This thesis reports the optical properties of InAlAs QDs and InGaAsN QWs grown by Molecular Beam Epitaxy (MBE) on both the conventional (100) and high Miller index surfaces. InAlAs QDs on AlGaAs matrix are grown by MBE on the conventional (100) and non-(100) GaAs substrates using different growth conditions, namely, growth temperature, different confinement barriers, and amount of deposited material. PL measurements revealed differences in the optical properties that are caused by substrate orientation effects. The PL emission energies of QDs grown on high Miller index surfaces such as (311)A and (311)B are found to be strongly dependent on the atomic terminated surface [A (Ga face) or B (As face)] of the substrate. The QDs grown on (311)B plane show superior optical properties over QDs grown on (311)A and (100) planes. The optimum structure to achieve the highest optical efficiency of QDs emitting in the visible red part of electromagnetic spectrum ( ̴ 666 nm) consisted of 4.4MLs Al0.35In0.65As/Al0.45Ga0.55As QDs grown on (311)B plane at a growth temperature of 550 0C. In addition, a further investigation was carried out to study the effect of post-growth thermal annealing on the optical properties of InAlAs QDs grown on (100), (311)A, and (311)B planes. A noticeable enhancement of the PL intensity at 10 K for all planes was observed by increasing the annealing temperature up to 700 0C. Thermal annealing of (311)A InAlAs/GaAlAs QDs resulted in a negligible blue shift, while a large blue shift was observed from (311)B and (100) QDs. This is explained by the smaller size of QDs, smaller strain, and lower In segregation from (311)A GaAs orientation. PL and Transmission Electron Microscopy (TEM) have been used to investigate the optical and structural properties, respectively, of In0.36Ga0.64As1-yNy/GaAs double quantum wells (QWs) grown both on the conventional (100) and non-(100) GaAs substrates. These include In0.36Ga0.64As1-yNy/GaAs QWs with three different compositions of nitrogen, namely, 0%, 1%, and 2%. QWs grown on (311)A GaAs plane show higher nitrogen incorporation over all the other planes. TEM measurements show that (311)B QWs have inferior structural properties than QWs grown on (311)A and (100). TEM images demonstrated that the (311)B QWs interfaces are undulated and not uniform. In contrast QWs grown on (311)A and (100) display very uniform and very flat interfaces. The effect of thermal annealing on the optical properties of In0.36Ga0.64As1-yNy/GaAs double QWs grown on different planes was investigated for two sets of samples having 0% and 1% nitrogen. It was found that annealing at 700 0C for 30 seconds is the optimum annealing temperature which improves the PL efficiency for all QWs. The PL enhancement is larger in samples with 1% nitrogen than 0%.

In this thesis photovoltaic and photocatalytic water-splitting dye complexes have been studied adsorbed onto the rutile TiO2(110) surface. The photovoltaic dye-sensitizer N3 (cis-bis(isothiocyanato)bis(2,2’-bipyridyl-4,4’-dicarboxylato)-ruthenium(II)) was studied along with Ru 455 (cis-bis(2,2’-bipyridyl)-(2,2’-bipyridyl-4,4’-dicarboxylic acid) ruthenium(II)) and Ru 470 (tris(2,2’-bipyridyl-4,4’-dicarboxylic acid) ruthenium(II)) which have very similar chemical structures. Dipyrrin-based dye complexes PY1 bis(5-(4-carboxyphenyl)-4,6-dipyrrin)bis(dimethylsulfoxide)Ruthenium(II)) and PY2 (bis(5-(4-carboxyphenyl)-4,6-dipyrrin)(2,2’-bipyridine) Ruthenium(II)) were also studied which should have different bonding geometries on the TiO2 surface. A single centre water-splitting dye complex (aqua(2,2’-bipyridyl-4,4’-dicarboxylic acid)-(2,2’:6’,6”-terpyridine) Ruthenium(II)) was studied along with a chloride containing analog ((2,2’-bipyridyl-4,4’-dicarboxylic acid)-(2,2’:6’,6”-terpyridine)chloride Ruthenium(II)). The molecules studied here would have been damaged using traditional UHV deposition techniques so electrospray deposition was used to deposit intact molecules in situ for experiments in UHV. Adsorption geometries of the molecules on the TiO2 surface were investigated using experimental photoemission data supported by density functional theory (DFT) calculations. Dipyrrin-based dye complexes were found to bond with both available carboxylic acid groups to the TiO2 surface. Also the results suggest that Ru 470 is most likely to bond to the TiO2 surface with a different bonding geometry to other bipyridine-based complexes with very similar chemical structures. The molecular orbitals of the dye complexes were investigated using near-edge x-ray absorption fine structure spectroscopy (NEXAFS). DFT calculations provided possible spatial distributions of the molecular orbitals involved in charge transfer. Energetic alignments were performed using data from visible light spectroscopy to compare energetics for core and valence-hole excitation. The core-hole clock implementation of resonant photoemission spectroscopy was used to measure upper limits on the timescale of charge transfer from the excited adsorbate molecules to the TiO2 surface. The results show charge transfer timescales mostly within the low-femtosecond timescale. The Ru 470 complex was found to be relatively slow at charge transfer possibly due to the different bonding geometry it appears to adopt on the TiO2 surface.

