High temperature rearrangement of disordered nanoporous carbon at the interface with single wall carbon nanotubes

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High temperature rearrangement of disordered nanoporous carbon at the interface with single wall carbon nanotubes Bo Yia, Ramakrishnan Rajagopalanb, Christopher L. Burketa, Henry C. Foleya,b,c,*, Xiaoming Liud, Peter C. Eklundc,d a

Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, United States Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, United States c Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, United States d Department of Physics, The Pennsylvania State University, University Park, PA 16802, United States b

A R T I C L E I N F O

A B S T R A C T

Article history:

Composites of nanoporous carbon and single wall carbon nanotubes were heat treated in

Received 21 April 2008

vacuum at temperatures ranging from 1200 to 2000 °C. The resultant interface between

Accepted 27 March 2009

the two allotropes of carbon was characterized using high resolution transmission electron

Available online 5 April 2009

microscopy and Raman spectroscopy. At the interface between the nanoporous carbon and the nanotube, the nanotube served as a template for ordering and orientation of the normally disordered nanoporous carbon along the nanotube axis during high temperature treatment. When annealed at 2000 °C, the nanoporous carbon transformed to graphitic nanoribbon which in turn crushed the nanotube to form a nanoscale carbon ‘‘bulb’’. This result is interesting since at these temperatures, the native nanoporous carbon is well known to resist ordering and is therefore referred to as being a ‘‘non-graphitizing’’ carbon. That the nanotube should act as a template for the incipient graphitization suggests that bonding and strength for load transfer may be developed at these interfaces. Ó 2009 Elsevier Ltd. All rights reserved.

1.

Introduction

Carbon nanotubes, especially single wall carbon nanotubes (SWCNT), exhibit unusual mechanical and electronic properties [1,2]. These have made them the focus of both fundamental and applied research [3,4]. In the past, there have been only a few studies devoted to thermal stability of SWCNT. These studies show that smaller diameter SWCNT coalesce into larger diameter nanotubes at temperatures between 1300 and 1400 °C, and that they gradually transform to even more stable structures – especially multi-wall nanotubes (MWCNT) [5–7]. When treated at temperatures in excess of 1600 °C, MWCNT or graphitic nanoribbons (GNR) are also observed to form from the nanotubes [8–10]. Although weak overall, the van der Waals

force between nanotubes play the important role of providing proximity in the initial stages of the processes of deformation and coalescence [11]. Formation of pregraphitic (ribbons) or graphitic structures at temperatures below 2000 °C from nanotubes is interesting because it does not occur with all forms of carbon. For example, carbons derived from polymers that tend to crosslink upon heating, such as those derived by pyrolysis of polyfurfuryl alcohol (PFA), tend to form nanoporous, disordered solid structures and they resist graphitization to temperatures well above 2000 °C. The contrast in behavior begs the question as to what would happen at an interface between the SWCNT and non-graphitizing carbon. To our knowledge, such studies have been only done on carbon fiber/glassy carbon composites [12,13]. Hishiyama et al. made carbon composites

* Corresponding author: Address: Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, United States. Fax: +1 814 865 5604. E-mail addresses: [email protected], [email protected] (H.C. Foley). 0008-6223/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.03.061

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by mixing unidirectionally aligned carbon fibers with polyfurfuryl alcohol and heat treating up to 2800 °C [12]. The authors noticed that due to stress accumulation at the boundary between the fibers and glassy carbon matrix, the graphitization of the composites was found to initiate at the interface between the fibers and the matrix. A question is of practical as well as fundamental importance is what kind of interface will exist in SWCNT-carbon composites. Because of their axial strength, SWCNT are of considerable interest for preparing carbon–carbon (CC) nanocomposites [4,14]. A strong interface between SWCNT and the host matrix will be necessary for good load transfer [15]. We have previously reported that a continuous interface can be obtained between PFA-derived nanoporous carbon (NPC) and SWCNT through pyrolysis at 600 °C of the poly (furfuryl alcohol) (PFA) functionalized SWCNT [16]. Thus in this paper, we report our findings on the interaction of SWCNT with NPC at their interface when they are subjected to much higher temperature treatment. The microstructural evolution of the interface and local regions was elucidated using HRTEM and Raman Spectroscopy.

2.

