Magister Scientiae - MSc (Physics)
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Item Hybrid two-dimensional perovskite semiconductor layers for electronic applications(Universty of the Western Cape, 2024) Mercuur, James Winston; Arendse, ChristopherPerovskite materials have emerged as a frontier in advanced functional materials, offering a versatile platform for various applications ranging from solar cells to light-emitting devices. Among the different fabrication techniques, Chemical Vapor Deposition (CVD) stands out for its ability to produce high-quality thin films with precise compositional control. This study delves into synthesising a two-dimensional (2D) hybrid perovskite thin film (PEA)2PbCl4 on diverse substrates, marking a significant advancement in the scalable production of perovskite-based devices. Optimising CVD parameters enabled the deposition of a PbCl2 precursor thin film and its subsequent conversion to the perovskite through an intercalation reaction with PEACl. X-ray diffraction (XRD) analysis, complemented by Rietveld refinement and VESTA simulations, confirmed the hybrid 2D structure of the (PEA)2PbCl4 with an inorganic octahedral layer number (n) equal to one. Scanning electron microscopy revealed a uniform distribution of columnar structures (~120 nm in diameter) for PbCl2 and a woven-like morphology for the perovskite. Optical properties assessed via UV-Vis spectroscopy indicated a band gap of 3.47 eV (Tauc plot) and 3.644 eV (Elliott model) for the perovskite. The perovskite Urbach energy (EU) was estimated at ~236 meV (Urbach method), suggesting significant disorder within the film. However, exciton binding energies were approximately 20 meV, as per the Elliott fit. Photoluminescence spectroscopy showed broadband self-trapped exciton emissions centred at ~532 nm with a full width at half maximum (FWHM) of 140 nm, attributed to intrinsic lattice distortions. These findings contribute to understanding the structural and optical properties of 2D hybrid perovskites and the potential of CVD synthesis, with implications for their application in optoelectronic device fabrication.Item Detecting anomalous transients in meertrap data(Universty of the Western Cape, 2024) Petersen-Charles, Jade Lindsay; Lochner, MichelleIn an era distinguished by significant technological progress, the prevalence of large and complex datasets characterizes the "big data" era across various disciplines. With improved telescopes being built aimed at generating datasets of unprecedented volumes, there is incredible potential for discovery. The MeerKAT radio telescope in South Africa has proven to be an excellent telescope to search for fast radio transients such as pulsars and fast radio bursts (FRBs). MeerTRAP (more TRAnsients and Pulsars), which commensally uses MeerKAT to search for fast radio transients, detects tens of thousands of candidate objects daily (on average), although the vast majority are not of astrophysical origin. Automated techniques such as machine learning are routinely used to identify targeted astrophysical transients. However, an emerging application of machine learning is to aid the detection of unidentified or rare sources, referred to as anomalies.Item The effects of ion beams on nonlinear electrostatic waves in space plasmas(Universty of the Western Cape, 2024) Maxengana, Mihlali; Bharuthram, RamashwarThe effect of ion beams on linear and nonlinear low frequency electrostatic ion-acoustic waves is examined in multi-species plasma models, which are regularly observed in the terrestrial magnetosphere. These models comprise two thermal (adiabatic) ions and one or two Boltzmann electron species, with different temperatures in the latter case. One or both ions are regarded as drifting (beam) species. The linear study reveals the existence of two linear ion-acoustic modes: slow and fast ion-acoustic modes. The slow ion-acoustic modes emerge in models with at least one beam, while fast modes occur in beam and beamless plasma models. The slow ion-acoustic modes are found to be highly sensitive to variations in ion beam speed, becoming unstable to the ion beam instability for intermediate speeds. In the nonlinear regime, the ion-acoustic modes evolve into nonlinear structures such as solitons and double layers, which are pulse like disturbances in potential, commonly known as electrostatic solitary waves. A critical value of the beam speed must be exceeded for the existence of the slow ion-acoustic solitons