Nanotechnology – Cheol-jin Lee, Jae-eun Yoo, Iljin Nanotech Co Ltd
Abstract for “Mass synthesis of high-purity carbon nanotubes vertically aligned on large-sized substrates using thermal chemicalvapor deposition”
A method for synthesizing high-purity carbon nanotubes vertically aligned on a large substrate using thermal chemical vapor desposition (CVD). The synthesis method involves the formation of nano-sized catalytical metal particles over a substrate. After that, purified carbon nanotubes are grown vertically from the catalytical metal particles using thermal CVD using carbon source gases.
Background for “Mass synthesis of high-purity carbon nanotubes vertically aligned on large-sized substrates using thermal chemicalvapor deposition”
“1. “1.
“The invention is a method for synthesizing carbon nanotubes and, more specifically, a mass-synthesis method for synthesizing high purity, vertically aligned carbon nanotubes over a large substrate.”
“2. “2.
“Carbon nanotubes have conductivity in an armchair structure and semiconductivity within a zigzag structure. They can be used as electron emission sources for field emission devices. It is possible to synthesize carbon nanotubes of high purity over large areas in vertically aligned forms. It is also important to note that carbon nanotube synthesis can be controlled easily in terms of the size and length of the carbon nanotubes and their uniformity and density over the substrate.
“Carbon nanotube synthesis methods exist in arc discharge, laser vaporization, gas phase synthesis and thermal chemical vapor deposition (CVD). Plasma CVD is also an option.”
The arc discharge method (C. Journet, Nature 388, 756 (1997), and D. S. Bethune, Nature 363, 605 (1993), respectively) and laser vaporization (R. E. Smally, Science 273, 483(1996)) cannot control the size or length of carbon nanotubes. This results in a low yield. Additionally, carbon nanotubes can also be produced with excess amorphous carbon lumps. This requires complicated purification processes. It is therefore difficult to grow carbon nanotubes on large substrates using these methods.
“Meanwhile the gas phase synthesis method (R. Andrews and al., Chem. Phys. Lett., 334, 468 (1999), which is suitable for mass synthesis, produces carbon nanotubes from carbon source gases in a gas phase without the use of a substrate. This method is not able to control the size or length of carbon-nanotubes and can cause adhering metal catalyst lumps on the inner or outer sides of carbon nanotubes. The method is not able to produce high-purity carbon nanotubes, and it cannot align carbon nanotubes vertically over a substrate.
The thermal CVD method is a technique for growing carbon nanotubes on porous silica (W. Z. Li and al. Science, 274, 1701 (1996))), or zeolite. (Shinohara and al. Japanese J. of Applicationl. Phys., 37, 1357 (1998)) substrate. Filling the pores of the substrate with a metallic catalyst can be a time-consuming and complicated process. The control of the size of carbon nanotubes can be difficult and yields are low. The thermal CVD method is limited in the ability to grow massive carbon nanotubes on a large substrate.
The plasma CVD method (Z. F. Ren and al. Science, 282,1105 (1998) is an excellent technique to vertically align carbon nanotubes. The problem is that plasma energy can damage carbon nanotubes, and the structure of carbon nanotubes is instabile due to low-temperature synthesis. Many carbon particles stick to carbon nanotubes’ surfaces.
“To address the above problems, the present invention provides a mass-synthesis method for high purity carbon nanotubes vertically aligned across a large substrate.”
The present invention relates to a method for synthesizing carbon-nanotubes. This involves forming a layer of metal catalyst on a substrate. The metal catalyst layer is etched in order to create isolated nano-sized catalytical metal particles. Carbon nanotubes are vertically aligned on the substrate from each isolated nano-sized Catalytic Metal particle.
“Preferably, the formation of the isolated nano-sized catalytical metal particles is done by a gasetching method. In which one etching agent from the group consisting ammonia, hydrogen, and hydride gases is thermally decomposed for use during etching. Plasma etching or wet etching with a hydrogen fluoride series of etchant can be used to form the nano-sized catalytical metal particles.
“Preferably, the formation of the catalytic metal particles as well as the carbon nanotubes is done in-situ in the same thermalCVD apparatus.”
“Preferably, when forming carbon nanotubes, one gas from the group consisting ammonia, hydrogen, and hydride gases is supplied to the thermal CVD apparatus together with the carbon source gaz.”
“Preferably after the formation of carbon nanotubes, synthesis also includes exhausting the carbon source gases using an inert gaz from the thermal CVD apparatus.”
“Preferably, after the formation of carbon nanotubes is completed, the synthesis process also includes in-situ purification of carbon nanotubes using the same thermal CVD apparatus. In-situ purification of carbon nanotubes should be done with a purification gas from the following group: hydrogen gas, ammonia gas, oxygen gas, or a mixture thereof.
“Preferably after in-situ purification of carbon nanotubes,” the synthesis method also includes exhausting the purification gases using an inertgas from the thermal CVD device.
