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  • (2021) Guan, Xinwei
    Thesis
    In the last decades, resistive switching (RS) has burgeoned as a promising option for next-generation non-volatile memory applications. RS devices generally have a two-terminal metal-insulator-metal structure, where a variety of novel materials have been employed as the insulator layer. Among them, halide perovskites have drawn extensive attention owing to their superb physical and electronic properties, including ambipolar carries transport, low defect density, tunable bandgap, long diffusion length, and so on. In this thesis, halide perovskites-based RS devices have been developed, and their electrical properties are tuned by engineering the intrinsic defects and interfaces. The thesis includes the following two parts: (1) The first ion-redistribution-induced interface-type memory based on hybrid perovskite is developed and fabricated. Owing to the movable vacancies in the perovskite film, we can reliably modulate the height of the Schottky barrier at the MAPbBr3/ITO interface, leading to an interface-type RS memory with better stability compared to filament counterpart. (2) Conventional photodetectors are only able to record a temporary optical signal but require additional memory devices to further store the output. In our work, an artificial iconic memory device is fabricated with a multiplayer structure of ITO/MAPbBr3/Au/MAPbBr3/Ag, composed of the series-connected photodetector and RS devices. The incident light can modulate the voltage distribution, and then the information is stored as the states of the RS memory. Overall, this thesis presents facile and cost-efficient methods to fabricate halide perovskites devices for potential RS applications. The systematic study on the modification of RS behavior of halide perovskites devices might give a better understanding of the RS mechanisms and provide routes to enhancing the device performance.

  • (2021) Zhang, Wei
    Thesis
    Climate change and energy crisis have become a serious concern because of the rapid industry development associated with large consumption of fossil resource. As an important energy storage device in renewable energy technologies, supercapacitors exhibit several competitive advantages over the lithium-ion batteries in high power density, rapid charge-discharge rate, and long cycle life. As a common electrode material of supercapacitor, activated carbon is generally produced from biomass carbonization followed by physical or chemical activation that involves either high-temperature treatments or toxic and corrosive activating agents, resulting in higher energy-consumption and environmental pollution. In this thesis, several new green and cost-effective methodologies are developed to synthesize hierarchical porous carbons for the high-performance supercapacitors. We experimentally demonstrate a new one-step strategy of mild Na2CO3-NaCl molten salt assisted activation using glucose as the carbon precursor. During the pyrolysis, Na2CO3 acts as the activating agent and NaCl served as the reaction media, where the porous texture of the activated carbon can be manipulated by optimizing the synthesis parameters including pyrolysis temperature, composition of the molten salt and their mass ratio with the carbon precursor materials. In contrast, the activated carbon pyrolyzed from the mixture of sodium carboxymethylcellulose (CMC-Na) and NaCl exhibits superior capacitive performance in the aqueous supercapacitor. The experimental results show that the self-templated Na2CO3 facilitates the formation of hierarchical porous structure. Without NaCl additive, the porous carbons directly pyrolyzed from CMC-Na exhibit inferior capacitive performance, demonstrating the importance of NaCl addition in producing capacitive carbons. We also report the high-performance ionic liquid pouch-type supercapacitors fabricated with the green aqueous binder and defect engineered graphene nanoparticles through a controllable ball-milling of pristine graphite. The parameters of the milling process, such as filling ratio, ball to graphite mass ratio, rotational speed, milling time, are systematically optimized to achieve the excellent gravimetric, volumetric and specific areal capacitances. Additionally, the high packing density of carbon electrodes and the post heat-treatment on ball-milled graphite can improve the cycle stability significantly. Our work provides a new insight to produce eco-friendly porous carbons for high throughout manufacturing production of large-scaled supercapacitors.

