Abstract:
An emitter includes an electrode, and a number of carbon nanotubes fixed on the electrode. The carbon nanotubes each have a first end and a second end. The first end is electrically connected to the substrate and the second end has a needle-shaped tip. Two second ends of carbon nanotubes have a larger distance therebetween than that of the first ends thereof, which is advantageous for a better screening affection. Moreover, the needle-shaped tip of the second ends of the carbon nanotube has a lower size and higher aspect ratio than the conventional carbon nanotube, which, therefore, is attributed to bear a larger emission current.

Description:
[0001]    This application is related to commonly-assigned applications entitled, “FIELD EMISSION CATHODE AND FIELD EMISSION DISPLAY EMPLOYING WITH SAME”, filed ______ (Atty. Docket No. US 21523). The disclosure of the above-identified application is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates to an emitter and, in particular, to an emitter employed with the carbon nanotubes and a method for manufacturing the same. 
         [0004]    2. Description of the Related Art 
         [0005]    Carbon nanotubes (CNTs) are widely used as field emitters for field emission displays (FEDs) and liquid crystal displays (LCDs). Such CNTs have good electron emission characteristics, and chemical and mechanical durability. 
         [0006]    Conventional field emitters are typically micro tips made of a metal such as molybdenum (Mo). However, the life span of such a micro tip is shortened due to effects of atmospheric environment, such as non-uniform electric field, and the like. A somewhat viable alternative has been carbon nanotubes having a high aspect ratio, high durability, and high conductivity preferably adopted as field emitters. 
         [0007]    In order to obtain a high current density from carbon nanotube emitters, carbon nanotubes must be uniformly distributed and arranged perpendicular to a substrate. The carbon nanotube emitters are generally grown from a substrate using a chemical vapor deposition (CVD). However, the carbon nanotubes formed by this process may be entangled with each other on the top thereof, which result in a poor morphology of CNTs and poor performance on emitting. Alternatively, the carbon nanotube emitters may also be manufactured by printing a paste obtained by combining carbon nanotubes with a resin to a substrate. This method is easier and less costly than CVD and thus preferred to CVD. However, the carbon nanotubes formed by this process are too dense to emit electrons effectively because of the strong screening effect generated between adjacent carbon nanotubes. 
         [0008]    What is needed, therefore, is a carbon nanotube emitter and a method for manufacturing the same that can overcome the above-described shortcomings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The present emitter and method for manufacturing the same are described in detail hereinafter, by way of example and description of an exemplary embodiment and with references to the accompanying drawings, in which: 
           [0010]      FIG. 1  is a schematic view of an emitter provided with a number of carbon nanotubes each having a needle-shaped tip according to an exemplary embodiment; 
           [0011]      FIG. 2  is a scanning electron microscope (SEM) image of the carbon nanotubes of  FIG. 1 ; 
           [0012]      FIG. 3  is a scanning electron microscope (SEM) image of the needle-shaped tip of the carbon nanotubes of  FIG. 1 ; 
           [0013]      FIG. 4  is a Raman spectrum view of the emitter of  FIG. 1 ; 
           [0014]      FIG. 5  is a voltage-current graph showing the electron emission characteristic of the emitter of  FIG. 1 ; 
           [0015]      FIG. 6  is a flow chart of steps for manufacturing the emitter of  FIG. 1 ; 
           [0016]      FIG. 7  is a schematic view of the manufactured emitter in steps of  FIG. 6 ; 
           [0017]      FIG. 8  is a flow chart of steps for growing a carbon nanotube array on a substrate; and 
           [0018]      FIG. 9  is a flow chart of steps for selecting a number of carbon nanotubes from the carbon nanotube array of  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    A detailed explanation of an emitter and method for manufacturing the same according to an exemplary embodiment will now be made with references to the drawings attached hereto. 
         [0020]    Referring to  FIGS. 1-3 , an emitter  100  according to the present embodiment is shown. The emitter  100  includes a substrate  10 , and a number of carbon nanotubes  11  disposed on the substrate  10 . 
