Patent Publication Number: US-2012045385-A1

Title: Systems and Methods for Controlling Chirality of Nanotubes

Description:
RELATED U.S. APPLICATIONS 
     The present application is a Divisional application of U.S. patent application Ser. No. 12/180,300, filed Jul. 25, 2008, which claims priority to U.S. Provisional Application Ser. No. 60/961,887, filed Jul. 25, 2007, each of which is hereby incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to systems and methods for manufacturing nanotubes, and more particularly, to systems and methods for controlling the chirality of nanotubes during manufacturing. 
     BACKGROUND ART 
     Carbon nanotubes have anisotropic structures with a variety of shapes, including single-walled, multi-walled, and bundled into rope-like multi-tube structures, among others. Carbon nanotubes typically range in diameter from about fractions of a nanometers to several tens of nanometers, and range in length from about several microns to several millimeters. Carbon nanotubes also exhibit conductive or semiconductive properties depending on their chirality. For example, it is generally recognized that carbon nanotubes having an arm-chair structure exhibit metallic properties, whereas carbon nanotubes having a zig-zag structure exhibit semiconductive or metallic properties depending on diameter. 
     In addition, it has been observed that the electronic and, perhaps, the mechanical characteristics of carbon nanotubes, such as single wall carbon nanotubes, may be governed by their chirality, and that their chirality may in turn be governed by the diameter of the catalysts from which the nanotubes are grown (Nasibulin et. al.,  Carbon  43 (2005), 2251-2257). Chirality often refers to the roll-up vector for the nanotube. Chirality has been described extensively in the literature (Satto et al.,  Physical Properties of Carbon Nanotubes , Imperial College Press (2004) pg 37), and may be specified by a vector C h  represented as: 
     
       
      
