Preserving Both Anion and Cation Sublattice Features during a Nanocrystal Cation-Exchange Reaction: Synthesis of Metastable Wurtzite-Type CoS and MnS.

The ability to selectively synthesize one particular polymorph in a solid-state system having multiple crystal structures with the same composition is important for accessing desired properties. Solution-mediated reactions, including anion and cation exchange, that chemically transform colloidal nanoparticles with pre-programmed structural features into targeted products have emerged as a powerful platform for predictably accessing metastable polymorphs. While nanocrystal ion-exchange reactions that retain anion sublattice features are well known, analogous reactions that preserve cation sublattice features are much less common, and guidelines for predictably targeting such sublattice features are not well established. Here, we report that both anion and cation sublattice features--hexagonal close-packed anions and tetrahedrally coordinated cations--can be preserved during cation exchange of roxbyite-type Cu2-xS nanocrystals to selectively produce wurtzite-type CoS and MnS. These polymorphs, which are metastable in bulk systems, form relative to other accessible structures having cubic close-packed anions and/or octahedrally coordinated cations. To facilitate these transformations, the scope of existing nanocrystal cation-exchange reactions was expanded to include 3d transition metal systems that previously have not been investigated in depth.