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  • (2019) Du, Haojin
    Thesis
    This work provides a systematic study on engineering nickel hydroxide-based nanomaterials with enhanced electrochemical properties for the applications of supercapacitor and overall water splitting catalyst. Silver nanowires (Ag NWs) and Ni(OH)2 nanosheets based hybrid materials were prepared, which can simultaneously realize bifunctions of flexible transparent electrodes and all-solid supercapacitors. Ag NWs provide high conductivity while ensuring flexibility of the electrodes. Ni(OH)2 also has a variety of functions, one of which can reduce the contact resistance by compacting Ag NWs, and the second one is to act as a collector as a pseudo-capacitor. Through the combination of two multifunctional materials, all-solid-state supercapacitor with high cycle stability and flexibility has been demonstrated, which may have potential applications for flexible supercapacitor applications. Bifunctional overall water splitting catalysts based on Ni(OH)2 nanomaterials have been fabricated by the corporation of the hydrothermal method with calcination, which can enhance both oxygen evolution reaction and hydrogen evolution reaction capability. The morphology and crystal structure of hexagonal Ni(OH)2 nanosheets at different temperatures were investigated. It has proved that the content of nickel is an important factor for the improvement of electrochemical activity. Graphene contains a large number of defects which provided sufficient electrochemical active sites and improve the adhesion of the Ni(OH)2 nucleation process to the graphene substrate. It solves the poor catalytic stability attributed to the weak adhesion between catalysts and the conductive substrate.

  • (2018) Cheung, Keng Ho
    Thesis
    TiO2 coatings were fabricated by anodizing unpolished or polished Ti6Al4V in 1 M H3PO4 or 1-3 M H2SO4 at room temperature at 120 V for 10-20 min and selected coatings were annealed at 300-500C for 8 h. Analyses included mineralogy (GAXRD, Raman), morphology (FESEM, AFM), topography (3D confocal microscopy), microstructure (FIB), chemistry (XPS), optical (UV-Vis), thermodynamic, human osteoblast-like cellular adhesion and proliferation, UV- or X-Ray-activated photocatalytic performance (MB degradation), UV- or X-Ray-activated antibacterial performance (Staphylococcus aureus). Anatase was the principal polymorph in the anodized TiO2 coatings although rutile was observed as a minor phase that resulted from dielectric breakdown and associated heating. The surface preparation of substrates determined not only the grain and pore sizes but also the morphology and topography of the coatings. Consequently, the presence of the amorphous TiO2 passivating layer, which is inherent to the unpolished substrates, resulted in thicker and more irregular anodized coatings compared to those of the anodized metal surfaces exposed by polishing. Large-scale delamination steps were observed in coatings on unpolished substrates while localized delaminations occurred on polished substrates with longer anodization times. 3D confocal microscopy allowed distinction between the fine-scale morphology (large open pores) and the coarse-scale topography (delaminations). These data showed that increasing time and acid concentration resulted in similar trends of increasing roughness (morphology) and unevenness (topography). The influence of the oxidation strength of the acid was pervasive in that it impacted on the crystallinity, microstructural homogeneity, coating thickness, Ti3+ concentration, gas generation during arcing to form pores, and resultant pore size and distribution density. The pores formed a subsurface network of variable continuity, which has a significant impact on the surface area and associated density of photocatalytically active sites, access by liquids and gases to the coating interior, penetration depth of incident radiation, gas condensation, and residual liquid trapping. These data and related thermodynamic analyses of the acids, anodization processes, and oxidation processes revealed that surface, bulk, and microstructural effects governed the photocatalytic performance. Nanostructural analysis allowed differentiation between fine-scale (small open pores from arcing) and coarse-scale (large open pores from arcing and oxidation eruptions) features. While the anodized coatings were biocompatible, cellular adhesion was dominated by the large pores and proliferation depended largely on the small pores. The anatase crystallinity was enhanced by increasing H2SO4 concentration, which increased the S-based species content, but most by annealing, which decreased it. Photocatalytic performance revealed the opposing effects of anatase crystallinity, which enhanced the performance, and blockage of active sites, which reduced the performance. The antibacterial performances against Staphylococcus aureus followed the same trends for unannealed coatings but the annealed coatings showed inferior performance owing to S depletion and associated reduction of availability for bacterial consumption. The performances of the X-irradiated coating were inferior to those of the UV-irradiated coatings because the absorption of the characteristic X-rays of the W source was very low.

