JP2004217456A - Method of forming carbon nano-thread - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for forming a carbon nano-thread to prepare a carbon nano-thread excellent in linearity. <P>SOLUTION: A carbon nanotube 3 held in a space is irradiated with gallium ion beam going through the carbon nanotube 3. The carbon nanotube 3 held in the space bridges finely structured micropillars 2 and grows. A carbon nanotube 3 other than the one held in the space is removed by the irradiation of gallium ion beam. The carbon nanotube 3 is heated during or after the irradiation of gallium ion beam. As a result, the carbon nanotube 3 is made amorphous and improved in linearity by irradiating the carbon nanotube 3 held in the space with gallium ion beam. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for forming a carbon nanowire that forms a nanoscale wire made of carbon.
[0002]
[Prior art]
By placing carbon nanotubes having a diameter of nanometer scale on a substrate on which a pattern is formed, it is expected to be applied as a structural material or wiring, or as an electronic element itself. Further, carbon nanotubes are used for manufacturing fine elements and micromachines.
[0003]
When considering such applications, it is important to control the position and size on the substrate to form carbon nanotubes. As a technique for growing carbon nanotubes on a substrate, a chemical vapor deposition (CVD) method in which a hydrocarbon is reacted at a temperature of about 600 to 1000 ° C. using a transition metal as a catalyst is used. This includes a CVD method using only thermal decomposition of hydrocarbons and a plasma CVD method using plasma in combination. In each case, carbon nanotubes are obtained. A CVD method using only thermal decomposition of hydrocarbons is described in the following document 1, and a plasma CVD method using plasma is described in the following document 2.
Reference 1: M. S. Dresselhaus, G .; Dresselhaus, Ph. Avouris (Eds.) "Carbon Nanotubes, Synthesis, Structures, properties, and Applications", Springer, Berlin, 2000, pp 32-39.
Document 2: Japanese Patent Application Laid-Open No. 2000-57934
[Problems to be solved by the invention]
However, the above-described method of forming carbon nanotubes by the CVD method has the following problems. In the CVD method, since the growth temperature is relatively low, the grown carbon nanotube has many defects and has a meandering shape. For this reason, even if the position and size are controlled and carbon nanotubes are formed on the substrate with great effort, linear carbon nanowires cannot be obtained, which is not suitable for use as a structural material or wiring.
[0005]
Further, when a junction structure in which two or more carbon nanotubes intersect is formed, there is a problem that electrical coupling between them is weak. For this reason, it has been difficult to form a circuit by connecting a plurality of carbon nanotubes as wiring.
[0006]
An object of the present invention is to solve the above-mentioned problems in conventional carbon nanotubes and to provide a method of forming carbon nanowires for producing carbon nanowires having excellent linearity.
[0007]
[Means for Solving the Problems]
In order to solve the above problem, the invention of claim 1 is a method for forming a carbon nanowire, which comprises irradiating a carbon nanotube held in a space with an ion beam penetrating the carbon nanotube.
According to a second aspect of the present invention, in the method for forming a carbon nanowire according to the first aspect, the ion beam is a gallium ion beam.
According to a third aspect of the present invention, in the method of forming a thin carbon nanowire according to the first or second aspect, the carbon nanotubes held in a space are grown by crosslinking between fine structures.
According to a fourth aspect of the present invention, in the method for forming a thin carbon nanowire according to the third aspect, the carbon nanotubes other than the carbon nanotubes held in the space are removed by ion beam irradiation.
According to a fifth aspect of the present invention, in the method for forming a carbon nanowire according to any one of the first to fourth aspects, the carbon nanotubes are heated during or after the irradiation of the ion beam.
[0008]
According to the above invention, a carbon nanowire is formed as follows. When carbon nanotubes held in space are irradiated with high-speed ions, if the energy of the ions is sufficiently high, the carbon nanotubes are a thin substance consisting of one or more graphite layers. Without penetrating this. In the process in which the ions interact with the atoms in the solid, the energy of the ions is transferred to the solid atoms, and the position of the solid atoms is greatly changed, thereby causing ion damage. This damage is greatest just before the ions are slowed down and slowed down before colliding with the atoms and stopping.
[0009]
However, since the total number of atoms in the carbon nanotube is small, the fast ions pass through the carbon nanotube with little deceleration. That is, ionic damage is much less than bulk solids. However, the interaction is not completely absent, and the carbon atom is strongly shaken by the Coulomb interaction between the electron of the carbon atom constituting the carbon nanotube and the charge of the ion. For this reason, part or all of the graphite structure of the carbon nanotube is broken, and the broken part becomes amorphous. During this process, carbon atoms are transferred, which acts to minimize the length of the bent tube.
