Nanocrystal Symmetry Breaking and Accelerated Solid-State Diffusion in the Lead–Cadmium Sulfide Cation Exchange system

The phenomenology of solid-state transformations in nanoparticles is important for applications utilizing their reactivity and for investigations into how nearby interfaces interact with the defects responsible for mass transport. We directly interrogate the structure and reaction kinetics of lead sulfide (PbS) nanocrystals undergoing cation exchange in organic solution to cadmium sulfide (CdS) via X-ray diffraction. The epitaxial relationship of zincblende CdS to rocksalt PbS breaks the overall symmetry of the core–shell nanocrystal without requiring the loss of unit cell symmetry, leading to anomalous peak shifts in the diffraction pattern. Conversion occurs in three stages: (1) surface exchange to form a metastable rocksalt CdS shell, (2) crystallization of this shell to zincblende, and (3) diffusive transport of ions through the completed shell. The interdiffusion coefficient, D, for ions diffusing through the shell follows the Arrhenius relationship with an activation energy of 160–180 kJ mol–1, whi...