We report measurements of electrical resistivity, thermopower, and thermal conductivity of highly C 60 -filled single-wall carbon nanotubes and unfilled controls, from 1.5 to 300 K. The data suggest that the C 60 chains provide additional conductive paths for charge carriers, increase the rate of phonon scattering, and block interior sites from sorbing other gas molecules. Single-wall carbon nanotubes ͑SWNT͒ filled with C 60 were discovered by Smith et al. 1 These "peapods" generally occur as

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... ghly ordered long chains with the same ϳ10 Å intermolecular spacing as in the fcc solid. Short clusters of 2-20 fullerenes are sometimes found. C 60 -C 60 and C 60 -SWNT interactions are expected to influence structure, dynamics, and electronic properties. 2,3 Elucidation of the growth mechanism 4 provided the basis for rational synthesis of highly filled samples. 5 This in turn enables the study of their physical properties, about which little is known. C 60 is a strong electrophile, so one might expect the surrounding tubes to become hole doped. However, theory predicts only weak occupancy of a quasi-one-dimensional ͑1D͒ band derived from a chain of C 60 lowest unoccupied molecular ͑LUMO͒ orbitals. 3 In the only experiment published to date, Hirahara et al. found that the electrical resistance diverged more steeply with decreasing T than for unfilled SWNT. 6 This was attributed to carrier scattering by the local electrostatic potential from ͑presumably charged͒ fullerenes. No absolute values were given. Here we report temperature dependence of electrical resistivity , thermal conductivity , and thermoelectric power or Seebeck coefficient S, measured on buckypapers of highly filled and control samples, 5 the only difference being the absence of C 60 in the reaction tube during the filling step. The average tube diameter is around 1.3 nm. Transmission electron micrographs of the filled sample show a preponderance of well-ordered nanotube ropes filled with C 60 molecules. Weight uptake measurements reveal a filling fraction of 90%. 5 Measurements of (T) were carried out from 1.5 to 300 K in helium vapor. Filled and control samples for all measurements were cut from the same buckypaper. These were vacuum degassed at 800°C for 1 h in dynamic vacuum (5 ϫ10 Ϫ6 Torr) and then exposed to air for about an hour during transfer to the cryostat. Adsorbed oxygen acts as a p-type dopant in nanotubes. [7] [8] [9] The effect on at saturation is only about 25% 7 but much more dramatic on S ͑see later͒. The results are shown in Fig. 1 . The overall T dependence is nonmetallic for both, while the low T upturn in is strongly reduced in the filled sample. The ratio (empty)/(filled)