JP2006315882A - Carbon nanotube assembly and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide carbon nanotube assemblies each having a new structure. <P>SOLUTION: A buffer layer composed of TiN is formed on an Si-substrate. Co nano-particles having particle diameters of ≤5 nm are deposited on the buffer layer by generating pulse arcs in a range of about 3-150 times by using pulse arc plasma at a vacuum degree of 1×10<SP>-5</SP>Torr. Thereafter, carbon nanotubes are grown to form carbon nanotube assemblies assembled in a dotted form, each of which has a pyramid shape having a cone shape at its tip end and comprises carbon nanotubes having an average outer diameter of 4 nm and an average inner diameter of 3 nm are formed. These assemblies exhibit high electric field electron emitting efficiency. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an aggregate of carbon nanotubes and a method for producing the same.

  Conventionally, many studies have been made on the use of carbon nanostructured materials such as carbon nanotubes as conductors of microelectronic elements, field emission electrodes, electrical wiring, microstructures, high-strength material adsorbents, and the like.

  This carbon nanotube has a shape in which a graphite sheet is rounded into a cylindrical shape, and can be classified into a single-walled carbon nanotube having a single sheet and a multi-walled carbon nanotube in which a plurality of sheets are nested.

  Patent Documents 1 and 2 below disclose the use of carbon nanotubes for wiring between each stage of an electronic device. As a method for forming carbon nanotubes, in Patent Document 1, transition metal fine particles having a particle diameter of 0.4 to 20 nm are formed on a bottom surface of a via hole by electroless plating, and this is used as a catalyst to form a via hole. It is disclosed that carbon nanotubes are grown by a plasma CVD method or the like.

  Patent Document 2 below discloses a method for selectively forming carbon nanotubes at the bottom of a high aspect ratio via hole. In this method, after generating metal fine particles by laser ablation and selecting the particle size, a flow of fine particles is obtained parallel to the height direction of the via hole, and Ni nanoparticles having a particle size of 5 nm are deposited on the bottom of the via hole. Then, using this as a catalyst, the carbon nanotubes are grown.

  Further, Patent Document 3 below discloses a method of forming an ultrafine particle film of Si by sputtering using helicon plasma, although it is not a method of forming carbon nanotubes.

Patent Document 4 below discloses a radial aggregate of sharp end multi-walled carbon nanotubes in which a plurality of spire-shaped multi-walled carbon nanotubes are gathered radially with a common root portion.
JP-A-2005-72171 JP 2005-22886 JP 2002-167671 A JP 2003-206116 A

However, the carbon nanotubes of Patent Documents 1 and 2 are formed in parallel and can be used as wiring, but cannot be used as a field emission electrode. Moreover, although the said patent document 4 is disclosing using as a field emission electrode, the carbon nanotube of this literature is essentially a multi-walled carbon nanotube with many layers, and the tip is a sharp end. is there.
However, development of other structures having a high field emission effect is expected.
Unlike the conventional structure, the present invention is an aggregate of carbon nanotubes with a completely new structure that has never existed so far, in which a plurality of single-walled or double-walled carbon nanotubes are gathered together at their tips.
By using this structure, for example, the field emission effect can be enhanced, and application to other fields is expected.
Accordingly, an object of the present invention is to provide a carbon nanotube aggregate having a novel structure.

The invention of claim 1 is an aggregate of carbon nanotubes in which tips of a plurality of carbon nanotubes are aggregated in a cone shape.
When a large number of carbon nanotubes are grown on the substrate, the tips of the carbon nanotubes contained in each region are gathered in a conical shape for each region. Accordingly, a large number of carbon nanotube aggregates are formed. However, by limiting the growth region, the number of aggregates of carbon nanotubes can be controlled, and one aggregate of carbon nanotubes with one aggregated tip is within the scope of the present invention.

The invention according to claim 2 is the carbon nanotube aggregate according to claim 1, wherein a portion of the carbon nanotube excluding the tip is formed substantially perpendicular to the substrate.
In other words, the roots and intermediate portions of the plurality of carbon nanotubes are formed substantially perpendicular to the substrate and are formed in parallel with each other, and the tip portions of the plurality of carbon nanotubes are gathered in a dot shape. .

A third aspect of the present invention is the carbon nanotube aggregate according to the first or second aspect, wherein the carbon nanotube is single-walled or double-walled.
The present invention is characterized in that each carbon nanotube constituting the aggregate is a single wall or a double wall. With this structure, it becomes easy to gather the tip portions in a dot shape.

The invention of claim 4 is characterized in that the carbon nanotube is formed by a particulate catalyst having a particle diameter of 5 nm or less formed on a substrate. The aggregate of carbon nanotubes described in 1.
The particulate catalyst formed on the substrate is characterized by having a particle size of 5 nm or less. With this configuration, it is easy to gather the tip portions in a dot shape.
The invention of claim 5 is characterized in that the carbon nanotube is formed by a particulate catalyst having a particle diameter of 2 nm or more and 4 nm or less formed on a substrate. 2. The aggregate of carbon nanotubes according to item 1.
With this configuration, it is easy to gather the tip portions in a dot shape.

