Synthesis of CdSe/ZnSe Core/Shell and CdSe/ZnSe/ZnS Core/Shell/Shell Nanocrystals: Surface-Ligand Strain and CdSe–ZnSe Lattice Strain

Size- and shape-controlled CdSe/ZnSe core/shell and CdSe/ZnSe/ZnS core/shell/shell nanocrystals in a zinc-blende structure with (zinc) alkanoates as the ligands are synthesized by managing two types of strains. The surface-ligand strain, strain between the inorganic crystal surface and organic ligands─plays a key role for the ZnSe epitaxy and can be relieved via a two-step epitaxy. Small (acetate) ligands are necessary for both steps and branched (2-hexyl decanoate) ligands are needed for growing thick (2–16 monolayers) ZnSe shells in step 2. The CdSe–ZnSe interface results in strong/asymmetric tensile strain on the inner portion (within ∼7 monolayers) of the ZnSe shells and weak/hydrodynamic compression strain on the CdSe core QDs. The CdSe–ZnSe lattice strain does not noticeably affect the ZnSe epitaxy but significantly alters photoluminescence (PL) of CdSe/ZnSe/ZnS core/shell/shell QDs. With a proper thickness of both ZnSe and ZnS shells, CdSe/ZnSe/ZnS core/shell/shell QDs in a green-emitting window are synthesized with near-unity PL quantum yield, mono-exponential PL decay dynamics, and improved photophysical/photochemical stability.