Doctoral Degrees (Physics)

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Browsing Doctoral Degrees (Physics) by Author "Dhlamini, Mokhotjwa Simon"

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    Luminescent properties of synthesized PbS nanoparticle phosphors
    (University of the Free State, 2008-08) Dhlamini, Mokhotjwa Simon; Swart, H. C.; Terblans, J. J.
    Luminescent lead sulphide (PbS) nanoparticles embedded in an amorphous silica (SiO2) matrix were synthesized at room temperature by a sol-gel process. The prepared nanocomposite materials were crushed into powders and annealed in air at 200oC. The chemical composition of the powders was analyzed with an energy dispersive x-ray spectrometer. Particle sizes, crystalline structure and morphology of the PbS nanoparticles were determined with transmission electron microscopy (TEM) and x-ray diffraction (XRD). The crystal particle sizes estimated from the XRD peaks and the TEM images were in the range of 10 to 50 nm in diameter. The SiO2:PbS powders were then irradiated with 325 nm (He-Cd) and 458 nm (Ar+) lasers for photoluminescence (PL) measurement. PL spectra were obtained for pure SiO2 as well as the encapsulated PbS nanoparticles at room temperature. Two strong broad bands, blue (450 nm) and yellow-orange (560 nm) from bulk SiO2 and PbS nanoparticles, respectively, were observed. The PL data show a blue shift from the normal emission wavelength at 3200 nm in bulk PbS to 560 700 nm in nanoparticulate PbS powders. The blue-shift of the emission wavelengths is attributed to quantum confinement of charge carriers in the restricted volume of nanoparticles. Energy transfer from ZnO nanoparticles to PbS nanoparticles was also observed. The possible mechanism for the energy transfer is reported. The powders were also subjected to prolonged 2 keV electron beam irradiation in a vacuum chamber at and different O2 pressures (5 × 10-8 2 × 10-7 Torr O2). The cathodoluminescence (CL) was measured with Ocean Optics S2000 spectrometer, and showed the emission peak to be at a wavelength of 680 nm. Changes in the CL brightness and the corresponding change in the surface chemical composition were investigated with Ocean Optics S2000, Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS). The oxygen Auger peak-to-peak height decreased simultaneously with the CL intensity. XPS analysis on the degraded spot showed the development of characteristic SiO, SiOx (0