Catalyst Distribution and Carbon Nanotube Morphology in Multilayer

1 Jul 2009 - of multilayer CNT forests where the first forest is generated either by catalyst coinjection (CCI) of ferrocene with hydrocarbon (xylene)...
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Catalyst Distribution and Carbon Nanotube Morphology in Multilayer Forests by Mixed CVD Processes Stephen C. Hawkins,* Jacinta M. Poole, and Chi P. Huynh CSIRO Materials Science and Engineering, PriVate Bag 10, BayView AVenue, Clayton, Victoria, Australia 3168 ReceiVed: NoVember 16, 2008; ReVised Manuscript ReceiVed: February 10, 2009

Carbon nanotubes can be grown as forests of aligned fibers on a substrate with a catalyst coated prior to or added during synthesis. A major process interruption can initiate the growth of second and successive layers of forest on top or at the base of the existing layers which are thereby lifted up. We report on the generation of multilayer CNT forests where the first forest is generated either by catalyst coinjection (CCI) of ferrocene with hydrocarbon (xylene) or by catalyst predeposition (CPD) of iron followed with hydrocarbon (acetylene). Subsequent layers are then produced by CCI alone to give uniform (all CCI) or mixed (CPD and CCI) structures to study the distribution of the iron catalyst and CNT morphology and to determine whether the CPD forest templates or otherwise influences the growth of subsequent CCI forests. The bottom-up base growth of second and subsequent CCI forests is reaction rate controlled. CCI multilayer forests accumulate catalyst (iron) in a variety of distinct locations. A pre-existing CPD forest modifies subsequent CCI forest initiation, morphology, and catalyst distribution but does not itself accumulate catalyst or change appearance. 1. Introduction Carbon nanotubes (CNTs) have many excellent properties and potential applications. To realize these requires a wide range of characteristics such as diameter, length, and orientation, which can be achieved by chemical vapor deposition (CVD) synthesis.1,2 Flow-through or floating-catalyst CVD growth of CNTs, which uses coinjection of catalyst and carbon source, produces short randomized fibers in the gas stream that may self-assemble into bundles3 or spinnable aerogels.4 However, the brief transit time limits the scope for producing highly specified, ordered CNTs and complex structures. Growth of CNTs on flat substrates held within the reaction zone uses a hydrocarbon and either a predeposited catalyst layer5,6 (the catalyst predeposition or CPD CVD process) or the capture of a coinjected catalyst (the catalyst co-injection or CCI CVD process).7 Such substrate growth produces uniform highly aligned CNTs as extensive forests or complex 2D structures. It allows the CNTs to be grown for seconds to hours and for the growth process to be halted and restarted or to be changed during execution. One interesting consequence of the flexibility of the substrate CVD process is the ability to produce ‘multilayer’ CNT forests. First reported in a 1999 patent as multilayer ‘films’,8 synthesis of multilayer or ‘stacked’ forests offers the potential for templating catalyst arrays,9 for more efficient use of substrates and the creation of novel structures8 and provides insights into the mechanisms of CNT growth.10-15 In the CPD CVD process, CNT growth can continue until the initially deposited catalyst is inactivated, through graphite overgrowth,16 encapsulation within the CNTs or formation of noncatalytic compounds such as carbides, silicides, etc.17-20 Growth can be perpetuated by continuous activation of the catalyst by water11 injection but once terminated, any further growth of CNT on the substrate occurs as a separate and distinct forest rather than by extension of the existing tubes unless * Author to whom correspondence should be addressed. Phone: +61 3 95452397. Fax +61 3 95452109. E-mail: [email protected].

