Bidirectional-growth of carbon nanofibers is a rare phenomenon found on free-standing catalyst particles, in contrast to the most commonly studied tip- and base-growth mechanisms for carbon nanostructures synthesized through thermal chemical vapor deposition. We reveal the underlying mechanisms of collective bidirectional growth in NixPd1-x-catalyzed carbon nanofiber carpets grown on a palladium substrate with varying nickel film thickness by monitoring the fiber growth evolution. The results show that the collective bidirectional growth is promoted and controlled by the chemical and physical restructuring of the sub-surface portion of the metal stack which undergoes micro-fragmentation as a result of the incorporation, diffusion, and precipitation of carbon. Carbon nanofiber growth can be controlled by engineering the catalyst-underlayer materials properties such as grain size, chemical composition and alloying. Since the determining factor whether carbon nanofibers or nanotubes are obtained is a strong function of catalyst size, the understanding of this growth mechanism can be transferred to the field of carbon nanotube synthesis. By keeping the grain size small enough to ensure carbon nanotube instead of carbon nanofiber growth, achieving dense, vertically aligned carbon nanotube carpets on metallic substrates might be possible, which is a prerequisite for carbon nanotube integration in integrated circuits.
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The authors would like to thank the electron microscopy center at ETH Zürich (EMEZ) for their support, Dr. A. Sologubenko and M. Süess for TEM imaging and helpful discussions, Y. Fleger for SE characterization at Bar Ilan University as well as M. Schamel and A. Furrer for FIB sample characterizations. Dr. G.D. Nessim was partially funded by a FP7 Marie Curie Reintegration Grant.