Patent Publication Number: US-2011052479-A1

Title: Method for preparing carbon nanotubes, carbon nanotube films, and electronic devices

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present application claims priority to Chinese Priority Patent Application CN 200910171619.8 filed in the Japan Patent Office on Aug. 31, 2009, the entire content of which is hereby incorporated by reference. 
     BACKGROUND 
     The present application relates to a method for preparing carbon nanotubes and carbon nanotube films, as well as a method for fabricating electronic devices, and is applicable to, for example, the fabrication of various electronic devices utilizing carbon nanotubes or carbon nanotube films. 
     Carbon nanotubes, especially, single-walled carbon nanotubes (SWNTs) have been arduously investigated and developed due to its excellent properties in electricity, machinery, magnetism, and configuration. To date, a plurality of methods, such as laser ablation process (non-patent document 1), arc discharge process (non-patent document 2), chemical vapor deposition (CVD) process (non-patent document 3) and the like have been employed as the method for preparing single-walled carbon nanotubes. Inevitably, there are considerable amount of carbon or metal impurities left in the resulting single-walled carbon nanotubes prepared by these processes. 
     In prior art, the resulting single-wall carbon nanotubes are refined with an acid to remove the impurities, by chemical oxidation which involves liquid oxidization (acid treatment, reflux, etc.) and electrochemical oxidization (non-patent document 4). During the refining process, the structure of SWNTs is often damaged due to the chemical defects resulted from the refining (non-patent document 5). Moreover, when the SWNTs are applied to nanotechnology, in order to disperse or dissolve the SWNTs, post-treatments such as ultrasonic treatment or drastic chemical reaction are often necessary (non-patent document 3, 6, 7). During the post-treatment, additional damages happen to the SWNTs. 
     Liu (non-patent document 8), Wang (non-patent document 9), etc., have reported the method for repairing the structure of SWNTs having defects by treating the SWNTs in ammonia (NH 3 ) atmosphere at 1000° C. After this treatment process, it can be found that the ratio I D /I G  of the intensity of band D (I D ) to that of band G (I G ), which is detected by Raman spectroscopy, is reduced to not more than 0.01, which shows that the defects in the side walls of SWNTs have been repaired. 
     PRIOR ART DOCUMENTS 
     Non-Patent Documents 
     Non-patent document 1: A. Thess, R. Lee, P. Nikolaev, H. J. Dai, P. Petit, J. Robert, C. H. Xu, Y. H. Lee, S. G. Kim, A. G. Rinzler, D. T. Colbert, G. E. Scuseria, D. Tomanek, J. E. Fischer and R. E. Smalley, Science, 1996, 273, 483-487 
     Non-patent document 2: S. Iijima, Nature, 1991, 354, 56-58 
     Non-patent document 3: J. L. Bahr, J. P. Yang, D. V. Kosykin, M. J. Bronikowski, R. E. Smalley and J. M. Tour, Journal of the American Chemical Society, 2001, 123, 6536-6542 
     Non-patent document 4: P. X. Hou, C. Liu and H. M. Cheng, Carbon, 2008, 46, 2003-2025 
     Non-patent document 5: J. Zhang, H. L. Zou, Q. Qing, Y. L. Yang, Q. W. Li, Z. F. Liu, X. Y. Guo anf Z. L. Du, Journal of Physical Chemistry B, 2003, 107, 3712-3718 
     Non-patent document 6: D. Tasis, N. Tagmatarchis, A. Bianco and M. Prato, Chemical Reviews, 2006, 106, 1105-1136 
     Non-patent document 7: D. Tasis, N. Tagmatarchis, V. Georgakilas and M. Prato, Chemistry, 2003, 9, 4000-4008 
     Non-patent document 8: Y. Q. Liu, L. Gao, J. Sun, S. Zheng, L. Q. Jiang, Y. Wang, H. Kajiura, Y. M. Li and K. Noda, Carbon, 2007, 45, 1972-1978 
     Non-patent document 9: Y. Wang, L. Gao, J. Sun, Y. Q. Liu, S. Zheng, H. Kajiura, Y. M. Li and K. Noda, Chemical Physics Letters, 2006, 432, 205-208 
     Non-patent document 10: J. Chen, M. A. Hamon, H. Hu, Y. S. Chen, A. M. Rao, P. C. Eklund and R. C. Haddon, Science, 1998, 282, 95-98 
     Non-patent document 11: C. Montalbetti and V. Falque, Tetrahedron, 2005,61, 10827-10852 
     Non-patent document 12: J. Jang, J. Bae and S. H. Yoon, Journal of Materials Chemistry, 2003, 13, 676-681 
     Non-patent document 13: P. S. Wharlton and D. H. Bohlen, Journal of Organic Chemistry, 1961, 26, 3615-&amp; 
     SUMMARY 
     However, the above existing method, which repairs the defects of the SWNTs by treating in ammonia (NH 3 ) atmosphere at 1000° C., is limited in application because a high temperature treatment at 1000° C. is necessary. Further, as the ammonia is decomposed into nitrogen gas and hydrogen gas, there is a risk of explosion due to the hydrogen gas. 
     Therefore, the present application is directed to provide a method for preparing carbon nanotubes, which can easily repair the defects in the side walls of the acid-refined SWNTs in a stable condition, and can prepare carbon nanotubes of excellent properties. 
     The present application is further directed in an embodiment to provide a method for preparing carbon nanotube films, which can be used to prepare carbon nanotube films of excellent properties, with carbon nanotubes whose defects in the side walls of the acid-refined carbon nanotubes are repaired, and who has excellent properties. 
     The present application is still further directed in an embodiment to provide a method for fabricating electronic devices, which can fabricate electronic devices of high performance by utilizing the carbon nanotubes prepared by the above-mentioned preparation method, or by utilizing the carbon nanotube films formed of the carbon nanotubes. 
