No description available.

No description available.

We developed a technique for measuring the focal properties and the magnification behaviour oi electrostatic lenses with cylindrical geometry. We applied the method to the study of three-element lenses of various proportions, and we found excellent agreement between our experimental results and a simple theoretical model of lenses with centre elements longer than 1.25 internal lens diameters. We extended our study to lenses with centre element 1 and 0.5 lens diameters where the simple theoretical model is inappropriate but there are detailed calculations on the focal properties and the magnification of such lenses. Again we found very good agreement. Having established a sound measuring technique we studied the properties of lenses with controlled magnification. We constructed a five-element lens whose linear magnification can be varied over a range of 9:1 at a given overall voltage ratio. The same lens can also be operated to give a constant magnification over an extremely wide range (10000:1) of overall voltage ratios. Such a lens can prove a powerful tool in electron spectroscopy for uses in electron guns and as the output lens of monochromators. The potential distribution required for a specific purpose can be obtained simply from design curves. By imposing certain constraints on the focusing voltages of the five-element lens we can form a system with a magnification that depends only on the overall voltage ratio as (Vs/V1)-0.2S and with the property of forming an image at a fixed distance from an object. We have made an experimental study of such a lens and found that a prediction of a simple scaling law for the potentials was valid. We developed two independent methods for measuring the spherical aberration of some of the above lenses. We have measured the third order coefficients Cs, Cso, Cs4 for two and three-element lenses of various proportions and the coefficient Cs for a five-element afocal lens. We have observed fifth order effects for large launching angles. The object of this thesis was to develop and use a technique to investigate the focal properties, magnification and spherical aberration of electrostatic lenses.

InP quantum dot (QDs) lasers grown on GaAs substrates have potential applications in photodynamic therapies, as multi-wavelength sources and for biophotonic sensing. However, to make these devices practical, further improvements are required in threshold current at elevated temperature. The main reason for this study is to identify the factors in the improved performance of lasers with respect to lowering the threshold current density and lowering the temperature dependence of threshold current density for samples with different Ga composition in the upper confining layer (UCL). A new way of determining the mode loss per unit length (αi) was introduced by extracting the peak net modal gain (G - αi) value of 6 cm-1 for a 2-mm-long laser from the averaged value of the modal gain (G), which is more accurate and significant than determining αi just at the value which loss (at net modal absorption or A + αi) and gain (at net modal gain or G - αi) spectra tend to at low photon energy. The highest αi value is 2.30 cm-1 for Ga = 0.54, 1.10 cm- 1 for Ga = 0.52, and Ga = 0.56 and 0.58 have almost zero αi values. I show that to maintain the same peak modal gain at 300 K at a higher temperature, for instance 360 K, one will need to compensate for two situations. First, increasing the current density to achieve 300 K inversion level (or the difference between the quasi-Fermi level separation and the absorption edge) to compensate for the increased nonradiative recombination processes and secondly adding more current density on top of that to compensate for the carrier spreading to higher energy states, in order to reach the peak net modal gain required at 360 K. Spontaneous emission rate spectra measured at J6 cm -1 show more filled QW states for Ga = 0.54 compared to Ga = 0.58 and compared with data taken at constant inversion level indicates that more carriers are supplied to the Ga = 0.54 to compensate for its high optical mode loss (αi), when compared to Ga = 0.58. As the temperature increases, some of the energetic carriers from the QW escape to the lower confining layer (LCL) and spontaneous emission measurements show this happens more in the Ga = 0.58 than in the Ga = 0.54. Absorption measurements indicate this is because QD and QW states move closer to the LCL states as the Ga composition in the well increases. Three series of samples grown at different times but with similar designs were compared in the study. Lowering the Al composition in the cladding layer, tends to lower the optical confinement factor (Γ), which causes the threshold current density to be increased in Series 1. The results show that αi plays the dominant role, not only in lowering the Jth but also lowering the threshold current temperature-dependence of these series.

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.

