Abstract:
A method is provided for selectively placing catalytic nanoparticles ( 22 ) for growing one dimensional structures ( 28 ) including nanotubes and nanowires. The method comprises providing a solution ( 23 ) including a plurality of catalytic nanoparticles ( 28 ) suspended therein. An alternating current is applied between two electrodes ( 12, 14 ) submersed in the solution ( 23 ), thereby positioning the plurality of catalytic nanoparticles ( 22 ) contiguous to the two electrodes ( 12, 14 ). A one dimensional nanostructure ( 28 ) is then grown from each of the catalytic nanoparticles ( 22 ).

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to growing one dimensional nanostructures, and more particularly to placing catalytic nanoparticles for the growth of one dimensional nanostructures.  
       BACKGROUND OF THE INVENTION  
       [0002]     One-dimensional nanostructures, such as belts, rods, tubes and wires, have become the latest focus of intensive research with their own unique applications. One-dimensional nanostructures are model systems to investigate the dependence of electrical and thermal transport or mechanical properties as a function of size reduction. In contrast with zero-dimensional, e.g., quantum dots, and two-dimensional nanostructures, e.g., GaAs/AlGaAs superlattice, direct synthesis and growth of one-dimensional nanostructures has been relatively slow due to difficulties associated with controlling the chemical composition, dimensions, and morphology. Alternatively, various one-dimensional nanostructures have been fabricated using a number of advanced nanolithographic techniques, such as electron-beam (e-beam), focused-ion-beam (FIB) writing, and scanning probe.  
         [0003]     Carbon nanotubes are one of the most important species of one-dimensional nanostructures. Carbon nanotubes are one of four unique crystalline structures for carbon, the other three being diamond, graphite, and fullerene. In particular, carbon nanotubes refer to a helical tubular structure grown with a single wall (single-walled nanotubes) or multiple wall (multi-walled nanotubes). 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. As used herein, a “carbon nanotube” is any elongated carbon structure.  
         [0004]     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, 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. Therefore, carbon nanotubes are potential building blocks for nanoelectronic and sensor devices because of their unique structural, physical, and chemical properties.  
         [0005]     Another class of one-dimensional nanostructures is nanowires. Nanowires of inorganic materials have been grown from metal (Ag, Au), elemental semiconductors (e.g., Si, and Ge), III-V semiconductors (e.g., GaAs, GaN, GaP, InAs, and InP), II-VI semiconductors (e.g., CdS, CdSe, ZnS, and ZnSe) and oxides (e.g., SiO 2  and ZnO). Similar to carbon nanotubes, inorganic nanowires can be synthesized with various diameters and length, depending on the synthesis technique and/or desired application needs.  
         [0006]     A carbon nanotube is also known to be useful for providing electron emission in a vacuum device, such as a field emission display. The use of a carbon nanotube as an electron emitter has reduced the cost of vacuum devices, including the cost of a field emission display. The reduction in cost of the field emission display has been obtained with the carbon nanotube replacing other electron emitters (e.g., a Spindt tip), which generally have higher fabrication costs as compared to a carbon nanotube based electron emitter.  
         [0007]     Both carbon nanotubes and inorganic nanowires have been demonstrated as field effect transistors (FETs) and other basic components in nanoscale electronic such as p-n junctions, bipolar junction transistors, inverters, etc. The motivation behind the development of such nanoscale components is that “bottom-up” approach to nanoelectronics has the potential to go beyond the limits of the traditional “top-down” manufacturing techniques.  
         [0008]     Another major application for one-dimensional nanostructures is chemical and biological sensing. The extremely high surface-to-volume ratios associated with these nanostructures make their electrical properties extremely sensitive to species adsorbed on their surface. For example, the surfaces of semiconductor nanowires have been modified and implemented as highly sensitive, real-time sensors for pH and biological species.  
         [0009]     Some of the challenges faced in forming one-dimensional nanostructures are (1) the selection of an appropriate catalyst, (2) size of the catalyst nanoparticle, (3) placement of the catalyst nanoparticles in desired locations, and (4) precise control over the growth condition parameters.  
