Mechanism of cone-shaped carbon nanotube bundle formation by plasma treatment

Formation of the cone-shaped multi-walled carbon nanotube (MWCNT) bundles was investigated with the consideration of the induced dipole moments of the MWCNTs interaction under the ion irradiation which is accelerated by the applied sheath electric field for the various argon, hydrogen, nitrogen, and oxygen plasmas. Vertically grown MWCNTs were irradiated by energetic ion whose energy and dose were controlled by the sheath formed on the MWCNT substrate. Plasma irradiation was carried out in a downstream region separated from the plasma source region, providing that the irradiated ion density and energy could be controlled precisely with the sheath electric field. In argon and hydrogen plasmas, the cone-shaped MWCNT bundle was not fabricated, while it was formed successfully in nitrogen and oxygen plasmas. Especially, the oxygen plasma was the most effective in the formation of the bundle. The mechanism of the bundle formation could be explained by a model explaining the interaction between the induced dipole moment of the MWCNT and the sheath electric field. For the nitrogen and oxygen plasma irradiated MWCNT, the induced dipole moment could be enhanced by C-N and C-O bonds so the size of the bundle is proportional to the ion irradiation and the sheath electric field. (C) 2010 Elsevier Ltd. All rights reserved.