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Carbon cage structures in single wall carbon nanotubes: a new class of materials
PERGAMON
Carbon 38 (2000) 1751–1756
Carbon cage structures in single wall carbon nanotubes: a new class of materials David E. Luzzi*, Brian W. Smith Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104 -6272, USA
Abstract A new class of materials, carbon cage structures contained within single-wall carbon nanotubes (SWNTs) has been discovered. This class of hybrid materials could form the basis of functional devices for application in electronics, biomedicine and microelectromechanical systems. The cage structures internal to the SWNTs are found to be C 60 fullerenes resembling nanoscopic peapods. Peapods are found to coalesce into capsules and interior tubes under prolonged exposure to a 100 keV electron beam. Through in-situ experiments, it is found that peapods form via the motion of C 60 molecules to the nanotubes. The fullerenes enter the SWNTs, presumably through open ends and / or sidewall defects and are found to cluster in chains. In this paper, the structure, some properties and synthesis routes of these hybrid structures will be explained. 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon nanotubes, Fullerene; B. Intercalation; C. Electron microscopy (TEM), Radiation damage
1. Introduction Since their discovery [1–3], single-wall carbon nanotubes (SWNTs) have stimulated widespread scientific interest due to their promising electronic and mechanical properties. However, the processes by which SWNTs are synthesized are still unrefined, and work on the functionalization of nanotubes is just beginning. One possible path towards the production of functionalized SWNTs is by filling the core with a material with useful properties. The nanotube can then function as a container with good stability or also contribute to the properties of the final structure. Soon after the first production of multi-wall carbon nanotubes (MWNT) methods were sought to fill the tube cores with other materials. The earlier work in this area has produced notable success in the opening and filling of MWNTs [4–12]. This prior work has involved the capillarity-based uptake of low surface tension liquids into MWNTs with large inner diameters produced using the carbon arc (CA) technique. The length of MWNT that has been continuously filled using these methods was short. Given the ease and control with which they are now *Corresponding author. Tel.: 11-215-898-8366; fax: 11-215573-2128. E-mail address: [email protected] (D.E. Luzzi).
produced, it is of interest to determine if single-wall carbon nanotubes (SWNT) can be efficiently filled. Soon after the production of single wall carbon nanotubes (SWNTs) [13], it was noted that the diameter of the most abundant nanotube produced by the pulsed laser vaporization method was correctly-sized to encapsulate a C 60 molecule [14]. Recently, we discovered that these unique hybrid structures (peapods) are a natural product in PLV-synthesized material that has been acid purified and annealed at 11008C, albeit at low concentrations [15]. In this paper, we review these findings including the methods that can be used to synthesize these unique structures. We find that the filling of SWNTs with C 60 (peapods) can be efficiently accomplished. The synthesis methods described herein provide a means to produce bulk quantities of these interesting materials for further study and point to the possibility of the general functionalizing of carbon nanotubes.
2. Experimental methods The starting material for this study was acid-purified carbon nanotubes produced by the pulsed laser vaporization (PLV) technique. The PLV material had been synthesized by the laser ablation of a graphitic target impregnated with 1.2 at% each Ni / Co catalyst. This raw
0008-6223 / 00 / $ – see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 00 )00088-9
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nanotube ‘‘felt’’ was refluxed in HNO 3 for 48 h, rinsed and neutralized, suspended in surfactant, and filtered to form a thin paper [16]. Such wet chemical etching is known to open the ends of MWNTs [11] as well as attack the sidewalls of SWNTs [17]. One additional batch of PLV material had received an anneal of 11008C in vacuo prior to being received. The as-purified materials were annealed under vacuums ranging from 20 to 40 mPa at temperatures between 100 and 12008C. These annealing treatments were carried out in a vacuum furnace or in-situ in a JEOL 2010F fieldemission-gun transmission electron microscope (FEGTEM). During in-situ anneals, temperature was monitored continuously via thermocouples. Only a few minutes were required to ramp between temperatures due to the small thermal mass of the heater. In addition, some PLV samples were annealed in a flowing Ar atmosphere at ambient pressure to a temperature of 6008C. The structure of both in-situ and ex-situ specimens was examined via TEM phase contrast imaging in either the FEG-TEM or in a JEOL 4000 high-resolution TEM (HRTEM). All microscopy was performed at an accelerating voltage of 100 kV to minimize the electron beam induced modification of the material. Electron dose to the specimen in the JEOL 4000 was approximately 3.4(10)19 electrons cm 22 s 21 , which was used for the study of the effects on the material of extended exposure to the electron beam. Magnification was determined using polyaromatic carbon shells present in the specimen, which originate from the decomposition of carbide crystals. It is known that the strong lattice fringes from these turbostratic shells have a well defined spacing of 0.34 nm. Microscopy specimens were prepared from nanotube paper by tearing away a small sliver and fixing it inside an oyster TEM grid, thereby foregoing additional chemical or thermal processing.
