JP7848909B2 - Quantum dot aggregate - Google Patents

Description

This invention relates to a quantum dot assembly containing quantum dots undergoing indirect transitions.

Quantum dots are nanoparticles composed of hundreds to thousands of atoms, with a particle size of several nanometers to tens of nanometers. Quantum dots are also known as fluorescent nanoparticles, semiconductor nanoparticles, or nanocrystals.

Quantum dots can have their emission wavelengths varied depending on the particle size and composition of the nanoparticles. Furthermore, the performance of quantum dots can be expressed by their fluorescence quantum yield (QY), fluorescence full width at half maximum (FWHM), absorption wavelength, and fluorescence wavelength.

The following patent and non-patent documents describe solar cells using _AgBiS2 quantum dots.

Japanese Patent Publication No. 2017-28267

ACS Applied Nano Materials, 2020, 3, 5, pp 4014 Cubic AgBiS2 Colloidal Nanocrystals for Solar Cells Nature Photonics, 2016, 10, pp 521 Solution-processed solar cells based on environmentally friendly AgBiS2 nanocrystals J. Mater. Chem. C, 2018, 6, pp 731 Enhanced optoelectronic performance in AgBiS2 nanocrystals obtained via an improved amine-based synthesis route

However, no research and development activities related to the practical application of silver bismuth chalcogenide quantum dots have been reported. For example, in the synthesis of silver bismuth chalcogenide quantum dots described in Non-Patent Documents 1-3 and Patent Document 1, S(TMS) ₂ (hexamethyldisilatian) is used as a sulfur source, but this is a reagent that readily reacts with moisture in the air when used in the atmosphere, releasing toxic _H2S .

Given the above background, there is a strong need for the synthesis of silver-bismuth chalcogenide quantum dots using simple and easily handled raw materials, and for a method that allows for mass production, as well as for the elucidation of the physical properties of silver-bismuth chalcogenide quantum dots synthesized by such a method.

This invention has been made in view of the above points, and aims to provide a quantum dot assembly containing silver bismuth chalcogenide quantum dots that can achieve uniform particle size.

Furthermore, the present invention aims to provide a method for producing silver bismuth chalcogenide quantum dots using simple, highly safe, and easy-to-handle raw materials.

The quantum dot aggregate in the present invention is a nanocrystal represented by _AgBiE2 (where E is at least one of tellurium, selenium, or sulfur) containing silver, bismuth, and chalcogen, and is characterized in that it contains a large number of quantum dots with an average particle size of 3 nm or more and 5 nm or less , and in the particle size distribution in STEM, more than two-thirds of the quantum dots are contained within ±20% of the average particle size.

In this invention, it is preferable that the surface of the quantum dot is covered with a ligand.

In this invention, it is preferable that the ligand is selected from at least one or two of the following: phosphine-based, aliphatic thiol-based, aliphatic amine-based, and aliphatic carboxylic acid-based ligands.

In this invention, it is preferable to obtain an optical band gap of 0.90 to 1.075 eV in the TauC plot.

The quantum dots of this invention allow for a narrower particle size distribution in STEM and enable the synthesis of silver-bismuth chalcogenide quantum dots with uniform particle sizes.

Furthermore, the quantum dot manufacturing method of the present invention allows for the simple, safe, and mass production of silver-bismuth chalcogenide quantum dots directly, without the need for intermediates, using easily handled reagents.

This is a schematic diagram of a quantum dot in an embodiment of the present invention. This is the absorption spectrum of _AgBiS2 in Example 1. This is the TauC plot of _AgBiS2 in Example 1. These are scanning transmission electron microscope (STEM) images and particle size analysis diagrams of _AgBiS2 in Example 1. This is a differential thermal analysis diagram of _AgBiS2 in Example 1. This is the X-ray diffraction (XRD) spectrum of _AgBiS2 in Example 1. This is the absorption spectrum of _AgBiS2 in Example 2. This is the TauC plot of _AgBiS2 in Example 2. These are scanning transmission electron microscope (STEM) images and particle size analysis diagrams of _AgBiS2 in Example 2. This is a differential thermal analysis diagram of _AgBiS2 in Example 2. This is the X-ray diffraction (XRD) spectrum of _AgBiS2 in Example 2. This is the absorption spectrum of _AgBiS2 in Example 3. This is the TauC plot of _AgBiS2 in Example 3. These are scanning transmission electron microscope (STEM) images and particle size analysis diagrams of _AgBiS2 in Example 3. This is a differential thermal analysis diagram of _AgBiS2 in Example 3. This is the X-ray diffraction (XRD) spectrum of _AgBiS2 in Example 3. This is the absorption spectrum of _AgBiS2 in Example 4. This is the TauC plot of _AgBiS2 in Example 4. These are scanning transmission electron microscope (STEM) images and particle size analysis diagrams of _AgBiS2 in Example 4. This is a differential thermal analysis diagram of _AgBiS2 in Example 4. This is the X-ray diffraction (XRD) spectrum of _AgBiS2 in Example 4. This is the absorption spectrum of _AgBiS2 in Example 5. This is the TauC plot of _AgBiS2 in Example 5. These are scanning transmission electron microscope (STEM) images and particle size analysis diagrams of _AgBiS2 in Example 5. This is a differential thermal analysis diagram of _AgBiS2 in Example 5. This is the X-ray diffraction (XRD) spectrum of _AgBiS2 in Example 5.

