This thesis describes a theoretical and numerical study of quantum transport and optical effects through an array of self-assembled InAs quantum dots grown in the intrinsic region of a GaAs p-i-n junction. We present a numerical simulation of this system and compare the generated transport and elecroluminescence results to recent experimental data. The simulation first calculates the quantum tunnelling, excitonic recombination, and relaxation rates within the dots, and then uses a stochastic model to simulate carriers entering and leaving the array. We highlight a number of features within the simulation, which shed light on similar features seen in experimental data. In particular, we demonstrate the importance of including the effects of the Coulomb interactions between the carriers, as this is shown be necessary for the simulated and experimental results to match closely. We also investigate a model of Auger processes which is shown to produce up-conversion luminescence, and study the effect of varying the location of the array within the intrinsic region. Additionally we present a master equation approach, which we use to describe a correlated tunnelling regime, in which the Coulomb interaction between an electron and a hole forces them to tunnel alternately onto a single dot before recombination. We produce current and photon noise predictions for both tunnelling and recombination limited regimes. We also investigate this phenomena for a pair of interacting dots, and find a number of two dot configurations which are able to produce current and electroluminescence. We present current and photo-current rate predictions for each case, and associated current and photon noise results.

This thesis describes the design, construction and testing of a dome coil. The dome coil is hemispherical in shape and is intended to be used within a set of hemispherical gradient coils in a seven tesla magnetic resonance imaging magnet. The dome coil has eight independent elements and is designed to be used for parallel transmission and reception. It is shown that the dome coil produces less specific absorption rate than a conventional birdcage coil and is suitable for head imaging. A study of travelling wave magnetic resonance imaging is also presented. In this study two different methods of generating a travelling wave (using a patch antenna and an end-fire helix antenna) are compared, and the power requirements and specific absorption rate of the travelling wave approach are considered. It is concluded that travelling wave magnetic resonance imaging is best performed using a local receive coil and with the travelling wave antenna placed at least 50 cm from the subject. All theory relevant to the design, construction, testing and use of these coils and antennas is also presented.

The work presented in this thesis describes the development and implementation of a number of ideas and methods that allow fMRI to be carried out using echo-planar imaging at ultra high field strength, despite the significant problems associated with this. In the first study, EPI is used to probe how the gradient echo (GE) and spin echo (SE) BOLD responses relate to the underlying neurological processes, whilst the brain is in both its active and resting states. These finding show that SE BOLD contrast is harder to detect but less localised to areas around large draining veins than GE BOLD contrast and thus potentially more localised to sites that represent true functional areas of activation. The second study describes how dynamic delta B0 mapping can be performed during fMRI experiments with a hyperoxic challenge in order to assess the magnitude and extent of delta B0 effects that arise due to susceptibility differences between air and tissue. Developing on this, this work describes the steps involved in the design and implementation of a dual echo GE/SE EPI sequence and how it can be used to enable off-resonance effects, such as image distortion and signal concentration/dilution, to be corrected on a dynamic basis for, simultaneously acquired, GE and SE data. The final study demonstrates how such a sequence can be used to detect resting state networks. Showing that the correspondingly low temporal separation of the GE and SE data allows GE and SE BOLD contrast mechanisms to be compared in a number of novels ways in different resting state networks.

Hyperpolarised 3He has been used extensively in the neutron community and for magnetic resonance imaging (MRI) of the lungs. 129Xe has many desirable advantages compared to 3He for NMR, namely its chemical sensitivity, solubility, adsorption capability and atmospheric abundance. In this work we present the design and construction of a 129Xe spin exchange polariser that operates at gas pressures between 0.6 and 3 bar. The design incorporates a novel Volume Holographic Grating (VHG) spectrally narrowed, high power laser diode device as the optical pump source for a continuous gas flow cylindrical polarisation cell. Nuclear polarisations of 34 +/- 3 % are achieved in 129Xe gas. Magnetisation production factors in excess of 80 % cm^3 min^-1 are achieved using a pseudo batch-flow operation method. Major parameters affecting 129Xe gas polarisation are discussed and the first measurements of cell heating from laser light absorption by Rb vapour are made. Hyperpolarised 129Xe gas produced from our system is imaged inside a low field, whole body MRI scanner. NMR measurements from inside a human lung are made using hyperpolarised 129Xe. Preliminary work on dissolved solution NMR is presented as are studies on the feasibility of using hyperpolarised 129Xe for porosity characterisation of porous media in low magnetic fields.

