High-Quality Single-Walled Carbon Nanotubes Synthesized by

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J. Phys. Chem. C 2007, 111, 12954-12959

High-Quality Single-Walled Carbon Nanotubes Synthesized by Catalytic Decomposition of Xylene over Fe-Mo/MgO Catalyst and Their Field Emission Properties Hui Zhang,† Dong Hoon Shin,† Heon Sang Lee,‡ and Cheol Jin Lee*,† School of Electrical Engineering, Korea UniVersity, Seoul 136-713, Korea, and LG Chem. Ltd. Technology Center, Daejeon 305-343, Korea ReceiVed: March 9, 2007; In Final Form: June 28, 2007

High-quality single-walled carbon nanotubes (SWCNTs) were synthesized over Fe-Mo/MgO catalyst by catalytic decomposition of xylene. The synthesized carbon material revealed bundled SWCNTs with few defects and amorphous carbon and revealed that the SWCNTs having diameters of about 0.84-1.60 nm dominated in the product. The product yield of the high-quality SWCNTs was about 63 wt % relative to the weight of the catalyst. The SWCNTs showed the low turn-on field 1.0 V/µm at a current density of 10-8 A/cm2 and the high emission current density of 2.0 mA/cm2 at an applied field of about 3.0 V/µm. The lifetime measurement of SWCNT emitter revealed good emission stability for 20 h during constant DC applied bias. It is considered that the SWCNTs produced by xylene can be used as effective field emitters.

Introduction Single-walled carbon nanotubes (SWCNTs) have generated much interest because of their unique physical and chemical properties.1-4 Substantive effort has been expended in searching for potential applications of SWCNTs, such as nanoscale transistors,5 field emission displays,6 sensors,7 supercapacitors,8 and SWCNT composites.9 Many prospects of potential applications of SWCNTs rely on the development of a cost-efficient large-scale production of high-purity SWCNTs. To synthesize SWCNTs, various methods including arc discharge,10 laser ablation,11 and catalytic chemical vapor deposition (CCVD)12-14 have been carried out. In particular, the CCVD method, which can promise controllable growth, high-purity growth, and efficient cost, has been paid much attention to investigate the effect of catalyst composition, carbon source, and support material. For the synthesis of SWCNTs by the CCVD method, many kinds of catalysts such as Fe, Co, Ni, and Mo either in monometallic form or in bimetallic form supported by Al2O3 or/and SiO2 were generally used.14-22 Interestingly, the combination of Fe and Mo greatly enhanced the yield of SWCNTs compared with single-component Fe catalyst.14 The increase in the yield of SWCNTs can be attributed to the synergistic effects between Fe and Mo since Mo stabilizes nanoparticles with 1-2nm diameters and/or acts as a carbon supplier.18,23 Additionally, to obtain SWCNTs using the CCVD method, support materials that have nanometer-scaled pores on the surface are inevitably necessary.24 In general, catalyst particles are confined to the nanometer-sized pores of the support materials, resulting in the growth of SWCNTs. To synthesized SWCNTs, support materials such as Al2O3, SiO2, and MgO have been widely used. However, the removal of Al2O3 or SiO2 support material from produced carbon material is laborious work. Recent studies showed that MgO was a more efficient support material for the synthesis of SWCNTs than Al2O3 or SiO2.25-28 In addition, since MgO is readily dissolved in hydrochloride acid, the purification * Corresponding author. Telephone: +82-2-3290-3216. Fax: +82-2921-4722. E-mail: [email protected]. † Korea University. ‡ LG Chem. Ltd. Technology Center.

