JP2000313608A - Production of carbon nanotube and carbon nanotube produced thereby - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable the formation of carbon nanotube at a low temperature and the control of its orientation by locally heating the part between a substrate and a target composed mainly of carbon in a reaction vessel and irradiating the target with a laser beam while keeping the above state to effect the ablation. SOLUTION: An optical system composed of a YAG laser 20, a mirror 19 and a condenser lens 18 is placed out of a reaction vessel 1. The laser beam is introduced through a quartz window 17 and radiated to a carbon target 2 in a lower radiation plate 3 having a triple-layer structure. The area in the radiation plate 3 is heated with two electrode rods 10 and two tungsten filaments 4. When a bias is applied in a plume space generated between a substrate holder 8 as a cathode and the carbon target 2 as an anode, the C+ ion in the plume generated between the electrodes is attracted to the cathode and, accordingly, a large electric current becomes unnecessary in contrast with conventional arc discharge and the excessive heating of the cathode to a high temperature and the damage of the substrate 7 can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a carbon nanotube and a carbon nanotube device, and can be expected to be applied to the fields of electronics such as an electron source, an electrochemical sensor, and a nanodevice.

[0002]

2. Description of the Related Art Carbon nanotubes are fullerenes having a cylindrical structure formed by winding several or more layers of graphite, and are new carbon materials discovered in 1991 (Nature, 354, (1991) 56). ).

[0003] Carbon nanotubes are thermally and chemically stable, and have mechanical strength that is about three orders of magnitude higher than commercially available carbon fibers. Furthermore, since nanotubes have the characteristic that they become conductors or semiconductors depending on their structure, application to electronic devices and the like can be expected by combining, for example, concentric cylindrical metal tubes and semiconductor tubes.

[0004] In the years from the discovery of carbon nanotubes to the present, one of the main production methods is the arc discharge method. In the arc discharge method initially used, DC arc discharge was performed using a carbon rod electrode in an atmosphere of 100 Torr of argon, but soot recovered by the method contained only a small amount of carbon nanotubes. . After that, the conditions of the arc discharge method for generating a large amount of carbon nanotubes were found. Use a carbon rod with a diameter of 9 mm for the cathode and a diameter of 6 mm for the anode.
This is a condition in which an arc discharge of about 18 V and 100 A was caused in a state where they were opposed to each other at a distance of m. In this method, the yield of carbon nanotubes in the product reaches 75% at a helium atmosphere gas pressure of 500 Torr.

Recently, R.S. Smallley et al. Reported that a Co-Ni alloy (Co: Ni = 0.
(6: 0.6 at%), laser ablation was performed in a quartz tube inserted into an electric furnace using a carbon target mixed with 0.6 at%. They report that this laser ablation method was able to produce a rope-shaped single-layer tube with high efficiency (Chem. Phys. Let.).
t. 243, (1995) 49).

[0006]

In the above arc discharge method, the current flowing through the carbon rod electrode is large, so that the temperature of the carbon rod on the cathode side where carbon nanotubes are formed becomes high. Since a small amount of the catalyst metal buried in the carbon rod has a higher vapor pressure than carbon, it is preferentially evaporated, and it is difficult to obtain a sufficient carbon nanotube generation rate with good reproducibility. Further, the carbon rods evaporate each time discharge is performed, so that the distance between the carbon rods increases. Unless the distance is set to be constant, the carbon nanotube growth is affected. Further, since a substrate such as Si cannot be attached to the evaporating high-temperature carbon rod, carbon nanotubes cannot be grown on the Si substrate.

Next, in the electric furnace type laser ablation method described above, the space of the portion of the quartz tube inserted into the electric furnace had to be set to a high temperature (1200 ° C.), but synthesis at a lower temperature is desired. It is. Furthermore, in this laser ablation method, the space where the quartz tube is inserted into the electric furnace is too large compared to the size of the carbon target, and heat is supplied to portions other than the target region, so that there is a large thermal loss. Was.

Regarding the orientation of the carbon nanotubes, the arc discharge allows the carbon nanotubes to grow to some extent in the direction perpendicular to the carbon rods, but the laser ablation method mainly produces rope-like carbon nanotubes, and grows in a desired direction. I can't do things.

Therefore, an object of the present invention is to lower the temperature for forming carbon nanotubes and localize the heating. Another object of the present invention is to control the orientation of carbon nanotubes.

[0010]

The configuration of the present invention which has been made to achieve the above object is as follows.

That is, a first aspect of the present invention is to dispose a target mainly composed of carbon and a base in a reaction vessel, and irradiate the target with a laser while locally heating the target and the base. A process for ablating the target by using the method.

A second aspect of the present invention is to dispose a target mainly composed of carbon and a substrate in a reaction vessel, and irradiate the target with a laser while applying a voltage between the substrate and the target. The present invention relates to a method for producing carbon nanotubes, comprising a step of ablating the target.

A third aspect of the present invention is that a target mainly composed of carbon and a base are arranged in a reaction vessel, and an extraction electrode is provided between the base and the target. Or applying a voltage between the extraction electrode and the base or between the target and the extraction electrode and the base, and irradiating the target with a laser to ablate the target. The present invention relates to a method for producing carbon nanotubes.

In the second or third manufacturing method of the present invention, it is preferable that the space between the carbon target and the substrate is locally heated.

A fourth aspect of the present invention is the second aspect of the present invention.
Alternatively, the present invention relates to a carbon nanotube device having a carbon nanotube produced by the third method, wherein the carbon nanotube is grown vertically from a substrate.

According to the first manufacturing method of the present invention, it is possible to generate carbon nanotubes without setting the substrate temperature and the plume space temperature to a high temperature (about 1200 ° C.). That is, since the plume is said to reach several thousand K instantaneously, the auxiliary and local heating of the plume space makes it possible to sufficiently produce carbon nanotubes at a relatively low temperature.

Further, according to the second or third manufacturing method of the present invention, the base holder or the lead electrode on which the base is attached is used as a cathode, and the carbon target is used as an anode.
By performing the bias application (voltage application), particles such as C ⁺ ions in the plume generated between the base and the carbon target can be attracted to the cathode. By using such a means, the substrate does not reach a high temperature, so that any material can be applied. Further, by performing the above-described bias application, the carbon nanotubes can be grown in a direction perpendicular to the substrate, and the orientation of the nanotubes can be controlled.

[0018]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows an example of a carbon nanotube (CNT) producing apparatus that can be used in the present invention. This apparatus uses a heating resistor in which two tungsten (W) filaments are connected in parallel, and is set so that a plume comes to a space between the two filaments.

A YAG laser 2 is provided outside the reaction vessel 1.
An optical system of a mirror 19 and a condenser lens 18 is additionally provided. The laser wavelength is 53, which is the second harmonic of YAG.
2 nm. Laser light is incident from the quartz window 17 at the upper right of the figure and is irradiated on the carbon target 2 in the triple structure lower radiation plate 3 arranged in the reaction vessel 1. Further, a heating mechanism is arranged in the radiation plate 3 so that only the periphery of the plume can be heated to a high temperature. The heating mechanism uses two electrode rods 10 and connects two tungsten filaments 4 in parallel as shown in FIG. However, the electrode rod 10 and the radiation plate 3 are insulated by the electric insulator 5. The electrode rod 10 is connected to an AC power supply 13 through an electric terminal 12. In addition, a pressure gauge 11, a leak valve 16, a variable leak valve 14 of an inert gas supply system 15, and a vacuum valve 21 of a vacuum exhaust system 22 are also connected to the reaction vessel 1.

In order to prevent the substrate 7 attached to the substrate holder 8 from being heated while the temperature is raised to the target temperature,
Shutter 2 between substrate holder 8 and filament 4
3 is installed. The substrate holder 8 is made of a heat-resistant material such as stainless steel.

The target 2 is placed directly below the tungsten filament 4 so that the plume generated from the target 2 comes to a space where heat is generated between the two filaments 4.

A feature of the present apparatus is that local heating can be performed unlike an electric furnace method, so that products during the reaction can be collected. In addition, carbon nanotubes can be generated with the substrate temperature kept low.

The apparatus shown in FIG. 1 is provided with a bias applying mechanism. The bias applying mechanism is designed to apply a bias to a plume space generated between the electrodes by using the substrate holder 8 as a cathode and the carbon target 2 as an anode.
The carbon target 2 is connected to a DC power supply 27 through a positive terminal 26. However, the base holder 8 is insulated by the electrical insulator 9 and the carbon target 2 is insulated by the electrical insulator 6.

