Effect of catalyst layer thickness and Ar dilution on the plasma deposition of multi-walled carbon nanotubes

The growth behaviour of carbon nanotubes (CNTs), produced by radio-frequency plasma enhanced chemical vapour deposition, is studied here as a function of a CH4/Ar ratio and Ni catalyst layer thickness. The composition of the plasma mixture was observed as being crucial for the morphology of the nanotubes, indicating a transition from a random to a more uniform orientation, when argon is added to the plasma atmosphere. Scanning tunnelling microscopy shows, as a result of the argon dilution, the formation of a defective structure (i.e. pentagons) at the tip of vertically aligned CNTs while hexagonal atomic arrangement was detected on the sidewall of randomly oriented CNTs deposited by pure methane. The electronic structure of CNTs was then investigated by C Is' photoemission spectroscopy. The results show a shift of the overall spectral to the higher-binding-energy side, indicating the formation of metallic aligned tubes when argon is added to the plasma atmosphere. Experimental results are applied to develop a coherent picture of the relationship between the deposition parameters and the microstructural features, as well as to check the relationship of the electronic properties predicted for nanotubes with the plasma chemical composition. (C) 2003 Elsevier Science B.V. All rights reserved.