of SWCNTs can be easily achieved by mild acid treatments without the damage of graphene layers. Recently, some research groups reported that Fe-Mo bimetallic catalyst was useful to synthesize high-purity SWCNTs over MgO support material using gas-phase carbon sources.28-31 However, in addition to the gas-phase carbon sources, it is considered that the liquid carbon source might be an important candidate to realize a mass production of CNTs at low cost using the CCVD method. Among the various liquid carbon sources, we have focused on xylene for the synthesis of SWCNTs because xylene is a stable and relatively cheap chemical source. Moreover, in previous work, although xylene was proved to be a good carbon source for the synthesis of high-purity multiwalled carbon nanotubes (MWCNTs)32,33 and double-walled carbon nanotubes (DWCNTs),34 thus far there are few reports on the synthesis of SWCNTs using xylene as carbon source. Cao et al. reported the synthesis of SWCNTs by the catalytic pyrolysis of ferrocene and xylene.35 Unfortunately, their experimental results showed a high percentage of MWCNTs from the produce carbon material. Keskar et al. announced the synthesis of isolated SWCNTs on Fe/quartz and Fe/SiO2-Si substrates by catalytic decomposition of xylene.36 According to our knowledge, up to the present, there have been no reports on the synthesis of SWCNTs over the Fe-Mo/MgO catalyst system using xylene as a carbon source. Therefore, further study on the synthesis of SWCNTs over Fe-Mo/MgO catalyst using xylene is much desired to open a large-scale synthesis of high-quality SWCNTs at low cost. Here, we demonstrated the large-scale synthesis of highquality SWCNTs by catalytic decomposition of xylene over FeMo/MgO catalyst. It is well-known that SWCNTs are considered to be excellent field emitters because of their small radius, chemical stability, and good electrical conductivity. Thus, we also evaluated the field emission properties of the high-quality SWCNTs produced by xylene. Our results showed that xylene, combined with Fe-Mo/MgO catalyst, might be an ideal carbon source for the large-scale synthesis of high-quality SWCNTs, which will greatly contribute to the development of CNT applications.

10.1021/jp071948j CCC: $37.00 © 2007 American Chemical Society Published on Web 08/11/2007

SWCNT Synthesized by Decomposition of Xylene

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Figure 1. Fe-Mo/MgO catalyst after calcination at 700 °C for 7 h in ambient air. (a) TEM image and (b) XRD pattern.

Experimental Section Preparation of Catalyst. To make an Fe-Mo solution, a mixture of Fe(NO3)3‚9H2O (99.99%, Aldrich) and Mo solution (9.8 mg/mL of Mo in H2O, Aldrich, ICP/DCP standard solution) was dissolved in deionized (DI) water for 1 h. Then the obtained Fe-Mo solution was introduced to the suspension of MgO and DI water followed by sonication for 1 h using an ultrasonicator. In this work, the mole ratio of Fe/Mo/MgO was 17.1:1:287. After drying, the resulting material was baked at 150 °C for 15 h in ambient vacuum and then ground in a mortar to break chunks into a powder. Finally, the ground material was calcined at 700 °C for 7 h in ambient air. Synthesis of SWCNTs. In a typical growth experiment, 200 mg of catalyst was put into a quartz boat at the center of a quartz tube (20-mm i.d. and 500-mm long) mounted in a tube furnace. The xylene was placed in a stainless steel bubbler at room temperature. The quartz tube reactor was heated to 800 °C in Ar atmosphere. Subsequently, a mixture of Ar gas (100 sccm) passing through xylene and Ar gas (1000 sccm) was introduced into the reactor. Xylene was carried into the reactor maintained at 800 °C for 10 min for the growth of SWCNTs. After the growth, the reactor was allowed to cool to room temperature in Ar atmosphere. Characterization. The samples were characterized by scanning electron microscopy (SEM, Hitachi S-4700), transmission electron microscopy (TEM, JEOL JEM-3011, 300 kV) with energy-dispersive X-ray spectroscopy (EDS) analyzer, Raman spectroscopy (Horiba Jobin-Yvon HR-800 UV), and thermogravimetric analysis (TGA, TA instrument, SDT 2960). The X-ray diffraction (XRD) pattern was recorded using a Rigaku D/MAX Rint 2000 diffractometer with Cu KR irradiation. Field Emission Studies. To evaluate genuine field emission properties from SWCNTs, the as-synthesized SWCNTs were purified by air oxidation at 350 °C for 30 min, followed by acid treatment (stirred in 10% HCl solution for 30 min) to eliminate the catalyst. Field electron-emission measurement was performed in a vacuum chamber at a pressure of 2.0 × 10-7 torr. First, the SWCNTs were dipped in ethanol solution and dispersed by sonication. Then, the SWCNT suspension was sprayed on the Ag/Ti (200 nm)/Cr (300 nm)/n-Si substrate using a spray gun. To enhance vertical alignment of SWCNTs on the substrate, a mechanical taping method was used. The substrate was dried in air, followed by baking at 400 °C for 20 min using a rapid thermal annealing in an Ar atmosphere to obtain good mechanical adhesion and ohmic contact property between the SWCNTs and the substrate. The cathode was the Ag/Ti film on the silicon substrate, and the anode was a stainless steel head positioned about 300 µm above the SWCNTs, and the measured

