Patent Application
20100171409
The present invention relates to carbon nanotubes which are densely aligned and highly oriented, a substrate and an electron emission device with such carbon nanotubes, and a substrate on which to synthesize the carbon nanotubes as well as methods of and apparatus for making them.
In a conventional micro fabricated electron emission device, metals, silicon and their compounds as for materials of the device have been used. Such a material on its uppermost surface as the electron emission face becomes easily oxidized in air, forming an oxide in which work function is high and has made the electron emission device with high work function. Also, an oxide such as of metal or silicon by its insulating nature is hard to emit electrons.
On the other hand, if a nanocarbon material such as carbon nanotubes having a size in the order of nanometers (nm) is utilized, there can be no surface oxidation, avoiding a rise in work function. Further, the nanocarbon material has a high conductivity and also has a high thermal conductivity. Therefore, it can readily emit electrons and its failure by Joule heat can be prevented.
By the way, many attempts have been made to use nanocarbon material as the material for electron emission devices. As is well known, however, many conventional nanocarbon materials made by utilizing plasma contain a lot of impurity components such as amorphous substances and there has been no method that can reproducibly make highly aligned carbon nanotubes especially suitable to emit electrons uniformly.
Since carbon nanotubes have an extremely small radius of curvature at their apical ends and are a material extremely stable mechanically and chemically for use in electron emission device, use in an electrode of a double layer capacitor as they have graphite edges at high density and use in an electrode of a fuel cell as they can adsorb and emit ions at high density, investigations have been advanced to put carbon nanotubes to practical use.
By the way, if used in an electron emission device, an double layer capacitance electrode or a fuel cell electrode, carbon nanotubes are used in a bundle in which they are aligned unidirectionally and at high density and namely as high-density and highly oriented carbon nanotubes. It is evident that their performances are enhanced as their density and orientation are increased. To with, the higher the density and orientation, the higher the electron emission efficiency per unit area for the electron emission device and the larger the output of power storage or generation per unit area for the double layer capacitor or fuel cell electrode. Consequently, densely aligned and highly oriented carbon nanotubes that are much higher in density and orientation are being needed.
Since carbon nanotubes are a nano material having a radius in the order of nanometers, it is impossible to synthesize carbon nanotubes randomly aligned and then to mechanically orient the individual carbon nanotubes and align them densely. Accordingly, in making densely aligned and highly oriented carbon nanotubes, a synthesizing method that carbon nanotubes are automatically aligned onto the substrate has been used. As one of such methods, there is known a method of synthesizing densely aligned and highly oriented carbon nanotubes by heating a substrate carrying a metal catalyst in an organic liquid (See Patent Reference 1.). This method, which is based on a peculiar interfacial cracking reaction that occurs when a solid substrate and an organic liquid contact having a high temperature difference, is called solid-liquid interfacial contact cracking in organic liquid. This method, which is based on a peculiar interfacial cracking reaction that occurs when a solid substrate and an organic liquid contact each other having a high temperature difference, is called solid-liquid interfacial contact cracking in organic liquid. According to this method, the highly pure carbon nanotubes can be synthesized without forming an impurity such as amorphous carbon as in arc discharge and chemical vapor deposition methods (See Non-patent Reference 1.).
The method according to Patent Reference 1 is mentioned referring to
To perform the solid-liquid interfacial contact cracking in the organic liquid with the synthesis apparatus above, a thin film of metal catalyst of a transition metal such as Fe, Co or Ni is layered on a silicon or diamond substrate and the substrate is exposed to hydrogen plasma to cause the substrate to carry fine particles of the metal catalyst densely distributed thereon. This substrate 33 is held by the substrate holder 35 and an electrical current is flown through the substrate 33 via the substrate holder 35 to heat the substrate 33. This causes carbon nanotubes to be synthesized and built on the fine particles of metal catalyst by a peculiar interfacial cracking reaction brought about when the substrate 33 and the organic liquid 30 come into contact with each other having a severe temperature difference. According to this method, carbon nanotubes which are highly oriented perpendicular to the substrate and densely aligned can be synthesized wherein making the thin film of metal catalyst thinner allows the fine particles of metal catalyst to be finer in particle size and more densely distributed. As shown in
Patent Reference 1: Japanese patent laid open application, JP 2003-12312 A
Patent Reference 2: Japanese patent laid open application, JP 2004-327085 A
Non-patent Reference 1: National Institute of Advanced Industrial Science and Technology, Research Center for Nanocarbon editing: Corporation's Series Nanocarbon Materials “Dream Expanding New Materials” (in Japanese), May 25, 2004, published by Maruzen Co., Ltd. pp. 155-157, p. 198, and Table 2
However, the conventional solid-liquid interfacial contact cracking method fails to produce carbon nanotubes which are sufficient in their crystal quality and with good reproducibility on their orientation. For this reason, it has the problem that it fails to realize an electron emission device which makes the best use of the excellent properties peculiar to carbon nanotubes.
Specifically, this method under identical conditions for manufacture such as thickness of a metal catalyst thin film, hydrogen plasma conditions, rate of temperature rise and heating temperature of a substrate in an organic liquid does not necessarily result in the synthesis of oriented and densely aligned carbon nanotubes which are identical in density and orientation, but those which are thicker and thinner than about 20 nm in radius and at a density lower than about 300 tubes/μm. In fact, it is only at one of ten times that highly evenly oriented and aligned carbon nanotubes with a desired radius and at a density are obtained, posing the problem that reproducibility is poor.
The cause for poor reproducibility is considered to be due to such as the fact that when a substrate on which a thin film of metal catalyst of a transition metal such as Fe, Co or Ni is applied is exposed to hydrogen plasma, forming fine particles of metal catalyst on the substrate so that they are densely distributed thereon or when the temperature of the substrate is rising to heat it in an organic liquid, fine particles of metal catalyst agglomerate together forming larger fine particles or in the case of a silicon substrate, fine particles of transition metal form silicon and silicide and become sunken in the silicon substrate or in the case of a diamond substrate, fine particles of transition metal become sunken in diamond lattices, which in turn makes fine particles of metal catalyst uneven in radius and distribution.
Also, whether or not fine particles of metal catalyst agglomerate together and whether or not fine particles of metal catalyst become sunken are considered to depend on subtle surface state differences of the silicon substrate or diamond substrate which are not controllable at all under the existing state of technology.
Since a carbon nanotube consists of Graphene sheets made up of sp2 hybridized orbital of carbon, it has a mechanical strength and conductivity which exceed those of diamond. If there exists lattice defects such as disorders in carbon arrangement or carbon vacancies in Graphene sheet, namely if the crystal quality is insufficient in Graphene sheet, it is not possible to exhibit the excellent mechanical property and electric conductivity of carbon nanotubes sufficiently.
