JP7770657B2 - Semiconductor nanoparticles based on AgCu chalcogen compounds - Google Patents

Description

The present invention relates to semiconductor nanoparticles primarily composed of an AgCu chalcogen compound. More specifically, the present invention relates to semiconductor nanoparticles composed of an AgCu chalcogen compound that contains Te as an essential chalcogen element and has favorable photoresponsiveness in the long wavelength region.

When semiconductors are made into nanoscale particles, they exhibit a quantum confinement effect and exhibit a band gap that corresponds to their particle size. Therefore, by adjusting the band gap through controlling the composition and particle size of semiconductor nanoparticles, it becomes possible to set the emission wavelength and absorption wavelength as desired. Semiconductor nanoparticles that utilize this property are also known as quantum dots (QDs), and are expected to be used in a variety of technical fields. Applications of semiconductor nanoparticles under consideration include, for example, light-emitting elements used in display devices and marker substances for detecting biological substances, as well as fluorescent materials.

This is because semiconductor nanoparticles not only allow the emission wavelength to be controlled by adjusting the particle size as mentioned above, but also have a sufficiently narrow and stable emission peak width compared to organic dyes. Furthermore, in addition to being able to control the absorption wavelength, semiconductor nanoparticles also have the properties of high quantum efficiency and a high absorption coefficient. Due to these properties, semiconductor nanoparticles are also being considered for use in photoelectric conversion elements and light-receiving elements installed in solar cells and various optical sensors.

In particular, semiconductor nanoparticles are expected to be applied to the light-receiving elements of optical sensors that respond to light in the near-infrared (NIR) and short-wave infrared (SWIR) regions. Optical sensors that can respond to light in these long-wavelength regions are installed in LIDAR (Light Detection and Ranging) and SWIR image sensors. LIDAR is a remote sensing system used in autonomous driving automobiles, drones, ships, etc., and has become an important device in the recent development of autonomous driving technology. Recently, LIDAR has also been applied to facial recognition technology and augmented reality (AR) technology in smartphones and tablets. Furthermore, demand for SWIR image sensors is expected to increase in the future in fields such as food inspection, agriculture, and drones.

Until now, silicon thin films have often been used as light-receiving elements in sensors for optical devices such as those described above. However, sensors based on silicon thin films have a significant drop in sensitivity in the long wavelength range of 900 nm or longer, making them unsuitable for the above applications. Therefore, there is hope for the development of light-receiving elements that use semiconductor nanoparticles.

Several semiconductor compounds have been investigated for the composition of semiconductor nanoparticles. Metal chalcogenide compounds such as PbS, PbSe, CdHgTe, Ag ₂ S, Ag ₂ Se, Ag ₂ Te, AgInSe 2 , AgInTe ₂ , CuInSe ₂ , CuInTe ₂ , and InAs are known as semiconductor compounds that are photoresponsive in the long wavelength region of the near-infrared region (NIR ₎ and short-wave infrared region (SWIR) (Patent Documents 1 to 4). The applicant of the present application also disclosed semiconductor nanoparticles mainly composed of AgAuS-based compounds in Patent Document 5.

Japanese Patent Application Laid-Open No. 2004-243507 Japanese Patent Application Laid-Open No. 2004-352594 Japanese Patent Application Laid-Open No. 2017-014476 International Publication No. WO2020/054764 Patent No. 7269591

The above-mentioned semiconductor compounds have photoresponsiveness in a desired wavelength region, but many of them have reasons that can be an obstacle when considering applications such as the display devices and markers for detecting biological substances mentioned at the beginning. For example, Pb is a hazardous substance that is prohibited by the European RoHS Directive (Restriction of the use of certain Hazardous Substances).
The EPA (Regulations on Electrical and Electronic Equipment) stipulates restrictions on the use of semiconductor nanoparticles in electrical and electronic equipment from the viewpoint of environmental impact. Therefore, it is difficult to expect that semiconductor nanoparticles made of compounds containing Pb as a metal component will be widely used in the electrical and electronic fields. Furthermore, when considering the use of semiconductor nanoparticles in bio-related fields, it is also difficult to use compounds containing heavy metals such as Cd and Hg.

In addition, while the semiconductor compounds mentioned above are responsive to light in the long-wavelength range, there is a growing demand for compounds that can exhibit light responsiveness at even longer wavelengths. Autonomous driving technology has made remarkable progress in recent years, and it is necessary to support autonomous driving levels (levels 4 and 5) that do not require a human driver to drive. In this case, it is important for LIDAR to have a responsiveness in the longer wavelength range, which is less susceptible to the influence of sunlight and natural light. For this reason, quantum dot technology is still in the research stage. Therefore, semiconductor nanoparticles that can exhibit responsiveness in the longer wavelength range while taking practicality into consideration are needed.

The present invention was made against the background described above and proposes semiconductor nanoparticles made of novel semiconductor compounds that take into consideration practicality in compliance with various regulations and have suitable photoresponsiveness. In particular, the present invention proposes semiconductor nanoparticles that exhibit suitable light absorption characteristics in the long wavelength regions of the near-infrared region (NIR) and short-wave infrared region (SWIR), and are also capable of emitting light.

In considering solutions to the above problems, the present inventors focused on semiconductor nanoparticles made of AgAuS compounds, which are related to the applicant's prior art (Patent Document 5). Ag and Au are metals that can be used favorably in terms of complying with regulations regarding environmental impact and toxicity, and AgAuS compounds are semiconductor compounds that can exhibit photoresponsiveness. However, conventional AgAuS compound nanoparticles have issues with photoresponsiveness in the near infrared (NIR) and shortwave infrared (SWIR) regions. In the applicant's prior art, the absorption wavelength of the semiconductor nanoparticles is shifted to longer wavelengths by adding a metal such as In to the AgAuS compound to create an AgAuS-based multi-component compound.

