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INTRODUCTION The carbon nanotube has the uncommon ability to serve as a molecular container, hermetically encapsulating atoms or even other small molecules within the linear confines of its cavity. For example, the most abundant nanotube produced by laser ablation with a Ni-Co catalyst is optimally sized to encapsulate a C60 cage [1]. Using high resolution transmission electron microscopy (HRTEM), we have previously shown the existence of such structures, with the encapsulated fullerenes typically assembled into chains that consist of hundreds of Van der Waals interacting members [2-4]. Such 'peapods' are often found in conjunction with coaxial tubes (CATs), comprised of nested 0.7 nm and 1.4 nm diameter graphene cylinders [4]. These molecular assemblies could conceivably exhibit novel properties. Electronic band structure calculations suggest that a peapod could exhibit a superconducting transition [5], and mechanics theory predicts that a CAT could have a superior resistance to bending than an empty nanotube [6], whose elastic modulus (-1 TPa) is already the largest that has ever been measured [7]. For these reasons, it is important to determine how peapods and their derivative structures can be synthesized. EXPERIMENT Spartan quantities of these unique hybrids were first observed in nanotube material that had been subjected to a complex purification scheme of acid reflux, filtration, and annealing [8]. Accordingly, we examined nanotube materials by HRTEM in the as-synthesized condition, after acid purification, and after subsequent ex situ and in situ anneals under a vacuum of 20-40 ltPa at temperatures of 100-1200O C. Two materials were employed. The first was synthesized by the pulsed laser vaporization (PLV) of a graphitic target infused with 1.2 at.% ea. Ni/Co catalyst. This 'felt' was then refluxed in concentrated HNO 3 for 48 hours, rinsed and neutralized, suspended in surfactant, and filtered to form a thin paper [8]. The second was produced by carbon arc (CA) discharge using -5 at.% ea. Ni/Y catalyst [9] and then similarly purified [10]. 21 Mat. Res. Soc. Symp. Proc. Vol. 593 ©2000 Materials Research Society

Specimens were prepared by tearing small pieces away from each paper and fixing these inside TEM oyster grids. At the torn edges, the SWNTs that are pulled away from the bulk are suitable for imaging. This preparation technique does not subject the specimen to any additional chemical or thermal processing and enables a substrate-free analysis. Each specimen was examined in a JEOL 2010F field emission TEM. A tension of 100 kV was chosen in order to minimize the rate of specimen damage [11]. In-situ anneals were accomplished with a heating stage. During these, temperature was monitored continuously via thermocouples, and the time required to ramp between temperatures was only a few minutes due to the small mass of the heater. Many of the structures under study can be treated as electron-optical weak phase objects. In this case, the HRTEM image is a two dimensional projection of the three dimensional