By the way, while the solid-liquid contact cracking method is capable of synthesizing carbon nanotubes which is densely aligned and highly oriented and high in purity, there has been the problem that carbon nanotubes made thereby are insufficient in their crystal quality.
Also, the solid and liquid interfacial contact cracking method under identical conditions for manufacture such as thickness of a metal catalyst thin film, hydrogen plasma conditions, rate of temperature rise and heating temperature of a substrate in an organic liquid does not necessarily result in the synthesis of oriented and densely aligned carbon nanotubes which are identical in density and orientation, thereby carbon nanotubes have non-uniform alignment once in several manufactures, thus posing the problem that its reproducibility is poor.
The present inventors have found out that if in place of a conventional substrate on which a thin film of metal catalyst of a transition metal such as Fe, Co or Ni is applied and which is then exposed to hydrogen plasma to form fine particles of metal catalyst thereon, a substrate is used on which a Co metal thin film is deposited and which is then thermally oxidized to form ultra-fine particles of cobalt oxide thereon, carbon nanotubes can reproducibly be made which are uniformly oriented highly and aligned densely having a desired radius and density and which are about three times as dense as carbon nanotubes obtainable conventionally. Further, as a result of their zealous investigations on the cause for poor reproducibility in orientation, the present inventors have discovered that it is due to the thermal stress that develops when the synthesis reaction is terminated and have arrived at the present invention.
With the abovementioned problems taken into account, it is a first object of the present invention to provide densely aligned and highly oriented carbon nanotubes. A second object of the present invention is to provide a substrate with densely aligned and highly oriented carbon nanotubes. A third object of the present invention is to provide an electron emission device using a substrate with densely aligned and highly oriented carbon nanotubes. A fourth object of the present invention is to provide a carbon nanotube synthesizing substrate for making densely aligned and highly oriented carbon nanotubes. A fifth object of the present invention is to provide a method of making a carbon nanotube synthesizing substrate for making densely aligned and highly oriented carbon nanotubes. A sixth object of the present invention is to provide a method of making densely aligned and highly oriented carbon nanotubes. A seventh object of the present invention is to provide a method of making an electron emission device as mentioned above. An eighth object of the present invention is to provide an apparatus for making densely aligned and highly oriented carbon nanotubes.
In order to achieve the first object mentioned above, there is provided in accordance with the present invention a carbon nanotube characterized in that it has a highest Raman scattering intensity in the vicinity of 1580 cm−1 in its Raman spectrum. The Raman scattering intensity in the vicinity of 1580 cm−1 is preferably two times or more as high as that in the vicinity of 1350 cm−1.
In order to achieve the second object mentioned above, the present invention provides a substrate with carbon nanotubes, characterized in that it comprises: a conductive substrate; fine particles of catalyst formed on a surface of the conductive substrate and a carbon nanotube grown on the fine particles of catalyst and oriented perpendicular to the surface of said conductive substrate, wherein the fine particles of catalyst consist of an ultra-fine particle of cobalt oxide catalyst.
The conductive substrate mentioned above is preferably a conductive silicon substrate or a conductive diamond substrate. The conductive substrate may comprise a substrate and a conductive film deposited onto said substrate wherein the substrate is a glass substrate, a semiconductor substrate, a metal substrate or an oxide substrate and said conductive film consists of any one of a silicon film or a diamond film. The diamond film preferably consists of any one of polycrystalline diamond, nanocrystalline diamond, diamond-like carbon and monocrystalline diamond.
In their Raman spectrum, the carbon nanotubes preferably have a highest Raman scattering intensity in the vicinity of 1580 cm−1. The Raman scattering intensity in the vicinity of 1580 cm−1 is preferably two times or more as high as that in the vicinity of 1350 cm−1.
According to the structure mentioned above, the substrate with carbon nanotubes which are high in crystal quality and are densely aligned and highly oriented can be obtained.
In order to achieve the third object mentioned above, the present invention provides an electron emission device, characterized in that it emits electrons by applying a voltage to apical ends of the carbon nanotubes in a substrate with carbon nanotubes as mentioned above.
According to this structure, the electron emission device in which carbon nanotubes are high in crystal quality and are densely aligned and highly oriented can reduce its driving voltage and have a current value obtained that is sufficient to emit a fluorescent material on the market for slow electrons. Moreover, the electron emission device needs no gate and can thus simplify the structure and reduce the cost of a surface light-emitting device for which the device is used. Furthermore, the carbon material remains unoxidized even in a low vacuum and thus allows realizing a light emitting device that can be paneled more easily, and is longer in operating life than those using a conventional electron emission device.
In order to achieve the fourth object mentioned above, the present invention provides a densely aligned and highly oriented carbon nanotube synthesizing substrate, characterized in that it comprises a substrate and ultra-fine particles of cobalt oxide formed on the substrate.
Said ultra-fine particles of cobalt oxide preferably have a degree of oxidation such that the binding energy of a core electron in cobalt shifts positively in a range of 1 eV to 3 eV. The substrate preferably is a silicon substrate or a diamond substrate. Also, the substrate may comprise a glass substrate, a semiconductor substrate, a metal substrate or an oxide substrate and a silicon film or a diamond film formed on the substrate.
The substrate of this structure to synthesize carbon nanotubes allows densely aligned and highly oriented carbon nanotubes to be synthesized reproducibly, at a density of about 1000 tubes per μm2 and in orientation normal to the substrate. Also, especially if a diamond substrate or a substrate with a diamond film is used, then between carbon atoms in an area of junction between a carbon nanotube and the substrate there is formed an orbital hybridized of sp2 hybridized orbital of carbon nanotube and spa hybridized orbital of diamond, thereby serving to firmly bond the carbon nanotube and the substrate together. This substrate is optimum for use in an application where a high strength of bond of carbon nanotube to a substrate is required.
In order to achieve the fifth object mentioned above, the present invention provides a method of making a densely aligned and highly oriented carbon nanotube synthesizing substrate, characterized in that it comprises: depositing a cobalt thin film on a silicon or diamond substrate, or depositing a cobalt thin film on a substrate which comprises a silicon film or a diamond film formed on a glass substrate, a semiconductor substrate, a metal substrate or an oxide substrate; and thermally oxidizing the deposited cobalt thin film to a degree of oxidation such that the binding energy of a core electron of cobalt in the deposited cobalt thin film shifts positively in a range of 1 eV to 3 eV.
According to this method, ultra-fine particles of cobalt oxide of substantially an identical particle size can be formed on a silicon or diamond substrate or on a silicon or diamond film layered on a glass, semiconductor, metal or oxide substrate and moreover so that they are substantially uniformly distributed over the substrate. Also, the ultra-fine particles of oxide can keep their form and shape of distribution when carbon nanotubes are synthesized and further has a catalytic activity in synthesizing carbon nanotubes, thereby making it possible for densely aligned and highly oriented carbon nanotubes to be synthesized on the ultra-fine carbon oxide particles.
