Carbon nanotubes synthesis in microwave plasma torch at atmospheric pressure

Well aligned multi-walled carbon nanotubes were synthesized at atmospheric pressure using a microwave plasma torch on silicon substrates with silicon oxide buffer layer and catalyst overlayer in the mixture of argon, hydrogen and methane. Iron or nickel was used as catalysts. The optimum substrate temperature for the deposition on Si/SiO2/Fe substrates was about 970 K. In this case SEM micrographs of the deposits revealed a presence of vertically aligned nanotubes with the diameters around 15 nm. TEM micrographs showed a presence of amorphous carbon particles in the samples and some defects in the wall structure of the produced nanotubes. In Raman spectra two peaks at 1332 and 1584 cm−1 were observed. The CNTs were also synthesized on the substrates without SiO2 buffer layer but their quality was lower. The synthesis with Ni instead of Fe catalyst required lower temperature and the alignment of the nanotubes was worse. The deposition process was monitored by optical emission spectroscopy. Atomic lines of hydrogen and argon, an emission of CN due to a presence of nitrogen impurities from atmosphere, a weak molecular band of CH and strong C2 emission were detected in the spectra.