JP2014084255A - Carbon nanotube thin film, transparent electrode and electrode for lithography - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbon nanotube thin film having high transmittance and low sheet resistance.SOLUTION: There is provided a dispersion liquid of a carbon nanotube which contains a carbon nanotube having an average length of 10 μm or more.

Description

  The present invention relates to a carbon nanotube thin film, a transparent electrode, and an electrode for photolithography.

  A carbon nanotube (hereinafter, referred to as “CNT” in some cases) has a cylindrical structure in which planar graphite (graphene sheet) is rounded. Due to their nanostructure specificity, CNTs exhibit various properties. In particular, CNTs are excellent in properties such as high current density resistance 1000 times or more that of copper, high thermal conductivity about 10 times that of copper, and tensile strength about 20 times that of steel. These characteristics are considered to greatly contribute to the improvement of characteristics of electronic materials, heat conductive materials, composite materials, etc., and in recent years, application of CNTs to various fields has been attempted.

  Single-walled carbon nanotubes (SWCNT) composed of a single layer of graphene sheets are classified into three types of carbon nanotubes with different characteristics depending on the geometric structure of the graphene sheet (armchair tube, zigzag tube and chiral tube) ). In addition, the band structure of the single-walled carbon nanotube varies depending on the arrangement of the six-membered carbon rings constituting the single-walled carbon nanotube. Therefore, single-walled carbon nanotubes exhibit semiconducting properties (semiconductor CNT) or metallic properties (metal CNT).

  The electrical resistance of the metal CNT is lower than that of the semiconductor CNT. If a carbon nanotube thin film can be produced only from metal-type CNTs with low electrical resistance, the electrical characteristics of the carbon nanotube thin film can be greatly improved. That is, a reduction in sheet resistance of the carbon nanotube thin film is expected.

  However, it is not easy to selectively synthesize only metal CNTs in a large amount. Non-Patent Document 1 below discloses a method for selectively synthesizing metal CNTs, but has not yet achieved mass production of metal CNTs. The content of metallic CNT in the CNT produced by the conventional synthesis method is about 30% of the total CNT. Therefore, a method for separating metal CNT and semiconductor CNT is required.

  For example, Non-Patent Document 2 below discloses a method for separating metal-type CNT and semiconductor-type CNT by electrophoresis. In this method, a sugar derivative gel is used for the carrier layer. The mobility of metallic CNT in the gel when an electric current is applied to the gel is different from the mobility of semiconductor CNT. Therefore, the metal-type CNT and the semiconductor-type CNT after electrophoresis are separated from each other in the gel. A section in which each CNT is unevenly distributed is cut out from the gel, and the metal CNT is eluted from a section in which the metal CNT is unevenly distributed to obtain a dispersion liquid of the metal CNT. The semiconductor-type CNTs are eluted from the section where the semiconductor-type CNTs are unevenly distributed to obtain a semiconductor-type CNT dispersion. Thus, it is possible to separate both CNTs using the difference in mobility during electrophoresis.

Succeeded in high-purity synthesis of metallic carbon nanotubes that open up new possibilities for electronics materials [online]. Honda Motor Co., Ltd. 2009. [retrieved on 2009-10-2]. Retrieved from the Internet: <URL: http://www.honda.co.jp/news/2009/c091002.html>. T.A. Tanaka et al. : Appl. Phys. Express 2,125002? 1-3 (2009).

  However, in addition to the extremely small amount of CNT that can be processed by the electrophoresis method of Non-Patent Document 2, the concentration of CNT in the obtained CNT dispersion is extremely low. Therefore, in order to produce an industrial product such as a transparent electrode using CNTs separated by electrophoresis, it is necessary to concentrate the CNT dispersion before using it, or to recoat the dispersion. For these reasons, when the electrophoresis method is used for mass production of industrial products using CNTs, the number of processes increases and the cost increases.

  As described above, it has been difficult to selectively synthesize a large amount of metal-type CNTs or to separate a large amount of metal-type CNTs using electrophoresis. Therefore, it has been difficult to improve various characteristics of industrial products (for example, transparent electrodes) by using only metal CNTs. Because of this background, it is difficult to apply high-quality CNTs to transparent electrodes. In order to improve and secure the characteristics of electrodes formed from CNTs, the thickness of the conductive portions formed from carbon nanotubes must be reduced. The thickness must be increased, and as a result, the transmittance of the electrode is lowered. This has been a barrier to the practical use of transparent electrodes.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a carbon nanotube thin film having high transmittance and low sheet resistance, an electrode including the thin film, and an electrode for photolithography.

  One aspect of the carbon nanotube thin film according to the present invention contains carbon nanotubes having an average length of 10 μm or more.

  In one aspect of the carbon nanotube thin film according to the present invention, the carbon nanotube is preferably a single-walled carbon nanotube.

  In one aspect of the carbon nanotube thin film according to the present invention, the carbon nanotube may be a single-walled carbon nanotube or a multi-walled carbon nanotube.

  In one aspect of the carbon nanotube thin film according to the present invention, it is preferable that an average thickness of a bundle composed of a plurality of carbon nanotubes and the carbon nanotube present alone is 150 nm or less.

One aspect of the carbon nanotube thin film according to the present invention includes a deposition layer on which carbon nanotubes are deposited, and the thickness of the deposition layer is preferably 200 nm or less.

