Elastic Forces Drive Nonequilibrium Pattern Formation in a Model of Nanocrystal Ion Exchange

TL;DR

This paper investigates how elastic forces influence nonequilibrium pattern formation during ion exchange in nanocrystals, revealing mechanisms behind observed compositional patterns and guiding nanomaterial design.

Contribution

It introduces an idealized model showing elastic deformations promote spatially modulated patterns during ion exchange, linking nonequilibrium dynamics to equilibrium structures in nanocrystals.

Findings

Elastic deformations promote pattern formation.

Core/shell nanocrystals exhibit phase transitions.

Patterns observed in experiments can be explained by the model.

Abstract

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.