  • (2021) Hou, Fei
    Thesis
    Scanning probe microscopy (SPM) is a powerful technique to investigate the surface properties at the nanoscale such as topography, ferroelectricity, piezoelectricity, and conductivity. In this thesis, these properties are studied in bulk WTe2, La-doped ZnO (ZnO:La) thin films, Highly Oriented Pyrolytic Graphite (HOPG) and Nb-doped SrTiO3. HOPG is firstly studied as the standard calibration material for scanning tunnelling microscopy (STM) and observed Moire patterns of twisted layers with a rotation angle of 5.4 degrees. To utilizing cross-sectional STM (XSTM), the fractured Nb-doped SrTiO3 (001) reveals different termination terraces of TiO2 and SrO on cleaved surfaces. Surface oxidation of WTe2 surfaces in different environments reveals non-uniform surface oxide dynamics, which upon saturation after several hours of atmospheric exposure yields a self-limiting approximately 2.5 nm-thick amorphous surface layer. Also, in a controlled environment involving a continuous flow of nitrogen, the surface oxide layer forms dendritic surface texture and is considerably impeded. WTe2 is a semimetal in which ferroelectricity was observed for the first time. SPM allows resolving spontaneously formed domains as well as so far un-detected stripe and ripple structures in bulk WTe2. The stripes are pushed or dragged by the electric field generated by the STM tip, and they can be bent or pinned by a non-uniform external field and defects on the surface. The ripples present merging and splitting of lines and, to some degree, are crystallographically oriented and have a concentration of point defects at the valleys of the ripples. Both defects and intrinsic strains of the material likely drive the formation of the ripple structure. Another material system investigated in this thesis is ZnO, which is highly promising for electronic and photonic devices. SPM study of spray pyrolysis grown ZnO:La thin films reveal that as-grown in-plane piezoelectric domains independent of topographic properties. At the same time, conductance correlates to topography and piezoelectric domains.

  • (2019) Soeriyadi, Angela
    Thesis
    Microorganisms are a valuable source of natural products with medically and industrially relevant activities. Cyanobacteria are one of the most chemically diverse microbial phyla, but have been largely underexplored due to their slow growth and intractability to genetic engineering. In recent years, genomic and metagenomic investigations of the aquatic environment have uncovered the untapped diversity of cyanobacterial nonribosomal peptide and polyketide biosynthetic gene clusters. This dissertation describes the application of emerging synthetic biology techniques using Escherichia coli as a heterologous host, focussing on translating the bioactive potential of cyanobacteria into industrial applications, while simultaneously characterising and tailoring this biochemical capacity. E. coli GB05-MtaA was previously shown to be a suitable host for the relatively simple cyanobacterial nonribosomal peptide synthetase (NRPS) pathway lyngbyatoxin (LTX). A synthetic biology approach for characterising and expanding lyngbyatoxin chemical diversity was explored. This thesis reports an in vitro investigation of wild-type LtxA NRPS activity that unravelled the multispecificity of the first adenylation domain to L-Val related amino acids, which correlates with the formation of novel lyngbyatoxin analogues in vivo. Efficient site-directed mutagenesis of the adenylation domain to modulate substrate specificities was performed through Red/ET-based recombineering, resulting in a library of mutated LTX pathways. Investigation of in vitro and in vivo pathway activity revealed the complexity of LTX biosynthesis, dictated by tailoring enzymes. The benefits of using this approach to probe LTX biosynthesis motivated the use of a similar application for the directed production of neosaxitoxin (neoSTX). NeoSTX is a paralytic shellfish toxin (PST) which has medically-relevant activities. The polyketide synthase (PKS)-like enzyme SxtA, which initiates PST biosynthesis, was expressed in E. coli to assess the suitability of this host. Efficient production of PST intermediates by heterologously expressed SxtA paved the way for the expression of an engineered neoSTX biosynthetic pathway in E. coli. Several variants of E. coli strains were successfully constructed to produce neoSTX. This synthetic biology process, with an E. coli expression system, is a feasible strategy to facilitate the commercial production of neoSTX.