of which the one slow mode propagates above and the second below the critical acoustic speed associated with the corresponding linear wave.Item Unsupervised machine learning applied to radio data(Universty of the Western Cape, 2023) Mohale, Koketso; Lochner, MichelleThis thesis presents work motivated by the belief that the next generation of discoveries in the field of astronomy will be made by the marriage of advanced data analysis algorithms in the form of unsupervised learning techniques, and the unprecedented volumes and complexities of data from the next generation of surveys. For several years, computers have been governed by Moore’s law, which posited that computing power would double every two years. The consequence was that computing has also become increasingly cost-effective, which has been a driving force in the ability to generate and analyse large volumes of datasets. These include machine learning advances like the use of deep learning and scalable techniques such as self-supervised learning which have been revolutionising areas of research, for example, natural language processing and computer vision. Similarly, astronomy is also met with a rapid growth in the availability of large datasets. Morden sky observing instruments such as the radio telescope MeerKAT and the optical telescope Blanco (which was used for the Dark Energy Survey) are already producing data volumes at unprecedented scales. The next generation of instruments like the Square Kilometre Array (SKA) and the Vera C. Rubin Observatory are expected to produce orders of magnitude more astronomical data at higher resolution and sensitivity. Ongoing efforts in the form of surveys and data analysis techniques in astronomy are motivated in part by outstanding questions in galaxy evolution and cosmology as well as the potential to discover new unknown phenomena.Item Production and evaluation of capacitive deionization electrodes earmarked for the removal of ions from winery wastewater(University of the Western Cape, 2023) Theron, Cleeve; Bladergroen, BernardMembrane capacitive deionization (MCDI) is a desalination technology considered to be cost effective specifically for salt bearing solutions with concentrations below 2000 mg‧L-1, which includes general winery wastewater. To consider winery wastewater to irrigate vineyards, the water needs to be desalinated to some extent. This study aims to establish whether in-house developed electrodes could be applied in MCDI to reduce the concentration of ions contained in winery wastewater model solutions. This would allow wineries to recycle wastewater for irrigation purposes and reduce their reliance on fresh water intake, increasing the winery industry’s drought resilience. Electrode optimization was done by systematically varying electrode production parameters and targeting several specific electrode variables which influence the performance of the electrode in terms of, maximum salt adsorption capacity (mSAC), practical salt adsorption capacity (pSAC), average salt adsorption rate (ASAR), wettability, through-plane electrical conductivity and surface area and pore size distribution with diameters in the mesopore size range.Item Impact of different chemical synthesis techniques on the structural and photoluminescence properties of Zn4B6O13: RE3+ (where RE3+ = Sm3+, Eu3+, and Dy3+) nanophosphors(University of the Western Cape, 2023) Siyalo, Athenkosi; Halinditwali, SylvainDifferent chemical synthesizing techniques, such as sol-gel, combustion, and solid-state reaction routes, offer the unique possibility of generating high-purity nanophosphors with different morphologies. In this project, we intend to do a comparative study on the structural and photoluminescence properties of Zn4B6O13:RE3+ (where RE3+ = Sm3+, Eu3+, and Dy3+) nanophosphors using the above-mentioned techniques. Up to now, the comparative study based on the synthesis techniques has not yet been reported. As it will be shown in the literature survey’s section, these methods were found to drastically affect the structural and photoluminescence characteristics of phosphor materials, which directs us to implement such studies on highly crystalline Zn4B6O13: RE3+ (where RE3+ = Sm3+, Eu3+, and Dy3+) nanophosphors. In this work, we intend to use different characterization techniques such as X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FT-IR), ultraviolet visible spectroscopy (UV-Vis), and photoluminescence (PL) spectroscopy. In this