     To these ends, the present application provides in an embodiment a method for preparing carbon nanotubes, which comprises a step of treating acid-refined carbon nanotubes with a solution of a compound containing an amino group or ammonia water. 
     The present application also provides in an embodiment a method for preparing carbon nanotube films which prepares the carbon nanotube films from the carbon nanotubes obtained by treating acid-refined carbon nanotubes with a solution of compound containing an amino group or ammonia water. 
     Typically, the method for preparing carbon nanotube films comprises a step of treating acid-refined carbon nanotubes with a solution of compound containing an amino group or ammonia water. 
     Moreover, the present application provides in an embodiment a method for fabricating an electronic device, in which carbon nanotubes or carbon nanotubes films formed of the same are used, and wherein the carbon nanotubes are prepared by treating the acid-refined carbon nanotubes with a solution of a compound containing an amino group or ammonia water. 
     The method for preparing the electronic devices comprises, for example, a step of treating the acid-refined carbon nanotubes with a solution of a compound having an amino group or ammonia water; or comprises a step of preparing carbon nanotube films from the carbon nanotubes which are treated with a solution of a compound containing an amino group or ammonia water; or comprises both of the two steps. 
     In the present application, the acid-refined carbon nanotubes refer to those carbon nanotubes which are treated by refluxing with acid, such as nitric acid, and the carbon nanotubes are featured with carboxyl group (—COOH) bonded to the side walls of the acid-refined carbon nanotubes. Basically, the carbon nanotubes can be synthesized by any suitable method. Specifically, various methods such as laser ablation process, arc discharge process, and chemical vapor deposition (CVD), etc., can be used. Compounds containing an amino group which reacts with a carboxyl group (—COOH) to substitute the hydroxyl group (—OH) of the carboxyl group are typically used as the compounds containing an amino group (—NH 2 ). Thus the amino group of the compound containing an amino group needs to be easily dissociated in the solution of the compound containing the amino group. As described above, —COOH is boned to the side walls of the acid-refined carbon nanotubes. Thus, if the carbon nanotubes are treated with the solution of the compound containing an amino group, then the amino group reacts with the carboxyl group bonded to the carbon nanotubes and substitutes the hydroxyl group of the carboxyl group, and the —COOH is transformed into —CONH 2 . During this process, the defects of the side walls of carbon nanotubes can be repaired. The compound containing an amino group is preferably, for example but not limited to, carbamide (NH 2 CONH 2 ). Ammonia water is the aqueous solution of gaseous ammonia (NH 3 ), and has the same function as the solution of the compound containing an amino group. 
     Preferably, the present application further includes a step of treating carbon nanotubes with a solution of a compound, which reacts with carboxyl to form —COCl, before treating the carbon nanotubes with the solution of a compound containing an amino group in an embodiment. If the carbon nanotubes bonded with carboxyl are treated with the solution, the carboxyl bonded to the carbon nanotubes transforms into —COCl. If the resulting carbon nanotubes boned with —COCl are treated with the solution of a compound containing an amino group, then —COCl boned to the carbon nanotubes transforms into —CONH 2 . The compound which reacts with carboxyl group to produce —COCl is preferably at least one selected from the group consisting of thionyl chloride (SOCl 2 ), oxalyl chloride ((COCl) 2 ), phosphorus trichloride (PCl 3 ), phosphorus oxychloride (POCl 3 ), and phosphorus pentachloride (PCl 5 ). 
     Generally, the treatment with the solution of a compound containing an amino group is performed at a temperature equal to or higher than 25° C. and lower than the boiling point of the solution of a compound containing an amino group, and typically not less than 25° C. and not higher than 90° C., but not limited thereto. Generally, the treatment with the solution of a compound which reacts with a carboxyl group to form —COCl is performed at a temperature equal to or higher than 25° C. and lower than the boiling point of the solution of a compound containing an amino group. Typically, the temperature for treating with ammonia water is preferably set at a comparative low temperature for inhibiting ammonia from evaporating. 
     Typically, the carbon nanotubes are single-walled carbon nanotubes, but also can be multilayer carbon nanotubes. And typically, the carbon nanotube films are single-walled carbon nanotube films, but also can be multilayer carbon nanotube films. The diameter and length of the carbon nanotubes forming the carbon nanotube films are not specifically limited. 
     The carbon nanotubes or carbon nanotube films can be applied to various electronic devices. As such electronic devices, field emission components, field effect transistors (FETs) (including thin film transistors (TFTs)), single electron transistors, molecular sensors, solar cells, photovoltaic components, light emitting components, memories, etc. Also, carbon nanotube films can be used, for example, as thin film electrodes or transparent electrodes. 
