The general aims of the work in this thesis are to locate and quantify magnetic dipoles using a Magnetoencephalography (MEG) system based on Superconducting Quantum Interference Device (SQUID) sensors, and to generate various target magnetic fields using magnetic dipoles. MEG provides direct, real-time measurements of magnetic fields at sub-millisecond temporal resolution and femtoTesla sensitivity. It is typically used to describe sources in terms of current dipoles, but here we adapt a different approach and use it to characterise magnetic dipoles. In the first part of this thesis, we describe initial experiments which were carried out in order to demonstrate the feasibility of using the high sensitivity of MEG SQUID sensors to detect extremely small magnetic field shifts due to magnetised samples, and to then locate and quantify the magnetic dipoles. We show that a standard MEG system can be used to measure magnetic field shifts due to susceptibility effects from samples exposed to an Ultra Low Field (ULF), as well as to detect and image the distribution of decaying longitudinal nuclear magnetisation from pre-polarised samples. During our experiments, we also identified a long-lived magnetisation in biological samples, whose magnetisation orientation is fixed by the sample orientation. This finding led us to carry out experiments on samples including human tissue (the hand, wrist, and foot) using MEG, and to characterise the magnetisation behaviour. Even though ULF Magnetic Resonance Imaging (MRI) has several benefits, it is difficult for it to compete with Ultra High Field (UHF) MRI since the higher the field is, the larger does the SNR tend to be. Yet, higher fields increase the effects of intrinsic magnetic susceptibility differences, which in turn leads to field inhomogeneities. Thus, in the second part of this thesis, we aim at improving the quality of high field MR images. We show how magnetic dipoles can be used to generate different target fields that can be used to shim different inhomogeneous magnetic fields at UHF. These magnetic dipoles can be realised using either an array of orthogonal coils or pieces of strongly diamagnetic material.

This thesis reports an investigation of deep level defects in narrow bandgap semiconductors, namely GaAs and GaAsN, and wide-gap GaN materials and devices that have potential applications in photovoltaics and betavoltaic microbatteries. Indeed, for such applications it is of paramount importance to determine the characteristics of the defects present in the materials, which will help understand their effects on the quality of the materials and the performance of devices. In particular, the investigation is done on: (i) a set of GaAs (311)A solar cell structures gown by molecular beam epitaxy (MBE); (ii) dilute GaAsN epitaxial layers containing different nitrogen concentrations grown by MBE; and (iii) betavoltaic microbattery based on a GaN p–i–n homojunction structures grown by metal-organic vapour phase epitaxy (MOVPE) technique using current-voltage (I-V), capacitance-voltage (C-V), deep level transient spectroscopy (DLTS), and Laplace DLTS measurements. The results of this study show that the defects affected significantly the electrical properties of different advanced semiconductor structures and devices. In particular, InGaAs Quantum Wires (QWr) Intermediate Band Solar Cells based nanostructures grown by MBE were studied. The DLTS and Laplace DLTS results showed that the efficiency measurements and external quantum efficiency (EQE) at different temperatures correlated with the appearance of defect peaks in QWr devices in the same temperature ranges. Additionally, this thesis reports the effect of a high dose of gamma (γ-) irradiation on MBE grown dilute GaAsN epilayers with nitrogen concentrations ranging from 0.2 to 1.2% with post-irradiation stability. The DLTS measurements revealed that after irradiation the number of traps either decreased, remained constant, or new traps are created depending on the concentration of nitrogen. Moreover, this thesis reports the effect of beta particle irradiation on the electrical properties of a betavoltaic microbattery based on a GaN p–i–n homojunction with 200 nm and 600 nm thicknesses of undoped layer (i-GaN). The experimental studies demonstrate that, only the sample with thinner i-GaN layer shows the creation of new shallow donor traps upon irradiation on the p-side of the p-i-n junction. While the sample with thicker i-GaN is more resistant to irradiation.

No description available.

