Separation of Metallic and Semiconducting Single-Wall Carbon

Oct 15, 2011 - Single-wall carbon nanotubes (SWCNTs) have attracted much ... Furthermore, because this method does not use ionic substances (e.g., Na+...
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Separation of Metallic and Semiconducting Single-Wall Carbon Nanotube Solution by Vertical Electric Field Kazuki Ihara,*,†,‡,§ Hiroyuki Endoh,‡ Takeshi Saito,† and Fumiyuki Nihey*,†,‡,§ †

Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Central, 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ‡ Green Innovation Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan § Technology Research Association for Single Wall Carbon Nanotubes (TASC), c/o National Institute of Advanced Industrial Science and Technology, Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan

bS Supporting Information ABSTRACT: A novel method to separate metallic and semiconducting single-wall carbon nanotubes (SWCNTs) by free solution electrophoresis was investigated. A vertical electrophoretic cell was used to separate SWCNT aqueous solution by the application of electric field along the vertical cell. In contrast with conventional electrophoresis, electric-field-induced layer formation was found in the cell with clear horizontal boundaries. Both the optical absorbance spectra and resonant Raman spectra of these layers revealed the enrichment of metallic and semiconducting SWCNTs with 95% semiconducting SWCNTs in a layer. This method was successfully applied for SWCNTs with diameters from 1.0 to 1.7 nm. Thin-film transistors fabricated by the direct deposition of the semiconductorenriched nanotubes onto the substrate showed excellent transport properties with an on/off current ratio of 4.4  105 and a mobility of 6.8 cm2 V 1 s 1.

I. INTRODUCTION Single-wall carbon nanotubes (SWCNTs) have attracted much attention for their peculiar mechanical and electrical properties1 and for possible applications such as flexible, printable thin-film transistors (TFTs).2 However, because their electronic properties are determined to be either semiconducting or metallic by their diameters and chiralities,3,4 and as the currently employed synthesis methods generally produce a mixture of metallic and semiconducting SWCNTs, postsynthesis methods to separate metallic and semiconducting nanotubes have been intensively studied. To date, several methods5 such as amine extraction,6 dielectrophoresis,7 selective oxidation,8 density-gradient ultracentrifugation,9 DNA separation,10 gel-based extraction,11 and free solution electrophoresis12,13 have been reported. Free solution electrophoresis12,13 is a simple and scalable method in which the difference in the electrophoretic mobilities of metallic and semiconducting SWCNTs in aqueous solution is utilized. Kim et al.12 developed covalent sidewall chemistry for SWCNTs and employed free solution electrophoresis for separation. Following covalent functionalization with p-hydroxybenzene diazonium salt, a negative charge is induced on the metallic SWCNTs, thus enabling subsequent separation by using free solution electrophoresis. Wakizaka et al.13 reported electrophoretic separation of SWCNTs dispersed with a nonionic surfactant. They dispersed raw materials of SWCNTs into deuterium oxide (D2O) with r 2011 American Chemical Society

polyoxyethylene stearyl ether and resorted free solution electrophoresis without sidewall treatment. With this method, metallic and semiconducting SWCNTs were successfully enriched without covalent functionalization. They ascribed this separation to the differences in the inherent zeta potentials of semiconducting and metallic nanotubes. With this method, elimination of sidewall functionalization is advantageous to preserve the material quality. Furthermore, because this method does not use ionic substances (e.g., Na+ ions) that degrade the performance of electronic devices14 but uses only a single kind of nonionic surfactant, it does not require tedious processes to remove unnecessary substances after the separation. However, because this method could not produce high-purity semiconducting SWCNTs, improvements in this method have been anticipated. In this Article, we propose a distinct metal-semiconductor separation by using modified free solution electrophoresis for SWCNT solution dispersed with a nonionic surfactant. Handmade vertical electrophoretic cells were used to apply vertical electric field to the SWCNT solution. After several hours of voltage application, distinct layers were formed in the cell with clear horizontal boundaries. This phenomenon, which we call Received: July 27, 2011 Revised: October 14, 2011 Published: October 15, 2011 22827

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Figure 1. (a) Schematic of a vertical electrophoretic cell. (b) Optical images of SWCNT solution in a vertical cell before and 4, 16, and 24 h after the voltage application (30 V). The diameter of nanotubes is 1.0 nm. (c,d) Solution of SWCNTs with nanotube diameters of (c) 1.4 and (d) 1.7 nm 16 h after the voltage application.

