Abstract:
The present invention relates to a long-life carbon nanotube field emitter with a three-dimensional structure and method for fabricating the same. Since the emitter having an extended area according to the design of the present invention can minimize the current density flowing per single wire of the carbon nanotube, it can be expected that the damage of the carbon nanotube is minimized so that the lifetime of the field emitter can be significantly improved and the commercialization of the carbon nanotube field emitter will be advanced.

Description:
[0001]    This application claims priority to Korean Patent Application No. 2006-0133799, filed on Dec. 26, 2006, in the Korean Intellectual Property Office, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a carbon nanotube field emitter and a method for fabricating the same, and in particular to a long-life carbon nanotube field emitter with a three-dimensional structure and a method for fabricating the same. 
         [0004]    2. Description of the Related Art 
         [0005]    A carbon nanotube has large aspect ratio, high electrical conductivity, and physicochemical stability so that it is very ideal as a material of the field emitter. It has been known that the field emitter using the carbon nanotube has higher efficiency than that of the field emitter using existing metal and silicon. Therefore, much research has been attempted to fabricate the field emitter using the carbon nanotube, wherein the field emitter has been fabricated in a type such as a diode type (see  FIG. 1 ), a triode type (see  FIGS. 2 to 4 ), etc., using a chemical vapor deposition method or a screen printing process, and the like. 
         [0006]      FIG. 1  is a cross-sectional view for explaining the concept of a diode type carbon nanotube field emitter of the prior art. There is also shown an electrical connection in order to explain the operation of the carbon nanotube field emitter. The same reference numerals indicate the same components throughout the following drawings and a duplicated description thereof will be omitted. It can be appreciated from  FIG. 1  that the carbon nanotube  30  is formed from a cathode electrode  10  on a substrate  5 . An anode electrode  20  and a phosphor  21  are disposed to be opposite each other at a position spaced by a predetermined distance above the carbon nanotube  30 . Voltage is applied between the cathode electrode  10  and the anode electrode  20  to operate the carbon nanotube field emitter. In such a diode type carbon nanotube field emitter, since the carbon nanotube  30  is formed on the cathode electrode  10  that is a two-dimensional plane, it has a problem that it is difficult to increase the number of carbon nanotubes per unit area. 
         [0007]      FIG. 2  is a cross-sectional view for explaining the concept of a triode type carbon nanotube field emitter of the prior art using a metal grid gate. The difference between  FIG. 1  and  FIG. 2  is that the triode carbon nanotube field emitter of  FIG. 2  is further provided with the metal grid gate  40 . Even in this case, the triode carbon nanotube field emitter also has a problem that it is difficult to increase the number of carbon nanotubes per unit area. 
         [0008]      FIG. 3  is a cross-sectional view for explaining the concept of the triode type carbon nanotube field emitter of the prior art, wherein the metal gate is positioned at a side of a substrate. It can be appreciated from  FIG. 3  that the metal gate  40  is formed on the substrate  5  such as the cathode electrode  10 . Such a structure has an advantage that it minimizes the collision of gases/ions moving in a vertical direction so that the damage of the carbon nanotube  30  can be reduced. However, such a structure has a disadvantage that it is difficult to increase the number of carbon nanotubes per unit area as in  FIGS. 1 and 2 . 
         [0009]      FIG. 4  is a cross-sectional view for explaining the concept of the triode type carbon nanotube field emitter of the prior art, wherein the metal gate is positioned below a cathode. It can be appreciated from  FIG. 4  that the metal gate  40  is positioned below the cathode  10 , interposing an insulation layer  50  therebetween. Such a structure does not have a problem that the number of the carbon nanotubes per unit area is not reduced as compared to the triode type carbon nanotube field emitter shown in  FIG. 3 . However, such a structure has a problem that it is difficult to increase the number of carbon nanotubes per unit area as in  FIGS. 1 and 2 . 
         [0010]    Various structures as above have been proposed, however, since they have a disadvantage that the lifetime of the field emitter is shortened due to the problems such as separation of carbon nanotubes, evaporation due to high current density, etc., the carbon nanotube has not yet been commercialized. Therefore, a core technology in a technical field of the field emitter using the carbon nanotube may be said to be a technology of improving the lifetime of the carbon nanotube. 
