  Understanding nanomechanical response of materials represents a scientific
challenge. Here, we have used in-situ electron microscopy to reveal drastic for
the first time changes of structural behavior during deformation of 1-nm-wide
metal rods as a function of temperature. At 300 K, stretched nanowires stay
defect-free, while at 150 K, elongation is associated with planar defects. As
size is reduced, energy barriers become so small that ambient thermal energy is
sufficient to overcome them. Nanorods display an elastic regime until a
mechanism with high enough blocking barrier can be nucleated. Ab-initio
calculations revealed that contribution from surface steps overrule stacking
fault energetics in nanorods, in such a way that system size and shape
determines preferred fault gliding directions. This induces anisotropic
behavior and, even large differences in elastic or plastic response for
elongation or compression. These results provide a new framework to improve
theoretical models and atomic potentials to describe the mechanical properties
at nanoscale.
