Synthesis, Core-Shell Growth, and Surface Chemistry of 2-Dimensional Cadmium Telluride and Cadmium Selenide Nanocrystals

This dissertation presents the synthesis, shell deposition, and surface ligand-exchange chemistry of 2-dimensional cadmium telluride and cadmium selenide nanocrystals. Flat, colloidal 2-dimensional CdTe and CdSe nanoplatelets are promising materials for optical and electronic applications. The research presented here will enable better understanding of their properties and facilitate further applications.First, a new tellurium precursor, tris(dimethylamino)phosphine telluride is readily prepared and is found to have superior reactivity than the trialkylphosphine tellurides that are commonly used for nanocrystal synthesis. Wurtzite CdTe nanoplatelets, (CdTe)13 magic-size nanoclusters, and CdTe quantum wires are synthesized with this precursor, and are fully characterized. It is demonstrated that in amine solvent, this tellurium precursor undergoes transamination reactions, affording (Me2N)x(RHN)3−xPTe. A reaction mechanism is proposed. Second, CdS and CdSe shells are deposited on wurtzite CdTe nanoplatelets in a cyclic manner, allowing fine tuning of the absorption spectrum of the nanoplatelets across the visible spectrum, while maintaining the morphology, lattice structure, and the smooth surface texture of the nanoplatelets. The CdTe nanoplatelets having an initial thickness of 1.9 nm are converted to CdTe−CdS and CdTe−CdSe core−shell nanoplatelets having maximum thicknesses of 3.0 and 6.3 nm, respectively. Shell deposition is found to greatly improve the stability of the nanoplatelets. CdS shells are also deposited on CdSe nanoplatelets. Third, surface ligand-exchange chemistry of CdSe and CdTe nanoplatelets is investigated. Surface ligands are important in determining the optical and electronic properties of semiconductor nanocrystals. We demonstrate that the Z-type ligation on CdSe nanoplatelets having the zinc-blende structure, large surface areas, and minimal strain distortions can be exchanged for L-type ligation by ethylenediamine (en), affording nanoplatelets having a composition of (CdSe)3[en]0.67, and this process can be reversely exchanged by Z-type ligands such as CdCl2 and Zn(oleate)2, hence significantly expanding the types of available surface ligand exchange reactions on CdSe nanoplatelets. Fourth, we also compare and contrast the ligand exchange behavior on nonpolar, wurtzite and polar, zinc-blende CdTe nanoplatelet surfaces. We show that wurtzite CdTe nanoplatelets of composition (CdTe)[(n-octylamine)0.15(oleylamine)0.27] undergo reversible L-type to Z-type ligand exchange with Cd(oleate)2, Zn(oleate)2, CdCl2, and ZnCl2. However, the wurtzite CdTe nanoplatelets are unstable with anionic X-type ligation. In contrast, zinc blende CdTe nanoplatelets of composition (CdTe)3[Cd(carboxylate)2]0.74 undergo semi-reversible X-type to X’-type ligand exchange with CdCl2, CdBr2, and ZnCl2, but do not exhibit Z-type to L-type ligand exchange. The wurtzite nanoplatelets with nonpolar surfaces prefer neutral L- or Z-type ligation, whereas the zinc-blende nanoplatelets with polar surfaces prefer anionic X-type ligation