emission area was 0.19625 cm2. Emission current was monitored with a Keithley 6517 A, and DC power was supplied with a constant power voltage and current controller. Before measuring field emission properties of SWCNTs, we performed the electrical annealing to create the even surface of SWCNTs. To obtain reliable experimental results, we measured field emission properties at several sites of the SWCNTs. Results and Discussion For CNT growth, we used Fe-Mo/MgO catalyst calcined at 700 °C for 7 h in ambient air. In the TEM observation of the Fe-Mo/MgO catalyst after calcination shown in Figure 1a, there are some nanoparticles (dark contrast) with various diameters present on the support material (light contrast). It is noteworthy that the shape of these nanoparticles cannot be distinguished clearly from the MgO support material, suggesting that the metal species have been dispersed in the MgO lattice at a certain extent because of the calcination treatment.37 Figure 1b shows the XRD pattern of the Fe-Mo/MgO catalyst where the five characteristic peaks of MgO were well-resolved. A minor phase corresponding to MgFe2O4 is also detected,37,38 indicating the presence of MgFe2O4-like particles with relatively small size, which is consistent with the result from the TEM observation. However, it was difficult to detect the molybdenum oxide peaks in XRD analysis because of the small amount of Mo in the Fe-Mo/ MgO catalyst. Nevertheless, the oxidized Mo species may in fact be a combination of oxides and molybdates.18 A similar result was reported by the Ni group in the Co-Mo/MgO catalyst (i.e., MgMoO4 and CoMoO4).39 From the XRD pattern, we suggest that metal oxide catalyst plays a key role in yielding a large-scale synthesis of SWCNTs in our work. When the reaction starts from the oxidized metal catalyst, a reduction of Fe oxide and Mo oxide and/or a transformation of Mo species to Mo carbide take place during the decomposition of the hydrocarbon precursor.40 From the high-resolution TEM (HRTEM) observation of our high-purity SWCNTs, we could find only a few catalyst particles in the bundles or in the hollow structure. Here, we got an estimation of catalyst composition after SWCNTs growth by EDS. As shown in Figure 2c, both Fe and Mo element was detected on a discrete metal particle and the atomic ratio of Fe/Mo is 16.6, which is approximately consistent with the initial ratio of 17.1 in metal impregnation solutions. Nevertheless, this result indicates that the Fe-Mo bimetallic catalyst had taken part in the growth of CNTs together. Figure 3 shows the typical SEM images of the carbon material produced by catalytic decomposition of xylene. A lowmagnification SEM image of the as-synthesized product (Figure

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Figure 2. TEM image and EDS spectrum of the catalyst in the as-synthesized SWCNT sample. (a) TEM image of the catalyst particle and SWCNTs. (b, c) EDS result from catalyst metal particle.

Figure 3. SEM images of the as-synthesized carbon filaments by catalytic decomposition of xylene. (a) Low-, (b) medium-, and (c) highmagnification SEM image.

3a) shows a large amount of entangled carbon filaments having a length of over several tens of micrometers. In addition, it is also considered that the entire surface of the catalyst is fully covered with weblike carbon filaments. It is noteworthy that the image was obtained from the as-synthesized carbon material without any purification. Therefore, the low-magnification SEM image clearly illustrates the remarkable abundance of carbon filaments, which indicates a high purity of the as-synthesized carbon filaments. Furthermore, from a medium-magnification SEM image (Figure 3b) and a high-magnification SEM image

