Abstract:
The present invention addresses the problem of conveniently and efficiently decorating nanostructures such as carbon nanotubes with aerosol nanoparticles using electrostatic force directed assembly (“ESFDA”). ESFDA permits size selection as well as control of packing density spacing of nanoparticles. ESFDA is largely material independent allowing different compositions of such nanoparticle-nanotube structures to be produced.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application 60/710,642 filed on Aug. 23, 2005, hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     N/A 
     BACKGROUND OF THE INVENTION 
     The present invention relates to the assembly of nanoparticles, and, in particular, to a method of using electrostatic force to assemble nanoparticles onto nanostructures. 
     The manufacture of nanostructures from carbon nanotubes and nanoparticles may be useful in a broad range of applications including: nanoelectronics, chemical sensors, biosensors, catalysis, fuel cells, and hydrogen storage. Current methods for assembling these components are primarily based on “wet-chemical” techniques in which the components are created or manipulated with chemical reactions taking place in solution. These methods, however, are generally very slow and the associated interfacial chemistry is material dependent, limiting their ability to create nanostructures of arbitrary composition. Furthermore, although the size of the nanoparticles may be controlled in the solution prior to assembly, there is very limited control over the assembly process. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the problem of conveniently and efficiently decorating both single-walled and multi-walled carbon nanotubes with nanoparticles by using electrostatic force directed assembly (“ESFDA”). Using ESFDA, the packing density, spacing, and size distribution of nanoparticles on nanotubes can be controlled during the assembly process. Due to the inherent material independent nature of electrostatic force, nanoparticle-nanotube structures can be produced from a variety of different materials and different combinations of materials. 
     Specifically then, the present invention provides a method of coating various nanostructures with a variety of nanoparticles in which a plurality of electrically conductive nanostructures is created. A plurality of charged aerosol nanoparticles is then produced, and an electrical field is applied to the nanostructures. The charged nanoparticles introduced to the electrical field are then attracted to the nanostructures and bond to the nanostructures. 
     Thus, it is an object of at least one embodiment of the present invention to provide a simple method of assembling nano-sized particles into structures without the need for a liquid environment. 
     It is another object of the invention to provide an assembly technique that is largely indifferent to the chemical composition of the nanoparticles and nanostructures and that thus may be used to assemble a variety of novel structures of arbitrary composition. 
     The nanostructures may be either single-walled or multi-walled in nature. 
     The bonding between the nanostructures and nanoparticles may be non-covalent in nature so as to preserve the sp 2  hybridization of carbon atoms. 
     It is thus another object of the invention to preserve desirable electronic properties of carbon in the nanostructure. 
     The assembly time may be controlled so as to control the packing density of the nanoparticles on the nanostructures. In addition or alternatively, the aerosol flow rate and/or electrical field may also be controlled so as to control the size of the nanoparticles that attach to the nanostructures. 
     Thus, it is yet another object of at least one embodiment of the present invention to provide an assembly technique that allows multidimensional control of the assembly process. 
     The nanostructures used in the invention may be nanotubes. 
     Thus, it is another object of at least one embodiment of the present invention that the nanoparticles introduced to the electrical field may comprise a mixture of two different types of nanoparticles. 
     It is thus another object of the invention to provide an assembly method that may work with multiple nanoparticles of different chemical compositions. 
     Thus, it is another object of at least one embodiment of the present invention that the nanoparticles used are catalysts, photoelectric materials, or semiconductors. 
     It is thus another object of the invention to provide an assembly technique that by allowing great flexibility in the types and composition of nanoparticles being used is widely applicable to applications such as: filtration, sensing, purification, generation of materials, catalyzation, hydrogen storage, fuel cell components, discharge electrodes, spintronics, Raman scattering, wave guides, solar energy harvesting, nanometrology, and marking the nanostructure to study nanomechanics. 
     The assembled nanoparticles and nanostructure may be attached to a substrate and the nanostructure removed to transfer the nanoparticles to the substrate. 
     Thus, it is another object of the invention to provide a method of organizing nanoparticles on a microscopic scale for transfer to another object. A carbon nanotube used as the substrate organizes the nanoparticles along one dimension, while mutual electrostatic repulsion of the nanoparticles organizes them with a regular spacing defined by the number of nanoparticles deposited. 