The invention according to claim 6 is characterized in that the density of the particulate catalyst is 1 × 10 ¹² / cm ^{2 to} 5 × 10 ¹³ / cm ^2. This is an aggregate of carbon nanotubes. A desirable range of the density of the particulate catalyst is 3 × 10 ¹² / cm ^{2 to} 3 × 10 ¹³ / cm ² . A more desirable range is 1 × 10 ¹³ / cm ^{2 to} 3 × 10 ¹³ / cm ² . In this desirable range, a high-quality carbon nanotube aggregate in which the tip portions of a plurality of carbon nanotubes are gathered in a cone shape can be obtained with certainty.
The invention of claim 7 is the aggregate of carbon nanotubes according to claims 1 to 6, wherein the diameter of the carbon nanotube is 2 nm or more and 5 nm or less.
The invention according to claim 8 is the aggregate of carbon nanotubes according to any one of claims 1 to 7, wherein the carbon nanotube has an average outer diameter of 4 nm and an average inner diameter of 3 nm. .

In the invention of claim 9, a particulate catalyst having a particle size of 5 nm or less is deposited on a substrate in a density range of 1 × 10 ¹² / cm ^{2 to} 5 × 10 ¹³ / cm ² , and thereafter, by plasma CVD, This is a method for producing a carbon nanotube aggregate in which double-walled or single-walled carbon nanotubes are grown and the tips of a plurality of carbon nanotubes are gathered in a cone shape. By setting the particle size of the particulate catalyst to 5 nm or less and the density in the range of 1 × 10 ¹² / cm ^{2 to} 5 × 10 ¹³ / cm ² , it is possible to collect the tip of multiple carbon nanotubes in a cone shape. It becomes. A desirable range of the density of the particulate catalyst is 3 × 10 ¹² / cm ^{2 to} 3 × 10 ¹³ / cm ² . Furthermore, the desirable range of the density of the particulate catalyst is 1 × 10 ¹³ / cm ^{2 to} 3 × 10 ¹³ / cm ² .
The invention according to claim 10 is the method for producing an aggregate of carbon nanotubes according to claim 9, wherein the particulate catalyst is generated by pulsed arc plasma at a vacuum of 1 × 10 ⁻⁴ Torr or less. is there.
The invention of claim 11 is the method for producing an aggregate of carbon nanotubes according to claims 9 to 10, wherein the particulate catalyst is cobalt or a cobalt alloy.
According to the invention of claim 12, the gas phase density above the substrate of the particulate catalyst formed by pulsed arc plasma is measured by absorption spectroscopy, and the particulate matter deposited on the substrate is determined from the measured gas phase density. 12. The method for producing a carbon nanotube aggregate according to claim 10, wherein the number of pulses of the pulse arc plasma is controlled so that the density of the catalyst becomes a desired value.
A laser or a hollow cathode lamp can be used as a light source for absorption spectroscopy. The density of the particulate catalyst deposited on the substrate and the density and number of pulses of the gas phase particulate catalyst scattered in the atmosphere by the pulsed arc plasma (generally, the density of the gas phase particulate catalyst x the number of pulses) The relationship is measured in advance. When the particulate catalyst is actually deposited on the substrate, the gas phase particulate catalyst density is measured by absorption spectroscopy, and the number of pulses is controlled so that a predetermined density of the particulate catalyst is obtained on the substrate. To do. Thereby, the desired optimum density of the particulate catalyst can be obtained on the substrate. At this time, the pulse width and the magnitude of the pulse voltage may be controlled.

  Plasma CVD is a method in which a raw material gas of carbon nanotubes is turned into plasma to form a plasma atmosphere, and carbon nanotubes are grown on the substrate using a particulate catalyst deposited on the substrate. A plasma atmosphere is a state in which at least a part of the material that constitutes the atmosphere is ionized (that is, a state in which charged particles such as atoms and molecules, ions and electrons, and neutral particles such as atoms and molecules are mixed) (Plasmaized state))).

According to the first and second aspects of the invention, the carbon nanotube aggregate in which the tip portions of a plurality of carbon nanotubes are gathered in a conical shape makes it possible to use the field electron emission efficiency from the tip portion when used for a field emission electrode. Will improve. In addition, since a large number of carbon nanotubes are gathered in the form of dots, the mechanical strength of the field emission electrode is improved.
According to the invention of claim 3, since the carbon nanotubes are single-walled or double-walled, the electrical characteristics of each carbon nanotube are uniform, and a carbon nanotube aggregate with uniform characteristics can be obtained.