The inventors, while taking inspiration from the AgAuS compound, a transition metal chalcogen compound, which is a conventional technology mentioned above, decided to impart photoresponsiveness in the long wavelength region to transition metal chalcogen compounds through a two-pronged approach.

The first approach is to use Te (tellurium), an element with a larger mass than S (sulfur), as an essential component of the chalcogen element. This is because by increasing the mass of the chalcogen element that bonds with the transition metal, the orbital energy difference between each transition metal element and the chalcogen element is reduced when a compound is formed, which is presumed to shift the photoresponse to longer wavelengths.

The second approach is to optimize the transition metal component of the chalcogen compound, specifically by replacing the combination of Ag and Au with a combination of Ag and Cu. Cu is a Group 11 element, just like Au, and has a similar electronic structure, making it potentially suitable as an optical semiconductor when made into a chalcogen compound. Furthermore, Cu is one of the essential elements of the human body, and therefore has good biocompatibility.

The inventors therefore investigated the feasibility of synthesizing nanoparticles of an AgCu chalcogen compound containing Te as the essential chalcogen element, and their photoresponsiveness (hereinafter, the chalcogen element will be referred to as Ch). As a result, they discovered an AgCu chalcogen compound that has optical absorption characteristics with an absorption edge in the long wavelength region of 1200 nm or more, and is also capable of exhibiting luminescence, leading to the present invention.

That is, the present invention provides semiconductor nanoparticles containing an AgCu chalcogen compound represented by the following formula, which is composed of Ag, Cu, and a chalcogen element, and in which the chalcogen element contains Te as an essential chalcogen element, and the semiconductor nanoparticles contain 90 atomic % or more of the AgCu chalcogen compound.

(In the formula, Ch is a chalcogen element. x, y, and z are the atomic numbers of Ag, Cu, and the chalcogen element, respectively, and 0.2≦z/(x+y)≦1.0. Also, 1.0≦x/y≦10.0.)

The following describes the structure and manufacturing method of semiconductor nanoparticles based on an AgCu chalcogen compound containing Te as an essential chalcogen element according to the present invention.

A. Structure of the Semiconductor Nanoparticles According to the Present Invention A-1. Chemical Composition of the Semiconductor Nanoparticles As described above, the semiconductor nanoparticles according to the present invention are primarily composed of an AgCu chalcogen compound. This AgCu chalcogen compound can be expressed as Ag _x Cu _y Ch _z , where x, y, and z are the atomic numbers of Ag, Cu, and the chalcogen element Ch, respectively. With respect to the composition of the AgCuCh compound, the ratio (atomic ratio) of the number of Ag atoms x to the number of Cu atoms y is 1.0≦x/y≦10.0. Within the range investigated by the present inventors, AgCu chalcogen compound nanoparticles within this composition range exhibit favorable light absorption characteristics in the long-wavelength region of 1200 nm or more. Furthermore, AgCu chalcogen compound nanoparticles within this composition range can also emit light. Furthermore, by changing the atomic ratio x/y of Ag to Cu, the photoresponse characteristics of the AgCu chalcogen compound nanoparticles change. The atomic ratio x/y of Ag to Cu is more preferably 3.0 or more and 5.0 or less, which allows an emission peak to be observed in the long wavelength region of 1200 nm or more.

The chalcogen element (Ch) of the AgCu chalcogen compound used in the present invention is a chalcogen element that requires Te. As described above, by using Te, which has a relatively large mass among chalcogen elements, the wavelength at which the photoresponse is exhibited can be shifted to longer wavelengths. In the AgCu chalcogen compound of the present invention, the chalcogen element is bonded to the transition metals Ag and Cu so as to achieve charge compensation. The ratio of the number of atoms z of the chalcogen element to the number of atoms (x + y) of the transition metal components (Ag and Cu) satisfies 0.2≦z/(x + y)≦1.0. This atomic ratio z/(x + y) is preferably 0.3 to 0.7 (0.3≦z/(x + y)≦0.7), and more preferably 0.5 to 0.7 (0.5≦z/(x + y)≦0.7).

Furthermore, since Te is an essential chalcogen element in the AgCu chalcogen compound of the present invention, Te may be the only element, but other chalcogen elements other than Te may also be included. The reason why other chalcogen elements are included in the AgCu chalcogen compound of the present invention in addition to Te is that when chalcogen elements are contained in protective agents, solvents, etc. used in the synthesis process, these elements bond with the transition metal component together with Te. The other chalcogen elements that may be included together with Te in this process are preferably S or Se. When the AgCu chalcogen compound of the present invention contains other chalcogen elements other than Te, and the number of Te atoms is ^zTe , the ratio ^zTe /z of the number of Te atoms (zTe) to the number of all chalcogen atoms ( ^z ) is preferably 0.2 or more and 0.4 or less, more preferably 0.25 or more and 0.35 or less. As will be described later, the inventors have found that when other chalcogen elements are contained in addition to Te as a chalcogen element, the light absorption characteristics change depending on the atomic ratio of Te.

