Abstract:
An apparatus ( 50 ) and method is provided for growing a network of common diameter nanotubes ( 24 ). The apparatus comprises chemically functionalizing a portion ( 16 ) of a substrate ( 12 ); anchoring catalyst nanoparticles ( 22 ), each having substantially the same diameter, on the portion ( 16 ) of the substrate ( 12 ); and growing overlapping carbon nanotubes ( 24 ), each having substantially the same diameter, on the catalyst nanoparticles ( 22 ).

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to a carbon nanotubes and more particularly to a network of single walled carbon nanotubes.  
       BACKGROUND OF THE INVENTION  
       [0002]     Carbon is one of the most important known elements and can be combined with oxygen, hydrogen, nitrogen and the like. Carbon has four known unique crystalline structures including diamond, graphite, fullerene and carbon nanotubes. In particular, carbon nanotubes refer to a helical tubular structure grown with a single wall or multi-wall, and commonly referred to as single-walled nanotubes (SWNTs), or multi-walled nanotubes (MWNTs), respectively. These types of structures are obtained by rolling a sheet formed of a plurality of hexagons. The sheet is formed by combining each carbon atom thereof with three neighboring carbon atoms to form a helical tube. Carbon nanotubes typically have a diameter in the order of a fraction of a nanometer to a few hundred nanometers.  
         [0003]     Carbon nanotubes can function as either a conductor, like metal, or a semiconductor, according to the rolled shape and the diameter of the helical tubes. With metallic-like nanotubes, it has been found that a one-dimensional carbon-based structure can conduct a current at room temperature with essentially no resistance. Further, electrons can be considered as moving freely through the structure, so that metallic-like nanotubes can be used as ideal interconnects. When semiconductor nanotubes are connected to two metal electrodes, the structure can function as a field effect transistor wherein the nanotubes can be switched from a conducting to an insulating state by applying a voltage to a gate electrode. It has been shown that carbon nanotubes yield a transconductance per unit channel width greater than that of silicon transistors. Therefore, carbon nanotubes are potential building blocks for nanoelectronic devices because of their unique structural, physical, and chemical properties.  
         [0004]     Existing methods for the production of nanotubes, include arc-discharge and laser ablation techniques. These methods typically yield bulk materials with bundles of nanotubes. Recently, reported by J. Kong, A. M. Cassell, and H Dai, in Chem. Phys. Lett. 292, 567 (1988) and J. Hafner, M. Bronikowski, B. Azamian, P. Nikoleav, D. Colbert, K. Smith, and R. Smalley, in Chem. Phys Lett. 296, 195 (1998) was the formation of high quality individual single-walled carbon nanotubes (SWNTs) demonstrated via thermal chemical vapor deposition (CVD) approach, using Fe/Mo or Fe nanoparticles as a catalyst. The CVD process has allowed selective growth of individual SWNTs, and simplified the process for making SWNT based devices. Typically, the choice of catalyst materials that can be used to promote SWNT growth in a CVD process comprises iron, cobalt, and nickel particles.  
         [0005]     A network of nanotubes has been shown as a field effect transistor by placing source and drain electrodes at opposed sides of the network and a gate electrode positioned adjacent the nanotubes therebetween. The network of nanotubes has obvious advantages since it allows multiple current paths. The nanotube network acts like a semiconducting channel even if some of the nanotubes in the network are metallic as long as they do not short out the entire channel. A network of carbon nanotubes are easily produced by growth on a catalyzed substrate or by suspending a substrate in a solution of carbon nanotubes. However, results are poor due to the inconsistency in nanotube diameter and density. The physical and chemical properties of carbon nanotubes vary with their diameter (current carrying capability) and helicity (determines whether metallic or semiconductor). Different nanotube diameters result in variable bandgaps of individual nanotubes leading to non-uniform electrical properties of the nanotube network.  
         [0006]     Accordingly, it is desirable to provide a carbon nanotube network having improved electrical consistency. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     An apparatus and method is provided for growing a network of common diameter nanotubes. The apparatus comprises chemically functionalizing a portion of a substrate; anchoring catalyst nanoparticles, each having substantially the same diameter, on the portion of the substrate; and growing overlapping carbon nanotubes, each having substantially the same diameter, on the catalyst nanoparticles. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0009]      FIGS. 1-3  are a top view and cross sections of a structure being prepared for the growth of carbon nanotubes;  
         [0010]      FIG. 4  is the structure of  FIG. 2  having catalytic nanoparticles positioned thereon in accordance with a first embodiment of the present invention;  
         [0011]      FIG. 5  is an isometric view of the structure of  FIG. 4  having carbon nanotubes grown thereon in accordance with the first embodiment of the present invention;  
         [0012]      FIG. 6  is an isometric view of the first embodiment of  FIG. 4  having conducting electrodes deposited thereon;  
         [0013]      FIG. 7  is a cut-away isometric view of a second embodiment of the present invention; and  
         [0014]      FIG. 8  is a block diagram of a third embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.  
