TWI838568B - Quantum dots and methods for making the same - Google Patents

Abstract

The purpose of the present invention is to provide a Ag ₂ E (E is at least one of Te, Se, and S) quantum dot and a method for manufacturing the quantum dot. The Ag ₂ E (E is at least one of Te, Se, and S) quantum dot can show a clear absorption peak in the near-infrared region and obtain an absorption wavelength based on the purpose. The method for manufacturing the quantum dot can be mass-produced, and the absorption wavelength can be freely controlled in the near-infrared region according to the purpose, and the safety is high. The quantum dot of the present invention is characterized in that it is a nanocrystal represented by Ag ₂ E (E is at least one of tellurium, selenium, or sulfur) containing silver and chalcogen elements, and the average particle size is greater than 1 nm and less than 15 nm. In the present invention, the absorption wavelength is preferably in the range of 800nm to 1700nm in the near-infrared region.

Description

Quantum dots and methods for making them

The present invention relates to a quantum dot that absorbs or emits light in the near-infrared region and a method for manufacturing the same.

Quantum dots are nanoparticles containing hundreds to thousands of atoms. Quantum dots are also called fluorescent nanoparticles, semiconductor nanoparticles or nanocrystals.

For quantum dots, the emission wavelength can be changed in various ways by changing the particle size or composition of the nanoparticles. In addition, the performance or characteristics of quantum dots can be expressed as: fluorescence quantum yield (Quantum Yield: QY), fluorescence half-height width (Full Width at Half Maximum: FWHM), absorption wavelength or fluorescence wavelength.

For example, the following non-patent documents 1 and 2 describe silver chalcogenide quantum dots.

Non-patent document 1 reports a method for synthesizing Ag ₂ Te quantum dots by a cation exchange method using CdTe. Non-patent document 2 reports a method for synthesizing Ag ₂ Te quantum dots via silver silylamide.

However, the synthesis method of non-patent document 1 is through a cadmium intermediate, which is a heavy metal subject to toxic regulation, and lacks practical application. In addition, the silver silylamide used in non-patent document 2 is a reactant with high reactivity when used in the atmosphere, and its operation requires attention. Prior art document Non-patent document

Non-patent literature 1: ACS Appl. Mater. Interfaces 2013, VOL. 5, pp 1149-1155 Cation Exchange-Based Facile Aqueous Synthesis of Small, Stable, and Nontoxic Near-Infrared Ag ₂ Te/ZnS Core/Shell Quantum DotsEmitting in the Second Biological Window Non-patent literature 2: ACS NANO 2011, VOL. 5, NO. 5, pp 3758-3765 Infrared Emitting and Photoconducting Colloidal SilverChalcogenide Nanocrystal Quantum Dots from a Silylamide-Promoted Synthesis

[The problem the invention is trying to solve]

Therefore, it is required to develop a practical and safer production method for Ag ₂ E (E is at least one of Te, Se, and S) quantum dots. In addition, there are the following issues.

That is, when quantum dots are used as an absorption material in the near-infrared region, it is important that a clear absorption peak exists in the near-infrared region and that the absorption wavelength can be freely adjusted according to the purpose. However, silver chalcogenide quantum dots having a clear absorption peak in the near-infrared region and being able to obtain an absorption wavelength appropriately based on the purpose, and a method for producing the same have not yet been established.

Furthermore, it is believed that the near-infrared fluorescence spectra of the silver chalcogenide quantum dots described in Non-Patent Documents 1 and 2, when judged based on the spectra that are not clearly described but revealed, have a fluorescence half-width (Full Width at Half Maximum: FWHM) exceeding 200 nm.

Based on the above, there is a strong demand for developing a practical and safer manufacturing method for silver chalcogenide quantum dots, as well as analyzing the physical properties of silver chalcogenide quantum dots manufactured by such a method.

The present invention is completed in view of the above aspects, and its purpose is to provide a kind of _Ag2E (E is at least any one of Te, Se, S) quantum dot and the manufacturing method of quantum dot. The above-mentioned _Ag2E (E is at least any one of Te, Se, S) quantum dot can show a clear absorption peak in the near-infrared region and obtain the absorption wavelength based on the purpose. The manufacturing method of the above-mentioned quantum dot can be mass-produced, and the absorption wavelength in the near-infrared region can be freely controlled according to the purpose, and it is highly safe.

Furthermore, the present invention is made in view of the above aspects, and its purpose is to provide a Ag ₂ E (E is at least one of Te, Se, S) quantum dot that exhibits high brightness near-infrared fluorescence, and a method for manufacturing quantum dots that can be mass-produced and has high safety. [Technical means for solving the problem]

The quantum dots of the present invention are characterized in that they are nanocrystals represented by Ag ₂ E (E is at least one of tellurium, selenium or sulfur) containing silver and chalcogen elements, and have an average particle size of not less than 1 nm and not more than 15 nm.

The characteristic of the method for manufacturing quantum dots in the present invention is that quantum dots represented by Ag ₂ E (E is at least one of tellurium, selenium or sulfur) are synthesized by using silver raw materials and trioctylphosphine containing a chalcogen element.

Furthermore, the method for producing quantum dots in the present invention is characterized in that quantum dots represented by Ag ₂ E (E is at least one of tellurium, selenium or sulfur) are synthesized from silver raw materials and dichalcogen compounds. [Effects of the Invention]

The quantum dots according to the present invention can show a clear absorption peak in the near-infrared region and have an absorption wavelength based on the purpose.

Alternatively, the quantum dots of the present invention can narrow the half-width of fluorescence in the near-infrared region, and can display near-infrared fluorescence with high brightness.

Furthermore, the method for producing quantum dots according to the present invention can be synthesized in an atmosphere without using highly reactive reagents or toxic heavy metals, and can be mass-produced.

In the present invention, as a method for synthesizing quantum dots, a method of mixing a silver raw material with trioctylphosphine containing a chalcogen or a method of mixing a silver raw material with a dichalcogen compound can be used. The former synthesis method is suitable for a method for producing quantum dots that can show a clear absorption peak in the near-infrared region and can freely control the absorption wavelength according to the purpose. In addition, the latter synthesis method is suitable for a method for producing quantum dots that can narrow the half-width of fluorescence in the near-infrared region and can show high-brightness near-infrared fluorescence.