electric-field-induced layer formation (ELF), contrasts with conventional horizontal electrophoresis. Optical absorbance spectra and resonant Raman spectra revealed that semiconducting and metallic SWCNTs were enriched in the layers with a purity of 95% for the semiconductor-enriched ones. This method was successfully applied for SWCNTs with nanotube diameters from 1.0 to 1.7 nm. TFTs were successfully fabricated by the direct deposition of the enriched nanotube solutions onto the substrates. Devices with semiconductor-enriched nanotubes showed excellent transport properties with an on/off current ratio of 4.4  105 and a mobility of 6.8 cm2 V 1 s 1, possibly due to the elimination of pathways by metallic nanotubes.

II. EXPERIMENTAL SETUP SWCNTs used in this study were synthesized by an enhanced direct-injection pyrolytic synthesis (eDIPS) method.15 Three kinds of SWCNTs with different average diameters (d), 1.0, 1.4, and 1.7 nm, were prepared. Each SWCNT sample was mixed into deuterium oxide (D2O) solution of 1 wt % polyoxyethylene (100) stearyl ether (Brij 700, Sigma Aldrich).16 The amount of SWCNTs was adjusted to be 1 mg/mL in the solution. The mixture was sonicated by using a horn-type ultrasonic homogenizer (Sonifier 450D, Branson) for 1 h (50% of duty cycle) with a power of 300 W to disperse homogeneously. The solution was then ultracentrifuged at 250 000g using a fixed angle rotor (S58A, Hitachi Koki) for 1 h to precipitate bundles and impurities. After the ultracentrifugation, the upper 50% of the supernatant was collected as a monodispersed SWCNT solution. The supernatant was mixed with the D2O solution of 1.5 wt % Brij 700 so that its optical absorbance corresponding to the second interband transition of semiconducting SWCNT (denoted as S22) as well as that corresponding to the first interband transition of metallic SWCNT (M11) were below unity. Figure 1a shows the schematic of one of our vertical electrophoretic cells used in this study. The cells were made from polyvinyl chloride or acrylic tubes. Looped-platinum wires were attached at the top and bottom side of the cell as a cathode and anode, respectively. Following the introduction of monodispersed SWCNT solution into the cell, a constant voltage of 30 V was applied by using a voltage source (Cross Power 500, ATTO) between the anode and cathode for >16 h. During the voltage application, the optical images of the cell were taken to record the electrophoretic behaviors of the constituents. After the voltage application, the SWCNT solution was fractionated from the top of the cell by a pipet. Additionally, a standard electrophoretic cell, in which the electric field is applied in the

horizontal direction, was used to observe the electrophoretic behaviors of separated metallic and semiconducting SWCNT solutions. Optical absorbance spectra of the fractionated samples were measured by a UV vis NIR spectrophotometer (UV3150, Shimadzu) in the range of 310 1600 nm. Resonant Raman spectra were measured by a Raman spectrometer (HR-800, Horiba Jobin Yvon) in the range of 150 350 and 1400 1700 cm 1. An Ar ion laser (514 nm) and a He Ne laser (633 nm) were used as excitation sources for the resonant Raman experiments. To demonstrate the advantages of this semiconductor-metal separation, TFTs were fabricated with semiconductor-enriched (referred as S-TFTs) and metal-enriched (M-TFTs) SWCNTs and compared with reference devices (R-TFTs) fabricated from the nanotube solution before separation. All devices were fabricated on heavily doped silicon substrates (Sb doped, 0.02 Ω 3 cm) covered by a 100-nm-thick SiO2 layer as a gate dielectric. The surface of the substrate was cleaned by oxygen plasma and functionalized by aminopropyltriethoxysilane (APTES, Sigma Aldrich) prior to nanotube deposition. To create SWCNT random networks, the functionalized substrates were soaked by the SWCNT solution for 10 120 min, followed by rinsing with deionized water and drying with nitrogen gas. This deposition process was repeated as needed. The density of the network was controlled by the period of deposition and the number of repetition. After the deposition process, substrates were annealed at 200 °C for 30 min in air. Source and drain electrodes were defined by Au evaporation through a metal mask. The channel width and length were 500 and 200 μm, respectively. Morphology of the nanotube random network was measured by an atomic force microscope (AFM, Nanoscope IV, Veeco). Electrical measurements for TFTs were carried out by using a semiconductor parametric analyzer (4156C, Agilent Technologies) at room temperature in dry air.

