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The remarkable electronic and mechanical properties of carbon nanotubes are widely acclaimed, but these molecules also have a unique structure that encapsulates a one-dimensional volume of space. The promise afforded by this structure is often overlooked. Carbon nanotubes have the capability to contain or transport other atoms, molecules, or crystalline materials inside. The constrained lumen of a nanotube could be a novel reaction chamber, catalyzing the synthesis of new compounds by forcing reactants and products to adopt specific conformations. Perhaps most importantly, the intrinsic properties of nanotubes could be modified by filling with species that have opportune magnetic, electronic charge transfer, absorption, mechanical, catalytic, or thermal characteristics. Such nanoscale heterostructures could have broadly tunable properties, but the components are noncovalently associated, so the chemistry and mechanical integrity of the underlying nanotube is preserved. Because of this potential, filling methods are certain to play a critical role in the evolution of carbon nanotubes as engineering materials. Initial efforts to create nanotube heterostructures were focused on filling multiwall carbon nanotubes (MWNTs) with metal salts and low-surface-tension melts by capillarity. However, this approach has proven limited in terms of yield, homogeneity of filling, and the types of materials that can be encapsulated. More recently, the 1998 discovery of a new class of supramolecular materials based on single-wall carbon nanotubes (SWNTs) introduced new methods for efficient filling. The first of these materials, descriptively called a peapod, was a one-dimensional crystal of C 60 molecules assembled inside a SWNT, and was obtained by a vapor phase process. Syntheses of many other peapod materials have been enabled by this discovery. SWNTs have been filled with various fullerenes and metallofullerenes, fullerene adducts, metal complexes, and other small molecules. This growing area of research is rapidly expanding the scope of nanotube-based materials. In this review, we discuss advances in the creation of nanotube heterostructures. Specifically, we will explore the synthesis of these one-dimensional materials and how their structures are manifest in their unique properties. In addition, we will discuss recent advances in the synthesis and characterization of related double-wall carbon nanotube materials. 2748_C003.fm Page 52 Tuesday, March 28, 2006 10:47 AM © 2006 by Taylor & Francis Group, LLC Chapter three: Carbon nanotube peapod materials 53 3. |