Colloidal quantum dots (QDs) allow for the tuning of dopant concentration as well as flexibility in the engineering of the surrounding medium. This thesis explores the use of magnetic resonance techniques and the development of hardware in order to characterize paramagnetically doped materials, in particular Mn-doped PbS colloidal QDs, and assess their potential for applications in quantum technologies such as quantum information processing (QIP). Colloidal PbS:Mn QDs capped with thioglycerol/dithiolglycerol ligands were synthesised in aqueous solution. Methods of tailoring the Mn-Mn and Mn-1H interactions, with the aim of maximizing phase memory times, were investigated. The distance between spins was optimized by initially, overgrowing the QDs with an undoped shell and secondly, by dispersing the QDs in solution. The use of a deuterated solution was found to further reduce the dephasing effects of Mn-1H interactions. This resulted in unprecedentedly long phase memory (TM ~ 8 μs) and spin–lattice relaxation (T1 ~ 10 ms) time constants for Mn2+ ions at T= 4.5 K, and in the observation of electron spin coherence (TM ~ 1 μs) near room temperature. Further improvements to relaxation times, as well as enhanced optical properties useful for the initialization and readout of spin qubits, were also studied by embedding the QDs in photonic crystals. Magnetic resonance techniques combined with paramagnetic Mn-impurities in PbS QDs are used for sensitive probing of the QD surface and environment. We report inequivalent proton spin relaxations of the capping ligands and solvent molecules. We determine the strengths and anisotropies of the Mn-1H spin interactions, and establish Mn-1H distances with ~1 Å sensitivity. These findings demonstrate the potential of magnetically doped QDs as sensitive magnetic nano-probes and the use of electron spins for surface sensing. We explore a means of characterizing mechanisms responsible for the functionality of paramagnetically doped materials. The development of instrumentation to identify and quantify interactions between paramagnetic and ordered magnetic phases is described. A probe was designed and built with a fast response time and with the aim of facilitating fast field jump experiments to identifying interactions between the different magnetic phases by correlating the response of a sample to mw irradiation with its response to a field jump.

In this thesis, the growth of metal-free naphthalocyanine (Nc) and copper naphthalocyanine (CuNc) on both bare Si/SiO2 and octadecyltrichlorosilane (OTS) modified Si/SiO2 surface were studied. The effects of the substrate temperature on morphology and structure of Nc and CuNc thin film growth were presented. For these purposes thin films of Nc and CuNc prepared by thermal vacuum evaporation were studied using atomic force microscopy (AFM) and X-ray diffraction (XRD). We observed that the increase of substrate temperature during growth affects the morphology, preferential molecular orientation and degree of crystallinity of both Nc and CuNc thin film, which were used as active layers in organic field effect transistor (OFET) devices. Organic thin film transistors (OFETs) were fabricated using these molecules as the active layers and their electrical characteristics were measured under both vacuum and atmospheric conditions and they were found to exhibit p-type transistor action. A series of samples of the Nc and CuNc thin films were grown on Si/SiO2 and OTS-modified oxide surface at different substrate temperature but fixed equivalent deposited thickness. The growth conditions, particularly the substrate temperature strongly affect nucleation size and shape of the organic thin film. In general, the thin film morphology shows a near circular grain and elongated grain shape at low substrate temperature, while the thin Nc film shows small needle-like structure and extended needle-like crystalline structures with large gaps at high substrate temperature. The optimum substrate temperature during the growth of Nc on both surfaces is achieved at 200 °C, and this occurs for growth of CuNc at 180 °C and 160 °C on Si/SiO2 and OTS surfaces, respectively, for which the naphthalocyanine thin film shows the best morphological and electrical properties. We used Nc and CuNc thin films prepared at different substrate temperatures as active layers to fabricate bottom and top-contact organic field effect transistors. Their electrical characteristics were measured at room temperature in vacuum and air in the dark. We plotted the output characteristic and transfer characteristic of all OFET devices so that the effects of grain size and crystal structure on the performance characteristic of Nc OFET device could be investigated. Then we studied the effects of hysteresis and charge traps on device performance when exposed to air. We found that the changes generated by exposure of the device to atmosphere may be reversed by annealing the thin film to ∼100 °C in vacuum. We reported the highest mobility of (5.16 ± 0.23) × 10-2 cm2 /Vs for top-contact Nc device prepared at 200ºC on SiO2 after annealing in vacuum, and also we reported the highest mobility of (3.56 ± 0.14) × 10-2 cm2 /Vs for top-contact CuNc device prepared at 180ºC on SiO2 after annealing in vacuum. We found that the top-contact device always performs better than the bottom-contact device. We attributed this to the change of morphology of active layer in the interface between contact metal and SiO2. Solvent induced self-assembly, self-trapping, and self-organizing of c-P30 cyclic porphyrin polymers on the Au surface that are deposited from two solutions and various concentrations in ambient condition was also studied. This results in the arrangement of cyclic polymers in different configurations such as stacking columnar, supramolecular nesting and uniform height hexagonal close packed structure. These conformations are observed using scanning tunnelling microscopy. Highly covered surface stacking columnar like porous array is also observed. We show that toluene:methanol mixture can play a crucial role in self-assembly of supramolecular structure in two dimensions, π-π stacking conformation perpendicular over surface in three dimensions and single in double nested nanoring conformation. Cyclic porphyrin polymers deposited from toluene shows nested nanorings structure, such as single nanoring self-trapped inside a near-circular shape single ring on surface. Diluted solutions using a large volume of methanol relative to the toluene can suppress the adsorption of nanorings to the surface. Interestingly, adsorption of the cyclic polymer from toluene:methanol 3:5 can result in the formation of uniformly height hexagonal close packing on surface, where nanorings aggregate as columnar stacks in two layers, dependent on concentration. Our results show that the self-assembly of artificial cyclic polymers is dependent on solvent and concentration provides a significant step towards control of the three-dimensional arrangement of supramolecular conformation on surfaces using non-covalent interactions.