  In one aspect of the carbon nanotube thin film according to the present invention, it is preferable that the ratio G / D between the G band and the D band in the Raman spectrum of the carbon nanotube is 0.5 or more.

  In one aspect of the carbon nanotube thin film according to the present invention, the haze value is preferably 2% or less.

  In one embodiment of the carbon nanotube thin film according to the present invention, the transmittance is preferably 70% or more, and the sheet resistance value is preferably 10 kΩ / □ or less.

  In one embodiment of the carbon nanotube thin film according to the present invention, it is preferable to transmit far-infrared light having a wavelength of 1 to 10 μm.

  In one aspect of the carbon nanotube thin film according to the present invention, the carbon nanotube is preferably synthesized by a chemical vapor deposition method.

  One aspect of the carbon nanotube thin film in the present invention is preferably formed through a step of applying a dispersion liquid in which carbon nanotubes having an average length of 10 μm or more are dispersed on a substrate.

  One aspect of the transparent electrode according to the present invention includes the carbon nanotube thin film and a transparent base material on which the carbon nanotubes are laminated.

  One aspect of the electrode for photolithography according to the present invention includes the carbon nanotube thin film and a photosensitive resin film in which the carbon nanotube thin films are stacked.

  According to the present invention, it is possible to provide a carbon nanotube thin film having high transmittance and low sheet resistance, a transparent electrode including the thin film, and a photolithography electrode.

It is the image of the surface of the carbon nanotube thin film (VF film: Vacuum Filtration film | membrane) of Example 1 of this invention image | photographed with the scanning electron microscope (SEM: Scanning Electron Microscope). It is an enlarged view of FIG. It is the image of the surface of the carbon nanotube thin film (VF film) of the comparative example 1 image | photographed with the scanning electron microscope. FIG. 4A is a schematic diagram of a transparent electrode according to an embodiment of the present invention, and FIG. 4B is a schematic diagram of a photolithographic electrode according to an embodiment of the present invention.

  Hereinafter, preferred embodiments of the present invention will be described. However, the present invention is not limited to the following embodiment.

(Carbon nanotube thin film)
The carbon nanotube thin film according to the present embodiment contains carbon nanotubes having an average length of 10 μm or more. The average length of the CNT is preferably 12 μm or more, and more preferably 15 μm or more. The upper limit of the average length of the CNT is the maximum length that can be technically manufactured, and is, for example, about 10 mm. In the thin film, a long network of CNTs forms a network (electron conduction path). Further, since CNTs are long, junctions (bonding sites) between CNTs and CNTs that cause an increase in sheet resistance are difficult to form in the thin film. Therefore, high transmittance of the thin film and low sheet resistance (electric resistance) are achieved. When the average length of CNTs is short, the number of CNT-CNT junctions (joining portions) that cause an increase in sheet resistance increases. Therefore, in order to achieve a low sheet resistance value, the amount of CNTs constituting the thin film It is necessary to increase. As a result, the film thickness increases and the transmittance of the thin film decreases. Since the CNT thin film according to this embodiment has high conductivity, it can be used as a conductive member or an antistatic layer for electronic devices. The CNT thin film may consist only of CNT having an average length of 10 μm or more. In addition to the CNT, the CNT thin film contains an additive in an amount that does not impair the effects of the present invention (for example, a hydrophilic polymer such as Nafion (registered trademark), a water-soluble polymer such as sodium carboxymethyl cellulose). Also good.

  As shown in FIG. 4A, the transparent electrode according to the present embodiment includes the carbon nanotube thin film 2 and a transparent substrate 4 on which the carbon nanotube thin film 2 is laminated. Since the transparent electrode of the present embodiment includes the CNT thin film 2 having high transmittance and low sheet resistance, it is suitable as a display electrode or an optical sensor electrode.

  The transparent substrate 4 is not particularly limited as long as it is a transparent substrate having a high transmittance, and examples thereof include a PET (polyethylene terephthalate) film and a PEN (polyethylene naphthalate) film.

  As shown in FIG. 4B, the photolithography electrode according to the present embodiment includes the carbon nanotube thin film 2 and the photosensitive resin film 6 laminated on the carbon nanotube thin film 2. By exposing the surface of the photosensitive resin film 6 in a pattern with ultraviolet rays or the like, a pattern composed of an exposed portion and an unexposed portion can be formed.

  Examples of the resin constituting the photosensitive resin film 6 include polyfunctional acrylates, urethane acrylates, photosensitive epoxy resins, and photosensitive polyimides.

  The carbon nanotubes constituting the carbon nanotube thin film are preferably single-walled carbon nanotubes. However, in one aspect of the carbon nanotube thin film, the carbon nanotube may be a single-walled carbon nanotube or a multi-walled carbon nanotube.

  As the number of CNT layers increases, the transmittance of the thin film tends to decrease and the sheet resistance tends to increase. However, if the number of multilayer CNT layers is 2 to 5, it is possible to suppress the influence of the number of layers on the transmittance and sheet resistance to a low level.

  In one aspect of the carbon nanotube thin film according to the present invention, it is preferable that the average thickness of a bundle composed of a plurality of carbon nanotubes and a carbon nanotube present alone is 150 nm or less. The average thickness is more preferably 10 to 100 nm, and particularly preferably 20 to 50 nm or less. When CNT exists as a bundle having a thickness equal to or smaller than the above upper limit value, almost all the bundles contain metallic CNT, and the sheet resistance of the carbon nanotube thin film can be reduced. The average thickness depends on the dispersibility of CNTs in a CNT dispersion which is a raw material of the CNT thin film. Therefore, the average thickness can be controlled by the content of CNT in the dispersion, the composition and content of the dispersant (surfactant), and the like.