Experimental

For this work SWCNT produced by high-pressure CO disproportion (HiPco) (CNI Inc.) were used in preparing the NPC/SWCNT. The details of preparation and characterization of this nanocomposite carbon can be found elsewhere [16]. The diameter of the purified nanotubes ranged between 0.87 and 1.3 nm.

The samples of the NPC/SWCNT were heat treated in a hightemperature vacuum furnace (‘‘Red Devil’’ R.D. Webb Inc.). In a typical process of heat treatment, 5 mg of NPC/SWCNTwere placed in the furnace and degassed at 105 Torr and 200 °C overnight. After degassing, the furnace was heated to the target temperature at a rate of 10 °C/min and kept at that target temperature for 2 h. Samples were prepared in this way with final heat treatment temperatures (HTT) of 1200, 1400, 1600, 1800 and 2000 °C. Pure NPC samples prepared from PFA were also treated at 1200, 1800 and 2000 °C under similar conditions. After heating, the samples were allowed to cool under vacuum and were then taken out of the furnace and dispersed in ethanol (Aldrich) for high resolution transmission microscopy (HRTEM) imaging and Raman spectral studies. Samples for HRTEM were made by dropping the dilute suspension on a mesh of copper lacey carbon grids and drying. The HRTEM images were taken with a JEOL 2010F operating at 200 kV. ˚. The point to point resolution was 2 A Samples for Raman spectroscopy were prepared as thin films formed by their dispersion on glass slides. Raman spectra were taken with a Jabin-Yvon Horiba T64000 micro-Raman spectrometer. Excitation was provided by an Ar-Kr laser at 1.5 mW incident power. An excitation wavelength of 514.5 nm was used.

3.

Results and discussion

We have previously shown that NPC derived from PFA pyrolyzed at 600 °C showed no long range order, while the NPC/

Fig. 1 – HRTEM images of (a) pure NPC treated at 1200 °C; (b)–(d) NPC–SWCNT treated at 1200 °C.

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SWCNT composite pyrolyzed under similar conditions was composed of thin bundles of SWCNT uniformly covered with disordered carbon [17]. Fig. 1a shows the HRTEM image of NPC (only) heat treated to 1200 °C. The sample had at most short ˚ with no specific orientation or structural range order of 5 A habit. When the NPC/SWCNT composite was treated at this temperature (Fig. 1b), no long range ordering took place, but most of the graphene sheets of the NPC did orient along the nanotube axis and around its circumference. Higher magnification images (Fig. 1c) showed evidence for a nascent templating effect of SWCNT. The blurring of the sidewall of the SWCNT suggests some structural disruption of the SWCNT when it was surrounded by NPC. Significantly, in the same sample, those SWCNT which were not interacting with the NPC remained fully intact at 1200 °C (Fig. 1d). When NPC/SWCNT was heated at 1400 °C, SWCNT with diameters >2 nm appeared (Fig. 2a) for the first time, showing that the expected coalescence of neighboring SWCNT had indeed been initiated at this temperature. Within the same sample, however, multilayered graphene stacking was also observed. The multilayered graphene stacks are formed due to increased orientation in the NPC which is in contact with the SWCNT. The extent of local ordering beginning at the interface between the NPC and the SWCNT increases with HTT (Fig. 2a–c). Some double-wall carbon nanotubes (DWCNT) and MWCNT appeared when HTT was conducted at temperatures higher than 1600 °C. Pure NPC also showed the tendency towards slightly increasing order

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with temperature, but the structure is clearly very different from that of graphite as NPC structure remains turbostratic in this range of temperature. When compared with the randomly aligned graphene sheets present in the pure NPC treated at 1800 °C (Fig. 2d), the NPC in contact with the SWCNT within the composite exhibited an increased tendency to align along the SWCNT axis with increasing temperature. Noticeable changes in the NPC were obtained when it was heated at 2000 °C (Fig. 3a). Although it did not graphitize, graphene layers are noticeably less wrinkled; distortions are annealed due to the release of intra-layer defects, so the layers become suddenly stiff and flat inside a pore wall. However, the graphene sheets of the NPC in contact with the SWCNT in the composite carbon were even more oriented and ordered. They displayed orientation along the nanotube axis (Fig. 3b and c). These graphene sheets have coalesced into graphitic nanoribbons (GNR). When confined by the GNR, the walls of the large SWCNT (and some DWCNT) collapsed into graphene sheets, and their ends formed bulbs. By contrast, when the same purified HiPco SWCNT were treated at the same conditions but in the absence of the NPC carbon, only multi-wall nanotubes (MWCNT) were formed [9]. However, in the carbon composite, since the SWCNT bundles were segregated by NPC, this resulted in less formation of MWCNT. The stress accumulated at the interface of the SWCNT and the NPC resulted in stress graphitization. By contrast, it is noteworthy that within the same sample those

