Doctoral Degrees (Physics)

Permanent URI for this collectionhttps://hdl.handle.net/10413/6603

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Metal plasmonic as a mechanism to improve the performance of thin film polymer solar cell : device fabrication and characterization.
(2024) Ike, Jude Nodebechukwunso.; Mola, Genene Tessema.
This thesis discusses the results of the investigations on the use of plasmon metal nanoparticles (NPs) to enhance the performance of polymer solar cells, which are promising alternative solar energy converters to silicon-based solar cells. Polymer solar cells offer a cost-effective, flexible solar panel using a solution processing method for the generation of power from solar sources. Several key factors are considered to achieve this goal, including optical absorption, nano-morphology, and charge carrier mobility. The thesis focuses on investigating the potential of various dopants, such as solvent additives, thermal annealing, and metal nanoparticles (NPs), to improve the performance of thin-film organic solar cells (TFOSCs) by enhancing the charge transport processes. This research employed conventional device architectures to study the effectiveness of the active and buffer layers on charge transport and stability. The results have already been published in several internationally referred journals. In this thesis, the synthesized metal NPs were employed as mechanisms to improve the performance of TFOSC incorporated with poly-3-hexylthiophene (P3HT) as the donor material and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the acceptor material. The popular metal NPs such as nickel, (Ni), zinc (Zn), silver (Ag), calcium (Ca), sulfur (S), and cobalt (Co) were used to synthesize various bimetallic composites. Bimetallic nanoparticles such as nickel-doped with zinc bimetallic (Ni/Zn), silver-doped with calcium (Ag/Ca), silver-doped with cobalt (Ag/Co), and silver-doped zinc sulfide (ZnS/Ag) were employed at different functional layers of solar cell structure. Hence, the study employed various spectrometers such as high-resolution transmission and scanning electron microscopy (HRTEM and HRSEM), X-ray diffraction, Ultraviolet-Visible (UV-Vis) spectroscopy to investigate the size, morphology, elemental mapping, and optical properties of synthesized metal NPs. HRTEM is indeed a powerful technique for characterizing nanoscale materials, and it can provide valuable insights for confirming the presence of core-shell structures in NPs. Compared to the pristine reference, the blend of metal NPs with active and buffer layers at different concentrations plays a crucial role in augmenting the optical and electrical properties in TFOSC devices. Such increment of optical and electrical properties in this thesis is due to improved short-circuit current (Jsc), fill factors (FFs), and charge carrier mobility, which are significant to enhance the power conversion efficiency (PCE) values in the polymer solar cells. These prominent improvements are due to the presence of localized surface plasmonic resonance (LSPR) effect of TFOSCs. Finally, this thesis provides a series of experimental works fabricated with several metal NPs in TFOSCs at different concentrations.
Application of machine learning in cosmology.
(2024) Nagarajappa, Chandan Ganjigere.; Ma, Yin-Zhe.
Abstract available in PDF.
The effect of metal sulfides on hole and electron transport buffer layers in organic photovoltaics: experimental and numerical device simulation investigations.
(2022) Adedeji, Michael Adepelumi.; Mola, Genene Tessema.
Organic solar cells (OSCs) are promising alternative renewable energy sources that often suffer from insufficient absorption of solar radiation, short exciton lifetimes and small diffusion length of their charge carriers. Several strategies are being investigated to overcome these challenges in a move towards the commercialization of this solar cell technology. Increasing the path-length of incident electromagnetic radiation within the photo-absorbing layer of the solar cell, may elongate the time that light spends within the solar cell, thereby increasing the light-matter interaction time and consequently the photo-absorption within the photo-active material of the solar cell. The process described may be accomplished if suitable plasmonic metal nano-structures are added into the solar cell matrix. This intervention may also enhance the collection of the photo-generated charge carriers. The effects of metal sulphide nanoparticles incorporation in organic solar cells were studied and presented in this thesis. The metal sulphide nanoparticles were characterized and introduced into the hole- and electron- transport layers of fullerene and non-fullerene electron acceptors based solar cells, to elicit improved photoabsorption via the localized surface plasmon effects and facilitate better charge collection at the electrodes. Devices were fabricated both in an ambient environment and in a controlled environment (Nitrogen filled glovebox). The metal sulphide nanoparticles were incorporated in the fabricated solar devices using both the conventional and inverted device architectures. The power conversion efficiencies of the devices improved significantly after the incorporation of these nanoparticles. Device numerical simulation studies were also performed to reproduce some of the experimental results with a view to further investigating the devices and discussing their charge transport characteristics. The simulation results show improved charge carrier characteristics from the metal-sulphide doped devices by way of improved conductivity and shifted Fermi level offsets which were aided by the presence of the metal-sulphides. Asides from successfully achieving improved device performances in the investigations and simulations carried out in this thesis, this thesis successfully demonstrated the incorporation of nano-composites in non-fullerene acceptors-based organic solar cells for the first time to the best of our knowledge.
