School of Physics (ETDs)

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Optimization of gallium oxide (ga2o3) nanomaterials for gas sensing applications
(University of the Witwatersrand, Johannesburg, 2024) Gatsi, Nyepudzai Charsline
Gas sensors are needed for monitoring different gases in indoor and outdoor environments, food quality assessment, and health diagnostics. Among materials studied for these applications, semiconducting metal oxides (SMOs) have generated a lot of interest due to their excellent sensitivity, simple circuit, and low cost. One-dimensional (1𝐷) 𝐺𝑎2𝑂3 nanomaterials are part of the promising candidates explored for the sensing of different gases due to their excellent electrical conductivity, high catalytic behavior, and chemical and thermal stability. This study reports the optimization of crystal structure, morphology, and surface chemistry of 𝐺𝑎2𝑂3 nanostructures for use in the detection of various gases. A set of unmodified and noble metal modified 1𝐷 𝐺𝑎2𝑂3 nanomaterials were synthesized by microwave-assisted hydrothermal method followed by heat-treatment at different temperatures and their gas sensing performances were systematically studied. The samples were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), Raman analysis, scanning electron microscope (SEM), transmission electron microscope (TEM), Brunauer-Emmett-Teller (BET), photoluminescence (PL), diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS) methods. The effects of heat-treatment temperatures on phase transformations and gas sensing performances of various 𝐺𝑎2𝑂3 polymorphs were investigated. The 𝛼 − 𝐺𝑎2𝑂3, 𝛽 − 𝐺𝑎2𝑂3 and 𝛼/𝛽 − 𝐺𝑎2𝑂3 crystal structures were synthesized and evaluated for gas sensing. The 𝛽 − 𝐺𝑎2𝑂3 sensing layers presented selective response coupled with fast response/recovery times towards carbon monoxide (𝐶𝑂) compared to the 𝛼 − 𝐺𝑎2𝑂3 and 𝛼/𝛽 − 𝐺𝑎2𝑂3 crystal structures. The observed variations in the gas sensing performances of these three crystal structures were attributed to controlled properties of different 𝐺𝑎2𝑂3 polymorphs. Furthermore, the 𝛽 − 𝐺𝑎2𝑂3 polymorph was prepared in the form of regular and hierarchical nanorod-based morphological features which demonstrated different gas sensing behaviors. The 𝛽 − 𝐺𝑎2𝑂3 regular nanorods showed better capabilities of detecting isopropanol than the nanobundle-like and nanodandelion-like features, and these differences were attributed to changes in textural, porosity, and compositional properties related to different morphologies. The effects of incorporating 𝐴𝑔 and 𝐴𝑢 noble metal nanocrystals on regular 𝛽 − 𝐺𝑎2𝑂3 nanorods surfaces on their gas sensing behaviour were also investigated. The results revealed that surface modification of 𝛽 − 𝐺𝑎2𝑂3 nanorods with 0.5 and 1.0 𝑚𝑜𝑙% 𝐴𝑔 and 𝐴𝑢 noble metals significantly lowered the sensor operating temperature compared to that of unmodified 𝛽 − 𝐺𝑎2𝑂3 nanorods towards the detection of ethylene. In addition, surface incorporation of 1.0 𝑚𝑜𝑙% 𝐴𝑔 dramatically increased the sensor sensitivity and selectivity and reduced the response/recovery times towards ethylene gas, and these positive changes were attributed to the electronic and chemical sensitization effects stimulated by the catalytic activity of 𝐴𝑔 nanocrystals incorporated on the surface of 𝛽 − 𝐺𝑎2𝑂3 nanorods. This study unambiguously optimized the crystal structure, morphology, and surface chemistry of 𝐺𝑎2𝑂3 nanostructures for the detection of carbon monoxide, ethylene and isopropanol gases. These sensors may potentially be used in real-time detection of carbon monoxide and isopropanol for indoor air quality monitoring to improve human health. In additional they have also demonstrated capabilities for the precise and economical detection of ethylene around plants and fruits, which could be beneficial to the horticultural and agricultural industries
Solar cell simulation using ab initio methods
(University of the Witwatersrand, Johannesburg, 2024) Zdravkovi´c, Milica; Quand, Alexander; Warmbier, Robert
Solar cells are a great source of renewable energy, but they are yet to reach their thermodynamic efficiency limits. Common commercial solar cells run at approximately 20% power conversion efficiency, and almost all efficiency loss comes from thermalisation. Ab initio simulations can reduce the need for physical experiments to quantify these losses while also providing insights into the quantum mechanical properties of materials. Note that density functional theory reformulates the expression for the ground state energy of a many particle system such that it is a functional of the electron density, thereby allowing the electronic energy to be solved for numerically. But the underlying mechanism behind thermalisation is the electron-phonon interaction. Using the theory of Green’s functions, the electron-phonon interaction self-energy and charge-carrier life times can be calculated. A method of approximating the charge-carrier lifetimes using the hydrostatic deformation potential interaction, which is only valid for longitudinal acoustic phonons, is presented. Deformation potentials of -10.125eV for Silicon and 18.663eV for Gallium Arsenide, commonly used solar cell materials, are calculated in good agreement with literature. Furthermore, the electron-phonon interaction life- times were calculated to be in the order of 2.0 × 10−15s for Si and 4.0 × 10−16s for GaAs, which could have indications that the optimal thickness of a GaAs absorption layer is much thinner than for Si. Thus the deformation potential method provides a satisfactory approximation for the electron-phonon quasiparticle lifetimes based on ab initio methods
The development of a burn-in test station for the ATLAS Tile Calorimeter Low Voltage Power Supplies for phase II upgrades
(University of the Witwatersrand, Johannesburg, 2022) Lepota, Thabo James; Mellado, Bruce
It is planned that the High Luminosity (HL) function of the Large Hadron Collider (LHC) will begin operation in 2027 with an integrated luminosity of 4000 fb−1.By using the HL-LHC scientists will be able to investigate new physics beyond the Standard Model (SM), examine electroweak symmetry in more detail, and examine the characteristics of the Higgs boson. The ATLAS Tile Calorimeter’s on and off detector electronics will be completely redesigned during phase II upgrade, run 3. Due to the high radiation levels, trigger rates, and high pile-up conditions associated with the HL-LHC era, it will be necessary to accommodate its acquisition system. The Institute of Collider Particle Physics is responsible for developing and manufacturing over a thousand transformer-coupled buck converters, known as Bricks, for the Low Voltage Power Supply (LVPS) system. The LVPS is critical to the TileCAL on detector electronics as it powers them by converting 200 V high voltage to 10 V. The Bricks are located within the inner barrel, they can only be accessed once a year. If an LVPS box fails, it can be down for up to a year, causing the Front-End electronics it supports to remain offline for the same amount of time. As a result, the Bricks’ reliability is of critical concern that must be addressed throughout their manufacturing process. In addition to the burn-in test station, the Bricks that pass the quality assurance tests are sent to the European Organization for Nuclear Research (CERN), to be installed in the ATLAS detector. In this manuscript, we show how we programmed the Peripheral Interface Controller (PIC) firmware, which is an integral part of the Brick Interface board functionality in the burn-in test station. We further give detail as to how the software framework (LabVIEW) used as a control program was modified and used to operate the burn-in test station during the burn-in process. The purpose of the test results discussed is to demonstrate the burn-in test station is functional according to the preliminary protocols prescribed for Bricks

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