<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>The study presented in this thesis was motivated by the large uncertainty on the concentration of atmospheric electrical discharges to the global nitrogen budget. This uncertainty is partly due to the fact that information concerning the <i>NO</i><i>x</i> production efficiency of electrical discharges having current signatures similar to those of lightning flashes is not available in the literature. Another reason for this uncertainty is the fact that energy is used as a figure of merit in evaluating <i>NO</i><i>x</i> production from lightning flashes even though insufficient knowledge is available concerning the energy dissipation in lightning flashes. The third reason for this uncertainty is the lack of knowledge concerning the contribution of discharge processes other than return strokes to the <i>NO</i><i>x</i> production in the atmosphere. Lightning is not the only process in the atmosphere that causes ionisation and dissociation of atmospheric air. Cosmic rays continuously bombard the Earth with high energetic particles and radiation causing ionization and dissociation of air leading to the production of <i>NO</i><i>x</i> in the atmosphere. The work carried out in this thesis is an attempt to improve the current knowledge on the way in which these processes contribute to the global <i>NO</i><i>x</i> production. Experiments have been conducted in this thesis to estimate the <i>NO</i><i>x</i> production efficiency of streamer discharges, laser-induced plasma, laboratory sparks having current signatures similar to those of lightning flashes, alpha particle impact in air and finally with the lightning flash itself. The results obtained from laboratory electrical discharges show the following: (a) The <i>NO</i><i>x</i> production efficiency, in terms of energy, of positive streamer discharges is more or less similar to those of hot discharges. (b) The <i>NO</i><i>x</i> production efficiency of an electrical discharge depends not only on the energy but also on the peak and the shape of the current waveform. (c) The current signature is a better figure of merit in evaluating the <i>NO</i><i>x</i> yield of electrical discharges. As a part of this thesis work a direct measurement of <i>NO</i><i>x</i> generated by lightning flashes was conducted and the results show that slow discharge processes such as continuing currents could be the main source of <i>NO</i><i>x</i> in lightning flashes. Concerning <i>NO</i><i>x</i> production by other ionisation processes such as alpha particle impacts in the atmosphere, the data gathered in this thesis show that each ionising event in air leads to the creation of one <i>NO</i><i>x</i> molecule. In terms of energy the <i>NO</i><i>x</i> production efficiency of alpha particles is similar to that of electrical discharges. The theoretical studies conducted within this thesis indicate that M-components contribute more than the return strokes to the <i>NO</i><i>x</i> production. The calculations also show that the contribution to the global <i>NO</i><i>x</i> budget by return stroke is not as high as that assumed in the current literature.</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>

This thesis concerns three-dimensional finite element modelling of Biot's equations for elasto-acoustic modelling of wave propagation in layered media including porous elastic materials. The concept of hierarchical (p-version) finite elements are combined with various weak forms of Biot's equations. Computationally efficient methods providing accurate solutions of sound propagation in layered porous media are discussed. The research falls within the areas linear acoustics and numerical acoustics. Important applications of the developed methods may be found within vehicle interior acoustics, e.g. engineering design of damping treatment based on multiple layers of porous materials. / QC 20100618

In several x-ray related areas there is a need for high-precision elements for x-ray generation and focusing. An elegant way of realizing x-ray related elements with high precision and low surface roughness is by the use of microfabrication; a combination of semiconductor processing techniques and miniaturization. Photolithographic patterning of silicon followed by deposition, etching, bonding and replication is used for batchwise fabrication of small well-defined structures. This thesis describes microfabrication of a miniature x-ray source and a refractive x-ray lens. A miniature x-ray source with diamond electrodes has been tested for x-ray fluorescence. Another version of the source has been vacuum encapsulated and run at atmospheric pressure. One-dimensionally focusing saw-tooth refractive x-ray lenses in silicon, epoxy, and diamond have been fabricated and tested in a synchrotron set-up. Sub-micron focal lines and gains of up to 40 were achieved. The conclusion of the thesis is that the use of microfabrication for construction of x-ray related components can not only improve the performance of existing components, but also open up for entirely new application areas.

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.

