Title:
Carbon nanotube and method for producing same
Kind Code:
A1


Abstract:
Carbon nanotubes are provided which are made to grow on a plane, on surfaces of which particulates are supported. The particulates generated by particulate generator, after sizes of the particulates are selected by a particulate size selector, are sprayed, together with a carrier gas, from a nozzle, onto a carbon nanotube-mounted substrate in a particulate deposition chamber. When the particulates are sprayed, a Stokes number is controlled according to particulates being supported. This enables the particulates to be supported, in a desired dispersed state, on the carbon nanotubes made to grow in a vertically orientated manner.



Inventors:
Sato, Shintaro (Kawasaki, JP)
Application Number:
11/079532
Publication Date:
04/06/2006
Filing Date:
03/15/2005
Assignee:
FUJITSU LIMITED (Kawasaki, JP)
Primary Class:
International Classes:
D01F9/12
View Patent Images:



Primary Examiner:
SMITH, JENNIFER A
Attorney, Agent or Firm:
ARMSTRONG, KRATZ, QUINTOS, HANSON & BROOKS, LLP (1725 K STREET, NW, SUITE 1000, WASHINGTON, DC, 20006, US)
Claims:
What is claimed is:

1. A carbon nanotube supporting particulates, wherein said carbon nanotube is made to grow on a plane and said particulates are supported on a surface of said carbon nanotube.

2. The carbon nanotube according to claim 1, wherein said particulates adhere directly to said surface of said carbon nanotube.

3. The carbon nanotube according to claim 1, wherein two or more kinds of particulates are used as said particulates to be supported.

4. The carbon nanotube according to claim 1, wherein said particulates are a catalyst to be used for growth of another carbon nanotube.

5. The carbon nanotubes according to claim 1, wherein said particulates are a catalyst to be used in an electrode of a fuel cell.

6. The carbon nanotube according to claim 1, wherein another carbon nanotube is made to grow using one of said particulates being supported as a growth starting point.

7. A method for producing a carbon nanotube supporting particualtes, comprising the step of: spraying said particulates onto a carbon nanotube made to grow on a plane.

8. The method for producing the carbon nanotube, further comprising the step of: spraying said particulates, together with gas, onto said carbon nanotube.

9. The method for producing the carbon nanotube according to claim 7, wherein sizes of said particulates are selected before said particulates are sprayed onto said carbon nanotube.

10. The method for producing the carbon nanotube according to claim 7, wherein a distance between a spraying port to spray said particulates onto said carbon nanotube and said plane is within 10 times longer than an internal diameter of said spraying port.

11. The method for producing the carbon nanotube according to claim 7, wherein Stokes number of said particulates is controlled when said particulates are sprayed onto said carbon nanotube.

12. The method for producing the carbon nanotube according to claim 11, wherein said Stokes number is controlled to be 0.1 or less when said particulates are to be selectively supported on an end portion of said carbon nanotube.

13. The method for producing the carbon nanotube according to claim 11, wherein said Stokes number is controlled to be one or more when said particulates are to be supported entirely on a surface of said carbon nanotube.

14. The method for producing the carbon nanotube according to claim 7, wherein another carbon nanotube is made to grow using said particulates being supported as a growth starting point.

15. The method according to claim 14, wherein the number and size of said another carbon nanotube is controlled by the number and size of said particulates.

16. The method according to claim 14, wherein, after the growth of said another carbon nanotube, new particulates are sprayed.

17. The method according to claim 16, wherein a new carbon nanotube is made to grow using one of said particulates as a growth starting point.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefits of priority from the prior Japanese Patent Application No. 2004-294011, filed on Oct. 6, 2004, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a carbon nanotube and a method for manufacturing the same, and particularly to a carbon nanotube which supports particulates and to a method for manufacturing the carbon nanotube.

