Blue-UV-Emitting ZnSe(Dot)/ZnS(Rod) Core/Shell Nanocrystals Prepared from CdSe/CdS Nanocrystals by Sequential Cation Exchange

Great control over size, shape and optical properties is now possible in colloidal Cd-based nanocrystals, which has paved the way for many fundamental studies and applications. One popular example of such class of nanocrystals is represented by CdSe(spherical core)/CdS(rod shell) nanorods. These can be nearly monodisperse in size and shape and have strong and stable photoluminescence that is tunable in the visible range (mainly by varying the size of the CdSe core). The corresponding Zn-based core/shell nanorods would be good candidates for tunable emission in the blue-UV region. However, while the synthesis of ZnS nanocrystals with elongated shapes has been demonstrated based on the oriented-attachment mechanism, elongated ZnS shells are difficult to fabricate because the more common cubic phase of ZnS has a highly symmetric crystal structure. We report here a procedure based on a sequence of two cation exchange reactions, namely, Cd²⁺Cu⁺ and then Cu⁺Zn²⁺, by which we transform colloidal CdSe(core)/CdS(shell) nanorods first into into Cu₂Se/Cu₂S nanorods, which are then converted into blue-UV fluorescent ZnSe(core)/ZnS(shell) nanorods. The procedure transfers the morphological and structural information of the initial Cd-based nanorods to the Zn-based nanorods. Therefore, the final nanoparticles are made by a ZnSe dot embedded in a rod-shaped shell of wurtzite ZnS. Since in the starting Cd-based nanorods the size of the CdSe core and the length of the CdS shell can be well controlled, the same holds for the final Zn-based rods. In the second step of the exchange reaction (Cu⁺Zn²⁺), a large excess of Zn²⁺ ions added over the Cu⁺ ions present in the Cu₂Se/Cu₂S nanorods is the key requisite to obtain bright, band-edge emission (with quantum yields approaching 15%) with narrow line widths (approaching 75 meV). In these ZnSe/ZnS nanorods, photogenerated carriers appear to be more confined in the core region compared to their parent CdSe/CdS nanorods.