O objetivo deste trabalho é demonstrar por que a cogeração não se desenvolve no Brasil, mesmo apresentando-se como alternativa para produção de eletricidade. Entretanto existem vários estímulos por parte do Estado para a implantação de grandes termoelétricas no país. Será apresentada a análise dos processos de cogeração, envolvendo: as alternativas tecnológicas, a viabilidade econômica, e as questões ambientais, chegando-se a um quadro indicando a sensibilidade aos diversos parâmetros econômicos que fazem parte dos projetos de cogeração. Serão mostrados cenários com a visão de futuro, indicando diferentes opções que permitam o crescimento do mercado de energia no Brasil, de uma forma mais segura. No momento em que o Brasil passa por profundas mudanças no setor energético, incluindo a necessidade da expansão do parque gerador e considerando que parte importante dessa expansão será através de grandes centrais termoelétricas, torna-se importante apresentar uma alternativa de produção de eletricidade, no Brasil ainda pouco explorada, que é a cogeração. Este trabalho irá mostrar que com maior incentivo aos projetos de cogeração, através de uma política energética nacional adequada, se pode produzir parcela importante da energia elétrica para o atendimento das necessidades crescentes da sociedade brasileira, com menor impacto ambiental e econômico, refletindo em menor custo final da energia produzida. / The aim of this paper is to show why the CHP does not develop in Brazil, even presenting itself as an alternative to producing electricity. However there are several incentives from the state for the deployment of large power plants in the country. You will see the analysis of cogeneration processes, involving: the technological alternatives, the economic viability and environmental issues, coming to a table showing the sensitivity to various economic parameters that are part of cogeneration projects. Scenarios are shown with the vision of the future, indicating different options that allow for growth of the energy market in Brazil, a more secure way. At the moment Brazil is going through profound changes in the energy sector, including the need for expansion of generating facilities and considering that an important part of this expansion will be through large power plants, it is important to present an alternative electricity production in Brazil still little explored, which is cogeneration. This paper will show that with greater encouragement to cogeneration projects through an adequate national energy policy, it can produce substantial amount of electricity to meet the growing needs of Brazilian society, with less environmental impact and economic development, reflecting a lesser cost final energy produced.