         [0011]    In the prior arts, a technology of making field emission from the carbon nanotube field emitter uniform (Taping Technique—J. M. Kim (SAIT) et al.,  Diamond and Related Materials,  2000, 9, 1184, Cyclic electrical aging-Y. C. Kim (LG FED Group) et al.,  Applied Physics Letters,  2004, 84, 5350), a technology of lowering field emission threshold voltage (Plasma Treatment—C. Y. Zhi ( Chinese Academy of Sciences ) et al.,  Applied Physics Letters,  2002, 81, 1690, Doping elements—J. C. Charlier et al.,  Nano Letters,  2002, 2, 1191.), etc., have been intensively studied. Also, a method for preventing the separation of the carbon nanotubes and improving the electrical conductivity of the carbon nanotube using a metal binder in order to improve the lifetime of the field emitter (S. H. Hong et al.,  Advanced Materials,  2006, 18, 553, J. M. Kim et al.,  Applied Physics Letters,  2005, 87, 063112.), a method for forming a metal layer in the phosphor in order to minimize the contamination of the carbon nanotube due to the vaporization/ionization of the phosphor (J. Li (Southeast Univ. China) et al.,  Applied Surface Science,  2003, 220, 96), etc., have been studied. 
         [0012]    An improvement in lifetime of the carbon nanotube and an improvement in brightness of the field emitter are contrary to each other. In order to improve the brightness of the field emitter, current density should be increased or kinetic energy of electron should be increased. However, in order to improve the brightness of the field emitter, the method for increasing the kinetic energy of electron should apply high acceleration voltage and widen an interval between a cathode and an anode so that it has problems that energy efficiency is low and electrical stability is low since arcing is easily produced. In order to improve the brightness of the field emitter, the method for increasing the current density has a problem that the current density flowing per a single wire of the carbon nanotube should be increased so that the carbon nanotube is easily damaged due to heat generation caused by the increased current density. Furthermore, since the existing carbon nanotube field emitter has the two-dimensional structure (see  FIGS. 1 to 4 ) so that the emitter area is restricted, the damage of the carbon nanotube is serious as the current density is increased. Also, in the carbon nanotube field emitter with the two-dimensional structure, since its structure is destroyed or its surface is contaminated due to the collision of gas and/or ionized particles from phosphor, so there is a disadvantage that the lifetime of the field emitter is shortened. 
         [0013]    Consequently, since the existing carbon nanotube field emitter with the two-dimensional structure cannot easily increase the number of the carbon nanotubes per unit area so that the current density per the carbon nanotube is high, it cannot solve the problem that the carbon nanotube is damaged and since the existing carbon nanotube field emitter is spread in the two-dimensional structure, it cannot protect the carbon nanotube from the collision of gases/ions. 
       SUMMARY OF THE INVENTION 
       [0014]    Accordingly, it is a technical problem of the present invention to provide a carbon nanotube field emitter and a method for fabricating the same capable of maximizing its emitter area by designing the carbon nanotube field emitter in a three-dimensional structure to maximize the emitter area and improving its lifetime by protecting the carbon nanotube from the collision of gases/ions. 
         [0015]    In order to solve the technical problems, a carbon nanotube field emitter of the present invention has a three-dimensional structure. 
         [0016]    More specifically, a carbon nanotube field emitter of the present invention comprises: at least two paired electrode plates whose wide surfaces are faced with each other; carbon nanotubes formed on each of both surfaces of the electrode plates; a substrate vertically fixing the electrode plates in a state where the sides of the respective electrode plates contact each other; an anode electrode mounted in parallel with the substrate in a state spaced therefrom and having a phosphor facing the substrate; a direct current power supply applying direct voltage between the anode electrode and the electrode plates; and a pulse wave supplier periodically applying pulse waves indicating an opposite sign of voltage to any one of the paired electrode plates and the other thereof to allow them to alternately perform the role of the cathode electrode and the gate. 
         [0017]    In the present invention, the ratio of length, which is the ratio of the height to the thickness of the electrode plate, is 1 or more. 
