JP2019218527A - Nanoparticle aggregate and production method of the same - Google Patents

Abstract

To provide quantum dots having different particle sizes.SOLUTION: A nanoparticle aggregate is provided, in which each nanoparticle includes a light-emitting layer and a barrier layer and one nanoparticle includes a plurality of light-emitting layers having different thicknesses. The light-emitting layer and the barrier layer in the nanoparticle aggregate are made of a nitride semiconductor, and each nanoparticle includes a base crystal formed of a II-VI semiconductor containing Zn.SELECTED DRAWING: Figure 1

Description

  The present invention relates to fine solid particles called nanoparticles and a method for producing the same.

  As the size of the solid particles is reduced in the nanometer (nm) region of micrometer (μm) or less, a phenomenon called quantization or the like, which changes the properties of the substance, occurs. For example, the forbidden band (band gap) width between the valence band and the conduction band changes. Fine solid semiconductor particles that cause quantization are called quantum dots. Various researches and developments have been made on such fine solid particles.

  As an application of the solid particles containing a semiconductor material, there is a phosphor. The phosphor particles emit high-energy light or particle beams and emit fluorescence of a predetermined wavelength. Lattice defects that cause non-radiative recombination will reduce the efficiency of the phosphor. In order to form an efficient phosphor, it may be desirable to form phosphor particles having good crystallinity. As a method of forming phosphor particles having good crystallinity, growth of a phosphor crystal from a liquid phase material, epitaxial growth of a phosphor using a base crystal, formation of a protective layer for protecting the phosphor crystal, and the like can be considered.

  There has been proposed a phosphor particle having a core-shell structure in which a core such as CdSe, CdS, InP, and GaP is covered with a shell layer such as ZnS or ZnSe (for example, Patent Document 1). Further, a structure has been proposed in which a core of a III-V semiconductor (InGaN) is covered with a shell of a II-VI semiconductor (ZnO, ZnS, ZnSe, ZnTe, etc.) (for example, Patent Document 2).

  At present, CdSe / ZnS, InP / ZnS, and the like, in which a shell having a large energy gap is laminated on a core having a small energy gap, are used as the phosphor particles in the visible light region. When the core-shell structure is composed of different compound materials, lattice mismatch (CdSe / ZnS: 11.1%, InP / ZnS: 7.8%) occurs. Lattice mismatch will distort the crystal lattice and reduce luminous efficiency and reliability.

  Lattice matching can be performed by selecting the mixed crystal composition. For example, it has been proposed to form a quantum dot with reduced strain by using a ZnOS mixed crystal as a base crystal and growing a lattice-matched AlGaInN mixed crystal thereon (for example, Patent Document 3).

JP 2011-76827 A Japanese Patent Application Laid-Open No. 2010-155873 (registered No. 4936338) JP-A-2006-145972

  When quantization occurs by reducing the size of the quantum dot, the energy gap changes and the emission wavelength changes. For example, in order to form white light having good color rendering properties, it may be preferable to use white light containing as much wavelength light as possible. A phosphor capable of emitting light of many wavelengths is desired. In such a case, quantum dots having different particle sizes are desired.

According to the embodiment,
An aggregate of nanoparticles is provided, wherein each nanoparticle includes a light-emitting layer and a barrier layer, and includes a plurality of light-emitting layers having different thicknesses within one nanoparticle.

