Nanotube solution, carbon nanotube fabric, and method for producing a nanotube solution

Abstract

Carbon nanotube solution, carbon nanotube fabric, and method for producing the solution are provided. Carbon nanotube solution comprising: a liquid medium; and one or more carbon nanotubes in the liquid medium; wherein the nanotube solution is substantially free of amorphous carbon. A carbon nanotube fabric comprised of a densified carbon nanotube wherein the carbon nanotube fabric is substantially free of amorphous carbon. A method for producing a nanotube solution with significantly low amount or controlled amount of amorphous carbon in nanotube solution, the method comprising: mixing a raw carbon nanotube material and a liquid medium to prepare a carbon nanotube dispersion in the liquid medium; adjusting a pH of the mixture by adding a pH adjuster to be 10 or more 13 or less; and thereafter, purifying the carbon nanotube solution to obtain a purified nanotube solution, wherein the purified nanotube solution is substantially free of amorphous carbon impurities.

Description

NANOTUBE SOLUTION, CARBON NANOTUBE FABRIC, AND METHOD FOR PRODUCING A NANOTUBE SOLUTIONTECHNICAL FIELD

[0001] The present disclosure relates to carbon nanotube (CNT) solution, a nanotube fabric formed from the nanotube solution, and methods for forming the nanotube solution and the nanotube film or fabric. More particularly, the present disclosure relates to solutions or dispersions of carbon nanotubes with high concentrations and low contaminants, and methods for purifying the CNT solution or dispersions.BACKGROUND

[0002] Recently, carbon nanotubes (hereinafter also referred to as “CNTs”) have attracted attention as materials that are lightweight as well as have excellent electrical conductivity and mechanical properties. CNTs are a sheet of carbon atoms rolled in a cylindrical shape, having diameters measured in nanometers which is about 100,000 times thinner than a human hair. These tiny cylinders are 50 times stronger than steel, half the density of aluminum, and have better properties (i .e., chemical, electrical, mechanical, and thermal) than other materials. Because of their size and exotic properties, they became novel materials for various applications. CNTs can be made in a single sheet to make up a single-walled carbon nanotubes (SWCN) or multi-walled walled carbon nanotubes (MWNTs). Due to their electrical properties, such as conducting and semi-conducting capabilities, CNTs may be used for various electronic elements.

[0003] For example, a carbon nanotube solution may be used to manufacture CNT films or CNT fabrics that may be used to manufacture various semiconductor devices, such as memory devices and logic devices. The nanotube-based random access memory i s a type of memory based on the position of carbon nanotubes deposited on a chip-like substrate, and is called to be the next-generation memory technology that i s fast, and capable of packing massive amounts of memory.

[0004] The common methods of manufacturing CNTs include various chemical vapor deposition (CVD) methods or other vapor phase growth techniques (electric-arc discharge, laser ablation, etc.).

[0005] The CVD process involves taking a carbon source in the gas phase andusing an energy source, such as a plasma or a resistively heated coil, to transfer energy to a gaseous carbon molecule. The CVD process uses hydrocarbons for the carbon source including methane, carbon monoxide, and acetylene. The hydrocarbons are pumped into a reaction chamber at a high temperature. The heat can be anywhere between 200 and 2200 °C . At a high temperature, the bond between carbon and hydrogen is broken, producing pure carbon molecules. Then, the carbon will diffuse toward the substrate, which is heated and coated with a catalyst where it will bind, creating a thin layer of film. It produces CNT if the proper parameters are maintained.

[0006] The electric-arc discharge method can be applied by resulting energy from DC between the graphitic anode and cathode with a chamber filled with helium gas. The arc occurred creates a high-temperature plasma, which sublimes the carbon precursors in the anode. The arc di scharge method provides fast production of CNTs with few defects.

[0007] Laser ablation is a method that prepares CNTs at high temperatures using a laser, in which the process was generally conducted in an inert atmosphere. This method transforms carbon species into graphitic materials with a metal catalyst, using a laser applied to vaporize the carbon. This method provides CNTs with higher purity than those generated by arc discharge.

[0008] S ome patents relate to methods of manufacturing a carbon nanotube solution, a carbon nanotube film or fabric formed from the carbon nanotube solution, and methods for forming the nanotube solution and the nanotube film or fabric.

[0009] For example, US20130243954A1 discloses CNT solutions and methods of purifying the solution. The methods include mixing, at least one complexing agents, at least one ionic species, and / or at least one buffer oxide etch (BOE) with a liquid medium containing CNTs and different types of contaminants, such as metal impurities, amorphous carbon, and / or silica particles, and performing a filtration process to the liquid medium to reduce the contaminants in the liquid medium. As a result, carbon nanotube solutions of low contaminants are produced.

[0010] Moreover, US 1 1072714 di scloses a nanotube solution being treated with a molecular additive, a nanotube film having enhanced adhesion properties due to the treatment of the molecular additive, and methods for forming the nanotube solution and the nanotube film. The nanotube solution includes a liquid medium, nanotubes in the liquid medium, and a molecular additive in theliquid medium, wherein the molecular additive includes molecules that provide source elements for forming a group IV oxide within the nanotube solution. The molecular additive can introduce silicon (Si) and / or germanium (Ge) in the liquid medium, such that nominal silicon and / or germanium concentrations of the nanotube solution ranges from about 5 ppm to about 60 ppm.

[0011] Furthermore, US20180186644A1 discloses the preparation method for purified carbon nanotubes having low carboneous impurities (amorphous carbon) nor inorganic impurities (metal and metal oxides). The CNT starting material is preferably in the form of water, or film for efficient diffusion of purifying media. The highly pure CNT is prepared by heat treating a CNT starting material in a specific amount of oxygen, then treating the CNT in a solution comprising water and acid, or further heat treating the CNT in an atmosphere comprising chlorine. Removal of chloride from purified CNT may be achieved by further treating the chlorine-treated material in the atmosphere comprising hydrogen.

[0012] However, conventionally proposed sheets such as those disclosed above have rooms for improvement since the sheets still contain a significant amount of amorphous carbon as one of the contaminants. Since amorphous carbon may have irregular shapes and / or sizes, with ununiform distribution in the CNT, their existence may cause defects in the semiconductor devices and may render unexpected behavior in the circuitry. Some of the undesirable features affect structures (uneven thickness of the nanotube fabric layer, and / or protrusions in the nano-tube fabric layer, etc.), or electric components (unexpected sheet resistance, etc.). Thus less amount or a controlled amount of amorphous carbon in CNTs is desirable.

[0013] An obj ect of the present di sclosure is to provide a carbon nanotube solution with excellent properties by reducing and controlling amorphous carbon in the carbon nanotube solution, as well as its manufacturing process. The present disclosure also provides a carbon nanotube film or fabric that may be used in semiconductor devices, such as nanotube-based random access memory.SUMMARY

[0014] While prior arts relate to general carbon nanotube products and manufacture thereof, the present disclosure provides a carbon nanotube solution, and carbon nanotube fabric with significantly low or controlled amount of amorphous carbon using a novel technique. Another obj ect of the presentdisclosure is to provide such a method for producing a nanotube solution that allows removal of amorphous carbon while leaving high concentration of carbon nanotubes in the solution.

