Plasma composition during plasma-enhanced chemical vapor deposition of carbon nanotubes

Neutral species and positive ions were extracted directly from a $C2H2:NH3$ plasma used to grow vertically aligned carbon nanotubes (CNTs) and analyzed by mass spectrometry. We observe that $NH3$ suppresses $C2H2$ decomposition and encourages CNT formation. We show that the removal of excess carbon, essential for obtaining nanotubes without amorphous carbon deposits, is achieved through gas phase reactions which form mainly HCN. We determine an optimum $C2H2:NH3$ gas ratio which is consistent with previous observations based upon postdeposition analysis. We find, in contrast to thin film growth by plasma-enhanced chemical vapor deposition, that the optimum condition does not correspond to the highest level of ionization. We also provide evidence that $C2H2$ is the dominant precursor for CNTs in our experiments.