FIGS. 1A, 1B, and 1C are schematic side views showing a process of manufacturing a nanocrystal by epitaxially growing a plurality of secondary crystals on a primary crystal according to the first embodiment. FIGS. FIG. 9 is a schematic side view showing a process of manufacturing a nanocrystal, in which a primary crystal is grown on a preliminary crystal according to a second embodiment, and after removing the preliminary crystal, a plurality of secondary crystals are epitaxially grown on the primary crystal. . FIG. 2 is a graph showing the energy gaps of ZnO _x S _1-x mixed crystal system, Al _1-x In _x N mixed crystal system, and In _x Ga _1-x N mixed crystal system as a function of lattice constant. 3A and 3B are a side view and a cross-sectional view illustrating an example of a normal pressure vessel and a pressure-resistant vessel for performing liquid phase growth. FIG. 4A shows that a ZnOS nanoparticle providing a growth surface having a predetermined area or more is put into a liquid phase raw material capable of growing a predetermined amount of InGaN crystal, and a limited number of InGaN layers are formed on the ZnOS nanoparticle. FIG. 4B is a graph showing the relationship between the number of layers of the InGaN layer that can be grown and the amount of ZnOS nanoparticles when grown, and FIG. 4B provides a growth surface having a predetermined area or more in the same amount of the InGaN liquid phase raw material as described above. 9 is a graph showing the relationship between the number of InGaN layers that can be grown and the amount of InAlN nanoparticles charged when InAlN nanoparticles are charged and a limited number of InGaN layers are grown on the InAlN nanoparticles. FIG. 5A is a graph showing a wavelength distribution of photoluminescence emission intensity obtained from a nanoparticle aggregate obtained by growing an InGaN layer on ZnOS nanoparticles according to the first embodiment and further forming an InAlN barrier layer, and FIG. 9 is a graph showing a wavelength distribution of photoluminescence emission intensity obtained from a nanoparticle aggregate obtained by growing an InGaN layer on InAlN nanoparticles and further forming an InAlN barrier layer according to Example 2.

11 primary crystal, 12 secondary crystal, 13 tertiary crystal, 15 preliminary crystal,
16 primary crystal, 17 secondary crystal, 26 port, 27 syringe,
28 temperature measuring unit, 29 heater, 33 temperature sensor, 34 heater,
35 stirrer, 36,37 inlet, 38 outlet, 39 flask,

  For example, it is assumed that a light emitting layer is formed on nanoparticles serving as a base material (seed, seed crystal), and the surface is covered with a protective layer that transmits light emitted from the light emitting layer. In order to extract light emitted from the light emitting layer to the outside, the protective layer has a property of transmitting light emitted from the light emitting layer. Such a protective layer is also called a barrier layer. If the base material is also formed of a material that transmits light emitted from the light emitting layer, the base material can be called a barrier layer.

1A to 1C are side views schematically illustrating a method for manufacturing nanoparticles according to a first embodiment. In the first embodiment, as shown in FIG. 1A, first, ZnO _0.8 S _0.2 nanocrystals 11 in which crystal growth is relatively easy are grown by liquid phase growth. ZnO _0.8 S _0.2 nanocrystals grown under the same conditions have almost the same size. The ZnOS nanocrystal grown in the liquid phase is called a primary crystal 11. The surface of the primary crystal is used as a growth surface, and a secondary crystal is grown thereon in a liquid phase. ZnOS, which is relatively easy to grow, can be grown in liquid phase at normal pressure. When growing a lattice-matched AlGaInN crystal on a ZnOS crystal, high-pressure liquid phase growth is performed.

As shown in FIG. 1B, a plurality of smaller In _0.5 Ga _0.5 N nanocrystals are grown on the surface of each ZnO _0.8 S _0.2 primary crystal 11. A plurality of nanocrystals grown on the surface of the primary crystal 11 are called secondary crystals 12.

FIG. 2 is a graph showing the relationship between the lattice constant and the energy gap of a ZnOS mixed crystal system, an AlInN mixed crystal system, and an InGaN mixed crystal system. “Mixed crystal system” is a term that indicates a system that includes a mixed crystal that is intermediate with both materials. The horizontal axis indicates the lattice constant in units of Å (angstrom, A) (= 1 nm / 10), and the vertical axis indicates the energy gap in units of eV (electron volts). The energy gap that determines the emission wavelength is ZnO: 3.2 eV, ZnS: 3.8 eV, AlN: 6.2 eV, GaN: 3.4 eV, and InN: 0.64 eV. The lattice constant of ZnO _0.8 S _0.2 is 3.37 A (0.0337 nm) and the lattice constant of In _0.5 Ga _0.5 N is 3.37 A (0.0337 nm). Align.

