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

Abstract

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, monoexponential PL decay dynamics, and improved photophysical/photochemical stability.

Selective Formation of Monodisperse Right Trigonal-Bipyramidal and Cube-Shaped CdSe Nanocrystals: Stacking Faults and Facet-Ligand Pairing

“…Growth of the zinc-blende CdSe seeds (∼3 nm) is carried out in an octadecene (ODE) solution of a mixture of Cd­(Fa) 2 , free fatty acids, and Cd­(Fa)Cl formed in situ by addition of palmitoyl chloride into the mixture, into which selenium dissolved in ODE (Se-ODE) and Cd­(Fa) 2 dispersed in a Vaseline-ODE gel (Cd­(Fa) 2 -Gel) precursor , are dropped through two separate syringes (see the Experimental Section for details). Chloride ions in the form of Cd­(Fa)Cl were confirmed as small negatively charged ligands to release surface–ligand strain yet retaining an ideal facet-ligand pairing for the (100) facets of zinc-blende nanocrystals. ,, Vaseline-ODE gel enables automatic and quantitative addition of insoluble precursors into the reaction system required for controlled growth of the faceted nanocrystals.…”

“…Chloride ions in the form of Cd(Fa)Cl were confirmed as small negatively charged ligands to release surface−ligand strain yet retaining an ideal facet-ligand pairing for the (100) facets of zinc-blende nanocrystals. 13,26,38 Vaseline-ODE gel 37 enables automatic and quantitative addition of insoluble precursors into the reaction system required for controlled growth of the faceted nanocrystals.…”