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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.
81

The development of quasi-elastic helium-3 spin-echo spectroscopy as a tool for the study of surface dynamics

Hedgeland, H. January 2007 (has links)
The first chapter of the work introduces the basic principles of the spin-echo technique and the spectrometer instrumentation, establishing it within the context of both neutron and helium atom scattering. The following two chapters describe the ongoing development of the spectrometer to increase its signal level and the range of surface science systems accessible to study. Chapter two details the development of the detector. The approach taken is a combination of progressive experimental improvements to the existing electron-impact ionisation detector, resulting in an order of magnitude increase in efficiency, as well as simulations modelling an alternative design for the next generation of ioniser. In chapter three, we move from the detector to the helium beam source, modelling from first principles anomalous attenuation seen in its performance. We develop an understanding of the behaviour seen which allows us to suggest improvements to the source design. The remaining chapters present studies of three surface science systems: potassium on copper, benzene on copper and benzene on graphite. Chapter four concerns the diffusion of potassium on copper, developing from previous studies of sodium. We find coverage dependent behaviour and establish an anisotropic potential energy surface with barriers varying from 80 to 40 meV and a friction coefficient of 0.25 ps<sup>-1</sup> for low coverages. We further discuss the deviations seen from these parameters at higher coverages. In chapter five we study a more complex molecule with the diffusion of benzene on copper. A simple jump diffusion mechanism is found with diffusion barriers of the order of 125 meV. Molecular dynamics are again used in combination with the data to study the potential energy surface. The final chapter consists of detailed observations of the diffusion of benzene on a graphite substrate. From quasi-elastic helium atom scattering data we establish a continuous diffusion mechanism with a friction coefficient of 5.5 ps<sup>-1</sup> and a maximum surface corrugation of 30 meV. Complementary neutron spin-echo measurements are also presented, enabling for the first time a direct comparison between the two techniques. Broad agreement is found with the helium spin-echo benefiting from higher signal levels.
82

Secondary electron imaging of liquid droplets on insulating surfaces in the ESEM

Craven, J. P. January 2001 (has links)
The Environmental Scanning Electron Microscope (ESEM) has rapidly become established as a new generation of scanning electron microscopes. The sustainability and utilisation of a gaseous atmosphere in these microscopes has opened the way for new electron microscopy studies of insulating, biological and even fully hydrated systems. Recently there has been an increasing drive to push the limits of the technique further to perform more <i>in situ</i> dynamic experiments and extract more quantitative information. However before this potential can be realised, some fundamental groundwork must be done. In this thesis we focus on the interpretation of secondary electron contrast obtained from liquid water droplets, with the aim to make a quantitative interpretation of contact angle. We develop and present two experimental methodologies to explore this parameter: 1) by direct imaging of water droplet profiles on flat insulating substrates and, ii) indirectly by quantitative interpretation of contrast profiles obtained across the surface of water droplets. In developing a procedure to measuring contact angle directly in the ESEM we focus on the practical implications of depositing and stabilising regular droplets and correctly mounting the substrates. We also compare the ESEM contact angle measurements to macroscopic optical measurements performed on the same surfaces to ensure the accuracy of the results. In extracting contact angle from contrast profiles we develop a general model of SE topographic contrast for the ESEM, considering the relative dominance of the different signal contributions that form the final image and how they are processes into an image. We relate the contrast profiles to the geometric shape of the water droplets and find a correlation between the breakdown of the model and the size of the interaction volume of the primary electron beam.
83

Probing Mott delocalisation using the de Haas-van Alphen effect

Goh, S. K. January 2009 (has links)
The physics of Mott delocalisation is investigated from the perspective of Fermiology through a series of high resolution de Haas-van Alphen experiments. Two systems in which some or all electrons can be forced to Mott localise by an experimental tuning parameter were chosen. The first system is CeRh<sub>1-<i>x</i></sub>Co<i><sub>x</sub></i>In<sub>5</sub> where the 4<i>f</i> electron of CeRhIn<sub>5</sub> can be driven into a delocalised state by Co substitution. The Fermi surface of CeRh<sub>1-<i>x</i></sub>Co<i><sub>x</sub></i>In<sub>5</sub> was studied for six different values of <i>x</i>. By measuring the angular dependence of de Haas-van Alphen frequencies, a Fermi surface sheet with <i>f</i>-electron character was observed to undergo an abrupt change in topology as <i>x</i> is varied. This reconstruction does not occur at the quantum critical concentration <i>x<sub>c</sub></i>, where antiferromagnetism is suppressed to <i>T = </i>0. Instead this sudden change occurs well below <i>x<sub>c</sub></i>, deep inside the antiferromagnetic state. Across all concentrations, the quasiparticle effective mass of this sheet does not diverge, suggesting this critical behaviour is not exhibited equally on all parts of the Fermi surface. The second system of interest is the Mott insulator Ca<sub>2</sub>RuO<sub>4</sub>, which can be metallised at 0.6 GPa. A completely new setup, utilising a 10-turn signal pick-up coil in an anvil cell for field modulation measurements, was developed for performing de Haas-van Alphen experiments under pressure. This novel setup thus has the potential to reach much higher pressures than the existing piston-cylinder type setup, opening up a much bigger phase space for future exploration in materials physics. The newly developed method was tested using Sr<sub>2</sub>RuO<sub>4</sub> and the results are in excellent agreement with a broad body of literature. Subsequently, the method was applied to study the metallic state of Ca<sub>2</sub>RuO<sub>4</sub>. De Haas-van Alphen signals were successfully recorded at high pressure using both the cryomagnetic system in Cambridge up to 18 T and a resistive magnet in National High Magnetic Field Laboratory in Tallahassee up to 31 T. Comparisons to band structure calculations were made.
84