The nature of gas and electricity and the methods of distribution, billing and payment all contribute to the 'invisibility' of much domestic energy consumption in industrial and post-industrial societies. For the householder, understanding how to invest and behave in ways that will give affordable comfort with minimum environmental impact involves making sense of a hidden set of processes. This poses a major challenge, one that a range of energy advice programmes is attempting to meet. The main focus to date has been on the actions taken as a result of advice, with little attention paid to teaching and learning processes or to context. This thesis explores formal and informal processes by which householders learn about their energy use in order to develop a theoretical framework. Constructivist learning theory guides the investigation and a variant of the 'conscious competence' model of learning is used as a starting point. The concept of'tacit knowledge' (foundational knowledge, usually acquired informally) is used in tracing the development of energy literacy. Empirical data come from householder surveys and from interviews of householders and advisers in five contrasting locations in the UK. Interpretation of this material demonstrates the construction of meaning through experience and interaction with others, and the potential role of the energy adviser as a trusted and knowledgeable person. The building of tacit knowledge is crucial to the development of energy literacy and the householder's ability to absorb and evaluate new information. Energy advisers need to be able to identify and develop existing knowledge, and to form effective networks with social welfare programmes. The need to build awareness by following up advice wherever possible is stressed. There also needs to be a supportive learning infrastructure that includes easily accessible feedback on consumption, and the availability of accurate information for those who are knowledgeable and confident enough to teach themselves.

Over the last decade, phase measurements derived from gradient echo MRI have increasingly been used as a source of quantitative information, allowing tissue composition and microstructure to be probed in vivo and opening up many new avenues of research. However, the non-local nature of phase contrast and the complexity of the underlying sources of phase variation mean that care must be taken in the interpretation and exploitation of phase information. The work described in this thesis explores the application of phase-based quantitative susceptibility measurements in vivo, and uses theory, experiment, and simulation to investigate the contribution of local structural effects to measurements of MRI signal phase. In initial work, the use of phase imaging and quantitative susceptibility mapping (QSM) is compared in the analysis of white matter lesions in multiple sclerosis, demonstrating in vivo the dipolar distortions inherent in phase images, and the correction of such artefacts through the application of QSM, based on a thresholded k-space division method . Visual analysis of the lesions with a focus on the presence of the peripheral rings that occur in some white matter lesions allows comparison of our data with previous studies. A theoretical description of effects of magnetic susceptibility anisotropy using a susceptibility tensor model is then presented, and its predictions tested using macroscopic phantoms composed of pyrolytic graphite sheet, a highly anisotropic and diamagnetic material. The results of these experiments confirm that the full tensor model must be used to predict the effects of structures composed of such materials on the magnetic field. Finally, Monte Carlo simulation is used to demonstrate the effects of perturber shape and diffusion on the MRI signal phase measured from a volume containing oriented, NMR-invisible, spheroidal perturbers with constant bulk magnetic susceptibility. The rate of phase accumulation over time is shown to be highly dependent on perturber shape and diffusion, and the possible implication of these results on real MRI measurements are discussed.

