Colloidal CdSe/Cu₃P/CdSe nanocrystal heterostructures and their evolution upon thermal annealing

We report the synthesis of colloidal CdSe/Cu₃P/CdSe nanocrystal heterostructures grown from hexagonal Cu₃P platelets as templates. One type of heterostructure was a sort of "coral", formed by vertical pillars of CdSe grown preferentially on both basal facets of a Cu₃P platelet and at its edges. Another type of heterostructure had a "sandwich" type of architecture, formed by two thick, epitaxial CdSe layers encasing the original Cu₃P platelet. When the sandwiches were annealed under vacuum up to 450 °C, sublimation of P and Cd species with concomitant interdiffusion of Cu and Se species was observed by in situ HR- and EFTEM analyses. These processes transformed the starting sandwiches into Cu ₂Se nanoplatelets. Under the same conditions, both the pristine (uncoated) Cu₃P platelets and a control sample made of isolated CdSe nanocrystals were stable. Therefore, the thermal instability of the sandwiches under vacuum might be explained by the diffusion of Cu species from Cu ₃P cores into CdSe domains, which triggered sublimation of Cd, as well as out-diffusion of P species and their partial sublimation, together with the overall transformation of the sandwiches into Cu₂Se nanocrystals. A similar fate was followed by the coral-like structures. These CdSe/Cu ₃P/CdSe nanocrystals are therefore an example of a nanostructure that is thermally unstable, despite its separate components showing to be stable under the same conditions.