Physics

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A study on the atmospheric and environmental impacts of aerosol, cloud and precipitation interaction.
(2022) Yakubu, Abdulaziz Tunde.; Chetty, Naven.
Understanding the mechanisms and processes of aerosol-cloud-precipitation interactions (ACPI) is essential in the determination of the specific role of aerosols in modulating extreme weather events and climate change in the long run. Atmospheric aerosols are mainly of various types and are emitted from differing sources. Considering they commonly exist in the heterogeneous forms in most environments, they significantly influence the incoming solar energy and the general perturbation of the clouds depending on their constituents. Thus, a systemic identification and characterisation of these particles are essential for proper representation in climate models. To better understand the process of climate change, this research explores the climate diversity of South Africa to examine aerosol sources and types concerning the atmospheric aerosol suspension over the region and their role in clouds and precipitation formation. The study further provided answers to the cause of extreme precipitation events, including drought and occasional flooding experienced over the region. Also, an insightful explanation of the process of ACPI is provided in the context of climate change. Furthermore, the research found that the effective radiative forcing (RF) over South Africa as monitored in Cape Town and Pretoria is negative (i.e., cooling effect) and provided an analysis of the cause. Similarly, the validation of some satellite datasets from MISR (Multiangle Imaging Spectroradiometer) and MODIS (Moderate Resolution Imaging Spectroradiometer) instruments against AERONET (Aerosol Robotic Network) is conducted over the region. Although a significant level of agreement is observed for the two instruments, intense improvements are needed, especially regarding measurements over water surfaces. Finally, the study demonstrated the proficiency of effective rainfall prediction from satellite instrument cloud datasets using machine learning algorithms.
Design and fabrication of tissue-like phantoms for use in biomedical imaging.
(2022) Ntombela, Lindokuhle Charles.; Chetty, Naven.; Adeleye, Bamise.
The continuous need for tissue-like samples to understand biological systems and the development of new diagnostic and therapeutic applications has led to the adoption of tissue models using potential materials. This work presents a low-cost method for manufacturing PVAslime glue-based phantoms to replicate diseased and healthy biological tissues’ optical, mechanical, and structural properties. The deformable phantoms with complex geometries are vital to model tissues’ anatomic shapes and chemical composition. Absorption and scattering properties were set by adding black India ink and aluminium oxide (Al2O3) particles in varying quantities to obtain slime phantom tissues with optical properties of the brain, malignant brain tumour, lung carcinoma, and post-menopausal uterus. The phantom properties were characterized and validated using a He-Ne laser emitting at 532 nm and 630 nm wavelengths propagated through various thicknesses of the fabricated phantom. The incident and transmitted intensity were measured to determine the absorption coefficient (a) and scattering coefficient (s). Furthermore, the effective attenuation coefficient (eff ) and penetration depth () were deduced from the reduced scattering coefficient (0s) and the anisotropy factor (g) obtained through the scattering phase function and Wolfram Mathematica. The anisotropy factor demonstrated a forward scatter, typical of strongly scattering media as real tissues. Such geometrically and optically realistic phantoms would function as effective tools for developing techniques in diagnostic and therapeutic applications such as laser ablation and PDT cancer treatment.
Improved collection of photogenerated current using bi-metal nanoparticles.
(2024) Jili, Ncedo.; Mola, Genene Tessema.