In the invention of claim 4, the carbon nanotube can be formed into a single-walled carbon nanotube or a double-walled carbon nanotube by forming it with a particulate catalyst having a particle diameter of 5 nm or less formed on the substrate. The outer diameter of each carbon nanotube can be made 5 nm or less, and the tips can be assembled in a dot shape.
In the invention of claim 5, by forming the carbon nanotubes with a particulate catalyst having a particle size of 2 nm or more and 4 nm or less formed on the substrate, the ratio of single-walled or double-walled carbon nanotubes can be increased. In addition, it is possible to make the electrical characteristics of the aggregate of carbon nanotubes whose tips are gathered in a dot shape uniform.

According to the sixth aspect of the present invention, the density of the particulate catalyst is 1 × 10 ¹² / cm ^{2 to} 5 × 10 ¹³ / cm ² , so that the tip portion can be assembled in a dot shape. By adjusting the distance between the target and the substrate, it is possible to form a carbon nanotube aggregate in which tips of a plurality of carbon nanotubes are aggregated in a cone shape. Desirably, it is 3 × 10 ¹² / cm ^{2 to} 3 × 10 ¹³ / cm ² . Further, it is desirably 1 × 10 ¹³ / cm ^{2 to} 3 × 10 ¹³ / cm ² . In this range, the tips can be surely assembled in a dot shape.
In the invention of claim 7, the diameter of the carbon nanotubes is 2 nm or more and 5 nm or less, so that the ratio of single-walled or double-walled carbon nanotubes can be increased, and the tips are gathered in a dot-like manner. The electrical characteristics of the carbon nanotube aggregate can be made uniform.

In the invention of claim 9, a particulate catalyst having a particle size of 5 nm or less is deposited on a substrate in a density range of 1 × 10 ¹² / cm ^{2 to} 5 × 10 ¹³ / cm ² , and then by plasma CVD, By growing double-layer or single-layer carbon nanotubes, it is possible to produce a carbon nanotube aggregate in which tips of a plurality of carbon nanotubes are aggregated in a cone shape. The larger the diameter of the particulate catalyst, the more multilayered, and the smaller the particle diameter, the single layer. In the present invention, when the number of layers is two or less, it becomes easy to produce a carbon nanotube aggregate having a pyramid shape with aggregated tips.
By this method, an aggregate of carbon nanotubes whose tips are gathered in a dot shape can be easily and uniformly produced. When the density of the particulate catalyst is in the range of 1 × 10 ¹² / cm ^{2 to} 5 × 10 ¹³ / cm ² , the tips of the plurality of carbon nanotubes are conical by adjusting the distance between the target and the substrate. It becomes possible to form an aggregate of carbon nanotubes assembled together. Desirably, it is 3 × 10 ¹² / cm ^{2 to} 3 × 10 ¹³ / cm ² . Further, it is desirably 1 × 10 ¹³ / cm ^{2 to} 3 × 10 ¹³ / cm ² . In this range, the tips can be surely assembled in a dot shape.

In the invention of claim 10, the particle size of the particulate catalyst can be reduced to 5 nm or less by being generated by pulsed arc plasma at a vacuum degree of 1 × 10 ⁻⁴ Torr or less. Each carbon nanotube can be made into a single layer or a double layer, and the tips thereof can be assembled in a dot shape.
In the invention of claim 11, by using cobalt or a cobalt alloy as the particulate catalyst, it becomes possible to produce a carbon nanotube aggregate in which the tip portions are gathered in a dot shape uniformly and at high speed.
According to the invention of claim 12, since the gas phase density of the particulate catalyst above the substrate is measured by absorption spectroscopy, and the number of pulses is controlled from the measured value, the particulate matter deposited on the substrate The density of the catalyst can be accurately controlled, and high-quality carbon nanotubes can be produced.

  Hereinafter, preferred embodiments of the present invention will be described in detail. It should be noted that technical matters other than the contents particularly mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art. The present invention can be implemented based on the technical contents disclosed in the present specification and the common general technical knowledge in the field.

As a raw material used for the production of carbon nanotubes, various materials having at least carbon as a constituent element can be selected. Examples of elements that can form the raw material together with carbon include one or more selected from hydrogen, fluorine, chlorine, bromine, nitrogen, oxygen, and the like. Examples of preferable source materials include source materials substantially composed of carbon and hydrogen, source materials substantially composed of carbon and fluorine, and source materials substantially composed of carbon, hydrogen and fluorine. . A saturated or unsaturated hydrocarbon (for example, CH ₄ , C ₂ H ₂ ), a fluorocarbon (for example, C ₂ F ₆ ), a fluoro hydrocarbon (for example, CHF ₃ ), or the like can be preferably used. A linear, branched or cyclic molecular structure can be used. Usually, it is preferable to use a source material (source gas) that exhibits a gaseous state at normal temperature and pressure. Only one type of material may be used as the source material, or two or more types of materials may be used in any proportion. The type (composition) of the raw material to be used may be constant throughout the manufacturing process (for example, the growth process) of the carbon nanotubes, or may be different depending on the manufacturing process. Depending on the properties and / or characteristics (eg, electrical characteristics) of the target carbon nanostructure, the type (composition) of the raw material used, the supply method, and the like can be selected as appropriate.

