Possible interplay of tangential and perpendicular modes in the growth of Fe-filled carbon nanotubes

Boundary layer chemical vapour synthesis is a fabrication approach which exploits random fluctuations within the viscous boundary layer established between a laminar flow of pyrolyzed ferrocene and a rough substrate, to yield iron filled radial carbon nanotubes (CNTs). The key feature of this method is the significant enhancement of the encapsulated nanowire-length, which has been explained in terms of the diffusion gradient established at the open-tip of growing CNTs. In this work, we carried out a statistical investigation on the possible relationship between the width of the inner-CNT-cavity and the nanowire diameter within radially grown iron filled CNTs. A systematic analysis of nanowire-continuity allowed for the identification of particularly high filling rates, with nanowire-lengths up to 20 mu m and an almost complete filling of the CNT-capillary. Two types of contact-modes could be identified between the catalyst particles and the CNTs: the first type, in which the catalyst-diameter is comparable with the width of the inner CNT-cavity (full catalyst encapsulation), and the second type, in which the base particle is found to be not fully encapsulated (i.e. only partially encapsulated within the inner CNT-cavity). The observed CNT/catalyst-base interactions resemble those reported in recent studies for tangential and intermediate-tangential growth-modes occurring after nanotube-cap nucleation. Comparative statistical analyses performed on other types of Fe-filled CNT films (flower-like structures and buckypaper) obtained by pyrolysis of ferrocene or ferrocene/dichlorobenzene mixtures are further discussed.