III. RESULTS AND DISCUSSION The electrophoretic behavior of SWCNT solution (d = 1.0 nm) during the voltage application (30 V) is shown in Figure 1b. The inner diameter and the electrode distance of the cell were 20 and 50 mm, respectively. Before voltage application, the SWCNT solution was homogeneously dispersed in the cell. By applying voltage, electric current of ∼0.1 mA flowed through the cell. After 4 h of voltage application, a couple of layers with slightly different colors appeared with a horizontal boundary in-between. These colors gradually became clearer 16 h later, with the upper 22828

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Figure 2. Absorbance spectra for fractions of SWCNTs with diameters of (a) 1.0, (b) 1.4, and (c) 1.7 nm. Blue and red curves correspond to the fractions from the lower and upper layers, respectively. Green curves correspond to the solution before separation. Peaks in the light blue and pink regions originate from the absorption by the second interband transition (S22) for semiconducting SWCNTs and the first interband transition (M11) for metallic SWCNTs, respectively.

and lower layers being dark brown and blue, respectively. Between the upper and lower layers, a thin transparent intermediate layer was observed. After 24 h of voltage application, the intermediate layer became thicker and divided the upper and lower layers. The MPEG file made from sequential optical images is provided as Supporting Information. Separations were also observed for SWCNT solutions with d = 1.4 and 1.7 nm. As shown in Figure 1c,d, by using smaller cells (12 mm in inner-diameter, 50 mm in electrode distance), these solutions were successfully separated after 16 h of voltage application. It was found that the separation proceeded faster for smaller electrophoretic cells. Figure 2a shows the optical absorbance spectra of the upper layer and lower layer of SWCNTs with a diameter of 1.0 nm, together with those for nanotube solution before separation. The absorbance peaks in the range of 550 900 (indicated by the light-blue region) and 400 550 nm (pink region) originate from the second interband transition of semiconducting SWCNTs (S22) and the first interband transitions of metallic SWCNTs (M11) with d = 1.0 nm,17 respectively. For the upper layer, the intensities of peaks in the M11 region increased, and those in the S22 region decreased as compared with the spectra for the

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solution before separation, suggesting the enrichment of metallic SWCNTs in the upper layer. For the lower layer, the opposite tendency was observed for both of M 11 and S22 regions, suggesting the enrichment of semiconducting SWCNTs in the lower layer. The purities of separated layers were estimated from the intensities of the specific peaks in the S22 and M11 regions. Corresponding peaks were fitted with Lorentzian curves on a linear background, and the ratio of the summated intensity of metallic peaks to that of semiconducting ones was obtained.17,18 To estimate the purities, we assumed that the solution before separation contains 67% of semiconducting and 33% of metallic nanotubes. From this assumption, it was revealed that the upper layer contains 70% of metallic SWCNTs and the lower layer contains 95% of semiconducting ones. It should be noted that the purity of 95% for semiconductor-enriched SWCNTs is competitive with those obtained by density gradient ultracentrifugation, DNA separation, or gel-based extraction.9 11 Such a high purity of semiconducting SWCNTs is promising for applications such as flexible, printable TFTs. This method was revealed to be effective for nanotubes with diameters of 1.4 and 1.7 nm. Figure 2b,c shows the absorbance spectra of these samples, in which the S22 and M11 regions are also indicated by colored areas. For each diameter, the increase (decrease) in absorbance in the M11 regions and the decrease (increase) in the S22 region for the upper (lower) layer were similarly observed. From the relative change in the absorbance for the S22 and M11 regions, the ratios of semiconducting SWCNTs in lower fractions were estimated to be >80% for both d = 1.4 and 1.7 nm. These results indicate that this method can be successfully applied to SWCNTs with diameters in the range from 1.0 to 1.7 nm. Enrichments are also confirmed by resonant Raman measurements. Excitation sources at 633 and 514 nm were used because these preferentially excite semiconducting and metallic SWCNTs with d = 1.0 nm, respectively.17 Raman spectra in the range of 150 350 cm 1 with the excitation at 633 nm are shown in the left-hand side of Figure 3a, where peaks corresponding to radial breathing mode (RBM) were observed. For d = 1.0 nm, RBM peaks in the range of 160 230 cm 1 (indicated by the pink region) and those in 240 320 cm 1 (light blue region) are assigned to metallic and semiconducting SWCNTs, respectively.17 The increase (decrease) in RBM intensities at 197 and 222 cm 1 indicates the increase (decrease) in metallic content in the upper (lower) layer. Raman spectra with the excitation at 514 nm are shown in the left-hand side of Figure 3b, where RBM peaks in the range of 160 230 cm 1 and those in 240 300 cm 1 are assigned to semiconducting and metallic SWCNTs, respectively. Changes in the intensity of metallic peaks are also consistent with former results. The purities of semiconducting SWCNTs estimated from the intensities of these RBM peaks for 633 and 514 nm spectra were 95 and 91%, respectively, consistent with the results from the optical absorbance measurements (95%). Raman spectra in the range of 1400 1700 cm 1 also show the changes in the metallic content. In the right-hand side of Figure 3b, a large peak was observed at 1590 cm 1, which corresponds to the tangential mode of graphitic planes (G-band). It is well known that G-band peak for metallic SWCNTs accompanies an asymmetric and broad peak around 1530 1560 cm 1, called Breit Wigner Fano (BWF) line shape.19 In this Figure, decrease in the BWF line shape for lower fraction is apparent, 22829

