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.

Photovoltaic cells and fluorescence sensing are two important areas of research in chemistry. The combination of photon-activated electron donors with electron acceptors provides a strong platform for the study of optical devices. A series of four oligothiophene-ruthenium complexes has been synthesized. Variation in oligothiophene length and bipyridine substitution allowed comparison of these variables on electronic properties. The longer oligothiophenes display lower energy absorption and emission compared to the shorter ones. Aromatic conjugation appears more complete with para-, rather than meta-, substitution. Oligothiophenes and Ru(bpy)32+ are highly fluorescent individually, but fluorescence is quenched when connected. Bonds of carbon to fluorine are among the strongest single bonds. Single bonds between carbon and hydrogen are also very strong and are ubiquitous. The ability to manipulate these bonds is of great interest to chemists. Two tungsten metal complexes, [6 (perfluorophenyl)bipyridyl] tetracarbonyltungsten and [6-(phenyl)bipyridyl]tetracarbonyltungsten, were prepared for mechanistic C-F and C-H bond activation studies, respectively. These compounds were synthesized through Stille and Suzuki coupling of commercial reagents. Ligands were then bound to tungsten to form the tetracarbonyl complexes.

This dissertation combines laboratory experiments and analysis of field samples to examine tungsten (W) geochemistry. Data from low ionic strength experimental solutions at room temperature containing between 0.01 M to 0.0002 M total sulfide and 0.0027 M - 0.0001 M tungstate were analyzed using UV/VIS spectrophotometry. Stability constants have been determined for the formation of mono-thiotungstate log K01= 3.43 ± 0.61, di-thiotungstate log K12 = 3.02 ± 0.61, tri-thiotungstate log K23 = 2.82 ± 0.02, and we estimated the tetra-thiotungstate log K34 ~ 2.34. Analysis of W, Mo, Mn, and Fe concentrations in estuarine surface and pore waters and sediments captured environmental samples from oxic and sulfidic conditions. Both surface waters and sediments demonstrated a positive correlation between W and Fe. Unlike Mo, which was depleted in sulfidic salt marsh pore waters, W was enriched in all pore waters in comparison to overlying waters. Thermodynamic modeling of W and Mo thioanion species in sulfidic pore water samples predicts ≤ 50% of tungstate (WO42-) forms thiotungstate species and complete conversion of molybdate (MoO42-) to tetrathiomolybdate (MoS42-). Unlike tetrathiomolydate that is known to be more particle reactive than molybdate, increases in dissolved W coincide with increases in dissolved sulfide in pore waters, suggesting thiotungstates are less particle reactive than thiomolybdates at circum-neutral pH. Finally, sediment analysis suggests sequestration of W is dependent on surface water salinity in the intermediate marsh sediments, and long-term W entrapment occurs in sulfidic salt marsh sediments. / acase@tulane.edu

<p>SiC is an attractive material since it has remarkable properties. For several years efforts have been put primarily in electronic applications. High power and high frequency devices can be fabricated on SiC due to its wide band gap, high breakdown field and high thermal conductivity. SiC devices can be used in harsh environment since its operation temperature is significantly high (about 1200 ). SiC bulk growth has been improved by seeded physical vapour transport (PVT) during last decades. However, the quality and doping concentration of SiC bulk are not good enough to be used as an active layer for devices. SiC epilayer growth by chemical vapour deposition (CVD) was established in the last three decades. Only about 5 µm/h growth rate is achieved by CVD with a standard process. Long deposition time is required to grow ≥100µm thick epilayer for high voltage devices. The main problem in standard CVD is the formation of silicon (Si) droplets due to supersaturation of Si-species on the growth surface or in the gas-phase, which is detrimental for devices performance. To solve the problem of Si-droplets, chloride-based CVD was introduced. Chlorinated species can dissolve the silicon aggregates through the formation of strong bonds to silicon species compared to Si-Si bonds. Typical chlorinated precursors are hydrogen chloride (HCl) and methyltrichlorosilane (MTS). In this thesis study, HCl was mainly used as chlorinated precursors. Distinct chlorinated precursors result in different chemical reactions which affect the epilayer growth appreciably. The Cl/Si ratio, which is the ratio of the amount of chlorinated precursors to silicon precursors, is a very critical growth parameter for morphology, growth rate and background doping concentration. The C/Si ratio and Si/H₂ ratio also affect the epilayer growth appreciably. Besides, growth temperature, growth pressure and temperature ramp up condition are other important growth parameters. In the CVD reaction chamber, the temperature profile and gas species distribution are not uniform along the whole susceptor length, which leads to different thickness of epilayer, morphology and doping concentration at different area of the reaction chamber. The polarity and off-angle of substrates can bring about complete different grown epilayers. Epitaxial defects are mainly replicated from the substrate. Therefore, the quality of substrates is very important as well. Deep energy levels can be introduced by adding transition metal such as vanadium (V), chromium (Cr) or tungsten (W). There are some limits which are needed to be overcome for a complete development of SiC. 4” SiC wafers are commercially available on the market, larger diameter would be very useful for the industrial development of SiC. High growth rate and good quality with controlled uniformity are desired for electronic applications. In this thesis, the influences of growth parameters such as C/Si and Cl/Si ratios, comparison between different precursors, growth condition in different areas of reaction chamber and effects of substrate polarity are discussed. Intentional incorporation of tungsten atoms is investigated by deep-level transient spectroscopy measurement and thermodynamic analysis.</p>

