Abstract:
Particles, which may include nanoparticles, are mixed with carbon nanotubes and deposited on a substrate to form a cold cathode. The particles enhance the field emission characteristics of the carbon nanotubes. An additional activation step may be performed on the deposited carbon nanotube mixture to further enhance the emission of electrons.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application claims priority to the following U.S. Provisional Patent Applications, Serial Nos. 60/343,642, 60/348,856, and 60/369,794. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates in general to carbon nanotubes, and in particular, to the utilization of carbon nanotubes in field emission applications.  
         BACKGROUND INFORMATION  
         [0003]    Carbon nanotubes have been used by many for field emission applications. Carbon nanotubes (CNTs) come in two families, single wall nanotubes (SWNTs) and multi-wall nanotubes (MWNTs). Both materials are long (11-10,000 microns) and thin (0.001-0.1 microns in diameter). This high aspect ratio and the fact that they are semiconducting or metallic makes them ideal candidates for field emission applications. One problem, however, is if the CNTs are too densely packed, the CNTs shield each other from the strong electrical fields needed to extract the electrons from the material. The field emission from these materials is further improved if the CNT fibers are aligned in parallel to the applied electrical field. Also desired is an inexpensive way of applying the CNT material onto suitable substrate materials at low temperature and aligning these materials using methods that are suitable for large-scale manufacturing.  
           [0004]    By growing CNTs directly on a catalyst, some success has been achieved in growing the CNT materials with acceptable density and alignment, but not always in a predictable fashion. Furthermore, the growth temperatures are high, too high for using low-temperature sodalime glass that is commonly used in the display industry.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0006]    [0006]FIG. 1 illustrates a process in accordance with an embodiment of the present invention;  
         [0007]    [0007]FIG. 2 illustrates a process for applying carbon nanotubes to a substrate;  
         [0008]    FIGS.  3 A- 3 G illustrate a process in accordance with an embodiment of the present invention;  
         [0009]    [0009]FIG. 4 illustrates a field emission image of a sample made with a process in accordance with the present invention;  
         [0010]    [0010]FIG. 5 illustrates a field emission image of a sample made with one of the embodiments in accordance with the present invention;  
         [0011]    FIGS.  6 A- 6 E illustrate a process in accordance with the present invention for applying carbon nanotubes to a substrate;  
         [0012]    [0012]FIG. 7 illustrates a data processing system configured in accordance with the present invention;  
         [0013]    [0013]FIG. 8 illustrates another process for activating an electron source material in accordance with an embodiment of the present invention;  
         [0014]    [0014]FIG. 9 illustrates carbon nanotubes on a silicon wafer applied in a paste;  
         [0015]    [0015]FIG. 10 illustrates carbon nanotubes on a silicon wafer applied in a paste and activated;  
         [0016]    [0016]FIG. 11 illustrates carbon nanotubes applied using a spray method;  
         [0017]    [0017]FIG. 12 illustrates a graph of current versus electrical field for activated and non-activated carbon nanotubes;  
         [0018]    [0018]FIG. 13 illustrates an image of emission sites of non-activated pixels; and  
         [0019]    [0019]FIG. 14 illustrates an image of emission sites from activated pixels in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0020]    In the following description, numerous specific details are set forth such as specific cathode configurations to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known circuits have been shown in block diagram form in order not to obscure the present invention in unnecessary detail. For the most part, details concerning timing considerations and the like have been omitted in as much as such details are not necessary to obtain a complete understanding of the present invention and are within the skills of persons of ordinary skill in the relevant art.  
         [0021]    Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views.  
         [0022]    The present invention provides a method of applying CNT materials onto almost any substrate material and activating for field emission the CNT fibers in a reproducible and inexpensive manner.  
         [0023]    The source of carbon nanotube powders can be purified single-wall carbon nanotube (SWNT) powders from Carbon Nanotechnologies, Inc. (Part # HPR92 S13). These SWNTs were 1 nm in diameter and 100˜1000 nm in length. But, any other kinds of single wall or multiwall carbon nanotubes can also be used in this method. There is no need to purify the CNT materials to eliminate the catalyst from the carbon.  
