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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
91

Aspects of atom beam microscopy and scattering from surfaces

Bacon, R. T. January 2007 (has links)
Specifically, I describe the design and construction of the vibrational isolation system, and an ex-vacuo manipulator with micron-level accuracy used to position key atom-optical components.  Both aspects of the scattering helium microscope (SHeM) are shown to be successful in design and operation. In chapter 5 we introduce and develop a one and two-dimensional simulation of atom scattering from surfaces.  Novel aspects of the simulation include the ability to treat different scattering elements assembled in an arbitrary fashion. In chapter 6, we apply the novel one-dimensional simulation to homoepitaxy and ion erosion of the Pt(12 12 11) surface. Secondly, we examine the one-dimensional strain relief dislocation networks in the growth of lithium on the Cu(100) surface, between 0.5 and 0.6ML coverage. The observed diffraction patterns are shown to arise from a degree of disorder, which explains the broadening and shifting of the diffraction peaks. Chapter 7 contains a two-dimensional analysis of a dilute overlayer structure formed by low coverage of Li/Cu(100), and an incommensurate island structure for thin film growth greater than one monolayer. We also analyse a two-dimensional strain relief dislocation network formed five monolayer growth of Cu/Pt(12 12 11), finding a triangular dislocation network accounts for the diffraction spot profiles occurring.
92

Probing electron-electron interactions with a quantum antidot

Bassett, L. C. January 2009 (has links)
In the integer quantum Hall (IQH) regime, an antidot provides a finite, controllable ‘edge’ of quantum Hall fluid which is an ideal laboratory for investigating the collective dynamics of large numbers of interacting electrons. Transport measurements of single antidotes probe the orbital energy spectra of the antidot edge, and gate-defined antidot devices offer the flexibility to vary both the dimensions of the antidot and the couplings to the extended IQH edge modes which serve as leads. We can also use the spin-selectivity of the IQH edge modes to perform spin-resolved transport measurements, from which we can infer the antidot spin-structure. This thesis describes a combination of such experimental measurements and related computational models designed to investigate the effects of electron-electron interactions in quantum antidotes, with general implications for the physics of spin and charge in IQH systems. Our work provides a powerful example of the practical applications of IQH edge modes for selective transport in mesoscopic quantum electronics, which we have used to perform the first spin-resolved measurements of V<sub>AD </sub>= 2 transmission resonances. Our discovery of spin-charge separation in the low-field antidot excitation spectrum paints a picture of the antidot as a finite droplet of interacting IQH fluid in the LLL, with all of the rich physics of exchange, collective modes, spin textures, etc., which this entails. Our results are therefore relevant not only for the physics of antidots, but more broadly for the understanding of interacting electronic systems of many particles in the IQH regime.
93

Photon emission from metals in the Scanning Tunnelling Microscope

Downes, A. R. January 1997 (has links)
The Scanning Tunnelling Microscope is an established surface science tool, combining unprecedented resolution with real space mapping. One of its major drawbacks, however, is that it gives no chemical information, but Photon Emission is able to probe the inelastic channel, which for metals contain invaluable chemical information. It has already been shown that flat metal surfaces produce differing emission spectra. Real surfaces may not be flat, so it is important to known how curvature changes the yield of photons, and the emission spectrum. For this reason, a two sphere model was developed. The mechanism for Photon Emission, that of excitation of localised surface plasmons followed by either radiation or dielectric loss, was split into separate problems so that the dependence of the overall photon emission on materials and curvature is clear. Experiments were performed on small silver particles, and for what was later believed to be a silver tip it was found that the Photon Emission was approximately proportional to both the tin and particle radius. Light emission was observed from clusters containing as few as ˜30 atoms, and the first chemically specific photon maps were presented, which distinguished silver particles from carbide deposits. Emission was also seen on the Si(111)-7x7 surface. The new theory compared well with spectra from flat surfaces, and when extended to two spheres it also showed that the Photon Emission was approximately proportional to both the tip and particle radius. It was predicted that the onset of emission would occur at a lower tip bias for gold particles than for silver particles, and this was confirmed experimentally. Metal-specific spectra will be produced if the plasmon modes do not move in energy when curvature varies, staying close to the travelling surface plasmon energy. This should occur for small radius tips and particles, and the movement of modes should be reduced for tungsten tips.
94