study, we aim to justify the appropriate technique that will produce the best PL properties for Zn4B6O13: RE3+ (where RE3+ = Sm3+, Eu3+, and Dy3+) nanophosphors, which could be used as a benchmark for future researchers in solid-state lighting systems. The incorporation of the Sm3+, Eu3+, and Dy3+ dopants did not change the cubic crystal structure of the Zn4B6O13 host material, as confirmed by the XRD results. However, they caused a slight deviation to higher 2𝜃 values, which confirms the charge exchange between the dopants and the host.Item Influence of synthesis method on structural(Springer, 2023) Noukelag, Sandrine Kamdoum; Noukelag, Sandrine Kamdoum; Ngqoloda, Siphelo; Kotsedi, LebogangThis contribution reports on the development of two versatile and efcient methods, namely the green and gamma radiolysis for Fe-Ag nanoparticles (NPs) synthesis, characterization, and further their growth inhibition potential on some spoilage microorganisms. Green Ag/Fe2O3 NPs were obtained at Fe-Ag [3:1], annealing temperature of 800 °C for 2 h, and gamma irradiated Ag/Fe3O4 NPs were obtained at Fe-Ag [7:1], a 50 kGy dose. The characterization techniques were performed with these two samples whereby the sizes from crystallographic and microscopic analyses were 39.59 and 20.00 nm for Ag/Fe2O3 NPs, 28.57 and 15.37 nm for Ag/Fe3O4 NPs, respectively. The polycrystallinity nature observed from X-ray diffraction was in accordance with the selected area electron difraction. The vibrational properties confrmed the presence of bimetallic Fe-Ag NPs with the depiction of chemical bonds, Fe–O and Ag–O from attenuated total refection-Fourier transform infrared spectroscopy and elements Ag, Fe, O from energy-dispersive X-ray spectroscopy analyses. The magnetic properties carried out using a vibrating sample magnetometer suggested a superparamagnetic behavior for the Ag/Fe2O3 NPs and a ferromagnetic behavior for the Ag/Fe3O4 NPs. Overall, the green Ag/Fe2O3 NPs successfully inhibited the growth of spoilage yeasts Candida guilliermondii, Zygosaccharomyces fermentati, Zygosaccharomyces forentinus, and spoilage molds Botrytis cinerea, Penicillium expansum, Alternaria alstroemeriae.Item Sample selection effects in laduma stacking experiments(University of the Western Cape, 2023) Gwebushe, Lwandile; Elson, EdwardThis study explores the impact of selection effects introduced by input optical cata-logues on high-redshift stacking experiments. Using the Hi stacking technique, the Hi content of LADUMA synthetic data cubes is analyzed within the redshift range of 0.7 < z < 0.758. The focus is on galaxies with stellar masses of M⋆/M⊙ ≥ 108.5. The investigation reveals several key findings. Firstly, the input optical catalogues displays biases towards specific galaxy types, including those with lower stellar masses, lower star-formation rates, lower specific star-formation rates, galaxies in groups, and red galaxies. These biases can lead to higher confusion rates in stacked spectra, emphasizing the need for appropriate correction methods to avoid overestimation in subsequent quantitative analyses. Secondly, the environmental impact on Hi gas properties is examined by comparing group galaxies and non-group galaxies. Non-group galaxies exhibit higher Hi richness and Hi gas fractions compared to group galaxies. Correlations between Hi properties and star-formation/stellar mass properties are observed, indicating that lower mass galaxies are relatively inactive within their environments and are at an earlier stage of evolution. The study also addresses the correction of confusion rates in stacked spectra. While a correction method is implemented, it is found to result in overestimated Hi deple-tion timescales. Criticism is directed towards the use of this method, stressing the importance of employing accurate and robust approaches to correct for confusion rates and ensure precise estimation of galaxy properties.Item Structural features of air-processible methyl ammonium lead triiodide (MAPbI3) perovskite thin films grown on Al-doped ZnO Nanowire Arrays(University of the Western Cape, 2023) Phakoe, Mpho; Cummings, FransciousThe performance of air stable, mixed halide (MAPbI3-xClx) perovskite based solar cells is highly dependent on the quality and stability of the perovskite thin film, which in turn, is dependent on the substrate on which it is deposited. ZnO presents excellent optoelectronic properties such as high electron mobility and diffusion length, direct band gap with