       C 
       h 
       =ma 
       1 
       +na 
       2  
      
     
     where a 1  and a 2  are real vectors of a hexagonal sublattice of graphite constituting the surface of the carbon nanotube. 
     In such a vector, when n-m is divisible by 3, the carbon nanotube is believed to exhibit metallic properties. Otherwise, the carbon nanotube is believed to exhibit semiconductive properties. For those carbon nanotubes exhibiting semiconductive properties, their band gap may also be affected by and change with the chiral vector. For certain applications, such as photonic detectors, or for transistor synthesis, control of the chiral vector (i.e., chirality) can be critical. Given that the diameter of carbon nanotubes can be expressed as 
         d   t   =a[n   2   +m   2   +nm]   1/2 , 
     it should be appreciated that with a very small change in the nanotube diameter, i.e., d t , there can be a significant effect on electronic character of the nanotube. 
     As an example, if “m” and “n” determine the metallic or semiconductor characteristic of the carbon nanotube, then changes in the electronic character of the carbon nanotube can occur with changes in its diameter “d”. The sensitivity, at the level of an individual carbon nanotube, can be in fractions of a nanometer. As a result, chirality control through control of the diameter, at present, can be difficult, if not impossible. For instance, if n=6 and m=5, then by definition d=0.948 a in nanometers (nm). However, if “n” were maintained such that n=6 and “m” were changed so that m=3, then d=0.793 a nm. The latter nanotube, with a slight change in “m”, becomes a metallic conductor, whereas the former is a semiconductor. 
     It is well accepted that applications using carbon nanotubes can be wide-ranging, including those in connection with memory devices, electron amplifiers, gas sensors, microwave shields, electrodes, electrochemical storage, field emission displays, and polymer composites among others. Specifically, semiconducting carbon nanotubes can be used, for instance, in memory devices, sensors, etc., while metallic carbon nanotubes may be used in electrode materials of cells, electromagnetic shields, etc. To make these applications practical using carbon nanotubes, it will, therefore, be necessary to obtain and/or created carbon nanotubes with a specific diameter or diameter range, in order to obtain carbon nanotubes with a specific chirality. 
     Selection between the metallic and semiconductive characteristics, therefore, requires a substantially precise ability control of the catalyst diameter. In some instances, the accuracy needs to be better than about 0.155 nm. However, it should be noted that this difference can become even closer, as “m” and “n” become large. Adding to the difficulty is the ability to precisely control the catalyst diameter during the growth process. In particular, if the catalysts are in a molten state ( Applied Physics Letters  87, 051919 — 2005_), the presence of droplet vibrations can likely introduce considerable diameter variations in the resulting carbon nanotube generated. If, on the other hand, the catalysts are in a crystalline state, these catalysts are likely formed from metallic clusters that also vary in diameter. As a result, carbon nanotubes generated from such metallic clusters can also vary in diameter. 
     There exist several historic approaches that have been taken to select, for example, single wall carbon nanotubes of a given chirality. These include: (1) attempts to control diameter of the catalyst particle (Katauraa et al.,  Diameter Control of Single - walled Carbon Nanotubes, Carbon  38 (2000) 1691-1697), (2) epitaxial growth of nanotubes on fragments of known chirality (U.S. Pat. No. 7,052,668), (3) using electric discharging or laser deposition to produce nanotubes having specific chirality, and (4) selection of only those tubes meeting the desired chirality after a batch of tubes have bee made and processed (Fering a et al.,  Molecular Chirality Control and Amplification by CPL: Correction, Science  276 (5311) 337-342). Of these, the last one seems to offer the most promise. However, it has been observed that such an approach can be destructive, may not let an operator preselect chirality with great accuracy (http://www.fy.chalmers.se/conferences/nt05/abstracts/P357), and can also be time consuming. 
     Accordingly, it would be desirable to provide an approach that can permit a predetermined chirality to be specified or defined substantially precisely, so that nanotubes with such specified chirality can subsequently be fabricated, and which approach can permit a volume of substantially uniform nanotubes with substantially uniform chirality to be obtained. 
     SUMMARY OF THE INVENTION 
     The present invention can be adapted to provide, among other things, (1) an approach that permits the chirality of the nanotubes to be specified or defined substantially precisely prior to fabrication, so that the fabricated nanotubes can be provided with the specified chirality, and (2) an approach that can precisely select nanotubes having a specific or defined chirality during fabrication. 
     In accordance with one embodiment of the present invention, a system for manufacturing nanotubes is provided. The system includes, in one embodiment a synthesis chamber within which nanotubes growth can be initiated. In an embodiment, the chamber includes an inlet through which reactive gas necessary for nanotube growth can be introduced. The synthesis chamber may also include an inlet through which a catalyst precursor may be introduced. The system also includes a cavity positioned within the synthesis chamber designed to resonate at a selected resonant frequency keyed to a radial breathing mode unique to a desired chiral nature of a nanotube to be manufactured. The system further includes a source for generating the selected resonant frequency in the cavity, such that the selected resonant frequency can be imposed on a plurality of catalyst particles situated within the synthesis chamber and from which nanotubes growth can occur, so as to permit nanotubes of exhibiting a resonant frequency substantially similar to the selected resonant frequency, and thus similar to the unique desired chiral nature, to be grown. In an embodiment, the source can provide one of an electromagnetic field, and electric field or a magnetic field for generating the resonant frequency. 
     In another embodiment, an alternate system for manufacturing nanotubes is provided. The system includes a synthesis chamber within which nanotubes growth can be initiated. In an embodiment, the synthesis chamber may be provided with an inlet through which reactive gas necessary for nanotube growth can be introduced. The synthesis chamber may also be provided with an inlet through which a catalyst precursor may be introduced. The system also includes a radiating plate positioned within the synthesis chamber and having thereon at least one preselected nanotube having a desired chiral nature. The preselected nanotube on the radiating plate, in and embodiment, is capable of re-radiating at its natural frequency which approximate the diameter of the preselected nanotube. The system further includes a heat source in communication with the synthesis chamber and designed to generate sufficient heat energy, so as to cause the preselected nanotube on the radiating plate to re-radiate at its natural frequency. In the presence of the re-radiating preselected nanotube, other nanotubes growing within the synthesis chamber adjacent to the preselected nanotube can be stimulated to resonate at a similar frequency and grow with a substantially similar chirality as that exhibited by the preselected nanotube. 