Modelling of quantum phase transitions in Dirac materials.
(2022) Hussien, Musa Alnour Musa.; Ukpong, Aniekan Magnus.
In this thesis, first-principles computations of the electronic ground state are used to investigate the underlying nature of the quantum phase transitions in selected Dirac materials and their associated hybrid materials. Various methods are used to break the symmetry of the ground state electronic structure and to tune the emergent phases. These involve the application of an external electric, magnetic or electromagnetic fields, and the manipulation of the internal intrinsic fields of materials, such as the introduction of strong spin orbit coupling, electron (or hole) doping, including a strong short-ranged disorder potential. The findings show that applying a perpendicular electric or magnetic field with a staggered potential in the underlying lattice allows for a tunable electronic transition between trivial and nontrivial quantum states. Signatures of the near-field electrodynamics of carriers in nanoclusters reveal the appearance of a quantum fluid phase at the distinct energies where topological quantum phase transitions occur. Emergence of the field-induced carrier density wave phase shows that the collective excitation mode is a distinct potential for carrier transmission in spintronic, optoelectronic, and photonic technologies. Furthermore, two types of insulating Dirac material are used as the tunnel barrier region of the perpendicular tunnel junction architecture. The resulting heterostructure is an artificially assembled metal-insulator-metal multilayer, and this serves as a generic platform for characterizing spin transport properties in spintronic devices. The dependence of the emergent phenomenon of proximity induced magneto-electronic coupling on the tunnel barrier material is unraveled. By analyzing the effect of the changes in the electronic structure on the spin transmission properties, it is found that the metal-insulator-metal platform exhibits a quantum phase transition by responding sensitively to both the tunnel barrier material and the applied perpendicular electric field during in-service operation of a spintronic device. The results show that when the electric field approaches its critical amplitude, the spin density of states exhibits a discontinuous change from half-metallic to metallic transport character in the presence of monolayer hexagonal boron nitride as a tunnel barrier material, contrary to when the monolayer molybdenum disulphide is inserted in the tunnel barrier region. The role of the applied electric field in the phase transition is characterized in terms of a spin-flip transition and an induced interfacial charge transfer. It is also found that the abrupt discontinuity in the changes in the spin-flip energy with increase in applied electric field provides the necessary and sufficient evidence of a first-order quantum phase transition in the spin transport phase. These findings show that the material of the tunnel barrier layer creates a non-trivial function in defining the magnetoelectric couplings that occur dynamically during spin tunneling.
Design and fabrication of tissue-like phantoms for use in biomedical imaging.
(2022) Ntombela, Lindokuhle Charles.; Chetty, Naven.; Adeleye, Bamise.
The continuous need for tissue-like samples to understand biological systems and the development of new diagnostic and therapeutic applications has led to the adoption of tissue models using potential materials. This work presents a low-cost method for manufacturing PVAslime glue-based phantoms to replicate diseased and healthy biological tissues’ optical, mechanical, and structural properties. The deformable phantoms with complex geometries are vital to model tissues’ anatomic shapes and chemical composition. Absorption and scattering properties were set by adding black India ink and aluminium oxide (Al2O3) particles in varying quantities to obtain slime phantom tissues with optical properties of the brain, malignant brain tumour, lung carcinoma, and post-menopausal uterus. The phantom properties were characterized and validated using a He-Ne laser emitting at 532 nm and 630 nm wavelengths propagated through various thicknesses of the fabricated phantom. The incident and transmitted intensity were measured to determine the absorption coefficient (a) and scattering coefficient (s). Furthermore, the effective attenuation coefficient (eff ) and penetration depth () were deduced from the reduced scattering coefficient (0s) and the anisotropy factor (g) obtained through the scattering phase function and Wolfram Mathematica. The anisotropy factor demonstrated a forward scatter, typical of strongly scattering media as real tissues. Such geometrically and optically realistic phantoms would function as effective tools for developing techniques in diagnostic and therapeutic applications such as laser ablation and PDT cancer treatment.
A study on the atmospheric and environmental impacts of aerosol, cloud and precipitation interaction.
(2022) Yakubu, Abdulaziz Tunde.; Chetty, Naven.
