Abstract:
An isotope-doped carbon nanotube ( 40 ) includes a plurality of first carbon nanotube segments ( 402 ) having carbon-12 isotopes and a plurality of second carbon nanotube segments ( 404 ) having carbon-13 isotopes. The first and second carbon nanotube segments are alternately arranged along a longitudinal direction of the carbon nanotube. Three preferred methods employ different isotope sources to form isotope-doped carbon nanotubes. In a chemical vapor deposition method, different isotope source gases are alternately introduced. In an arc discharge method, a power source is alternately switched between different isotope anodes. In a laser ablation method, a laser is alternately focused on different isotope targets. In addition, an apparatus for implementing the preferred methods is provided.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to nano-materials and methods and apparatuses for forming nano-materials, and more particularly to isotope-doped carbon nanotubes and a method and an apparatus for forming the same.  
           [0003]    2. Description of the Prior Art  
           [0004]    Carbon nanotubes were discovered by S. Ijima (Nature, vol. 354, pp. 56-58, 1991) and synthesized by T. W. Ebbesen and P. M. Ajayan (Nature, vol. 358, pp. 220-222, 1992). Theoretical studies showed that carbon nanotubes exhibit either metallic or semiconductive behavior depending on the radii and helicity of the tubules. Carbon nanotubes have interesting and potentially useful electrical and mechanical properties, and offer potential for use in electronic devices. Carbon nanotubes also feature high aspect ratios (&gt;1000) and atomically sharp tips, which make them ideal candidates for electron field emitters, white light sources, lithium secondary batteries, hydrogen storage cells, transistors and cathode ray tubes (CRTs).  
           [0005]    Carbon nanotubes are currently being produced by a variety of different techniques such as arc discharge (See S. lijima et al, Nature, Helical Microtubules of Graphitic Carbon, vol. 354, pp. 56-58, 7 Nov. 1991), laser ablation (See T. W. Ebbesen and P. M. Ajayan, Large-scale Synthesis of Carbon Nanotubes, Nature, vol. 358, pp. 220-222, 16 Jul. 1992) and chemical vapor deposition (CVD) (See W. Z. Li et al., Large-scale Synthesis of Aligned Carbon Nanotubes, Science, vol. 274, pp. 1701-1703, Dec. 6, 1996). All of the above-mentioned disclosures are incorporated herein by reference.  
           [0006]    Isotope labeling is a powerful tool in the study of nano-material growth mechanisms and in nano-sized isotope junction synthesis. Methods of isotope labeling use reactants containing different isotopes of a special element (usually light elements such as carbon, boron, nitrogen and oxygen), which are fed in designated concentrations (pure or mixed) and sequences into nano-material synthesis process to provide in situ isotope labeling of nano-materials.  
           [0007]    None of the three above-described techniques for producing carbon nanotubes, namely arc discharge, laser ablation and CVD, provides isotope-doped carbon nanotubes or a method for making isotope-doped carbon nanotubes.  
         SUMMARY OF THE INVENTION  
         [0008]    Accordingly, an object of the present invention is to provide isotope-doped carbon nanotubes.  
           [0009]    Another object of the present invention is to provide various methods for forming isotope-doped carbon nanotubes.  
           [0010]    A further object of the present invention is to provide at least one apparatus for forming isotope-doped carbon nanotubes.  
           [0011]    In order to achieve the first above-mentioned object, an isotope-doped carbon nanotube in accordance with the present invention comprises a plurality of first carbon nanotube segments having carbon-12 isotopes and a plurality of second carbon nanotube segments having carbon-13 isotopes. The first and second carbon nanotube segments are alternately arranged along a longitudinal direction of the carbon nanotube.  
           [0012]    In order to achieve the second above-mentioned object, a first preferred method of the present invention for forming isotope-doped carbon nanotubes comprises: providing a first and a second carbon source gas respectively comprising carbon-12 and carbon-13 isotopes; putting a substrate having a catalyst film deposited thereon into a reaction chamber; creating a vacuum in the reaction chamber, introducing a protecting gas at a predetermined pressure therein, and heating the reaction chamber up to a predetermined temperature; introducing the carbon-12 source gas into the reaction chamber, whereby first carbon nanotube segments are formed on the catalyst film; after a given time, shutting off the flow of carbon-12 source gas and introducing the carbon-13 source gas into the reaction chamber, whereby second carbon nanotube segments are formed on the first carbon nanotube segments; and after a given time, shutting off the flow of carbon-13 source gas and cooling the reaction chamber down to room temperature, whereby isotope-doped carbon nanotubes are formed.  