         [0021]    The substrate  10  may be an electrode made of copper, tungsten, aurum, gold, molybdenum, platinum, ITO glass, and combinations thereof. Alternatively, the substrate  10  may be an insulating substrate, such as a silicon sheet, coated with a metal film with a predetermined thickness. The metal film maybe one of an aluminum (Al) film, silver (Ag) film or the like. In the present embodiment, the substrate  10  is a silicon sheet coated with an Al film and configured for supporting and electrically connecting to the carbon nanotubes  11  and may function as a cathode of a field emission display (FED) (not shown). If necessary, a gate insulating layer and a gate electrode may be optionally formed on the conductive substrate  10 . 
         [0022]    The carbon nanotubes  11  may be conductive single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), or multi-walled carbon nanotubes (MWCNT), or their mixture. The carbon nanotubes  11  are parallel to each other. Each of the carbon nanotubes  11  has the approximately same length and includes a first end  111  and a second end  112  opposite to the first end  111 . The first end  111  is electrically connected to the conductive substrate  10  by Van der Waals Force. For enhancing a fastening force between the first end  111  and the conductive substrate  10 , the first end  111  can be connected to the conductive substrate  10  via a conductive adhesive or by metal-bonding. The second end  112  extends away from the conductive substrate  10  and has a needle-shaped tip (not labeled). The needle-shaped tip is employed as an electron emitting source of the carbon nanotube emitter  100  for emitting electrons. The carbon nanotubes  11  each may have a diameter in a range from about 0.5 nm to about 50 nm and a length in a range about 100 μm to about 1 mm. The distance between the second ends  112  of the two adjacent carbon nanotubes  11  ranges from about 50 nm to about 500 nm. In the present embodiment, the carbon nanotubes  11  are SWCNTs having a diameter of about 1 nm and a length of about 150 mm. As shown in  FIG. 3 , two adjacent second ends  111  of carbon nanotubes  11  are spaced from each other by a distance greater than that between the first ends  112 , thereby diminishing influence from the screening effect between the adjacent carbon nanotubes. 
         [0023]    Referring to  FIGS. 4-5 , in use, when the emitter  100  of the present embodiment is employed in the FED, the second end  112  can emit electrons when a low voltage is applied to the FED, because of the good electron emission characteristics of the needle-shaped tips. In the present embodiment, the emitter  100  starts to emit electrons when the applied voltage is about 200V or more. Understandably, as the applied voltage is increased, the current density increases accordingly. As shown in  FIG. 4 , defect analysis in Raman spectrum for the field emission affect of the carbon nanotubes  11  is shown. It can be seen that the carbon nanotubes  11  of the present embodiment have a lower defect peak than typical carbon nanotube. Therefore, it is possible to provide better field emission effect for the FED as desired. 
         [0024]    Referring to  FIG. 6  and  FIG. 7 , a flow chart of an exemplary method for manufacturing the above-described emitter  100  is shown. The method includes: 
         [0025]    step S 101 : providing two conductive substrates  20  spaced apart from each other and a carbon nanotube array (not shown); 
         [0026]    step S 102 : selecting one or more carbon nanotubes  21  from the carbon nanotube array; 
         [0027]    step S 103 : fixing each end of the one or more carbon nanotubes  21  on one of the two conductive substrates  20 ; and 
         [0028]    step S 104 : supplying a voltage sufficient to break the one or more carbon nanotubes  21  for forming two emitters  100 . 
         [0029]    In step S 101 , the carbon nanotube array may be acquired by the following method. The method may employ chemical vapor deposition (CVD), Arc-Evaporation Method, or Laser Ablation, but not limited to those method. In the present embodiment, the method employs high temperature CVD. Referring also to  FIG. 8 , the method includes: 
         [0030]    step S 201 : providing a substrate; 
         [0031]    step S 202 : forming a catalyst film on the surface of the substrate; 
         [0032]    step S 203 : treating the catalyst film by post oxidation annealing to change it into nano-scale catalyst particles; 
         [0033]    step S 204 : placing the substrate having catalyst particles into a reaction chamber; and 
         [0034]    step S 205 : adding a mixture of a carbon source and a carrier gas for growing the carbon nanotube array. 
         [0035]    In step S 201 , the substrate maybe a silicon wafer or a silicon wafer coated with a silicon oxide film on the surface thereof. In one embodiment, the silicon wafer has flatness less than 1 μm, for providing flat for the formed carbon nanotube array. 
         [0036]    In step S 203 , the catalyst film may have a thickness in a range from about 1 nm to about 900 nm and the catalyst material may be selected from a group consisting of Fe, Co, Ni, or the like. 
