Near-infrared photoresponsive Ag(Bi,Sb)S-based multi-quantum dots

Abstract

The present invention provides Ag(Bi,Sb)S-based multi-quantum dots that are responsive to near-infrared light, have low environmental and biological impact, and are capable of absorbing light of a wide wavelength range. The near-infrared light-responsive Ag(Bi,Sb)S-based multi-quantum dots are made of a compound containing Ag, Bi, Sb, and S as constituent elements.

Description

[Technical Field]

[0001] The present invention relates to near-infrared light-responsive Ag(Bi,Sb)S-based multi-quantum dots. [Background technology]

[0002] Semiconductor nanoparticles with a particle size of several to several dozen nanometers exhibit a band gap that corresponds to their particle size due to the quantum size effect. These semiconductor nanoparticles, also known as quantum dots, are attracting attention as optically functional materials because their optical absorption and emission properties can be controlled by adjusting the band gap.

[0003] Non-Patent Document 1 discloses quantum dots made of PbS, a group IV-VI semiconductor. The quantum dots exhibit optical properties that correspond to their particle size in the visible to near-infrared region due to the quantum size effect. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Gomaa Mohamed Gomaa Khalaf, et al., Small Science, Volume 3, Issue 11, 2300062 Summary of the Invention [Problem to be solved by the invention]

[0005] The quantum dots described in Non-Patent Document 1 can absorb a wide range of light from the visible light region to the near-infrared region, but are highly toxic, so there are limitations on the amount that can be used.

[0006] The present invention has been made in view of the above problems, and its object is to provide Ag(Bi,Sb)S-based multi-quantum dots that have low environmental and biological impact and are capable of absorbing light of a wide wavelength range. [Means for solving the problem]

[0007] The near-infrared light responsive Ag(Bi,Sb)S-based multi-quantum dots according to the present invention are characterized by the fact that they are made of a compound containing Ag, Bi, Sb, and S as constituent elements.

[0008] In recent years, ternary compounds such as II-IV-V2 and I-III-VI2 types, which are relatively easy to synthesize and can absorb light in the visible to near-infrared wavelength range, have attracted attention. As a result of extensive research into such ternary compounds, the inventors succeeded in nanoparticle formation of solid solutions consisting of IV-VI group semiconductors AgBiS2 and AgSbS2, thereby completing the present invention. Multi-nary quantum dots of Ag(Bi,Sb)S2 solid solution, a compound containing Ag, Bi, Sb, and S, do not contain toxic elements such as lead and can absorb light with wavelengths from the visible to near-infrared range. In other words, this configuration provides novel Ag(Bi,Sb)S-based multi-nary quantum dots that are environmentally and biologically friendly, can absorb light over a wide wavelength range, and are responsive to near-infrared light. Furthermore, the band gap of these multi-nary quantum dots, i.e., their optical properties, can be controlled by varying the Bi and Sb composition ratio in the compound. Therefore, by changing the composition ratio of Bi and Sb, it is possible to easily fabricate two or more types of multi-quantum dots with different optical properties. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing an outline of the manufacturing process of multi-quantum dots of Ag(Bi,Sb)S2 solid solution. [Figure 2] This is a TEM image of multi-component quantum dots of Ag(Bi,Sb)S2 solid solution. [Figure 3] This is the result of XRD measurement of Ag(Bi,Sb)S2 solid solution. [Figure 4] FIG. 1 is a diagram showing the lattice spacing on the (200) plane of an Ag(Bi,Sb)S2 solid solution. [Figure 5] This is the measurement result of the absorption spectrum of Ag(Bi,Sb)S2 solid solution. [Figure 6]FIG. 1 shows the band gap and average grain size of Ag(Bi,Sb)S2 solid solution. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of near-infrared light-responsive Ag(Bi,Sb)S-based multi-quantum dots (hereinafter sometimes referred to as multi-quantum dots) according to the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the present invention.

[0011] The Ag(Bi,Sb)S-based multi-quantum dots according to the present invention are nanoparticles made of a compound containing Ag, Bi, Sb, and S as constituent elements. The compound is a solid solution made of ternary compounds, AgBiS2 and AgSbS2. In the following description, the compound may be referred to as Ag(Bi,Sb)S2 solid solution.