As the radical source material, a material having at least hydrogen as a constituent element can be preferably used. It is preferable to use a radical source material (radical source gas) that exhibits a gaseous state at normal temperature and pressure. A particularly preferred radical source material is hydrogen gas (H ₂ ). In addition, a substance that can generate H radicals by decomposition, such as hydrocarbon (CH ₄ or the like), can be used as the radical source substance. Only one type of material may be used as the radical source material, or two or more types of materials may be used in any proportion.

In one preferable embodiment of the manufacturing method, the plasma atmosphere is formed by converting the raw material into plasma in the reaction chamber. Alternatively, the source material may be converted into plasma outside the reaction chamber, and the plasma may be introduced into the reaction chamber to form a plasma atmosphere in the reaction chamber.
It is desirable to inject radicals into the plasma atmosphere from the outside of the atmosphere. It is preferable to generate radicals by decomposing radical source materials in a radical generation chamber outside the chamber forming the reaction chamber, and to inject it into a plasma atmosphere in the reaction chamber. Alternatively, the radical generation material in the same chamber as the reaction chamber may be decomposed outside the plasma atmosphere, and radicals generated thereby may be injected into the plasma atmosphere. In short, radicals are generated in a region different from the processing region where film formation or processing is performed with the plasma of the raw material, and only this radical is injected into the processing region to control the film formation and processing and thereby carbon. Nanotubes may be grown.

  A preferred method for generating radicals from a radical source material includes a method of irradiating the radical source material with electromagnetic waves. As the electromagnetic wave used in this method, either microwave or high frequency (UHF wave, VHF wave or RF wave) can be selected. It is particularly preferable to irradiate VHF waves or RF waves. According to such a method, for example, by changing the frequency and / or input power, the decomposition strength (radical generation amount) of the radical source material can be easily adjusted. Therefore, there is an advantage that the production conditions of carbon nanotubes (such as the amount of radicals supplied into the plasma atmosphere) can be easily controlled.

Here, as is well known, “microwave” refers to an electromagnetic wave of about 1 GHz or more. The “UHF wave” refers to an electromagnetic wave of about 300 to 3000 MHz, the “VHF wave” refers to an electromagnetic wave of about 30 to 300 MHz, and the “RF wave” refers to an electromagnetic wave of about 3 to 30 MHz.
As another preferred method for generating radicals from a radical source material, a method of applying a DC voltage to the radical source material can be mentioned. It is also possible to employ a method of irradiating the radical source material with light (eg, visible light or ultraviolet light), a method of irradiating an electron beam, a method of heating the radical source material, or the like. Alternatively, a member having a catalytic metal may be heated, and a radical source material may be brought into contact with the member (that is, by heat and catalytic action) to generate radicals. As the catalyst metal for generating radicals, one or more selected from Pt, Pd, W, Mo, Ni, etc. can be used.

The radical injected into the plasma atmosphere preferably contains at least a hydrogen radical (that is, a hydrogen atom; hereinafter, sometimes referred to as “H radical”). It is preferable to decompose a radical source material containing at least hydrogen as a constituent element to generate H radicals and inject the H radicals into a plasma atmosphere. Particularly preferred as such a radical source material is hydrogen gas (H ₂ ).
In particular, when only H radicals are supplied, carbon nanotubes can be generated satisfactorily. In addition, when OH radicals or O radicals are present appropriately, the formation of carbon nanotubes is likely to be facilitated.

  It is desirable to adjust at least one of the production conditions of the carbon nanotube based on the concentration of at least one radical in the reaction chamber (for example, the concentration of at least one radical of carbon radicals, hydrogen radicals, and fluorine radicals). . Examples of production conditions that can be adjusted based on such radical concentration include the amount of raw material supplied, the intensity of plasma of the raw material (the severity of the plasma conditions), the amount of radicals (typically H radicals) injected, etc. Is mentioned. Such production conditions are preferably controlled by feeding back the radical concentration. According to such a production method, it is possible to more efficiently produce carbon nanotubes having properties and / or characteristics according to the purpose.

  As a method for measuring radicals, a radical emission line (that is, a carbon atom emission line) is emitted into the reaction chamber, the emitted emission line is received, and the radical concentration can be measured from the light absorption spectrum. Therefore, it is possible to efficiently produce carbon nanotubes having properties and / or characteristics according to the purpose. The emission line unique to the carbon radical (carbon atom) can be obtained, for example, by applying an appropriate energy to a gas containing at least carbon as a constituent element. An emission line unique to the carbon radical (carbon atom) can be emitted.

Monitors and control targets are not limited to C, H, and F radicals, and other target radicals may be C ₂ , CF, CF ₂ , CF ₃ , and C _x F _y (x ≧ 1, y ≧ 1). . Examples of manufacturing conditions that can be adjusted based on the measurement results include: the amount of raw material supplied, the plasma intensity of the raw material, the amount of radicals (typically H radicals) injected, the amount of radical source material supplied, the radicals Examples include the radicalization strength of the source material. Such production conditions are preferably controlled by feeding back the measurement result of the radical concentration. According to such a production method, carbon nanotubes having properties and / or characteristics according to the purpose can be produced more uniformly and efficiently.

