Computer simulations on mechanical and electrical properties of nanoscale materials
Yakobson, Boris I.
Doctor of Philosophy
Nanoscale materials have highly regular atomistic structures with very few defects due to their small sizes. The small size and near-perfect structure give such materials unique properties compared with materials at a larger scale. This work investigates the structures and properties of several nanoscale materials using various computer simulation methods. The great strength of carbon nanotubes comes from the strong covalent bonding between carbon atoms, and has been of great interest in research, however both the theoretical and experimental results obtained are in a wide range. In this work, different atomic mechanisms about the nucleation of structural failure are proposed and analyzed, revealing the competition of two routes of forming defects--brittle bond breaking and plastic yield. The relevance of these two routes are shown to be dependent on nanotube symmetry, test time, and temperature. The nanotube strength is decided by the dominant route chosen under these parameters. Helical symmetry exists in many nanoscale structures, but it's far less utilized in computer simulations compared with translational and rotational symmetry. In this work a model for helical symmetry in tight-binding computational method is developed, then the implemented code are used to calculate the structure of thin silicon nanowires, as well as the properties of twisted armchair graphene nanoribbons, such as their deformation energy, band gap, and electrical conductance. Inspired by carbon nanotube, this work also investigates very thin silicon nanotubes. They are shown to have stable structures when filled with various metal atoms along the axis. They can also go through significant structural changes from one stable atomistic configuration to another. Such thin metal-endohedral silicon nanotubes can then combine to form thicker silicide wires that are morphologically identical to experimental disilicide wires synthesized from epitaxial growth.
Carbon nanotube strength; Objective method