Theses and Dissertations (Physics)

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Optical and structural properties of hematite for photoelectrochemical water splitting
Maabong Kelebogile, D. (University of Pretoria, 2018-01)
The continuous energy and climate change issues has motivated the urgent need to explore and develop green and renewable energy technologies, worldwide. Photoelectrochemical (PEC) water splitting offers great potential to convert water into solar fuels using sunlight. Hematite is a prospective photoanode material for solar water oxidation half reaction because of its favorable optical energy band gap of 2.1 eV. However, its practical performance is greatly limited by low efficiencies, mainly due to rapid recombination of the photogenerated electron-hole pairs. To address the problem of high charge recombination, many research efforts have been adopted such as morphology control, doping and heterostructuring. Doping remains one of the effective strategies to suppress charge recombination through increasing conductivity and charge mobility. Among several investigated dopants, titanium substitution of Fe has been shown to effectively enhance the performance of hematite. In this study, we investigated Ti-doped hematite thin films prepared by a simple and cost effective chemical solution methods, varying the Ti concentration from 0.1 at% to 20 at%. A thin film photoanode produced by spin coating showed a significantly enhanced photoelectrochemical performance with increased photocurrent density and reduced current onset potentials compared to pristine hematite. We also determined the effect of electrochemical oxidation (anodization) on microstructural properties at the surface of pristine hematite photoanode under PEC environment conditions. Anodization is considered an operational step in the functionality of a photoelectrochemical cell and may have influence of functionality of the photoanode in a PEC cell. Hematite thin film were anodized at 500 and 700 mV versus Ag/AgCl in KOH under illumination, for various anodization times. XRD diffractometry revealed an increase in the average crystallite size upon anodization. Microscopic surface scanning analyses also revealed an increase in the average particle size at the surface upon anodization. The increased particle size, particularly at longer exposure times, could contribute to poor photoelectrochemical effect due to increased minority carrier diffusion distances. This dissertation is one of the first steps into an attempt to investigate the effect of anodization on the microstructural properties of hematite in a PEC cell, for solar energy conversion applications.
Widening the field-of-view of Very Long Baseline Interferometric surveys through advanced calibration techniques
Coetzer, Llewellyn Victor (University of Pretoria, 2025-05)
Very long baseline interferometry (VLBI) is a vital tool in astronomy that unlocks extremely high resolutions with high sensitivity. The fields of view (FoVs) of VLBI observations historically were limited by bandwidth and time smearing, as well as signal attenuation experienced by the array elements due to their primary beams. Technological upgrades and improved correlation and calibration techniques have largely eliminated smearing, leaving the primary beams as the last major FoV restriction. In this thesis, we derive primary beam models for a large subset of antennas in the European VLBI Network (EVN) and enhanced Multi-Element Radio Linked Interferometer Network (e-MERLIN) using an observation scheme similar to out-of-focus holography (OOF). These models are also applied to a wide-field observation of the Hubble Deep Field-North (HDFN) to test their efficacy in removing beam-related attenuation. Obtaining accurate primary beam models of the antennas in VLBI networks would allow for wide-field observations’ development to accelerate and unlock many new facets of science, including active galactic nuclei (AGN) surveys, AGN feedback studies, gravitational lens surveys, and interstellar medium density studies, to only mention a few. In this project, we also face the additional challenge of modelling heterogeneous arrays, meaning that different primary beams are present in the data.