  Similarly, by measuring radicals, particularly H radicals, in radical generating chambers that generate radicals to be injected or injection ports that inject radicals into the reaction chamber, the amount of radicals injected into the reaction chamber is set to a predetermined value. It is desirable to control the supply amount of the radical source material and the power applied to the radical source material. In this way, the amount of radicals injected into the reaction chamber, particularly the amount of H radicals, can be controlled in real time during the growth process, and high-quality carbon nanotubes can be generated.

A member having a metal catalyst (Pt, Pd, W, Mo, Ni, etc.) for generating radicals is arranged facing the radical generating chamber, and the radical generating means is configured so that the metal catalyst can be heated. May be. For example, it can be set as the structure which has arrange | positioned the wavy Ni wire (catalyst metal member) inside the radical generation chamber. H ₂ as a radical source material is introduced and brought into contact with a heater in which a current is passed through the wire. Thereby, H radicals can be generated by the catalytic action of Ni. The heating temperature of the catalyst metal can be, for example, about 300 to 800 ° C., and is usually preferably about 400 to 600 ° C. The plasma discharge means is preferably configured as a capacitively coupled plasma (CCP) generating mechanism.

Examples of particulate catalysts for growing carbon nanotubes include transition metals such as Ni, Fe, Co, Pd, and Pt, alloys of these transition metals, and alloys of these transition metals with other metals and semiconductors. Can be used. As a method for depositing the particulate catalyst on the substrate, it is desirable to use a pulsed arc plasma deposition method. For example, in a vacuum of 10 ⁻⁴ Torr or less, an arc is generated on a target made of a transition metal such as Co to generate a transition metal plasma, and a particulate catalyst having a particle size of 5 nm or less on the substrate. Can be deposited. The reason why it is 10 ⁻⁴ Torr or less is to reduce the particle diameter by decreasing the collision probability of atoms and molecules. Further, when the substrate is made of Si, the catalyst particles and Si are alloyed to generate silicide, so it is desirable to form a buffer layer of TiN, Al ₂ O ₃ or the like. Further, when CoTi is used for the catalyst particles, it does not react with Si, so that the particulate catalyst can be deposited directly on the Si substrate.

  EXAMPLES Hereinafter, although this invention is demonstrated based on a specific Example, this invention is not limited to a following example.

  First, a Si substrate was used as a substrate as a base. A TiN buffer layer and a particulate catalyst made of Co were deposited on the TiN buffer layer on the Si substrate by the coaxial vacuum arc deposition apparatus shown in FIG. In FIG. 1, a susceptor 11 is provided in a reaction chamber 10, and a Si substrate 12 is provided thereon. A halogen lamp 13 for heating the Si substrate 12 is provided under the susceptor 11. A plasma gun 14 is provided above the reaction chamber 10. FIG. 2 is a principle diagram of the plasma gun 14. A columnar cathode 15 is provided at the center, a cylindrical insulator 16 is provided around the cathode 15, and a ring-shaped trigger electrode 17 is provided outside thereof. A cylindrical anode 18 is provided coaxially with the cathode 15 and outside the insulator 16. A target 19 made of Co is provided on the end face of the cathode 15, and a cap 30 is provided on the end face of the target 19. A plate-like insulator 31 is provided on the end face of the trigger electrode 17. By applying an electric field between the cathode 15 and the anode 18 and applying a pulse voltage to the trigger electrode 17, a pulse arc is generated and constituent atoms of the target 19 are scattered. In this embodiment, Ti was used for the target 19 when the buffer layer was formed, and metal Co was used when the particulate catalyst was formed.

FIG. 3 shows the TiN buffer layer and the conditions for forming the particulate catalyst. To form the TiN buffer layer, the distance between the target 19 and the Si substrate 12 is 10 cm, the temperature of the Si substrate 12 is 400 ° C., the pressure in the reaction chamber 10 is 3 × 10 ⁻² Torr, and N ₂ is reacted. While flowing through the chamber 10, a pulse voltage was applied to generate 900 times of arc plasma. As a result, a TiN buffer layer having a thickness of 20 nm was formed on the Si substrate 12. The buffer layer is used in order to prevent the particulate catalyst and Si from reacting to become silicide.

For depositing the particulate catalyst composed of Co, the temperature of the Si substrate 12 is set to room temperature, the pressure in the reaction chamber 10 is applied at 1 × 10 ⁻⁵ Torr, and the pulse voltage is applied 30 to 100 times without flowing gas. A pulsed arc plasma was generated. As a result, Co nanoparticles having a particle size of 5 nm or less were deposited on the TiN buffer layer at a density of 1 × 10 ¹² / cm ^{2 to} 5 × 10 ¹³ / cm ² .
FIG. 4 shows an atomic force microscope image (AFM image) of the substrate surface when Co particles are deposited on the substrate using 10 pulse arcs. From this image, the particle size was determined to be 2-3 nm and the density was 3 × 10 ¹² / cm ² . Therefore, it was found that the density of Co nanoparticles deposited in one pulse arc was 3 × 10 ¹¹ / cm ² .