  • (2018) Wan, Tao
    Thesis
    Recently, resistive switching devices have emerged as promising candidates for next generation non-volatile memory and neuromorphic computing applications. In general, resistive switching device consists of a two-terminal metal-insulator-metal structure, in which metal oxide is widely employed as the insulator. Among a variety of metal oxides, SrTiO3 has attracted extensive attention owing to its superior physical and chemical properties. In this dissertation, novel SrTiO3 resistive switching devices through solution processed approaches have been developed and their electrical properties are tuned through engineering the defects and interfaces. The thesis includes the following parts: (1) Specific cation doping is utilized to modulate the electrical properties of SrTiO3 nanoparticles film. In Cr-doped SrTiO3 device, oxygen vacancies are induced since the Ti4+ is partly replaced by Cr3+, leading to reversible hysteresis loops upon application of voltage, while negligible resistance change is observed in the undoped device. (2) By insertion of a reduced graphene oxide layer between SrTiO3 and the bottom electrode, a transition from digital switching to analog switching is demonstrated. Typical potentiation and depression behaviours are implemented towards neuromorphic computing applications. (3) To explore the potential applications of silver nanowires as electrodes and artificial filaments, controlled fragmentation of silver nanowires has been realized through ultraviolet (UV)/ozone irradiation and a low-temperature annealing process. Based on the fragmented silver nanowire network, the device exhibits a reliable threshold switching effect with a selectivity of 5 × 105. (4) A facile sol-precipitation method is introduced to prepare SrTiO3 nanocubes and thin films. Using silver top electrode deposited by ink-jet printing, the device shows a typical bipolar switching behaviour. After deposition of silver nanowires onto the bottom electrode, unipolar switching with a high on/off ratio (~105) is demonstrated. This work provides facile and cost-effective solution-based methods to fabricate SrTiO3 devices for potential resistive switching applications. Meanwhile, the systematic study on modification of switching behaviour of SrTiO3 devices may give a better understanding of switching mechanism and offer ways to improve the device performance.

  • (2019) Bahman Rokh, Ghazaleh
    Thesis
    Undoped, single Ce3+- and Fe3+-doped, and Ce3+-Fe3+-codoped TiO2 thin films were deposited on unpolished fused SiO2 substrates by sol-gel spin coating. All of the samples were annealed in air at 450ᵒC for 2 h. New formalism for defect equilibria is presented, which are contrasted with those of the conventional Kröger-Vink scheme. This has been developed in order to compensate for the discrepancy between the conventional assumption of compensated stoichiometry and the potential for uncompensated stoichiometry. All of the data are mutually correlative and consistent with the effects of a series of progressive solubility mechanisms and structural responses with increasing doping. The initial solubility mechanisms in the single Ce-doped TiO2 and Fe-doped TiO2 are interstitial and substitutional, respectively involving dissolution of Ce3+ and Fe3+ followed by formation of a Frenkel pair by transposition of the central ion to the interstitial site and formation of a central Ti vacancy; accompanied by stress minimisation by integrated solid solubility. In single Ce-doped TiO2, at 0.09-0.10 mol% Ce, substitutional solubility occurs, which alters the properties significantly. This process involves occupation of the Ti lattice site by Ce3+, which causes maximal a-b plane expansion, followed by intervalence charge transfer (Ti4+ + Ce3+ → Ti3+ + Ce4+). At higher doping levels, substitutional solubility of Ce3+ for Ti3+ in the interstice occurs, initiating gradual structural collapse. The photocatalytic performance increases significantly from the introduction of the shallow donor defect 〖"Ce" 〗_"Ti" ^x. In single Fe-doped TiO2, at 0.10 mol% Fe, substitutional solubility occurs, which alters the properties significantly. This process involves occupation of the Ti lattice site by Fe2+, which causes maximal a-b plane and c axis expansion, followed by intervalence charge transfer (Ti4+ + Fe2+ → Ti3+ + Fe3+). At higher doping levels, similar valence repulsions (Fe3+, Fe3+, Ti3+) result in gradual structural destabilisation. The photocatalytic performance enhances significantly from the introduction of the acceptor defect 〖"Fe" 〗_"Ti" ^''. In codoped Ce-Fe- TiO2, the coexistence of Ce3+ and Fe3+ substitutional solubilities on the Ti lattice sites, respective Frenkel pair formation, and respective substitutional solid solubilities are the progressive mechanisms. While the mechanism at the highest doping levels retains Fe3+ in the substitutional site, when Ce3+ is in the substitutional site, multivalence charge transfer (Ti4+ + Ce3+ → Ti3+ + Ce4+ and Ce3+ + Fe3+ → Ce4+ + Fe2+) also occurs. The doping concentrations of 0.09 mol% Ce + 0.10 mol% Fe cause the photocatalytic performance to increase significantly from the combined effect of Ce3+ substitutional + Fe3+ interstitial solid solubility and introduction of the relatively deep midgap state 〖"Ce" 〗_"Ti" ^' and midgap state 〖"Fe" 〗_"i" ^"•••" .