2. Description of the Related Art

Carbon nanotubes due to their unique machanical, thermal, and electrical properties, are being studied to widen their applicability in various fields at present. Also, attempts to modify the carbon nanotubes in a wide variety of forms and to add new functions to the carbon nanotubes are being made. One of the attempts is to make the carbon nanotubes support particulates. An example has been so far reported in which carbon nanotubes are doped with nitrogen and support metal particulates by using a chemical modification in liquid (see “Nano Letters,” Vol. 3, p. 275, 2003). Also, another example has been reported in which platinum particulates are made to be supported on cup-stack type carbon nanotubes by impregnating the cup-stack type carbon nanotubes with a platinum salt solution and by reducing in an atmosphere of high-temperature hydrogen (see “Nano Letters,” Vol. 3, p. 723, 2003). In this report in particular, mention is made of a possiblity that the carbon nanotubes supporting platinum can be used as a catalyst for an electrode of a fuel cell.

Thus, it is expected that, by making the carbon nanotubes support particultes, characteristics of the carbon nanotube are improved and new additional capabilities are added to the carbon nanotubes, as a result, making the carbon nanotubes be used in various fields including a fuel cell field.

Moreover, in recent years, carbon nanotubes are being produced which are formed by combining a plurality of carbon nanotubes; that is, branched carbon nanotubes called a carbon nanotube network or a carbon nanotube junction are also formed. Carbon nanotubes are conventionally used as a one-dimensional material. However, such branched carbon nanotubes are expected to widen their applicable fields as a three-dimensional material (see Japanese Unexamined Patent Publication No. 2004-18328, “Applied Physics Letters,” Vol. 79, p. 1879, 2001, and “Nature,” Vol. 402, p. 253, 1999).

Additionally, a graphite structure having organization being similar to branched carbon nanotubes are disclosed in Japanese Unexamined Patent Publication No. 2003-238123, which is obtained by forming a three-dimensional structure of amorphous carbon in which a catalyst metal such as iron is accumulated in a partial portion of the amorphous carbon by using a method of focused ion beam scanning in hydrocarbon gas and by treating the three-dimensional amorphous carbon thermally at temperatures of about 700° C. to 900° C.

Conventional technology of forming carbon nanotubes has some problems as shown below.

First, the conventional method for making carbon nanotubes support particulates has a problem. For example, when the carbon nanotubes are to support metal particulates in liquid after having been doped with nitrogen, the composition of the carbon nanotubes themselves changes due to the doping with nitrogen. The carbon nanotubes having such changed compositions can be used only for limited applications. Moreover, since it is thought that, when the above method is used, kinds of particulates that can be supported on the carbon nanotubes are limited, a problem in terms of general versatility remains unsolved accordingly.

Also, in the case of the method in which the platinum particulates are made to be supported on the above cup-stack carbon nanotubes by impregnating the cup-stack carbon natotubes with a platinum salt solution and by reducing in an atmosphere of high-temperature hydrogen, the carbon nanotubes that can be produced by the method is limited only to the cup-stack type carbon nanotubes. Moreover, in the above method, there is a risk that substances other than a substance that is desired to be deposited as particulates may reside on the carbon nanotubes.

Furthermore, since reactions in a liquid are used in these two methods, aggregation among the carbon nanotubes occurs, as a result, causing the produced carbon nanotubes to have a bundle form. It is difficult to separate the bundled carbon nanotubes into individual carbon nanotubes and, since this problem is combined with such problems as the change in compositions of the carbon nanotubes, adhesion of impurities to the carbon nanotubes described above, or the like occurs and, if the bundled carbon nanotubes are employed as-is, their application fields are very limited.

On the other hand, it is thought that, if the particulates are supported on carbon nanotubes being in an erected state which are obtained by a vertical orientation growth in a specified area on a plane of a substrate or the like, the carbon nanotubes can have their wide application fields and can be used in various fields. However, so far, a proper method for letting particulates be supported on carbon nanotubes made to grow on such a plane has not yet been available.

Moreover, the conventional method of forming branched carbon nanotubes has some problems. For example, a conventional method in which Y junction carbon nanotubes are formed by using an alumina fine hole as a template has been proposed. However, in this method, a structure of the Y junction carbon nanotubes is determined by the alumina template and controllability of the alumina template is not completely good.