(Figure 3c) of the as-synthesized carbon material, it can be found that the produced carbon filaments have diameters of 9-22 nm and have a uniform shape with smooth and clean surfaces; this verifies the high quality of the pure CNTs grown by the proposed method. TEM observation was performed to further study the morphology and microstructure of the carbon filaments. A typical TEM image of the as-synthesized carbon filaments is shown in Figure 4a, which reveals that the filaments are almost completely composed of CNT bundles. Figure 4b,c shows the representative HRTEM images of the as-synthesized CNT material. In addition to SWCNT bundles, it is obvious that some isolated SWCNTs are also observed in the carbon product. Occasionally, DWCNTs were also detected by HRTEM, but the amount of DWCNTs was very small compared with the abundant SWCNTs. In the present work, HRTEM images show that only a slight amount of amorphous carbon appears on the surface of the SWCNTs, indicating the high purity of the as-synthesized SWCNTs. Moreover, from HRTEM observation, the diameter of SWCNTs can be measured. In this work, the diameters of the SWCNTs in the bundles are mostly in the range of 0.9-1.3 nm. However, interestingly, isolated SWCNTs normally exhibit larger diameters about 1.6-2.0 nm in comparison with individual SWCNTs within the bundles. Raman spectroscopy is a convenient and powerful tool for the characterization of SWCNTs. Figure 5 shows the Raman spectrum measured with Ar laser of wavelength 514.5 nm on the as-synthesized SWCNT material. It clearly exhibits the typical bands of SWCNTs, characterized by the narrow and strong G-band around 1604.7 cm-1 with a left shoulder at 1583.4 cm-1 and radial breathing mode (RBM) vibration in the lowfrequency region. The peak around 1350.4 cm-1 (D-band) is associated with the defects, impurities, or lattice distortions in the carbon material. An intensity ratio between the D-band and the G-band, ID/IG, is a good indicator to discuss the purity and defects of the SWCNTs. In this work, the small ratio of ID/IG indicates that the produced carbon material contains relatively few amorphous carbon impurities and has a low defect level in the graphene structure of SWCNTs. A high-resolution Raman spectrum in low-frequency region (RBM vibration) exhibits the presence of four components as shown in the inset of Figure 5. It has been well-known that the RBM corresponds to the atomic vibration of carbon atoms in the radial direction as if the tube were breathing. Theoretical predication suggests that the vibration frequency of RBM is inversely proportional to the diameter of SWCNTs. Therefore, on the basis of the RBM peaks, the diameter of SWCNTs can be estimated. Considering that van der Waals interactions exist within SWCNTs because the assynthesized SWCNTs were packed into bundles, the expression

SWCNT Synthesized by Decomposition of Xylene

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Figure 6. TGA curve of the as-synthesized SWCNTs by catalytic decomposition of xylene.

Figure 4. TEM images of the as-synthesized CNTs by catalytic decomposition of xylene. (a) TEM image showing CNT bundles. (b) HRTEM image showing SWCNT bundles. (c) HRTEM image showing isolated SWCNTs.

Figure 5. Raman spectrum of the as-synthesized SWCNTs by catalytic decomposition of xylene.

ω ) 6.5 + 223.75/d is used to calculate the diameter of SWCNTs, where ω is RBM frequency per centimeter and d is the diameter of SWCNTs in nanometers.21 According to the above relation between SWCNT diameter and RBM frequency, the observed peaks at 146.3, 174.7, 190.7, and 274.3 cm-1

correspond to SWCNTs with a diameter of 1.60, 1.33, 1.21, and 0.84 nm, respectively. The Raman analysis result indicates that the SWCNTs with various diameters (ranging from 0.84 to 1.60 nm) are present in the sample, which is consistent with that roughly evaluated from the HRTEM observation. TGA was carried out to obtain the carbon content information of the as-synthesized SWCNTs. Figure 6 shows a plot for the weight percentage vs oxidation temperature, measured by heating the as-synthesized SWCNTs in the air ambient. The percentage weight curve between 100 and 800 °C was plotted by adjusting 100% for the weight of the as-synthesized SWCNTs at 100 °C. In this work, the as-synthesized SWCNTs start to burn near 400 °C and finish gasification near 650 °C, showing a weight loss around 38 wt %. The weight gain of the SWCNTs is calculated by the weight loss mentioned above divided by the weight left at 650 °C, which is presumably the weight of the catalyst and support materials.17 As a result, a carbon yield of 63 wt % is achieved, which indicates a higher carbon yield compared with that of SWCNTs produced by gas carbon sources over Fe-Mo/MgO catalyst, for the product yield of SWCNTs synthesized by gas carbon sources generally showed less than 47 wt %.28-30 The differential thermogravimetric curve shows a single, narrow width peak at 566 °C corresponding to SWCNTs, indicating that the as-synthesized SWCNTs have a relatively high purity and contain little other unwanted carbon species. Thus, the TGA result of the SWCNTs, together with their TEM result and Raman scattering characterization, shows that SWCNTs with high purity, high product yield, and fairly few defects were synthesized by catalytic decomposition of xylene over Fe-Mo/MgO catalyst. Figure 7 shows field emission properties of the SWCNTs produced by xylene. In this work, the field emission was measured more than five times for each sample. The distance between the anode and cathode (exactly, tip of SWCNTs) was about 300 µm. Even though the length of protruding SWCNTs is about several micrometers, we neglect the length of SWCNTs when we calculate the distance between the anode and the cathode. When we evaluate the field emission properties of CNTs, we generally use the purified CNTs because the as-synthesized SWCNTs indicate poor emission repeatability due to the existence of amorphous carbon covered on the CNTs, catalyst particles, and support material. After removing the amorphous carbon, catalyst particles, and support material from CNT products, we can get stable and repeatable field emission information. In this work, we evaluated the field emission properties of the purified SWCNTs and the as-synthesized SWCNTs at the same time. From the as-synthesized SWCNTs, we found unstable and unrepeatable field emission properties.