[0012] The Bi and Sb contents in the Ag(Bi,Sb)S2 solid solution are preferably greater than 0 atomic % and less than or equal to 30 atomic %. If the Bi and Sb contents are greater than 0 atomic %, a solid solution can be formed, and if the Bi and Sb contents are less than or equal to 30 atomic %, it is easy to improve the properties by quantum efficiency.

[0013] The properties of the solid solution multi-quantum dots vary depending on the ratio of Bi to Sb in the compound. The smaller the ratio of Bi in the compound, the closer the properties of the multi-quantum dots are to those of AgSbS2, and the smaller the ratio of Sb in the compound, the closer the properties of the multi-quantum dots are to those of AgBiS2. In the present invention, the ratio (x / (x+y)) of the number of Bi atoms to the sum of the number of Bi atoms (x) and the number of Sb atoms (y) in the compound is preferably 0.05 or more and less than 1.00. By keeping the ratio of the number of Bi atoms in the above range, multi-quantum dots capable of absorbing light of a wide wavelength can be obtained.

[0014] The Ag content in the Ag(Bi,Sb)S2 solid solution is preferably 10 atomic % or more and 40 atomic % or less, and the S content in the solid solution is preferably 30 atomic % or more and 60 atomic % or less. Furthermore, elements other than Ag, Bi, Sb, and S that may be contained in the solid solution include O, Se, Te, etc., and these elements are acceptable if they are less than 5 mass %. However, the total content of Ag, Bi, Sb, and S in the solid solution is preferably 99 mass % or more, and more preferably 99.9 mass % or more.

[0015] The multi-quantum dots according to the present invention preferably have an average particle size of 2 nm or more and 20 nm or less. The particle size of the multi-quantum dots is related to the band gap adjustment function due to the quantum size effect. In order to achieve favorable light emission and light absorption properties by adjusting the band gap, it is preferable to set the average particle size as described above. The average particle size of the multi-quantum dots can be obtained by observing a plurality of particles (preferably 100 or more) using an electron microscope such as a TEM, measuring the particle size of each particle, and calculating the particle number average.

[0016] Furthermore, the multi-quantum dots according to the present invention preferably have at least one of the following protective agents bonded to the particle surface: alkylamines having alkyl chain carbon atoms of 4 to 20; alkenylamines having alkenyl chain carbon atoms of 4 to 20; alkylcarboxylic acids having alkyl chain carbon atoms of 3 to 20; alkenylcarboxylic acids having alkenyl chain carbon atoms of 3 to 20; alkanethiols having alkyl chain carbon atoms of 4 to 20; trialkylphosphines having alkyl chain carbon atoms of 4 to 20; trialkylphosphine oxides having alkyl chain carbon atoms of 4 to 20; triphenylphosphine; and triphenylphosphine oxide. When handling multi-quantum dots, they are often dispersed in an appropriate dispersion medium to form a solution (sometimes referred to as a slurry or ink). The protective agent is useful for suppressing aggregation of the multi-quantum dots in the solution, thereby forming a uniform solution. Furthermore, when the protective agent is added to the reaction system together with the raw materials in the synthesis process of the Ag(Bi,Sb)S2 solid solution, it also acts to form multi-quantum dots with a suitable average particle size. The protecting agent may be any of the above alkylamines, alkenylamines, alkylcarboxylic acids, alkenylcarboxylic acids, alkanethiols, trialkylphosphines, trialkylphosphine oxides, triphenylphosphine, and triphenylphosphine oxides, which may be used alone or in combination.

[0017] A transmission electron microscope (TEM) can be suitably used to analyze the particle size of the multi-quantum dots according to the present invention. The particle composition can be determined by an energy dispersive X-ray spectrometer (EDX) or the like to grasp the overall composition of the multi-quantum dots.

[0018] The multi-quantum dots according to the present invention have a band gap that is adjusted according to the ratio of Bi and Sb, and thus have variable light absorption characteristics. The multi-quantum dots according to the present invention preferably have an absorption edge wavelength on the long-wavelength side of the absorption spectrum of 850 nm or more and 1150 nm or less. This allows the multi-quantum dots to have absorptivity and responsiveness to light in the visible to near-infrared regions. Furthermore, the multi-quantum dots according to the present invention have low biological and environmental impacts, making them highly versatile.

