JP2006188380A - Method for converting characteristic of assembly of single walled carbon nanotubes - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new method by which carbon nanotubes exhibiting semiconductive nature are efficiently and selectively removed with a sufficient rate from an assembly of single walled carbon nanotubes, thereby the carbon nanotubes exhibiting metallic nature are allowed to remain with a sufficient ratio. <P>SOLUTION: The method includes a step of applying oxidation treatment to the assembly of the single walled carbon nanotubes, and the carbon nanotubes exhibiting semiconductive nature in the assembly of the single walled carbon nanotubes are selectively oxidized and removed of the nature. Thereby, the ratio of the carbon nanotubes exhibiting metallic nature in the assembly of the single walled carbon nanotubes is increased. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a characteristic conversion method for reducing semiconducting properties of a single-walled carbon nanotube aggregate and increasing metallic properties.

  A single-walled carbon nanotube (SWNT) is a substance made of carbon, and has a shape in which a sheet (graphene sheet) made of a hexagonal ring of carbon is wound into a cylindrical shape. The SWNT aggregate has a unique property that conductivity and light absorption spectrum differ depending on how the graphene sheet is wound, and is expected to be applied as a functional material in the future. On the other hand, in an actual SWNT aggregate, there are SWNTs (metal SWNTs) whose electronic structure is metallic and exhibits metallic properties, and SWNTs (semiconductor SWNTs) whose electronic structure is semiconducting and exhibits semiconducting properties. Mixed.

  At present, SWNTs can be formed mainly using a laser evaporation method, an arc discharge method, and a vapor phase chemical vapor deposition method (CVD method), and many SWNTs produced by the CVD method are currently commercially available. However, when such a generation technique is used, the SWNT aggregate described above inevitably contains a mixture of metal SWNTs and semiconductor SWNTs. Such a characteristic of the SWNT aggregate becomes a fatal defect when various applications (for example, FET, optical element, conductive wire, electrode, FED, etc.) are considered for the SWNT aggregate.

  In particular, in the use of nano wires used in semiconductor devices, sufficient electrical conduction cannot be ensured with copper wires because of their size, and therefore, the use of metal single-walled carbon nanotubes has been studied. The above properties are fatal.

  In view of the problems as described above, at present, it is a big problem to separate the metal SWNT and the semiconductor SWNT in the SWNT aggregate. For example, in JP-A-2004-210608 and M. Yudasaka et al. Chem. Phys. Lett., 374 (2003) pp. 132-136, the SWNT aggregate is selectively removed by light irradiation. It is disclosed. In these techniques, the semiconductor SWNT or the metal SWNT is selectively removed by selecting the wavelength of light.

  In addition, Z. Chen et al. Nano Lett., 3 (2003) pp. 1245-1249 uses the difference in interaction between bromine and semiconductor SWNTs and metal SWNTs, and finally the metal SWNTs by centrifugation. Has been disclosed to selectively remove the semiconductor SWNT. Furthermore, in R. Krupke et al. Science, 301 (2003) pp. 344-347, by isolating SWNTs with a surfactant and selectively orienting metal SWNTs between electrodes using electrophoresis, It is disclosed that the semiconductor SWNT is selectively removed.

  However, in any of the above-described methods, the semiconductor SWNTs are efficiently and selectively removed from the SWNT aggregate at a sufficient rate, and the metal SWNTs are not left at a sufficiently high rate.

JP 2004-210608 A M. Yudasaka et al. Chem. Phys. Lett., 374 (2003) pp. 132-136 Z. Chen et al. NanoLett., 3 (2003) pp. 1245-1249 R. Krupke et al. Science, 301 (2003) pp. 344-347

  An object of the present invention is to provide a novel method capable of efficiently removing semiconductor SWNTs from SWNT aggregates efficiently and at a sufficient rate and easily leaving metal SWNTs at a sufficiently high rate. To do.

In order to achieve the above object, the present invention provides:
A step of oxidizing the single-walled carbon nanotube aggregate to selectively oxidize the carbon nanotubes exhibiting the semiconducting properties of the single-walled carbon nanotube aggregate to eliminate their functions, and the single-walled carbon tube The present invention relates to a method for converting characteristics of a single-walled carbon nanotube aggregate, wherein the proportion of carbon nanotubes exhibiting metallic properties in the aggregate is increased.