  The carbon nanotube thin film includes a deposition layer on which carbon nanotubes are deposited, and the average thickness of the deposition layer is preferably 200 nm or less. More preferably, the average thickness of the deposited layer is 100 nm or less, and particularly preferably 20 nm or less. The transmittance can be reduced by reducing the thickness of the deposited layer. In addition, when a CNT thin film consists only of CNT, a deposition layer is a CNT thin film itself. The thickness of the deposited layer may be measured by SEM or the like.

The ratio G / D between the G band and the D band in the Raman spectrum of the carbon nanotubes contained in the carbon nanotube thin film is preferably 0.1 or more. More preferably, G / D is 0.5 or more. The G band is a Raman peak (Raman scattering intensity) due to the graphite structure, which appears in the vicinity of 1590 cm ⁻¹ of the Raman spectrum of CNT. The D band is a Raman peak that appears in the vicinity of 1350 cm ⁻¹ of the CNT Raman spectrum due to a point defect or crystal edge of the CNT. G / D is an index indicating the crystallinity of CNT, and the larger the numerical value, the higher the crystallinity of CNT. In a thin film composed of CNTs having G / D of 0.5 or more, since there are few defect structures of CNTs, sheet resistance (electric resistance) and thermal resistance can be significantly reduced. What is necessary is just to measure the Raman spectrum and G / D of CNT with a normal Raman spectrometer.

  Since the carbon nanotube thin film according to the present embodiment contains carbon nanotubes having an average length of 10 μm or more, the carbon nanotube thin film can have a haze value of 2% or less. That is, in the CNT thin film of the above aspect, the ratio (haze value) of diffuse transmitted light to total transmitted light can be reduced to 1% or less, more preferably 0.5% or less. A CNT thin film having such a haze value is suitable for a transparent electrode for a display that requires translucency. In addition, what is necessary is just to measure the haze value of a CNT thin film with a normal haze meter.

  Since the carbon nanotube thin film according to this embodiment contains the carbon nanotube having the above characteristics, it can have a transmittance (total light transmittance of visible light) of 70% or more. Preferably, the transmittance is 80% or more, more preferably 90% or more. Here, the transmittance is the ratio of light (radiant divergence of transmitted light) that is transmitted through the CNT thin film to light incident on the surface of the CNT thin film (radiant divergence of incident light). A CNT thin film having the above-described transmittance hardly absorbs light and is excellent in transparency. You may measure the transmittance | permeability of a CNT thin film with a normal spectrophotometer or a haze meter. Further, the transmittance of the CNT thin film may be calculated by binarizing the image data of the CNT thin film.

  Since the carbon nanotube thin film according to the present embodiment contains the carbon nanotube having the above characteristics, 10 kΩ / sq. The following sheet resistance can be provided. Therefore, the CNT thin film according to this embodiment is suitable as a material that requires high electrical conductivity. Preferably, the sheet resistance is 1 kΩ / sq. Or less, more preferably 0.5 kΩ / sq. It is as follows.

  Since the carbon nanotube thin film according to this embodiment contains the carbon nanotube having the above characteristics, it can have a refractive index of visible light of 1.4 to 1.6. Unlike the ITO transparent electrode, the refractive index of such a CNT thin film is almost the same as the refractive index of general glass. Therefore, when the transparent electrode having the CNT thin film according to the present embodiment is used as an LCD (liquid crystal display) material, reflection due to a difference in refractive index is suppressed, and the light transmittance in the LCD panel is reduced due to the reflection. Can be suppressed. In addition, the refractive index of CNT should just be measured with a normal Abbe refractometer etc.

  Since the carbon nanotube thin film according to the present embodiment contains the carbon nanotube having the above characteristics, it can transmit far-infrared light having a wavelength of 1 to 10 μm. That is, since the CNT thin film of this embodiment does not absorb light in the far-infrared light wavelength region, it can be used as an antistatic layer or a wiring layer for a far-infrared sensor or the like.

(Method for producing carbon nanotube thin film)
The carbon nanotube thin film according to the present embodiment can be produced by directly spraying the CNT itself having an average length of 10 μm or more onto the substrate surface, as in the aerosol deposition method. The CNT thin film can also be produced by once dispersing CNTs having an average length of 10 μm or more in a dispersion medium and applying this dispersion to the surface of the substrate. Filtrate (CNT self-supporting film) obtained by filtering a CNT dispersion can also be used as a CNT thin film.

  The transparent electrode according to this embodiment can be manufactured by laminating the CNT thin film obtained by the above manufacturing method on a transparent substrate. You may manufacture a transparent electrode by apply | coating the dispersion liquid of CNT directly on a transparent base material. Moreover, the electrode for photolithography according to this embodiment can be manufactured by laminating a photosensitive resin film on the CNT thin film obtained by the above manufacturing method.

  Among the production methods described above, a method of forming a CNT thin film by applying a CNT dispersion on the surface of the substrate and removing the dispersion medium is preferable. In this method, since the conventional printing method can be easily applied, the manufacturing cost is reduced and the productivity is improved. Below, the dispersion liquid of CNT suitable for manufacture of a CNT thin film is demonstrated.