[0015] In order to achieve the obj ect, one aspect of the present di sclosure is directed to a nanotube solution comprising: a liquid medium; and one or more carbon nanotubes in the liquid medium; wherein the nanotube solution is sub stantially free of amorphous carbon.

[0016] In one embodiment, the liquid medium is an aqueous solvent, or a nonaqueous solvent, or any mixture thereof.

[0017] In addition or alternatively, the carbon nanotubes are one of single-wall carbon nanotubes, multi-wall carbon nanotubes, or any mixture thereof.

[0018] In addition or alternatively, the length of the carbon nanotube ranges between 5 nm to 5 pm.

[0019] In addition or alternatively, a concentration of the carbon nanotubes ranges between 10 mg and 900 mg with respect to 1 liter of the nanotube solution.

[0020] In addition or alternatively, the nanotube solution is substantially free of impurities having a diameter greater than 500 nm .

[0021] In addition or alternatively, the nanotube solution comprises less than 500 parts per billion of metallic impurities.

[0022] As used herein, the term “nanotube solution” generally includes both solution and dispersion. Moreover, as used herein, the term “ substantially free” generally refers to a concentration of less than or equal to 1 part-per-billion (ppb). Further, as used herein, the term “diameter” generally refers to a straight line passing through the center of a circle or sphere and meeting the circumference or surface at each end. Furthermore, as used herein, the term “impurity” generally refers to the substance inside a confined amount of liquid, gas, or solid, which differs from the chemical composition of the material .

[0023] In another embodiment, the present disclosure provides a carbon nanotube fabric having a carbon nanotube fabric comprised of a densified carbon nanotube wherein the carbon nanotube fabric is substantially free of amorphous carbon.

[0024] In one embodiment, the length of the carbon nanotube ranges between 5 nm to 5 pm.

[0025] In addition or alternatively, the carbon nanotube fabric is substantiallyfree of impurities having a diameter greater than 500 nm .

[0026] In addition or alternatively, the carbon nanotube fabric comprises less than about 15 * 1010atoms / cm2of metallic impurities.

[0027] In addition or alternatively, the carbon nanotubes are one of single-wall carbon nanotubes, multi-wall carbon nanotubes, or any mixture thereof.

[0028] As used herein, the term “nanotube solution” generally includes both solution and dispersion. Moreover, as used herein, the term “ substantially free” generally refers to a concentration of less than or equal to 1 part-per-billion (ppb). Further, as used herein, the term “diameter” generally refers to a straight line passing through the center of a circle or sphere and meeting the circumference or surface at each end. Furthermore, as used herein, the term “impurity” generally refers to the substance inside a confined amount of liquid, gas, or solid, which differs from the chemical composition of the material .

[0029] Another obj ect of the present disclosure is to provide a method for producing a nanotube solution with significantly low amount or controlled amount of amorphous carbon in nanotube solution, the method comprising: mixing a raw carbon nanotube material and a liquid medium to prepare a carbon nanotube di spersion in the liquid medium; adjusting a pH of the mixture by adding a pH adjuster to be 10 or more 13 or less; and thereafter, purifying the carbon nanotube solution to obtain a purified nanotube solution, wherein the purified nanotube solution i s substantially free of amorphous carbon impurities.

[0030] In one embodiment, the pH adjuster is a hydroxide base.

[0031] In addition or alternatively, the pH adjuster promotes separation of carbon nanotube from bound carbon fragments and free amorphous carbon.

[0032] In addition or alternatively, the purifying process comprises filtering by a hollow fiber filter.

[0033] In addition or alternatively, the hollow fiber filter comprising membrane formed of modified polyethersulfone, polysulfone, or mixed ester cellulose.

[0034] In addition or alternatively, the liquid medium is an aqueous solvent, a non-aqueous solvent, or any mixture thereof.

[0035] In addition or alternatively, a concentration of the carbon nanotubes ranges between 10 mg and 900 mg with respect to 1 liter of the nanotube solution.

[0036] In addition or alternatively, the nanotube solution is substantially free of impurities having a diameter greater than 500 nm .

[0037] As used herein, the term “nanotube solution” generally includes both solution and dispersion. Moreover, as used herein, the term “ substantially free” generally refers to a concentration of less than or equal to 1 part-per-billion (ppb). Further, as used herein, the term “diameter” generally refers to a straight line passing through the center of a circle or sphere and meeting the circumference or surface at each end. Furthermore, as used herein, the term “impurity” generally refers to the substance inside a confined amount of liquid, gas, or solid, which differs from the chemical composition of the material .

[0038] These and other aspects may be understood more readily from the following description and the appended drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIG. l shows a flow diagram illustrating a method for producing or purifying a CNT solution in accordance with one embodiment consistent with the present disclosure.

[0040] FIG. 2 illustrates a sectional view of a filtration device for performing a cross flow filtration process in accordance with one embodiment consistent with the present disclosure.

[0041] FIG. 3A and 3B are graphs providing measurements for amorphous carbon via TGA for prefilter and postfilter CNT solutions.

[0042] FIG. 4 is a diagram illustrating amorphous carbon fractionation process to determine the amount of carbon nanotube, free amorphous carbon, and bound carbon fragments.

[0043] FIG. 5 is a scanning electron micrograph (SEM) image showing results of the fractionation process of a CNT solution treated with tetramethylammonium hydroxide (TMAH), in accordance with one embodiment consistent with the present disclosure.DETAILED DESCRIPTION

[0044] Hereinafter, the present di sclosure will be described in detail with reference to embodiments and examples. However, the present disclosure is not limited to the following embodiments and examples, and may be freely modified for implementation without departing from the scope of claims of the present invention and the scope of their equivalents.

[0045] A novel method is described below that allows the removal of amorphous carbon in the carbon nanotube solution without damaging the CNT. The present disclosure also teaches the production of carbon nanotube solutions or application solutions that can be used for various applications of CNTs. Some of the materials produced from the carbon nanotube solutions may include a fabric of single or multiple, interconnected carbon nanotubes. A fabric of nanotubes in the present disclosure may have a structure of multiple entangled nanotubes that are irregularly arranged relative to one another. Alternatively, the fabric of nanotubes for the present disclosure may possess some degree of positional regularity of the nanotubes, e.g., some degree of parallelism along their long axes. Such positional regularity may be found, for example, on a relatively small scale wherein flat arrays of nanotubes are arranged together along their long axes in rafts on the order of one nanotube long and ten to twenty nanotubes wide. In other examples, such positional regularity may be found on a larger scale, with regions of ordered nanotubes, in some cases, extended over substantially the entire fabric layer.

[0046] CNT Solution

[0047] For developing various applications involving high-density carbon nanotube (i .e., solutions, fabrics, memories, etc.) having a highly concentrated carbon nanotube in the solution while minimizing the presence of impurity is necessary.

