Semiconductor nanocrystal colloids : manganese doped cadmium selenide, (core)shell composites for biological labeling, and highly fluorescent cadmium telluride

Abstract

This thesis describes the characterization and applications of nanometer sized semiconductor (or quantum dot) colloids produced by chemical means. The nanocrystals are synthesized by pyrolysis of organometallic precursors in the coordinating solvent trioctylphosphine oxide (TOPO). The important developments that have contributed to this method are discussed. Manganese doped CdSe nanocrystals are synthesized using a manganese and selenium containing organometallic compound. Chemical etching and electron paramagnetic resonance (EPR) experiments reveal that most of the dopant atoms lie near the surface within the inorganic lattice. Results from fluorescence line narrowing (FLN) and photoluminescence excitation (PLE) spectroscopies show that doped nanocrystals behave as if they were undoped nanocrystals in an external magnetic field. The nanocrystal surface is initially passivated by dative organic ligands. Better passivation and optical properties are achieved by growth of a large band gap semiconductor shell that provides both a physical and an energetic barrier between the exciton and the surface. (CdSe)ZnS (core)shell are prepared with control over both core and shell sizes. The composite nanocrystals are characterized by absorption, emission, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), and wide angle X-ray scattering (W AXS). The maximum quantum yield is achieved when the core is protected from oxidation by a complete shell; thicker shells show no further increase in quantum yield values, due to defects caused by the large lattice mismatch. Exchange of surface TOPO ligands for mercaptocarboxylic acids produces (core)shell nanocrystals that, when treated with base, are soluble in water and remain fluorescent. Established protocols are used to link these water-soluble nanocrystals to the biomolecules avidin or biotin, producing useful fluorescent labels. Stable phosphine tellurides are prepared using hexapropylphosphorus triamide (HPPT). This precursor is used to prepare CdTe nanocrystals that display room temperature quantum yields up to 70%. The CdTe growth is investigated by absorption and emission spectroscopy. CdTe nanocrystals are characterized by TEM and WAXS.