high exciton binding energy. An array of ZnO nanowires (NWs) grown vertically on a conducting substrate, benefits from a large surface area, direct electron transport pathway and reduced recombination rate of carriers when used in a solar cell. These arrays of ZnO NWs may be synthesised by a wide range of methods, with the chemical bath deposition (CBD) method considered to be the most simple and cost-effective.Item Accelerating reionization simulations using machine learning(University of the Western Cape, 2023) Masipa, Mosima Portia; Sultan, HassanEpoch of Reionization (EoR) refers to the time in the history of the universe when the appearance of the first luminous sources reionized the intergalactic medium (IGM). The EoR carries a wealth of information regarding structure formation and evolution. Ongoing and planned 21cm experiments such as the Hydrogen Epoch of Reionization Array (HERA) and the Square Kilometre Array (SKA) are expected to generate huge amounts of high dimensional datasets, and hence a new generation of efficient simulations and tools are required in order to maximize their scientific return. While Convolutional neural networks (CNNs) achieve the state-of-the-art performance to extract information from large scale fields, generating large training datasets and fully exploring the cosmological and astrophysical parameter space require fast simulations.Item Machine learning approaches to study star formation and black hole accretion in the Meerkat/MIGHTEE survey(University of the Western Cape, 2023) Silima, Walter; Vaccari, MattiaGalaxy formation and evolution are driven by two main physical processes: star formation and black hole accretion. Both processes can be traced via the synchrotron emission at radio wavelengths. However, a reliable classification of radio sources as star-formation-dominated sources (or Star-Forming Galaxies, SFGs) and blackhole- accretion-dominated sources (or Active Galactic Nuclei, AGN) is non-trivial and often requires extensive use of multi-wavelength data. Although significant effort has been put into classifying radio sources as SFGs or AGN over the decades, the rapid growth of radio data available from facilities such as the South African MeerKAT telescope, the Australian Square Kilometre Array Pathfinder (ASKAP), and eventually the Square Kilometre Array (SKA) requires the development of efficient and reliable automated classification techniques.Item Inclusive (p, α) and (p,³He) cross sections induced by 100 MeV protons on ⁴⁰Ca(University of the Western Cape, 2006) Masikhwa, Tshiftriwa Moureen; Fortsch, S.V.Alpha clusteri ng in nuclei is studied with quasi - free knockout reactions. A beam of 100 MeV polarized protons was used in the quasi-free knockout reaction 40 ~a( p,p' a)36 Arat iThemba LABS. While the scattered protons were detected in the K600 magnetic spectrometer, the alpha particles were measured with a detector telescope consisting of three silicon surface barrier detectors. Both coinc idence and pre-scaled singles events were recorded to extract cross sections as well as analysing powers. The inclusive spectra of 40 Ca(p, a) and 4° Ca(p,3 He) were extracted from the prescaled-s ingles data. The prescaled single events were analyzed in order to test the efficiency of the silicon detector telescope and to obtain the double differential cross sections of the emitted 3He and 4 He particles.Item Study of the use of Electret Ion chambers as radom dosimeters in South Africa(University of the Western Cape, 2002) Vuza, Hurnphrey Monwabisi; Lindsay, RRadon is a naturally occuring radioactive gas which has been shown to be a major contribution to background dose. The underground mines in South Africa present a major occupational hazard. A need for cheap personal monitoring has been identified. In this thesis, commercially available electret ion chambers (EICs) are studied, with a view to use them as personal monitors. These detectors were developed to measure radon activity concentrations in houses. Their use in the harsh underground mining environments has shown some complications such as water entering the detectors and their response time to a rapid change in radon concentration has been questioned. Several measurements are discussed in this thesis relevant to solve these problems. Firstly, the EIC's are checked for accuracy in the laboratory using a radon source, as well as the radon chamber at the CSIR in Pretoria. Comparisons with alpha track detectors are also carried