Image quality is degraded by involuntary movement of the subject in an MRI scanner. It is fairly challenging in MRI of the brain to monitor the involuntary head movement accurately. Though there are a few techniques to monitor head movement of the subject for prospective motion correction, it is still an unsolved problem in MRI. In this study, head movement inside an MR scanner is monitored via measurement of changes in the voltage induced in head mounted coils by switched magnetic field gradients. The motion of a rigid body such as the human head is decomposed into two components: namely translation and rotation. There are three degrees of freedom (DOFs) for translational motion i.e. translation along the x, y and z axes and three rotational degrees of freedom for rotational motion i.e. rotation about the x, y and z axes. Head movement is monitored in a gradient field by measuring the change in induced voltage in head mounted coils. To calculate the change in induced voltage I follow two approaches: circular loops simulation, analytical as well as numerical calculations. I show that by using a standard method one can form a linear model to identify the position and orientation of the coils. An experimental arrangement is set up to check the validity of the analytical and numerical calculations. Experiments carried out with a rig of five coils verified that the changes in induced voltage in the coils is linear with respect to the changes in position of the coils. The linear model is also verified by comparing estimated positions obtained by using the coils to those found by image realignment of fast field echo (FFE) images using Statistical Parametric Mapping (SPM). We experimentally evaluate the new approach for monitoring head movement inside an MR scanner, which exploits the linear variation of the voltages induced in a set of coils by time-varying magnetic field gradients with respect to small changes in position/orientation. This approach was tested by attaching five coils to a structured agar phantom and a healthy volunteer's head. The results suggest that it is possible to estimate the position and orientation with 0.22 mm and 0.24˚ root-mean-square error using this set-up. The new approach could be used for prospective or retrospective motion correction. An experiment is also carried out by using free running EPI (Echo Planar Imaging) to track the head movement inside an MR scanner. There is a strong relation between head movement and EPI waveforms, the central point of the experiment is to track the head displacements via measuring induced voltage in the coils by using EPI waveforms during execution of free running EPI. The results obtained from the experiment reveal that the method is promising.

Research into antiferromagnetic materials for application in spintronics has rapidly expanded in recent years. The prediction and observation of spin based phenomena with antiferromagnets as the active components, has expanded the field and there is a need for high quality materials that are compatible with existing III-V semiconductor systems to expand this research. Copper manganese arsenide is one such material and will be the subject of this thesis. Early studies had shown that this material grows epitaxially on both gallium arsenide and gallium phosphide substrates by molecular beam epitaxy. This thesis builds on this early work by further characterising CuMnAs, improving the techniques used to grow it, and enhancing our understanding of the material. A key result of this thesis is that the Néel temperature of CuMnAs can be studied using temperature dependent transport measurements. This method allows for a range of layer thickness (from between 5 and 140 nm) to be studied. We find that the Néel temperature of CuMnAs is suppressed by around 100K when the layer thickness is less than 10nm. At the thicknesses studied there is agreement (around (480±5)K) with the more established neutron diffraction technique for measuring Néel temperature, which was also used to determine the magnetic structure of the CuMnAs studied. In addition to measurement of the Néel temperature of CuMnAs, a detailed study is made in this thesis of the ideal growth conditions for ultrathin (sub 10nm) films of CuMnAs. Post-growth examination of ultrathin layers of CuMnAs showed that significant portions of material were missing due to poor adhesion. This thesis shows the results of the development of several different nucleation and growth methods, which were used to improve the adhesion of the CuMnAs layer to the substrate. These methods are evaluated using atomic force microscopy, x-ray diffraction, magnetometry and transport measurements. CuMnAs has previously shown to strongly prefer growth under stoichiometric conditions, as non-stoichiometric conditions have tended to favour the formation of clusters of the excess material. In excess Mn conditions these clusters are ferromagnetic MnAs inclusions that are conducting and contribute to the magnetic behaviour. This thesis presents the results of a simulation study of the conductivity of ferromagnetic elements in a non-ferromagnetic medium. This approach could be extended to allow the number of inclusions in a CuMnAsl layer to be approximated from transport measurements. Finally, this thesis will also look at the effects of alloying CuMnAs with phosphorous. This reduces the lattice constants of the material while retaining the same crystal and magnetic structure. In thick films of the alloy the Néel temperature increases from that of CuMnAs.

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.

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.

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.

This thesis presents a study of the electron-phonon interaction in an n-doped weakly coupled semiconductor superlattice (SL). Two experiments were performed which studied different aspects of this interaction. Firstly, a coherent phonon optics chip was designed. This was used in an experiment where a phonon beam was passed through the SL while an electrical bias was applied to it. The experiment provided a sensitive measurement of the effects caused by bias in the SL on the phonon beam. Secondly, a train of strain pulses was passed through the SL and the charge transferred in the device due to the strain was investigated. A coherent phonon optics chip was formed using a semiconductor superlattice as a transducer structure and a p-i-n photodiode as a coherent phonon detector on the opposite side of the substrate. The doped weakly coupled superlattice structure, which is the main subject of investigation in this thesis was grown between the transducer and detector structures. Optical access mesas were processed on both sides of the substrate to allow the application of bias to both the doped superlattice and the p-i-n structures. A photocurrent pump-probe experiment was then performed using a femtosecond laser to excite the transducer structure and activate the detection mechanism. The application of bias to the weakly coupled SL was found to cause a small attenuation to the 378 GHz phonon beam passing through it. An investigation of the possible causes of this attenuation ruled out several trivial explanations, suggesting that it was caused by the interaction between electrons and phonons in the structure. The active control of phonon amplitude by electrical means has not previously been demonstrated and may offer exciting new prospectives for phonon devices and experiments. The coherent phonon optics technique was shown to be very sensitive and it will be a useful technique to increase our understanding of future acousto-electric devices. The electrical signal that acoustic excitation caused in the SL device was investigated using a pulse shaping technique in combination with an amplified femtosecond laser. A Fabry-Perot cavity was used in the laser path to create a train of equally spaced laser pulses with an adjustable pulse spacing. Focusing these pulses on an aluminium film transducer creates a train of equally spaced acoustic pulses simulating a monochromatic acoustic wave packet. The SL was processed and electrically contacted so that the charge transferred through it due to the acoustic pulse train could be monitored using a 12.5 GHz-bandwidth digital oscilloscope. The variation in charge transfer seen as a function of the DC bias applied to the device and as a function of the total energy of the acoustic pulse train was investigated. The behavior was compared to a theoretical model developed in the style of previous theories of electrical conversion in SLs excited by electromagnetic waves. The dependencies of the charge transfer on the bias and energy of the pulse train were well reproduced in the theory. The theory predicted that magnitude of the signal in the superlattice was independent of the frequency of the acoustic pulse train. This was verified by measuring the frequency dependence of the signal seen for a variety of transducer films. The frequency dependencies seen were well explained through simulations presuming the device response was independent of train frequency. This confirms the predictions of the theory. Both the experiments detailed in this thesis have helped increase our understanding of the nature of electron-phonon interactions in superlattices. It is hoped that a fuller understanding of these interactions may be instrumental in the creation of exciting new acousto-electrical devices.