Browsing by Subject "phosphorene"

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    Atomic scale electro-mechanics of two dimensional materials using modeling and simulations
    (2018-06) Verma, Deepti
    Objective molecular dynamics (OMD) is a generalization of the universally adopted periodic boundary conditions (PBCs) used in atomic simulations. By taking advantage of the translational symmetry, molecular dynamics under PBCs reduces the overall number of atoms that need to be simulated in crystalline bulk materials. Unfortunately, PBCs are not generally applicable to structures having helical and rotational symmetries. OMD replaces the use of translational symmetry with helical and rotational symmetries, which are widely present in a number of nanostructures and nano materials. Such structures include carbon nanotubes, mechanically deformed nanomaterials such as 2D films subjected to bending and torsion. The recent capability resulted from the coupling of self-consistent charge density functional tight binding (SCC-DFTB) with OMD enables unprecedented calculations on helical and mechanically deformed two-dimensional materials with quantum mechanical accuracy. This coupling is especially important for simulating materials with complex bonding such as zinc oxide thin films, which contains charged species. It also captures the charge redistribution under mechanical deformation in layer bending. In this thesis, we use this capability to simulate bending of 2D materials and build continuum elastic models based on the atomistic data. These structures of interest include phosphorene, boron nitride and zinc oxide. Mechanical properties of 2D materials are studied mainly via bending deformations. Bending deformations lead to charge distribution across the thickness in two-dimensional materials. This effect can produce structural effects in nano-films. Zinc oxide layers are important because of their properties of piezo-electric effects with potential applications as important energy materials. In addition, we also study bending and in-plain deformation of boron nitride. This understanding of bending deformations will help advance the development of strain engineering technology for these 2D materials.
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    Evaluating Graphene as a Channel Material in Spintronic Logic Devices
    (2016-03) Anugrah, Yoska
    Spintronics, a class of devices that exploit the spin properties of electrons in addition to the charge properties, promises the possibility for nonvolatile logic and memory devices that operate at low power. Graphene is a material in which the spin orientation of electrons can be conserved over a long distance, which makes it an attractive channel material in spintronics devices. In this dissertation, the properties of graphene that are interesting for spintronics applications are explored. A robust fabrication process is described for graphene spin valves using Al2O3 tunnel tunnel barriers and Co ferromagnetic contacts.  Spin transport was characterized in both few-layer exfoliated and single-layer graphene, and spin diffusion lengths and spin relaxation times were extracted using the nonlocal spin valve geometry and Hanle measurements. The effect of input-output asymmetry on the spin transport was investigated.  The effect of an applied drift electric field on spin transport was investigated and the spin diffusion length was found to be tunable by a factor of ~8X (suppressed to 1.6 µm and enhanced to 13 µm from the intrinsic length of 4.6 µm using electric field of ±1800 V/cm). A mechanism to induce asymmetry without excess power dissipation is also described which utilizes a double buried-gate structure to tune the Fermi levels on the input and output sides of a graphene spin logic device independently. It was found that different spin scattering mechanisms were at play in the two halves of a small graphene strip. This suggests that the spin properties of graphene are strongly affected by its local environment, e.g. impurities, surface topography, defects. Finally, two-dimensional materials beyond graphene have been explored as spin channels.  One such material is phosphorene, which has low spin-orbit coupling and high mobility, and the interface properties of ferromagnets (cobalt and permalloy) with this material were explored.  This work could potentially enable spin injection without the need for a physical tunnel barrier to solve the conductivity mismatch problem inherent to graphene.