Lattice mismatched heterostructures grown by Interfacial Misfit (IMF) technique, which allows the strain energy to be relieved both laterally and perpendicularly from the interfaces, are investigated. However, electrically active defects are created at the interface and away from the interface with energy levels deep in the bandgap of the host materials. These defects dramatically affect the optical and electrical properties of the devices. In this thesis, an investigation of deep level defects is carried out on GaSb/GaAs uncompensated and Te compensated heterostructures grown by the IMF method using DLTS, Laplace DLTS, I-V, C-V, C-F and C-G-F measurements. Furthermore, the effect of thermal annealing treatments on the defect states is also studied on both types of samples. It was found that the well-known EL2 electron trap is commonly observed near to the interface of both uncompensated and Te compensated GaSb/GaAs IMF samples. However, several additional electron defects are detected in Te compensated samples. Rapid thermal annealing performed on uncompensated samples resulted in the annihilation of the main electron trap EL2 at a temperature of 600 oC. On the other hand rapid thermal annealing and conventional furnace annealing were carried out on Te compensated samples, and it was observed that rapid thermal annealing process is more effective in terms of defects reduction. The density of interface states is determined from C-G-F and forward bias DLTS measurements. Te compensated samples exhibit the highest density of interface states and have additional hole traps as compared to uncompensated samples. The electrical properties of p-i-n GaInAsSb photodiodes grown on uncompensated and Te compensated GaSb/GaAs templates on GaAs substrates using special growth mode are investigated. The non-radiative defects which could have detrimental effects on the performance of these photo diodes are studied here for the first time. Both electron and hole defects are detected, and their capture cross-section measurements reveal that some of defects originate from threading dislocations. The double pulse DLTS measurements are performed and the concentration distributions of the detected defects are determined.

Dynamic nuclear polarization (DNP) is a technique for enhancing the sensitivity of nuclear magnetic resonance (NMR) experiments by increasing the polarization of the nuclear spins. DNP experiments are often characterized by the enhancement which is achieved, and the enhancement build-up rate. These parameters are strongly influenced by electronic and nuclear relaxation processes. The purpose of the work presented in this thesis was to develop hardware and analysis methods for studying relaxation in electron paramagnetic resonance (EPR) and NMR. Two novel probes for combined NMR and longitudinally detected EPR experiments have been built. The first of these probes is designed for low temperature NMR, EPR and DNP experiments, with the main focus being relaxation studies in typical low-temperature DNP conditions. The second probe is designed for high temperature cryoporometry studies of lignin degradation. Relaxation data in magnetic resonance often exhibits multi-exponential decay. An algorithm for performing a Laplace inversion of multi-exponential relaxation data, and extracting distributions of time constants, is described. The algorithm, which is based on Tikhonov regularization, uses a uniform penalty and a zero-crossing penalty to stabilize the solution, but does not use a non-negativity constraint, so relaxation spectra with both positive and negative peaks can be produced. Two experiments to study relaxation and other dynamic processes in samples containing organic radicals have been performed. The first of these was designed to study saturation, relaxation and spectral diffusion processes in EPR during continuous microwave irradiation. The second experiment measured relaxation of DNP-enhanced nuclear polarization, and revealed an offset dependence of the relaxation behaviour. Both experiments were performed using the low-temperature probe, and the data was analysed using the Laplace inversion algorithm.

This thesis describes the development and applications of quantitative MRI and combined EMG and MRI study of Brachial Plexus. The protocols developed in this thesis have been used on normal healthy subjects, aiming at characterizing the tissues based on their MR and EMG parameters. The Brachial Plexus is the upper portion of the peripheral nervous system and controls the movements of shoulder and arms. Neurological disorders in the brachial plexus can result from cervical spondylotic neuropathy due to compression of nerve roots exiting from vertebra or compression of the spinal cord due to bulging discs. MRI provides the opportunity to obtain precise information on the location of these disorders and to provide quantitative biomarkers. EMG in the form of the distribution of F-latency (DFL) is a recently introduced nerve conduction parameter that can detect functional symptoms with such disorders. To study the brachial plexus the diffusion weighted MRI with body signal suppression (DWIBS) technique was used to highlight the nerves from surrounding tissues. This technique was then used to investigate the diffusivity of water molecules in the peripheral nerve axon. The diffusion time dependency of the diffusion coefficient was used to study the presence of restricted diffusion in the brachial plexus. A clear reduction of the apparent diffusion coefficient was observed with long diffusion times and confirmed the restricted diffusion in nerves and cord. The T2 relaxation was used to investigate the properties of intercellular and intracellular space in peripheral nerves. Diffusion weighting dependency of T2 and echo time dependency of apparent diffusion coefficient (ADC) was observed in initial studies. The magnetisation transfer (MT) and z-spectra were used to study macromolecular characteristics and exchange mechanisms. Asymmetry in z-spectra both for nerves and spinal cord was observed, this relates to possible detection of the nuclear overhauser effect (NOE) in the brachial plexus. Quantitative MRI studies showed that these parameters can be used as important biomarkers for neurological studies in the brachial plexus. The DFL, representing the motor nerve fibres conduction characteristics, was measured for normal healthy nerves and combined with MR parameters. Correlation between DFL and MR parameters was observed for the first time.