Computational insights into charged dopant-vacancy defect complexes in graphane for nanotechnology applications
Mapingire, Hezekia (University of Pretoria, 2025-02)
In this contribution, we present a detailed analysis of the effects of the presence of substitutional nitrogen-vacancy complexes in the two-dimensional material- graphane. We critically examine the derived formation energies, transition energy levels and U-parameters. In order to do this, we commence by systematically characterizing substitutional nitrogen point defects of the form NC, NH and NCH in graphane. We also do a detailed investigation of vacancy point defects of the type VC, VH and VCH in this graphene derivative two-dimensional material. We comprehensively derive the formation energies of these point defects giving the material science research community invaluable information about the stability aspects of these point defects in graphane. This investigation extends to fundamental aspects of density of states, defect level diagrams and activation energies, leading to a deeper understanding of the stability landscape of the point defects as well as the host material at play. In the second part of this investigation, we thoroughly examine the intricate relationship that exist when we combine these point defects to form the nitrogen-vacancy complexes of the form NCVH, NCVCH, NCHVH and NCHVCH. We unravel the paramount information and the subtle influences that nitrogen-vacancy complexes have on graphane. We meticulously explore the fundamental effects of the presence of nitrogen-vacancy complexes on the structural and electronic properties of hydrogenated graphene. Our detailed analysis provides a pivotal groundwork on the potential applications of point defect modified graphane in nanotechnology. Information on the defect energy levels are scrutinized to unravel the electronic dynamics while the calculated defect induced band gaps offer valuable insights into graphane’s potential applications in band gap engineering as well as in quantum computing. Furthermore, this investigation sheds light on the intricate stability patterns of point defect modified graphane. Our findings contribute to the critical comprehension of the interplay that exist between fundamental defect parameters of formation energies, defect transition energy levels, U-parameters as well as binding energies. Our results are of critical importance in terms of paving the way for technological advancement in the use of two-dimensional materials for nano-technology applications
Investigating star-formation and supernovae in Messier-82
Shungube, Sibongumusa Wiseman (University of Pretoria, 2025-04)
Context: Messier 82 (M82) is a well-studied nearby starburst galaxy, home to numerous compact radio sources, including supernova remnants (SNRs) and HII regions. The high star formation rate (SFR) in M82 leads to frequent supernovae (SNe), making it an ideal laboratory for studying compact radio sources in a dense starburst environment. Aims: This thesis aims to catalog and analyze compact radio sources in the central region of M82 using new 1.5 GHz e-MERLIN observations. The study provides updated classifications for previously known sources and identifies new objects, contributing to our understanding of supernova (SN) activity and star formation (SF) in M82. Methods: The dataset was obtained from e-MERLIN L-band observations in 2016, processed using CASA for calibration and imaging. Compact sources were detected using PyBDSF. Sources were selected based on a signal-to-noise ratio (S/N) threshold of 6, ensuring reliable detections. Spectral indices were derived from multi-frequency subband imaging and combined with existing C-band (5 GHz) flux measurements to refine classifications. Morphological analysis was also performed to distinguish between SNRs and HII regions. Results: A total of 36 compact sources were identified, including 31 previously known objects and four newly detected sources. A key finding of this study is the reclassification of source 38.76+53.5, previously thought to be an HII region, as an SNR based on its steep spectral index (-1.054). This demonstrates the importance of multi-frequency spectral analysis in accurately identifying radio sources in starburst environments. Additionally, comparisons with previous studies from 2008 C-band observation confirmed the persistence of known sources while revealing discrepancies in classification. The spectral properties of 41.95+57.5 were further analyzed, contributing to the ongoing debate about its nature, while observations of SN 2008iz suggest it has now transitioned into a SNR. These findings refine existing classifications and contribute to a deeper understanding of the radio source population in M82. While this study does not establish a complete catalogue, it provides an updated dataset that enhances our knowledge of the physical processes shaping radio emission in starburst galaxies. Future multi-wavelength and long-term monitoring studies will be essential for further constraining the nature and evolution of these sources.
The application and analysis of single-molecule spectroscopy data in photosynthesis
Botha, Joshua Leon (University of Pretoria, 2024-12)
Most of the energy that sustains life on earth can be traced back to photosynthesis, and so its importance is difficult to overstate. Our understanding of this crucial process is still growing as we develop ever-advanced techniques to investigate the world of proteins and their interactions with light. Single-molecule spectroscopy (SMS) has emerged as a transformative technique for studying molecular systems at the nanometre scale, offering unparalleled insights into dynamic processes and heterogeneity at the individual molecule level. However, effective utilisation of SMS data often requires sophisticated analytical tools and stable experimental setups, which can pose significant challenges to researchers. This thesis addresses these challenges through the development of Full SMS, a versatile analysis software suite and a custom-designed sample cell holder, which together provide an integrated framework to advance SMS-based studies. Full SMS is a comprehensive graphical user interface (GUI)-driven software package for processing and analysing multiparameter SMS datasets. These datasets typically include fluorescence intensity and lifetime, as well as spectral properties of single dye molecules, quantum dots, or fluorescent active biomolecules. The software enables detailed analysis through tools for statistical evaluation of fluorescence intensity, clustering, and level identification; lifetime decay fitting; and second-order correlation function calculations. Visualisation features include the display of fluorescence intensity and spectral traces, as well as raster-scan images, with robust filtering options to tailor data processing. A custom HDF5-based file structure ensures efficient storage, while flexible export capabilities facilitate integration into broader research work flows. Written in Python, Full SMS is open source and accessible to users without programming expertise, leveraging a multiprocess architecture for enhanced computational performance. Full SMS is used to analyse three sample types as case studies to showcase its utility and ease-of-use. Complementing the software, this thesis also presents a custom-designed sample cell holder, which extends the capabilities of the SMS experimental setup designed and assembled in-house at the University of Pretoria. This hardware innovation enhances stability and versatility, allowing for more robust and reproducible single molecule measurements under a wider range of experimental conditions. Together, these advancements provide a powerful platform for SMS, bridging the gap between complex experimental data and resulting insights, and significantly contributing to the field of single-molecule biophysics.