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. 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. 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>Resonant-cavity-enhanced (RCE) photodetectors have recently attracted attention due to their wavelength selectivity and high efficiency in comparison to conventional photodetectors. The goal of this ongoing research initiative is to develop a Si-based RCE infrared photodetector using inductively coupled plasma chemical vapor deposition (ICP-CVD) as the primary fabrication method. At the current stage of the project, wavelength-selective optical structures have been successfully fabricated using Si/SiO₂ layer pairs. These structures demonstrate sharp transmission peaks at their intended wavelength, making them potentially useful for efficient photodetection. The next phase of the photodetector development process involves using ion implantation to introduce dopants and create the bias.The project also explores the temperature sensing capability of the resonant-cavity structures. The temperature sensitivity tests indicate that the specific type of structure fabricated in this project may be relevant for fiber-optic temperature sensing applications. Additional testing is required to evaluate the performance characteristics of such structures as Fabry-Perot sensors capable of wavelength-encoded temperature measurement.</p> / Master of Applied Science (MASc)

<p>Single junction silicon photovoltaic cells (SJSPVCs) are unable to transform all the energy in the solar spectrum into electricity, due to the broad nature of the solar spectrum and the limits imposed by a single bandgap. Furthermore, high surface recombination velocity reduces the SJSPVC external quantum efficiency response, particularly to ultraviolet photons. It is the goal of spectral engineering to optimize the light that is incident on the cell, by down-shifting high energy photons to lower energies, for example, to improve the performance of photovoltaic cells.</p> <p>This thesis represents a study into the luminescence of ion implanted films, involving silicon nanocrystals (Si-NCs) and rare-earth ions in fused silica or silicon nitride. Quantum efficiency measurements taken with an integrating sphere were used to characterize some of the samples. Other photoluminescence (PL) characterization work was carried out with a single-wavelength laser and a collection lens normal to the sample. Variable angle spectroscopic ellipsometry (VASE) was used to estimate the optical constants of the implanted films. In secondary work, Rutherford backscattering spectrometry, time-dependent PL, infrared-PL measurements, and electrical conductivity measurements were used to characterize select samples.</p> <p>It was found that the conversion efficiency of Si-NCs in fused silica was about 1% – too low to be useful according to modeled results. However, considerable variation in the peak wavelength of the Si-NC PL was obtained, depending on the peak concentration of implanted silicon. Si-NC-type PL was also produced by low-energy implantation of oxygen into a Czochralski silicon wafer.</p> <p>Oxygen was also implanted into films of cerium-doped high-purity silicon nitride, and it was shown that the photoluminescence from these films is largely dependent on the level of oxygen doping. The internal conversion efficiency of a cerium-doped fused silica sample was found to approach 20%, which indicates that this is a promising avenue for future research.</p> <p>Finally, energy transfer was demonstrated between Si-NCs and erbium ions. The lifetime of the erbium PL appears to increase with increasing implanted silicon fluence.</p> / Master of Applied Science (MASc)

<p>An ammonium polysulfide (NH₄)₂S_x solution was optimized through a series of experiments to be used for surface passivation of III-V nanowires . The effectiveness of sulfur passivation was investigated by measuring the photoluminescence from p-InP nanowires before and after passivation. The optimized parameters included solvent type, molarity and passivation time. According to the experiments, passivation of nanowires in 0.5 M solution diluted in isopropyl alcohol for 5 min produced the maximum photoluminescence improvement. It was also demonstrated that the whole surface passivation of vertical nanowires in ensemble samples caused a 40 times increase in the photoluminescence intensity while top surface passivation of individual nanowires resulted in a 20 times increase of photoluminescence intensity. A model was developed to calculate the photoluminescence from single nanowires under different surface recombination and surface potential. The model showed that the 40 times increase in the photoluminescence is mainly due to the reduction of surface state density from 10¹² cm^-2before passivation to 5×10¹⁰ cm^-2after passivation.</p> <p>The effect of sulfur passivation on core-shell p-n junction GaAs nanowire solar cells has been investigated. The relative cell efficiency increased by 19% after passivation.</p> / Doctor of Philosophy (PhD)