  An InGaN crystal having an excellent light emitting function is caused to function as a light emitting layer of the phosphor particles. When the crystallinity of the light emitting layer is impaired, the luminous efficiency may be reduced. It is preferable to provide a protective layer on the light emitting layer so as to protect the light emitting layer.

As shown in FIG. 1C, a tertiary crystal 13 of In _0.6 Al _0.4 N nanocrystals serving as a protective layer is liquid-phase grown on the surface of the In _0.5 Ga _0.5 N secondary crystal 12. The tertiary crystal 13 is formed of In _0.6 Al _0.4 N having the same lattice constant of 3.37 A so as to lattice match with the secondary crystal of the In _0.5 Ga _0.5 N light emitting layer. As can be seen from FIG. 2, for the same lattice constant, the energy gap of InAlN is larger than the energy gap of InGaN, and InAlN transmits light emitted from InGaN.

  In FIG. 2, if a mixed crystal having the same lattice constant is selected and a base layer and a growth layer epitaxially grown thereon are formed, lattice matching is established. By reducing distortion, crystal defects will be reduced, and quantum dots with high efficiency will be realized.

  In Example 2, as shown in FIG. 1L, the ZnOS nanoparticles 15 are grown as normal crystals under normal pressure. As shown in FIG. 1M, a primary crystal 16 of InAlN is grown on the preliminary crystal 15 by high-pressure liquid phase growth. As shown in FIG. 1N, the preliminary crystal 15 is removed by etching or the like. The remaining InAlN becomes the primary crystal 16 on which a light emitting layer is grown. As shown in FIG. 10, an InGaN secondary crystal 17 is grown as a light emitting layer on the primary crystal 16 as in the first embodiment. Further, it is preferable to form an InAlN protective layer covering the InGaN light emitting layer. The InAlN primary crystal 16, the InGaN secondary crystal, and the InAlN protective layer are grown by high-pressure liquid phase growth. The entire manufactured nanoparticles are formed of an AlGaInN-based material, and stable and excellent characteristics can be expected.

  With reference to FIGS. 3A and 3B, a growth vessel used for liquid phase growth will be briefly described. FIG. 3A shows a growth vessel for normal pressure. The flask 39 has, in addition to the outlet, a port 26 that can be replaced with an inert gas, a plurality of dedicated ports provided with a syringe 27 that can inject a reaction precursor, and a temperature measuring unit 28 equipped with a thermocouple. Will be installed. As the inert gas, for example, argon (Ar) is used.

FIG. 3B shows a pressure growing container. The outside of the reaction vessel 32 is made of stainless steel, and the inside is made of Hastelloy. The reaction vessel 32 is provided with at least two intake ports 36 and 37 and an exhaust port 38. The intake ports 36 and 37 are connected to, for example, an Ar gas supply source and an N ₂ gas supply source via valves, respectively, and supply Ar gas and N ₂ gas into the reaction vessel 32 from the intake ports 36 and 37. can do. The exhaust port 38 is connected to an exhaust pump via a valve, and can exhaust the atmosphere (gas) in the reaction vessel 32. By adjusting each valve, the partial pressure of various gases in the reaction vessel 32, particularly the partial pressure of N ₂ gas, can be accurately controlled.

  The reaction container 32 is provided with a temperature sensor 33, a heater 34, a stirring mechanism 35, and the like. The temperature sensor 33 can measure the temperature of the contents in the reaction container 32. The heater 34 can heat the contents. The stirring mechanism 35 (rotary blade) can stir the contents. All operations relating to sample preparation are preferably carried out in a glove box using vacuum-dried (140 ° C.) instruments and equipment.

  In Examples 1 and 2, an InGaN secondary crystal is grown on a ZnOS primary crystal or an InAlN primary crystal. In order to prevent the secondary crystal from growing on the front surface of the primary crystal, a primary crystal which provides an excessive surface with respect to the amount of the raw material of the secondary crystal is introduced. That is, the thickness of the secondary crystal when the secondary crystal grows on the front surface of the surface of the primary crystal is set to one atomic layer or less. The area and thickness of the growing secondary crystal will be distributed over a wide range. The amount of the group III material used as the raw material of the secondary crystal was set to 0.2 mmol, and the input amount of the primary crystal at which the thickness of the light emitting layer became one layer or less was calculated.