3. Experimental results An HRTEM image of an isolated SWNT is shown in Fig. 1a. Under the imaging conditions used in this work, atoms appear dark against a bright background. The image is darkest where the electron beam encounters the most atoms, which occurs where the beam is tangent to a nanotube or a fullerene wall. The parallel lines in the image are thus opposing walls of the tubule. The lines are separated by 1.4 nm, which is the expected diameter of a typical SWNT. In comparison, Fig. 1b shows a 1.4-nm diameter SWNT from the 11008C specimen containing a self-assembled chain of collinear C 60 molecules similar to a nanoscopic peapod. A number of observations support this interpretation of the image. Each circle in the image is 0.7 nm in diameter and is separated from the tube by 0.3 nm at the closest point. These are the expected measurements of
Fig. 1. HRTEM micrographs of (a) a typical 1.4 nm diameter SWNT, and (b) a 1.4 nm diameter SWNT that contains a selfassembled chain of C 60 molecules [22]. There is variability in the separations of adjacent molecules in (b), although they are frequently arranged in easily distinguished pairs. One such pair is indicated with an arrow. Both tubes are surrounded by vacuum. The scale bar is 2 nm.
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contained C 60 separated from the tubule walls by a graphitic Van der Waals spacing. The contrast of each circle is similar to the contrast of the tubule walls, which supports that the constituent atoms are carbon and not a more strongly scattering element. Each molecule must be a closed cage because an unclosed structure with fewer bonding constraints should not appear circular in cross section and is expected to fluctuate between many different conformations. Finally, if the C 60 molecules were on the exterior surface of the tube, they should be randomly positioned around the circumference, and their images should sometimes intersect the images of the tubule walls. However, even in imaging many such chains, this was not observed. This supports that the C 60 is contained. The number of peapods in this material is quite small. The peapods tend to be segregated on both the microscopic and macroscopic scales. On the microscopic scale, segregation is observed as regions in which few peapods can be found in the microscope adjacent to regions in which approximately 10% of observed SWNTs that are isolated or in small ropes contain C 60 . Macroscopic segregation has been seen in UV–VIS experiments in which separate milligram weight samples were found to contain between 0.8 and 5% of SWNTs filled with C 60 [18]. There is obvious variability in the separations of adjacent C 60 molecules of Fig. 1b within the nanotube. The images of some are distinct, while others are so close that they appear to be tangent or paired. An example of the latter case is indicated with an arrow in Fig. 1b. Pairing of C 60 is commonly observed in chains. The chronological sequence of images in Fig. 2 shows a chain of C 60 molecules coalescing inside a 1.4 nm diameter SWNT as a consequence of 100 keV electron irradiation. Images were taken at approximately 300 s intervals. Before extensive irradiation (Fig. 2a), each molecule is easily distinguished from its neighbors. The separation between some adjacent molecules is decreased after moderate irradiation (Fig. 2b) such that pairs are formed (cf. Section 3.1). This contraction becomes more pronounced during observation, and noticeable gaps are formed between groups of coalescing molecules (Fig. 2c). Coalescence occurs by structural damage to the C 60 followed by the formation of inter-cage covalent bonds. This is seen in the image as a blurring of contrast between the closely separated cages. Prolonged irradiation (Fig. 2d) ultimately produces pill shaped capsules that are a few nanometers in length. Irradiation damage to the surrounding nanotube is also apparent in one location beginning in Fig. 2b but occurs at a slower rate than damage to the C 60 . In the as-received acid-purified PLV material, no peapods have been detected during thousands of observations of individual SWNTs or of SWNTs bundled in ropes. However, the as-received material is coated with surfactant, which obscures the nanotubes and makes it difficult to observe peapods. We have found that baking in vacuum for at least 24 h at 2258C removes the surfactant without
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modifying the nanotube material. Several hundred tubes were observed after such cleaning and were found to be empty of C 60 . Thus, by patient collection of a large number of HRTEM images of the material in the aspurified condition and after 2258C anneal, it is apparent that peapods do not exist in the material following these processing steps. In-situ annealing experiments were conducted in the FEG-TEM at temperatures between 2258C and 4508C on specimens that were prepared from PLV material and subsequently cleaned by baking for 24 h at 2258C. The image of Fig. 3 was recorded at 3508C. C 60 molecules have adsorbed to the surface of the nanotubes, which were originally clean. The molecules adsorb to the surface and are present for only short times before vanishing. The process is quite dynamic with many molecules appearing, briefly residing on the surface and then disappearing so quickly as to be undetectable to the eye. This rapid activity was seen at 3508C but not at 3258C. Thus the onset temperature lies between these limits. Annealing experiments were carried out at various temperatures on a number of specimens in order to determine the formation mechanism of the peapods. In Fig. 4, a sample is shown after annealing at 4508C for 2 h in vacuum. The walls of the SWNTs are partially healed. Many peapods are seen to be present indicating that the formation of peapods occurred during the annealing treatment following acid purification and the 2258C cleaning anneal. These results confirm that C 60 is entering the nanotubes during these relatively low temperature anneals.