     According to the present application as described above in an embodiment, the defects in the side walls of the carbon nanotubes during acid refining can be repaired by treating with a solution of a compound containing an amino group or ammonia water. Typically, the treatment is performed at a temperature low to 100° C. or below, and a stable condition without a strong acid, thus the carbon nanotubes are prevented from being severed or the like. Furthermore, the treatment can be easily and efficiently performed. Especially, by treating the carbon nanotubes with a solution containing the compound which reacts with carboxyl group on the side walls of the carbon nanotubes to form —COCl before treating with the solution of a compound containing an amino group, the reaction utilizing the compound containing an amino group is made happen more easily, and lowering of the treatment temperature and shortening of the treatment time for treating with the solution of a compound containing an amino group can be realized. 
     According to the present application in an embodiment, the defects in the side walls of the acid-refined carbon nanotubes can be repaired easily under a stable condition, and it is easy to fabricate nanotubes with excellent properties and without defects. Moreover, the carbon nanotube films with excellent properties can be easily fabricated with those carbon nanotubes. Further, these carbon nanotubes or carbon nanotube films can be used to fabricate high performance electronic devices. 
     Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a photograph in substitution for a picture showing a TEM image of the SWNTs which have been acid-refined by refluxing in nitric acid according to Example 1 corresponding to the first embodiment; 
         FIG. 2  is a photograph in substitution for a picture showing a TEM image of the SWNTs which have been treated directly with NH 2 CONH 2  solution according to Example 1 corresponding to the first embodiment; 
         FIG. 3  is a photograph in substitution for a picture showing a TEM image of the SWNTs which have been treated with NH 2 CONH 2  solution after treated with SOCl 2  solution according to Example 2 corresponding to the second embodiment; 
         FIG. 4  is a schematic figure showing the results obtained by measuring the Raman spectra of the acid-refined SWNTs, the SWNTs treated directly with NH 2 CONH 2  solution, and the SWNTs treated with NH 2 CONH 2  solution after treated with SOCl 2  solution according to Examples 1 and 2 respectively corresponding to the first and second embodiments; 
         FIG. 5  is a schematic figure showing the TG-DTA curves of the acid-refined SWNTs according to Example 2 of the second embodiment; 
         FIG. 6  is a schematic figure showing the TG-DTA curves of the SWNTs treated with NH 2 CONH 2  solution after treated with SOCl 2  solution according to Example 2 of the second embodiment; 
         FIG. 7  is a schematic figure showing the results obtained by measuring the FT-IR spectra of the acid-refined SWNTs, the SWNTs treated directly with NH 2 CONH 2  solution, and the SWNTs treated with NH 2 CONH 2  solution after treated with SOCl 2  solution according to Examples 1 and 2 respectively corresponding to the first and second embodiments; 
         FIG. 8  is a schematic figure showing the results obtained by measuring the dispersion concentration of the acid-refined SWNTs, the SWNTs treated directly with NH 2 CONH 2  solution, and the SWNTs treated with NH 2 CONH 2  solution after treated with SOCl 2  solution according to Examples 1 and 2 respectively corresponding to the first and second embodiments; and 
         FIG. 9  is a schematic figure showing the defect repair mechanism of the SWNTs of NH 2 CONH 2  solution treatment according to Example 1 corresponding to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The present application will be disclosed below with reference to the figures according to an embodiment as follows: 
     1. the first embodiment (carbon nanotubes and the method for preparing the same) 
     2. the second embodiment (carbon nanotubes and the method for preparing the same) 
     3. the third embodiment (carbon nanotube films and the method for preparing the same) 
     1. The First Embodiment 
     Carbon Nanotubes and the Method for Preparing the Same 
     In the first embodiment, carbon nanotubes are synthesized with known methods of prior art. In particular, the carbon nanotubes can be synthesized with, for example but not limited to, laser ablation process, arc discharge process, CVD process, etc. 
     Then the carbon nanotubes synthesized as described above are refined with acid by known methods. In particular, the acid refining is performed, for example, by refluxing the carbon nanotubes with nitric acid (HNO 3 ). During the acid refining, the circlewise arranged carbon atoms in the side walls of carbon nanotubes are oxidized into carboxyl groups (—COOH), which lead to defects. 