“The present invention will be described in greater detail with the help of the accompanying drawings. These drawings show preferred embodiments. The invention can be expressed in many forms, and should not be taken to mean that it is limited to the ones shown herein. These embodiments are intended to be comprehensive and complete and fully communicate the concept of this invention to all who are skilled in the art. Schematically, the appended figure shows a thermal chemical deposition (CVD), apparatus. The thickness and proportions for a substrate, catalytic metal particles, and catalyst metal layer are exaggerated in the drawings. You should also note that similar reference numbers can be used throughout the drawings to denote identical or corresponding parts.
“Embodiment 1”
FIG. 1 will describe the method for synthesizing carbon nanotubes using the present invention. 1. This is a flowchart that illustrates the synthesis process. FIGS. 2A and 2B are sections of the substrate upon which carbon nanotubes will be formed. FIG. 3 is a schematic view showing the thermal chemical vapor desposition (CVD), apparatus that is used in the synthesis. The flowchart shows the basic steps of the synthesis in solid-line boxes. Optional steps are shown in dashed boxes.
“Referring back to FIG. Referring to FIG. 1, a metal catalyst (130 of FIG. 2A) is applied to a substrate (110 in FIG. 2A) is the substrate on which carbon nanotubes will be made (step 20). A substrate 110 can be made from glass, quartz or silicon, as well as alumina (A2O3). The metal catalyst layer 130 can be made of cobalt, nickel (Ni), iron or an alloy of the two (Co?Ni or Co?Fe), or a mixture of them. The substrate 110 is covered with the metal catalyst layer 130. It can be deposited to a thickness of several nanometers to a few hundreds nanometers or, more preferably, to a thickness between 2 and 200 nm by either thermal deposition, electron beam deposition or sputtering.
“In the event that the substrate 110 is made from silicon and the metal catalyst 130 is made of Co, Ni, or an alloy thereof, an insulating (120) layer (FIG. 2B is formed on the substrate 100 prior to the formation of the metal catalytic layer 130. This prevents the formation of silicide films by reaction between the substrate 110 and the metal catalyst 130 (step 10). As the insulating layer 120, a silicon oxide or alumina can be formed.
“Following that, the metal catalyst layer 130 is etched to separate nano-sized catalytical metal particles (step 30-).
“In particular, refer to FIG. “In particular, referring to FIG. The boat 310 is loaded so that the metal catalyst layer 130, which has been placed on top of the substrate, faces in the opposite direction of the flow of gas (arrow 315 in FIG. 3. Substrates 110 are placed so that the metal catalyst layer 130 is not facing the flow of gas. This allows for uniform reaction of substrates 110 and 130 coated with the metal catalyst layers 130. Also, the mass flow of the etching gas can be evenly controlled. The substrate 110 should be inserted into the boat 310 so that the metal catalyst layer 130’s surface faces downwards to avoid any defects caused by unstable reaction products or carbon particles falling from the furnace 300 wall.
“As shown at FIG. 4. The reaction furnace 300 injects the etching gas 200 into the reaction furnace. This removes the metal catalyst layer 130 from the grain boundaries to form nano-sized catalytical metal particles 130P that are isolated and uniformly distributed over the substrate 110. “Nano size” is a term that refers to the size of a nanometer. The size ranges from a few nanometers up to several hundred nanometers. The etching conditions can affect the size and shape of isolated nanosized catalytical metal particles. The shape of catalytical metal particles also affects the form of carbon nanotubes that are produced in a subsequent step.
“Then, a carbon source gases is introduced into the thermal CVD apparatus in order to grow carbon nanotubes on the substrate 110 (step40).
“To control the time and growth rate of carbon nanotubes, a gas carrier (inert gas like hydrogen or argon) along with the carbon source can be supplied to the reaction furnace 300 by opening a third valve.
The etching gas can be used to control the density and growth patterns of carbon nanotubes that are synthesized on substrates. It can be supplied in an appropriate ratio with the carbon source gases. It is preferred that the ratio between the carbon source gas (ammonia gas, hydrogen gas or hydride gas) and the etching gases be between 2:1 and 3:1.
“As shown at FIG. 5. The carbon source gas is supplied into the reaction furnace 300 and pyrolized to produce carbon nanotubes that protrude from the nano-sized catalytical metal particles 130P.
“FIGS. 6A-6C are schematic views of the base growth model. Refer to FIGS. for details on the growth mechanism. 6A-6C. As shown in FIG. 6A shows that a carbon source gas, such as acetylene (C2H2)), is supplied to the reaction furnace 300. The gas phase is then pyrolized into carbon units (C?C, C or C) and hydrogen (H2). The carbon units are absorbed onto the catalytical metal particles 130P, and then diffuse into the catalytical metal particles 130P. The catalytic metal particles 130P become supersaturated with the dissolved Carbon units. This initiates the growth of the carbon nanotubes 150. The carbon nanotubes 150 begin to grow like a bamboo as the carbon units are incorporated into the catalytical metal particles 130P. 6C by the catalytic function o the catalytical metal particles 130P. The catalytic metal particles 130P can have blunt or round tips. Carbon nanotubes 150 will grow with blunt or round tips. Carbon nanotubes can be grown with sharp tips, even though it is not shown in the drawings.