Hereinafter, an embodiment of the present invention (hereinafter referred to as "embodiment") is described in detail. Furthermore, the present invention is not limited to the following embodiment, and can be implemented in various modifications within the scope of its subject matter. In this specification, the notation "～" includes a lower limit and an upper limit.

FIG1A and FIG1B are schematic diagrams of quantum dots in this embodiment. The quantum dot 1 shown in FIG1A is a nanocrystal containing silver (Ag) and a chalcogen (referring to at least one of tellurium (Te), selenium (Se) and sulfur (S)). Hereinafter, the chemical formula of the quantum dot in this embodiment is represented by Ag ₂ E (E is at least one of Te, Se, S).

The quantum dots in this embodiment have fluorescence properties based on band edge luminescence and exhibit quantum size effects according to the size of the particles.

The average particle size of the quantum dots of this embodiment is characterized by being greater than 1 nm and less than 15 nm. In addition, in this embodiment, the above average particle size can be met, and multiple quantum dots can be generated with uniform particle sizes. The so-called "uniform" refers to a state where more than 90% of the particles are contained within ±30% of the average particle size. In this way, in this embodiment, fine and uniform high-quality quantum dots can be mass-produced.

The quantum dots of the present embodiment contain Ag and Te, Ag and Se, or Ag and S as the main components, and may also contain elements other than these elements. However, as described below, in the method for preparing the quantum dots of the present embodiment, reactants such as metal amides or organic lithium compounds that show higher reactivity when operated under the atmosphere are not used, and the reaction intermediates do not contain regulated heavy metals represented by Cd or Pb. Cadmium/lead-containing compounds are toxic regulated heavy metals that can easily lead to increased costs, restricted operations, or complicated manufacturing steps. The quantum dots of the present embodiment do not contain substances derived from highly reactive reactants or regulated heavy metals. In addition, in the present embodiment, two or more of Te, Se and S may also be included.

As shown in FIG1A , it is preferred that a plurality of organic ligands 2 are coordinated on the surface of the quantum dots. This can inhibit the aggregation of quantum dots and exhibit the target optical properties. The ligands that can be used for the reaction are not particularly limited, and the following ligands can be listed as representative ones.

(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 ; capric} acid: _C9H19COOH ; _{caprylic acid: C7H15COOH ( 3} ) Thiols _{: octadecylmercaptan} : _{C18H37SH ; hexadecanethiol} : _C16H33 SH; Tetradecyl mercaptan: C ₁₄ H ₂₉ SH; Dodecanethiol: C ₁₂ H ₂₅ SH; Decyl mercaptan: C ₁₀ H ₂₁ SH; Octyl mercaptan: C ₈ H ₁₇ SH (4) Phosphine series Trioctylphosphine: (C ₈ H ₁₇ ) ₃ P; Triphenylphosphine: (C ₆ H ₅ ) ₃ P; Tributylphosphine: (C ₄ H ₉ ) ₃ P (5) Phosphine oxide series Trioctylphosphine oxide: (C ₈ H ₁₇ ) ₃ P=O; Triphenylphosphine oxide: (C ₆ H ₅ ) ₃ P=O; Tributylphosphine oxide: (C ₄ H ₉ ) ₃ P=O

In this embodiment, as shown in Fig. 1B, the quantum dot 1 may be a core-shell structure having a core 1a and a shell 1b covering the surface of the core 1a. As shown in Fig. 1B, it is preferred that a plurality of organic ligands 2 are coordinated on the surface of the quantum dot 1.

The core 1a shown in FIG1B is Ag ₂ E. The shell 1b, like the core 1a, does not contain regulated heavy metals such as Cd, Hg, or Pd, or substances derived from highly reactive reagents such as metal amides or organic lithium compounds.

In addition, the shell 1b may be in a state of being dissolved in the surface of the core 1a. In FIG1B , the boundary between the core 1a and the shell 1b is shown by a dotted line, but this means that the boundary between the core 1a and the shell 1b can be confirmed by analysis or cannot be confirmed by analysis.

However, in the present embodiment, the quantum dot 1 having only the core 1a, i.e., the core of FIG. 1A , without the shell 1b may have the absorption wavelength shown below, or may have the fluorescence wavelength and fluorescence half-width shown below.

[First Quantum Dot] <Absorption Characteristics> The inventors synthesized nanocrystals represented by Ag ₂ E (E is at least one of tellurium, selenium or sulfur) containing silver and chalcogen elements by a novel manufacturing method that can be mass-produced and has excellent safety, and analyzed the physical properties. The results showed that the nanocrystals have an absorption wavelength in the near-infrared region of 800 nm to 1700 nm.

The so-called "absorption wavelength" refers to the wavelength of the peak of the absorption spectrum, that is, the state where the absorption peak appears in the near-infrared region. The inventors of the present invention have mainly conducted the following research: (1) Free control of the absorption peak observed in the near-infrared region according to the purpose; (2) Clarifying the contrast between the peak and the valley of the absorption peak observed in the near-infrared region. Regarding the above (1), it is to respond to the demand for adjusting the absorption peak wavelength in the near-infrared region according to the purpose. In addition, regarding the above (2), it is to make the conduction/cutoff of absorption clear when used as a near-infrared absorbing material.

The quantum dots according to the present embodiment have an absorption wavelength in the near-infrared region of 800 nm to 1700 nm, and the absorption wavelength can be freely controlled according to the purpose. As shown in the following production method, as an example, the amount of solvent (also known as the concentration of silver and chalcogen element), the ratio of octadecene (ODE) used as solvent, and trioctylphosphine (TOP) can be adjusted.

In this embodiment, the absorbance ratio between the peak and valley of the absorption peak can be set to 1.0 or more. The absorbance ratio is preferably set to 1.5 or more, and more preferably set to 2.0 or more. Thereby, when used as a near-infrared absorbing material, the on/off of the absorption can be made clear.

In this embodiment, it is preferred that the longest absorption wavelength peak having the above absorbance ratio exists in the near-infrared region of 1000-1500 nm. This can further clarify the on/off of the absorption corresponding to the desired purpose.