The energy demand has been continuously growing owing to the shortage of sources of traditional energy (such as fossil fuels), due to the growing population of the world, and increased industrialization, which prompted the need for more energy. However renewable energy (such as photovoltaics) has attained attention due to its reliance on the infinite energy source (sun) which provides an hour long energy flow that fulfil the yearly energy of the glob. Not only that, renewable energy sources offer clean energy, that is meant to contribute to decarbonization in the future and reduce environmental changes. Solar cell materials that can effectively capture photons and conduct charges are continuously investigated for the last six decades. Contrarily to silicon based solar cells, organic solar cells are among the most promising solar cells in terms of offering cheap device fabrication, flexibility, high absorption, etc. However, these solar cells still suffer from low efficiency compared to traditional silicon solar cells due to poor absorption, low mobility, and poor stability. Numerous strategies have been employed to improve the efficiency of OSC devices, these include Ternary OScs, Tandem OSCs, and the inclusion of nanoparticles in OSC devices. Nanoparticles remain the best candidate to feature in OSC devices because Tandem OSCs require multi-absorber layers which leads to high device cost, whereas nanoparticles can be produced at a small scale and still offer good results. This study takes advantage of the features offered by the nanoparticles and uses them to investigate the effect of Nickel doped with cobalt bi-metal nanoparticles(Ni/Co BMNPs) in the PEDOT:PSS buffer layers of the P3HT: PCBM-based devices. Solar cells were successfully fabricated with four different concentrations of Ni/Co BMNPs as 0.05 %(0.5 mg), 0.15 %(1.5 mg), and 0.25 %(2.5 mg). Significant improvements were achieved for the 0:05% with the Fill factor of 58:52 %, and current density of 15.31 mA/cm2, and maximum efficiency of 5:05 % which displayed 67:8 % improvement from the undoped device. The investigation was further conducted by simulation program called SCAPS to confirm the contribution of the metal nanoparticles on the device performance. The results were reproduced in SCAPS where the energy band gap of the P3HT:PCBM and the shallow conduction density of electrons of the PEDO:PSS were simultaneously varied. All results are comparable with the experimental results and found to be similar. The device that was made to mimic the 0:05 % device produced a FF of 57:76 %, Jsc of 15.76 mA/cm2, and maximum efficiency of 5:76 % which displayed 88 % improvement from the undoped device. This study further provides factors that contributed to the high/low device performance due to the inclusion of the BMNPs in the OSC device and some of the necessary background and theory are provided to support these findings.
Metal plasmonic as a mechanism to improve the performance of thin film polymer solar cell : device fabrication and characterization.
(2024) Ike, Jude Nodebechukwunso.; Mola, Genene Tessema.
This thesis discusses the results of the investigations on the use of plasmon metal nanoparticles (NPs) to enhance the performance of polymer solar cells, which are promising alternative solar energy converters to silicon-based solar cells. Polymer solar cells offer a cost-effective, flexible solar panel using a solution processing method for the generation of power from solar sources. Several key factors are considered to achieve this goal, including optical absorption, nano-morphology, and charge carrier mobility. The thesis focuses on investigating the potential of various dopants, such as solvent additives, thermal annealing, and metal nanoparticles (NPs), to improve the performance of thin-film organic solar cells (TFOSCs) by enhancing the charge transport processes. This research employed conventional device architectures to study the effectiveness of the active and buffer layers on charge transport and stability. The results have already been published in several internationally referred journals. In this thesis, the synthesized metal NPs were employed as mechanisms to improve the performance of TFOSC incorporated with poly-3-hexylthiophene (P3HT) as the donor material and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the acceptor material. The popular metal NPs such as nickel, (Ni), zinc (Zn), silver (Ag), calcium (Ca), sulfur (S), and cobalt (Co) were used to synthesize various bimetallic composites. Bimetallic nanoparticles such as nickel-doped with zinc bimetallic (Ni/Zn), silver-doped with calcium (Ag/Ca), silver-doped with cobalt (Ag/Co), and silver-doped zinc sulfide (ZnS/Ag) were employed at different functional layers of solar cell structure. Hence, the study employed various spectrometers such as high-resolution transmission and scanning electron microscopy (HRTEM and HRSEM), X-ray diffraction, Ultraviolet-Visible (UV-Vis) spectroscopy to investigate the size, morphology, elemental mapping, and optical properties of synthesized metal NPs. HRTEM is indeed a powerful technique for characterizing nanoscale materials, and it can provide valuable insights for confirming the presence of core-shell structures in NPs. Compared to the pristine reference, the blend of metal NPs with active and buffer layers at different concentrations plays a crucial role in augmenting the optical and electrical properties in TFOSC devices. Such increment of optical and electrical properties in this thesis is due to improved short-circuit current (Jsc), fill factors (FFs), and charge carrier mobility, which are significant to enhance the power conversion efficiency (PCE) values in the polymer solar cells. These prominent improvements are due to the presence of localized surface plasmonic resonance (LSPR) effect of TFOSCs. Finally, this thesis provides a series of experimental works fabricated with several metal NPs in TFOSCs at different concentrations.
Modelling of quantum phase transitions in Dirac materials.
(2022) Hussien, Musa Alnour Musa.; Ukpong, Aniekan Magnus.