[0019] The multi-quantum dots of the present invention can be suitably used in devices utilizing light absorption properties, such as solar cells and optical sensors. In particular, the multi-quantum dots of the present invention are absorbing and responsive to light in the near-infrared region, and by combining two or more types of Sb and Bi with different abundance ratios, they can efficiently absorb light over a wide wavelength range. Therefore, they are expected to be used in photoelectric conversion devices operating in the near-infrared region and to realize high-efficiency solar cells. To fabricate solar cells and the like, the multi-quantum dots can be coated or supported on an appropriate substrate or carrier. The structure, shape, and dimensions of the substrate or carrier are not particularly limited. Examples of plate-, foil-, or film-like substrates include glass, quartz, silicon, ceramics, and metals. Examples of granular or powder-like carriers include inorganic oxides such as ZnO, TiO2, WO3, SnO2, In2O3, and Al2O3. Alternatively, the multi-quantum dots may be supported on the inorganic oxide carrier and then fixed to the substrate.

[0020] Furthermore, when applying or supporting multi-quantum dots on a substrate or carrier, as described above, a solution, slurry, or ink in which the multi-quantum dots are dispersed in an appropriate dispersion medium is often used. Chloroform, toluene, cyclohexane, hexane, etc. can be used as the dispersion medium for this solution. Dipping and spin coating methods can be used as the application method for the multi-quantum dot solution, and various methods such as dropping, impregnation, and adsorption can be used as the support method.

[0021] Next, a method for producing multi-quantum dots according to the present invention will be described. Multi-quantum dots can be produced by mixing an Ag precursor, an Sb precursor, a Bi precursor, and a sulfur source in a reaction solvent, and heating the reaction system formed from these at a temperature of 100°C or higher and 200°C or lower.

[0022] The Ag precursor, Sb precursor, and Bi precursor can be an Ag salt or Ag complex, an Sb salt or Sb complex, or a Bi salt or Bi complex, respectively. The Ag precursor is preferably a salt or complex containing monovalent Ag, and the Sb precursor and Bi precursor are preferably a trivalent Sb or Bi salt or trivalent Sb or Bi complex. Complexes having a ligand containing a sulfur (S) atom may also be used as the Ag precursor, Sb precursor, and Bi precursor. In this case, the compound can be synthesized using the sulfur atom contained in the ligand of the Ag complex, Sb complex, and / or Bi complex as the sulfur source for the Ag(Bi,Sb)S2 solid solution.

[0023] Examples of Ag precursors that can be used include silver acetate (Ag(OAc)), silver nitrate, silver carbonate, silver oxide, silver oxalate, silver chloride, silver iodide, and silver(I) cyanide salts. Examples of Sb precursors include antimony acetate (Sb(OAc)3) and antimony chloride (SbCl3), and examples of Bi precursors include bismuth acetate (Bi(OAc)3) and bismuth chloride (BiCl3). Examples of sulfur sources that can be used include elemental sulfur, as well as compounds such as thiourea, alkylthiourea, thioacetamide, and alkanethiol, as well as compounds such as β-dithiones, dithiols, xanthates, and diethyldithiocarbamates. Even when the Ag complex, Sb complex, or Bi complex has a ligand containing an S atom, a sulfur source may be added separately.

[0024] The contents of Bi and Sb in the Ag(Bi,Sb)S2 solid solution can be adjusted by the amounts (feed amounts) of the Sb precursor and Bi precursor used. The ratio of Bi to Sb (feed ratio) in the amounts of the Sb precursor and Bi precursor charged should be set between 0.01 and 0.90. The amount of S in the reaction system is preferably set to an atomic ratio of 1 to 1.5 relative to the total number of Ag, Sb, and Bi atoms in the reaction system. However, with regard to the sulfur source, even if excess S is present in the reaction system, it has little effect on the composition of the Ag(Bi,Sb)S2 solid solution.

[0025] The reaction system for synthesizing multi-quantum dots can be produced without a solvent, or a solvent can be used. When a solvent is used, octadecene, tetradecane, oleic acid, oleylamine, dodecanethiol, or a mixture thereof can be used.