[0048] With respect to 1 liter of the nanotube solution, a concentration of the carbon nanotube may be 1 mg or more, preferably 10 mg or more, preferably 50 mg or more, preferably 75 mg or more, preferably 100 mg or more, preferably 250 mg or more, preferably 500 mg or more, preferably 750 mg or more and 1000 mg or less, preferably 900 mg or less, preferably 750 mg or less, preferably 500 mg or less, preferably 250 mg or less, preferably 100 mg or less, preferably 75 mg or less, or preferably 50 mg or less. Keeping the concentration of CNT within the ranges may provide various advantages. For example, it may increase the stability of the CNT solution that may prolong the life of the solution. It may also l ead to reliable thickness of the CNT fabric which would be important during a spin coating process, which will be discussed below. A common concentration used for memory and logic applications is about 100 mg with respect to 1 liter of the nanotube solution. Such a concentration is exemplary; various useful concentrations ranges depend upon the application. For example, in the case where a monolayer fabric isdesired, one could use a more diluted concentration with a single or a few applications of the applicator liquid, e.g. , by spin coating, to the sub strate. In the event that a thick multilayer fabric is desired, a spraying technique could be employed with a nearly saturated composition of nanotube in the applicator liquid. The concentration is, of course, dependent upon the specific liquid medium choice, method of nanotube di spersion and type of nanotube used, e.g. , single-walled nanotubes (SWNT), double-walled nanotubes (DWNT) or multiwalled nanotubes (MWNT) and / or mixture thereof.

[0049] As the concentration of CNT solution increases to a higher level, several significant challenges arise. For example, when a higher purity level of the CNT solution is requested, it would require an increased amount of nanotube concentrations, thereby increased levels of impurities may be accompanied into the CNT solution. For example, in order for nanotube concentrations higher than 333 mg / L to be useful in semiconductor processing, the nanotube dispersions may have to be purified such that the impurity level i s reduced to 0.01 wt % or less. Moreover, as the concentration of nanotubes increases in the nanotube dispersion with increasing purity levels, the stability of the solution can become problematic. These impurities or contaminants may be inorganic (metal impurities) or carboneous (amorphous carbon).

[0050] For example, the initial CNT materials may contain metal contaminants as metal catalysts are typically used in CNT production. These metal catalysts are difficult to be completely removed as some are embedded in the CNTs. Typical CNT raw materials contain about 5% of iron (Fe) and other residual metal catalysts remain even after extensive acid washing with HC1. On the other hand, in the semiconductor industry, there is a stringent requirement for the maximum amount of metal contaminants allowed in the CNT solution. In certain manufacturing processes, the concentration of metal contaminants in the CNT solution must be less than 25 parts per billion (ppb). Accordingly, it is necessary to purify the CNT solution such that the metal impurities contained in the CNT solution can be reduced to an amount below the required maximum value.

[0051] The CNT raw materials may contain amorphous carbon as one of the contaminants. Because amorphous carbon may have irregular shapes and / or sizes, their existence in the CNT solution, and ultimately in the various applications involving CNTs, they may cause defects in the devices and thus renders unexpected behavior, especially in the circuitry. For example, non-uniform di stribution of amorphous carbon and other impurities in the fabric may result in an irregular distribution of electrical current, which can prevent the reliable performance of the semiconductor device. In other situations, the defects may be structural, such as uneven thickness of the nanotube fabric layer since the nanotube fabric layer would be slightly thicker with the presence of amorphous carbon, and / or protrusions in the nanotube fabric layer. Another consideration due to the presence of amorphous carbon may be the interruption of CNT-CNT wall contact. Accordingly, the presence of amorphous carbon in CNT solution needs to minimized, or at least controlled.

[0052] Moreover, single-walled nanotubes (SWNT) may typically have a diameter of about 1 -3 nm, and multi-walled nanotubes (MWNT) may typically have a diameter of about 3 -30 nm . Also, the mean or median length of the carbon nanotube may be 5 nm or more, preferably 10 nm or more, preferably 20 nm or more, preferably 50 nm or more, preferably 0. 1 pm or more, preferably 0.5 pm or more, preferably 1 pm or more and 5 pm or less, preferably 1 pm or less, preferably 0.5 pm or less, preferably 100 nm or less, preferably 50 nm or less, preferably 20 nm or less, preferably 10 nm or less. Keeping the length of CNT within such a range would be important because, as the length of CNT increases, the dispersibility of in CNT solution decreases. The length of CNT may also affect the electrical performance when CNT product is implemented within a device. Moreover, the carbon nanotube may be single-walled nanotubes, multi-walled nanotubes or a mixture of both and can be controlled as is necessary for a particular application such as memory, switches, relays, chemical sensors, biosensors and resonators.

[0053] The CNT synthesis method usually begins with raw nanotube materials in dry powder form . In some embodiments, it may be necessary to suspend, disperse, solvate, or mix the CNT powder in a liquid medium. The liquid medium may be an aqueous solvent, a non-aqueous solvent, or mixture thereof that would be used to form the carbon nanotube solution. To form an aqueous CNT solution, it is necessary to dissolve the CNT powder in water. Water i s readily available and may be selected based on its compatibility or compli ance with the manufacturing industry standards. Water can be deionized or di stilled depending on the intended application and / or preference. Another consideration is the purity of water. The water may be of high purity and its use of it may depend on the intended application. The intended application may include the formation of carbon nanotube layers, carbon nanotube films and / or carbonnanotube fabrics.

[0054] Moreover, the CNT solution of the present disclosure need not necessarily be homogeneously distributed in a liquid medium containing water, or containing substantially water, or even containing only water. For example, the liquid medium contain or be predominantly organic solvents such as ethyl lactate, dimethyl sulfoxide (DMSO), monomethyl ether, 4-methyl-2 pentanone, N-methylpyrrolidone (NMP), t-butyl alcohol, methoxy propanol, propylene glycol, ethylene glycol, gamma-butyrolactone, benzyl benzoate, salicylaldehyde, tetramethyl ammonium hydroxide, and esters of alpha-hydroxy carboxylic acids. In other situations, the liquid medium may contain or be predominantly a nonhalogenated solvent.

[0055] F or heavy metals, for example, metals having a specific gravity of greater than about 5 g / ml, are generally toxic in relatively low concentrations to plant and animal life and tend to accumulate in the food chain. Examples include lead, mercury, cadmium, chromium, and arsenic. Such compounds are carefully regulated in the industry and are desirably maintained at minimum levels. In one or more embodiments, the CNT solution of the present disclosure, when placed on a surface, may include less than about 500 ppb, or less than about 200 ppb, or less than about 50 ppb, or less than about 25 ppb, or about 0.1 to 10 ppb of trace heavy metal impurities.

[0056] S imilarly, the concentration of group I and group II elements is regulated due to the deleterious effect of elements such as sodium, potassium, magnesium and calcium, and the like, on the performance characteristics of the electronic device. For example, the CNT solution of the present di sclosure, when placed on a surface, may include less than about 500 ppb, or less than about 200 ppb, or less than about 50 ppb, or less than about 25 ppb, or about 1 to 25 ppb of trace alkali (group I element) and alkaline earth (group II element) impurities.