The composition of the AgCu chalcogen compound described above refers to the overall composition of the AgCu chalcogen compound in the semiconductor nanoparticles. The AgCu chalcogen compound applied to the present invention may be composed of a single phase or multiple phases. For example, when the chalcogen element is only Te, the AgCu chalcogen compound (Ag _x Cu _y Te _z ) can have several specific chemical compositions depending on the valences of Ag, Cu, and Te, including chalcogen compounds with stoichiometric compositions such as AgCuTe (x = 1, y = 1, z = 1) and AgCuTe ₂ (x = 1, y = 1, z = 2). Furthermore, when the chalcogen element contains S and/or Se in addition to Te, chalcogen compounds with stoichiometric compositions for these elements (such as AgCuS (x = 1, y = 1, z = 1) and AgCuSe (x = 1, y = 1, z = 1)) are also available. The AgCu chalcogen compound in the semiconductor nanoparticles of the present invention is composed of these stoichiometric compounds in a single phase or a mixed state of multiple phases. It may also contain compounds that do not have the above-mentioned stoichiometric composition. It is sufficient that x, y, and z are within the above-mentioned ranges for the entire AgCu chalcogen compound in the semiconductor nanoparticles.

The semiconductor nanoparticles of the present invention are primarily composed of AgCu chalcogen compounds, with at least 90 atomic % of the AgCu chalcogen compounds being the primary component. The semiconductor nanoparticles may be composed solely of AgCu chalcogen compounds. More preferably, the semiconductor nanoparticles contain at least 95 atomic % of AgCu chalcogen compounds. The semiconductor nanoparticles of the present invention may contain elements other than the essential chalcogen elements Ag, Cu, and Te that constitute the AgCu chalcogen compounds. For example, the semiconductor nanoparticles may contain elements contained in the solvent used to synthesize the AgCu chalcogen compounds or in the precursors of the raw materials Ag, Cu, and Te. Elements other than the essential constituent elements Ag, Cu, and chalcogen elements that may be contained include C, P, Cl, Br, I, etc., and the content of these elements in the semiconductor nanoparticles is acceptable as long as they are less than 10 mass %. The compositional values of the compounds and elements shown here are those for the semiconductor nanoparticles and do not include the content of the protective agent and its constituent elements, as described below.

A-2. Structure of the Semiconductor Nanoparticles According to the Present Invention As described above, the AgCu chalcogen compound applied to the present invention is composed of a single phase or multiple phases. Semiconductor nanoparticles composed of multiple phases can have a so-called core-shell structure. An example of a core-shell structure is a structure comprising a core (core compound) composed of an AgCu chalcogen compound containing Ag, Cu, and Te, and a shell (shell compound) composed of an AgCu chalcogen compound or a compound not containing Ag, Cu, or Te, whose composition is different from that of the core compound, and the shell compound covers at least a portion of the surface of the core compound. Furthermore, instead of a regular combination of multiple phases like the core-shell structure, multiple phases with different compositions may be randomly distributed. Cu-rich, Ag- and Te-rich, and the terms "Cu-rich" and "Ag- and Te-rich" mean that the composition ratio of Cu, Ag, and Te in the phase is greater than 50 atomic %.

The semiconductor nanoparticles according to the present invention may be spherical, cubic, or rod-shaped. Spherical and cubic semiconductor nanoparticles preferably have an average particle size of 2 nm or more and 20 nm or less. The particle size of the semiconductor nanoparticles is related to the band gap adjustment effect due to the quantum confinement effect. The above average particle size is preferable for achieving favorable light absorption characteristics through band gap adjustment. The average particle size of the semiconductor nanoparticles can be obtained by observing multiple semiconductor nanoparticles (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. The particle size can be measured as the average value of the major diameter (long axis) and minor diameter (short axis).

In addition, a scanning transmission electron microscope (scanning TEM) can be suitably used to analyze the composition and structure of the semiconductor nanoparticles according to the present invention. In particular, a high-angle annular dark-field scanning transmission microscope (HAADF-STEM) can obtain scattering images that reflect the compositional information of the nanoparticles, and when combined with an energy-dispersive X-ray spectrometer (EDS, EDX), it is possible to understand the distribution of Ag, Cu, and Te and the overall composition of the nanoparticles.

A-3. Photoresponsiveness of semiconductor nanoparticles according to the present invention As described above, the band gap of semiconductor nanoparticles is adjusted by the quantum confinement effect according to the particle size, and the photoresponsiveness changes. Regarding the light absorption characteristics of the semiconductor nanoparticles according to the present invention, the absorption edge wavelength on the long wavelength side of the absorption spectrum is preferably 1200 nm or more. This allows the semiconductor nanoparticles to exhibit absorption for light in the visible light region to the near-infrared region. In a more preferred embodiment, the semiconductor nanoparticles according to the present invention can have an absorption edge wavelength on the long wavelength side of 1400 nm or more.

The semiconductor nanoparticles of the present invention can also exhibit a luminescence phenomenon. The emission spectrum exhibits an emission peak in the wavelength region of 1000 nm or more. In a preferred embodiment, the semiconductor nanoparticles of the present invention can exhibit an emission peak in the long wavelength region of 1200 nm or more, more preferably 1400 nm or more.

A-4. Use of the Semiconductor Nanoparticles According to the Present Invention The semiconductor nanoparticles according to the present invention can be applied to various applications such as the above-mentioned optical sensor elements by coating and supporting them on an appropriate substrate or carrier. There are no particular limitations on the configuration, shape, or dimensions of this substrate or carrier. Examples of plate-shaped or foil-like substrates include glass, quartz, silicon, ceramics, and metals. Examples of granular or powdered carriers include inorganic oxides such as ZnO, TiO ₂ , WO ₃ , SnO ₂ , In ₂ O ₃ , and Al ₂ O _3. Alternatively, the semiconductor nanoparticles may be supported on the inorganic oxide carrier and then fixed to a substrate.