Preferably, the cobalt thin film is deposited to a film thickness of not more than 10 nm and the thermal oxidation is effected in air at a temperature in a range of 800° C. to 1000° C. for a time period in a range between 5 minutes and 20 minutes.
If the shift of a core electron of cobalt in binding energy is less than 1 eV, variations in radius and distribution of carbon nanotubes are then liable to occur, preventing reproducible synthesis of densely aligned and highly oriented carbon nanotubes. Conversely, if the shift of a core electron of cobalt in binding energy is more than 3 eV, then the ultra-fine particles of cobalt oxide lose their catalytic activity so that carbon nanotubes may not grow.
In order to achieve the sixth object mentioned above, the present invention provides a method of synthesizing densely aligned and highly oriented carbon nanotubes, characterized in that it comprises: maintaining a carbon nanotube synthesizing substrate at a temperature ranging from 900° C. to 400° C.; and synthesizing carbon nanotubes on the substrate.
If the temperature at synthesis exceeds 900° C., then the radius and shape of distribution of ultra-fine cobalt oxide particles change so that it becomes for densely aligned and highly oriented carbon nanotubes which are uniform in radius and density not to be synthesized reproducibly. If it is 400° C. or less, carbon nanotubes will no longer grow.
The carbon nanotubes may be synthesized by any one of thermal chemical vapor deposition, plasma chemical vapor deposition and solid-liquid interfacial contact cracking in organic liquid which allow manufacturing at low cost and make it possible to synthesize carbon nanotubes in high density and at high orientation.
The present invention also provides a second method of synthesizing carbon nanotubes by solid-liquid interfacial contact cracking wherein a substrate carrying a catalyst composed of a transition metal or a catalyst composed of a transition metal oxide is heated in an organic liquid of alcohol to synthesize carbon nanotubes, characterized in that it comprises: a first synthesis process in which they are synthesized on the substrate at a low substrate temperature; and a second synthesis process split from the first synthesis process in which they are synthesized on the substrate at a high substrate temperature.
According to this method, in the first synthesis process at the low temperature, carbon nanotubes oriented normal to the substrate and insufficient in crystal quality are grown. In the second synthesis process at the high temperature, carbon nanotubes having excellent crystal quality are grown continuously on carbon nanotubes grown in the first synthesis process.
Making the synthesis time period of the first synthesis process short and making the synthesis time period of the second synthesis process long allows synthesizing carbon nanotubes most part of which is accounted for by those excellent in crystal quality.
The first synthesis process is preferably performed at a substrate temperature of 500° C. to 700° C. The second synthesis process is preferably performed at a substrate temperature of 750° C. to 1100° C. If the first synthesis process is carried out for a synthesis time period of not less than 10 seconds, then carbon nanotubes that are excellent in crystal quality are grown continuously on carbon nanotubes grown in the first synthesis process.
The present invention provides a third method of synthesizing carbon nanotubes by solid-liquid interfacial contact cracking wherein a substrate carrying a catalyst composed of a transition metal or a transition metal oxide is heated in an organic liquid to synthesize a carbon nanotube on the substrate, characterized in that it comprises a step of slowly cooling the substrate, thereby terminating a reaction to synthesize.
The slow cooling step is preferably a step in which in terminating the synthetic reaction, the energy for heating the substrate is switched off and in a predetermined time period thereafter the substrate is removed from the organic liquid into a gaseous atmosphere and is then held in the gaseous atmosphere. Alternatively, the slow cooling step may comprise a step in which in terminating the synthetic reaction, the substrate is removed from the organic liquid into a gaseous atmosphere and in a predetermined time period thereafter the energy for heating the substrate is switched off and then the substrate is maintained in the gaseous atmosphere.
If the organic liquid is alcohol, the predetermined time period is then preferably not more than 10 seconds. Also, the slow cooling step may be a step in which in terminating the synthetic reaction, the substrate is removed from the organic liquid into a gaseous atmosphere and in a predetermined time period thereafter, the energy for heating the substrate is decreased gradually and then switched off.
The method mentioned above operates as follows. In synthesizing carbon nanotubes by solid-liquid interfacial contact cracking for supplying the substrate with energy to heat it by such as electrical heating, the substrate is maintained at a high temperature of 500 to 1000° C. to hold carbon nanotubes grown on the substrate at a high temperature close to the substrate temperature. On the other hand, the organic liquid surrounding the substrate is held at a low temperature of not more than its boiling temperature (e.g., 64.7° C. if the organic liquid is methanol).
In terminating the synthetic reaction in the conventional solid-liquid interfacial contact cracking method, it has been the practice to terminate the energy to heat the substrate by way of terminating the substrate heating current while the substrate is held in the organic liquid and to remove the substrate from the organic liquid after the substrate is sufficiently cooled in the organic liquid. Since the substrate and carbon nanotubes growing on the substrate are in contact with the organic liquid high in thermal conductivity, terminating the energy to heat the substrate rapidly cools the substrate and the carbon nanotubes growing on the substrate from a high temperature of 500 to 1000° C. to the boiling point of the organic liquid. Also, since the substrate (e.g., silicon substrate) and the carbon nanotubes vary in thermal conductivity or heat capacity to the organic liquid, a large temperature difference develops between the substrate and the carbon nanotubes while they are cooled and a large difference in heat shrinkage is created between the substrate (e.g., silicon substrate). Between the substrate and the carbon nanotubes, there develops a thermal stress that is sufficient to reorient the carbon nanotubes grown normal to the substrate. This is considered to be the reason why the conventional solid-liquid interfacial contact cracking method was low in reproducibility on orientation.
To meet with this problem, the present invention in the method cuts off the energy to heat the substrate, quickly removes the substrate from the organic liquid low in temperature and high in thermal conductivity and then holds the substrate in a gas low in thermal conductivity. Consequently, the substrate and the carbon nanotubes are placed in a state close to their thermal equilibrium state in which no large temperature difference develops between the substrate and the carbon nanotubes while they are cooled and no large difference in heat shrinkage is created between the substrate and the carbon nanotubes. Thus, no such thermal stress develops as to reorient the carbon nanotubes grown normal to the substrate. Hence, there becomes higher the reproducibility on orientation. By removing the substrate from the organic liquid low in temperature and high in thermal conductivity and thereafter cutting off the energy to heat the substrate and slowly cooling the substrate, or by removing the substrate from the organic liquid low in temperature and high in thermal conductivity and thereafter gradually decreasing the energy to heat the substrate to cool the substrate, the effect is likewise brought about and the reproducibility on orientation becomes higher.
