130 research outputs found
Electrochemical detection of 4-chlorophenol on phthalocyanine-metal organic framework sensors
No full textDissertation (MSc (Chemistry))--University of Pretoria, 2022.Metal organic frameworks (MOFs) have been used as electrochemical sensors owing to their high surface area and porosity. Unfortunately, the low conductivity and redox activity of MOFs limit their application in electrochemical sensing. To address this problem redox-active materials can be incorporated in MOFs to produce MOF composites. In this work we report on the synthesis and characterization of Zeolitic imidazolate framework-8 (ZIF-8), Zirconium based MOF (UiO-66) and their corresponding phthalocyanine (Pc) composites. Solvothermal synthesis was used to obtain ZIF-8, UiO-66, and their composites in powder form. The morphology and elemental composition were attained using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis, respectively. The composites were composed of agglomerated rods surrounded by irregular polyhedral structures. The presence of MOF and Pc elements confirmed that the Pcs were within the MOFs. The crystallographic properties of the composites were probed using powder X-ray diffraction (PXRD) which showed that the composites were more crystalline than the pristine MOFs. The increase in order of the structure of the composites confirmed incorporation of the Pcs onto the MOFs. The surface area and pore size distribution of the composites were determined using Brunauer-Emmett-Teller (BET) analysis. The pore volume ranged from 0.10 to 0.37 cc/g. The decrease in surface area and pore volume of the composites indicated that the Pcs were adsorbed onto the MOF pores. The optical properties of the composites were obtained using ultraviolet-visible (UV-vis) spectroscopy. The composites exhibited the characteristic B and Q bands of phthalocyanines and showed metal-to-metal charge transfer between MOFs and Pcs confirming that the phthalocyanines were incorporated within the MOF structure. ZIF-8, UiO-66 and their corresponding phthalocyanine composites were immobilized on a glassy carbon electrode (GCE) using the drop drying method. The modified electrodes were used for the detection of 4-chlorophenol, and the electrochemical responses were recorded using cyclic voltammetry. The electrocatalytic behaviour of the electrodes modified with MOF composites showed improved oxidation compared to the ZIF-8 or UiO-66 modified electrodes. The FePc@ZIF-8 modified electrode showed the best catalytic behaviour with an anodic peak potential at 0.54 V while FePc@UiO-66 behaved best with respect to enhancing catalytic currents. The limit of detection ranged from 0.05 mM to 2.8 mM for all redox active composites. This work shows that phthalocyanine-MOF composites are effective electro-catalysts and may be developed for the detection of other environmental pollutants.National Research Foundation (NRF) of South Africa.ChemistryMSc (Chemistry)Unrestricte
Computational studies of doped tin disulphide monolayer for photoelectrochemical water splitting
No full textDissertation (MSc (Physics))--University of Pretoria, 2023.In this ab initio study, density functional theory, including a Hubbard U correction term (DFT+U), calculations were performed to investigate the photoelectrochemical (PEC) water splitting possibility of SnS2 monolayer. Initially, the thermodynamic stability and photocatalytic (PC) properties of pristine SnS2 were studied and it was found that it suffers poor electrical conductivity and the bottom of conduction band minimum (CBM) is below the reduction potential of H^(+/) H_2 level, limiting the PC performance. To improve performance for PEC water splitting, various doping strategies were performed on a large 5×5 SnS2 supercell. These doping strategies are: C adsorbed onto an interstitial position, C adsorbed on a S atom, C adsorbed on a Sn atom, C substituting a S atom and C substituting a Sn atom.Department of Physics (University of Pretoria)National Research Foundation (NRF)PhysicsMSc (Physics)UnrestrictedFaculty of Natural and Agricultural Science
Two-dimensional semiconductor transition metal dichalcogenide nanosheets as charge carriers in metallophthalocyanine based solar cells
No full textDissertation (MSc (Chemistry))--University of Pretoria, 2020.Two-dimensional (2D) materials have received widespread attention as prime candidates for scientific research in the fields of optoelectronics and photonic applications. This thesis focuses on the use of transition metal dichalcogendes (TMDs) for solar cell application.
TMDs are compounds with the general formula MX2, where M is transition metal and X is chalcogen and most of them are intrinsically n-type. These materials are arranged as layers stacked together by weak van der Waals forces. They have found application in solar cells due to their high carrier mobility, chemical stability and flexibility. Exfoliation of the bulk TMDs maintains these properties and further imparts more properties such as switching the bandgap from indirect to direct and this further increases the carrier mobility of the material.