  FIG. 4A is a graph showing a case where ZnOS nanoparticles are charged as a primary crystal into an InGaN raw material liquid according to Example 1. The horizontal axis indicates the amount of primary crystal (ZnOS nanoparticle) input in units (mg), and the vertical axis indicates the number of InGaN nitride layers to be grown. If the input amount of ZnOS nanoparticles is 85 mg or more, the number of grown InGaN layers is less than one.

  FIG. 4B is a graph showing a case where InAlN nanoparticles are charged as a primary crystal into an InGaN raw material liquid according to Example 2. The horizontal axis indicates the amount of primary crystal (InAlN nanoparticles) charged in units (mg), and the vertical axis indicates the number of InGaN nitride layers to be grown. If the input amount of the InAlN nanoparticles is 65 mg or more, the number of the grown InGaN layers is less than one.

Example 1
Step a. Synthesis of ZnOS Crystal (S Composition: 0.2) For example, a ZnOS crystal is produced by liquid phase growth by a thermal decomposition method using a container shown in FIG. 3A. Zinc acetate (2.0 mmol) as a Zn material and sulfur (0.4 mmol) as an S material are added to oleylamine (10 mL) as a solvent, and the mixture is heated and heated at a synthesis temperature of 130 ° C. for 1 hour, 250 ° C. for 1 hour, and 300 ° C. The synthesis was performed at a temperature of 1 ° C. for 1 hour under an N ₂ atmosphere. After the synthesis, the precipitate was washed by centrifugation with toluene and ethanol to prepare a ZnOS crystal having an S composition of 20%, a lattice constant a of 3.37 A, and a particle size of 10 nm. The ZnOS crystal is used as a base crystal in the subsequent steps.

Step b. Synthesis of InGaN light emitting layer (Ga composition: 0.5) on ZnOS crystal For example, in a high-pressure container shown in FIG. 3B, a solvent benzene (6 mL), InI ₃ (0.1 mmol) as an In material, and GaI ₃ (Ga as a Ga material) 0.1 mmol), NaNH ₂ (2.0 mmol), hexadecylamine (0.2 mmol), and ZnOS base crystal (85 mg) as a nitrogen material, and an InGaN light emitting layer is synthesized at a synthesis temperature of 300 ° C. for one hour. Was. After the synthesis, the resultant was washed by centrifugation with toluene and ethanol to form an InGaN light emitting layer (Ga composition: 0.5) on the ZnOS base crystal. The light emitting layer did not cover the entire surface of the ZnOS base crystal and existed as independent particles having a size of about 2 to 5 nm.

Step c. Synthesis of InAlN barrier layer (Al composition: 0.4) on particles synthesized in step b In a high-pressure vessel, solvent benzene (6 mL), InI ₃ (0.12 mmol) as an In material, AlI ₃ (Al as an Al material) 0.08 mmol), NaNH ₂ (2.0 mmol) as a nitrogen material, and ZnOS crystal (100 mg) with an InGaN light-emitting layer created in step b, and synthesis was performed at a synthesis temperature of 300 ° C. for 1 hour. After the synthesis, the resultant was washed by centrifugation with toluene and ethanol to form an InAlN barrier layer having an Al composition of 40% and a lattice constant a of 3.37 A on the InGaN light emitting layer.

  FIG. 5A shows the emission spectrum of the synthesized nanoparticles. 365 nm was used as excitation light.

  The emission spectrum is distributed over a wide wavelength range. The emission spectrum includes a component having a peak near 540 nm and a component having a peak near 640 nm. It is considered that the component around 540 nm is derived from the light emitting layer on the m-plane whose thickness is suppressed, and the component around 640 nm is derived from the light emitting layer on the c-plane.