Neutral hydrogen lensing simulations in the hubble frontier fields
Blecher, Tariq; Deane, Roger; Obreschkow, Danail; Heywood, Ian (Oxford University Press, 2024-07)
Cold gas evolution ties the formation of dark matter haloes to the star formation history of the universe. A primary component of cold gas, neutral atomic hydrogen (HI), can be traced by its 21-cm emission line. However, the faintness of this emission typically limits individual detections to low redshifts ( z 0 . 2). To address this limitation, we investigate the potential of targeting gravitationally lensed systems. Building on our prior galaxy–galaxy simulations, we have developed a ray-tracing code to simulate lensed HI images for known galaxies situated behind the massive hubble frontier field galaxy clusters. Our findings reveal the existence of high HI mass, high HI magnification systems in these cluster-lensing scenarios. Through simulations of hundreds of sources, we have identified compelling targets within the redshift range z ≈0 . 7 −1 . 5. The most promising candidate from our simulations is the Great Arc at z = 0.725 in Abell 370, which should be detectable by MeerKAT in approximately 50 h. Importantly, the derived HI mass is predicted to be relatively insensitive to systematic uncertainties in the lensing model, and should be constrained within a factor of ∼2 . 5 for a 95 per cent confidence interval.
Investigating the stability and photophysics of organic solar cell active layers based on multicomponent polymer materials
Nchinda, Leonato Tambua (University of Pretoria, 2025-02-20)
Organic solar cells (OSCs) are considered strong contenders for next-generation renewable energy solutions due to their cost-effectiveness and flexible design. Significant advancements in the synthesis of innovative active-layer materials and enhancements in device fabrication have brought their power conversion efficiency (PCE) up to approximately 20% in both single- and multi-junction configurations. While some studies suggest OSCs may achieve a lifespan of up to 20 years, their current stability remains a barrier to full commercial deployment. Random terpolymerization has emerged as a promising approach to enhance the photovoltaic performance and stability of polymer donors. The molecular engineering of these terpolymers has allowed for simple morphological control in binary devices over ternary blends. However, the stability and photophysics of terpolymers have seldom been investigated, despite the promise of terpolymer materials in addressing the morphological instability found in bulk heterojunctions (BHJ) OSCs. This thesis presents a systematic investigation into the stability and photophysical properties of a series of terpolymers, aimed at uncovering specific underlying molecular mechanisms. Stability tests on terpolymers were conducted under thermal stress, and the terpolymers demonstrated exceptional stability under elevated temperatures of 85 degrees Celsius. In the first part, we explored the thermal stability of three terpolymers (P1, P2, and P3) made from one donor (thienyl-substituted benzodithiophene, BDTT) and different ratios of two types of electron acceptors, namely fluorobenzotriazole (FTAZ) and thienothiophene-capped diketopyrrolopyrrole (TTDPP), blended with the fullerene acceptor PC71BM (regioregular [6,6]-phenyl-C71-butyric acid methyl ester). The study found that the terpolymers exhibited broad absorbance from 350 nm to 900 nm, and thermal degradation had minimal effect on the pristine films. Incorporation of the FTAZ acceptor in the terpolymerization approach served as a suitable strategy in enhancing the thermal stability of the active layers. However, the BHJ films showed significant morphological changes due to PC71BM aggregation. Prolonged annealing resulted in PC71BM aggregation and terpolymer decomposition, but without affecting their molecular structure. The results highlight that controlled annealing can regulate PC71BM diffusion, improving the nanostructure crucial for efficient OSCs. In the second part, we investigated the photophysics of the three DPP-based terpolymers with varying acceptor ratios (FTAZ and TTDPP), blended with PC71BM. Increasing the TTDPP ratio in P1 improved its molecular structure, leading to better intermolecular interactions, enhanced pi-conjugation, and a red shift in absorption by 13 nm into the near-infrared range. Transient absorption spectroscopy showed efficient charge carrier dynamics and intermolecular charge transfer in the P1 blend. However, thermal annealing reduced long-lived charge carriers across all blends due to aggregation of the fullerene acceptor, disrupting phase separation.
Microstructural and mechanical characterization of silicon carbide irradiated with 158 MeV xenon swift heavy ions.