<CNT dispersion>
The dispersion of carbon nanotubes according to the present embodiment includes a dispersion medium (solvent), carbon nanotubes having an average length of 10 μm or more, and two or more kinds of surface activity having a hydrophilic structure portion and a hydrophobic structure portion in the molecule. It is preferable to contain an agent.

  In this embodiment, the hydrophobic structure part of 2 or more types of surfactants chemisorbs on the surface of CNT, and CNT is covered with these surfactants. As a result, the solvophilicity (particularly hydrophilicity) of CNT is increased, and the dispersibility of CNT is improved as compared with the case where only one kind of surfactant is used. That is, two or more surfactants function as a dispersant. Examples of the characteristics of the dispersant necessary for the dispersion of CNT include permeability between CNTs, adsorption to the CNT surface, and solubility in a solvent. However, it is difficult for the chemical structure of one type of surfactant to have all these characteristics. Therefore, in this embodiment, by using two or more types of surfactants according to the respective properties, the surfactants complement each other's properties, compared with the case where only one type of surfactant is used, High CNT dispersibility is achieved. When two or more kinds of surfactants have a hydrophilic structure part having the same polarity charge, the surfactant not only enhances the solvophilicity but also utilizes the repulsive force between hydrophilic structure parts having the same polarity charge. Thus, it is possible to suppress reaggregation of dispersed CNTs and stabilize the dispersion state. The reason why the dispersibility of CNT is improved as compared with the case of using only one kind of surfactant is that the properties of the dispersant necessary for the dispersion of CNT include the permeability between CNTs and the adsorptivity to the CNT surface. Although dispersion state stabilization can be mentioned, it is difficult to have all of them in one type of chemical structure. However, by using two or more kinds of dispersants according to the respective properties in combination, the dispersants complement each other's properties.

  The average length of the CNTs in the dispersion is preferably about 10 μm to 10 mm. More preferably, the average length of CNT is 12 μm or more and 15 μm or less. When the average length of the CNT is within the above numerical range, excellent physical properties unique to the nanostructure of the CNT, such as optical characteristics, conductivity, mechanical strength, or thermal conductivity, can be effectively utilized. If the CNTs are too short, these physical properties are not sufficiently expressed. Moreover, the coating property of a dispersion liquid improves by making the average length of CNT into 15 micrometers or less. Note that the upper and lower limits of the length of the CNT contained in the dispersion are substantially the same as the upper and lower limits of the CNT contained in the CNT thin film.

  CNT is composed of single-layer (single-wall) carbon nanotubes (SWCNT), double-walled double-wall carbon nanotubes (DWCNT), and three or more layers, based on the number of layers (graphene sheets) that compose them. Multi-walled carbon nanotubes (MWCNT) etc. In other words, the MWCNT has a structure in which a plurality of graphene sheets closed in a cylindrical shape are stacked in a nested manner. Any of SWCNT, DWCNT, and MWCNT can be used for the dispersion of this embodiment. In the present embodiment, the number of MWCNT layers may be 2-5, and the average number of all carbon nanotubes may be about 1-5. The dispersion may contain two or more CNTs selected from the group consisting of SWCNT, DWCNT and MWCNT.

  The size and shape of the CNT in the dispersion may be examined by analyzing a sample (spin coat film) obtained by spin-coating a diluted solution of the dispersion on a smooth substrate by various methods. The reason why the dispersion is diluted here is to prevent the CNTs in the spin coat film from reaggregating in the course of the analysis and to keep the CNTs in an isolated state. The average length of the CNTs in the dispersion may be measured by observing the CNTs in the spin coat film with an SEM or the like. The average length of CNTs in the dispersion is an average value of the lengths of arbitrary CNTs in the spin coat film. The average length of CNT contained in the CNT thin film is equal to the average length of CNT in the dispersion used for the production of the CNT thin film.

  It is preferable that the average value (CNT average thickness) of the bundle composed of a plurality of carbon nanotubes in the dispersion liquid and the plurality of carbon nanotubes in the dispersion liquid is 0.5 nm or more and 150 nm or less. , 100 nm or less is more preferable, and 80 nm or less is particularly preferable. If the CNT average thickness is 150 nm or less, the haze value when the visible light transmittance is 80% in the CNT thin film produced from the CNT dispersion of this embodiment can be 10% or less. The average CNT thickness is an average value of the thickness of any isolated CNT and isolated bundle in the spin coat film. The method for measuring the average CNT thickness is substantially the same as the method for measuring the average length of CNTs, except that a transmission electron microscope (TEM) is used instead of SEM.

  The content of carbon nanotubes in the dispersion is preferably 0.0001 to 10% by weight. In this case, it is easy to use the dispersion as a coating material, and it is easy to form a composite material from the dispersion and another substrate.

  The ratio of the number of carbon nanotubes having an absolute length of 10 μm or more and 10 mm or less is preferably 50% or more, more preferably 60% or more, based on the total number of carbon nanotubes in the dispersion. 70% or more is more preferable. That is, the dispersion liquid according to this embodiment is suitable for manufacturing a CNT thin film because it contains a long proportion of long CNTs as compared with a conventional dispersion liquid.