     Next, the carbon nanotubes refined with acid as described above are treated with the solution of a compound containing an amino group (—NH 2 ) or ammonia water. In particular, the solution or ammonia water is mixed with the carbon nanotubes. With such a treatment, the carboxyl group bonded to the side walls of the carbon nanotubes reacts with the amino group of the compound in the solution or the ammonia in the ammonia water, and the hydroxyl group (—OH) of the carboxyl group is substituted by the amino group, thus the carboxyl group transforming into —CONH 2 , which will be removed from the carbon nanotubes eventually. Thus, the defects in the side walls of the carbon nanotubes are repaired by the above series of processes. The solvent of said solution can be selected from known solvents according to requirements. In particular, for example but not limited to, water can be employed as the solvent. The treatment temperature is selected according to requirements, typically a temperature equal to or higher than the room temperature (for example, 25° C.) and lower than the boiling point of the solvent of the solution, and preferably a temperature at least 10° C.˜20° C. lower than the boiling point. In particular, the treatment temperature ranges, for example, from 25° C. to 90° C., The treatment time is selected according to requirements, usually in connection with the treatment temperature and between 1 day and 10 days. 
     Next, the carbon nanotubes treated with the solution of a compound containing an amino group or ammonia water are washed with water, separated by centrifugal separation or filtration separation, and then dried. 
     Thus the desired carbon nanotubes are obtained. 
     According to the first embodiment, the defects occurred in the side walls of the acid-refined carbon nanotubes can be easily and almost completely repaired under a stable condition, and the carbon nanotubes of excellent properties and without defects can be obtained. The carbon nanotubes can be used to, for example, various electronic devices, thereby electronic devices of high performance can be realized. 
     2. The Second Embodiment 
     Carbon Nanotubes and the Method for Preparing the Same 
     In the second example, firstly, the carbon nanotubes are synthesized and refined with acid in the same way as that of the first embodiment. 
     Next, the carbon nanotubes refined with acid as described above are treated with the solution containing a compound which reacts with the carboxyl group to produce —COCl. In particular, for example, the solution is mixed with the carbon nanotubes. With the treatment, the carboxyl group (—COOH) bonded to the side walls of the carbon nanotubes reacts with the compound contained in the solution, and the hydroxyl group (—OH) contained in the carboxyl group is substituted by Cl, producing —COCl. The solvent of the solution can be selected from various known solvents according to requirements. The treatment temperature is selected according to requirements, typically a temperature equal to or higher than the room temperature (for example, 25° C.) and lower than the boiling point of the solvent of the solution, and preferably is a temperature at least 10° C.˜20° C. lower than the boiling point. In particular, the treatment temperature ranges, for example, from 25° C. to 90° C., and generally, a commonly low temperature is enough. The treatment time is selected according to requirements, in connection with the treatment temperature, and generally between 1 day and 10 days. 
     Next, after being treated with the solution containing a compound which reacts with a carboxyl group to produce —COCl, the carbon nanotubes are treated, in the same manner as in the first embodiment, with the solution of a compound containing an amino group or ammonia water. With the treatment, the —COCl bonded to the side walls of the carbon nanotubes reacts with the compound contained in the solution or the ammonia of the ammonia water, and the Cl in the —COCl is substituted by the amino group, which produces —CONH 2 . Thus, before being treated with the solution of a compound containing an amino group or ammonia water, the carbon nanotubes are treated with the solution containing the compound which reacts with the carboxyl group to produce —COCl, facilitating the reaction of the solution of a compound containing an amino group or the ammonia water (non-patent document 10, 11). 
     Next, in the same manner as in the first embodiment, the carbon nanotubes treated with the solution of a compound containing an amino group or ammonia water are washed with water, separated by centrifugation or filtration, and then dried. 
     Thus, the desired carbon nanotubes are obtained. 
     According the second embodiment, apart from the advantages of the first embodiment, other advantages are also obtained. For instance, before being treated with the solution of a compound containing an amino group or ammonia water, the carbon nanotubes are treated with the solution containing a compound which reacts with carboxyl group to produce —COCl, so as to realize the lowering of the temperature or the shortening of the time of the treatment of the solution of a compound containing an amino group or ammonia water. 
     Example 1 
     Example 1 is an example corresponding to the first embodiment. 
     The SWNTs synthesized by chemical vapor deposition (CVD) process are used as the SWNTs. The SWNTs used in the present invention are available from Chengdu Institute of Organic Chemistry of Chinese Academy of Sciences (CIOC), which are synthesized by CVD process at 1000° C. using methane (CH 4 ) as the raw materials and CoMo as the catalyst. 
     The SWNTs are refined with acid by refluxing with 2.6 M nitric acid (HNO 3 ) at 140° C. for 48 hours to remove impurities. 
     Next, 1 g NH 2 CONH 2  is dissolved in 2 ml water to prepare NH 2 CONH 2  solution. 
     Then, 20 mg of SWNTs refined with acid as described above is mixed with the NH 2 CONH 2  solution, then is treated with ultrasound in water bath for 1 min. Then the SWNTs are treated in the NH 2 CONH 2  solution at 25° C.˜90° C. for not less than 2 days. 
     Next, thus obtained admixture is filtered, rinsed and dried. 