In this embodiment, it is preferred that the longest absorption wavelength is in the near-infrared region of 1100 nm to 1600 nm. Moreover, in this embodiment, it is more preferred that the longest absorption wavelength is in the near-infrared region of 1300 nm to 1500 nm.

In addition, the average particle size of the quantum dots of the present embodiment is not less than 1 nm and not more than 15 nm, and is uniform. However, in terms of obtaining an absorption wavelength in the near-infrared region, a smaller particle size is also a factor.

<Ligand exchange> In this embodiment, for the organic ligand 2, it is preferred to use a shorter ligand. The above situation is not limited, and for the organic ligand 2, 3-methylpropionic acid (MPA) can be used.

For example, quantum dots are dispersed in large quantities in the light absorption layer (or quantum dot layer) of the infrared sensor. In this case, the ligands of the quantum dots contained in the light absorption layer are preferably shorter than the ligands when the quantum dots are formed by synthesis.

Thus, by using shorter ligands for the quantum dots contained in the light absorbing layer, the roughness of the light absorbing layer can be reduced, and the extraction efficiency of electrons and holes can be improved. On the other hand, by using longer ligands when forming quantum dots by liquid phase synthesis, the dispersion film-forming property can be improved.

In this embodiment, quantum dots with longer ligands can be synthesized by a liquid phase synthesis method and then exchanged for shorter ligands (such as 3-hydroxypropionic acid) before or after coating the composition containing quantum dots.

Although not particularly limited, in this embodiment, it is preferred to add dodecanethiol (DDT) in the final step of the synthesis stage, so that ligand exchange can be performed rapidly.

[Second quantum dot] <Fluorescence properties> The fluorescence wavelength of the quantum dot of this embodiment is in the range of 800 nm to 1700 nm in the near-infrared region, and the fluorescence half-width is less than 200 nm. Here, the so-called "fluorescence wavelength" refers to the wavelength of the peak of the fluorescence spectrum. The quantum dot of this embodiment can control the fluorescence wavelength within the range of 800 nm to 1700 nm, preferably 1000 nm to 1700 nm, and more preferably 1200 nm to 1700 nm. In addition, the so-called "fluorescence half-width" refers to the half-peak full width (Full Width at Half Maximum) of the expansion of the fluorescence wavelength at half the intensity of the peak value of the fluorescence intensity in the fluorescence spectrum. Furthermore, the fluorescence half-width is preferably 150 nm or less, and more preferably 100 nm or less.

In this embodiment, as described below, a dichalcogenide compound is used as a precursor as a reaction system for synthesizing quantum dots, and Ag is introduced into the precursor. By manufacturing quantum dots based on such a direct and simple synthesis reaction, the fluorescence half-width can be narrowed. As shown in the following experimental results, specifically, a fluorescence half-width of less than 150 nm can be obtained.

The quantum yield of the quantum dots in this embodiment is 5% or more. Furthermore, the quantum yield is more preferably 10% or more, further preferably 20% or more, and further preferably 30% or more. Thus, in this embodiment, the quantum yield of the quantum dots can be increased.

In this embodiment, the fluorescence wavelength can be freely controlled within the range of 800 nm to 1700 nm. The quantum dots in this embodiment are solid solutions based on Ag ₂ E, which is a chalcogen element other than silver. In this embodiment, the fluorescence wavelength can be appropriately controlled by adjusting the average particle size of the quantum dots and the composition of the quantum dots. The fluorescence wavelength is preferably 1000 nm or more, and more preferably 1200 nm or more.

The quantum dots of this embodiment can also satisfy both the absorption characteristics shown in the first quantum dots and the fluorescence characteristics shown in the second quantum dots.

[First quantum dot production method] Next, the first quantum dot production method of this embodiment is described. In this embodiment, quantum dots represented by Ag ₂ E (E is at least one of tellurium, selenium, or sulfur) are synthesized using a silver raw material and trioctylphosphine containing a chalcogen element.

In the present embodiment, the Ag raw material is not particularly limited, and for example, the following organic silver compounds or inorganic silver compounds can be used. That is, as acetate, silver acetate (I): Ag(OAc) can be used; as fatty acid salt, silver stearate: Ag(OC(=O)C ₁₇ H ₃₅ ); silver oleate: Ag(OC(=O)C ₁₇ H ₃₃ ); silver myristate: Ag(OC(=O)C ₁₃ H ₂₇ ); silver laurate: Ag(OC(=O)C ₁₁ H ₂₃ ); silver acetylacetonate: Ag(acac); as halides, monovalent compounds can be used; and silver chloride (I): AgCl; silver bromide (I): AgBr; silver iodide (I): AgI, etc. can be used.

In the present embodiment, it is preferred to dissolve the silver raw material in octadecene (ODE) and trioctylphosphine (TOP) as solvents. Furthermore, oleylamine can also be used instead of TOP for synthesis, but in order to obtain an excellent P/V ratio (peak/trough ratio), it is preferred to use TOP. In the present embodiment, by adjusting the amount of ODE and TOP (solvent amount) and the ratio of ODE/TOP, the absorption peak wavelength can be freely controlled in the near-infrared region according to the purpose. Although not limited, it is preferred to adjust the solvent amount (also known as the concentration of silver and chalcogen element) within the range of 5 mL to 30 mL. Moreover, it is more preferred to adjust the solvent amount within the range of 10 mL to 25 mL. Furthermore, although not limited thereto, it is preferred that the ratio of ODE/TOP be in the range of 0.5 to 1.5, and it is more preferred that the ratio of ODE/TOP be in the range of 0.8 to 1.0.

Examples of trioctylphosphines containing sulfur elements include trioctylphosphine telluride (Te-TOP), trioctylphosphine selenide (Se-TOP) and trioctylphosphine sulfide (S-TOP).

In this embodiment, trioctylphosphine containing a sulfide element is mixed with a solution obtained by dissolving a silver raw material in a solvent, and then heated for synthesis. In this embodiment, the heating temperature can be lowered. Specifically, the heating temperature can be set to 200°C or less. Preferably, it can be set to 150°C or less. In addition, the lower limit of the heating temperature is about 70°C.