When applying or supporting semiconductor nanoparticles on a substrate or carrier, as mentioned above, a solution, slurry, or ink in which the semiconductor nanoparticles 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 to apply the semiconductor nanoparticle solution, and various methods such as dropping, impregnation, and adsorption can be used to support the semiconductor nanoparticles.

The semiconductor nanoparticles according to the present invention preferably contain a protective agent to suppress aggregation during their synthesis or when dispersed in a dispersion medium as described above. This protective agent is preferably at least one of the following: alkylamines with alkyl chain carbon atoms of 4 to 20; alkenylamines with alkenyl chain carbon atoms of 4 to 20; alkylcarboxylic acids with alkyl chain carbon atoms of 3 to 20; alkenylcarboxylic acids with alkenyl chain carbon atoms of 3 to 20; and alkanethiols with alkyl chain carbon atoms of 4 to 20. These protective agents bond to the surface of the semiconductor nanoparticles to coat at least a portion of them, suppressing aggregation of the semiconductor nanoparticles in the dispersion and forming a uniform solution. Furthermore, adding the protective agent to the reaction system along with the raw materials during the semiconductor nanoparticle synthesis process allows the synthesis of nanoparticles with an appropriate average particle size. The protective agent can be any of the alkylamines, alkenylamines, alkylcarboxylic acids, alkenylcarboxylic acids, and alkanethiols, either singly or in combination.

B. Method for Producing Semiconductor Nanoparticles According to the Present Invention Next, a method for producing semiconductor nanoparticles according to the present invention will be described. The present inventors have discovered that, in the synthesis of an AgCu chalcogen compound having the above-described composition, it is preferable to use compounds containing Ag, Cu, and Te as precursors (Ag precursor, Cu precursor, Te precursor), introduce these precursors into the same reaction system, and heat and react them simultaneously. Below, a method for producing semiconductor nanoparticles using this AgCu chalcogen compound synthesis method will be described.

B-1. Raw materials (Ag precursor, Cu precursor, Te precursor)
The Ag precursor, Cu precursor, and Te precursor used as raw materials are, respectively, an Ag salt or Ag complex, a Cu salt or Cu complex, and a Te compound. The Ag precursor and Cu precursor are preferably salts or complexes containing monovalent Ag and monovalent Cu.

The Ag precursor is preferably an Ag salt or Ag complex. The Ag precursor is preferably a salt or complex containing monovalent Ag. Specific examples of suitable Ag precursors include silver acetate (Ag(OAc)), silver nitrate, silver carbonate, silver oxide, silver oxalate, silver chloride, silver iodide, silver(I) cyanide, and silver diethyldithiocarbamate.

A Cu salt or Cu complex is used as the Cu precursor. The Cu precursor is preferably a salt or complex containing monovalent Cu. However, a salt or complex containing divalent Cu can also be used as the Cu precursor. This is because, during the synthesis of semiconductor nanoparticles, divalent Cu is reduced to monovalent Cu by the solvent or the coexisting Te precursor. Specific examples of suitable Cu precursors include copper acetate, copper chloride, copper iodide, and copper bromide.

As the Te compound that serves as the Te precursor, tellurium oxide (TeO ₂ ), telluric acid (Te(OH) ₆ ), sodium tellurite (Na ₂ TeO ₃ ), and the like can be used.

B-2. Formation of a reaction system of an AgCu chalcogen compound When synthesizing an AgCu chalcogen compound, the above-mentioned Ag precursor, Cu precursor, and Te precursor are mixed to form a single reaction system, which is then reacted. At this time, the Ag precursor, Cu precursor, and Te precursor that have been prepared separately may be mixed sequentially, and the order of mixing is not particularly limited. Alternatively, a mixture of an Ag precursor and a Cu precursor that will become the transition metal component of the AgCu chalcogen compound may be prepared as a metal source precursor, and the metal source precursor and Te precursor may be mixed to form a reaction system.

The atomic ratio (x/ _y ) of Ag to Cu in the synthesized AgCu _{chalcogen compound (AgxCuyChz )} can be adjusted by the ratio of the amounts of the Ag precursor and the Cu precursor charged. The ratio of the amounts charged for synthesizing the AgCu chalcogen compound of the present invention, in which the atomic ratio (x/y) of Ag to Cu falls within a predetermined range, is preferably set so that the ratio (a/b: hereinafter referred to as the Ag charge ratio) is 1 to 10, more preferably 2 to 4, where a is the number of Ag atoms in the Ag precursor and b is the number of Cu atoms in the Cu precursor.

Furthermore, the amount of Te precursor ^added to the reaction system can affect the number of Te atoms ( _zTe ) in the chalcogen elements of the AgCu _chalcogen compound ( _AgxCuyChz ) being synthesized. According to the studies of the present inventors, when the number of Te atoms in the Te precursor is c and the number of Cu atoms in the Cu precursor described above is b, the ratio (c/b: hereinafter, this ratio may be referred to as the Te addition ratio) is preferably set to 0.1 or more and 2 or less, more preferably 0.1 or more and 1 or less.

As described above, the semiconductor nanoparticles of the present invention preferably have a protective agent bonded to the AgCu chalcogen compound. Furthermore, when this protective agent contains a chalcogen element, the chalcogen element can form part of the chalcogen element of the AgCu chalcogen compound of the present invention. Therefore, it is preferable to add a protective agent to the above-described reaction system along with the Ag precursor, Cu precursor, and Te precursor. It is preferable to add at least one of the following protective agents: an alkylamine having an alkyl chain carbon number of 4 to 20, an alkenylamine having an alkenyl chain carbon number of 4 to 20, an alkylcarboxylic acid having an alkyl chain carbon number of 3 to 20, an alkenylcarboxylic acid having an alkenyl chain carbon number of 3 to 20, or an alkanethiol having an alkyl chain carbon number of 4 to 20.