     Thus the desired SWNTs is obtained. 
     The recovery ratio of the SWNTs is not less than 95%. 
     Example 2 
     Example 2 is an example corresponding to the first embodiment. 
     The SWNTs synthesized by chemical vapor deposition (CVD) process are used as the SWNTs. The SWNTs used in the present invention are available from Chengdu Institute of Organic Chemistry of Chinese Academy of Sciences(CIOC), which are synthesized by CVD process at 1000° C. using methane (CH 4 ) as the raw materials and CoMo as the catalyst. 
     The SWNTs are refined with acid by refluxing with 2.6 M nitric acid (HNO 3 ) at 140° C. for 48 hours to remove impurities. 
     Next, 20 ml SOCl 2  and 1 ml DMF (N,N-dimethylformamide) are mixed to prepare the SOCl 2  solution. 
     Next, 100 mg of SWNTs refined with acid as described above is mixed with SOCl 2  solution and stirred at 25° C. for 24 hours, so that the SWNTs react with the SOCl 2  solution (See non-patent Document 10) 
     Next, the obtained admixture is rinsed with THF (tetrahydrofuran), then dried in vacuum at room temperature, thus the SWNTs whose side walls are bonded with —COCl are obtained. 
     Next, NH 2 CONH 2  solution is obtained by dissolving 1 g NH 2 CONH 2  in 2 ml water. 
     Next, 20 mg of SWNTs whose side walls are bonded with —COCl obtained as described above are mixed with the NH 2 CONH 2  solution, and is treated with ultrasound in water bath for 1 min. Then the SWNTs are treated in the NH 2 CONH 2  solution at 25° C.˜90° C. for not less than 2 days. 
     Next, thus obtained admixture is washed with water, separated by centrifugation or filtration, and then dried. 
     Thus the desired SWNTs is obtained. 
     The recovery ratio of the SWNTs is not less than 95%. 
     In order to evaluate the effects of SOCl 2  solution-based pre-treatment, two SWNTs samples are compared, with one obtained by treating with SOCl 2  solution and then dipping in the solution of NH 2 CONH 2  contained in a transparent vessel at 60° C., the other obtained by directly dipping and treating in the solution of NH 2 CONH 2  contained in a transparent vessel at 60° C. As a result, the volume of the first sample is three times as that of the second one. This is because the NH 2 CONH 2  treatment after the SOCl 2  treatment results in sufficient stripping of the SWNT bundles, thus reducing the diameter of the bundles (non-patent document 10). The stripping of the SWNT bundle is considered to enable to increase the whole surface area of the SWNTs in NH 2 CONH 2  solution, as a result, the contact area between the acid-refined SWNTs and NH 2 CONH 2  solution is increased, and the reaction is sped up. Moreover, in the case where the acid-refined SWNTs are treated with SOCl 2  solution, and then dipped and treated in NH 2 CONH 2  solution of 60° C. in a transparent vessel, the SWNTs exist in the whole volume of NH 2 CONH 2  solution. Obviously, the contact area between the acid-refined SWNTs and NH 2 CONH 2  solution increases and thus the reaction becomes faster as the temperature of NH 2 CONH 2  solution treatment, which is performed after treating the SWNTs with SOCl 2  solution, becomes higher. Moreover, in the case where the acid-refined SWNTs are directly dipped and treated in NH 2 CONH 2  solution, the color of the supernatant in the transparent vessel becomes dark yellow, on the contrary, in the circumstance where the acid-refined SWNTs are treated with SOCl 2  solution, and then dipped and treated in NH 2 CONH 2  solution at 60° C., the color of the supernatant in the transparent vessel does not change. This is because the pretreatment of SOCl 2  solution makes the reaction of SWNTs with NH 2 CONH 2  solution accelerated, which prevents the generation of the intermediate products. 
     Next, the evaluation results of the SWNTs prepared by Examples 1 and 2 will be described. 
     Now, the SWNTs just being refined with acid by refluxing with 2.6 M HNO 3  at 140° C. for 48 hours are referred to as “newly acid-refined SWNTs”. And the SWNTs of Example 1, which are treated with NH 2 CONH 2  solution after being refined with acid by refluxing with 2.6 M HNO 3  at 140° C. for 48 hours, are referred to as “Sample 1”. the SWNTs of Example 2, which are treated with SOCl 2  solution, and then NH 2 CONH 2  solution after being refined with acid by refluxing with 2.6 M HNO 3  at 140° C. for 48 hours, are referred as “Sample 2”. 
       FIGS. 1-3  show the TEM images of newly acid-refined SWNTs, sample 1 and sample 2 respectively. According to  FIGS. 1-3 , impurities (amorphous carbon, multi-layer carbon nanotubes, nanometer graphite particles, metal impurities, etc.) can be found in the newly acid-refined SWNTs, and on the contrary, almost no impurity can be found in sample 1 and sample 2. Thus it can be considered that the impurities of newly acid-refined SWNTs are substantially removed by the treatment of Examples 1 and 2. Moreover, it can be found there is almost no change in the length of SWNTs of samples 1 and 2, compared with the newly acid-refined SWNTs. This means that the SWNTs of Examples 1 and 2 have not be severed because the treatment of Examples 1 and 2 employs chemically stable NH 2 CONH 2  and is conducted at low temperature and stable condition. 
     Table 1 shows the results of element analysis of the newly acid-refined SWNTs, sample 1 and sample 2 by energy dispersive X-ray spectroscopy (EDS). 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 element 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 sample 
                 Carbon (mol %) 
                 Oxygen (mol %) 
               
