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No description available.

The work in this thesis is to understand, through theory and simulation, a guidance mechanism due to the weak interaction of modes in photonic crystal fibres (PCFs). Firstly, two common kinds of PCFs, that guide light by total internal reflection and by photonic bandgaps, are reviewed. Several typical PCF structures for which light propagation is governed by weak mode interaction are then discussed and particularly compared with bandgap-guiding PCFs. Two independent methods are developed to model a set of related rectangular hollow-core PCF structures. The boundary element method is derived for a general PCF configuration and applied to our model structures. This method numerically provides some basic features about the guided modes, such as the propagation constant and field profile. The calculations show an ideal confinement in our model structure by considering a scalar wave equation and a high dielectric constant at the glass intersections. However, in realistic guidance, both confinement loss and the field of the guided modes indicate a raised leakage due to mode interactions. The analytic methodology starts by solving the ideal case considered in boundary element calculations and leads to analytic solutions for the perfectly guided modes. A perturbation method corresponding to the realistic guidance is then applied to these analytic solutions. This method can provide insight into understanding the formation of leakage through an analysis of mode interactions. An approximate analytic method for obtaining the attenuation of guided modes from the perturbation interaction is demonstrated. Attenuations calculated in this way give good agreement with boundary element results in magnitude and trends in variation. The influences of frequency and fibre parameters on features of the attenuation are also investigated. An overall interpretation of this guidance mechanism and suggestions for fibre optimisation are made in the final chapter, where further development of this work is also proposed.

The technique of single crystal synchrotron X-ray diffraction is applied in the study of solid state proton transfer processes in hydrogen bonded molecular complexes. Proton transfer processes are of interest where they are responsible for a number of physical and chemical properties within solid state functional materials; their study gives insight into the occurrence of such properties and where they may be targeted and tuned in future materials. The synchrotron X-ray diffraction technique has been trialled with respect to the potential it offers for high throughput capability for studying proton transfer processes as a function of an external stimulus or across a number of molecular systems. Chapter 1 contains a review of the literature of the hydrogen bond, including its role in crystal engineering and proton transfer effects. In Chapter 2, the theory behind the analytical techniques used in the study of hydrogen bonded molecular complexes, in which crystallographic methods are fundamental, are described. In Chapter 3 the research project aims and objectives are presented; these objectives are targeted at the use of single crystal synchrotron X-ray diffraction in the study and rationalisation of solid state proton transfer processes. In Chapter 4, the experimental methods implemented in this research project to achieve these research goals are reported. Chapter 5 is the first of the result chapters and applies the synchrotron single crystal X-ray diffraction technique in the study of variable temperature proton disorder in centrosymmetric hydrogen bonded carboxylic acid dimers. Chapter 6 focuses on the design of proton transfer systems implementing a number of crystal engineering strategies in the design of short strong hydrogen bonds (SSHBs) for potential proton migration studies. Chapter 7 applies a combination of diffraction methods (synchrotron and laboratory X-ray diffraction) and refinement strategies in the study of temperature dependent proton migration across SSHBs, allowing the potential of these methods in the study of proton migration to be assessed. Chapter 8 is the final application of the synchrotron technique in studies of proton transfer behaviour investigating static proton transfer behaviour in molecular complexes of the proton sponge 1,8-bis(dimethylamino)naphthalene with organic acids. The urea-acid inclusion materials presented in Chapter 9 additionally allow the investigation of the hydrogen bond as a crystal engineering tool in the design of hydrogen bonded solvent-inclusion networks. In the last chapter, Chapter 10, conclusions from the findings in Chapters 5 to 9 are pulled together and patterns explored. Drawing on these overall findings, some suggestions for future work are also made.

In this thesis basic methods for the fabrication and characterisation of several nano-silicon containing systems are presented. Due to their morphology, these systems are highly reactive. Silicon wafers were used to prepare layers of porous silicon via electrochemically etching and micro– and nano– sized silicon powders were chemically etched in order to yield silicon nanoparticles. Dependent on the fabrication, particle size of the nanocrystals and porosity of the assemblies can be tailored over a wide range: mean particle sizes can be between 3 to 20 nm and porosities can be varied from 10 to 90 %. A huge surface area of up to 500m2/g which is in addition, due to the fabrication process, hydrogen terminated, entail the outstanding chemical and photo-chemical properties of nanocrystalline silicon. Both, chemical and photo-chemical properties of silicon nanocrystal structures are investigated. The emphasis lies on optical spectroscopy. The indirect band gap structure of silicon in combination with quantum confinement effects are the origin of the interesting luminescence properties of nano-silicon. The energy transfer process from photo-excited excitons confined in silicon nanocrystals to molecules present in the surrounding ambient, like oxygen or a variety of organic substances, has been studied. Measurements demonstrated that long-living excitons very efficiently transfer their energy to surrounding molecules. The low probability of creating excitons which can persist for a long time, from μs to ms, by a photon and structural properties of porous silicon, or rather its reactive surface, however, seem to be the reason for a low total quantum yield of sensitised excited singlet state oxygen.

