We propose, based on recent experimental results, a polyyne ring nucle
us (PRN) growth model for the synthesis of single-layer nanotubes (SLN
's). The PRN model assumes that (i) the critical nuclei are the planar
carbon polyyne rings that are observed to be most stable for sizes in
the range C-10 to C-40; (ii) ComCn clusters (possibly charged) play t
he role of a catalyst by serving to add C-2 or other gas phase species
into the growing tube; (iii) promoters such as S, Bi, and Pb serve to
modify the rates for these processes by stabilizing the ring structur
e. We suggest experiments to test and amplify this PRN model, includin
g a flow tube arrangement that might be useful for synthesizing more u
niform SLN's.