Masters Degrees (Physics)

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Browsing Masters Degrees (Physics) by Author "Dejene, F. B."

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    Growth and characterization of ZnO nanoparticles by sol-gel process
    (University of the Free State (Qwaqwa Campus), 2015-01) Ungula, Jatani; Dejene, F. B.
    Solid state lighting technology is of particular interest in application of semiconductors. To this end, ZnO nanostructures have gained great attention in the research community, in part because of its requisite large direct band gap. The stability of the exciton (binding energy 60 meV) in this material, can lead to lasing action based on exciton recombination and possibly exciton interaction, even above room temperature. Therefore, it is very important to realize an optimized growth of ZnO nanostructures and investigate their properties. The main motivation for this thesis is not only to successfully realize the controllable growth of ZnO nanoparticles by sol-gel method, but also to investigate the structure, optical and electrical properties in detail by means of scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy), UV-Vis spectroscopy, X-ray diffraction (XRD) and other techniques. The influence of various growth parameters on the morphology, optical and electrical properties of the nanoparticles were also systematically studied. These include the growth temperature, volume ratios of water to ethanol solvent and different dopants effects. By controlling these parameters different shapes of nanoparticles, like spherical particles, nanorods and nanoflowers are demonstrated. XRD indicated that all the as-grown and annealed nanoparticles produced at temperatures between room temperature and 75 °C crystallize in the wurtzite structure and post growth annealing enhanced the crystalline quality of the materials while the band gap energy reduces. The crystallite size, obtained from XRD analysis, of as prepared ZnO nanostructures was found to decrease from 24 to 12 nm with the increase in volume ratio of ethanol in the solvent as peak intensities and sharpness increase with volume ratio of water. Thus in order to have smaller particles more volume ratios of ethanol solvent is favourable at growth temperature of 35 °C. The dopants were also observed to have slight effect on the grain sizes .No traces of zinc hydroxide were observed even in materials grown at lower temperature as reported by some authors. The optical quality of the nanostructures was investigated using PL. Both UV and defect related emissions have been observed for all as-grown and annealed samples of nanostructures. Photoluminescence spectra showed a strong ultra-violet emission, for annealed ZnO nanoparticles, which was centred on 385 nm and weak green emission at 550 nm confirming that the samples possess good optical properties with less structural defects and impurities. The effect of post-growth annealing on the optical quality of the nanostructures was carefully examined. Annealing at a temperature of 600 °C enhances the UV emission and suppresses defect related deep level emission for all samples. The PL spectra showed strong, broad and intense emission in visible region for Ce-doped ZnO samples while other dopants suppressed this green emission. The reflectance spectra of the annealed products show that the percentage absorption in visible range increases with annealing temperature. UV measurements depict a shift in absorption edge confirming the changes in particle sizes with varying ratios of solvents (water and ethanol). The band gap decreased from 3.31 to 3.17 eV with an increase in the ethanol composition in the solvent, implying that the optical properties of these materials are clearly affected by the precursor compositions. The SEM micrograph of ZnO revealed that the surface aspect depends on both the dopant used and annealing temperature. The characterization of the nanoparticles with Scanning Electron Microscopy (SEM) showed that at low temperatures (35 °C and 45ºC) clearly defined spherical particles are formed while at higher temperatures agglomerated irregular and diminished nanoparticles were observed.
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    ItemOpen Access
    Preparation and properties of long afterglow CaAl₂O₄ phosphors activated by rare earth metal ions
    (University of the Free State, 2011) Wako, Ali Halake; Dejene, F. B.; Swart, H. C.
