Abstract:
An emission device is provided for extracting electrons onto an anode of a visual display. The emission device ( 10 ) includes a conductivity limited material ( 18 ) positioned between first and second electrodes ( 14, 16 ) and having a surface ( 26 ). A plurality of catalytic nanoparticles ( 22 ) are distributed throughout the conductivity limited material ( 18 ), wherein some of the catalytic particles ( 22 ) are contiguous to the surface ( 26 ). A plurality of nanostructures ( 24 ), such as carbon nanotubes, are grown from the catalytic nanoparticles ( 22 ) contiguous to the surface ( 26 ). A voltage is applied across the conductivity limited material ( 18 ) having a plurality of catalytic particles ( 22 ) embedded therein, thereby causing the electrons to tunnel between the catalytic particles ( 22 ). An anode ( 28 ) is spaced apart from the nanostructures ( 24 ) for extracting and receiving electrons emitted from the nanostructures ( 24 ) when a first potential is applied across the first and second electrodes ( 14, 16 ) and a second potential is applied to the anode ( 28 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to carbon nanotube visual displays, and more particularly to electron source structures involving the combined mechanism of electron conduction and electron extraction. 
       BACKGROUND OF THE INVENTION 
       [0002]    A nanotube, and more specifically a carbon nanotube, is 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 by replacing other electron emitters (e.g., a Spindt tip), that generally have higher fabrication costs with a carbon nanotube based electron emitter. 
         [0003]    One approach for fabricating nanotubes includes depositing 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 integration of carbon nanotubes growth with other device fabrication processes. 
         [0004]    Ni has been used as one of the catalytic materials for the formation of single walled nanotubes during a laser ablation and arc discharge process as described by A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y. H. Lee, S. G. Kim, A. G. Rinzler, D. T. Colbert, G. E. Scuseria, D. Tomanet, J. E. Fischer, and R. E. Smalley in Science, 273, 483 (1996) and by D. S. Bethune, C. H. Kiang, M. S. de Vries, G. Gorman, R. Savory, J. Vazquez, and R. Beyers in Nature, 363, 605 (1993). 
         [0005]    Field effect devices typically comprise a metal cathode on a substrate, with carbon nanotubes grown on the cathode. A metal catalyst may be positioned between the cathode and the carbon nanotubes for facilitating carbon nanotube growth. A gate electrode is positioned between an anode and the tops of the carbon nanotubes for controlling electron emission from the carbon nanotubes. Electrons flow from the metal cathode through the metal catalyst if present, and out the carbon nanotubes to the anode spaced therefrom. 
         [0006]    A different approach comprises an electron emitting structure including a thin film containing fine particles between opposing electrodes. Voltage is applied across the thin film to impart a surface conduction current. Islands of the spatially discontinuous film serve as electron emitting regions. A microcrack is formed in the film for effectively emitting electrons. 
         [0007]    However, the microcrack process requires multiple process steps that are costly and difficult to control. Furthermore, the conversion of the conduction current to emission current efficiency is low. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    An emission device is provided for extracting electrons onto an anode of a visual display. The emission device includes a conductivity limited material positioned between first and second electrodes and having a surface. A plurality of catalytic nanoparticles are distributed throughout the conductivity limited material, wherein some of the catalytic particles are contiguous to the surface. A plurality of nanostructures, such as carbon nanotubes, are grown from the catalytic nanoparticles contiguous to the surface. A voltage is applied across the conductivity limited material having a plurality of catalytic particles embedded therein, thereby causing the electrons to tunnel between the catalytic particles. An anode is spaced apart from the nanostructures for extracting and receiving electrons emitted from the nanostructures when a first potential is applied across the first and second electrodes and a second potential is applied to the anode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
           [0010]      FIG. 1  is a cross section of a first embodiment of the present invention; 
           [0011]      FIG. 2  is a cross section of a second embodiment of the present invention; 
           [0012]      FIG. 3  is a cross section of a third embodiment of the present invention; and 
           [0013]      FIG. 4  is a schematic of an array of one of the embodiments of the present invention; and 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    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. 
         [0015]    Referring to  FIG. 1 , an emission device  10  is illustrated for forming a catalyst  18  on a substrate  12  that can be used to grow nanostructures  24 , and more preferably carbon nanotubes according to a preferred exemplary embodiment of the present invention. The device  10  may, for example, be a display, and the nanostructures  24  may be any type of structure capable of conducting electrons, for example, carbon nanotubes or carbon fibers. Using masking techniques well known in the industry, metal electrodes  14  and  16  are deposited on the substrate  12 . The substrate  12  comprises silicon; however, alternate materials, for example, glass, ceramic, metal, a semiconductor material, or a flexible material, are anticipated by this disclosure. Substrate  12  can include control electronics or other circuitry, which are not shown in this embodiment for simplicity. Also, substrate  12  may include an insulating layer, such as silicon dioxide, silicon nitride, or the like between the substrate  12  and the electrodes  14 ,  16 . The metal electrodes  14 ,  16  comprise molybdenum, but may comprise any metal with a high melting temperature, for example, Nobium, Hafnium, tungsten or iridium, and are deposited at room temperature up to 500° C. The electrodes  14 ,  16  are spaced between 20 and 100 micrometers apart, and more preferably 50 micrometers apart. The thickness of the electrodes  14 ,  16  is between 0.01 and 100 micrometers, and would preferably be 1.0 micrometers. 
