Doctoral Degrees (Physics)

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

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    ItemOpen Access
    A combined ab initio and experimental study of lanthanides and/or transition metal doped oxides
    (University of the Free State (Qwaqwa Campus), 2017-01) Mulwa, Winfred Mueni; Dejene, B. F.; Ouma, C. N. M.; Onani, Martin
    Ab initio modelling techniques have produced a notable contribution in analysing semiconductor metal oxides properties by use of first principles. These techniques have transformed to a high level of accuracy, owing to the development in algorithms and improved computational ability. In the study of structural, electronic and optical properties of metal oxides, ab initio techniques have been used with a lot of success to illustrate these properties. Ab initio studies therefore can complement experimental findings or even provide reliable results on properties which have not yet been experimentally investigated. Properties which can be calculated with the use of density functional theory (DFT) include spectroscopic, energetic, electronic and geometric properties. In this combined experimental and ab initio work on metal oxides doped with transition metals, the used of local density approximation with the Hubbard U correlation to compute the structural, electronic and optical properties of ZnA12O4 and Cu2+:ZnA12O4 was used. The powders of doped and undoped ZnA12O4 were effectively synthesized by use of the sol-gel technique. The X-ray diffraction (XRD) pattern for ZnA12O4 displayed crystalline peaks corresponding to cubic structure and phase dissociation was not observed. It also showed negligible lattice distortion and a slight shift to higher angles with increase of Cu2+ percentage doping. Energy dispersive X-rays spectroscopy (EDS) confirmed pure samples of ZnA12O4 components. Scanning electron microscopy (SEM) micrographs showed a uniform, well distributed and spherical morphology. The high resolution transmission electron microscopy (HRTEM) showed the influence of varying Cu2+ concentration on the particle agglomeration as well as on the crystallite sizes. The average crystallite sizes of ZnA12O4 powders almost remained constant with the increase of Cu2+ doping concentration. The lattice spacing approximated from selected area electron diffraction (SAED) was 0.242 nm corresponding to (311) lattice of ZnA12O4. Setting excitation at 283 nm, the photoluminescence (PL) emission peaks were at 388 nm, 425 nm and 480 nm in undoped ZnA12O4 which was due to oxygen vacancies while the peak at 586 nm was due to Cu2+ ions. Computationally, introduction of Cu2+ ions did not lead to significant lattice distortion and the PL emission peak was at 435 nm with a transition from Cu_3d to Cu_4p. The substitutional energies in Cu2+:ZnA12O4 predicted negative formation energies for oxygen vacancies suggesting that these vacancies are easily formed in ZnA12O4. The two point defects (oxygen vacancy and Cu2+ dopant) existed singly as the binding energies were found to be negative. Both experimental and computational work were carried out on lanthanide-doped metal oxide (ꝩ-A12O3 in this case). The powders of doped and undoped (ꝩ-A12O3 were successfully prepared using the sol-gel technique. The A12O3 as well as Ce3+: A12O3 were modelled where the Kohn- Sham equations were solved by the use of local density approximation with the Hubbard U correction. In ꝩ-A12O3:Ce3+, introduction of the dopant caused lattice strain as well as reduction in band gap. The formation energies in all the charge states were negative, suggesting that the ꝩ - A12O3 lattice could easily accommodate Ce3+. The PL emission peak was reported to be at 502 nm with a transition from O_2p to Ce_4f. The X-ray diffraction (XRD) pattern exhibited crystalline peaks corresponding to cubic structure. Due to difference in ionic radius between A13+ and Ce3+, lattice distortion was realized. As the doping concentration increased, there was a slight shift to lower angles. Only aluminium and oxygen elements were detected in the EDS analysis. SEM analysis revealed agglomeration on doping. From the HRTEM findings, the crystallite size of 16.0 nm was realized. The lattice spacing approximated from SAED was 0.138 nm corresponding to (440) lattice plane of  -A12O3. With excitation at 240 nm, the PL emission peaks at 440 nm and 462 nm were due to oxygen vacancies while the peak at 560 nm was due to Ce3+ doping. This result shows that Ce3+ doping of  -A12O3 improves its luminescence property therefore making it a possible candidate for blue light emitting diodes application. DFT work on both transition metal and lanthanide-doped metal oxides was investigated in undoped TiO2, lanthanides-doped TiO2 as well as transition metal (Cr3+) doped TiO2 by the use of local density approximation with the Hubbard U correlation to compute the substitutional energies, thermodynamic transition levels, optical properties and magnetic properties of Cr3+:TiO2 and lanthanide-doped TiO2. Unlike ZnAl2O4 and  -A12O3, TiO2 was not experimentally synthesized but was modelled theoretically. Lanthanide doping was found to cause red shift of the band gap from the ultraviolet region to the visible region of the optical absorption spectra in TiO2. The value of the computed substitutional energy implied that lanthanide ions are easily incorporated in TiO2 crystal lattice. The most favorable doping percentage was anticipated to be approximately 3%. On doping TiO2 with chromium, a transition was observed from paramagnetism to ferromagnetism at 6% doping. The magnetic moment per chromium atom was 2.59 μB for rutile phase of TiO2 and 2.49 μB for anatase phase. This result makes Cr3+ doped TiO2 a possible candidate for application in memory devices.