The reaction system used to synthesize semiconductor nanoparticles can produce nanoparticles without a solvent, but it is preferable to use a solvent. If a solvent is used, octadecene, tetradecane, oleic acid, oleylamine, dodecanethiol, or a mixture of these can be used.

B-3. AgCu chalcogen compound synthesis conditions AgCu chalcogen compound nanoparticles are synthesized by heating a reaction system consisting of an Ag precursor, a Cu precursor, a Te precursor, and a protective agent. The heating temperature (reaction temperature) at this time is 100°C or higher and 200°C or lower. If the temperature is lower than 100°C, the synthesis reaction does not proceed easily. On the other hand, if the temperature exceeds 200°C, nanoparticles with unstable particle shapes may be formed. A more suitable reaction temperature is 100°C or higher and 150°C or lower.

The reaction time (heating time) can be adjusted depending on the amount of raw materials charged, etc., but is preferably 5 minutes or more and 120 minutes or less. The reaction time is more preferably 10 minutes or more, and even more preferably 15 minutes or more. It is preferable to stir the reaction system while the synthesis reaction of AgCu chalcogen compound nanoparticles proceeds.

After the synthesis reaction of AgCu chalcogen compound nanoparticles is complete, the reaction system can be cooled as needed, and the resulting semiconductor nanoparticles can be recovered. At this point, the nanoparticles can be precipitated by adding a poor solvent, such as alcohol (ethanol, methanol, etc.), or the semiconductor nanoparticles can be precipitated and recovered by centrifugation or other methods, and the particles can then be washed with alcohol (ethanol, methanol, etc.) and then uniformly dispersed in a good solvent such as chloroform.

As described above, the present invention relates to semiconductor nanoparticles whose main component is an AgCu chalcogen compound (Ag _x Cu _y Ch _z ) containing Te as an essential chalcogen element. The semiconductor nanoparticles according to the present invention have favorable photoresponsiveness and are also practical in consideration of usage regulations, etc. Furthermore, the semiconductor nanoparticles according to the present invention are applicable to applications such as light receiving elements and light emitting elements in the long wavelength region of the near infrared region (NIR) and short wave infrared region (SWIR).

1 is a TEM image of semiconductor nanoparticles made of an AgCu chalcogen compound (Ag _x Cu _y Ch _z ) synthesized in the first embodiment. 10 shows the results of measuring the absorption spectrum of semiconductor nanoparticles made of an AgCu chalcogen compound (Ag _x Cu _y Ch _z ) synthesized in the first embodiment. 1 shows the results of measuring the emission spectrum of semiconductor nanoparticles made of an AgCu chalcogen compound (Ag _x Cu _y Ch _z ) synthesized in the first embodiment. 10 shows the results of measuring the absorption spectrum of semiconductor nanoparticles made of an AgCu chalcogen compound (Ag _x Cu _y Ch _z ) synthesized in the second embodiment. 10 shows the results of measuring the emission spectrum of semiconductor nanoparticles made of an AgCu chalcogen compound (Ag _x Cu _y Ch _z ) synthesized in the second embodiment. 10A and 10B are TEM images of semiconductor nanoparticles made of an AgCu chalcogen compound (Ag _x Cu _y Ch _z ) synthesized by changing the heating temperature in the third embodiment. 10 shows the results of measuring the absorption spectrum of semiconductor nanoparticles made of an AgCu chalcogen compound (Ag _x Cu _y Ch _z ) synthesized by changing the heating temperature in the third embodiment. 10 shows the results of measuring the emission spectrum of semiconductor nanoparticles made of an AgCu chalcogen compound (Ag _x Cu _y Ch _z ) synthesized by changing the heating temperature in the third embodiment. 10A to 10C are TEM images of semiconductor nanoparticles made of an AgCu chalcogen compound (Ag _x Cu _y Ch _z ) synthesized by changing the reaction time in the third embodiment. 10 shows the results of measuring the absorption spectrum of semiconductor nanoparticles made of an AgCu chalcogen compound (Ag _x Cu _y Ch _z ) synthesized by changing the reaction time in the third embodiment. 10 shows the results of measuring the emission spectrum of semiconductor nanoparticles made of an AgCu chalcogen compound (Ag _x Cu _y Ch _z ) synthesized by changing the reaction time in the third embodiment.

First Embodiment : Hereinafter, an embodiment of the present invention will be described. In this embodiment, semiconductor nanoparticles made of an AgCu _chalcogen compound ( _AgxCuyChz ) were synthesized, and their appearance and photoresponsive properties were evaluated. In this embodiment, nanoparticles of an AgCu chalcogen compound were synthesized with different atomic ratios of Ag and Cu (x _/ y).

In this embodiment, semiconductor nanoparticles made of an AgCu chalcogen compound were synthesized by first preparing a mixture of Ag precursor and Cu precursor, which serve as transition metal components, as a metal source precursor. The metal source precursor was then mixed and reacted with a Te precursor to synthesize the semiconductor nanoparticles.

A metal source precursor was prepared by dissolving and mixing silver acetate (Ag(OAc)) as an Ag precursor and copper acetate (Cu(OAc)) as a Cu precursor in 3 mL of 1-dodecanethiol (DDT), a solvent and protective agent. The total content of Ag and Cu in the solution was set to 0.08 mmol in terms of metal. The silver acetate and copper acetate were mixed so that the Ag charging ratios (a/b) were 3/1, 2.5/1.5, 2/2, and 1.5/2.5. While this metal source precursor was being prepared, 0.2 mmol of tellurium oxide (TeO ₂ ) was dissolved in 2 mL of DDT, a solvent and protective agent, and heated at 100°C for 5 minutes under a nitrogen atmosphere to prepare a Te precursor solution.

