Nanomechanics: Full recovery takes time.

W ith the advent of near-equilibrium growth techniques, it is now possible to synthesize virtually any type of crystalline nanostructure. Researchers can exert a fine control over size, shape and surface chemistry to tailor the properties of their nanomaterial to specific applications relevant to energy conversion, sensing, electronics and photonics. Once in a device, these nanostructures are frequently subjected to intense mechanical duress, so structural integrity becomes a key factor for ensuring robust performance. Unfortunately, the mechanisms that govern irreversible deformation and, ultimately, failure of nanomaterials are a far cry from the mechanisms at play in their bulk counterpart and are still largely unclear. now report another remarkable behaviour of nanoscale materials, showing that seemingly irreversible, plastic deformations in ZnO and p-doped Si nanowires can gradually recover over time 1. The ability of a material to recover plastic deformations in a measurable timescale is called anelasticity and it is usually very small in bulk inorganic materials at room temperature. Two notable characteristics that affect the mechanical properties of nanoscale materials underpin the profound difference with their bulk counterpart: the prevalent role of surface and near-surface regions and the scarcity of defects (high crystalline quality). In the case of an elastic strain imposed on a material, for example, deformation is entirely reversible. However, nanomaterials can exhibit different elastic moduli, mainly because surface atoms have truncated chemical coordination therefore possessing a unique electronic structure, which make them prone to relaxation and reconstruction dynamics. At vanishingly small length scales, the unique mechanical properties of the surface layer tend to dominate the total elastic response of the material 2. Depending on the nature of the surface stiffness and the surface stress state, nanowires may exhibit either size-dependent elastic stiffening or softening 3,4. Once the elastic limit is reached, a material without the capacity to sustain plastic deformations fractures in a brittle fashion under an applied load. But if the material has the intrinsic ability to sustain plasticity, specific mechanisms that can cope with very high stresses come into play 5. Dislocations — quasi-1D defects delineating slipped and unslipped regions of a crystal — are the vehicles for plastic deformation in bulk materials. However, nanomaterials synthesized by bottom-up techniques contain a paucity of dislocations 6 and thus nucleation of new defects is needed to commence plastic deformation. Stress-driven nucleation requires subjecting a nanomaterial to stresses approaching the theoretical strength for perfect crystals (often more than …