In this thesis, first-principles computations of the electronic ground state are used to investigate the underlying nature of the quantum phase transitions in selected Dirac materials and their associated hybrid materials. Various methods are used to break the symmetry of the ground state electronic structure and to tune the emergent phases. These involve the application of an external electric, magnetic or electromagnetic fields, and the manipulation of the internal intrinsic fields of materials, such as the introduction of strong spin orbit coupling, electron (or hole) doping, including a strong short-ranged disorder potential. The findings show that applying a perpendicular electric or magnetic field with a staggered potential in the underlying lattice allows for a tunable electronic transition between trivial and nontrivial quantum states. Signatures of the near-field electrodynamics of carriers in nanoclusters reveal the appearance of a quantum fluid phase at the distinct energies where topological quantum phase transitions occur. Emergence of the field-induced carrier density wave phase shows that the collective excitation mode is a distinct potential for carrier transmission in spintronic, optoelectronic, and photonic technologies. Furthermore, two types of insulating Dirac material are used as the tunnel barrier region of the perpendicular tunnel junction architecture. The resulting heterostructure is an artificially assembled metal-insulator-metal multilayer, and this serves as a generic platform for characterizing spin transport properties in spintronic devices. The dependence of the emergent phenomenon of proximity induced magneto-electronic coupling on the tunnel barrier material is unraveled. By analyzing the effect of the changes in the electronic structure on the spin transmission properties, it is found that the metal-insulator-metal platform exhibits a quantum phase transition by responding sensitively to both the tunnel barrier material and the applied perpendicular electric field during in-service operation of a spintronic device. The results show that when the electric field approaches its critical amplitude, the spin density of states exhibits a discontinuous change from half-metallic to metallic transport character in the presence of monolayer hexagonal boron nitride as a tunnel barrier material, contrary to when the monolayer molybdenum disulphide is inserted in the tunnel barrier region. The role of the applied electric field in the phase transition is characterized in terms of a spin-flip transition and an induced interfacial charge transfer. It is also found that the abrupt discontinuity in the changes in the spin-flip energy with increase in applied electric field provides the necessary and sufficient evidence of a first-order quantum phase transition in the spin transport phase. These findings show that the material of the tunnel barrier layer creates a non-trivial function in defining the magnetoelectric couplings that occur dynamically during spin tunneling.
Synthesis and characterization of magnetic iron oxide nanoparticles.
(2022) Ndlovu, Nkanyiso Linda.; Masina, Colani John.
Three high-purity cubic spinel-type crystalline magnetic iron oxides i.e. Fe3O4, CoFe2O4, and NiFe2O4 nanoparticles were successfully synthesized by co-precipitation method. X-ray diffraction (XRD) showed the formation of stoichiometric phases with average particle size of 11.7 nm, 23.6 nm, and 16.4 nm for the as-prepared Fe3O4, CoFe2O4, and NiFe2O4 nanoparticles, respectively. Transmission electron microscopy (TEM) observation for all three samples revealed spherical morphology with single magnetic domain structure. From high resolution TEM (HR-TEM) imaging, lattice fringes with d-spacing of 0.473 nm and 0.248 nm corresponding to (111) and (311) reflections planes, were observed for both the Co-doped and Ni-doped samples. Energy-dispersive x-ray spectroscopy (EDX) analysis showed the presence and homogeneous distribution of main elements Fe, O, Co, and Ni in the samples. Quantitative EDX results confirmed the formation of stoichiometric CoFe2O4 and NiFe2O4 phases with the experimentally measured weight wt% of the samples closely equal to the theoretical calculated wt% values i.e. Fe = 46.35 wt%, O = 26.79 wt%, and Co = 26.87 wt% for CoFe2O4, and Fe = 47.02 wt%, O = 27.27 wt%, and Ni = 24.75 wt% for NiFe2O4. The magnetic properties of these nanoparticles were investigated by 57-Fe Mossbauer spectroscopy (MS) and Vibrating Sample Magnetometer (VSM) techniques. Room temperature MS spectrum for the pure Fe3O4 phase consist of two superimposed sextets with isomer shifts (0.321, 0.463) mm/s and hyperfine field (57.3, 43.4) T attributed to tetrahedral (A-sites) and octahedral (B-sites). The CoFe2O4 and NiFe2O4 samples both showed room temperature MS spectra consisting of two sextets and a single central paramagnetic doublet. The two sextets in each sample had almost equal isomer shifts for both A- and B-sites i.e. 0.2956 & 0.3247 mm/s and 0.3784 & 0.2761 mm/s for each of the sites of the CoFe2O4 and NiFe2O4 sample, respectively. The paramagnetic doublet was fitted with isomer shift of 0.3272 mm/s for the CoFe2O4 sample and 0.3249 mm/s for the NiFe2O4 sample. Temperature dependence M-T magnetization curves measured at H = 500 Oe inthe zero-field-cooled (ZFC) and field-cooled (FC) conditions