         [0018]    Also, a glass substrate can be used as the substrate. 
         [0019]    In order to solve the technical problem, a method for fabricating a carbon nanotube field emitter according to a first aspect of the present invention comprises the steps of: (a) fabricating a plurality of electrode plates whose at least one surface is formed with carbon nanotubes; (b) arranging the paired electrode plates whose wide surfaces are formed with the carbon nanotubes and are faced with each other; (c) mounting an anode electrode having a phosphor to be spaced from the electrode plates; (d) mounting a pulse wave supplier periodically applying pulse waves indicating an opposite sign of voltage between the paired electrode plates facing each other to allow them to alternately perform the role of the cathode electrode and the gate; and (e) mounting a direct current power supply applying direct voltage between the paired electrode plates facing each other and the anode electrode for acceleration of electrons emitted from the cathode plates toward the anode. 
         [0020]    In this case, the step (a) comprises the steps of: (a-1) applying the mixture of the carbon nanotubes and carbon nanotube composite powders and organic binders to a plurality of predetermined regions on at least one surface of a base of the electrode plates; (a-2) forming the carbon nanotubes only on the applied region by calcinating the applied resultant products in vacuum; and (a-3) obtaining the plurality of electrode plates formed with the carbon nanotubes by cutting the base of the electrode plates to include the regions formed with the carbon nanotubes. 
         [0021]    In order to solve the technical problems as mentioned above, a method for fabricating a carbon nanotube field emitter according to a second aspect of the present invention comprises the steps of: (a) allowing paired electrode plates whose at least one surface is formed with carbon nanotubes to be formed in plural in an arrangement state where the wide surfaces formed with the carbon nanotubes are faced with each other; (b) mounting an anode electrode having a phosphor to be spaced from the electrode plates; (c) mounting a pulse wave supplier periodically applying pulse waves indicating a different magnitude of voltage between the paired electrode plates facing each other to allow them to alternately perform the role of the cathode electrode and the gate; and (d) mounting a direct current power supply applying direct voltage between the paired electrode plates facing each other and the anode electrode for acceleration of electrons emitted from the cathode plates toward the anode. 
         [0022]    In this case, the step (a) comprises the steps of: (a-1) film-forming a metal-based composite material layer including the carbon nanotube on a substrate; and (a-2) forming the plurality of paired electrode plates in an arrangement state where wide surfaces formed with the carbon nanotubes are faced with each other, by allowing the carbon nanotubes in a constant interval pattern to remain and removing only the metal-based composite material layer through etching. 
         [0023]    Also, the step (a-2) of removing only the metal-based composite material layer through etching can be preformed using physical etching by laser irradiation or using chemical etching by chemical liquid. 
         [0024]    On the other hand, in another case, the step (a) comprises the steps of: (a-1) forming a metal film on the substrate; (a-2) forming the plurality of paired electrode plates in an arrangement state where wide surfaces are faced with each other, by etching the metal film in a constant interval pattern; (a-3) applying a catalyst forming carbon nanotube to the side of the etched metal film; and (a-4) forming the carbon nanotube on the side of the etched metal film using the catalyst for growth of carbon nanotubes. 