  • (2021) Zhang, Ji
    Thesis
    This thesis focuses on the investigations of magnetic thin film structure of cubic and perovskite transition metal oxide magnets, particularly when two different magnetic orderings are competing at such interface. The goal is to understand the various effects of an interfaces that exert on the neighboring materials; to understand the role of the layer structure and microstructure on the overall magnetic properties; and to investigate the feasibility to control such interfacial phenomenon by means of strain, ion implantation, and crystal orientation. Four thin film systems are included: MnxOy/Ni80Fe20, La0.7Ca0.3MnO3/CaMnO3, La0.7Ca0.3MnO3 and BiFeO3/SrRuO3. Oxygen ion implantation was performed on MnxOy/Ni80Fe20 bilayers, and the results show an enhancement of exchange bias and coercivity after the implantation. Polarized neutron reflectometry study reveals a significant change in magnetic spin reversal mechanism due to chemical modification. In the La0.7Ca0.3MnO3/CaMnO3 system exchange bias is assisted by the magnetic frustration at the layer interface. Results suggest, the strength of exchange bias is strongly related to the degree of frustration. Controlling the strain state of La0.7Ca0.3MnO3 shows an effective method to alter the frustration property. Further, the magnetic glassy spins in the La0.7Ca0.3MnO3 epitaxial thin film is studied. Combining DC magnetization, AC susceptibility, and polarized neutron reflectometry measurements, the spin-glass nature of the sample is obtained which spins are freeze around 139 K which just below the ferromagnetic transition of the bulk sample. Finally, for the BiFeO3/SrRuO3 system, an enhancement of net magnetization of the canted antiferromagnetic BiFeO3 was probed in (111)-orientated sample. There is possible exchange interaction of Ru4+ and Fe3+ and the strong orbital p-d hybridization of SrRuO3 which could contribute to the enhanced magnetization.

  • (2021) Karamiqucham, Behrooz
    Thesis
    By their proximity, jovian planets provide the best lab to analyse their unique spectral features contributing further to the improvement of both planetary and exoplanetary atmospheric sciences. In this thesis, VSTAR and ATMOF codes have been modified and performed to suit the spectral modelling of the jovian planets producing accurate telluric models for line removal and fitting the modelled spectra. The spectral model fits here, used for analysing different spectral features in accordance with P-T profiles, chemical composition and cloud parameters, focus on cloud base pressures and opacities in the atmosphere of jovian planets. The line-by-line radiative transfer atmospheric models presented here are innovative and novel in their concurrent analysis of the transmitted, emitted and reflected light capable of using the most powerful modelling software. The models specified here relate to the atmospheric characterisation of the giant planetary worlds in the Solar System, with potential for expanded application to planetary worlds beyond our own neighbourhood. My models spectrally characterised the upper atmosphere of the ice giants in the NIR bands (R ~2400, 5100 and ~17800), determining their cloud optical parameters by applying three of the latest theoretical and empirical methane line datasets. Meanwhile, by calculating their D/H ratios in both methane and hydrogen, I was able to reassess their formation scenario, suggesting the formation of Uranus at a greater distance from the Sun than Neptune. I also analysed and modelled Jupiter’s planetary disk in three NIR bands of J, H and K (R ~2400) using its various cloud particle distribution through their optical depths and base pressure ranges. In total, 153 NIR spectra indicative of 51 spectral regions on Jupiter’s disk have been modelled and characterised. The spectral regions correspond to 9 designated latitudes and 7 specified longitudes, as a function of their cloud opacity and base pressure changes. The analysis produced the global map of clouds/haze variations on Jupiter’s disk resulting in better understanding of its atmospheric characteristics, dynamics and features capable of creating a broader spectral view to contribute to the planet’s future atmospheric studies along with its giant remote cousins.