out.Item Intense pulsed neutron generation based on the principle of plasma immersion ion implantation technique (PIp3s) technique(University of the Western Cape, 2006) Victor, Motloung, Setumo; Franklyn, Chris. B.; Lindsay, RThe development of a deuterium deuterium/ tritium-deuterium (D-D/ D-T) pulsed neutron generator based on the principle of the Plasma Immersion Ion Implantation 1Pl) technique is presented, in terms of investigating development of a compact system to generate an ultra short burst of mono-energetic neutrons (of order 1910 per second) during a short period of time (.20ps) at repetition rates up to 1 kHz. The system will facilitate neutron detection techniques, such as neutron back-scattering, neutron radiography and time-of-flight activation analysis. Aspects addressed in developing the system includes (a) characterizing the neutron spectra generated as a function of the target configuration/ design to ensure a sustained intense neutron flux for long periods of time, (b) the system was also characterised as a function of power supply operating conditions such as voltage, current, gas pressure and plasma density.Item Cross-sectional transmission electron microscopy of nickel silicide formation(University of the Western Cape, 1993) Julies, Basil Allen; Knoesen, D; Pretorius, RSilicides play a significant role in modern device technology. The operation of electronic devices heavily relies on the specific properties of metal-semiconductor interfaces. Although semiconductor technology has proven very successful in utilizing the properties of materials the knowledge on formation, structure and electrical behaviour of interfaces is still far from complete. In this study an investigation into the Ni-Si binary system was made. Several techniques namely Rutherford Backscattering Spectroscopy, Transmission Electron Microscopy, Scanning Electron Microscopy, Auger Emission Spectroscopy and X-ray Diffraction were employed in the characterization of nickel thin films on silicon and the respective silicides which were formed. Special attention was given to the phase transition from NiSi to NiSi2. First phase formation, namely Ni2Si, was investigated at a vacuum furnace temperature of 290°C. This phase was found to be polycrystalline and grew in layers of uniform thickness with sharp Si-silicide and Ni-silicide interfaces. Growth continued until all the Ni-metal was consumed. Second phase formation (NiSi) was observed at 330°C only after the Ni2Si has grown to its full thickness. This polycrystalline phase also grows in layers. These layers however, are not of uniform thickness, the interfaces between the silicide and silicon substrate therefore being less regular. It was found that NiSi grains could assume one of two crystal structures, orthorhombic or a FeSi cubic structure. Generally it seems as if NiSi initially crystallizes into an orthorhombic crystal structure, before undergoing an allotropic transformation to the FeSi cubic structure. Micro-diffraction was used to characterize individual grains. Final phase formation (NiSi2) was mainly examined at 750°C. A scanning electron microscopy investigation showed that after 5 minutes of annealing islands of NiSi2 was observed in a NiSi matrix. With longer annealing times these islands grew laterally and eventually joined up with others. Cross-sectional transmission electron microscopy very firmly confirms the presence of NiSi2 surrounded by NiSi. Rutherford backscattering, X-Ray diffraction and Auger electron spectroscopy complement these results. Scanning electron microscopy shows that after the coalescence of individual NiSi2 islands, holes appear on the grain boundaries. These holes probably result from an accumulation of vacancies on the grain boundaries during NiSi2 formation which occurs via Ni diffusion in NiSi into the underlying silicon. As the NiSi2 phase continues to grow these holes increase in size and later take on the same crystal structure as the surrounding cubic NiSi2 grains. Although the reaction: NiSi + Si => NiSi2 is thermodynamically favourable to occur at 750°C, it was found that even after 15 minutes of annealing at 750°C, some grains were still NiSi while many others had switched to NiSi2. Identification was once again done by micro diffraction. This means that there is more than just the thermodynamic aspect involved in deciding when NiSi should transform to NiSi2. A model has therefore been proposed in which the major factors in determining the time