4. Discussion It is now well established that peapods contain C 60 . This is based on the observed size of features and the contrast in HRTEM micrographs, on UV–VIS experiments [18], and on the presented evidence that the motion of external C 60 in SWNT containing material is coincident with the formation of peapods. C 60 is contained almost exclusively by 1.3 nm to 1.4 nm diameter SWNTs. Tubes of this size are the only tubes that allow contained C 60 to everywhere maintain a preferred graphitic 0.3 nm Van der Waals separation from the tubule walls. Thus it is possible that the filling of nanotubes with C 60 is strongly correlated to the diameter of a SWNT. Furthermore, contained C 60 is observed only in chains, some of which have 30 or more members. This observation may be explained by first considering that solid C 60 exists as an FCC structure with a lattice parameter of 1.41 nm [19]. Each C 60 is spaced approximately 1.0 nm center-tocenter from 12 nearest neighbors. An isolated C 60 molecule contained in a SWNT is thus under-coordinated in the direction of the tubule axis, and its stability may be increased if it is coordinated with neighbors on one or both sides. Alternatively, the activation energy required for the
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Fig. 2. A chain of C 60 molecules coalescing inside a 1.4 nm diameter SWNT under 100 keV electron irradiation (a) after minimal irradiation time, (b) after |300 s, (c) after |600 s, and (d) after |900 s. In (a), each molecule is distinct from its neighbors. Pairing is apparent in (b) as coalescence is initiated. In (c), gaps form between groups of coalescing molecules. In (d), the chain is clearly transformed into longer capsules. Radiation damage to the surrounding SWNT is visible in the top right of each image beginning with (b). The scale bar is 2 nm.
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Fig. 3. An in-situ HRTEM image at 3508C showing the adsorption of C 60 molecules onto the surface of SWNTs.
diffusion of an individual C 60 might be very low such that they are always moving and are unable to be clearly imaged. The physical center-to-center separation between C 60 molecules in a chain is difficult to determine experimentally because of the depth of field of the microscope. The depth of field is approximately 10 nm, so a well focused longitudinal section of tube does not necessarily have its axis aligned normal to the electron beam. Because the
Fig. 4. Formation of peapods. A rope of PLV material is shown after further annealing treatment for 1 h at 4508C. Many of the SWNTs are now filled with chains of C 60 molecules.
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HRTEM image is a projection, the apparent separation between molecules in a chain in this condition would be less than the physical separation. However, such error in measurement is minimized for very long longitudinal sections that are completely in focus. Using this criterion, the physical center-to-center separation of C 60 molecules in a chain is often 1 nm. However, observations of smaller intermolecular separations invite the question of whether C 60 can exist in a dimerized or polymerized state. Since it is known that thin C 60 films photopolymerize under UV/ visible light [20], it is possible that dimerization could have occurred during normal handling of the specimen outside of the microscope. The center-to-center separation between C 60 molecules in a pair is often 0.9 nm, which is consistent with a (C 60 ) 2 dimer to within the resolution of the microscope [21]. Irradiation-induced coalescence is interpreted as a consequence of damage to contained C 60 molecules. Consider that a graphene sheet may be curved into a spherical molecule only with the introduction of pentagonal defects. This requires that the preferred bond angles cannot be maintained at the pentagon, and the corresponding carbon atoms are less tightly bound to the lattice. Such is the case for C 60 , and so it is expected that contained C 60 will be damaged by the electron beam at a faster rate than the SWNT where the carbon–carbon bond angles are less distorted. The displacement of carbon atoms from C 60 leaves unsatisfied bonds in the essentially hermetic nanotube cavity. These dangling bonds can be satisfied if covalent bonds are formed between adjacent damaged C 60 molecules. Because each fullerene cap requires six pentagons, the coalescence of C 60 molecules allows the total number of pentagons in the isolated molecules to be reduced by as much as 12 (n21). Although the coalesced state is energetically favorable, it is supposed that the incident electron beam is required to damage the C 60 molecules and form unsatisfied bonds before coalescence can begin. It is clear by inspection that the SWNTs in the current work are filled either via a vapor phase process or via the surface diffusion of individual molecules, or a combination of both. The source of C 60 is presumably residual crystallites left in the material after the synthesis and purification processes. It is known from prior analysis that acid purified PLV material contains residual crystallites of C 60 [16]. It is therefore likely that C 60 enters the nanotube by transporting to open ends or sidewall defects by surface diffusion or in the gas phase. If the mechanism of C 60 transport is via sublimation and via the vapor phase, then the onset temperature should be a strong function of vacuum system pressure. On the other hand, a surface diffusion mechanism should be less sensitive to pressure. This remains to be tested. Once the fullerene arrives on the nanotube surface, the large amount of filling that is seen argues for surface diffusion to open end or sidewall defects. Under a vapor phase transport
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mechanism, as the temperature is increased, the residence time of C 60 on the surface of the SWNT, and thus the probability of entering the SWNT will decrease. At high temperatures, the defects in the SWNTs will anneal thereby blocking the entrance of C 60 molecules into the tubes. Thus it would appear that there will be a critical temperature window within which peapods can be efficiently formed.