A bias is applied (voltage applied) between the substrate 7 and the carbon target 2 using the above-described bias applying mechanism.
Is performed, plumes are generated in the carbon target 2 used for the anode by laser ablation, so that the C ⁺ ions in the plume are attracted to the cathode during the application of the bias, so that it is necessary to flow a large current like an arc discharge. Absent. Therefore, the temperature of the cathode does not become high and the substrate 7 is not damaged, and a nanotube can be formed even when a small amount of current is applied or an electric field is applied.

Another example of the bias applying mechanism used in the present invention will be described with reference to the carbon nanotube manufacturing apparatus shown in FIG.

The bias applying mechanism in the present apparatus applies a bias using the extraction electrode 29 as a cathode. The extraction electrode 29 is attached to the shutter 23 and is insulated by the electrical insulator 28.

According to the structure of the bias applying mechanism,
It is possible to produce nanotubes on the TEM grid placed in front of the extraction electrode 29 at the same time as producing nanotubes on the base 7.

Further, when the carbon target 2 is irradiated with a laser to perform ablation in a state where a bias is applied by using the above-described bias applying mechanism, the carbon nanotubes are perpendicular to the substrate 7 or the extraction electrode 29. Can be grown, that is, the orientation can be controlled in the vertical direction.

[0030]

The present invention will be described in more detail with reference to the following examples.

Example 1 Carbon nanotubes were produced using a device schematically shown in FIG. 1 without applying a bias. The conditions for producing the carbon nanotubes used in this example are shown below.

<Preparation conditions> Laser light: YAG laser (wavelength 523 nm, pulse width 15 ns, 10 Hz, 500 mJ / pulse) Energy density: 2.7 J / cm ² (focal length 700 mm condenser lens) 21 J / cm ² ( (Focal distance: 350 mm condenser lens) ・ Target: carbon pellet containing Co-Ni catalyst ・ Ablation time: 10, 20 sec ・ Plume space temperature: 600 to 1000 ° C. ・ Base: Grid for Si, TEM ・ Base-target distance: 55 mm ・Atmosphere: Argon gas 500 Torr

The laser energy density was controlled by changing the focal length of the condenser lens to change the ablation area. The target has a carbon / cobalt / nickel ratio of 98.8: 0.6: 0.6 at.
%, 90.0: 5.0: 5.0 at%, and two types of carbon targets containing a catalytic metal were used.

FIG. 3 shows an argon gas of 500 Torr, a plume space temperature of 1000 ° C., and an energy density of 2.7 J /
FIG. 3 shows a schematic view of a sample surface on which nanotubes 30 were obtained by performing laser ablation at cm ² . Ablation experiments were performed on the TEM observation samples using the substrate as a TEM grid. Ablation time is 20 seconds. Although the diameter of the nanotube 30 in FIG. 3 is estimated to be several nm, there is a possibility that several tubes grow as a bundle due to the presence of the branched tube. Amorphous carbon 31 is attached around the nanotube 30.

FIG. 4 is a schematic diagram showing a sample subjected to an ablation experiment with a plume space temperature of 600 ° C. and an argon gas of 500 Torr with a laser intensity as high as 21 J / cm ² . Ablation time is 10 seconds. As shown in FIG. 4, a filament-shaped nanotube 30 is formed, and has a length of about 200 to 300 nm.
The difference from the above low energy ablation is
That the nanotubes 30 grew straight and the thickness was 1
0 to 20 nm, and the growth length was shortened.

In the present embodiment, the plume space is locally and supplementarily heated, so that the R.P. Since the carbon nanotubes can be produced at a lower production temperature than the electric furnace type laser device developed by Smallley et al., The thermal power supplied from the prior art is consequently reduced and saved.

Example 2 In this example, carbon nanotubes were produced by a laser ablation method with bias application. The conditions for producing the carbon nanotubes in this example were the same as those in Example 1, except that the distance between the target and the substrate was changed to 5 to 55 mm, and the DC power supply used for bias application was controlled to the maximum voltage.

FIG. 5 shows a substrate holder 8 according to this embodiment.
1 shows a schematic view of a carbon nanotube growing on a substrate 7 attached to FIG.