Fig. 2 – HRTEM images of NPC/SWCNT treated at (a) 1400 °C; (b) 1600 °C; (c) 1800 °C and (d) pure NPC annealed at 1800 °C. The arrows are pointed to the multilayer carbon shells formed due to increased orientation of NPC.

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SWCNT which are not surrounded by NPC increased in diameter and their walls appear to be severely damaged (Fig. 3d). The HRTEM images show that the NPC alone behaved as expected that is as a ‘‘non-graphitizing’’ carbon with only low levels of ordering and no preferential orientation occurring even at high temperatures. However, in the presence of the SWCNT, at high temperature (2000 °C) the NPC did begin to order and orient and did so with a strong preference along the tube wall and parallel to the tube axis. The new phenomenon of nanobulb formation at the tube end was also observed. The imaging indicates that solid state reactions between NPC and SWCNT do take place at the elevated temperatures. Although HRTEM imaging is quite powerful, it is not a means for obtaining information on bulk or mean behaviors; for that we turned to Raman spectroscopy. Raman spectroscopy is especially important because it is quite sensitive to the subtle changes in the average environment of the SWCNT. The Raman spectra were collected from bulk samples of the NPC/SWCNT material after HTT and these results are shown in Fig. 4. For comparison, all the spectra were normalized by the highest intensity of the G band at 1590 cm1. The band in the range of 100–300 cm1 is a signature of the SWCNT, and it is related to its radial breathing mode (RBM) [17]. The intensity of the RBM band is sensitive to bonding, defects and charge transfer. In the pristine NPC/SWCNT nanocomposite, there are 3 main peaks at 250, 273, 275 cm1 and a relatively weak peak at 190 cm1. From the Kataura plot,

a relation between the RBM frequency x and nanotube diameter dt: has been derived whereby xðcm1 Þ  223:7=d t ðnmÞ þ 12 [18,19]. On this basis, the three main Raman peaks arise from the metallic SWCNT with 0.85–1 nm diameters, while the weak peak arises from semi-conducting SWCNT with 1.3 nm diameter. RBM of SWCNT was noted to have decayed dramatically in the NPC/SWCNT sample that was heated at 1200 °C. This result is quite different from that which is observed with the pure SWCNT. The Raman spectrum of the pure SWCNT display strong RBM line intensities even after high temperature heat treatment at 1200 °C; in fact RBM decay became evident only when HTT was higher than 1800 °C [7,8]. A new band in the Raman spectrum of NPC/SWCNT sample at 120 cm1 corresponding to the double-sized SWCNT was observed when the HTT reached 1800 °C. This line was indistinct in the spectra of the sample treated at lower temperatures because the intensities of the RBM decrease with tube diameter, which makes the RBM of the coalesced tubes too weak to observe until they reach significant concentration within the material. The RBM disappeared completely from the spectrum when the sample was treated at 2000 °C. Only small numbers of double-sized SWCNT survived at this temperature as shown in HRTEM images, and make little or no contribution to RBM. In the high frequency region of the Raman spectra, from 1250 to 1800 cm1, there are two bands that are associated with the tangential C–C stretching modes of the SWCNT, a

Fig. 3 – HRTEM images of (a) pure NPC and (b), (c) and (d) NPC–SWCNT each annealed at 2000 °C. The arrows are pointed to the ‘‘neck’’ between GNC and nanobulbs.

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s

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m

600 ºC

Intensity (a. u.)