Understanding the mechanisms and processes of aerosol-cloud-precipitation interactions (ACPI) is essential in the determination of the specific role of aerosols in modulating extreme weather events and climate change in the long run. Atmospheric aerosols are mainly of various types and are emitted from differing sources. Considering they commonly exist in the heterogeneous forms in most environments, they significantly influence the incoming solar energy and the general perturbation of the clouds depending on their constituents. Thus, a systemic identification and characterisation of these particles are essential for proper representation in climate models. To better understand the process of climate change, this research explores the climate diversity of South Africa to examine aerosol sources and types concerning the atmospheric aerosol suspension over the region and their role in clouds and precipitation formation. The study further provided answers to the cause of extreme precipitation events, including drought and occasional flooding experienced over the region. Also, an insightful explanation of the process of ACPI is provided in the context of climate change. Furthermore, the research found that the effective radiative forcing (RF) over South Africa as monitored in Cape Town and Pretoria is negative (i.e., cooling effect) and provided an analysis of the cause. Similarly, the validation of some satellite datasets from MISR (Multiangle Imaging Spectroradiometer) and MODIS (Moderate Resolution Imaging Spectroradiometer) instruments against AERONET (Aerosol Robotic Network) is conducted over the region. Although a significant level of agreement is observed for the two instruments, intense improvements are needed, especially regarding measurements over water surfaces. Finally, the study demonstrated the proficiency of effective rainfall prediction from satellite instrument cloud datasets using machine learning algorithms.
Synthesis and characterization of boron nanotubes and other related boron nanomaterials by dual pulsed laser ablation.
(2019) Adebisi, Mufutau Amobi.; Moodley, Mathew Kisten.
Developments in nanotechnology in the last 30 years have provided the increasing usage of nanostructured particles in many applications. Boron nanomaterials (BNMs) are a good example of such nanoparticles. The uniqueness of boron nanomaterials set it apart from other similar nanomaterials. It has a wide field of promising applications ranging from quantum computing to materials science and medicine. Quasi-one-dimensional nanomaterials like nanowires, nanotubes, bamboo-like – nanotubes, nanosheets, nanoribbons, and nanorods are of great interest in fundamental research as well as their potential uses in many technological applications, such as field emitters, electronics, pharmaceutical, and novel circuitry elements, sensor devices and after appropriate functionalization in biomedical due to their applications. In this dissertation, the laser furnace technique was utilized to study the synthesis of boron nanomaterials. In the study, we achieved the synthesis of single-walled boron nanotubes and among other nanostructured materials of boron such as bamboo like – nanotubes, nanowires, and nanorods. These kinds of nanomaterials were synthesized through the use of the dual pulsed laser ablation technique. Experiments were conducted in a tube furnace. The boron composite target was ablated by sequential 1064 and 532 nm laser pulses at a furnace temperature of 1000 °C or 1100 °C in argon/nitrogen gas using different pressure and flow rates. The investigation involved varying the parameters such as gas pressure, gas flow rate, and furnace temperature. We observed the effect of varying these parameters on the synthesis of boron nanomaterials. To verify these effects, the as – prepared boron nanomaterials were characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy (RS), ultraviolet-visible (UV- VIS), photoluminescence (PL) and vibration sample magnetometer (VSM). It was found that the variation of the argon/nitrogen pressure and flow rate influence the quantity, quality and type of the as-prepared boron nanomaterials, since these parameters affected the plasma dynamics. The low flow rate and low pressure reduced the cross section for a collision between the plasma constituents, in particular, between the boron atoms and the metal catalysts which affected the probability of nucleation and growth of boron nanomaterials.The temperature was found to be the critical process parameter in the nucleation and the growth of boron nanomaterials. It was found that as the synthesis temperature increased to 1100 °C, there is also an increase in the nucleation and the growth of boron nanomaterials. It was discovered that an argon/nitrogen pressure of 400 Torr, the flow rate of 200 sccm and temperature of 1100 °C produced more boron nanomaterials at a rate of 100 mg/h with the highest quality of single-walled boron nanotubes (SWBNTs) with diameters ranging from 0.4 to 2.0 nm and 2.0 microns in length. The density gradient ultracentrifugation was carried out on boron as – prepared nanomaterials which resulted in the separation into different diameters of single-walled boron nanotubes. It was discovered that SWBNTs with smallest diameters of about 1.43 nm were found at the lower density, while the SWBNTs with larger diameters of 2.05 nm were obtained at higher density. These diameters were obtained by HRTEM analyses and the length up to one micrometer was also observed. The lattice fringes of 0.34 nm were found by HRTEM imaging in bamboo like-nanotubes, nanowires and nanorods. The lattices spacing from the fringes is consistent with the recent theoretical calculations of bulk boron of 0.35 nm. SEM analyses revealed the tubular and spherical structures of SWBNTs synthesized. XRD results showed that α – boron and β – boron were the solid phases formed in the products. It was observed that the crystallinity and size of the materials increased with an increase in furnace temperature. Raman analyses showed certain peaks below 500 cm-1, which is attributed to tubular structures similar to the peaks observed in a single-walled carbon nanotube. Raman peaks attributed to α – boron cluster was also observed at 788, 878 and 965 cm-1, confirming that SWBNTs formed from most stable sheets of α – boron. The statistical analyses of width distributions of the synthesized SWBNTs revealed variations in their diameters obtained according to each sample layer. The strong exciton absorption peak SWBNTs around 279 nm was revealed by UV-VIS results, while the luminescence of SWBNTs was found around 332 nm with photoluminescence analyses. Ferromagnetic properties of SWBNTs materials at room temperature were determined through VSM measurements. Finally, a simple model of vapour liquid solid (VLS) mechanism process has been developed to describe the formation of the SWBNTs.
