Cation exchange reactions in colloidal branched nanocrystals

Octapod-shaped colloidal nanocrystals composed of a central "core" region of cubic sphalerite CdSe and pods of hexagonal wurtzite CdS are subject to a cation exchange reaction in which Cd²⁺ ions are progressively exchanged by Cu⁺ ions. The reaction starts from the tip regions of the CdS pods and proceeds toward the center of the nanocrystals. It preserves both the shape and the anionic lattices of the heterostructures. During the exchange, the hexagonal wurtzite CdS pods are converted gradually into pods of hexagonal Cu₂S chalcocite. Therefore, the partial cation exchange reactions lead to the formation of a ternary nanostructure, consisting of an octapod in which the central core is still CdSe, while the pods have a segmented CdS/Cu₂S composition. When the cation exchange reaches the core, the cubic sphalerite CdSe core is converted into a core of cubic Cu _2-xSe berzelianite phase. Therefore fully exchanged octapods are composed of a core of Cu_2-xSe and eight pods of Cu₂S. All these structures are stable, and the epitaxial interfaces between the various domains are characterized by low lattice mismatch. The Cu_2-xSe(core)/ Cu₂S(pods) octapod represents another example of a nanostructure in which branching is achieved by proper organization of cubic and hexagonal domains in a single nanocrystal.