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- Genre: Doctoral Dissertation
Description
Fluctuation Electron Microscopy (FEM) has become an effective materials' structure characterization technique, capable of probing medium-range order (MRO) that may be present in amorphous materials. Although its sensitivity to MRO has been exercised in numerous studies, FEM is not yet a quantitative technique. The holdup has been the discrepancy between the computed kinematical variance and the experimental variance, which previously was attributed to source incoherence. Although high-brightness, high coherence, electron guns are now routinely available in modern electron microscopes, they have not eliminated this discrepancy between theory and experiment. The main objective of this thesis was to explore, and to reveal, the reasons behind this conundrum.
The study was started with an analysis of the speckle statistics of tilted dark-field TEM images obtained from an amorphous carbon sample, which confirmed that the structural ordering is sensitively detected by FEM. This analysis also revealed the inconsistency between predictions of the source incoherence model and the experimentally observed variance.
FEM of amorphous carbon, amorphous silicon and ultra nanocrystalline diamond samples was carried out in an attempt to explore the conundrum. Electron probe and sample parameters were varied to observe the scattering intensity variance behavior. Results were compared to models of probe incoherence, diffuse scattering, atom displacement damage, energy loss events and multiple scattering. Models of displacement decoherence matched the experimental results best.
Decoherence was also explored by an interferometric diffraction method using bilayer amorphous samples, and results are consistent with strong displacement decoherence in addition to temporal decoherence arising from the electron source energy spread and energy loss events in thick samples.
It is clear that decoherence plays an important role in the long-standing discrepancy between experimental FEM and its theoretical predictions.
The study was started with an analysis of the speckle statistics of tilted dark-field TEM images obtained from an amorphous carbon sample, which confirmed that the structural ordering is sensitively detected by FEM. This analysis also revealed the inconsistency between predictions of the source incoherence model and the experimentally observed variance.
FEM of amorphous carbon, amorphous silicon and ultra nanocrystalline diamond samples was carried out in an attempt to explore the conundrum. Electron probe and sample parameters were varied to observe the scattering intensity variance behavior. Results were compared to models of probe incoherence, diffuse scattering, atom displacement damage, energy loss events and multiple scattering. Models of displacement decoherence matched the experimental results best.
Decoherence was also explored by an interferometric diffraction method using bilayer amorphous samples, and results are consistent with strong displacement decoherence in addition to temporal decoherence arising from the electron source energy spread and energy loss events in thick samples.
It is clear that decoherence plays an important role in the long-standing discrepancy between experimental FEM and its theoretical predictions.
ContributorsRezikyan, Aram (Author) / Treacy, Michael M.J. (Thesis advisor) / Smith, David J. (Committee member) / McCartney, Martha R. (Committee member) / Rez, Peter (Committee member) / Arizona State University (Publisher)
Created2015
Description
A novel Monte Carlo rejection technique for solving the phonon and electron
Boltzmann Transport Equation (BTE), including full many-particle interactions, is
presented in this work. This technique has been developed to explicitly model
population-dependent scattering within the full-band Cellular Monte Carlo (CMC)
framework to simulate electro-thermal transport in semiconductors, while ensuring
the conservation of energy and momentum for each scattering event. The scattering
algorithm directly solves the many-body problem accounting for the instantaneous
distribution of the phonons. The general approach presented is capable of simulating
any non-equilibrium phase-space distribution of phonons using the full phonon dispersion
without the need of the approximations commonly used in previous Monte Carlo
simulations. In particular, anharmonic interactions require no assumptions regarding
the dominant modes responsible for anharmonic decay, while Normal and Umklapp
scattering are treated on the same footing.
This work discusses details of the algorithmic implementation of the three particle
scattering for the treatment of the anharmonic interactions between phonons, as well
as treating isotope and impurity scattering within the same framework. The approach
is then extended with a technique based on the multivariable Hawkes point process
that has been developed to model the emission and the absorption process of phonons
by electrons.