To synthesize AgCu chalcogen compound nanoparticles, first, a solution of metal source precursors (Ag, Cu) was heated at 150°C for 5 minutes under a nitrogen atmosphere. The Te precursor solution was then injected into the solution using a syringe. The reaction temperature was then increased to 150°C under a nitrogen atmosphere, and the mixture was stirred while being heated for 15 minutes. After the reaction was completed, the mixture was allowed to cool and then centrifuged at 4000 rpm for 5 minutes to separate the supernatant and precipitate. 4 ^cm3 of ethanol/toluene was added to the supernatant as a poor solvent to generate a precipitate, which was then centrifuged at 4000 rpm for 5 minutes to recover the precipitate, yielding AgCu chalcogen compound nanoparticles.

The AgCu chalcogen compound nanoparticles obtained by the above procedure were dispersed in ³ cm3 of chloroform to obtain a dispersion of semiconductor nanoparticles made of an AgCu chalcogen compound. This dispersion was transferred to a sample bottle and purged with nitrogen, and then stored in a refrigerator in a dark place.

[TEM Observation of Semiconductor Nanoparticles]
TEM observation was performed on semiconductor nanoparticles composed of the AgCu chalcogen compound synthesized in this embodiment (Ag charge ratio: 3/1, 2.5/1.5, 2/2, 1.5/2.5). Figure 1 shows TEM images of each semiconductor nanoparticle produced in this embodiment (see the scale bar in each photograph for magnification). Referring to Figure 1, in the AgCu chalcogen compound synthesized with an Ag charge ratio of 3/1, many nearly spherical nanoparticles are observed. As the Ag charge ratio decreases (the proportion of Cu increases), there is a tendency for the number of amorphous nanoparticles to increase accordingly.

[Composition analysis of semiconductor nanoparticles]
Composition analysis was performed by SEM-EDS analysis on semiconductor nanoparticles composed of an AgCu chalcogen compound (Ag charging ratios: 3/1, 2.5/1.5, 2/2, 1.5/2.5) synthesized in this embodiment. The results of measuring the composition of each semiconductor nanoparticle are shown in Table 1. In this embodiment and each of the following embodiments, the results of the composition analysis are expressed in atomic % relative to the total nanoparticles. Table 1 also shows the ratio x/y of the number of Ag atoms (x) to the number of Cu atoms (y), and the ratio z/(x+y) of the number of atoms of the chalcogen element (z) to the number of atoms of the transition metal component (x+y), calculated based on the composition analysis results.

As can be seen from Table 1, an increase in the Ag charge ratio (a/b) naturally increases the ratio (x/y) of the number of Ag atoms (x) to the number of Cu atoms (y) in the AgCu chalcogen compound. It was also confirmed that the ratio of the number of atoms (z) of the chalcogen element to the number of atoms (x + y) of the transition metal component (Ag and Cu) does not vary significantly even when the Ag charge ratio is changed. It was also confirmed that the AgCu chalcogen compound synthesized in this embodiment contains Te as an essential chalcogen element, while also containing S as another chalcogen element. Furthermore, except for No. 4, the ratio of the number of Te atoms to the total number of chalcogen elements (z ^Te /z) does not vary significantly even when the Ag charge ratio is changed. The AgCu chalcogen compound No. 4 has the smallest Ag content ratio (a/b: 1.5/2.5), and when the TEM observation results (FIG. 1) are referred to, two-phase separation is observed, and this is thought to be the only difference in tendency.

[Measurement of absorption spectrum and emission spectrum]
Next, absorption and emission spectra were measured to evaluate the photoresponsiveness of each semiconductor nanoparticle. The absorption spectrum was measured using a UV-visible spectrophotometer (Agilent Technologies, Inc., Agilent 8453) in the wavelength range of 400 nm to 1600 nm. The emission spectrum was measured using a diode array spectrophotometer (PMA-12, C10027-02) manufactured by Hamamatsu Photonics K.K. The sample was dissolved in chloroform (n = 1.4429) and adjusted so that the absorbance at 365 nm was 0.1, and then the measurement was performed.

The measurement results of the absorption spectrum of each semiconductor nanoparticle produced in this embodiment are shown in Figure 2, and the measurement results of the emission spectrum are shown in Figure 3. Furthermore, the absorption edge wavelength on the long wavelength side and the peak wavelength of the emission spectrum of each semiconductor nanoparticle measured based on these are shown in Table 2.

Referring to the absorption spectrum measurement results in Figure 2 and Table 2, all semiconductor nanoparticles have optical absorption characteristics that show an absorption edge in the region of 1400 nm or more. Furthermore, all of them exhibit a small peak thought to be an exciton peak at wavelengths of 800 nm to 950 nm, while an absorption peak appears near 1400 nm. Regarding the composition of the AgCu chalcogen compound (Ag _x Cu _y Ch _z ), when the atomic ratio of Ag to Cu (x/y) is examined, a shift in the absorption edge toward longer wavelengths is observed as x/y increases and the atomic ratio of Ag becomes higher.

All of the semiconductor nanoparticles of this embodiment were confirmed to emit light. Referring to the emission spectrum measurement results in Figure 3 and Table 2, an emission peak was confirmed in the wavelength region of 1000 nm or more. In particular, the emission peak wavelength of the semiconductor nanoparticles (No. 1) with a high Ag atomic ratio is on the long wavelength side. An emission peak was observed in the region of 1200 nm or more for these semiconductor nanoparticles.

