Elastic forces drive nonequilibrium pattern formation in a model of nanocrystal ion exchange

Chemical transformations, such as ion exchange, are commonly employed to modify nanocrystal compositions. Yet the mechanisms of these transformations, which often operate far from equilibrium and entail mixing diverse chemical species, remain poorly understood. Here we explore an idealized model for ion exchange in which a chemical potential drives compositional defects to accumulate at a crystal's surface. These impurities subsequently diffuse inward. We find that the nature of interactions between sites in a compositionally impure crystal strongly impacts exchange trajectories. In particular, elastic deformations which accompany lattice-mismatched species promote spatially modulated patterns in the composition. These same patterns can be produced at equilibrium in core/shell nanocrystals, whose structure mimics transient motifs observed in nonequilibrium trajectories. Moreover, the core of such nanocrystals undergoes a phase transition-from modulated to unstructured-as the thickness or stiffness of the shell is decreased. Our results help explain the varied patterns observed in heterostructured nanocrystals produced by ion exchange and suggest principles for the rational design of compositionally patterned nanomaterials.

Automated summary

This study explores an idealized model for ion exchange and finds that the nature of interactions between sites in compositionally impure crystals strongly impacts exchange trajectories. Elastic deformations promote spatially modulated patterns, which can be observed in core/shell nanocrystals. The core of such nanocrystals undergoes a phase transition from modulated to unstructured when the thickness or stiffness of the shell is decreased.