[0026] The multi-quantum dots are preferably Ag(Bi,Sb)S2 solid solutions bonded with a protective agent. Therefore, it is preferable to add a protective agent to the above-mentioned reaction system together with the Ag precursor, Sb precursor, Bi precursor, etc. The protective agent is preferably at least one of alkylamines having alkyl chain carbon atoms of 4 to 20, alkenylamines having alkenyl chain carbon atoms of 4 to 20, alkylcarboxylic acids having alkyl chain carbon atoms of 3 to 20, alkenylcarboxylic acids having alkenyl chain carbon atoms of 3 to 20, alkanethiols having alkyl chain carbon atoms of 4 to 20, trialkylphosphines having alkyl chain carbon atoms of 4 to 20, trialkylphosphine oxides having alkyl chain carbon atoms of 4 to 20, triphenylphosphine, and triphenylphosphine oxide.

[0027] The heating temperature (reaction temperature) of the reaction system consisting of the Ag precursor, Sb precursor, Bi precursor, and sulfur source is 50°C or higher and 200°C or lower. At temperatures below 50°C, the synthesis of the Ag(Bi,Sb)S2 solid solution is difficult. On the other hand, at temperatures above 200°C, there is a risk that a compound with the desired composition may not be produced. The average particle size of the multi-quantum dots increases with increasing reaction temperature, but it rarely exceeds the preferred average particle size within the temperature range. A more preferred reaction temperature is 100°C or higher and 170°C or lower. The heating time (reaction time) can be adjusted by the amount of raw materials charged, but is preferably 1 minute or higher and 60 minutes or lower. It is preferable to stir the reaction system during the synthesis reaction of the multi-quantum dots.

[0028] After the synthesis reaction of the multi-quantum dots is completed, the reaction system is cooled as necessary, and the multi-quantum dots are recovered. At this time, the multi-quantum dots are precipitated by adding a non-solvent alcohol (ethanol, methanol, etc.), or the multi-quantum dots are precipitated and recovered by centrifugation, and the particles are further washed with alcohol (ethanol, methanol, etc.) and then uniformly dispersed in a good solvent such as chloroform.

[0029] [Example] Examples of the present invention will be described below, but the present invention is not limited to the descriptions of these examples.

[0030] 1 is a diagram showing an outline of the manufacturing process of multi-quantum dots according to an embodiment. Silver acetate (Ag(OAc)) as an Ag precursor, antimony acetate (Sb(OAc)3) as an Sb precursor, bismuth acetate (Bi(OAc)3) as a Bi precursor, and sulfur powder as a sulfur source were weighed and placed in a test tube. 1-dodecanethiol was added as a protective agent in an amount of 0.3 cm. 3 and 2.7 cm of oleylamine as a solvent. 3 Added.

[0031] In this example, as shown in Table 1, Ag(OAc) was 0.03 mmol, Sb(OAc) was 0.05 mmol, and Bi(OAc) was 1.25 × 10 -3 The composition ratio of Bi and Sb was controlled by changing the concentration in the range of 0.05 mmol to 0.03 mmol. The ratio of Bi to Sb in the metal precursor was 0.025, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6, and multi-quantum dots according to the example were synthesized at eight different ratios. In addition, multi-quantum dots according to AgBiS2 and AgSbS2 were synthesized as comparative examples in the same manner. [Table 1]

[0032] Each precursor, protective agent, solvent, and stirrer were placed in a test tube, and the atmosphere was replaced with nitrogen three times, followed by stirring with a hot stirrer for 10 minutes while heating at a reaction temperature of 150°C. After the reaction was completed, the mixture was allowed to cool for 20 minutes, then transferred to a test tube and centrifuged at 4000 rpm for 5 minutes to separate the supernatant and precipitate.

[0033] Then, 4 cm of methanol was added to the supernatant as a non-solvent. 3 The mixture was added to form a precipitate, which was then centrifuged at 4000 rpm for 5 minutes to recover the precipitate. 3 After adding the precipitate and dispersing it, the mixture was centrifuged under the same conditions. 3 Additionally, the same procedure was repeated to remove by-products and solvents for purification.

[0034] The precipitate obtained by the above procedure was dissolved in 3 cm of chloroform. 3 The dispersion liquid of the multi-quantum dots according to the examples and comparative examples was obtained by dispersing the dispersion liquid in the sample bottle, which was then purged with nitrogen and stored in a refrigerator in a dark place.

