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In this work a series of new semiconducting liquid crystals (LCs), which are applicable for organic light emitting diodes (OLEDs), were investigated. Semiempirical calculations were carried out on monomers and anti-cofacial dimers built from our molecules, representing molecules in solution and thin film respectively. Compared to the monomer a doubling of the oscillator strength in the dimer was found for longitudinal offsets larger than 20 A. Smaller shifts showed a forbidden absorption transition from ground to the lowest excited state. Assuming that the absorption transition is equivalent to the emissive transition, this might explain the reduced optical quantum efficiency observed for all of our materials in the solid state. OLEDs made from blends of three different blue/green emitters with a red component showed white light emission with voltage independent CIE coordinates close to the ideal white. With polarised microscopy nematic phases frozen in a glassy state at room temperature were observed for all blends. Thus the blends were homogeneous and no phase separation occurred. This is important for homogeneous white emission and the alignment of the LCs due to a rubbed alignment layer below. Polarised white electroluminescence with an average polarisation ratio of 8:1 was shown from an OLED made with a blend deposited onto an alignment layer. Polarised background light for LC displays is desirable as this minimises the losses at the polarisers in the display and thus increases its brightness or lowers the power consumption. The low efficiency of the red emitter however limited the OLED performance. Surface relief gratings (SRGs) with periods of a few hundred nm and a maximum depth of 66 nm and periods in the nm-range with a depth of 140 nm were spontaneously induced on our films. They were formed through molecular mass transport from the dark to bright regions during crosslinking by irradiation with a sinusoidal light pattern created by a phase mask. The anisotropic properties of LCs are shown to enhance transport. SRGs were formed at room temperature and an elevated sample temperature of 65deg. They are suitable feedback structures for optically pumped organic lasers and can also be employed to enhance the outcoupling of OLEDs.

Electron paramagnetic resonance (EPR) has been used to study point defects in synthetic single crystal diamond. Newly observed defects are reported in high pressure high temperature (HPHT) and chemical vapour deposition (CVD) diamond. HPHT diamond doped with 15N has been used to investigate the g = 2 region of the EPR spectrum which is obscured when natural isotopic abundances are present. Two previously unreported defects labelled WAR9 and WAR10 are reported. The EPR data has been shown to be consistent with the neutral nitrogen interstitial, N0 I (WAR9), and neutral nitrogen di-interstitial, NI-I001 (WAR10), defects respectively. Two further defects observed in CVD diamond are reported here. The first labelled WAR2 is preferentially aligned with the direction of growth, [001]. The EPR data is consistent with a (V-(CH)-V)0 structure although theoretical studies suggest that this structure is unstable at CVD growth temperatures. Growth mechanisms are suggested that would account for the observed preferential alignment. The second defect labelled WAR5, has been observed exclusively in samples grown using an experimental CVD chemistry containing oxygen. The EPR data is consistent with the OV0 defect, although no confirming 17O hyperfine structure has been observed. 13C hyperfine data is also reported for the KUL1/VnH− defect (n = 1 or 2) but the new data is not sufficient to conclusively discount either the n = 1 or n = 2 models suggested for this defect. Changes in defect concentrations in CVD diamond with thermal and illumination treatments has been investigated. Experimental data has indicated the presence of an unseen trap, common to CVD diamond, with concentrations comparable to that of N0S, and levels in the band-gap 0.5{1.2 eV above the top of the valence band. The difficult quantification of sub part per billion defect concentrations, as observed in electronic grade material, is tackled with the use of rapid passage EPR. It is shown that with this technique it is possible to detect concentrations of single nitrogen in diamond at tens of parts per trillion, close to a factor of 100 improvement on the currently used slow passage EPR.

