The effects of ammonia on the growth of large-scale patterned aligned carbon nanotubes using thermal chemical vapor deposition method

A systematic study on the effects of NH3 on the growth of carbon nanotubes in atmospheric thermal-CVD was undertaken. Large-scale patterned aligned CNTs were prepared by thermal-CVD method in temperature of 900 degreesC, The effects of NH3 on the growth stage were investigated. Patterned Ni-catalyst of thickness 10 nm was prepared by optical lithography and e-gun evaporation on p-Si(100) substrates lift-off process. Following a heating process, the CNTs were then grown at 900 degreesC by thermal-CVD method using argon (Ar), NH3 and methane (CH4) as the diluting, reactive and carbon source gases, respectively. In the experimental results, aligned CNTs could be synthesized with mixing ratio of CH4/NH3 larger or less than 1 varying the fraction of Ar dilution. The role of NH3 was considered to enhance the catalyst activity by preventing it from being passivated. Furthermore, vertically aligned free-standing CNTs were observed at low CNT density when dealing the introduction of NH3; meanwhile, a transition of growth characteristics from spaghetti-like to vertically-aligned was observed when the growth condition was switched from low NH3 to high NH3 ratio. Without Ar dilution, high NH3 flow relative to CH4 seemed to hamper the CNT growth as well as the degree of straightness and vertical alignment. A kinetics-based growth mechanism, either on carbon supply limited kinetics or on carbon diffusion limited kinetics, in combination with the proposed role of NH3, can successfully explain the effect of NH3 in different experimental conditions. Furthermore, the base growth mode was observed on all process conditions of thermal CVD in this work, and a model based on the effect of surface radius of curvature of catalyst particles on the surface energy based on broken bond model was adopted to explain the formation of bamboo-like structure and the correlation between the spacing of cone-shaped graphite and its straightness. (C) 2004 Elsevier B.V. All rights reserved.