         [0016]    A material  18  is deposited between the electrodes  14  and  16  and on the substrate  12 . The material  18 , for example, comprises a conductivity limited material  20  of, for example, oxides of silicon, aluminum, or zirconium, and would preferably be approximately 1 micron thick. The material  18  also is immiscible with catalytic particles  22  of metal. Examples of suitable catalytic particles  22  include titanium, vanadium, chromium, manganese, copper, zirconium, niobium, molybdenum, silver, hafnium, tantalum, tungsten, rhenium, gold; and preferably, ruthenium, rhodium, palladium, osmium, iridium, platinum; and more preferably nickel, iron, cobalt, or a combination thereof. The catalytic particles  22  may have a radius in the range of 0.5 to 100 nanometers, and preferably 2.5 nanometers. The catalytic particles  22  may be spaced apart in the range of 1 to 100 nanometers, and preferably 5.0 nanometers. 
         [0017]    The process used to place the material  18  on the substrate may comprise any of several known processes. For example, the dielectric material  20  and the metal for forming the catalytic particles  22  may be co-evaporated onto the substrate  12 . As the material  18  forms, the metal coalesces into the catalytic particles  22 . Alternatively, the material  18  may be formed by combining two salts of a dielectric and a metal and applying to the substrate. Yet another alternative would be to ink jet print the material  18  onto the substrate using methods known to those in the industry. 
         [0018]    Nanostructures  24  are then grown from the catalytic particles that have formed in the dielectric material  20  at its surface  26  by exposing the device  10  to a carbon containing gas at less than 550° C. for a few minutes, for example. It should be understood that the nanostructures  24  may be grown by any method known in the industry. The nano-composite material  18  allows for both the direct and selective growth of nanostructures  24  by CVD techniques, e.g., thermal CVD, HF-CVD, and PE-CVD, at low temperature and a controlled electron transport and injection in the nanostructures  24  modulated by the voltage applied between the two electrodes  14 ,  16 . The nanostructures  24  will emit electrons toward the anode  28  for illuminating phosphors (not shown) positioned on the anode  28  as is well known in the industry. 
         [0019]    The material  18  (nano-composite thin film) displays unique electron transport properties. The characteristic of the conduction current depends on both the concentration and the dimension of the catalyst particles  22  embedded in the dielectric material  20 . The nano-structure of the material  18  depends on the metal catalyst concentration (particle dimension and distribution) in the material  18  (dielectric matrix), a critical concentration usually called “percolation threshold” (PT). This nano-structure may vary from metal nano-particles in the dielectric material  20 , to a filamentary metal network structure containing insulator nano-cluster. A metal catalyst content higher than the percolation threshold leads to an electrical conduction in the material  18 , which is typical of metals, and for the growth of large carbon nanotubes  24  densely packed. For a metal catalyst content lower than the percolation threshold, the electron conduction occurs by activated mechanisms such as electron tunneling and/or electron hopping and the dependence of the catalyst electrical conductivity on temperature becomes non linear. 
         [0020]    When a potential is applied across the electrodes  14  and  16  in the range of 5 to 20 volts, and more preferably of 10 volts, electrons will percolate through the material  18 , generally from one electrode to the other. The percolation of electrons may be thought of as “hopping”, or tunneling through the dielectric material  20 , from one catalytic particle  22  to another, following the path of lowest activation energy, or least resistance. Some of the electrons will be “hopping” to the catalytic particles  22  that are positioned on or at the surface  26 . Current transport across the electrode  14  and  16  within the material  18  may be explained by substrate assisted tunneling. The electrons transfer between metal catalyst particles  22  under an activated mechanism. Because the electron tunneling occurs over the whole surface  26  of the material  18 , the current is conductivity-limited. This conduction-limited current involves an energy activation process which is associated with the dimension of catalytic particles  22 . The activation energy of the conductivity-limited material  18  (nano-composite thin film) depends on the electrostatic energy of the conductive catalyst particles  22  of a radius with respect to distance to a neighbor catalytic particle  22 . When a voltage is applied between electrodes  14  and  16 , the activation energy becomes field dependent. The bias voltage would preferable be 5 volts, but may be in the range of 3 to 10 volts. Once the electrons reach the catalytic particle  22  from which a nanostructure  24  has been grown, and due to the positive bias on the anode  28 , the electrons will continue along the nanostructure  24  and exit towards the anode  28 . 