1200 ºC

1400 ºC

1600 ºC

1800 ºC

2000 ºC 50

100

150

200

250

1200

1400

1600

Raman Shift (cm-1) Fig. 4 – Raman spectra of NPC/SWCNT after HTT. All the spectra were normalized with the strongest G bands. The RBM features from metallic tubes and semi-conducting tubes are marked as ‘‘m’’ and ‘‘s’’, respectively. RBM of NPC/SWCNT treated at 2000 °C was not shown because it was too weak to recognize.

stronger band at 1590 cm1, the G band, and a weaker band near 1340 cm1, the D band. Disorder and defects in the periodic sp2 network of SWCNT walls strengthen the intensity of the D band scattering [20]. The G band of the SWCNT actually contains several components. In order to evaluate the thermal evolution of the NPC/SWCNT nanocomposite in detail, curve fitting of the band intensities with Lorentzians was carried out for the G band of each of the heat treated samples. The resultant Raman parameters are listed in Table 1. A minimum number of Lorentzian curves was used to fit the data in each case. The most intensive component of the G band near 1590 cm1 is associated with the ordered sp2 hybridized carbon vibration along the nanotube axis [20]. Broadening of this component was observed after heat treatment, even for an HTT of 1200 °C, which was a sign of increased disorder in the graphene structure of SWCNT walls. At the same time, there was a significant rise in the intensity of the D band after heat treatment. Both the broadening of the G band and the intensity increase in the D band were consistent with the decrease in the intensity of the RBM band that took place with higher temperature heat treatment. We believe that close contact of SWCNT with neighboring NPC causes significant

disruptions in the wall of the nanotube, thereby forming more ‘‘defective’’ SWCNT. Disruptions of the SWCNT walls at the interface with the NPC were also found in HRTEM images. Lopez et al. simulated the thermal stability of SWCNT and found that SWCNT with high concentration of defects were produced upon coalescence [10]. In our experiments, the decrease in the RBM intensity and the rise of G band intensities, which indicate reaction between the NPC and the SWCNT, were observed only when the HTT was higher than 1800 °C. In contrast SWCNT–SWCNT coalescence was observed to commence at a much lower temperature (1200 °C) [7,8]. However, for the NPC–SWCNT interaction only the formation of SWCNT defects is evident at 1200 °C and higher temperatures were required to cause coalescence of the SWCNT and NPC. Another change in the G band component near 1590 cm1 was its shift to higher frequency with high temperature treatment. It has been well established that both the tangential mode and the RBM shift toward higher frequency with increasingly higher pressure imposed on the nanotube [21,22]. Empirically, we know that there is significant volume shrinkage and densification in the NPC upon heating above 800 °C since the material diminishes significantly in porosity.

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Table 1 – Raman factors of NPC/SWNT treated at different HTT temperature. G band (cm1)

HTT temperature (°C)

Before annealing 1200 1400 1600 1800 2000

D band (cm1)

x

FWHM

X

1591 1596 1595 1596 1594 1584

16.2 35.1 28.9 29.1 25.0 30.9

1335 1340 1339 1343 1345 1350

ID/IG

0.04 0.18 0.22 0.36 0.33 0.54

x: Raman frequency. FWHM: frequency width at half maximum. ID/IG: Intensity ratio of D band over G band.

~1200 ºC

~1400-1800 ºC

~2000 ºC

Fig. 5 – Scheme of NPC/SWCNT structural transformation under HTT.

Volume shrinkage will cause stress accumulation at the boundary of the SWCNT and the NPC. At the nanoscale, with higher temperature this increasing deformation in the NPC surrounding the SWCNT will apply increasing pressure directly to the SWCNT. This is then consistent with the observation of the G band shifting to higher frequency – it is due to stress accumulation at the interface of SWCNT and the NPC. When NPC/SWCNT was heated at 2000 °C, there was only one component in the G band at 1584 cm1, and this indicates that graphitic-like carbon was dominant in the sample. The FWHM of the G band is 30 cm1. Usually, the G band of the well-ordered graphite, or large MWCNT with inner diameters of 3–5 nm, has a FWHM of 18 cm1 [20]. We also note that there is a small shoulder located at 1620 cm1 on the main G band. This shoulder is usually assigned as the D 0 or G* band and it is associated with the end planes of the graphene sheets [23]. The broadening of the G band and the appearance of the D 0 band, together with the increase in the intensity of the D band, confirmed the finite lateral size of the graphitic-like carbon which we have ascribed to GNR layers and to the limited number of layers in any GNR stack. Both of these conclusions taken from the analysis of the Raman spectrum of the material treated at 2000 °C were consistent with the observations made independently from the HRTEM images. The narrow shoulder of the G band – in the vicinity of 1570 cm1 is related to the vibration around the circumference of the SWCNT and this frequency is diameter dependent [24]. We observed that upon heating to higher temperature the band arising from the circumferential vibration of the SWCNT in our samples shifted to slightly lower frequency. This is an observation similar to one that was made earlier