           [0013]    In order to achieve the second above-mentioned object, a second preferred method of the present invention for forming isotope-doped carbon nanotubes comprises: providing first and second carbon rods respectively comprising carbon-12 and carbon-13 isotopes, and respectively connecting the first and second carbon rods through a multi-position switch to a positive terminal of an electric arc discharge supply; connecting a pure graphite rod to a negative terminal of the electric arc discharge supply; placing the first and second carbon rods adjacent the pure graphite rod to create an arc gap, putting all the rods into an arc discharge reaction chamber, creating a vacuum in the reaction chamber, and introducing a protecting gas at a predetermined pressure therein; applying a discharge current between the first carbon rod and the graphite rod, whereby first carbon nanotube segments are formed on the graphite rod; after a given time, applying a discharge current between the second carbon rod and the graphite rod, whereby second carbon nanotube segments are formed on the first carbon nanotube segments; and after a given time, switching off the electric arc discharge supply, whereby isotope-doped carbon nanotubes are formed.  
           [0014]    In order to achieve the second above-mentioned object, a third preferred method of the present invention for forming isotope-doped carbon nanotubes comprises: providing first and second carbon targets respectively comprising carbon-12 and carbon-13 isotopes; providing a carbon nanotube accumulator; putting the first and second carbon targets and the accumulator into a laser ablation reaction chamber, with the accumulator placed behind the first and second carbon targets; creating a vacuum in the reaction chamber, and introducing a protecting gas at a predetermined pressure therein; heating a region in the vicinity of the first and second carbon targets up to a predetermined temperature; focusing a laser beam on the first carbon target, whereby first carbon nanotube segments are formed on the accumulator; after a given time, focusing the laser beam on the second carbon target, whereby second carbon nanotube segments are formed on the first carbon nanotube segments; and after a given time, switching off the laser beam, whereby isotope-doped carbon nanotubes are formed.  
           [0015]    In order to achieve the third above-mentioned object, an apparatus of the present invention for forming isotope-doped carbon nanotubes comprises a reaction chamber with at least one gas supply conduit and at least one gas exhaust conduit, at least one energy supply device, first and second carbon sources respectively comprising first and second carbon isotopes, a carbon nanotube forming medium, and a switching device. The switching device can selectively switch between the first carbon source and the second carbon source in order to make the first carbon source and the second carbon source deposit on the carbon nanotube forming medium alternately.  
       
    
    
       [0016]    Other objects, advantages and novel features of the present invention will be drawn from the following detailed description of preferred embodiments of the present invention with the attached drawings, in which:  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a schematic side elevation view of an isotope-doped carbon nanotube of the present invention;  
         [0018]    [0018]FIG. 2 is a schematic diagram of an apparatus used to form isotope-doped carbon nanotubes in accordance with a first preferred method of the present invention;  
         [0019]    [0019]FIG. 3 is a schematic diagram of an apparatus used to form isotope-doped carbon nanotubes in accordance with a second preferred method of the present invention; and  
         [0020]    [0020]FIG. 4 is a schematic diagram of an apparatus used to form isotope-doped carbon nanotubes in accordance with a third preferred method of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    The present invention is further described below and by reference to the figures.  
         [0022]    Referring to FIG. 1, an isotope-doped carbon nanotube  40  in accordance with the present invention comprises a plurality of first carbon nanotube segments  402  having carbon-12 isotopes and a plurality of second carbon nanotube segments  404  having carbon-13 isotopes. The first and second carbon nanotube segments  402 ,  404  are alternately arranged along a longitudinal direction of the carbon nanotube  40 . In a preferred embodiment of the present invention, the carbon nanotube  40  has a length of 10˜1000 μm and a diameter of 0.5˜50 nm.  
         [0023]    A first preferred method of the present invention for forming a plurality of the carbon nanotubes  40  involves chemical vapor deposition. Referring to FIG. 2, said chemical vapor deposition method comprises the following steps. By performing the following steps, isotope-doped carbon nanotubes each having only one first carbon nanotube segment  402  and only one second carbon nanotube segment  404  can be formed:  
         [0024]    (1) providing two different ethylene gases respectively comprising carbon-12 isotopes and carbon-13 isotopes;  
         [0025]    (2) putting a substrate  132  into a reaction chamber  110 , the substrate  132  having an iron thin film  134  deposited thereon, the iron thin film  134  being 5 nm thick and functioning as a catalyst;  
         [0026]    (3) creating a vacuum in the reaction chamber  110  via a gas exhaust conduit  116 , introducing argon gas at a pressure of 1 atmosphere into the reaction chamber  110  through a gas supply conduit  118 , and heating the reaction chamber  110  up to 650˜750° C. using a reaction furnace  106  disposed around the reaction chamber  110 ;  
         [0027]    (4) opening a valve  112  and introducing ethylene gas having carbon-12 isotopes into the reaction chamber  110  through a gas supply pipe  102  at a flow rate of 120 sccm (standard cubic centimeters per minute), leaving first carbon nanotube segments (not shown) having carbon-12 isotopes being formed on the iron thin film  134 ;  
         [0028]    (5) after a given time, closing the valve  112 , and opening a valve  114  and introducing ethylene gas having carbon-13 isotopes into the reaction chamber  110  through a gas supply pipe  104  at a flow rate of 120 sccm, leaving second carbon nanotube segments (not shown) having carbon-13 isotopes being formed on said first carbon nanotube segments;  
         [0029]    (6) after a given time, closing the valve  114  to stop the flow of ethylene gas having carbon-13 isotopes and cooling the reaction chamber  110  down to room temperature, leaving isotope-doped carbon nanotubes being formed on the substrate  132 .  