         [0016]     Referring to  FIG. 1 , a resist  14  is formed on a substrate  12  of the device  10 . The substrate  12  preferably comprises silicon dioxide on silicon, but may alternatively comprise, for example, glass, ceramic or a flexible substrate. The resist would comprise any resist typically used in the semiconductor industry. Optionally, the layer  18  may be formed by a stamping technique known to those skilled in the industry without using the resist  14 , as discussed below.  
         [0017]     Referring to  FIG. 2 , some of the resist  14  is lifted, e.g., by a photo etch, to expose a portion  16  of the substrate  12 . While only one portion  16  of the substrate  12  is exposed in the device  20  of  FIG. 2 , it should be understood that many portions  16 , perhaps many thousands or more, could exist on a single substrate  12 .  
         [0018]     Referring to  FIG. 3 , the portion  16  is chemically functionalized by exposing to radiation, or submerging the device  20  in a wet solution, or exposing to a vapor, of aminopropyltriethoxysilane (APS), thereby forming a layer  18  on the portion  16  of the substrate  12 . While APS is the preferred solution, any chemical or multilayers of chemicals that create a charged surface on the substrate to allow electrostatic interaction with the oppositely charged catalytic nanoparticles. The electrostatic interaction between the chemically functionalized surface and the nanoparticles will immobilize the nanoparticles in the selected region. The layer  18  would have a thickness, for example, in the range of 5.0 to 1000 Angstroms.  
         [0019]     Referring to  FIG. 4 , catalyst nanoparticles  22  of a fixed diameter are anchored on the layer  18  by submerging the device  30  in a wet solution containing the catalyst nanoparticles  22 . APS has an affinity (an electrostatic attraction) for the catalyst nanoparticles  22 . The catalyst nanoparticles  22  preferably comprise nickel, iron, cobalt, or any combination thereof, but could comprise any one of a number of other materials including a transition metal or alloys thereof, for example, Fe/Co, Ni/Co or Fe/Ni. The wet solution containing the catalyst nanoparticles  22  may comprise any solvent that allows monodisperse suspension of the catalytic nanoparticles. The nanoparticles would have a diameter in the range of 0.5 nanometers to 5 nanometers, but preferably would be approximately 1.0 to 2.0 nanometers thick for transistor or sensor applications discussed later. The resist  14  is then removed by either a wet or dry etch. Alternatively, the resist  14  may be removed prior to submerging the device  30  in the wet solution.  
         [0020]     Referring to  FIG. 5 , a chemical vapor deposition (CVD) is performed by exposing the device  40  to hydrogen (H 2 ) and a carbon containing gas, for example methane (CH 4 ), between 450° C. and 1000° C., but preferably at 850° C. CVD is the preferred method of growth because the variables such as temperature, gas input, and catalyst may be controlled. Carbon nanotubes  24  are thereby grown from the nanoparticles  22  forming a network  26  of connected carbon nanotubes  24 . Although only a few carbon nanotubes  24  are shown, those skilled in the art understand that a large number of carbon nanotubes  24  could be grown. By using nanoparticles  22  having a common diameter, the nanotubes  24  will grow with a similar common diameter. The desired diameter of the carbon nanotubes may be selected by depositing catalytic nanoparticles  22  having the desired diameter. The carbon nanotubes  24  may grow as either a metallic or semiconducting. The nanotubes  24  may be grown in any manner known to those skilled in the art, and are typically 100 nm to 1 cm in length and less than 1 nm to 100 nm in diameter.  
         [0021]     Referring to  FIG. 6 , conductive electrodes  28  are placed on the carbon nanotubes  24  at the sides of the network  26  of device  50 . The conductive electrodes  28  may comprise any conductive material, but preferably would comprise layers of chromium and gold, titanium and gold, palladium, or gold. Contact between the nanotubes  24  and conductive electrodes  28  are made during fabrication, for example, by any type of lithography, e-beam, optical, soft lithography, or imprint technology.  
         [0022]     In one embodiment, the conductive electrodes  28  of device  60  may be used as a source and a drain, respectively. A gate electrode  32  may be either buried in the substrate, for example, below the portion  16  of the substrate  12  (not shown), or it may be placed above the carbon nanotubes  24 , separated therefrom by a dielectric layer  34  as shown in device  70  of  FIG. 7 .  
         [0023]      FIG. 8  illustrates an embodiment wherein the device of  FIG. 6  is used as a sensor. For example, when a molecule attaches itself to a nano-structure, such as the carbon nanotube  24 , a characteristic of the material changes, such as the change in a current flowing in the nanotube  24  that is measurable in a manner known to those skilled in the art. By measuring this change in the current, it is known that a determination may be made as to the number of molecules that have attached to the carbon nanotube  24 , and therefore, a correlation to the concentration of the molecules in the environment around the carbon nanotube  24 . Additionally, the nano-structure may be coated with a substance for determining specific environmental agents. And while a change in current is the preferred embodiment for the measurable material characteristic, other embodiments would include, for example, magnetic, optical, frequency, and mechanical. The exemplary system  80  includes the device  60 , for example, having one of its electrodes  28  coupled to a power source  36 , e.g., a battery. A circuit  38  determines the current between the electrodes  28  and supplies the information to a processor  42 . The information may be transferred from the processor  42  to a display  44 , an alert device  46 , and/or an RF transmitter  48 , for example.  
         [0024]     While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.