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    ItemOpen Access
    Material properties of semiconducting nanostructures synthesized using the chemical bath deposition method
    (University of the Free State (QwaQwa Campus), 2013-11) Koao, Lehlohonolo Fortune; Dejene, B. F.; Swart, H. C.
    The recent global research interest in wide band gap semiconductors has been focused on zinc oxide (ZnO) due to its excellent and unique properties as a semiconductor material. The high electron mobility, high thermal conductivity, good transparency, wide and direct band gap (3.37 eV), large exciton binding energy (60 meV) at room temperature and easiness of growing it in the nanostructure form, has made it suitable for wide range of applications in optoelectronics, piezoelectric devices, transparent and spin electronics, lasing and chemical sensing. PbS nanostructures is a narrow energy gap material which have relevance for optical applications in the near-IR region of the electromagnetic spectrum such as telecommunications, photovoltaics and bioimaging. It has similar electron and hole effective masses hence the exciton, can be strongly confined which is not always feasible in other semiconductors. Thus the PbS system provides an ideal platform to investigate the exciton in the strong confinement regime. In this thesis, structural and luminescence properties of undoped and doped ZnO and PbS nanostructures (nanorods, nanoflakes, nanoparticles, and nanoflowers) are investigated by different approaches for possible future application of these nanostructures as solar cells and light emitting diodes. Undoped and doped ZnO and PbS nanostructures were grown by chemical bath deposition process. Still it is a challenge for the researchers to produce a stable, reproducible high quality and homogeneously doped ZnO/PbS materials and this seriously hinders the progress of ZnO and PbS nanostructcures to be utilized in various applications. The first part of the thesis includes synthesis of undoped ZnO nanostructures by controlling the growth parameters such as concentrations of precursors (zinc acetate) and synthesis time. Crystalline zinc oxide (ZnO) flower-like nanostructures were synthesized by the chemical bath deposition (CBD) method. The X-ray diffraction (XRD) pattern for the ZnO flower-like microstructures showed crystalline peaks corresponding to a hexagonal wurtzite ZnO structures. Scanning electron microscopy (SEM) observations showed the presence of microcrystallites forming microflower-like aggregates. In the case where a higher molar concentration of zinc acetate was used in the preparation process the microflower-like structures were larger in size than that of the lower mol% used. The shape however did not change. The absorption edges red shifted slightly with an increase in the molar concentration of the zinc acetate and in synthesizing time. The band gap energies decreased slightly with an increase in the molar concentration of the zinc acetate and again in synthesizing time. PL showed that the maximum luminescence intensity was reached at the ZnO synthesized for 5 minutes, any further increase in the synthesizing time resulted into the luminescence intensity decrease. An increase in zinc acetate mol% resulted only in a decrease in luminescence intensity. Controlling growth parameters is important in the sense of controlling the physical, electronic, and chemical properties of materials. In order to understand how to tune these properties in the nanostructure, it is necessary to have an understanding of the growth mechanism that dictates the morphology, structure, and rate of growth of the nanomaterial. The ZnO nanostructures (flower-like rods) were later doped with rare-earth elements (e.g. Ce3+ and Eu3+) and transition metal (e.g. Cu2+). Flower-like hexagonal ZnO:Ce3+ nanostructures obtained for undoped and low mol% of Ce3+. ZnO changed into mixed structure with emergence of pyramids for higher mol% Ce3+. The absorption edges showed that as the molar concentration of Ce3+ ions increases the optical absorption edge shift to a higher. The band gap energies decreased linearly with Ce Concentration. The luminescence bands of undoped ZnO nanoflower-like was quenched