In this work we explore the use of molybdenum sulfide (MoS2) and tungsten (WS2) TMDs as electron transporting layers for solar cell application. The TMDs were successfully synthesised using the chemical exfoliation method and they were further doped with gold (Au) in order to enhance charge carrier mobility. The doped and undoped TMDs were characterised using X-ray diffraction (XRD), Raman spectroscopy, energy-dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM).
These TMDs were used together with metallophthalocyanines (MPc’s) as photon absorbers. MPc’s are known to be thermally and chemically stable in addition to having a relatively high extinction coefficient in the visible range. In our study we show the photovoltaic properties of TMDs combined with a variety of MPc’s. The utilisation of these TMDs with MPc’s has led to the generation of cost-effective solar cells that can be exploited for future technological applications.NRFChemistryMSc (Chemistry)Unrestricte
Plasmonic effects of gold nanostructures in organic solar cells
No full textDissertation (MSc (Physics))--University of Pretoria, 2022.Gold (Au) nanospheres, nanorods and nanoprisms were synthesized using the seed-mediated growth method. The structural, optical, and morphological properties of Au nanostructures were explored. Transmission electron microscope (TEM) and scanning electron microscope (SEM) were used to identify the shape and size of Au nanostructures. The average diameters of Au nanospheres (AuNSs) and Au seeds were 6 and 4 nm, respectively. Au nanoprisms with average edge length of 68 nm and Au nanorods (AuNRs) with an average length of 70 and width of 40 nm (aspect ratio of 1.9) were obtained. In UV-Vis spectra, plasmon absorption peaks of these nanostructures were located in the range of 395 to 629 nm for Au seeds (with the smallest sizes) to AuNRs (with largest sizes). X-ray diffraction (XRD) confirmed the face-centered cubic crystalline structure of Au, while Raman showed strong vibrational modes. Additionally, the structural, optical, and morphological properties of Poly[N-9'-heptadecanyl-2,7-carbazole-alt- 5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]:[6,6]-Phenyl-C61-butyric acid methyl ester (PCDTBT: PC70BM) layers with AuNSs, AuNRs, and Au nanoprisms were characterized.UV-Vis spectra were obtained within the absorption range of 378 – 564 nm. PCDTBT: PC70BM with Au nanoprisms absorbed at high intensity in comparison to the other samples. SEM micrographs revealed compact morphology with pin-hole defects. XRD and Raman analysis showed no crystalline domains. The electron transport layer, titanium dioxide (TiO2) and hole transport layer, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) were spin-coated and characterized. PCDTBT: PC70BM based organic solar cells (OSCs) were fabricated with Au nanostructures were incorporated between the electron-hole transport layer and the active layer. Analysis of the electrical characteristics of the devices was done using current density voltage (J-V). Power conversion efficiency (PCE) for the pristine device was 2.04 %, while AuNSs with plasmonics achieved 1.19%. A month after fabrication, the PCE for pristine and plasmonic devices with AuNSs increased to 2.08 and 1.72%, respectively. When compared to the other plasmonic devices, the device with AuNSs performed the best.NRF postgraduate scholarshipPhysicsMSc (Physics)Unrestricte
Optimization of halide perovskite thin films by sequential physical vapour deposition for solar cell applications
No full textThesis (PhD (Physics))--University of Pretoria, 2020.In this thesis, we have developed a reproducible, safe, and scalable sequential thermal vapour deposition (STVD) method for the growth of quality 3D halide perovskite (HaP) thin films. High-quality methylammonium lead tri-bromide (MAPbBr3), methylammonium lead tri-iodide (MAPbI3), and methylammonium lead bromide-iodide (MAPb(I1-xBrx)3) thin films have been optimised using the STVD technique. The structural, optical, morphological, and electrical properties were tuned by varying the thicknesses of the organic (MAI, MABr) and inorganic (PbI2, PbBr2) precursor thin films and post-annealing times of the HaP. X-ray diffractograms confirmed the cubic MAPbBr3 structure with the Pm¯3 m space group, tetragonal MAPbI3 crystal structure with I4/mcm space group, and the tetragonal MAPbI3 structure being transformed to cubic MAPbBr3 system as MAPb(I1-xBrx)3 (x=0.89-0.95) forms. UV-Vis spectra revealed broad absorption bands with a redshift in absorption onset from 540 to 550 nm for MAPbBr3 and 750 to 780 nm for MAPbI3 as the thickness of respective organic precursors increased from 300 to 500 nm. The bandgap of MAPb(I1-xBrx)3 decreased from 2.21 to 2.14 eV as the thicknesses of MABr precursors increased from 300 to 500 nm. The crystallisation of the HaP started within the