JP2011185613A - Method and device for evaluating monodispersibility of carbon nanotube - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem wherein there is no evaluation method for efficiently evaluating the monodispersibility of a CNT suspension or CNT membrane and enabling quantitative evaluation. <P>SOLUTION: The CNT suspension wherein CNT (carbon nanotube) is dispersed in a dispersing medium or a CNT dispersed membrane formed by applying the CNT suspension to the surface of a substrate is irradiated with ultraviolet rays as incident light and absorbance is calculated from the transmitted light from the CNT suspension or the reflected light from the CNT dispersed membrane to evaluate the monodispersibility of CNT of the CNT suspension or CNT dispersed membrane. This evaluation method includes processes of: calculating a light absorption curve from the absorbance; respectively extracting the intensities of the absorption peaks of two places centering around two specific energy levels in the light absorption curve; and calculating the intensity ratio of the absorption peaks in two places. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method and an evaluation apparatus for evaluating the monodispersity of carbon nanotubes dispersed in a dispersion or on the surface of a substrate, and a carbon nanotube suspension or carbon selected by using this evaluation method or evaluation apparatus. The present invention relates to a nanotube thin film.

  A carbon nanotube (hereinafter also referred to as “CNT”) is a graphite material having a cylindrical structure with a diameter on the order of nanometers and a length on the order of microns. Due to its unique mechanical or electrical features, various applications such as thin film transistor materials and electron emission source materials are being studied.

  When CNT is used for a thin film transistor or an electron emission source, the CNT is often deposited on a substrate and used as a thin film. As deposition methods for thin film production, there are a method of directly growing CNTs from a catalyst formed on a substrate, a method of physically depositing CNTs generated in a growth furnace on a substrate, etc., but more generally A method of forming a CNT-dispersed thin film by preparing a CNT suspension in which CNTs are dispersed in a dispersion medium and applying and printing the suspension on a substrate is employed. In this case, an organic solvent such as N, N-dimethylformamide or N-methyl-2-pyrrolidone is used as the dispersion medium. Moreover, water, heavy water, etc. are used as a dispersion medium by using together surfactants, such as sodium dodecyl sulfate and carboxymethylcellulose.

  In the CNT suspension, the van der Waals force acting as an attractive force between the CNTs and the interaction between π orbitals exist, so that the CNT dispersibility needs to be kept good. When CNT is used as a thin film transistor material, it is particularly important to ensure monodispersity. Moreover, it is necessary to use semiconducting CNT for this application. CNT shows metallicity and semiconductivity depending on its diameter and winding method (chirality). Theoretically, it is considered that one-third of the whole exhibits metallic properties and the remaining two-thirds exhibit semiconducting properties. Currently, technologies for extracting semiconducting CNTs are progressing. However, as a premise for applying these technologies, it is necessary to prepare a CNT suspension ensuring monodispersity. This is because it becomes difficult to separate metallic CNTs and semiconducting CNTs if they aggregate together. Currently, density gradient centrifugal equilibrium sedimentation, agarose gel electrophoresis, and the like have been proposed as CNT separation techniques, but it is necessary to prepare a CNT suspension with high monodispersity in any method.

  As a method of dispersing CNTs in a dispersion medium, a combination of ultrasonic treatment and ultracentrifugation treatment is generally used. The CNTs aggregated by the ultrasonic treatment are dispersed in a dispersion medium, and the remaining aggregated CNTs are precipitated by the ultracentrifugation treatment. A CNT suspension with high monodispersity can be obtained by separating the supernatant from the suspension after ultracentrifugation.

  It is indispensable to quantitatively evaluate or inspect the monodispersibility of CNT in order to control various properties of the CNT separation process and the CNT-dispersed thin film. Again, this is because the semiconductor / metal separation rate is greatly influenced by the monodispersity of the prepared CNT suspension, and the electrical conduction characteristics in the case of a thin film are also greatly affected.

  A particle size distribution measuring device is generally used for evaluating the monodispersity of a dispersoid in a general suspension. This apparatus measures the particle size distribution of a dispersion by irradiating a suspension with laser or the like as incident light and measuring in detail the scattered light or diffracted light from the dispersion. Since the evaluation method using this measuring apparatus can be evaluated as it is in the state of a suspension, it has an advantage that it can be evaluated efficiently. For a dispersion having a nearly spherical shape, a high quantitative property can be obtained for the particle size distribution.