and shifted from the yellow region to the blue region when ZnO flower-like was doped with different molar concentration of Ce3+. Eu3+ doped ZnO flower-like structures were synthesized. The XRD spectra of the undoped and low concentration Eu3+ doped ZnO nanostructures correspond to the various planes of a single hexagonal ZnO phase. In contrast with Ce3+ doping, the morphology of the ZnO flower-like rods totally changed to large blocks shape when doped with Eu3+ ions. The effective band gap energy of the ZnO decayed exponentially with the addition of Eu3+. The maximum luminescence intensity was also measured for the same sample. Although weak luminescence was observed for excitation above the band gap at 300 nm the best results were obtained by exciting the Eu3+ directly through the 7F0 → 5L6 absorption band at 395 nm. Excitation at a wavelength of 395 nm produced the highest Eu3+ luminescence intensity without any noticeable ZnO defect emissions. In this work undoped and Cu2+-doped ZnO nanostructures were prepared by the chemical bath deposition (CBD) method. XRD analysis showed the sample prepared were hexagonal ZnO for undoped and Cu-doped. The presence of Cu2+ ions caused the particle size of ZnO flower-like structures to decrease. In the UV-Visible study the reflectance intensity decreased with an increase in the molar concentration of Cu2+ and there was no shift in the absorption edges. The luminescence intensity was found to be a maximum for the undoped ZnO flower-like structures and quenched after addition of Cu2+ ions. In the last part of the thesis, the influence of synthesis temperature and molar concentration of lead acetate on the structure, morphology and optical properties of PbS nanoparticles were investigated. The X-ray diffraction (XRD) peaks correspond to the various planes of a single phase cubic PbS. The surface morphology study revealed nanorod structures at low synthesis temperatures but a particulate structure at the high synthesis temperatures. It was also observed that an increase in the molar concentration of lead acetate has no significant influence on the morphology of the PbS nanorods and the crystallite sizes. The reflectance spectra showed a shift of the absorption edge to a higher wavelength with an increase in the synthesis temperature and molar concentration of Pb acetate. The luminescence intensity was found to decrease with an increase in the synthesis temperature and molar concentration of Pb acetates. The PbS nanoparticles were later doped with Tb3+ and co-doped with Ce3+ ions. When the Tb3+ concentration was increased to 2 mol%, the morphology of the PbS:Tb3+ changed to a mixture of spherical nanoparticles and nanorods. The absorption edges of these PbS nanoparticles slightly shifted to higher wavelength depending on the ionic strength of the precursors. The PL result show an increase in emission intensity with an increase in Tb3+ ions up to 0.3 mol% Tb3+ and decreased there after most probably due to luminescence concentration quenching. A new band at 433 nm was found to emerge as the Tb3+ ions increases. Co-doping PbS nanostructures with 0.3 mol% and 2 mol% Ce3+, the spherical nanoparticles changed the morphology to the nanorods surrounded by the spherical nanoparticle. It was also observed that the size of the nanorods increased with an increase in the molar concentration of Ce3+ ions. The nanoparticles showed good optical properties with high reflectance in the UV and visible regions. The absorption edges shifted to higher wavelength with the addition of Tb3+ and Ce3+, respectively. The photoluminescence results displayed an optimum increase in luminescence intensity when the ratio of Ce:Tb was 1:10 and further increase in cerium content quenched the luminous intensity. It was observed that as the molar concentration of co-dopant (Ce3+) increased the luminescence band at around 433 nm diminished.
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    ItemOpen Access
    Sol-gel synthesis and characterization of MAl2O4 (M = Zn or Mg) spinels doped, co-doped and triply doped nano-phosphors
    (University of the Free State (Qwaqwa Campus), 2014-12) Motloung, Setumo Victor; Dejene, B. F.; Swart, H. C.; Ntwaeaborwa, O. M.