By the above-mentioned manufacturing method, the following quantum dots can be obtained. The quantum dots are nanocrystals represented by Ag ₂ E (E is at least one of tellurium, selenium or sulfur) containing silver and chalcogen elements, and have an average particle size of 1 nm to 15 nm, and absorb wavelengths in the near-infrared region of 800 nm to 1700 nm. In addition, the manufacturing method of the quantum dots according to the present embodiment can be synthesized by a method that can be mass-produced without using highly reactive reactants or toxic heavy metals when operating under the atmosphere.

Furthermore, in this embodiment, dodecanethiol (DDT) can be added in the final step of the synthesis stage. By adding DDT, the ligand exchange of the quantum dots can be carried out quickly.

[Method for producing the second quantum dot] Next, the method for producing the second quantum dot of this embodiment is described. In this embodiment, quantum dots represented by Ag ₂ E (E is at least one of Te, Se, or S) are synthesized using a silver raw material and a dichalcogen compound.

In the present embodiment, the Ag raw material is not particularly limited, and for example, the following organic silver compounds or inorganic silver compounds can be used. That is, as acetate, silver acetate (I): Ag(OAc) can be used; as fatty acid salt, silver stearate: Ag(OC(=O)C ₁₇ H ₃₅ ); silver oleate: Ag(OC(=O)C ₁₇ H ₃₃ ); silver myristate: Ag(OC(=O)C ₁₃ H ₂₇ ); silver laurate: Ag(OC(=O)C ₁₁ H ₂₃ ); silver acetylacetonate: Ag(acac); as halides, monovalent compounds can be used; and silver chloride (I): AgCl; silver bromide (I): AgBr; silver iodide (I): AgI, etc. can be used.

In this embodiment, as the dichalcogen compound, an organic dichalcogen compound represented by _R1 - _E1 - _E2 - _R2 can be used. Here, _R1 and _R2 can be alkyl groups, and _E1 and _E2 can be at least one of Te, Se, or S. _R1 and _R2 can be the same or different. _E1 and _E2 can be the same or different.

In this embodiment, when tellurium is dissolved in a solid solution, an organic tellurium compound (organic chalcogen compound) or an inorganic tellurium compound dissolved in a high boiling point solvent can be used as a raw material. The structure of the compound is not particularly limited, and for example, diphenyl ditelluride: (C ₆ H ₅ ) ₂ Te ₂ and dialkyl ditelluride: R ₂ Te ₂ can be used.

In the present embodiment, when selenium is dissolved in a solid solution, an organic selenium compound (organic chalcogen compound) or an inorganic selenium compound dissolved in a high boiling point solvent can be used as a raw material. The structure thereof is not particularly limited, and for example, a solution obtained by dissolving selenium in diphenyl diselenide: (C ₆ H ₅ ) ₂ Se ₂ or other dialkyl diselenide: R ₂ Se ₂ at a high temperature, or a high boiling point solvent of a long chain hydrocarbon such as octadecene can be used.

In the present embodiment, when sulfur is dissolved in a solid solution, an organic sulfur compound (organic chalcogen compound) or an inorganic sulfur compound dissolved in a high boiling point solvent is used as a raw material. The structure thereof is not particularly limited, and for example, a solution obtained by dissolving sulfur in diphenyl disulfide: (C ₆ H ₅ ) ₂ S ₂ or other dialkyl disulfide: R ₂ S ₂ or a high boiling point solvent of a long chain hydrocarbon such as octadecene at a high temperature can be used.

In this embodiment, a dichalcogen compound as a precursor is obtained from the above-mentioned organic chalcogen or inorganic chalcogen. In this embodiment, it is preferred to synthesize the dichalcogen compound as a precursor within a range of 120°C to 250°C. In addition, it is preferred to set the reaction temperature to a lower temperature of 100°C to 220°C, more preferably to a lower temperature of 80°C to 200°C, and further preferably to a lower temperature of 60°C to 200°C.

Then, the silver raw material and the disulfide compound are mixed and dissolved. As the solvent, octadecene can be used as a high boiling point saturated hydrocarbon or unsaturated hydrocarbon. In addition, dodecylbenzene can be used as an aromatic high boiling point solvent, and butyl butyrate: C ₄ H ₉ COOC ₄ H ₉ ; benzyl butyrate: C ₃ H ₇ COOCH ₂ C ₆ H ₅ can be used as a high boiling point ester solvent; aliphatic thiol, aliphatic amine, or fatty acid compound or aliphatic phosphorus compound can also be used as a solvent.

In order to obtain Ag ₂ E with a higher fluorescence intensity, it is preferred to add 1 to 200 equivalents of thiol relative to Te, Se or S in the reaction of the dichalcogenide compound as a precursor and the silver raw material. In order to obtain quantum dots with a higher fluorescence intensity, it is more preferred to add 5 to 1000 equivalents, and further preferably to add 50 to 10000 equivalents. The thiol is not particularly limited, for example, octadecyl thiol: C ₁₈ H ₃₇ SH; hexadecanethiol: C ₁₆ H ₃₃ SH; tetradecyl thiol: C ₁₄ H ₂₉ SH; dodecanethiol: C ₁₂ H ₂₅ SH; decanethiol: C ₁₀ H ₂₁ SH; octanethiol: C ₈ H ₁₇ SH, etc.

In this embodiment, the reaction method is not particularly limited. To obtain quantum dots with narrow fluorescence half-width, it is important to synthesize Ag ₂ Te, Ag ₂ Se and Ag ₂ S with uniform average particle size. Therefore, it is preferred to quickly add the silver raw material to the dichalcogenide compound as a precursor in a heated solvent and heat it at 120°C to 250°C.

Furthermore, in the present embodiment, a compound having an auxiliary effect of freeing the metal of the precursor in the reaction solution by coordination or chelation is required for the reaction.

As compounds having the above-mentioned effects, there can be listed ligands that can complex with silver, for example, phosphorus-based ligands, amine-based ligands, thiol-based ligands, and carboxylic acid-based ligands are preferred, and thiol-based ligands are particularly preferred in terms of their high efficiency.

Thus, by appropriately carrying out the reaction between Ag and chalcogen, quantum dots based on Ag and chalcogen can be manufactured, which have luminescence in the near-infrared region and a narrow fluorescence half-width.

