Effects of oxygen on the growth characteristics of carbon nanotubes on conductive substrates

Abstract

The effects of oxygen on Fe-catalyzed carbon nanotube (CNT) growth on Ta substrates was studied. CNTs were grown on Fe thin-film catalysts deposited on silicon substrates via exposure to C₂H₄ in a thermal chemical vapor deposition (CVD) furnace. Heating for CVD growth causes the Fe film to dewet to form catalyst particles. During CVD, the sample was exposed to gas mixtures of Ar, Ar/O₂, H₂, and C₂H₄. Experiments were performed with varying amounts of oxygen from mixing of the Ar and Ar/O₂ carrier gas, as well as pre-annealing samples in oxygen or hydrogen-rich environments. Samples were characterized via scanning electron microscopy (SEM) and atomic force microscopy (AFM). It was found that when an optimum amount of oxygen was introduced, taller CNT carpets were observed. Pre-annealing samples in an oxygenrich environment shows additional benefits in carpet growth. In contrast, pre-annealing in a hydrogen-rich environment counteracts the benefits of introducing oxygen during the growth phase. Coarsening of the catalyst particles was suspected as a reason for the difference in growth patterns, and pre-annealed sample morphologies were characterized without C₂H₄ flow. AFM scans show apparent coarsening in samples exposed to hydrogen-rich environments, and reduced coarsening in the samples exposed to oxygen-rich environments.