Support-Catalyst-Gas Interactions during Carbon Nanotube Growth on Metallic Ta Films

We present a detailed study of processes and interactions occurring during the Fe-catalyzed chemical vapor deposition of carbon nanotubes on metallic Ta supports. In situ X-ray photoemission spectroscopy and X-ray diffraction show that the Fe catalyst increases the reactivity of Ta toward oxidation and carbide formation, whereas Ta promotes the reduction of Fe. This causes an unusual temperature dependence of carbon nanotube growth, where at low temperatures (similar to 550 degrees C) vertically aligned forests of carbon nanotubes with ohmic contacts grow readily on metallic Ta, whereas at high temperatures (>600 degrees C) nanotube growth is sparse because of the diffusion of Fe away from the surface through grain boundaries of in situ formed polycrystalline Ta2O5. The Fe-Ta model system highlights general material selection criteria for nanotube applications that require a conductive support.