JP4863590B2 - Carbon nanotube modification method, carbon nanotube and electron emission source - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for modifying a multi-walled carbon nanotube that removes a metal or a metal compound contained in the multi-walled carbon nanotube, a modified multi-walled carbon nanotube, and an electron emission source using the modified multi-walled carbon nanotube. .
[0002]
[Prior art]
In recent years, carbon nanotubes have attracted attention as field electron emission sources. The field electron emission source is superior in terms of energy saving, long life and the like as compared with an electron source (thermal electron emission source) using thermal energy.
The field emission phenomenon is a phenomenon in which when the applied voltage on the surface of the metal or semiconductor is set to about 10 ⁹ V / m, electrons are emitted into the vacuum even at room temperature through the barrier by the tunnel effect. The extraction current is determined by how high voltage is applied from the extraction electrode portion (hereinafter referred to as the gate electrode portion) to the emission portion (hereinafter referred to as the emitter).
[0003]
For this reason, it is known that the strength of the electric field applied to the emitter increases as the emitter has a sharper tip. In view of these points, carbon nanotubes are recently attracting attention as electron emission source materials.
Carbon nanotubes have an external shape of 10 to several tens of nanometers and a length of several μm, and have a structural form sufficient to cause field emission at a low voltage in terms of shape, and the material carbon is chemically stable, Since it is mechanically strong, it is an ideal material for a field emission source.
Known methods for producing carbon nanotubes include thermal CVD and plasma CVD, and carbon nanotubes can be produced by these methods.
[0004]
[Problems to be solved by the invention]
However, when producing carbon nanotubes by the above-described method, metals, alloys, metal compounds, etc. are used as catalysts. These metals are usually contained in the growth ends of carbon nanotubes grown from the substrate and become carbon nanotubes. Remains.
[0005]
As described above, when using these carbon nanotubes as an electron emission source, it is known that the sharper the tip of the carbon nanotube, the higher the applied electric field strength and the higher the current density obtained. If the metal or metal compound remains at the tip, it is a matter of course that the efficiency of electron emission deteriorates.
Therefore, when carbon nanotubes are used as an electron emission source, the metals used as a catalyst at the time of manufacturing the carbon nanotubes are contained at the tips of the carbon nanotubes, and thus these metals need to be removed.
[0006]
Conventionally, as a method for removing the metal at the tip of the carbon nanotube, a heating method, a method of treating with oxygen plasma, a method of treating with an acidic aqueous solution or an alkaline aqueous solution, a laser ablation method, and the like have been proposed. However, the heating method requires a considerably high temperature, and the removal selectivity is poor.
Further, it has been reported that the method of treating with oxygen plasma oxidizes the tip of the carbon nanotube and removes the metal, but oxygen plasma treatment is known to inhibit the electron emission ability of the carbon nanotube.
[0007]
Furthermore, the method of treating with an acidic aqueous solution or an alkaline aqueous solution is not a desirable method in terms of using an aqueous solution as a method for producing an electron emission source. In addition, the laser ablation method is difficult to achieve uniformity over a large area.
As described above, there has been a demand for a method for removing carbon nanotube metals by a relatively simple and efficient method. However, there has been an appropriate method for removing carbon nanotube metals relatively easily and efficiently. I did not.
[0008]
An object of the present invention is to easily and efficiently remove metals contained in carbon nanotubes.
Another object of the present invention is to provide a carbon nanotube excellent in electron emission characteristics.
Another object of the present invention is to provide an electron emission source having excellent electron emission characteristics.
[0009]
[Means for Solving the Problems]
According to the present invention, plasma is generated by supplying at least one of hydrogen and helium and a halogen-based gas between a pair of electrodes and supplying power between the electrodes, and is contained in the carbon nanotube by the plasma. There is provided a method of modifying carbon nanotubes characterized by removing the metals.
[0010]
Here, the halogen-based gas is preferably at least one of a fluorine-based gas and a chlorine-based gas.
The fluorine-based gas is preferably at least one gas selected from CF ₄ , SF ₆ , NF ₃ , and BF ₃ .
Further, the chlorine-based gas is preferably CCl ₄ .
[0011]
The carbon nanotubes supply aliphatic hydrocarbons and hydrogen having 1 to 5 carbon atoms or aromatic hydrocarbons and hydrogen between the pair of electrodes at a constant frequency of 13.56 MHz. Carbon nanotubes deposited on the substrate by generating plasma are preferred.
In addition, according to the present invention, a carbon nanotube modified by the method for modifying each carbon nanotube is provided.
[0012]
According to the present invention, in the electron emission source in which an emitter is disposed between the cathode conductor and the gate electrode and electrons are emitted from the emitter by applying a voltage between the cathode conductor and the gate electrode, An electron emission source comprising the carbon nanotube is provided.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a method for modifying a multi-walled carbon nanotube, a multi-walled carbon nanotube, and an electron emission source according to an embodiment of the present invention will be described.