  The haze value of the CNT thin film produced from the dispersion when the visible light transmittance is 80% is preferably 10% or less, and more preferably 2% or less. The haze value is a scale for evaluating the dispersibility of CNTs in the dispersion, and the larger the haze value, the coarser the aggregates of CNTs in the dispersion and the greater the number. The low haze value of the present embodiment means that the CNTs in the dispersion are highly dispersible and the CNTs are less likely to aggregate despite the fact that the dispersion contains CNTs that are longer and more easily aggregated than before. .

  The surfactant preferably has a hydrophobic structure portion and a hydrophilic structure portion in the molecule. Such a surfactant has affinity for both the dispersion medium and CNT, and is excellent in the action of dispersing CNT in the dispersion medium. The surfactant is not particularly limited as long as it has such characteristics. Surfactants are roughly classified into anionic surfactants, cationic surfactants, zwitterionic surfactants and nonionic surfactants based on the type of hydrophilic structure charge when dissolved in pure water. Is done. The dispersion contains two or more selected from the group consisting of these.

  Examples of the anionic surfactant include the following sulfonic acid derivatives and carboxylic acid derivatives, but are not limited thereto. Examples of the sulfonic acid derivatives include alkyl sulfonates such as sodium lauryl sulfonate, alkyl benzene sulfonates such as sodium dodecylbenzene sulfonate (hereinafter referred to as “SDBS”), and aromatic sulfones such as dodecyl phenyl ether sulfonate. Examples include acid surfactants. Examples of the carboxylic acid derivative include linear fatty acids such as sodium myristate and sodium stearate, sodium cholate which is a component of bile acid which is a biological substance, and the like.

  Examples of the cationic surfactant include quaternary ammonium salts. Specific examples include trimethylcetylammonium chloride, dodecyltrimethylammonium halide, coconut oil alkyltrimethylammonium halide, hexadecyltrimethylammonium halide, beef tallow alkyltrimethylammonium halide, octadecylammonium halide, alkylimidazolium halide and the like. However, the cationic surfactant is not limited to these.

  Examples of zwitterionic surfactants include amphoteric polymers such as 2-methacryloyloxyethyl phosphorylcholine polymer and polypeptide, 3- (N, N-dimethylstearylammonio) propanesulfonate, and 3- (N, N-dimethylmyristyl). Ammonio) propanesulfonate, 3-[(3-cholamidopropyl) dimethylammonio] -1-propanesulfonate (CHAPS), 3-[(3-cholamidopropyl) dimethylammonio] -2-hydroxypropanesulfonate ( CHAPSO), n-dodecyl-N, N′-dimethyl-3-ammonio-1-propanesulfonate, n-hexadecyl-N, N′-dimethyl-3-ammonio-1-propanesulfonate, 3- (tetradecyldimethylamino) Nio) propane-1-sulfonate , N-octylphosphocholine, n-dodecylphosphocholine, n-tetradecylphosphocholine, n-hexadecylphosphocholine, dimethylalkylbetaine, perfluoroalkylbetaine, lecithin, and other amphoteric small molecules (including amphoteric surfactants) Etc. However, the zwitterionic surfactant is not limited thereto.

  Nonionic surfactants include polyacrylic acid, polyethylene glycol, polyoxyethylene nonylphenyl ether, polyoxyethylene dodecyl ether, polyoxyethylene octylphenyl ether, polyoxyethylene lauryl ether, polyoxyethylene fatty acid ester, polyoxyethylene Examples include polyoxypropylene block polymers, polyoxyethylene alkylamines, alkyl polyglucosides, glycerin fatty acid esters, sorbitan fatty acid esters, sucrose fatty acid esters, and propylene glycol fatty acid esters. However, the nonionic surfactant is not limited to these.

  Some of the surfactants have a zwitterionic group having both a cation (positive charge) and an anion (negative charge) as a hydrophilic structure, and other surfactants have a hydrophilic structure other than the zwitterionic group. It is preferable to have a part. That is, it is preferable to use a zwitterionic surfactant in combination with another surfactant. Thereby, the dispersibility of CNT improves remarkably and reaggregation of the dispersed CNT is remarkably suppressed. When all of the two or more surfactants are zwitterionic surfactants, the cation (positive charge) portion and anion (negative charge) of the surfactant adsorbed on the CNT surface in the high concentration dispersion The portions are associated with each other between the CNTs, and the CNTs self-assemble to form aggregates, which tends to reduce the dispersibility of the CNTs.

  The present inventors consider that the dispersion mechanism of CNTs by zwitterionic groups is as follows. The hydrophobic structure portion of the surfactant having the zwitterionic group is bonded to the surface of the CNT or its aggregate. And the zwitterionic group which has a positive charge and a negative charge self-assembles on the surface of a CNT aggregate, and forms a zwitterionic molecular film (SAZM: Self-Assembled Zwitterionic Monolayer). SAZM covering CNT aggregates tends to electrostatically bond with SAZM covering other CNT aggregates due to strong electrostatic interactions between dipoles. The electrostatic force causes the CNT aggregates in the mixture to pull each other, causing the CNTs constituting the CNT aggregates to peel off, and the surface of the new CNT aggregates is exposed. The newly exposed surface is newly covered with SAZM. Since the above reaction is repeated until the CNTs constituting the CNT aggregate are isolated and dispersed, the CNTs are finally highly monodispersed.