Furthermore, according to the method for producing quantum dots of the present embodiment, it is possible to synthesize quantum dots without using regulated heavy metals represented by Cd, Hg, and Pb, and reactants that are highly reactive in the atmosphere represented by metal amides and organic lithium compounds as intermediates. Thus, quantum dots can be synthesized using a method that is practical, mass-producible, and highly safe. Example

Hereinafter, the effects of the present invention will be described by way of the embodiments and comparative examples of the present invention. Furthermore, the present invention is not limited in any way by the following embodiments.

[Synthesis of quantum dots as absorbing materials in the near-infrared region] <Raw materials> In the experiment, the following raw materials were used to synthesize silver chalcogenide (Ag ₂ E series) quantum dots that absorb in the near-infrared region. (Solvent) Octadecene: manufactured by Aldrich Co., Ltd., manufactured by Idemitsu Kosan Co., Ltd. Trioctylphosphine: Hokuko Industrial Co., Ltd. Dodecanethiol: manufactured by Arkema Co., Ltd. (Silver raw material) Silver acetate: manufactured by Kishida Chemical Co., Ltd. (Tellurium raw material) Tellurium (4 N: 99.99%): manufactured by Shinko Chemical Co., Ltd., or Aldrich Co. <Measurement equipment> UV-visible spectrophotometer: V-770 manufactured by Hitachi, Ltd. Scanning transmission electron microscope (STEM): SU9000 manufactured by Hitachi, Ltd. X-ray diffraction device (XRD): D2 PHASER manufactured by Bruker Corporation

[Example 1] 166.9 mg of silver acetate, 10.0 mL of ODE, and 10.0 mL of TOP were added to a 100 mL reaction container. Then, the raw materials were dissolved by heating while stirring under an inert gas (N ₂ ) atmosphere. Thus, the total solvent amount of ODE and TOP was 20 mL. The ratio of ODE/TOP was 1.0.

To the solution was added 2.0 mL of 0.25 M trioctylphosphine telluride (Te-TOP), and the solution was heated at 110° C. for 10 minutes while stirring.

To the solution was added 2.0 mL of DDT, and the mixture was heated at 190°C for 10 minutes while being stirred. The obtained reaction solution (Ag ₂ Te) was cooled to room temperature.

Ethanol was added to the obtained reaction solution to generate a precipitate, and the precipitate was recovered by centrifugal separation. Then, toluene was added to the precipitate to disperse the precipitate, thereby obtaining a dispersed solution of Ag ₂ Te quantum dots.

The obtained reaction solution was measured by a UV-visible spectrometer to obtain the UV-visible near-infrared absorption spectrum shown in Figure 2. As shown in Figure 2, an absorption peak appeared at 1150.0 nm. The absorbance ratio between the peak top and the valley of the absorption peak was about 2.1.

When the quantum dots obtained in Example 1 were observed by STEM, the average particle size of the plurality of quantum dots was 3.1 to 3.5 nm, and was 1 nm to 15 nm. Moreover, the particle sizes of more than 90% of the quantum dots were within the average particle size ±0.7 nm. That is, the particle sizes of more than 90% of the quantum dots were within the average particle size ±30%, and the particle sizes of the plurality of quantum dots were uniformly generated.

In the XRD spectrum of the quantum dots obtained in Example 1, as shown in FIG. 3 , a peak was observed near 40°, confirming the formation of Ag ₂ Te solid solution.

Next, a ligand exchange experiment was performed on the quantum dots obtained in Example 1. The results are shown in FIG. 10A and FIG. 10B . By adding 3-methylpropionic acid (MPA), a rapid migration from the upper hexane layer to the lower dimethyl sulfoxide (DMSO) layer occurred.

This confirms the rapid exchange of the ligands surrounding the quantum dots obtained in this example.

[Example 2] 166.9 mg of silver acetate, 8.5 mL of ODE, and 9.0 mL of TOP were added to a 100 mL reaction vessel. Then, the raw materials were dissolved by heating while stirring in an inert gas (N ₂ ) atmosphere. Thus, the total solvent amount of ODE and TOP was 17.5 mL. The ratio of ODE/TOP was about 0.94.

To the solution was added 2.0 mL of 0.25 M Te-TOP, and the solution was heated at 110° C. for 10 minutes while stirring.

To the solution was added 2.0 mL of DDT, and the mixture was heated at 190°C for 10 minutes while stirring. The resulting reaction solution (Ag ₂ Te) was cooled to room temperature. The resulting reaction solution (Ag ₂ Te) was cooled to room temperature.

Ethanol was added to the obtained reaction solution to generate a precipitate, and the precipitate was recovered by centrifugal separation. Then, toluene was added to the precipitate to disperse the precipitate, thereby obtaining a dispersed solution of Ag ₂ Te quantum dots.

In the ultraviolet-visible near-infrared absorption spectrum of the obtained reaction solution measured by an ultraviolet-visible spectrometer, as shown in FIG4 , an absorption peak appears at 1250.0 nm. The absorbance ratio between the peak top and the valley of the absorption peak is about 2.0.

When the quantum dots obtained in Example 2 were observed by STEM, the average particle size of the plurality of quantum dots was 3.1 to 3.7 nm, and was 1 nm to 15 nm. Moreover, the particle sizes of more than 90% of the quantum dots were within the average particle size ±0.7 nm. That is, the particle sizes of more than 90% of the quantum dots were within the average particle size ±30%, and the particle sizes of the plurality of quantum dots were uniformly generated.

Furthermore, in the XRD spectrum of the quantum dots obtained in Example 2, as shown in FIG. 5 , a peak was observed near 40°, confirming the formation of Ag ₂ Te solid solution.

[Example 3] 166.9 mg of silver acetate, 7.5 mL of ODE, and 7.5 mL of TOP were added to a 100 mL reaction vessel. Then, the raw materials were dissolved by heating while stirring in an inert gas (N ₂ ) atmosphere. Thus, the total solvent amount of ODE and TOP was 15 mL. The ratio of ODE/TOP was 1.0.

To the solution was added 2.0 mL of 0.25 M Te-TOP, and the solution was heated at 110° C. for 10 minutes while stirring.

To the solution was added 2.0 mL of DDT, and the mixture was heated at 190°C for 10 minutes while stirring. The resulting reaction solution (Ag ₂ Te) was cooled to room temperature. The resulting reaction solution (Ag ₂ Te) was cooled to room temperature.