Solid-state nuclear magnetic resonance studies of heterogeneous materials

Groom, N. S. January 2005 (has links)
Solid-state NMR has been applied to the study of two types of heterogeneous materials: bone and polymer-clay nanocomposites. Each has useful and unexpected properties that are largely determined by the interfacial region between the two components making up each material rationalization of these properties at a molecular level is essential if we are to intelligently design new materials, which might mimic bone for instance. In Chapter 3. 2D experiments that correlate the <i>T</i><sub>1</sub> relaxation time by the chemical shift are described and their application to the study of the interfacial region in Fe<sup>3+</sup> substituted polymer-clay nanocomposites is described. It is found that in polyhydroxyethylmethacrylate nanocomposites the polymer is significantly more ordered at the surface than in polymethylmethacrylate nanocomposites. In Chapter 5, a variety of techniques that explicit the dipolar coupling between <sup>31</sup>P in the inorganic phase and <sup>13</sup>C in the organic matrix of equine bone are used, with the aim of identifying the proteins and/or the amino acid residues responsible for the binding of the two phases and the nature of the interaction between the phases. Natural equine bone samples (i.e. non-isotopically enriched) are studied to give the first experimental evidence that (i) it is predominantly glutamate residues that are bound to the inorganic surface with a <i>d-</i><sup>13</sup>C-<sup>31</sup>P distance in the range 0.45-0.5 nm. (ii) no P-O-C bonds occur and (iii) that proline and hydroxyproline are not involved in binding. Finally, in Chapter 6, solid-state NMR is applied to the study of osteoarthritic equine bone samples with a view to characterizing the changes in bone composition and structure that occur with this disease.
95

Ionised magnetron sputter deposition

Chiu, K. F. January 2000 (has links)
The goal of this research work was to establish the Ionised Magnetron Sputter Deposition (IMSD) system and apply it to thin film fabrication. Using magnetron sputter deposition, with an additional built-in rf coil generating a rf coupled plasma to ionise sputtered atoms, the process provides a high level of control over the energy input to a growing film. The controllable parameters include ion flux, ion incident energy and the ratio of ions to neutrals of the depositing species. The possibility of depositing films and coatings with up to 85% of the depositing species as ions with energies controllable up to 150 eV offers a remarkable opportunity to engineer film growth and modify film properties by precisely controlled ion bombardment. The work presented here is concerned with the investigation and understanding of the basic properties of the IMSD process. It is composed of (1) a background introduction; (2) IMSD system characterisation; (3) characterisation of IMSD deposited metal thin films. Firstly, an introduction to magnetron sputtering and the effects of energetic bombardment on film properties, and a brief survey of ion assisted techniques are presented. The recently developed IMSD process is then introduced. The rf inductively coupled (RFI) plasma generated in the IMSD process was characterised using a single electrical probe. The RFI plasma is confined close to the substrate, and the bombarding ions are drawn directly from it, so that the plasma parameters are crucial to the ion bombardment on the substrate surface, in terms of ion flux and ion energy. It is found that the ion flux can be controlled by the power applied to the rf coil, which controls the ion density. The ion incident energy can be determined by the difference between plasma and substrate potentials. The ionisation fraction of the depositing flux (ratio of ions to total depositing atoms) has been measured by a parallel-plates method, which was developed here. The probe method was also employed later to confirm the measurement. Directionality of depositing flux was examined by depositing films into sub-micron vias and trenches.
96

Studies in kinematic relativity

Evans, G. W. January 1955 (has links)
No description available.
97

Development and characterisation of advanced field emission tips

James, E. M. January 1997 (has links)
This dissertation concerns the development and use of a novel form of field emission electron source in a medium-voltage (100 kV) scanning transmission electron microscope (STEM). It is the properties of the electron source, and particularly its brightness, which determine the extent to which interference effects are observable at the detection plane. These features directly contribute to high resolution image contrast. Interest was stimulated into the possibilities of improving the brightness of electron sources by at least an order of magnitude, by Hans-Werner Fink in the late eighties. He developed a process whereby a tungsten emitter could be produced which terminated in a single atom (a so-called nano-tip). The electron beam from such a source has been shown to exhibit high coherence, good current stability, a small cone angle and confinement of the emission surface to the atomically-sized apex. Experiments, however, were confined to tips in simple field emission chambers and the low-voltage point-projection microscope. In this dissertation, nanometre-sized protrusions have been grown at the apeces of tungsten emitters using a method whereby the surface metallic layer of atoms is melted under the action of strong electrostatic fields and applied heat. Such tips have shown emission characteristics, within an un-modified 100 kV STEM, which correspond with those previously observed from nano-tips. An evaluation of the suitability of the STEM for future use of these tips is included. In addition to requiring low levels of electrical instabilities, a mechanically reinforced tip base assembly was necessary. This was designed, constructed and shown to be an improvement over that of the commercially available source. A method to measure absolute beam coherence values at the specimen plane, and hence characterise tips, was also evaluated.
98