[0010]
By irradiating with high-speed ions in this way, the carbon nanotubes held in the space can be made amorphous to be converted into highly linear carbon nanowires. In addition, two or more carbon nanotubes that cross and contact each other are fused as amorphous, and change to a bonding structure that is in electrical contact.
[0011]
On the other hand, when the carbon nanotubes are present in contact with the substrate surface, ions passing through the carbon nanotubes are decelerated on the substrate surface to sputter the substrate atoms. Thereby, the carbon nanotubes are sputtered together with the substrate atoms by the atoms jumping out of the substrate. That is, the carbon nanotubes existing in contact with the substrate surface can be selectively removed, and only the carbon nanotubes held in the space can be left as carbon nanowires.
[0012]
Note that the lower limit of the energy of the fast ions that can penetrate the carbon nanotube depends on the number of layers of the carbon nanotube and the mass of the ions. As the number of layers of carbon nanotubes is smaller and the mass of ions is smaller, ions with lower energy can be used. For example, in the case of Ar ions, an effect of 1 keV can be expected for a single-walled carbon nanotube. An energy range of usually used is 3 keV to 50 keV for easy handling.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, embodiments of the present invention will be described in detail with reference to the drawings.
[0014]
[Embodiment 1]
A method for forming a carbon nanowire according to the present embodiment will be described with reference to FIG. In this embodiment mode, the carbon nanotubes are irradiated with ions.
[0015]
That is, a plurality of micro pillars 2 which are micro structures of silicon are formed on the substrate 1 by using the silicon process technology. Next, an iron thin film (having a thickness of 1 nm or less, preferably 0.5 nm or less) is deposited as a catalyst on the substrate 1 having the fine columns 2, and then CVD growth is performed at 900 to 950 ° C. using methane as a raw material. Thereby, the hollow carbon nanotubes 3 bridging between the fine columns 2 can be grown.
[0016]
Thus, the carbon nanotubes 3 are grown between the micro pillars 2 of silicon by the CVD method. Specific growth conditions are described in the following reference 3.
Reference 3: Y. Homma, T .; Yamashita, Y .; Kobayashi, and T.W. Ogino, "Growth of suspended carbon nanonetworks on 100-nm-scale silicon pillars" Appl. Phys. Lett. 81 (2002) 2261-2263
[0017]
These carbon nanotubes 3 mainly grow by connecting the tops of the fine columns 2. Under the above growth conditions, a single-walled carbon nanotube 3 or a bundle thereof grows. In order to increase the probability of cross-linking, it is preferable to make the height and the interval of the fine columns 2 approximately equal. In addition to the crosslinked carbon nanotubes 3, the carbon nanotubes 4 grow on the surface of the substrate 1. A structure 5 where a plurality of carbon nanotubes 3 grown from different microcolumns 2 intersect is also formed. Since the growth temperature is relatively low in the CVD method, these grown carbon nanotubes 3 have a meandering shape.
[0018]
Next, the sample is irradiated with a high-speed gallium ion beam. For this purpose, it is convenient to use a commercially available focused ion beam (FIB) device. If the location is specified using an attached scanning electron microscope or a secondary electron image generated by irradiation of a focused ion beam of gallium, gallium ions Ga ⁺ can be selectively irradiated to carbon nanotubes in a specific region. The irradiation conditions are, for example, an acceleration energy of 30 keV, a beam current of 1 to 3 pA, and an irradiation area of several μm × several μm.
[0019]
It can be observed as a secondary electron image that the bent and cross-linked carbon nanotubes 3 become linear as the irradiation amount of the gallium ions Ga ⁺ increases. Therefore, the irradiation amount can be adjusted while viewing the secondary electron image. The optimum ion irradiation amount varies depending on the diameter of the carbon nanotubes 3 or the bundle thereof, but in most cases, the total irradiation amount is appropriately in the range of 5 × 10 ¹⁴ to 1 × 10 ¹⁶ ions / cm ² . As a result, as shown in FIG. 2, the hollow carbon nanotubes 3 that have been bent and cross-linked after the CVD growth are made amorphous by being made amorphous, and linearized, and linear carbon nanowires connecting the fine columns 2 with straight lines. A network-like shape consisting of 3A is obtained. The intersection structure 5 in which the plurality of carbon nanotubes 3 intersect changes into a junction structure 5A of carbon nanowires 3A that are completely joined both structurally and electrically, due to the fusion of the intersections due to amorphization.