               
                   
                   
               
               
                   
                 newly acid-refined 
                 95.83 
                 4.17 
               
               
                   
                 SWNTs 
               
               
                   
                 Sample 1 
                 98.38 
                 1.62 
               
               
                   
                 Sample 2 
                 98.70 
                 1.30 
               
               
                   
                   
               
            
           
         
       
     
     According to table 1, the sum of carbon content (mol %) and oxygen content (mol %) is 100% for any of the newly acid-refined SWNTs, sample 1 and sample 2, and no other element besides carbon and oxygen is detected. This implies that NH 2 CONH 2  and SOCl 2  for the treatment in Examples 1 and 2 have neither bonded with the SWNTs by covalent bond or non-covalent bond, nor been adsorbed to the surfaces of SWNTs. The NH 2 CONH 2  and SOCl 2  are completely washed out and removed during the filtration. For the newly acid-refined SWNTs, sample 1 and sample 2, carbon contents respectively are 95.83 mol %, 98.38 mol %, 98.70 mol %, and oxygen content respectively are 4.17 mol %, 1.62 mol %, 1.30 mol %. Here, the oxygen comes from the carboxyl group bonded to the side walls of the acid-refined SWNTs. Compared to the newly acid-refined SWNTs, the oxygen content of sample 1 and sample 2 subjected to the treatment of Example 1 and Example 2 are greatly reduced, which implies that the carboxyl groups in Sample 1 and Sample 2 are greatly reduced, thus it can be concluded that the defects in Sample 1 and Sample 2 are significantly reduced. The oxygen content in Sample 2 is less than that in Sample 1, which indicates that SOCl 2  has an effect of further reducing the carboxyl groups bonded to the side walls of the acid-refined SWNTs. 
     To determine the differences between the newly acid-refined SWNTs and the SWNTs of Sample 1 and Sample 2, Raman spectroscopy is obtained, which is an effective method in detecting defects in SWNTs. In particular, Renishaw Micro Raman spectrometer having an excitation wavelength of 633 nm is used to record the Raman spectroscopy of the SWNTs. 
     Thus obtained Raman spectroscopy is shown in  FIG. 4 , in which, Curve (a) is the Raman spectra of the newly acid-refined SWNTs, Curve (b) is the Raman spectra of Sample 1, while Curve (c) is the Raman spectra of Sample 2. There are two main regions in the typical Raman spectra of the SWNTs, which respectively are D band near 1330 cm −1  corresponding to disordered graphite layers and the conjugated system destroy degree, and G band near 1590 cm −1  corresponding to the stretching vibration of tangential C—C bond. The ratio (I D /I G ) of D band intensity I D  to G band intensity I G  is a measurement of covalent bond shift or defects in the side walls, which is a common practice (non-patent document 8). Due to the damage in the side walls of the SWNTs caused by the oxidization effect of nitric acid which is a strong acid, I D  of newly acid-refined SWNTs is observable. The ratio I D /I G  for newly synthesized SWNTs is 0.03. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 sample 
                 I D /I G   
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Newly acid-refined SWNTs 
                 0.2719 
               
               
                   
                 Sample 1 
                 0.050 
               
               
                   
                 Sample 2 
                 0.010 
               
               
                   
                   
               
            
           
         
       
     
     Table 2 shows the I D /I G  values of newly acid-refined SWNTs, sample 1 and sample 2. 
     According to Table 2, I D /I G  is 0.2719 for the newly acid-refined SWNTs, on the contrary, the ratio is greatly reduced for sample 1 and sample 2, and is 0.050 for sample 1 and 0.010 for sample 2. Based on this, it can be concluded that the SWNTs of sample 1 and sample 2 subjected to the treatment of Example 1 and Example 2 almost have no defect, and the graphite structure with defects in the newly acid-refined SWNTs has almost been completely repaired. Moreover, I D /I G  of sample 2 is reduced more than that of sample 1, which indicates that SOCl 2  has effects in helping repair the defects of SWNTs with defects. The result of Raman spectra is consistence with the result of element analysis. 
     In Example 1 and Example 2, the influences of treatment temperature of NH 2 CONH 2  solution on the repair of SWNTs are investigated. To this end, the treatment is performed respectively at 25° C., 60° C., and 90° C., and the treatment time is 8 days. The results are shown in Table 3, wherein, sample 1 (90° C.), sample 1 (60° C.), and sample 1 (25° C.) denote samples that the SWNTs treatment using NH 2 CONH 2  solution as Example 1 are respectively performed at 90° C., 60° C., and 25° C., while sample 2 (90° C.), sample 2 (60° C.), and sample 2 (25° C.) denotes samples that the SWNTs treatment using NH 2 CONH 2  solution as Example 2 are respectively performed at 90° C., 60° C., and 25° C. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 sample 
                 I D /I G   
               
               
                   
                   
               
             
            
               
                   
                 Newly acid-refined SWNTs 
                 0.2719 
               
               
                   
                 Sample 1 (90° C.) 
                 0.0567 
               
               
                   
                 Sample 1 (60° C.) 
                 0.1952 
               
               
                   
                 Sample 1 (25° C.) 
                 0.2312 
               
               
                   
                 Sample 2 (90° C.) 
                 0.0100 
               
               
                   
                 Sample 2 (60° C.) 
                 0.0205 
               
               
                   