The characterisation of nanostructured magnetic materials using image spectroscopy and electron tomography

Chong, K. K. R. January 2007 (has links)
In this dissertation, two advanced TEM techniques, image spectroscopy and electron tomography, are applied to the quantitative structural and chemical characterisation of nanostructured magnetic materials, in both two and three dimensions (3D), at nanometre spatial resolution. In image spectroscopy, chemical information is obtained by acquiring extended energy-selected series of images, which are interpreted using a combination of computer processing and traditional electron energy-loss spectroscopy (EELS) analysis. In tomography, 3D microstructural and chemical information is obtained by acquiring ultra-high tilt series of high-angle-annular dark field (HAADF) or energy-filtered TEM (EFTEM) images, and subsequently applying tomographic reconstruction and visualisation algorithms to the data. The techniques are applied to the characterisation of materials that include closely-spaced FeNi nanoparticles coated in oxide shells, lithographically patterned Ni pillars, platinum nanoparticles, chromium carbides in stainless steel, titanomagnetite and an ALH meteorite sample. Biological samples include magnetotactic crystals and bacteria cells, greigite-containing bacterial cells and amyloid plaque cores. The chemical data from image spectroscopy and morphological data from electron tomography are cross-correlated. Furthermore, experimental tomographic data are compared with tomographic reconstructions of image simulations.
99

Design and construction of an instrument for scanning and conventional transmission electron microscopy at 10Å resolution

Beaumont, S. P. January 1979 (has links)
This dissertation is an account of the design and construction of an instrument which combines scanning and conventional transmission electron microscopy in a single column. The microscope incorporates a lanthanum hexaboride electron gun working at voltages up to 100kV, and was designed to be capable of examining the same specimen feature in either the scanning or the conventional transmission mode of operation, with a resolution of 10Å or better without realinement or adjustment of the specimen. A three lens electron optical column has been added to the projector lenses of an AEI EM6 electron microscope. All lenses were designed with the aid of Munro's finite element computer programs. Particular attention was paid to the objective lens, which is of the Riecke-Ruska saturating condenser objective type. Computations of the first order optical properties and the third order aberration coefficients were undertaken and are presented here in such a way as to enable the effects of changes in all the important design variables to be determined. The probe-forming properties of condenser objectives, including the limitations to field width arising from their off-axis aberrations, are considered and predictions made of the capabilities of the chosen design. Attention is paid to the particular problem of column layout in a microscope where the illumination and imaging optics are not independent. The difficulties of machining electron lenses with very small polepiece bores to high accuracy are examined and a method described which is capable of polishing 2mm bores to a roundness of 0.2 microns using simple apparatus. A quantitative theory of stigmator correction has been developed. Micrographic evidence is presented to show that the microscope is capable of a resolving power of 10Å in CTEM and of forming 10Å diameter STEM probes. The limitations to performance and possible improvements to the instrument are discussed.
100

Dynamics, stability and formation of amyloid fibrils : insights from mass spectrometry

Caddy, G. L. January 2006 (has links)
In this thesis I have used electrospray mass spectrometry (ESI-MS) to investigate various aspects of amyloid fibrils, including their mechanism of formation, their structure and dynamics, and approaches to inhibit fibril development. I have used hydrogen exchange methods coupled with ESI-MS to examine the differences in spontaneous protein unfolding between amyloidogenic and non-amyloidogenic variants of human lysozyme and thus determined a correlation between the ease with which the partially unfolded event occurs and the likelihood of amyloid deposition <i>in vivo</i>. I also used a similar approach to probe the interaction between the amyloidogenic lysozyme variant and a chaperone known to inhibit its fibril formation. I have developed a novel method using ESI-MS for the direct analysis of lysozyme enzyme function in real-time. This method was used to determine whether a small antibody fragment, which is known to prevent fibril formation in amyloidogenic lysozyme, has a detrimental effect on substrate binding and catalysis. The approach was validated using hen lysozyme and a corresponding antibody fragment which is known to bind in the active site of the enzyme. From the results generated, I can conclude that the antibody interactions do not ameliorate the enzymatic activity of lysozyme. I have also established a hydrogen exchange protocol to examine the structure and dynamics of a well-characterised amyloid fibril system. Under the rigorous experimental conditions used, I found that the exchange is dominated by a mechanism of dissociation and re-association that results in the recycling of molecules within the fibril population. Moreover, sing this protocol to examine the differences between seeded and unseeded fibrils, I found that changes in the fibril morphology are able to perturb this equilibrium.

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