The nature of ultrafast acoustic strain generation and effects in III-V semiconductor-based nanostructures is explored in this thesis via experimental observations that are supported by theoretical analysis. Specifically, coherent phonon generation processes in bulk gallium arsenide (GaAs) are investigated through remote hypersonic detection using a double quantum well-embedded p-i-n diode, after which strain-induced effects in a double barrier quantum well resonant tunnelling diode are examined. Finally, preliminary studies on acoustic modulation of a double barrier quantum dot resonant tunnelling diode are also considered, with recommendations for future experimentation. It was experimentally observed that the transduction of strain in bulk GaAs produces an initial acoustic wavepacket that is strongly asymmetric with a heavily damped leading edge. This was determined to be due to photogeneration of a supersonically expanding electron-hole plasma near the irradiated GaAs surface. Coupled with its propagation from the free surface, the plasma generates stress and therefore strain in the system that is caused by a combination of the deformation potential and thermoelasticity; the former and latter are shown to be dominant for low and high optical excitation densities, respectively. These acoustic waves cannot escape the plasma until it has decelerated to subsonic velocities, which is achieved in a finite time, thus resulting in the observed asymmetry and damped leading edge. This finite acoustic escape time was reduced at high optical excitation densities due to plasma expansion limitation by increased non-radiative Auger recombination of electron-hole pairs. This conclusion is substantiated by analytical expressions derived from the inhomogeneous wave equation, and analysis of the spatially- and temporally-expanding plasma density based on the deformation potential mechanism only. Numerical simulations based on these expressions are fitted to the experimental data, and the thermoelasticity contribution at high excitation densities is deduced from a non-linear deviation of the electron-hole recombination rate and a change in the duration of the leading edge. This contribution expressed a square-law behaviour in the former parameter, which is attributed to non-radiative Auger processes. Strain-induced effects on a double barrier quantum well resonant tunnelling diode resulted in the detection of current modulation on a picosecond timescale only when the device was biased within its resonance region, with the largest modulations at the resonance threshold and peak biases. Through analysis of the device structure and stationary current-voltage characteristics, it is demonstrated that the observed current changes are due to variations of the resonant tunnelling rate caused by acoustic modulation of the confined ground state energies in the diode itself. Numerical analysis of the tunnelling rates provided excellent agreement with the experimental data, particularly when comparing charge transfer rates, where the limited temporal response of the experimental device could be ignored. Furthermore, the charge transferred at the resonance threshold and peak has a set polarity regardless of optical excitation density, and therefore the device possesses “rectifying” behaviour. As such, it has been demonstrated that, by exploiting this acoustoelectronic pumping effect, control of picosecond charge transfer in a resonant tunnelling diode or its application as a hypersonic detector are possible. In closing, the mechanisms for strain generation in bulk GaAs and the utilisation of the acoustoelectronic pumping effect in a double barrier quantum well resonant tunnelling diode are both exhibited in this work, and provide promising evidence and novel hypersonic detection methods for future research into ultrafast acoustic effects in semiconductor nanostructures.

In this thesis, the probes were modelled and constructed at the SPMMRC. All measurements were performed on a 7T Philips scanner. The coils have been successfully evaluated. The dipole, loops, strip line and wrist probes for imaging the pelvis, knee and wrist were tested for clinical use at 7T. Two elements wrist coil can pick up signals from the whole region of interest. The advantage is more uniformity of field of view and better sensitivity. The in vivo MRI images acquired in the wrist showed the two elements provided the good quality images for the human wrist. The second study is microstrip line probe. The current flows over the flat-strip were computed, it showed that a significant increase of current close to the edges. This result agrees with theory. We did not use the strip line coil to image a human body, because the coil generated a high SAR/B1 +2 level in the region of interest. The third study was of a coil of two square loops. One way of achieving decoupling is to use the overlapping technique to decouple the coils in the simulation. It produced high signal-to-noise ratio and provides a large field of view. Finally, the dipole has been developed for in vivo MRI applications. We presented a novel model for determining the length of the PECs required for tuning the dipole at 298 MHz. The efficiency, field of view and homogeneity were improved by adding the flat strip, two strips and array strips dipole. The SAR/B1 +2 generated by the dipoles was much less than produced by the loop coil and strip line coil in the pelvis. The dipoles showed the desired improvement in SNR and homogenous coverage. Coverage goes much further into the pelvis and knee as well.