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 work outlines the process undertaken in the formulation and validation of a numerical model for analysis of practical microwave processing problems. The proposed model adopts a novel multi-domain Eulerian-Lagrangian approach to the problem, defining two discrete numerical domains coupled through a set of data transfer algorithms. One of the numerical domains is defined for analysis of electromagnetic field distribution while the other is used for analysis of the thermophysical aspects of the problem. The thermophysical domain is restricted to the load being processed and is discretised in an Eulerian manner using an unstructured mesh for solution using a finite volume approach. The electromagnetic domain is discretised using a tensor-product rectilinear structured mesh for solution of Maxwell’s equations using a Yee finite difference time domain approach. The thermophysical load is represented within the electromagnetic domain through a mapped Lagrangian complex permittivity distribution rather than being defined explicitly. The two domains are coupled through mapping routines capable of defining the complex permittivity distribution within the electromagnetic domain and transferring the calculated power density distribution into the thermophysical domain. This interdomain coupling allows the meshes in the two domains to be non coincident, enabling the discretisation of the two domains to be completely independent of each other. This approach to analysis of coupled microwave processing problems is novel and provides a number of significant benefits over conventional single-domain methods. The primary benefit of the approach is that the electromagnetic and thermophysical parts of the analysis can be handled by different solvers using differing meshes. This is a very significant advance as the optimal approach to solving one of the parts be be extremely inefficient or indeed unfeasible for the other. The approach allows electromagnetic fields irradiating complex geometries placed inside a rectilinear microwave ovens to be analyses using a tensor product solver. The solution of the electromagnetic field distribution is typically the most computationally expensive part of a coupled solution of microwave heating. The ability to use a Yee finite difference solver rather than a conformal FDTD or finite element approach provides a very significant reduction in computational expense, enabling more complex analyses to be performed. Solution of thermophysical aspects of the problem are most effectively tackled using an unstructured spatial discretisation in cases with complex geometries. The adoption of an unstructured finite volume approach for the thermophysical part of the analysis provides an analysis capability far beyond that of the finite difference approach typically used in analyses with a finite difference electromagnetic solver. Further benefits stem from the inherent capability to alter the discretisation of the electromagnetic domain independently of the thermophysical domain, enabling cases with advection and/or rotation of the load within the oven to be considered with relative ease. Analysis of this type of problem is highly complex when using a single domain approach as the mesh needs to be redefined at regular interval during the solution. The capability to refine the discretisation of the electromagnetic domain also improves efficiency in cases where dielectric properties vary significantly during the process as mesh resolution can be varied as the process progresses. The primary drawback with the adoption of the multidomain approach is that the load is represented in the electromagnetic domain as a mapped Langrian complex permittivity distribution rather than being explicitly defined as part of the domain discretisation. There are therefore issues relating to the smearing of material boundaries which may influence wave scattering across the boundary adversely affecting accuracy of the electric field solution. In order to study the efficiency and accuracy of the approach a series of tests were conducted to assess the performance of each individual component of the analysis framework to ensure that these had been implemented effectively and to determine the magnitude of any apparent errors. The model was subsequently applied to a simple test case to ensure that the components were coupled in an effective manner. This test and validation process showed that individual components to be accurate and fit for purpose with errors due to data transfer between the two computational domains shown to be small. The results obtained from the validation case agreed relatively closely with experimental data demonstrating the implementation and efficacy of the model. The model was subsequently validated against two practical microwave processing problems - thawing of food within a domestic microwave oven and polymer curing using a dual-section microwave system. In the food thawing study, the solution obtained by the numerical model was validated against data obtained during an experimental study. The study was intended to meet the requirements of an industrial partner in research work that eliminated a range of simplifications adopted in alternate studies. The analysis therefore focussed on thawing of a challenging ’real-world’ material, placed in a complex shaped container. The load was placed on the rotating turntable of a domestic microwave oven. Results obtained from the numerical simulations agree moderately well with experimentally derived data. Primary disparities between experimental data and numerical solutions would appear to stem from inaccuracies in modelling the solid-liquid phase change in a complex multi-component material coupled with the very significant variation in dielectric loss over the melting temperature range. The microelectronics study focussed on curing of polymer materials in a microelectronics package using a dual-section microwave oven system. The requirement for this study was to predict the optimal process parameters for operation of the system. Numerical assessment of the development of key variables such as temperatures, degree of cure and stresses during the process was critical to this problem. Experimental measurements of these parameters during microwave processing were not feasible. Numerical comparisons of the microwave system with a conventional convection oven process have additionally been carried out. Key results from the study include optimal temperature profiles, final degree of cure distribution and residual stress magnitudes. Numerical data from the analyses are being integrated into an experimental study as part of ongoing work. An overall assessment of the numerical approach would indicate that it is a viable method of efficiently obtaining solutions to practical microwave processing problems. Further research is required to assess the influence of the smeared dielectric boundary in the finite difference solver on reflection, refraction and focussing effects on the accuracy of the numerical solution.

This thesis describes the work undertaken in the design and construction of a millimetre wave, quasi-optical impedance bridge. The instrument operates as a nulling complex reflectometer, with nulling being implemented at the signal frequency by interferometric optical techniques. In principle, the quasi-optical components of the reflectometer are operable in the frequency range 80 GHz to 500 GHz, though the range of continuous coverage is determined by the waveguide components and antennas used to couple sources and detectors into the Gaussian beams. Gaussian beam mode theory has been employed in the design and measurement of the quasi-optical components, which behaved as expected. Phase and polarisation effects within the impedance bridge were modelled and the consequent behaviour of the impedance bridge predicted. A comparison of these predictions with early results led to refinement of the model. This model is based upon a more complete description of the transmissivities and reflectivities of the wire grid polarising beam splitters and accurately predicts the instrument behaviour. A data acquisition and control system has been built to facilitate automated operation of the impedance bridge. This system is interfaced to an Acorn® Archimedes microcomputer and the hardware and software has been developed to enable interrupt driven data acquisition. The reflectivity of a quasi-optical load has been measured at 85 GHz with an accuracy and standard deviation of better than 0.1 % In amplitude and a standard deviation of better than 0.10° in phase. The quasi-optical half cube and computer interface components developed in this work form the basis of a stock of general purpose modules now in everyday use at St. Andrews.