5. Conclusions Contained C 60 can be induced to enter SWNTs, forming nanoscopic peapods. Encapsulated C 60 is typically found in chains of 10 or more members. The separations between adjacent molecules are consistent with expected Van der Waals interactions. However, these separations are determined to vary, which suggests that other types of C 60 – C 60 bonding might be present as well. A tendency for C 60 molecules to be paired is apparent even in long chains. Some chains are observed to coalesce into pill shaped capsules under electron irradiation. The mechanism for the formation of encapsulated C 60 chains is now apparent, and we have successfully synthesized them in large fractions of the tubes that comprise our samples. In contrast to prior studies showing the uptake of liquid into large diameter MWNTs, the mechanism involves transport in the vapor phase or the surface diffusion of individual molecules. SWNTs with small interior diameters are completely filled over long lengths. The extent of filling argues for the entrance of C 60 through sidewall defects as well as open ends. A minimum temperature must be achieved in order to promote exterior C 60 to enter the tubes. Annealing at too high a temperature will limit the residence time of C 60 on the SWNT as well as cause the healing of the nanotubes walls, thereby eliminating access to the nanotube interiors. There is thus a critical temperature window required for the formation of bucky-peapods. The ability to synthesize these unique hybrid materials in bulk will now permit scaled-up empirical study. Interestingly, there is no obvious physical reason why SWNTs could not be filled with any appropriately-sized molecule with an affinity for nanotubes, and present in the gas phase or highly mobile at temperatures within the critical temperature window. Thus, the present work opens the possibility for the general functionalizing of the particular diameter SWNTs produced at highest concentration by PLV and CA methods.
als. This work benefited from discussions with colleagues Profs. J.E. Fischer, L.A. Girifalco and V. Vitek. This work was funded by the National Science Foundation with central facility support from the U. Penn MRSEC.
References [1] [2] [3] [4] [5] [6]
[7] [8] [9]
[10]
[11] [12] [13] [14] [15] [16]
[17] [18] [19] [20] [21] [22]
Acknowledgements We thank Profs. A.G. Rinzler, D.T. Colbert and R.E. Smalley for the acid purified and / or anealed PLV materi-
Ajayan PM et al. Chem Phys Lett 1993;215:509. Bethune DS et al. Nature 1993;363:605. Iijima S, Ichihashi T. Nature 1993;363:603. Ajayan PM, Iijima S. Capillarity-induced filling of carbon nanotubes. Nature 1993;361:333. Ajayan PM et al. Opening carbon nanotubes with oxygen and implications for filling. Nature 1993;362:522. Ajayan PM et al. Carbon nanotubes as removable templates for metal oxide nanocomposites and nanostructures. Nature 1995;375:564. Davis JJ et al. The immobilisation of proteins in carbon nanotubes. Inorg Chim Acta 1998;272:261. Dujardin E et al. Capillarity and wetting of carbon nanotubes. Science 1994;265:1850. Sloan J et al. Selective deposition of UCl 4 and (KCl) x (UCl4) y inside carbon nanotubes using eutectic and noneutectic mixtures of UCl 4 and KCl. J Solid State Chem 1998;140:83. Tsang SC, Harris PJF, Green MLH. Thining and opening of carbon nanotubes by oxidation using carbon dioxide. Nature 1993;362:520. Tsang SC et al. A simple chemical method of opening and filling carbon nanotubes. Nature 1994;372:159. Ugarte D et al. Filling carbon nanotubes. Appl Phys A 1998;67:101. Thess A et al. Crystalline ropes of metallic carbon nanotubes. Science 1996;273:483. Nikolaev P et al. Diameter doubling of single-wall nanotubes. Chem Phys Lett 1997;266:422. Smith BW, Monthioux M, Luzzi DE. Encapsulated C 60 in carbon nanotubes. Nature 1998;396:323. Rinzler AG et al. Large scale purification of single wall carbon nanotubes: process, product and characterization. Appl Phys A 1998;67:29. Monthioux M, Smith BW, Luzzi DE. Unpublished data, 1998. Burteaux B et al. Abundance of encapsulated C 60 in singlewall carbon nanotubes. Chem Phys Lett 1999;310:21. Heiney PA. J Phys Chem Solids 1992;53:1333. Rao AM et al. Photoinduced polymerization of solid C 60 films. Science 1993;259:955. Wang G-W et al. Synthesis and X-ray structure of dumb-bellshaped C 120 . Nature 1997;387:583. Smith BW, Monthioux M, Luzzi DE. Carbon nanotube encapsulated fullerenes: a unique class of hybrid materials. Chem Phys Lett 1999;315:31.