[0057] Transition metals may also be avoided due to their ready migration and the deleterious effect that such migration has on the device performance. See, Mayer, et al . Electronic Materials Science: For Integrated Circuits in Si and GaAs, 2nd Ed, Macmilliam, New York, 1988, which is incorporated herein by reference. As is the case for heavy metals and group I and group II metals, it may be desirable to maintain the impurity level of transition metals, such as copper, iron, cobalt, molybdenum, titanium and nickel, to less than preselected values. In one or more embodiments of the present invention, the applicatorliquids of the present invention, when placed on a surface, may include less than about 500 ppb, or less than about 200 ppb, or less than about 50 ppb, or less than about 25 ppb, or about 0.1 to 10 ppb of transition metal impurities.

[0058] The semiconductor industry has well-established testing protocols for monitoring the particulate level s at, for example, 5 pm, 3 pm, 1 pm, 500 nm, 300 nm and 100 nm. The metrology employed for detecting the particulate contaminate will have a resolution of 0.2 nm. Typical equipment includes Tencor surfscan® (KLA Corporation, CA) and the like. Such testing methods and equipment may be readily adapted for use in evaluating the particulate levels of nanotube compositions.

[0059] Furthermore, in some cases, substantially free of various impurities may refer to a concentration of less than or equal to 1 part-per-billion (ppb). Accordingly, extensive purification processes may be performed to the CNT solution. In some cases, the purification processes may include one or more of a cross-flow filtration (CFF) process, a filtration process, sonication, centrifugation, treatments with certain chemicals, and / or any combinations thereof, which will be discussed below.

[0060] CNT Fabric Layer

[0061] The fabrics of nanotubes may retain desirable physical properties of the nanotubes from which they are formed. For example, in some electrical applications, the fabric preferably has a sufficient amount of nanotubes in contact so that at least one ohmic (metallic) or semi-conductive pathway exists from a given point within the fabric to another point within the fabric. Singlewalled nanotubes (SWNT) may typically have a diameter of about 1 -3 nm, and multi-walled nanotubes (MWNT) may typically have a diameter of about 3 -30 nm. The nanotubes may curve and occasionally cross one another. Gaps in the fabric, i . e. , between nanotubes either laterally or vertically, may exist. Such fabrics may include single-walled nanotubes, double-walled nanotubes (DWNT), multi-walled nanotubes, and / or mixture thereof.

[0062] The fabric may have small areas of di scontinuity with no tubes present. The fabri c may be prepared as a l ayer or as multiple fabric layers, one formed over another. The thickness of the fabri c can be chosen as thin as substantially a monolayer of nanotubes or can be chosen much thicker, e.g. , tens of nanometers to hundreds of microns in thickness. The porosity of the fabrics can vary from low-density fabrics with high porosity to high-density fabrics with low porosity. The average length of a nanotube ranges between 50- 1000 nm and 1 - 100 pmincluding single-walled nanotubes, multi-walled nanotubes or a mixture of both and can be controlled as is necessary for a particular application such as memory, switches, relays, chemical sensors, biosensors and resonators.

[0063] High-density nanotube fabric layers may reveal a much lower surface resistance than that of the nanotube fabrics of normal density . For example, the carbon nanotube fabric density is approximately 10 nanotubes per 0.2 >< 0.2 um area. In addition, the high-density nanotube fabrics layer may be used to construct three-dimensional (3D) semiconductor memory devices. Because a high-density nanotube fabric layer may have a lower surface resistance, the 3D memory devices so constructed would consume less power than regular memory devices.

[0064] To fabricate memory and / or logic devices in an industrial scale, there is a need to develop a Complementary metal-oxide-semiconductor (CMOS) grade formulation of nanotube dispersions. The CMOS-grade formulation may require that the nanotube di spersions be of ultra-high purity, that is, substantially free from surfactants, trace metals, and / or other additives. In some cases, substantially free may refer to a concentration of less than or equal to 1 part- per-billion (ppb). In other situations, concentrations of metallic or carbonaceous contamination that are above those required for CMOS fabrication may be acceptable. The present disclosure serves to exemplify creation of CNT solution with stringent requirements that meet or exceed those of a CMOS process flow but can be modified in applications that have relaxed requirements.

[0065] Appropriate purification techniques may apply to the CNT solution to remove impurities to produce a purified nanotube solution without affecting the nanotube chemical structure or electronic properties in the CNT fabric layer. Impurities may be removed for example, by dispersing the nanotubes in dilute acid solution to dissolve metal impurities, followed by separation of the nanotubes from the metallic solution. A mild acid treatment with nitric acid or hydrochloric acid may be used. Other suitable methods for metal removal include magnetic purification. Amorphous carbon may be removed, for example, by a combination of high speed centrifugation using an ultracentrifuge and filtration techniques for example but not limited to gravity filtration, cross flow filtration, vacuum filtration and others. Other suitable purification techniques include the preferential oxidation of non-fullerenic carbonaceous materials. Multiple purification steps may be desired in order to obtain nanotubes of a purity for use in a CMOS-grade nanotube solution. See, for example, Chiang, etal. , J. Phys.ChemB 105, 1 157 (2001 ); and Haddon, et al ., MRS Bulletin, April 2004).

[0066] In one or more embodiments, the nanotubes composition contains less than about 1018atoms / cm3of metal impurities, or less than about 1016atoms / cm3of metal impurities, or less than about 1014atoms / cm3of metal impurities, or less than about 1012atoms / cm3of metal impurities, or less than about 1010atoms / cm3of metal impurities . Compositions having lower levels of metallic impurities, e.g. ca. 1010- l 012atoms / cm3, may be used in the manufacture of advanced devices having fine features, for example, devices having features of less than or equal to 250 nm .

[0067] Heavy metals, for example, having a specific gravity of 5 g / ml, are generally toxic in relatively low concentrations to plant and animal life and tend to accumulate in the food chain. Examples include lead, mercury, cadmium, chromium, and arsenic. Such compounds are carefully regulated in the semiconductor fabrication industry and are desirably maintained at minimum levels. For example, the nanotube composition may include less than about 1018atoms / cm3of heavy metal impurities, or less than about 1016atoms / cm3of heavy metal impurities, or less than about 1014atoms / cm3of heavy metal impurities, or less than about 1012atoms / cm3of heavy metal impurities or even less than about 15 * 1010atoms / cm3of heavy metal impurities.

[0068] S imilarly, the concentration of group I and group II elements is regulated due to the deleterious effect of elements such as sodium, potassium, magnesium and calcium, and the like, on the performance characteristics of the electronic device. For example, the nanotube composition may include less than about 1018atoms / cm3of group I and group II element impurities, or less than about 1016atoms / cm3of group I and group II element impurities, or less than about 1014atoms / cm3of group I and group II element impurities, or less than about 1012atoms / cm3of group I and group II element impurities or even less than about 15 x 1010atoms / cm3of group I and group II element impurities.