Perovskite-based materials are in the focus of research not only because of their excellent physical properties, but also because their relatively simple structure facilitates the understanding of structure-property relationships, which is crucial for developing novel materials with improved qualities. Recent research in the field of ferroelectric and piezoelectric materials is concerned with the development of eco-friendly lead-free materials. To achieve this goal, it is important to understand the fundamental correlation between the ‘Structure’ and the ‘Property’. In this work, the primary focus has been to elucidate the structural changes occurring as a function of doping in three different systems: (1) BiScO3-PbTiO3 (BS-PT), a recently developed system which has already attracted much interest because of its superior physical properties near the morphotropic phase boundary (MPB); (2) BiScO3-BaTiO3 (BS-BT), which can be considered as a lead-free analogue of the BS-PT family and lastly, (3) Na0.5Bi0.5TiO3-BaTiO3 (NBT-BT), which is a well-known lead-free material at the NBT-rich side of the phase diagram. Powder samples with a range of compositions for each system were prepared following the solid-state synthesis route and were investigated utilizing both neutron and x-ray powder diffraction and dielectric measurements. Detailed crystallographic information was obtained by Rietveld refinement against the neutron powder diffraction data. Structural phase transitions as a function of temperature were determined by nonambient x-ray powder diffraction and compared with the physical properties of the ceramics using high-temperature dielectric measurements. The significant outcomes are: 1. The best model to represent the so-called MPB of xBS-(1-x)PT system is found to be a mixture of a tetragonal and a monoclinic phases from the powder diffraction data. The structure beyond the MPB compositions is in better agreement for a single monoclinic model with the space group Cm than the accepted space group R3m. By contrast, single crystals with compositions around the MPB provide evidence for a model consisting of two primitive monoclinic cells. 2. The lead-free BS-BT system exhibits an extended phase boundary between tetragonal and pseudocubic phases, which can be modelled by a combination of tetragonal and rhombohedral phases. The incorporation of BS into BT also results in the suppression of the two low-temperature phase transitions of BT. 3. Samples with new compositions synthesized in the xNBT-(1-x)BT system demonstrate a rare enhancement in the tetragonality of the unit cell and an increase in the Curie temperature for compositions where x <= 0.40.

Defects in single crystal diamond grown by chemical vapour deposition (SC-CVD) have been studied using electron paramagnetic resonance (EPR) and optical techniques. Annealing studies in the temperature range of 900 - 1600°C have been performed and the changes in EPR and Fourier transform infrared (FTIR) spectra for nitrogen doped SC-CVD have been documented. It has been possible to model the annealing behaviour of several hydrogen related defects and the associated kinetics and annealing parameters have been determined. Previously unreported optical absorption lines have been detected in the as-grown and annealed nitrogen doped samples. FTIR spectroscopy has been performed in conjunction with an applied uniaxial-stress to investigate hydrogen related absorption features in diamond. Several absorption lines have shown to shift and split under the application of uniaxial-stress and the symmetry and stress-splitting parameters for these defects have been determined. Models for the defects responsible are proposed. The mid-infrared absorption line at 3123 cm-1 is suggested to originate from a C-H stretch mode at the paramagnetic negative nitrogen-vacancy-hydrogen complex. For a 3123 cm-1 absorption line with a Lorentz linewidth of 3.8(2) cm-1, 1 cm-1 of absorption at 3123 cm-1 corresponds to 25(10) ppm of NVH- centres. A previously unreported paramagnetic defect has been detected and is given the label WAR1. It has an S = 1 ground state and a pseudo-C3v symmetry; the g and D spin Hamiltonian parameters have been determined and suggest that the defect is vacancy related.

Using confocal Raman spectroscopy/microscopy, we have monitored pharmaceutical crystallisation 'in situ' in three model (well characterised polymorphic systems) Active Pharmaceutical Ingredients (APIs) and one previously unstudied system where polymorphism had not being reported prior to this study: flufenamic acid, a Non Steroidal Non-Inflammatory Drug (NSAID); nifedipine, an antihypertensive; tolbutamide, used in the treatment of type II diabetes; and imipramine hydrochloride, an antidepressant respectively. Constrained crystallisation from the solid amorphous state was utilised to kinetically trap polymorphs via the Ostwald's rule of stages. Particular emphasis was placed on the phonon-mode/low wavenumber region (4-400cm-1) of the Raman spectral window (this region provides useful information about lattice environment). In all cases our results from the Raman experiments were complemented with similar experiments using Differential Scanning Calorimetry (DSC) and Variable Temperature X-ray Powder Diffraction (VTXRPD). To reduce data complexity, principal component analysis was deployed and found to be extremely effective. In chapter two, a multi-technique study of flufenamic acid (FFA) was carried out which served as a groundwork for later chapters. A solid-solid transformation between two forms of FFA (forms I and III) was observed, due to the abrupt nature of this transition, the 'Lindemann vibrational catastrophe' was envisaged as a possible mechanism for the transformation. Using FFA as a test case in chapter three, polymorphic transformations was monitored in both FFA forms I and III using in situ Raman spectroscopy (as well as VTXRPD) by adopting the constrained crystallisation approach. The approach showed excellent promise (with the XRPD patterns of FFA form II and one unknown form uncovered) and was further explored in later chapters using a variety of pharmaceutical materials. While in chapter four, the interconversion between the different polymorphs of nifedipine was studied using the constrained crystallisation approach monitored using in situ Raman spectroscopy (together with VTXRPD and DSC), our results compared favourably well with those previously published in literature. We also reported for the first time the phonon-mode Raman spectral for this system as earlier publications focussed only on the 'traditional' fingerprint region. Similarly in chapter five, in situ Raman spectroscopy was also used to monitor the polymorphic transformations in tolbutamide (using the constrained crystallisation approach), results from the Raman analysis was compared with those obtained from VTXRPD and found to be in agreement. Thus further showing that Raman spectroscopy combined with the constrained crystallisation approach is a veritable tool for monitoring polymorphic transitions. In chapter six, preliminary results (Raman/XRPD/DSC) showed for the first time that imipramine hydrochloride exhibits polymorphism, with the possibility of at least two new polymorphs. Combination of state of the art spectroscopic techniques with appropriate statistical methods, X-ray powder diffraction and DSC was shown to be an extremely effective approach to investigating and characterising polymorphism in drugs, often using only milligram or sub-milligram sample quantities. Lastly in chapter seven, the novel technique of Transmission Raman Spectroscopy (TRS) was employed in carrying out a quantitative study of polymorphic content in a model pharmaceutical formulation and the results obtained compared with those from traditional backscattering geometry. The transmission method is shown to provide a true bulk measurement of the composition, being unaffected by systematic or stochastic sub-sampling issues that can plague traditional backscattering geometries.