special steps are taken.21 Initiation of new CNT growth requires either that the old catalyst be reactivated or that fresh catalyst be added. Second and successive layers of CNT forest have been observed to grow both on top9,10,16,22 of the existing layers (‘top growth’) and also below12-15 the existing layers (‘bottom growth’), which are thereby lifted off the substrate surface. Existing catalysts have been reactivated by oxidative processes at the top16,21 or base12-15 of the existing forest. Fresh catalyst has been added to the top8-10 or base12 of the existing forest by disrupting the catalyst and hydrocarbon supply alone8-10 or by terminating and restarting the entire reactor process.12 We now report a study of the base growth of multilayer forests achieved by briefly halting only the catalyst and hydrocarbon flow while holding all other parameters steady during multihour growth processes. We investigate the changing morphology of CNTs produced and the distribution of catalyst and show that growth under these conditions is reaction rate controlled. We also report on multilayer forests where the first layer is grown using the CPD CVD process (iron followed by acetylene)23 and subsequent layers by the CCI CVD process (ferrocene in xylene)7 to investigate whether the CPD forest or residual CPD catalyst behaves as a template to affect the growth of the CCI forest. 2. Experimental Section The standard conditions for the CCI CVD process (unless otherwise noted) entailed injection (3 mL/h) of o-xylene (‘Xyl’, Merck) containing 2% w/w ferrocene (‘Fer’, Aldrich) through a heated injection block (temperature 125 °C, stirred injection zone temperature 200 °C, exit block temperature 150 °C) with an argon carrier flow rate of 450 sccm at 1 atm into a quartz reactor (72 mm id) held at a plateau temperature of 750 °C in a three-zone furnace. Reactions were typically run for 1-8 h with injection of reactants for periods of 20 min to 5 h and halts of 5-20 min to

10.1021/jp810072j CCC: $40.75 Published 2009 by the American Chemical Society Published on Web 07/01/2009

Multilayer Forests by Mixed CVD Processes

J. Phys. Chem. C, Vol. 113, No. 30, 2009 12977 ature 670 °C). The catalyst was 5 nm iron, thermal vapor coated onto quartz glass slides or silicon wafers (N/PH 5-10 Ω · cm, 525 ( 25 µm Prime 50 nm thermal oxide buffer layer). Following heating in helium (700 sccm, 35 min, 670 °C), acetylene (5% v/v) was run in for 20 min. 3. Results and Discussion

Figure 1. Double-layer forest (5 + 1 h).

Figure 2. 5 × 1 h multilayer forest, individual layer heights measured.

Figure 3. Linear growth vs injection time in a multilayer forest.

initiate new forest layers. Carrier gas flow was maintained during the halts. Substrates used were silicon wafer and quartz glass as noted. At the completion of a reaction, inert gas flowed through for an additional 15 min to flush out residual reactants and by products. Standard conditions for the CPD CVD process used a quartz reactor (44 mm i.d.) and a three-zone furnace (plateau temper-

The base growth mode of the CCI CNT synthesis is demonstrated by injection of Fer/Xyl for 5 h, halting for 15 min while all other parameters are held steady then restarting injection and running for a further period of 1 h. The result (Figure 1) is a double-layer forest where the thin second layer appears below the thick first layer, with a clear break between the two. The ratio of forest heights (1305 and 249 µm) is ∼5:1 indicating that the growth rate for the final hour (second forest layer) is the same as for the first 5 h. (Note that absolute growth rates are affected by substrate material and treatment and while consistent within experiments, may vary between experiments). The growth rate of CNTs produced by redeposited catalyst bottom growth at 770 °C is reported12,15 to be independent of forest depth suggesting a reaction rate limited process, whereas by reinitiation13 at 775 °C growth rate is inversely proportional to the square root of the total running time, indicating a diffusion limited process, with reaction rate limitation predominating below 740 °C. Our observation at 750 °C is of linear growth with respect to time of layers in multilayer forests, and hence is reaction rate limited. This linear growth rate over time is illustrated by running the reaction for five periods of 1 h with 10 min breaks between. This produces a five-layer forest with the individual heights of 290, 264, 270, 268, and 272 µm, respectively (Figure 2). As the resulting layer heights are proportional to the injection time, CNT lengths can be easily controlled. Compared with the growth rate, there is little delay due to diffusion in the initiation of a new forest. This phenomenon is demonstrated where two 60 min injections are followed by a 20 min injection and followed again by two 60 min injections all with halt times of 10 min. The 20 min forest is one-third of the height to the adjacent 60 min forests (Figure 3). The generation of forest layers by halting only the hydrocarbon and catalyst rather than with a complete process break12 eliminates physical intervention and improves process stability. Our base growth mode for continuous reactor operation is in contrast to other results,9,10,22 where the Fer/Xyl is added dropwise to the reactor resulting in abrupt rises and falls in the reactant concentration. The fresh catalyst deposits on top of the existing CNT layers and initiates new top growth for as long as the hydrocarbon lasts, up to about 5 min.9 Variation of the halt time influences the completeness or clarity of the break. At 5 min, the break is indistinct and separable with difficulty. Below this time the break is undetectable or absent. A halt time of 10 min or more reduces the bridging between layers at the break, making the layers more

Figure 4. SEM of break zones for halt times of (a) 5 and (b) 10 min (c) shows the morphology of the forest across the break zone where CNTs appear less convoluted than those above and below the gap.

