Structural and Electrical Properties of Ozone Irradiated Carbon Nanotube Yarns and Sheets

Materials Express

Toru Iijima1, Yasuhiro Inagaki1, Hisayoshi Oshima2, Takuya Iwata1, Ryota Sato3, Golap Kalita1, Toru Kuzumaki3, Yasuhiko Hayashi1, 4, ∗, and Masaki Tanemura1 1Department of Frontier Materials, Nagoya Institute of Technology, Gokiso-cho Showa-ku, Nagoya, Aichi 466-8555, Japan 2Research Labolatory, DENSO CORPORATION, 500-1 Minamiyama Komenoki-cho, Nisshin, Aichi 470-0111, Japan 3Department of Materials Science, Tokai University, 1117 Kitakaname, Hiratsuka, Kanagawa 259-1292, Japan 4Department of Electrical and Electronic Engineering, Okayama University, 3-1-1 Tsushima-naka, Communication Kita, Okayama 700-8530, Japan

1. INTRODUCTION Poor electrical conductivity of carbon nanotube (CNT) Carbon nanomaterials like carbon nanotube1 (CNT), yarn and sheets has been one of the main hurdles in nanohorn,2 fullerene,3 and graphene4 are extremely their practical application. Here,IP: we 192.168.39.151 demonstrate thatOn: Sat, 02 Oct 2021 18:07:47 promising for many device applications. Especially, CNT the ozone irradiation can be an effectiveCopyright: approach American Scientific Publishers Delivered byis Ingenta a well known material with many exciting properties, to reduce the resistivity of spun CNTs bundle with such as high electrical conductivity,5 mechanical stability,6 oxidization and induced defects. The CNT yarn field emission7 and thermal conductivity.8 Applications of and sheet are fabricated from a spinnable vertically individual CNT have been demonstrated for fabrication aligned CNT forest synthesized by a thermal chem- of sensors,9 transistors,10 cantilevers,11 etc. On the other ical vapor deposition. The electrical resistance of a hand, the bundle of CNTs also has lot of significances CNT yarn and sheet reduces with a certain period and various applications have been explored by taking the of ozone irradiation; however for a long duration of advantage of their properties. Effort has been made to fab- ozone irradiation resistance increases with structural ricate yarns and sheets from a large scale spinnable ver- deformation of the CNTs. A comparative study on 12–15 the effect of shapes between the CNT yarn and tically aligned CNT forest (VACNF). There are two sheet with morphological change in the CNTs due to significant merits of using spinnable VACNFs. The most the ozone irradiation is performed. It was observed important aspect is easy and simple fabrication process of that the desorption energy of ozone molecules was the CNT yarn and sheet; it can be fabricated by only draw- slightly large for the CNT yarn due to the difference ing with or without twisting from a spinnable VACNF. of adsorption sites. The fraction of defects induced by The second merit is that CNT yarn and sheet are com- the ozone irradiated on the CNT yarn was estimated posed of only CNTs, whereas CNT fibers generally contain 16–18 from the ultimate resistance change with temperature binders such as a polymer. However, those composite and Raman spectroscopy. fibers do not show better properties than the pristine CNTs due to the limited electrical conductivity. In comparison Keywords: Carbon Nanotube, Chemical Vapor to the composite fibers, the CNT yarn and sheet can be Deposition, Spinning, Yarn, Sheet, Ozone Irradiation. more suitable for the fabrication of larger CNTs structure. CNT yarn and sheet are expected to be tough ropes,19 light cables,20 flexible and transparent loudspeakers,21 etc. Pre- viously, we have reported a fabrication of the CNT yarn from a spinnable VACNF.22–23 It is important to improve various properties of CNT yarns and sheets for their suit- ∗Author to whom correspondence should be addressed. able applications. The electrical conductivity of CNT yarn Email: [email protected] and sheet is reported to be significantly lower7 24 than

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that of the individual CNTs.5 25–26 Therefore, processing of CNTs bundle with the ozone27–29 or nitrogen dioxide9 was considered as possible ways to improve the electrical prop- erty of CNT yarns and sheets. However, the ozone irradiation process creates large induced defects at the CNTs surface.29 Hence, it is essential to carefully control the ozone oxidation to reduce resistance of CNTs yarn and sheets with limited induced defects. In addition, the oxidation process by ozone irradiation can be an effective tool to determine the shape of the CNTs bundle. The individual CNT inside a CNT yarn with larger mass density are found to be not affected by ozone irradiation whereas almost all CNTs of a sheet are oxidized. In the paper, we report the morphological change and improvement in the electrical properties of the CNTs yarn and sheets with the ozone treatment.