[0069] Lastly, transition metals are also avoided due to their ready migration and the deleterious effect of such migration to the device performance. See, Mayer, et al . Electronic Materials Science: For Integrated Circuits in Si and GaAs, 2nd Ed, Macmilliam, N.Y. , 1988, which is incorporated herein by reference. As is the case for heavy metal s and group I and group II metals, it is desired to maintain the impurity level of transition metals, such as copper, iron, cobalt, molybdenum, titanium and nickel, to less than preselected values. Forexample, the nanotube compositio r may include less than about 1018atoms / cm3of transition metal impurities, or less than about 1016atoms / cm3of transition metal impurities, or less than about 1014atoms / cm3of transition metal impurities, or less than about 1012atoms / cm3of transition metal impurities or even less than about 15 x l 010atoms / cm3of transition m etal impurities.

[0070] The impurity content of the nanotubes can be monitored using conventional methods, such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM) and using analytical techniques such as x- ray microanalysis (ED AX), or Vapor Phase Decomposition and Inductively- Coupled Plasma Mass Spectrometry (VPD, ICP / MS).

[0071] Metallic impurity levels may be measured using conventional methods such as EDAX and VPD, IPC / MS . If large quantities of solution (e g. , >about 1000 mL), are available for testing, direct volumetric concentration measurements (atoms / cm3) can be determined. Alternatively, a known volume of the composition may be deposited over a known surface area and the surface impurity concentration (atoms / cm2) can be determined.

[0072] The nanotubes are pretreated to reduce the particulate impurities levels to a preselected level . For example, the semiconductor industry has established standardized particulate impurity level s for particular processes, and the nanotubes may be pretreated to reduce the nanotube particulate levels to below the accepted level s. In one or more embodiments, the composition is substantially free of particle impurities having a diameter of greater than about 5 micron (pm), or greater than about 1 pm, or greater than about 3 pm, or greater than about 500 nm, or greater than about 300 nm, or greater than about 100 nm, or greater than about 45 nm.

[0073] Nanotube-Based Random Access Memory

[0074] The CNT solution produced can also be used to form a component of memories such as described in U. S . Pat. No. 6,919, 592, entitled“El ectromechanical Memory Array Using Nanotube Ribbons and Method for Making Same,” filed Jul . 25 , 2001 ; U. S . Pat. No. 6,643, 165, entitled “El ectromechanical Memory Having Cell Selection Circuitry Constructed with Nanotube Technology,” filed Jul . 25, 2001 ; U. S . patent application Ser. No. 10 / 810,962, entitled “NRAM Bit Selectable Two-Drive Nanotube Array,” filed Mar. 26, 2004; and U. S. patent application Ser. No. 10 / 810,963 , entitled “Non- Volatile RAM Cell and Array Using Nanotube Switch Position for InformationState,” filed Mar. 26, 2004. The disclosures of these patents and patent applications are incorporated herein by reference. The CNT solution holds potential as a stand-alone commercial product to serve the research and development laboratories that work on single-walled carbon nanotubes as well other applications.

[0075] Manufacturing Process

[0076] The CNT synthesis method usually begins with raw nanotube materials in dry powder form . In some embodiments, it may be necessary to suspend, disperse, solvate, or mix the CNT powder in a liquid medium. The liquid medium may be an aqueous solvent, a non-aqueous solvent, or mixture thereof that would be used to form the carbon nanotube solution. To form an aqueous CNT solution, it is necessary to dissolve the CNT powder in water. Water i s readily available and may be selected based on its compatibility or compli ance with the manufacturing industry standards. Water can be deionized or di stilled depending on the intended application and / or preference. Another consideration is the purity of water. The water may be of high purity and its use of it may depend on the intended application. The intended application may include the formation of carbon nanotube layers, carbon nanotube films and / or carbon nanotube fabrics.

[0077] Moreover, the CNT solution of the present disclosure need not necessarily be homogeneously distributed in a liquid medium containing water, or containing substantially water, or even containing only water. For example, the liquid medium contain or be predominantly organic solvents such as ethyl lactate, dimethyl sulfoxide (DMSO), monomethyl ether, 4-methyl-2 pentanone, N-methylpyrrolidone (NMP), t-butyl alcohol, methoxy propanol, propylene glycol, ethylene glycol, gamma-butyrolactone, benzyl benzoate, salicylaldehyde, tetramethyl ammonium hydroxide, and esters of alpha-hydroxy carboxylic acids. In other situations, the liquid medium may contain or be predominantly a nonhalogenated solvent.

[0078] In the event that a thick multilayer fabric is desired, a spraying techni que could be employed with a nearly saturated composition of nanotube in the applicator liquid. The concentration depends upon the specific liquid medium choice, method of nanotube di spersion and type of nanotube used (e.g., SWNT, DWNT, or MWNT and / or a mixture thereof) that would be appropriate for the intended applications. For example, the CNT solution may be used as an applicator liquid (or an application solution) to apply carbon nanotubes on asemiconductor substrate to form nanotube films.

[0079] A spin coating process, for example, may be used to evenly apply the nanotube elements over the sub strate. In the spin coating process, an excess amount of carbon nanotube solution is applied to a spinning substrate, often a silicon wafer on a rotating plate. The rotation is typically conducted at high speed and it spreads the fluid by centrifugal acceleration. The spinning is continued while the liquid is spun off the edge of the substrate and a thin uniform film of carbon nanotubes is left on the sub strate. The solvent on or in the film is typically volatile and evaporates from the film. One of the benefits of the spin coating process is that the carbon nanotube solution is applied to the substrate element evenly, thereby creating a substantially uniform layer of carbon nanotube elements. Other coating processes may involve spraying, dipping, silk screening, and gravure printing the CNT elements over the substrate.

[0080] Other well-known manufacturing embodiments may include chemical vapor deposition (CVD) or other vapor phase growth techniques (electric-arc discharge, laser ablation, combustion, and the like).

[0081] Moreover, in order to apply the nanotube solution to a sub strate uniformly, the nanotubes in the solution need to be distributed homogeneously in a liquid medium without substantial precipitation, flocculation or other macroscopic interaction, because the occurrence of these incidences would lead to nanotubes clumping together and form a non-uniform film which is undesirable.

[0082] There are several methods to detect precipitation, flocculation or other macroscopic interactions in the nanotube solution. In certain situations, visual observation at high magnification (greater than about 20,000x) may detect precipitation or flocculation in the solution. Another technique is called dynamic light scattering. The dynamic light scattering technique allows fast, non-invasive analysi s of many materials by analyzing the light scattered by a sample with a frequency change. Dynamic light scattering is measured by using a means of light traveling through the solution, and a change of light scattering intensity over time i s recorded by a detector placed normally to the beam direction or from multiple detectors placed at various angles.

