Theory of growth and mechanical properties of nanotubes

We have investigated the growth and mechanical properties of carbon nanotubes using a variety of complementary theoretical techniques. Ab initio molecular dynamics calculations show that the high electric field present at the tube tips in an are-discharge apparatus is not the critical factor responsible for open-ended growth. We then show by explicit molecular dynamics simulations of nanotube growth that tubes wider than a critical diameter of approximate to 3 nm, that are initially open, can continue to grow straight and maintain an all-hexagonal structure. Narrower tubes readily nucleate curved, pentagonal structures that lead to tube closure with further addition of atoms. However, if a nanotube is forced to remain open by the presence of a metal cluster, defect-free growth can continue. For growth catalyzed by metal particles, nanometer-sized protrusions on the particle surface lead to the nucleation of very narrow tubes. Wide bumps lead to a strained graphene sheet and no nanotube growth. We have also simulated the growth and properties of double-walled nanotubes with the aim of investigating the role of lip-lip interactions on nanotube growth. Surprisingly, the lip-lip interaction by itself does not stabilize open-ended growth, but rather facilitates tube closure by mediating the transfer of atoms between inner and outer shells. Furthermore, a simulation of growth on a wide double-wall nanotube leads to considerable deviations from the ideal structure, in contrast to corresponding simulations for single-wall tubes that result in nearly perfect structures. As regards mechanical properties, carbon nanotubes, when subjected to large deformations, reversibly switch into different morphological patterns. Each shape change corresponds to an abrupt release of energy and a singularity in the stress-strain curve. The transformations, observed in molecular dynamics simulations, are explained well by a continuum shell model. With properly chosen parameters, the model provides a remarkably accurate "roadmap" of nanotube behavior beyond Hooke's law. We have also investigated static and dynamical proper ties of carbon nanotubes under uniaxial tension by quantum and classical simulations. In strained nanotubes at high temperatures double pentagon-heptagon defect pairs are spontaneously formed. Their formation is energetically favorable at strains greater than 5%. They act as nucleation centers for the formation of dislocations in the originally ideal graphite network and lead to the onset of a plastic deformation of the carbon nanotube.