  The inventors of the present invention have intensively studied to achieve the above object. As a result, by subjecting single-walled carbon nanotube (SWNT) aggregates to oxidation treatment, various conditions such as oxidation time, temperature and type of oxidant to be used are appropriately controlled and selected, so that semiconductor properties can be selected. SWNTs (semiconductor SWNTs) are selectively oxidized and their functions disappear, the semiconductor SWNTs are substantially selectively removed, and SWNTs having metallic properties in the SWNT aggregate It has been found that the ratio of (metal SWNT) is relatively increased.

  Therefore, the SWNT aggregate exhibits metallic properties as a whole, and can be applied to various uses such as FETs, optical elements, conductive wires, electrodes, FEDs, and is difficult in the past. It is also possible to realize nanowires that were

  In addition, the said oxidation process can be implemented by arrange | positioning a SWNT aggregate | assembly in the atmosphere containing this using a predetermined | prescribed oxidizing agent. Under the present circumstances, the said atmosphere can also be heated as needed.

  Moreover, in the preferable aspect of this invention, the process of removing catalyst components, such as a metal catalyst contained in a SWNT aggregate | assembly, is included. Furthermore, it includes a step of removing carbon impurities other than the SWNT structure contained in the SWNT aggregate. If such a residual catalyst component or carbon impurity is contained in the SWNT assembly, the semiconductor SWNT is oxidized to generate its function, and even if it is substantially selectively removed, the residual catalyst component, carbon impurity, etc. As a result, the expression of metallic properties in the SWNT aggregate may be reduced. Therefore, it is preferable to remove residual catalyst components and carbon impurities in accordance with a preferred embodiment of the present invention.

  As described above, according to the present invention, the semiconductor SWNT can be selectively removed from the SWNT aggregate efficiently and at a sufficient rate, and the metal SWNT can be easily left at a sufficiently high rate. A method can be provided. Therefore, the SWNT aggregate can be applied to various uses such as FETs, optical elements, conducting wires, electrodes, FEDs, and further, nanowires that have been difficult in the past can be realized.

  Hereinafter, details of the present invention, other features and advantages will be described based on the best mode for carrying out the invention.

  In the property conversion method of the present invention, a single-walled carbon nanotube (SWNT) aggregate is first prepared, and the SWNT aggregate is subjected to oxidation treatment. The oxidation treatment is preferably performed by using an oxidizing agent and placing the SWNT aggregate in an atmosphere containing the oxidizing agent.

  As the oxidizing agent, any kind can be used as long as the object of the present invention can be realized, but hydrogen peroxide is preferably used. As a result, only the semiconductor SWNT in the SWNT aggregate can be efficiently oxidized, and the remaining ratio of the metal SWNT can be efficiently increased, thereby realizing the object of the present invention more effectively. It becomes like this.

  If the oxidant is solid at room temperature, the SWNT aggregate is brought into direct contact with the oxidant, or the oxidant is dissolved in a predetermined solvent to form a solution and immersed in the solution for a predetermined time. By performing this, oxidation treatment can be performed. Furthermore, if the oxidizing agent is liquid at room temperature, the oxidation treatment can be performed by immersing the SWNT aggregate directly in the oxidizing agent or by immersing it in a diluted oxidizing agent. Further, if the oxidizing agent is gaseous at room temperature, the SWNT aggregate is placed in a gas atmosphere containing the oxidizing agent.

  Since hydrogen peroxide is a liquid at room temperature, the oxidation treatment is performed according to the oxidation treatment operation in the case of using the liquid oxidant.

  In the present invention, in addition to the oxidation treatment described above, it is preferable to remove catalyst components such as a metal catalyst contained in the SWNT aggregate to remove carbon impurities other than those constituting the SWNT. If such a residual catalyst or carbon impurity is contained in the SWNT aggregate, the semiconductor SWNT is oxidized to produce its function, and even if it is substantially selectively removed, the residual catalyst component, carbon impurity, etc. Due to the influence, the expression of metallic properties in the SWNT aggregate may be reduced.

  When removing the remaining metal catalyst, it is dissolved and removed using an acid such as hydrochloric acid or sulfuric acid. Moreover, when removing carbon impurities, it can be removed by burning in an oxygen-containing atmosphere.

  The SWNT aggregate subjected to the above-described operation can be manufactured by a known method, for example, using a laser vapor deposition method, an arc discharge method, a CVD method, or the like.

  First, a SWNT assembly purchased from CNI was prepared. This SWNT aggregate is obtained by the HiPco method (High pressure carbon monoxide method) using a thermal CVD method in which a carbon monoxide gas is brought into contact with an iron catalyst at a high temperature and thermally decomposed to produce a SWNT aggregate. is there.

  Next, the SWNT aggregate (SWNT1) was placed in air kept at 300 ° C. for about 30 minutes, and the carbon material around the iron fine particles (iron catalyst) mixed therein was burned. Next, the SWNTs were dispersed in hydrochloric acid, and the mixed iron fine particles were dissolved in a hydrochloric acid solution. Thereafter, the hydrochloric acid solution in which the iron fine particles were dissolved was filtered through a membrane filter, so that only the SWNTs from which the iron fine particles had been removed remained on the membrane filter and recovered (SWNT2).

  Next, 4 mg of the recovered SWNT2 was ultrasonically dispersed in 30 mL of a 30% aqueous hydrogen peroxide solution for about 1 hour. Next, the obtained SWNT dispersion was heated to 90 ° C. to promote the oxidation of SWNT2 with hydrogen peroxide. After an oxidation treatment with hydrogen peroxide for about 52 minutes, the dispersion was cooled to room temperature and dispersed in a hydrochloric acid solution, and the remaining iron fine particles were removed again. Then, the final SWNT aggregate (SWNT3) was obtained by filtering the obtained SWNT dispersion.

  FIG. 1 is a light absorption spectrum of three SWNTs (SWNTs 1 to 3) in the above-described oxidation treatment process. In this figure, the absorption peaks appearing between 0.5 eV and 1.25 eV of SWNTs 1 and 2 correspond to the optical transition between the Van Hove singularities (vHS) of the semiconductor SWNT, and 1.96 eV to 2 The absorption peak appearing between .6 eV corresponds to the optical transition between the vHS of the metal SWNTs. The absorption peak appearing between 0.5 eV and 1 eV of SWNT3 corresponds to the optical transition between vHS of the semiconductor SWNT, and the absorption peak appearing between 1.7 eV and 2.6 eV is Supports optical transition between vHS.

  As is apparent from FIG. 1, it can be seen that the optical transition between vHS in SWNT3 metal SWNT is larger than that in SWNT1 and 2, compared with the optical transition between vHS in semiconductor SWNT. That is, it shows that the proportion of the metal SWNT in SWNT3 is increasing. This indicates that the semiconductor SWNT in the SWNT2 can be selectively removed by treating the SWNT2 with hydrogen peroxide.

  The present invention has been described in detail with specific examples. However, the present invention is not limited to the above contents, and various modifications and changes can be made without departing from the scope of the present invention.

It is a light absorption spectrum of the single-walled carbon nanotube (SWNT) in the oxidation process.

Claims (9)

  A step of oxidizing the single-walled carbon nanotube aggregate to selectively oxidize the carbon nanotubes exhibiting the semiconducting properties of the single-walled carbon nanotube aggregate and extinguishing their functions; A method for converting characteristics of a single-walled carbon nanotube aggregate, wherein the proportion of carbon nanotubes exhibiting metallic properties in the aggregate is increased.   The method for converting characteristics of a single-walled carbon nanotube aggregate according to claim 1, wherein the oxidation treatment is performed in an atmosphere containing an oxidizing agent.   The method for converting characteristics of a single-walled carbon nanotube aggregate according to claim 2, wherein the oxidizing agent is hydrogen peroxide.   The method for converting characteristics of a single-walled carbon nanotube aggregate according to claim 2 or 3, wherein the oxidation treatment is performed in a temperature range of 0 to 150 ° C.   The method for converting characteristics of a single-walled carbon nanotube aggregate according to any one of claims 1 to 4, further comprising a step of removing a catalyst component in the single-walled carbon nanotube aggregate.   6. The method of converting characteristics of a single-walled carbon nanotube aggregate according to claim 5, wherein the catalyst component is a metal catalyst, and the removal of the metal catalyst is performed by dissolving and removing with an acid.   The single-walled carbon nanotube aggregate according to any one of claims 1 to 6, further comprising a step of removing carbon impurities other than constituting the carbon nanotube in the single-walled carbon nanotube aggregate. Characteristic conversion method.   The method of claim 7, wherein the carbon impurities are removed by burning in an oxygen-containing atmosphere. A nanowire comprising an aggregate of carbon nanotubes with an increased proportion of carbon nanotubes exhibiting metallic properties by the method according to claim 1.

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