  As a method for evaluating the monodispersity of CNT, there is an observation method using a transmission electron microscope or a scanning probe microscope. In the case of using a transmission electron microscope, the CNT suspension is dropped on a transmission electron microscope microgrid or the like, dried, and observed under an electron microscope. Since the transmission electron microscope can obtain a very high resolution CNT image, the aggregation state can be evaluated in detail. In the case of using a scanning probe microscope, the CNT suspension is dropped on the surface of a silicon substrate or the like, dried, and observed with a scanning probe microscope. The aggregation state of CNTs can be evaluated from an image obtained by a scanning probe microscope, and a statistical distribution regarding the diameter of the CNTs can be obtained from the image analysis.

  As another method for evaluating the monodispersity of CNT, there is a method using an ultraviolet visible near infrared spectrophotometer. In UV-visible-near-infrared spectroscopy, a method for evaluating dispersibility by the absorbance or the half-value width of an absorption peak peculiar to CNT is taken. When the dispersibility is low, the absorbance changes according to the dispersion state. Therefore, the monodispersity is evaluated based on this change. Further, when the monodispersity is high, a sufficient change in absorbance cannot be obtained, and therefore the sharpness of the absorption peak peculiar to CNT is evaluated. The absorption peak here is observed from 1 electron volt to 4 electron volt as an energy region, and is caused by light absorption accompanying electron excitation (interband transition) from the valence band to the conduction electron band of CNT. This spectrophotometer method is efficient because it can be evaluated as a suspension.

  Patent Document 1 discloses an apparatus and method for measuring the dispersibility of CNT from the photoluminescence intensity from a sample excited by irradiation of excitation light. Patent Document 2 discloses an apparatus for measuring the dispersibility of toner in ink with high accuracy. Patent Document 3 discloses a method for evaluating the dispersibility of a granular material dispersed in a solvent, a production method, and the like.

JP 2009-31114 A JP-A-7-140071 Japanese Patent Laid-Open No. 11-201927

The following analysis has been made from the viewpoint of the present invention.
In the monodispersity evaluation method using the particle size distribution measuring device, high quantitativeness can be obtained for the particle size distribution for a dispersion having a nearly spherical shape. However, since the CNT has a very high aspect ratio with a diameter on the order of nanometers and a length on the order of micrometers, it cannot be quantitatively evaluated with a particle size distribution measuring apparatus assuming spherical particles.

  Further, in the method using a transmission electron microscope or a scanning probe microscope, since the CNT suspension is dropped and dried, there is a possibility that the CNT suspension is aggregated during the processing.

  In addition, in UV-Vis near-infrared spectroscopy, the method of evaluating dispersibility by the absorbance or the half-value width of the absorption peak peculiar to CNT is used to evaluate the sharpness of the absorption peak due to transition between bands such as impurities such as amorphous or CNT defects. Quantitative evaluation by this method is difficult because it is greatly affected.

  In order to solve these problems, there is a demand for an evaluation method that can be efficiently evaluated as a CNT suspension and can be quantitatively evaluated. Moreover, an evaluation apparatus that mechanically executes the evaluation method is desired. In addition, the CNT suspension selected by the evaluation method is very monodisperse, so it can be applied to thin film transistors and electron emission sources, and it is a necessary material as a preparation sample for semiconductor / metal CNT extraction. is there. In addition, if the evaluation method and the evaluation device are effective not only for the CNT suspension but also for the CNT-dispersed thin film formed on the substrate, quantitative evaluation in a state closer to the applied product is possible. The CNT-dispersed thin film selected by the method and the evaluation apparatus is effectively used for various applications.

