Superelasticity and nanofracture mechanics of ZnO nanohelices.

A superelasticity (shape memory) behavior has been discovered for the superlattice-structured ZnO nanohelices. By in situ manipulation using a nanoprobe, the nanohelix could elastically recover its shape after an extremely large axial stretching to a degree close to the theoretical limit, while suffering little residual plastic deformation. As a result, its spring constant can be increased continuously for up to 300-800%. A shape memory/recovery of the nanohelix was observed after subjecting to a buckling deformation. The superelastic deformation and fracture process of a nanohelix have been studied by transversely compressing under an AFM tip. A two-step mechanism is suggested for explaining the measured force-displacement curve. It is suggested that the small thickness and the superlattice structure of the nanohelix might be the keys for the observed superelasticity. The ZnO nanohelices may be a new category of shape-memory ceramic nanostructures, which could be of great interest for investigating nanoscale fracture process and application in MEMS and NEMS. The elastic recovery of the nanohelix after extremely large deformation makes it a potential structure for nanoscale elastic energy storage.