“The first embodiment is described using a horizontal type thermal (CVD), but it is possible to use a conveyer, in-line, or vertical type CVD apparatus.”
“The first embodiment of the synthesis process can produce carbon nanotubes with a diameter of just a few nanometers up to a few hundreds nanometers, such as 1 to 400 nm or a length of a couple of micrometers up to a few hundred, such as 0.5 to 300?m.
The carbon nanotubes 10, which have been synthesized, can be purified in-situ (step 60) after the process is complete. The carbon particles and carbon lumps that are found on the surface of the 150-grown carbon nanotubes 150 are removed in situ during the growing (step 40).
“In particular, FIG. 3. is closed to stop the supply of carbon source gas. A fourth valve 460 opens to supply purification gases from a purification gaz supply source 470 to reaction furnace 300 through gas inlet 322. The purification gas can be either hydrogen gas, ammonia gas, oxygen, or a combination of these gases. If ammonia or hydrogen gas are selected as the purification gases, they can be supplied by the etching gas source 410, or the carrier gas/or diluent gaz supply source 455.
“Hydrogenions (H+), which are formed by thermal decomposition ammonia gas/hydrogen gas, remove carbon lumps and carbon particles. O2 is the oxygen ions that are used to purify oxygen gas. These ions, which are formed by the thermal decomposition oxygen gas, can be used to combust carbon lumps and carbon particles. Purified carbon nanotubes are obtained by purification.
“It is preferred that an inert gaz is supplied to the reaction furnace 300 before the purification (step60), as illustrated in FIG. 7. At a rate of 200-500 sccm, exhaust carbon source gas from reaction furnace 300 via a gas outlet. 340 (step 50 in FIG. 1). Argon gas is preferable as an inert gas. This allows for precise control of the length of carbon nanotubes that are grown and prevents any unwanted reactions to carbon source gas after the synthesis of carbon nanotubes.
“It is also preferred that an inert gaz is added to the reaction furnace 300 at 200 to 500 sccm after purification (step 60) to exhaust any purification gas remaining from the reaction furnace 30, through the gas outlet 34 (step 70 of FIG. 1). Preferably, the temperature of reaction furnace 300 should be lowered during exhaustion of purification gases. The purification of the purification gases (step 70), is necessary to prevent partial damage of carbon nanotubes 150 caused by the purificationgas.
“According the first embodiment, the nano-sized catalyst metal particles that are suitable for the growth of carbon Nanotubes are separated from one another with a high density, without agglomerating. Thus, amorphous carbon lumps are not created in the synthesis carbon nanotubes. The carbon nanotubes can then be vertically aligned with the substrate of high purity.
The metal catalyst layer is removed and the nano-sized catalytical metal particles are etched over the substrate to form a uniform layer. Carbon nanotubes can then be grown vertically over large substrates.
“Also the control of the size and density of catalytic metal particles is possible by changing the etching conditions. For example, flow rate and temperature of etching gases such as ammonia, or etching time and temperature.
The first embodiment of the invention offers an advantage because the flow conditions of carbon source gases can be changed to alter the length of carbon nanotubes, such as the flow rate and reaction temperatures.
“In addition, the thermal CVD apparatus allows batch-type synthesis, in which multiple substrates can simultaneously be loaded into the apparatus to synthesise carbon nanotubes. This increases yield.”
“The formation and purification of catalytic metal particles and the formation carbon nanotubes from carbon source gases are both in-situ at the same temperature. In-situ purification is also possible for carbon nanotubes. Consider other synthesis methods that require different chambers for each process. This reduces the time needed for chamber-to chamber substrate transfer and allows for faster ramp-up to the appropriate temperature in each chamber. The purification process is also simple. This allows for increased yields of carbon nanotubes purified.
“Embodiment 2”
The second embodiment differs from the first in that the formation of small-sized catalytical metal particles (step 30), is done by plasma etching and not thermally decomposed gases. Plasma etching has the advantage of being able to etch at low temperatures and allowing for easy control over the reaction.
Plasma etching can either be done in one plasma etching device or in combination with a thermal UVD apparatus for the subsequent formation of carbon-nanotubes. Combination type systems can be multi-chamber systems in which a plasma and thermal CVD apparatus are combined in one cluster or a combination of remote plasma and thermal CVD. Combination type systems are preferred because they reduce the time required to transfer substrate and prevent it from being exposed to airborne contaminants.
Plasma is most preferred when it is made with ammonia.