[0083] Since carbon nanotubes tend to attract each other due to van der Waals interactions (atomic level forces between the individual nanotube elements) or through 7t-7i interactions (a stacking effect due to the presence of free electronsin the 7r-orbitals along the nanotube structure), carbon nanotubes may bundle together along their sidewalls. Amorphous carbon may be attracted by the carbon nanotubes through similar mechanisms. Further, amorphous carbon may be trapped in the bundles of carbon nanotube elements. Appropriate purification techniques are desired to remove impurities without affecting the nanotube’ s chemical structure or electronic properties

[0084] The nanotube solution, fabric, memory, or other article may contain less than about 1018atoms / cm2of metal impurities, or less than about1016atoms / cm2of metal impurities, or less than about 1014atoms / cm2of metal impurities, or less than about 1012atoms / cm2of metal impurities, or less than about 1010atoms / cm2of metal impurities.

[0085] Moreover, the nanotube solution, fabric, memory, or other article may include less than about 1018atoms / cm2of metallic impurities, or less than about 1016atoms / cm2of metallic impurities, or less than about 1014atoms / cm2of metallic impurities, or less than about 1012atoms / cm2of metallic impurities or even less than about 15 * 1010atoms / cm2of metallic impurities. For example, devices having fine features may be devices having features of less than or equal to 250 nm .

[0086] Moreover, the nanotube solution, fabric, memory, or other article may include less than about 1018atoms / cm2of heavy metal impurities, or less than about 1016atoms / cm2of heavy metal impurities, or less than about1014atoms / cm2of heavy metal impurities, or less than about 1012atoms / cm2of heavy metal impurities or even less than about 15 * I O10atoms / cm2of heavy metal impurities.

[0087] Furthermore, the nanotube solution, fabric, memory, or other article may include less than about 1018atoms / cm2of group I and group II element impurities, or less than about 1016atoms / cm2of group I and group II element impurities, or less than about 1014atoms / cm2of group I and group II element impurities, or less than about 1012atoms / cm2of group I and group II element impurities or even less than about 15 >< 1010atoms / cm2of group I and group II element impurities.

[0088] For heavy metals and group I and group II metals, it may be desirable to maintain the impurity level of transition metals, such as copper, iron, cobalt, molybdenum, titanium; and nickel, to less than preselected values. In one or more embodiments of the present invention, the nanotube film, fabric or other article may include less than about 1018atoms / cm2of transition metalimpurities, or less than about 1016atoms / cm2of transition metal impurities, or less than about 1014atoms / cm2of transition metal impurities, or less than about 1012atoms / cm2of transition metal impurities or even less than about 15 x l O10atoms / cm2of transition metal impurities.

[0089] Due to the requirement of low metal contaminants in the CNT solution, the CNT solution can be kept in containers, lines, or tubes made of non-metal materials, such as quartz or Teflon. Although metal or stainless steel containers are more robust and are generally more desirable in the fabrication process, using metal or stainless steel containers for the CNT solution may introduce metal contaminants into the CNT solution.

[0090] Removal of Amorphous Carbon

[0091] A pH adjuster may be added to the CNT solution. The pH adjuster includes, but not limited to, alkyl ammonium bases such as tetramethylammonium hydroxide (TMAH), hydroxide bases such as potassium hydroxide, sodium hydroxide, ammonia hydroxide, and any other known-pH adjusters. The pH adjuster may be added several times until the pH of the CNT solution i s adjusted preferably between 10 and 13 , more preferably 10.5 and 12.5 , and still more preferably at 12. The increase in pH of the CNT solution may promote separation of the free amorphous carbon (FAC) and / or bound carbon fragments (BCF) that are attached to carbon nanotube. The addition of the pH adjuster may render amorphous carbon particles or other impurities more soluble or dispersed in the liquid medium, thereby separating the amorphous carbon from the CNT solution via subsequent filtration and / or centrifugation processes. The addition of the pH adjuster may be performed in a single step or in multiple steps.

[0092] Even after the addition of the pH adjuster, binary metal ions (e.g. , Fe++, Ca++, etc. ) may still exist as counter ions for surface carboxylic acid groups. To remove these binary metal ions, complexing agents may be reintroduced in the CNT solution. The complexing agents may chelate the binary metal ions from the CNT surfaces. Then, the chelated binary metals may depart from the CNT sidewalls and become soluble in the CNT solution .

[0093] The ionic species (e.g. , the nitrate salts) may be added in the CNT solution before, after or during the addition of pH adjuster. The ionic species may be added in the CNT solution and introduce charges in the CNT solution which increase the conductivity (ionic strength) of the CNT solution. For example, when nitrate salts are used and dissolved in the CNT solution,negatively charged nitrate ions (NO3 ~) are introduced in the CNT solution. The presence of charged ions or particles may interfere with the van der Waals and the 7t-7t interactions among the nanotubes. As a result, the nanotubes may repel each other at locations where charged particles are present and attract each other at locations where charged particles are absent. Thus, a high concentration of nitrate ions within the CNT solution may separate nanotubes apart from each other and screen the amorphous carbon from the nanotubes.

[0094] In addition, complexing agents may be added before, after, or simultaneously with the addition of the ionic species for the removal of metal impurities. The complexing agent (or chelating agent) i s defined as chemicals that form soluble, complex molecules with certain metal ions, inactivating the ions so that they cannot normally react with other elements or ions to produce precipitates or scale. Exampl es of such a complexing agents can include chemicals such as ethylenediaminetetraacetic acid (EDTA), diethylene triamine pentaacetic acid (DTPA), etidronic acid, or combinations thereof. In addition, the complexing agents may be added to the first CNT solution before, after, and / or between the sonicating, grinding, and / or agitating processes.

[0095] Thereafter, a sonication process may be performed to the CNT solution which contains the ionic species and / or the complexing agents to enhance the homogeneity of the nanotube elements dispersed in the CNT solution. The initial CNT material may also contain silica (SiCh) as one of the contaminants. An extensive sonication process may also transport silica into the CNT solution, if the CNT solution i s contained in a quartz- or glass-type container during the sonication process. Silica in the CNT solution appears to be in particle form. Accordingly, if the silica-contaminated CNT solution is used to form a CNT film, the exi stence of silica particles of sufficient size may become defects in the CNT film. Therefore, a purification process that controls the level s of silica in the CNT solution is desirable. One alternative approach to prevent silica contaminants is to carry out the processing of CNT solutions in metallic vessels. However, such an alternative approach may introduce other impuriti es, such as metal .

[0096] Opti onally, other carbon materials may be added to or mixed with the first CNT solution.

[0097] In one embodiment, after the CNT solution is treated with various techniques described above, one may perform a cross flow filtration (CFF) process to the CNT solution. The CFF is performed using 100 nm hollow fiberfilter to remove the amorphous carbon. The membrane material options for the filtration may include modified polyethersulfone, polysulfone, mixed ester cellulose, and others. Because amorphous carbon is screened from the nanotube elements due to the presence of nitrate ions, it becomes easier to remove the amorphous carbon from the CNT solution through filtration.