         [0025]    In this case, the step (a-4) of forming the carbon nanotube may be (1) a step of growing the carbon nanotube in a vacuum furnace by injecting gas having any one component selected from a group consisting of CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , and CO, (2) a step of growing the carbon nanotube by placing the resultant products applied with the catalyst forming carbon nanotube into any one of a solvent group including carbon consisting of Co(CO) 8 , Fe(CO) 5 , Fe(C 5 H 5 ) 2 , Ethanol, Methanol, Xylene or mixed solvents thereof and then performing ultrasonic treatment thereon, and (3) a step of placing the resultant products into carbon nanotube solution formed of the carbon nanotube or the composite material including the carbon nanotube and a solvent whose boiling point is 300° C. or less or spraying the solution on the resultant products. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    These and other objects, features, and advantages of preferred embodiments of the present invention will be more fully described in the following detailed description, taken in conjunction with the accompanying drawings. In the drawings: 
           [0027]      FIG. 1  is a cross-sectional view for explaining the concept of a diode type carbon nanotube field emitter of the prior art; 
           [0028]      FIG. 2  is a cross-sectional view for explaining the concept of a triode type carbon nanotube field emitter of the prior art using a metal grid gate; 
           [0029]      FIG. 3  is a cross-sectional view for explaining the concept of the triode type carbon nanotube field emitter of the prior art, wherein the metal gate is positioned at a side of a substrate; 
           [0030]      FIG. 4  is a cross-sectional view for explaining the concept of the triode type carbon nanotube field emitter of the prior art, wherein the metal gate is positioned below a cathode; 
           [0031]      FIG. 5  is a view showing a schematic construction of a carbon nanotube field emitter according to the present invention; 
           [0032]      FIGS. 6A to 6E  are process views for explaining a first embodiment of a method for forming a carbon nanotube array structure in a three-dimensional structure applied to the carbon nanotube field emitter shown in  FIG. 5 ; 
           [0033]      FIGS. 7A to 7C  are process views for explaining a second embodiment of a method for forming a carbon nanotube array structure in a three-dimensional structure applied to the carbon nanotube field emitter shown in  FIG. 5 ; and 
           [0034]      FIGS. 8A to 8E  are process views for explaining a third embodiment of a method for forming a carbon nanotube array structure in a three-dimensional structure applied to the carbon nanotube field emitter shown in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]    Hereinafter, the preferred embodiments of the present invention will be described with reference to the accompanying drawings. The present embodiments do not limit the scope of the present invention but are proposed only by way of example. The same parts in different embodiments are indicated by the same signs and terms. 
         [0036]      FIG. 5  is a view showing a schematic construction of a carbon nanotube field emitter according to an embodiment of the present invention. Referring to  FIG. 5 , an anode electrode  20  formed with a phosphor  21  faces an insulating substrate  15 . Also, both sides of cathode electrodes  10  with a three-dimensional structure are formed with carbon nanotubes  30 , wherein the plurality of cathode electrodes  10  are vertically arranged in a state where one side of each cathode electrode  10  contacts the insulating substrate  15 . Herein, the reason for referring to be the three-dimensional structure is that in the prior art, the carbon nanotube is two-dimensionally formed on the cathode electrode and is used for the carbon nanotube field emitter as it is, however, in the present invention, the carbon nanotube three-dimensionally formed considering a height due to the vertical arrangement of the plurality of cathode electrodes  10 , is used for the carbon nanotube field emitter. The plurality of cathode electrodes  10  are arranged in a row so that their wide surfaces formed with the carbon nanotubes  30  face each other or their narrow surfaces are parallel with each other. Direct current voltage is supplied between the cathode electrode  10  and the anode electrode  20 . The cathode electrodes  10  whose wide surfaces formed with the carbon nanotubes  30  face each other are supplied with pulse waves by means of a pulse wave supplier  60 . When the pulse waves are supplied, gates  40  and the cathode electrodes  10 , which make pairs and face each other, alternately perform their roles. Preferably, the ratio of length of the cathode electrode  10  (the height  80  of the cathode/the thickness  70  of the cathode) is 1 or more. Theoretically, the ratio of length of the cathode electrode can be large without any restriction. However, the height  80  of the cathode electrode is subject to the limitation by means of an interval between the anode electrode  20  formed with the phosphor  21  and the insulating substrate  15 . The reason for making the ratio of length 1 or more is that the large number of cathode electrodes  10  and gates  40  can be formed on the insulating substrate  15  with a predetermined area so that the carbon nanotube field emitter with excellent performance can be fabricated. The carbon nanotube field emitter according to the embodiment of the present invention having such a structure has advantages as follows: 
         [0037]    (1) Since the carbon nanotube field emitter of the present invention has the three-dimensional structure, as the ratio of length of the cathode electrode is getting higher, the formation area (hereinafter, referred to as “emitter area”) of the carbon nanotube serving as the field emitter becomes wider so that the efficiency of the carbon nanotube field emitter can be high. 