Another conventional method in which cobalt is supported on magnesium oxide, from which Y junction carbon nanotubes are made to grow, has been also proposed. However, in this method, it is difficult to control the number of branches of the carbon nanotubes, sizes of the carbon nanotubes, or the like.

Also, in the method in which a graphite structure is formed by thermally treating amorphous carbon having accumulated catalyst metal, if only one structure is formed, the method is thought to be effective. However, in a case where many structures are to be formed, for example, over an entire surface of a substrate, enormous time is required for the formation of the graphite structure.

Thus, so far, no proper method of forming such branched carbon nanotubes as described above with excellent controllability and effectiveness has been available. Similarly, no proper method of forming the branched carbon nanotubes having erected shapes on a plane or the branched carbon nanotubes that support particulates with excellent controllability and effectiveness has been available.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention to provide a carbon nanotube that supports particulates in a state in which the carbon nanotube is erected on a plane and a method for producing the carbon nanotube.

According to one aspect of the present invention, there is provided a carbon nanotube supporting particulates, wherein the carbon nanotube is made to grow on a plane and the particulates are supported on a surface of the carbon nanotube.

According to another aspect of the present invention, there is provided a method for producing a carbon nanotube supporting particualtes, including the step of spraying the particulates on a carbon nanotube made to grow on a plane.

The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically and briefly a particulate deposition system.

FIG. 2 shows schematically flows of gas and trajectories of particulates in the vicinity of carbon nanotubes occurring when inertia of a particulate is small.

FIG. 3 shows schematically flows of gas and trajectories of particulates in the vicinity of the carbon nanotubes when inertia of the particulate is large.

FIG. 4 shows one example of a scanning electron microscopic photograph of particulate-supporting carbon nanotubes.

FIG. 5 shows schmematically and briefly a gas sensor using semiconductor-particulate-supporting carbon nanotubes.

FIG. 6 shows one example of a scanning electron microscopic photograph of branched carbon nanotubes formed by using the supported particulate as a catalyst.

FIG. 7 shows an example of configurations of a particulate deposition system.

FIG. 8 shows a schematic cross-sectional view of a DMA (Differential Mobility Analyzer).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described below with reference to the accompanying drawings, wherein like reference numerals refer to like elements throughout.

First, a method for letting particulates be supported on carbon nanotubes made to grow on a plane of a substrate or the like is described. Moreover, the growth of the carbon nanotubes on a plane can be achieved also by a known method, for example, a method in which catalyst metal such as cobalt is deposited on a plane and vertical-orientation growth of carbon nanotubes is made to occur using the deposited catalyst metal, by a CVD (Chemical Vapor Deposition) method and, its description is omitted accordingly.

FIG. 1 shows schematically and briefly a particulate deposition system. As shown in FIG. 1, a particulate deposition system 1 includes a particulate generator 2, a particulate size selector 3, and a particulate deposition unit 4. The particulate generator 2 generates particulates, for example, by laser abrasion of a target. The particulates generated by the particulate generator 2 is carried to the particulate size selector 3 by inert carrier gas. The particulate size selector 3 is provided with an aim of making uniform sizes of particulates generated by the particulate generator 2. The particulate size selector 3 may have a DMA to select sizes of particulates, for example, according to electric mobility of the particulates, an impactor being operated using inertia of particulates, or the like. The particulate deposition unit 4 is fitted with a particulate deposition chamber 6 to house a carbon nanotube-mounted substrate 5 in which vertical orientation growth of a plurality of carbon nanotubes 5a is made to occur and with a nozzle 7 facing the carbon nanotube-mounted substrate 5. Particulates selected by the particulate size selector 3 are sprayed onto the carbon nanotube-mounted substrate 5, together with the carrier gas, through the nozzle 7. An exhaust system is connected to the particulate deposition chamber 6 so that its internal pressure can be controlled. During the processing of letting particulates be deposited on the carbon nantotubes 5a, a pressure within the particulate deposition chamber 6 is controlled to be, for example, 10 Torr or less. Moreover, a pressure within the particulate generator 2, the particulate size selector 3, or the like may be either at a low level or at a normal level.