The signal-to-noise ratio available in Magnetic Resonance Imaging (MRI)is determined by the static magnetic field strength, causing a continued drive toward higher fields to enable faster image acquisition at finer spatial resolution. The work in this thesis is primarily concerned with the development of sequences for Ultra High Field Magnetic Resonance Imaging (7T) which allow the acquisition of images with high spatial resolution for study of the structure and function of the brain. The methods developed here for high spatial resolution structural imaging allow the identification of regions of the cortex which exhibit layers of high myelin concentration within the cortical strip. This permits the investigation of the correspondence of functional regions in the visual cortex to their underlying structure 'in vivo'. A robust methodology for high resolution functional mapping over a restricted field of view is presented and the results of fMRI studies demonstrating 1 mm isotropic resolution in the primary somatosensory cortex S1 using this methodology are shown. BOLD responses to vibrotactile digit stimulation were investigated using a travelling wave paradigm to measure the topographic representation of the digits in S1 and an event related paradigm for characterization of the haemodynamic delay. A spin-echo EPI acquisition has been optimized and tested to compare the BOLD response in GE and SE echo planar images by employing visual and motor tasks. The specificity of the BOLD responses of SE and GE data was found to be similar using a travelling wave paradigm.

In this thesis the underlying mechanisms behind the phenomenon known as the BOLD response were investigated. Functional imaging of the brain relies on the oxygenation level of blood, therefore the relaxation properties of blood were measured at different oxygenations. The relaxation properties of blood are also modified by introduction of a paramagnetic contrast agent and are dependent on the static magnetic field of the MRI system. These dependencies were also characterised. This led to the discovery that the transverse relaxation rate is non-linearly related to contrast agent concentration. This result was confirmed by performing a Monte-Carlo simulation. A measurement of total cerebral blood volume (CBV) change, during neural activity, was performed at high temporal resolution (TR = 300 ms). This was achieved by infusing a contrast agent, whilst a visual stimulus was presented to the volunteer. This technique also provided an interesting and novel method to test models of the BOLD response. The temporal characteristics of the BOLD response were differentially effected by the contrast agent, reflecting the dependency of each feature on blood volume change. This allowed a qualitative understanding of the volume contribution to each of these characteristics to be gathered. Two models of the BOLD haemodynamic response were constructed to describe the contrast agent infusion experiment. Each model was developed from several existing models of the BOLD response, with the aim of comparing empirical and biomechanical model elements. As part of this work an existing model of BOLD signal change was extended to include the arterial and venous vasculature. Both models were separately fitted to the experimental total CBV and BOLD signal data. The results of fitting the data show that existing haemodynamic models cannot fully describe the measured results.

The increased signal-to-noise ratio available at high magnetic field makes possible the acquisition of clinically useful MR images either at higher resolution or for quantitative methods. The work in this thesis is focused on the development of quantitative imaging methods used to overcome difficulties due to high field MRI systems (> 3T). The protocols developed and presented here have been tested on various studies aiming at discriminating tissues based on their NMR properties. The quantities of interest in this thesis are the longitudinal relaxation time T1, as well as the magnetization transfer process, particularly the chemical exchange phenomenon involving amide protons which is highlighted particularly well at 7T under specific conditions. Both quantities (T1 and amide proton transfer) are related to the underlying structure of the tissues in-vivo, especially inside the white matter of the brain. While a standard weighted image at high resolution can provide indices of the extent of the pathology, a robust measure of the NMR properties of brain tissues can detect earlier abnormalities. A method based on a 3D Turbo FLASH readout and measuring reliably the T1 in-vivo for clinical studies at 7T is first presented. The other major part of this thesis presents magnetization transfer and chemical exchange phenomena. First a quantitative method is investigated at 7T, leading to a new model for exchange as well as contrast optimization possibility for imaging. Results using those methods are presented and applied in clinical setting, the main focus being to image reliably the brain of both healthy subjects and Multiple Sclerosis patients to look at myelin structures.