2. EXPERIMENTAL DETAILS

The spinnable VACNF was synthesized by the thermal chemical vapor deposition (CVD) method with hydrogen as the process gas and acetylene as a carbon source at 680 C. A thermally oxidized SiO2/Si wafer coated with Fe as a catalyst and alumina as a supporting layer was used Fig. 1. Typical photograph of the fabricated (a) CNT yarn and (b) CNT IP: 192.168.39.151 On: Sat,sheet. 02 Oct As shown2021 in 18:07:47 the photograph, the yarn is fabricated by twisting as a substrate. The height of a VACNF was controlled with and pulling the CNTs from the substrate; while the sheet is fabricated Copyright: American Scientific Publishers the synthesis time (typically 7 min for 250 m ofDelivered height). by byIngenta pulling the CNTs without twisting. Spinnability was obtained only if the height of VACNF was in the range of 200 to 400 m in the performed experiment. Figures 1(a) and (b) show photographs of the 3. RESULTS AND DISCUSSION fabricated typical CNT yarn and sheet. Scanning and trans- Figures 2(a) and (b) show the SEM images of the VACNF mission electron microscope (SEM, HITACHI S-3000H under spinning from the substrate. The edge of the VACNF and TEM, JEOL JEM-z2500) were used to analysis the was pulled horizontally to fabricate the CNT yarn and morphology of the VACNF and each individual CNTs. The

Communication sheet. The pulled CNTs bundles were connected to the CNT yarn and sheet were fabricated from the spinnable adjoining CNTs due to which the continuous spinning was the VACNF with a width of 6 mm. The diameter of the possible. Figure 2(a) shows that the CNTs were pulled CNT yarn was measured as 13.5 m fabricated by pulling from the spinnable VACNF with a height of 250 m. at a speed of 10 mm/min and twisting at the 250 revolu- Figure 2(b) is a magniﬁed SEM image at the top of tions/min. In this condition, the mass density of the CNT the spinnable VACNF. The image clearly shows that the 3 yarn was 420 mg/cm . The CNT yarn and sheet with a pulled CNTs are connected to adjoining CNTs at the length of about 2 cm were ﬁxed on the glass substrates by spinnable VACNF (arrows inset). The CNTs are connected silver paste for the current–voltage (I–V ) measurement. at their top or root part by Van der Waals force. To obtain Ozone irradiation was carried out with a commer- spinnable VACNFs, the condition of the bundled CNTs cial UV/ozone cleaner system (Filgen UV253E). The gas is an extremely important aspect; several thick bundles of inside the system was exchanged for oxygen (99.99%) CNTs are consisting of the thin CNTs bundle.23 The thin before irradiating ozone. The electrical resistance of the bundles of CNTs connect the pulled CNTs and the adjoin- ozone irradiated CNT yarn and sheet was measured from ing CNTs under spinning. TEM study shows that the diam- the I–V characteristic. Again, the temperature dependent eter and number of graphene layers of the CNTs in the resistance was measured to study the effect of the adsorbed spinnable VACNF are of 4–7 nm and 2–3 layers, respec- ozone molecules with the ozone irradiation on the resis- tively. TEM study also shows that the sample consists of tance by heating the samples on a hotplate up to 470 K and more than 60% of double wall CNTs. Figure 2(c) shows a then cooling to room temperature. Raman spectroscopy typical TEM image of an individual double wall CNTs. (JASCO NRS-1500 W) was used to evaluate the quality First, we compared the effect of ozone irradiation on of ozone irradiated CNTs. the resistance change of the CNT yarn and sheet. It has