Moreover, the particulate deposition system 1 does not necessarily require the particulate size selector 3. However, in the particulate deposition system 1, particulates are sprayed from the nozzle 7 onto the carbon nanotubes 5a in a dispersed state by a mechanism corresponding to inertia of the particulates as described later and, therefore, in order to exercise good control on the dispersed state, it is preferable for the particulate deposition system 1 to be fitted with the particulate size selector 3.

In the case where particulates are made to be supported on the carbon nanotubes 5a by the particulate deposition system 1 having the above configurations, the dispersed state of the particulates on the carbon nanotube-mounted substrate 5 is controlled by a specified kind of a parameter.

FIGS. 2 and 3 are diagrams explaining a deposition mechanism of the particulates. FIG. 2 shows schematically flows of gas and trajectories of particulates in the vicinity of carbon nanotubes occurring when inertia of the particulates is small. FIG. 3 shows schematically flows of gas and trajectories of particulates in the vicinity of the carbon nanotubes occurring when inertia of the particulates is large. Dotted lines in FIGS. 2 and 3 show typical flows of gas occurring when particulates 8 are sprayed from the nozzle 7 serving as a spraying port on the carbon nanotube-mounted substrate 5. As shown in FIGS. 2 and 3, the gas ordinarily flows approximately along upper ends of the plurality of carbon nanotubes 5a. Solid lines in FIGS. 2 and 3 show a trajectories of particulates occurring when the particulates 8 contained in the gas sprayed from the nozzle 7 go off from the flow of the gas.

In the case where the inertia of the particulate 8 is small, as shown in FIG. 2, the particulates 8, when flows of gas curve, almost follow the gas flow. Therefore, in this circumstance, only the particulates 8 that go off from the flows of gas due to the Brownian motion are deposited on the carbon nanotubes 5a. As a result, such particulates 8 as described above are selectively supported on end portions of the carbon nanotubes 5a.

On the other hand, in the case where the inertia of the particulates 8 is large, the particulates 8, when the flows of gas curve, go off from portions of the curved flows. Thus, the particulates 8 that have gone off from the flows of gas enter from shallow portions to deep portions in clearances among the carbon nanotubes 5a and then come to cover wide regions of the carbon nanotubes 5a and finally are supported on entire surfaces of the exposed carbon nanotubes 5a.

A Stokes number “Stk” as a paremeter representing a magnitude of the inertia of the particulate expressed by a following equation (1) can be used (see Hinds, W. C., Aerosol Techonology, John Wiley & Sons, New York, 1982).
Stk=τU/(Dn/2) (1)
where “τ” denotes relaxation time of a particulate, “U” denotes a gas speed at a spraying port, and “Dn” denotes an internal diameter of the spraying port. The relaxation time is expressed by the following equation (2).
τ=ρpdp2Cc/18 μ (2)
where “ρp” denotes the density of a particulate, “dp” denotes the diameter (average particle diameter) of the particulate, “Cc” denotes the Cunningham correction coefficient, and “μ” denotes the viscosity coefficient of gas.

In general, it is said that, if the Stokes number “Stk” is about one or more, the inertia of the particulates in the system is sufficiently large and an effect caused by the inertia is not neglible, however, the concrete number differs depending on each system. For example, it is reported that, in an impactor which collects particulates on a substrate using the inertia, the Stokes number “Stk” of about 0.2 is a threshold value of the particulate (see above reference).

In the particulate deposition system 1, too, the Stokes number “Stk” serves as an important parameter. Devoted studies have come to reveal that, if the Stokes number “Stk” is 0.1 or less, the particulates 8 are supported mainly on end portions of the carbon nanotubes 5a and that, if the Stokes number “Stk” is one or more, the particulates 8 enter deeper regions in clearances among the carbon nanotubes 5a, which enables the particulates to be supported on the carbon nanotubes 5a in a highly dispersed state in deeper regions. Moreover, a distance “D” between an end of the nozzle 7 and the substrate 5b is an important parameter and this distance “D” is preferably within 10 times longer than the inner diameter of the nozzle 7. However, this is not applied to a case where the Stokes number “Stk” is very large, for example, to a case where the Stokes number “Stk” is 10 or more.