Studies of indirect methods in high-resolution electron microscopy

Chang, L.-Y. January 2004 (has links)
Since accurate image restoration relies on a detailed understanding of imaging models, this thesis commences with a discussion of the partial coherence effects in an electron microscopy. The various approximations which have been developed to describe the effects of partial coherence in image formation are reviewed, and the validity of these expressions when applied to imaging at high resolution is examined. Accuracies and efficiencies of different calculation approaches, are also compared. Hence a computationally efficient alternative algorithm for incorporating the effects of partial coherence in calculations of high-resolution electron images has been developed. Both linear (the Wiener filter) and nonlinear (the MAL algorithm) image restoration methods are then discussed and compared. The significance of the nonlinear contributions to image intensities in a linear restoration is firstly investigated and the performance of the linear Wiener filter in dealing with the nonlinear contribution is assessed. Finally, the nonlinear image restoration method of the MAL algorithm has been developed using a general underlying imaging model. The performance of the MAL restoration method is then investigated through both qualitative and quantitative comparisons. Comparisons between a linear restoration and a nonlinear MAL restoration are also discussed for both simulated and experimental defocus series.
85

Structure determination using electron diffraction and microscopy

Haydon, S. K. January 2001 (has links)
This thesis presents a program of work initiated to implement, assess and test a difference potential methodology of structure refinement using quantitative electron diffraction. The results of a parallel synthetic and structural study of the pentagonal tunnel (PT) structure, tetragonal lead tungsten bronze (lead-TTB), and the unexplored quaternary Pb-Nb-W-O system are also presented. Quaternary Pb-Nb-W-O phases are synthesised via fully oxidised and reduced reaction mechanisms. The phases observed in both experiment types are similar and based on superstructures of basic TTB, formed by the ordering of pentagonal columns (PCs) within the host framework. The formation of a fully oxidised quaternary analogue of lead-TTB is discussed. Reduced phase preparations prove it is possible to incorporate lead into PC structures whilst maintaining the parent framework PC distribution. In other experiments an alternative PT/PC ordering scheme is found. A model for this phase and its hidden supercell is presented and discussed. A new TTB related phase is also observed possessing a tetragonal cell repeat five times the (210)<SUB>TTB</SUB> plane spacing. A possible model for this phase is proposed. Methods of extracting experimental intensities for the difference potential calculation from an exposure series of diffraction patterns are discussed. A software package (<I>EDIM</I>) for performing the intensity extraction automatically is presented. Tests performed to investigate the characteristics of the difference potential are described, and the results of a limited refinement of lead-TTB using <I>EDIM</I> generated experimental electron diffracted intensities are presented. These calculations confirm that the electron difference potential is a viable means of determining light atom positions in structures containing heavy atom components.
86

Atom optics and surface growth studies using helium atom scattering

Holst, B. January 1997 (has links)
The first part of thesis investigates the application of electrostatically deformed single crystal membranes as focusing elements for a neutral atomic beam in an ultra high resolution diffractometer. The second part demonstrates the versatility of helium as a surface science tool in a study of the growth of ultra thin Cu-films on Pt(111). Chapter 1 gives an introduction to the subject of atom focusing with an overview of the various methods hitherto applied. Chapter 2 presents a theoretical framework for the application of a focusing element in an ultra high resolution diffractometer. An off axis reflected ray equation is derived from which aberration coefficients are obtained and used to determine the best obtainable performance. Chapter 3 presents equations for calculating deformation of radically symmetric thin plates (membranes). Furthermore interferometry experiments are presented which investigate the deformation of single crystal Au(100) and Si(100) in varying fields. The films mechanical properties are discussed, and contourmaps obtained from the interferometry experiments used in computer simulations to evaluate their focusing properties. Chapter 4 contains the experimental methods and sample preparation of the two types of samples used here. The Pt(111) and the Si(111)-H(1 x 1) eventually chosen for the mirror. In particular a new mount specially designed for the focusing mirror is described in detail.
87