chamber, and prolonged post-annealing times exceeding 10 min caused the transformation of MAPbI3 to PbI2. Scanning Electron Micrographs show pin-hole-free and densely packed grains with an average size that increases as thicknesses increase. The charge carrier mobility increases while trap density decreases as the thickness of organic precursors increased. Besides, the thesis investigated the growth and stability of thin MAPbBr3 films at metal/MAPbBr3 interfaces. We studied the structure, morphology, and stability of the optimised MAPbBr3 perovskite on aluminium (Al), tin (Sn), silver (Ag), gold-zinc (Au-Zn) and gold (Au) electrodes, immediately and 60 days later. FE-SEM images show an average grain size that increased linearly with the work function from 294 nm for Al to 850 nm for Au. The MAPbBr3 grains remain glued to Sn, Ag, Au-Zn but delaminate quickly on Al. X-ray analysis of MAPbBr3 reveals variable crystallographic texturing for various metals and loss in intensity of prominent peaks at different rates over time. We found that the best thicknesses of 100 nm PbI2 and 500 nm MAI, and 100 nm PbBr2 and 500 nm MABr are needed for the preparation of quality MAPbI3 and MAPbBr3 thin films for solar cells, respectively. Quality thin MAPb(I0.11Br0.89)3 film is formed by inter-diffusion and halide exchange processes when optimised MAPbBr3 is grown on optimised MAPbI3 as a bottom layer. Al speeds up the degradation of MAPbBr3 at Al/MAPbBr3 while MAPbBr3 is relatively stable at Au-Zn/MAPbBr3 interfaces.University of Pretoria, the National Research Foundation/The World Academy of
Sciences (NRF-TWAS), and NRF grant no N0115/115463 of the SARChIPhysicsPhD (Physics)Restricte
Optimization of processing parameters in the preparation of hematite thin films for photoelectrochemical water splitting
No full textDissertation (MSc (Physics))--University of Pretoria, 2022.In this study, the influence of coating techniques and layering of hematite (α-Fe2O3) thin films were studied. Two colloidal-based coating techniques namely dip coating and thermal spray pyrolysis were used to synthesize nanostructured hematite thin films. All films were annealed at 500 °C for I hr to perform the calcination from ɣ- Fe2O3 (magnetite) to α-Fe2O3 (hematite).
X-ray diffraction confirmed the calcination to hematite, it furthermore confirmed the hexagonal corundum structure of hematite. Raman spectroscopy confirmed the polycrystallinity of hematite with seven optical vibrational modes (two Eg and five A1g) observed in the first Brillouin zone. Ultraviolet-visible spectroscopy showed good absorbance in the visible region with absorbance onset ranging from 596.75 – 608.75 nm. Scanning electron microscopy depicted various nanoparticles ranging from agglomerated nanodiscs and nanorods to nanospheres. This study showed that coating techniques could attribute to the optical and structural properties of hematite thin films for photoelectrochemical water splitting. Furthermore, a modified annealing approach was used to determine the effects of annealing time and multiple layer formation on the structural, optical and electronic properties of hematite thin films. Scanning electron microscopy revealed an increase in film thickness with increased annealing time. Furthermore, it depicted the formation of four single layers when annealing between each coating. UV-Vis indicated a decrease in bandgap with prolonged annealing. Additionally, UV-Vis revealed better absorption for coherent layers when compared to the single layers annealed four times. The photocurrent increased two-fold from 1.65 x 10-4 Acm-2 to 4.77 x 10-4 Acm-2 when annealing time was increased from 30 mins to 1 hr. Similarly, a significant improvement from 2.26 x 10-4 Acm-2 to 4.35 x 10-4 Acm-2 was found with a decrease of multiple layer formation and annealing frequency. From this work it was determined that annealing time and multiple layer formation by increased annealing frequency influences the optical, structural and electrical properties of nanostructured hematite thin films.NRF (National Research Foundation)PhysicsMSc (Physics)Unrestricte
Fabrication and characterization of Si-based MOS devices
No full textDissertation (MSc)--University of Pretoria, 2021.Silicon is a lead component in modern technologies due to its relatively low cost, and stability under various conditions. Silicon-based metal oxide semiconductor (MOS) structures have applications in elds varying from solar cells to gas sensors. Currently, silicon dioxide (SiO2) is preferred as there is high control over the oxide quality. As the demand for smaller scaled devices increases, the inability to form very thin SiO2 layers without inadverse e ects to device performance has become apparent. Aluminium oxide (Al2O3) is a high-k dielectric which has recently received renewed interest as a possible replacement for SiO2.