Additionally, in the particulate deposition system 1, a process of letting particulates be supported can be performed in a high vacuum. However, the parameter changes depending on whether the process of letting particulates be supported is performed in a low pressure of about 10 Torr or in a high vacuum of about 10−6 Torr. In such a high vacuum, the concept of the Stokes number itself can not be employed and, instead of this, kinetic energy of the particulates serves as an important parameter.

Moreover, the particulate deposition system 1 is so constructed that the gas flow rate to and the pump speed from the particulate deposition chamber 6 can be adjusted and the Stokes number “Stk” can be controlled in a manner to correspond to an internal diameter of the nozzle 7.

FIG. 4 shows one example of a scanning electron microscopic photograph of particulate supporting carbon nanotubes.

The condition of forming particulate-supported carbon nanotubes is described in detail later. When a process of letting particulates be supported on the carbon nanotube is performed by using the above particulate deposition system 1, as shown in FIG. 4, it is possible to let particulates be supported on surfaces of the carbon nanotubes in a highly dispersed state.

Thus, in the particulate supporting method described above, gas containing particulates is sprayed onto the carbon nanotubes made to grow on the substrate. As a result, by appropriately setting kinds and sizes of particulates, a shape of the spraying port, conditions of spraying gas, or the like, it is possible to let particulates be easily supported in a desired dispersed manner on the carbon nanotubes in an erected state on a plane.

Furthermore, in the above particulate supporting method, since particulates are carried by inert gas and are directly adhered to the carbon nanotubes, the problems of the change in the composition caused by doping of the carbon nanotubes, adherence of impurities that easily occurs when the particulates are supported in a state of liquid phase, or the like can be avoided, which enables formation of the excellent particulate supporting carbon nanotubes having stable entire compositions.

The particulate deposition system 1 can be used for particulates of various kinds or various sizes, and therefore the particulate supporting carbon nanotubes produced by using the particulate deposition system 1 can be used in a variety of applications. One of examples of such applications is a sensor that uses semiconductor particulates.

FIG. 5 shows schmematically and briefly a gas sensor using semiconductor particulate supporting carbon nanotubes. When a gas sensor 10 is formed by using semiconductor particulate supporting carbon nanotubes 11 as shown in FIG. 5, bridge growth of carbon nanotubes 11a is first made to occur between two electrodes 12a and 12b and then semiconductor particulates 11b are made to be supported on the carbon nanotube 11a by using the above particulate deposition system 1.

In the gas sensor 10 having such the configurations as above, when the semiconductor particulates 11b adsorb a gas molecule, for example, an oxygen molecule 13, an electron moves from the semiconductor particulate 11b to a side of the oxygen molecule 13 and, as a result, conductance of the semiconductor particulate supporting carbon nanotubes 11 changes. Therefore, by providing a circuit that can measure conductance between the two electrodes 12a and 12b, it is possible to measure the amount of the oxygen molecule 13.

Another example of applications of the particulate supporting carbon nanotubes is a catalyst for an electrode of a fuel cell. At present, in general, a catalyst being used for an electrode of the fuel cell is platinum particulates dispersed in carbon black. However, the platinum particualtes have a problem. That is, aggregation of platinum particulates occurs at time of formation of a catalyst, of an operation of a cell, or the like in some cases, which causes no full use of a function of the platinum particulates to be made. To solve such a problem, by using, as a supporter, carbon nanotubes instead of carbon black and by having platinum particulates be supported in a highly dispersed manner using the particulate deposition system 1 described above so that the shape shown, for example, in FIG. 4 is obtained, the high-quality catalyst for an electrode of a fuel cell is realized.

Moreover, in the case of the particulate supporting carbon nanotubes which are formed by using the above particulate deposition system 1, by using the particulate being supported as a catalyst to make another carbon nanotube grow, branched carbon nanotubes can be formed.