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Figure 8. Lifetime property of field emission from the purified SWCNTs at a constant DC bias voltage of 2500 V for 20 h.

emission performance of the SWCNTs is higher than that of the previously reported SWCNTs.41,42 To investigate the emission uniformity of the SWCNT emitter, we evaluated an emission pattern from the SWCNT emitter that indicated a circle with a diameter of 5 mm (a circular area of 0.19625 cm2). In this work, we used the phosphor-coated indium-tin-oxide/glass substrate as an anode and applied the electric field of 2.5 V/µm between the cathode and the anode. As shown in Figure 7c, the SWCNT emitter shows a very uniform emission pattern in bright mode. We consider that the high emission performance may be caused by high purity and good crystallinity of the produced SWCNTs by xylene. This result indicates that the SWCNTs produced by xylene can be used as effective field emitters. The Fowler-Nordheim plot is shown in the inset of Figure 7b. To calculate the field enhancement factor (β), we use the FowlerNordheim eq:

J ) A(β2V2/φd2) exp(-Bφ3/2d/βV)

Figure 7. Field emission characteristics of the SWCNTs. (a) Emission current density of the as-synthesized SWCNT emitter. (b) Emission current density of the purified SWCNT emitter. (c) Emission pattern from the SWCNT emitter with a diameter of 5 mm. The inset shows the corresponding Fowler-Nordheim plots.

As shown in Figure 7a, the as-synthesized SWCNTs reveal the low turn-on field of 1.5 V/µm at a current density of 10-8 A/cm2 and the high emission current density of 1.0 mA/cm2 at an applied field of 3.0 V/µm. However, to evaluate genuine field emission properties from SWCNTs, we focused on the field emission properties of the purified SWCNTs. As shown in Figure 7b, the purified SWCNTs show the low turn-on field of 1.0 V/µm at a current density of 10-8 A/cm2 and the high emission current density of 2.0 mA/cm2 at an applied field of 3.0 V/µm. This result reveals that the obtained

where J is the current density, A ) 1.54 × 10-6 (A V-2 eV), B ) 6.83 × 109 (eV-3/2 V m-1), β is a field enhancement factor, φ is the work function, d is the distance between the anode and the cathode, and V is the applied voltage. When assuming the work function to be 5.0 eV, the SWCNTs indicated the high field enhancement factor (β) of 3613. The above results reveal that the SWCNTs produced by catalytic decomposition of xylene over Fe-Mo/MgO catalyst indicate lower turn-on voltage and a higher field enhancement factor (β) than those of the previously reported SWCNTs.42-44 Figure 8 shows lifetime stability of field emission from the purified SWCNTs, indicating fairly stable emission current at high current density with a constant DC bias voltage of 2500 V for 20 h. At the accelerated lifetime measurement of the emission current density of 1 mA/cm2, the current density decreased from 1.0 to 0.8 mA/cm2 for 10 h but reduced to 0.6 mA/cm2 for 20 h during constant DC applied bias. This result reveals that the SWCNTs have an emission stability better than that of the previously reported SWCNTs, although they still show poor emission stability compared with that of MWCNTs.6,42 Conclusion Large quantities of high-quality SWCNTs have been produced by catalytic decomposition of xylene over Fe-Mo/MgO catalyst. The diameters of the produced SWCNTs are mostly in the range of 0.84-1.60 nm based upon Raman analysis and HRTEM observation. Our studies demonstrate that Fe-Mo/ MgO is an efficient catalyst for the SWCNT synthesis and