[0035] Each of the manufactured multi-quantum dots according to the examples was observed using a TEM (Hitachi, H-7650). Figure 2 shows TEM images of multi-quantum dots with a Bi / Sb ratio of 0.025, 0.05, 0.2, and 0.4 (see the scale bar in each photograph for magnification). Each TEM image confirmed that nearly spherical multi-quantum dots were synthesized. Based on the TEM images, the average particle size of the multi-quantum dots of each composition was measured and calculated. In particle size measurement, the particle sizes of all measurable multi-quantum dots included in the TEM images were determined, and the average particle size was calculated.

[0036] As shown in Figure 2, the average particle sizes of the multi-quantum dots with a Bi / Sb ratio of 0.025, 0.05, 0.2, and 0.4 were 6.05 nm, 5.14 nm, 4.76 nm, and 4.46 nm, respectively. It can be seen that the larger the Bi / Sb ratio, the smaller the average particle size of the multi-quantum dots.

[0037] To clarify the particle composition, EDX analysis (Horiba, Ltd., Emax Energy EX-250) was performed to analyze the composition of the multi-quantum dots. Table 2 shows the contents of each element, Ag, Sb, Bi, and S, in eight types of multi-quantum dots with different Bi to Sb loading ratios. The results of the composition analysis are shown in atomic % relative to the total number of multi-quantum dots. Table 2 also shows the ratio (x / (x+y)) of the number of Bi atoms to the sum of the number of Bi atoms (x) and the number of Sb atoms (y), calculated based on the composition analysis results. [Table 2]

[0038] From Table 2, it can be seen that the ratio of the number of Bi atoms (x / (x+y)) in the multi-quantum dots according to the examples does not completely match the ratio of Bi to Sb in the metal precursor, but tends to approach it as the ratio of Bi to Sb increases.

[0039] XRD measurements (Rigaku, SmartLab-3K) were performed on the multi-quantum dots according to the examples and comparative examples. Figure 3 shows the results. The diffraction peaks of the multi-quantum dots according to the examples were located between the diffraction peaks of cubic bulk AgBiS2 and cubic bulk AgSbS2, and shifted to lower angles as the Bi atomic ratio (x / (x+y)) increased. This indicates that the multi-quantum dots according to the examples are Ag(Bi,Sb)S2 solid solution multi-quantum dots with a cubic crystal structure, and that the larger the Bi atomic ratio (x / (x+y)), the larger the generated multi-quantum dots.

[0040] 4 is a diagram in which the horizontal axis represents the ratio of the number of Bi atoms (x / (x+y)) and the vertical axis represents the interplanar spacing of the (200) plane of the multi-quantum dots according to the examples and comparative examples. The interplanar spacing was determined from the diffraction peak obtained by XRD measurement. The fact that the interplanar spacing of each multi-quantum dot is intermediate between that of AgBiS2 and AgSbS2 also indicates that the multi-quantum dots according to the examples are Ag(Bi,Sb)S2 solid solutions.

[0041] Next, the absorption spectrum of each Ag(Bi,Sb)S2 solid solution multi-component quantum dot was measured using a UV-Vis-NIR spectrophotometer (JASCO Corporation, V-770) in the wavelength range of 300 nm to 2000 nm.

[0042] Figure 5 shows the absorption spectra of multi-quantum dots of Ag(Bi,Sb)S2 solid solutions, AgSbS2, and AgBiS2. The long-wavelength absorption edge wavelengths of the multi-quantum dots of each Ag(Bi,Sb)S2 solid solution were longer than that of AgSbS2, reaching 850 nm or more. Furthermore, as the ratio (x / (x + y)) of the number of Bi atoms to the sum of the number of Bi atoms (x) and the number of Sb atoms (y) in the Ag(Bi,Sb)S2 solid solution increased, the absorption edge wavelength shifted from 850 nm to 1150 nm, approaching the absorption edge wavelength of AgBiS2.