  In the first aspect of the present invention, ultraviolet light is incident as incident light on a CNT suspension in which CNT (carbon nanotube) is dispersed in a dispersion medium or a CNT dispersion thin film formed by applying the CNT suspension to a substrate surface. And calculating the absorbance from the transmitted light from the CNT suspension or the reflected light from the CNT-dispersed thin film, and evaluating the CNT monodispersity of the CNT suspension or the CNT-dispersed thin film. A step of obtaining an absorption curve from absorbance, a step of extracting intensities of two absorption peaks centered at two specific energy levels in the absorption curve, and an intensity ratio of the absorption peaks of the two locations. An evaluation method for evaluating the monodispersity of the CNT, characterized by comprising the step of:

  In the second aspect of the present invention, ultraviolet light is incident as incident light on a CNT suspension in which CNTs (carbon nanotubes) are dispersed in a dispersion medium or a CNT dispersion thin film formed by applying the CNT suspension to a substrate surface. Is a CNT monodispersity evaluation apparatus that measures transmitted light from the CNT suspension or reflected light from the CNT-dispersed thin film, and calculates absorbance from the measured transmitted light or reflected light. And calculating the absorption curve from the absorbance, and extracting the intensity of the absorption peak centered around 5.9 eV and the intensity of the absorption peak centered around 5.0 eV in the absorbance curve, respectively. An evaluation apparatus is provided, comprising: an extraction unit; and an evaluation unit that evaluates monodispersity of CNTs from the intensity ratio of the extracted absorption peaks.

  In a third aspect of the present invention, there is provided a method for producing a CNT suspension or a CNT-dispersed thin film, characterized by performing evaluation and selection using the above method.

  According to the first aspect of the present invention, it is possible to efficiently evaluate the monodispersity of a CNT suspension or a CNT-dispersed thin film as it is in suspension, and according to the second aspect, the evaluation An evaluation device can be obtained that mechanically executes the method. According to the third aspect, a CNT suspension or a CNT-dispersed thin film having a predetermined monodispersed state can be obtained.

  The CNT suspension selected by this evaluation method or evaluation device has a very high monodispersity, can be applied to thin film transistors and electron emission sources, and is an optimal material as a preparation sample for semiconductor / metal CNT extraction. I will provide a. This evaluation method and evaluation apparatus are effective not only for the CNT suspension but also for the CNT-dispersed thin film formed on the substrate, and can be evaluated in a state closer to the applied product. The CNT-dispersed thin film obtained by screening with this evaluation method and evaluation device can be used effectively for various applications.

It is a block diagram of the evaluation apparatus which concerns on one Example of this invention.

  In the first aspect, the two specific energy levels are preferably energy levels around 5.9 eV and 5.0 eV in the absorption curve, respectively.

  The extraction of the intensity of the absorption peak is preferably performed by fitting each absorption peak according to the Lorentz curve, and the integrated value is used as the peak intensity.

When the intensity of the absorption peak centered around the 5.9 electron volt is A (5.9 eV) and the intensity of the absorption peak centered around the 5.0 electron volt is A (5.0 eV), It is preferable to evaluate the monodispersity of CNT by the CP value calculated by
(Formula) CP = 40 × A (5.0 eV) / A (5.9 eV)
Thereby, quantitative evaluation becomes possible.

  Depending on whether the CP value is 20 or less, it is particularly preferable to perform selection based on the monodispersity of CNTs. This gives specific evaluation criteria.

  As the two absorption peaks, an ultraviolet absorption peak derived from π plasmon absorption in completely monodispersed CNT and an ultraviolet absorption peak derived from π plasmon absorption in aggregated CNT can be used.

Example 1
Embodiments of the present invention will be described below with reference to the drawings as necessary. In the first embodiment of the present invention, a CNT suspension in which CNTs are dispersed in a dispersion medium or a CNT-dispersed thin film in which CNTs are dispersed on a substrate surface is prepared. The target CNTs are single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes. Even if the CNT used is chemically modified with a carboxyl group or the like, the quantitativeness of this evaluation method is not affected.

