Starting from an amorphous C film, single-walled carbon nanotubes were obtained in situ in a high-resolution electron microscope by the combined effect of irradiation and axial strain. Ductile nanotubes developed either a junction or a linear chain of C atoms before failure. These facts have been put in direct evidence for the first time. Tight-binding calculations indicate that the bonding in the linear chain is of a cumulene type.

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