wherein the particle size of the Group I-III-VI semiconductor nanocrystal core is 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, and thereby completed the present invention.

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

[0032] [Quantum Dots] First, the core-shell quantum dots according to the present invention will be described. As shown in Fig. 1, the core-shell quantum dots 10 according to the present invention are core-shell quantum dots 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 described above, after extensive research into the issue of improving the photostability of quantum dots containing core semiconductor nanocrystals made of Group I-III-VI chalcopyrite-type semiconductors, the researchers discovered that if a structure is created in which 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, thereby 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 method. In the effective mass approximation method, 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 potential V(r) can be expressed as a time-independent equation as follows:

[0037] Here, the confining potential of V(r) is ｃｏｒｅ 0 inside the quantum dot and V in the shell ｓｈｅｌｌ Assuming that, 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 into spherically symmetric polar coordinates as follows:

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

[0040] The above equations (2) and (3) are the first kind of spherical Bessel function J ０ and the Hankel function of the first or second kind h ０ （１） Since it has a solution of the form

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

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

[0043] When the above equation (8) is solved numerically, multiple solutions may be obtained. In this case, focusing on excited electrons, if the excited electron level in the core-shell quantum dot 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, 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 (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) are possible, then as one rises from the level with the lowest binding energy, the electron distribution will have more nodes, resulting in a wider range of leakage and making it impossible to effectively bind electrons. Therefore, when 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) A semiconductor nanocrystal core made of a group I-III-VI element has a larger effective mass than InP, and tends to have a relatively large band gap, so the particle size is also large and it is easy to take on multiple bound states. However, a large effective mass also leads to a large electron density and a large absorption coefficient, making it useful as a light conversion material. In particular, when the semiconductor nanocrystal core is CuIn Ｘ Ga １－Ｘ Se ２ , CuIn ＸGa １－Ｘ S ２ , AgIn Ｘ Ga １－Ｘ Se ２ , AgIn Ｘ Ga １－Ｘ S ２ (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 "Group I-III-VI semiconductor nanocrystal core" 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 Group 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 Se ２ , CuIn Ｙ Ga １－Ｙ S ２ , AgIn Ｙ Ga １－Ｙ Se ２ , AgIn Ｙ Ga １－Ｙ S ２ It is preferable to use a semiconductor nanocrystal shell material containing one or more layers of semiconductor nanocrystals selected from 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 Se ２ , CuIn Ｘ Ga １－Ｘ S ２ , AgIn Ｘ Ga １－Ｘ Se ２, AgIn Ｘ Ga １－Ｘ S ２ (where 0≦X≦1) from a semiconductor nanocrystal or a mixed crystal thereof, CuIn having a band gap larger than that of the 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 By growing a crystal of Y (where 0≦Y≦1) or a mixed crystal thereof, the crystal system between the core and shell becomes a 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 is made into 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 by a transmission electron microscope (TEM), measuring the increase in particle size, performing elemental analysis by 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, and these may be used alone or in combination.

[0050] [Wavelength conversion member] There is also 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 that has 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 appropriately selected depending on the desired properties. These resins preferably have high transmittance to enhance 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 appropriately selected depending on the application.

[0055] [Backlight unit] There is also provided a backlight unit comprising 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 in the core-shell quantum dot according to the present invention is one within the range of excitation energy of light from the excitation light source, based on the top of the valence band of the Group I-III-VI 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, which has improved light stability.

[0058] [Quantum Dot Manufacturing Method] The quantum dot manufacturing method will be described in detail below. As described above, the core-shell quantum dot manufacturing method 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 conditions 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 enables the manufacture of quantum dots with improved photostability. The method for estimating the constraint conditions using the effective mass approximation method is as described above.

[0059] As mentioned above, 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 (where 0≦X≦1) and a semiconductor nanocrystal core made of a semiconductor nanocrystal or a mixed crystal thereof, and CuIn coating the core. Ｙ Ga １－Ｙ Se ２ , CuIn Ｙ Ga１－Ｙ S ２ , AgIn Ｙ Ga １－Ｙ Se ２ , AgIn Ｙ Ga １－Ｙ S ２ ZnSe, ZnS (where 0≦y≦1) or a mixed crystal thereof.

[0060] (Core Synthesis Step) 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 constraint conditions 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, thereby synthesizing 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 low-reactivity Se=TOP, it is advisable to use an appropriate halide, etc. It is also known that Group III-VI semiconductor nanocrystal cores uniformly doped with Group VI elements 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 III precursor in which an organic acid such as gallium acetylacetonate is reacted when Se or S is dissolved in dodecanethiol as a Group VI precursor.

[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 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 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 the precursor by heating to 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 may be selected that will give the desired particle size and composition in accordance with the reactivity of the Group III-VI nanoparticles to be reacted, and may be selected appropriately.

[0069] The method for dissolving the Group I 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 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 can be suitably used.