     The present invention also provides a method for manufacturing nanotubes. The method includes initially exposing a plurality of catalyst particles from which nanotubes can grow to a substantially high frequency field. Such a high frequency field can be an electromagnetic field, an electric field, or a magnetic field. The catalyst particles may in addition be exposed to a substantially high temperature and a reactive gas necessary to permit nanotube growth. Next, the catalyst particles may be resonated at a selected resonant frequency that can be keyed to a radial breathing mode to a diameter unique to a desired chiral nature of a nanotube to be manufactured. The catalyst particles, in an embodiment, may be in a laminar flow, a fluidized bed, or seeded on a substrate. Thereafter, those nanotubes having a resonant frequency substantially similar to the selected resonant frequency, and thus the unique desired chiral nature, are allowed to be grown. The grown nanotubes, in one embodiment, may be substantially uniform in their chiral nature. 
     The present invention further provides another method for manufacturing nanotubes. The method includes initially exposing a preselected nanotube having a desired chiral nature in an environment having a heat source with sufficient heat energy to radiate the preselected nanotube. The preselected nanotube, in an embodiment, may be secured to a substrate, and the heat source may emit a substantially high temperature exceeding about 1250° C. In addition, a catalyst precursor may be introduced into the environment in the presence of the preselected nanotube to permit subsequent nanotube growth. Next, the radiated nanotube may be allowed to re-radiate at its natural frequency in the presence of the heat source. The natural frequency, in one embodiment, approximates the diameter of the re-radiating nanotube. Thereafter, the re-radiating nanotube may be permitted to stimulate nanotubes growing adjacent thereto to grow with a substantially similar chirality as that exhibited by the re-radiating nanotube. The grown nanotubes, in an embodiment, may be substantially uniform in their chiral nature. 
     A system for manufacturing nanotubes can also be provided by the present invention. The system includes a first furnace for generating radiant energy, for instance, exceeding about 1250° C., and within a terahertz frequency or small band of frequencies. The system also includes a filter positioned within the first furnace to select for energy within a particular resonant frequency or small band of frequencies corresponding to a chiral nature of a desired nanotube. The filter, in an embodiment, includes a frequency selected surface having one or more slots dimensioned to permit energy within a selected resonant frequency or small band of frequencies to pass therethrough. The system further includes a second furnace in fluid communication with the first furnace for receiving the selected energy within a particular resonant frequency or small band of frequencies. The second furnace may be provided with an inlet for introducing reactive gas and an inlet for introducing a catalyst precursor into the second furnace for use in the growth of nanotubes. A template may be situated in the second furnace for providing a footprint from which nanotubes can grow. In an embodiment, the template may be capable of being stimulated in the presence of the selected energy, so as to permit nanotubes exhibiting a resonant frequency substantially similar to the selected resonant frequency or small band of frequencies, and thus the desired chiral nature, to grow. 
     A method of manufacturing nanotube, based on the above system, is further provided, in accordance with an embodiment of the present invention. The method includes initially generating within a first environment radiant energy, for instance, exceeding about 1250° C., and within a terahertz frequency or small band of frequencies. Next, the radiant energy may be filtered within the first environment, so as to select for energy within a particular resonant frequency or small band of frequencies corresponding to a chiral nature of a desired nanotube. Thereafter, the selected energy within a particular resonant frequency or small band of frequencies may be directed from the first environment into a second environment. A reactive gas may also be introduced into the second environment for use in growing nanotubes. A template positioned in the second environment to the selected energy, may subsequently be exposed to the selected energy, and reactive gas, so that the template can be stimulated in the presence of the selected energy to a footprint from which nanotubes can grow. Once growth is initiated, nanotubes exhibiting a resonant frequency substantially similar to the selected resonant frequency or small band of frequencies, and thus the desired chiral nature, are permitted to grow from the template. 
     In another embodiment of the invention, a radiating energy generator is provided. The generator includes a housing having a first end, an opposite second end, and reflective interior surfaces extending between the first end and the second end. The housing, in embodiment, may be sufficient small or portable in size. The generator also includes a heat source positioned at the first end of the housing for generating radiant energy. The heat source may be designed to generate pulses of energy, such as a flash lamp, and may include a capacitor to provide sufficient power to permit heat generation. The generator further includes a filter positioned at the second end of the housing to allow only energy within a terahertz range to pass. The filter, in an embodiment, may include a frequency selective surface that includes one or more slots dimensioned to permit energy within a terahertz range to pass through. An exit port may be provided at the second end of the housing and adjacent the filter through which only the energy within the terahertz range can leave the housing. 
     In a further embodiment of the invention, a method for generating power is provided. The method includes initially providing a reflective pathway. Next, radiant energy may be directed from one end of the reflective pathway towards an opposite end of the reflective pathway. Thereafter, the radiant energy may be filtered at the opposite end of the reflective pathway to allow only energy within a terahertz range to be selected. Subsequently, only the energy within the terahertz range may be allowed to exit the reflective pathway. 
     Various uses and applications by the radiant energy generator of the present invention are also provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A-B  illustrates a Chemical Vapor Deposition system for fabricating nanotubes, in accordance with one embodiment of the present invention. 
         FIG. 2  illustrates a microwave cavity for use in connection with the system shown in  FIG. 1  for passively controlling the chirality of the carbon nanotubes being fabricated. 
         FIG. 3  illustrates a radiator plate for use in connection with the system shown in  FIG. 1  for passively controlling the chirality of the carbon nanotubes being fabricated. 
         FIG. 4  illustrates a Frequency vs. Diameter curve for the carbon nanotubes generated using the system of  FIGS. 2 and 3 , in connection with one embodiment of the present invention. 
         FIG. 5  illustrates a schematic view of a terahertz (THz) filter for use in connection with the system shown in  FIG. 1  for actively selecting carbon nanotubes with a specific chirality during fabrication. 
     