Atom diffractometry from nanostructured surfaces

Huang, C. January 2006 (has links)
Chapter 1 introduces the basic principles of atom diffractometry and scattering from surfaces. I discuss the experimental and theoretical methods employed to determine the atom-surface interaction potential. In addition, a brief description of the apparatus and typical measurements are given. In Chapter 2, a new method is developed for determining the atom-surface interaction potential by interference effects. Based on a 0.5 ML Ni-Cu(100) surface, the helium interaction potential on an unreconstructed Ni(100) overlayer is determined experimentally, which would not have been previously possible due to the absence of diffractive and resonant scattering intensity. A detailed investigation of growth mechanism and surface structure of Ni-Cu(100) is presented in Chapter 3. It is observed that the nickel growth occurs by forming ordered overlayer structures at and below room-temperature, whilst proceeds via alloying processes at high temperatures. The second growth system, Li-Cu(100), is studied in Chapter 4. A sequence of ordered overlayer structures with unusual electronic corrugations are observed in the growth at low temperatures. The surface corrugation for helium scattering from a complex structure, <i>c</i>(5V 2 x v 2)<i>R</i>45° Li-Cu (100), is derived through a diffraction analysis using the exact, close-coupled channels method. Finally, a comparison between neon and helium scattering from an ordered <i>c</i>(2x2) Li-Cu(100) structure is presented in Chapter 5. The surface corrugation for neon scattering is determined on a metallic overlayer for the first time. it is found that the neon corrugation amplitude is at least one order of magnitude larger than for helium. Such a difference exceeds those observed previously on low-index transition metal surfaces.
88

The effects of the encapsulation of inorganic halides within carbon nanotubes

Bendall, J. S. January 2006 (has links)
This thesis is concerned with the encapsulation of inorganic material within the cavity of single-walled carbon nanotubes, with an ultimate aim of modifying their properties at an atomic level. Nanotubes are expected to constrain the atomic lattice positions of internal crystals as well as protecting them from the external environment. Furthermore, the crystals could potentially adopt a structure that is completely unrelated to that observed in the bulk, and it is a good way to produce long continuous nanowires. Following on from the synthesis of high quality material, characterisation of these encapsulated crystals will be carried out using electron microscopy and related techniques. Once characterisation of the structure has been achieved, further analysis will be used to investigate what effect the filling of carbon nanotubes has on the geometrical, electronic and vibrational structure of the nanotube, as well as its reactivity. This will be achieved using, amongst other techniques, Raman and optical spectroscopy, scanning probe methods and thermal decomposition methods. Observed changes in the carbon nanotube can then be related to the specific encapsulated structures observed, as well as the general properties of the host material. From the observations, these materials will then begin to be used for specific applications.
89

A new approach to the theory of renormalised fields

Edwards, S. F. January 1954 (has links)
No description available.
90

Nanometre structures for electron optics

Ito, Y. January 1997 (has links)
Manipulation of the phase of an electron wave has been demonstrated by three dimensional nanometer scale solid state diffraction gratings. The diffraction gratings with varying hole depths (three dimensional nanostructures) were created by controlling the dwell time of an electron probe. The gratings were directly made by using the computer controlled electron probe (diameter < 0.5nm) of a dedicated scanning transmission electron microscope (STEM) with a 100keV field emission gun. These structures were made in a thin film of amorphous (a-) AIF<SUB>3</SUB> evaporated on a thin a-C or a-Si<SUB>3</SUB>N<SUB>4</SUB> film. The electron optical properties of these devices were investigated in a conventional transmission electron microscope. Other instrumentation and technique developments included an improved electron energy loss (EEL) spectrometer and an energy filtered selected area electron diffraction mode in STEM. The most significant result is that a "wedge" depth profiled grating exhibited an asymmetrical diffraction pattern (a violation of Friedel's law), demonstrating conclusively that the grating acted as a strong phase object. Two practical electron optical devices were demonstrated, i.e. "biprism" profiled gratings and pixelated Fresnel phase (PFP) lenses. The "biprisms" exhibited overlap of two waves passing through each side of the biprism and possible electron interference fringes by the biprism action. Both convex and concave PFP lenses have been demonstrated for the first time. The designs showed that they are robust to the random phase error and the absorption effect was small. A simulation successfully explained the experimental observations. This simulation incorporated the proximity effect as an additional concave phase retardation of up to approximately 0.2π in the convex lenses and a variable hole diameter. The FWHM of the peak at the designed focal point for the large PFP lens was approximately 8nm, which matched well with the simulation.

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