Ethanol was added to the obtained reaction solution to generate a precipitate, and the precipitate was recovered by centrifugal separation. Then, toluene was added to the precipitate to disperse the precipitate, thereby obtaining a dispersed solution of Ag ₂ Te quantum dots.

In the ultraviolet-visible near-infrared absorption spectrum of the obtained reaction solution measured by an ultraviolet-visible spectrometer, as shown in FIG6 , an absorption peak appears at 1350.0 nm. The absorbance ratio between the peak top and the valley of the absorption peak is about 1.8.

When the quantum dots obtained in Example 3 were observed by STEM, the average particle size of the plurality of quantum dots was 3.1 to 3.7 nm, and was 1 nm to 15 nm. Moreover, the particle sizes of more than 90% of the quantum dots were included in the average particle size ±0.7 nm. That is, the particle sizes of more than 90% of the quantum dots were included in the average particle size ±30%, and the particle sizes of the plurality of quantum dots were uniformly generated.

Furthermore, in the XRD spectrum of the quantum dots obtained in Example 3, as shown in FIG. 7 , a peak was observed near 39.9°, confirming the formation of Ag ₂ Te solid solution.

[Example 4] 166.9 mg of silver acetate, 5.0 mL of ODE, and 6.0 mL of TOP were added to a 100 mL reaction vessel. Then, the raw materials were dissolved by heating while stirring in an inert gas (N ₂ ) atmosphere. Thus, the total solvent amount of ODE and TOP was 11 mL. The ratio of ODE/TOP was about 0.83.

To the solution was added 2.0 mL of 0.25 M Te-TOP, and the solution was heated at 110° C. for 10 minutes while stirring.

To the solution, 2.0 mL of DDT was added, and the mixture was heated at 190°C for 10 minutes, usually with stirring. The obtained reaction solution (Ag ₂ Te) was cooled to room temperature.

Ethanol was added to the obtained reaction solution to generate a precipitate, and the precipitate was recovered by centrifugal separation. Then, toluene was added to the precipitate to disperse the precipitate, thereby obtaining a dispersed solution of Ag ₂ Te quantum dots.

In the ultraviolet-visible near-infrared absorption spectrum of the obtained reaction solution measured by an ultraviolet-visible spectrometer, as shown in FIG8 , an absorption peak appears at 1450.0 nm. The absorbance ratio between the peak top and the valley of the absorption peak is about 1.6.

When the quantum dots obtained in Example 4 were observed by STEM, the average particle size of the plurality of quantum dots was 3.1 to 3.7 nm, and was 1 nm to 15 nm. Moreover, the particle sizes of more than 90% of the quantum dots were within the average particle size ±0.7 nm. That is, the particle sizes of more than 90% of the quantum dots were within the average particle size ±30%, and the particle sizes of the plurality of quantum dots were uniformly generated.

Furthermore, in the XRD spectrum of the quantum dots obtained in this example, as shown in FIG. 9 , a peak was observed near 40.1°, confirming the formation of Ag ₂ Te solid solution.

[Synthesis of quantum dots as a fluorescent material in the near-infrared region] <Raw materials> In the experiment, the following raw materials were used to synthesize silver chalcogenide (Ag ₂ E series) quantum dots that emit light in the near-infrared region. (Solvent) Octadecene: manufactured by Aldrich Co., Ltd., manufactured by Idemitsu Kosan Co., Ltd. Dodecanethiol: manufactured by Arkema Co., Ltd. (Silver raw material) Silver acetate: manufactured by Kishida Chemical Co., Ltd. (Dichalcogenide compound) Diphenyl ditelluride: manufactured by Tokyo Chemical Industry Co., Ltd. (Tellurium raw material) Tellurium (4 N: 99.99%): manufactured by Shinko Chemical Co., Ltd., or Aldrich Co. <Measurement equipment> Fluorescence spectrometer: NIRQuest512-1.9 manufactured by Ocean Optics UV-visible spectrophotometer: V-770 manufactured by Hitachi, Ltd. X-ray diffraction device (XRD): D2 PHASER manufactured by Bruker Co., Ltd. Scanning transmission electron microscope (STEM): SU9000 manufactured by Hitachi, Ltd.

[Example 5] 123.0 mg of diphenyl ditelluride, 15.0 mL of dodecanethiol (DDT), and 15.0 mL of octadecene (ODE) were added to a 100 mL reaction container, and then heated while stirring in an inert gas (N ₂ ) atmosphere to dissolve the raw materials.

To the solution was added 204.0 mg of silver acetate, and the mixture was heated at 185°C for 10 minutes while being stirred. The obtained reaction solution (Ag ₂ Te) was cooled to room temperature.

Ethanol was added to the obtained reaction solution to generate a precipitate, and the precipitate was recovered by centrifugal separation. Then, toluene was added to the precipitate to disperse the precipitate, thereby obtaining a dispersed solution of Ag ₂ Te quantum dots.

The obtained reaction solution was measured by UV-Vis spectrometer. As shown in the measurement results of UV-Vis near-infrared absorption spectrum in Figure 11, absorption maximums were obtained at 1219.0 nm, 1400.0 nm, and 1420.0 nm. Also, as shown in the measurement results of near-infrared fluorescence spectrum in Figure 12, fluorescence maximum was obtained at 1322.5 nm. Moreover, based on Figure 12, the fluorescence half-width is about 150 nm, which is less than 200 nm.

Furthermore, from the STEM photos shown in FIG. 13A and FIG. 13B , it can be seen that the average particle size of the plurality of quantum dots is 2.4 to 2.7 nm, and is between 1 nm and 15 nm. Furthermore, the particle sizes of more than 90% of the quantum dots are included in the average particle size ±0.7 nm. In other words, it can be seen that the particle sizes of more than 90% of the quantum dots are included in the average particle size ±30%, and the particle sizes of the plurality of quantum dots can be uniformly generated.

Furthermore, based on the peaks of the XRD spectrum of Ag ₂ Te shown in FIG. 14 , it can be confirmed that a Ag ₂ Te solid solution is generated.