  Next, CNT is dispersed using an organic solvent such as N, N-dimethylformamide as a dispersion medium. A dispersion medium generally used for dispersing CNTs such as an organic solvent such as N-methyl-2-pyrrolidone can be used as the dispersion medium. Moreover, water, heavy water, etc. can be used as a dispersion medium by using together surfactants, such as sodium dodecyl sulfate and carboxymethylcellulose. The CNT is subjected to ultrasonic treatment using a horn type ultrasonic crusher or the like. For example, the processing condition may be an output of 300 W and a processing time of 1 hour. Next, in order to fractionate the monodispersed CNTs, ultracentrifugation is performed with an ultracentrifuge to precipitate the aggregated CNTs. The processing conditions may be, for example, a rotation speed of 60,000 rpm and a processing time of 1 hour. The supernatant of the suspension after ultracentrifugation is collected and used as a monodispersed CNT suspension.

  The CNT-dispersed thin film may be formed by dropping and drying this CNT suspension on the substrate surface. Silicon, sapphire, mica, or the like is used as the substrate. In addition, the substrate surface may be chemically modified to improve monodispersity. For example, APTES can be used.

  The CNT suspension is irradiated with ultraviolet rays as incident light, and the absorbance is calculated from the transmitted light or reflected light from the CNT suspension or CNT dispersed thin film. Here, an existing ultraviolet visible near infrared spectrophotometer may be used. However, it is necessary to be a photometer capable of measuring a region from 4.5 electron volts to 6.5 electron volts as an energy region.

  In the absorption curve obtained by an ultraviolet-visible near-infrared spectrophotometer or the like, the intensity of the absorption peak centered around 5.9 eV and the intensity of the absorption peak centered around 5.0 eV are extracted. In extraction, fitting is performed assuming that each absorption peak follows the Lorentz curve, and the integrated value is used as the peak intensity.

In this treatment, the monodispersity of CNT can be evaluated from their strength ratio. The following values are defined as indicators for quantifying the dispersibility of CNTs.
CP = 100 × standard deviation of CNT diameter / average value of CNT diameter (1)

  The CNT diameter here is an apparent diameter when the CNT becomes an aggregate and forms a so-called bundle. If the CNTs are sufficiently monodispersed, this diameter matches the diameter of the single CNT. At this time, the CP value gradually approaches zero.

The above-described absorption peak intensity centered around 5.9 eV is A (5.9 eV), and the absorption peak intensity centered around 5.0 eV is A (5.0 eV). The CP value was evaluated by carefully evaluating the CNT diameter distribution with a scanning probe microscope or the like, and the intensity ratio of the absorption peak was calculated separately with a photometer. The inventors discovered that the following relationship exists between the CP value and the peak intensity ratio by repeating many experiments.
CP = 40 × A (5.0 eV) / A (5.9 eV) (2)
By using this formula, monodispersity can be quantitatively evaluated.

  Here, the peak at 5.9 eV is derived from π plasmon absorption in completely monodispersed CNT. The peak at 5.0 eV is derived from π plasmon absorption in the agglomerated CNT.

(Example 2)
The second embodiment of the present invention is a mechanized evaluation method according to the first embodiment, and a block diagram thereof is shown in FIG. Measuring the transmitted light (or reflected light) from the CNT suspension 1 (or CNT-dispersed thin film) and the irradiation unit 2 that irradiates ultraviolet rays as incident light and the CNT suspension 1 (or CNT-dispersed thin film) Measuring unit 3 for calculating the absorbance from the transmitted light (or reflected light), the intensity of the absorption peak centered around 5.9 eV in the absorption curve, and about 5.0 eV The extraction unit 5 extracts the absorption peak intensity, and the evaluation unit 6 evaluates the CNT monodispersity from the intensity ratio.

  The CNT suspension 1 (or CNT-dispersed thin film) is irradiated with ultraviolet rays as incident light from the irradiation unit 2. Ultraviolet rays are separated by a spectroscope. As the light energy region, it is desirable to be able to irradiate the region of 4.5 to 6.5 electron volts. Transmitted light (or reflected light) from the CNT suspension 1 (or CNT-dispersed thin film) is measured by the measuring unit 3. The measurement unit has a light receiving element as a main component, and measures transmitted light (or reflected light) that varies with the energy of incident light. The calculation unit 4 obtains the absorbance from the transmitted light (or reflected light). Absorbance is calculated for each change in the energy of the incident light, resulting in an absorption curve. The extraction unit 5 extracts the intensity of the absorption peak centered around 5.9 eV and the intensity of the absorption peak centered around 5.0 eV. In extraction, fitting is performed on the assumption that each absorption peak follows a Lorentz curve. In the evaluation unit 6, the CNT monodispersity is evaluated from the intensity ratio obtained in the extraction unit 5.