Carbon 38 (2000) 1751–1756
Carbon cage structures in single wall carbon nanotubes: a new class of materials David E. Luzzi*, Brian W. Smith Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104 -6272, USA
Abstract A new class of materials, carbon cage structures contained within single-wall carbon nanotubes (SWNTs) has been discovered. This class of hybrid materials could form the basis of functional devices for application in electronics, biomedicine and microelectromechanical systems. The cage structures internal to the SWNTs are found to be C 60 fullerenes resembling nanoscopic peapods. Peapods are found to coalesce into capsules and interior tubes under prolonged exposure to a 100 keV electron beam. Through in-situ experiments, it is found that peapods form via the motion of C 60 molecules to the nanotubes. The fullerenes enter the SWNTs, presumably through open ends and / or sidewall defects and are found to cluster in chains. In this paper, the structure, some properties and synthesis routes of these hybrid structures will be explained. 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon nanotubes, Fullerene; B. Intercalation; C. Electron microscopy (TEM), Radiation damage
1. Introduction Since their discovery [1–3], single-wall carbon nanotubes (SWNTs) have stimulated widespread scientific interest due to their promising electronic and mechanical properties. However, the processes by which SWNTs are synthesized are still unrefined, and work on the functionalization of nanotubes is just beginning. One possible path towards the production of functionalized SWNTs is by filling the core with a material with useful properties. The nanotube can then function as a container with good stability or also contribute to the properties of the final structure. Soon after the first production of multi-wall carbon nanotubes (MWNT) methods were sought to fill the tube cores with other materials. The earlier work in this area has produced notable success in the opening and filling of MWNTs [4–12]. This prior work has involved the capillarity-based uptake of low surface tension liquids into MWNTs with large inner diameters produced using the carbon arc (CA) technique. The length of MWNT that has been continuously filled using these methods was short. Given the ease and control with which they are now *Corresponding author. Tel.: 11-215-898-8366; fax: 11-215573-2128. E-mail address: [email protected] (D.E. Luzzi).
produced, it is of interest to determine if single-wall carbon nanotubes (SWNT) can be efficiently filled. Soon after the production of single wall carbon nanotubes (SWNTs) [13], it was noted that the diameter of the most abundant nanotube produced by the pulsed laser vaporization method was correctly-sized to encapsulate a C 60 molecule [14]. Recently, we discovered that these unique hybrid structures (peapods) are a natural product in PLV-synthesized material that has been acid purified and annealed at 11008C, albeit at low concentrations [15]. In this paper, we review these findings including the methods that can be used to synthesize these unique structures. We find that the filling of SWNTs with C 60 (peapods) can be efficiently accomplished. The synthesis methods described herein provide a means to produce bulk quantities of these interesting materials for further study and point to the possibility of the general functionalizing of carbon nanotubes.
