Nucleation and Growth of Monodisperse and Monocrystalline Wurtzite CdSe Nanocrystals: Zinc Alkanoates as Neutral Ligands

Here, we demonstrate that monocrystalline (free of stacking faults) wurtzite CdSe nanocrystals with monodisperse size, shape (dots, rods, or wires), and facet structure are synthesized in both strongly confined and weakly confined size regimes. Considering the unique c-axis of wurtzite CdSe, we introduce a new type of neutral ligand (e.g., zinc-alkanoate ones) to pair with their dominating nonpolar low-index facets. Nucleation of the stacking fault-free wurtzite seeds instead of zinc-blende tetrahedrons is identified as the key step, which is optimized by a set of conditions matching the neutral zinc-alkanoate ligands. In the following growth stage, conditions are much less stringent, although the neutral zinc-alkanoate ligands are still critical in achieving nearly atomically flat facets of those monocrystalline nanocrystals. In the strongly confined size regime, the ensemble photoluminescence (PL) full-width-at-half-maximum (FWHM) of wurtzite CdSe nanocrystals reaches a record low (59 meV). In the weakly confined size regime, dual-peak PL caused by thermal population is observed. Monodisperse and monocrystalline wurtzite CdSe nanocrystals show distinctively size-dependent optical properties, in comparison with their zinc-blende counterparts. Results here suggest that the atomically precise synthesis of colloidal semiconductor nanocrystals is feasible, implying an advanced class of nanomaterials for exploring various optical and optoelectronic applications.