The development of a low-cost, handheld quantum key distribution device.
(2017) Pillay, Sharmini.; Petruccione, Francesco.; Mariola, Marco.
Quantum Key Distribution (QKD) is an emerging field of information security. To date, this technology has been implemented for large scale financial and voting purposes, but QKD is a versatile solution which can also be utilised to secure personal transactions. The development of low cost, portable QKD devices can further promote the use of quantum encryption in commercial security systems. Research has been done to design hand-held QKD devices for personal use with ATMs. These devices use a short-range free space channel to produce a secret key using the polarisation of single photons as qubits. Free space applications of QKD usually utilise polarisation encoding of single photons since the polarisation states do not deteriorate in the turbulent atmosphere. Recent research also shows the feasibility of using quantum coherent states with continuous variable QKD in free space. The proposed device uses the Coherent One Way (COW) protocol to exchange a secret key between the two authenticated parties. The COW protocol is a simple, practical protocol which uses the time of arrival of consecutive weak coherent pulses as the bit encoding. The security of this protocol lies in preserving the coherence between consecutive laser pulses. Should decoherence be observed in the monitoring line, the presence of an eavesdropper is inferred. An advantage of using the COW protocol is the small size and low cost of the setup. This is ideal for a hand-held device used for short-range QKD. The COW protocol is not traditionally used for a free space channel due to the fragility of coherence in a turbulent medium. Since this is a short-range device which will not encounter any turbulence, the coherence of the laser beam is not compromised. It is therefore suitable to use the COW protocol under these conditions. We present in this thesis, the design of the system, in particular, the conversion from a fibre channel to a free space channel. A low cost optical synchronisation system is presented for use in a laboratory environment and the system is characterised with respect to the efficiency of the source, synchronisation and detection components. The bit generation rate and quantum bit error rate of the system are measured and discussed. Synchronisation techniques for long range free space implementation of the COW protocol, using radio transmission, are presented with a simulation. The simulation is used to demonstrate the compensation for Doppler effects required for communication between a Low Earth Orbit satellite and a ground station.
Characterisation of quantum channels in plasmonic metamaterials and bulk optical systems.
(2018) Uriri, Solomon Akpore.; Tame, Mark Simon.
Quantum channels are key to our understanding of how quantum information can be processed and transmitted. In this respect, over the past decade light has become an important carrier of quantum information. More recently, metamaterials have opened up many new exciting ways of controlling and manipulating light in the quantum regime, and in particular, controlling the polarisation and orbital angular momentum of light. In this work, we undertake an indepth characterisation of quantum channels made from plasmonic metamaterials and bulk optical systems by probing them with quantum states of light. We rst experimentally demonstrate the active control of a plasmonic metamaterial operating in the quantum regime. Using an external laser, we control the temperature of the metamaterial and carry out quantum process tomography on single-photon polarization-encoded qubits sent through, characterizing the metamaterial as a variable quantum channel. We nd that the overall polarization response can be tuned by up to 33%. Second, we experimentally realise a more complicated type of quantum channel in the form of a non-Markovian process made from the sum of two Markovian processes, and a Markovian process from two non-Markovian processes in a comparable bulk optical system. We perform quantum process tomography, and obtain high process delities. We discuss how these more complex types of quantum channel may be implemented using metamaterials.
Mathematical and numerical analysis of the discrete fragmentation coagulation equation with growth, decay and sedimentation.
(2018) Ogunniyi, Jeremiah Ayodele.; Sivakumar, Venkataraman.