[0043] The band gaps of the Ag(Bi,Sb)S2 solid solution multi-component quantum dots were calculated from the measured absorption spectra using the Tauc plot method. Figure 6 shows the band gap and average particle size plotted against the Bi atomic ratio (x / (x+y)). As the Bi atomic ratio (x / (x+y)) increased, the band gap and average particle size decreased. Furthermore, when the Bi atomic ratio (x / (x+y)) was 0.07, the band gap was 1.36 eV, and when the atomic ratio was 0.54, the band gap was 1.12 eV. This indicates that the band gap of the Ag(Bi,Sb)S2 solid solution multi-component quantum dots can be controlled by adjusting the Bi atomic ratio (x / (x+y)).

[0044] In the above-described embodiment, the following configurations are envisioned. (1) Near-infrared light-responsive Ag(Bi,Sb)S-based multi-quantum dots, which are composed of compounds containing Ag, Bi, Sb, and S as constituent elements.

[0045] According to this configuration, a novel Ag(Bi,Sb)S-based multi-quantum dot can be provided as a material containing Ag(Bi,Sb)S2 solid solution, a compound containing Ag, Bi, Sb, and S. The Ag(Bi,Sb)S-based multi-quantum dot has low environmental and biological impact, can absorb light of a wide wavelength range, and is responsive to near-infrared light. Furthermore, by changing the composition ratio of Bi and Sb in the compound, the band gap of the multi-quantum dot can be adjusted, i.e., its optical properties can be controlled. Therefore, by changing the composition ratio of Bi and Sb, two or more types of multi-quantum dots with different optical properties can be easily produced.

[0046] (2) In the near-infrared light-responsive Ag(Bi,Sb)S-based multi-quantum dots of (1), it is preferable that the absorption edge wavelength on the long wavelength side of the absorption spectrum is 850 nm or more and 1150 nm or less.

[0047] According to this configuration, light in the visible to near-infrared region can be absorbed, so that it can be used in photoelectric conversion elements that operate in the near-infrared region, and energy can be used efficiently.

[0048] (3) In the near-infrared light-responsive Ag(Bi,Sb)S-based multi-quantum dots of (1) or (2), it is preferable that the ratio of the number of Bi atoms to the total number of Bi atoms and Sb atoms in the compound is 0.05 or more and less than 1.00.

[0049] According to this configuration, since the ratio of the number of Bi atoms is 0.05 or more, the absorption edge wavelength of the multi-quantum dot can be shifted to a longer wavelength side than AgSbS2. Furthermore, since the ratio of the number of Bi atoms is less than 1.00, the absorption edge wavelength of the multi-quantum dot can be shifted to a shorter wavelength side than AgBiS2. In other words, according to this configuration, by adjusting the ratio of the number of Bi atoms to the sum of the number of Bi atoms and the number of Sb atoms, it is possible to provide multi-quantum dots with various optical properties different from those of AgSbS2 and AgBiS2.

[0050] (4) In the near-infrared light-responsive Ag(Bi,Sb)S-based multi-quantum dots of any one of (1) to (3), it is preferable that the Bi content in the compound is more than 0 atomic % and not more than 30 atomic % and the Sb content in the compound is more than 0 atomic % and not more than 30 atomic %.

[0051] According to this configuration, it is possible to suitably provide multi-quantum dots that have low environmental and biological impact and are capable of absorbing light of a wide range of wavelengths. [Industrial Applicability]

[0052] The present invention can be used for Ag(Bi,Sb)S-based multi-quantum dots that are made of compounds containing Ag, Bi, Sb, and S as constituent elements and that respond to near-infrared light.

Claims

1. Ag(Bi, Sb)S-based multi-quantum dots that respond to near-infrared light and are made of a compound containing Ag, Bi, Sb, and S as constituent elements.

2. 2. The near-infrared light responsive Ag(Bi, Sb)S-based multi-quantum dot according to claim 1, wherein the long-wavelength absorption edge wavelength of the absorption spectrum is 850 nm or more and 1150 nm or less.

3. 3. The near-infrared light responsive Ag(Bi, Sb)S-based multi-quantum dot according to claim 1, wherein the ratio of the number of Bi atoms to the total number of Bi atoms and Sb atoms in the compound is 0.05 or more and less than 1.

00.

4. 4. The near-infrared light responsive Ag(Bi, Sb)S-based multi-quantum dot according to claim 3, wherein the Bi content in the compound is more than 0 atomic % and not more than 30 atomic % and the Sb content in the compound is more than 0 atomic % and not more than 30 atomic %.

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