School of Physics - Theses
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ItemAdvances in atomic resolution imaging using scanning transmission electron microscopy( 2017)Scanning transmission electron microscopy (STEM) is capable of imaging at sub-Ångström resolution, simultaneously acquiring multiple signals resulting from the elastic and inelastic scattering of the electron probe. In this thesis theoretical advances are made, in tandem with experiment, to develop novel imaging techniques in STEM: the characterisation of surface reconstructions using secondary electrons, a method for elemental mapping, a method for studying electric and magnetic fields in a specimen and an investigation of specimen mis-tilt in annular bright-field imaging.
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ItemThe role of thermal scattering in the imaging of condensed matter at atomic resolution( 2014)Transmission electron microscopy is a technique capable of determining specimen composition and structure at atomic resolution. Since the invention of the first microscopes in the 1930’s, a series of instrumentation and theoretical advances have led to a mature field where one can routinely obtain images of individual columns of atoms with high precision. Despite the extraordinary success of the microscope and its prevalence in laboratories around the world, interpretation of experimental images is a non-trivial matter. The strong interaction of electrons with the charged particles making up the target specimen (~10^4 times greater than that for x-rays) results in interesting and complicated scattering dynamics which can make direct interpretation of experimental images difficult. A particularly important scattering mechanism at high incident energies is thermal scattering, whereby the incident electron excites a phonon (that is, a vibrational mode) within the specimen. In this thesis we will present a new model for thermal scattering which affords new physical insights as compared with previous models. In particular the new model distinguishes between elastic and thermal scattering of the fast electron and can predict the individual contributions to the scattered intensity from both types of scattering. We will use this model to gain new insights into the role of thermal scattering in a number of different imaging modes.
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ItemA generalised holographic approach to coherent diffractive imaging( 2013)This thesis examines methods for obtaining the full complex wave-field from measured probability intensities in microscopy experiments, otherwise known as the phase problem. We present a new theoretical framework for solving the phase problem. From this, previously intractable problems may be solved via direct methods. In addition, we present a method for greatly increasing the potential output of single particle diffraction measurements in free-electron laser science (FELS) experiments. This method extracts many single particle diffraction measurements from a single many particle diffraction measurement. We investigate the feasibility for performing coherent diffractive imaging (CDI) in the scanning transmission electron microscopy (STEM) geometry using standard methods. Despite some success in ideal cases, we find that the inversions (from the measured diffraction intensities to the complex wave-field) are very sensitive to imperfections in the data. Consequently we investigate the degree to which various sources of error, such as detector noise and spatial incoherence, affect the convergence properties of the inversions. Difficulties in the application of single-shot CDI in STEM may be overcome by measuring many diffraction patterns from the same sample and combining these data for a ptychographic method. We performed a ptychographic reconstruction of a complex sample transmission function. This was done by combining several diffraction measurements from overlapping regions of a boron-nitride cone. We present the subsequent atomic resolution retrieval and the algorithm used. We develop and implement new holographic algorithm to obtain a high resolution reconstruction of the complex exit surface wave formed by a gnat’s wing. In doing so we derive a new error metric applicable in both holographic and non-linear CDI. It is also found to be a more reliable measure for the fidelity of the retrieval. By extending the previous holographic method to accommodate diffraction data taken in the near-field (or the Fresnel regime) we find that previous restrictions on the illumination and the position of the sample in the beam are greatly reduced. We demonstrate the experimental feasibility of this method by recombining the complex exit surface wave emanating from a microfibre. Here the specimen was illuminated by a plane wave while the diffraction data was taken in a plane centimetres from the object. We show how the requirement for full coherence of the imaging system can be removed by incorporating the partially coherent modes of the imaging probe into the formalism of the holographic method. A new iterative linear algorithm is developed, extending the applicability of the algorithm to higher resolutions and greatly increasing the computational speed of the retrieval. We present a direct single-shot sub-Angström retrieval from the edge structure of a CeO2 nano-particle, using electrons. This retrieval is improved by the inclusion of the entire autocorrelation function of the exit surface wave. Using the new iterative linear algorithm we are able to include a subsequent iteration, correcting for the non-linear contribution to the autocorrelation function by the object. Finally, we use the techniques developed in this thesis to extend the new iterative linear algorithm to include ptychographic data-sets.