The densely aligned and highly oriented carbon nanotubes of the present invention may consist of densely aligned and highly oriented carbon nanotubes synthesized by any one of the synthesis methods described above on a substrate as described above.
Since these densely aligned and highly oriented carbon nanotubes of the present invention are constituted by a low cost substrate and carbon nanotubes oriented normal to the substrate and aligned at an unprecedentedly high density and synthesized by a low cost synthesis method, they are capable of improving at low cost the performance of a device such as an electron emission device, an electrode of a double layer capacitor or an electrode of a fuel cell using densely aligned and highly oriented carbon nanotubes.
Also, while carbon nanotubes can be applied to not only such devices as mentioned above but varieties of technical fields, densely aligned and highly oriented carbon nanotubes of the present invention which can be manufactured at a low cost and has a form commonly used in various technical fields. Namely, they are aligned densely perpendicular to and firmly bonded to their substrate, can serve in varieties of applied researches and commercialization studies requiring them, and can thus contribute to promoting a variety of such researches and studies.
In order to achieve the seventh object mentioned above, the present invention also provides a method of making an electron emission device, characterized in that it comprises: a first step of forming a cobalt thin film on a conductive substrate and thermally oxidizing the cobalt thin film to form ultra-fine particles of cobalt oxide on said conductive substrate for use as a carbon nanotube synthesizing substrate; a second step of synthesizing carbon nanotubes on said carbon nanotube synthesizing substrate resulting from the first step, by solid-liquid interfacial contact cracking in an organic liquid of alcohol in which they are synthesized at a low substrate temperature in a first synthesis process and at a high substrate temperature in a second synthesis process; and a third step of slowly cooling the carbon nanotube synthesizing substrate when the second step is terminated, whereby an electron emission device is made.
The first step preferably comprises forming the cobalt thin film onto the conductive substrate having a film thickness of not more than 10 nm and thermally oxidizing said cobalt thin film in air at a temperature ranging between 800° C. and 1000° C. for a time period ranging between 5 minutes and 20 minutes. It is preferred that the first synthesis process be performed at a substrate temperature of 500° C. to 700° C. and the second synthesis process be performed at a substrate temperature of 750° C. to 1100° C. The first synthesis process is preferably carried out for a synthesis time period of not less than 0 seconds but not more than 3 minutes.
The third step comprises preferably a step in which in terminating the second step, the energy for heating the carbon nanotube synthesizing substrate is switched off and in a predetermined time period thereafter the substrate is removed from the organic liquid into a gaseous atmosphere and is then held in the gaseous atmosphere. The organic liquid may comprise alcohol and the predetermined time period is then preferably not more than 10 seconds.
The third step may comprise a step in which in terminating the second step, the substrate is removed from the organic liquid into a gaseous atmosphere and in a predetermined time period thereafter the energy for heating the carbon nanotube synthesizing substrate is switched off and then the substrate is held in the gaseous atmosphere. The organic liquid may comprise alcohol and the predetermined time period is then preferably not more than 10 seconds.
The third step may alternatively comprise a step in which in terminating the second step, the carbon nanotube synthesizing substrate is removed from the organic liquid into a gaseous atmosphere and in a predetermined time period thereafter the energy for heating the substrate is gradually decreased and switched off.
An electron emission device of the present invention is characterized in that it uses carbon nanotubes synthesized according to a method of the present invention. Since the crystal quality of carbon nanotubes are higher than those of the prior art, the electron emission efficiency of this electron emission device is extremely higher than that of the prior art.
According to the method configured as mentioned above, the electron emission device comprising carbon nanotubes with high purity can be manufactured using solid-liquid interfacial contact cracking method. Also, according to the method, the electron emission device comprising carbon nanotubes, which are densely aligned and highly oriented and have high crystal quality, can be manufactured reproducibly at an extremely high yield.
In order to achieve the eighth object mentioned above, the present invention further provides an apparatus for synthesizing carbon nanotubes by solid-liquid interfacial contact cracking synthesis wherein a substrate carrying a catalyst composed of a transition metal or a transition metal oxide is heated in an organic liquid to synthesize carbon nanotubes onto said substrate, characterized in that it comprises a means for removing the substrate from the organic liquid into a gaseous atmosphere in a predetermined time period. According to this apparatus, since the substrate on which carbon nanotubes are grown can be quickly removed from the organic liquid low in temperature and high in thermal conductivity, it is possible to reproducibly synthesize carbon nanotubes oriented normal to the substrate.
According to carbon nanotubes and their synthesizing methods of the present invention, carbon nanotubes having an excellent crystal quality, that there are little lattice defects such as disorders in carbon lattices arrangement and carbon vacancies, can be synthesized. It is also possible to realize providing an electron emission device extremely higher in electron emission efficiency than those in the prior art.
According to a substrate with carbon nanotubes, an electron emission device using the substrate and methods of making them, a substrate with carbon nanotubes which are highly oriented, densely aligned and have high a crystal quality can be used to realize providing an electron emission device which is extremely higher in electron emission efficiency than those in the prior art. According to an electron emission device of the present invention, the driving voltage therefor can be reduced and a current value that is sufficient to emit a fluorescent material on the market for slow electrons can be obtained. Moreover, an electron emission device of the present invention needs no gate electrode and can thus simplify the structure and reduce the cost of a surface light-emitting device for which the device is used. Further, the carbon material remains unoxidized even in a low vacuum and thus allows realizing a light emitting device that can be paneled more easily, and is longer in operating life than those using a conventional electron emission device.
According to a carbon nanotube synthesizing apparatus of the present invention, carbon nanotubes oriented normal to the substrate can be synthesized reproducibly and an electron emission device having high electron emission efficiency can be manufactured.
Hereinafter, best modes for carrying out the present invention will be described in details with reference to the Drawing Figures in which the same reference characters are used to designate the same or corresponding components.
At the outset, mention is made of a substrate for synthesizing carbon nanotubes which are densely aligned and highly oriented.
As shown in
The ultra-fine particles 3 of cobalt oxide preferably have a degree of oxidation such that the binding energy of core electrons in cobalt is shifted positively in a range of 1 eV to 3 eV. If the shift of a core electron of cobalt in binding energy is less than 1 eV, variations in radius and distribution of carbon nanotubes are then liable to occur, preventing reproducible synthesis of densely aligned and highly oriented carbon nanotubes. Conversely, if the shift of a core electron of cobalt in binding energy is more than 3 eV, then the ultra-fine particles of cobalt oxide lose their catalytic activity so that carbon nanotubes may not grow. The ultra-fine particles of cobalt oxide 3 preferably have a grain size of not more than 10 nm.
b) shows an instance where the substrate consists of a plurality of materials. In this instance, the synthesizing substrate 1 comprises a base substrate 6 that consists of a substrate 4 and a silicon film 5 or a diamond film 5 formed on the substrate 4, and ultra-fine particles 3 of cobalt oxide 3 substantially uniformly distributed over the base substrate 6 and having substantially a uniform particle size. The preferred degree of oxidation and the particle size there of ultra-fine particles of cobalt oxide are as mentioned in connection with
Mention is next made of a method of making a substrate for synthesizing carbon nanotubes which are densely aligned and highly oriented.