         [0024]    One method is to grind the CNT materials into shorter lengths. This allows better control of material properties. In some cases, satisfactory results may be achieved without grinding. A typical ball mill was used to grind CNT bundles. FIG. 1 is the schematic diagram of such a ball mill. The rate of this machine is about 50˜60 revolutions per minute. In this method, 0.2 g CNT bundles as well as 40-100 Al 2 O 3  balls (5˜10 mm in diameter) were mixed into 200˜300 ml IPA (Isopropyl alcohol). The mixture was ground for 1˜7 days in order to disperse the CNTs. A surfactant (Triton® X-100, about 1 drop per 100 ml IPA) or other kind of materials can also be added to the mixture in order to achieve better dispersion of CNTs.  
         [0025]    Other solvents can be used instead of IPA (e.g., acetone). Mixtures of solvents can also be used. Water or mixtures of water and solvent may also be used. IPA is inexpensive, is not extremely hazardous or toxic, and can be dried at relatively low temperatures.  
         [0026]    Because the CNTs can easily agglomerate (stick to each other), an ultrasonic mixing process was applied to the CNT solution to disperse the CNTs again before spraying them onto the substrates. An ultrasonator made by (Sonics and Materials Inc., Danbury, Conn.) was used to further disperse the carbon nanotubes. Full power for 3-5 minutes, until the IPA starts to warm to about 40 C. Other means of applying ultrasonic energy to the solution may also be tried.  
         [0027]    Next, the process involves a spraying of the CNT mixture onto the substrate. In this method, the CNT mixture can be sprayed on various kinds of substrates such as metal, ceramic, glass, plastics, organic and semiconductors. The substrates can be coated with conducting, insulating or semiconducting patterned layers to provide electrical conductivity to some areas and electrical isolation or selected electrical resistance to other areas. These layers can be deposited using printing methods (thick film) or by evaporation, sputtering or other thin film methods. Standard photolithography patterning and/or etching processes may be needed for additional patterning of the added layers.  
         [0028]    Referring to FIG. 2, in order to get more uniform and well dispersed CNT solution coating on the substrates, more IPA can be added into the above solution before spraying. In this method, the CNT solution  201  for spray can be approximately 0.05 g CNT in 1000 ml IPA. Condensed gas  203  can charge an atomizer  202  to create the spray. CNT mixture  206  can be sprayed on selected areas by using a shadow mask  205 . In order to prevent the solution  206  from flowing to unexpected areas, the substrate  204  can be heated up to 50 C.-100 C. both on the front side and back side during the spray process. The substrate  204  can be sprayed back and forth or up and down several times until the CNT mixture  206  covers the entire surface uniformly. The thickness of the CNTs  206  may be about 1˜2 μm. Then they are dried in air naturally or using a heat lamp  207 .  
         [0029]    Ink jet printing or other printing techniques (or any other deposition process) may also be used to apply the CNT mixture to the substrate. Ink jet processes have advantages in a large scale manufacturing environment.  
         [0030]    After the CNTs are sprayed on the substrate, a taping process may be used to remove some of the CNTs from the surface. In this method, 3M Scotch tape may be used to remove CNTs from the surface. But, many other varieties of tape can be used in this process. The tape is adhered on the CNT coating. It is important to be sure that there is no air between the tape and the CNT coating. If air exists between them, the CNTs at that area will not be removed. A rubber roller can be used to further press the tape in order to eliminate air gaps in the interface. Finally, the tape is removed by pulling up at one end. A very thin CNT layer is left on the substrate.  
         [0031]    FIGS.  3 A- 3 G illustrate in further detail the foregoing process. In FIG. 3A, a substrate  301  is cleaned. In FIG. 3B, conductive (e.g., feedlines  302  are added to substrate  301  by using printing methods. In FIG. 3C, a shadow mask  303  is added, wherein the holes in the shadow mask  303  are aligned to the areas of the substrate where it is desired to deposit the CNT material. A magnet  304  may be used to hold the shadow mask  303  to the substrate  301 . In FIG. 3D, the foregoing spraying process (see FIG. 2) is used to spray on the CNT mixture  305 . The solvent in the mixture  305  evaporates leaving the CNT material  305 . A heater  306  is applied to the back of the substrate  301  and magnet  304 , and alternatively, a heat lamp (not shown) may also be used on the front, to speed the evaporation process and to keep the mixture from running under the mask  303 .  
         [0032]    [0032]FIG. 3E shows the cold cathode after the mask  303 , magnet  304  and heater  306  are removed. The CNT material  305  is patterned on the feedlines  302 . The CNT material  305  may be left to dry further if required.  