The presence of an oxide layer on silicon introduces traps at the oxide-silicon interface. Characterization of these defects is a necessary step to building a complete understanding of a device's performance. Manual characterization would be a time-consuming process which is prone to human error. A fully automated characterization system has been developed using LabVIEW. This system was used to characterize n-type silicon with SiO2 and Al2O3 interfacial layers over a large range of temperatures, voltages and frequencies. Traps at the interface have been found to have a large e ect on the capacitance and conductance of these devices, especially at high frequencies.National Research FoundationPhysicsMScUnrestricte
Effects of irradiation-induced defects in germanium
No full textDissertation (MSc (Physics))--University of Pretoria, 2023.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 differentPhysicsMSc (Physics)RestrictedFaculty of Natural and Agricultural Science
Schottky barrier diode fabrication on n-GaN for altraviolet detection
No full textThesis (PhD)--University of Pretoria, 2010.There are many potential areas for the utilization of GaN-based nitride materials, including ultraviolet photodetectors. Ultraviolet photodetectors are used in the military for missile plume detection and space communications. Medically, ultraviolet photodiodes are used in monitoring skin cancer. Schottky barrier metal-semiconductor contacts are choice devices for the manufacture of ultraviolet photodiodes due to higher short wavelength sensitivity and fast response. They also require simple fabrication technology; suffer lower breakdown voltages, and record larger leakage currents at lower voltages as compared to p-n structures of the same semiconductor material. Thus the formation of a Schottky contact with high barrier height, low leakage current, and good thermal stability in order to withstand high temperature processing and operation are some of the most important factors in improving the performance of Schottky barrier photodiodes to be used for ultraviolet detection. The first stage of this study was to establish a chemical cleaning and etching technique. It was found that KOH was suitable in reducing C from the surface and that (NH4)2S further reduced the surface oxides. The next phase of the work was to select a metal that will allow UV light to pass through at a high transmission percentage: a combination of annealed Ni/Au was found to be ideal. The transmission percentage of this alloy was found to be above 80%. The next phase was the fabrication of Ni/Au Schottky barrier diodes on GaN to study the electrical characteristics of the diodes. Electrical characterization of the diodes showed that the dominant current transport mechanism was thermionic emission, masked by the effects of series resistance, which resulted from the condition of the GaN surface. Finally, we fabricated GaN UV photodiodes and characterized them in the optoelectronic station designed and produced during this research. Device responsivity as high as 31.8 mA/W for GaN and 3.8 mA/W for AlGaN were recorded. The calculated quantum efficiencies of the photodiodes were 11 % for GaN and 1.7 % for AlGaN respectively.Physicsunrestricte
The study of ion migration in methylammonium lead bromide crystals
No full textDissertation (MSc (Physics))--University of Pretoria, 2020.The Inverse Temperature Crystallization (ITC) and seed-induced method were used to grow high-quality cubic samples within hours using a 1M solution of methylammonium lead bromide (MAPbBr3) samples. The current-voltage (I-V) hysteresis observed within the MAPbBr3 perovskite sample demonstrates
anomalous dependence on scan rate and various preconditioning pulses. We investigate this dependence and the relationship of current-transient with slow migrating ions. The current transients fitted using a bi-exponential decay model produced two distinct time constants t1 = 38.4 s and t2 = 6.49 s associated with migrating ionic species. From the Arrhenius plot an activation energy of Ea = 0.410 associated with migrating Br ions was extracted. Future research is required towards the understanding of I-V hysteresis and the link to ion migration in MAPbBr3 perovskite.NRF FundingPhysicsMSc (Physics)Restricte
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