“Then, follow the first embodiment to make carbon nanotubes.”
“Embodiment 3”
The third embodiment differs from the first and second in that nano-sized catalytical metal particles can be obtained by wet etching rather than dry. To form the nano-sized catalytical metal particles, the substrate with the metal catalyst layer is placed in an etchant. This can be a hydrogen fluoride series or mixed solution of HF, NH4F, or deionized water. The immersion time may be between 1 and 5 minutes. This wet etching method has the advantage that it can be done at low temperatures.
“Then, follow the first embodiment to make carbon nanotubes.”
“Embodiment 4”
“The fourth embodiment is a combination between the first and third embodiments. The first is wet etching as in the third embodiment. Then, dry etching with gas is done as in the original embodiment. A substrate with a metal catalyst layer is etched using an etchant (HF solution dilute with deionized waters) for between 1 and 5 minutes. The substrate is then dried. The substrate is then loaded into the thermal CVD device. Ammonia gas is introduced at a rate of 60-300 sccm for 5-20 minutes to form small catalytic metal particles on the substrate.
“Then, follow the first embodiment to make carbon nanotubes.”
“Embodiment 5”
“The first embodiment differs in that the formation nano-sized catalytical metal particles (step 30), is done by photolithography rather than thermally decomposed gases.
“In particular, FIG. 8A shows the coated metal catalyst layer 130 being photoresist coated and exposed to development to form a photoresist pattern PR. This can be, for example, a few nanometers up to a few hundred.
“Following that, the metal catalyst layer 130 can be etched using the photoresist patterns PR as an etching pattern to form nano-sized catalytical metal particles 130P as shown in FIG. 8B. 8B. 8C.”
“In the current embodiment, where catalytical metal particles are formed using photolithography, it is possible to control the size and densities of these catalytical metal particles by controlling the density and size of the photoresist patterns. It is therefore possible to control arbitrarily the size and density of carbon nanotubes.
The following examples will provide a more detailed description of the invention. These examples are intended as illustrations and do not limit the invention’s scope.
“EXPERIMENTAL EXAMPLE I”
“EXPERIMENTAL EXAMPLE #2”
“To synthesize carbon-nanotubes, the procedure of Experimental Example 1 was applied except that a nickel film (Ni), was used instead of the Fe film as the metal catalyst layer. SEM has shown that carbon nanotubes can be grown vertically and uniformly over the substrate. The obtained carbon nanotubes are approximately 50 nm in diameter and 80?m long, according to TEM.
“EXPERIMENTAL EXAMPLE #3”
“To synthesize carbon-nanotubes, the procedure of Experimental Example 1 was applied except that a cobalt film (Co) was used instead of the Fe film as the metal catalyst layer. SEM shows that carbon Nanotubes are uniformly and vertically grown on top of the substrate. The obtained carbon nanotubes are approximately 70 nm in diameter and 30?m long, according to TEM.
“EXPERIMENTAL EXAMPLE”
“To synthesize carbon-nanotubes, the procedure of Experimental Example 1 was used except that a Co?Ni alloy film was used instead of the single Fe layer as the metal catalyst. SEM shows that carbon Nanotubes are uniformly and vertically grown on top of the substrate. The obtained carbon nanotubes are approximately 90 nm in diameter and about 100 mm long, according to TEM.
“EXPERIMENTAL EXAMPLE”
“To synthesize carbon nanotubes, the procedure of Experimental Example 4 was followed, except that a Co?Fe alloy film was used instead of the Co?Ni alloy film as the metal catalyst layer. SEM clearly shows that carbon nanotubes grow vertically and uniformly over the substrate. The obtained carbon nanotubes are approximately 90 nm in diameter and 80?m long, according to TEM.
“EXPERIMENTAL EXAMPLE SIX”
“To synthesize carbon-nanotubes, the process described in Experimental Example 4 was followed, except that a Ni?Fe alloy film was used instead of the Co?Ni film as the metal catalyst. SEM shows that carbon Nanotubes are uniformly and vertically grown on top of the substrate. The TEM results show that the carbon nanotubes obtained have a diameter around 80 nm, and a length about 80?m.
“EXPERIMENTAL EXAMPLE 7”.
“EXPERIMENTAL EXAMPLE 8)”
“In the synthesis of carbon nanotubes according the present invention, high-density catalytic metal particles are formed from one another without agglomerating. This allows high purity carbon nanotubes to be vertically aligned on a substrate. The nano-sized catalytical metal particles can also be obtained by uniformly etching over a layer of metal catalyst. This allows carbon nanotubes to be evenly distributed across large substrates. You can also adjust the flow rate and processing time, as well as the density and diameter of the carbon nanotubes. The present invention allows for batch-type synthesis, where carbon nanotubes can be grown simultaneously on a variety of substrates. Vertically aligned carbon nutubes can be made over large substrates with high purity and high yield. Carbon nanotubes can also be easily purified in situ during the synthesis process to ensure maximum efficiency.