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Figure 5. Iron distribution across a multilayer CNT forest by EDXS (a) and backscatter (b).

Figure 6. Top and bottom of first forest layer (a, b) and second forest layer (c, d) top and bottom, respectively.

distinct and cleanly and easily separable, despite the amount of bridging that still appears to be present. Increasing the halt time to 15 and 20 min further reduces the bridging. Figure 4 shows the break zone structure when injection was halted for 5 (Figure 4a) and 10 min (Figure 4b). Asa halt time of 10 min produced a break zone between the forest layers that was always clearly defined and separable, it was chosen as the process standard. The morphology of the forest across the break zone (Figure 4c) appears to be slightly different, with more alignment of the fibers than in the forests above or below the zone. No effect of halt time on subsequent growth rate was observed. The effect of halt time illustrates the interaction of a number of variables. Cao et al.9 suggest that after each drop, (and hence delivery of a bolus of catalyst and hydrocarbon), CNT growth

can continue for up to about 5 min. The catalyst must remain active for at least this length of time, and the hydrocarbon supply presumably derives from the slowly evaporating droplet residue. No significant reservoir of hydrocarbon could be adsorbed on or trapped among the CNTs themselves, as growth does not increase as a function of the amount of CNT present. In our system, halting the injection immediately cuts off the supply of catalyst and hydrocarbon so growth should cease almost immediately (although the reactor volume and gas flow rate are such that it would take several minutes for the concentration to fall to zero). Recommencement of injection within an ∼5 min time frame would both rescue catalyst particles that would otherwise terminate and hence keep some of the existing CNTs growing, and initiate a new growth phase,

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Figure 7. (a, b) EDXS and detailed backscatter (c, d) analysis of first two CCI forest layers.

Figure 8. (a, b) Original CPD CNTs before being placed in the CCI reactor.

Figure 9. Mixed structure multilayer forests.

resulting in significant intergrowth of the layers and hence loss of definition of the layer break (Figure 4a). Cao9 found with the dropwise CCI CVD process that iron particles settled at the top of the existing forest and initiated the next forest, with the iron remaining at the interface of the two. This generated distinct layers of catalyst in a threedimensional array. In our system, catalyst vapor or particles must diffuse downward between the existing CNTs to the substrate surface for bottom up growth to occur after injection is restarted. The distribution of iron was analyzed to clarify the behavior of this catalyst. Energy dispersive X-ray spectroscopy (EDXS). was used to measure the iron content at multiple points in of each of 5 ×

1 h layers of forest produced by the CCI CVD process and the average for each layer plotted (Figure 5a). In each layer of the forest, three measurements at the top, middle, and base of each layer were made. The analysis was completed on an intact forest. The layers were not separated before analyzing. Carbon comprises ∼98-99% of the content, with iron making up the remainder. Iron concentration in the top (initial) layer is highest, with a gradual decline toward the base (layer 5), as might be anticipated for a system where the gaseous constituents diffuse freely from the top to the substrate and there is little selective retention at other points. However, analysis using SEM backscatter imaging, which probes a shallower sample depth and hence is more sensitive than EDXS to surface composition, indicates (Figure 5b) that much more of the iron in the top layer is present on the surface, as opposed to the interior of the CNTs, compared to that in the lower layers. SEM shows the top part of the first layer of CNTs to have a high density of surface ‘nodules’ (Figure 6a) compared with that of the lower part where the CNTs have cleaner and smoother edges and surfaces (Figure 6b). There appears to be a slight increase in nodule density (although not to the level seen for the top of the first layer) from the bottom of the first to the top of the next layer (Figure 6c), with a slight decline toward the bottom of that layer and this pattern occurs in each layer. However, this appearance does not show clearly in photographs (e.g., Figure 6d). Detailed EDXS analysis of the first layer (Figure 7a) shows a small increase in overall iron content from the top to the base, with the next layer (Figure 7b) being lower but similar in pattern, possibly reflecting accumulation of iron associated with the base

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Figure 10. Ordered structure of the top CPD forest layer compared with the CCI forest layer.

