Electromechanical characterization of quasi-one dimensional nanostructures of silicon, carbon, and molybdenum disulfide via symmetry-adapted tight-binding molecular dynamics.

University of Minnesota Ph.D. dissertation. November 2010. Major: Material Science and Engineering. Advisors: Traian Dumitrica, William W. Gerberich. 1 computer file (PDF); xxii, 139 pages, appendix p. 138-139.With a newly developed symmetry-adapted tight-binding molecular dynamics (MD) capability, we performed microscopic calculations on a variety of quasi-one dimensional silicon, carbon, and molybdenum disulfide nanostructures. In symmetry-adapted MD the helical symmetry instead of the standard translational symmetry is used. In the considered nanostructures, equivalent calculations can now be performed with a substantial smaller, in terms of the number of atoms, repeating domain. The symmetry-adapted method was utilized in the studied highlighted below. The stability of the most promising ground state candidate silicon nanowires with less than 10 nm in diameter was comparatively studied with with nonorthogonal tight-binding and classical potential models. The computationally expensive tight-binding treatment becomes tractable due to the substantial simplifications introduced by the presented symmetry-adapted scheme. It indicates that the achiral polycrystalline of fivefold symmetry and the hexagonal (wurtzite) wires of threefold symmetry are the most favorable quasi-one-dimensional silicon arrangements. Quantitative differences with the classical model description are noted over the whole diameter range. Using a Wulff energy decomposition approach it is revealed that these differences are caused by the inability of the classical potential to accurately describe the interaction of Si atoms on surfaces and strained morphologies. The elastic response for a large catalog of carbon nanotubes subjected to axial and torsional strain was next derived from tomistic calculations that rely on an accurate tight-binding description of the covalent binding. The application of the computationally expensive quantum treatment is possible due to the simplification in the number of atoms introduced by accounting for the helical and ngular symmetries exhibited by the elastically deformed nanotubes. The elasticity of nanotubes larger than 1.25 nm in diameter can be represented with an isotropic elastic continuum. The torsional plastic response of single-walled carbon nanotubes is studied with tight-binding objective molecular dynamics. In contrast with plasticity under elongation and bending, a torsionally deformed carbon nanotube can slip along a nearly axial helical path, which introduces a distinct (+1,−1) change in wrapping indexes. The low energy realization occurs without loss in mass via nucleation of a 5-7-7-5 dislocation dipole, followed by glide of 5-7 kinks. The possibility of nearly axial glide is supported by the obtained dependence of the plasticity onset on chirality and handedness and by the presented calculations showing the energetic advantage of the slip path and of the initial glide steps. Symmetry-adaptedMD combined with density-functional-based tight-binding made possible to compute chiral nanotubes as axial-screw dislocations. This enabled the surprising revelation of a large catalog of MoS2 nanotubes that lack the prescribed translational symmetry and exhibit chirality-dependent electronic band-gaps and elastic constants. Helical symmetry emerges as the natural property to rely on when studying quasi-one dimensional nanomaterials formally derived or grown via screw dislocations. The nonlinear elastic response of carbon nanotubes in torsion was derived with the symmetry-adapted MD and a density-functional-based tight-binding model. The critical strain beyond which tubes behave nonlinearly, the most favorable rippling morphology, and the twist- and morphology-related changes in fundamental band gap were identified from a rigorous atomistic description. There is a sharply contrasting behavior in the electronic response: while in single-walled tubes the band-gap variations are dominated by rippling, multiwalled tubes with small cores exhibit an unexpected insensitivity. Results are assistive for experiments performed on nanotubes-pedal devices. Despite its importance, little is known about how complex deformation modes alter the intrinsic electronic states of carbon nanotubes. We considered the rippling deformation mode characterized by helicoidal furrows and ridges and elucidate that a new intralayer strain effect rather than the known bilayer coupling and &sigma-&pi orbital mixing effects dominates its gapping. When an effective shear strain is used, it is possible to link both the electrical and the mechanical response of the complex rippled morphology to the known behavior of cylindrical tubes. Moving on to graphene, to describe the strain stored in helical nanoribbons, we supplement the standard elasticity concepts with an effective tensional strain. Using &pi -orbital tight binding and objective molecular dynamics coupled with density functional theory, we show that twisting couples the frontier conduction and valence bands, resulting in band-gap modulations. In spite of the edges and ridges of the helical nanoribbons, from the effective strain perspective these band-gap modulations appear strikingly similar with those exhibited by the seamless carbon nanotubes