  The two or more kinds of surfactants are the same or similar hydrophobic structures, such as chain aliphatic hydrocarbons, chain aliphatic hydrocarbons, aromatic hydrocarbons, and cyclic hemiacetals (pyranose, furanose, etc.) It is preferable to have at least one selected from the group consisting of Examples of the combination of surfactants having the same or similar hydrophobic structure include, for example, CHAPS having a cholic acid group and sodium cholate, and 3- (tetradecyldimethylaminio) propane-1- having a saturated fatty acid group. Examples include sulfonate and sodium myristate. In the case where the hydrophobic structure part of the surfactant is the same or similar, the same or similar hydrophobic structure part has a small difference in adsorptive power to CNT and does not eliminate the other surfactant. There is a tendency that it is difficult to inhibit the expression of each other's dispersibility.

  The molecular weight of the surfactant (particularly the amount of nonionic surfactant) is preferably 1000 or less, and more preferably 400 to 600. The molecular weight here is a weight average molecular weight. As the molecular weight of the surfactant is smaller, the surfactant tends to enter the gaps between the CNTs constituting the bundle (aggregate), and the CNTs are likely to be dispersed. Further, the smaller the molecular weight of the surfactant, the easier it is to remove the surfactant from the coating film prepared from the dispersion.

  The viscosity of the dispersion at 25 ° C. is preferably 10 mPa · s or less. The low viscosity of the dispersion is due to the low molecular weight of the surfactant. The lower the viscosity of the dispersion, the easier it is to dissolve, wash, remove, etc. the surfactant, and the easier it is to isolate CNTs. Note that the viscosity of the CNT dispersion may be measured with a vibration viscometer or the like.

  The surfactant is preferably soluble in the solvent. Surfactants tend to increase the thermal resistance and electrical resistance of CNTs by binding to CNTs. If the surfactant is soluble in the solvent, the surfactant can be easily removed from the CNT surface by washing, so that the characteristics such as thermal resistance and electrical resistance of the CNT obtained simply from the dispersion tend to be improved.

  The dispersion medium of the CNT dispersion is preferably a polar solvent, and more preferably water. Examples of the dispersion medium other than water include alcohol solvents such as methanol, ethanol, propanol, butanol and isopropyl alcohol, ketone solvents such as acetone, 2-butanone, methyl isobutyl ketone and N-methylpyrrolidone, tetrahydrofuran, gamma butyrolactone, Examples include dimethylformamide. Further, a mixed solution of these plural dispersion media may be used.

  The concentration of the surfactant can be appropriately set according to the amount of CNT. For example, when the total weight of CNTs contained in the dispersion is 1 part by weight, the weight of the surfactant contained in the dispersion is preferably 1-1000 parts by weight, and more preferably 5-100 parts by weight. . Thereby, there exists a tendency for the dispersibility of CNT to improve.

  The dispersion may contain a stabilizer in addition to the surfactant. The combined use of the stabilizer tends to make the CNT dispersion state more stable. Stabilizers include substances that form hydrogen bonds. Specific examples include glycerol, polyhydric alcohol, polyvinyl alcohol, alkylamine, acidic polymer, basic polymer, and the like. Examples of the acidic polymer include κ-carrageenan, DNA, Nafion (registered trademark), cellulose acetate, cellulose phosphate, cellulose sulfonate, gellan, arabian gum, and polyphosphoric acid.

  If the dispersion contains many impurities that cause deterioration of the CNT characteristics, the quality of the CNT thin film may deteriorate. Moreover, when there are many impurities in a dispersion liquid, it will be necessary to refine | purify CNT in a dispersion liquid further, and the production cost of a CNT thin film may rise. Therefore, the smaller the amount of impurities in the dispersion, the better.

<Method for producing CNT dispersion>
The method for producing a dispersion of CNTs according to this embodiment includes a step (stirring step) of stirring a mixed solution containing carbon nanotubes, the dispersion medium, and the surfactant using a stirring ball mill. Thereby, the dispersion liquid of CNT which concerns on the said this embodiment can be manufactured.

  As a raw material of the dispersion liquid, for example, CNT having an average length of 10 μm or more in an as-grown state (a state in which the CNT is grown on the substrate) can be used. By applying such a dispersion containing CNTs to the substrate, the average length of the CNTs in the coating film can be 10 μm or more. The CNT used as a raw material may be manufactured by a method such as an arc discharge method, a CVD method, a laser ablation method, or a high PCO method (high pressure carbon monooxide method). Of these, the CVD method is preferable.

  Conventionally, in order to reduce the sheet resistance of a CNT thin film, it has been necessary to use CNT produced by an arc discharge method or HiPCO method, or metal-type CNT purified by electrophoresis or the like. However, CNTs obtained by purification by arc discharge method, HiPCO method, electrophoresis method or the like are expensive, and it is difficult to mass-produce thin films using the CNTs. On the other hand, the CVD method is preferable because long CNTs can be easily synthesized as compared with CNTs synthesized by other methods, and stable mass production of CNTs and reduction of production costs are possible. The average length of CNTs and the number of layers are, for example, the amount of raw material gas (hydrogen, methane, acetylene, etc.) used in the CVD method supplied to the reaction field, the composition of the metal catalyst for growing CNTs (Fe, Al, etc.) And the amount thereof, or various reaction conditions such as the temperature of the reaction field can be adjusted. It is also possible to appropriately adjust the average value of the length of the CNTs depending on the selection of the stirring process or the dispersion process method for the dispersion, the execution time of each process, and the like.