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hole injection but also the induction of defects at the same time. Therefore, the resistance of ozone irradiated CNTs initially decreases because the hole injection is dominant rather than the induction of defects. However, when the time length of ozone irradiation is prolonged, the effect of induced defects becomes dominant, and resistance begins to increase. The relation between the ozone irradiation time and change in resistance of the CNT yarn and sheet were plotted in Figure 3(a). The changes in resistance for both the cases were normalized with resistance of pristine R/R CNTs, as 0. In case of the CNT sheet, the resistance dropped to 85%, which was the minimum, for 5 min of ozone irradiation as shown in the inset of Figure 3(a). Then, the resistance of the CNT sheet returned to the initial value and remained constant, an unusual behavior for the CNT sheet which did not appear for an individual CNT.29 The induction of large defects on the outermost graphitic walls of individual CNTs by ozone irradiation increases the contact resistance of each CNTs. Therefore, the resistance of CNT sheet also increased up to the initial value Communication in this experiment. Again, even if the effect of defects became dominant at a site, the hole injection surpassed the effect of defects to neutralize the change in resistance and as a result the resistance remains constant. The resistance of the CNT sheet increased drastically after the CNTs IP: 192.168.39.151 On: Sat,sample 02 Oct was 2021 exposed 18:07:47 to ozone irradiation for 60 min. In Copyright: American Scientificcomparison, Publishers the resistance of the CNT yarn continued to Delivered bydecrease Ingenta until 50 min, which survived increase in resistance for ten time longer duration than that of the CNT sheet. The drop of resitance with the ozone irradiation was obtained as 83% that of the pristine CNTs. In case of the CNT yarn, we consider that the injected holes diffuse to the inside of the CNT yarn, which is not oxidized. As the hole injection was predominance over the effect of defects which were induced at the surface of CNT yarn only, the resistance continued to decrease. The CNTs at the surface of the CNT yarn contributed to a slight increase of resistance after reaching the minimum. The surface of CNT yarn protected the CNTs inside the CNT yarn from degradation by the oxidation. Meanwhile, we could observe all CNTs were degraded in the CNT sheet because all were Fig. 2. (a) SEM image of spinning from VACNF. (b) Magniﬁed SEM exposed to ozone irradiation. image at the top part of VACNF under spinning. The pulled CNTs are Figure 3(b) shows the temperature dependent resistance connected to the adjoining CNTs at the spinnable VACNF as shown of the pristine CNT yarn and ozone irradiated (20 min) by the arrow marks. (c) Typical TEM image of individual CNT which CNT yarn and sheet. It can be observed that for the pris- composes spinnable VACNF. More than 60% of the observed CNTs were double walled CNTs. tine CNTs yarn and sheet, the change in resistance during heating and cooling traced each other without any significant change (black dashed line). However, the resistance been reported that the ozone irradiation decreases the increased for the ozone irradiated yarn and sheets dur- resistance of CNTs, which is a simple approach for the ing heating cycle having a nonlinear behavior due to des- CNTs oxidation.27–29 The CNTs have p-type conductiv- orption of the ozone molecules (red solid circle for yarn ity and gain holes due to charge transfer between CNTs and blue solid triangle for sheet). Again, the temperature and the adsorbed ozone molecules with the ozone irradi- dependent resistance during cooling of the ozone irradiation, thereby reducing the resistance of CNTs. However, ated samples shows similar characteristic as that of pris- the ozone irradiation on the CNTs causes not only the tine CNTs yarn and sheet with a higher resistance. This is

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due to the fact that the ozone molecules get release from the CNTs surface during the heating cycle and only the induced defects increase overall resistance of the yarn and sheet. Therefore, the increase in resistance during heating can be determined from desorption of ozone molecules. The Arrhenius equation as following was used to derive the activation energy of desorption; −Ea R = A exp T (1) kB