Observation of the surface of the sample using an FE-SEM (field emission scanning electron microscope) shows that the orientation of the carbon nanotubes 30 to be produced tends to be vertical as the bias voltage is higher. Was. In other words, a significant improvement in the orientation was observed compared to the case where no bias was applied in Example 1. Further, even when a bias was applied, there was a region where the carbon nanotubes grew without orientation as in the case of Example 1. However, if the applied bias voltage was kept increasing, such a region was reduced. Finally, as shown in FIG. 5, the carbon nanotubes were grown in the vertical direction in almost all the regions on the substrate, and the density of the vertically aligned carbon nanotubes was about 2-3 times.

In the local heating method without bias application in Example 1, no vertically aligned carbon nanotubes were observed, but oriented carbon nanotubes could be produced by bias application.

Example 3 A carbon nanotube sample device oriented vertically by applying a bias, that is, a carbon nanotube device, was used, and an anode having a phosphor was provided at a distance of 1 mm from the device. Further, it was set in a vacuum device, and a voltage of 1 kV was applied to the anode. As a result, since the phosphor emitted light and the electron emission current was confirmed, it was confirmed that the carbon nanotube device of this example had good electron emission functionality.

[0042]

As described above, according to the present invention, the following effects can be obtained. (1) By performing local heating of the plume space in addition to laser ablation, carbon nanotubes can be formed at a lower temperature than in the prior art. (2) By performing a predetermined bias application (voltage application) in addition to laser ablation, carbon nanotubes can be formed at a lower temperature than in the prior art. Further, carbon nanotubes can be grown in a direction perpendicular to the substrate, and the orientation of the nanotubes can be controlled. (3) By further controlling the orientation of the nanotubes, there is a possibility that the nanotubes are grown while being bonded to the substrate, and application to nanodevices such as single electron transistors as electronic functional devices can be expected.

[Brief description of the drawings]

FIG. 1 is a schematic view showing one example of a carbon nanotube production apparatus that can be used in the present invention.

FIG. 2 is a schematic diagram showing another example of a carbon nanotube manufacturing apparatus that can be used in the present invention.

FIG. 3 is a schematic view of a sample surface from which nanotubes are obtained in Example 1 of the present invention.

FIG. 4 is a schematic view of another sample surface from which nanotubes were obtained in Example 1 of the present invention.

FIG. 5 is a schematic diagram of carbon nanotubes growing on a substrate attached to a substrate holder when a bias is applied in Example 2 of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Reaction container 2 Carbon target 3 Triple structure lower radiation plate 4 Tungsten filament 5, 6, 9, 28 Electrical insulator 7 Substrate 8 Substrate holder 10 Electrode rod 11 Pressure gauge 12 AC terminal 13 AC power supply 14 Variable leak valve 15 Inactive Gas supply system 16 Leak valve 17 Quartz window 18 Condenser lens 19 Mirror 20 YAG laser 21 Vacuum valve 22 Vacuum exhaust system 23 Shutter 24 Drawer rod 25 Negative terminal 26 Positive terminal 27 DC power supply 29 Extraction electrode 30 Carbon nanotube 31 Amorphous carbon

Continued on the front page (72) Inventor Toru Tada 3-30-2 Shimomaruko, Ota-ku, Tokyo F-term in Canon Inc. (reference) 4G046 CC06 4K029 BA01 BA24 BA34 BD00 CA01 CA13 DB17 DB20 DC05

Claims (6)

[Claims] 1. A target mainly composed of carbon and a substrate are placed in a reaction vessel, and the target is irradiated with a laser while locally heating the target and the substrate to ablate the target. A method for producing a carbon nanotube, comprising the steps of: 2. A step of arranging a target mainly composed of carbon and a substrate in a reaction vessel and irradiating the target with a laser while applying a voltage between the substrate and the target to ablate the target. A method for producing a carbon nanotube, comprising: 3. A target mainly composed of carbon and a base are arranged in a reaction vessel, and an extraction electrode is provided between the base and the target. The extraction electrode is provided between the extraction electrode and the target, or between the extraction electrode and the target. A method for producing carbon nanotubes, comprising a step of irradiating a laser to the target to ablate the target while a voltage is applied between the substrates or between the target, the extraction electrode, and the substrate. . 4. The method according to claim 2, wherein the space between the carbon target and the substrate is locally heated. 5. The method according to claim 1, wherein the target is a carbon target mixed with a metal catalyst.
The method for producing a carbon nanotube according to any one of the above. 6. A carbon nanotube device having carbon nanotubes produced by the method according to claim 2, wherein the carbon nanotubes are grown vertically from a substrate. Nanotube device.