The importance and need for solar energy man and living organisms cannot be overemphasised. It is therefore necessary to estimate the amount of solar radiation incident on South African surfaces and how it can be harnessed for solar energy application. The amount of solar radiation received by a surface depends on solar elevation, weather patterns, geographical location, cloud cover, aerosols, time of the day as well as surface reflectivity. Some of these factors influencing the amount of solar radiation are studied in this thesis. We made use of measurements from the South African Universities Radiometric Network (SAURAN), Ozone Monitoring Instrument (OMI), Clouds and the Earth’s Radiant Energy System (CERES) and the Modern Era Retrospective Analysis for Research and Application (MERRA) over different South African surfaces by extension, Africa. Measurements from the SAURAN network were used to study the seasonal variation in global horizontal irradiance (GHI), direct normal irradiance (DNI) and diffuse horizontal irradiance (DHI) over South African cities and Namibian city from 2013 to 2017. The selected South African cites are Alexander Bay, Durban, Bloemfontein, Pretoria and Port Elizabeth while Windhoek was selected in Namibia. Over a longer period, we examined the trend in shortwave flux and total cloud fraction over the same surfaces using MERRA data. The results reveal summer maximum and winter minimum for GHI, DNI and DHI. We observed that measurements of GHI, DNI and DHI from Durban and Port Elizabeth were much higher than other locations. For shortwave flux over the surfaces, the result showed that the North-western region received the highest amount of solar radiation while the South-eastern region received the least. This decrease down east was linked with low cloud fraction in the Northwest while the cloud fraction in the Southeast was very high. However, the amount of solar energy received in the South-eastern part of South Africa is still higher than the amount received in the United States and several European countries who are maximizing solar energy potential. The result revealed that South Africa received good amount of solar radiation throughout the year and this must be harnessed more. We obtained a statistically significant trend in shortwave flux of 2.65 Wm-2 per decade over Alexander Bay between 2009 and 2017 while a significant trend of 2.34 Wm-2 per decade was obtained in Port Elizabeth between 2000 and 2009. We then investigated the seasonal variation in temperature, ultraviolet aerosol index (UVAI) and total column ozone over different geographic zones of South Africa using datasets from OMI from 2004 to 2016. Cape Town was selected for the southern region, Springbok in the west, Durban for east while Irene and Johannesburg were selected for the Northern region. The results indicated the influence of warm Agulhas current on Durban temperature and cold Benguela current on Cape Town temperature. The result of seasonal variation in UVAI showed a spring time maximum attributed to biomass burning. High UVAI in Durban was linked to significant emissions from local sugar cane burning while that of Cape Town was attributed to marine aerosols. Ozone seasonality shows the well-established spring timemaximum and autumn minimum for southern mid-latitudes. We then used second order Fourier decomposition to determine the increasing and decreasing trend in UVAI and total ozone. This study was then extended over Africa as we estimated the interannual and seasonal variation in shortwave flux, ozone and aerosol index over Africa. The results reveal that Eastern, Central and Southern Africa received more solar radiation compared to Northern and Western Africa. However, all regions received good amount of radiation which makes Africa a potential place for solar energy exploration. In the eastern part of Africa, high ozone was observed in Malindi which was attributed to its unique location while a systematic increase in total ozone was observed in Ethiopia. For western regions of Gambia and Senegal, ozone trends were similar and its precursors were suggested to be from natural sources with little anthropogenic contributions. High ozone in Mozambique was attributed to the atmosphere being rich in carbon dioxide, carbon monoxide, methane, organic and inorganic particles as well as biomass burning. Total ozone in Algeria was low compared to other North African regions. This was attributed to the injection of cold and dry air from higher latitudes to southern Spain and Algeria which creates vortex on the Algerian coast and the formation of cyclone. However, in Morocco, ozone was very high and it was linked to the presence of high-level volatile organic compounds from forests made up of Eucalyptus and Pine trees. The result of the Aerosol Index (AI) showed very high absorption properties in Namibia being one of the three places on earth with persistent low-level cloud and the only location with steady supply of tiny aerosol particles from inland fires. High aerosols index observed in Algeria linked to high desert fires resulting in pollutions while high aerosol index in Congo was attributed to high mineral dust concentration from the Sahara and high nitric acid concentrations from biomass burning in the Congo basin. We also assessed the effect of aerosols and clouds on surface energy budget over eight South African regions using MERRA and CERES datasets. Both datasets showed similar pattern in climatology and interannual variability of both shortwave and longwave radiation. We determined aerosol forcing on both shortwave and longwave flux and showed that it had more effects on shortwave compared to longwave which results in dimming. We obtained the Bowen ratio and it showed a strong negative correlation with aerosol effect anomaly in winter and spring which indicates that as aerosol effect increases, less radiation reaches the earth.
Mode control with a diode-pumped solid-state digital laser.
(2017) Bell, Lebohang Teboho.; Ngcobo, Sandile.; Moodley, Mathew Kisten.
Abstract available in PDF.
Molecular sensors for evaluating substandard anti-retroviral medication using surface-enhanced raman spectroscopy.
(2023) Thobakgale, Setumo Lebogang.; Mthunzi-Kufa, Patience Thenjiwe.; Ismail, Yaseera.; Ombinda-Lemboumba, Saturnin.