[Example 6] 123.0 mg of diphenyl ditelluride and 30.0 mL of dodecanethiol (DDT) were added to a 100 mL reaction container, and then heated while stirring in an inert gas (N ₂ ) atmosphere to dissolve the raw materials.

To the solution was added 204.0 mg of silver acetate, and the mixture was heated at 175°C for 20 minutes while being stirred. The obtained reaction solution (Ag ₂ Te) was cooled to room temperature.

Ethanol was added to the obtained reaction solution to generate a precipitate, and the precipitate was recovered by centrifugal separation. Then, toluene was added to the precipitate to disperse it, thereby obtaining a dispersed solution of Ag ₂ Te quantum dots.

In the ultraviolet-visible near-infrared absorption spectrum of the obtained reaction solution measured by an ultraviolet-visible spectrometer, absorption maxima exist at 1219.0 nm, 1400.0 nm, and 1420.0 nm, similar to FIG. 11 of Example 5. In addition, in the near-infrared fluorescence spectrum of the reaction solution, similar to FIG. 12 of Example 5, a fluorescence maxima exists at 1322.5 nm, and the fluorescence half-width is about 150 nm, which is less than 200 nm.

When the quantum dots obtained in this embodiment are observed by STEM, the average particle size of the plurality of quantum dots is 2.4 to 2.7 nm, and is 1 nm to 15 nm, as in FIG. 13 of Embodiment 5. Moreover, the particle sizes of more than 90% of the quantum dots are included in the average particle size ±0.7 nm. That is, the particle sizes of more than 90% of the quantum dots are included in the average particle size ±30%, and the particle sizes of the plurality of quantum dots can be uniformly generated.

In addition, in the XRD spectrum of the quantum dots obtained in this example, the same peak as that in FIG. 14 of Example 5 was observed, confirming the formation of Ag ₂ Te solid solution.

[Example 7] 41.0 mg of diphenyl ditelluride, 10.0 mL of dodecanethiol (DDT), and 5 mL of octadecene (ODE) were added to a 100 mL reaction container, and then heated while stirring in an inert gas (N ₂ ) atmosphere to dissolve the raw materials.

To the solution was added 68.0 mg of silver acetate, and the mixture was heated at 180°C for 15 minutes while being stirred. The obtained reaction solution (Ag ₂ Te) was cooled to room temperature.

Ethanol was added to the obtained reaction solution to generate a precipitate, and the precipitate was recovered by centrifugal separation. Then, toluene was added to the precipitate to disperse the precipitate, thereby obtaining a dispersed solution of Ag ₂ Te quantum dots.

In the ultraviolet-visible near-infrared absorption spectrum of the obtained reaction solution measured by an ultraviolet-visible spectrometer, absorption maxima exist at 1219.0 nm, 1400.0 nm, and 1420.0 nm, similar to FIG. 11 of Example 5. In addition, in the near-infrared fluorescence spectrum of the reaction solution, similar to FIG. 12 of Example 5, a fluorescence maxima exists at 1322.5 nm, and the fluorescence half-width is about 150 nm, which is less than 200 nm.

When the quantum dots obtained in this embodiment are observed by STEM, the average particle size of the plurality of quantum dots is 2.4 to 2.7 nm, and is 1 nm to 15 nm, as in FIG. 13 of Embodiment 5. Moreover, the particle sizes of more than 90% of the quantum dots are included in the average particle size ±0.7 nm. That is, the particle sizes of more than 90% of the quantum dots are included in the average particle size ±30%, and the particle sizes of the plurality of quantum dots can be uniformly generated.

In addition, in the XRD spectrum of the quantum dots obtained in this example, the same peak as that in FIG. 14 of Example 5 was observed, confirming the formation of Ag ₂ Te solid solution.

[Comparative Example 1] 143.3 mg of silver chloride, 10.0 mL of ODE, and 10.0 mL of TOP were added to a 100 mL reaction container, and then heated while stirring in an inert gas (N ₂ ) atmosphere to dissolve the raw materials.

To this solution, 63.8 mg of tellurium powder was added, and the solution was heated at 115° C. for 10 minutes while stirring.

2.0 mL of DDT was added to the solution, and the solution was heated at 200°C for 10 minutes while being stirred. The obtained reaction solution (Ag ₂ Te) was cooled to room temperature. The reaction solution became a black suspension, and no absorption peak was observed in the obtained solution in the near-infrared region.

[Comparative Example 2] 231.7 mg of silver oxide, 10.0 mL of ODE, and 10.0 mL of TOP were added to a 100 mL reaction container, and then heated while stirring in an inert gas (N ₂ ) atmosphere to dissolve the raw materials.

To this solution, 63.8 mg of tellurium powder was added, and the solution was heated at 115° C. for 10 minutes while stirring.

2.0 mL of DDT was added to the solution, and the solution was heated at 200°C for 10 minutes while being stirred. The obtained reaction solution (Ag ₂ Te) was cooled to room temperature. The reaction solution became a black suspension, and no absorption peak was observed in the obtained solution in the near-infrared region.

[Comparative Example 3] 38.3 mg of tellurium, 15.0 mL of dodecanethiol, and 15 mL of octadecene were added to a 100 mL reaction container, and then heated while stirring in an inert gas (N ₂ ) atmosphere to dissolve the raw materials.

To the solution was added 204.0 mg of silver acetate, and the mixture was heated at 185°C for 10 minutes while being stirred. The obtained reaction solution (Ag ₂ Te) was cooled to room temperature. The reaction solution became a black suspension, and no fluorescence was observed in the obtained solution in the near-infrared region.

Based on the above experimental results, it can be seen that according to Examples 1 to 7, the quantum dots obtained are all nanocrystals represented by Ag ₂ Te, and the average particle size can be adjusted to be within the range of 1 nm to 15 nm. In addition, it can be seen that the average particle size can be preferably adjusted to be 1 nm to 10 nm, and more preferably to be 1 nm to 5 nm. Regarding the use of Ag ₂ Se or Ag ₂ S as Se or S as the chalcogen element, based on chemical insights, it is also inferred that the same results are obtained.