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION 
     Presently, there exist multiple processes and variations thereof for growing nanotubes. These include: (1) Chemical Vapor Deposition (CVD), a common process that can occur at near ambient or at high pressures, and at temperatures above about 400° C., (2) Arc Discharge, a high temperature process that can give rise to tubes having a high degree of perfection, and (3) Laser ablation. 
     The present invention, in one embodiment, employs a CVD process or similar gas phase pyrolysis procedures known in the industry to generate the appropriate nanostructures, including carbon nanotubes. It should be noted that although reference is made throughout to nanotube synthesized from carbon, other compound(s) may be used in the synthesis of nanotubes in connection with the present invention. For instance, it should be understood that boron nanotubes may also be grown, but with different chemical precursors. Other methods, such as plasma CVD or the like can also be used. 
     Growth temperatures for a CVD process can be comparatively low ranging, for instance, from about 400° C. to about 1300° C. Carbon nanotubes, both single wall (SWNT) or multiwall (MWNT), may be grown, in an embodiment of the present invention, by exposing nanostructural catalyst particles in the presence of reagent carbon-containing gases (i.e., gaseous carbon source). In particular, the nanostructural catalyst particles may be introduced into the reagent carbon-containing gases, either by addition of existing particles or by in situ synthesis of the particles from a metal-organic precursor, or even non-metallic catalysts. Although both SWNT and MWNT may be grown, in certain instances, SWNT may be selected due to their relatively higher growth rate and tendency to form rope-like structures, which may offer advantages in handling, thermal conductivity, electronic properties, and strength. 
     The strength of the individual carbon nanotubes generated in connection with the present invention may be about 30 GPa or more. Strength, as should be noted, is sensitive to defects. However, the elastic modulus of the carbon nanotubes fabricated in the present invention may not be sensitive to defects and can vary from about 1 to about 1.2 TPa. Moreover, the strain to failure of these nanotubes, which generally can be a structure sensitive parameter, may range from a few percent to a maximum of about 15% in the present invention. 
     Furthermore, the nanotubes of the present invention can be provided with relatively small diameter, so that relatively high double layer capacitance can be generated when these materials are used in the form of an electrode. In an embodiment of the present invention, the nanotubes fabricated in the present invention can be provided with a diameter in a range of from less than 1 nm to about 10 nm. 
     The individual carbon nanotubes generated in connection with the present invention, in particular, single-walled carbon nanotubes, can also be substantially uniform in their chirality. In an embodiment of the invention, the radial breathing mode (RBM) of a particular nanotube diameter may be identified and utilized, so that single-walled nanotubes with a specific diameter and, thus, chirality can subsequently be selected during fabrication. In particular, since the RBM can be unique to the chiral nature of the nanotube, the RBM can provide a footprint from which the chirality can be selected prior to fabrication, as well as maintained throughout the growth process, so long as the catalyst particle from which each carbon nanotubes may be grown substantially maintains its diameter throughout the growth process. It is recognized that the growth of a nanotube from a catalyst may have to be initiated in order for the imposed electromagnetic radiation to act on that nanotube and affect its chirality. 
     Carbon nanotubes having a particular diameter range, including a diameter size of about 0.948 a nm, exhibit semiconductive properties, while those having a relatively smaller diameter range, including a diameter size of about 0.793 a nm, can be metallic conductors. The uniformity of the specific diameter, and thus chirality, allows the carbon nanotubes of the present invention to be use in connection with particular applications. As an example, the semiconducting single-walled carbon nanotubes of the present invention can be used, for instance, in memory devices, sensors, etc., while the metallic single-walled carbon nanotubes of the present invention can be used, for instance, in electrode materials of cells, electromagnetic shielding, microwave antennas, electrical conductors, etc. 
     Systems for Fabricating Nanotubes 
     With reference now to  FIG. 1A , there is illustrated a system  10 , similar to that disclosed in U.S. patent application Ser. No. 11/488,387 (incorporated herein by reference), for use in the fabrication of nanotubes. System  10 , in an embodiment, may be coupled to a synthesis chamber  11 . The synthesis chamber  11 , in general, includes an entrance end  111 , into which reaction gases may be supplied, a hot zone  112 , where synthesis of extended length nanotubes  113  may occur, and an exit end  114  from which the products of the reaction, namely the nanotubes and exhaust gases, may exit and be collected. The synthesis chamber  11 , in an embodiment, may include a quartz tube  115  extending through a furnace  116 . The nanotubes generated by system  10 , on the other hand, may be individual single-walled nanotubes, bundles of such nanotubes, and/or intertwined single-walled nanotubes (e.g., ropes of nanotubes). 
     System  10 , in one embodiment of the present invention, may also include a housing  12  designed to be substantially airtight, so as to minimize the release of potentially hazardous airborne particulates from within the synthesis chamber  11  into the environment. The housing  12  may also act to prevent oxygen from entering into the system  10  and reaching the synthesis chamber  11 . In particular, the presence of oxygen within the synthesis chamber  11  can affect the integrity and compromise the production of the nanotubes  113 . 
     System  10  may also include a moving belt  120 , positioned within housing  12 , designed for collecting synthesized nanotubes  113  made from a CVD process within synthesis chamber  11  of system  10 . In particular, belt  120  may be used to permit nanotubes collected thereon to subsequently form a substantially continuous extensible structure  121 , for instance, a non-woven sheet. Such a non-woven sheet may be generated from compacted, substantially non-aligned, and intermingled nanotubes  113 , bundles of nanotubes, or intertwined nanotubes (e.g., ropes of nanotubes), with sufficient structural integrity to be handled as a sheet. 