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Figure 3. Resonant Raman spectra for SWCNTs (1.0 nm in diameter) with excitations at (a) 633 and (b) 514 nm. For each, spectra in the radial-breathing-mode (RBM) region and those in the G-band region are shown in the left-hand side and right-hand side of each figure, respectively. Blue and red curves correspond to the fractions from the lower and upper layers, respectively. Green curves correspond to the solution before separation. In the light blue (S) and pink (M) regions, RBM peaks for semiconducting and metallic SWCNTs should be observed, respectively.

suggesting the decrease in metallic content in consistency with the absorbance and RBM results. The mechanism of the separation reported here is still unclear at present. Electrophoretic behavior in this study is unusual, as shown in Figure 1b, where ELF in the solution was clearly observed. As described, after a few hours of voltage application, a couple of layers appeared with a horizontal boundary. Their colors gradually became clearer with the appearance of a transparent intermediate layer between the upper and lower layers. Finally, the upper and lower layers were spatially separated by the intermediate layer. This behavior is completely different from conventional electrophoresis in which gradual (not abrupt) changes in the densities of constituents should be observed. Because this separation of metallic and semiconducting SWCNTs is induced by the application of electric field, the zeta potentials of semiconducting and metallic SWCNT micelles seem to account for the separation mechanism. From our observation, the zeta potential of metallic SWCNT micelles should be positive, whereas that of semiconducting ones should be negative. To verify our intuition, the electrophoretic behavior of the metal- and semiconductor-enriched fractions was observed by using a standard horizontal electrophoretic cell (50 mm in electrode distance). Metal-enriched nanotube solution was introduced into the cell, as shown in Figure 4a, and a constant voltage (30 V) was applied. Nanotubes accumulated to the anode 16 h after the voltage application, as shown in Figure 4b. Similar experiments were also conducted for semiconductor-enriched solution, and it was found that they also accumulated to the same electrode (anode), as shown in Figure 4c. These results

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Figure 4. (a,b) Metal-rich SWCNT solution in a horizontal electrophoretic cell (top view) (a) before and (b) 16 h after the voltage application (30 V). Marks of minus ( ) and plus (+) indicate the position of the cathode and anode, respectively. (c) Semiconductorenriched SWCNT solution 16 h after the voltage application (30 V). (d) SWCNT solution in a tilted electrophoretic cell after the voltage application. (e) Solution in a large-diameter cell after the voltage application. The cathode and anode are diagonally located at the upper-left and lower-right corner of the cell, respectively.

surprisingly contradicted our intuition and indicated that the zeta potentials of metallic as well as semiconducting nanotubes are negative, suggesting that the separation mechanism cannot be explained simply by the electrophoretic mobilities of semiconducting and metallic SWCNTs. We suggest that the changes in specific gravities of metallic and semiconducting solutions by electric field play an important role. Figure 4d shows the separation of SWCNT solution by using a tilted vertical cell. Interestingly, the boundaries of the layers remained horizontal, although the direction of electric field should be accordingly tilted. Figure 4e shows the separation of SWCNT solution by using a vertical cell with a large cell diameter (40 mm). The cathode and anode are diagonally attached at the upper-left and lower-right corner of the cell. Irrespective of the direction of the electric field, separation occurred with the boundaries of layers keeping horizontal. These observations suggest that layers with different specific gravities were constructed by the application of electric field, where the specific gravity of the metal-enriched layer is smaller than that of semiconductor-enriched layer. We found that the specific gravity gradient also occurred for Brij 700 solution without nanotubes. We performed the electrophoresis of 1 wt % Brij 700/D2O solution without nanotubes and measured the specific gravities of fractions corrected from the cell after the voltage application. As a result, we found the variation of the specific gravity along the cell. (For details of the experiments, 22830