    This work comprises of several aspects of calcium-aluminate phosphor activated with rare earth metal ions i.e. (CaAl2O4:Eu2+, Nd3+, and Dy3+). In particular the luminescent and structural properties of the long afterglow CaAl2O4:Eu2+,Nd3+,Dy3+ phosphors prepared by urea-nitrate solution-combustion method were investigated. The solution-combustion method is more efficient because phosphors with high efficiency were obtained at low temperature (500 oC) in a very short period of time (5 min). The effects of varying concentration of host matrix composition (Ca:Al), flux i.e. boric acid (H3BO3), activator (Eu2+) and co-activator (Nd3+/Dy3+) mass ratios and urea ((NH2)2CO) on the structural, luminescent, and thermoluminescent(TL) properties of the CaAl2O4:Eu2+, Nd3+, Dy3+ phosphors were studied. It was observed that Ca:Al mass ratios greatly affect the crystalline structure of the material. The results of the X-ray diffraction (XRD) analysis reveal that the formation of several crystalline phases depends on the ratios of the host material. The XRD peaks show the presence of other phases such as Ca3Al2O6 and CaAl4O7 but the predominant phase formed was that of CaAl2O4. However it was found that the crystalline structure is generally not affected by the variation of the co-dopants concentration. Photoluminescence (PL) studies revealed a general rise in intensity with an increase in the mass ratio of Ca:Al. The highest PL intensity was observed with 0.7% Ca. The luminescent intensities vary from each other when co-doped with various proportions of Nd3+ and Dy3+. The addition of H3BO3 favored the formation of pure monoclinic CaAl2O4 phase while the variation of the amount of ((NH2)2CO) showed mixed phases although still predominantly monoclinic. Both boric acid and urea to some extent influence the luminescence intensity of the obtained phosphor but unlike the case of CO(NH2), the emission peak for H3BO3, does not shift evidently because the energy level difference of 4f-5d does not change obviously. The broad blue emissions consisting mainly of symmetrical bands having maxima between 440–445 nm originate from the energy transitions between the ground state (4f7) and the excited state (4f65d1) of Eu2+ ions while the narrow emissions in the red region 600-630 nm arise from the f-f transitions of the remnant unreduced Eu3+ions. High concentrations of H3BO3 generally reduce both intensity and lifetime of the phosphor powders. The optimized content of H3BO3 is 5.8 mol % for the obtained phosphor with excellent properties. XRD analysis of the influence of Eu2+ and Nd3+ doping concentrations on the morphological, structural and PL properties of the CaAl2O4: Eu2+; Nd3+ phosphor, depict a dominant monoclinic phase that indicates no change in the crystalline structure of the phosphor even with high concentration of Eu2+ or Nd3+. The Energy Dispersive x-ray Spectroscopy (EDS) and Fourier Transform Infra-Red Spectroscopy (FTIR) spectra showed the expected chemical components of the phosphor. The excitation spectra show one broadband from 200 nm to 300 nm centered around 240 nm corresponding to the crystal field splitting of the Eu2+ d-orbital. The prepared phosphor compositions exhibit PL emission in the blue region with a maximum around 440 nm. This is a strong indication that there was dominantly one luminescence centre, Eu2+ which represents emission from transitions between 4f7 (8S7/2) ground state and the 4f6-5d1 excited state configuration. Two other, minor peaks, at 580 and 614 nm indicate the presence of remnants of Eu3+ ions as a result of incomplete reduction during sample preparation. High concentrations of Eu2+ and Nd3+ generally reduce both intensity and lifetime of the phosphor powders. The optimized content of Eu2+ is 0.36 mol % and for Nd3+ is 0.09 mol % for the obtained phosphors with good properties. The decay characteristics exhibit a significant rise in initial intensity with increasing Eu2+ doping concentration while the decay time increased with Nd3+ co-doping. Analysis of the TL glow curves is one of the most significant ways to measure the number of traps and also the activation energy of the trap levels in luminescent materials. In the present study TL properties of the CaAl2O4:Eu2+, Nd3+,Dy3+ phosphors were investigated above room temperature by use of Nucleonix 1009I TL reader. The trap depths were estimated with the aid of the peak shape method. The glow curve of CaAl2O4:Eu2+ with a first peak at 50 °C was found to correspond to several traps. The ratio of Nd3+:Dy3+ ions were observed to influence the position, concentration and type of traps formed. The observed afterglow can be ascribed to the generation of suitable traps due to the presence of the Nd3+ trap levels. Trivalent rare earth ions (Nd3+/Dy3+) are thought to play the role of hole traps in calcium aluminate phosphors (CaAl2O4:Eu2+). In these phosphors, Eu2+ ions act as luminescent centre emitting in the blue (λ max = 440 nm) region. Despite a large number of research on the phenomenon the mechanism of the persistent luminescence of CaAl2O4:Eu2+,Nd3+,Dy3+ has not been well presented. A proper understanding of the exact luminescence mechanisms and the identification of trap levels or locations in long phosphorescent materials is required for their use in areas such as detection of radiation, sensors for cracks in buildings, fracture of materials and temperature among others.