         [0038]    (2) Although the present invention and the prior art is the same in efficiency, since the present invention has a wider emitter area than that of the prior art, the carbon nanotube field emitter of the present invention can lower the current density flowing per single wire of the carbon nanotube to ½ or less as compared to the carbon nanotube field emitter of the prior art with the two-dimensional structure. Therefore, the present invention minimizes the damage of the carbon nanotube so that the lifetime of the carbon nanotube field emitter can be improved. 
         [0039]    (3) Since the carbon nanotube  30  of the present invention is formed to be substantially horizontal to the surface of the anode electrode  20  or the phosphor  21 , the collision of gases/ions moving in a vertical direction is minimized so that the damage of the carbon nanotube  30  can be prevented, making it possible to improve the lifetime of the carbon nanotube field emitter. 
         [0040]    The following embodiments are to explain a method for fabricating the carbon nanotube field emitter of the embodiments of the present invention shown in  FIG. 5 . Since the most important thing in the method of the present invention is a method for forming the carbon nanotube array structure with the three-dimensional structure in the entire structure of the carbon nanotube field emitter, the description will be made up to this fabricating step through the drawings and the description of the method for forming the remaining components will be described with reference to  FIG. 5 . 
       EMBODIMENT 1 
       [0041]    The carbon nanotubes and carbon nanotube composite powders are mixed with organic binders formed of ethylcellulose and terpineol using a 3-roll mill and a duplication screen printing is then performed on a base  11  for conductive cathode electrodes (or gates) using the mixture. Thereafter, application should be performed only on defined regions by means of masks regularly exposing regions A where the cathode electrodes (or gates) will be made. In the present embodiment, the regions A where the cathode electrodes (or gates) will be made have a rectangular shape and are two-dimensionally arranged at a constant interval. Then, they are calcinated at 100 to 500° C. in a vacuum of 1 mTorr or less so that the carbon nanotubes  30  with the two-dimensional structure as shown in  FIG. 6A  are formed. 
         [0042]    Next, glass spacers  31  are mounted along short sides of the circumferences of the regions formed with the carbon nanotubes  30  with the two-dimensional structure to complete the structure shown in  FIG. 6B . The glass spacers  31  are mounted by the screen printing of glass frit or the application of insulating adhesives to glass plates cut at a constant thickness or glass beads with a constant diameter. In the case of the screen printing of the glass frit, on the contrary to the screen printing performed in  FIG. 6A , masks not to apply glass to the regions A where the cathode electrodes (or gates) will be made are used. 
         [0043]    Subsequently, as the resultant products of  FIG. 6B , the base  11  for the conductive cathode electrodes (or gates) and the glass spacers  31  are cut at a constant width by means of a laser cutter or a diamond cutter (not shown) along a cutting line C-C′, as shown in  FIG. 6C . The width is not particularly limited, but can be finely cut to be 10 μm to several millimeters. The cathode electrodes (or gates) formed with the cut carbon nanotubes with the two-dimensional structure can be lifted up by means of a pincette or a robot arm  32  and can then be moved. 
         [0044]    Furthermore, as shown in  FIG. 6D , assembly grooves  34  are prepared at a constant interval on the insulating substrate  15 , for example, the glass substrate so that cutting bodies  33  of the cathode electrodes (or gates) formed with the carbon nanotubes can be arranged and the cutting bodies  33  of the cathode electrodes (or gates) formed with the carbon nanotubes are mounted to be fitted in the assembly grooves  34  by means of the pincette or the robot arm  33  so that the carbon nanotube array structure with the three-dimensional structure is completed as shown in  FIG. 6E . 
         [0045]    From after the carbon nanotube array structure with the three-dimensional structure is completed, the process of completing the entire structure of the carbon nanotube field emitter will be described with reference to  FIG. 5 . As shown in  FIG. 5 , the anode electrode  20  formed with the phosphor  21  is mounted to be faced with the insulating substrate  15  and the direct current voltage is supplied between the cathode electrodes or the gates  10  or  40  and the anode electrode  20 . Also, the gates  40  and the cathode electrodes  10  whose wide surfaces facing each other formed with the carbon nanotubes  30  are supplied with the pulse wave by means of the pulse wave supplier  60  so that the gates  40  and the cathode electrodes  10  whose wide surfaces face each other alternately perform their roles so that the carbon nanotube field emitter is completed. 