  As described above, the CP value calculated by the above equation (1) is defined as an index for quantifying the dispersibility of CNTs. The intensity of the absorption peak centered around 5.9 eV is A (5.9 eV), the intensity of the absorption peak centered around 5.0 eV is A (5.0 eV), and the CP value is (2) Calculate by the formula. The calculation result of this formula is displayed as information by various displays and communications.

  In the third embodiment of the present invention, ultraviolet light is irradiated as incident light, the absorbance is calculated from the transmitted light (or reflected light), the intensity of the absorption peak centered around 5.9 eV in the absorption curve, and It is a CNT suspension or a CNT-dispersed thin film selected by extracting the intensity of absorption peaks centered around 5.0 electron volts and evaluating the monodispersity of CNTs from their intensity ratios. If a CNT material of 20 or less is selected as the CP value obtained by this evaluation method, a material having sufficient monodispersity can be obtained.

  Although the present invention has been described with reference to the above-described embodiments, the present invention is not limited to the configurations of the above-described embodiments, and various modifications that can be made by those skilled in the art within the scope of the present invention. Of course, including modifications.

1: CNT suspension 2: Irradiation unit 3: Measurement unit 4: Calculation unit 5: Extraction unit 6: Evaluation unit

Claims (8)

A CNT suspension in which CNT (carbon nanotubes) are dispersed in a dispersion medium or a CNT dispersion thin film formed by applying the CNT suspension to a substrate surface is irradiated with ultraviolet rays as incident light, and the CNT suspension Calculating the absorbance from the transmitted light from or reflected light from the CNT-dispersed thin film, and evaluating the CNT monodispersity of the CNT suspension or the CNT-dispersed thin film,
Obtaining an absorption curve from the absorbance;
Extracting the intensity of two absorption peaks centered around two specific energy levels in the absorption curve, respectively;
Calculating the intensity ratio of the two absorption peaks;
An evaluation method for evaluating monodispersity of CNTs, comprising:   The evaluation method according to claim 1, wherein the two specific energy levels are energy levels near 5.9 eV and 5.0 eV, respectively, in the absorption curve.   The evaluation method according to claim 1 or 2, wherein the extraction of the intensity of the absorption peak is performed by fitting each absorption peak according to a Lorentz curve, and using the integrated value as the peak intensity. When the intensity of the absorption peak centered around the 5.9 electron volt is A (5.9 eV) and the intensity of the absorption peak centered around the 5.0 electron volt is A (5.0 eV), The evaluation method according to claim 2 or 3, wherein the monodispersity of the CNT is evaluated by the CP value calculated by:
(Formula) CP = 40 × A (5.0 eV) / A (5.9 eV).   The evaluation method according to claim 4, wherein sorting based on monodispersity of CNTs is performed depending on whether the CP value is 20 or less.   The two absorption peaks are an ultraviolet absorption peak derived from π plasmon absorption in completely monodispersed CNT and an ultraviolet absorption peak derived from π plasmon absorption in aggregated CNT, The evaluation method according to any one of Items 1 to 5. A CNT suspension in which CNT (carbon nanotubes) are dispersed in a dispersion medium or a CNT dispersion thin film formed by applying the CNT suspension to a substrate surface is irradiated with ultraviolet rays as incident light, and the CNT suspension A CNT monodispersity evaluation apparatus for measuring transmitted light from or reflected light from the CNT-dispersed thin film, and calculating absorbance from the measured transmitted light or reflected light,
A calculation unit for obtaining an absorption curve from the absorbance;
An extraction unit for extracting the intensity of the absorption peak centered around 5.9 eV and the intensity of the absorption peak centered around 5.0 eV in the absorption curve,
An evaluation unit for evaluating the monodispersity of CNT from the intensity ratio of the extracted absorption peak;
The evaluation apparatus characterized by including.   A method for producing a CNT suspension or a CNT-dispersed thin film, characterized by performing evaluation and selection using the evaluation method according to any one of claims 1 to 6.

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