     The above method may be used to produce a sensing electrode having a conductive nanowire extending between a first and second electrode to conduct electricity therethrough and nanoparticles of a different material than the conductive nanowire responsive to an environment of the conductive nanowire to change the properties of conduction of the conductive nanowire. 
     Thus, it is another object of at least one embodiment of the invention to provide a novel sensing device making use of the ability to attached materials of arbitrary composition, in a regular manner, to a conductive nanowire. 
     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of the electrostatic assembly apparatus of the present invention showing a nanoparticle generating arc and a carrier gas system flowing aerosol nanoparticles over a grid of nanotubes charged to attract and bond to the nanoparticles; 
         FIG. 2  is a simplified depiction of a TEM image of the assembled nanostructures of  FIG. 1 ; 
         FIG. 3  is a cross-sectional elevation view of the flow of nanoparticles over the nanostructures per  FIG. 1  showing the perpendicular forces of gas flow and electrostatic attraction that can be used to sort nanoparticle size; 
         FIG. 4  is an elevation view of a nanotube disposed between electrodes and having reactive nanoparticles on its surface to provide a novel sensing electrode; 
         FIGS. 5   a  and  5   b  are elevation fragmentary views of an assembled nanotube and nanoparticles showing steps of destroying the nanotube to transfer the ordered nanoparticles to a secondary substrate; 
         FIG. 6  is a fragmentary view similar to that of  FIGS. 4 and 5  showing a sensor in which nanoparticles attracting environmental agents, affect current flow in the underlying nanotube to create a sensor; 
         FIG. 7  is a fragmentary view similar to that of  FIG. 6  showing the same principle used to detect complex molecules such as DNA or RNA; 
         FIG. 8  is a fragmentary view similar to that of  FIG. 7  showing the structure sued as a photocell or light sensor by providing a closely coupled path between a photoelectric material and a conductive wire; 
         FIG. 9  is a fragmentary elevation view of a complex nanotube structure generated by seeding nanotubes with catalyst nanoparticles to create sites from which other nanotubes can be grown. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an electrostatic force directed assembly (ESFDA) device  10  provides an arc plasma source chamber  18  enclosing an arc cathode  14  opposed to an arc anode  20 , the later holding a precursor material  22  from which nanoparticles will be created. A plasma arc voltage source  12  couples the arc anode  20  to the arc cathode  14  to create the arc  16  which strikes the precursor material  22  for the production of the nanoparticles  42 . The arc cathode  14  and arc anode  20  may be, for example, tungsten and graphite respectively. 
     The application of the arc  16  to the precursor material  22  creates an aerosol of nanoparticles  42  through physical vaporization of the solid precursor material  22 . This generation of nanoparticles creates a relatively broad size distribution of nanoparticles  42 . A significant fraction of the nanoparticles  42  are charged by the arc  16  or through plasma or thermionic emission, which makes ESFDA feasible without the use of further nanoparticle  42  charging device. The charging of the nanoparticles  42  may prevent their agglomeration. Alternatively, the nanoparticle  42  production means may use aerosol reactors or may aerosolize colloidal nanoparticles  42 , in which case additional charging means may be required in such assemblies, or example using corona discharge electrodes. 
     Multiple precursor materials  22  may be used, for example, silver (Ag) and tin oxide (SnO 2 ) for example from different arc assemblies or using a mixture of precursor materials  22 . The gaseous aerosol may thus comprise a mixture different nanoparticles  42 , including generally catalysts, photo-catalysts, or semiconductors. The nanoparticles  42  will typically be less than 100 nanometers in diameter and may have a mean diameter of less than 20 nanometers. 
     The ESFDA device  10  further provides a room temperature carrier gas source  28  that is connected to the plasma source chamber  18  by way of a first inlet tube  26  and a second inlet tube  24 . The first and second inlet tubes are coupled together by a metering valve or flow meter  30  so that flow of the gas into the chamber  18  may be precisely controlled. The gas from the carrier gas source  28  is applied to the chamber  18  to carry the nanoparticles  42  created by the arc plasma source down a flow tube  38 . The flow tube  38  generally acts as an electrode and may comprise grounded metal conductor. A bypass tube  25  branches from the flow tube to the flow out of the flow tube  38  independent of the flow of carrier gas into the chamber  18 . The bypass tube  25  leads to a metering valve  34  and then connects to an exhaust tube  27  leading to a filter  32  disposed at the end of the exhaust tube  27  to catch any nanoparticles  42  that are diverted from the flow tube  38 . 