These photoresponsiveness evaluation results indicate that the semiconductor nanoparticles composed of the AgCu chalcogen compound synthesized in this embodiment have an absorption edge of 1200 nm or more and exhibit good light absorption in the long-wavelength region. In particular, No. 1 AgCu chalcogen compound nanoparticles with an Ag loading ratio of 3/1 (x/y = 4) had a long-wavelength absorption edge wavelength of 1500 nm or more and exhibited good emission characteristics, with a peak wavelength of 1200 nm or more.

Second Embodiment : In this embodiment, semiconductor nanoparticles composed of AgCu chalcogen compounds of various compositions were synthesized by adjusting the Te charge ratio. The semiconductor nanoparticle synthesis method applied in this embodiment is basically the same as that in the first embodiment. A solution of a Te precursor was added to a solution of a metal source precursor, and the solution was heated and reacted to obtain nanoparticles of an AgCu chalcogen compound. In this embodiment, the amount of Te precursor added was adjusted to perform the synthesis reaction so that the Te charge ratio (c/b), which is the ratio of the number of Te atoms in the Te precursor, c, to the number of Cu atoms in the metal source precursor, b, was 1/4, 1/2, 1/1, or 2/1. After synthesis, the nanoparticles were separated and purified in the same manner as in the first embodiment to obtain nanoparticles of an AgCu chalcogen compound. The Ag charge ratio (a/b) was 3/1, and the other reaction conditions were the same as in the first embodiment.

[Composition analysis of semiconductor nanoparticles]
The composition of the semiconductor nanoparticles (Te content ratio: 1/4, 1/2, 1/1, 2/1) composed of the AgCu chalcogen compound synthesized in this embodiment was analyzed by SEM-EDS. The results of the composition measurement for each semiconductor nanoparticle are shown in Table 3.

Referring to Table 3, even if the Te content ratio (c/b) increases, there is no significant change in the ratio (z/(x+y)) of the number of atoms of the chalcogen element (z) to the number of atoms of the transition metal component (Ag, Cu) (x+y). An increase in the Te content ratio leads to an increase in the ratio (z ^Te /z) of the number of Te atoms to the total number of atoms of the chalcogen element. It also appears that an increase in the Te content ratio also tends to lead to a decrease in the atomic ratio x/y of Ag to Cu.

[Measurement of absorption spectrum and emission spectrum]
Next, absorption spectrum and emission spectrum measurements were performed to evaluate the photoresponsiveness of each semiconductor nanoparticle produced in this embodiment. These measurement methods were the same as those in the first embodiment. The results of these measurements are shown in Figures 4 and 5, and the absorption edge wavelength on the long wavelength side and the peak wavelength of the emission spectrum of each semiconductor nanoparticle are shown in Table 4.

Referring to the absorption spectrum measurement results in Figure 4 and Table 4, all of the semiconductor nanoparticles with different Te loading ratios exhibit an absorption peak near 1400 nm, while also exhibiting an exciton peak. Furthermore, the long-wavelength absorption edge is in the region of 1500 nm or higher in all cases. In particular, the semiconductor nanoparticles (No. 5) with a low Te loading ratio (low atomic ratio of Te in the AgCu chalcogen compound) exhibited a further shift in the absorption edge wavelength.

Referring to the emission spectrum measurement results in Figure 5 and Table 4, the emission spectrum curves of semiconductor nanoparticles No. 5 to No. 7 (c/b: 1/4 to 1/1) show similar trends with little difference. Semiconductor nanoparticle No. 8, which has a high Te loading ratio (c/b: 2/1), did not show a clear peak.

Third Embodiment : In this embodiment, the photoresponsiveness of semiconductor nanoparticles obtained when the synthesis conditions for AgCu chalcogen compound nanoparticles were changed was examined.

First, the effects of temperature before and after the formation of a reaction system were investigated. Here, in the synthesis process for AgCu chalcogen compound nanoparticles of the first embodiment, the heating temperature (T1) of the metal source precursor solution before adding the Te precursor solution to the metal source precursor solution and the heating temperature (reaction temperature: T2) after adding the Te precursor solution were adjusted. Specifically, AgCu chalcogen compound nanoparticles were synthesized under three conditions: (i) T1 = T2 = 150°C, (ii) T1 = 150°C and T2 = 120°C, and (iii) T1 = T2 = 120°C. The heating times for each of these conditions were the same as in the first embodiment.

In this embodiment, the effect of the heating time (reaction time) after adding the Te precursor solution to the metal source precursor solution was also investigated. In this study, AgCu chalcogen compound nanoparticles were synthesized using reaction times of 5, 10, 20, 40, and 80 minutes. The heating temperatures in this study were T1 = T2 = 120°C.

In each study, the synthesis method and conditions other than the above-mentioned reaction conditions were the same as those in the first embodiment, and the Ag feed ratio (a/b) was 3/1.

Regarding the results of this study, we will first explain the temperatures before and after forming the reaction system. Figure 6 is a TEM image of AgCu chalcogen compound nanoparticles synthesized under the heating conditions (i) to (iii) described above. This TEM image shows that nanoparticles with a stable particle shape are synthesized under condition (iii), in which the heating temperatures (T1 and T2) before and after forming the reaction system are both 120°C. Furthermore, the shape of the nanoparticles tends to become unstable when heating at 150°C is included.