Furthermore, it can be seen that according to Examples 1 to 4, Ag ₂ Te is synthesized by using silver raw materials and Te-TOP, and the absorption wavelength is in the near-infrared region of 800 nm to 1700 nm. Furthermore, it can be seen that in this embodiment, it is preferred to have an absorption wavelength in the near-infrared region of 1100 nm to 1500 nm, and more preferably, it is preferred to have an absorption wavelength in the near-infrared region of 1300 nm to 1450 nm. It can be seen that in this embodiment, the absorbance ratio of the peak top and valley of the absorption peak can be set to about 1.5 or more, which can further clarify the contrast.

Furthermore, it can be seen that in the experiments of Examples 1 to 4, the amounts of ODE and TOP (amount of solvent) and the ratio of ODE/TOP were variously changed, thereby making it possible to freely control the absorption peak wavelength observed in the near-infrared region according to the purpose.

In this embodiment, the amount of solvent is adjusted within the range of 5 mL to 30 mL. Furthermore, it is more preferred to adjust the amount of solvent within the range of 10 mL to 25 mL. In addition, the ratio of ODE/TOP is adjusted within the range of 0.5 to 1.5. Furthermore, it is more preferred to set the ratio of ODE/TOP within the range of 0.8 to 1.0.

Furthermore, it can be seen that according to Examples 5 to 7, quantum dots having a luminescent wavelength in the near-infrared region can be synthesized. [Industrial Applicability]

According to the present invention, it is possible to synthesize silver chalcogenide quantum dots that have an absorption band in any range of 800 nm to 1700 nm and exhibit near-infrared fluorescence with high brightness without using a reactant that exhibits high reactivity when operated under the atmosphere and without using an intermediate containing a heavy metal that is a toxic regulation target. Moreover, by applying the quantum dots of the present invention to optical communication devices, etc., excellent near-infrared absorption and near-infrared luminescence characteristics can be obtained in the device. In particular, the quantum dots of the present invention can be applied to the quantum dot layer of the light absorption layer of an infrared sensor.

This case is based on Japanese Patent Application No. 2019-149026 filed on August 15, 2019. The contents of this application are hereby incorporated by reference in their entirety.

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

FIG. 1A is a schematic diagram of a quantum dot in an embodiment of the present invention. FIG. 1B is a schematic diagram of a quantum dot in an embodiment of the present invention. FIG. 2 is an absorption spectrum of Ag ₂ Te in Embodiment 1. FIG. 3 is an X-ray Diffraction (XRD) spectrum of Ag ₂ Te in Embodiment 1. FIG. 4 is an absorption spectrum of Ag ₂ Te in Embodiment 2. FIG. 5 is an X-ray Diffraction (XRD) spectrum of Ag ₂ Te in Embodiment 2. FIG. 6 is an absorption spectrum of Ag ₂ Te in Embodiment 3. FIG. 7 is an X-ray Diffraction (XRD) spectrum of Ag ₂ Te in Embodiment 3. FIG8 is an absorption spectrum of Ag ₂ Te in Example 4. FIG9 is an X-ray Diffraction (XRD) spectrum of Ag ₂ Te in Example 4. FIG10A is an image showing the state change of the solution in the ligand exchange experiment of the quantum dots obtained in this example. FIG10B is a schematic diagram showing a part of FIG10A. FIG11 is an absorption spectrum of Ag ₂ Te in Example 5. FIG12 is a fluorescence (Photoluminescence: PL) spectrum of Ag ₂ Te in Example 5. FIG13A is a scanning transmission electron microscope (STEM) photograph of Ag ₂ Te in Example 5. FIG13B is a schematic diagram showing a part of FIG13A. FIG. 14 is an X-ray Diffraction (XRD) spectrum of Ag ₂ Te in Example 5.

1: Quantum dots

2: Organic ligand

Claims (12)

A quantum dot is characterized in that it is a nanocrystal represented by Ag ₂ E (E is at least one of tellurium, selenium or sulfur) containing silver and a chalcogen element, has an average particle size of 1 nm to 15 nm, and has a core-shell structure, wherein the core-shell structure comprises a core composed of the Ag ₂ E and a shell covering the surface of the core, and the shell does not contain regulated heavy metals such as Cd, Hg or Pd, or substances derived from highly reactive reactants such as metal amides or organic lithium compounds. For example, the quantum dots in Request 1 have an absorption wavelength in the near-infrared region of 800nm~1700nm. For quantum dots in claim 1 or 2, the fluorescence wavelength is in the near-infrared range of 800~1700nm, and the fluorescence half-width is less than 200nm. As in claim 1, the surface of the quantum dot is covered by a ligand. As in claim 4, the quantum dots, wherein the ligands are selected from at least one or two of the phosphine series, aliphatic thiol series, aliphatic amine series and aliphatic carboxylic acid series. A method for producing quantum dots is characterized in that quantum dots represented by Ag ₂ E (E is at least one of tellurium, selenium or sulfur) are synthesized at a temperature below 200° C. using a silver raw material and trioctylphosphine containing a chalcogen element, and formed into a core-shell structure, wherein the core-shell structure comprises a core composed of the Ag ₂ E and a shell covering the surface of the core, and the shell does not contain regulated heavy metals such as Cd, Hg or Pd, or substances derived from highly reactive reactants such as metal amides or organic lithium compounds. The method for manufacturing quantum dots as claimed in claim 6, wherein the above-mentioned silver raw material is reacted with the above-mentioned trioctylphosphine containing a sulfur element at a temperature below 180°C. A method for manufacturing quantum dots as claimed in claim 6 or 7, wherein dodecanethiol (DDT) is added to a solution mixed with the above-mentioned silver raw material and the above-mentioned trioctylphosphine containing a sulfide element. A method for manufacturing quantum dots is characterized in that quantum dots represented by Ag ₂ E (E is at least one of tellurium, selenium or sulfur) are synthesized from silver raw materials and dichalcogenide compounds. The method for manufacturing quantum dots as claimed in claim 9 is to synthesize the above-mentioned quantum dots in a solvent containing thiol. The method for manufacturing quantum dots as claimed in claim 9 or 10 is to react the disulfide compound with the silver raw material at a temperature of 120°C or more and 250°C or less. The method for manufacturing quantum dots as claimed in claim 6 does not include regulated heavy metals represented by cadmium, mercury and lead, and reactants that are highly reactive in the atmosphere represented by metal amides and organic lithium compounds as intermediates to synthesize the above-mentioned quantum dots.