[0098] Because carbon nanotubes may have a length of about 1.2 microns or more, pores of membrane may have a diameter smaller enough to prevent carbon nanotubes from permeating out of membrane. In the present disclosure, for the filtration process, the pore size of hollow fiber membrane is 650 nm .

[0099] Then centrifugation process may be used to remove the acidic waste, as well as some metal ions until all of the nanotube sediments are at the bottom of the centrifuge tube.

[0100] Removing all amorphous carbon from the CNT solution, however, may alter the properti es of the CNT solution or fabric (e g. , ordering of the fabric into colloidal nematic crystal structure thereby forming rafting structures, etc.). If necessary, the amorphous carbon may be added to the CNT solution in a controlled manner in order to prevent the property alteration of the CNT fabric. Moreover, with the controlled amount of amorphous carbon in the CNT solution would lead to very smooth and randomly oriented CNT fabric.

[0101] In developing dispersions with this novel method, the manufacture of CNT solution may retain control over the system electrostatics and interactions in the coated film / substrate while minimizing the amount of amorphous carbon.

[0102] EXAMPLE 1

[0103] Referring to FIG. 1 , there is illustrated a method for producing or purifying a CNT solution in accordance with one embodiment consistent with the present disclosure. While different contaminants may be removed in one or more steps consistent with the present disclosure, this particular example focuses on a method that can remove amorphous carbon from the CNT solution.

[0104] In Step S 101 , a first CNT solution is provided. The first CNT solution may be prepared by dispersing acid oxidized CNT raw materials in water. It has been discovered that carbon nanotubes may form stable dispersions in water. For example, raw CNT material may be used as a starting point. The first CNT solution likely contains high levels of various impurities, including amorphous carbon. The first CNT solution may be contained in a stainless steel container.

[0105] In Step S 102, the pH adjuster is added to the first CNT solution. The pH of the CNT solution was adjusted to 10- 12 by addition oftetramethylammonium hydroxide (TMAH). As shown in Table 1 below, the increase in pH of the CNT solution may be beneficial in removing the Free Amorphous Carbon (FAC) and / or Bound Carbon Fragments (BCF) which are attached to CNTs. It is important to note that different molar concentrations lead to different levels in pH: Samples 1 - 6 involves pH 1 1 to pH 12.7; Samples Al - A8 involves repeated test and involves pH 10 to pH 12.7; and Samples B l - B8 involves in process solution (pre-slurry) and involves pH 10 to pH 12.7.[01061 TABLE 1FOD Test with Slurry and Pre-Slurry

[0107] In Step S 104, 35ppm nitrate was added to dilute the CNT solution to OD 0.7 with Sol . pH 12.0.

[0108] In Step S 105, after mixing the ionic species with the first CNT solution, the first CNT solution may be sonicated for about 30 minutes in a chilled sonication bath at about 15° C. (or at any temperature within the range of about 5-25 ° C ). The sonication process may enhance the homogeneity of the nanotube dispersions and, at the same time, allow the complexing agent to effectively mix with and therefore chelate the binary metal remaining on the CNT surfaces. Normally, the complexing agent can form complexes with binary metal ions such that they cannot react with other elements.

[0109] Once the sonication process is completed, a cross flow filtrati on or other suitable filtration processes that allows for the separation of the CNTs from the contaminant-laden liquid may be performed in Step S 106. In one embodiment, the cross flow filtration process is performed with a l OOnm 47mm polyethersulfone (PES) membrane syringe filter, such as the filtration device 200 illustrated in FIG. 2. The filtration device 200 includes an inlet 210, an outlet 220, an exhaust 230, and a membrane 240 having a plurality of pores.

[0110] In Step S 106, the cross flow filtration process is performed by feeding the first CNT solution into inlet 210. Some liquid medium of the first CNT solution may pass through pores of membrane 240. The liquid medium that passes through pores is called a permeate. The permeate flows out of filtration device 200 from exhaust 230 and is rej ected in Step S 106. Because complexing agents are soluble in water, the permeate may include the complexing agents that have reacted with the binary metal from the CNT surfaces.

[0111] The liquid medium that does not pass through pores is called a retentate. The retentate exits filtration device 200 through outlet 220 and is recovered. Because binary metals may be removed in the permeate, the recovered retentate may include fewer metal impurities as compared with the first CNT solution before adding the ionic species and before performing the cross flow filtration process. Subsequently, Step S 106 was repeated several times so as to produce a second CNT solution with a desired purity level for certain impurities. As a result, the second CNT solution contained significantly fewer metal impurities than the first CNT solution. Steps S 104, S 106, and / or S 107 may be repeated for multiple times until a concentration of each of the individual impurities become lower than a predetermined concentration (e.g. , 25 ppb). In one embodiment,Steps S 104, S 106, and S 107 are repeated for two or more times. In another embodiment, only Steps S 105 and S 106 are repeated for two or more times. Although Step S 106 has been described as a process to remove amorphous carbon, it is to be understood that Step S 106 may also remove other impurities and other small particles in the first CNT solution.

[0112] The second CNT solution obtained from Steps S 104, S 106, and S 107 may still include larger particles (e g. , molecular weight species). Accordingly, in Step S 108, centrifugation may be performed to the second CNT solution in order to remove the larger particles in the liquid. An initial centrifugation step S 108 was performed at 25,000 rpm for 2 hours. The remaining liquid was then manually (by hand) transferred from the centrifugation vial into another vial . The sediment was di scarded. To ensure that the second CNT solution is not contaminated again after this centrifugation step, the solution was transferred to a container through a pump process, thereby avoiding any possible human contamination or other extraneous contaminants.

[0113] Subsequently, in Step S 109, a third CNT solution is obtained. The third CNT solution includes a low level of metal contaminants and is substantially free from amorphous carbon and other large and small particles. Although Step S 106 may be performed several times to permeate the ionic species, complexing agents, and binary metals, once added, it is very difficult to exhaust all of the complexing agents from the CNT solution. Accordingly, the third CNT solution may still contain a very small amount of residual complexing agents. Regardless of the amount, the existence of residual complexing agents in the third CNT solution may indicate that the third CNT solution has been treated with complexing agents. In one embodiment, a residual complexing agent contained in the third CNT solution may have a concentration of less than 25 ppb .

[0114] Amorphous Carbon Characterization Process

[0115] The effectiveness of the above process for removing amorphous carbon from the CNT solution is discussed below.

[0116] The thermogravimetric analysis (TGA) is performed on the CNT solution before and after the filtration process S 106. The purpose of conducting TGA is to find out the decomposition temperature of amorphous carbon and to estimate the wt% of the amorphous carbon in the CNT samples. Perkin Elmer Hyphenated TGA-FTIR-GCMS Tl-900 system was used in this process.

[0117] The testing sample was prepared with the following procedure. First, TGA pan was tagged in the TGA instrument and the loading sample was takenout. A hot plate with a clean pre-weighted TGA crucible was heated to 100°C . Then droplets of CNT formulation was placed were placed on the hot TGA crucible. The droplets were allowed to dry (20-30 minutes) to form thin films. This process was repeated to load the maximum amount of sample. Then, the pan was dried on the hot plate at 100°C for 15 minutes and then transferred to the TGA for running the experiment. Starting sample weight was typically 0.9 to 1 .7 mg.

