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Unformatted text preview: Energetics and packing of fullerenes in nanotube peapods Mina Yoon,* Savas Berber, ² and David Tománek Physics and Astronomy Department, Michigan State University, East Lansing, Michigan 48824-2320, USA s Received 31 January 2005; published 12 April 2005 d We use structure-optimization techniques to study the equilibrium packing of fullerenes in carbon nanotube peapods. Our results for nanotubes containing C n fullerenes with 60 l n l 84 atoms indicate that the fullerenes are more densely packed in the nanotubes than in the bulk crystal, in agreement with experimental data. We find that the reduction of the interfullerene distance, as well as a structural relaxation of fullerenes and nanotubes, results from a high internal pressure within the peapods, suggesting the use of nanotubes as nanoscale autoclaves for chemical reactions. DOI: 10.1103/PhysRevB.71.155406 PACS number s s d : 81.05.Tp, 61.46. 1 w, 61.48. 1 c, 68.65. 2 k Following the discovery of fullerenes 1 and carbon nanotubes 2 s NTs d , nanotube peapods emerged as very inter- esting nanostructures. 3 The first observation of the hierarchi- cal self-assembly of C 60 molecules and single-walled carbon nanotubes s SWNTs d to peapods by high-resolution transmis- sion electron microscopy 3 s HRTEM d was followed by re- ports of other fullerenes and metallofullerenes being encap- sulated in single-walled carbon nanotubes. 4,5 Some of the structural aspects of carbon nanotube pea- pods are intriguing and even appear counterintuitive. In par- ticular, the equilibrium structure of fullerenes in peapods is quantitatively different from that in bulk solids. HRTEM images, 6 electron diffraction, 7 and Raman measurements 4 suggest that the equilibrium spacing between fullerenes in peapods is smaller by 3–4 % than in three-dimensional mo- lecular crystals, but larger than in solids based on polymer- ized fullerenes. This finding is unexpected, since all inter- fullerene distances should be equal and independent of dimensionality in close-packed systems with only pairwise nearest-neighbor interactions. Elastic deformations, associ- ated with a fullerene-to-nanotube charge transfer in the pea- pod, have been offered as a tentative explanation for the reduction of the interfullerene distance. 4,7 Unfortunately, there is no independent evidence for such a charge transfer in these all-carbon systems. Also, like charges on adjacent fullerenes should enhance the interfullerene repulsion, thus increasing the interfullerene distance. Here we investigate the energetics and packing of fullerenes upon their encapsulation in nanotubes. We find a net energy gain associated with fullerene encapsulation in nanotubes, giving rise to a “capillary force.” In nanotube peapods, we find this force to compress encapsulated fullerenes with an effective pressure of the order of GPa, inducing strain in the nanotube wall. Our results indicate that the encapsulation energy of fullerenes depends only on the diameter and not on the chirality of the enclosing nanotube.diameter and not on the chirality of the enclosing nanotube....
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This note was uploaded on 03/18/2012 for the course PHYSICS 303 taught by Professor Ihn during the Spring '12 term at Swiss Federal Institute of Technology Zurich.
- Spring '12