Browsing by Author "Blumenthal, Mark"

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    CdSe based nanowires for the photocatalytic production of hydrogen gas
    (2018) Abdullah, Ilyaas; Levecque, Pieter; Blumenthal, Mark
    Photocatalytic production of hydrogen was investigated towards achieving a decarbonized supply of hydrogen gas for clean energy conversion technologies such as the proton exchange membrane fuel cell (PEMFC). This study uses a template-directed electrodeposition technique to synthesize multi-segmented CdSe based nanowires for use as a photocatalyst device for hydrogen production. CdSe, Ni, Au and Pt nanowires were successfully synthesized with dimensions ranging from 100 nm to 350 nm in diameter and up to 10 µm long. The CdSe stoichiometry was not easily controlled despite following literature protocols and requires a more systematic investigation. The electrodeposition of Ni nanowires was found to be most effective with very few problems encountered. Improvements in the morphology of Au and Pt nanowires were made by using a constant current as opposed to constant potential electrodeposition techniques. Multi-segmented nanowire devices were prepared with nanowires left embedded in a porous anodized aluminium oxide (AAO) template. Polymer PEDOT: PSS and noble metal Pt was used as an anode and cathode electrocatalyst materials respectively. A prototype photocatalytic testing system was set-up using a 1600 W xenon arc lamp as a light source, an in-house made photoreactor as the device holder, and a mass spectrometer for online gas detection measuring ionic currents of evolved species. The set-up was able to successfully detect hydrogen evolved during the tests but does require further development if more complete photocatalytic testing is to be conducted in future. Photocatalytic hydrogen production from the irradiated devices was inconclusive, but hydrogen detection from devices was observed in an 80 % MeOH solution with no irradiation. Through these tests it was learned that photocatalytic activity needs to be differentiated from regular catalytic activity. This is particularly the case if testing is conducted in organic media and if the photocatalytic phenomena is to be properly isolated and understood correctly
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    Electrochemical synthesis of functional nanowires
    (2016) Fawzy, Mirette; Blumenthal, Mark
    Semiconducting, metal, and multi-segmented nanowires of 200 nm diameters with lengths up to 10_m were synthesized through electrochemical deposition in the pores of anodic aluminium oxide (AAO) and polycarbonate track-etched (PCTE) templates. With the work focusing on CdSe nanowires, results from the cyclic electrodeposition/stripping method was reported with different deposition conditions(potential range, solution, and scan rate). The stoichiometric deposition of CdSe nanowires was confirmed by energy dispersive spectroscopy (EDS) analysis, with an average Cd:Se atomic percentage of0.92. Structural characterizations were done on the synthesized nanowires via scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) measurements. Devices with a single CdSe nanowire were made by standard electron beam lithography techniques. Electrical transport measurements were performed on the nanowire arrays as well as single contacted CdSe nanowires. Photo response measurements were also carried out on the samples in the dark and under illumination. The nanowire arrays showed pronounced visible light photoconductivity. As a direct application, a preliminary proof-of-concept photoelectrochemical activity test for water splitting using the synthesized CdSe nanowires was performed in a water/methanol mixture.