Figure 11. (a) Morphology of CPD (top) layer of CNTs and (b) top of the first subsequent layer of CCI CNTs.

Figure 12. Relative levels of surface iron present in a mixed structure multilayer forest by SEM backscatter analysis.

growth mode. However, backscatter analysis indicates that the majority of the excess surface iron in the first layer (Figure 5b) is at the very top of it (Figure 7c). This pattern is not repeated for subsequent layers (e.g., Figure 7d). As the surface iron distribution by backscatter does not markedly distort the overall iron distribution as shown by EDXS, it appears to represent only a small proportion of the catalyst total. It possibly derives from a population of particles that have formed by agglomeration in the reactor atmosphere and are subject to sedimentation rather than diffusion and hence settle in the forest canopy.9 However, as noted below, it is clear that there is an active process involved in capturing the particles as they do not adhere to every available surface. Despite the

differences in catalyst distribution and CNT morphology, an SEM study of the mean diameter across a multilayer forest (5 × 1 h) indicated no significant difference in CNT diameter between successive forests. Neither the backscatter nor the EDXS appear to correlate well with the (subjective) estimate of nodule number and distribution. This may indicate that the backscatter analysis is not seeing their iron content due to a graphitic shell, just as it is not seeing the iron in the CNT cores, and the EDXS is not distinguishing between the iron in the nodules and CNT cores. The EDXS represents the total iron content (i.e., settled at the top, diffusely adsorbed, and trapped in nodules and cores), which appears to be uniform or weighted slightly to the base in each forest (Figure 7a,b). The core quantity appears to be substantially greater than the other contributions as it overwhelms their distributions. Mixed Structure Multilayer Forests. Multilayer forests wherein individual layers are produced by different CVD processes, as anticipated by Dai,8 were grown by treating wafers bearing a CPD grown forest in the CCI reactor. Thus, substrates were first coated with iron (5 nm) and exposed to acetylene at 670 °C and then moved to another reactor for treatment with ferrocene/xylene at 750 °C. Multiple CCI layers were then grown below this initial CPD layer, producing a mixed structure forest. The morphology of the CCI CNTs is relatively tortuous and branched with a broad spread of diameters (Figure 6a-d). CNTs produced by the CPD process are straighter, smooth, and unbranched (Figure 8) and with a slightly lower and less variable mean diameter. The catalyst remains firmly attached to the substrate when the CNT forest is removed. It was of particular interest to determine whether the pre-existing CPD catalyst or forest structure acts as a template to influence the subsequent CCI CNT or forest structure. It was thought that this could occur

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Figure 13. (a) TEM of CCI forest showing the presence of iron in nodules and general iron distribution. (b) TEM of CPD forest showing smoother and iron free.

TABLE 1: Effect of Substrate on Diameters of CNT after Growth of a CCI (5 × 1 h) Multilayer Forest

S1 S2

S3 S4

S5 S6

substrate preparation prior to growth of CCI CNTs

mean CNT diameter (nm)

std dev

Si wafer, iron coated then CCI forest grown standard CPD process (CPD forest present) followed by CCI process (a) standard CPD CNTs (after further CCI process) (b) CCI process CNTs standard CPD process, CPD CNTs removed. followed by CCI process standard CPD process, CPD CNTs removed, wafer oxidized followed by CCI process untreated Si wafer subjected to CCI process untreated quartz glass slide subjected to CCI process

38

17

32

10

34 38

12 15

34

10

39

13

45

17

in a variety of ways, such as by the CPD catalyst structure constraining subsequent CCI catalyst binding and agglomeration and hence CNT growth, the CCI CNTs growing directly as a continuation of the CPD CNTs, or the presence of the CPD forest modifying the catalyst agglomeration and deposition process, for example, by capturing the larger iron particles. Such templating would be manifested perhaps as finer, smoother, straighter CCI CNTs. The most striking effect of having the CPD forest present is that when 5 × 1 h CCI layers are then grown, the first CCI layer grows to only 25% of the height of the next four (Figure 9). It would appear that the CPD forest delays initiation of the first forest only, with later forests initiating normally. It cannot be acting as a simple diffusion barrier as it would influence all forests equally, and indeed the effect could be exacerbated as growth continued. This delay is not seen (i.e., the five layers grow equally) when the CPD forest is manually removed prior to CCI forest growth and the substrate is otherwise unchanged. This suggests that initiation is inhibited by the binding of the CPD forest to the substrate. This could be due to the force required to lift the