[0118] The experiment was conducted with the following procedure. First, the CNT samples were heated up in TGA instrument to a pre-determined temperature at a constant heating rate in air. The weight loss of the sample was measured as a function of temperature / time. The temperature was programmed to hold at 100°C for 15 minutes, then increased from 100°C to 750°C at a rate of 5°C / minute. The sample and the TGA balance were exposed to air.

[0119] FIG. 3 A and 3B are graphs provide measurements for amorphous carbon via TGA in the CNT solution before and after the filtration step S 106. The prefilter measurement was taken after the TMAH was added to the CNT solution (dilution to OD 0.7; Sol . pH 12.0, 35 ppm Nitrate). FIG. 3 A shows the measurement of weight % loss of nitrate, amorphous carbon and carbon nanotube: 60 wt% for nitrate at 100 to 200 °C; 30 wt% for amorphous carbon at 200 to 350 °C; and 10 wt% for carbon nanotube at 500 to 650 °C . In contrast, FIG. 3B shows the measurements for nitrate, amorphous carbon and carbon nanotubes after the 2.5 mL filtration (through 100 nm 47mm PES membrane syringe filter). The results are: 69 wt% for nitrate at 100 to 200 °C; 24 wt% for amorphous carbon at 200 to 350 °C; and 3 wt% for carbon nanotube at 500 to 650 °C. The results of TGA show that the amount of amorphous carbon in the post filter CNT solution i s substantially zero.

[0120] FIG. 4 illustrates amorphous carbon fractionation process applied to the purified CNT solution obtained from Step S 109. Fractionation is a process of classification of an analyte or a group of analytes from a certain sample according to physical or chemical properties. The purpose of fractionation in this application is to measure the amount of carbon nanotube, FAC (free amorphous carbon) and BCF (bound carbon fragments) in the purified CNT solution that was obtained. FIG. 5 illustrates a scanning electron micrograph (SEM) image showing results of the fractionation process of a CNT solution treated with TMAH. The image shows that the output has very low amorphous carbon content.

[0121] Because the present disclosure provides a method for efficiently removing amorphous carbon and other impurities in the CNT solution, one advantage of the present di sclosure is that more types of containers (especially metal containers) may be used in the manufacturing process. Further, because fewer process steps are required for the purification process consistent with the present disclosure, another advantage of the present di sclosure is the reduction of fabrication cost.

[0122] Although examples and / or embodiments consistent with the present disclosure have been described in detail with respect to the removal of individual types of contaminants and / or impurities, it is to be understood that these embodiments are provided for illustrative and explanatory purposes only . Other embodiments may be apparent to those skilled in the art from consideration of thi s disclosure and practice of the embodiments described herein. For example, in certain embodiments, two or more types of contaminants and / or impurities may be removed in a singl e step, in consecutive steps, or in non-consecutive steps. Further, the sequence of the process steps described in this disclosure by no means imply or suggest that these process steps must be performed in accordance with the disclosed order. Other orders of the process steps may be possible and apparent to those skilled in the art. Accordingly, it is intended that the true scope of the present disclosure be defined by the appended claims and their equivalents.

[0123] The present disclosure uses the terms “applicator liquids,” “application solutions,” “carbon nanotube solutions,” “nanotube solutions” interchangeably to refer to the same thing. The present di sclosure also uses the terms “layers” and “fabrics” interchangeably to refer to the same thing. The terms “carbon nanotube” or “CNT” is used for the purpose of enhancing readability of this disclosure. Thus, although the term “carbon nanotube” or “CNT” has been used, it is intended to cover not only “carbon” nanotubes, but also any tubular structure in the nanometer scale made of other material . Furthermore, the terms “solution” and “di spersion” may be used interchangeably in this di sclosure to refer to the same thing.

Claims

CLAIMSWhat is claimed i s:

1. A nanotube solution comprising: a liquid medium; and one or more carbon nanotubes in the liquid medium; wherein the nanotube solution is sub stantially free of amorphous carbon.

2. The nanotube solution of claim 1 , wherein the liquid medium is an aqueous solvent, or a non-aqueous solvent, or any mixture thereof.

3. The nanotube solution of claim 1 , wherein the carbon nanotubes are one of single-wall carbon nanotubes, multi-wall carbon nanotubes, or any mixture thereof.

4. The nanotube solution of claim 1 , wherein the length of the carbon nanotube ranges between 5 nm to 5 pm.

5. The nanotube solution of claim 1 , wherein a concentration of the carbon nanotubes ranges between 10 mg and 900 mg with respect to 1 liter of the nanotube solution.

6. The nanotube solution of claim 1 , wherein the nanotube solution is substantially free of impurities having a diameter greater than 500 nm.

7. The nanotube solution of claim 1 , wherein the nanotube solution comprises less than 500 parts per billion of metallic impurities.

8. A carbon nanotube fabric having a carbon nanotube fabric compri sed of a densified carbon nanotube wherein the carbon nanotube fabric is substantially free of amorphous carbon.

9. The carbon nanotube fabric of claim 8, wherein the length of the carbon nanotube ranges between 5 nm to 5 pm.

10. The carbon nanotube fabric of claim 8, wherein the carbon nanotubefabric is substantially free of impurities having a diameter greater than 500 nm.1 1. The carbon nanotube fabric of claim 8, wherein the carbon nanotube fabric comprises less than about 15 >< 1 O10atoms / cm2of metallic impurities.

12. The carbon nanotube fabric of claim 8, wherein the carbon nanotubes are one of single-wall carbon nanotubes, multi-wall carbon nanotubes, or any mixture thereof.

13. A method for producing a nanotube solution, comprising steps of: mixing a raw carbon nanotube material and a liquid medium to prepare a carbon nanotube di spersion in the liquid medium; adjusting a pH of the mixture by adding a pH adjuster to be 10 or more 13 or less; and thereafter, purifying the carbon nanotube solution to obtain a purified nanotube solution, wherein the purified nanotube solution i s substantially free of amorphous carbon impurities.

14. The method of claim 13, wherein the pH adj uster is a hydroxide base.

15. The method of claim 13, wherein the pH adj uster promotes separation of carbon nanotube from bound carbon fragments and free amorphous carbon.

16. The method of claim 13, wherein the purifying process comprises filtering by a hollow fiber filter.

17. The method of claim 16, wherein the hollow fiber filter comprising membrane formed of modified polyethersulfone, polysulfone, or mixed ester cellulose.

18. The method of claim 13, wherein the liquid medium is an aqueous solvent, a non-aqueous solvent, or any mixture thereof.

19. The method of claim 13, wherein a concentration of the carbon nanotubes ranges between 10 mg and 900 mg with respect to 1 liter of thenanotube solution.20 / The method of claim 13, wherein the nanotube solution is substantially free of impurities having a diameter greater than 500 nm.

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