  • (2021) Zheng, Xiaoran
    Thesis
    CeO2-x and CeO2-x-based catalysts are emerging as important functional materials in many energy- and environment-related applications. However, there remain uncertainties and misconceptions in the interpretation of the fundamental function of defects in determining the characteristics of materials. The present work explores this relationship in detail by considering the critical role of defect equilibria in terms of the effects of solid solubility and charge compensation mechanisms on the resultant physicochemical properties and catalytic performance of bulk CeO2-x as well as MoO3-CeO2-x and RuO2-CeO2-x heterojunctions. Electrodeposition was used to synthesise holey nanosheets and heterojunctions were created using wet chemistry. Analyses consisted of XRD, Raman, SEM, HRTEM, EDS, SAED, AFM, XPS, EPR, PL, KPFM, and UV-Vis. DFT was used to calculate the optical indirect band gap (Eg) values for the different solubility mechanisms for the dopant valences. The catalytic performance was assessed by HER and ozonation testing. The combination of XPS data, their detailed and extensive analyses, and consideration of all possible defect equilibria represents a powerful tool to interpret the physicochemical properties and catalytic performance of bulk materials and heterojunction nanostructures based on them. With this information, it is possible to decouple multifarious data for disparate materials such as bulk materials, chemisorbed heterojunction nanostructures, and physisorbed heterojunction nanostructures. A key outcome of the present work is that the primary factor in both the properties and performance unambiguously is Ce3+ ions, not oxygen vacancies. This is manifested through the solubility mechanisms of the dopants, which are interstitial, and the charge compensation mechanisms, which are ionic for Mo doping and ionic + redox for Ru doping. The latter mechanisms may be altered by three F centres (viz., colour centres), which derive from oxygen vacancies, and intervalence charge transfer (IVCT) in the case of Mo doping. The F centres and metal interstitials also are key factors in raising the Fermi level (Ef) of the doped materials, effectively reducing the Eg, particularly for Mo doping. The hydrogen evolution reaction (HER) performance was dominated by the heterojunctions, where the strong bonding from chemisorption, IVCT, and homogeneous and high distribution density of small heterojunction particles with Mo doping resulted in enhancement such that this performance is the best yet reported for CeO2-x-based materials. In contrast, the HER with Ru doping was relatively poor owing to the weak bonding from the inhomogeneous and low distribution density of large physisorbed heterojunction particles. The ozonation performance was outstanding but adversely affected by cerium vacancies. While this performance for Mo doping was improved by reduction owing to IVCT, that for Ru was uniformly poor owing to the high cerium vacancy concentration. The performance for bulk CeO2-x was poor owing to structural destabilisation during reduction, thus suggesting stabilising effects from the heterojunction particles.

  • (2021) Al Mahmood, Md. Abdullah
    Thesis
    Multi-layered waste packaging materials are widely used in the packaging industries due to their flexibility of applications, superior properties, and relatively lower cost. Despite the advantages accomplished by the polymer-metal multilayers packaging materials, recycling the waste in a traditional method is a very difficult task due to the complexities of multi-materials intrinsic behaviours during the processing of cast-off materials. In this study, a newly developed microrecycling technique (thermal disengagement technology, TDT) has been introduced and briefly demonstrated. Several outcomes are: (i) Polymer laminated Al packaging materials available from the local market was thermally disengaged by TDT into several useful products including ~98% pure Al, and graphitic C without any major emission (ii) laminated polymers in multi-layered packaging materials can be degraded into the graphitic C in an inert atmosphere. Degraded C can stay on Al surface to provide well protection against surface oxidation, (iii) TDT was utilised for different types of multilayer packaging materials consisting of multiple polymers and metallic contents (Al, Cu, & Fe) available in the local market. TDT is highly capable to recycle all different types of packaging materials irrespective to their inclusive materials. (iv) Al-containing packaging material was recycled in different media (air, nitrogen, & argon). Argon media was suitable to recycle Al into its original form and polymers into degraded graphitic C. Recycled Al was transformed into microparticles by non-traditional mechanical milling at cryogenic temperature (-196°C) created by liquid N2. Synthesised flake shaped, and micro-sized carbonaceous Al microparticles are contamination-free and thermally stable and can be useful in the fields of additive manufacturing, (v) A rapid transformation process was introduced where thermally disengaged Al from the TDT subsequently thermally transformed in an arc furnace at a very high temperature (~2000°C) in a short time period (~20s) in an inert and vacuum condition. As a result of this rapid transformation, ceramic reinforced Al alloy with an enhanced physical, microstructural, and mechanical properties was synthesised. The overall project of recycling polymer-metal multilayer packaging materials can be concluded with numerous green materials output along with some co-products and metallic alloys.