2. Experimental methods The starting material for this study was acid-purified carbon nanotubes produced by the pulsed laser vaporization (PLV) technique. The PLV material had been synthesized by the laser ablation of a graphitic target impregnated with 1.2 at% each Ni / Co catalyst. This raw
0008-6223 / 00 / $ – see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 00 )00088-9
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nanotube ‘‘felt’’ was refluxed in HNO 3 for 48 h, rinsed and neutralized, suspended in surfactant, and filtered to form a thin paper [16]. Such wet chemical etching is known to open the ends of MWNTs [11] as well as attack the sidewalls of SWNTs [17]. One additional batch of PLV material had received an anneal of 11008C in vacuo prior to being received. The as-purified materials were annealed under vacuums ranging from 20 to 40 mPa at temperatures between 100 and 12008C. These annealing treatments were carried out in a vacuum furnace or in-situ in a JEOL 2010F fieldemission-gun transmission electron microscope (FEGTEM). During in-situ anneals, temperature was monitored continuously via thermocouples. Only a few minutes were required to ramp between temperatures due to the small thermal mass of the heater. In addition, some PLV samples were annealed in a flowing Ar atmosphere at ambient pressure to a temperature of 6008C. The structure of both in-situ and ex-situ specimens was examined via TEM phase contrast imaging in either the FEG-TEM or in a JEOL 4000 high-resolution TEM (HRTEM). All microscopy was performed at an accelerating voltage of 100 kV to minimize the electron beam induced modification of the material. Electron dose to the specimen in the JEOL 4000 was approximately 3.4(10)19 electrons cm 22 s 21 , which was used for the study of the effects on the material of extended exposure to the electron beam. Magnification was determined using polyaromatic carbon shells present in the specimen, which originate from the decomposition of carbide crystals. It is known that the strong lattice fringes from these turbostratic shells have a well defined spacing of 0.34 nm. Microscopy specimens were prepared from nanotube paper by tearing away a small sliver and fixing it inside an oyster TEM grid, thereby foregoing additional chemical or thermal processing.
3. Experimental results An HRTEM image of an isolated SWNT is shown in Fig. 1a. Under the imaging conditions used in this work, atoms appear dark against a bright background. The image is darkest where the electron beam encounters the most atoms, which occurs where the beam is tangent to a nanotube or a fullerene wall. The parallel lines in the image are thus opposing walls of the tubule. The lines are separated by 1.4 nm, which is the expected diameter of a typical SWNT. In comparison, Fig. 1b shows a 1.4-nm diameter SWNT from the 11008C specimen containing a self-assembled chain of collinear C 60 molecules similar to a nanoscopic peapod. A number of observations support this interpretation of the image. Each circle in the image is 0.7 nm in diameter and is separated from the tube by 0.3 nm at the closest point. These are the expected measurements of
Fig. 1. HRTEM micrographs of (a) a typical 1.4 nm diameter SWNT, and (b) a 1.4 nm diameter SWNT that contains a selfassembled chain of C 60 molecules [22]. There is variability in the separations of adjacent molecules in (b), although they are frequently arranged in easily distinguished pairs. One such pair is indicated with an arrow. Both tubes are surrounded by vacuum. The scale bar is 2 nm.
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contained C 60 separated from the tubule walls by a graphitic Van der Waals spacing. The contrast of each circle is similar to the contrast of the tubule walls, which supports that the constituent atoms are carbon and not a more strongly scattering element. Each molecule must be a closed cage because an unclosed structure with fewer bonding constraints should not appear circular in cross section and is expected to fluctuate between many different conformations. Finally, if the C 60 molecules were on the exterior surface of the tube, they should be randomly positioned around the circumference, and their images should sometimes intersect the images of the tubule walls. However, even in imaging many such chains, this was not observed. This supports that the C 60 is contained. The number of peapods in this material is quite small. The peapods tend to be segregated on both the microscopic and macroscopic scales. On the microscopic scale, segregation is observed as regions in which few peapods can be found in the microscope adjacent to regions in which approximately 10% of observed SWNTs that are isolated or in small ropes contain C 60 . Macroscopic segregation has been seen in UV–VIS experiments in which separate milligram weight samples were found to contain between 0.8 and 5% of SWNTs filled with C 60 [18]. There is obvious variability in the separations of adjacent C 60 molecules of Fig. 1b within the nanotube. The images of some are distinct, while others are so close that they appear to be tangent or paired. An example of the latter case is indicated with an arrow in Fig. 1b. Pairing of C 60 is commonly observed in chains. The chronological sequence of images in Fig. 2 shows a chain of C 60 molecules coalescing inside a 1.4 nm diameter SWNT as a consequence of 100 keV electron irradiation. Images were taken at approximately 300 s intervals. Before extensive