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    Growth and characterisation of platinum and palladium catalysed silicon based nanostructures for nano-device fabrication
    (2018) Khumalo, Zakhelumuzi Mesuli; Blumenthal, Mark; Topic, Miroslava
    This thesis first describes the application of ion beam techniques for the analysis of synthesised metal silicide nano-droplets which are used to synthesis silicon nanowires. This analysis was performed for both Molecular Beam Epitaxy (MBE) and Electron Beam Evaporator (EBE) deposited metal coated silicon substrates. The formation of platinum silicide phases as a function of annealing temperature and time was investigated using in situ real-time Rutherford Backscattering Spectrometry (RBS). In situ real-time RBS revealed the reaction of platinum and silicon to start at about 200 °C and 230 °C, for MBE and EBE coatings respectively, forming platinum silicide phases (Pt2Si and PtSi) in sequence. Further analysis on the EBE deposited coatings using a scanning electron microscope confirmed the formation of droplets at 800 °C. Particle induced X-ray emission analysis showed the variation of platinum concentration, in droplets regions, between 1600 and 2000 counts. The surrounding regions were left almost uncovered due to platinum coating dewetting. Work was then carried out to investigate the growth of silicon nanowires from the produced droplets. Silicon nanowires were observed to form at 800 °C and 1000 °C using pulsed laser ablation and thermal annealing techniques, respectively, for the EBE deposited coatings. Furthermore, MBE was shown to produce self-aligned platinum silicide (PtSi) nanoclusters and nanowires on Si (111) substrates near the eutectic point (T = 978 °C and 67 at% Si) of the platinum silicon system. With an added silicon source in the MBE annealing chamber, the formation of two different silicon steps (straight and wave-like) is reported. The steps determine the position and the shape of the thermomigrating PtSi droplets. Nanoclusters preferentially form in the triple point of the wave-like steps while nanoclusters that form on the straight steps combine and grow into silicide nanowires. We report, for the platinum (MBE deposited) catalysed core-shell silicon nanowires, a reproducible method for eliminating hazardous acids during lithography device preparation with the use of silicide forming metal. With regard to palladium coatings, self-aligned polycrystalline palladium-silicide nanowires and palladium catalysed crystalline core-amorphous shell silicon nanowires were synthesised. The catalyst (palladium) was deposited on a silicon substrate using MBE. As a result of thermal annealing in the silicon flux, self-aligned and fully crystalline palladium-silicide nanowires were synthesised while crystalline core-amorphous shell silicon nanowires were grown when an argon gas was introduced. These results highlight the mechanisms to synthesise specific types of nanowires to be used in a variety of electronic devices depending on their applications. Furthermore, for the application of the produced core-shell nanowires in nanodevice fabrication, we present two approaches to reduce the oxidation of the nanowires during the thermal annealing growth method. The ratios of the amorphous shell to crystalline core on the nanowires produced, from the two methods, are compared and show a remarkable drop in oxide thickness when compared to nanowires fabricated using the current techniques available. In addition, focused ion beam was utilised to contact the oxide reduced nanowires, without first removing the thin oxide shell, for transport measurements. The oxygen reduced core-shell silicon nanowires showed very low electrical resistivity (4×10-1 Ω.cm). The techniques employed presents a new alternative to the production of low cost, high yield, high conducting silicon nanowires that will find use in a range of semiconductor based technology.
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    Implementation of the Quantum Hall Effect based precision resistance measurement system
    (2019) Maboko, Mporome Brian; Blumenthal, Mark; Matlejoane, Alexander
    The integer Quantum Hall Effect (QHE) occurs when a two-dimensional electron gas (2DEG) is subjected to a strong perpendicular magnetic field and when the system is cooled to low temperatures. The QHE harbours a wealth of unique phenomena. Of interest is the existence of the Quantum Hall Resistance (QHR) which had found to be related to two fundamental constants of nature via the von Klitzing constant h e 2 , where e is the charge of the electron and h Planck’s constant. This thesis investigates the properties of the QHE in a low dimensional electron gas system. The von Klitzing constant is determined as well as the electron density n2D and mobility µ of the material measured. The results are compared to the accepted value of the von Klitzing constant as determined by the metrological community. The average von Klitzing constant obtained is 25 783.637 Ω within an accuracy of 1.13 × 10−12. Our results are further interpreted using the Landau quantum mechanical model of electron transport in perpendicular magnetic field. The measurement of standard resistances utilising a standard DC resistance measurement system were also undertaken at the National Metrology Institute of South Africa (NMISA). This ties in with the ongoing project of NMISA to develop an in-house quantum Hall measurement system to provide the full traceability for resistance standard measurements in the Republic of South Africa. The device measured utilised a GaAs/AlGaAs heterostructure structure, grown via Molecular Beam Epitaxy (MBE). A micron sized Hall bar with Ohmic contacts was patterned using standard clean room procedures. Magnetotransport measurements at low temperatures, sub 200 mK were carried out on the device. The transverse and longitudinal resistances were obtained and plotted against the perpendicular magnetic field. Quantum Hall plateaus and Shubnikov de-Haas (SdH) oscillations were observed. Properties of the heterostructure such as the electron density (n2D) and mobility (µ) were determined. The n2D obtained was 2.27 × 1011 cm−2 with µ at 3.5 × 105 cm2V−1 s −1 . All results were compared to current literature values.
