Theses and Dissertations from UMD

Permanent URI for this communityhttp://hdl.handle.net/1903/2

New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

More information is available at Theses and Dissertations at University of Maryland Libraries.

Browse

Subject: search.filters.subject.phonon

Search Results

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    NOVEL QUASI-FREESTANDING EPITAXIAL GRAPHENE ELECTRON SOURCE HETEROSTRUCTURES FOR X-RAY GENERATION
    (2024) Lewis, Daniel; Daniels, Kevin M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Graphene, the 2D allotrope of carbon, boasts numerous exceptional qualities like strength, flexibility, and conductivity unmatched for its scale, and amongst its lesser-known capabilities is electron emission at temperatures and electric fields too low to allow for conventional thermionic or field emission sources to function. Driven by the mechanism of Phonon-Assisted Electron Emission (PAEE), planar microstructures fabricated from quasi-freestanding epitaxial graphene (QEG) on silicon carbide have exhibited emission currents of up to 8.5 μA at temperatures and applied fields as low as 200 C and 1 kV/cm, orders of magnitude below conventional electron source requirements.These emission properties can be influenced through variations in microstructure design morphology, and performance is controllable via device temperature and applied field in the same manner as thermionic or field emission sources. As 2D planar devices, graphene microstructure electron emitters can also be encapsulated with a thermally evaporated oxide, granting electrical isolation and environmental resistance, and can even exhibit emission current enhancement under these conditions. Graphene electron emitters expressed as heterostructure material stacks could see implementation as electron emission sources in environments or devices where conventional thermionic or field emission sources can’t be supported due to thermal, power system, or physical size limitations, the presence of contaminants, or even poor vacuum containment. An explorable application could see an oxide-encapsulated graphene electron source paired with a layered interaction-emission anode to create a micron-scale vertical alignment x-ray source with no need of vacuum containment. We investigate these properties with using hydrogen-intercalated quasi-freestanding bilayer epitaxial graphene, a rare and difficult to manufacture formulation that allows the graphene to behave as if it were a freestanding structure, while still benefiting from the macro-scale mechanical strength and fabrication process compatibility afforded by its silicon carbide substrate. The quasi-freestanding nature of the graphene limits substrate phonon interactions, allowing the graphene phonon-electron interactions to dominate, in turn empowering the PAEE mechanic. Our devices benefit from an ease of interaction that is untenable for processes not employing QEG, with the speed and simplicity of fabrication being a hallmark of our investigations. We begin our exploration of how the PAEE mechanism itself can be influenced in our designs, and how process and fabrication optimizations can be leveraged for device applications. Graphene’s role in the fields of microelectronics, condensed matter physics, and materials science is still novel, and rapidly expanding, and our investigations explore a unique facet of this wonder material’s capabilities.
  • Thumbnail Image
    Item
    Phonon Modeling in Nano- and Micro- scale Crystalline Systems
    (2018) VanGessel, Francis; Chung, Peter; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Submicrometer phonon systems are becoming increasingly relevant in modern day technology. Phonon mechanisms are notably relevant in a number of solid-state devices including lasers, LEDs, transistors, and thermoelectrics. Proliferation of these devices has been driven by advancements in silicon-on-insulator technology. These advancements have allowed for the manufacture of devices with complex nanostructures and dimensions deep in the sub-microscale regime. However, accompanying improvements in the manufacture and design of novel crystalline systems is the requirement for accurate computational approaches for phonon modeling in nanostructured, anisotropic, and complex materials. The phonon Boltzmann transport equation is uniquely well suited to modeling energy transfer at the nano- and micro- meter length scales and is therefore an excellent candidate for this simulation task. However, current Boltzmann modeling approaches utilize a range of assumptions and simplifications that restrict their validity to isotropic, nominally one or two dimensional, or compositionally simple systems. In this dissertation we present an original finite volume-based methodology for the solution of the three dimensional full Brillouin zone phonon Boltzmann transport equation. This methodology allows for separate