     A substrate electrode  40  is provided beneath the flow tube  38  and a voltage source  36  is connected between the substrate electrode  40 , and the flow tube  38  to provide an electrical field therebetween. The voltage source  36  may be either positive or negative in nature, depending on the charge of the nanoparticles, so as to attract the nanoparticles to the substrate electrode  40 . Supported by the substrate electrode  40  and in electrical communication with the substrate electrode  40 , are nanostructures  44  to which the nanoparticles will be assembled. In one embodiment, the nanostructures  44  are carbon nanotubes (CNT) coating a substrate electrode  40  that is a perforated copper grid. The holes in the grid appear to enhance the effect of the electrical field. A gap distance (e.g., 2-0.5 mm) is maintained between the metal flow tube  38  and the substrate electrode  40  using, for example, a precision-machined ceramic spacer. The larger the gap between the flow tube  38  and the substrate electrode  40 , the higher applied voltage necessary to sufficiently attract the nanoparticles  42  to the nanostructures  44 . 
     The presence of the voltage source  36  creates an electrical field in the neighborhood of the nanostructures  44  on the substrate electrode  40 . Thus, after the nanoparticles  42  flow through the flow tube  38 , they are preferentially attracted to the nanostructure  44  surfaces on the substrate electrode  40 . This attraction results in nanoparticle  42  decoration of the nanostructures  44 , which will be discussed further below. The voltage source may provide a voltage of 2 kV-500 V depending on the gap distance. The voltage is limited only by the breakdown voltage of the carrier gas (about 3×10 6  V/m for dry air). Calculation from experiments show the maximum electric field near the surface of a 20-nm carbon nanotube reaches 2.45×10 6  V/m for a voltage source  36  of 2 kV and a gap of 2 mm. 
     In the absence of an electrical field, it has been determined that the nanoparticles  42  are not appreciably attracted to the nanostructures  44  and do not bond to the surfaces of the nanostructures. 
     Various different nanostructures  44  such as carbon nanotubes, nanorings, nanorods, and nanowires may be used in the present invention. The nanostructures  44  are produced using known methods in the relevant technological field or, alternatively, may be bought from known commercial sources, e.g., Carbon Nanotechnologies, Inc. and Alfa Aesar. 
     Referring now to  FIG. 2  a cross-sectional simplified TEM image of a nanostructure  44  shows its surface decorated with a plurality of nanoparticles  42 . The nanoparticles  42  are bonded to the exterior surface of the nanostructures  44 . The adhesion between the nanoparticles  42  and the nanostructures  44  appears to be non-covalent in nature and thus preserves the sp 2  hybridization of the carbon atoms in that may be present in the nanostructures  44 . By maintaining this sp 2  hybridization, the unique intrinsic properties of the nanostructures  44  are preserved. The bonding between the nanostructures  44  and nanoparticle  42  may provide for good electrical conduction between the two. 
     The nanoparticles  42  are generally evenly spaced across the external surface of the nanostructure  44  because of the electrical charge carried on each of the nanoparticles  42  which cause them to mutually repel one another leading to a minimum energy configuration of substantially uniform distribution. 
     The average separation of the nanoparticles  42  bonded to the external surfaces of the nanostructures  44  can be controlled by adjusting the assembly conditions. For example, adjusting the duration of the flow of nanoparticles  42  over the substrate electrode  40  will control the packing density of the nanoparticles  42  on the nanostructures  44  as the packing density increases with increased assembly time. If the assembly time is sufficiently long, the entire surface of the nanostructure  44  may be coated with nanoparticles  42 . 