FIG. 6 shows one example of a scanning electron microscopic photograph of branched carbon nanotubes formed by using the supported particulates as a catalyst. The condition of forming the branched carbon nanotubes is described in detail later. The carbon nanotubes shown in FIG. 6 are obtained by having iron particulates be supported on carbon nanotubes that have grown on a plane using the particulate deposition system 1 and then by making branches of the carbon nanotubes grow using the iron particulates as a growth starting point by a CVD method. In the above method, a place where branches of the carbon nanotubes grow, the number of the branches, and sizes of the branches can be controlled by a place of the iron particulates supported by the carbon nanotubes, the number of the supported iron particulates, sizes of the supported iron particulates. Also, a length of a branch of the carbon nanotube can be controlled by the growth time. As a result, according to the method, it can be said that controllability required for the formation of the branched carbon nanotubes is greatly improved when compared with the conventional method.

Such branched carbon nanotubes as described above can find various applications in electrical wirings, sensors, or the like. In particular, by configuring a sensor so that particulates are further supported on the branched carbon nanotubes, a surface area of a sensing portion is increased and, therefore, a detecting sensitivity can be improved more.

Hereinafter, a specified example of the method for forming the particulate supporting carbon nanotubes is described. FIG. 7 shows an example of configurations of a particulate deposition system. A particulate deposition system 20 shown in FIG. 7 includes a particulate generating chamber 22 to house a target 21 and a laser radiator 23 to radiate the target 21 with a laser. The particulate generating chamber 22 is fitted with a mechanism to guide a specified amount of a flow of helium (He) gas, to which a supplying tube 25 to feed the guided helium gas, together with particulates generated internally, is connected. In an outer portion surrounding the supplying tube 25 is provided an electric furnace 26 operated so as to suppress the aggregation of particulates, in the supplying tube 25, to be fed from the particulate generating chamber 22 to a DMA 24. The DMA 24 guides the gas containing the particulates fed from the supplying tube 25 into the inside of the DMA 24 to select the sizes of the particulates and feeds the gas containing the particulates obtained after the size selection to the side of a particulate deposition chamber 27.

FIG. 8 shows a schematical cross-sectional view of the DMA 24. The DMA 24, as shown in FIG. 8, has a double-cylinder structure made up of an outer tube 24a and an inner tube 24b. A sheath gas Qs is made to flow in a laminar flow state into the inside of the outer tube 24a. Gas Qp containing particulates is guided from the supplying tube 25 through the outer surrounding portion. Then, when a DC (Direct Current) voltage is applied between the outer tube 24a and inner tube 24b, particulates 24c in the gas Qp flows downward, while being drawn toward the inner tube 24b by the Coulomb force, with a flow of the sheath gas Qs. Here, a speed at which the particulates 24c cross a flow of the sheath gas Qs is determined by a balance between the Coulomb force and a resistive force that the particulates 24c receive from the Sheath gas Qs and large particulates 24c cross the flow slowly and small particulates 24c cross the flow rapidly. Thus, since a position in which the particulates 24c reach the inner tube 24b differs depending on sizes of the particulates 24c, by providing a slit 24d in the inner tube 24b and taking out only the particulates 24c that have reached the slit 24d, the particulates 24c having a specified size can be selected.

As shown in FIG. 7, the gas containing particulates obtained after having passed through the DMA 24 and having been selected in the DMA 24 is guided through the nozzle 28 into the particulate deposition chamber 27. In the particulate deposition chamber 27, a carbon nanotube-mounted substrate 30 is provided on the stage 29 in a manner to face the nozzle 28 and is so constructed that the gas containing particulates obtained after having been selected in the DMA 24 is sprayed onto the carbon nanotube-mounted substrate 30 from the nozzle 28. Furthermore, to the particulate deposition chamber 27 is connected a pump 31, which enables an internal pressure of the particulate supporting chamber 27 to be controlled.