where, A, Ea, and kB are the Arrhenius prefactor, activation energy, and Boltzmann constant, respectively. The calculated activation energy of desorption from the CNT yarn and sheet at heating were found as 37 and 30 meV, respectively (solid lines). Here, if the ozone molecules were chemisorbed to the CNTs, binding energy was larger than 190 meV.30 Therefore, the ozone molecules which contributed to decrease in the resistance were found to be physisorbed to the CNTs in this experiment. The slightly larger value for the CNT yarn was due to the difference of the adsorption sites. There are four possible adsorption sites in a CNT bundle31–32 as shown in Figure 3(c); (1) surface; (2) pore; (3) groove; (4) interstitial, and adsorption energy at each sites becomes large in the order.32 The dense CNT yarn has a larger number of interstitial sites than that of the CNT sheet, leading to larger activation IP: 192.168.39.151 On: Sat, 02 Oct 2021 18:07:47 Copyright: American Scientificenergy. ThePublishers gradients of the resistances of both samples Delivered byduring Ingenta cooling (red and blue dashed lines) showed the same values as the previous report.13 Moreover, the ultimate changes in resistance of the CNT yarn and sheet from the pristine were 1.25 and 1.39, respectively. Defects were induced to all the CNTs of a CNT sheet, whereas defects were induced to the ∼64% CNTs of the CNT yarn as evaluated from the ultimate changes in resistance. Raman studies were performed to evaluate the morpho- Communication logical change of the CNTs with induced defects due to the ozone irradiation. Figure 4(a) shows normalized Raman spectra of the pristine and ozone irradiated CNTs yarn and sheet. The CNT yarn and sheet were fabricated from the same VACNF, hence the Raman spectra of pristine CNTs were similar for the cases. The intensity ratio between the graphite-like G-band at 1590 cm−1 and the disorder- −1 induced D-band at 1350 cm (IG/ID ratio) was evaluated, which gives the information about the quality of CNTs samples. The IG/ID ratio for the pristine, the ozone irradiated yarn and sheet of CNTs were measured to be 0.953, 0.833, and 0.759, respectively. The degradation degree in Raman spectrum of the CNT yarn was perhaps due to sig- Fig. 3. (a) The relations between ozone irradiation time and change nals from CNTs at surface and inside of CNT yarn. We in resistance. The inset indicates the change in resistance of the CNT attempted to estimate the ratio of defect induced CNTs in sheet for short ozone irradiation time. (b) Arrhenius plot for the yarn I /I (circle for cooling; solid circles for heating) and sheet (triangle for the CNT yarn. If the G D ratio of the CNT yarn varies cooling; solid triangle for heating) in comparison with pristine CNTs. linearly from the value of the pristine to that of CNT sheet (c) A schematic illustration of the adsorption sites in CNT bundles; depending on the fraction of defects induced in the CNT (1) surface; (2) pore; (3) groove; (4) interstitial. yarn, a following equation is considered; − xZ + xZ = Z 1 pristine sheet yarn (2)

360 Mater. Express, Vol. 2, 2012 Structural and Electrical Properties of Ozone Irradiated CNT Yarns and Sheets Materials Express Iijima et al.

the TEM study, it was observed that the induced defects appeared in the side walls of the CNTs (arrow inset) cor- relating the Raman studies of the CNTs yarn and sheets.

4. CONCLUSION Electrical conductivity of the CNT yarn and sheet fabricated from the spinnable VACNF was studied with ozone irradiation. It was observed that with a controlled ozone irradiation, the resistance of the CNT yarn and sheet decreased with hole injection to the CNTs. The resistance of the CNT yarn reduces to 83% for a 50 min ozone irradiation, while for the sheet the resistance reduces to 85% for a 5 min ozone irradiation. After reaching to the minimum, the resistance increased with the longer duration of ozone irradiation due to structural deformation of the CNTs. The temperature dependence resistance studies showed that the resistance of the ozone irradiated CNT yarn and sheet increased with heating, corresponding to

desorption of ozone molecules. The activation energy of Communication the CNT yarn at heating was slightly larger than that of the CNT sheet, this may be due to the difference of adsorption sites. Raman studies revealed that the defects are induced only in the surface of a CNT yarn, whereas all the CNTs were degraded for a CNTs sheet almost. Our ﬁnding shows that the electrical resistance of CNTs yarn and sheets can IP: 192.168.39.151 On: Sat,be 02 reduced Oct 2021 with 18:07:47 controlled ozone irradiation depending on Copyright: American Scientific Publishers Delivered bythe Ingenta induced defects and degradation. Acknowledgment: This research was partially sup- ported by AIXTRON K.K. in Japan and AIXTRON Ltd. in UK.

