The mechanical behavior of nanostructures is known to
transition
from a Hall-Petch-like “smaller-is-stronger” trend,
explained by dislocation starvation, to an inverse Hall-Petch “smaller-is-weaker”
trend, typically attributed to the effect of surface diffusion. Yet
recent work on platinum nanowires demonstrated the persistence of
the smaller-is-stronger behavior down to few-nanometer diameters.
Here, we used in situ nanomechanical testing inside of a transmission
electron microscope (TEM) to study the strength and deformation mechanisms
of platinum nanoparticles, revealing the prominent and size-dependent
role of surfaces. For larger particles with diameters from 41 nm down
to approximately 9 nm, deformation was predominantly displacive yet
still showed the smaller-is-weaker trend, suggesting a key role of
surface curvature on dislocation nucleation. For particles below 9
nm, the weakening saturated to a constant value and particles deformed
homogeneously, with shape recovery after load removal. Our high-resolution
TEM videos revealed the role of surface atom migration in shape change
during and after loading. During compression, the deformation was
accommodated by atomic motion from lower-energy facets to higher-energy
facets, which may indicate that it was governed by a confined-geometry
equilibration; when the compression was removed, atom migration was
reversed, and the original stress-free equilibrium shape was recovered.