Utilization of catalyst deactivation for the growth of aligned and random carbon nanotubes by a single-step process

We have synthesized hybrid carbon nanotubes (CNTs) i.e. random secondary carbon nanotubes, on top of the aligned primary CNTs grown on Fe deposited n-type (1 1 1) silicon substrate by thermal chemical vapor deposition technique utilizing the concept of catalyst deactivation. The transmission electron microscopy and Raman spectroscopy results reveal that the primary CNTs are highly pure and crystalline and the secondary CNTs contain defects and impurities with a low degree of crystallinity. It is observed that by providing Fe catalyst during deposition, vertically aligned primary CNTs are synthesized for a shorter duration of growth. Random and entangled secondary CNTs on top of primary tubes are found to grow if the duration of growth is increased. It is suggested that termination of primary CNTs after a short time duration is because of the deactivation of the Fe catalyst deposited on the substrate, whereas Fe catalyst provided in the precursor solution initiates secondary growth of CNTs. The present work also suggests that base and tip growth mechanisms are responsible for the growth of primary and secondary CNTs, respectively.