Next, carbon nanotubes were grown on the Si substrate 12 on which the TiN buffer layer was formed using the microwave plasma CVD apparatus of FIG. A susceptor 21 made of Mo is provided in the reaction chamber 20, and the substrate 12 is provided thereon. A carbon heater 22 for heating the substrate 12 is provided under the susceptor 21. A microwave of 2.45 GHz is introduced into the reaction chamber 20 from the upper part of the reaction chamber 20. The reaction chamber 20 is provided with an exhaust port 24, which is evacuated by a vacuum pump so that a certain degree of vacuum is obtained in the reaction chamber 20. Further, from the intake port 23 provided in the reaction chamber 20, H ₂ and CH ₄ gas are introduced into the reaction chamber 20 via the mass flow controllers 25 and 26, respectively.

Next, when depositing the particulate catalyst made of Co on the substrate 12, various samples were prepared in which the number of pulse arcs was changed. Carbon nanotubes were grown on each sample. It was found that the self-organized carbon nanotube aggregate in which the tips of a plurality of carbon nanotubes aggregated into a pyramidal shape was obtained when the number of pulse arcs was about 3 to 150 times. It was. That is, the density of Co nanoparticles depends on the distance between the substrate and the target, but when it is 1 × 10 ¹² / cm ^{2 to} 5 × 10 ¹³ / cm ² , pyramidal self-assembly occurs. I understood. Further, when the number of pulse arcs is 30 to 100 times, that is, when the density of the particulate catalyst is 1 × 10 ¹³ / cm ^{2 to} 3 × 10 ¹³ / cm ² , the tips of the plurality of carbon nanotubes are surely attached. A self-organized carbon nanotube aggregate in an aggregated pyramid shape was obtained. Further, when the number of pulse arcs is 10 to 100 times, that is, when the density of the particulate catalyst is 3 × 10 ¹² / cm ^{2 to} 3 × 10 ¹³ / cm ² , the pyramid-shaped self-assembly is good in quality. The obtained carbon nanotube aggregate was obtained. FIG. 6 shows the relationship between the number of pulse arcs and the growth rate of carbon nanotubes.

FIG. 7 shows the conditions of Co particles deposited on the substrate by 50 pulse arcs, substrate temperature 700 ° C., microwave power 900 W, pressure 70 Torr, CH ₄ flow rate 50 sccm, H ₂ flow rate 70 sccm, growth time 5 sec. FIG. 6 is an image (SEM image) of the surface when a carbon nanotube is grown using a scanning electron microscope. It can be seen that the tips of the plurality of carbon nanotubes are gathered in the form of dots in a pyramid shape. The density of this pyramid is 3 × 10 ⁸ / cm ² and the average spacing is about 0.5 μm. An enlarged image of FIG. 7 is shown in FIG. A side SEM image is shown in FIG. It can be seen that the carbon nanotubes grow substantially perpendicular to the substrate and the tips are gathered in a cone shape. In addition, as a result of imaging the surface SEM image while changing the growth time, it was found that self-organization in which the tips gather in a cone shape can be seen from the initial stage of growth. Therefore, it was found that pyramid-like self-organization occurred in the early stage of growth, and thereafter, the carbon nanotubes grew perpendicularly to the substrate from the root.

  Next, the number of pulse arcs for depositing Co nanoparticles, that is, the density of Co nanoparticles was changed, and an SEM image of the surface when carbon nanotubes were grown was imaged. As a result, it was found that as the density of Co nanoparticles increases, the carbon nanotubes grow more densely, and the self-organization in which the tips of the plurality of carbon nanotubes gather in a cone shape is less likely to occur. 10 and 11 show SEM images at that time. 10 shows a case where the pulse arc is 250 times, and FIG. 11 shows a case where the pulse arc is 50 times.

  Next, FIG. 12 shows an SEM image of the side surface of the carbon nanotube aggregate with the tips assembled in a pyramid shape. It can be seen that the tip part forming the pyramid grows straight, but the other intermediate part and root part grow perpendicular to the substrate, but are twisted.

  Next, the growth temperature of the carbon nanotube was changed. The sample whose SEM image is shown in FIG. 7 has a growth temperature of 700 ° C., but only the growth temperature was set to 600 ° C., and the other growth conditions were exactly the same, and the carbon nanotubes were grown. The SEM image of the surface at that time is shown in FIG. It will be understood that a carbon nanotube aggregate having tips gathered in a pyramid shape is obtained uniformly.

  Next, the growth time of the carbon nanotube was changed. FIG. 14 shows an SEM image of the surface when carbon nanotubes were grown at a growth time of 5 sec to 5 min, and FIG. 15 shows an image (TEM image) of the side surface by a transmission electron microscope. Even if the growth time is lengthened, the formed pyramid does not disappear once, and a carbon nanotube aggregate is obtained in which the tip with the long middle part and the root part gathered in the shape of a cone. Is understood.