         [0037]    In step S 203 , the treatment is carried out at temperatures ranging form about 500° C. to about 700° C. for anywhere from about 5 hours to about 15 hours. 
         [0038]    In step S 204 , the reaction chamber is heated up to about 500° C. to about 700° C. and filled with protective gas, such as inert gas or nitrogen for maintaining purity of the carbon nanotube array. 
         [0039]    In step S 205 , the carbon source may be selected from acetylene, ethylene or the like, and have a velocity of about 20 sccm (Standard Cubic Centimeter per Minute) to about 50 sccm. The carrier gas may select from insert gas or nitrogen, and have a velocity of about 200 sccm to about 500 sccm. 
         [0040]    In step S 102 , the two conductive substrates  20  are spaced apart from each other to apply tension to the carbon nanotubes  21  selected from the carbon nanotube array. The distance between the two conductive substrates  20  is limited by the length of the carbon nanotubes. 
         [0041]    In step S 103 , the number of carbon nanotubes  21  are selected and drawn out form the carbon nanotube array provided in step S 101  and opposite ends of the carbon nanotubes  21  are fixed onto the two conductive substrates  20 , respectively. Referring to  FIG. 9 , the method for selecting the carbon nanotubes  21  includes; 
         [0042]    step S 301 : providing a metal thread having a diameter of about 20 nm to about 100 nm; 
         [0043]    step S 302 : bringing the metal thread towards the carbon nanotube array and contacting the carbon nanotube array; 
         [0044]    step S 303 : pulling out the metal thread away from the carbon nanotube array for obtaining a number of carbon nanotubes  21 . 
         [0045]    In described method above, the metal may be selected from the following materials: copper, silver, and gold, or an alloy thereof. In the step S 302 , because of the strong molecular force between the carbon nanotube and the metal thread, some carbon nanotubes  21  can be adsorbed onto the metal thread. In step S 303 , a single segment of carbon nanotubes  21  is acquired. In the present embodiment, the acquired carbon nanotubes  21  have a length of about 2 μm to about 200 μm. 
         [0046]    In step S 104 , the two conductive substrates  20  and the carbon nanotubes  21  are placing into a reaction chamber (not shown) for ensuring purity of the obtained carbon nanotubes  21  before supplying the voltage on the carbon nanotubes. The reaction chamber may be a vacuum chamber having pressure intensity less than 1×10−1 Pa or is filled with inert gas or nitrogen to prevent the carbon nanotubes  21  from oxidizing during breaking. In the present embodiment, the reaction chamber is a vacuum chamber having a pressure intensity of 2×10 −5  Pa. As well known in the art, the voltage applied between the two conductive substrates  20  is determined according to the dimension of the carbon nanotubes  21 . The supplied voltage may have a range from about 7V to about 10V. In the present embodiment, the applied voltage is 8.25V. When the current flows through the carbon nanotubes  21 , heat, known as joule heat, can be generated. The joule heat can break the carbon nanotubes  21 . After breaking, the current is turned off and the joule heat disappears quickly, thus annealing the formed carbon nanotubes  11 . The anneal, which is advantageous for improving mechanical strength of the carbon nanotubes  11 , can be carried out in a vacuum chamber for preventing the carbon nanotubes  11  from oxidizing. Thus, two emitters  100  are obtained. The obtained emitters  100  have an approximately as many second ends  112  each having a needle-shaped tip as there are carbon nanotubes. 
         [0047]    The described method above for manufacturing the carbon nanotubes  11  of the emitter  100  can prevent pollutant entering the carbon nanotubes  11  as the second ends  112  are closed and have a substantially uniform length, which can provide substantially uniform electron emitting characteristics. Moreover, the second ends  112  of the two adjacent carbon nanotubes  11  are spaced from each other by a distance greater than that of the first ends  111 , thereby diminishing influence from the screening effect between adjacent carbon nanotubes. 
         [0048]    It is to be understood that the above-described embodiments are intended to illustrates, rather than limit the invention. Variations may be made to the embodiments without departing from the spirit of the invention as claimed. The above-described embodiments illustrate the scope of the invention but do not restrict the scope of the invention. 
         [0049]    It is to be understood that the above description and the claims drawn to a method may include some indication in reference to certain steps. However, the indication used is only to be viewed for identification purposes and not as a suggestion as to an order for the steps.