SWCNT Synthesized by Decomposition of Xylene xylene might be an ideal carbon source for the synthesis of SWCNTs over Fe-Mo/MgO catalyst. This process shows great promise for a large-scale production of SWCNTs at low cost because of high catalytic performance of Fe-Mo/MgO catalyst, convenient purification, and cheap carbon source. For field emission properties, the SWCNTs showed low turn-on field of 1.0 V/µm at a current density of 10-8 A/cm2 and high emission current density of 2.0 mA/cm2 at an applied field of about 3.0 V/µm. The SWCNTs also indicated a fairly stable emission current at a constant DC bias voltage of 2500 V for 20 h. Thus, we consider that the SWCNTs produced by xylene can be used as effective field emitters. Acknowledgment. This work was supported by the Center for Nanotubes and Nanostructured Composites at Sungkyunkwan University, by the National R&D Project for Nano Science and Technology, by the Ministry of Commerce, Industry, and Energy of Korea through a Components and Materials Technology Development Project (No. 0401-DD2-0162), by Korea University, and by the BK 21 Program of the Ministry of Education. References and Notes (1) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (2) Bethune, D. S.; Kiang, C. H.; Devries, M. S.; Gorman, G.; Savoy, R.; Vazquez, J.; Beyers, R. Nature 1993, 363, 605. (3) Treacy, M. M. J.; Ebbesen, T. W.; Gibson, J. M. Nature 1996, 381, 678. (4) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: London, 1998. (5) Tans, S. J.; Verschueren, A. R. M.; Dekker, C. Nature 1998, 393, 49. (6) Bonard, J.-M.; Salvetat, J.-P.; Sto¨ckli, T.; de Heer, W. A.; Forro´, L.; Chaˆtelain, A. Appl. Phys. Lett. 1998, 73, 918. (7) Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K. J.; Dai, H. J. Science 2000, 287, 622. (8) An, K. H.; Kim, W. S.; Park, Y. S.; Choi, Y. C.; Lee, S. M.; Chung, D. C.; Bae, D. J.; Lim, S. C.; Lee, Y. H. AdV. Mater. 2001, 13, 497. (9) Hu, Y. H.; Shenderova, O. A.; Hu, Z.; Padgett, C. W.; Brenner, D. W. Rep. Prog. Phys. 2006, 69, 1847. (10) Journet, C.; Maser, W. K.; Bernier, P.; Loiseau, A.; dela Chapelle, M. L.; Lefrant, S.; Deniard, P.; Lee, R.; Fischer, J. E. Nature 1997, 388, 756. (11) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H. J.; Petit, P.; Robert, J.; Xu, C. H.; Lee, Y. H.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Tomanek, D.; Fischer, J. E.; Smalley, R. E. Science 1996, 273, 483. (12) Satishkumar, B. C.; Govindaraj, A.; Sen, R.; Rao, C. N. R. Chem. Phys. Lett. 1998, 293, 47. (13) Nikolaev, P.; Bronikowski, M. J.; Bradley, R. K.; Rohmund, F.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Chem. Phys. Lett. 1999, 313, 91. (14) Cassell, A. M.; Raymakers, J. A.; Kong, J.; Dai, H. J. J. Phys. Chem. B 1999, 103, 6484. (15) Dal, H. J.; Rinzler, A. G.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1996, 260, 471.