The ferromagnetic semiconductor gallium manganese arsenide is an important test-bed material for spintronics applications. Whilst a Curie temperature anywhere close to room temperature has yet to be demonstrated, the excellent micromagnetic properties, simple band structure and unusual combination of having both low moment densities and high spin-orbit coupling make this an interesting material to study from both theoretical and experimental perspectives. This thesis reports some experimental studies into the magnetic and magnetoresistive anisotropies in gallium manganese arsenide. In the first main chapter a study of the Anisotropic Magnetoresistance in thin (Ga,Mn)As films is reported, based on transport measurements of micro-scale devices, contributing to the first systematic study in this material. The Anisotropic Magnetoresistance comprises crystalline and non-crystalline components; this study shows that a uniaxial crystalline component can dominate over the whole range of temperatures from 2K up to the Curie temperature, the first time this has been seen in any material system to our knowledge. The following chapter shows that the magnetic anisotropy of gallium manganese arsenide thin films can be engineered by lithographically patterning the material into structures on length scales of a micron or less. Using electron beam lithography to define the structures and SQUID magnetometery to study the resulting magnetic configuration, it is shown that the magnetic anisotropy can be greatly modified, even resulting in a switching of the easy- and hard-axis directions. Finally a new technique based on Anisotropic Magnetoresistance measurements is presented to locate the crossover of competing magnetic anisotropy coefficients in the temperature domain. Conventionally performed by SQUID magnetometry, this new technique is cheaper and simpler whilst qualitatively reproducing the main features of the SQUID measurements.

This thesis describes an experimental investigation of the resonant injection of carriers into self-assembled indium arsenide (InAs) quantum dots incorporated in the intrinsic region of gallium arsenide (GaAs) p-i-n resonant tunneling diodes, and of the resulting electroluminescence spectrum associated with carrier recombination in the quantum dots, wetting layer and GaAs matrix. A series of devices of different designs have been measured and it is shown that bipolar resonant injection, i.e. resonant injection of both electrons andholes, into the zero-dimensional states provided by the InAs quantum dots is possible. It is shown that bias-tunable tunneling of carriers into the dots provides a means of controlling injection and light emission from a small number of individual dots within a large ensemble. Magnetotunneling spectroscopy is used to investigate the possibility that fluctuations in the potential profile of the GaAs emitter layer play a significant role in the carrier dynamics of such devices. We also show that the extent of carrier energy relaxation prior to recombination can be controlled by tailoring the morphology of the quantum dot layer. Additionally, a study into the phenomenon of low-temperature up-conversion electroluminescence (UCEL) is presented. Injection of carriers into the quantum dot states at an applied bias well below the GaAs flat-band condition results in near-band-edge GaAs electroluminescence, i.e., emission of photons with energies much larger than that supplied by the applied bias and the thermal energy. The origin of this UCEL is discussed and is attributed to carrier excitation resulting from (non-radiative) Auger recombination of electron-hole pairs in the quantum dot ground states.

Similarities in the form of the Schrodinger equation that governs the behaviour of electronic wavefunctions, and Maxwell’s equations which govern the behaviour of electromagnetic waves, allow ideas that originated in solid state physics to be easily applied to electromagnetic waves in photonic structures. While electrons moving through a semiconductor experience a periodic variation in charge, in a photonic crystal electromagnetic waves experience a periodic variation in refractive index. This leads to ideas such as bandstructure being applicable to the one and two dimensional photonic crystals used in this work. The following work will contain theoretical and experimental studies of the transmission through, and electric fields within, one dimensional photonic crystals. A slow variation in the structure of these crystals will lead to the bandstructure shifting, with an photonic analogy of electronic Bloch oscillations and Wannier-Stark ladders being seen in these structures. The two dimensional photonic crystals will be shown, through Hamiltonian ray tracing, to support both stable and chaotic ray paths. Examination of the phase space reveals the existence of ‘Dynamical Barriers’, regions in phase space supporting stable ray trajectories that divide separate regions in which the ray trajectories are chaotic. Various manners in which the bandstructure may be varied will be presented, along with a proposed switch that may be made using these structures. While the ray tracing will be carried out in photonic crystals in the limit of infinitesimally thin dielectric sheets, the model will then be developed to show the bandstructure of a photonic crystal made from finite width dielectric sheets, with examples of dispersion surfaces for these structures being presented.