In recent years, near-infrared emitting quantum dots that do not contain toxic heavy metals such as Cd and Pb have attracted attention. The inventors of this invention focused on silver bismuth chalcogenide ( _AgBiE2 (where E is at least one of tellurium, selenium, or sulfur)) ternary quantum dots, and have successfully synthesized silver bismuth chalcogenide quantum dots mildly and directly in an atmospheric environment using an easily handled reagent, without the need for intermediates, and have elucidated their physical properties.

The following describes in detail one embodiment of the present invention (hereinafter abbreviated as "Embodiment"). However, the present invention is not limited to the following embodiment and can be implemented in various modifications within the scope of its gist.

Figures 1A and 1B are schematic diagrams of quantum dots in this embodiment. The quantum dots 1 shown in Figures 1A and 1B are nanocrystals synthesized directly using easily handled reagents and without the need for intermediates.

In this embodiment, the quantum dot 1 is preferably a nanocrystal represented by _AgBiE2 (where E is at least one of tellurium (Te), selenium (Se), or sulfur (S)) containing silver (Ag), bismuth (Bi), and chalcogen. This compound undergoes indirect transitions and emits very little light.

Here, "nanocrystal" refers to nanoparticles having a particle size of approximately 1 nm to several tens of nm. In this embodiment, a large number of quantum dots can be produced with a uniform particle size. "Uniform" means that more than two-thirds of the particles are contained within ±20% of the average particle size. Thus, in this embodiment, fine and uniform high-quality quantum dots can be mass-produced. In this embodiment, the particle size of the quantum dots can be adjusted within the range of 1 nm to 15 nm. Preferably, it is 2 nm to 10 nm, more preferably 3 nm to 7 nm, and even more preferably 4 nm to 5 nm.

The main components of quantum dots are Ag, Bi, and chalcogen, but other elements may also be present. However, it is preferable that the reactants used in the production of quantum dots are easy to handle, do not require intermediates, and allow for the synthesis of quantum dots by sequentially adding the raw materials and reacting them in a high-boiling point solvent at a temperature of approximately 100°C to 150°C.

By using this synthesis method, quantum dots can be mass-produced stably without increasing manufacturing costs, imposing restrictions on handling reagents, or complicating the manufacturing process.

In this embodiment, as described later, the reaction system for synthesizing quantum dots involves sequentially adding the raw materials—Ag raw material, Bi raw material, and ligand—to a high-boiling point solvent, and finally adding the chalcogenide raw material. The reaction is then carried out at approximately 100°C to 150°C. By producing quantum dots based on such a direct and simple synthesis reaction, the particle size of the quantum dots can be standardized. Specifically, more than two-thirds of the total quantum dot particles can be contained within ±20% of the average particle size.

As shown in Figure 1A, it is preferable that numerous organic ligands 2 are coordinated to the surface of the quantum dots 1. This suppresses aggregation of the quantum dots 1 and allows the desired optical properties to be achieved. While the ligands that can be used in the reaction are not particularly limited, the following ligands are representative examples.