Africa has the highest number of people living with HIV and AIDS, with South Africa housing the largest Anti-retroviral treatment (ART) program in the world. In addition, the continent is troubled by the continuing growth of substandard ART medication which is imported from external continents. The World Health Organization also states that due to the limited information on this issue, adequate remedial measures cannot be put into place. As such, this study proposed the application of surface-enhanced Raman spectroscopy (SERS) as a drug screening method for ART. Sensing platforms were synthesized using a combination of metals, crosslinker organic molecules, deposition, and self-assembly methods. The platforms were used for tailored adsorption of three ART medications in their active pharmaceutical ingredient (API) form: Tenofovir (TDF), Lamivudine (LAM) and Dolutegravir (DLG) prior to evaluation with Raman spectroscopy. Molecular interactions, signal enhancement and statistical methods such as linear regression were carried out on the analytes and data from the SERS analysis showed significant differences in the sensing capabilities of the platforms based on the calibration sensitivity, analytical sensitivity, and limit of detection. The molecular composition and chemical functionality of the sensors allowed specific adsorption and preference to the complementary functional groups of the API samples which led to enhanced Raman signals on each platform. From the results obtained, it was concluded that the synthesis of tailored platforms for molecular sensing of ART medication was successful, providing potential application of these sensors in the quality control of anti-retroviral medication. Future work will entail routine molecular screening of ARVs to monitor changes in ART quality with respect to geographical location, shelf life and formulation methods.
Application of machine learning techniques to the description of open quantum systems.
(2023) Naicker, Kimara.; Petruccione, Francesco.; Sinayskiy, Ilya.
This work focuses on using classical machine learning (ML) models to study the quantum dynamics of excitation energy transfer (EET) within strongly coupled open quantum systems relevant to light harvesting complexes (LHCs). Direct evidence for long-lived quantum coherence has been found to play an important role in EET processes during the first step of photosynthesis in certain LHCs where excitation energy is transmitted from the antenna pigments to the reaction center in which photochemical reactions are initiated [1–3]. The numerically exact method used to simulate the dynamics in this work is the hierarchical equations of motion (HEOM) adapted by Ishizaki and Fleming to suit the quantum biological regime [4–6]. In the case of an open quantum system, such as a photosynthetic pigment-protein complex, evolving over time we can generate a set of time dependent observables that depict the coherent movement of electronic excitations through the system by solving a suitable set of quantum dynamic equations such as the HEOM. We have focused on solving two problems, the first being the inverse problem. That is, the objective is to determine whether a trained ML model can perform Hamiltonian tomography by using the time dependence of the observables as inputs. We demonstrate the capability of the convolutional neural network (CNN) to solve the inverse problem. That is, the trained CNN can accurately describe the system under study by predicting the parameters of the system Hamiltonian when given the aforementioned time dependent data. The models developed can predict Hamiltonian parameters such as excited state energies and inter-site couplings of a system up to 99.28% accuracy. The second use of the same data set of observables involves time-series analysis. Although various analytical solutions for the dynamics of open quantum systems such as the HEOM have been developed, these often require immense computational resources. We demonstrate that models such as SARIMA, CatBoost, Prophet, convolutional and recurrent neural networks can predict the long-time dynamics provided that the initial short-time dynamics is given. Our results suggest that SARIMA can serve as a computationally inexpensive yet accurate way to predict long-time dynamics.
The effects of traveling ionospheric disturbances on SuperDARN near range echoes.
(2022) Hiyadutuje, Alicreance.; Kosch, Michael Jurgen.; Stephenson, Judy Ann Elizabeth.
Traveling Ionospheric Disturbances (TIDs) and Near Range Echoes (NREs) are both natural phenomena observed by SuperDARN High Frequency (HF) radars. This study presents for the first time observations of NREs in the lower E-region whose amplitudes are moderately modulated by medium-scale TIDs propagating in the F-region that have been observed by the same radar at another time in the far ranges. Two events during geomagnetic storms in winter recorded by the SANAE radar and two events during quiet times in summer recorded by the Zhongshan radar, both radars in the southern hemisphere, are described. The Gradient Drift Instability (GDI) proved to be the likely mechanism. The GDI is driven by the velocity difference between neutrals and plasma in the E-region ionosphere, due to the global convection electric field, and can be modulated by the polarization electric field of a passing TID via the near-vertical equipotential magnetic field lines.
The effect of metal nano-composites on the performance of thin film organic solar cells.
(2021) Mbuyise, Xolani Goodboy.; Mola, Genene Tessema.