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Figure 5. (a) Tapping-mode atomic force microscopy (AFM) image of the random network of semiconductor-rich SWCNTs on a SiO2/Si substrate. (b) Drain current ID as a function of gate voltage VGS with a drain voltage of VDS = 1 V for semiconductor-enriched (S-TFT, blue) and metal-enriched (M-TFT, red) thin-film transistors. Green curve corresponds to the device (R-TFT) made from the solution before separation. (c) Plots of on/off current ratios (IDmax/IDmin) versus mobilities (μ) obtained from 80 devices in total. The definitions of colors are the same as panel b. Thin curves are drawn to guide the eye.

see Supporting Information.) We estimate that the specific density gradient was caused by the accumulation of Brij 700 micelles (which have small but negative zeta potential, see Supporting Information) as a result of their electrophoretic behavior. As for the electrophoresis of nanotubes, it is still unclear why the metallic (semiconducting) nanotubes tend to move to upper (lower). Further experiments should be required to understand fully the mechanism behind the separation. As shown in Figure 1a, this method utilizes a simple vertical cell with just a pair of electrodes. We expect that the prolonged duration (16 h or more) of voltage application can be shortened by optimizing experimental conditions (e.g., adequate increase in the strength of electric field and/or improving the cell structure to suppress turbulent flow). The energy consumption during the separation process is as low as a few milliwatts, much lower than that consumed during ultracentrifugation (several hundred watts). The cell can be scaled to larger volume as shown in Figure 4e, from which several tenths of a milligram of semiconductor-rich nanotubes can be obtained. The present method provides semiconductorrich nanotubes with a purity of 95% for d = 1.0 nm, which is

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competitive with those obtained by density-gradient ultracentrifugation, DNA separation, or gel-based extraction. The separation efficiency is limited by semiconducting nanotubes that remain in the metal-rich counterpart. We estimate that >80% of semiconducting nanotubes were collected in semiconductor-rich layer. Both the purity and separation efficiency can be improved further by repeated application of electrophoretic processes to the separated samples. We also performed similar experiments with other nonionic surfactants (e.g., polyoxyethylene (23) lauryl ether (Brij 35, Sigma Aldrich)) in the place of Brij 700 and obtained successful results. Because simple, scalable, and efficient characteristics of this method are well competitive with other methods such as density-gradient ultracentrifugation, we believe it will offer an opportunity for practical use. Finally, to demonstrate the advantage of this method, TFTs were fabricated by the direct deposition of the separated SWCNT solution onto the substrate. Figure 5a shows the tapping -mode atomic microscopy image of the random network of semiconducting SWCNTs (d = 1.4 nm) on a SiO2/Si substrate. By rinsing with deionized water, followed by thermal treatment, surfactants are completely removed. Figure 5b shows the drain current ID as a function of gate voltage VGS with a drain voltage of VDS = 1 V for S-TFT (semiconductor-rich), M-TFT (metal-rich), and R-TFT (reference, before separation). For each type, we selected a device with a drain current ∼5 μA at VGS = 10 V. All devices showed p-type transistor characteristics, in which |ID| increased as VGS decreases for VGS < 0 V. Mobilities were obtained from the maximum transconductance, giving 6.0, 6.8, and 7.1 cm2 V 1 s 1 for M-TFT, S-TFT, and R-TFT, respectively. On/off current ratios, defined by the maximum of ID divided by the minimum of ID in this gate range, were 2.5, 4.4  105, and 25 for M-TFT, S-TFT, and R-TFT, respectively. Figure 5c shows the plots of on/off current ratios versus mobilities with their network density controlled as previously described. For S-TFTs, high on/off ratios are preserved, possibly due to the elimination of pathways by metallic SWCNTs. These results clearly demonstrate the effectiveness of the separation method.