(1) Aliphatic primary amines: _Oleylamine : _{C18H35NH2 ,} Stearyl( _{octadecyl ) amine} : _C18H37NH2 , Dodecyl( _{lauryl )} amine: _{C12H25NH2 , Decylamine} : _C10H21NH2 , _Octylamine : _C8H17NH2

₍ 2) Fatty acids: Oleic acid: _C17H33COOH , _Stearic acid: _{C17H35COOH ,} Palmitic acid: _C15H31COOH , Myristic acid: _{C13H27COOH , Lauric} acid: _C11H23COOH , _{Decanoic acid: C9H19COOH , Octanoic} acid: _C7H15COOH

(3) Thiol compounds: _{Octadecanethiol} : _C18H37SH , _{Hexanedecanethiol} : _C16H33SH , _{Tetradecanethiol} : _C14H29SH , _{Dodecanethiol} : _C12H25SH , Decanethiol _{: C10H21SH} , _Octanethiol : _C8H17SH
( 4) _Phosphine -based trioctylphosphine: ( _C8H17 ) _3P , triphenylphosphine: _{( C6H5} ) _3P , tributylphosphine: ( _C4H9 ) _3P

(5) _Phosphine oxide system Trioctylphosphine oxide: ( _C8H17 ) _{3P =} O, Triphenylphosphine oxide: ( _C6H5 ) _3P =O, Tributylphosphine oxide: ( _C4H9 ) _{3P =} O

In this embodiment, as shown in Figure 1B, the quantum dot 1 may have a core-shell structure comprising a core 1a and a shell 1b covering the surface of the core 1a. Preferably, as shown in Figure 1B, a large number of organic ligands 2 are coordinated to the surface of the quantum dot 1.

The core 1a shown in Figure 1B is AgBiE _2. The shell 1b, like the core 1a, does not contain regulated heavy metals such as Cd, Hg, and Pb, nor substances derived from highly reactive reagents such as silicon compounds.

Furthermore, the shell 1b may be in a solid solution state on the surface of the core 1a. However, in this embodiment, an optical band gap of 0.90 to 1.10 eV can be obtained in the TauC plot using only the core 1a, i.e., the quantum dot 1 of the core alone as shown in Figure 1A, without using the shell 1b.

Next, the method for manufacturing quantum dots in this embodiment will be described. In this embodiment, quantum dots represented as _AgBiE2 (where E is at least one of tellurium, selenium, or sulfur) are synthesized from a silver raw material, a bismuth raw material, and a chalcogenide raw material (a chalcogenide is at least one of tellurium, selenium, or sulfur).

In this embodiment, the Ag raw material for _AgBiE2 is not particularly limited, but for example, the following organosilver reagents and inorganic silver reagents can be used. Specifically, as the acetate salt, silver(I) acetate: Ag(OAc), as the fatty acid salt, silver stearate: Ag(OC(= _O ) _{C17H35 )} , silver oleate: Ag(OC(=O) _C17H33 ), silver myristate: Ag(OC(=O) _C13H27 ), silver _dodecanoate : Ag(OC(=O) _C11H23 ), silver acetylacetonate: Ag( _acac ), and as the halide, monovalent compounds can be used, such as silver(I) chloride: AgCl, silver(I) bromide: AgBr, silver(I) iodide: AgI, etc.

In this embodiment, the Bi raw material for _AgBiE2 is not particularly limited, but for example, the following organic bismuth reagents or inorganic bismuth reagents can be used. Specifically, as fatty acid salts, bismuth(III) oxidized acetate: BiO(OC(=O) _CH3 ), bismuth(III) acetate: Bi(OC(=O) _CH3 ) ₃ , bismuth( _III ) 2-ethylhexanoate: Bi(OC(=O) _C21H45 ) ₃ , bismuth(III) neodecanoate: Bi(OC(=O) _C27H57 ) ₃ , bismuth( _III ) _subgallate : Bi( _{C7O6H5 )} , trivalent compounds can be used as halides: bismuth(III) fluoride: _BiF3 , bismuth(III) chloride: _BiCl3 , bismuth(III) bromide: _BiBr3 , and inorganic salts such as bismuth nitrate pentahydrate (III): Bi( _NO3 ) ₃・_5H2 O, bismuth(III) oxycarbonate (BiO) _₂CO₃ , bismuth(III) oxide ( _Bi₂O₃ ) _{, etc.} can be used.

In this embodiment, tellurium (Te) is used as a raw material either in solid form or dissolved in a high-boiling point solvent, either as an organic tellurium compound (organic chalcogen compound) or an inorganic tellurium compound. While the structure of the compound is not particularly limited, examples _include trioctylphosphine telluride ( _C8H17 ) _3P =Te, obtained by dissolving tellurium in trioctylphosphine; tributylphosphine telluride ( _C4H9 ) _3P =Te, obtained by dissolving tellurium in tributylphosphine; or a solution obtained by dissolving tellurium at high temperature in a high-boiling point solvent that is _a long-chain hydrocarbon such as octadecene.