         [0033]    [0033]FIG. 3F shows the application of tape  309  to the surface of the cathode with the adhesive of the tape in contact with the CNT material  305 . The tape  309  may be applied to a tape substrate  308 . Rolling of the tape may be used to further press the tape  309  onto the CNT material  305  using a compliant roller  307 .  
         [0034]    [0034]FIG. 3G shows the removing of the tape  309 . This can be done by pulling up from one end to the other of the substrate backing  308 . Portions  310  of the CNT material  305  are thus removed with the tape  309  leaving the CNT materials  305  on the feedlines  302  aligned. The tape  309  can be discarded.  
         [0035]    A field emission image of a sample cold cathode created by this process is shown in FIG. 4.  
         [0036]    The technique of mixing carbon nanotubes with host materials such as adhesives of all kind is known (sometime this is called “carbon nanotubes in a paste”). This paste is generally printed (for example, screen printed) on a substrate to define localized emission spots. In these emission spots, carbon nanotubes are homogeneously mixed with the paste. In the virgin situation after printing, the carbon nanotubes possess a random orientation on the paste, meaning that a large part of the nanotubes are oriented at different angles with respect to the vertical of the substrate, but also many other carbon nanotubes are distributed similarly around a line parallel to the substrate. As a result, the contribution to field emission of these carbon nanotubes that are not oriented vertically with respect to the substrate is minimal or null. Furthermore, the existence of a high concentration of carbon fibers in the material and the random orientation can create non-optimized electric field distribution in the paste including the carbon nanotubes and as a result shielding effects between neighboring nanotubes.  
         [0037]    It is desirable to have a process whereby one can re-align these nanotubes, mechanically or otherwise, and also would be very important to lower in some cases the density of the carbon nanotubes in order to lower the shielding effect in an active device. This process can be implemented utilizing existing soft adhesives in the sense that a sustaining substrate that is coated with these soft adhesives can be applied to the surface of the printed paste including carbon nanotubes such that in a pulling process using the above soft adhesives, one can exercise suitable force on the carbon nanotubes to achieve the following results:  
         [0038]    a) Increase the concentration of carbon nanotubes aligned vertically or with the small distribution with respect to the normal to the substrate;  
         [0039]    b) Pull some of the carbon nanotubes totally out of the mixture to achieve optimal carbon nanotube surface density; and  
         [0040]    c) By using an optimal soft adhesive minimizing the surface contamination of the emissive area still achieving results a and b above.  
         [0041]    Excellent emission results can be obtained utilizing this technology. FIG. 5 illustrates a field emission image of a cold cathode sample created with the “carbon nanotubes in a paste” process described in further detail with respect to FIGS.  6 A- 6 B. In FIG. 6A, a substrate  601  is cleaned. In FIG. 6B, conductive (e.g., metal) feedlines  602  are deposited on substrate  601  using printing methods. FIG. 6C shows the mixture of CNT material mixed with a paste  603  printed in a pattern on the feedlines  602 . The paste  603  may consist of CNT material, silver paste, glass frit, a glass frit vehicle, and a glass frit thinner. An example of this paste is 0.5 grams CNT material, 1.4 grams frit vehicle, and 1.25 grams of silver paste (silver paste may be a Dupont product 7713, Conductor Composition; frit vehicle may be a Daejoo Vehicle DJB-715 from Daejoo Fine Chemical Co., Ltd., or from Pierce and Stevens F1016A02; CNT material may be provided by Carbon Nanotechnologies, Inc., purified or unpurified (CNT material may be multi-wall or single-wall)).  
         [0042]    In FIG. 6D, an adhesive tape  604  applied to a backing  605  may be applied to the surface of the cathode such that the adhesive  604  of the tape is in contact with the CNT paste material  603 . A roller  606  may be used to apply uniform contact pressure. FIG. 6E shows the tape  604  being removed by peeling from one side to the other. Some of the CNT material paste  607  is pulled off with the tape  604 . In this process, the CNT fibers are aligned in the vertical direction, and the density of the CNT fibers  603  not aligned is reduced.  
         [0043]    An alternative process is the utilization of single wall or multi-wall or a mixture of single wall and multi-wall carbon nanotubes in IPA (alcohol or other solvent) host. Furthermore, in order to homogenize the solution of carbon nanotubes and IPA, certain chemicals are added to the mixture in order to diminish the surface forces between the carbon nanotubes and obtain isolated carbon nanotubes in a homogeneous mixture with the IPA with minimal bundles (aggregates or clusters of carbon nanotubes all together).  