This thesis examines the role of plasmonic nano-particles in the fabrication of thin film organic solar cells (OSCs). Organic solar cells are made up of conducting polymers blend that are formed as ultrathin layers (a few tens of nanometers thick) on various types of substrates. These conducting polymers, unlike their inorganic counterparts, have high optical absorption coefficients, allowing for the fabrication of ultra-thin solar cells (100–200 nm) in thickness. Moreover, they offer several advantages over other solar cell technologies in terms; mechanical flexibility, cheap device processing using roll-to-roll printing methods and etc. Organic photovoltaics (OPV) is a sector that has been steadily increasing over the last past two decades, with a justifiable power conversion efficiency (PCE) of 17 % to date. However, organic photovoltaic cells whose photo-active material is sandwiched between two differing work functions, are exhibiting relatively low PCE due to poor charge carrier generation and charge transport processes. Several factors can be considered in improving the overall performance of organic solar cells. These include enhancing photon harvesting ability of the absorber layer, reducing energy losses through recombination processes and so on. In recent years, significant advancements in OSCs have been made through the use of metal nano-composites or nano-particles in the solar absorber and transport buffer layers. It is to be noted that the shape and size of the metal nano-composite play an important role to achieve the required impact in OSCs. This investigation emphasizes on the use of tri-metallic nano-composites to assist in improving optical absorption, free charge carrier generation and charge transport processes. The goal of the research was to improve the power conversion efficiency of thin-film organic solar cells by using a trimetal nano-composite in the active layer (P3HT:PCBM). Based on Ce:Co:Ca nano-composites (NCs), the best device enhancements of PCE value of 5.3 % were discovered. The PCE of Ag:Zn:Ni NCs increased by up to 84 % from an initial value of 1.8 %, while Ag:Fe:Ni NCs improved by up to 3.83 % from an initial value of 2.70 %. Metal NCs feature local surface plasmon resonance (LSPR), which improves the power conversion efficiency of solution produced thin film organic solar cells. Because of the interaction with illumination, LSPR creates strong electromagnetic fields in the region of the particles on the one hand, and scattering effects in solar cell devices. However, high concentration of nanoparticles is found to be counter productive in the performance of OSC.
Plasmon as a mechanism to improve performance of bulk-heterojunction organic solar cells.
(2022) Dlamini, Wiseman Mpilo.; Mola, Genene Tessema.
Narrow optical absorption band of fullerene based organic photovoltaics (OPVs) is one of the challenges to produce efficient polymer based solar panels that compete with inorganic counter part. There have been efforts to mitigate the challenges in the past but not enough to overcome all the problems. Plasmon light trapping using metal nano-particles (NPs) incorporated into the organic films is one of the mechanisms being investigated intensively in recent years. Excited plasmon meta nano-partices can dephase in number of ways that could assist in improving photon harvesting as well as charge transport processes in thin film organic solar cells without compromising the optimum thickness of the photoactive medium. The most investigated metal NPs are gold (Au) and silver (Ag) NPs due to their excellent plasmonic and light scattering properties. However, due to their scarcity and cost, several other metallic NPs have also been considered alternative options. Furthermore, mono metal NPs tend to possess high scattering coefficients but low absorption coefficients or vice versa. As a result, several nano-composites of NPs with differing scattering and absorption coefficients have been synthesized and studied. In this study, we have investigated the effect of inexpensive, solution processed and environmentally friendly metal NPs in organic solar cell based poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend solar absorber. Metal nanocomposites using metals such as zinc (Zn), copper (Cu), Ag, manganese (Mn) and sulphur (S) were synthesized via simple low temperature colloidal chemistry. These nano-composites were bimetallic (copper coated with silver, Cu@Ag), zinc sulphide (ZnS), zinc oxide (ZnO) coated with Ag (ZnO:Ag) and zinc sulphide doped with manganese (ZnS:Mn). The size, chemical composition and morphology of the synthesized NPs were studied using high resolution scanning electron microscopy (HRSEM), high resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS/EDX). X-ray diffraction (XRD) was used to study their crystalline structure. The optical properties of the NPs in deionized water were studied using UV spectroscopy (UV-Vis). SEM analysis of the NPs showed various structures which mainly consisted of core shell type particles agglomerated together to form large clusters of nano-composites. EDX chemical composition analysis showed no significant impurities. This was consistent with the XRD which showed no diffraction peaks corresponding to impurity. HRTEM analysis confirmed the formation of a core-shell type structure for Cu@Ag with Cu core. Inter-planar (d-spacing) obtained via HRTEM compared very well with those obtained via XRD analysis. The NPs were incorporated either within the photoactive layer or the hole transport layer (HTL) of the solar structure. Significant enhancements on the optical and electrical properties of the OPV devices with NPs were observed when compared to pristine devices. Different NP concentrations were investigated ranging between (1 - 10 ) wt% relative to the absorber blend molecules. In some cases, the effect of solvent additives such as dimethyl sulfoxide (DMSO) at 5 wt% was used together with NPs in the HTL to boost the charge transport processes. The enhanced optical absorption, and electrical properties observed as increased current-densities (J), fill-factors and charge carrier mobilities resulted to improved power conversion efficiencies (PCEs) with increases exceeding 100 % when compared to pristine devices. The open-circuit voltages for all devices remained relatively unchanged. The enhancements in the optical and electrical properties which resulted to improved PCEs were attributed to strong light trapping through scattering by NPs and local surface plasmon resonance (LSPR) excitation on the metal-semiconductor interface. Light scattering at different angles into or within the photo-active layer increases its effective optical path length and hence more photons are absorbed. The thesis presents a series of experimental investigations on recently fabricated thin film organic solar cells with/without metal NPs at various concentration.
High-resolution geophysical imaging and characterization of severe landslides vicinities at South-Eastern Nigeria for uni-vario-seasonal degradation monitoring and civil engineering remediation.