Furthermore, in this embodiment, when solid-solution selenium (Se) is used, the selenium used as a raw material is either an organic selenium compound (organo-chalcogen compound) or an inorganic selenium compound, either in its solid state or dissolved in a high-boiling point solvent. While the structure is not particularly limited, for example, trioctylphosphine selenide ( _C8H17 ) _3P =Se, obtained by dissolving selenium in trioctylphosphine, tributylphosphine _selenide ( _C4H9 ) _3P =Se, obtained by dissolving selenium in tributylphosphine, or a solution obtained by dissolving selenium at high temperature in a high-boiling point solvent that is _a long-chain hydrocarbon such as octadecene can be used.

Furthermore, in this embodiment, when sulfur (S) is dissolved in a solid solution, the sulfur used as a raw material is either an organic sulfur compound (organic chalcogen compound) or an inorganic sulfur compound, either in its solid form or dissolved in a high-boiling point solvent. While the structure is not particularly limited, for example, trioctylphosphine sulfide ( _C8H17 ) _3P =S, obtained by dissolving sulfur in trioctylphosphine, or _{tributylphosphine} sulfide ( _C4H9 ) _3P =S, obtained by dissolving sulfur in tributylphosphine, or a solution obtained by dissolving sulfur at high temperature in a high-boiling point solvent that is _a long-chain hydrocarbon such as octadecene can be used.

In this embodiment, an organic bismuth compound or an inorganic bismuth compound is added to a high-boiling point solvent and dissolved. As the solvent, octadecene can be used as a saturated or unsaturated hydrocarbon with a high boiling point of 150°C or higher. In addition, as an aromatic high-boiling point solvent, _{dodecylbenzene} : _{C6H5 ( CH2 ) 11CH3 , and} as a high-boiling point ester solvent, _butyl butyrate: _C4H9COOC4H9 , _benzyl butyrate: _{C6H5CH2COOC4H9} can be used, but aliphatic thiol _{, aliphatic} amine, fatty acid, or aliphatic phosphorus compounds can also be used as solvents.

At this time, the reaction temperature is set to a range of 100°C to 200°C, and the silver compound is dissolved. Preferably, the reaction temperature is lower, between 100°C and 175°C, and even more preferably between 100°C and 150°C.

Furthermore, in this embodiment, there are no particular limitations on the reaction conditions, but in order to obtain quantum dots with uniform particle sizes, it is important to react at a low temperature of about 100°C to a high temperature of about 140°C when synthesizing _AgBiTe₂ , _AgBiSe₂ , and _AgBiS₂ . For this reason, it is preferable to dissolve one or two types of raw materials in a high-boiling point solvent heated to about 100°C, sequentially add the other raw materials to the solution, and then continue the reaction at the same temperature to synthesize quantum dots.

Furthermore, in this embodiment, in order to obtain _AgBiE2 with uniform particle size, it is preferable to add 1 to 200 equivalents of thiol relative to Te, Se, or S in the reaction of the precursor silver raw material, bismuth raw material, and chalcogen raw material, more preferably 5 to ₁₀₀₀ equivalents, and even more preferably 50 to 10000 equivalents. In particular, the thiol is not limited, but for example, octadecanethiol: _C18H37SH , _{hexanedecanethiol} : _C16H33SH , tetradecanethiol: _C14H29SH , _{dodecanethiol} : _C12H25SH , _decanthiol : _C10H21SH , octanthiol: _{C8H17SH , etc.} can be used.

Furthermore, in this embodiment, when adding each raw material and proceeding with the reaction, a compound is needed that plays an auxiliary role in releasing the precursor metal into the reaction solution through coordination or chelation.

Compounds having the aforementioned roles include ligands capable of forming complexes with silver. For example, phosphorus-based ligands, amine-based ligands, thiol-based ligands, and carboxylic acid-based ligands are preferred, with thiol-based ligands being particularly preferred due to their high efficiency.

This allows for the proper reaction between Ag, Bi, and chalcogen, enabling the production of AgBiE ₂ quantum dots based on Ag, Bi, and chalcogen, with an optical band gap of 0.90 to 1.10 eV and a uniform particle size.