As shown in
As shown in
Also, a polycrystalline diamond film, nanocrystalline diamond film, diamond-like carbon film or monocrystalline diamond film can be synthesized on a glass substrate, semiconductor substrate, metal substrate or oxide substrate by chemical vapor deposition or plasma chemical vapor deposition using hydrogen and hydrocarbon gases as the source gases (See Non-patent Reference 1.).
A method of synthesizing carbon nanotubes using a substrate for making densely aligned and highly oriented carbon nanotubes will be mentioned next.
In synthesizing carbon nanotubes by solid-liquid interfacial contact cracking in an organic liquid, the apparatus mentioned in connection with
Mention is next made of densely aligned and highly oriented carbon nanotubes.
The densely aligned and highly oriented carbon nanotubes 11 can be synthesized from the low-cost substrate 1 and by the low-cost synthesis method mentioned above. And, they are of the highest density so far made, i.e. about two or three times as dense as those densely aligned and highly oriented carbon nanotubes which are synthesized using the conventional method of solid-liquid interfacial contact cracking in organic liquid (See Patent Reference 1.) and hence are capable of improving the performance and reducing the cost of a device using densely aligned and highly oriented carbon nanotubes, such as an electron emission device, the electrode of a double layer capacitor or that of a fuel cell.
While carbon nanotubes can be applied not only to a device as mentioned above but also to a variety of technical fields and have seen progresses of their applied and commercialization researches in various fields of their applications. Densely aligned and highly oriented carbon nanotubes according to the present invention can be manufactured at a low cost and has a form commonly used in various technical fields. Namely, they are aligned densely perpendicular to their substrate, can be used conveniently in varieties of applied researches and commercialization studies requiring them and can thus promote a variety of such researches and studies.
Mention is next made of Example 1.
As the base substrate, a n-type Si (100) oriented substrate (having a specific resistance of 0.0026 Ω·cm and a size of 7×22×0.5 mm) was used.
Initially, a cobalt thin film was deposited onto this substrate to a thickness of about 6 nm by magnetron sputtering and the substrate was then thermally oxidized in air at 900° C. for 10 minutes.
Then, in the apparatus shown in
Synthesis conditions used for solid-liquid interfacial contact cracking in an organic liquid are as follows:
Source organic liquid: methanol (purity of 99.9% and boiling point of 64.7° C.);
Synthesis temperature: 600° C.; and
Synthesis time: 5 minutes.
From
From these results, it is considered that while thermally oxidizing a cobalt thin film at a high temperature of 900° C. causes ultra-fine particles of cobalt oxide extremely small in particle size to be formed on a substrate, distributing evenly over its surface, the oxidation weakens the reactivity of cobalt and silicon with each other while restraining cobalt oxide ultra-fine particles from agglomerating with one another on the silicon surface or from sinking in the silicon substrate. It is also considered that if the degree of oxidation is excessively high, then the cobalt oxide ultra-fine particles lose their catalytic activity so that no reaction for synthesis of carbon nanotubes is brought about. While if the substrate temperature in synthesizing carbon nanotubes exceeds 900° C., then densely aligned and highly oriented carbon nanotubes cannot be synthesized reproducibly. It is considered that this is because at a temperature in excess of 900° C., it is no longer negligible for cobalt oxide and silicon to react with each other and then cobalt oxide ultra-fine particles are likely to coalesce with one another and to sink into the substrate.
Comparing
A method of synthesizing carbon nanotubes according to the present invention will be mentioned next.
First, a thin film of metal catalyst of a transition metal such as Fe, Co or Ni was deposited onto a conductive silicon substrate or diamond substrate. The substrate is then exposed to hydrogen plasma or thermally oxidized to have fine particles of catalyst carried thereon distributed at a high density.
Next, the substrate 33 mentioned above is held on the substrate holder 35 in an synthesis apparatus as shown in
Next, the electric current flown through the substrate 33 is altered to heat the substrate to a temperature in the second synthesis process to cause the carbon nanotubes grown in the first synthesis process to continue to grow. Then, when the substrate temperature in a range between 750° C. and 100° C., carbon nanotubes excellent in crystal quality allow to grow as shown in
The carbon nanotubes 1 obtained by the first synthesis process has not sufficient crystal quality and is desirably as short as possible. However, the synthesis time period of 10 seconds or more in the first synthesis process makes it possible for carbon nanotubes 1 grown in the first synthesis process to continue to be grown by the second synthesis process. If it is less than 10 seconds, carbon nanotubes grown in the first synthesis process cannot be continued to grow by the second synthesis process.
Example 2 on a method of synthesizing carbon nanotubes in accordance with the present invention will be mentioned.
First, a cobalt thin film of 5 nm was deposited on a conductive silicon substrate by sputtering a cobalt target in an argon (Ar) atmosphere of 7 Pa with a discharge current of 35 mA for 3 minutes.
Then, the substrate was thermally oxidized to make a silicon conductive substrate carrying ultra-fine particles of cobalt oxide. The conditions for thermal oxidation were 900° C. and 15 minutes of heating by current passage in air. This thermal oxidation step was carried out in a way as mentioned in connection with
Next, carbon nanotubes were synthesized in a method of the present invention using the synthesis apparatus shown in
Source organic liquid: methanol (purity of 99.9% and boiling point of 64.7° C.);
First synthesis process: substrate temperature of 600° C. and synthesis time period of 1 minute; and
Second synthesis process: substrate temperature of 900° C. and synthesis time period of 5 minutes.
As for a comparative specimen, carbon nanotubes were synthesized according to the conventional method at a substrate temperature of 700° C. for a synthesis time period of 6 minutes.
In
From
Example 3 on an electron emission device of the present invention will be mentioned next.
An electron emission device using carbon nanotubes made in the Example of the carbon nanotube synthesizing according to the method of the present invention and an electron emission device using carbon nanotubes according to the conventional method were compared on their electron emission efficiencies.
As is apparent from
An apparatus of the present invention for use in the method of synthesizing carbon nanotubes of the present invention will be described.
