Physical Properties and Transformations of Low-Dimensional Systems.

다국어 초록 (Multilingual Abstract)

Evolving from the macroscopic scale to the nanometer scale, in particular by reducing the dimensionality, fundamental properties (such as electronic and mechanical properties) of certain systems exhibit dramatic changes, which not only give rise to a wide range of emergent phenomena, but also boost technology development including nanoelectronics, optoelectronics and catalysis. In this thesis, I utilized combined techniques including density functional theory (DFT), molecular dynamic simulations (MD), continuum elasticity approach, and the tight-binding model to conduct a systematic study on low-dimensional nanostructures regarding their electronic and mechanical properties as well as underlying microscopic transformation mechanisms between different structural allotropes.First, I briefly introduce the motivation and background of this thesis. Then, in Chapter 2, I describe the computational techniques, mainly the DFT approach, on which most of my thesis is based.In Chapters 3 and 4, I apply the continuum elasticity method to study the phonon spectrum of two-dimensional (2D) and one-dimensional (1D) systems. My results highlight advantages of the continuum elasticity approach especially for the flexural acoustic phonon modes close to the Γ point, which are otherwise extremely hard to converge in atomistic calculations that use very large supercell sizes.From Chapter 5 to Chapter 7, I focus on allotropes of group III, V and VI elements and study both their stability and microscopic transformation mechanisms from one allotrope to another. First, I predicted a stable phosphorus coil structure, which may form by reconstruction of red phosphorous, and which was synthesized by filling a carbon nanotube with sublimed red phosphorus. Second, I proposed two stable 2D allotropes of Se and Te. I also suggested and evaluated a promising fabrication approach starting from natural 1D structures of these elements. After considering low-dimensional charge neutral systems, I changed my focus to study the effect of net charge on the equilibrium structure. Considering a heterostructure of alternating electron donor layers an monolayers of boron, I have identified previously unknown stable 2D boron allotropes that may change their structure under different levels of charge transfer.From Chapter 8 to Chapter 10, I focus mainly on carbon-based nanomaterials and their properties. In Chapter 8, I proposed a way to enhance the density of states at the Fermi level in doped C60 crystals in order to increase their superconducting critical temperature to room temperature. In Chapter 9, I have investigated a shear instability twisted bilayer graphene using the tight binding model. This system is susceptible to very small structural changes, since it becomes superconducting in a very narrow range of twist angles near the 'magic angle'. In Chapter 10, I introduced the cause of an unusual negative Poisson ratio and a shape-memory behavior in porous graphene with an artificially designed pattern.In Chapter 11, I finally present general conclusions of my PhD Thesis.