  • (2021) Govinden, Vivasha
    Thesis
    This thesis explores topological defects and topological defect transitions in epitaxial ultrathin ferroelectric heterostructures. Geometrical confinements in ultrathin films has enabled the realisation of several nontrivial topological polarisation arrangements in ferroelectrics, categorised as a range of topological defects such as bubble, meron, vortex, flux-closure domain, etc. These ferroelectric topological defects can be engineered by tuning depolarisation field, mechanical and electrical boundary conditions. Our model system is an ultrathin (001) oriented PbZrxTi1-xO3/ SrTiO3/ PbZrxTi1-xO3 heterostructure fabricated on La0.67Sr0.33MnO3 buffered SrTiO3 substrates. Several topological defects are realised under specific mechanical and electrical boundary conditions. Topological defect transitions are also achieved using different routes such as electric field, thickness variation, mechanical pressure and thin film milling. These topological defects have also gained immense technological interest on account of their emergent properties. This thesis further studies the functional properties in topological defects such as electrical conductivity in bubble domains. The motion of these bubble domains is also investigated. The results herein offer new insights on how to engineer topological defects and topological defect transitions in order to design multifunctional ferroelectric/multiferroic devices with enhanced operational speed, sensitivity and energy-efficiencies.

  • (2021) Wang, Yiping
    Thesis
    Single enzyme nanoparticles (SENs), which encapsulate individual enzymes in a thin permeable polymer network offer great control over the chemical and physical environment directly around the enzyme. SENs have exhibited great enzyme stability by restricting enzyme extensive unfolding motion under extreme conditions, like extreme pH and high temperature. However, up to date, the control over the chemistry of the shell is still quite limited. In this thesis, a new SEN formation strategy has been explored. In order to minimize the risk of enzyme deactivation during synthesis of the SENs, the weak electrostatic interaction was utilized to assemble charged polymers around the enzyme. Different lengths of charged polymers were pre-prepared via reversible addition−fragmentation chain-transfer polymerization (RAFT) and then attached to the surface of enzyme via electrostatic interactions. This strategy has been investigated for the different enzyme, including lysozyme, trypsin, protease, horseradish peroxidase, and glucose oxidase. Isothermal titration calorimetry (ITC) and asymmetric flow field-flow fraction (AF4) in combination with multiangle light scattering (MALS) reveal the binding number and strength of polymer chains / enzyme. The strength of binding can be tuned based on the charge density of the bound polymer. In this method, the trithiocarbonate group of a RAFT agent was placed close to the surface of the enzyme and the initiation of a free radical acrylamide / bisacrylamide polymerisation in solution can result in chain extension of the RAFT polymer and direct the formation of the newly formed hydrogel around the outside of the enzyme. AF4-MALS and small-angle X-ray scattering (SAXS) confirm the formation of a thin cross-linked shell around the enzyme. The mild conditions of this method of SEN formation, which avoids any covalent modification of the enzyme, results in no loss in activity on our model enzyme (glucose oxidase), and four-fold increase in thermal stability. The method is then utilized to probe the protective effect of trehalose close to the enzyme. Trehalose is generally assumed to be the most effective sugar to use as a protein stabilizer. In this method, trehalose molecules were placed close to enzyme surface by either assembling enzyme with charged trehalose polymers or crosslinking with trehalose monomer. In order to evaluate the effect of the trehalose in SENs on stabilizing enzyme, another disaccharide sucrose was treated in the same way for comparison. It was found that the core-shell structure, instead of the chemistry of the shell, was more important for stabilizing enzyme structure under heat treatment. This study offers a new technique for synthesis of SENs with ease of design and control of the shell chemistry of SENs, opening up new pathways for enzyme stabilization and application.