     To collect the fabricated nanotubes  113 , belt  120  may be positioned adjacent the exit end  114  of the synthesis chamber  11  to permit the nanotubes to be deposited on to belt  120 . In one embodiment, belt  120  may be positioned substantially parallel to the flow of gas from the exit end  114 , as illustrated in  FIG. 1A . Alternatively, belt  120  may be positioned substantially perpendicular to the flow of gas from the exit end  114  and may be porous in nature to allow the flow of gas carrying the nanomaterials to pass therethrough. Belt  120  may be designed as a continuous loop, similar to a conventional conveyor belt. To that end, belt  120 , in an embodiment, may be looped about opposing rotating elements  122  (e.g., rollers) and may be driven by a mechanical device, such as an electric motor. Alternatively, belt  120  may be a rigid cylinder. In one embodiment, the motor may be controlled through the use of a control system, such as a computer or microprocessor, so that tension and velocity can be optimized. 
     In an alternate embodiment, instead of a non-woven sheet, the fabricated single-walled nanotubes  113  may be collected from synthesis chamber  11 , and a yarn  131  may thereafter be formed. Specifically, as the nanotubes  113  emerge from the synthesis chamber  11 , they may be collected into a bundle  132 , fed into intake end  133  of a spindle  134 , and subsequently spun or twisted into yarn  131  therewithin. It should be noted that a continual twist to the yarn  131  can build up sufficient angular stress to cause rotation near a point where new nanotubes  113  arrive at the spindle  134  to further the yarn formation process. Moreover, a continual tension may be applied to the yarn  131  or its advancement into collection chamber  13  may be permitted at a controlled rate, so as to allow its uptake circumferentially about a spool  135 . 
     Typically, the formation of the yarn  131  results from a bundling of nanotubes  113  that may subsequently be tightly spun into a twisting yarn. Alternatively, a main twist of the yarn  131  may be anchored at some point within system  10  and the collected nanotubes  113  may be wound on to the twisting yarn  131 . Both of these growth modes can be implemented in connection with the present invention. 
     As provided hereinafter in more detail, system  10  of the present invention can be adapted to provide (1) an approach that permits the chirality of the nanotubes to be specified or defined substantially precisely prior to fabrication, so that the fabricated nanotubes can be provided with the specified chirality, and (2) an approach that can precisely select nanotubes having a specific or defined chirality during fabrication. 
     Passive Control of Chirality 
     System  10  of the present invention can be designed to include a device for passive control of the chirality of the nanotubes fabricated during growth. In particular, system  10  can use such a device to permit the chirality of the single-walled nanotubes being generated to be specified or defined substantially precisely prior to fabrication. As such, only single-walled nanotubes with the specified chirality can subsequently be fabricated. System  10  can, therefore, can be utilized to produce substantially uniform carbon nanotubes of the same chirality. 
     In accordance with an embodiment of the present invention, as illustrated in  FIG. 2 , a resonant cavity  21  may be established within a CVD synthesis chamber  11  of furnace  116  (i.e., reactor). The resonant cavity  21 , in an embodiment, may be designed so that in the presence of a substantially high frequency field, it can be caused to resonate at a desired frequency, for instance, a frequency at or close to the resonant frequency of the desired nanotube. In one embodiment, the resonant oscillation may generated by a source  22 , such as a microwave source, similar to that available from Techtrol Cyclonetics in New Cumberland, Pa. Of course, the resonant oscillation may also be generated by any other mechanisms known to create THz EM frequencies, such as an induced current from a coil surrounding the synthesis chamber  11 , or from an oscillating magnetic field situated circumferentially about the synthesis chamber  11 . 
     Although illustrated as being substantially entirely within the synthesis chamber  11 , it should be appreciated that the resonant cavity  21  may be positioned either within the synthesis chamber  11  or partially within the synthesis chamber  11 . To the extent that the resonant cavity  21  may be placed entirely within the synthesis chamber  11 , the resonant cavity  21  may be exposed to substantially high temperature, i.e., temperature necessary to permit nanotube growth. However, when positioned partially within the synthesis chamber  11 , the resonant cavity  21  may be placed about an entrance portion  23  of the furnace  116 , so that the cavity  11  does not substantially “see” the highest temperature within the synthesis chamber  11 . 
     In an embodiment, the resonant cavity  21  may be made from a high temperature metal, such as molybdenum or other similar metals, and the system  10  can be one that utilizes a gas phase pyrolysis system. In an alternate embodiment, system  10  may be designed to use a microwave plasma to induce nanotube growth. 
     Still looking at  FIG. 2 , system  10  may include individual inlets  24  and  25  through which a reactive gas (i.e., carbon source) and a suitable catalyst precursor (i.e., catalyst particle source) may be introduced into the heated synthesis chamber  11 , respectively. The catalyst precursor  25  provides a source from which a catalyst particle can be generated for subsequent growth of the single-walled nanotube thereon. The reactive gas  24 , on the other hand, provides a carbon source for depositing carbon atoms onto the catalyst particle in order to grow the nanotube. In certain instances, it may be desirable to also introduce a conditioner compound into the synthesis chamber  11 . In an embodiment, the conditioner compounds can act to control size distribution of the catalyst particles generated from the catalyst precursor  25 , and thus the diameter of the nanotubes growing on each of the catalyst particles. Although the system  10  provides individual inlets for the reactive gas  24  and the catalyst precursor  25 , it should be appreciated that if a mixture of the reactive gas  24  and catalyst precursor  25 , along with the conditioner compound, is provided, such a mixture can be introduced into the synthesis chamber  11  through a single inlet. 
     Examples of a reactive gas  24  for use in connection with the present invention include, but are not limited to, ethanol, methyl formate, propanol, acetic acid, hexane, methanol, or blends of methanol with ethanol, or any combination thereof. Other carbon sources may also be used, including C 2 H 2 , CH 3 , and CH 4 , or a combination thereof. 
     Examples of a catalyst precursor  25  from which catalyst particles may be generated includes ferrocene, nickelocene, cobaltocene, materials such as iron, iron alloy, copper, gold, nickel or cobalt, their oxides or their alloys, a combination of any of these, or a combination of any of these with other metals or ceramics compounds, such as aluminum oxide, MnO, or other similar oxides. Alternatively, the catalyst particles may be made from metal oxides, such as Fe 3 O 4 , Fe 2 O 4 , or FeO, or similar oxides of cobalt or nickel, or a combination thereof. Another alternative is carbonyl compounds of iron, cobalt or nickel. 
