Abstract Submitted to the    NANOTUBE'04 Conference:

Using in situ x-ray photoemission spectroscopy (XPS) we have determined that particles of both pure Fe and iron oxide (FeO) are efficient catalysts for carbon nanotube growth, although the growth mechanism could be radically different, judging from the growth rates and characteristics of the resulting nanotubes. The changing chemical composition of the Fe catalyst has been followed during the different steps of the CVD growth. The interaction of Fe with the substrate is modified by the deposition of different intermediate layers (Al₂O₃, TiN or TiO₂) between the catalyst and the Si substrate. After annealing and hydrocarbon exposure, the originally pure Fe layer suffers a series of structural and chemical modifications that depend on the nature of the buffer layer employed: Strongly oxidized particles form onto Al₂O₃ layers, while bigger metallic islands form onto TiN and TiO₂. The formation of FeO particles ont o Al₂O₃ is correlated with fast and dense growth of straight and thin (1 to 5 walls) CNTs. On the other hand, nanotubes grown from the metallic particles have a bigger number of walls (more than 20), and show many metallic inclusions and structural defects. With this work we want to emphasize the importance of a precise characterization of the catalyst as a previous step to understanding the growth mechanism involved in carbon nanotube growth.