| Metallic nanowires have been intensively studied because of the fundamental interests in low-dimensional physics and technological applications as molecular electronic devices. Recently, Kondo and Tagayanagi provided evidence that thin gold wires form a helical structure, akin in some respects to carbon nanotubes. The suspended gold nanowires with 6 nm in length and diameters down to 0.6 nm were successfully prepared and characterized by electron-beam irradiation on a gold thin film in an ultrahigh vacuum transmission electron microscopy (UHVTEM). It was found that gold (110) nanobridges, which possess a regular crystalline fcc structure when sufficiently thick or short, can transform at diameters below ~ 1.5 nm into regular but noncrystalline wires several nanometers long, hanging between tips. These nanowires appear to form mostly discrete, helical multishell structures of specific"magic"sizes and shapes, and each tube consists of a triangular sheet folded cylindrically onto itself. The helical multishell metallic nanowires represent a novel organization of matter, similar but not identical to clusters, which is yet to be fully understood. What determines the magic helical multishell nanowire shapes and sizes? What about nanowires made of other metals besides gold? The physical properties of metallic nanowires, such as mechanical, thermodynamic, electronic, magnetic, optical, conductance, etc., and the evolution of the wire structure and properties under external agents, including temperature, stress, and fields, is potentially interesting. In this thesis, we have systematically studied the novel structures of gold, zirconium, rhodium, and titanium nanowires by using genetic algorithm (GA) based on molecular dynamics (MD). Based on the optimized structures of these metallic nanowires, their novel physical properties have been investigated.The structures of free-standing gold nanowires with diameters from 0.4 to 3.0 nm are systematically studied by using molecular-dynamics-based genetic algorithm simulations with glue potential. Helical and multiwalled cylindrical structures are found for the thinner nanowires. As the wire diameter increase up to about 3nm, the emerge of fcc crystal-like structures in the Au nanowires are observed. This noncrystalline-crystalline transition starts from the core region of the nanowires. The... |