pintronics is a rapidly developing field in solid state physics based on the quantum property of spin angular momentum. It has the potential to offer a new generation of electronic devices exploiting spin properties instead of, or in addition to, charge. Such quantum-based devices are expected to demonstrate significant advantages over traditional charge based electronics with a promise of faster data processing speeds and lower power consumption. One of the most widely studied spintronic materials is the dilute magnetic semiconductor gallium manganese arsenide ((Ga,Mn)As). This continues to be a valuable test ground for spintronics applications due to its close relation to the traditional, and well-characterised, semiconductor GaAs, and its relatively high Curie temperature despite values remaining some way off the much sought-after room temperature. The two primary focuses of this thesis are phase-coherent transport and critical phenomena, both of which whilst well understood in metals have seen limited work in (Ga,Mn)As. Critical behaviour in particular has not been extensively studied despite continued disputes over theoretical models and resistance peak positions relative to Curie temperature. Studies of both these areas are presented within this thesis split over four main chapters. The first of these chapters acts as a general introduction to spintronics, and includes both a brief history of the subject, and a theoretical overview focused on the structure and properties of (Ga,Mn)As. This introductory chapter also includes an in-depth review of nanofabrication including typical processing techniques and their applications to the study of spintronics in Nottingham. The second chapter presents a comprehensive study of critical phenomena within (Ga,Mn)As, showing how the behaviour of magnetic properties close to Tc are strongly correlated between samples. Both magnetisation and susceptibility are found to demonstrate behaviour very close to that predicted by the Heisenberg model; a result in strong agreement with theoretical work. The study of critical behaviour is carried over into the third chapter with transport measurements showing that resistance data can be directly used to accurately measure sample Curie temperature by finding the peak in the derivative deltaR/deltaT. This potentially offers an alternate approach to calculating Tc that is faster and cgeaper than the more conventional magnetometry or Arrott plot methods. Analysis is also carried out on the resistance peak which is expected to follow the critical behaviour of the specific heat. The final experimental chapter focuses on the development of nanoring fabrication processes in (Ga,Mn)As including the difficulties associated with fabricating nanoscale structures, the testing performed to achieve high quality, reproducible structures, and the final adopted recipe. This chapter then details early test measurements on these devices including an initial study on the first structures within a dilution refrigerator, and prelimenary work on a second improved batch at 4He temperatures. This work will act as a foundation for the future aim of conducting a full phase-coherence phenomena study in highly optimised (Ga,Mn)As samples grown in Nottingham.

