Acetylene: A Key Growth Precursor for Single-Walled Carbon Nanotube Forests

The growth of tall, single-walled carbon nanotube (SWNT) forests is often believed to require the use of specific feedstocks (e.g., ethanol) or etchants (e.g., water). Here, on the basis of mass spectrometry for hydrogendiluted methane in a microwave plasma-enhanced chemical vapor deposition (PECVD) reactor, we develop a purely thermal CVD process for SWNT forests without an etchant gas. The thermally grown SWNT forests are found to have a similar high quality, purity, growth rate, areal density, diameter, and chirality distribution as in the PECVD case. We show that C2H2 is the main growth precursor, and suggest more generally that feedstock conversion to C2H2 is of key importance to SWNT forest CVD. A reactive etchant, such as water, atomic hydrogen, or hydroxyl radicals, can widen the SWNT forest deposition window but is not required in cold-wall reactors at low pressures.