Mashabela, Tshegofatso Boys (University of Pretoria, 2025-01-20)
The structural and mechanical properties of 3C-SiC substrates subjected to 158 MeV Xe²⁶⁺ swift heavy ion (SHI) irradiation at different fluences were investigated. To study the changes induced by irradiation 3C-SiC was irradiated with swift heavy ions (SHI) to fluences of 1×10¹⁰, 1×10¹¹, and 1×10¹³ ions/cm² at room temperature (RT). To achieve this, Stopping and ion range in matter (SRIM) simulations will be employed to predict the behavior of SiC under irradiation conditions, providing insight into defect formation and energy deposition profiles. Raman spectroscopy will be utilized to analyze structural changes at the atomic level, while Vickers’ hardness testing will evaluate changes in the material's hardness. Additionally, Atomic Force Microscopy (AFM) will be used to study surface structural modifications and to extract mechanical properties such as Young's modulus and compressibility. Results revealed defect-induced hardening at lower fluences and significant degradation of mechanical properties at the highest fluence, attributed to defect accumulation and clustering. The findings indicated that while 3C-SiC functions effectively as barrier material, it may degrade once it reaches its irradiation threshold. This enhances the understanding of SiC’s behavior under irradiation, crucial for its application in nuclear reactors and aerospace technologies.
Microstructural characterization of zirconium carbide implanted with europium ions and annealed in a hydrogen atmosphere.
Kadi, Tshepiso Bridget (University of Pretoria, 2024-12)
This study investigates the effect of europium (Eu) ion implantation and heat treatment on the microstructure of ZrC along with the diffusion behaviour of Eu ions in the ZrC substrate. ZrC samples were implanted with Eu ions at room temperature with a fluence of 1×10¹⁶ cm⁻² and an energy of 360 keV, then annealed from 600°C to 800°C. Structural changes, elemental composition, strain, phase identification, and diffusion were analyzed using Scanning Electron Microscopy (SEM), EDS, X-ray Diffraction (XRD), and Rutherford Backscattering Spectroscopy (RBS). XRD analysis indicated implantation-induced structural damage, as evidenced by increased FWHM, suggesting lattice strain and defects. RBS revealed a europium penetration depth of ~76 nm, while annealing at 700°C caused Eu ions to migrate closer to the surface, and at 800°C, they diffused outward, with SEM showing larger surface cracks. Oxidation began at 600°C, with ZrO₂ formation confirmed by XRD and EDS, peaking at 700°C, and partially reducing at 800°C in a hydrogen-rich atmosphere. XRD data also showed recrystallization during annealing, with enhanced peak intensities and increased crystallite sizes, highlighting the interplay between Eu diffusion, surface oxidation, and microstructural recovery in ZrC.
Tracing morphological indicators of radio galaxy interaction with ambient medium using MeerKAT data
Mogamisi, Katlego (University of Pretoria, 2025-02)
As radio galaxies interact with their gaseous environments, their morphologies are affected by the environment and the environment in turn is affected by the jet-mode feedback. These effects caused by the environment can be seen in the morphologies of the radio galaxies, which allows us to use radio galaxies as probes to estimate the properties of their environment. In effect, this allows us a view of the components of the Universe which are only accessible through such specialised observation: the non-thermal Universe. In this work, using sensitive MeerKAT observations of radio galaxies in various environments, we endeavour to estimate the properties of galaxy cluster environments, galaxy group environments and filamentary Warm-Hot intergalactic environments. Using the interferometric images from the MeerKAT Galaxy Cluster Legacy Radio Survey (MGCLS) we selected a subset of radio galaxies which are associated with various media, supplementing the data with other MeerKAT images in the case of Intra-Group Media, which are not captured in MGCLS images. In this dissertation, we present the analysis of the sample sources to estimate their physical and morphological properties and link them to possible environmental influences. For the first time, we estimate the WHIM filamentary pressure and density of the filaments towards the direction of Abell 22 cluster using three Bent-tailed galaxies and compare our results with those available in the literature. We also report the discovery of relic emission in the bridge between A3391 and A3395 and use other galaxies, including G4Jy 917, in different cluster environments to put constraints on Intra-Cluster Media. Finally, we look at the faint, relic cocoon emission of the giant X-shaped radio galaxy PKS 2014-55 to help us understand Intra-Group media better. This study paves the way towards targeted radio observations of radio galaxies in diffuse gas media, in particular WHIM filaments.