References and Notes

1. S. Iijima; Helical microtubules of graphitic carbon; Nature 354, 56 (1991). 2. S. Iijima, M. Yudasaka, R. Yamada, S. Bandow, K. Suenaga, F. Kokai, and K. Takahashi; Nano-aggregates of single-walled Fig. 4. (a) Raman spectra of pristine, 20 min ozone irradiated CNT graphitic carbon nano-horns; Chem. Phys. Lett. 309, 165 (1999). yarn and sheet. (b) A TEM image of 20 min ozone irradiated CNT sheet. 3. H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl, and R. E. The arrow indicates a destruction of side walls of the double walled Smalley; C : Buckminsterfullerene; Nature 318, 162 (1985). CNT at a surface of bundled CNTs. 60 4. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov; Electric field where, x and Z are the fraction of defects induced in effect in atomically thin carbon films; Science 306, 666 (2004). I /I 5. T. W. Ebbesen, H. J. Lezec, H. Hiura, J. W. Bennett, H. F. Ghaemi, the CNT yarn and G D ratio, respectively. Based on the and T. Thio; Electrical conductivity of individual carbon nanotubes; equation, we found that ∼62% of CNTs in the CNT yarn Nature 382, 54 (1996). were induced defects by the ozone irradiation. This eval- 6. G. Gao, T. Çagin,˘ and W. A. Goddard; Energetics, structure, mechan- uated ratio of defects induced CNTs in the CNT yarn ical and vibrational properties od single-walled carbon nanotubes; conformed closely to the result of Arrhenius plots. The Nanotechnology 9, 184 (1998). 7. W. A. de Heer, A. Châtelain, and D. Ugarte; A carbon nanotube calculated depth of ozone-irradiated CNT yarn for 20 min field-emission electron source; Science 270, 1179 (1995). was 2.63 m. The Raman studies show that that the 8. M. Akoshima, K. Hata, D. N. Futaba, K. Mizuno, T. Baba, and D band intensity increases (IG/ID ratio decreases) for the M. Yumura; Thermal diffusivity of single-walled carbon nano- ozone irradiated CNTs yarn and sheet, as the highly active tube forest measured by laser flash method; Jpn. J. Appl. Phys. 48, 05EC07 (2009). ozone molecule can chemically react with the CNTs sur- 9. J. Kong, N. R. Franklin, C. Zhou, M. G. Chapline, S. Peng, K. Cho, face. Figure 4(b) shows a TEM image at the edge of a and H. Dai; Nanotube molecular wires as chemical sensors; Science fabricated CNT sheet after the ozone irradiation. From 287, 622 (2000).

Mater. Express, Vol. 2, 2012 361 Materials Express Structural and Electrical Properties of Ozone Irradiated CNT Yarns and Sheets Iijima et al.