         [0010]     In the case of carbon nanotubes, various catalytic material processes have been invoked even for a similar growth technique such as thermal chemical vapor deposition (CVD). For example, a slurry containing Fe/Mo or Fe nanoparticles served as a catalyst to selectively grow individual single walled nanotubes. However the catalytic nanoparticles usually are derived by a wet slurry route which typically has been difficult to use for patterning small features.  
         [0011]     Another approach for fabricating nanotubes is to deposit metal films using ion beam sputtering to form catalytic nanoparticles. In an article by L. Delzeit, B. Chen, A. Cassell, R. Stevens, C. Nguyen and M. Meyyappan in Chem. Phys. Lett. 348, 368 (2002), CVD growth of single walled nanotubes at temperatures of 900° C. and above was described using Fe or an Fe/Mo bi-layer thin film supported with a thin aluminum under layer. However, the required high growth temperature prevents simple integration of carbon nanotube growth with other device fabrication processes.  
         [0012]     Ni has been used as one of the catalytic materials for the bulk formation of single walled nanotubes during laser ablation and arc discharge processes as described by Thess et al. in Science, 273, 483 (1996) and by Bethune et al. in Nature, 363, 605 (1993). Thin Ni layers have been widely used to produce multiwalled carbon nanotubes via CVD. The growth of single walled nanotubes using an ultrathin Ni/Al bilayer film as a catalyst in a thermal CVD process has been demonstrated. The Ni/Al film deposited by electron-beam evaporation allows for easier control of the thickness and uniformity of the catalyst materials (U.S. Pat. No. 6,764,874). When the substrate is heated, the Al layer melts and forms small droplets which absorb the residual oxygen inside the furnace and/or from the underlying SiO 2  layer and oxidize quickly to form thermally stable Al 2 O 3  clusters. This in turn provides the support for the formation of Ni nanoparticles which catalyze the growth of single walled nanotubes.  
         [0013]     The diameters of single walled nanotubes and inorganic nanowires are proportionally related to the sizes of the catalytic nanoparticles used in CVD processes (L. An et al., “Synthesis of nearly uniform single-walled carbon nanotubes using identical metal containing molecular nanoclusters as catalysts”, J. Amer, Chem. Soc., Vol. 124, pp. 13688-13689, 2002). However, consistently uniform nanotubes and nanowires have not been produced because of the fairly broad diameter distributions of the nanoparticles used as catalysts.  
         [0014]     Accordingly, it is desirable to provide a simple yet reliable technique to assemble catalytic nanoparticles selectively in desired locations for device applications. 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  
       [0015]     A method is provided for selectively placing catalytic nanoparticles for growing one dimensional structures including nanotubes and nanowires. The apparatus comprises providing a solution including a plurality of catalytic nanoparticles suspended therein. An alternating current is applied between two electrodes submersed in the solution, thereby positioning the plurality of catalytic nanoparticles contiguous to the two electrodes. A one dimensional nanostructure is then grown from each of the catalytic nanoparticles. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0017]      FIG. 1  is a simplified cross-sectional view of an apparatus on which the exemplary method of the present invention may be applied;  
         [0018]      FIG. 2  is a simplified isometric view of the apparatus of  FIG. 1 ;  
         [0019]      FIG. 3  is a simplified cross-sectional view of an apparatus on which an exemplary embodiment of the method has been applied;  
         [0020]      FIG. 4  is a simplified cross-sectional view of an apparatus on which another exemplary embodiment of the method has been applied;  
         [0021]      FIG. 5  is a simplified cross-sectional view of an apparatus on which yet another exemplary embodiment of the method has been applied; and  
         [0022]      FIG. 6  is a simplified flow chart of the steps of an exemplary embodiment of the present invention; and 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     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.  