The density of the particulate catalyst was varied. The number of pulse arcs when depositing Co fine particles is 30 times (density 9 × 10 ¹² / cm ² ), 50 times (density 1.5 × 10 ¹³ / cm ² ), and 70 times (density 2.1 × 10 ¹³ / cm ² ), 100 times (density 3 × 10 ¹³ / cm ² ), and 200 times (density 6 × 10 ¹³ / cm ² ), SEM images of the respective surfaces when carbon nanotubes were grown are shown in FIG. To FIG. In each case where the number of pulse arcs is 30 to 100, an aggregate of carbon nanotubes in which the tips are gathered in a conical shape in a dot shape is observed. It can be seen that the density of the aggregate also increases as the density of the Co nanoparticles increases. However, as shown in FIG. 20, when the number of pulse arcs is 200, it can be seen that pyramid-like self-organization does not occur.

  In this way, the number of pulse arcs was changed in various ways, and the growth rate of carbon nanotubes and the SEM image of the surface were imaged. As a result, Co particles were deposited by about 3 to 150 pulse arcs. In this case, it was found that pyramid-like self-organization occurs.

  Next, the electron field emission characteristics of the carbon nanotube aggregates thus produced were measured using the apparatus shown in FIG. The results are shown in FIGS. A characteristic of the aggregate of carbon nanotubes in the shape of a pyramid whose tips are gathered in a conical shape is shown as A, and a characteristic of a high-density carbon nanotube grown in parallel without a pyramid is shown as B. In the case of the pyramidal carbon nanotube aggregate of this example, it can be seen that a current of 5 μA flows at an electric field of 1 V / μm and a current of 400 μA flows at an electric field of 2 V / μm. On the other hand, in the case of the high-density carbon nanotube in which no pyramid is formed, it can be seen that a current of 4 μA flows in an electric field of 3 V / μm and a current of 400 μA flows in an electric field of 6.5 V / μm. Obviously, the electron field emission characteristics are improved.

Next, the Raman spectroscopic measurement of the pyramidal carbon nanotube aggregate of this example was performed. The results are shown in FIG. G band and D band are observed.
[Considerations about the principle]

  The reason why the pyramid-shaped carbon nanotube aggregate in which the tips are gathered in the shape of a cone is formed as follows. If the density of the Co nanoparticles is low, the particle diameter is as small as 2 to 3 nm, and the diameter of the carbon nanotubes that grow using it as a catalyst is also small, so that each carbon nanotube cannot stand independently, In the initial stage, it seems that the tip is integrated into a cone shape under the Van der Waals force. Then, in the subsequent growth, the aggregate stands on the substrate with a large number of carbon nanotubes as a large number of legs, so that the mechanical strength of the aggregate increases, and the tip grows from the root portion with the tip integrated. Continuing, it seems that an aggregate of carbon nanotubes whose tips are integrated into a pyramid shape is formed. A schematic diagram of the growth mechanism at this time is shown in FIG.

  In addition, the change in the shape of the growing carbon nanotubes is considered as follows according to the number of pulse arcs when depositing Co nanoparticles, and therefore the density of Co nanoparticles. If the density of the Co nanoparticles is too low, the distance between adjacent carbon nanotubes is too large and there is no interaction, so that it grows randomly and does not grow orderly and perpendicular to the substrate. On the other hand, if the density of the Co nanoparticles is too high, the fine particles themselves continuously become large lumps, and thus do not grow in an orderly manner with respect to the substrate. When the density of Co nanoparticles is appropriate, it is considered that adjacent carbon nanotubes complement each other in an interactive manner and grow orderly in parallel with each other perpendicular to the substrate. A schematic diagram of the growth mechanism at this time is shown in FIG.

  A collection of carbon nanotubes in the shape of pyramids with their tips gathered in the shape of cones, such as field emission electrodes, field emission electrode arrays, next-generation VLSI interstage wiring, planar wiring, fine capacitance, diodes, transistors, etc. It can also be applied to.

  The aggregate of carbon nanotubes of the present invention can be used, for example, as a field electron emission electrode, and can be used for a display and other electronic devices.

