Metal nanocrystals: Deforming like liquid droplets.

T he properties of materials often change drastically when their size is decreased to the nanometre range. For example, as the surface-to-volume ratio of a material is increased, typically its melting temperature depresses and its chemical reactivity tends to increase. Also, as the size of a crystal shrinks down to submicrometre sizes, it becomes more difficult to deform it plastically (that is, the material becomes harder). This occurs because of the increasing difficulty for dislocations in the crystal to nucleate and move as crystal size is decreased. Yet Ju Li, Ze Zhang, Litao Sun and colleagues report in Nature Materials that a 10-nm silver nanocrystal can be deformed at room temperature in the absence of any dislocation activity 1 by compressing or stretching it using the tip of a scanning tunnelling microscope located inside a high-resolution transmission electron microscope (TEM), which allows for the deformation of the nanocrystal to be visualized at atomic resolution. Li and co-authors used a setup where the nanocrystal was partially bonded to a tungsten tip that approached a counter ZrO 2 surface (Fig. 1a). The authors observed that when the nanocrystal was compressed against the oxide, it continuously flattened (Fig. 1b–d), and that it continuously elongated when the tip was retracted (Fig. 1e,f; during tip retraction, the contact area between the top of the nanocrystal and the oxide surface decreased until complete detachment). They also observed that when the nanocrystal detached from the surface it rapidly recovered the original facetted shape. Visualization of the lattice planes in the TEM images demonstrated that, during the whole process, the nanocrystal remained as a perfect single crystal — that is, it did not contain dislocations. Importantly, the authors demonstrated that the nanocrystal deformed by surface diffusion, as indicated by the growth of atomic planes on its external surface. They confirmed this by means of molecular dynamics simulations (Fig. 1g). Surface diffusion as a deformation mechanism is of a different nature than classical plastic deformation, which involves the nucleation and migration of dislocations. In fact, nanocrystal deformation in the experiments by Li and colleagues was not purely elastic but pseudoelastic, and differed fundamentally from classical plastic deformation by the fact that in the absence of any applied constraint the nanocrystal returned spontaneously to its rest shape (to minimize its total surface and interface energy). Phenomenologically, the nanocrystal behaved like a liquid droplet, yet remained a crystalline solid. The rest …