Effect of oxygen addition to methane on growth of vertically oriented carbon nanotubes by radio-frequency plasma-enhanced chemical-vapor deposition

We have investigated the effect of oxygen addition to methane on the growth of vertically oriented carbon nanotubes (VCNTs) by radio-frequency plasma-enhanced chemical-vapor deposition (RFCVD). By adding moderate concentrations of oxygen to methane during RFCVD, highly crystalline VCNTs were densely grown at 430 ° C . With increasing oxygen concentration, the length of the VCNTs was increased and the linearity was degraded, whereas when excess oxygen was added to methane, no VCNTs were grown. Plasma states were investigated by optical emission spectroscopy during RFCVD. In the case of RFCVD with oxygen added to methane, peaks due to CO and OH radicals were dominant. The CO species appear to be one of the key factors in the growth of well-crystallized VCNTs. Moreover, VCNTs were grown at temperatures as low as 350 ° C with the addition of moderate amounts of oxygen to methane in RFCVD.