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

Abstract

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.

Separating Geometric and Diffusive Contributions to the Surface Nucleation of Dislocations in Nanoparticles

“…As the particle size decreases for a specific material, the melting temperature decreases and thermal vibration increases; therefore, surface diffusion will play an increasingly strong role in nucleating dislocations. In addition, prior work by some of the present authors demonstrated both “displacive” and “diffusive” behavior of platinum nanoparticles, with a strong decrease in strength due to decreasing size for the surface-dislocation-nucleation regime (displacive deformation); 52 however, that work, and the others mentioned, could not distinguish the mechanism of the weakening.…”

“…For nanostructures, this is often called “diffusive deformation”. Prior work by the present authors using a similar apparatus 52 have explicitly distinguished homogeneous deformation in the “diffusive regime” from heterogeneous deformation in the “displacive regime”, with a transition occurring at approximately 9 nm. Therefore, that prior work, and also prior simulations of the so-called “Coble-creep-like behavior”, 51 confirm that the present 12 and 16 nm nanoparticles should undergo displacive (defect-based) deformation.…”

“…As described previously, there are two different factors governing the surface nucleation of dislocations in small nanostructures: geometric factors, where smaller particles have larger curvature and therefore a reduced activation barrier for a dislocation to nucleate, and surface diffusion, where small particles have lower effective melting temperatures and higher surface mobility, by which the random thermal motion causes kinks to form, nucleating dislocations at the reduced activation barrier. It should be noted that surface diffusion, ,, or grain-boundary sliding which occurs between two adjacent grains in a polycrystalline material, has also been suggested to cause gross atom migration, resulting in significant material deformation without dislocations. For nanostructures, this is often called “diffusive deformation”.…”