irradiation (Fig. 2a), each molecule is easily distinguished from its neighbors. The separation between some adjacent molecules is decreased after moderate irradiation (Fig. 2b) such that pairs are formed (cf. Section 3.1). This contraction becomes more pronounced during observation, and noticeable gaps are formed between groups of coalescing molecules (Fig. 2c). Coalescence occurs by structural damage to the C 60 followed by the formation of inter-cage covalent bonds. This is seen in the image as a blurring of contrast between the closely separated cages. Prolonged irradiation (Fig. 2d) ultimately produces pill shaped capsules that are a few nanometers in length. Irradiation damage to the surrounding nanotube is also apparent in one location beginning in Fig. 2b but occurs at a slower rate than damage to the C 60 . In the as-received acid-purified PLV material, no peapods have been detected during thousands of observations of individual SWNTs or of SWNTs bundled in ropes. However, the as-received material is coated with surfactant, which obscures the nanotubes and makes it difficult to observe peapods. We have found that baking in vacuum for at least 24 h at 2258C removes the surfactant without
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modifying the nanotube material. Several hundred tubes were observed after such cleaning and were found to be empty of C 60 . Thus, by patient collection of a large number of HRTEM images of the material in the aspurified condition and after 2258C anneal, it is apparent that peapods do not exist in the material following these processing steps. In-situ annealing experiments were conducted in the FEG-TEM at temperatures between 2258C and 4508C on specimens that were prepared from PLV material and subsequently cleaned by baking for 24 h at 2258C. The image of Fig. 3 was recorded at 3508C. C 60 molecules have adsorbed to the surface of the nanotubes, which were originally clean. The molecules adsorb to the surface and are present for only short times before vanishing. The process is quite dynamic with many molecules appearing, briefly residing on the surface and then disappearing so quickly as to be undetectable to the eye. This rapid activity was seen at 3508C but not at 3258C. Thus the onset temperature lies between these limits. Annealing experiments were carried out at various temperatures on a number of specimens in order to determine the formation mechanism of the peapods. In Fig. 4, a sample is shown after annealing at 4508C for 2 h in vacuum. The walls of the SWNTs are partially healed. Many peapods are seen to be present indicating that the formation of peapods occurred during the annealing treatment following acid purification and the 2258C cleaning anneal. These results confirm that C 60 is entering the nanotubes during these relatively low temperature anneals.
4. Discussion It is now well established that peapods contain C 60 . This is based on the observed size of features and the contrast in HRTEM micrographs, on UV–VIS experiments [18], and on the presented evidence that the motion of external C 60 in SWNT containing material is coincident with the formation of peapods. C 60 is contained almost exclusively by 1.3 nm to 1.4 nm diameter SWNTs. Tubes of this size are the only tubes that allow contained C 60 to everywhere maintain a preferred graphitic 0.3 nm Van der Waals separation from the tubule walls. Thus it is possible that the filling of nanotubes with C 60 is strongly correlated to the diameter of a SWNT. Furthermore, contained C 60 is observed only in chains, some of which have 30 or more members. This observation may be explained by first considering that solid C 60 exists as an FCC structure with a lattice parameter of 1.41 nm [19]. Each C 60 is spaced approximately 1.0 nm center-tocenter from 12 nearest neighbors. An isolated C 60 molecule contained in a SWNT is thus under-coordinated in the direction of the tubule axis, and its stability may be increased if it is coordinated with neighbors on one or both sides. Alternatively, the activation energy required for the
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Fig. 2. A chain of C 60 molecules coalescing inside a 1.4 nm diameter SWNT under 100 keV electron irradiation (a) after minimal irradiation time, (b) after |300 s, (c) after |600 s, and (d) after |900 s. In (a), each molecule is distinct from its neighbors. Pairing is apparent in (b) as coalescence is initiated. In (c), gaps form between groups of coalescing molecules. In (d), the chain is clearly transformed into longer capsules. Radiation damage to the surrounding SWNT is visible in the top right of each image beginning with (b). The scale bar is 2 nm.
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Fig. 3. An in-situ HRTEM image at 3508C showing the adsorption of C 60 molecules onto the surface of SWNTs.
diffusion of an individual C 60 might be very low such that they are always moving and are unable to be clearly imaged. The physical center-to-center separation between C 60 molecules in a chain is difficult to determine experimentally because of the depth of field of the microscope. The depth of field is approximately 10 nm, so a well focused longitudinal section of tube does not necessarily have its axis aligned normal to the electron beam. Because the
Fig. 4. Formation of peapods. A rope of PLV material is shown after further annealing treatment for 1 h at 4508C. Many of the SWNTs are now filled with chains of C 60 molecules.