     Referring now to  FIG. 3 , similarly, controlling the flow rate of the nanoparticles  42  or the strength of the electrical field will control the size of the nanoparticles  42  assembled on the nanostructures  44 . As the nanoparticles  42  approach the substrate electrode  40 , they are held in a laminar flow represented by flow lines  46  applying an airflow force  50  on the nanoparticles  56  and  58  that is dependant on the size of the nanoparticles  56  and  58 . In contrast, an electrostatic force  48  perpendicular to the airflow force  50  and toward the substrate is dependent on the charge of the nanoparticles  56  and  58  which will be largely independent of their size. These competing forces create trajectory lines  52  and  54  that sort nanoparticles  42  of different sizes. Trajectory line  52  is traveled by representative small nanoparticle  56  and depicts the result of a proportionally larger electrostatic force  48  than airflow force  50  while trajectory line  54  is traveled by relatively larger nanoparticle  58  represents a proportionally smaller electrostatic force  48  than airflow force  50 . Larger nanoparticles  58  tend to be carried along further by the airflow force  50  due to their size relative to that of the smaller nanoparticles  56  allowing size distribution to be controlled. Nanoparticle size can affect the properties of the nanoparticles  42 . 
     To the extent that the amount of charge can be controlled on different nanoparticles  42 , this same effect may be used to sort nanoparticle materials or provide different size ratios among nanoparticles  42  of different materials. 
     Nanoparticles  42  reaching the nanostructures  44  are selected through their electrical mobility, the ability of a particle to move in an electrical field, characterized by the following equation: Z p =ν p /E=neC c /3πμD p , wherein ν p  is the nanoparticle  42  velocity along electric field lines, E is the electrical field, n is the number of elementary charges carried by nanoparticles, e is the elementary charge, C c  is the Cunningham slip correction factor, D p  is the diameter of the nanoparticles, and μ is the flow viscosity. The electric field need not be homogenous but can be further altered to control the distribution of the particles for example with electrode shapes, shields or photoelectric dissipation. 
     Some size selection is also intrinsic to the process of generating and conveying the aerosolized nanoparticles  42 . 
     Referring now to  FIG. 4 , the present invention may be used to construct a novel device in which a single nanotube  60  is disposed between a first electrode  64  and a second electrode  66 . An ohmmeter  62  or similar current sensitive device is placed between the two electrodes to measure the resistance of the nanotube  60 . Nanoparticles  42  are lined up across the surface of the nanotube  60  using the assembly procedure discussed herein. Alternatively, the nanotube  60  could be replaced by another type of conductive nanostructure  44 . 
     As shown in  FIG. 6 , the nanoparticles  42  may be selected to attract other particles  70  in the environment and bond to them. This bonding creates a region of increased resistance along the nanotube  60  that may be measured by the ohmmeter  62  (shown in  FIG. 4 ) to detect the presence of the particles  70 . 
     As shown in  FIG. 7 , in an alternative embodiment, strands of nucleotides  72  may be attached to the nanoparticles (before or after assembly) to hybridize with complimentary nucleotides  74  in the environment. Again, the result of this bonding of nucleotides  74  may be detected in changes in the flow of electrons  68  within the nanotube  60  near the bonded nucleotides  72 . This particular embodiment is useful in the field of biosensors sensitive to the presence of a particular biological agent. The nucleotide  72  is selected to bond only to the particular agent to be detected. 
     Referring to  FIG. 8 , a photoelectric nanoparticle  42  attached to the surface of a carbon nanotube  60  may be struck by a light ray  74  to eject electrons  68  collected by the nanotube  60  to create an improved photocell or photo sensor. 
     Referring now to  FIGS. 5   a  and  5   b , a carbon nanotube  60  is placed on the surface of a substrate  80 . Using the disclosed assembly procedure, nanoparticles  42  are lined up across the surface of the nanotube  60 . By applying a high voltage or heat or oxidizing chemical to the nanotube  60 , the nanotube  60  can be destroyed leaving behind only the nanoparticles  42  which remain lined up in a row on the substrate as shown in  FIG. 5   b . This technique can be used with conductive or semiconductive nanoparticles  42  to create fine conductive or semiconductive paths (for example, for integrated circuits) or to create fiducial marks on the substrate  80  for studies of microscopic strain or the like. 
     Referring now to  FIG. 9  the nanoparticles  42  may be a catalyst that is used to grow additional nanotubes  60  branching from the nanotubes  60  on which the nanoparticles  42  were originally deposited. The resultant structure may have a relatively large surface area while having a relatively small volume and may, in turn, be coated with different nanoparticles  42  to provide for catalytic structures, photocells, or filters or the like. 
     Various alternatives are contemplated as being within the scope of the following claims, particularly pointing out and distinctly claiming the subject matter regarded as the invention.