“While the preferred embodiments of this invention have been shown and described, it will be apparent to those skilled in art that many modifications in form and details can be made without departing from its spirit and scope as defined in the appended claims.”
Summary for “Mass synthesis of high-purity carbon nanotubes vertically aligned on large-sized substrates using thermal chemicalvapor deposition”
“1. “1.
“The invention is a method for synthesizing carbon nanotubes and, more specifically, a mass-synthesis method for synthesizing high purity, vertically aligned carbon nanotubes over a large substrate.”
“2. “2.
“Carbon nanotubes have conductivity in an armchair structure and semiconductivity within a zigzag structure. They can be used as electron emission sources for field emission devices. It is possible to synthesize carbon nanotubes of high purity over large areas in vertically aligned forms. It is also important to note that carbon nanotube synthesis can be controlled easily in terms of the size and length of the carbon nanotubes and their uniformity and density over the substrate.
“Carbon nanotube synthesis methods exist in arc discharge, laser vaporization, gas phase synthesis and thermal chemical vapor deposition (CVD). Plasma CVD is also an option.”
The arc discharge method (C. Journet, Nature 388, 756 (1997), and D. S. Bethune, Nature 363, 605 (1993), respectively) and laser vaporization (R. E. Smally, Science 273, 483(1996)) cannot control the size or length of carbon nanotubes. This results in a low yield. Additionally, carbon nanotubes can also be produced with excess amorphous carbon lumps. This requires complicated purification processes. It is therefore difficult to grow carbon nanotubes on large substrates using these methods.
“Meanwhile the gas phase synthesis method (R. Andrews and al., Chem. Phys. Lett., 334, 468 (1999), which is suitable for mass synthesis, produces carbon nanotubes from carbon source gases in a gas phase without the use of a substrate. This method is not able to control the size or length of carbon-nanotubes and can cause adhering metal catalyst lumps on the inner or outer sides of carbon nanotubes. The method is not able to produce high-purity carbon nanotubes, and it cannot align carbon nanotubes vertically over a substrate.
The thermal CVD method is a technique for growing carbon nanotubes on porous silica (W. Z. Li and al. Science, 274, 1701 (1996))), or zeolite. (Shinohara and al. Japanese J. of Applicationl. Phys., 37, 1357 (1998)) substrate. Filling the pores of the substrate with a metallic catalyst can be a time-consuming and complicated process. The control of the size of carbon nanotubes can be difficult and yields are low. The thermal CVD method is limited in the ability to grow massive carbon nanotubes on a large substrate.
The plasma CVD method (Z. F. Ren and al. Science, 282,1105 (1998) is an excellent technique to vertically align carbon nanotubes. The problem is that plasma energy can damage carbon nanotubes, and the structure of carbon nanotubes is instabile due to low-temperature synthesis. Many carbon particles stick to carbon nanotubes’ surfaces.
“To address the above problems, the present invention provides a mass-synthesis method for high purity carbon nanotubes vertically aligned across a large substrate.”
The present invention relates to a method for synthesizing carbon-nanotubes. This involves forming a layer of metal catalyst on a substrate. The metal catalyst layer is etched in order to create isolated nano-sized catalytical metal particles. Carbon nanotubes are vertically aligned on the substrate from each isolated nano-sized Catalytic Metal particle.
“Preferably, the formation of the isolated nano-sized catalytical metal particles is done by a gasetching method. In which one etching agent from the group consisting ammonia, hydrogen, and hydride gases is thermally decomposed for use during etching. Plasma etching or wet etching with a hydrogen fluoride series of etchant can be used to form the nano-sized catalytical metal particles.
“Preferably, the formation of the catalytic metal particles as well as the carbon nanotubes is done in-situ in the same thermalCVD apparatus.”
“Preferably, when forming carbon nanotubes, one gas from the group consisting ammonia, hydrogen, and hydride gases is supplied to the thermal CVD apparatus together with the carbon source gaz.”
“Preferably after the formation of carbon nanotubes, synthesis also includes exhausting the carbon source gases using an inert gaz from the thermal CVD apparatus.”
“Preferably, after the formation of carbon nanotubes is completed, the synthesis process also includes in-situ purification of carbon nanotubes using the same thermal CVD apparatus. In-situ purification of carbon nanotubes should be done with a purification gas from the following group: hydrogen gas, ammonia gas, oxygen gas, or a mixture thereof.
“Preferably after in-situ purification of carbon nanotubes,” the synthesis method also includes exhausting the purification gases using an inertgas from the thermal CVD device.
