Study Of Structure And Properties Of GaN And SiC With One Dimensional Nanostructures By Atomistic Simulations

Low-dimensional nanostructures have become the focus of intensive research owing to their novel physical, chemical, and electro-optical properties as well as their potential applications in nanodevices. It is worthy of study not only for understanding the fundamental physical phenomena, but also for their promising applications such as functional building blocks for novel electrical, optical and magnetic nanodevices. In this dissertation the physical properties, such as thermal, mechanical and electrical properties, of GaN and SiC one dimensional nanostructures are investigated using classic molecular dynamics, first principles and ab initio molecular dynamics methods.1. Molecular dynamics methods with a Stilling-Weber potential are used to investigate the thermal stability, thermal conductivity and mechanical properties of wurtzite-type single GaN nanotubes and nanowires. Nanowires with axial orientations along the [001], [100] and [110] crystallographic directions and nanotubes with axial orientations along the [001] direction are studied.The melting temperature of GaN nanowires and nanotubes increases with the size (the diameter of nanowires and the thickness of nanotubes) to a saturation value, which is close to the melting temperature of bulk GaN. Melting of the [1-10] and [110]-oriented nanowires is initiated at the surface edges formed by the triangular shape and then spreads across the nanowire surfaces. The melting of the [001]-oriented nanowires and nanotubes starts from the surface, and rapidly extends to the inner regions as temperature increases.The thermal conductivity of GaN nanowires and nanotubes is smaller than that of the bulk GaN single crystal. The thermal conductivity is also found to decrease with temperature and increase with the size of the nanostructures.The simulation results show that (1) under axial tension, the nanotubes exhibit brittle properties, whereas at high temperatures, they behave as ductile materials. The brittle to ductile transition temperatures have been determined which generally increases with increasing wall thickness. The nanowires with different axial orientations show distinctly different deformation behavior under loading. The brittle to ductile transition is observed in the nanowires oriented along the [001] direction. The nanowires oriented along the [110] direction exhibit slip in the {010} planes, whereas the nanowires oriented along the [100] direction fracture in a cleavage manner. (2) Under axial compression, the buckling critical stress decreases with the increase of wire length, which is in agreement with the Euler theory.2. Molecular dynamics methods using the Tersoff bond-order potential are performed to study the nanomechanical behavior of [111]-orientedβ-SiC nanowires and nanotubes under axial tension, compression, torsion, combined tension-torsion and compression-torsion. Under axial tensile strain, the bonds of the nanowires are just stretched before the failure of nanowires and nanotubes by bond breakage. Under axial compressive strain, the collapse of the SiC nanowires by yielding or column buckling mode depends on the lengthes and diameters of the nanowires and nanotubes. Under torsional strain, the nanowires buckle with bond breaking and rearrangement. The combined loading causes the decrease of the critical stress.3. Electronic band structures of single-walled SiC nanotubes are studied under uniaxial strain using first principles calculations. The band structure can be tuned by mechanical strain in a wide energy range. The band gap decreases with uniaxial tension strain, but increases firstly with uniaxial compression strain and then decreases with further increasing compression strain.4. First-principles molecular dynamics is used to study the radiation-induced defect formation and displacement threshold energy of single-walled SiC nanotubes. The simulation results reveal a strongly anisotropic threshold for atomic displacement. The displacement threshold energy also shows size dependence, which increases with the diameter of SiC nanotubes. Three types of defects are observed after the irradiation, the vacancy, the Stone-Walse defect, and the antisite defects.