by Kim et al. during the HTT of DWCNT [25]. The broad shoulder of the G band with a maximum at 1520 cm1 is associated with the energy separation (E11 = Ec1Ev1) between the first pair of electronic density of states (DOS) singularities in the valence (v) and conduction (c) bands of the metallic nanotubes [26]. This feature arising from the metallic tubes in our NPC–SWCNT samples decreased gradually with increasing HTT and vanished completely when the HTT was 1800 °C and higher. This change therefore is consistent with the change in the intensity of the RBM band, since the signature of metallic tubes also disappeared with an HTT of 1800 °C. Ultimately, these results all indicate that smaller nanotubes are less thermally stable and more reactive than larger nanotubes, a conclusion that is consistent with thermodynamics and is kinetically consistent as well. From a thermodynamic point of view, both the NPC and the SWCNT are less stable than graphite. Thus there is a free energy driving force for transforming either of them into more stable structures at higher temperature, The free energy, however, can be released only if there is a pathway with a low enough activation barrier for the transformation to happen in experimental time. By creating the interface between the NPC and the SWCNT, we see that transformation does occur; this is a transformation that would not occur if either, alone, were treated in the same way. We infer then that the creation of the interface opens up new mechanistic pathways for this transformation to proceed. It is interesting to note that the final structures of the high temperature heat treatment of NPC and SWCNT, although slightly more ordered, are still noticeably different from graphite. The 1D SWCNT transform to 1D MWCNT and 1D partially graphitized

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nanoribbons, while the NPC transforms to 3D partially graphitized nanoribbons. Hence, the deformations and structural changes that take place in the combined SWCNT and NPC at high temperature lead to new and different intermediate carbon morphologies.

4.

Conclusion

In summary, the presence of NPC surrounding the SWCNT resulted in significant rearrangement of disordered NPC during high temperature treatment. Fig. 5 briefly summarizes schematically the structural changes of the NPC/SWCNT nanocomposite upon high temperature treatment. When the NPC/SWCNT was heated at 1200 °C, the NPC began to form graphene sheets and densified. The orientation of the NPC graphenes was only slightly affected by SWCNT along both the neighboring nanotube’s axial and circumferential directions. Although the SWCNT could remain intact when heated alone at this temperature, the SWCNT covered with NPC begins to react and to form high concentrations of defects. With a further increase in the HTT(1400–1800 °C), the extent of orientation of NPC increased and orientation of the graphenes along nanotube axis became preferred. At 2000 °C, the NPC graphenes were completely converted into GNR due to the template effect of SWCNT. At the same time, the forces induced by NPC shrinkage and confinement collapsed the SWCNT and DWCNT into GNR, while the ends of the tubes not in contact with the GNR bulged to produce the nanobulbs that were observed we believe for the first time.

Acknowledgements This project is funded by NSF NIRT DMR01-03585. The authors would thank Dr. Humberto Gutierrez for helpful discussion.

R E F E R E N C E S

[1] Treacy MMJ, Ebbesen TW, Gibson JM. Exceptionally high Young’s modulus observed for individual carbon nanotubes. Nature 1996;381:678–80. [2] Tans SJ, Devoret MH, Dai HJ, Thess A, Smalley RE, Geerligs LJ, et al. Individual single-wall carbon nanotubes as quantum wires. Nature 1997;386:474–7. [3] Dresselhaus MS, Dresselhaus G, Saito R, Jorio A. Raman spectroscopy of carbon nanotubes. Phys Rep 2005;409:47–99. [4] Thostenson ET, Ren ZF, Chou TW. Advances in the science and technology of carbon nanotubes and their composites: a review. Comp Sci Tech 2001;61:1899–912. [5] Nikolaev P, Thess A, Rinzler AG, Colbert DT, Smalley RE. Diameter doubling of single-wall nanotubes. Chem Phys Lett 1997;266:422–6. [6] Metenier K, Bonnany S, Beguin F, Journet C, Bernier P, de la Chapelle ML, Chauvet O, Lefrant S. Coalescence of singlewalled carbon nanotubes and formation of multi-walled carbon nanotubes under high-temperature treatments. Carbon 2002;40:1765–73.