  The purity of the CNT used as the raw material for the CNT thin film is preferably 90% by mass or more. Impurities in the CNT may cause an increase in sheet resistance of the CNT thin film. Therefore, when using CNT containing a large amount of impurities, it is necessary to remove the impurities from the CNTs by refining in order to prevent CNT quality deterioration. As a result, the production cost of the CNT thin film may increase.

  In the stirring step, a shearing force can be applied to the CNTs by a stirring ball mill to eliminate the aggregation. Note that the agitating ball mill is a container equipped with a structure capable of stirring the media filled in the container by rotating the stirring blade inserted in the container with external power or stirring the media inside. It has a mechanism for flowing the media inside the container by rotating it. The particle size of the media is preferably 10 mm or less in consideration of dispersion efficiency. In addition, the material of the media is preferably a hard ceramic, and more preferably zirconia, in order to suppress contamination due to wear.

  The manufacturing method according to the present embodiment may further include a step of treating the mixed solution with ultrasonic waves (ultrasonic treatment step). There exists a tendency for the dispersibility of CNT to improve more by an ultrasonic treatment process. The stirring step and the ultrasonic treatment step may be performed simultaneously or separately.

  In this embodiment, in addition to the stirring ball mill, a media-type dispersion step such as beads and rolls may be further performed. Further, a centrifugal separation step may be performed in order to remove aggregates and short CNTs (for example, CNTs shortened by receiving a shearing force in the dispersion step) from the CNTs after the dispersion step. Thereby, there exists a tendency for lengthening and monodispersion of CNT finally contained in a dispersion liquid to become easy.

  In the present embodiment, CNTs that are longer than conventional CNTs are used as raw materials. In this embodiment, since the aggregation of CNTs can be easily eliminated by the chemical action of two or more surfactants, the shearing force acting on the CNTs in the stirring step is reduced compared to the conventional case, and the CNTs are shortened. It is possible to sufficiently eliminate the aggregation of CNTs while suppressing the formation. Therefore, in this embodiment, short CNTs are hardly formed in the first place, and there is little need to remove short CNTs by a centrifugation process. Further, in this embodiment, since the aggregate of CNTs is easily eliminated by the action of the surfactant, it is not necessary to remove the aggregates of CNTs by a centrifugation step. For the reasons described above, in this embodiment, the centrifugation step is not necessarily required, and it is possible to reduce the loss of the CNT raw material due to the removal of aggregates and short CNTs. Therefore, according to this embodiment, the production cost of the CNT dispersion and the CNT thin film is reduced.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these Examples.

Example 1
[Synthesis of carbon nanotubes]
SWCNTs used as raw materials for the dispersions of Examples 1 and 2 and Comparative Examples 1 and 2 below were synthesized by the following procedure. First, an Al film (15 nm thickness) and an Fe film (0.6 nm thickness) were formed on a silicon substrate with a thermal oxide film by sputtering. This substrate was placed in a reaction furnace, and a mixed gas mainly composed of 800 ° C. Ar, H ₂ and C ₂ H ₂ was reacted in the furnace, and SWCNT having an average length of about 1 mm was placed on the substrate. Grown up.

[Preparation of CNT dispersion]
0.1 g of SWCNT having an average length of about 1 mm, 0.4 g of CHAPS, and 1.6 g of sodium cholate were weighed and diluted with 1 L of ultrapure water. This was allowed to adjust under high temperature and high pressure, and after cooling, the mixture was stirred for 2 weeks at a rotation speed of 200 rpm using a stirring ball mill (manufactured by Nippon Coke Co., Ltd., zirconia beads (φ = 5 mm)). The dispersion liquid of CNT of Example 1 was obtained by the above method. The content of CNT in the dispersion was 0.01 wt%.

<Measurement of average length of CNT>
In the CNT thin film, since the CNTs are intertwined in a complicated manner, it is difficult to directly measure the average length of the CNT in the CNT thin film. Therefore, instead of measuring the average length of CNT in the CNT thin film, the average length of CNT in the CNT dispersion was measured by the following method. A diluted solution of the dispersion was spin-coated on the substrate to prepare a sample in which each CNT alone was in an isolated state. A sample was observed with an SEM, arbitrary 10 CNTs were selected from the obtained image, and the length was calculated from the number of pixels. The average value of 10 calculated values was defined as the average length of the entire CNT. The average length of CNT in the CNT thin film is substantially equal to the average length of CNT in the CNT dispersion. As a result of the measurement, it was confirmed that the average length of the CNTs in the CNT dispersion used in Example 1 was 10 μm.

<Preparation of CNT thin film>
A filter folder was placed on the filtration bell. A filter for filtration was installed on the filter folder, and 20 ml of ultrapure water was spread in the filter folder. As a filter for filtration, an omnipore membrane (manufactured by Milipore) made of hydrophilic PTFE (polytetrafluoroethylene) and having a pore diameter of 0.1 μm was used. After 0.1 g of the CNT dispersion was diluted with ultrapure water, it was added to the ultrapure water stretched in the filter folder. The container of the CNT dispersion was also washed with ultrapure water, and the water used for the washing was added to the ultrapure water stretched in the filter folder. Subsequently, the inside of the filter bell was depressurized with a diaphragm pump and filtered. The deposited layer (filtration membrane) of CNT formed on the filter during filtration was washed with 20 ml of ultrapure water three times. The filtered membrane after washing was dried on a hot plate at 100 ° C. to obtain the CNT thin film of Example 1.