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    Structural and electrical characteristics of printed metal nanoparticle networks
    (2016) Van den Berg, Claire Barbara; Blumenthal, Mark; Topic, Mira; Härting, Margit
    The structural and electrical properties of metal nanoparticle (NP) networks and their dependence on the constituent phases have been investigated. Percolation and effective media theories have been used to describe the physical properties of disordered systems, as well as providing a link between their structural features and the corresponding electrical transport properties. Silver and palladium nanoparticulate layers in ethyl cellulose polymer binder (ETHOCELTM), were fabricated onto paper using the method of screen printing. The metal-binder ratios were varied in order to observe changes in the microstructure when a percolating network consisting of the metal NPs is formed through the layer.
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    The effect of a top gate voltage on the Quantum Hall measurement of a 2DEG InGaAs/InAlAs hetero-structure
    (2022) Maguim, Silatsa Carine; Blumenthal, Mark
    The main purpose of this thesis is to investigate the quantization of the Hall resistance "Rxy " as well as the Shubnikov De Haas oscillations of the longitudinal "Rxx " resistance in a two dimensional electron system at different gate voltages. The sample measured utilised the InGaAs/InAlAs heterostructure grown via Molecular Beam Epitaxy (MBE). The above-mentioned effects are investigated before and after exposure of the sample to UV light. To calculate the electron density and mobility of the sample on each gate voltage, two different methods are used. The Fast Fourier transform method and the minima method. The results show that illumination increase the electron density and mobility of the sample. Results further show that both methods found similar results. Another remarkable observation is that the as gate voltage becomes more negative the density of electrons decreases and so the mobility. The maximum electron density using the FFT method is 1.8560 1011cm−2 with the mobility of 363880cm2V −1 s −1 . This results fall within the standard errors calculated by the minima method and given by: 1.7889 ± 0.026727 1011cm−2 with a mobility of 378340cm2V −1 s −1 .
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    Towards A Single Electron Current On Superfluid Helium
    (2021) Funk, Oliver; Blumenthal, Mark; Nicolls, Frederick
    The aim of this dissertation was to investigate the application of a system of electrons floating above the surface of superfluid helium to the field of single electron transport. Previous work done by Dr Forrest Bradbury at Princeton University (now a collaborator in the group) demonstrated the highly efficient and precise control of packets of electrons floating on the surface of superfluid helium, localised to channels defined in a silicon substrate. Using similar devices and methodologies, the work done in this dissertation investigates whether this modality of electron transport can be effectively applied to deliver a current of single electrons. Single electron devices have numerous applications in the field of metrology and quantum information processing. They allow for measurements to be made of fundamental quantities, such as the charge of an electron, and further demonstrate various quantum mechanical properties of nature. Presented in this dissertation is the work completed to date, which includes: the design and fabrication of the nanoscale device used to conduct the electrons on superfluid helium experiments, the required electronics needed to control the device and the data acquisition system needed to read various signals off the device. The fabrication was done at Oak Ridge National Labs in the USA. Additionally, a hermetically sealed superfluid cell designed in collaboration with Dr Jay Amrit from Universit´e Paris-Sud, France used to house the device is presented, as well the probe needed to insert this cell into the dilution fridge. The theory behind the functionality of the device and the way in which it would work is developed. A simulation of working of the device is presented, as well as the expected measurement quantities. The outlook for continued work in this exciting and very novel physical system is also presented.