First, the outline of the present embodiment will be described. In the present embodiment, at least one of hydrogen gas and helium gas, and one or more kinds of gases (hereinafter referred to as halogen-based gases) among halogen gas and halogen compound gas are used. Gas is generated in a coexisting state, and metal, an alloy, or a metal compound (metals) is removed from the carbon nanotube by the plasma (hereinafter referred to as a halogen-based plasma). It is characterized by that.
[0014]
In particular, when multi-walled carbon nanotubes are grown on a substrate by plasma CVD, the catalyst metal used for the growth of carbon nanotubes remains in the tips of the carbon nanotubes, and this remains in the same reaction vessel. By treating with halogen-based plasma, metals can be easily removed from the tip of the carbon nanotube, and the tip of the carbon nanotube is modified into a sharp acute angle.
[0015]
That is, multi-walled carbon nanotubes are generated by a plasma reaction, and the catalyst residue metal or the like is removed with halogen-based plasma in the same reaction vessel, so that multi-walled carbon nanotubes having sharp edges and sharp edges are obtained.
The modified carbon nanotube is very suitable for use as a field emission source material, and the modification method of the carbon nanotube is a simple and efficient method as a process for producing a field emission source material. Therefore, it is very significant as an industrial production method of carbon nanotubes for field emission sources.
[0016]
FIG. 1 is a diagram showing an apparatus used in an embodiment of the present invention, and the apparatus shown in FIG. 1 is also used in each example described later. In FIG. 1, 101 is a reaction vessel (chamber), 102 is an anode, 103 is a stainless steel ring, 104 is a stainless steel sample stage, 105 is a substrate, 106 is a Teflon ring, 107 is a cathode, 108 is a permanent magnet, 109 is a frequency 13 .56MHz AC power supply. An AC power source 109 is a power source for generating plasma between the anode 102 and the cathode 107, and a permanent magnet 108 is for generating a magnetic field for generating high-density plasma in the vicinity of the substrate 105. The source gas is supplied from the pipe 110, passes through the reaction vessel 101, passes through the pipe 111, and is led to a vacuum pump (not shown).
[0017]
When producing carbon nanotubes in the reaction vessel 101, the anode electrode 102 is applied by applying AC power from the AC power source 109 while supplying a carbon-containing gas from the tube 110 to the reaction vessel 101 and exhausting it from the tube 111. And a cathode electrode 107, thereby generating carbon nanotubes on the substrate 105.
[0018]
Further, when the carbon nanotubes generated on the substrate 105 are modified, that is, when the metal of the catalyst residue is removed, a predetermined halogen-based gas coexisting with hydrogen and / or helium is supplied from the tube 110 to the reaction vessel 101. As a result, the anode 111 and the cathode electrode 107 are discharged from the tube 111 while being supplied to a predetermined halogen-based gas atmosphere, and the AC power is supplied from the AC power source 109 to the pair of electrodes (the anode 102 and the cathode 107). To generate plasma, thereby removing the metal or metal oxide deposited on the carbon nanotubes and modifying the carbon nanotubes.
[0019]
As the halogen-based plasma in the present embodiment, at least one of hydrogen gas and helium gas and one or more kinds of gases of fluorine gas and fluorine compound gas (hereinafter referred to as fluorine-based gas) are used. Plasma generated in the coexisting state (hereinafter referred to as fluorine-based plasma), at least one of hydrogen gas and helium gas, and at least one of chlorine gas and chlorine compound gas (hereinafter, Plasma (hereinafter referred to as chlorine-based plasma), at least one of hydrogen gas and helium gas, bromine gas and bromine compound gas. Or a plasma (hereinafter referred to as bromine-based plasma) generated in the state of coexistence with one or more kinds of gases (hereinafter referred to as bromine-based gas). Referred.) And the like can be used, say from the efficiency of removal of metals, fluorine-based plasma, chlorine-based plasma is preferred.
[0020]
More preferable as the fluorine-based plasma is plasma using CF ₄ , SF ₆ , NF ₃ , BF ₃ or the like as the fluorine compound gas. Further, more preferable as the chlorine-based plasma is plasma using CCl ₄ , CHCl ₃ or the like as the chlorine compound.
In the halogen-based plasma treatment of carbon nanotubes, it is important to use a mixed gas of a halogen-based gas and hydrogen gas and / or helium gas. This is because, in the halogen-based plasma treatment, when a halogen-based gas is used alone, the halogen-based gas is plasma-polymerized to generate a polymer, and the polymer adheres to the carbon nanotubes. As a diluent for the halogen-based gas, It is necessary to coexist hydrogen gas or helium gas, or both hydrogen gas and helium gas.
