Softening to hardening of stretched diamondene nanotubes

Diamondene, a carbon nanomaterial containing both sp2 and sp3 carbon atoms, is obtained by compressing two or more layers of graphene. By curving rectangular diamondene and matching the unsaturated C-C bonds on the two unbent edges, a nanotube is built. We build two diamondene nanotubes (DNTs) with different radii and test their strengths under uniaxial tension. From the stress-strain curves, we discover that DNTs exhibit softening followed by hardening. The mechanism is as follows: the bond lengths and bond angles impart different stiffnesses to the tube at different axial strains. Molecular dynamics simulations demonstrate that the feature of the softening-hardening process is independent of either the tube radii or the system temperature. The critical strain for the tensile strength of a DNT becomes lower at a higher temperature. This is caused by thermal vibration of the atoms in the tubes. At the same temperature, for a DNT with a larger radius, the value of critical strain is higher. These properties will be beneficial for the potential applications of DNTs in nanodevices.