  • (2019) Burns, Stuart Robin
    Thesis
    Ferroelectric materials are characterised by spontaneous polarization that is switchable by an electric field. Although they have been studied for nearly one hundred years, there is a growing research community learning more about these complex materials and their potential applications - many of which have only come to light in recent times. Two reoccurring themes in the community are i) the influence of dimensionality reduction on the physics of ferroelectrics (and multiferroics - which simultaneously exhibit ferroelectric and magnetic order), and ii) the functional response in the GHz regime and faster. These themes tie together when considering future device applications, where densely packed ferroic elements would interface with industry standard high frequency electronics. This thesis attempts to further define the prerequisites needed for such devices. The first topic of the thesis concerns the effect of geometric constraints on the crystallographic and magnetic structures in a model multiferroic. Neutron diffraction experiments were carried out on a series of bismuth ferrite (BFO) films to determine the influence of thickness on the stability of the spin structure. The results demonstrated a thickness-dependent expansion of the period of the spin cycloid in BFO, suggesting size effects can alter the magnetic order. Reduced dimensionality is equally important to the growth of new phases of thin film materials. Hence, strain engineering of BFO and the fine thermodynamic energy balance in mixed phase films was explored. An empirical evaluation of the energetics involved in forming strain-relieving secondary phases in BFO grown on lanthanum aluminate (LAO) substrates enabled the extraction of explicit energy costs for forming inter-phase boundaries. The second topic of the thesis evaluated the local behaviour of ferroelectrics in the high frequency regime. Scanning microwave impedance microscopy (sMIM) was used to probe the dielectric response of mixed-phase BFO thin films, as well as the high frequency AC domain wall conductivity of lead zirconate titanate (PZT) films. The results implied quasi-continuous tunable domain wall conduction in the microwave regime. The results herein offer new insights into the coupling of ferroelectric thin films with magnetism, self-strain, and externally applied high frequency electric fields.

  • (2019) Xu, Yuwen
    Thesis
    The morphologies of ceria nanocrystals play an essential role in determining their redox performance in many applications, yet the effect of synthesis variables on the formation of ceria nanoparticles of different morphologies and their related growth mechanisms are poorly understood. The present thesis investigates the morphological development of nanoceria through the examination of the key processing variables for precipitation and hydrothermal synthesis at high Ce and NaOH concentrations. The characterization included the analytical techniques XRD, TEM, HRTEM, XPS, BET, and laser Raman microspectroscopy. A comprehensive survey of the effects of experimental conditions on the resultant morphologies of nanoceria during precipitation and hydrothermal synthesis is presented. The key experimental variables [Ce], [NaOH], temperature, and time are observed to have significant effects on the morphological evolution and grain growth. Detailed schematic mapping of nanoceria in the form of nanorods and nanocubes indicates that high [Ce] and low temperature facilitate chain formation and subsequent coalescence into hexagonal nanorods. With increasing temperature and time, these structurally destabilise and re-form into nanospheres and nanocubes. The present work is elucidated through a range of mechanistic interpretations that underpin a new morphological map for high [Ce]; this incorporates not only morphological development but also grain growth effects of nanoceria. The experimental data and morphological map have allowed the derivation of the comprehensive schematic mapping that explains, for the first time, the effects of the four key experimental variables on the generation of the different morphologies that have been observed and reported in the literature.