[0071] Similarly, the synthesis temperature and retention time are not particularly limited, as they can be adjusted as appropriate to obtain the desired particle size and particle size distribution. The solvent is not particularly limited and may be selected as appropriate 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 Step) In the shell synthesis step, a semiconductor nanocrystal shell is formed on the surface of a Group I-III-VI semiconductor nanocrystal core. The structure, composition, etc. of the semiconductor nanocrystal shell are not limited as long as 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. The shell layer structure is CuIn 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 It is preferable to use a material containing one or more layers of semiconductor nanocrystals or mixed crystals thereof selected from 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 step, it is desirable to add and dissolve the Group I, III, II, and VI precursors in a solution in which a ligand has been dissolved in order to suppress aggregation after the synthesis of the shell layer, and the reaction is also carried out similarly to the core synthesis step by adding the Group I precursor solution to the III-VI semiconductor nanocrystal at a high temperature of 150°C or higher and 350°C or lower, whereby the Group I precursor diffuses by a cation exchange reaction using the III-VI semiconductor nanocrystal 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. Since high reactivity is not required for shell layer synthesis, zinc carboxylate, zinc acetate, and zinc halide are suitable for use due to their ease of handling and compatibility with the solvent. Furthermore, 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 desirable. 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.

[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 a solid Group VI precursor material in a solvent is not particularly limited, and a preferred method involves heating the material to a temperature of 100°C to 180°C.

[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.

[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) 0.8 mmol of gallium acetylacetonate, 16 mL of oleylamine, and 4 mL of dodecanethiol were added to a 100 mL flask, and the mixture was heated to 150 ° C. to dissolve the gallium precursor. When 1.5 mL of a 0.8 M solution of Se dissolved in dodecanethiol was added, the solution turned orange, confirming the synthesis of GaSe nanoparticles. Next, 2 mL of a 0.2 M solution of copper chloride dissolved in oleylamine was added, and the temperature was raised to 200 ° C. and stirred for 20 minutes, causing the solution to turn reddish-brown, confirming the formation of core particles.

[0079] (Shell Layer Synthesis Step) Next, after core synthesis, the solution was cooled to room temperature, and ethanol was added to it. The solution was then centrifuged at 8000 rpm for 3 minutes to precipitate the quantum dots. 20 mL of octadecene was added to the precipitated quantum dots to redisperse them, and the mixture was added to a 100 mL flask and degassed at room temperature for 20 minutes. After purging with nitrogen, the mixture was heated to 200°C.

[0080] Then, 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 cooled reaction solution, which was then separated using a centrifuge to precipitate the quantum dots. The precipitated quantum dots were then redispersed in toluene.

[0081] [Comparative Example 1] (Core synthesis step) 0.8 mmol of gallium acetylacetonate and 2 mL of a 0.2 M solution of copper chloride dissolved in oleylamine were added to a 100 mL flask, followed by the addition of 16 mL of oleylamine and 4 mL of dodecanethiol, and the mixture 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. and the mixture was stirred for 20 minutes with heating. The solution was colored reddish-brown, and it was confirmed that core particles were produced.

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

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

[0084] (Shell Layer Synthesis Step) Next, after core synthesis, 4 mL of a 0.2 M solution of gallium acetylacetonate dissolved in oleylamine was added to the solution cooled to room temperature, followed by 2 mL of a 0.2 M solution of copper chloride dissolved in oleylamine, and then heated to 150 ° C. to dissolve the precursor. Next, 1.5 mL of a 0.8 M solution of S dissolved in dodecanethiol was added, heated to 200 ° C., and heated and stirred for 1 hour. After cooling to room temperature, ethanol was added to the resulting reaction solution, and the mixture was centrifuged at 8000 rpm for 3 minutes in a centrifuge to precipitate the quantum dots. 20 mL of octadecene was added to the precipitated quantum dots to redisperse them, and the mixture was added to a 100 mL flask and degassed at room temperature for 20 minutes. After that, nitrogen was purged and the mixture was heated to 200 ° C. Then, 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 cooled reaction solution, which was then separated using a centrifuge to precipitate the quantum dots. The precipitated quantum dots were then redispersed in toluene.

[0085] [Example 3] (Core synthesis process) 0.8 mmol of gallium acetylacetonate, 16 mL of oleylamine, and 4 mL of dodecanethiol were added to a 100 mL flask, and the mixture 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 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 dissolved in oleylamine was added and heated and stirred for 20 minutes, causing the solution to turn reddish-brown, confirming the formation of core particles.