The simulation code was validated by comparison with both analytical, numerical,
and experimental results; in particular, simulation results show close agreement with
a wide range of experimental data such as the thermal conductivity as function of the
isotopic composition, the temperature and the thin-film thickness.
Boltzmann Transport Equation (BTE), including full many-particle interactions, is
presented in this work. This technique has been developed to explicitly model
population-dependent scattering within the full-band Cellular Monte Carlo (CMC)
framework to simulate electro-thermal transport in semiconductors, while ensuring
the conservation of energy and momentum for each scattering event. The scattering
algorithm directly solves the many-body problem accounting for the instantaneous
distribution of the phonons. The general approach presented is capable of simulating
any non-equilibrium phase-space distribution of phonons using the full phonon dispersion
without the need of the approximations commonly used in previous Monte Carlo
simulations. In particular, anharmonic interactions require no assumptions regarding
the dominant modes responsible for anharmonic decay, while Normal and Umklapp
scattering are treated on the same footing.
This work discusses details of the algorithmic implementation of the three particle
scattering for the treatment of the anharmonic interactions between phonons, as well
as treating isotope and impurity scattering within the same framework. The approach
is then extended with a technique based on the multivariable Hawkes point process
that has been developed to model the emission and the absorption process of phonons
by electrons.
The simulation code was validated by comparison with both analytical, numerical,
and experimental results; in particular, simulation results show close agreement with
a wide range of experimental data such as the thermal conductivity as function of the
isotopic composition, the temperature and the thin-film thickness.
ContributorsSabatti, Flavio Francesco Maria (Author) / Saraniti, Marco (Thesis advisor) / Smith, David J. (Committee member) / Wang, Robert (Committee member) / Goodnick, Stephen M (Committee member) / Arizona State University (Publisher)
Created2018
Description
Producing a brighter electron beams requires the smallest possible emittance from the cathode with the highest possible current. Several materials like ordered surface, single-crystalline metal surfaces, ordered surface, epitaxially grown high quantum efficiency alkali-antimonides, topologically non-trivial Dirac semimetals, and nano-structured confined emission photocathodes show promise of achieving ultra-low emittance with large currents. This work investigates the various limitations to obtain the smallest possible emittance from photocathodes, and demonstrates the performance of a novel electron gun that can utilize these photocathodes under optimal photoemission conditions. Chapter 2 discusses the combined effect of physical roughness and work function variation which contributes to the emittance. This is particularly seen in polycrystalline materials and is an explanation for their higher than expected emittance performance when operated at the photoemission threshold. A computation method is described for estimating the simultaneous contribution of both types of roughness on the mean transverse energy. This work motivates the need for implementing ordered surface, single-crystalline or epitaxially grown photocathodes. Chapter 3 investigates the effects of coulomb interactions on electron beams from theoretically low emittance, low total energy spread nanoscale photoemission sources specifically for electron microscopy applications. This computation work emphasizes the key role that image charge effects have on such cold, dense electron beams. Contrary to initial expectations, the primary limiter to beam brightness for theoretically ultra-low emittance photocathodes is the saturation current. Chapters 4 and 5 describe the development and commissioning of a high accelerating gradient, cryogenically cooled electron gun and photoemission diagnostics beamline within the Arizona State University Photoemission and Bright Beams research lab. This accelerator is unique in it's capability to utilize photocathodes mounted on holders typically used in commercial surface chemistry tools, has the necessary features and tools for operating in the optimal regime for many advanced photocathodes. A Pinhole Scan technique has been implemented on the beamline, and has shown a full 4-dimensional phase space measurement demonstrating the ability to measure beam brightness in this gun. This gun will allow for the demonstration of ultra-high brightness from next-generation ultra-low emittance photocathodes.
ContributorsGevorkyan, Gevork Samvelovich (Author) / Karkare, Siddharth (Thesis advisor) / Padmore, Howard (Committee member) / Alarcon, Ricardo (Committee member) / Kaindl, Robert (Committee member) / Graves, William (Committee member) / Arizona State University (Publisher)
Created2023