       EMBODIMENT 2 
       [0046]    A metal-matrix composite material layer  36  including the carbon nanotubes with a thickness of 10 μm to several millimeters is film-formed on the insulating substrate  15 , for example, the glass substrate so that a structure as shown in  FIG. 7A  is formed. 
         [0047]    Next, as shown in  FIG. 7B , the composite material layer  36  including the carbon nanotubes is irradiated with a CO 2  laser  38  beam having an output power of 1 to 5 W at a scanning speed of 0.1 to 100 mm per second so that the metal is selectively etched and the cathode electrodes  10  (or gates  40 ) and the carbon nanotubes  30  remain. 
         [0048]    By repeating such an etching, the metal-matrix composite material layer  36  is formed with stripe patterns having a width of 0.1 to 500 μm at an interval of 0.1 to 500 μm so that the carbon nanotube array structure with the three-dimensional structure is completed as shown in  FIG. 7C . 
         [0049]    From after the carbon nanotube array structure with the three-dimensional structure is completed, the process of completing the entire structure of the carbon nanotube field emitter is the same as the embodiment 1 and the further description thereof will thus be omitted. 
         [0050]    In the present embodiment 2, in order to etch the composite material layer  36  including the carbon nanotubes, physical etching using laser is used. In addition to the physical etching, chemical etching using a mask pattern and chemical liquid may be applied. 
       EMBODIMENT 3 
       [0051]    As shown in  FIG. 8A , a metal layer  39  of a thickness of 10 μm to several millimeters is first formed on the insulating substrate  15 , for example, the glass substrate and a photoresist  51  is then applied thereon. 
         [0052]    Next, as shown in  FIG. 8B , UV is exposed through the mask with the stripe patterns having a width of 5 to 500 μm and the photoresist is then removed to obtain a photoresist pattern  51   a . Thereafter, the metal layer  39  is etched to obtain the metal cathode electrode  10  or the gate  40 . 
         [0053]    Next, a carbon nanotube growth catalyst  52  is applied to obtain a structure shown in  FIG. 8C . The carbon nanotube growth catalyst  52  includes at least one of Fe, Co, and Ni. 
         [0054]    Next, as shown in  FIG. 8D , the photoresist pattern  51   a  is removed so that the carbon nanotube growth catalyst  52  remains only on the side of the metal cathode electrode  10  or the gate  40 . 
         [0055]    Next, the carbon nanotube array structure with the three-dimensional structure as shown in  FIG. 8E  is completed by putting the resultant products of  FIG. 8D  into a vacuum furnace at 100 to 900° C. and growing the carbon nanotube  30  while gas having any one component selected from a group consisting of CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , and CO flows. 
         [0056]    From after the carbon nanotube array structure with the three-dimensional structure is completed, the process of completing the entire structure of the carbon nanotube field emitter is the same as the embodiment 1 and the further description thereof will thus be omitted. 
         [0057]    In the present embodiment 3, the method for performing vacuum heat treatment under gas atmosphere for the carbon nanotube growth in order to form the carbon nanotube is described. In addition to the method, there are methods for forming the carbon nanotube as follows: 
         [0058]    (a) The carbon nanotube can be formed by putting the resultant products of  FIG. 8D  into a solvent including carbon such as Co(CO) 8 , Fe(CO) 5 , Fe(C 5 H 5 ) 2 , Ethanol, Methanol, Xylene or mixed solvents thereof and then performing ultrasonic treatment thereon. 
         [0059]    (b) The carbon nanotube can be formed by putting the resultant products of  FIG. 8D  into carbon nanotube solution formed of the carbon nanotube or the composite material including the carbon nanotube and a solvent whose boiling point is 300° C. or less or by spraying the solution on the resultant products. 
         [0060]    With the present invention as above, the damage of the carbon nanotube is minimized so that the lifetime of the carbon nanotube field emitter can be remarkably improved as well as the carbon nanotube field emitter with excellent performance can be fabricated. Also, the carbon nanotube field emitter with such a structure can be widely applied to the most advanced fields such as a field emission display, a backlight unit, an X-ray source, a field emission scanning microscope/a field emission tunneling microscope, a sensor, etc.