In situ control of the catalyst efficiency in chemical vapor deposition of vertically aligned carbon nanotubes on predeposited metal catalyst films

Premature termination of growth, presumably because of catalyst deactivation, is an undesirable side effect of chemical vapor deposition of vertically aligned carbon nanotubes on predeposited metal catalyst films. The addition of ferrocene, an effective precursor for in situ Fe formation, was found to enhance carbon nanotube growth rates and extend growth to 3.25 mm thick carbon nanotube films. Ferrocene was introduced into the gas stream by thermal evaporation concurrently with acetylene using a specially constructed source. The key factor facilitating the growth of thick carbon nanotube films was the independent and precise control of the ferrocene amount in the feedstock. The carbon nanotube films were characterized by scanning and transmission electron microscopy, and Raman spectroscopy. The temperature dependence of the carbon nanotube growth with ferrocene exhibits a steep drop at high substrate temperatures and a loss of vertical alignment at 900 °C. The negative temperature coefficient of the growth rate suggests that the reaction mechanism of vertically aligned carbon nanotube growth is governed by a heterogeneous intermediate step.