Materials design and band gap engineering of complex nanostructures using a semi-empirical approach : low dimensional boron nanostructures, h-BN sheet with graphene domains and holey graphene.

Semi-empirical approach;Materials design;Low Dimensional Boron Nanostructures;Band Gap Engineering

Cherno Baba Kah

University:University of Louisville

Department:Physics and Astronomy

关键词: Semi-empirical approach;    Materials design;    Low Dimensional Boron Nanostructures;    Band Gap Engineering;   

Others  :  https://ir.library.louisville.edu/cgi/viewcontent.cgi?article=3622&context=etd

美国|英语

来源: The Universite of Louisville's Institutional Repository

【 摘 要 】

This dissertation will explore the potential of a semi-empirical Hamiltonian, developed by the research group at the University of Louisville, in predicting the existence of new families of low-dimensional boron nanostructures based on icosahedral α-B12 clusters, and in tuning the band gaps of h-BN sheets with graphene domains and holey graphene. This semi-empirical Hamiltonian models electron-electron and electron-ion interactions using environment-dependent (ED) functions, and ion-ion interactions via usual pairwise terms. Additional features of our approach are that it uses a linear combination of atomic orbitals (LCAO) framework to describe the Hamiltonian and it calculates the charge distribution around a site self-consistently (SC). Throughout this dissertation, we will refer this semi-empirical Hamiltonian using the acronym SCED-LCAO. Our first application on boron nanostructures using SCED-LCAO revealed that one and two-dimensional nanostructures (referred as α, δ4 and δ6 sheets) based on icosahedral α-B12 clusters were structurally stable. A relative stability with respect to δ6 was also determined for the two-dimensional sheets with the strength of the stability in the order of �4 < � < �6. The infinite one-dimensional chain (which is the least stable among the low dimensional Boron structures predicted) as well as δ4 and δ6 sheets are found to have semiconducting properties while α sheet has metallic properties. With recent reports on the synthesis of an ultra-thin layer of α-tetragonal B50 structure, we delved into a second project that focused on investigating the structural stabilities and properties of a single layer of α-tetragonal B50. We found that, the α-tetragonal B50 does not keep its two-dimensional nature but prefers to exhibit symmetry breaking. Our prediction is inconsistent with experimental observations but this may be due to experiments discerning double or multi-layer structures of α-tetragonal B50. We note that the stability of multi-layer α-tetragonal B50 structure requires further investigation. A third application studied includes the band gap engineering on h-BN sheet by creating in it graphene domains of different shapes (triangular, circular, hexagonal and rectangular) and sizes with the aim of reducing the energy gap of pristine h-BN. For this project, the parametrization of the SCED-LCAO Hamiltonian corresponding to the nitrogen element was developed as a first step towards the investigation of pristine h-BN sheets and h-BN sheets embedded with graphene domains. The results of our study of h-BN sheets embedded with graphene domains reveal that the density of states are dependent on the shapes and sizes of the graphene domains and