In the quantum dot manufacturing method of this embodiment, it is preferable to have the following steps: dissolving one or two of the above-mentioned silver raw material, bismuth raw material, chalcogenide raw material, and ligand in the high-boiling point solvent heated to 100°C to 150°C; then sequentially adding the other raw materials; and after adding all the raw materials, synthesizing quantum dots at a reaction temperature of approximately the same magnitude.

This allows for the safe and rapid mass production of silver bismuth chalcogenide with a uniform particle size, using easily handled reagents and without the need for intermediates.

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

<Raw materials>
In this invention, the following raw materials were used to synthesize silver chalcogenide compound ( _AgBiE2 system) quantum dots having a uniform particle size.
(solvent)
Octadecene: Manufactured by Aldrich Co., Ltd. and Idemitsu Kosan Co., Ltd. (as a raw material for chalcogen)
Sulfur powder (99.0% or higher): Manufactured by Kishida Chemical Co., Ltd. (Bismuth raw material)
Bismuth acetate oxide: Manufactured by Kishida Chemical Co., Ltd. Bismuth nitrate pentahydrate: Manufactured by Fujifilm Wako Pure Chemical Corporation Bismuth oxide: Manufactured by Mitsuwa Chemical Co., Ltd. (silver raw material)
Silver acetate: Manufactured by Kishida Chemical Co., Ltd. (ligand)
Trioctylphosphine: Manufactured by Hokko Chemical Co., Ltd. Dodecanethiol: Manufactured by Arkema <Measuring Instruments>
Ultraviolet-Visible Spectrophotometer: Hitachi, Ltd. V-770
X-ray diffraction device (XRD): Bruker D2 PHASER
Scanning transmission electron microscope (STEM): Hitachi, Ltd. SU9000
Differential thermal balance (TG-DTA): Rigaku Corporation, Thermo Plus EVO2

[Example 1]
In a 300 mL reaction vessel, 200.2 mg of silver acetate (Ag(OAc)), 30.0 mL of dodecanethiol (DDT), and 582.2 mg of bismuth nitrate pentahydrate (Bi( _NO₃ ) _₃ · _5H₂O ) were added. The mixture was then heated at 145°C for 5 minutes with stirring under an inert gas ( _N₂ ) atmosphere to dissolve the starting materials.

To this solution, 13.0 mL of 0.20 M octadecensulfide (S-ODE) was added, and the mixture was heated with stirring at the same temperature for a further 10 minutes.

The resulting reaction mixture was cooled to room temperature, toluene and ethanol were added to generate a precipitate, and the precipitate was collected by centrifugation. Then, hexane was added to the precipitate to disperse it and obtain a dispersion solution of AgBiS ₂ particles.

The obtained dispersion solution was measured using an ultraviolet-visible spectrometer. As a result, the ultraviolet-visible near-infrared absorption spectrum shown in Figure 2 was obtained. Furthermore, it was found that the quantum dots obtained in this synthesis have an optical band gap of 0.90 eV, as shown in the analysis results of the TauC plot in Figure 3. In addition, the average particle size of these quantum dots was approximately 4.43 nm, as shown in Figure 4, and it was found that more than two-thirds of the total number of particles fell within ±20% of the average particle size. Furthermore, differential thermal analysis (TG) revealed that approximately 15% of the entire quantum dot was coordinated with ligands, as shown in Figure 5.
Furthermore, the peak values and peak patterns of the XRD spectrum of _AgBiS2 particles shown in Figure 6 demonstrate that an _AgBiS2 solid solution is being formed.

[Example 2]
In a 300 mL reaction vessel, 200.2 mg of silver acetate (Ag(OAc)), 30.0 mL of dodecanethiol (DDT), and 582.2 mg of bismuth nitrate pentahydrate (Bi( _NO₃ ) _₃ · _5H₂O ) were added. The mixture was then heated at 145°C for 5 minutes with stirring under an inert gas ( _N₂ ) atmosphere to dissolve the starting materials.

To this solution, 13.0 mL of 0.20 M octadecensulfide (S-ODE) was added, and the mixture was heated with stirring at the same temperature for a further 20 minutes.

The resulting reaction mixture was cooled to room temperature, toluene and ethanol were added to generate a precipitate, and the precipitate was collected by centrifugation. Then, hexane was added to the precipitate to disperse it and obtain a dispersion solution of AgBiS ₂ particles.

