In situ diagnosis of chemical species for the growth of carbon nanotubes in microwave plasma-enhanced chemical vapor deposition

A synthesis of multi-wall carbon nanotubes (MWNTs) by microwave plasma-enhanced chemical vapor deposition using CH4/H-2/NH3 gases on Ni/Cr-coated glass at low temperature, was investigated by optical emission spectroscopy (OES) and quadrupole mass spectroscopy (MS). It was observed that the MWNTs were gown within a very restrictive range of the gas compositions. Optical emission lines were observed primarily from atomic hydrogen H-alpha, molecular hydrogen, and CN radicals. The quadrupole mass spectrum also showed the formation of C2H2 and HCN. An addition of a small amount of NH3 resulted in a decrease of C2H2, which could be used to estimate amounts of carbon sources present in the plasma for the growth of MWNTs, and increases of CN and H-alpha radicals acting as etching species of amorphous carbon. These results show that the etching species of amorphous carbon as well as the growth species are necessary and the ratio between two species have to be in appropriate condition for the synthesis of carbon nanotubes at low temperature. The optimum C2H2/H-alpha ratio in the gas mixture for the growth of MWNTs at low temperature was found to be 1:5 in this study. (C) 2002 Elsevier Science B.V. All rights reserved.