         [0044]    An advantage of this method is that by obtaining this homogeneous mixture, a spraying process can be utilized through a mechanical or other kind of mask such that spraying this mixture directly onto the active substrate through the mask will localize the carbon nanotubes on the future emission sites, and the fixation of these carbon nanotubes will be achieved by spraying onto the substrate where the substrate temperature is 50-100 degrees C.  
         [0045]    Furthermore, after the fixation of the carbon nanotubes on the desired emissive locations, the same pulling technique can be used, but this time the pulling forces will be exercised more uniformly on all the carbon nanotubes that are exposed in the first layers on the sprayed material. As a result the aligning process and the decrease in the density of the carbon nanotubes is more efficient, more effective and more controllable.  
         [0046]    Referring to FIG. 8, there is illustrated an alternative embodiment whereby a patterned, or embossed, activation surface is used to activate the electron source material in a manner as described previously. Often the cathode plate with the electron source is not a flat surface, which lends itself to activation from a flat activation surface (e.g., adhesive tape). Therefore, when the tape is applied, it may not be able to adequately activate the electron source material, which in this example is the carbon nanotubes. To address the problem, an embossed activation surface can be used so that the adhesive is able to reach down to the electron source material at each pixel site. Moreover, areas that do not need to be activated are not subject to contact with the adhesive material. Note, such a patterned activation surface can be used without embossing where only certain areas need to be activated. In FIG. 8, a substrate  801  has a conductive cathode  802  pattern thereon, and patterned insulators  804  with metal gates  805  deposited thereon. Carbon nanotubes  803  are then deposited onto the cathodes  802 . To activate the carbon nanotubes  803 , the adhesive  807  may be embossed onto a backing film  806  so that the adhesive  807  reaches down to the carbon nanotubes  803  within the insulator  804  walls.  
         [0047]    A representative hardware environment for practicing the present invention is depicted in FIG. 7, which illustrates an exemplary hardware configuration of data processing system  713  in accordance the subject invention having central processing unit (CPU)  710 , such as a conventional microprocessor, and a number of other units interconnected via system bus  712 . Data processing system  713  includes random access memory (RAM)  714 , read only memory (ROM)  716 , and input/output (I/O) adapter  718  for connecting peripheral devices such as disk units  720  and tape drives  740  to bus  712 , user interface adapter  722  for connecting keyboard  724 , Mouse  726 , and/or other user interface devices such as a touch screen device (not shown) to bus  712 , communication adapter  734  for connecting data processing system  713  to a data processing network, and display adapter  736  for connecting bus  712  to display device  738 . CPU  710  may include other circuitry not shown, which will include circuitry commonly found within a microprocessor, e.g., execution unit, bus interface unit, arithmetic logic unit, etc. CPU  710  may also reside on a single integrated circuit. Display device  738  can implement the display technology described herein.  
         [0048]    Either utilizing CVD to grow nanotubes or spraying or mixing nanotubes into a paste, then applying to a substrate for electron field emissions, it appears that electron emission current is strongly related to carbon nanotube density when applied onto substrates. It has been found that by activating the surface (e.g., by using adhesive tape to remove some carbon nanotube material) better electron emission characteristics can be achieved. For example, four samples of spray and paste carbon nanotubes on silicon wafers were made, one of the wafers activated from each group and one wafer kept as control. These were then inspected and nanotubes counted per square area from high power SEM pictures. FIG. 9 illustrates the SEM picture from the paste control wafer, while FIG. 10 illustrates the paste-activated wafer. FIG. 11 illustrates the spray-activated wafer. The following table shows the CNT density per square centimeter for each of the samples.  
                                                 CNT DENSITY PER SQUARE CENTIMETER            Paste-   Paste-               Control   Activated   Spray-Control   Spray-Activated       Wafer   Wafer   Wafer   Wafer               ˜1 × 10 10     ˜1 × 10 9     ˜1 × 10 10  − 1 × 10 11     ˜1 × 10 8  − 1 × 10 9                    
 
         [0049]    Referring to FIG. 12, the emission from the activated devices was much better than that of the non-activated devices at a given electric field. The SEM pictures have shown that the carbon nanotubes density after activated is around 1%-10% of the non-activated samples. The field emission is improved if the CNT density per square centimeter is less than 1×10 10 . Thus, the emission current is inversely proportional to the carbon nanotube density. FIG. 13 shows field emission from sites of non-activated pixels, while FIG. 14 shows light emission from sites of activated pixels.