Enhancing the conductivity of bacterial cellulose/polyvinyl alcohol composite for the development of flexible transparent electrodes
Ntobeng, Modisa Maatseke (University of Pretoria, 2024-11)
This study addresses the key components required to enhance the power conversion efficiency of organic solar cells. The first part of the research focuses on incorporating silver nanoparticles into the PEDOT:PSS layer to enhance light absorption. The silver nanoparticles were synthesised using the chemical reduction method. Then incorporated into the PEDOT:PSS liquid to make a blend of PEDOT:PSS and silver nanoparticles. The blend was coated on top of a glass substrate using the spin coating method. Characterisation techniques such as XRD, TEM, SEM, Raman, and UV-vis were used. TEM analysis revealed that the silver nanoparticles synthesised were spherical and ranged in size from 10 to 70 nm. The UV-visible spectroscopy confirmed the presence of the silver nanoparticles, showing an absorption peak at 389 nm. Furthermore, UV-vis analysis was conducted to evaluate the absorption of both the pristine PEDOT:PSS and the PEDOT:PSS with silver nanoparticles. The findings showed an enhanced absorption in the PEDOT:PSS blend with silver nanoparticles, demonstrating that the incorporation of silver nanoparticles into the PEDOT:PSS improved its light absorption properties. The second part is directed towards fabricating a transparent, flexible, conductive substrate that will serve as an anode for the organic solar cell. Bacterial cellulose synthesised using kombucha tea through static cultivation was combined with polyvinyl alcohol to make flexible, lightweight and transparent substrates. The composite substrates were made conductive by adsorbing multi-walled carbon nanotubes onto the substrate using the adsorption method. Different concentrations of multi-walled carbon nanotubes were explored on the composite films. Characterisation techniques such as UV-vis spectroscopy, SEM, TEM, XRD, TGA and electrical conductivity measurements of the individual components and the final films were assessed. The substrate with a 0.05% concentration of multi-walled carbon nanotubes showed the highest conductivity. The TGA results showed that the addition of polyvinyl alcohol to bacterial cellulose resulted in composite substrates with lower degradation at temperatures 213- 467 ℃, as compared to pure bacterial cellulose which is due to the structural degradation of the composite substrates. The UV-vis transmittance spectra indicated that using a higher concentration of multi-walled carbon nanotube dispersion during fabrication resulted in electrically conductive transparent substrates with reduced transparency.
Extending quantum detailed balance through optimal transport
Skosana, Samuel (University of Pretoria, 2024-11)
We develop a general framework for studying classes of quantum dynamical systems that are close to and structurally similar to other systems satisfying specified properties, in particular, quantum detailed balance conditions. This is done in terms of optimal transport plans and Wasserstein distances between systems on possibly different observable algebras. Basic metric properties of Wasserstein distances are proven. As a possible application of our framework to non-equilibrium statistical mechanics, bounds on deviations from detailed balance are derived.
Very Large Array broad-band monitoring of the flux-ratio anomalous lens system B2045+265 as a probe for dark matter
Lentz, Zane Deon (University of Pretoria, 2024-12-08)
In this thesis, we investigate the flux-ratio anomaly of the cusp gravitational lens sys-tem CLASS B2045+265 with the aim of gaining insight into dark matter distributions within the galactic halo. CLASS 2045+265 has a radio-loud background galaxy that is being gravitationally lensed by a radio-loud foreground galaxy to form four lensed images. The flux-ratios of the four lensed images are known from previous observa-tions to be inconsistent with the predictions from a simple singular isothermal ellip-soidal model for the foreground mass distribution. Here, we investigate the possible causes of the flux-ratio anomaly within CLASS B2045+265, which include a popu-lation of low mass dark matter haloes, as predicted by various dark matter models, variability from the background active galactic nucleus (AGN), or a propagation ef-fect as the light passes through the foreground lensing galaxy. For this, we have used monitoring data taken with the Karl G. Jansky Very Large Array at frequencies between 12 and 18 GHz. Firstly, we introduce a pipeline for use with the Common Astronomy Software Applications (CASA) package, which allowed us to calibrate large quantities of radio interferometric data. We have also modified a python script to automate the process of self-calibration and model fitting using the Difference Mapping (DIFMAP) package. We use the forementioned pipelines to perform a spectral analysis of CLASS B2045+265 over a 63-day monitoring period between 2022 March and May. The spectral indices of the three brightest lensed images A, B and C between 12 and 18 GHz are found to be α A = 0.945 ± 0.003, α B = 0.954 ± 0.006 and αC = 0.962 ± 0.005, respectively, which are consistent at the 2σ-level. We find the flux densities of these components to be varying over time, but the change is not significant enough to contribute to the anomalous flux ratios. We find a 0.97% variation in the Rcusp parameter (0.5137 ± 0.0003) between day 0 and day 63 which quantifies the deviation from perfect symmetry in the ratios of a cusp lens system where a system with perfect symmetry has Rcusp = 0 . Based on our an analysis of the radio spectra, we do not observe any variations as a function of frequency, which allows us to rule out any frequency dependent effects, such as free-free ab-sorption. Therefore, we believe the flux-ratio anomaly present in CLASS B2045+265 is most likely caused by a perturbation to the mass model by means of a combination between complex mass structures and sub-haloes present within the system. How-ever, given the large mass-fraction in sub-haloes that we find is needed to explain the extreme Rcusp parameter for this system, it is most likely that these sub-haloes are along the line-of-sight towards the distant AGN, as opposed to being purely within the lensing galaxy. Further analysis with high angular resolution telescopes or next gen (ngVLA, SKA, etc.) will be needed to determine whether this is the cause of the flux-ratio anomaly in CLASS B2045+265.
