Catalyzed growth of carbon nanostructures occurs mainly through two modes, i.e., base growth when the metal nanoparticle remains at the bottom of the nanotube, or when it is lifted by the growing carbon nanostructure due to tip growth. A correct prediction of the dominant growth mode depends on the energy gain due to the addition of C atoms from the carbon-metal catalyst solution to the graphene sheets forming the carbon nanostructures. We determine this energy gain through atomistic scale

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... ular dynamics simulations. Our results suggest tip growth for Ni and base growth for Fe catalysts.