     Examples of a conditioner compound for use in connection with the fluid mixture of the present invention include Thiophene, H 2 S, other sulfur containing compounds, or a combination thereof. 
     In one embodiment of the invention, system  10  can be designed so that the catalyst precursors  25  introduced into system  10  can subsequently provide a batch of catalyst particles within the synthesis chamber  11 , whether it be a laminar flow of catalyst particles or fluidized bed of catalyst particles, and from which nanotubes may be grown. 
     In an alternate embodiment, rather than creating a batch of catalyst particles, a substrate preseeded with catalysts may be positioned either horizontally or vertically, relatively to the direction of gas flow, within the synthesis chamber  11  to provide a base from which nanotubes may be grown. To the extent that the substrate may be positioned vertically, the substrate may be porous to permit the gas flow to pass therethrough. When using a substrate preseeded with catalysts, system  10  may not need to introduce a catalyst precursor  25  into the synthesis chamber  11 . 
     In either the fluidized bed or the preseeded substrate embodiments, the catalyst particles may be exposed, within system  10 , to a substantially high frequency field, such as an electromagnetic field, an electric field or a magnetic field within cavity  21  in the synthesis chamber  11 . The high frequency field, as noted above, can be generated by source  22 . The selected resonant frequency to be imposed by source  22 , in one embodiment, may be keyed into the radial breathing mode (RBM) of a particular or predetermined nanotube diameter. Since this RBM can be unique to the chiral nature of a nanotube, the chirality for the nanotubes to be fabricated can, therefore, be preselected prior to growth and maintained throughout the growth process. In an embodiment, a narrow band of electromagnetic radiation in the terahertz (THz) region, corresponding to the natural radial breathing mode of a certain chiral nanotube, may be selected for the particular chirality to be generated. 
     Thereafter, because the nanotube being fabricated and the catalyst particle from which nanotube growth occurs both resonate at the same frequency, as imposed by source  22 , any nanotube being initiated but not be at the resonant frequency being imposed can be suppressed. As a result, only those nanotubes growing at the imposed resonant frequency, and thus the specified chirality, can continue to grow. 
     In another embodiment of the present invention, looking now at  FIG. 3 , system  10  may be modified to provide an alternative approach to passively control the chirality of the nanotubes being fabricated. In particular, since the furnace  116  in which the nanotubes may be fabricated also operate at a substantially high temperature level, once the single-walled nanotubes initiate growth within the high temperature environment of the synthesis chamber  11 , these nanotubes may re-radiate at their own natural frequency. Such a frequency, in an embodiment, can approximate the diameter of the re-radiating nanotubes. As such, a these nanotubes re-radiate, they can stimulate adjacent nanotubes to resonate at a similar frequency, so as to cause the adjacent nanotubes to grow at or near a similar diameter, and thus similar chirality. To that end, system  10  may be provided with at least one radiator plate  31  that can be stimulated by the heat radiation of furnace  116  to re-radiate at the natural frequency of the radiator plate  31 . In one embodiment, radiator plate  31  may be positioned vertically or horizontally to the direction of gas flow within synthesis chamber  11 . Such a radiator plate  31  may be a seeded substrate provided with preselected individual nanotubes having the desired chirality, for example (10,10), at about 1.4 nm in diameter. Of course, other chiral characteristics and diameters may be used. If vertically positioned to the direction of gas flow, radiator plate  31  may be porous to permit reactive gas to flow therethrough. 
     With reference now to  FIG. 4 , it should be appreciated that typically, the catalyst particles used in connection with the present invention may be relatively larger than the carbon nanotubes fabricated by a factor that can be as high as about 1.6 times. It is possible, therefore, to have a range of catalyst diameters all producing tubes of a substantially similar diameter, and substantially uniform chirality. Such a possibility can result when the catalysts, in an embodiment, may be molten, as expected to be the case, at temperatures exceeding 1250° C. within the synthesis chamber  11 . 
     System  10  of the present invention, as provided, can therefore be adapted to provide a protocol to passively control the chirality of the nanotubes fabricated. As a result of such a capability, system  10  can be utilized to produce substantially uniform carbon nanotubes of the same chirality. 
     Active Control of Chirality 
     System  10  can also be adapted to provide a protocol to actively control the chirality of the nanotube being fabricated. In this approach, system  10  may include a first furnace for generating energy (i.e., heat radiation) and a filter within the first furnace to select for energy within a particular frequency or a small band of frequencies, while blocking or shunting the others frequencies to prevent them from passing through. The selected energy, in an embodiment, may be within a particular THz frequency or within a small band of THz frequencies. The system may also include a second furnace for receiving the selected energy, and either (i) a batch of particles, for instance, a laminar flow of catalyst particles, or a fluidized bed of catalyst particles, or (ii) a seeded substrate, for instance, seeded with catalyst particles, or seeded with nanotubes of a desired chirality. The batch of particles or seeded substrate, in an embodiment, in the presence of the selected energy within the particular radiation band of interest may be directly stimulated and may provide a template (i.e., footprint) to initiate nanotube growth at or near the selected/desired diameter, and thus chirality. 
     In one embodiment, as illustrated in  FIGS. 5 and 6 , system  10  may be provided with a first furnace  50  and a frequency selective filter  51  positioned within synthesis chamber  52 . Frequency selective filter  51  may be designed to permit energy, for instance, within a particular THz frequency or a small band of frequencies to pass therethrough. Filter  51 , as shown in  FIG. 6 , includes outer members  61 , an inner member  62 , and a filtering member  63  situated between inner member  62  and each of outer members  61 . 