The block diagram which showed the apparatus for depositing a particulate catalyst. The block diagram which showed the principle of the arc gun of the apparatus. The table which showed the conditions in the case of depositing a buffer layer and a particulate catalyst. The AFM image of the substrate surface on which Co nanoparticles were deposited. The block diagram which showed the apparatus which grows a carbon nanotube. The measurement figure which showed the relationship with the deposition rate of the carbon nanotube with respect to the frequency | count of the pulse arc at the time of depositing Co nanoparticle. The SEM image which shows the surface structure of the carbon nanotube aggregate grown on the board | substrate in which the particulate catalyst was formed by 50 times of pulse arcs. The enlarged image of the SEM image of FIG. The SEM image which showed the side structure of the carbon nanotube aggregate grown on the board | substrate in which the particulate catalyst was formed by 50 times of pulse arcs. The SEM image which shows the surface structure of the carbon nanotube aggregate grown on the board | substrate which formed the particulate catalyst by 250 times of pulse arcs. The SEM image which showed the side structure of the carbon nanotube aggregate grown on the board | substrate in which the particulate catalyst was formed by 50 times of pulse arcs. The SEM image of the side surface of the carbon nanotube aggregate which the tip gathered in the shape of a pyramid. The SEM image which shows the surface structure of the carbon nanotube aggregate grown at 600 degreeC on the board | substrate in which the particulate catalyst was formed by 50 times of pulse arcs. The SEM image which shows the surface structure of the carbon nanotube aggregate grown for 5 minutes at 700 degreeC on the board | substrate which formed the particulate catalyst by 50 times of pulse arcs. A TEM image of the side surface of the carbon nanotube aggregate. The SEM image which shows the surface structure of the carbon nanotube aggregate grown on the board | substrate in which the particulate catalyst was formed by 30 times of pulse arcs. The SEM image which shows the surface structure of the carbon nanotube aggregate grown on the board | substrate in which the particulate catalyst was formed by 50 times of pulse arcs. The SEM image which shows the surface structure of the carbon nanotube aggregate grown on the board | substrate in which the particulate catalyst was formed by 70 times of pulse arcs. The SEM image which shows the surface structure of the carbon nanotube aggregate grown to the board | substrate which formed the particulate catalyst by 100 times of pulse arcs. The SEM image which shows the surface structure of the carbon nanotube aggregate grown on the board | substrate in which the particulate catalyst was formed by 200 times of pulse arcs. The block diagram which showed the apparatus for measuring the field electron emission effect from the front-end | tip of the pyramid-shaped carbon nanotube aggregate of a present Example. The measurement figure which showed the relationship between the electric field and emission current which were measured with the apparatus. The measurement figure which showed the relationship between the electric field and emission current which were measured with the apparatus. The measurement figure by the Raman spectroscopy of the pyramid-shaped carbon nanotube aggregate of a present Example. The schematic diagram explaining the principle in which the pyramid-shaped carbon nanotube aggregate of a present Example is formed. The schematic diagram which showed the relationship between the density of a particulate catalyst, and the property of the carbon nanotube formed.

Explanation of sign

DESCRIPTION OF SYMBOLS 10 ... Reaction chamber 11 ... Susceptor 12 ... Si substrate 13 ... Halogen lamp 15 ... Cathode 16 ... Insulator 17 ... Trigger electrode 18 ... Anode

Claims (12)

A carbon nanotube aggregate in which the tips of a plurality of carbon nanotubes are gathered in a cone shape. The carbon nanotube aggregate according to claim 1, wherein a portion of the carbon nanotube other than the tip is formed substantially perpendicular to the substrate. The aggregate of carbon nanotubes according to claim 1 or 2, wherein the carbon nanotubes are single-walled or double-walled. The carbon nanotube according to any one of claims 1 to 3, wherein the carbon nanotube is formed by a particulate catalyst having a particle diameter of 5 nm or less formed on the substrate. Aggregation. 4. The carbon nanotube according to claim 1, wherein the carbon nanotube is formed by a particulate catalyst having a particle diameter of 2 nm to 4 nm formed on the substrate. 5. Carbon nanotube aggregate. 6. The aggregate of carbon nanotubes according to claim 1, wherein the density of the particulate catalyst is 1 × 10 ¹² / cm ^{2 to} 5 × 10 ¹³ / cm ² . The aggregate of carbon nanotubes according to claim 1, wherein a diameter of the carbon nanotube is 2 nm or more and 5 nm or less. The aggregate of carbon nanotubes according to any one of claims 1 to 7, wherein the carbon nanotube has an average outer diameter of 4 nm and an average inner diameter of 3 nm. A particulate catalyst having a particle size of 5 nm or less is deposited on a substrate in a density range of 1 × 10 ¹² / cm ^{2 to} 5 × 10 ¹³ / cm ² , and then two or one layer of carbon is formed by plasma CVD. A method for producing an aggregate of carbon nanotubes, in which nanotubes are grown and the tips of a plurality of carbon nanotubes are aggregated into a cone shape. The method for producing a carbon nanotube aggregate according to claim 9, wherein the particulate catalyst is generated by pulsed arc plasma at a vacuum degree of 1 × 10 ⁻⁴ Torr or less. The method for producing a carbon nanotube aggregate according to claim 9, wherein the particulate catalyst is cobalt or a cobalt alloy. The gas phase density of the particulate catalyst formed by the pulsed arc plasma is measured by absorption spectroscopy, and from the measured gas phase density, the particulate catalyst deposited on the substrate is measured. The method for producing a carbon nanotube aggregate according to claim 10 or 11, wherein the number of pulses of the pulsed arc plasma is controlled so that the density becomes a desired value.