The obtained dispersion solution was measured using a UV-Vis spectrometer. As a result, the UV-Vis near-infrared absorption spectrum shown in Figure 7 was obtained. Furthermore, the quantum dots obtained in this synthesis were found to have an optical band gap of 1.075 eV, as shown in the analysis results of the TauC plot in Figure 8. The average particle size of these quantum dots was approximately 4.53 nm, as shown in Figure 9, and it was found that more than two-thirds of the total particles fell within ±20% of the average particle size. Additionally, analysis using a differential thermal balance revealed that approximately 18% of the total mass of the quantum dots was coordinated by ligands, as shown in Figure 10.

Furthermore, the peak values and peak patterns of the XRD spectrum of _AgBiS2 particles shown in Figure 11 demonstrated that an _AgBiS2 solid solution was formed.

[Example 3]
200.2 mg of silver acetate (Ag(OAc)), 30.0 mL of dodecanethiol (DDT), and 340.8 mg of bismuth acetate (BiO(OC(=O) _CH3 )) were placed in a 300 mL reaction vessel. The mixture was then heated at 140°C for 5 minutes with stirring under an inert gas ( _N2 ) atmosphere to dissolve the starting materials.

To this solution, 19.5 mL of 0.20 M octadecensulfide (S-ODE) was added, and the mixture was heated with stirring at the same temperature for a further 15 minutes.

The resulting reaction mixture was cooled to room temperature, toluene and ethanol were added to generate a precipitate, and the precipitate was collected by centrifugation. Then, hexane was added to the precipitate to disperse it and obtain a dispersion solution of AgBiS ₂ particles.

The obtained dispersion solution was measured using a UV-Vis spectrometer. As a result, the UV-Vis near-infrared absorption spectrum shown in Figure 12 was obtained. Furthermore, the quantum dots obtained in this synthesis were found to have an optical band gap of 1.065 eV, as shown in the analysis results of the TauC plot in Figure 13. The average particle size of these quantum dots was approximately 4.23 nm, as shown in Figure 14, and it was found that more than two-thirds of the total particles fell within ±20% of the average particle size. Additionally, analysis using a differential thermal balance revealed that approximately 18% of the total mass of the quantum dots was coordinated by ligands, as shown in Figure 15.

Furthermore, the peak values and peak patterns of the XRD spectrum of AgBiS2 _particles shown in Figure 16 demonstrated that an _AgBiS2 solid solution was formed.

[Example 4]
In a 300 mL reaction vessel, 200.2 mg of silver acetate (Ag(OAc)), 30.0 mL of dodecanethiol (DDT), and 340.8 mg of bismuth acetate (BiO(OC(=O) _CH3 )) were added. The mixture was then heated at 100°C for 5 minutes with stirring under an inert gas ( _N2 ) atmosphere to dissolve the starting materials.

To this solution, 13.0 mL of 0.20 M octadecensulfide (S-ODE) was added, and the mixture was heated with stirring at the same temperature for a further 15 minutes.

The resulting reaction mixture was cooled to room temperature, toluene and ethanol were added to generate a precipitate, and the precipitate was collected by centrifugation. Then, hexane was added to the precipitate to disperse it and obtain a dispersion solution of AgBiS ₂ particles.

The obtained dispersion solution was measured using a UV-Vis spectrometer. As a result, the UV-Vis near-infrared absorption spectrum shown in Figure 17 was obtained. Furthermore, it was found that the quantum dots obtained in this synthesis have an optical band gap of 1.10 eV, as shown in the analysis results of the TauC plot in Figure 18. The average particle size of these quantum dots was approximately 4.82 nm, as shown in Figure 19, and it was found that more than two-thirds of the total particles fell within ±20% of the average particle size. Additionally, analysis using a differential thermal balance revealed that approximately 19% of the total mass of the quantum dots was coordinated by ligands, as shown in Figure 20.

Furthermore, the peak values and peak patterns of the XRD spectrum of _AgBiS2 particles shown in Figure 21 demonstrated that an _AgBiS2 solid solution was formed.

[Example 5]
200.2 mg of silver acetate (Ag(OAc)), 30.0 mL of dodecanethiol (DDT), and 582.2 mg of bismuth nitrate pentahydrate (Bi( _NO₃ ) _₃ · _5H₂O ) were placed in a 300 mL reaction vessel. The mixture was then heated at 100°C for 5 minutes with stirring under an inert gas ( _N₂ ) atmosphere to dissolve the starting materials.