                 Sample 2 (25° C.) 
                 0.1256 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 3, for sample 1, I D /I G  of newly acid-refined SWNTs is 0.2719, on the contrary, the ratio is greatly reduced to for sample 1 (25° C.), sample 1 (60° C.) and sample 1 (90° C.), and is 0.2312 for sample 1 (25° C.), 0.1952 for sample 1 (60° C.) and 0.0567 for sample 1 (90° C.). And for sample 2, I D /I G  of newly acid-refined SWNTs is 0.2719, on the contrary, the ratio is greatly reduced to for sample 2 (25° C.), sample 2 (60° C.) and sample 2 (90° C.), and is 0.1256 for sample 2 (25° C.), 0.0205 for sample 2 (60° C.) and 0.0100 for sample 2 (90° C.). Based on this, it can be concluded that the higher the treatment temperature is, the better the repair effect is. Moreover, it can be seen that I D /I G  of sample 2 is much smaller than that of sample 1 when comparing at the same treatment temperature. This is because the SOCl 2  solution treatment performed before the NH 2 CONH 2  solution treatment increases the defect repair effects for the SWNTs. Or, by treating with SOCl 2  solution before treating with NH 2 CONH 2  solution, the treatment temperature of NH 2 CONH 2  solution required for achieving an identical I D /I G  value is significantly reduced. 
     In Example 2, the influences of treatment time of NH 2 CONH 2  solution on the defect repair of SWNTs are investigated. To this end, the treatment temperature is set at 90° C., while treatment times respectively are 2 days, 4 days, 6 days and 8 days. The results are shown in Table 4, wherein sample 2 (2 days), sample 2 (4 days), sample 2 (6 days) and sample 2 (8 days) respectively denote the samples that the SWNTs treatment using NH 2 CONH 2  solution as Example 2 are respectively performed for 2 days, 4 days, 6 days and 8 days. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 sample 
                 I D /I G   
               
               
                   
                   
               
             
            
               
                   
                 Newly acid-refined SWNTs 
                 0.2719 
               
               
                   
                 Sample 2 (2 days) 
                 0.1387 
               
               
                   
                 Sample 2 (4 days) 
                 0.0598 
               
               
                   
                 Sample 2 (6 days) 
                 0.0194 
               
               
                   
                 Sample 2 (8 days) 
                 0.0100 
               
               
                   
                   
               
            
           
         
       
     
     As shown in  FIG. 4 , I D /I G  of newly acid-refined SWNTs is 0.2719, on the contrary, the ratio is greatly reduced for sample 2 (2 days), sample 2 (4 days) and sample 2 (6 days), and is 0.1387 for sample 2 (2 days), 0.0598 for sample 2 (4 days), 0.0194 for sample 2 (6 days), and almost 0 for sample 2 (8 days). Based on this, it can be concluded that the longer the treatment time is, the better the repair effect is. 
     The qualities of the newly acid-refined SWNTs and the SWNTs of sample 2 are evaluated by differential thermal analysis and thermogravimetry (TG-DTA).  FIG. 5  shows TG and DTA results of newly acid-refined SWNTs.  FIG. 6  shows TG and DTA results of sample 2 (90° C.). The TG-DTA measurement is performed as follows: 10 mg of the obtained sample is heated at speed of 5° C./min, air is used as ambient atmosphere, and an empty platinum tray is used as reference. 
     According to  FIGS. 5 and 6 , the newly acid-refined SWNTs begin to oxidize at T=380° C., while the sample 2 (90° C.) treated with SOCl 2  solution begins to oxidize at T=485° C. Based on this, it can be concluded that the SWNTs, which are treated with SOCl 2  solution before being treated with NH 2 CONH 2  solution, becomes difficult to be oxidized. This implies that the defects of the side walls of the SWNTs of sample 2 (90° C.) are more sufficiently repaired. 
     Fourier Transform Infrared Spectroscooy (FT-IR) is employed to evaluate the newly acid-refined SWNTs, sample 1 and sample 2, wherein the FT-IR is conducted by using FTIR spectrometer (Bio-Rad FTS-185). The obtained FTIR spectra are shown in  FIG. 7  and the intensities of FTIR spectra are normalized so as to obtain identical I G . Table 5 shows the vibration frequencies of the peaks of FTIR spectra of newly acid-refined SWNTs (non-patent document 5, 12). 
     
       
         
           
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Absorption 
                   
               
               
                 band/cm −1   
                 attributes 
               
               
                   
               
             
            
               
                 1060 
                 C—C—O ring stretching vibration, C—C—C asymmetric 
               
               
                   
                 stretching vibration 
               
               
                 1540 
                 —C═C—stretching vibration 
               
               
                 1650 
                 stretching vibration of C═O boned with H 
               
               
                 1699 
                 C═O 
               
               
                   
               
            
           
         
       
     