“The present invention will be described in greater detail with the help of the accompanying drawings. These drawings show preferred embodiments. The invention can be expressed in many forms, and should not be taken to mean that it is limited to the ones shown herein. These embodiments are intended to be comprehensive and complete and fully communicate the concept of this invention to all who are skilled in the art. Schematically, the appended figure shows a thermal chemical deposition (CVD), apparatus. The thickness and proportions for a substrate, catalytic metal particles, and catalyst metal layer are exaggerated in the drawings. You should also note that similar reference numbers can be used throughout the drawings to denote identical or corresponding parts.
“Embodiment 1”
FIG. 1 will describe the method for synthesizing carbon nanotubes using the present invention. 1. This is a flowchart that illustrates the synthesis process. FIGS. 2A and 2B are sections of the substrate upon which carbon nanotubes will be formed. FIG. 3 is a schematic view showing the thermal chemical vapor desposition (CVD), apparatus that is used in the synthesis. The flowchart shows the basic steps of the synthesis in solid-line boxes. Optional steps are shown in dashed boxes.
“Referring back to FIG. Referring to FIG. 1, a metal catalyst (130 of FIG. 2A) is applied to a substrate (110 in FIG. 2A) is the substrate on which carbon nanotubes will be made (step 20). A substrate 110 can be made from glass, quartz or silicon, as well as alumina (A2O3). The metal catalyst layer 130 can be made of cobalt, nickel (Ni), iron or an alloy of the two (Co?Ni or Co?Fe), or a mixture of them. The substrate 110 is covered with the metal catalyst layer 130. It can be deposited to a thickness of several nanometers to a few hundreds nanometers or, more preferably, to a thickness between 2 and 200 nm by either thermal deposition, electron beam deposition or sputtering.
“In the event that the substrate 110 is made from silicon and the metal catalyst 130 is made of Co, Ni, or an alloy thereof, an insulating (120) layer (FIG. 2B is formed on the substrate 100 prior to the formation of the metal catalytic layer 130. This prevents the formation of silicide films by reaction between the substrate 110 and the metal catalyst 130 (step 10). As the insulating layer 120, a silicon oxide or alumina can be formed.
“Following that, the metal catalyst layer 130 is etched to separate nano-sized catalytical metal particles (step 30-).
“In particular, refer to FIG. “In particular, referring to FIG. The boat 310 is loaded so that the metal catalyst layer 130, which has been placed on top of the substrate, faces in the opposite direction of the flow of gas (arrow 315 in FIG. 3. Substrates 110 are placed so that the metal catalyst layer 130 is not facing the flow of gas. This allows for uniform reaction of substrates 110 and 130 coated with the metal catalyst layers 130. Also, the mass flow of the etching gas can be evenly controlled. The substrate 110 should be inserted into the boat 310 so that the metal catalyst layer 130’s surface faces downwards to avoid any defects caused by unstable reaction products or carbon particles falling from the furnace 300 wall.
“As shown at FIG. 4. The reaction furnace 300 injects the etching gas 200 into the reaction furnace. This removes the metal catalyst layer 130 from the grain boundaries to form nano-sized catalytical metal particles 130P that are isolated and uniformly distributed over the substrate 110. “Nano size” is a term that refers to the size of a nanometer. The size ranges from a few nanometers up to several hundred nanometers. The etching conditions can affect the size and shape of isolated nanosized catalytical metal particles. The shape of catalytical metal particles also affects the form of carbon nanotubes that are produced in a subsequent step.
“Then, a carbon source gases is introduced into the thermal CVD apparatus in order to grow carbon nanotubes on the substrate 110 (step40).
“To control the time and growth rate of carbon nanotubes, a gas carrier (inert gas like hydrogen or argon) along with the carbon source can be supplied to the reaction furnace 300 by opening a third valve.
The etching gas can be used to control the density and growth patterns of carbon nanotubes that are synthesized on substrates. It can be supplied in an appropriate ratio with the carbon source gases. It is preferred that the ratio between the carbon source gas (ammonia gas, hydrogen gas or hydride gas) and the etching gases be between 2:1 and 3:1.
“As shown at FIG. 5. The carbon source gas is supplied into the reaction furnace 300 and pyrolized to produce carbon nanotubes that protrude from the nano-sized catalytical metal particles 130P.
“FIGS. 6A-6C are schematic views of the base growth model. Refer to FIGS. for details on the growth mechanism. 6A-6C. As shown in FIG. 6A shows that a carbon source gas, such as acetylene (C2H2)), is supplied to the reaction furnace 300. The gas phase is then pyrolized into carbon units (C?C, C or C) and hydrogen (H2). The carbon units are absorbed onto the catalytical metal particles 130P, and then diffuse into the catalytical metal particles 130P. The catalytic metal particles 130P become supersaturated with the dissolved Carbon units. This initiates the growth of the carbon nanotubes 150. The carbon nanotubes 150 begin to grow like a bamboo as the carbon units are incorporated into the catalytical metal particles 130P. 6C by the catalytic function o the catalytical metal particles 130P. The catalytic metal particles 130P can have blunt or round tips. Carbon nanotubes 150 will grow with blunt or round tips. Carbon nanotubes can be grown with sharp tips, even though it is not shown in the drawings.