The carbon nanotube synthesizing apparatus 40 of the present invention has a conductive substrate 33 held fast and, for flowing current through the substrate 33, is provided with an electrode 42 and an electrodes 43 which are clamped together by means of an insulating clamping member 44. A vertical part of the electrode 42 is connected to an electrode moving equipment 45 disposed on the cover 39. The electrode moving equipment 45 is functioned to move the electrode 42 at a desired speed and, as illustrated, vertically. The electrode moving equipment 5 may be provided at least, e.g., with a pulse motor and a gear assembly for converting a rotation of the pulse motor to a vertical movement of the electrode 42 wherein the interval between pulses applied to the pulse motor are controlled to move the electrode 42 at a desired speed vertically. The electrodes 42 and 43 have their lengths such that when removed together with the substrate 33 as shown in
A carbon nanotube synthesizing method of the present invention using the synthesis apparatus of the present invention will be mentioned next. In this connection, note that the method of the present invention is identical to the conventional solid-liquid interfacial contact cracking method described in Patent Reference 1 except for the slow cooling step and its detailed explanation is therefore omitted.
A thin film of transition metal such as Fe, Co or Ni is deposited onto a conductive silicon or diamond substrate. The substrate is exposed to hydrogen plasma or the thin film of transition metal is thermally oxidized to cause fine particles of catalyst to be carried on and distributed at a high density on the substrate. The substrate 33 is held fast between the electrodes 42 and 43 and immersed in the organic liquid 30 as shown in
In terminating the synthetic reaction, one of three alternative slow cooling steps stated below can be adopted.
At first, a first possible slow cooling step will be explained.
With the current between the electrodes 42 and 43 cut off, in a predetermined time period thereafter the electrode moving equipment 45 is driven to remove the electrodes 42 and 43 and the substrate 33 held by the electrodes 42 and 43 from the organic liquid 30 and to maintain them in a gaseous atmosphere 46 of the organic liquid as shown in
Next, a second possible slow cooling step will be explained.
As shown in
Next, a third possible slow cooling step will be explained.
As is shown in
An electron emission device according to the present invention will be mentioned next.
Example 4 will be mentioned below.
Initially, a thin film of cobalt of 5 nm thick was deposited on a conductive silicon substrate by sputtering a cobalt target with a discharge current of 35 mA for 3 minutes in an argon (Ar) atmosphere of 7 Pa.
Next, the substrate was thermally oxidized to make a conductive silicon substrate having ultra-fine particles of cobalt oxide carried thereon. The thermal oxidation condition were 900° C., 15 minutes by electrical current conduction in the atmosphere of air. This step of thermal oxidation is carried out in a way as mentioned above in connection with
In this thermal oxidation step, ultra-fine particles of cobalt oxide which are extremely small in particle size are formed and uniformly distributed on the substrate surface and, with oxidized cobalt weakening its reactivity with silicon, they are inhibited from coalescing together on the silicon surface and from sinking in the silicon substrate. Hence, there result densely aligned and highly oriented carbon nanotubes. Also, in this thermal oxidation step the degree of oxidation of cobalt is so apt that it retains its catalytic activity in cobalt oxide in the form of ultra-fine particles and sufficient to bring about a reaction to synthesize carbon nanotubes.
Next, carbon nanotubes were synthesized according to the method of the present invention with the first cooling step, using the synthesis apparatus shown in
Source organic liquid: methanol (purity of 99.9% and boiling point of 64.7° C.);
Carbon nanotube synthesis conditions: substrate temperature of 600° C. and synthesis time period of 5 minutes; and
Slow cooling step conditions: In 10 minutes after the current cutoff, the substrate was removed from the organic liquid and held in gaseous atmosphere of the organic liquid for 30 minutes.
For comparison with the conventional method, a comparative specimen was also prepared which was under the same carbon nanotube synthesis conditions above but was held in the organic liquid until it was cooled down to the methanol boiling point. The tests were repeated several times.
All the specimens which were repeatedly made according to the method of the present invention showed a SEM image of (a) while the specimens made according to the conventional method included ones showing a SEM image of (b). It is thus shown that carbon nanotubes which are oriented perpendicular to the substrate can be synthesized reproducibly according to the method of the present invention.
An electron emission device of the present invention will be mentioned.
The conductive substrate 62 if composed of a single material is preferably a conductive silicon or conductive diamond substrate. The conductive substrate 62 is more preferably made of a conductive silicon.
A polycrystalline diamond, nanocrystalline diamond, diamond-like carbon or single crystalline diamond film can be synthesized on a glass, semiconductor, metal or oxide substrate by chemical vapor deposition or plasma chemical vapor deposition using hydrogen and hydrocarbon as source gases (See Non-patent reference 1.).
The fine catalyst particles can be constituted by ultra-fine particles of cobalt oxide. Ultra-fine cobalt oxide particles preferably have a particle size of 10 nm or less. Such ultra-fine cobalt oxide particles are obtainable, preferably by a process of thermal oxidation to be described later. According to the process of thermal oxidation, ultra-fine cobalt oxide particles extremely small in particle size can be produced as evenly distributed on a surface of the conductive substrate 62 and cobalt by being oxidized becomes less reactive with a conductive substrate 62. As a result, ultra-fine particles of cobalt oxide evenly distributed on the surface of the conductive substrate 2 are inhibited from agglomerating together and from sinking into the conductive substrate 62, and densely aligned and highly oriented carbon nanotubes are thereby obtained.
Carbon nanotubes 64 have a feature that they can be synthesized by the solid-liquid interfacial contact cracking method to be described yet of the present invention and they are extremely high in crystal quality without containing any carbonaceouse products such as amorphous carbon. As a criterion to show a high crystalline perfection of the carbon nanotubes, it can be cited that in their Raman scattering spectrum they have a highest Raman scattering intensity in the vicinity of 1580 cm−1. This Raman scattering in the vicinity of 1580 cm−1 is that based on lattice vibrations of a lattice composed of sp2 hybridized orbital of carbon. The higher the Raman scattering intensity in the vicinity of 1580 cm−1, the higher is the perfection of the lattice based on the sp2 hybridized orbital of carbon. Namely, it is shown that there are little disorder of periodicity of carbon lattices and only a small number of carbon vacancies in the Graphene sheets that constitute carbon nanotubes.
In the Raman scattering spectrum of carbon nanotubes 4 of the present invention, the Raman scattering intensity in the vicinity of 1580 cm−1 is more than twice as high as the Raman scattering intensity in the vicinity of 1350 cm−1. Carbon nanotubes 4 that satisfy this relationship are higher in crystal quality than those in the prior art with a consequential rise in electron emission efficiency of the electron emission device 60 with such carbon nanotubes 64.