[0020]
Even after the carbon nanotubes 3 are linearized, if the irradiation amount of gallium ions Ga ⁺ is further increased, the carbon nanowires 3A start to be cut. This is because the carbon atoms are sputtered to a small extent, and the surface of the fine column 2 is sputtered at the portion where the fine column 2 and the carbon nano fine line 3A are in contact with each other. It is because it is lost. On the other hand, since all the carbon nanotubes 4 existing on the surface of the substrate 1 and the side walls of the fine columns 2 are sputtered, only the crosslinked carbon nanowires 3A can be left.
[0021]
After the carbon nanowires 3A linearized by ion irradiation are obtained, if the mechanical properties, electrical properties, and the like are not sufficiently improved as they are, the carbon nanowires 3A subjected to ion irradiation are heat-treated. Thereby, these characteristics of the carbon nanowires 3A can be improved. For example, heat treatment at 800 to 1200 ° C. may be performed for several minutes to several tens of minutes in a vacuum or a rare gas atmosphere such as argon. Alternatively, heat treatment can be performed by selectively irradiating a specific carbon nanowire 3A with a laser. The heat treatment can be performed during ion irradiation. In this case, even at a relatively low temperature of about 500 ° C., the effect of improving the characteristics of the carbon nanowires 3A may be remarkably exhibited.
[0022]
[Embodiment 2]
A method for forming a carbon nanowire according to the present embodiment will be described with reference to FIG. In this embodiment, an example is described in which carbon nanotubes are placed on a microgrid and ion irradiation is performed.
[0023]
That is, the carbon nanotubes 12 grown by CVD are placed on the microgrid 11 which is a thin film mesh as used in transmission electron microscope observation. For example, there is a method of scraping the carbon nanotubes 12 from the substrate or chemically dissolving the substrate and scooping the carbon nanotubes 12 from the solution onto the microgrid 11. In the case of a CVD method using no substrate, the grown carbon nanotubes 12 may be dispersed on the microgrid 11 as they are. Since the growth temperature is relatively low in the CVD method, the grown carbon nanotubes 12 have a meandering shape.
[0024]
The ion irradiation after disposing the carbon nanotubes 12 is performed by a high-speed gallium ion beam in the same manner as in the first embodiment. As a result of the ion irradiation, as shown in FIG. 4, the carbon nanotube 12 does not adhere to the grid even on the portion bridged by the opening portion 11A, which is the hole of the microgrid 11, and even on the microgrid 11. The defect in the portion that is raised above is removed, and it can be changed to a linear carbon nanowire 12A.
[0025]
As described above, the first and second embodiments of the present invention have been described in detail. However, the specific configuration is not limited to this embodiment, and there are design changes and the like within a range not departing from the gist of the present invention. Are also included in the present invention. For example, in the above embodiment, a gallium ion beam is used for ion beam irradiation, but the ions are not limited to gallium ions, and rare gas ions such as helium, neon, argon, xenon, and krypton, hydrogen, nitrogen, oxygen, Other ionic species can be used, including gaseous ions such as carbon monoxide.
[0026]
【The invention's effect】
As described above, by irradiating the carbon nanotube held in the space with the ion beam penetrating the carbon nanotube, the carbon nanotube can be made amorphous and the linearity can be improved. Thereby, a nanometer-scale fine wire made of carbon can be formed by cross-linking, and a great advance can be brought about industrial application of the carbon nanowire.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram for explaining ion irradiation in a method of forming a carbon nanowire according to a first embodiment of the present invention.
FIG. 2 is an explanatory diagram for explaining after ion irradiation in the method for forming carbon nanowires according to the first embodiment of the present invention.
FIG. 3 is an explanatory diagram for explaining ion irradiation in a method for forming a carbon nanowire in Embodiment 2 of the present invention.
FIG. 4 is an explanatory diagram for explaining after ion irradiation in a method of forming a carbon nanowire according to a second embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Substrate 2 Fine pillar 3, 4 Carbon nanotube 3A Carbon nanowire 5 Cross structure 5A Joining structure 11 Micro grid 11A Opening 12 Carbon nanotube 12A Carbon nanowire

Claims (5)

A method for forming a carbon nanowire, comprising irradiating a carbon nanotube held in a space with an ion beam penetrating the carbon nanotube. The method according to claim 1, wherein the ion beam is a gallium ion beam. The method for forming a carbon nanowire according to claim 1 or 2, wherein the carbon nanotubes held in a space are grown by crosslinking between fine structures. The method for forming carbon nanowires according to claim 3, wherein carbon nanotubes other than the carbon nanotubes held in a space are removed by ion beam irradiation. The method for forming a carbon nanowire according to any one of claims 1 to 4, wherein the carbon nanotube is heated during or after the irradiation of the ion beam.

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