     The outer members  61  and the inner member  62 , in an embodiment, may be made from a dielectric material, and may be designed to selectively permit energy at a substantially constant frequency bandwidth and at a particular angle of incidence to pass therethrough. The filtering members  63 , similarly, may be made from a dielectric material, and may include a frequency selective surface  64  embedded therein. The frequency selective surface  64 , in an embodiment, may include one or more slots  65 , each being similarly dimensioned to permit energy at a desired frequency or small band of frequencies to pass therethrough. As illustrated, slots  65  may be positioned in a desired geometric pattern. In addition, each of slots  65  may include three lobes  66  (i.e., tri-lobe). According to one embodiment, lobes  66  may be substantially equidistant from one another. 
     System  10  may also include a second furnace  53  in fluid communication with the first furnace  50 . In an embodiment, the second furnace  53  may be similar to the furnace shown in  FIG. 2  and may include a laminar flow of catalyst particles, a fluidized bed of catalyst particles, or a substrate seeded with catalyst particles (not shown) within synthesis chamber  54 . Alternatively, the second furnace  53  may be similar to the furnace shown in  FIG. 3  and may include a substrate seeded with nanotubes (not shown) having a desired chirality within synthesis chamber  54 . The second furnace  53 , in fluid communication with the first furnace  50 , may be designed to direct the selected energy from first furnace  50  into synthesis chamber  54  to directly stimulate the catalyst particles or the nanotubes on the substrate within synthesis chamber  54  to initiate nanotube growth at or near the selected or desired diameter, and thus chirality. 
     To initiate nanotube growth, taking advantage of the ability of the furnace of the present invention to generate heat radiation (i.e., energy) in the terahertz (THz) range, especially when temperature within the furnace reaches above about 1250° C., furnace  50  may be permitted to generate the necessary heat radiation within the synthesis chamber  52  in the THz range. The heat radiation, may then be directed downstream in synthesis chamber  52  toward filter  51 , where heat radiation of a particular frequency or a small band of frequencies can be allowed to pass through the frequency selective surface  64 . The selected radiation, thereafter, may be directed into the second furnace  53  through a pathway  55 . 
     In the embodiment where the second furnace  53  may be similar to the furnace in  FIG. 2 , since the selected radiation resonates at a frequency or small band of frequencies within a particular THz range, the selected radiation can similarly impose such resonant frequency or frequencies on the catalyst particles from which nanotube growth occurs and any nanotube which has initiated growth. In the presence of the imposed resonant frequency, any nanotube being initiated but not be at the resonant frequency being imposed can be suppressed. As a result, only those nanotubes growing at the imposed resonant frequency, and thus the specified chirality, can continue to grow. 
     In the embodiment where the second furnace  53  may be similar to the furnace in  FIG. 3 , since the selected radiation resonates at a frequency or small band of frequencies within a particular THz range, the selected radiation can similarly impose such resonant frequency or frequencies on the substrate having nanotubes with the desired chirality. In the presence of the imposed resonant frequency, once a single-walled nanotube initiate growth, the nanotube may re-radiate at their own natural frequency to stimulate adjacent nanotubes to also grow at or near a similar diameter, and thus similar chirality. 
     Although reference is made to energy/radiation within the terahertz range, it should be appreciated that furnace  50  of system  10  may be modified to select for energy/radiation within other frequency ranges. 
     THz Generator 
     Taking advantage of the ability of the furnace  50  of the present invention to generate heat radiation (i.e., energy) in the terahertz (THz) range, especially when temperature within the furnace reaches above about 1250° C., and the fact that the design of filter  51  makes it substantially ideal as a notch filter for selectively allowing only energy of within the THz range to pass, furnace  50  may be modified along with filter  51  to provide a THz generator  70 , as illustrated in  FIG. 7 , capable of yielding significant power. Such a THz generator  70  can be utilized in a number of different applications. 
     THz generator  70 , in an embodiment, includes a housing  71  having sides  711 , a heat source  72 , such as a flash lamp, positioned at one end of housing  71 , and frequency selective filter  51  positioned at an opposite of housing  71 . Heat source  72 , in an embodiment, may be used to generate short pulses of the necessary heat radiation (e.g., above 1250° C.) within the terahertz range. As such, heat source  72  may be coupled to a capacitor  73  capable of providing the sufficient power to permit the heat source to generate such a level of heat radiation. Generator  70  may also include an exit port  74  adjacent filter  51  to permit heat radiation at the selected frequency by filter  51  to exit housing  71 . 
     Filter  51 , as indicated above, includes the filtering members  63 , which may be designed, so that each of slots  65  on frequency selection surface  64  may be capable of selecting a center frequency (i.e., targeted frequency). In an embodiment, the center frequency may be 5 THz, or any other desired frequency. The outer members  61  and the inner member  62  of filter  51 , on the other hand, may be designed to selectively permit energy at a substantially constant frequency bandwidth of about ±0.3, relative to a center frequency, at an angle of about 45° of incidence to an angle normal incidence. In an example, if the center frequency (i.e. targeted frequency) that is permitted to get through is, for instance, 5 THz, then the energy striking the outer members  61  and inner member  62 , at angles of 45° or greater, may pass through at frequencies of from about 3.5 THz to about 6.5 THz. This bandwidth can decrease toward a single frequency of 5 THz as the angle of incidence approaches normal striking. It should be appreciated that given the design of the filtering member  63  and their dielectrics, the bandwidth can further decrease to from about 4.55 THz to about 5.45 THz. In particular, each of slots  65  on frequency selection surface  64  may be responsible for the peak of the transmission/receive curve for a particular center frequency selected. In addition, filtering members  63  can produce curves for angles of incidence from about 0° to 60° for each of the frequency selective surfaces  64  that are substantially identical. To this end, the bandwidth approaches an ideal notch filter. In particular, the curves rise quickly to the frequency desired, then fall off substantially sharply after that frequency. 
     Example 
     To illustrate that generator  70  may be designed to provide sufficient heat radiation within the THz range, assuming the temperature generated by heat source  72  inside of housing  71  of generator  70  may be about 1250° C., then using Weins Displacement law, the wavelength at which maximum energy can be emitted, as defined by: 
         Tλ   max =2.898×10 6  nmK
 
