Controlling Ion-Exchange Balance and Morphology in Cation Exchange from Cu3-xP Nanoplatelets into InP Crystals

Synthesis of colloidal nanocrystals (NCs), which are not readily available via the wet-chemical approach based on arrested precipitation, has often relied on templated growth. Cation exchange, in which guest cations in bulk solution replace host cations in template NCs, has evolved as one of the most powerful examples. Despite its versatility and facileness, there are caveats because most of the cation-exchange processes presuppose the formation of crystalline defects, which are more or less uncontrolled in terms of population and locations. The defect formation is a consequence of the imbalance between extraction and incorporation of host and guest cations. Here, we demonstrate the controlling of ion-exchange balance in the Cu+-to-In3+ cation-exchange reaction of Cu3-xP nanoplatelets (NPLs), which triggers the nanoscale Kirkendall effect, a representative phenomenon of crystal defect generation, clearly shown by morphology change of NPLs. Cation-exchanged NPLs exhibit various morphologies depending on the ligand composition introduced in the cation-exchange reaction. For example, Kirkendall voids appear inside NPLs when Cu+ is expected to undergo solvation more than In3+ is dissolved. High-resolution transmission electron microscopy images of partially cation-exchanged NPLs show separate steps of Kirkendall void nucleation and growth at the Cu3-xP/InP interface. Elemental analysis combined with a mathematical description of stoichiometry of the Cu3-xP segment enables the quantification of Cu+ vacancy concentration, which has to do with Kirkendall void nucleation. Estimation of ion solvation energetics reveals that the composition of solvating ligands is responsible for imbalance between cation solvation and desolvation, resulting in an increase in Cu+ vacancy. In addition to defect generation inside a crystal, defect removal is visualized via the transformation of cracked InP NPLs into hollow NPLs after annealing at high temperature. Our in-depth study on ion-exchange balance and morphology change in cation-exchange reaction not only provides mechanistic insights into internal defect formation and removal process accompanied with cation exchange but also opens up the possibility of diverse morphology control of NCs.