[0021]
General halogen-based plasma generation conditions include a halogen-based gas flow rate of 1 to 300 sccm / s and a pressure in the reaction chamber of 0.1 to 50 Pa. The mixing ratio of hydrogen gas and / or helium gas and halogen-based gas is 1: 1 to 1: 100 in volume ratio.
[0022]
As described above, according to the embodiment of the present invention, at least one of hydrogen and helium gas and a halogen-based gas are allowed to coexist between a pair of electrodes (anode 102 and cathode 107), and power is supplied between the electrodes. The plasma is generated by removing the metal, the alloy or the metal compound contained in the carbon nanotube by the plasma, and the carbon nanotube is subjected to a modification treatment. It is suitable as a source material. In addition, since the reforming process can be performed using the carbon nanotube manufacturing apparatus, it is not necessary to provide a dedicated reforming apparatus, and the reforming process can be performed at low cost.
[0023]
Here, the halogen plasma is preferably fluorine plasma or chlorine plasma.
Further, as the fluorine-based plasma, plasma using CF ₄ , SF ₆ , NF ₃ , or BF ₃ gas as the fluorine-based gas is suitable. As the chlorine-based plasma, plasma using CCl ₄ gas as the chlorine-based gas is suitable.
[0024]
In addition, as the multi-walled carbon nanotube, the same apparatus as that used for the reforming process is used, and a constant frequency power of 13.56 MHz supplied from the AC power source 109 is applied between a pair of electrodes (the anode 102 and the cathode 107). It is preferably a multi-walled carbon nanotube deposited on the substrate 105 by introducing an aliphatic hydrocarbon having 1 to 5 carbon atoms and hydrogen or an aromatic hydrocarbon and hydrogen and generating plasma. .
[0025]
Also, an emitter is disposed between the cathode conductor and the gate electrode. For example, the cathode conductor and the gate electrode are stacked via an insulating member, and an emitter is disposed on the cathode conductor, and a voltage is applied between the cathode conductor and the gate electrode. Thus, in the electron emission source that emits electrons from the emitter, the emitter can use the multi-walled carbon nanotube modified as described above. In this case, the multi-walled carbon nanotube has excellent electron emission characteristics since the metal, metal oxide, etc. are removed and the tip is sharpened.
[0026]
【Example】
The first embodiment of the present invention will be described below. First, carbon nanotubes were produced as follows using the reaction apparatus shown in FIG.
A constant frequency power of 13.56 MHz output from the AC power supply 109 is supplied between the two electrodes (the anode 102 and the cathode 107) and a mixed gas of acetylene and hydrogen is introduced from the tube 110, thereby generating plasma, Carbon nanotubes were deposited on the substrate 105.
At this time, a magnet 108 was placed under the sample stage 104 and a constant magnetic field was applied. This magnetic field is not necessarily required for the production of carbon nanotubes, but the plasma current density can be increased by applying the magnetic field.
[0027]
Table 1 shows the production conditions of the carbon nanotubes. As the substrate 105, aluminum was vapor-deposited on soda glass and then nickel was vapor-deposited, and carbon nanotubes were vapor-deposited for 60 minutes. The bias potential is the potential of the cathode 107 with respect to the anode 102, and positive plasma is generated. Therefore, the cathode 107 side on which the substrate 105 is disposed is set to a negative potential so that plasma molecules gather on the substrate 105 side.
[0028]
[Table 1] Carbon nanotube production treatment conditions Hydrogen gas flow rate (sccm / s): 23
Acetylene gas (sccm / s): 0.4
RF power (W) of AC power supply 109: 360
Pressure in reaction vessel 101 (Pa): 10
Bias potential (V) between anode 102 and cathode 107: −50
Processing time (min): 60
An SEM photograph of the obtained multi-walled carbon nanotube is shown in FIG. It can be seen that nickel used as a catalyst is contained at the tip of the multi-walled carbon nanotube.
[0029]
Next, a method for modifying the multi-walled carbon nanotube will be described. The modification treatment of the multi-walled carbon nanotubes by the halogen plasma treatment was performed using the apparatus of FIG. As the halogen gas, a mixed gas of CF ₄ and hydrogen was used. Table 2 shows the conditions of the halogen plasma. The bias potential is the potential of the cathode 107 with respect to the anode 102, and positive plasma is generated. Therefore, the cathode 107 side on which the substrate 105 is disposed is set to a negative potential so that plasma molecules gather on the substrate 105 side. The modification treatment time was 5 minutes.
[0030]
[Table 2] Carbon nanotube reforming treatment conditions Hydrogen gas flow rate (sccm / s): 0.8
CF ₄ gas flow rate (sccm / s): 17
RF power (W) of AC power supply 109: 400
Pressure in reaction vessel 101 (Pa): 7.5
Bias potential (V) between anode 102 and cathode 107: −50
Processing time (min): 5
An SEM photograph of the multi-walled carbon nanotube after the halogen plasma treatment is shown in FIG. It can be seen that the tips of the multi-walled carbon nanotubes after the halogen plasma treatment are sharply sharpened, and the metal has been removed.
