The research contained in this thesis focus on low dimensional semiconductor nanostructures: morphology of core-shell nanowires, vapor-liquid-solid growth of Si nanowires, and assembly of single-walled carbon nanotubes. The morphological instability of low dimensional epitaxial core-shell nanowires consisting dislocation-free and misfit-strained core/shell is investigated using a linear stability analysis. Molecular dynamics and Monte Carlo simulations are performed to study the surface segregation-induced freezing in Au-Si alloy system and the Au-catalyzed vapor-liquid-solid growth of Si nanowire based on the angular-dependent embedded atom method. The assembly of carbon nanotube networks and vertically aligned carbon nanotubes are investigated using coarse-grained molecular dynamics simulations.

The morphological stability against azimuthal, axial, and general helical perturbations for epitaxial core-shell nanowires in the growth regimes limited by either surface diffusion or evaporation-condensation surface kinetics. For both regimes, we find that geometric parameters (i.e., core radius and shell thickness) play a central role in determining whether the nanowire remains cylindrical or its shell breaks up into epitaxial islands similar to those observed during Stranski-Krastanow growth in thin epilayers. The combination of small cores and rapid growth of the shell emerges as the key factor leading to stable shell growth. In addition, the effect of surface stress is considered, the morphological stability is affected by the sign of product of mismatch strain and surface stress. Our results provide an explanation for the different core-shell morphologies reported in the Si-Ge system experimentally and also identify a growth-induced intrinsic mechanism for the formation of helical nanowires.

We report an atomic simulation study of the surface profile and surface tension of liquid AuSi alloys using the angular-dependent embedding atom method. The surface-induced atomic layering is clearly in the AuSi alloy system, a crystalline monolayer is predicted at the surface of the eutectic liquid above the eutectic temperature. The freezing of crystalline monolayer appears to be a first order phase transition, the pre-frozen layer progressively decreases its width with increasing temperature. The calculations of surface tension are carried out using the mechanical definition and capillary fluctuation method, the surface stress decreases with decreasing temperature and Au composition. No observable size effect is found on surface tension even for droplets down to 10 nanometers in diameter. Monte Carlo simulation with random Si depositions is performed to investigate the atomic mechanism of Si nanowire vapor-liquid-solid growth. The nanowire sidewalls, which are wetted by Au atoms from the catalyst droplets, exhibit faceting without discernible rough region. Volume diffusion is dominant compared with surface diffusion due to surface segregation even in nano-size droplet, the non-planar nucleation at the catalyst-nanowire interface is observed with steady nucleation rate.

The aligned topology of ultra-thin single-walled carbon nanotube networks is investigated using Molecular Dynamics simulations with angular-dependent interaction. Our results show that the effectiveness of geometrical confinement induced by controlling the channel geometries, mainly their width and depth. The networks show a marked reduction in metallic transport as the effect of decreasing width enhances the alignment along the channel. The thin networks have a higher probability for semiconduction, and the probability decreases rapidly within the first few mono-layers. The coarse-grained molecular dynamics simulation with Van der Waals interaction is employed to investigate the mechanical assembly of vertically aligned carbon nanotubes during stretching and densifying processes. The space limitations in different directions are the key factors controlling the characteristic curvatures of vertically aligned carbon nanotubes, so the strain-dependent electrical resistance can be achieved under mechanical loads.