We have performed atomistic simulations for helical multi-shell (HMS) Cu nanowires and nanotubes. Our investigation on HMS Cu nanowires and nanotubes has revealed some physical properties that were not dealt in previous works that considered metal nanowires. As the diameter of HMS nanowires increased, their cohesive energy per atom and optimum lattice constant decreased. As the diameter of HMS nanotubes increases, their cohesive energy per atom decreased but optimum lattice constant increased. Shell-shell or core-shell interactions mainly affected on the lattice constant and the diameter of HMS nanowires or nanotubes. This study showed that HMS nanotubes for materials of fcc metal crystals can be maintained when forces exerted on atoms of inner shell of the HMS nanotubes are zero or act on the direction of the outside.

We have performed atomistic simulations for cylindrical multi-shell (CMS)-type Cu nanowires containing defects. Our investigation has revealed some physical properties that have not been detected in previous studies that have considered defect-free nanowires. Since the vacancy formation energy is lowest in the core of a CMS-type nanowire, a vacancy formed in the outer shell of a CMS-type nanowire naturally migrates toward the core. The maximum of the formation energy of an adhered atom on the surface of a CMS-type nanowire was modeled using a 16-11-6-1 nanowire. The formation energy of an adhered atom decreased when the diameter of the CMS-type nanowire was either above or below the diameter of the peak energy maximum. This investigation found three recombination mechanisms for the vacancy-adhered atom pairs: (i) by direct recombination, (ii) by a kick-in recombination, and (iii) by a ring recombination. Vacancy formation energy calculations show that an onion-like cluster with a hollow was formed, and molecular dynamics simulations for various CMS-type nanowires found that vacancies migrated towards the core. From these, we obtained basic information on the formation of hollow CMS-type metal nanowires (metal nanotubes) [Y. Oshima, et al., Phys. Rev. B 65, 121401 (2002)].

We have performed classical molecular dynamics simulations for hypothetical silicon nanotubes using the Tersoff potential. Our investigation presented a systematic study about the thermal behavior of hypothetical silicon nanotubes and showed the difficulty in producing silicon nanotubes or graphitelike sheets. However, since the elastic energy per atom to curve the sheet into cylinders for silicon atoms is as low as that for carbon atoms, if graphitelike sheets of silicon are formed, the extra cost to produce the tubes is of a similar order to that in carbon. Through the investigations on the structure and properties of a double-wall silicon nanotube, we concluded that quasi-one-dimensional structures consisting of silicon atoms become nanowires rather than nanotubes in order to minimize the number of sp2 bonds.

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