    The luminescent and structural properties of the MAl2O4 (M = Zn, Mg) (which are thereafter referred as hosts) phosphors prepared by sol-gel methods at a relatively low temperature (~80 °C) are discussed. Zinc, magnesium and aluminium nitrates and citric acid were used as the starting materials for the hosts preparations. The prepared gels were dried in an oven and subsequently annealed in air either at 800 0C for an hour. In order to study the effects of the different dopants into the hosts matrix, the dopants concentration were varied. The host material was either singly doped or co-doped or triply doped. Furthermore, in order to study the effects of the catalyst content on the prepared powders, the optimum concentrations for the singly doped phosphors were prepared and the catalyst content was varied during synthesis. Generally, the surface morphologies, surface topographies, crystal structure, photoluminescence (PL), Ultraviolet-visible (UV-Vis) properties were influenced by the dopant concentration and catalyst content. The incorporation of the foreign atoms seems to populate the hosts with more defects. For the ZnAl2O4: x% Pb2+ samples, the Thermo gravimetric analysis (TGA) showed that the minimum annealing temperature required to obtain single phase ZnAl2O4 is above 400 °C. Undoped and Pb2+-doped ZnAl2O4 nanoparticles exhibit the violet emission at slightly different positions, which suggests the possibilities that the luminescence centre can either be due to the defects level in the host or Pb2+ ions. The emission peaks at 390 and 399 nm are ascribed to the typical ultra-violet (UV) transitions 3P0,1 → 1S0 in Pb2+ ions. On the study of ZnAl2O4: x% Cr3+ (0 ≤ x ≤0.3), Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) analysis confirmed the presence of all expected ions in the powder material. The results showed that Cr3+ can occupy multiple-sites in the host matrix. It was interesting to see, once again, that the PL results showed that the host and the Cr3+-doped exhibit violet emission slightly at different peak positions, which suggests that the luminescence can originate from the host or Cr3+ ion. Emission from the host is attributed to the band-gap defects in the host material, while the emission from the Cr3+ is attributed to the 4T1 → 4A2 transition. At the higher mol% there is an emission at 692 nm, which is attributed to the 2E → 4A2 transition in Cr3+. On the case-study of the co-doped ZnAl2O4: 0.1% Ce3+, x% Eu3+ (0 ≤ x ≤ 2mol%), the results showed that the nanopowders microstructure consists of non-uniform sizes and the loss in lattice fringes as the Eu3+ mol% increase suggest the increase in strain or disorder. The incorporation of the co-activator (Eu3+) at the higher mol% resulted in the radiative energy transfer from Ce3+→ Eu3+. The International Commission on Illumination (CIE) color coordinates show the shift from the blue to orange visible region as the Eu3+ concentration is increased. From the triply doped MgAl2O4: 0.1% Ce3+, 0.1% Eu2+, x% Tb3+ (0 ≤ x ≤ 2%) study, the PL results revealed the existence of the energy transfer from Eu2+ → Tb3+ → Ce3+. CIE colour chromaticity showed that the colour can be tuned from bluish → greenish by changing the Tb3+ mol% and the excitation wavelength. In both studies of the effects of the catalyst content in ZnAl2O4:1.5% Pb2+ and ZnAl2O4:0.01% Cr3+, the results showed that the increase in the catalyst content lead to the morphological evolution and transformation from small particles to rods-like-needles. In addition, at the higher catalyst content, the extra peak associated with the ZnO impurities are observed. The emission intensity was influenced by the catalyst content. The catalysts content does not affect the emission colour in the case of ZnAl2O4:1.25% Pb2+. However, in the case of ZnAl2O4:0.01% Cr3+, the results revealed the possibilities of tuning the emission colour by varying the catalyst content.
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    ItemOpen Access
    Study on luminescence and structural properties of vanadates phosphors
    (University of the Free State, 2016-01) Foka, Kewele Emily; Dejene, B. F.; Swart, H. C.
