Role of the Crystal Structure in Cation Exchange Reactions Involving Colloidal CuSe Nanocrystals.

Stoichiometric CuSe nanocrystals were synthesized in either cubic or hexagonal (metastable) crystal structures and used as the host material in cation exchange reactions with Pb ions. Even if the final product of the exchange, in both cases, was rock-salt PbSe nanocrystals, we show here that the crystal structure of the starting nanocrystals has a strong influence on the exchange pathway. The exposure of cubic CuSe nanocrystals to Pb cations led to the initial formation of PbSe unselectively on the overall surface of the host nanocrystals, generating CuSe@PbSe core@shell nanoheterostructures. The formation of such intermediates was attributed to the low diffusivity of Pb ions inside the host lattice and to the absence of preferred entry points in cubic CuSe. On the other hand, in hexagonal CuSe nanocrystals, the entrance of Pb ions generated PbSe stripes "sandwiched" in between hexagonal CuSe domains. These peculiar heterostructures formed as a consequence of the preferential diffusion of Pb ions through specific (a, b) planes of the hexagonal CuSe structure, which are characterized by almost empty octahedral sites. Our findings suggest that the morphology of the nanoheterostructures, formed upon partial cation exchange reactions, is intimately connected not only to the NC host material, but also to its crystal structure.