Microwave-assisted hydrothermal synthesis and energy storage application of nickel-aluminum layered double hydroxide-graphene foam nanocomposites
Taghizadeh, Fatemeh (University of Pretoria, 2015-10)
The graphene foam (GF) was synthesized by chemical vapor deposition (CVD) and different mass of GF were added to the LDH. The morphological, structural and compositional properties of LDH and LDH/GF composites were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy and Brunauer - Emmett – Teller (BET). The results show the presence of interlaced sheets of LDH/GF composites. The electrochemical properties of the synthesized composite electrode system (with Ag/AgCl as reference electrode) displayed excellent electrochemical performance. All results clearly show and demonstrated excellent potential of graphene based composites electrode materials for energy storage applications.
Synthesis and characterization of transparent conducting oxides for Stark e ect applications in single molecule spectroscopy
Cronje, Abraham Johannes (University of Pretoria, 2019-09)
The desire to study the characteristics of transparent conducting oxides (TCOs) for Stark application in single molecule spectroscopy (SMS) stemmed from our interest in the dark states of light harvesting complex II (LHCII) of plants. Investigation of this mechanism requires a Stark cell. The high optical quality required by the SMS procedure demands a very speci c thickness microscope slide at the objective to sample interface. Since no commercial TCOs of this standard were available, we had to synthesize our own and spincoat our SMS setup's stock microscope slides. As a rst attempt, uorine doped tin oxide (FTO) thin lms were synthesized at 0 to 3 percent atomic doping levels. The optical quality of the lms were low, as they appeared almost burned (blackish). X-ray di raction (XRD) con rmed the lms to be FTO by the FTO powder di raction le (PDF 00-041-1445). The average optical band gap achieved was 3.860 eV, in good agreement to literature. A minimum resistivity of 0.37 :cm􀀀�1 was obtained for the 2% sample. Scanning electron microscopy revealed that the poor optical quality of the lms was due to agglomeration of tin, most likely due to a too high tin concentration in the solution. Due to availability and experience in the department with zinc oxide thin lms, the following study was on the synthesis of aluminium doped zinc oxide (AZO). Thin lms of atomic doping percentages ranging from 0 to 5 percent in steps of 1 percent were fabricated. A new deposition of layer procedure was also used, described in the next paragraph. AZO was successfully synthesized, con rmed by XRD correspondence to zinc oxide's PDF (00- 036-1451) and the average optical band gap of 3.2873 eV agreement to literature. SEM revealed polycrystalline morphology in all lms. All lms had high transparency in the visible, as showcased by ultraviolet to visible (UV-Vis) spectroscopy. A minimum resistivity of 8.61 :cm􀀀�1 was obtained for the 2% sample. From experience gained in the FTO study, we hypothesized that a di erent Deposition of thin lm layer technique could have a signi cant impact on the quality of TCO lms. Simply drying a layer before addition of a new layer would not solidify the layer in a signi cant way. Addition of new solution would then solubilize a good amount of dried particles of the previous layers. This would result in a new pseudo-solution of much higher concentration, resulting in aggregation and low reproducibility. Our results indicated that the Deposition of thin lm layer technique had little impact on the optical quality of the lms, however, the crystalline and electrical qualities were both signi cantly better for TCO application. This procedure was used for the AZO study.