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[7] Yudasaka M, Ichihashi T, Kasuya D, Kataura H, Iijima S. Structure change of single-wall carbon nanotubes and singlewall carbon nanohorns caused by heat treatment. Carbon 2003;41:1273–80. [8] Gutierrez HR, Kim UJ, Kim JP, Eklund PC. Thermal conversion of bundled carbon nanotubes into graphitic ribbons. Nano Lett 2005;11:2195–201. [9] Kim UJ, Gutierrez HR, Kim JP, Eklund PC. Effect of the tube diameter distribution on the high-temperature structural modification of bundle single-walled carbon nanotubes. J Phys Chem B 2005;109:23358–65. [10] Lopez MJ, Rubio A, Alonso JA. Deformation and thermal stability of carbon nanotube ropes. IEEE Trans Nanotech 2004;3:230–6. [11] Ruoff RS, Tersoff J, Lorents DC, Shekhar S, Chan B. Radial deformation of carbon naotubes by van-der-Waals forces. Nature 1993;364:514–6. [12] Hishiyama Y, Inagaki M, Kimura S, Yamada S. Graphitization of carbon fibre/glassy carbon composites. Carbon 1974;12:249–58. [13] Inagaki M, Meyer RA. Stress graphitization. In: Thrower PA, Radovic LR, editor. Chemistry and Physics of Carbon, vol. 26. CRC Press; 1999. p. 195-232. [14] Coleman JN, Khan U, Blau WJ, Gun’ko YK. Small but strong: a review of the mechanical properties of carbon nanotubepolymer composites. Carbon 2006;44:1624–52. [15] Qian D, Dickey EC, Andrews R, Rantell T. Load transfer and deformation mechanism in carbon nanotube-polystyrene composites. Appl Phys Lett 2000;76:2868–70. [16] Yi B, Rajagopalan R, Foley HC, Kim UJ, Liu X, Eklund PC. Catalytic polymerization and facile grafting of poly(furfuryl alcohol) to single-wall carbon nanotube: preparation of nanocomposite carbon. J Am Chem Soc 2006;128:11307–13. [17] Rao AM, Richter E, Bandow S, Chase B, Eklund PC, Williams KA, et al. Diameter-selective Raman scattering from vibrational modes in carbon nanotubes. Science 1997;275:187–91. [18] Strano MS. Probing chiral selective reactions using a revised Kataura plot for the interpretation of single-walled carbon nanotube spectroscopy. J Am Chem Soc 2003;125:16148–53. [19] Saito R, Dresselhaus MS, Dresselhaus G. Physics of carbon nanotubes. London: Imperial College Press; 1998. [20] Eklund PC, Holden JM, Jishi RA. Vibrational-modes of carbon nanotubes-spectroscopy and theory. Carbon 1995;33:959–72. [21] Venkateswaran UD, Rao AM, Richter E, Menon M, Rinzler A, Smalley RE, et al. Probing the single-wall carbon nanotube bundle: Raman scattering under high pressure. Phys Rev B 1999;59:10928–34. [22] Merlen A, Toulemonde P, Bendiab N, Aouizerat A, Sauvajol JL, Montagnac G, et al. Raman spectroscopy of open-ended single wall carbon nanotubes under pressure: effect of the pressure transmitting medium. Phys Status Solidi B 2006;243:690–9. [23] Katagiri G, Ishida H, Ishitani A. Raman-spectra of graphite edge planes. Carbon 1988;26:565–71. [24] Kasuya A, Sasaki Y, Saito Y, Tohji K, Nishina Y. Evidence for size-dependent discrete dispersion in single wall nanotubes. Phys Rev Lett 1997;78:4434–7. [25] Kim YA, Muramatsu H, Hayashi T, Endo M, Terrones M, Dresselhaus MS. Thermal stability and structural changes of double-walled carbon nanotubes by heat treatment. Chem Phys Lett 2004;398:87–92. [26] Pimenta MA, Marucci A, Empedocles SA, Bawendi MG, Hanlon EB, Rao AM, et al. Raman modes of metallic carbon nanotubes. Phys Rev B 1998;58:16016–9.