Molecular packing of fullerenes inside single-walled carbon nanotubes

We report a first electron diffraction study of the C60s packing inside individual single-walled carbon nanotubes (C60s at SWCNT or peapod). Diffraction patterns from a single peapod were recorded using a nanometer-sized electron beam at 80 kV. Determination of the peapod structure, including the diameter (D) and chiral angle of the SWCNT, and the average spacing between C60s along the tube direction (ZC60ZC60) are performed. The determined ZC60ZC60 increases from around 9.03 to 9.69 Å as D increases from around 14.17 to 14.38 Å, while in large diameter SWCNTs with D larger than 14.5 Å, a significant reduction in ZC60ZC60 takes place. Molecular dynamics (MD) simulations reproduce the same reduction in ZC60ZC60, due to the well-known linear-to-zigzag transition. More importantly, prior to the linear-to-zigzag transition, a 1D-to-3D transition of C60s configuration is predicted. The resulting expansion of accessible volume is likely to contribute to the observed increasing ZC60ZC60 as a function of increasing SWCNT diameter before D reaching 14.5 Å.