  • (2019) Theska, Felix
    Thesis
    More than 37,000 new aircraft are required by 2037 as air traffic has been predicted to double every two decades. Improved fuel efficiency is crucial for next generation aircraft engines to extend aircraft range and speed. Aircraft engine turbine discs are commonly made of Alloy 718, a polycrystalline, precipitation-strengthened nickel-based superalloy. High-temperature strength, creep resistance and corrosion resistance allow operational temperatures of 650 °C for service cycles of several 100,000 h under high mechanical workloads. Conventional processing of Alloy 718 turbine discs involves multi-step forging, solution annealing, and dual ageing. Direct ageing is an alternative process to achieve yield strength increments of > 10 % by omitting solution annealing. However, the detailed processing-microstructure-property relationships during direct ageing of Alloy 718 are not fully understood. The microstructure of Alloy 718 contains a γ- (FCC) matrix with a grain size of ASTM 10 for grain boundary and solid solution strengthening. Micronscale δ- (Ni3(Nb), D0a) phase platelets decorate grain- and twin-boundaries for optimal creep strength. High dislocation densities in the grain interior provide work hardening. Nanoscale γ'- (Ni3(Al,Ti), L12) and γ''- (Ni3(Nb),D022) precipitates in duplet and triplet morphology provide the high-temperature strength of Alloy 718. This study provides a correlative high-resolution microscopy approach to reveal the microstructural evolution in Alloy 718. Comparing conventional versus direct ageing demonstrates that high retained dislocation densities and Nb concentrations in solid solution yield inverted precipitate morphologies. Atom probe microscopy techniques such as the enhanced iso-surface method and the interface method are developed to quantify γ'- and γ''-precipitate volume fractions, particle radii, number densities, and co-precipitate morphologies. Slices of forged turbine disc materials are characterized to clarify the occurrence of circumferential yield strength variations. Finally, the study on the early stages of precipitation illustrates the detailed nucleation, growth and coarsening mechanisms of γ'- and γ''-precipitates during direct ageing of Alloy 718.

  • (2019) Huang, Bosi
    Thesis
    Photocatalytic and photoelectrochemical water splitting can play an important role in future renewable energy systems by allowing solar energy to be stored in the form of hydrogen. Tungsten oxide (WO3) has some promising properties as a photocatalytic material, but as a simple binary oxide, is has limitations on its optoelectronic properties. In order to improve the visible-light absorption efficiency for photocatalytic applications, the band gap should be further reduced. Moving to ternary compounds can be a strategic method to achieve this, as ternary compounds provide a much greater variety in compositions and hence properties. In this work, the effect of introducing a second metal cation into tungsten oxide is studied by Density Functional Theory (DFT) calculations using the HSE06 functional. The compounds investigated include AWO4 tungstates (A = Sn, Fe), M2WO6 tungstates (M = Bi, Sb), tungstite (WO3·H2O) and hydrotungstite (WO3·2H2O). The band gaps are found to be 2.98 eV (WO3), 1.65 eV (SnWO4), 2.03 eV (FeWO4), 2.83 eV (Bi2WO6), 2.91 eV (Sb2WO6), 2.27 eV (WO3·H2O), 2.51 eV (WO3·2H2O). The band gaps of all the tungstates studied in this work are lower than that of binary WO3. The valence bands of these ternary oxides are formed by the oxygen 2p orbitals and the s orbitals of the second metal cations, which contribute to the band gap narrowing compared to WO3. Structural properties are also strongly correlated with the band energies; the spacing between layers of WO6 octahedra was found to be related to the conduction band edge, with larger spacing causing a shift in the band edge to a more negative potential compared to WO3. The tungstates studied are found to have either a small band gap (SnWO4, FeWO4, WO3·H2O and WO3·2H2O), and hence can potentially give a high efficiency for visible-light absorption, or a more negative conduction band edge than WO3 (Bi2WO6, Sb2WO6), which means they may be able to achieve the overall water splitting. The defect properties of several intrinsic defects in α-SnWO4 are also evaluated; a SnW antisite defect and oxygen vacancy (VO) are found to be the most two probable defects to form in α-SnWO4. Furthermore, SnW defects may lead to an enhancement of the photocatalytic performance under sunlight, while VO are expected to decrease the performance.