         [0030]    It is to be understood that after performing step (5), step (4) may be repeated to form carbon nanotubes each having two said first carbon nanotube segments and one said second carbon nanotube segment. Similarly, steps (4) and (5) may respectively be repeated a desired number of times to form the carbon nanotubes  40  each having the first and second carbon nanotube segments  402 ,  404  alternately arranged therein.  
         [0031]    In alternative embodiments of the first preferred method, other metals such as cobalt, nickel or the like can be used as the catalyst instead of iron. Other carbon hydrogen compounds such as methane, ethyne or propadiene can be used as the carbon source gas instead of ethylene. Other gases such as helium, nitrogen or hydrogen can be used as the protecting gas instead of argon.  
         [0032]    A second preferred method of the present invention for forming a plurality of the isotope-doped carbon nanotubes  40  involves arc discharge. Referring to FIG. 3, said arc discharge method comprises the following steps. By performing the following steps, isotope-doped carbon nanotubes each having only one first carbon nanotube segment  402  and only one second carbon nanotube segment  404  can be formed:  
         [0033]    (1) providing a first carbon rod  202  comprising carbon-12 isotopes and having a diameter of 8˜12 mm, the first carbon rod  202  being formed by pressing a composite powder and high purity graphite particles at a pressure of 3300˜3800 atmosphere, each particle having a diameter of 5 μm and being carbon-12 isotope graphite, the composite powder functioning as a catalyst and comprising nickel powder (0˜13% by weight) and/or ytterbia powder (0˜48% by weight), providing a second carbon rod  204  comprising carbon-13 isotopes and having a diameter of 8˜12 mm, the second carbon rod  204  being formed in the same way with the same composite powder as the first carbon rod  202  is formed but using high purity graphite particles of carbon-13 isotopes, bonding the first and second carbon rods  202 ,  204  with an adhesive insulator  203  therebetween, and respectively connecting the first and second carbon rods  202 ,  204  to two load-side contact of a switch  212 , a supply-side contact of the switch  212  being connected to a positive terminal  214  of an electric arc discharge supply, the first and second carbon rods  202 ,  204  functioning in turn as an anode;  
         [0034]    (2) connecting a pure graphite rod  208  to a negative terminal  215  of the electric arc discharge supply, the pure graphite rod  208  functioning as a cathode  208 ;  
         [0035]    (3) placing the anodes  202 ,  204  adjacent the cathode  208  to create an arc gap of 1.5˜2 mm, putting the anodes  202 ,  204  and cathode  208  into an arc discharge reaction chamber  210 , creating a vacuum in the reaction chamber  210  via a gas exhaust conduit  216 , and introducing helium gas at a pressure of 100˜500 Torr into the reaction chamber  210  through a gas supply conduit  218 ;  
         [0036]    (4) switching the switch  212  to connect the first carbon rod  202  with the positive terminal  214 , and applying a discharge voltage of 20˜40V and a discharge current of 90˜110A between the anode  202  and the cathode  208 , leaving first carbon nanotube segments (not shown) having carbon-12 isotopes being formed on the cathode  208 ;  
         [0037]    (5) after a given time, switching the switch  212  to disconnect the first carbon rod  202  while at the same time connecting the second carbon rod  204 , and applying a discharge voltage of 20˜40V and a discharge current of 90˜110A between the anode  204  and the cathode  208 , leaving second carbon nanotube segments (not shown) having carbon-13 isotopes being formed on said first carbon nanotube segments; and  
         [0038]    (6) after a given time, switching off the electric arc discharge supply, leaving isotope-doped carbon nanotubes being formed on the cathode  208 .  