     can, therefore, be 
       λ max =2.898×10 6 /(1250° C.+273.15° C.)
 
       λ max =1902.635 nm
 
       or, 
       Frequency max =157.67 THz 
     Now using Plank&#39;s Law to find the power at, for example, 5 THz, that can be emitted from generator  70 , using 
         I (λ, T )=(2 hc   2 /λ 5 )·(1/ e   hc/λkT −1)
 
         h= 6.626 068 96(33)×10 −34  J·s=4.135 667 33(10)×10 −15  eV·s, and
 
         k= 1.380 6504(24)×10 −23  J/K
 
     it is calculated that 
         I (λ, T )=˜903,000
 
     in units of energy per unit time per unit surface area per unit solid angle per unit wavelength. 
     It should be noted that the heat radiation generated by heat source  72  within housing will be reflected by the surfaces of sides  711  and along the length of housing  71  before it exits through the end of housing  71  with exit port  74 . In an example where each of these sides  711  is about 0.7 m, then the surface area of housing  71  that can act to reflect the heat radiation is about 0.5 m 2 . If I(λ,T) is multiplied by the surface area of housing  71 , while assuming the solid angle to be a sphere, which is about 47π radians, and that the wavelength may be about 60 μm, then the wattage should be: 
       Watts(5 Terahertz)=340 W 
     This is a great deal of wattage for the given frequency generated by generator  70 . However, in the presence of the frequency selective filter  51 , the amount of energy and thus power that can pass through and exit housing  71  will decrease. With the configuration and parameters noted above, filter  51  should permit energy at frequencies from about 4.55 THz to about 5.45 THz, to pass therethrough, while highly attenuating all other frequencies. 
     To find out how much power will exit though exit port  74  of housing  71 , if it is assumed that sides  711  completely reflect the energy along the interior of housing  71 , the transmission coefficients can then be calculated for the material that comprises each side  711  of housing  71 , and a determination can be made as to what makes it through exit port  74 . 
     Assuming parallel polarization of energy, and about 0° angle of incidence, if this power is divided by the area of the end through energy exits through port  74 , power density can be obtained for generator  70 . In practice, there will be both parallel and perpendicular polarizations and many angles of incidence. As such, these angles of incidence can act to decrease the total power transmission. Although what can be calculated is the maximum transmitted power, it nevertheless can provide an idea of at least the order of magnitude of power. Using the equations below, 
         T   π =2ε 2   k   x /(ε 2   k   x +ε 1   k   tz )
 
       where 
         k   tz =( k   2   2   k   x   2 ) 0.5    
     the following can be calculated: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Permittivity 
                 Transmission coefficients 
               
               
                   
                   
               
             
            
               
                   
                 Air to 1.3 
                 0.95792135  
               
               
                   
                 1.3 to 3   
                 0.579501721 
               
               
                   
                   3 to 1.3 
                 0.730682314 
               
               
                   
                   
               
            
           
         
       
     
     Next, based on the above results, the total power emanating from exit port  74  can be calculated to be: 
       58 Watts, 
       or 
       116 W/m 2    
     This would be the maximum power output due to the considerations already stated. However, the materials that make up the permittivity surfaces in filter  51  can act to further decrease power output. In particular, as the critical angle through filter  51  may be about 41.2°, it can act to further decrease power output. Critical angles occur when going from a more dense the dielectric material in filter  51 , for example, with relative permittivity of 3, to a less dense the dielectric material in filter  51 , for example, with relative permittivity of 1.3. As a result, the maximum power that could be transmitted from generator  70  through exit port  74  can be: 
       (41.2/90)·58.4623 Watts=26.7425 Watts
 
     It is understood that in order to keep the notch filter cool, that a separation between the filter  51  and the heat generator  72  may be required, and this separation can further reduce the energy output. 
     Applications 
     To the extent that a THz generator capable of yielding significant power can be provided, such a THz generator can be utilized in a number of different applications. In addition, since housing  71  can be modified to be of a small or portable size, the THz generation technology may also be used commercially in a manner that may otherwise not be possible at present. In particular, the THz generation technology of the present invention may be used in connection with (1) radar sensing, including better weather penetration, along with higher angular resolution normally associated with EO systems, (2) remote detection of chemical and biological agents that might resonate with a desired wavelength, (3) detection of cracks in space shuttle foam, (4) tumor imaging, such as that in breast tissue, (5) counterfeit detection, such as detecting counterfeit watermarks on paper currency, and (6) providing valuable spectroscopic information about the composition of a material, especially in chemical and biological species, all of which may resonate at a frequency similar to the energy leaving the THz generator. 
     While the invention has been described with reference to particular embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the scope of the invention.