Size-Dependent Role of Surfaces in the Deformation of Platinum Nanoparticles

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 HallPetch 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.

Automated summary

The strength and deformation mechanisms of platinum nanoparticles were studied using in situ nanomechanical testing inside a transmission electron microscope. Surface curvature was found to play a prominent and size-dependent role, leading to a smaller-is-weaker trend in larger particles. However, particles below 9 nm showed a saturation in weakening and exhibited homogeneous deformation with shape recovery after load removal.