         [0039]    It is to be understood that after performing step (5), step (4) may be repeated to form carbon nanotubes each having two said first carbon nanotube segments and one said second carbon nanotube segment. Similarly, steps (4) and (5) may respectively be repeated a desired number of times to form the carbon nanotubes  40  each having the first and second carbon nanotube segments  402 ,  404  alternately arranged therein.  
         [0040]    In alternative embodiments of the second preferred method, other suitable materials such as pure cobalt powder, pure nickel powder or the like can be used as the catalyst and pressed with the graphite particles. Other gases such as argon, nitrogen or hydrogen can be used as the protecting gas instead of helium. Furthermore, a cooling pipe can be attached around the arc discharge reaction chamber  210  to avoid excessive build-up of heat therein.  
         [0041]    A third preferred method of the present invention for forming a plurality of the isotope-doped carbon nanotubes  40  involves laser ablation. Referring to FIG. 4, said laser ablation method comprises the following steps. By performing the following steps, isotope-doped carbon nanotubes each having only one first carbon nanotube segment  402  and only one second carbon nanotube segment  404  can be formed:  
         [0042]    (1) providing a first carbon target  302  comprising carbon-12 isotopes, the first carbon target  302  being formed by pressing a composite powder together with a high purity graphite powder of carbon-12 isotopes, the composite powder functioning as a catalyst and comprising cobalt powder (2.8% by weight) and nickel powder (2.8% by weight), and providing a second carbon target  304  comprising carbon-13 isotopes, the second carbon target  304  being formed in the same way with the same composite powder as the first carbon target  302  is formed but using high purity graphite powder of carbon-13 isotopes;  
         [0043]    (2) providing a carbon nanotube accumulator  308 ;  
         [0044]    (3) putting the first and second targets  302 ,  304  and the accumulator  308  into a laser ablation reaction chamber  310 , with the accumulator  308  being placed behind the first and second targets  302 ,  304 ;  
         [0045]    (4) creating a vacuum in the reaction chamber  310  via a gas exhaust conduit  316 , and introducing argon gas at a pressure of 500˜760 Torr into the reaction chamber  310  through a gas supply conduit  318 ;  
         [0046]    (5) heating a region in the vicinity of the first and second targets  302 ,  304  up to 1000˜1200° C. using a heater  306 ;  
         [0047]    (6) focusing a pulsing laser beam  314  of a laser (not shown) on the first target  302  using a lens  312  located in front of the first and second targets  302 ,  304 , the pulsing laser beam  314  having a wavelength of 532 nm and a single pulsing energy of 250 mJ, a diameter of a spot of irradiation on the first target  302  being 5 mm, leaving first carbon nanotube segments (not shown) having carbon-12 isotopes being formed on the accumulator  308 ;  
         [0048]    (7) after a given time, switching the lens  312  to focus the laser beam  314  on the second target  304 , leaving second carbon nanotube segments (not shown) having carbon-13 isotopes being formed on said first carbon nanotube segments; and  
         [0049]    (8) after a given time, switching off the pulsing laser beam  314 , leaving isotope-doped carbon nanotubes being formed on the accumulator  308 .  
         [0050]    It is to be understood that after performing step (7), step (6) may be repeated to form carbon nanotubes each having two said first carbon nanotube segments and one said second carbon nanotube segment. Similarly, steps (6) and (7) may respectively be repeated a desired number of times to form the carbon nanotubes  40  each having the first and second carbon nanotube segments  402 ,  404  alternately arranged therein.  
         [0051]    In alternative embodiments of the third preferred method, other suitable materials such as pure cobalt powder, pure nickel powder or the like can be used as the catalyst and pressed with the graphite powder. Other gases such as helium, nitrogen or hydrogen can be used as the protecting gas instead of argon. In addition, the laser beam may be focused on the respective first and second targets  302 ,  304  by mounting the first and second targets  302 ,  304  on a rotatable member and rotating the rotatable member to exchange locations of the first and second targets  302 ,  304 .  
         [0052]    The preferred methods of the present invention can form multiple isotope-doped carbon nanotubes  40 , each comprising the first and second carbon nanotube segments  402 ,  404  alternately arranged along the longitudinal direction of the carbon nanotube  40 . Accordingly, the growth pattern of different carbon isotopes can be recorded in situ by micro-Raman spectroscopy. Further, the growth mechanisms of carbon nanotubes can be investigated in this way. Moreover, the preferred methods can be employed to form one-dimensional nano-materials containing isotopes other than those of pure carbon; for example, isotopes of light element compositions including boron, nitrogen or oxygen.  
         [0053]    It will be understood that the particular methods of the present invention are shown and described by way of illustration only, and not as limiting the invention. The principles and features of the present invention may be employed in various and numerous embodiments thereof without departing from the scope of the invention.