Therefore, composition analysis was performed using EDS on semiconductor nanoparticles prepared under conditions (i) (T1 = T2 = 150°C) and (iii) (T1 = T2 = 120°C), in which the heating temperatures before and after reaction system formation were the same. The results are shown in Table 5. Table 5 shows that although there were differences in particle shape, there were no significant differences in composition between these AgCu chalcogen compound nanoparticles.

The measurement results of the absorption and emission spectra of these two semiconductor nanoparticles are shown in Figures 7, 8, and Table 6.

Looking at the absorption spectrum measurement results in Figure 7 and Table 6, the absorption characteristics of semiconductor nanoparticles under heating conditions (i) and (iii) form similar curves overall. However, for semiconductor nanoparticles (iii) where the heating temperature was 120°C before and after reaction system formation, the absorption peak intensity is clearly stronger and the absorption edge wavelength is also longer. Furthermore, looking at the emission spectrum measurement results in Figure 8 and Table 6, the emission peak wavelength for semiconductor nanoparticles (iii) where the heating temperature was 120°C before and after reaction system formation is significantly shifted (by 200 nm or more) to the longer wavelength side.

Next, we will explain the results of an investigation into the effect of reaction time. Figure 9 is a TEM image of AgCu chalcogen compound nanoparticles synthesized over reaction times of 5 to 40 minutes (T1 = T2 = 120°C). Figure 9 shows that increasing the reaction time results in particle shapes that approach spheres. Table 7 also shows the results of compositional analysis of these semiconductor nanoparticles by EDS. Table 7 shows that increasing the reaction time does not result in a significant difference in the atomic ratio (z/(x + y)) of the transition metal components (Ag, Cu) to the chalcogen elements. Furthermore, the atomic ratio (x/y) of Ag in the transition metal components does not vary significantly within the range of 3.0 to 5.0.

For AgCu chalcogen compound nanoparticles synthesized with reaction times of 5 to 40 minutes (T1 = T2 = 120°C), the absorption spectrum measurement results are shown in Figure 10, and the emission spectrum measurement results are shown in Figure 11. These results are summarized in Table 8.

Looking at the absorption spectra in Figure 10, there is no significant difference in the absorption characteristics of each semiconductor nanoparticle, except for the 80-minute reaction time. Furthermore, looking at the emission spectrum measurement results in Figure 11, as with the absorption spectrum results, there is no difference in the emission spectrum curves, and the emission peak wavelengths are also roughly the same.

Summarizing the results of the study on synthesis conditions investigated in this embodiment, it can be said that a relatively low heating temperature and reaction temperature of 120°C for the metal source precursor is more preferable than 150°C (first embodiment). Furthermore, a longer reaction time is preferable because it results in spherical particles, but there is little difference in properties other than the particle shape. It is believed that the reaction time can be set relatively freely. The synthesis conditions for semiconductor nanoparticles should be adjusted depending on the concentrations of the transition metal and chalcogen elements in the metal source precursor and Te precursor, as well as the scale of the reaction system, but the results of this embodiment can be used as a reference.

As described above, semiconductor nanoparticles made of the AgCu chalcogen compound according to the present invention can exhibit excellent photoresponsiveness. This AgCu chalcogen compound is also designed to comply with usage regulations and avoid the use of heavy metals. The semiconductor nanoparticles according to the present invention are expected to be applied to light-emitting elements and fluorescent materials used in display devices and marker substances for detecting biological substances, as well as photoelectric conversion elements and photodetectors mounted in solar cells and optical sensors. In particular, the present invention aims to improve light absorption characteristics in the long wavelength range of the near-infrared region (NIR) and shortwave infrared region (SWIR). Therefore, the present invention is particularly useful for photodetectors used in LIDAR and SWIR image sensors, among the above-mentioned optical elements, where responsiveness in the near-infrared region is important.

Claims (8)

Semiconductor nanoparticles containing an AgCu chalcogen compound represented by the following formula, which is composed of Ag, Cu, and a chalcogen element:
the chalcogen element contains Te as an essential chalcogen element,
The semiconductor nanoparticles contain 90 atomic % or more of the AgCu chalcogen compound.
(In the formula, Ch is a chalcogen element; x, y, and z are the atomic numbers of Ag, Cu, and the chalcogen element, respectively, and 0.2≦z/(x+y)≦1 is satisfied. Also, 1.93 ≦x/y≦10.0 is satisfied.) The semiconductor nanoparticles according to claim 1, wherein the chalcogen elements constituting the AgCu chalcogen compound include chalcogen elements other than Te, and the other chalcogen elements are S and Se. The semiconductor nanoparticles according to claim 2, wherein the ratio of the number of Te atoms to the number of chalcogen element atoms constituting the AgCu chalcogen compound is 0.2 or more and 0.4 or less. The semiconductor nanoparticles according to claim 1 or 2 contain 99 atomic % or more of the AgCu chalcogen compound. The semiconductor nanoparticles according to claim 1 or 2, having an average particle size of 2 nm or more and 20 nm or less. The semiconductor nanoparticles according to claim 1 or 2, wherein at least one of the following protective agents is bonded to the surface: alkylamines having an alkyl chain carbon number of 4 to 20, alkenylamines having an alkenyl chain carbon number of 4 to 20, alkylcarboxylic acids having an alkyl chain carbon number of 3 to 20, alkenylcarboxylic acids having an alkenyl chain carbon number of 3 to 20, and alkanethiols having an alkyl chain carbon number of 4 to 20. The semiconductor nanoparticles according to claim 1 or 2, wherein the absorption edge wavelength on the long wavelength side of the absorption spectrum is 1200 nm or longer. 3. The semiconductor nanoparticles according to claim 1, wherein the peak wavelength of the emission spectrum is 1000 nm or more.