     It is evident that the stretching vibration of C═O boned with H and the stretching vibration of C═O originates from the carbonyl group (C═O), which is caused by the chemical oxidization due to the nitric acid used for acidifying the SWNTs. As seen in  FIG. 7 , there is no peak corresponding to the stretching vibration of C═O boned with H and the stretching vibration of C═O in sample 1 and sample 2, thus it can be concluded that there is no C═O. Therefore, no C═O group exists after the treatment of example 1 and 2, which means that the C═O in sample 1 and sample 2 has been reduced by NH 2 CONH 2 . The conclusion is consistent with those of EDS analysis and Raman analysis. Moreover, according to  FIG. 7 , there is no other peak observed in sample 1 and sample 2, which implies that the carbonyl group bonded to the newly acid-refined SWNTs has been removed from the SWNTs without transforming into other organic groups. 
     In order to evaluate the integrity of the SWNTs of sample 1 and sample 2 treated with NH 2 CONH 2  solution, the newly acid-refined SWNTs, sample 1 and sample 2 are dispersed in 1 wt % sodium dodecyl benzene sulfonate (SDBS) solution, treated with horn ultrasound for 1 hour, then centrifugated for 2 times (13000 rpm, 30 mins). UV-Vis-NIR absorbances at 550 nm are measured, and the values for newly acid-refined SWNTs, sample 1 and sample 2 are respectively 0.31, 0.12 and 0.10. The result indicates that for the SWNTs treated with NH 2 CONH 2  solution, the concentration of SWNTs is reduced by at least three times, in other words, the SWNTs of sample 1 and sample 2 treated with NH 2 CONH 2  solution are difficult to be dispersed. Obviously, it is resulted from the reduction of defects in the SWNTs of sample 1 and sample 2. Treating the SWNTs in ammonia at 1000° C. will repair the structure of the SWNTs with defects and confirm the reduction of the dispersancy of SWNTs (non-patent documents 8 and 9). The concentration of the SWNTs of sample 2 is less than that of sample 1, thus it can be reconfirmed that the pre-treatment with SOCl 2  solution is helpful in reducing the defects in SWNTs. 
     As described above, in Embodiments 1 and 2, the defects occurred in the acid-refined SWNTs are repaired by amino group (—NH 2 ). Although the mechanism for reducing the defects of SWNTs according to the treatment of Example 1 and 2 is still unveiled, the present invention gives the following explanation. 
       FIG. 9  shows a simple and reliable reaction path of the carboxyl reductive reaction using NH 2 CONH 2  solution. It is well known that the carboxyl group reacts with NH 2 CONH 2  to produce acidamide by heating. In the process that acid-refined SWNTs reacts with NH 2 CONH 2  solution, SWNT-CONH 2  is firstly formed as an intermediate. The existence of the amino group explains the color change of the supernatant, since the aroma amine shows a color. The color of the acid-refined SWNTs treated with NH 2 CONH 2  solution at 90° C. is much clearer than that of the sample treated with NH 2 CONH 2  solution at 60° C., which is possibly resulted from the accelerated decomposition of SWNT-CONH 2  through a further reaction when treating with NH 2 CONH 2  solution at a higher temperature. Because no intermediate is detected after rinsing and drying, it is considered to be instable. 
     The further amido-based reductive reaction of SWNT-CONH 2  to desorb organic groups and obtain SWNTs of intact structure happens in the way of well known Wharton reaction (non-patent document 13). That is to say, the ketone group of the intermediate is firstly reduced by the amino group generated from the hydrolyzation of NH 2 CONH 2  solution. The new intermediate decomposes and releases nitrogen gas (N 2 ), and the SWNTs are finally obtained. In which case, SOCl 2  not only activates the carboxyl group but also accelerates the reaction between the acid and the amine. 
     3. The Third Embodiment 
     Carbon Nanotube Film and the Method for Preparing the Same 
     In the third embodiment, the carbon nanotubes obtained from the first embodiment and the second embodiment are used to prepare carbon nanotube films. 
     That is to say, in the third embodiment, the carbon nanotubes obtained from the first embodiment and the second embodiment are used to prepare carbon nanotube films on a substrate (not shown) according to the well known method. In particular, as the method for fabricating carbon nanotube films, many methods can be used, such as drop casting, spin casting, air brushing, dip casting, Langmuir-Blodgett method and filtration method. 
     The carbon nanotubes dispersion, which utilizes a surfactant to make the carbon nanotubes separated from each other and well dispersed in the liquid, is typically used during preparing the carbon nanotube films. As the surfactant, anion surfactants, cation surfactants, amphoteric surfactants, and nonionic surfactants can be utilized, wherein, anion surfactants are preferred. The anion surfactants can be, for example, sodium dodecylsulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), sodium dodecanesulfonate (SDSA), etc. 
     As the substrate for preparing carbon nanotube films, various substrates can be selected according to requirements. In particular, for example, the glass substrates, quartz substrates, silicon substrates (especially those have oxidation film (SiO 2  film) formed on the surface) can be use as the substrates. And as the flexible substrates, various plastic substrates can be used. The plastic substrates formed from, for example but not limiting to, polyethylene terephthalate, polyethylene, polypropylene, polystyrene, polycarbonate or the like can be used as the plastic substrates. As the transparent substrates, the transparent plastic substrate made of polyethylene terephthalate or the like can be used. 
     According to the third embodiment, the carbon nanotubes of excellent properties prepared by the first embodiment and the second embodiment can be utilized to prepare carbon nanotube films of excellent properties. The carbon nanotube films can be used, for example, as conductive films or transparent conductive films. The sheet resistance of the conductive films or the transparent conductive films can be reduced to at most 60% of that of prior art. The conductive films and transparent conductive films can be applied to, for example, the thin film electrodes or transparent electrodes of various electronic devices, so as to realize high performance electronic devices. 
     It should be appreciated, for example, that the values, raw materials, equipments and processes given in the foregoing embodiments and examples are simply exemplary and can be changed according to requirements. 
     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.