To this solution, 13.0 mL of 0.20 M octadecensulfide (S-ODE) was added, and the mixture was heated with stirring at the same temperature for a further 15 minutes.

The resulting reaction mixture was cooled to room temperature, toluene and ethanol were added to generate a precipitate, and the precipitate was collected by centrifugation. Then, hexane was added to the precipitate to disperse it and obtain a dispersion solution of AgBiS ₂ particles.

The obtained dispersion solution was measured using a UV-Vis spectrometer. As a result, the UV-Vis near-infrared absorption spectrum shown in Figure 22 was obtained. Furthermore, it was found that the quantum dots obtained in this synthesis have an optical band gap of 1.10 eV, as shown in the analysis results of the TauC plot in Figure 23. The average particle size of these quantum dots was approximately 4.52 nm, as shown in Figure 24, and it was found that more than two-thirds of the total particles fell within ±20% of the average particle size. Additionally, analysis using a differential thermal balance revealed that approximately 22% of the total mass of the quantum dots was coordinated by ligands, as shown in Figure 25.

Furthermore, the peak values and peak patterns of the XRD spectrum of _AgBiS2 particles shown in Figure 26 demonstrated that an _AgBiS2 solid solution was formed.

[Comparative Example 1]
200.2 mg of silver acetate (Ag(OAc)), 30.0 mL of octadecene (ODE), and 582.2 mg of bismuth nitrate pentahydrate (Bi( _NO₃ ) _₃ · _5H₂O ) were placed in a 300 mL reaction vessel. The mixture was then heated at 150 °C for 5 minutes with stirring under an inert gas ( _N₂ ) atmosphere.

To this solution, 13.0 mL of 0.20 M octadecensulfide (S-ODE) was added, and the mixture was heated with stirring at the same temperature for a further 15 minutes.

The resulting reaction solution ( _AgBiS₂ ) was cooled to room temperature. The reaction solution showed no change in color, and XRD confirmed that the starting materials remained intact in the resulting solution.

[Comparative Example 2]
200.2 mg of silver acetate (Ag(OAc)), 30.0 mL of dodecanethiol (DDT), and 582.2 mg of bismuth nitrate pentahydrate (Bi( _NO₃ ) _₃ · _5H₂O ) were placed in a 300 mL reaction vessel. The mixture was then heated at 250 °C for 5 minutes with stirring under an inert gas ( _N₂ ) atmosphere to dissolve the starting materials.

To this solution, 13.0 mL of 0.20 M octadecensulfide (S-ODE) was added, and the mixture was heated with stirring at the same temperature for a further 15 minutes.

The resulting reaction solution ( _AgBiS₂ ) was cooled to room temperature. The reaction solution changed to a black suspension, and no reflection peaks were observed in the resulting solution by XRD.

In Comparative Example 1, it was found that the AgBiE ₂ -based quantum dots could not be properly synthesized because the ligand dodecanethiol was not present in the system, and in Comparative Example 2, it was found that the reaction temperature was high at 250°C.

Based on the above, it was found that _AgBiE2- based quantum dots can be synthesized according to Examples 1 to 5.

According to the present invention, _AgBiS2 quantum dots with a uniform particle size and an optical band gap of 0.90 to 1.10 eV can be synthesized using an easily handled reagent and in a method that allows for direct mass production without the need for intermediates. Furthermore, by applying the quantum dots of the present invention to light absorption devices and the like, excellent solar cell performance can be obtained in the apparatus.

1: Quantum dot 1a: Core 1b: Shell 2: Organic ligand

Claims (4)

A quantum dot aggregate characterized by being a nanocrystal represented by _AgBiE2 (where E is at least one of tellurium, selenium, or sulfur) containing silver, bismuth, and chalcogen, containing a large number of quantum dots with an average particle size of 3 nm to 5 nm , and in the particle size distribution in STEM, more than two-thirds of the quantum dots are contained within ±20% of the average particle size. The quantum dot assembly according to claim 1, characterized in that the surface of the quantum dot is covered with a ligand. The quantum dot assembly according to claim 2, characterized in that the ligand is selected from at least one or two of the phosphine-based, aliphatic thiol-based, aliphatic amine-based, and aliphatic carboxylic acid-based systems. A quantum dot assembly according to any one of claims 1 to 3, characterized in that an optical band gap of 0.90 to 1.075 eV can be obtained in a TauC plot.