(2021) Chikwelu, Edward Emenike.; Chetty, Naven.
This research work used high-resolution geophysical methods to explore the near surface of severe gully-erosion sites. The erosion that mesmerized the environment as landslides is the major landscaping challenge facing Anambra Basin in the South Eastern Nigeria. The geophysical techniques used include electrical resistivity tomography (ERT), induced polarization (IP), vertical electrical sounding (VES), and geotechnical analysis to study the subsurface conditions of severe gullies. Principally Wenner, Schlumberger, and dipole-dipole are the geophysical tomography technique used during the survey, depending on the peculiarity of each selected site. Geotechnical data analysis was used to confirm the results of vertical electrical sounding at a specific location, with the aid of resistivity formations. The geodynamics of the sites as related to rocks’ susceptibility to failure, and the mechanisms of slope failure was investigated, and foundation depths of the immediate surroundings of the Nanka gully were studied using geotechnical data. The surrounding and the subsurface of the eroded portions were monitored through imaging and analysis across the basin to measure hydrological contribution to the gully risks and other prevailing factors. The geoelectrical data was acquired with the ABEM Terrameter SAS 4000 and the ABEM LUND ES464 electrode selection system (using resistivity method) and processed with the RES2DINV software to produce 2D subsurface images. The VES resistivity curve matching was developed by a partial curve matching technique; and interpreted by Minitab 18 software to produce subsurface contour maps. In addition, the subsurface contour maps were qualitatively analysed with mapped surface geology and information on current geological typical rocks. In the geotechnical analysis, Spangler and Handy sampling techniques were used to collect eighteen (18) samples from dug gully walls, and the laboratory tests were to ascertain the soil properties index. The results of the models indicate that the study area is mainly clayey and sandstone formations that exhibited low resistivity values corresponding to the shale layers and groundwater zones. Many features that may lead to slope failure are present in the study areas, such as fractures, boulders, weak zone, and saturated zone. The results also showed that the soils in the study areas are friable hence easily washed off whenever there is storm water runoff from the surface, making the landslide active over the years despite every protective precaution put in place. In conclusion, this research work has identified lithologies, structural deformations, and distinguishing clayey zones from water-saturated zones, proving that the geophysical technique is the most successful tool in the landslide investigation.
Computational study of the structure and optical properties of perovskite solar cells materials.
(2021) Kheralla, Adam Abdallah.; Chetty, Naven.
The effects of nano-composites in bulk heterojunction thin-film organic solar cells.
(2020) Hamed, Mohammed Saeid Gebreel.; Mola, Genene Tessema.
An investigation into the radioprotective potential of Costus afer and Drymaria cordata extracts on whole-body irradiated mice.
(2021) Akomolafe, Idowu Richard.; Chetty, Naven.
The need for effective and non-toxic radioprotectors has shifted researchers' attention to plants and natural products as an alternative to synthetic compounds. This study investigated the radioprotective potential of Costus afer (CAE) and Drymaria cordata (DC) extracts on mice's survival, haematological and histopathological parameters following X-ray irradiation. One hundred and fourteen (54 male & 60 female) mice with total body masses between 38-45g and aged between 10-12 weeks old were used for this study. The mice were divided into twelve groups containing six and ten mice, respectively, for experiments CAE and DC. Animals were further sub-divided into irradiated and un-irradiated groups. The animals in both experiments received 250mg/kg extract of CAE and DC by oral gavage for six days and thirteen days, respectively, in addition to feeding and water ad libitum. Exposure of mice to radiation was done at the Radiotherapy and Oncology Department, Grey's Hospital using a linear accelerator. Blood samples were collected at different time intervals for the haematology test. Harvesting of kidney and liver for histopathology examination also occurred. Post-irradiation monitoring then continued for 30 days. Data were analysed by a one-way ANOVA test, followed by Tukey's multiple comparison test. Our findings revealed that the mice irradiated with 3Gy, 4Gy, 6Gy and 8Gy doses of X-ray radiation experienced a significant reduction in their White Blood Cell, Packed Cell Volume, Haemoglobin, Neutrophils, Lymphocytes, Eosinophils, and Platelet counts when compared with the control group in both experiments. In both experiments, CAE and DC extract offered protection against the radiation-induced haematological alterations by elevating all the blood parameters, except red blood cells and monocyte in the CAE treatment groups. In addition, the pre-treatment of mice with DC delayed the onset of mortality, thereby increasing the mice's survival rate. Histopathological changes in the CAE treatment groups' kidney and liver sections revealed no visible lesion in the pre-treated mice. Hepatocytes seem to be within normal histological limits. Although it is evident that the CAE and DC extracts protect against radiation-induced haematological damage and increases survival rate, no significant improvement in the histopathological parameters was recorded. Thus, further research is needed to prove the CAE and DC radioprotective potential on histopathological variables.

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