Single molecule spectroscopy on photosynthetic light-harvesting complexes
Kyeyune, Farooq (University of Pretoria, 2020-02)
Single molecule spectroscopy (SMS) is a powerful approach to study subtle, fundamental properties of biological systems generally obscured by the ensemble average. SMS allows for a detailed understanding of the molecular mechanisms underlying the biological function of many systems. In this thesis, SMS was used to investigate the photophysical properties of photosynthetic light-harvesting complexes (LHCs) under different environments. The two LHCs studied are LHCII, the major lightharvesting complex of higher plants (specifically Spinacia oleracea), and LH2, one of the major light-harvesting complexes of purple bacteria (specifically Rhodopseudomonas acidophila). In the first part, the photodynamics of LHCII in two different oxygen-depleted environments, i.e., in the presence of enzymatic oxygen scavengers and under nitrogen gas purging, were investigated. In the presence of oxygen scavengers, we observed at least two distinct states, which are characterized as unquenched and quenched, where quenching refers to energy dissipation in the form of heat. Under the nitrogen gas atmosphere, the majority of LHCII complexes exhibited only an unquenched state, with a negligible probability of switching to the quenched stated. Moreover, we found that the rate at which LHCII switches between the unquenched and quenched states was two orders of magnitude lower compared to that in the presence of oxygen scavengers. We speculate that the quenched state in LHCII could be activated by molecular oxygen, which, in turn, might play a key role in regulating light harvesting in oxygenic photosynthesis. Surprisingly, LH2, a pigment-protein from an anoxygenic organism, was also found to be incredibly stable under nitrogen gas purging. Overall, these results will help to increase our understanding of the photophysical mechanisms underlying the regulation of light harvesting, with a view of developing robust bio-solar devices as well as improving biomass yields. In the second part, the effects of plasmonic coupling on the fluorescence dynamics of LHCII were explored. We demonstrated that the brightness (fluorescence intensity) of a single LHCII can be significantly enhanced when coupled to a gold nanorod (AuNR). The increase in brightness is due to the enhanced rate of excitation and increased decay rate of LHCII placed near the nanorod. The AuNRs utilized in this study were chemically synthesized, and the LHCII/AuNR hybrid system was constructed using a simple and economical spin-assisted layer-by-layer technique. A fluorescence brightness increment of up to 240-fold was obtained, accompanied by a two orders of magnitude decrease in the average (amplitude-weighted) fluorescence lifetime down to a few picoseconds. This large fluorescence enhancement is explained by the strong spectral overlap of the longitudinal localized surface plasmon resonance of the utilized AuNRs and the absorption or emission bands of LHCII. In principle, these results provide an effective strategy to study the fluorescence dynamics of weakly emitting photosynthetic LHCs, especially at the single-molecule level where the fluorescence signal is usually overwhelmed by the background noise.
The study of transparent hematite films using ultrafast and Raman spectroscopies
Congolo, Sipho (University of Pretoria, 2019-11)
Hematite (Fe2O3) is a promising photoanode material that is being studied immensely for its application in solar water splitting to produce hydrogen and oxygen as fuels. It has attractive properties such as a narrow bandgap that allows for absorption of visible light, it is earth-abundant and is an easily processable photocatalytic material. In this study, we report on hematite thin films prepared by spray pyrolysis on fluorine-doped tin oxide (FTO) coated glass substrates. The samples were prepared by spray pyrolysis and treated with tetraethoxysilane as well as post-annealed. We employed ultrafast transient absorption spectroscopy and high-resolution confocal Raman microscopy for analysis of the hematite thin films. For all the films, Raman spectroscopy confirmed the characteristic spectrum of the hematite. The high-resolution Raman mapping showed a uniform intensity over the analyzed areas which suggests a uniform coating of the hematite films on the FTO substrates. Ultrafast transient absorption spectroscopy was used to investigate the effect of three experimental parameters; the effect of the spray volume, tetraethoxysilicate treatment of the hematite and post-annealing at 500 ºC for 2 hours with 10 ºC/min ramping. All three parameters gave a positive result. Ultrafast transient absorption spectroscopy indicates that all three experimental parameters slowed down electron-hole recombination. Global analysis of the difference absorption data resolved the spectra and associated decay lifetimes of three distinct processes, operating on the ultrafast, tens of picoseconds and hundreds of picoseconds timescales. Thus, understanding these properties will aid in the engineering of this material to prolong recombination and, as a result, improve its solar to hydrogen conversion efficiency in photoelectrochemical cells.