CPD forest off the substrate, though very little force is required to remove it or to a change in the substrate surface on removal of the CPD forest, for example, by exposure of CPD catalyst or substrate surfaces to which CCI catalyst can bind. The CPD forest has a more highly aligned and ordered structure than the CCI forests as a result of the different processes used. The structural difference between CNTs from the two processes is evident (Figure 10) where the CPD forest meets the first CCI forest layer. However, despite being exposed to a subsequent CCI process, there was no change in the CPD forest, in particular there is no accumulation of nodules (Figure 11a). The morphology of the CCI CNTs immediately below the CPD forest (Figure 11b) is similar to that seen without the CPD forest present (Figure 6a) although slightly less nodular. Despite any effect the CPD forest has on the subsequent CCI forest growth and morphology, the CPD forest does not accumulate any detectable iron by EDXS (not shown). Instead, backscatter analysis shows that the surface iron concentrates in the first CCI forest below the CPD forest (Figure 12) with a relatively large amount also accumulating in the lowermost forest. The absence of iron in or at the top of the CPD forest suggests that the accumulation mechanism (be it sedimentation or agglomeration) is dependent upon the surface character of the CNTs or an interaction with the substrate. By TEM analysis, many small traces of iron can be seen (Figure 13a) in the CCI CNT surfaces that may act as anchor points for subsequent iron attachment, whereas the CPD CNTs present a pure graphitic surface (Figure 13b). The TEM study also shows that the nodules are comprised of an iron seed and a graphitised shell (Figure 13a inset) and the catalyst particles at the CNT cores captured during growth. TEM of the CPD CNTs shows no iron at all (Figure 13b inset). The analyses and observations indicate that there are several distinct populations of iron in the CNTs. The very high concentration at the top of the uppermost CCI layer appears to derive from particles settling out of the gas stream, although failure to accumulate on the CPD forests suggests that a more active binding process is present in the CCI CNTs. The nodule distribution appears to be slightly skewed to the tops of each layer, possibly representing a population of particles that has reached the substrate but failed to bind and hence initiate new growth, instead being captured by the successfully growing

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CNTs. The particles would gradually accumulate a graphite shell and be deactivated. This may also explain the small but consistent decline in backscatter signal in going from the newest (i.e., layer 5) to older forests. The general diffuse surface iron would be as expected from adsorptive processes in an atmosphere near equilibrium, while the core iron particles would be developed from accumulation at the substrate surface. Work to further clarify the behavior and utilization of the catalyst is continuing. A series of substrates (Table 1) was prepared to clarify whether the CPD forest or the residual catalyst structure, after removal of the CPD forest, influences the diameter of subsequently grown CCI CNTs to be either larger, smaller or more or less uniform than would otherwise occur. The diameters of the CNTs were measured at three levels (top, middle, bottom) in each forest layer by SEM. The mean and standard deviations were calculated. Anomalously high values due to bundled tubes (>2 sd from mean and >1 sd from next nearest value) were excluded and the means and standard deviations recalculated. The substrates compared were (Sample 1) a silicon (Si) wafer coated with 5 nm iron; (Sample 2) a Si wafer bearing a forest grown by the standard CPD process; (Sample 3) a Si wafer with a CPD forest grown then carefully removed; (Sample 4) a Si wafer with a CPD forest grown and removed and the wafer oxidized in air (700 °C, 60 min); (Sample 5) an untreated Si wafer; and (Sample 6) an untreated quartz glass slide. The various substrates were placed in the CCI reactor and a 5 × 1 h multilayer CCI forest grown. The diameters of the topmost CCI layer were measured in each sample. In sample 2, both the CPD forest and the topmost CCI forest were measured. The large standard deviations for the measured diameters indicate the broad range of CNT diameters produced by the CVD methods used. As a consequence, comparisons are only indicative of the potential influences of the various substrates on the resulting CNT diameters and distributions. This notwithstanding, it is surprising that no clear effect on the CCI CNT diameters (Table 1) by the substrates or, particularly, the CPD forest can be seen, given the effect the CPD forest has on CCI CNT morphology and initiation, and warrants further study. 4. Conclusions The phenomenon of multilayer CNT forests was identified and patented by Dai8 who anticipated that the structures, including those produced by mixed processes, would have a number of potential applications and be a route to novel properties. Various mechanisms of multilayer formation have been further explored by subsequent researchers as summarized