J. Phys. Chem. C, Vol. 111, No. 35, 2007 12959 (16) Hafner, J. H.; Bronikowski, M. J.; Azamian, B. R.; Nikolaev, P.; Rinzler, A. G.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Chem. Phys. Lett. 1998, 296, 195. (17) Su, M.; Zheng, B.; Liu, J. Chem. Phys. Lett. 2000, 322, 321. (18) Alvarez, W. E.; Kitiyanan, B.; Borgna, A.; Resasco, D. E. Carbon 2001, 39, 547. (19) Harutyunyan, A. R.; Pradhan, B. K.; Kim, U. J.; Chen, G. G.; Eklund, P. C. Nano Lett. 2002, 2, 525. (20) Hornyak, G. L.; Grigorian, L.; Dillon, A. C.; Parilla, P. A.; Jones, K. M.; Heben, M. J. J. Phys. Chem. B 2002, 106, 2821. (21) Lyu, S. C.; Liu, B. C.; Lee, T. J.; Liu, Z. Y.; Yang, C. W.; Park, C. Y.; Lee, C. J. Chem. Commun. 2003, 734. (22) Hart, A. J.; Slocum, A. H.; Royer, L. Carbon 2006, 44, 348. (23) Saito, T.; Xu, W. C.; Ohshima, S.; Ago, H.; Yumura, M.; Iijima, S. J. Phys. Chem. B 2006, 110, 5849. (24) Ago, H.; Nakamura, K.; Uehara, N.; Tsuji, M. J. Phys. Chem. B 2004, 108, 18908. (25) Flahaut, E.; Peigney, A.; Laurent, C.; Rousset, A. J. Mater. Chem. 2000, 10, 249. (26) Colomer, J.-F.; Stephan, C.; Lefrant, S.; Van Tendeloo, G.; Willems, I.; Konya, Z.; Fonseca, A.; Laurent, C.; Nagy, J. B. Chem. Phys. Lett. 2000, 317, 83. (27) Yan, H.; Li, Q. W.; Zhang, J.; Liu, Z. F. Carbon 2002, 40, 2693. (28) Ago, H.; Uehara, N.; Yoshihara, N.; Tsuji, M.; Yumura, M.; Tomonaga, N.; Setoguchi, T. Carbon 2006, 44, 2912. (29) Lyu, S. C.; Liu, B. C.; Lee, S. H.; Park, C. Y.; Kang, H. K.; Yang, C. W.; Lee, C. J. J. Phys. Chem. B 2004, 108, 1613. (30) Liu, B. C.; Lyu, S. C.; Jung, S. I.; Kang, H. K.; Yang, C. W.; Park, J. W.; Park, C. Y.; Lee, C. J. Chem. Phys. Lett. 2004, 383, 104. (31) Li, Y.; Zhang, X. B.; Shen, L. H.; Luo, J. H.; Tao, X. Y.; Liu, F.; Xu, G. L.; Wang, Y. W.; Geise, H. J.; Van Tendeloo, G. Chem. Phys. Lett. 2004, 398, 276. (32) Andrews, R.; Jacques, D.; Rao, A. M.; Derbyshire, F.; Qian, D.; Fan, X.; Dickey, E. C.; Chen, J. Chem. Phys. Lett. 1999, 303, 467. (33) Kichambare, P. D.; Qian, D.; Dickey, E. C.; Grimes, C. A. Carbon 2002, 40, 1903. (34) Wei, J. Q.; Jiang, B.; Wu, D. H.; Wei, B. Q. J. Phys. Chem. B 2004, 108, 8844. (35) Cao, A. Y.; Zhang, X. F.; Xu, C. L.; Liang, J.; Wu, D. H.; Chen, X. H.; Wei, B. Q.; Ajayan, P. M. Appl. Phys. Lett. 2001, 79, 1252. (36) Keskar, G.; Rao, R.; Luo, J.; Hudson, J.; Chen, J.; Rao, A. M. Chem. Phys. Lett. 2005, 412, 269. (37) Ning, G. Q.; Wei, F.; Wen, Q.; Luo, G. H.; Wang, Y.; Jin, Y. J. Phys. Chem. B 2006, 110, 1201. (38) Coquay, P.; De Grave, E.; Peigney, A.; Vandenberghe, R. E.; Laurent, Ch. J. Phys. Chem. B 2002, 106, 13186. (39) Ni, L.; Kuroda, K.; Zhou, L. P.; Kizuka, T.; Ohta, K.; Matsuishi, K.; Nakamura, J. Carbon 2006, 44, 2265. (40) Moisala, A.; Nasibulin, A. G.; Kauppinen, E. I. J. Phys.: Condens. Matter 2003, 15, S3011. (41) Kim, J. M.; Choi, W. B.; Lee, N. S.; Jung, J. E. Diamond Relat. Mater. 2000, 9, 1184. (42) Bonard, J.-M.; Salvetat, J.-P.; Sto¨ckli, T.; Forro´, L.; Chaˆtelain, A. Appl. Phys. A 1999, 69, 245. (43) Bonard, J.-M.; Kind, H.; Sto¨ckli, T.; Nilsson, L.-O. Solid-State Electron. 2001, 45, 893. (44) Sveningsson, M.; Morjan, R. E.; Nerushev, O. A.; Sato, Y.; Backstrom, J.; Campbell, E. E. B.; Rohmund, F. Appl. Phys. A 2001, 73, 409.