According to the electron emission device 60, its drive voltage can be reduced to obtain a current value that is sufficient to emit a fluorescent material on the market for slow electrons. Moreover, the electron emission device 60 needs no gate and can thus simplify the structure and reduce the cost of a surface light-emitting device for which the device is used. The carbon material remains unoxidized even in a low vacuum and thus allows realizing a light emitting device that can be paneled more easily, and is longer in operating life, than one with the conventional electron emission device used.
A method of making an electron emission device 60 of the present invention will be mentioned next. The method of making an electron emitting 1 comprises the following steps:
A first step, a cobalt thin film 7 is formed on a conductive substrate 2 and is then thermally oxidized to make a carbon nanotube synthesizing substrate 10;
A second step, carbon nanotubes 64 are synthesized onto the carbon nanotube synthesizing substrate 10 by solid-liquid interfacial contact cracking in an organic liquid of alcohol in which they are synthesized on the substrate at a low temperature in a first synthesis process and at a high temperature in a second synthesis process; and
A third step, the carbon nanotube synthesizing substrate is slowly cooled in terminating the second step.
The first step will be explained by referencing
As shown in
As shown in
While ultra-fine particles of cobalt oxide are shown in
The second and third steps with reference to
As shown in
In the second step, initially the liquid tank 31 is loaded with an organic liquid 30 of alcohol in an amount such that the carbon nanotube synthesizing substrate 10 can be immersed in the organic liquid 30. The carbon nanotube synthesizing substrate 10 is clamped on the electrodes 42 and 43 and left immersed completely in the organic liquid 30 as shown in
At this point, an electric current is flown through the carbon nanotube synthesizing substrate 10 via the electrodes 42 and 43 to heat the substrate at a temperature of the first synthesis process for a predetermined time period and thereby to perform a first synthesis of carbon nanotubes. The substrate temperature then is preferably set in a range between 500° C. and 700° C. If the substrate temperature exceeds 700° C., it is undesirably that particulate carbon nanotubes grow.
While the first synthesis process does not yield carbon nanotubes sufficient in crystal quality and is desirably as short as possible and preferably between 10 seconds and 3 minutes. A synthesis time period of 10 seconds or more in the first synthesis process makes it possible for carbon nanotubes 4 grown in the first synthesis process to continue to be grown by the second synthesis process. If it is less than 10 seconds, carbon nanotubes grown in the first synthesis process cannot be continued to grow by the second synthesis process. Carbon nanotubes grown in the first synthesis process that has low crystal quality needs to be brought through the second synthesis process to raise their crystal quality.
Next, the electric current flown through the carbon nanotube synthesizing substrate 10 is altered to heat the substrate to a temperature of the second synthesis process to cause the carbon nanotubes grown in the first synthesis process to continue to grow. Then, the substrate temperature is preferably set in a range between 750° C. and 1100° C. If it is lower than 750° C., carbon nanotubes synthesized will then has low crystal quality. Conversely, if it exceeds 1100° C., then carbon nanotubes continued to grow will not be oriented perpendicular to the substrate. The longer the time period in which the second synthesis process is performed the larger will be the length and radius of carbon nanotubes 4 in proportion to the time period in which it is carried out. The time period in which the second synthesis process is carried out is set according to a length and radius of carbon nanotubes as desired.
The effect of setting different substrate temperatures in two divided stages to synthesize carbon nanotubes will be explained with reference to
In the third step, the carbon nanotube synthesis substrate 10 is slowly cooled in terminating the synthesis of carbon nanotubes. Three alternative slow cooling ways stated below can be adopted.
A first possible way is initially mentioned.
With the current between the electrodes 42 and 43 cut off in a predetermined time period thereafter, the electrode moving part 25 is driven to remove the electrodes 42 and 43 and the carbon nanotube synthesizing substrate 10 held by the electrodes 42 and 43 from the organic liquid 30 and to maintain them in a gaseous atmosphere 46 of the organic liquid until the substrate is cooled to a temperature of 100° C. or lower.
A second possible way is mentioned next.
The electrode moving part 45 is driven to remove the electrodes 42 and 43 and the carbon nanotube synthesizing substrate 10 retained by the electrodes 42 and 43 from the organic liquid and to place them in a gaseous atmosphere 26 of the organic liquid in a predetermined time period. Thereafter, the current between the electrodes 42 and 43 is cut off and the substrate is maintained as it is until the substrate is cooled to a temperature of 100° C. or lower.
A third possible way is mentioned next.
The electrode moving equipment 45 is driven to remove the electrodes 42 and 43 and the carbon nanotube synthesizing substrate 10 retained by the electrodes 42 and 43 from the organic liquid 30 and to place them in a gaseous atmosphere 46 of the organic liquid in a predetermined time period. Thereafter, the current between the electrodes 42 and 43 is diminished gradually and then cut off.
Whichever of the ways mentioned above is adopted, the predetermined time period before the carbon nanotube synthesizing substrate 10 is removed from the organic liquid 30 is preferably 10 seconds or less in the case of alcohol as the organic liquid. If it exceeds 10 seconds, then carbon nanotubes oriented perpendicular to the substrate cannot be obtained reproducibly.
The effect of slow cooling in the third step is mentioned with reference to
As shown in
Slow cooling in the third step keeps the carbon nanotube synthesizing substrate 33 and the carbon nanotubes 11 close to their equilibrium state, preventing development of large temperature difference between the carbon nanotube synthesizing substrate 33 and the carbon nanotubes 11 and thus preventing development of large difference in thermal shrinkage between the carbon nanotube synthesizing substrate 33 and the carbon nanotubes 11.
Thus, since the orientation of the carbon nanotubes 4, which were grown perpendicular to the carbon nanotube synthesizing substrate 33, remain unaltered, the severe thermal stress is prevented from occurring between the carbon nanotube synthesizing substrate 33 and the carbon nanotubes 4. As a result, the carbon nanotubes 11 are prevented from coming off, and thus can be made hard to peel off from the carbon nanotube synthesizing substrate 33.
According to the manufacturing method of the present invention, the use of solid-liquid interfacial contact cracking allows manufacturing carbon nanotubes of high purity. Also, by controlling cobalt catalytic state, introducing a multistage process and slow cooling process, it is possible to manufacture densely aligned and highly oriented carbon nanotubes having high crystal quality at a very high yield.
Explanations are given of further details of the electron emission device and the method of its making according to the present invention with respect to an example thereof.
In the first step, an n-type Si (100) surface substrate (a specific resistance of 0.0026 Ω·cm and a size of 7 mm×22 mm×0.5 mm) was used as the conductive substrate 62 and carbon nanotubes were synthesized by the following steps below.
At first, a cobalt thin film 11 of 5 nm thick was deposited onto the conductive substrate 62 by sputtering a cobalt target with a discharge current of 35 mA in an argon (Ar) atmosphere of 7 Pa (Pascal). The substrate 62 was then thermally oxidized in air at 900° C. for 10 minutes to make a carbon nanotube synthesizing substrate 10.