No description available.

In this work we present electron beam techniques for probing the chiral structure of materials. The motivation for the work lies in two distinct aspects of chiral materials science. In the first case, chiral plasmonic excitations have recently been proposed for use in a variety of sensing technologies but require structural optimisation, in which modern electron microscopy techniques excel. In the second case, the recent development of intrinsically chiral electron ‘vortex beams’ suggests the prospect of being able to discriminate chiral crystal structures directly within the electron microscope. We were keen to explore this prospect because it could overcome a long-standing and fundamental limitation in TEM techniques. The introduction section of this thesis provides an overview of the essential background and theory underpinning the research, specifically considering the theory of surface plasmons, the properties of electron beams and a description of crystal chirality, all of which will be used in our work. We give a description of diffraction of electrons in a crystal and show the wavefunctions of electron vortex beams. Results show electron channeling in a simple cubic crystal, highlighting the impact of the position of the beam on the unit cell. When the beam is focused onto an atomic column the electrons are forward scattered along the column, whereas they are scattered outwards when the beam is centered between columns. In Chapter 2 we discuss the details of data acquisition and post processing techniques, namely Richardson-Lucy deconvolution and non-negative matrix factorisation, which together extract plasmonic modes from EELS data sets. We give a detailed analysis of the plasmonic excitations which exist on the surfaces of both a nanopatterned gold chiral nanoparticle and a hole in a continuous metallic film that also supports localised surface plasmon resonances. We show, using EELS in a scanning transmission electron microscope that we can map the resonance modes of the structure with high spatial resolution. We confirm the link between the modes supported by a plasmonic nanoparticle and a hole of similar shape, though we find that 3D roughness has an effect on the energy of the modes, shifting modes by around 0.3eV in some cases. The modes found are chiral leading to chiral fields, which have applications as sensors of biological molecules. In Chapter 3 we demonstrate the ability to simulate EELS, with accuracy, of realistic plasmonic nanoparticles with 3D shapes which extends beyond the usual two-dimensional or idealised simulations of the literature. We study the effects of inevitable manufacturing and other structural imperfections on the plasmonic response of real patterned structures and show that they lead to shifts in energy and spatial intensity of the modes due to these defects. We find that structural defects are enough to make so called dark modes (ie. those that do not have dipole character and are therefore not usually excited by incident photons) become bright, and shift the energy of modes between similar structures. We also illustrate the ability to predict the intensity distribution of plasmonic modes on structures using only symmetry arguments. This type of calculation gives a simple derivation of the modes which would exist on a plasmonic nanoparticle without requiring a more complicated eigenmode analysis. Using symmetry terms and irreducible representations the modes which appear on a nanoparticle can be grouped by symmetry terms, allowing the breaking of symmetry, via defects, to be better be visualised. In the latter part of this work we turn to a completely different experiment to consider the exploitation of newly-discovered vortex electron beams in assigning chirality to crystals within a simple electron microscopy experiment. With these beams, which posses a orbital angular momentum, will show that is possible (in favourable cases) to detect the handedness of the crystal via a difference in diffraction pattern intensity distribution for beams of opposite OAM, when scattering in opposite handed crystals. Our work demonstrates the ease of assigning chirality using convergent electron vortex diffraction for a crystals with a threefold, fourfold and sixfold screw axis. We present this work using modified multislice simulations of the diffraction of vortex electron beams from chiral potential layers. We demonstrate that the azimuthal phase component of electron vortex beams opens up new opportunities for rapid chiral discrimination and structural studies in electron diffraction. We show that the symmetry of the resulting convergent beam patterns matches the point symmetry of the crystal only when the handedness of both the impinging vortex probe and chirality of the crystal are congruent. This methodology was tested on the real crystal structures of α-quartz and the magnetic crystal Cr1/3NbS2. It was found that effects (due to the matching of the beam and crystal chirality) are most obvious in the overlap of diffraction disks, where interference effects allow the novel phase profile of vortex beams to produce intensity variations. The thickness of the crystal, the convergence angle and the energy of the beam were all parameters which need to be tuned in order to achieve chiral specific scattering. In thinner crystals the effects were less obvious, requiring a deduction in the beam energy in order to see a difference between CBED patterns of enantiomers. In cases where the convergence angle did not lead to overlapping disks in the CBED pattern the effects of chiral specific scattering were not obvious. Future work should include an experimental check of the feasibility of observing such weak diffraction effects in an electron microscope. Should this prove possible, this method could be of use to to discriminate chiral crystal structures directly within the electron microscope using only one set of diffraction patterns.