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HRTEM image is a projection, the apparent separation between molecules in a chain in this condition would be less than the physical separation. However, such error in measurement is minimized for very long longitudinal sections that are completely in focus. Using this criterion, the physical center-to-center separation of C 60 molecules in a chain is often 1 nm. However, observations of smaller intermolecular separations invite the question of whether C 60 can exist in a dimerized or polymerized state. Since it is known that thin C 60 films photopolymerize under UV/ visible light [20], it is possible that dimerization could have occurred during normal handling of the specimen outside of the microscope. The center-to-center separation between C 60 molecules in a pair is often 0.9 nm, which is consistent with a (C 60 ) 2 dimer to within the resolution of the microscope [21]. Irradiation-induced coalescence is interpreted as a consequence of damage to contained C 60 molecules. Consider that a graphene sheet may be curved into a spherical molecule only with the introduction of pentagonal defects. This requires that the preferred bond angles cannot be maintained at the pentagon, and the corresponding carbon atoms are less tightly bound to the lattice. Such is the case for C 60 , and so it is expected that contained C 60 will be damaged by the electron beam at a faster rate than the SWNT where the carbon–carbon bond angles are less distorted. The displacement of carbon atoms from C 60 leaves unsatisfied bonds in the essentially hermetic nanotube cavity. These dangling bonds can be satisfied if covalent bonds are formed between adjacent damaged C 60 molecules. Because each fullerene cap requires six pentagons, the coalescence of C 60 molecules allows the total number of pentagons in the isolated molecules to be reduced by as much as 12 (n21). Although the coalesced state is energetically favorable, it is supposed that the incident electron beam is required to damage the C 60 molecules and form unsatisfied bonds before coalescence can begin. It is clear by inspection that the SWNTs in the current work are filled either via a vapor phase process or via the surface diffusion of individual molecules, or a combination of both. The source of C 60 is presumably residual crystallites left in the material after the synthesis and purification processes. It is known from prior analysis that acid purified PLV material contains residual crystallites of C 60 [16]. It is therefore likely that C 60 enters the nanotube by transporting to open ends or sidewall defects by surface diffusion or in the gas phase. If the mechanism of C 60 transport is via sublimation and via the vapor phase, then the onset temperature should be a strong function of vacuum system pressure. On the other hand, a surface diffusion mechanism should be less sensitive to pressure. This remains to be tested. Once the fullerene arrives on the nanotube surface, the large amount of filling that is seen argues for surface diffusion to open end or sidewall defects. Under a vapor phase transport
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mechanism, as the temperature is increased, the residence time of C 60 on the surface of the SWNT, and thus the probability of entering the SWNT will decrease. At high temperatures, the defects in the SWNTs will anneal thereby blocking the entrance of C 60 molecules into the tubes. Thus it would appear that there will be a critical temperature window within which peapods can be efficiently formed.
5. Conclusions Contained C 60 can be induced to enter SWNTs, forming nanoscopic peapods. Encapsulated C 60 is typically found in chains of 10 or more members. The separations between adjacent molecules are consistent with expected Van der Waals interactions. However, these separations are determined to vary, which suggests that other types of C 60 – C 60 bonding might be present as well. A tendency for C 60 molecules to be paired is apparent even in long chains. Some chains are observed to coalesce into pill shaped capsules under electron irradiation. The mechanism for the formation of encapsulated C 60 chains is now apparent, and we have successfully synthesized them in large fractions of the tubes that comprise our samples. In contrast to prior studies showing the uptake of liquid into large diameter MWNTs, the mechanism involves transport in the vapor phase or the surface diffusion of individual molecules. SWNTs with small interior diameters are completely filled over long lengths. The extent of filling argues for the entrance of C 60 through sidewall defects as well as open ends. A minimum temperature must be achieved in order to promote exterior C 60 to enter the tubes. Annealing at too high a temperature will limit the residence time of C 60 on the SWNT as well as cause the healing of the nanotubes walls, thereby eliminating access to the nanotube interiors. There is thus a critical temperature window required for the formation of bucky-peapods. The ability to synthesize these unique hybrid materials in bulk will now permit scaled-up empirical study. Interestingly, there is no obvious physical reason why SWNTs could not be filled with any appropriately-sized molecule with an affinity for nanotubes, and present in the gas phase or highly mobile at temperatures within the critical temperature window. Thus, the present work opens the possibility for the general functionalizing of the particular diameter SWNTs produced at highest concentration by PLV and CA methods.
als. This work benefited from discussions with colleagues Profs. J.E. Fischer, L.A. Girifalco and V. Vitek. This work was funded by the National Science Foundation with central facility support from the U. Penn MRSEC.
References [1] [2] [3] [4] [5] [6]
[7] [8] [9]
[10]
[11] [12] [13] [14] [15] [16]
[17] [18] [19] [20] [21] [22]
Acknowledgements We thank Profs. A.G. Rinzler, D.T. Colbert and R.E. Smalley for the acid purified and / or anealed PLV materi-
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