  • (2019) Kim, Dohyung
    Thesis
    Visualizing polar and magnetic domains in recent years has become an important topic in the physical research field due to their potential applications in microelectronics such as data storage and photonic devices. Scanning probe microscopy (SPM) as a large group of the microscopy techniques has been a very useful instrument in studying micro to nano sized structures whereby resolution ranges from atomic level to the nanoscale. In this thesis, various techniques based on SPM were used to study piezoelectric, electrical, and magnetic properties of different materials. The materials were chosen based on their potential applications in science and technology. Each chapter in this dissertation is allocated to specific materials. Organic-inorganic metal halide perovskites have gained considerable attention for next-generation photovoltaic cells due to rapid improvement in power conversion efficiencies. In this dissertation, fundamental understanding of underlying mechanisms related to light and bias induced effects at the nanoscale is systematically studied in mixed halide perovskites and single crystal MAPbI3. Firstly, mixed halide perovskites show that periodically striped ferroelastic domains can be modulated significantly under illumination as well as by electric bias. This leads to strain disorder with changed external conditions such as light, bias, and temperature. Secondly, the role of domain walls is explicitly explored in mixed halide perovskites. The results show the observation of enhanced ion migration at the domain walls and its effect in local charge separation and collection. Thirdly, single crystal MAPbI3 is chosen to investigate types of ionic defects for specific (100) and (112) lattice facets. The results reveal significant anisotropic properties. These findings provide fresh microscopic insight for designing more efficient perovskite-based devices. Multiferroics have recently attracted considerable attention due to their unusual properties and for exploiting them in potential applications such as memories, sensors, field-effect transistors, transducers and actuators. First of all, among various multiferroics, iron vanadate (FeV2O4) spinel oxide is considered as an interestingly synthesized material as it undergoes various structural and magnetic phase transitions at low temperature (~below 140 K) with complex spin structures. Here, a cryogenic MFM is used to visualize temperature-dependent magnetic domains of (001)-oriented single crystalline epitaxial iron vanadate thin films. The results show that the magnetic domains are varied as a function of temperature accompanied by domain coarsening and shrinking. Secondly, conductivity of domains and domain walls in polycrystalline BTO ceramics is studied to observe conduction behaviour as a function of temperature. The results reveal distinct temperature-dependent electric conduction at certain grain boundaries and domains. These findings on magnetic and polar domain structures provide the materials to understand underlying mechanism regarding interesting phenomena. In summary, these studies suggest that SPM work provides highly detailed physical properties of the materials and an in-depth insight into novel functional properties in (FAPbI3)0.85(MAPbBr3)0.15, MAPbI3 single crystal and FeV2O4, BaTiO3.

  • (2019) Qin, Yiting
    Thesis
    Recent development of wearable devices in healthcare, wearable and soft robotics has generated increasing demand for memory devices which are more compact with higher data storage capacity and mechanical flexibility, as well as lower fabrication costs. Resistive random access memory (RRAM), which is an emerging technology, has unique advantage of high response speed, low power consumption, and 3D stack architecture. In this research, a sandwich structure (Au/CeO2/Au/Si) based RRAM device has been developed. Scanning electron microscope (SEM), transmission electron microscopy (TEM) were applied to analyze the phase and microstructure for both inks and the thin films. The electrical properties including current-voltage (I-V) response were systematically tested by auto-lab and source meter. A stretching test also was done with a thermoplastic polyurethane (TPU) based device. A suspension ink with controlled shape of CeO2 nanocrystals was prepared. A series of samples have been prepared with solvent, toluene or hexane, by using CeO2 as core material for the ink. The thin film was deposited by spin coating or drop-coating following with plasma treatment. Both silicon and TPU based device have been used as substrates for comparison. Shape controlled CeO2 nanocrystals and printable inks have been successfully fabricated. Different solvent was used to vary the properties of the ink and improve the cracking-free films. A large area cracking-free CeO2 thin film was obtained by plasma processing. Ink jet printing method was also involved as one method of film fabrication. The memory behaviour could be more clearly demonstrated with obvious characteristic from the I-V curve both at silicon and TPU elastic substrates. Thus, the plasma treatment with different treatment time and power could help improve the thin film performance. A simple flexible device had been manufactured with a basic memory behaviour. In summary, the project provided a systematic study on developing metal oxide based nanocrystals towards crack-free thin film and memory devices applications, which may have potential applications in future printed, flexible data storage devices.