    A self-activated yellow emitting zinc vanadate (Zn2 Y20 1) was synthesized by combustion method. The influence of the processing parameters such as synthesis temperature and dopants concentration on the structure, morphology and luminescence properties was investigated. The X-ray diffraction (XRD) analysis confirmed that the samples have a tetragonal structure and no significant structural change was observed in varying both the synthesis temperature and the dopants concentration. The estimated average grain size was 78 nm for the samples synthesized at different temperatures and 77 nm for the doped samples. Scanning electron microscope (SEM) images show agglomerated hexagonal-like shape particles with straight edges at low temperatures and the shape of the particles changed to cylindrical-like structures at moderate temperatures but were destroyed at higher temperatures. The microstructure retained its original structure when the phosphor was doped with Ba, Ca and Sr. The photoluminescence (PL) of the product exhibited broad emission bands ranging from 400 to 800 nm. The best luminescence intensity was observed for the undoped Zni Y20 1 samples and those synthesized at 600°C. Any further increase in synthesis temperature and concentration of dopants, respectively, led to a decrease in the luminescence intensity. The broad band emission peak of Zn2 Y201 consist of two broad band's corresponding to emission from the Em1 (3T2-1A1) and Em2(3T1 - 1A1) transitions. The Zn2 Y20 1 phosphor was prepared by a sol-gel method. The effect of annealing temperature on the structure and photoluminescence of Zn2 Y201 was investigated. The XRD results showed the single monoclinic phase of Zn2Y201. The crystallinity of the Zn2Y201 phosphor improved while the full width at half maximum of (022) XRD peak was decreased with the increase in annealing temperature. SEM showed that the grains size increased with the increase in annealing temperature, which is due to the improvement in crystallinity of Zn2 Y201. Thermal behaviour of the Zni Y20 1 phosphor was investigated by Thermogravimetric analysis (TOA) and differential scanning calorimetry (DSC), respectively. TOA results showed a total weight loss of 65.3% when temperature was tisen from 35 to 500°C. The photoluminescence emission spectra of annealed Zn2 V201 powders showed a broad band emission from 400 to 800 nm. The PL intensity enhanced as the annealing temperature was increased, resulting to an improvement of the crystallinity. PL emission peaks shift from green emission towards a yellow emission. Dy doped YVQ4:0y3+ phosphors were produced by the combustion method at 600°C. The structure and optical properties of the powders were investigated. The XRD patterns showed the tetragonal phase similar to the standard JCPD file (1 7-0341). SEM shows that the particle sizes were small and agglomerated, and the size increased with the o y3+ dopant concentration and its shape changed to bulk-like particles. In PL, the emission spectra exhibited a weak band at 663 nm for the 4F 912 - 6H1 1/2 transition and a peak at 483 nm (blue) for the4F912 - 6H1 s12 transition and a 574 run (yellow) peak with higher intensity for the 4F912 - 6H 1312 transition. The dependence of the properties of YV04:Dy3+ phosphor upon urea:nitrate concentration was investigated. The samples were synthesized by combustion method. The single tetragonal phase was observed by x-ray diffraction spectra. A highly crystalline YV04:Dy3+ sample was observed when increasing the ratio of the urea to 2. The estimated crystalline size were found to be 20, 39, 33, 30, and 27 nm for the sample prepared with the ratio of 1, 2, 2.5, 3 and 4, respectively. The formation of agglomerated particles was observed by SEM images and it was observed that when increasing the concentration of urea further the flake-like particles formed. The UV diffuse reflectance spectra of YVQ4:Dy3+ with various ratios of urea showed the determined optical band gap ranging from 3.3 to 2.3 eV. Luminescence properties of YV04:0y3+showed that the phosphor emit yellow colour at 573 nm and blue colour at 482 nm corresponding to 4 F912~6Hn12 and 4f912~6H1 s12. respectively. A very week band at 663 nm which correspond to 4 F912~6H1 1/2 transition was also observed. It was found that the PL emission intensity increases with an increase in the ratio of urea and reached maximum at 2 then decreases when increasing the ratio of urea further. YV04:Eu thin films were well deposited by pulse laser deposition at deposition temperature of 200, 300 and 400 °C. The oxygen pressure and deposition time were held constant. The films deposited at higher temperature showed a tetragonal phase. The XRD spectra for the sample deposited at 200 °C showed a very small peak at (200) orientation. Phosphor thin film showed a crystalline structure when the temperature increased. SEM images indicated larger particles at higher temperature. Atomic force microscopy (AFM) results showed the smooth surface with small particles at lower temperature and surface roughness at higher temperature due to the crystallinity. The PL shows the typical emission peaks of Eu in a red region at the 594 and 618 nm attributed to 5Do-7F 1 and 5Do-7F2, transitions. Also the peaks at 652 and 699 nm corresponding to 5Do-7F3 and 5Do-7f4 are observed. The spectra showed an increase in intensity when deposition temperature was increased. YV04:Eu3+ thin films were prepared by pulse laser deposition (PLD). YV04:Eu3+ thin films were deposited at room temperature by varying the deposition time from 30, 45 to 60 minutes. The XRD analysis confirmed that the samples have a tetragonal phase. The improved on crystallinity of the films was observed when increasing deposition time. The estimated grain particle size increased from 52 to 69 nm as the deposition time increased from 30 to 60 minutes, respectively. SEM images showed that when increasing the deposition time, particles were agglomerated and the formation of homogeneous surface was observed for a film deposited at 45 minutes. The rough surface with larger particles was observed for the sample deposited at 60 minutes. PL emission spectra of YV04:Eu3+ showed the main emission peaks which are due to the Eu3+ transition 5Dj-7fj. The strongest red emission peak at 618 nm is due to transition 5Do-7F2. The increased in deposition time showed the improvement in intensity of the thin films.