         [0021]    In summary, the material  18  (nano-structured catalyst thin film) at a certain percolation threshold varying between 40 to 60% allows both the growth of thin and dispersed carbon nanotubes  24  that field emit electrons under the influence of the electric field generated by a biased anode, and plays the role of electron valve which control the transport and the injection of the electron current in each carbon nanotube  24  forming the electron sources (extractors) of the emitting structure. The “tunneling and/or hopping” of the electron from one catalytic particle  22  to another one is an activated mechanism. The conduction current flowing between the electrodes  14 ,  16  is conductivity-limited. The material  18  (catalytic nano-composite conduction layer) formed between the two electrodes is preferably made of catalytic metal nano-particles such as Fe, Co, Ni or a mixture thereof embedded in an insulator matrix made of, for example, Al 2 O 3 , SiO 2 , MgO, Y 2 03, ZrO 2 , and diamond like carbon. The material  18  exhibits a thickness varying from 10 to 150 nm with catalytic nanoparticles  22  having a dimension of around 5 nm. The electrical conductivity is in the order of 10 −7  to 10 −3  Ohm −1  and is a function of the energy of activation and the percolation threshold. The percolation threshold is between 40-60% and depends on the metal-dielectric composition and the material thermal annealing temperature and time. The carbon nanotubes are grown on the material  18  and are anchored to the conductive small catalyst particles  22 . The material  18  can withstand higher electrical field strength without risk of electrical breakdown. 
         [0022]    Referring to  FIG. 2 , another embodiment comprises a structure  30  including an electrode  32  formed on the substrate using standard lithographic techniques. The electrode  32  comprises a conductive material, for example, one of the metals molybdenum, Nobium, Hafnium, tungsten or iridium similar to that of the electrodes  14 ,  16 . A dielectric layer  34  is formed over the substrate  12  and electrode  32 . However, it should be understood that the electrode  32  may be formed directly between the substrate  12  and the material  18 . A voltage of up to 5 volts is applied to the electrode  32 , which is negative with respect to the anode  28 , causing the electrons to further be encouraged to deflect towards the surface  26 . By placing the electrode  32  in the middle between the electrodes  14  and  16 , there is a tendency for the electrons to be emitted from the carbon nanotubes  24  also in the middle between the electrodes  14  and  16 , and thus provide a more focused beam toward the anode  28 . 
         [0023]    Referring to  FIG. 3 , yet another embodiment comprises a structure  40  similar to that of  FIG. 1 ; however; the nanoparticles  22  in the material  18  are terminated by carbon nanostructures  24  (e.g., carbon nanotubes) formed on catalyst nanoparticles  22 , serving as growth nuclei. In this embodiment the small catalytic nanoparticles  22  are preferably formed in the middle of the material  18  and larger catalytic nanoparticles  22  are formed on the sides of the material  18  near the electrodes  14  and  16 . Since the carbon nanotube  24  dimension depends on the particle size of the catalyst nanoparticles  22 , it is preferable that the catalyst nanoparticles  22  size and the density is 0.1-10 nm and 10 5 -10 11  nanoparticles/cm 2 , respectively, and the distance between nanoparticles  22  is at least equal to the particle size. The size, density and material of the nanoparticles  22  are appropriately set for localized growth. Field emission from material  18  requires that the increase in energy of the emitted electrons during tunneling must be greater than the work function of the nanoparticles  22 . To obtain a large gradient of the electric field directed into the vacuum space, the nanostructures  24  operate as electron emission extractors (antenna). Hence, the tunneling electrons between electrode  14  and  16  preferentially channel towards the nanoparticles  22  attached to the high field enhancement nanostructures  22  and are emitted into the vacuum space by the low electric field produced by electrode  28 . 
         [0024]    The present invention comprises a composite material  18  of metal-dielectric mixture including grains with nano-metric size which plays both the role of catalyst precursor for carbon nanotubes  24  synthesize and also as an electron regulation layer for electron injection into the carbon nanotubes  24 . Furthermore, the use of this nano-composite catalytic material  18  with the carbon nanotubes  24  permits fabrication of unique multi-emitting electron sources, for applications such as Field Emission Displays. 
         [0025]    Referring to  FIG. 4 , the structures  10 ,  30 , or  40  may easily be fabricated in an array  42 , or matrix of structures  10 ,  30 ,  40 , for use in an emissive display. The nanostructures  24  will emit electrons toward the anode  28  for illuminating RGB color phosphors (not shown) positioned on the anode  28  as is well known in the industry. Each sub-pixel of emitters  24  (not shown), included within each area of material  18 , are uniquely coupled to one of the column conductors  43 ,  44 ,  45  and row conductors  46 ,  47 ,  48 . For example, when a voltage is applied to column conductor  45  and row conductor  47 , the voltage is applied to electrodes  14  and  16  and through material  18 , as illustrated. While nine structures of sub-pixels of the display are shown, it should be understood that any number may be used with the present invention. For different size and different resolution displays, the sub-pixel length (in the Y direction) will be maintained, while the width (in the X direction) will vary. 
         [0026]    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.