Hawkins et al. in this paper. We have demonstrated a simple process for generating multilayer CNT forests, both of the pure CCI type and combined with the CPD type. We have found that the CPD forest influences the catalyst deposition and subsequent forest initiation and morphology without itself being significantly altered. Multilayer and mixed multilayer forests add another dimension in the control of CNT properties, the creation of novel nanostructures and provide insights into the mechanism of catalyst deposition and activation. References and Notes (1) Terrones, M. Annu. ReV. Mater. Res. 2003, 33, 419–501. (2) Teo, K.; Singh, C.; Chhowalla, M.; Milne, W. Encycl. Nanosci. Nanotechnol. 2004, 1, 665–686. (3) Bronikowski, M. J.; Willis, P.; Colbert, D.; Smith, K. A.; Smalley, R. E. J. Vac. Sci. Technol. A 2001, 19, 1800–1805. (4) Li, Y.-L.; Kinloch, I. A.; Windle, A. H. Science 2004, 304, 276. (5) Ren, Z. F.; Huang, Z. P.; Xu, J. W.; Wang, J. H.; Bush, P.; Siegal, M. P.; Provencio, P. N. Science 1998, 282, 1105–1107. (6) Fan, S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. Science 1999, 283, 512–514. (7) Andrews, R.; Jacques, D.; Rao, A.; Derbyshire, F.; Qian, D.; Fan, X.; Dickey, E.; Chen, J. Chem. Phys. Lett. 1999, 303, 467–477. (8) Dai, L.; Huang, S. Multilayer carbon nanotube films. WO00/63115. 1999. (9) Cao, A.; Zhang, X.; Wei, J.; Li, Y.; Xu, C.; Liang, J.; Wu, D.; Wei, B. J. Phys. Chem. B 2001, 11937–11940. (10) Deck, C. P.; Vecchio, K. S. J. Phys. Chem. B 2005, 12353–12357. (11) Li, Q.; Zhang, X.; DePaula, R. F.; Zheng, L.; Zhao, Y.; Stan, L.; Holesinger, T. G.; Arendt, P. N.; Peterson, D. E.; Zhu, Y. T. AdV. Mater. 2006, 18, 3160–3163. (12) Li, X.; Cao, A.; Jung, Y. J.; Vajtai, R.; Ajayan, P. M. Nano Lett. 2005, 5, 1997–2000. (13) Zhu, L.; Xiu, Y.; Hess, D. W.; Wong, C. P. Nano Lett. 2005, 5, 2641–2645. (14) Zhu, L.; Hess, D. W.; Wong, C. P. J. Phys. Chem. B 2006, 110, 5445–5449. (15) Zhu, L.; Xu, J.; Xiao, F.; Jiang, H.; Hess, D. W.; Wong, C. P. Carbon 2007, 344–348. (16) Sun, L.; Liu, Z.; Ma, X.; Zhong, Z.; Tang, S.; Xiong, Z.; Tang, D.; Zhou, W.; Zou, L. Y.; Tan, K.; Xie, S.; Lin, J. Chem. Phys. Lett. 2001, 222–226. (17) Nishimura, K.; Okazaki, N.; Pan, L.; Nakayama, Y. Jpn. J. Appl. Phys. 2004, 43, L471–L474. (18) Cao, A.; Ajayan, P. M.; Ramanath, G. Appl. Phys. Lett. 2004, 109– 111. (19) Wang, Y.; Li, B.; Ho, P.; Yao, Z.; Shi, L. Appl. Phys. Lett. 2006, 183113–3. (20) Sato, H.; Hori, Y.; Hata, K.; Seko, K.; Nakahara, H.; Saito, Y. J. Appl. Phys. 2006, 104321–6. (21) Wang, Y.; Kim, M. J.; Shan, H.; Kittrell, C.; Fan, H.; Ericson, L. M.; Hwang, W. F.; Arepalli, S.; Hauge, R. H.; Smalley, R. E. Nano Lett. 2005, 5, 997–1002. (22) Zhang, H.; Liang, E.; Ding, P.; Chao, M. Physica B 2003, 10–16. (23) Zhang, M.; Atkinson, K. R.; Baughman, R. H. Science 2004, 306.

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