Browsing by Subject "Objective molecular dynamics"

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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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    Understanding of edge and screw dislocations in nanostructures by modeling and simulations
    (2013-02) Dontsova, Evgeniya
    The role of the extended dislocation defects in nanostructures only recently began to be explored. In bulk materials, dislocations are modeled only away from their cores within the framework of the continuum mechanics. It is known that applying continuum modeling in the core region leads to divergences. In nanostructures, the core region dominates and new investigation methods are needed. This work contributes to the fundamental understanding of the role of dislocations in important carbon and zinc oxide nanostructures, by using atomistic investigation methods. In quasi-zero-dimensional structures, thesis describes the first attempt to rationalize dislocation processes in carbon nano-onions. Experiments show that carbon nano-onions exhibit an unusual dislocation dynamics with unexpected attraction of outer edge dislocation towards the core. Atomistic calculations combined with rigorous energy analysis attribute this behavior to an unusual inward driving force on the outer edge dislocation associated with a reduction in the number of dangling bonds. Moving on to quasi-one-dimensional nanostructures, we study the stability of screw-dislocated zinc oxide structures in the wurtzite phase with a symmetry-adapted molecular dynamics methodology, which introduces a significant simplification in the simulation domain size by accounting for the helical symmetry explicitly. The goal is to provide the theoretical support for a universal screw-dislocation-driven growth mechanism suggested by recent experiments. Moreover, the effects of axial screw dislocations on the electronic properties in helical zinc oxide nanowires and nanotubes are explored. We demonstrate significant screw-dislocation-induced band gap modifications that originate in the highly distorted cores. Finally, using the same objective technique, we investigate the stability against torsional deformations of quasi-one-dimensional graphene nanoribbons with bare, F-, and OH-saturated armchair edges. The prevalence of twisted nanoribbons prompted the construction of a simple phenomenological model inspired from the Landau phase transition theory, which is based on the atomistic data and gives the structural parameters of the nanoribbon as functions of its edge chemistry and axial strain.