real and reciprocal space discretization. By taking a sampling of vibrational modes throughout the first Brillouin zone our methodology captures three unique sources of phonon anisotropy. We investigate the effect of phonon anisotropy in a fin field effect transistor, calculating the effect that incorporating various sources of anisotropy has on the resultant temperature fields. In a second study, we consider phonon flow through silicon nanowires with a modified boundary geometry. The three-dimensional flow fields are calculated and thermal transport below the Casimir limit is observed. Reduction in thermal conductivity is a result of maximizing the phonon backscatter that occurs in our phononic system. The backscatter serves to create regions of highly misaligned phonon flux. In addition, our silicon nanowire geometry has properties analogous with a high-pass phonon filter. In the final study we apply our Boltzmann transport methodology to the simulation of phonon transport in the energetic material, RDX. We study phonon transport in the vicinity of a material hotspot, the location at which chemistry initiates in the material. By applying Boltzmann modeling, applied for the first time to this material, we gain valuable insights into the interplay between thermal transport and phonon modes linked with initiation.
  • Thumbnail Image
    Item
    Fully Anisotropic Solution of the Three Dimensional Boltzmann Transport Equation
    (2016) VanGessel, Francis; Chung, Peter W.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The development of accurate modeling techniques for nanoscale thermal transport is an active area of research. Modern day nanoscale devices have length scales of tens of nanometers and are prone to overheating, which reduces device performance and lifetime. Therefore, accurate temperature profiles are needed to predict the reliability of nanoscale devices. The majority of models that appear in the literature obtain temperature profiles through the solution of the Boltzmann transport equation (BTE). These models often make simplifying assumptions about the nature of the quantized energy carriers (phonons). Additionally, most previous work has focused on simulation of planar two dimensional structures. This thesis presents a method which captures the full anisotropy of the Brillouin zone within a three dimensional solution to the BTE. The anisotropy of the Brillouin zone is captured by solving the BTE for all vibrational modes allowed by the Born Von-Karman boundary conditions.
  • Thumbnail Image
    Item
    Ab initio Lattice Dynamics and Infrared Dielectric Response
    (2004-11-24) lawler, hadley Mark; Shirley, Eric L.; Drew, H. Dennis; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Methods for theoretically evaluating lattice dynamics, anharmonic effects and related optical properties from first principles are designed and implemented. Applications of density-functional theory and the pseudopotential approximation are adapted, via the Born-Oppenheimer approximation, the Hellmann-Feynman force theorem, and wave-commensurate supercells, to a direct calculation of the Born-von Karman force constants. With a symmetry analysis and interpolation of Born-von Karman force constants, the complete phonon spectra are obtained for the cubic systems Ar, Si, Ge, and diamond, and for the stacked hexagonal system, graphite. The phonon spectra for the polar materials GaAs and GaP, in which the degeneracy between longitudinal and transverse optical modes is lifted, are also calculated. The splitting is a consequence of the macroscopic field associated with long-range Coulombic interactions and longitudinal displacements. Diagramatically-derived expressions for the finite lifetime of the Raman mode arising from phonon-phonon interactions are calculated for Si, Ge, and diamond from first principles, and agree with experiment to within uncertainty. The infrared absorption spectra of GaAs and GaP are calculated from first principles through the phonon anharmonic self-energy (phonon-phonon interaction) and the Born effective charges (photon-phonon interaction). Several aspects of the spectra are in detailed agreement with the experimental spectra, including the strong temperature dependence of the far-infrared absorption due to the onset of difference processes; the linewidth and asymmetric lineshape of the reststrahlen; the spectral structure of the absorption by two-phonon modes, and overall oscillator strengths. The theory allows for the identification of narrow spectral transmission bands with an ionic mass mismatch in the case of GaP. Analytic and complete calculations are performed for the ion-ion displacement correlation function in solid Ar, and agree well. The correlations are evaluated for arbitrary lattice vector and Cartesian displacement directions, and their pressure dependence leads to the conjecture that anharmonic effects are less prominent at higher pressures.