Strongly correlated oxides are studied widely for the host of unique applications, such as hightemperature superconductivity, colossal magneto resistance, exotic magnetic, charge and orbital ordering, and insulator-to-metal transitions. Transitional metal oxides which form the majority of the correlated oxide systems and oxides of Vanadium, especially VO2 and V2O5 are the two most favourite systems among researchers for several applications. In this thesis, the growth and characterization of VO2 and V2O5 are discussed along with a special focus on the optical property, especially thermochromic properties. Traditionally SMT behaviour and Infrared reflectively was the focus area for VO2 research, and its only until recently that VO2 is being treated as a much more complex system and investigated as highly responsive naturally disordered metamaterial near the phase transition temperature where the material exhibits semiconducting and metallic phase co-existence. Since each phase of VO2 has a distinct optical and electrical properties, controlling the extent of phase transitions by accurate temperature modulation, enables exploitation of the material for new properties like emissivity modulation in the NIR region and for creating IR visible reversible and rewritable patterns. V2O5 is traditionally seen as a high TCR material and regarded as material of choice for application ranging from catalysis, gas sensors to lithium batteries. In this study, however we focus on the optical properties of the material, especially the visible range thermochromic nature of V2O5 coatings synthesised by oxidative annealing of MOCVD grown VOx coatings. The impact of doping and selective oxygen vacancy generation on the thermochromic property are discussed

Ice formed by water vapour deposition has been identified in different terrestrial environments: 1) in the atmosphere; 2) at the ground’s surface; 3) in caves; 4) in seasonally frozen ground; and 5) in perennially frozen ground (permafrost). Thus far, ground ice formed by diffusion and deposition of vapour in soils (types 4 and 5) has rarely been studied in a natural setting and remains one of the most poorly described ice types on Earth. This thesis focuses on the dynamics of deposition and sublimation of atmospheric water vapour into permafrost and the isotopic signature (D/H and 18O/16O) of the emplaced ground ice under different experimental conditions. Ground ice was produced in sediments with different thermo-physical characteristics (glass beads, JSC Mars-1 simulant). After a two-month growth period, the higher porosity sediments (JSC) had more than 7x the gravimetric water content than the lower porosity soil. Ground ice profiles had a distinct concave downwards shape due to the decrease in saturation vapour pressure with depth. Results also indicate that vapour deposited ground ice has a distinct δD-δ18O composition that plots near regression slope value of 8. Pore water isotopes plot below the global meteoric water line (GMWL) when the source of moisture is directly on top of the sediments. If an air gap is introduced between the source of moisture and the sediments, the pore water isotopes shift above the GMWL due to re-sublimation at the ground surface. Overall, this thesis addressed some fundamental knowledge gaps required to better understand the growth and isotopic evolution of ground ice emplaced by vapour deposition.

>Magister Scientiae - MSc / Perovskite solar cells have gained tremendous attention within the past decade, due to its rapid improvement in power conversion e ciency (PCE), with the current record cell at 25%. The aim of this study is to create a repeatable and scalable chemical vapour deposition technique that can be used to construct perovskite solar cells with a high PCE while maintaining long-term stability. The technique requires the formation of a uniform and compact lead halide layer, either PbI2 or PbCl2 that is sequentially converted into the perovskite structure with the exposure of Methylammonium iodide (MAI) vapour. The use of CVD with a 5 cm diameter quartz tube was successfully used to deposit uniform thin lms of both PbI2 and PbCl2 over an area of 6 cm2 with a thickness deviation of 5%. Thickness control was obtained by varying the amount of source material which allows for repeatable control within 5% error, without the need for a crystal thickness monitor.

No abstract provided. / Thesis / Master of Engineering (MEngr)