In the embodiment, the particulate deposition system 20 described as above was operated as below. First, a cobalt substrate was mounted as the target 21 in the particulate generating chamber 22 and an internal pressure was controlled to be about 10 Torr and then, by using the laser radiating device, the cobalt substratre was radiated with a pulsed laser; that is, in the embodiment, a second harmonic Nd:YAG laser having its repetition frequency of 20 Hz. The cobalt substrate was heated by the radiation of the laser and vapor was produced. The vapor was cooled by the helium gas guided, at a flow rate of about one standard liter per minute (slm), into the particulate generating chamber 22 and cobalt particulates were produced by nuclear condensation occurring at the time. The particulates produced as above were fed, together with helium gas, to the DMA 24 through the supplying tube 25 to select the particulates so that their average particulate diameters were about 4 nm. Then, the gas containing particulates obtained after the selection of particulate sizes was guided into the nozzle 28 having an internal diameter of 2 mm and, from the place, the gas was guided into the particulate deposition chamber 27. The particulate deposition chamber 27, before the gas containing the particulates is guided, was put into a state where the carbon nanotube-mounted substrate 30 obtained by making carbon nanotubes 30a grow in an orientated state on a substrate 30b in a direction being approximately parallel to an axial direction of the nozzle 28 was arranged so that a distance between an end of the nozzle 28 and the substrate 30b was about 4 mm and the internal pressure in the particulate supporting chamber 27 was controlled to be about 10 Torr.

Under this condition, the Stokes number “Stk” of the particulate being sprayed from the nozzole 28 was about 1.9. The particulates guided into the particulate deposition chamber 27 were supported 100% on the carbon nanotubes 30a. The helium gas was exhaused by the pump 31 out of the particulate deposition chamber 27. The particulate supported carbon nanotubes formed at this time point is shown in FIG. 4 and, by employing the particulate supporting method, particulates could be supported in a highly dispersed state on surfaces of independent carbon nanotubes 30a.

Next, a method for forming branched carbon nanotubes is described by using a concrete example.

First, by using the same method as described above, iron particulates having an average particulate diameter of about 4 nm were produced and made to be supported in a highly dispersed state on surfaces of carbon nanotubes which were made to grow on a substrate in an orientated state and in an approximately vertical direction.

Next, the carbon nanotube-mounted substrate on which particulates were supported by the carbon nanotubes was carried into the CVD chamber and carbon nanotubes were made to grow by the CVD method using the particulates being supported on the carbon nanotubes as a growth starting point. Here, the carbon nanotubes were made to grow at a substrate temperature of 670° C. and at a pressure of 100 Pa and for growth time of 10 minutes by a hot-filament CVD method using a flow rate of about 10 standard cubic centimeter per minute (sccm) of a mixture (1:9) of acetylene and argon as the source gas.

The branched carbon nanotubes formed by this method are shown in FIG. 6 and, by using the deposition method, branches of carbon nanotubes can be made to grow using, as a growth starting point, iron particulates serving as a catalyst. Moreover, it is possible to let the iron particulates be supported further on the carbon nanotube formed as above and to make the branches of the carbon nanotubes grow more.

The above method of forming the branched carbon nanotubes described above has big advantages over a conventional method of forming the branched carbon nanotubes in that branches of the carbon nanotubes can be made to grow using the particulates as a growth starting point, in that a large amount of branched carbon nanotubes can be formed in a short time, and in that variations in quality can be suppressed even in the repetitive formation.

Moreover, in the above embodiment, one kind of particulate to be supported on a carbon nanotube is used. However, two or more kinds of the particulates may be also employed. Also, in the above embodiment, a nozzle is used as a spraying port to be used when the particulates are sprayed onto the carbon nanotubes, however, a tube, orifice, or the like can be employed.

According to the present invention, particulates are supported on surfaces of carbon nanotubes made to grow on a plane. By this, non-bundle shaped particulate supporting carbon nanotubes with high quality can be realized in a stable manner and can be applied to various applications including electric wirings, sensors, fuel cells, or the like.

The foregoing is considered as illustrative only of the principles of the present invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and applications shown and described, and accordingly, all suitable modifications and equivalents may be regarded as falling within the scope of the invention in the appended claims and their equivalents.