  The base material is not limited to ZnOS, and may be any material having the same lattice number as InGaN of the light emitting layer. Further, the synthesis method and material of the base material are not limited to the above.

Example 2
Step a. Synthesis of ZnOS Crystal (S Composition: 0.2) A ZnOS crystal for a growth base is prepared by a thermal decomposition method. Zinc acetate (2.0 mmol) as a Zn material and sulfur (0.4 mmol) as a S material were added to the solvent oleylamine (10 mL), and the synthesis temperature was 130 ° C.-1 hour, 250 ° C.-1 hour, 300 ° C.-1 hour, N synthesis was carried out under ₂ atmosphere. After the synthesis, the precipitate was washed with toluene and ethanol by centrifugation to prepare a ZnOS crystal having an S composition of 20%, a lattice constant a of 3.37 A, and a particle size of 10 nm. Finally, methanol is added and the mixture is recovered as a dispersion.

Step b. Synthesis of InAlN barrier layer (Al composition: 0.4) separated into island shape on ZnOS crystal In a high-pressure vessel, solvent benzene (6 mL), InI ₃ (0.12 mmol) as In material, AlI ₃ (Al material as Al material) 0.08 mmol), NaNH ₂ (2.0 mmol) as a nitrogen material, and ZnOS crystal (65 mg) were added, and synthesis was performed at a synthesis temperature of 300 ° C. for 3 hours. After the synthesis, the resultant was washed by centrifugation with toluene and ethanol to form an independent InAlN barrier layer (Al composition: 0.4) on the ZnOS seed.

Step c. Removal of ZnOS seed The ZnOS crystal is removed from the gap between the InAlN islands formed on ZnOS by wet etching. As the etchant, for example, a mixture of pure water and hydrochloric acid (concentration: 36% by volume) at a volume ratio of 100: 1 (dilute hydrochloric acid) is used. As a result, the ZnOS crystal is removed, leaving only an InAlN barrier layer region (hereinafter, also referred to as nanoparticles) having a size of about 5 nm.

Step d. Synthesis of InGaN light emitting layer (Ga composition: 0.5) on InAlN barrier layer In a high-pressure container, solvent benzene (6 mL), InI ₃ (0.1 mmol) as an In material, GaI ₃ (0.1 mmol) as a Ga material, As a nitrogen material, NaNH ₂ (2.0 mmol), hexadecylamine (0.2 mmol), and the InAlN nanoparticles (65 mg) produced in the step c were charged, and at a synthesis temperature of 300 ° C. for 1 hour, an InGaN light emitting layer (Ga composition: 0.5). After the synthesis, the resultant was washed with toluene and ethanol by centrifugation to produce an InGaN light emitting layer (Ga composition: 0.5) on the InAlN nanoparticles. The light emitting layer did not cover the entire surface of the nanoparticle, and was present independently with a size of about 2-3 nm.

Step e. Synthesis of InAlN barrier layer covering InGaN light emitting layer (Ga composition: 0.5) In a high-pressure vessel, solvent benzene (6 mL), InI ₃ (0.1 mmol) as an In material, AlI ₃ (0.1 mmol) as an Al material, As a nitrogen material, NaNH ₂ (2.0 mmol), hexadecylamine (0.2 mmol), and InAlN nanoparticles (85 mg) having an InGaN light-emitting layer were charged, and an InAlN barrier layer was synthesized at a synthesis temperature of 300 ° C. for 1 hour. Was. After the synthesis, the resultant was washed with toluene and ethanol by centrifugation to prepare an InAlN barrier layer (Al composition 0.4) covering the InGaN light emitting layer.

  The seed is not limited to ZnOS, but may be any seed having the same lattice constant as InAlN of the barrier layer and InGaN of the light emitting layer. Further, the method and material for synthesizing the grown crystal serving as the base are not limited to those described above.

  FIG. 5B shows the emission spectrum of the synthesized nanoparticles. An emission spectrum distributed over a wider wavelength range than the emission spectrum of Example 1 is observed. The emission spectrum includes a component having a peak at around 500 nm and a component having a peak at around 550 nm. The component at 500 nm is derived from the light-emitting layer on the m-plane whose thickness is suppressed. The component at around 550 nm is a light-emitting layer on the c-plane. Probably originated.