Next, using the synthesis apparatus as shown in
In the second step, the carbon nanotube synthesizing substrate 10 prepared in the first step was clamped on the electrodes 42 and 43 and then submerged in the methanol 30. In the first synthesis process, current was flown between the electrodes 42 and 43 to heat the substrate until its temperature reached 600° C. at which it was maintained for 1 minute. In the second synthesis process, current was flown between the electrodes 42 and 43 to heat the substrate until its temperature reached 900° C. at which it was maintained for 5 minutes.
In the third step, the current flown between the electrodes 42 and 43 was cut off and at 10 seconds thereafter the carbon nanotube synthesizing substrate 10 was removed from the methanol 30 using the electrode moving equipment 45 into methanol gas atmosphere 46 where the substrate 10 was maintained for 30 minutes for slowly cooling the carbon nanotube synthesizing substrate 10.
Carbon nanotubes 4 were synthesized following the steps above. Also, the method of Example 1 was repeated several times to synthesize several sets of carbon nanotubes 4.
Carbon nanotubes were synthesized in the same manner as in Example 5 except that they were at a substrate temperature of 700° C. for a synthesis time period of 6 minutes, without the slow cooling and with the substrate in the organic liquid until it was cooled to the boiling point of methanol.
The cobalt oxide ultra-fine particles 3 formed on the carbon nanotube synthesizing substrate 10 prepared in Example 5 were measured using the XPS (X-ray photoelectron spectroscopy) equipment.
The cobalt oxide ultra-fine particles 3 formed on the carbon nanotube synthesizing substrate 10 prepared in Example 5 had the same X-ray photoelectron spectrum as shown in
In the manufacture of carbon nanotubes 64 of Example 5, thermally oxidizing a cobalt thin film 7 at a high temperature of 900° C. resulted in carbon nanotubes 64 in which ultra-fine particles of cobalt oxide 63 extremely small in particle size were formed on a substrate, distributing evenly over its surface as mentioned above. This appears to be due to the fact that the oxidation weakens the reactivity of cobalt and silicon with each other while restraining cobalt oxide ultra-fine particles 3 from agglomerating with one another on the silicon surface or from sinking in the silicon substrate. On the other hand, if the degree of oxidation was excessively high, then the cobalt oxide ultra-fine particles 3 lost their catalytic activity so that no reaction for synthesis of carbon nanotubes 64 was brought about. If the substrate temperature in synthesizing carbon nanotubes 64 exceeded 900° C., then, densely aligned and highly oriented carbon nanotubes 64 could not be synthesized reproducibly. It is considered that this is because at a temperature in excess of 900° C., it is no longer negligible for cobalt oxide and silicon to react with each other and then cobalt oxide ultra-fine particles 63 are likely to coalesce with one another and to sink into the substrate.
Carbon nanotubes 4 synthesized in Example 5 and those in Comparative Example 1 were observed using the scanning electron microscope (SEM). Carbon nanotubes 64 synthesized in Example 5 were confirmed to be carbon nanotubes 64 oriented perpendicular to the substrate surface as shown in
Thus, Example 5 in which the slow cooling step was included in making carbon nanotubes 64 confirmed that not only could carbon nanotubes 64 be obtained which were oriented perpendicular to the substrate 62 surface but also their reproducibility was extremely high in this orientation.
Carbon nanotubes synthesized in Example 5 and those in Comparative Example 1 were evaluated by Raman spectroscopy on their crystal quality completeness. For the measurement, the laser Raman spectrophotometer (made by JASCO corporation; model NR-1800) was used and the Argon (Ar) laser having an output power of 50 mW and with a wavelength of 514.5 nm was used as the excitation light source.
In the Raman spectrum of carbon nanotubes 4 synthesized in Example 5 as same as shown in
It has thus been found that carbon nanotubes 64 synthesized in Example 5 is higher in crystal quality than those in Comparative Example 1.
An electron emission device was made using carbon nanotubes synthesized in Example 5. The electron emission device 60 made in Example 6 was configured that a conductive substrate 62 having carbon nanotubes 64 formed thereon was opposed via a spacer 22, as shown in
An electron emission device was made in the same manner as in Example 6 except that carbon nanotubes synthesized in Comparative Example 1 was used.
The field emission characteristics of the electron emission device 60 of Example 6 and Comparative Example 2 were evaluated. The voltage is applied between the conductive substrate 62 and the transparent electrode 23a to measure electrical current flown between the substrate 62 and the transparent electrode 23a and thereby measuring the current-voltage characteristics as a function of applied voltage.
As a similar measuring system as shown in
As will be appreciated from the foregoing description, densely aligned and highly oriented carbon nanotubes of the present invention are unprecedentedly high in density of carbon nanotubes and of low cost. Since the carbon nanotubes have the high crystal quality and their electron emission efficiency is high, the carbon nanotubes are useful if used as the material of an electron emission device. Since the carbon nanotubes has the high crystal quality, they can reveal the mechanical strength that is intrinsic to sp2 hybridized orbit of carbon and hence are extremely useful if used as the material of the reinforcement for the mechanical strength.
According to the present invention, the substrate with carbon nanotubes which are high in orientation, density and crystal quality as well as the electron emission device using the same can be provided. Since the electron emission device of the present invention has a form commonly used in various technical fields, they can serve in varieties of applied researches and commercialization studies requiring carbon nanotubes and are thus expected to promote such researches and studies.
It is thus possible to improve at low cost not only the electron emission device but also devices such as an electrode of a double layer capacitor and an electrode of a fuel cell using densely aligned and highly oriented carbon nanotubes.
The use of a densely aligned and highly oriented carbon nanotube synthesizing substrate of the present invention allows reproducibly synthesizing densely aligned and highly oriented carbon nanotubes which are oriented perpendicular to the surface of the substrate and aligned at an unprecedentedly high density. Using a densely aligned and highly oriented carbon nanotube synthesizing substrate making method of the present invention allows obtaining a substrate with densely aligned and highly oriented carbon nanotubes. Using a densely aligned and highly oriented carbon nanotube synthesizing substrate of the present invention and a densely aligned and highly oriented carbon nanotube synthesizing method of the present invention allows reproducibly synthesizing densely aligned and highly oriented carbon nanotubes which are oriented perpendicular to the surface of the substrate and aligned at an unprecedentedly high density and a substrate with such densely aligned and highly oriented carbon nanotubes.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-341181 | Nov 2005 | JP | national |
| 2006-070030 | Mar 2006 | JP | national |
| 2006-070031 | Mar 2006 | JP | national |
| 2006-231314 | Aug 2006 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2006/323501 | 11/24/2006 | WO | 00 | 2/4/2010 |