10. M. H. Yang, K. B. K. Teo, W. I. Milne, and D. G. Hasko; Carbon 22. T. Iijima, H. Oshima, Y. Hayashi, U. B. Suryavanshi, A. Hayashi, nanotube Schottkey diode and directionally dependent field-effect and M. Tanemura; Morphology control of a rapidly grown verti- transistor using asymmetrical contacts; Appl. Phys. Lett. 87, 253116 cally aligned carbon-nanotube forest for fiber spinning; Phys. Status (2005). Solidi A 208, 2332 (2011). 11. S. Akita, H. Nishijima, Y. Nakayama, F. Tokumasu, and K. Takeyasu; 23. T. Iijima, H. Oshima, Y. Hayashi, U. B. Suryavanshi, A. Hayashi, Carbon nanotube tips for a scanning probe microscope: Their fabri- and M. Tanemura; In-situ observation of carbon nanotube fiber spin- cation and properties; J. Phys. D: Appl. Phys. 32, 1044 (1999). ning from vertically aligned carbon nanotube forest; Diam. Relat. 12. M. Zhang, K. R. Atkinson, and R. H. Baughman; Multifunctional Mater. 24, 158 (2012). carbon nanotube yarns by downsizing an ancient technology; Science 24. Y. Inoue, Y. Suzuki, Y. Minami, J. Muramatsu, Y. Shimamura, 306, 1358 (2004). K. Suzuki, A. Ghemes, M. Okada, S. Sakakibara, H. Mimura, and 13. M. Zhang, S. Fang, A. A. Zakhidov, S. B. Lee, A. E. Aliev, C. D. K. Naito; Anisotropic carbon nanotube papers fabricated from mul- Williams, K. R. Atkinson, and R. H. Baughman; Strong, transparent, tiwalled carbon nanotube webs; Carbon 49, 2437 (2011). multifunctional carbon nanotube sheets; Science 309, 1215 (2005). 25. C. L. Kane, E. J. Mele, R. S. Lee, J. E. Fischer, P. Petit, H. Dai, 14. X. Zhang, K. Jiang, C. Feng, P. Liu, L. Zhang, J. Kong, T. Zhang, A. Thess, R. E. Smalley, A. R. M. Verschueren, S. J. Tans, and Q. Li, and S. Fan; Spinning and processing continuous yarns from C. Dekker; Temperature-dependent resistivity of single-wall carbon 4-inch wafer scale super-aligned carbon nanotube arrays; Adv. Mater. nanotubes; Europhys. Lett. 41, 683 (1998). 18, 1505 (2006). 26. P. L. McEuen and J. Y. Park; Electron transport in single-walled 15. X. Zhang, Qi. Li, Y. Tu, Y. Li, J. Y. Coulter, L. Zheng, Y. Zhao, carbon nanotubes; MRS. Bull. 29, 272 (2004). Q. Jia, D. E. Peterson, and Y. Zhu; Strong carbon-nanotube fibers 27. S. Picozzi, S. Santucci, L. Lozzi, C. Cantalini, C. Baratto, spun from long carbon-nanotube arrays; Small 3, 244 (2007). G. Sberveglieri, I. Armentano, J. M. Kenny, L. Valentini, and 16. B. Vigolo, A. Pénicaud, C. Coulon, C. Sauder, R. Pailler, C. Journet, B. Delley; Ozone adsorption on carbon nanotubes: Ab initio calcu- P. Bernier, and P. Poulin; Macroscopic fibers and ribbons of oriented lations and experiments; J. Vac. Sci. Technol. A 22, 1466 (2004). carbon nanotubes; Science 290, 1331 (2000). 28. W. Wongwiriyapan, S. Honda, H. Konishi, T. Mizuta, T. Ikuno, 17. S. Kumar, T. D. Dang, F. E. Arnold, A. R. Bhattacharyya, B. G. T. Ohmori, T. Ito, R. Shimazaki, T. Maekawa, K. Suzuki, Min, X. Zhang, R. A. Vaia, C. Park, W. W. Adams, R. H. Hauge, H. Ishikawa, K. Oura, and M. Katayama; Ultrasensitive ozone detec- R. E. Smalley, S. Ramesh, and P. A. Willis; Synthesis, structure, tion using single-walled carbon nanotube networks; Jpn. J. Appl. and properties of PBO/SWNT composites; Macromelecules 35, 9039 Phys. 45, 3669 (2006). (2002). 29. S. Agrawal, M. S. Raghuveer, H. Li, and G. Ramanath; Defect- 18. D. Qian, E. C. Dickey, R. Andrews, and T. Rantell; Load transfer induced electrical conductivity increase in individual multiwalled and deformation mechanisms in carbon nanotube-polystyrene com- carbon nanotubes; Appl. Phys. Lett. 90, 193104 (2007). posites; Appl. Phys. Lett. 76, 2868 (2000). 30. B. Akdim, T. Kar, X. Duan, and R. Pachter; Density functional the- 19. B. C. Edwards; Design and deploymentIP: 192.168.39.151 of a space elevator; On:Acta Sat, 02ory Oct calculations 2021 18:07:47 of ozone adsorption on sidewall single-wall carbon Astronaut. 47, 735 (2000). Copyright: American Scientificnanotubes Publishers with Stone-Wales defects; Chem. Phys. Lett. 445, 281 20. Y. Zhao, J. Wei, R. Vajtai, P. M. Ajayan, and E. V. Barrera;Delivered Iodine by Ingenta(2007). doped carbon nanotube cables exceeding specific electrical conduc- 31. X. Ma, M. Tsige, S. Uddin, and S. Talapatra; Application of carbon tivity of metals; Sci. Rep. 1, 83 (2011). nanotubes for removing organic contaminants from water; Mater. 21. L. Xiao, Z. Chen, C. Feng, L. Liu, Z. Bai, Y. Wang, L. Qian, Express 1, 183 (2011). Y. Zhang, Q. Li, K. Jiang, and S. Fan; Flexible, stretchable, trans- 32. J. Zhao, A. Buldum, J. Han, and J. P. Lu; Gas molecule adsorption parent carbon nanotube thin film loudspeakers; Nano Lett. 8, 4539 in carbon nanotubes and nanotube bundles; Nanotechnology 13, 195 (2008). (2002).

Communication Received: 4 September 2012. Revised/Accepted: 30 October 2012.

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