[0031]
FIG. 4 shows electric field strength-current density characteristics comparing the characteristics of the carbon nanotubes before modification and the characteristics of the carbon nanotubes modified as described above, and 401 is a curve representing the characteristics before modification. Reference numeral 402 denotes a curve indicating the characteristics after the modification. As shown in FIG. 4, the maximum ultimate current density after the modification was 1.3 mA / cm ² , which was 20% higher than the maximum ultimate current measured using the carbon nanotubes before the halogen plasma treatment.
[0032]
Next, a second embodiment will be described. In the second example, the multi-walled carbon nanotube generation process was performed by the same method as in the first example using the apparatus of FIG.
However, SF ₆ gas was used for the modification treatment of the multi-walled carbon nanotube as the halogen plasma treatment. Table 3 shows the modification treatment conditions by the halogen plasma treatment.
[0033]
[Table 3] Carbon nanotube modification treatment conditions SF ₆ gas flow rate (sccm / s): 20
RF power (W) of AC power supply 109: 200
Pressure in reaction vessel 101 (Pa): 8
Bias potential (V) between the anode 102 and the cathode 107: −10
Processing time (min): 6
An SEM photograph after the halogen plasma treatment is shown in FIG. Also in the second example, it can be seen that the tips of the multi-walled carbon nanotubes after the halogen plasma treatment are sharply sharpened, and the metal contained in the tips has been removed.
[0034]
Next, a third embodiment will be described. In the third example, the multi-walled carbon nanotube generation process was performed in the same manner as in the first example, using the apparatus of FIG.
In addition, for the modification treatment of the multi-walled carbon nanotube, CF ₄ gas was used as the halogen plasma treatment using the apparatus of FIG. Halogen plasma treatment conditions were the same as in Table 2. However, the processing time was 2 minutes. The SEM photograph after the halogen plasma treatment was almost the same as in FIG. 3, and the tip of the carbon nanotube after the halogen plasma treatment was sharpened at an acute angle, and the metal contained in the tip was removed.
[0035]
【Effect of the invention】
According to the present invention, metals contained in carbon nanotubes can be easily and efficiently removed.
In addition, since the reforming process can be performed using the carbon nanotube manufacturing apparatus, the reforming process can be performed at low cost.
Moreover, according to the present invention, it is possible to provide an electron emission source having excellent electron emission characteristics.
[Brief description of the drawings]
FIG. 1 is a diagram showing an apparatus used in an embodiment of the present invention.
FIG. 2 is an SEM photograph of the carbon nanotube before modification according to the first example of the present invention.
FIG. 3 is an SEM photograph of the modified carbon nanotube according to the first example of the present invention.
FIG. 4 is a diagram showing electric field strength-current density characteristics of carbon nanotubes according to a first example of the present invention.
FIG. 5 is an SEM photograph of a modified carbon nanotube according to a second example of the present invention.
[Explanation of symbols]
101 ... Reaction vessel 102 ... Anode 103 ... Stainless steel ring 104 ... Sample stage 105 ... Substrate 106 ... Teflon ring 107 ... Cathode 108 ... Permanent magnet 109 ... AC Power supply 110, 111 ... tube

Claims (7)

Plasma is generated by supplying at least one of hydrogen and helium and a halogen-based gas between a pair of electrodes and supplying power between the electrodes, and metals contained in the carbon nanotube are removed by the plasma. A carbon nanotube modification method characterized by the above. 2. The carbon nanotube reforming method according to claim 1, wherein the halogen-based gas is at least one of a fluorine-based gas and a chlorine-based gas. _3. The carbon nanotube reforming method according to claim 2, wherein the fluorine-based gas is at least one kind of gas selected from CF ₄ , SF ₆ , NF ₃ , and BF ₃ . The method for reforming carbon nanotubes according to claim 2, wherein the chlorine-based gas is CCl ₄ . The carbon nanotubes are supplied with an aliphatic hydrocarbon and hydrogen having 1 to 5 carbon atoms or an aromatic hydrocarbon and hydrogen between the pair of electrodes at a constant frequency of 13.56 MHz to generate plasma. The carbon nanotube modification method according to any one of claims 1 to 4, wherein the carbon nanotube is deposited on a substrate by being generated. Carbon nanotubes modified by the carbon nanotube modification method according to any one of claims 1 to 5. In an electron emission source in which an emitter is disposed between a cathode conductor and a gate electrode and electrons are emitted from the emitter by applying a voltage between the cathode conductor and the gate electrode.
The electron emitter according to claim 6, wherein the emitter includes the carbon nanotube according to claim 6.

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