Energetics and stability of C 60 molecules encapsulated in carbon nanotubes P. Liu a , Y.W. Zhang b, *, H.J. Gao c , C. Lu a a Institute of High Performance Computing, Singapore 117602, Singapore b Department of Materials Science and Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore c Division of Engineering, Brown University, Providence, RI 02912, USA ARTICLE INFO Article history: Received 17 September 2007 Accepted 18 January 2008 Available online 2 February 2008 ABSTRACT An energetic analysis was performed to study the interactions of C 60 molecules encapsu- lated in carbon nanotubes. Both direct interaction between C 60 molecules through van der Waals forces and indirect interaction between encapsulated C 60 molecules through the elastic deformation of their host carbon nanotubes were considered. For C 60 s encapsu- lated in a (9,9) nanotube, the indirect interaction dominates and a relatively large energy barrier exists for the formation of a uniform, stable, one-dimensional (1-D) C 60 array. For a (10, 10) nanotube, the indirect interaction leads to a small energy barrier to form a 1-D C 60 array, while for a (11,11) nanotube the influence of the indirect interaction is negligible. Molecular dynamics simulations were performed to confirm the present energetic analysis, suggesting that the indirect interaction between encapsulated molecules/particles through the elastic deformation of their host nanotubes may affect the stability of nanotube-based structures. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Filling of the interior space of a carbon nanotube to make no- vel nano-structures has attracted much research interest [1– 10]. Various molecules encapsulated in carbon nanotubes have been reported, including C 60 ,C 70 ,C 80 and higher fuller- enes in carbon nanotubes (C n @nanotubes) and endohedral metallofullerenes in nanotubes (M l @C n @nanotubes). Such hy- brid structures, often called peapods, provide ideal templates for realizing one-dimensional crystalline nano-structures. Two issues are of fundamental importance to the applica- tions of these peapod structures: First, although the filling rate of fullerene into carbon nanotubes have improved signif- icantly over the last few years, the filling mechanisms are still not yet fully understood [11,12]. As a result, the full reproduc- ibility and controllability of the filling process is yet to be per- fected. Second, the regular arrangement of fullerene molecules inside carbon nanotubes is not always achievable. Experimental observations have shown that encapsulated C 60 molecules are often not uniformly distributed within nano- tubes [1–3,7]. van der Waals interactions have been used to interpret the equilibrium separation distance of the encapsu- lated molecules. However, since there is no metastable state in the energy profile, the van der Waals interaction alone is unable to explain the non-uniform distribution of encapsu- lated molecules within a nanotube. To explain this phenome- non, dimerization of C 60 molecules inside a carbon nanotube was proposed [10]. However the dimerization reaction re- quires a very short molecular distance involving a large en- ergy barrier. Although it is known that other factors such as molecular interactions of the encapsulated molecules with their host tubes, defects and surrounding environment may 0008-6223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.01.029 * Corresponding author: Fax: +65 67763604. E-mail address: msezyw@nus.edu.sg (Y.W. Zhang). CARBON 46 (2008) 649 – 655 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon