Theses and Dissertations (Physics)

Permanent URI for this collectionhttp://hdl.handle.net/2263/32485

Browse

Now showing 1 - 20 of 204

Widening the field-of-view of Very Long Baseline Interferometric surveys through advanced calibration techniques
(University of Pretoria, 2025-05) Radcliffe, Jack F.; ll3w3llyncoetzer@gmail.com; Coetzer, Llewellyn Victor
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.
Neutral hydrogen lensing simulations in the hubble frontier fields
(Oxford University Press, 2024-07) Blecher, Tariq; Deane, Roger; Obreschkow, Danail; Heywood, Ian
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
(University of Pretoria, 2025-02-20) Kruger, T.P.J. (Tjaart) ; Tegegne, Newayemedhin Aberra; u20815957@tuks.co.za; Nchinda, Leonato Tambua
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.
(University of Pretoria, 2025-01-20) Thabethe, Thabsile T.; Njoroge, E.G. (Eric G.); u16166176@tuks.co.za; Mashabela, Tshegofatso Boys
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.
(University of Pretoria, 2024-12) Thabethe, Thabsile T.; Langa, Dolly Frans; Ntsoane, Tshepo Paul; tshepisokadi1@gmail.com; Kadi, Tshepiso Bridget
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
(University of Pretoria, 2025-02) Thorat, Kshitij; Bonnassieux, Etienne; u17261598@tuks.co.za; Mogamisi, Katlego
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
(University of Pretoria, 2024-11) Diale, M. (Mmantsae Moche); Nolwazi, Nombona; modisamaatseke@gmail.com; Ntobeng, Modisa Maatseke
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
(University of Pretoria, 2024-11) Duvenhage, Rocco; s.skosana@tuks.co.za; Skosana, Samuel
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
(University of Pretoria, 2024-12-08) McKean, John; u23805804@tuks.co.za; Lentz, Zane Deon
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
(University of Pretoria, 2015-10) Manyala, Ncholu I.; Dangbegnon, Julien K.; fatemehtaghizadeh86@gmail.com; Taghizadeh, Fatemeh
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
(University of Pretoria, 2019-09) Diale, M. (Mmantsae Moche); Kruger, T.P.J. (Tjaart); joramskaap@gmail.com; Cronje, Abraham Johannes
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
(University of Pretoria, 2020-02) Kruger, T.P.J. (Tjaart); Diale, M. (Mmantsae Moche); farooq.kyeyune@up.ac.za; Kyeyune, Farooq
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
(University of Pretoria, 2019-11) Diale, M. (Mmantsae Moche); Kruger, T.P.J. (Tjaart); sipho.congolo@gmail.com; Congolo, Sipho
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
(University of Pretoria, 2023-12) Meyer, W.E. (Walter Ernst); Auret, F.D. (Francois Danie); abrahamwillembarnard@gmail.com; Barnard, Abraham Willem
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
(University of Pretoria, 2024-03) Thorat, Kshitij; Dean, Roger; toivosamuel@gmail.com; Mabote, Toivo Samuel
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
(University of Pretoria, 2023-12-15) Malherbe, Johan B.; Thabethe, Thabsile T.; Odutemowo, Ope; tasabeeh.jafer@gmail.com; Osman Jafer, Tasabeeh Alabid
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.
Studying single plant light-harvesting complexes in near-native environments
(University of Pretoria, 2023-12-14) Krüger, Tjaart P.J.; Kirchhoff, Helmut; bertus.mooikloof@gmail.com; van Heerden, Bertus
Photosynthesis in plants begins with light absorption by light-harvesting complexes, the main one being light-harvesting complex II (LHCII). This protein is a key component of a critical photoprotective process called non-photochemical quenching (NPQ). Single-molecule spectroscopy (SMS) has revealed that isolated LHCII complexes have complex, individual behaviour, and this behaviour can be directy related to NPQ. The physiological applicability of these findings is, however, limited by the artificial environment typically used for SMS. Applying SMS in vivo is immensely challenging, however, in part because the complexes are crowded and in constant motion. New approaches are therefore needed which enable the study of LHCII (and similar proteins) in controllable environments that mimic the native one. This thesis develops two such approaches, namely real-time feedback-driven single-particle tracking (RT-FD-SPT) and proteoliposomes, and applies them to LHCII. Different RT-FD-SPT methods were investigated using theoretical modeling, illuminating fundamental aspects of performance and aiding in the selection of an appropriate method. New data analysis code was developed for fluorescence lifetime analysis and applied to measurements on LHCII. The first measurements of photon antibunching from LHCII are reported. An RT-FD-SPT setup with unique spectroscopic measurement capabilities was constructed and used, for the first time, on light-harvesting complexes. LHCII aggregation was investigated using fluorescence correlation spectroscopy (FCS) and RT-FD-SPT. Lastly, a proteoliposome protocol was optimised which allows protein-lipid interactions to be studied at the single-molecule level.
Application of the Jost-matrix theory to the lambda-nuclear and multi-lambda systems
(University of Pretoria, 2024-02-28) Rakitianski, Sergei; u04361482@tuks.co.za; Gopane, Ishmael Mmusi
This study involves the investigation into the hypernuclear and multi-lambda systems using the Jost-function method, as well as the recovery of the two-body potential from the two and three-body systems using the approximate(guessed) wavefunction. The Schrodinger equation describing the quantum system of interest is solved by being replaced with a system of first-order differential equations, which enable one to obtain the Jost functions. These Jost functions are multi-valued energy functions which can be treated as single-valued functions defined on a complex energy surface called the Riemann surface. Direct calculations of the Jost functions, the S-matrix, for all complex momenta of physical interests including the spectral points corresponding to the bound states and resonance states can be obtained. In this work, this method was used to locate the spectral points for the wide range of Λ-nuclear systems within the two-body ΛA-model. The S-matrix residues as well as the corresponding Nuclear-Vertex and Asymptotic-Normalization constants (NVC’s and ANC’s) for the bound states are also found. For scattering parameters the Jost functions were factorized in such a way that they contain certain combination of the channel momenta times an analytic single-valued function of the energy E. The remaining energy-dependent factors were now defined on single energy plane which does not have any branching points anymore. For these energydependent functions, a system of first-order differential equations is obtained. Then, using the fact that the functions are analytic, they were expanded in the power series to obtain a system of differential equations that determine the expansion coefficients. When the expansion coefficients are obtained for the expansion around the energy E0 = 0,the coefficients are then used to calculate the effective range parameters. For the same hypernuclear systems, the scattering lengths, effective radii, and the other effective-range parameters (up to the order ∼ k8) for the angular momentum ℓ = 0, 1, 2 are calculated. Possible bound and resonant states of the multi-lambda systems ΛΛ(0+), ΛΛΛ(12 −) and ΛΛΛΛ(0+, 1+, 2+) are sought as zeros of the corresponding Jost functions calculated within the framework of the hyperspherical approach with local two-body S-wave potentials describing the ΛΛ interactions. Bound ΛΛ(0+), ΛΛΛ(1 2 −) and states only appears if the two-body potentials are multiplied by a minimum factor of ∼ 1.461 and 3.449. For ΛΛΛΛ(0+, 1+, 2+) systems the bound states appear when the two-body potentials aremultiplied by the factors ∼ 3.018, 4.360 and 3.419. A method for deducing the two-body potential from a given two- or three-body wave function is suggested. This method makes it possible to numerically obtain an unknown potential acting between the particles A and B when we know the potentials of their interaction with a third particle C and know the characteristics of the three-body bound state (ABC). The systems (nnp) and (ΛΛα) were used to show that a three-body wave functions can be constructed using the knowledge of the binding energies and sizes of these systems to deduce reasonable and realistic nn and ΛΛ potentials.
Effects of irradiation-induced defects in germanium
(University of Pretoria, 2023) Diale, Mmantsae; Auret, F.D. (Francois Danie); U20791152@tuks.co.za; Thaba, Cambel Rashia
In this study, we investigated the electrical characteristics of defects in germanium produced by alpha particle radiation using conventional deep level transient spectroscopy (DLTS) and Laplace deep-level transient spectroscopy (L-DLTS). Resistive deposition was used to fabricate palladium (Pd) and silver-gold (Ag/Au) Schottky contacts. I-V and C-V measurements were used to determine the suitability of the device by calculating the ideality factor (n) and carrier concentration (Nd). The Pd/n-Ge Schottky diodes were of high quality with ideality factor n = 1.159 before irradiation. After irradiation, ideality factor increased to n = 1.383, showing that exposing the device to irradiation, Schottky diodes of the device degraded. The carrier concentration of the devices from the C-V graphs, where the plot representing the samples before irradiation, was observed to be steeper compared to the plot before irradiation, indicating a decrease in free carrier density from 1.52×1016 cm-3 for an unirradiated sample to a 6.37×1015 cm-3 irradiated sample. A DLTS spectrum of the unirradiated germanium did not show any electrically active defects in detectable concentrations. After irradiation, DLTS spectrum illustrated the presence of several defects: E07+04, E10, E16+17, E23+25, and E37 (E+Eʹ). Peaks E and E’ were separated at 185 K by L-DLTS using manual regularization parameters, allowing the inversion routine to take into consideration the possibility of two or more closely spaced peaks. Conventional DLTS spectrum were also recorded for different pulse widths, and the peak height was reduced with a shorter pulse width, indicating that partial trap recharge was hardly observed for defects with pulse widths of 100 ns and 1 µs. The peak height started intensifying from 10 µs to 1 ms, which is a longer pulse width until it reaches saturation, meaning that all traps were now filled. The activation energies for all observed defects were calculated from the Arrhenius plot. Defects E25, E23, E10, E0.07, and E0.04 were observed after irradiation, but they were not all fully characterized. AuAg/Ge samples measurements illustrated the presence of 6 peaks after irradiation, which revealed an additional peak with an activation energy of 0.21 eV, defect E21 when compared to Pd/Ge samples. The electrical properties of E and E’ defects in germanium (Ge) introduced by alpha particle radiation were studied using high-resolution Laplace deep-level transient spectroscopy (L-DLTS). From the Arrhenius plot, it was observed that the E-centre consists of two components with similar DLTS signatures, but they have different properties, given that they have different activation energies of Et 0.375 eV and 0.370 eV. Electric field dependence of the E defect was measured at different temperatures to distinguish between Poole-Frenkel and phonon-assisted tunnelling, it was observed that the emission of carriers was described by phonon-assisted tunnelling for all measurements at different temperatures. The defect's depth profile measurements for the E-centre showed that as we probe deeper into the bulk of the semiconductor, the concentration of the E-centre defect decreases. The DLTS amplitude of the combined E and E' defects increased as the filling pulse width increased from a few microseconds to a millisecond. However, the DLTS amplitude of E was observed to be 4 times bigger than the DLTS amplitude of E' and both defects are structurally different
Laplace deep-level transient spectroscopy studies of the divacancy in alpha-particle irradiated silicon
(University of Pretoria, 2024-02-15) Meyer, W.E. (Walter Ernst); Auret, F.D. (Francois Danie); u10539362@tuks.co.za; Weideman, H.J.S.
Defects in semiconductors are usually detrimental to the device operation. Particularly in space, the high levels of radiation induce defects that damage electronics in satellites and space craft. However, in some devices defects are crucial to device operation and they are purposely introduced into the semiconductor during manufacture. In both cases it is important to be able to characterize these defects in order to find ways to remove defects that are detrimental and to introduce or keep those defects that are useful. During this research, the deep-level transient spectroscopy (DLTS) technique was performed on Schottky diodes fabricated on n-type silicon, which were irradiated by alpha particles from an Am241 source. In contrast to gamma and electron irradiation, which practically induce only point defects, alpha particles produce some defect clusters as well, especially in the region just before coming to rest. In particular, the two charged states of the divacancy were investigated. These investigations included the determination of the DLTS signature (the ionization enthalpy and apparent capture cross-section) of the observed defects. The depth profile and introduction rate of the defects were also determined. This was then compared to previously done research on electron irradiated silicon to determine if any other unknown defects arose from alpha-particle irradiation. The conventional deep-level transient spectroscopy spectrum showed three discrete peaks at 90 K, 125 K and 225 K, when recorded at a rate window of 80 s-1. By comparison with literature, it was determined that the peak at 90 K was due to both the CiCs defect and the VOi defect, while the peak at 125 K was due to V2 (=/-) defect level and the peak at 225 K was due to the PV defect and the V2 (-/0) defect level. The annealing profile of both charge states of the peaks due to the divacancy showed annealing in the range 350 K to 400 K, which was not observed in electron-irradiated diodes. We suggest that this is due to defect clusters annealing out, releasing interstitials that combine with the divacancies thereby converting them to highly mobile vacancies. It was also observed that, in the region 550 K to 620 K, where the divacancy anneals, the two peaks annealed by different amounts. This is not the case in electron-irradiated material. We therefore suggest that the V2 (=/-) charge transition level is suppressed by cluster effects.

Browse

Recent Submissions