Increase in demand for uniform ceramic coatings on larger industrial components have led to a need for a PP-MOCVD coating system scale up. The objective of this thesis is to develop a fully functional coating system operating in the PP-MOCVD regime that is able to deposit thin film ceramic coatings on commercial or industrial components with complex 3D geometries. This can be achieved by applying engineering and vacuum science theories, coupled with the established fundamentals of PP-MOCVD. A larger system was designed and assembled around the boundaries set by the dimensions and geometry of a stainless steel water pump impellor acting as the base substrate. Most of the components were sourced off the shelf from vacuum and fluid specialists. Components which were unavailable for various reasons were designed, and machined in-house by the departmental workshop. Initial test depositions were conducted using small stainless steel disk substrates, heated using a resistive heater similar to the one utilised on the research scale system. The test depositions were performed with the heater and substrate combination placed in strategic locations. This is to test the overall uniformity of precursor flux in the chamber volume. The resulting coating uniformity on the disk surfaces were fair but problems such as the large collection of unreacted precursor on the chamber viewport and valve timing issues had to be addressed. Before making any improvements to the system, each of the process areas leading to a successful deposition needed to be understood. Five process areas were developed: ‘Liquid Delivery’, ‘Atomization’, ‘Evaporation’, ‘Transport and Reactor Geometry’, and ‘Droplet Management’. Each of the process areas were analysed individually and changes were made to push for a maximum evaporation efficiency. xviii The improved system provided opportunities to perform depositions that were once not possible for PP-MOCVD. Two sets of deposition tests were designed and conducted. Firstly, the improvements were justified with a series of depositions using flat stainless steel plates with dimensions 65x65x5mm. The other set of 3D case study depositions involve observing the effects of the operational parameters of PP-MOCVD on the uniformity and penetration depths of the coatings into different sized macro blind trenches. Five geometric setup conditions were used to justify the improvements made to the system. These are: ‘Substrate positioned in the direct line of spray’, ‘Use of an unheated receptor’, ‘Use of a heated receptor’, ‘Use of an unheated receptor with a non-axial substrate setup’, and “Choked Flow’. As expected, the uniformity of the coatings on both sides of the plate varied significantly when the substrate is placed over the line of sight of the precursor spray. Similarly, the coating produced under the induced choked flow condition resulted in low conformality. The introduction of an unheated receptor plate resulted in an increase in uniformity on both sides of the plate. Further prove that PP-MOCVD is geometry independent is provided by the deposition made with the non-axial substrate placement resulting in a coating of similar result to the unheated receptor. The use of a heated receptor provided a source for a secondary evaporation of the larger precursor droplets collected resulting in an increase in coating thickness while maintaining good conformality. The effects of temperature, pressure, injection volume, and concentration were explored in the final case study. With maximum depths of 50mm, the macro blind trenches has an aspect ratio of 1:1 and cross-sectional areas of 3x3mm, 9x9mm, and 15x15mm. The final results show that as the temperature rises, the depth penetrated into the trench decreases. This could be due to the change in rate limiting steps as homogeneous reactions begin to increase at higher temperatures. Similar trends were observed with increasing pressure. As the pressure difference between the volume of the trenches and the rest of the chamber decreases, the push needed to xix force the precursor down the trench also decreases, resulting in less depth penetration. The effects of injection volume and concentration observed, can be explained by how much precursor molecules are present during one pulse cycle. The more that is available at any given time, the more likely a reaction will occur and deeper the penetration will get. Of course a ceiling or a limit exists where the molecules in the chamber will get evacuated without being reacted. The future work made possible as a result of the scaled up system are proposed. These include a scale up of the operational parameters to suit any given substrate geometry, improvements to the heating source to achieve greater thermal uniformity, further improvements to the overall system accessibility, and performing other depositions using different substrate materials and precursor types.

The Physical Vapour Deposition (PVD) magnetron sputtering process is one of the widely used techniques for depositing thin film coatings on substrates for various applications such as integrated circuit fabrication, decorative coatings, and hard coatings for tooling. In the area of coatings on cutting tools, tool life can be improved drastically with the application of hard coatings. Application of coatings on cutting tools for various machining techniques, such as continuous and interrupted cutting, requires different coating characteristics, these being highly dependent on the process parameters under which they were formed. To efficiently optimise and customise the deposited coating characteristics, PVD process modelling using RSM methodology was proposed. The aim of this research is to develop a PVD magnetron sputtering process model which can predict the relationship between the process input parameters and resultant coating characteristics and performance. Response Surface Methodology (RSM) was used, this being one of the most practical and cost effective techniques to develop a process model. Even though RSM has been used for the optimisation of the sputtering process, published RSM modelling work on the application of hard coating process on cutting tool is lacking. This research investigated the deposition of TiAlN coatings onto tungsten carbide cutting tool inserts using PVD magnetron sputtering process. The input parameters evaluated were substrate temperature, substrate bias voltage, and sputtering power; the out put responses being coating hardness, coating roughness, and flank wear (coating performance). In addition to that, coating microstructures were investigated to explain the behaviour of the developed model. Coating microstructural phenomena assessed were; crystallite grain size, XRD peak intensity ratio I111/I200 and atomic number percentage ratio of Al/Ti. Design Expert 7.0.3 software was used for the RSM analysis. Three process models (hardness, roughness, performance) were successfully developed and validated. The modelling validation runs were within the 90% prediction interval of the developed models and their residual errors compared to the predicted values were less than 10%. The models were also qualitatively validated by justifying the behaviour of the output responses (hardness, roughness, and flank wear) and microstructures (Al/Ti ratio, crystallographic peak ratio I111/1200, and grain size) with respect to the variation of the input variables based on the published work by researchers and practitioners in this field. The significant parameters that influenced the coating hardness, roughness, and performance (flank wear) were also identified. Coating hardness was influenced by the substrate bias voltage, sputtering power, and substrate temperature; coating roughness was influenced by sputtering power and substrate bias; and coating performance was influenced by substrate bias. The analysis also discovered that there was a significant interaction between the substrate temperature and the sputtering power which significantly influenced coating hardness, roughness, and performance; this interaction phenomenon has not been reported in previously published literature. The correlation study between coating characteristics, microstructures and the coating performance (flank wear) suggested that the coating performance correlated most significantly to the coating hardness with Pearson coefficient of determination value (R2) of 0.7311. The study also suggested some correlation between coating performance with atomic percentage ratio of Al/Ti and grain size with R2 value of 0.4762 and 0.4109 respectively.