<p>Tungsten trioxide is a material with a variety of application areas. For example, the material is used within thin film technologies as electrochromic material in smart windows, as electrochemically functional material in thermal control applications or as active layer in gas sensing application. Metal-oxide semiconductor gas sensors are of significant interest to detect toxic and hazardous gases. The use of small and cheep sensors is preferable since a large number of sensors easily can be placed at different sites to monitor the concentration of different species without involving huge investments.</p><p>In this work, WO₃ nanoparticle films were produced using an advanced gas deposition unit for gas sensing applications. The structure of the WO₃ nanoparticle films was determined using X-ray diffraction, neutron scattering, X-ray photoelectron spectroscopy, elastic recoil detection analysis and electron microscopy. The as deposited films consist of sub-stoichiometric WO₃ and exhibit a large degree of porosity, which together with the small particle size of about 5 nm results in a large surface area and therefore excellent prospects for gas sensor applications. </p><p>Investigations on the optical properties and temperature dependence of the resistance indicate hopping conduction in the WO₃ films. The bandgaps for tetragonal and monoclinic WO₃ were found to be direct, which is in accordance with band structure calculations.</p><p>Sensor properties were investigated using resistance measurements upon test gas exposures. The experiments were performed at fixed operating temperatures as well as on temperature modulated sensors. The films of WO₃ showed excellent sensitivity to H₂S gas and selectivity to other gases. The responses of temperature modulated sensors were further analyzed using mathematical transformations and pattern recognition methods whereby different gases could be distinguished.</p><p>We also present a sensing technique using conduction noise as a tool for detection of alcohol vapor. The relative change of the noise, due to the inserted alcohol, can be as large as two orders of magnitude. </p>

<p>Laser-induced chemical vapor deposition (LCVD) is a process enabling the deposition of solid material from a gas phase in the form of free-standing microstructures with high aspect ratios. The deposition rate, wire diameter, and material properties are sensitive to changes in temperature and gas pressure. Through experimentation these dependencies are clarified for carbon and tungsten-coated carbon microhelices to be used as heating elements in cold gas microthrusters for space applications. The integration of heaters into the thruster will raise the temperature of the gas; thus, improving the efficiency of the thruster based on specific impulse.</p><p>Deposition rate is measured during the fabrication process, and the geometrical dimensions of the spring are determined through microscopy analysis. By experimentally measuring the spring rate, material properties such as shear modulus and modulus of elasticity for LCVD-deposited carbon can be determined as a function of process parameters. </p><p>Electrothermal characterization of carbon and tungsten-coated microcoils is performed by resistively heating the coils and measuring their surface temperature and resistance in atmospheres relevant to their operating environments. Through high-resolution microscopy analysis, sources having detrimental effects on the coils are detected and minimized. The results gained from these experiments are important for efforts in improving the performance of cold gas microthrusters.</p>

<p>Nanoparticles of iron, cobalt and tungsten oxide were synthesised by photolytic laser assisted chemical vapour deposition (LCVD). An excimer laser (operating at 193 nm) was used as an excitation source. The LCVD process, was monitored <i>in situ</i> by optical emission spectroscopy (OES). The synthesised particles were further analysed using transmission electron spectroscopy (TEM), X-ray diffraction (XRD), high resolution scanning electron microscopy (HRSEM), X-ray fluorescence spectroscopy (XRF), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy.</p><p>Iron and cobalt single crystalline nanoparticles were synthesized using ferrocene and cobaltocene precursors. The diameter of the particles could be tailored by the experimental parameters (e.g., partial pressure and laser power) and were in the range 1 - 50 nm in diameter. In both cases, the particles were covered by a carbon shell, typically 7 nm thick. A thin graphitic layer was observed at the interface metal-carbon. Amorphous carbon was deposited on top of the graphitic carbon. Particle temperature, reaching the boiling point of the respective metal, was observed by OES of the thermal emission during the laser-induced particle formation process (and subsequent heating). Both bcc and fcc Fe phases were formed, both hcp and fcc for the Co phases. Size dependent magnetic properties were observed using superconducting quantum interference device (SQUID) measurements, where super-paramagnetic magnetic domains dominated for <i>d</i> < 10 nm. The iron particles were further processed, whereby the amorphous shell was removed by refluxing in nitric acid. In a subsequent step, the graphitic surface was functionalized by attaching an octyl ester, rendering the particles hydrophobic.</p><p>Tungsten oxides were synthesized from combinations of WF₆/H₂/O₂ as precursors. No particles could be deposited if H₂ was removed from the gas-mixture. The as-deposited oxide nanoparticle film was amorphous. A monoclinic WO₃ particle film could be achieved by annealing the amorphous oxide. Above 400°C, the oxide particles increased in size from ca. 20 nm to 60 nm through coalescence. The gas-sensing properties of the tungsten oxide were tested by conductance measurements using H₂S as analyte. The sensitivity of the amorphous oxide nanoparticle film was found to be superior to that of a crystalline oxide nanoparticle film. </p>