lapse for transformation to take place are presented. This model generally presents an atomistic approach which centres around the degree of Ni diffusion across the grain boundary of two individual grains. Observations also suggests that NiSi2 results from NiSi by a diffusion process although nucleation can take place at random. This model must not be seen as contradictory to models proposed in the literature which only allow for non-uniform growth at the Si-NiSi, interface, but must rather be seen as complementing it by allowing for diffusion processes as well.Item Antimicrobial activity of South African red algal secondary metabolites(UWC, 2001) Cameron D; Keats, DW; Cyster, LF; Leng, H; Green, IInfectious diseases, for example, measles, scarlet fever, malaria, tuberculosis, and cholera have claimed many lives. These infections are caused by pathogenic viruses, bacteria, or fungi that invade the body's tissues and multiply. According to the Online Medical Dictionary (2000), the multiplication may be clinically barely visible or result in local cellular injury because of competitive metabolism, toxins, intracellular replication, or antigen-antibody response. An infection may remain localized, sub-clinical, and temporary if the body's defensive mechanisms are effective or it could persist, spread, and become an acute, sub-acute, or chronic clinical infection or disease state.Item The determination of excitation functions for natRb+p up to 100 MeV with an emphasis on the production of 82Sr(UWC, 2000) Buthelezi, E.Z; Green, A; Nortier, F.MThe radioisotope strontiuur-S2 (T172= 25.55 days) decays to short-lived t"Rb 1T,, = 76 seconds) by l00o/o electron caphue @C). The short-lived t'*Rb decays by g5.SYopositron-ernissiou p* and 4.5%oEC to stable'2Kr. Rubidiurn-82g is used in Positron Enr.ission Tonrography (PET) applications, including studies of myocardial infarction, kidney tissue, and brain blood florv. In the present study the excitation fi,rnctions for the production of 82Sr, other radio-strontiums, and radio-contaminants produced in the proton bonrbardrnenl of ""'Rb rvere measured by ureans of the stacked-foil teclutiqueItem Spin and parity assignment in 152gd investigating octupole structures(University of the Western Cape, 2008) Bvumbi, S; Roux, DSpin and parity assignment in 152gd investigating octupole structures The high-spin states of the nucleus ls2cd have been populated via the ls2sm (cr, 4n) 152Gd fusion-evaporation reaction at abeam energy of 45MeV. The emitted y rays were observed with the AFRODITE spectrometer afiay. The previously known decay scheme of 1s2cd was extended. Directional Correlation from Oriented states of nuclei (DCO) and linear polarization measurements were performed in order to assign spins and parities in the 1s2cd decay scheme. An alternative interpretation of the first excited Kn: 0* band, previously considered to be a B-vibration is discussed.Item The influence of maternal nicotine exposure on neonatal rat trachea development and alveoli status(University of the Western Cape, 2000) Vergotine, Zelda; Maritz, G. S.The aim of this study was to determine the effect of maternal nicotine exposure on the status of the rat tracheal development as well as the rat lung of 7 to 42 day old offspring. Wistar dams were injected subcutaneously, using a dosage of 1 mg nicotine/kg body mass/day, and treatment commenced 7 days after conception. The morphological and morphometric data were used to evaluate any damage or change in the structure of the trachea and lungs due to maternal nicotine exposure. Three techniques were used namely, light microscopy, scanning electron microscopy and transmission electron microscopy. Both morphological status and morphometric data were obtained from the sample animals.Item Pre-equilibrium cross section calculations with a realistic effective nucleon-nucleon interaction(University of the Western Cape, 1995) Karriem, Zainuddin; Lindsay, RobertThe principle aim of this work is to improve the way in which pre-equilibrium double differential cross-sections are currently calculated [1, 2, 3, 4]. The theoretical basis for these calculations is the Multistep Direct Theory by Feshbach, Kerman and Koonin (FKK theory) [5]. This theory leads to the simplification of multistep reactions as a folding of single-step reactions through the implementation of the Distorted Wave Born Approximation (DWBA). The pre-equilibrium cross-sections are subsequently calculated with a multistep direct code.