IV. CONCLUSIONS In conclusion, a novel method to separate metallic and semiconducting SWCNTs by using vertical free solution electrophoresis was reported. In contrast with conventional horizontal electrophoresis, clear ELF, responsible for distinct separation, was observed. Optical absorbance spectroscopy and resonant Raman spectroscopy revealed that metallic and semiconducting SWCNTs were enriched at upper and lower layers, respectively. The analysis of the optical absorbance spectra revealed that the semiconducting SWCNTs were enriched up to 95% in the lower layer. This method is successfully applied for SWCNTs with diameters from 1.0 to 1.7 nm. TFT fabricated from semiconductor-enriched solution showed excellent device characteristics. Simple, scalable, and efficient characteristics of this method offer a promising opportunity for practical use. ’ ASSOCIATED CONTENT

bS

Supporting Information. Electrophoretic behavior of SWCNT solution and nonionic surfactant, specific gravity of fractions, and MPEG file made from sequential optical images. This material is available free of charge via the Internet at http:// pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected], [email protected].

’ ACKNOWLEDGMENT We thank Shigekazu Ohmori, Yuki Asada, Hideaki Numata, Kunihiro Hara, Shinichi Yorozu, and Sumio Iijima for useful discussions. This work is partially supported by the New Energy and Industrial Technology Development Organization (NEDO). ’ REFERENCES (1) Dresselhaus, M. S.; Dresselhaus, G.; Avouris, P. Carbon Nanotubes: Synthesis, Structure, Properties, and Applications; Springer: New York, 2001. (2) Cao, Q.; Rogers, J. A. Adv. Mater. 2009, 21, 29. (3) Hamada, N.; Sawada, S.; Oshiyama, A. Phys. Rev. Lett. 1992, 68, 1579. (4) Saito, R.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Appl. Phys. Lett. 1992, 60, 2204. (5) Hersam, M. C. Nat. Nanotechnol. 2008, 3, 387. (6) Chattopadhyay, D.; Galeska, I.; Papadimitrakopoulos, F. J. Am. Chem. Soc. 2003, 125, 3370. (7) Krupke, R.; Hennrich, F.; L hneysen, H.; Kappes, M. M. Science 2003, 301, 344. (8) Miyata, Y.; Maniwa, Y.; Kataura, H. J. Phys. Chem. B 2006, 110, 25. (9) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C. Nat. Nanotechnol. 2006, 1, 60. (10) Zheng, M.; Jagota, A.; Semke, E. D.; Diner, B. A.; McLean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. Nat. Mater. 2003, 2, 338. (11) Tanaka, T.; Jin, H.; Miyata, Y.; Fujii, S.; Suga, H.; Naitoh, Y.; Minari, T.; Miyadera, T.; Tsukagoshi, K.; Kataura, H. Nano Lett. 2009, 9, 1497. (12) Kim, W.-J.; Usrey, M. L.; Strano, M. S. Chem. Mater. 2007, 19, 1571. (13) Wakizaka, Y.; Nakayama, K.-i.; Tanaka, S.; Sakurai, Y. Separation of Metallic and Semiconducting Single-Walled Carbon Nanotubes by Electric Field, Proceedings of The 32nd Fullerenes-Nanotubes General Symposium, Nagoya, Japan, 2007. (14) Bradley, K.; Cumings, J.; Star, A.; Gabriel, J.-C. P.; Gr€uner, G. Nano Lett. 2003, 3, 639. (15) Saito, T.; Ohshima, S.; Xu, W.-C.; Ago, H.; Yumura, M.; Iijima, S. J. Phys. Chem. B 2005, 109, 10647. (16) Moore, V. C.; Strano, M. S.; Haroz, E. H.; Hauge, R. H.; Smalley, R. E.; Schmidt, J.; Talmon, Y. Nano Lett. 2003, 3, 1379. (17) Kataura, H.; Kumazawa, Y.; Maniwa, Y.; Umezu, I.; Suzuki, S.; Ohtsuka, Y.; Achiba, Y. Synth. Met. 1999, 103, 2555. (18) Haddon, R. C.; Sippel, J.; Rinzler, A. G.; Papadimitrakopoulos, F. MRS Bull. 2004, 29, 252. (19) Brown, S. D. M.; Jorio, A.; Corio, P.; Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Kneipp, K. Phys. Rev. B 2001, 63, 155414.

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