“The first embodiment is described using a horizontal type thermal (CVD), but it is possible to use a conveyer, in-line, or vertical type CVD apparatus.”
“The first embodiment of the synthesis process can produce carbon nanotubes with a diameter of just a few nanometers up to a few hundreds nanometers, such as 1 to 400 nm or a length of a couple of micrometers up to a few hundred, such as 0.5 to 300?m.
The carbon nanotubes 10, which have been synthesized, can be purified in-situ (step 60) after the process is complete. The carbon particles and carbon lumps that are found on the surface of the 150-grown carbon nanotubes 150 are removed in situ during the growing (step 40).
“In particular, FIG. 3. is closed to stop the supply of carbon source gas. A fourth valve 460 opens to supply purification gases from a purification gaz supply source 470 to reaction furnace 300 through gas inlet 322. The purification gas can be either hydrogen gas, ammonia gas, oxygen, or a combination of these gases. If ammonia or hydrogen gas are selected as the purification gases, they can be supplied by the etching gas source 410, or the carrier gas/or diluent gaz supply source 455.
“Hydrogenions (H+), which are formed by thermal decomposition ammonia gas/hydrogen gas, remove carbon lumps and carbon particles. O2 is the oxygen ions that are used to purify oxygen gas. These ions, which are formed by the thermal decomposition oxygen gas, can be used to combust carbon lumps and carbon particles. Purified carbon nanotubes are obtained by purification.
“It is preferred that an inert gaz is supplied to the reaction furnace 300 before the purification (step60), as illustrated in FIG. 7. At a rate of 200-500 sccm, exhaust carbon source gas from reaction furnace 300 via a gas outlet. 340 (step 50 in FIG. 1). Argon gas is preferable as an inert gas. This allows for precise control of the length of carbon nanotubes that are grown and prevents any unwanted reactions to carbon source gas after the synthesis of carbon nanotubes.
“It is also preferred that an inert gaz is added to the reaction furnace 300 at 200 to 500 sccm after purification (step 60) to exhaust any purification gas remaining from the reaction furnace 30, through the gas outlet 34 (step 70 of FIG. 1). Preferably, the temperature of reaction furnace 300 should be lowered during exhaustion of purification gases. The purification of the purification gases (step 70), is necessary to prevent partial damage of carbon nanotubes 150 caused by the purificationgas.
“According the first embodiment, the nano-sized catalyst metal particles that are suitable for the growth of carbon Nanotubes are separated from one another with a high density, without agglomerating. Thus, amorphous carbon lumps are not created in the synthesis carbon nanotubes. The carbon nanotubes can then be vertically aligned with the substrate of high purity.
The metal catalyst layer is removed and the nano-sized catalytical metal particles are etched over the substrate to form a uniform layer. Carbon nanotubes can then be grown vertically over large substrates.
“Also the control of the size and density of catalytic metal particles is possible by changing the etching conditions. For example, flow rate and temperature of etching gases such as ammonia, or etching time and temperature.
The first embodiment of the invention offers an advantage because the flow conditions of carbon source gases can be changed to alter the length of carbon nanotubes, such as the flow rate and reaction temperatures.
“In addition, the thermal CVD apparatus allows batch-type synthesis, in which multiple substrates can simultaneously be loaded into the apparatus to synthesise carbon nanotubes. This increases yield.”
“The formation and purification of catalytic metal particles and the formation carbon nanotubes from carbon source gases are both in-situ at the same temperature. In-situ purification is also possible for carbon nanotubes. Consider other synthesis methods that require different chambers for each process. This reduces the time needed for chamber-to chamber substrate transfer and allows for faster ramp-up to the appropriate temperature in each chamber. The purification process is also simple. This allows for increased yields of carbon nanotubes purified.
“Embodiment 2”
The second embodiment differs from the first in that the formation of small-sized catalytical metal particles (step 30), is done by plasma etching and not thermally decomposed gases. Plasma etching has the advantage of being able to etch at low temperatures and allowing for easy control over the reaction.
Plasma etching can either be done in one plasma etching device or in combination with a thermal UVD apparatus for the subsequent formation of carbon-nanotubes. Combination type systems can be multi-chamber systems in which a plasma and thermal CVD apparatus are combined in one cluster or a combination of remote plasma and thermal CVD. Combination type systems are preferred because they reduce the time required to transfer substrate and prevent it from being exposed to airborne contaminants.
Plasma is most preferred when it is made with ammonia.
“Then, follow the first embodiment to make carbon nanotubes.”
