Structural transformation from single-wall to double-wall carbon nanotube bundles

We report an x-ray diffraction (XRD) analysis of the structural transformation from single-wall carbon nanotubes (SWNTs) to double-wall carbon nanotubes (DWNTs) via ${\mathrm{C}}_{60}$-encapsulating SWNTs $({\mathrm{C}}_{60}$ peapods). It was shown that the bundle structure of the starting SWNTs retains on this process. The thermal expansion coefficients of DWNTs hardly changed from the pristine SWNTs. The intertube spacing between inner and outer tubes of DWNTs was obtained as $0.36\pm 0.01\mathrm{nm}.$ This spacing was shown to be determined by the discreteness of the SWNT diameter under a turbostratic constraint between the inner and outer tubes of DWNT at synthesis temperatures.