The focus of this thesis is to produce highly polarised Nuclear Magnetic Resonance (NMR) samples for use in vivo applications. This work focuses on using the brute force method to polarise relevant molecules, for example, 13C labelled pyruvic acid and 13C labelled sodium acetate. The brute force method uses the Boltzmann distribution to polarise a sample by exposing it to large magnetic fields, 15 T, and ultra-low temperatures, ~20 mK. The disadvantage of using this method is the long polarisation time. To counteract the long relaxation times, two sets of relaxation agents were assessed: paramagnetic lanthanides and nanoparticles. Chelated gadolinium is routinely used as a spin-lattice, T1, contrast agent in clinical Magnetic Resonance Imaging (MRI). It is known that when the electron spin flip time is similar to the Larmor frequency, the T1¬ time of the nuclei is reduced. Each lanthanide has a different electron spin flip time, therefore, one lanthanide may be effective at low temperatures. Unfortunately the lanthanides do not prove to be efficient in the millikelvin regime, where the brute force method is at its most effective, so the lanthanides are of limited use. Metals are known to have short T1 times in the millikelvin regime due to the Korringa effect. The conduction electrons of the metal can contribute or absorb energy from nuclei, resulting in a reduction of the T1 of relevant molecules. By having a strong interaction between conduction electrons and the nuclei of interest, it could be possible to reduce the T1¬ of any nuclei of interest. To maximise the contact between the metals and the nuclei, metal nanoparticles were used. Copper and platinum nanoparticle samples are shown to enhance the relaxation rate of nearby protons, however, aluminium and silver nanoparticle samples, which are also expected to be effective, are not. This contradicts the idea that the Korringa effect is the only relaxation mechanism which relaxes the nuclei. The magnetic properties of nanoparticles can be different from their bulk counterpart, therefore, could be contributing to the relaxation of nearby nuclei. It would therefore be advantageous to study the nanoparticle’s magnetisation in a Superconducting Quantum Interference Device (SQUID). Unfortunately, the interpretation of the magnetisation becomes very complicated, as the nanoparticles can react with the solvents. These reactions can result in a 1000-fold increase in the magnetisation of the sample. With the limited magnetic data collected in this work, it is difficult to correlate the nanoparticles magnetic properties with their effectiveness as a T1 relaxation agent.

This thesis describes the growth and characterisation of gallium nitride, indium nitride and indium gallium nitride semiconductors primarily carried out using a novel growth technique called Anion Modulation Epitaxy (AME) and also plasma-assisted MBE (PA-MBE). Characterisation was typically performed by x-ray diffraction, scanning electron microscopy and optical reflectance studies. All of the work in this thesis was carried out in the hope to improve layer structure and quality which in turn would create higher efficiency solar cells. Nanorods were grown using PA-MBE as these are known to form entirely defect-free material and this would be an attractive quality when trying to increase the efficiency. InN rods were grown at temperatures between 350 C and 450 C on SiC substrates of both Si- and C-polar faces at various indium fluxes to establish optimal growth conditions. It was found that a BEP flux of approximately 2x10-7 Torr and a growth temperature approximately 400 C provided a large array of rods. Samples produced tall, thin nanorods as well as short, fat ones. CBED analysis revealed that the tall nanorods were growing In-polar which mimics the behaviour seen for GaN. Photoluminscence (PL) data for the rods agrees with the bulk PL measurement of InN in the literature confirming that reasonable quality films have been produced. Coalescence of the rods was achieved by increasing the flux to 2x10-6 Torr. Also, p-n junctions were grown on both faces of SiC and preliminary tests have shown a response to light. A new growth method was developed from conventional PA-MBE known as Anion Modulation Epitaxy (AME) and gives rise to improved growth compared with equivalent samples by PA-MBE as the growth temperature is decreased. It also allows p-doping for GaN to be carried out at lower temperatures and more consistently. Direct comparison of GaN samples grown at equivalent temperatures by PA-MBE and AME show improved structural, electrical and optical properties for the samples grown using AME. It has also proven to be a useful tool for studying temperature changes at the substrate surface when using any pulsed growth technique. Substrate temperature was shown to vary by approximately 15 C each time the flow was interrupted. Slower, long-term trends were also monitored depending on the average nitrogen to metal ratio. An increase in overall temperature is derived from increasing metal rich growth, whereas the opposite effect is true for increased nitrogen rich growth. AME was also used for the growth of intermediate band solar cells (IBSC). The entire growth is easily monitored and altered using AME without altering the growth parameters drastically. Pulsing the nitrogen allows for variations in the metal cell fluxes to be kept under control at the surface. The discovery of `hidden' metal in the layer would have taken a lot longer to discover, and would have ruined the sample without utilising AME.