Tungsten carbide-based metal matrix composite coatings are deposited by PTAW (Plasma Transferred Arc Welding) on production critical components in oil sands mining. Homogeneous distribution of the reinforcement particles is desirable for optimal wear resistance in order to reduce unplanned maintenance shutdowns. The homogeneity of the coating can be improved by controlling the heat transfer, solidification rate of the process and the volume fraction of carbide. The degree of settling of the particles in the deposit was quantified using image analysis. The volume fraction of carbide was the most significant factor in obtaining a homogeneous coating. Lowering the current made a modest improvement in homogeneity. Changes made in other operational parameters did not effect significant changes in homogeneity. Infrared thermography was used to measure the temperature of the surface of the deposit during the welding process. The emissivity of the materials was required to acquire true temperature readings. The emissivity of the deposit was measured using laser reflectometry and was found to decrease from 0.8 to 0.2 as the temperature increased from 900C to 1200C. A correction algorithm was applied to calculate the actual temperature of the surface of the deposit. The corrected temperature did increase as the heat input of the weld increased. A one dimensional mathematical model of the settling profile and solidification of the coatings was developed. The model considers convective and radiative heat input from the plasma, the build-up of the deposit, solidification of the deposit and the settling of the WC particles within the deposit. The model had very good agreement with the experimental results of the homogeneity of the carbide as a function of depth. This fundamental model was able to accurately predict the particle homogeneity of an MMC deposited by an extremely complicated process. It was shown that the most important variable leading to a homogeneous coating is to operate at the packing saturation limit of the reinforcement. In the case of the MMC explored, a fully homogeneous coating was obtained with 50 vol% WC in a NiCrBSi matrix. / Materials Engineering

The ability to detect and quantify presence and concentration of unknown gasses is sought for applications ranging from environmental monitoring to medical analysis. Metal oxide based chemical sensing technology currently exists but the ability to provide a compositional gas breakdown reliably within a short time frame is not readily available. A very small sensor that can differentially identify the type and concentration of a gas is required. Novel methods of creating low cost and easily tuned one and two-dimensional gas sensing elements are explored. Tungsten trioxide has been thoroughly documented as an electrochromic coating, but highly sensitive WO3 elements with beam and nanowire structures have yet to be explored. Research of WO3 as a gas sensor encompasses three major components: A suitable sensing chamber with accurate analyte gas flow control and temperature control, a reliable method for WO3 deposition, and a high yield fabrication process. This thesis explores all three of these technologies. Chapter two starts with a summary of existing tungsten trioxide fabrication methods. An overview of WO3 processing follows. A comprehensive setup was designed and created to test the gas sensing response of a series of metal oxide based resistive elements through conductimetric analysis. Chapter three provides an in depth account of gas sensor test chamber design and testing. Critical test chamber aspects such as temperature control, precise gas flow control, highly efficient analyte gas switching and ease of use are presented. Chapter four outlines WO3 electrodeposition and the fabrication of beam structures for testing, while chapter five explores the templated electrodeposition of WO3 segments intercalated between gold nanowire segments. Finally, chapter six provides a summary of the research presented in this thesis as well as future directions and options available for further exploration of WO3 gas sensing elements. / Micro-Electro-Mechanical Systems (MEMS) and Nanosystems

Tungsten trioxide is a material with a variety of application areas. For example, the material is used within thin film technologies as electrochromic material in smart windows, as electrochemically functional material in thermal control applications or as active layer in gas sensing application. Metal-oxide semiconductor gas sensors are of significant interest to detect toxic and hazardous gases. The use of small and cheep sensors is preferable since a large number of sensors easily can be placed at different sites to monitor the concentration of different species without involving huge investments. In this work, WO3 nanoparticle films were produced using an advanced gas deposition unit for gas sensing applications. The structure of the WO3 nanoparticle films was determined using X-ray diffraction, neutron scattering, X-ray photoelectron spectroscopy, elastic recoil detection analysis and electron microscopy. The as deposited films consist of sub-stoichiometric WO3 and exhibit a large degree of porosity, which together with the small particle size of about 5 nm results in a large surface area and therefore excellent prospects for gas sensor applications. Investigations on the optical properties and temperature dependence of the resistance indicate hopping conduction in the WO3 films. The bandgaps for tetragonal and monoclinic WO3 were found to be direct, which is in accordance with band structure calculations. Sensor properties were investigated using resistance measurements upon test gas exposures. The experiments were performed at fixed operating temperatures as well as on temperature modulated sensors. The films of WO3 showed excellent sensitivity to H2S gas and selectivity to other gases. The responses of temperature modulated sensors were further analyzed using mathematical transformations and pattern recognition methods whereby different gases could be distinguished. We also present a sensing technique using conduction noise as a tool for detection of alcohol vapor. The relative change of the noise, due to the inserted alcohol, can be as large as two orders of magnitude.