Title

Property and structure evolution of CdSe nanocrystals in amines

Date of Completion

January 2006

Keywords

Chemistry, Inorganic|Chemistry, Physical|Chemistry, Polymer

Degree

Ph.D.

Abstract

Etching of CdSe NCs by 3-amino-1-propanol in the presence of oxygen was discussed as evidenced by the stepwise blueshift in UV/Vis absorption and PL emission along with the decreases in particle size. Such etching behavior is the result of two competitive processes taking place on the surface of these CdSe NCs: (i) oxidation of the exposed Se-sites to acidic SeO 2 entities that are readily solubilized in the basic APOL/H2O mixture, and (ii) chemical affinity of dissolved Cd2+ ions that together with cadmium hydroxide surface moieties temporally impede dissolution of the Cd-sites. This leads to Cd-terminated NC surfaces that minimize the number of exposed Se sites and account for the temporal etching resistance at the plateau regions. High-resolution electron microscopy data are consistent with NCs exhibiting mainly Cd-terminated polar facets by adopting pyramidal morphologies. These nanocrystalline objects appear to have a (0001) base and side facets parallel to either {011¯1} or {21¯1¯1}, which correspond to the three polar planes with the highest density of exposed Cd sites for the wurtzite structure. ^ The structure-PL property relationship of quantum dots was then investigated by a controlled chemical etching. Through this etching process, a discrete blueshift in UV/Vis and PL emission peaks was observed. Combined with the results from density functional theory calculations, a closed shell structure model was proposed for quantum dots of high PL efficiency. Based on this model, room temperature synthesized zinc blende CdSe NCs with high PL efficiency was realized by a chemical etching method. The etched CdSe NCs turns out to be single crystalline and more spherical in shape than the as-synthesized polycrystalline and irregular shaped NCs. ^ Annealing CdSe NCs with O.1M Cd2+ in 3-amino-propanol in the presence of oxygen, redox assisted asymmetric Ostwald ripening was observed rather than etching. During this asymmetric Ostwald ripening process, spherical CdSe NCs gradually evolve into CdSe nanorods. The oxidation of Se into SeO2 helps dissolution of CdSe NCs smaller than critical sizes, while the reduction of SeO2 back to Se by amine promotes the asymmetric growth of CdSe NCs larger than critical size.^