This work mainly investigates the use of aerosol assisted chemical vapour deposition (AACVD) process to fabricate thin film materials for energy efficient glazing and thin film solar cells applications. Ga-doped ZnO thin films were firstly deposited onto glass substrates by AACVD of zinc and gallium acetylacetonates in methanol. After optimizing the doping concentration, film thickness and heating temperature, ZnO:Ga coatings with high visible transparency (> 80 %) and infrared reflection (up to 48.9 % at 2500 nm) were obtained, which is close to the optical requirements for commercial energy saving glazing. Pyramid-shaped and plate-shaped zinc oxides films were then deposited on glass substrates by AACVD of zinc-acetate-dihydrate, acetic acid and deionized water in methanol. These surface-textured ZnO films exhibited good visible transparency (~70 %), low sheet resistance (~60 Ω sq-1) and ultra large haze factor (up to 98.5 %), which is the most hazy ZnO ever reported and can be potentially used as the front contact in thin-film solar cells. Finally, uniform compact CH3NH3PbI3 perovskite films with high phase purity and micron-sized pinhole-free grains were deposited on glass substrates by a novel two-step and three-step sequential AACVD process. In conclusion, AACVD shows a great potential for the scalable fabrication of ZnO-based and organometallic halide-based thin film materials.

This work investigates the use of the novel electric field assisted chemical vapour deposition (EACVD) process in the production of titanium dioxide thin films for photocatalytic applications on glass substrate. This work looks into the interaction of applied electric fields with the precursor species during the aerosol assisted chemical vapour deposition (AACVD) and atmospheric pressure chemical vapour deposition (APCVD) reaction of Titanium isopropoxide (TTIP) and Titanium (IV) Chloride (TiCl4) with different solvents. The electric field was generated by applying a potential difference between two fluorine-doped tin oxide glass sheets. The electric field was varied between 0 – 3000 Vm-1. The deposited films were analysed and characterized using scanning electron microscopy, X-ray diffraction, Raman spectroscopy, atomic force microscopy, UV-vis spectroscopy, water-contact angles and resazurin photcatalytic testing. It was observed that the application of electric fields produced changes in the morphology, particle size, growth rate, crystal orientation and crystal phases. Generally, films produced under the influence of the electric fields showed higher photo-activity than films produced in absence of electric fields. The deposited films produced from the electric field assisted aerosol chemical vapour deposition (EAACVD) showed higher photo-activity with applied AC electric fields than with applied DC electric fields. Likewise, they showed higher photo-activity than the deposited films produced from the electric field assisted atmospheric pressure chemical vapour deposition (EAAPCVD) with applied AC electric fields. The results obtained were explained by the interaction mechanisms between the electric fields and the precursor species, which differ depending on the CVD technique used. Although titanium dioxide photo-activity is comprised by a combination of factors, it was observed that an optimum can be obtained by varying both experimental conditions and field strength. In particular, optimum results were obtained for deposited films which showed long-shaped particles, reduced particle size and high preferential orientation in the anatase (004) plane. Electric field assisted chemical vapour deposition (EACVD) shows a great potential for the improvement of commercial products available in the market such as self-cleaning and antibacterial surfaces.

