Shape control of CdSe/CdS nanocrystals during shell formation and growth: Dominating effects of surface ligands over core crystal structure

CdSe/CdS nanocrystals (NCs) are among the most studied semiconductor nanocrystals, yet there is still much information to be gained. This work reveals that core@shell NCs with different shapes are more controlled by the interaction between the NC surface and the capping ligands than the core concentration, but not at all by the difference in the crystalline nature of the core. Among the precursors, cadmium carboxylates promote an isotropic structure, while conversely, long-chain cadmium phosphonates favor an anisotropic one. Cadmium carboxylates are critical in the formation of the headshell, while cadmium phosphonates play a role in the anisotropic tail growth. Against expectations, the CdSe-core crystal structure (zinc blende or wurtzite) plays very little role in determining the structure of the final shape, which may be due to the two-stage CdS shell formation process, and gives rise to a tadpole shape. With appropriate capping ligands, precise control of the CdSe/CdS structures can be achieved in both shape formation and growth process. We claim, here, that CdSe/CdS with morphologies as different as tadpoles, nanoflowers, dot-in-rods, and tetrapods are obtained with only varying surface ligand ratios. This unique crystal-growth mechanism can be applied to other seed-mediated methods to produce aniso-tropic nanostructures.