         [0024]     One dimensional nanostructures such as nanotubes and nanowires show promise for the development of molecular-scale sensors, resonators, field emission displays, and logic/memory elements. One dimensional nanostructures is herein defined as a material having a high aspect ratio of greater than 10 to 1 (length to diameter). Preparation of these nanostructures by chemical vapor deposition (CVD) has shown a clear advantage over other approaches. In addition, the CVD approach allows for the growth of fairly uniform one dimensional nanostructures by controlling the size of catalytic nanoparticles. For example, the diameters of single walled nanotubes are typically proportionally related to the sizes of the catalytic nanoparticles used in the CVD process. The positioning of the carbon nanotubes at specific locations has previously been challenging. The method disclosed herein positions catalytic nanoparticles at desired locations by the application of an alternating current (AC) field to conducting electrodes. Once the catalytic nanoparticles are positioned, carbon nanotubes may be grown using conventional CVD processes. Optionally, the size of the catalytic nanoparticles may be controlled by the frequency of the AC field, thereby controlling the size of the carbon nanotubes grown therefrom.  
         [0025]     A one dimensional nanostructures growth technique is disclosed wherein catalytic nanoparticles of selected sizes may be placed in a desired position. With the appropriate choice of amplitude and frequency, the use of an AC bias dramatically enhances the placement of desired catalytic nanoparticles sizes.  
         [0026]     Referring now to  FIG. 1 , illustrated in simplified cross-sectional views, and in  FIG. 2  in a partial perspective view, is an assembled structure utilized for selective placement of catalytic nanoparticles according to an exemplary embodiment of the present invention. More specifically, illustrated in  FIG. 1  is an apparatus for selectively positioning catalytic nanoparticles, wherein provided is an assembly  10  including two or more electrodes  12 ,  14 . Although electrodes  12 ,  14  are shown as positioned on insulating layer  18 , they could be recessed or buried. Assembly  10  in this particular embodiment includes a substrate  17 , comprising a semiconductor material  16  which has been coated with an insulating material  18 . It should be understood that anticipated by this disclosure is an alternate embodiment in which substrate  17  is formed as a single layer of insulating material, such as glass, plastic, ceramic, or any dielectric material that would provide insulating properties. By forming substrate  17  of an insulating material, the need for a separate insulating layer formed on top of a semiconductive layer, or conductive layer, such as layer  18  of  FIG. 1 , is eliminated.  
         [0027]     The semiconductor material  16  comprises any semiconductor material well known in the art, for example, silicon (Si), gallium arsenide (GaAs), germanium (Ge), silicon carbide (SiC), indium arsenide (InAs), or the like. Insulating material  18  is disclosed as comprising any material that provides insulative properties such silicon oxide (SiO 2 ), silicon nitride (SiN), or the like. The insulating material  18  comprises a thickness of between 2 nanometers and 10 microns. Semiconductor material  16  and insulating material  18  form substrate  17  as illustrated in  FIGS. 1 and 2 . In this specific example, assembly  10  includes a first electrode  12  and a second electrode  14  formed on an uppermost surface of insulating material  18 . Fabrication of metal electrodes  12  and  14  is carried out using any form of lithography, for example, photolithography, electron beam lithography, and imprint lithography on an oxidized silicon substrate  17 . In some embodiments, electrodes  12 ,  14  may comprise highly doped semiconductor material. Electrodes  12  and  14  comprise a thickness in the range of 1 nanometer to 5000 nanometers. Electrodes  12  and  14  are formed to define therebetween a gap  20  and provide for the application of an AC electric field (as illustrated in  FIG. 2 ). The gap  20  between electrodes  12  and  14  may be between 1 nanometer and 100 microns.  
         [0028]     The solution  23  is immiscible with catalytic particles  22  in a solution such as an aqueous environment (water based), or non-aqueous based on, for example, methanol, ethanol, or acetone. Examples of suitable catalytic particles  22  for nanostructure growth include titanium, vanadium, chromium, manganese, copper, zirconium, niobium, molybdenum, silver, hafnium, tantalum, tungsten, rhenium, gold, ruthenium, rhodium, palladium, osmium, iridium, platinum, nickel, iron, cobalt, or a combination thereof. More particularly for carbon nanotube growth, examples include nickel, iron, and cobalt, or combinations thereof. And for silicon nanowire growth, examples include gold or silver. The catalytic particles  22  may have a radius in the range of 0.5 to 100 nanometers, and preferably in the range of 1 to 5 nanometers for single walled nanotubes. The catalytic particles  22  may be spaced apart in the range of 1 to 100 nanometers, and preferably 5.0 nanometers.  