Solid state synthesis of layered, rotationally disordered intergrowths consisting of rock salt (MX) and hexagonal (TX2) constituents in various sequences [(MX)1+δ]m[TX2]n is carried out by developing structural and compositional prototypes of the desired product, using fine control of the elemental reactants and then annealing at low temperature to facilitate self-assembly. (M = Sn, La, Eu; T = V, Mo.) The remarkable rotational disorder in these systems - in contrast to traditional misfits - and their proven applications in thermal, electrical and thermoelectric disciplines make them a useful group of materials for demonstrating control of reaction pathways of solid state reactions using low temperatures and short times. The synthesized materials are structurally characterized using X-ray diffraction (XRD), X-ray reflectivity (XRR), and Scanning Transmission Electron Microscopy (STEM). Electrical characterization is carried out on patterned samples using the Van der Pauw method of resistivity and the Hall effect method. Composition of the samples is determined using wavelength dispersive electron probe microanalysis (EPMA). Time domain thermoreflectance is used to determine the cross plane thermal conductivity. The family of [(SnSe)1.05]m(MoSe2)n (m = n = 1, 2, 3, 4), which possess the same composition but different unit cell thicknesses, shows that there is no correlation between c-axis unit cell thickness and cross plane thermal conductivity. The family of structural isomers [(SnSe)1.05]4[MoSe2]4, [(SnSe)1.05]3[MoSe2]3[(SnSe)1.05]1[MoSe2]1, [(SnSe)1.05]3[MoSe2]2[(SnSe)1.05]1[MoSe2]2, [(SnSe)1.05]2[MoSe2]3[(SnSe)1.05]2[MoSe2]1,[(SnSe)1.05]2[MoSe2]1[(SnSe)1.05]1[MoSe2]2[(SnSe)1.05]1[MoSe2]1 and [(SnSe)1.05]2[MoSe2]2[(SnSe)1.05]1[MoSe2]1[(SnSe)1.05]1[MoSe2]1 have the same c-axis lattice thickness and absolute composition but have different numbers of [(SnSe)1.05]/[MoSe2] interfaces. Thermal conductivity studies carried out on these showed no correlation with the interface density. (LaSe1-x)1.17(VSe2-y)n (n = 2, 3, 4) feature a family of compounds that self-assemble at higher than usual temperatures. They form non-stoichiometric moieties with unique structural proclivities including La vacancies and V interstitials compared to other ferecrystals or previous misfits. The designable electrical properties show evidence of charge transfer. (EuSe)1+δ(VSe2)n (n = 1, 2, 3) is a family of materials that complements the investigation of Ln-based ferecrystals. They show evidence of multiple M oxidation states. These compounds highlight the use of rational design of structure and composition to tune properties. This dissertation includes previously published and unpublished co-authored material. / 10000-01-01

The production of large area, high quality two-dimensional (2D) materials using chemical vapour deposition (CVD) has been an important and difficult topic in contemporary materials science research, after the discovery of the diverse and extraordinary properties exhibited by these materials. This thesis mainly focuses on the CVD synthesis of two 2D materials; bilayer graphene and monolayer tungsten disulphide (WS2). Various factors influencing the growth of each material were studied in order to understand how they affect the quality, uniformity, and size of the 2D films produced. Following this, these materials were combined to fabricate 2D vertical heterostructures, which were then spectroscopically examined and characterised. By conducting ambient pressure CVD growth with a flat support, it was found that high uniform bilayer graphene could be grown on the centimetre scale. The flat support provides for the consistent delivery of precursor to the copper catalyst for graphene growth. These results provide important insights not only into the upscaling of CVD methods for growing large area, high quality graphene and but also in how to transfer the product onto flexible substrates for potential applications as a transparent conducting electrode. Monolayer WS2 is of interest for use in optoelectronic devices due to its direct bandgap and high photoluminescence (PL) intensity. This thesis shows how the controlled addition of hydrogen into the CVD growth of WS2 can lead to separately distributed domains or centimetre scale continuous monolayer films at ambient pressure without the need for seed molecules, specially prepared substrates or low pressure vacuum systems. This CVD reaction is simple and efficient, ideal for mass-production of large area monolayer WS2. Subsequent studies showed that hexagonal domains of monolayer WS2 can have discrete segmentation in their PL emission intensity, forming symmetric patterns with alternating bright and dark regions. Analysis of the PL spectra shows differences in the exciton to trion ratio, indicating variations in the exciton recombination dynamics. These results provide important insights into the spatially varying properties of these CVD-grown TMDs materials, which may be important for their effective implementation in fast photo sensors and optical switches. Finally, by introducing a novel non-aqueous transfer method, it was possible to create vertical stacks of mixed 2D layers containing a strained monolayer of WS2, boron nitride, and graphene. Stronger interactions between WS2 on graphene was found when swapping water for IPA, likely resulting from reduced contamination between the layers associated with aqueous impurities. This transfer method is suitable for layer by layer control of 2D material vertical stacks and is shown to be possible for all CVD grown samples, a result which opens up pathways for the rapid large scale fabrication of vertical heterostructure systems with large area coverage and controllable thickness on the atomic level.