The effect of palladium deposition on electrically active defects in irradiated silicon
Barnard, Abraham Willem (University of Pretoria, 2023-12)
DLTS was used to study the effect of resistive physical vapour deposition of Pd Schottky contacts on the defects observed in an n-type Si substrate that was irradiated before deposition (“pre-irradiated”) and compared to defects in a diode that was irradiated after deposition (“post-irradiated”). In the post-irradiated samples, the familiar radiation-induced defects were observed. However, in the pre-irradiated samples, 13 new defects were observed, with DLTS signatures differing from those of the defects in the post-irradiated diodes. Out of the 13 newly observed defects, four defects, with activation energy of 0.180, 0.220, 0.360 and 0.607 eV, had DLTS signatures corresponding to defects previously observed in Pt-containing Si, while no match was found for other defects. The effect (referred to as the Pd Effect) was carefully studied, and it was found that the effect was only observed with Pd, and not when other metals including Au, Ni, Al, Ag were used. Careful experiments ruled out annealing during evaporation of the contact as a possible cause. Different sources of Pd were used in un-used crucibles in an attempt to avoid contamination, but the effect was observed in all cases. It was found that this phenomenon was inhibited by the presence of a thin intermediate layer, irrespective of the layer being Pd or Au. We therefore conclude that the effect is only observed when Pd is deposited directly on the irradiated Si surface. We believe that these defects are produced by defect-enhanced diffusion of Pd. Overall, the study enhances our understanding of defect behaviour in silicon-based devices, particularly under irradiation and metal deposition conditions, and reveals the unique properties and effects of Pd.
Probing the Association of Galaxy Clusters with Bent-Tailed Radio Sources Using the MeerKAT Galaxy Cluster Legacy Survey
Mabote, Toivo Samuel (University of Pretoria, 2024-03)
A bent-tailed radio galaxy (BT) is an active radio-loud galaxy in which its double jet presents a bend in the same direction, possibly as a result of interaction with the intracluster medium (ICM). Our dataset was extracted from the MeerKAT Galaxy Cluster Legacy Survey (MGCLS). The MGCLS is a survey consisting of observations of 116 clusters at declinations ranging from -85° to ∼ 0°. Through visual inspection of the continuum images aided by an automated source finder, we have separated BT radio sources from other extended radio sources in the MGCLS images. As a result of the study, we present the largest catalogue of BTs to date using the MGCLS sample, from which we extracted 1292 BTs, of which 83% are wide-angle-tailed radio galaxies (WATs) and 17% are narrow-angle-tailed radio galaxies (NATs). As expected, most of the BT sources are estimated to fall below the traditional Fanaroff-Riley divide. We find that around 65% of BTs are associated with massive clusters (1014.5 to 1015 h−1 MJ) for a subsample of 37 galaxy clusters, within which we identified 401 BTs. Contrary to expectations, we do not observe a significant increase in the number of BTs in merging clusters; 45% of the clusters in MGCLS that we have confirmed as merging also contain 45% of the BTs. This may be due to MeerKAT presenting a more complete picture of the BT population. We observed a high number density of BTs in the cluster center region, with an exponential decrease as the distance from the cluster center increases. This pilot study provides the largest sample of BT sources and is well-suited for future studies, including machine-learning-based exploration, as well as highlighting the requirements of future studies using SKA precursors and pathfinders.
Migration and aggregation of ruthenium implanted in glassy carbon
Osman Jafer, Tasabeeh Alabid (University of Pretoria, 2023-12-15)
Glassy carbon is a continuous, isotropic and non-graphitizing carbon that combines the properties of glass and ceramic with those of graphite. It has excellent properties such as high tensile strength, high hardness, good thermal and electrical conductivity, and combined resistance to high temperatures, wear, and corrosion. Glassy carbon is also highly impermeable to gases and liquids. These outstanding properties of glassy carbon make it a good choice for nuclear applications. Glassy carbon has been proposed as a containment material for radioactive fission products. For glassy carbon to be considered a suitable candidate for fission products containment, it must be an effective diffusion barrier for fission products, such as ruthenium (Ru), and its microstructure should not change dramatically under ion bombardment and extreme heat conditions. In summary, this study thoroughly investigated the impact of implantation and annealing temperatures on the microstructure and migration behaviour of Ru implanted in glassy carbon, with a focus on assessing its suitability as a diffusion barrier for Ru fission products. Raman and XRD results revealed amorphization of glassy carbon and structural changes induced by ion bombardment and subsequent annealing, showcasing the transition from tensile to compressive stress. RBS and SIMS elucidated Ru migration, with notable aggregation and segregation at higher annealing temperatures. Remarkably, both low and high-temperature annealing did not lead to significant Ru loss, affirming glassy carbon's efficacy as a storage container for Ru. Surface analyses through SEM and AFM showed a reduction in roughness post-implantation, while annealing-induced variations in roughness were linked to Ru migration or aggregation, surface diffusion and cluster formation. This comprehensive investigation provides valuable insights into Ru migration in glassy carbon, laying the foundation for its potential application as an effective diffusion barrier for Ru fission products.

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