         [0029]     During operation in accordance with an exemplary embodiment of the present invention as illustrated in  FIG. 2 , an AC field is applied between electrodes  12  and  14  thereby causing movement of catalytic nanoparticles  22  suspended within an aqueous environment  23  toward gap  20  where the field and/or field gradient is the strongest. It should be understood that anticipated by this disclosure is the use of any environment, such as liquid or gaseous in which nanometer-scale components are contained. More specifically,  FIG. 2  illustrates catalytic nanoparticles  22  placed on electrodes  12  and  14  and on the insulating material  18 . The catalyst  20  preferably comprises for carbon nanotube growth, for example, nickel, cobalt, iron, and a transition metal or oxides and alloys thereof. The AC field may be applied for a duration of only a second or two up to several minutes depending on catalytic nanoparticles  22  concentration in the solution  23 , to position a desired number of the catalytic nanoparticles  22  in preferred locations. Optionally, a chemical functionalization step may be performed on the insulating layer  18  to immobilize, or attach, the catalytic nanoparticles  28  in preferred locations. Similarly, for positioning the catalytic nanoparticles  28  only on the electrodes  12  and  14 , a chemical functionalization step may be performed on the insulating layer  18  to repel the catalytic nanoparticles  28  from the insulating layer  18  ( FIG. 3 ).  
         [0030]     Immediately prior to the application of an AC field, substrate  17  is cleaned, followed by a 20 minute soak in ethanol to remove oxidized Au. It should be understood that the amplitude of the AC bias, frequency and trapping time may vary, dependent upon the nature, desired size, and concentration of the catalytic nanoparticles and the dielectric environment in which the catalytic nanoparticles are contained. Placement time in this particular example is typically between 5 and 30 seconds. In principle, one may use a direct current (DC) field to trap catalytic nanoparticles in the gap, but such DC field is not the field of choice herein as use of a DC field will result in a success rate that is much lower as compared to an AC field. Under the influence of an AC field, catalytic nanoparticles  22  experience a dielectrophoretic force that pulls them in the direction of maximum field gradient found in gap  20 .  
         [0031]     After catalytic nanoparticles  22  positioning and removal of the solution  23 , one dimensional nanostructures  28  are then grown from the catalytic nanoparticles  22  in a manner known to those skilled in the art, e.g., applying a gas comprising hydrogen and carbon for carbon nanotube growth. Although only a few catalytic nanoparticles  22  and one dimensional nanostructures  28  are shown, those skilled in the art understand that any number of catalytic nanotubes  22  and one dimensional nanostructures  28  could be formed.  
         [0032]     The one dimensional nanostructures  28  may be grown, for example, as a field effect transistor for use in sensors or electronic circuits, or as conductive elements, in which case a one dimensional nanostructures  28  will be grown from one catalytic nanoparticle  22  to an electrode or to another one dimensional nanostructures  28  to form a electrical connection between electrodes as shown in  FIGS. 2 and 3 .  
         [0033]     Alternatively, when used for a display device, the one dimensional nanostructures  28  may be grown in a vertical direction as illustrated in  FIG. 4 , for example. It should be understood that any one dimensional nanostructure  28  having a height to radius ratio of greater than 10, for example, would function equally well with some embodiments of the present invention.  
         [0034]     The process is further illustrated by the flow chart  40  in  FIG. 6  wherein a material  16  is provided  60  to form a substrate  17 . The material  16  may be coated  62  with an insulating material  18 . Two electrodes  12  and  14  are fabricated  64  on the substrate  17  surface. A solution  23  comprising catalytic nanoparticles  22  is applied  66  to the two electrodes  12  and  14 . An alternating current is applied  68  to the electrodes  12  and  14  causing the catalytic nanoparticles  22  to migrate to a position contiguous to the electrodes  12  and  14 .  
         [0035]     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.