Molecular packing of fullerenes inside single-walled carbon nanotubes

We report a first electron diffraction study of the C ₆₀s packing inside individual single-walled carbon nanotubes (C ₆₀s 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 C ₆₀s along the tube direction (Z _C60) are performed. The determined Z _C60 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 Z _C60 takes place. Molecular dynamics (MD) simulations reproduce the same reduction in Z _C60, due to the well-known linear-to-zigzag transition. More importantly, prior to the linear-to-zigzag transition, a 1D-to-3D transition of C ₆₀s configuration is predicted. The resulting expansion of accessible volume is likely to contribute to the observed increasing Z _C60 as a function of increasing SWCNT diameter before D reaching 14.5 Å.