Crystallisation is an important unit operation in the processing of pharmaceuticals and other fine chemicals. This thesis presents research in the area of crystallisation and solid-state chemistry and investigates methods towards optimising the physical and particle properties of organic solid forms. The work was carried out in the context of the EPSRC Centre for Innovative Manufacturing in Continuous Manufacturing and Crystallisation (CMAC). CMAC aims to accelerate the adoption of continuous manufacturing processes for the production of higher quality chemical products at lower cost and more sustainably. Chapters 1-3 present an introduction to the research area in crystallisation tackled, along with background theory and experimental methods employed. Crystal morphology is important for the ease of downstream processing in the production of pharmaceuticals and fine chemicals due to its impact on the bulk properties of a substance. Impurities can play a significant role in the final crystal morphology produced from crystallisation and can therefore be added intentionally (known as additives) to tune the morphology of a substance to a more desirable shape. Chapter 4 presents the discovery of a polymer-additive mediated effect on the crystal morphology of succinic acid; the effect of the additive is dependent on both the concentration of succinic acid and the polymer additive and a thorough investigation of these parameters at different scales is presented. A possible mechanism for the interaction of the polymer with the crystal surface is also described. Chapter 5 reports the transfer of the crystallisation of succinic acid carried out in Chapter 4 into the continuous environment using two different platforms. The system is used to characterise the design of a mesoscale segmented flow crystalliser as well as transferred to a well-established oscillatory flow platform. In the latter platform the additive effect of the polymer on the crystal morphology of succinic acid is successfully reproduced in the continuous environment. Chapter 6 explores the effect of a range of structurally similar additive candidates on the crystallisation of the Active Pharmaceutical Ingredient (API), isoniazid. As employed in Chapter 4, face indexing and analysis of the crystal structure are used to rationalise the effect of the additives. Chapter 7 and 8 detail small-scale molecular crystallisation studies in attempts to induce disordered or layered solid forms and understand orientational disorder. In Chapter 7 the production of a range of multi-component molecular complexes of the API piroxicam are reported. This includes a solvated system which shows interesting structural and thermal properties which have been studied in detail. Chapter 8 is concerned with studies of orientational disorder in 5-chlorouracil.

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Chirality in the bulk Ntb phase was confirmed directly by resonant X-ray scattering of 'hard' X-rays in a selenium-labelled binary mixture of bent liquid crystal (LC) dimers. Furthermore the high orientation of the resonant Bragg spots, corresponding to the pitch length of a well aligned helix, enabled estimation of the coherent Ntb domain size. Ntb phase chirality was also confirmed by circular dichroism measurements in various pure LC dimer compounds. In addition to resonant Bragg spots, non-resonant broad peaks at larger scattering angles were observed. These are attributed to localised cybotactic layering of the mesogens, which was found to depend on spacer length and mesogen composition. To account for simultaneous observation of sharp and broad X-ray peaks, a new two component orientational order parameter was introduced, which can be deconvoluted if the conical angle is known or if near perfect helical order is achieved. Based on these findings, a more quantitative molecular level model of the Ntb phase was geometrically constructed. In order to follow the local director the dimers or polymer chains adopt an average conformation resembling a helix (helical segment in dimer case). The helical conformation was supported by the findings of polarised infrared spectroscopy in the Ntb phase of a main-chain LC copolyether. Experimental evidence suggests that the Ntb phase is very similar in both polymer and dimer sample types. Throughout the project several banana phases in bent LC dimers were observed and characterised, some of which are new or previously misidentified. In the case of the misidentified phase, an alternative molecular level model was proposed based on polarisation splay of the molecules. A LC self-assembling 3D array of dendron functionalised CdS quantum dots (QDs) was investigated by a variety of experimental techniques including grazing incidence X-ray scattering and fluorescence spectroscopy. The structure was found to possess very low cubic symmetry and represents the lowest symmetry QD array to date. The strange fluorescence quenching properties of the material were found to be dependent on the annealing temperature, hence the degree of positional order among QDs inside the array.

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The main aim of this study is to explore the fundamentals of aperiodicity through the deposition of a single elemental aperiodic overlayer onto a quasicrystal surface. The main difficulty with understanding the effects of aperiodicity on the quasicrystal structure arises due to the chemical complexity associated with the surface, as such structures are only observed in metallic alloys containing two or more elements. The formation of a single element quasicrystalline structure has been achieved through the deposition of Pb on i-AI-Pd-Mn and d-AI-Ni-Co (results of the latter not included). This includes an initial study of the effects of aperiodicity on the electronic properties of the surface and the conclusion that the formation of a pseudogap at the Fermi level is due to the aperiodicity of the quasicrystalline structure and not the chemical complexity of the system. The effects of aperiodicity on the magnetic properties of such a system are not yet known, in particular for a single magnetic species which is ordered aperiodically. For this reason Fe was deposited onto two different quasicrystalline surfaces. They show the formation of such a quasiperiodic magnetic film was not observed, the Fe was in fact aligned periodically along the high symmetry axes of the substrates. This result, along with other work in the same field provides some guidance as to which atoms have the greatest chance of ordering aperiodically. Most of the research within the field involves thin film deposition in ultra high vacuum. This is a costly technique, and if films on quasicrystal surfaces were one day to be exploited by industry then production would be severely limited. It is for this reason that the electrochemical deposition of Cu onto AI-Pd-Mn has been studied. This work provides the starting point for further study into the electrochemical deposition on to the quasicrystal surface and comparison to those studies in Ultra Right Vacuum (URV).