[0086] (Shell Layer Synthesis Step) Next, after core synthesis, 4 mL of a 0.2 M solution of gallium acetylacetonate dissolved in oleylamine was added to the solution cooled to room temperature, followed by 2 mL of a 0.2 M solution of silver acetate dissolved in oleylamine, and then heated to 150 ° C to dissolve the precursor. Next, 1.5 mL of a 0.8 M solution of S dissolved in dodecanethiol was added, heated to 320 ° C, and heated and stirred for 1 hour. After cooling to room temperature, ethanol was added to the resulting reaction solution, and the mixture was centrifuged at 8000 rpm for 3 minutes to precipitate the quantum dots. 20 mL of octadecene was added to the precipitated quantum dots to redisperse them, and the mixture was added to a 100 mL flask and degassed at room temperature for 20 minutes. After that, nitrogen was purged and the mixture was heated to 200 ° C.

[0087] Then, 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 cooled reaction solution, which was then separated using a centrifuge to precipitate the quantum dots. The precipitated quantum dots were then redispersed in toluene.

[0088] [Comparative Example 2] (Core synthesis step) 0.8 mmol of gallium acetylacetonate and 2 mL of a 0.2 M solution in which silver acetate was dissolved in oleylamine were added to a 100 mL flask, followed by the addition of 16 mL of oleylamine and 4 mL of dodecanethiol, and the mixture was heated to 150 ° C. to dissolve the gallium precursor and silver precursor. 1.5 mL of a 0.8 M solution in which Se was dissolved in dodecanethiol was added, and the mixture was heated to 200 ° C. and stirred for 20 minutes with heating. The solution turned reddish-brown, and it was confirmed that core particles had been produced.

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

[0090] [Example 4] (Core synthesis process) In a 100 mL flask, 0.64 mmol of gallium acetylacetonate, 0.16 mmol of indium acetylacetonate, 16 mL of oleylamine, and 4 mL of dodecanethiol were added, and the mixture was heated to 150 ° C. to dissolve the gallium precursor. When 1.5 mL of a 0.8 M solution of Se dissolved in dodecanethiol was added, the solution turned orange, confirming the synthesis of GaInSe nanoparticles. Next, the temperature was raised to 320 ° C., and 2 mL of a 0.2 M solution of silver acetate dissolved in oleylamine was added and heated and stirred for 20 minutes, causing the solution to turn reddish-brown, confirming the formation of core particles.

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

[0092] [Measuring Average Particle Diameter] The average particle diameter was measured by directly observing at least 20 particles with 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 diameter. Based on the core particle diameter calculated by this method, the possible bound states were numerically determined by the effective mass approximation method.

[0093] [Measurement of Emission Wavelength, Emission Half Width, and Emission Efficiency] In the examples and comparative examples, the fluorescence emission properties of the quantum dots were evaluated by measuring 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 using a quantum efficiency measurement system (QE-2100) manufactured by Otsuka Electronics Co., Ltd.

[0094] [Evaluation of Light Stability] In the examples and comparative examples, the light stability 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 Corporation), and adjusting the output with an LED lighting power supply (PSB3-30024: manufactured by CCS Corporation) to 1000 cd / m ２ After irradiation with excitation light of 1000 kJ / s 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]

[0097] As described above, in Examples 1, 2, 3, and 4, the primary particle diameter of the core was controlled to be smaller than in the Comparative Example, and the particle diameter could be controlled (inhibited from increasing) by producing Group III-VI nanoparticles and then reacting with a Group I precursor. On the other hand, in the Comparative Example, the Group VI precursor was reacted in the presence of Group I and III, and the highly reactive Group I element increased 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 Example 1, Comparative Example 1, Example 3, and Comparative Example 2, each with a similar composition, were compared, it was found that the quantum yield was improved in the Examples, and excitons could be effectively trapped, resulting in improved quantum yield.

[0098] This specification includes the following aspects: [1]: A core-shell quantum dot comprising a Group I-III-VI semiconductor nanocrystal core and a single or multiple 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 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. [2]: The Group I-III-VI 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 ２The quantum dot of the above [1], which is a semiconductor nanocrystal or a mixed crystal thereof selected from the group consisting of CuIn (where 0≦X≦1), wherein the semiconductor nanocrystal shell has an energy gap larger than that of the I-III-VI group 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 , ZnSe, ZnS (where 0≦y≦1) or a mixed crystal thereof. [4]: ​​A wavelength conversion member in which the quantum dots of [1], [2], or [3] above 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 dots is one within a range of excitation energy of light from the excitation light source, with the top of the valence band of the I-III-VI group semiconductor nanocrystal core as the reference. [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 that 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.

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, characterized in that 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 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 material characterized in that the quantum dots according to any one of claims 1 to 3 are dispersed therein.

5. A backlight unit comprising the wavelength conversion material according to claim 4 and an excitation light source, wherein the excited electron level in the core-shell quantum dots is one within the 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; and a shell synthesis step of forming a semiconductor nanocrystal shell on the surface of the Group I-III-VI semiconductor nanocrystal core, wherein the particle size of the Group I-III-VI semiconductor nanocrystal core is adjusted in the core synthesis step so that the excited electron level in the core-shell quantum dot, which 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.

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