Browsing by Author "Mntungwa, Nhlakanipho"

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    A facile hybrid method to synthesize metal and metal chalcogenide nanoparticles using various capping groups
    (2014) Mntungwa, Nhlakanipho; Revaprasadu, N.; Pullabhotla, V.S.R. Rajasekhar
    The synthesis of bismuth (Bi), antimony (Sb) and tin (Sn) metal and metal chalcogenide nanoparticles is presented using a novel hybrid solution based high temperature method. Furthermore is the synthesis of zinc telluride (ZnTe) and cadmium telluride (CdTe) nanostructured materials also described. The nanomaterials are predominantly capped by organic ligands. The use of water soluble ligands such as triethanolamine (TEA) and cysteine is also reported. The synthetic method involves the reduction of a metal salt followed by the introduction of the chalcogenide source in the case of metal chalcogenide nanoparticle synthesis. The metal or metal chalcogenide prepared is thermolysed into a high boiling point ligand at high temperature. For the water soluble nanoparticles after reduction there is a simultaneous addition of a chalcogenide source and a ligand at room temperature. The Bi and Sb nanoparticles showed similar spherical morphology when capped with tri-noctylphosphine oxide (TOPO). Bismuth particles in the form of dots, branched nanorods and self assembled cubes were obtained at different reaction conditions. The cubed shaped Bi and Sb nanoparticles which appear to self-assemble were obtained when using oleylamine (OA) as a capping group. The Bi2S3, Bi2Se3 and Bi2Te3 nanoparticles were elongated when capped using alkylamines such as hexadecylamine (HDA) and OA, however at low temperature of 90oC they appeared to be close to spherical. The antimony chalcogenides, Sb2S3, Sb2Se3 and Sb2Te3 gave a similar rod shape morphology. The Sn nanoparticles appeared aggregated to some extent. A triangular shaped morphology is observed for the OA capped SnS nanoparticles. HDA capped ZnTe nanoparticles were synthesized using NaHTe and ZnCl2 as the tellurium and zinc sources respectively. The particles synthesized at reaction temperatures of 230 oC and 270 oC were spherical in shape. The blue shift in the absorption spectra confirms that the particles undergo quantum confinement. The photoluminescence studies show that the particles emit at a wavelengths close to their band edge. The water soluble cysteine and TEA capped ZnTe particles appear as nanorods which are aggregated. Finally water soluble CdTe nanoparticles were synthesized using TEA and cysteine as capping groups. The results showed that both cysteine and TEA are effective capping groups for water soluble CdTe nanoparticles. The optical properties of the particles synthesized in both capping groups revealed absorption features due to the Cd-ligand complex. The photoluminescence spectra showed reasonably narrow emission peaks. The cadmium salt was varied to study its effect on the morphology of the CdTe nanoparticles. The nitrate source gave well defined, crystalline, close to spherical nanoparticles. The FT-IR measurements confirmed the presence of the capping ligand on the surface of CdTe nanoparticles.
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    Synthesis and shape control of functionalized cadmium telluride nanoparticles
    (2010) Mntungwa, Nhlakanipho; Revaprasadu, N.
    The synthesis of organically and water soluble cadmium telluride nanoparticles by a facile hybrid solution high temperature method is presented. This method involves the reaction by the addition of an aqueous suspension or solution of a cadmium salt (chloride, acetate, nitrate or carbonate) to a freshly prepared NaHTe solution. The nanoparticles were prepared by passivating with organic surfactants such as hexadecylamine (HDA) and tri-n-octylphosphine oxide (TOPO) for their solubility and stability in organic solvents. To stabilize and make the nanoparticles water soluble, L-cysteine ethyl ester hydrochloride and triethanolamine were used as capping agents. The absorption and photoluminescence spectroscopy, fourier transform infrared spectroscopy, powder X-ray diffraction, transmission electron microscopy and high resolution transmission electron microscopy techniques were used to confirm the quality of the as-synthesized nanoparticles. All measurements were performed without any post preparative size separation of the nanoparticles. The dissertation comprises of four chapters. The first chapter is a brief description of the properties of nanomaterials and a literature review on some of the principal routes to synthesize nanomaterials. The mechanisms of particle growth and shape control are also discussed. Finally some key applications of CdTe are described. The second chapter deals with the synthesis of HDA and TOPO-capped CdTe nanoparticles. The reaction parameters such as reaction time, reduction time of tellurium, Cd:Te ratio, reaction temperature and cadmium source were varied to investigate their influence on the optical properties and morphology of the particles. Details of the absorption features such as excitonic peaks and band edges are presented. The emission properties are also discussed. The morphology of the particles was investigated by electron microscopy and powder X-ray diffraction techniques. It was found that the cadmium source (chloride, carbonate, acetate and nitrate) had a profound effect on the final morphology of the particles. A detailed mechanism for the particle growth is presented. The synthesis of cysteine and TEA-capped CdTe nanoparticles are described in Chapter three. A systematic study of the effects of the capping group, pH, reaction time and cadmium source was carried out for the water soluble CdTe nanoparticles. The absorption and emission data for the particles are discussed in detail. The electron microscopy images confirm the nanosize and crystalline nature of the particles. In addition fourier transform infrared spectroscopy was used to confirm the capping of the particles by cysteine and TEA. All the cadmium sources were found to be successful in producing high quality water soluble CdTe particles. The final chapter four is a summary of general conclusions of the study.