The light emitting layers of Example 1 and Example 2 are both In _0.5 Ga _0.5 N. Normally, compounds having the same chemical composition emit luminescence (fluorescence) having substantially the same emission spectrum. The size of the light emitting layer was reduced to a nano region where quantization occurs. When crystal growth is performed using a growth substrate such as the seed described above, it is considered that the growth rate of the crystal grown thereon may vary depending on the crystal plane of the seed. In the above example, since the m-plane and the c-plane are respectively exposed, it is considered that the thickness of each light-emitting layer is not uniform. It is thought that the resulting distribution of size and thickness may have contributed to the formation of light emission distributed over a wide wavelength range.

  The wavelength distribution shapes of the emission spectra of Example 1 and Example 2 are significantly different. Example 1 and Example 2 have substantially the same lattice constant of the underlying crystal, but differ in the composition of the underlying crystal serving as the growth substrate. How the composition of the underlying crystal affects the emission wavelength distribution is not yet known in detail.

  If a nanoparticle aggregate prepared by mixing the phosphor nanoparticles manufactured according to Example 1 and the phosphor nanoparticles manufactured according to Example 2 is prepared, it is possible to provide the phosphor nanoparticles distributed over a wider wavelength range. Would.

  By changing the lattice constants of the primary crystal, the secondary crystal, and the tertiary crystal constituting the phosphor nanoparticles, it is expected that the obtained emission spectrum will be modeled. By selecting various lattice constants, preparing various phosphor nanoparticles, and mixing them appropriately, phosphor nanoparticles having various emission spectra can be provided.

  As described above, the present invention has been described with reference to the examples. However, the numerical values, materials, and the like are not limited unless otherwise specified. It will be apparent to those skilled in the art that various modifications, substitutions, improvements, and the like can be made.

Claims (10)

  An aggregate of nanoparticles, wherein each nanoparticle includes a light-emitting layer and a barrier layer, and a single nanoparticle includes a plurality of light-emitting layers having different thicknesses.   The nanoparticle assembly according to claim 1, wherein the light emitting layer and the barrier layer are nitride semiconductors.   3. The nanoparticle assembly according to claim 1, wherein each of the nanoparticles includes a base crystal formed of a II-VI group semiconductor including Zn.   3. The nanoparticle assembly according to claim 1, wherein each of the nanoparticles includes a base crystal formed of a group III-V semiconductor having no light emitting function. 4. A first step of liquid phase growth of a number of primary nanocrystals;
A second step of liquid phase growing a III-V semiconductor secondary crystal having a light emitting function on each surface of the plurality of primary nanocrystals;
A third step of performing liquid phase growth of a protective layer covering each surface of the secondary crystal;
A method for producing a nanocrystal aggregate comprising:   The method for producing a nanocrystal aggregate according to claim 5, wherein the first step is a normal-pressure liquid phase growth step of growing a group II-VI semiconductor mixed crystal.   The method for producing a nanoparticle assembly according to claim 5 or 6, wherein the second and third steps are pressurized liquid phase growth steps using a pressure vessel.   The method for producing a nanocrystal aggregate according to claim 5, wherein the first step is a liquid phase growth step in which a group III-V semiconductor mixed crystal having no light emitting function is performed under pressure using a pressure vessel. The first step includes:
A first preliminary step of liquid phase growing a number of preliminary nanocrystals;
A second preliminary step of liquid-phase growing a primary crystal on each surface of said plurality of preliminary nanocrystals;
A third preliminary step of removing the preliminary nanocrystal from the primary crystal;
The method for producing a nanocrystal aggregate according to any one of claims 5 to 8, comprising:   The method of manufacturing a nanoparticle assembly according to claim 9, wherein the preliminary nanocrystal is a II-VI group semiconductor mixed crystal, and the primary crystal is a III-V semiconductor mixed crystal having no light emitting function.

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