<Calculation of transmittance>
The CNT thin film formed on the filter for filtration was moistened with ultrapure water, and then bonded to the quartz substrate so that the CNT thin film was in contact with the quartz substrate, and dried on a hot plate heated to 50 ° C. After drying, only the filter for filtration was peeled off to prepare a measurement sample in which the CNT thin film was transferred onto the quartz substrate. The total light transmittance (transmittance of light having a wavelength in the range of 380 to 760 nm) of this measurement sample was measured with a haze meter (NDH-5000 manufactured by Nippon Denshoku Industries Co., Ltd.).

<Measurement of sheet resistance>
The sheet resistance of the CNT thin film of Example 1 was measured by a 4-terminal 4-probe method. For measuring the sheet resistance, a resistivity meter Loresta GP manufactured by Mitsubishi Chemical Analytech Co., Ltd. was used.

(Example 2)
Using an applicator, the same CNT dispersion as in Example 1 was applied to a transparent PET film (Polyethylene terephthalate film). As the PET film, Cosmo Shine A4300 manufactured by Toyobo Co., Ltd. was used. The transmittance of the PET film was 91%. The coating film on the PET film was dried with an explosion-proof dryer at 90 ° C. for 5 minutes, and then the entire coating film was immersed in a large amount of ultrapure water together with the PET film for 10 minutes. After pulling up the PET film from the ultrapure water, water droplets on the coating film surface were blown off by air blow. Through these steps, a transparent electrode of Example 2 including the CNT thin film of Example 2 and a PET film on which the CNT thin film was laminated was obtained. Moreover, the transmittance | permeability of the transparent electrode of Example 2 was measured using the Nippon Denshoku Industries Co., Ltd. haze meter. The transmittance of the CNT thin film alone of Example 2 was determined by subtracting the transmittance of the PET film from the transmittance of the transparent electrode. In the same manner as in Example 1, the sheet resistance of the CNT thin film alone in Example 2 was measured.

(Comparative Example 1)
SWCNTs having an average length of about 1 mm synthesized by the same method as in Example 1 were processed by a wet jet mill to prepare a dispersion containing CNTs having an average length of 2.3 μm. The content of CNT in the dispersion was adjusted to 0.05 wt%. Z-314 (3- (N, N-dimethyltetradecylammonio) propanesulfonic acid) was used as the dispersant. A CNT thin film of Comparative Example 1 was produced in the same manner as in Example 1 using 0.1 g of this CNT dispersion. The transmittance and sheet resistance of the CNT thin film of Comparative Example 1 were measured in the same manner as in Example 1.

(Comparative Example 2)
SWCNT having an average length of about 1 mm, which was synthesized by the same method as in Example 1, was processed with a probe-type ultrasonic disperser (manufactured by Nippon Seiki Seisakusho), and the average length was 0.2 μm. A dispersion containing CNTs was prepared. The content of CNT in the dispersion was adjusted to 0.01 wt%. Z-314 (3- (N, N-dimethyltetradecylammonio) propanesulfonic acid) was used as the dispersant. A CNT thin film of Comparative Example 2 was produced in the same manner as in Example 1 using 0.5 g of this CNT dispersion. The transmittance and sheet resistance of the CNT thin film of Comparative Example 2 were measured in the same manner as in Example 1.

  Table 1 shows the average length of CNT, the transmittance of the CNT thin film, and the sheet resistance of each Example and Comparative Example.

Claims (13)

Containing carbon nanotubes having an average length of 10 μm or more,
Carbon nanotube thin film. The carbon nanotube is a single-walled carbon nanotube,
The carbon nanotube thin film according to claim 1. The carbon nanotubes are single-walled carbon nanotubes and multi-walled carbon nanotubes,
The carbon nanotube thin film according to claim 1. A bundle composed of a plurality of the carbon nanotubes, and the average thickness of the carbon nanotubes present alone is 150 nm or less,
The carbon nanotube thin film as described in any one of Claims 1-3. Comprising a deposited layer on which the carbon nanotubes are deposited;
The deposited layer has a thickness of 200 nm or less;
The carbon nanotube thin film as described in any one of Claims 1-4. The ratio G / D of G band and D band in the Raman spectrum of the carbon nanotube is 0.5 or more.
The carbon nanotube thin film as described in any one of Claims 1-5. The haze value is 2% or less,
The carbon nanotube thin film as described in any one of Claims 1-6. The transmittance is 70% or more,
Sheet resistance value is 10 kΩ / □ or less,
The carbon nanotube thin film as described in any one of Claims 1-7. Transmit far-infrared light having a wavelength of 1 to 10 μm,
The carbon nanotube thin film as described in any one of Claims 1-8. The carbon nanotubes are synthesized by chemical vapor deposition,
The carbon nanotube thin film as described in any one of Claims 1-9. It is formed through a step of applying a dispersion liquid in which carbon nanotubes having an average length of 10 μm or more are dispersed on a substrate.
The carbon nanotube thin film as described in any one of Claims 1-10. The carbon nanotube thin film according to any one of claims 1 to 11,
A transparent substrate on which the carbon nanotube thin film is laminated;
A transparent electrode comprising: The carbon nanotube thin film according to any one of claims 1 to 11,
A photosensitive resin film in which the carbon nanotube thin films are laminated;
Comprising
Electrode for photolithography.