The miniaturization trends on electronic components manufacturing, have challenged conventional knowledge on materials strength and deformation behavior. ”The smaller the stronger” has become a commonplace expression summarizing a multitude of experimental findings in micro-scale plasticity, and modelling tools capable of capturing this distinctive reality are in urgent demand. The thesis investigates the ubiquitous size effects in plastic deformation of micron-scale specimens. Tracing the source of such a behavior to the constituent elements of plastic deformation, we use as starting point the dynamics of discrete dislocations and try to embody them into a continuum framework. The thesis is structured in two independent parts. In the first part the question why size effects occur in constrained geometries is addressed. A systematic investigation of the connection between internal and external length scales is carried out in a system where dislocations, in the form of continuous lines embedded in a threedimensional isotropic medium, move, expand, interact, and thus create plastic distortion on the deforming body. Our modelling strategy utilizes a set of deterministic evolution equations on dislocation densities for describing the stress-driven evolution of the material’s internal state. These transport-like equations simultaneously serve the role of constitutive laws describing the deformation of the stressed body. Subsequent application to three benchmark problems is found to give good agreement both with experiment and discrete dislocation dynamics simulation. The second part of this thesis focuses on the heterogeneity and intermittency of deformation processes on the micro scale. Recent experimental results question the concept of smooth and homogeneous plastic flow with fluctuations that average out above a certain scale. Bursts of activity, which follow power-law size distributions and produce long-range correlated deformation patterns, seem to pertain even on scales far greater than the atomic one. In short, plasticity in this view appears as a ’crackling noise’ phenomenon similar to other irregular and burst-like processes such as earthquakes or granular avalanches. But then why do we witness smooth stress-strain curves on macroscopic sample testing? Concepts originating from Self Organized Criticality and pinning theories are employed for producing an efficient continuum description which is then used to study the effect of intrinsic and extrinsic deformation parameters on the fluctuation phenomena. It is deduced that hardening, load driving and specimen size, are all decisive on constraining fluctuating behavior, and limits of classical theory’s applicability can be drawn.

The current climate in research has focused on the rational design of new materials with desirable characteristics. The demand for their full characterisation has in turn placed a new importance on structural chemistry, and important developments have taken place as a consequence. For instance an important probe to understand the interactions between molecules is to use variable pressure, and this has been exploited experimentally through the design and implementation of the diamond anvil compression cell (DAC). Using a DAC to study molecular materials at high pressure can result in problems, however: in X-ray diffraction the physical presence of the cell restricts access to reciprocal space, resulting in experimental structures of lower precision and often missing hydrogen atom location data. Traditionally the solution has been sought in neutron diffraction where hydrogen (deuterium) atoms scatter more intensely and so contribute more to the scattering pattern. This introduces another set of problems, however, in that the assumption is made that the isotope substitution does not alter the overall structure. In addition expense and time delays are incurred through this protracted experimental route. This thesis reports the development of a computational technique which can be used to reliably locate hydrogen atoms without the need for neutron diffraction data. The project reports rigorous testing on cases of varying difficulty, from the simple to the more complex. The test cases selected were also of industrial and environmental importance, so determining their complete structures under high pressure conditions was in itself a desirable outcome. Computationally completed structures were then compared to neutron diffraction results or used as the model to be refined against the neutron diffraction pattern.