showed the superparamagnetic nature of all three particles. The MZFC magnetization curve showed a maximum (cusp) at 225 K, 300 K, and 228 K corresponding to blocking temperature (TB), for Fe3O4, CoFe2O4, and NiFe2O4, respectively. For the CoFe2O4 sample the irreversibility temperature (Tirr) was equal to the blocking temperature (TB). While measured Tirr for Fe3O4 and NiFe2O4 was 300 K for both samples. The M-H magnetization curves at 300 K for all three samples revealed the coexistence of ferrimagnetic and superparamagnetic behaviour of the nanoparticles. At 300 K all three samples exhibit symmetrical and almost "closed" hysteresis loops with coercivity approximately 36, 70, and 117 Oe and remanence magnetization of approximately 5, 3, and 4 emu/g, for Fe3O4, NFe2O4, and CoFe2O4, respectively. Furthermore, M-H measurements at 300 K showed a high saturation magnetization of 89 emu/g for the Fe3O4 sample compared to 37 emu/g and 26 emu/g for the CoFe2O4, and NiFe2O4, respectively. M-H measurements recorded at low temperatures showed rather "opened" hysteresis loops compared to loops measured at 300 K. In contrast to saturated magnetization M-H curves for the Fe3O4 and NiFe2O4 nanoparticles, unsaturated M-H loops were observed for CoFe2O4 sample in the temperature range 10 - 100 K. A significant increase in coercivity to 102 Oe, 391 Oe, and 2.4 kOe was observed for Fe3O4, NiFe2O4, and CoFe2O4, respectively, when the temperature was reduced from 300 K to 10 K. For the CoFe2O4 sample, a highest coercivity of 2.7 kOe was measured at 100 K. And finally, M-H data at 10 K showed high saturation magnetization of 100 emu/g, 51 emu/g, and 31 emu/g, for the pure magnetite, CoFe2O4, and NiFe2O4 samples, respectively.
The effect of metal sulfides on hole and electron transport buffer layers in organic photovoltaics: experimental and numerical device simulation investigations.
(2022) Adedeji, Michael Adepelumi.; Mola, Genene Tessema.
Organic solar cells (OSCs) are promising alternative renewable energy sources that often suffer from insufficient absorption of solar radiation, short exciton lifetimes and small diffusion length of their charge carriers. Several strategies are being investigated to overcome these challenges in a move towards the commercialization of this solar cell technology. Increasing the path-length of incident electromagnetic radiation within the photo-absorbing layer of the solar cell, may elongate the time that light spends within the solar cell, thereby increasing the light-matter interaction time and consequently the photo-absorption within the photo-active material of the solar cell. The process described may be accomplished if suitable plasmonic metal nano-structures are added into the solar cell matrix. This intervention may also enhance the collection of the photo-generated charge carriers. The effects of metal sulphide nanoparticles incorporation in organic solar cells were studied and presented in this thesis. The metal sulphide nanoparticles were characterized and introduced into the hole- and electron- transport layers of fullerene and non-fullerene electron acceptors based solar cells, to elicit improved photoabsorption via the localized surface plasmon effects and facilitate better charge collection at the electrodes. Devices were fabricated both in an ambient environment and in a controlled environment (Nitrogen filled glovebox). The metal sulphide nanoparticles were incorporated in the fabricated solar devices using both the conventional and inverted device architectures. The power conversion efficiencies of the devices improved significantly after the incorporation of these nanoparticles. Device numerical simulation studies were also performed to reproduce some of the experimental results with a view to further investigating the devices and discussing their charge transport characteristics. The simulation results show improved charge carrier characteristics from the metal-sulphide doped devices by way of improved conductivity and shifted Fermi level offsets which were aided by the presence of the metal-sulphides. Asides from successfully achieving improved device performances in the investigations and simulations carried out in this thesis, this thesis successfully demonstrated the incorporation of nano-composites in non-fullerene acceptors-based organic solar cells for the first time to the best of our knowledge.
Wigner functionals and ghost imaging.
(2023) Durgapersadh, Akshay.; Konrad, Thomas.; Roux, FiIippus Stefanus.
This dissertation discusses Wigner functionals and an application, namely ghost imaging. Wigner functionals aim to provide more accurate measurement results due to the inclusion of all the degrees of freedom of light. The main concepts discussed are spontaneous parametric down-conversion (SPDC), the evolution equation for light through the SPDC crystal, the probability distribution for the ghost image, conditional probability distribution, and the point spread function. The ghost imaging calculation is done for the rational thin crystal and extreme thin crystal limits. It is shown that the probability distribution produced is the same for the rational thin crystal and extreme thin crystal limits, and consequently, the point spread function, and conditional probability distributions are the same.

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