Doctor of Philosophy

dissertationIn this dissertation, our aim is to contribute to the understanding of edge properties of two-dimensional (2D) carbonic materials, including graphene and organometallic frame-works. A set of modeling and simulations, using rst-principles density functional theory (DFT), tight-binding (TB) method, and molecular dynamics (MD) method, have been performed to (1) investigate the structural edge stability of graphene from both thermo-dynamic and kinetic points of view and (2) explore the existence of nontrivial electronic edge states, which carry nonzero topological invariant, in 2D organometallic frameworks. Specically, this dissertation comprises the following four chapters of topics: (1) chemical versus thermal folding of graphene edges; (2) quantum manifestations of graphene edge stress and edge instability; (3) prediction of a two-dimensional organic topological insulator; (4) prediction of a large gap at Chern band in a two-dimensional organic framework. Our work presented in the rst two chapters not only has explained certain experimental observations on graphene edges, but also has been conrmed by other researchers' ndings, both experimentally and theoretically. The studies shown in the last two chapters predict the existence of quantum spin Hall phase, a physical phenomenon that has been an exciting area of recent research in condensed matter physics, in 2D organometallic frameworks, a class of materials that are used to be mostly of interest to chemists. Therefore, we hope that these new ndings could lead to a marriage of condensed matter physics and organic chemistry to foster an interdisciplinary research eld, which will broaden the scientic and technology impact of topological materials

Quantum Manifestations of Graphene Edge Stress and Edge Instability: A First-Principles Study

We have performed first-principles calculations of graphene edge stresses, which display two interesting quantum manifestations absent from the classical interpretation: the armchair edge stress oscillates with a nanoribbon width, and the zigzag edge stress is noticeably reduced by spin polarization. Such quantum stress effects in turn manifest in mechanical edge twisting and warping instability, showing features not captured by empirical potentials or continuum theory. Edge adsorption of H and Stone-Wales reconstruction are shown to provide alternative mechanisms in relieving the edge compression and hence to stabilize the planar edge structure.Comment: 5figure

Paenibacillus polymyxa Antagonism towards Fusarium: Identification and Optimisation of Antibiotic Production

An antibiotic produced by Paenibacillus polymyxa 7F1 was studied. The 7F1 strain was isolated from the rhizosphere of a wheat field. Response surface methodology was used to optimize the physicochemical parameters. The strain showed broad-spectrum activity against several plant pathogens. Identification of the strain was realized based on 16s rRNA gene and gyrB gene sequencing. The antibiotic was optimized by one-factor-at-a-time (OFAT) and response surface methodology (RSM) approaches. The suitable antibiotic production conditions were optimized using the one-factor-at-a-time method. The individual and interaction effects of three independent variables: culture temperature, initial pH, and culture time, were optimized by Box-Behnken design. The 16SrRNA gene sequence (1239 nucleotides) and gyrB gene (1111 nucleotides) were determined for strain 7F1 and shared the highest identities to those of Paenibacillus polymyxa. The results showed the optimal fermentation conditions for antibiotics produced by Paenibacillus polymyxa 7F1 were a culture temperature of 38 °C, initial pH of 8.0, and culture time of 8 h. The antibiotics produced by Paenibacillus polymyxa 7F1 include lipopeptides such as iturin A and surfactin. The results provide a theoretical basis for the development of bacteriostatic biological agents and the control of mycotoxins