Matching Items (3)
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- All Subjects: diffractive imaging
- Creators: Kirian, Richard
- Creators: Subramanian, Ganesh
Description
Topological insulators with conducting surface states yet insulating bulk states have generated a lot of interest amongst the physics community due to their varied characteristics and possible applications. Doped topological insulators have presented newer physical states of matter where topological order co&ndashexists; with other physical properties (like magnetic order). The electronic states of these materials are very intriguing and pose problems and the possible solutions to understanding their unique behaviors. In this work, we use Electron Energy Loss Spectroscopy (EELS) – an analytical TEM tool to study both core&ndashlevel; and valence&ndashlevel; excitations in Bi2Se3 and Cu(doped)Bi2Se3 topological insulators. We use this technique to retrieve information on the valence, bonding nature, co-ordination and lattice site occupancy of the undoped and the doped systems. Using the reference materials Cu(I)Se and Cu(II)Se we try to compare and understand the nature of doping that copper assumes in the lattice. And lastly we utilize the state of the art monochromated Nion UltraSTEM 100 to study electronic/vibrational excitations at a record energy resolution from sub-nm regions in the sample.
ContributorsSubramanian, Ganesh (Author) / Spence, John (Thesis advisor) / Jiang, Nan (Committee member) / Chen, Tingyong (Committee member) / Chan, Candace (Committee member) / Arizona State University (Publisher)
Created2013
Description
Phase problem has been long-standing in x-ray diffractive imaging. It is originated from the fact that only the amplitude of the scattered wave can be recorded by the detector, losing the phase information. The measurement of amplitude alone is insufficient to solve the structure. Therefore, phase retrieval is essential to structure determination with X-ray diffractive imaging. So far, many experimental as well as algorithmic approaches have been developed to address the phase problem. The experimental phasing methods, such as MAD, SAD etc, exploit the phase relation in vector space. They usually demand a lot of efforts to prepare the samples and require much more data. On the other hand, iterative phasing algorithms make use of the prior knowledge and various constraints in real and reciprocal space. In this thesis, new approaches to the problem of direct digital phasing of X-ray diffraction patterns from two-dimensional organic crystals were presented. The phase problem for Bragg diffraction from two-dimensional (2D) crystalline monolayer in transmission may be solved by imposing a compact support that sets the density to zero outside the monolayer. By iterating between the measured stucture factor magnitudes along reciprocal space rods (starting with random phases) and a density of the correct sign, the complex scattered amplitudes may be found (J. Struct Biol 144, 209 (2003)). However this one-dimensional support function fails to link the rod phases correctly unless a low-resolution real-space map is also available. Minimum prior information required for successful three-dimensional (3D) structure retrieval from a 2D crystal XFEL diffraction dataset were investigated, when using the HIO algorithm. This method provides an alternative way to phase 2D crystal dataset, with less dependence on the high quality model used in the molecular replacement method.
ContributorsZhao, Yun (Author) / Spence, John C.H. (Thesis advisor) / Schmidt, Kevin (Committee member) / Weierstall, Uwe (Committee member) / Kirian, Richard (Committee member) / Zatsepin, Nadia (Committee member) / Arizona State University (Publisher)
Created2016
Description
The structure-function relation in Biology suggests that every biological molecule has evolved its structure to carry out a specific function. However, for many of these processes (such as those with catalytic activity) the structure of the biomolecule changes during the course of a reaction. Understanding the structure-function relation thus becomes a question of understanding biomolecular dynamics that span a variety of timescales (from electronic rearrangements in the femtoseconds to side-chain alteration in the microseconds and more). This dissertation deals with the study of biomolecular dynamics in the ultrafast timescales (fs-ns) using electron and X-ray probes in both time and frequency domains.
It starts with establishing the limitations of traditional electron diffraction coupled with molecular replacement to study biomolecular structure and proceeds to suggest a pulsed electron source Hollow-Cone Transmission Electron Microscope as an alternative scheme to pursue ultrafast biomolecular imaging. In frequency domain, the use of Electron Energy Loss Spectroscopy as a tool to access ultrafast nuclear dynamics in the steady state, is detailed with the new monochromated NiON UltraSTEM and examples demonstrating this instrument’s capability are provided.
Ultrafast X-ray spectroscopy as a tool to elucidate biomolecular dynamics is presented in studying X-ray as a probe, with the study of the photolysis of Methylcobalamin using time-resolved laser pump – X-ray probe absorption spectroscopy. The analysis in comparison to prior literature as well as DFT based XAS simulations offer good agreement and understanding to the steady state spectra but are so far inadequate in explaining the time-resolved data. However, the trends in the absorption simulations for the transient intermediates show a strong anisotropic dependence on the axial ligation, which would define the direction for future studies on this material to achieve a solution.
It starts with establishing the limitations of traditional electron diffraction coupled with molecular replacement to study biomolecular structure and proceeds to suggest a pulsed electron source Hollow-Cone Transmission Electron Microscope as an alternative scheme to pursue ultrafast biomolecular imaging. In frequency domain, the use of Electron Energy Loss Spectroscopy as a tool to access ultrafast nuclear dynamics in the steady state, is detailed with the new monochromated NiON UltraSTEM and examples demonstrating this instrument’s capability are provided.
Ultrafast X-ray spectroscopy as a tool to elucidate biomolecular dynamics is presented in studying X-ray as a probe, with the study of the photolysis of Methylcobalamin using time-resolved laser pump – X-ray probe absorption spectroscopy. The analysis in comparison to prior literature as well as DFT based XAS simulations offer good agreement and understanding to the steady state spectra but are so far inadequate in explaining the time-resolved data. However, the trends in the absorption simulations for the transient intermediates show a strong anisotropic dependence on the axial ligation, which would define the direction for future studies on this material to achieve a solution.
ContributorsSubramanian, Ganesh (Author) / Spence, John (Thesis advisor) / Rez, Peter (Committee member) / Alford, Terry (Committee member) / Weierstall, Uwe (Committee member) / Kirian, Richard (Committee member) / Arizona State University (Publisher)
Created2016