Scanning probe microscopy techniques were employed to investigate C60 molecules adsorbed on Si(111)-(7x7) and Ag-Si(111)-(√3x√3)R30o using imaging, spectroscopy, and manipulation methods. First, dynamic scanning tunnelling microscopy revealed the lowest unoccupied molecular orbital features of C60 molecules adsorbed on Si(111)-(7x7) with extremely high resolution at 77 K. Experimental data were compared with Hückel molecular orbital theory simulations to determine the orientation of the molecules on these surfaces. Second, C60 molecules were imaged with a qPlus atomic force microscope, in the attractive force regime and appeared as bright spherical protrusions. The potential energy of interaction between the AFM tip and C60 molecules adsorbed on Si(111)-(7x7) was quantified by force spectroscopy. Furthermore, a C60 molecule was transferred to the scanning probe microscope tip and used as molecular probe to image the Si(111)-(7x7) surface and other C60 molecules. The on-tip C60 molecule was imaged with high precision. Hückel molecular orbital theory calculations accurately predicted the shape and characteristics of molecular orbitals observed with dynamic scanning tunnelling microscopy, which were strongly dependent on molecular symmetry, orientation, and adsorption angle. Using qPlus atomic force microscopy, chemical reactivity was probed close to or at the carbon atom positions in the C60 cage. Density functional theory simulations showed that an (iono)covalent bond formed between a carbon atom and the underlying Si adatom was responsible for contrast formation. The pair potential for two C60 molecules was also determined experimentally and found to be in very good agreement with the Girifalco potential (Girifalco, L.A., J. Phys. Chem., 1992. 96(2): p. 858). Using Hückel molecular orbital theory, the mutual orientation of a C60 molecule adsorbed on the STM/AFM tip and a C60 molecule adsorbed on the Si(111)-(7x7) surface was determined via comparison of simulated images to the experimental data. Individual C60 molecules were also manipulated with qPlus atomic force microscopy. Manipulation of single C60 molecules was performed on the Ag-Si(111)-(√3x√3)R30o surface using scanning tunnelling microscopy at room temperature and at 100 K. The interaction was predominantly attractive. Due to weak molecule-substrate interaction, a short-range chemical force between the C60 molecule and the tip was considered to be responsible for the manipulation process.

The relationship between meal structure and composition can modulate gastrointestinal processing and the resulting sense of satiety. This applies also to the fat component of meals and particularly to the surface area available for digestion. The main hypothesis underpinning this thesis work was that fat emulsion droplet size has a profound effect on fat digestion and, in turn, on the gastrointestinal and satiety responses. To test this hypothesis two fat emulsion meal systems were used. They had exactly the same composition but a small (termed the Fine emulsion, with a droplet size of 400 nm) or a large (termed the Coarse emulsion, with a droplet size of 8 μm) emulsified fat droplet size. The two fat emulsion systems were manufactured and characterised using a range of bench techniques, in vitro digestion models and MRI techniques in vitro. The difference in microstructure caused different temporal creaming characteristics for the emulsions and different percentage hydrolysis profiles in a gastric digestion model in vitro. The Fine emulsion showed initial rapid hydrolysis whilst the Coarse emulsion showed an initial slow hydrolysis phase with the hydrolysis rate increasing at later stages. This indicated that there was indeed a droplet size effect on fat hydrolysis whereby the smaller droplet size with a larger surface area hydrolysed faster than a larger droplet size. The emulsions’ performance was finally tested in vivo in healthy volunteers using MRI in a series of pilot studies leading to a main physiological study. Creaming differences in the gastric lumen were addressed by redesigning the meals using a locust bean gum (LBG) thickener that made them stable throughout the gastric emptying process. A main three-way physiological and satiety study in healthy volunteers showed that a highly emulsified, intragastrically stable emulsion delayed gastric emptying, increased small bowel water content and reduced consumption of food at the end of the study day. Finally, magnetic resonance imaging, relaxometry and spectroscopy were further evaluated to assess fat emulsion parameters in vitro and in vivo in the gastric lumen. Main static magnetic field and droplet size effects on T2 relaxation times of the Fine and the Coarse emulsions were observed. There was reasonable correlation between m-DIXON and spectroscopy methods to quantify fat fraction both in vitro and in vivo. Differences in T2 relaxation times for different droplet sizes of 20% fat emulsions were detected in vitro. These changes were however difficult to separate from creaming effects in vivo with a view of drawing meaningful inferences on droplet sizes. The main conclusion from this work was that manipulating food microstructure especially intragastric stability and fat emulsion droplet size can influence human gastrointestinal physiology and satiety responses and that MRI and MRS provide unique non invasive insights into these processes. This improved knowledge could help designing foods with desired health-promoting characteristics which could help to fight the rising tide of obesity.