“Embodiment 3”
The third embodiment differs from the first and second in that nano-sized catalytical metal particles can be obtained by wet etching rather than dry. To form the nano-sized catalytical metal particles, the substrate with the metal catalyst layer is placed in an etchant. This can be a hydrogen fluoride series or mixed solution of HF, NH4F, or deionized water. The immersion time may be between 1 and 5 minutes. This wet etching method has the advantage that it can be done at low temperatures.
“Then, follow the first embodiment to make carbon nanotubes.”
“Embodiment 4”
“The fourth embodiment is a combination between the first and third embodiments. The first is wet etching as in the third embodiment. Then, dry etching with gas is done as in the original embodiment. A substrate with a metal catalyst layer is etched using an etchant (HF solution dilute with deionized waters) for between 1 and 5 minutes. The substrate is then dried. The substrate is then loaded into the thermal CVD device. Ammonia gas is introduced at a rate of 60-300 sccm for 5-20 minutes to form small catalytic metal particles on the substrate.
“Then, follow the first embodiment to make carbon nanotubes.”
“Embodiment 5”
“The first embodiment differs in that the formation nano-sized catalytical metal particles (step 30), is done by photolithography rather than thermally decomposed gases.
“In particular, FIG. 8A shows the coated metal catalyst layer 130 being photoresist coated and exposed to development to form a photoresist pattern PR. This can be, for example, a few nanometers up to a few hundred.
“Following that, the metal catalyst layer 130 can be etched using the photoresist patterns PR as an etching pattern to form nano-sized catalytical metal particles 130P as shown in FIG. 8B. 8B. 8C.”
“In the current embodiment, where catalytical metal particles are formed using photolithography, it is possible to control the size and densities of these catalytical metal particles by controlling the density and size of the photoresist patterns. It is therefore possible to control arbitrarily the size and density of carbon nanotubes.
The following examples will provide a more detailed description of the invention. These examples are intended as illustrations and do not limit the invention’s scope.
“EXPERIMENTAL EXAMPLE I”
“EXPERIMENTAL EXAMPLE #2”
“To synthesize carbon-nanotubes, the procedure of Experimental Example 1 was applied except that a nickel film (Ni), was used instead of the Fe film as the metal catalyst layer. SEM has shown that carbon nanotubes can be grown vertically and uniformly over the substrate. The obtained carbon nanotubes are approximately 50 nm in diameter and 80?m long, according to TEM.
“EXPERIMENTAL EXAMPLE #3”
“To synthesize carbon-nanotubes, the procedure of Experimental Example 1 was applied except that a cobalt film (Co) was used instead of the Fe film as the metal catalyst layer. SEM shows that carbon Nanotubes are uniformly and vertically grown on top of the substrate. The obtained carbon nanotubes are approximately 70 nm in diameter and 30?m long, according to TEM.
“EXPERIMENTAL EXAMPLE”
“To synthesize carbon-nanotubes, the procedure of Experimental Example 1 was used except that a Co?Ni alloy film was used instead of the single Fe layer as the metal catalyst. SEM shows that carbon Nanotubes are uniformly and vertically grown on top of the substrate. The obtained carbon nanotubes are approximately 90 nm in diameter and about 100 mm long, according to TEM.
“EXPERIMENTAL EXAMPLE”
“To synthesize carbon nanotubes, the procedure of Experimental Example 4 was followed, except that a Co?Fe alloy film was used instead of the Co?Ni alloy film as the metal catalyst layer. SEM clearly shows that carbon nanotubes grow vertically and uniformly over the substrate. The obtained carbon nanotubes are approximately 90 nm in diameter and 80?m long, according to TEM.
“EXPERIMENTAL EXAMPLE SIX”
“To synthesize carbon-nanotubes, the process described in Experimental Example 4 was followed, except that a Ni?Fe alloy film was used instead of the Co?Ni film as the metal catalyst. SEM shows that carbon Nanotubes are uniformly and vertically grown on top of the substrate. The TEM results show that the carbon nanotubes obtained have a diameter around 80 nm, and a length about 80?m.
“EXPERIMENTAL EXAMPLE 7”.
“EXPERIMENTAL EXAMPLE 8)”
“In the synthesis of carbon nanotubes according the present invention, high-density catalytic metal particles are formed from one another without agglomerating. This allows high purity carbon nanotubes to be vertically aligned on a substrate. The nano-sized catalytical metal particles can also be obtained by uniformly etching over a layer of metal catalyst. This allows carbon nanotubes to be evenly distributed across large substrates. You can also adjust the flow rate and processing time, as well as the density and diameter of the carbon nanotubes. The present invention allows for batch-type synthesis, where carbon nanotubes can be grown simultaneously on a variety of substrates. Vertically aligned carbon nutubes can be made over large substrates with high purity and high yield. Carbon nanotubes can also be easily purified in situ during the synthesis process to ensure maximum efficiency.
“While the preferred embodiments of this invention have been shown and described, it will be apparent to those skilled in art that many modifications in form and details can be made without departing from its spirit and scope as defined in the appended claims.”
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