We report on some of the conditions during carbon sublimation which pr
omote single-shell carbon nanotube formation. Using as a catalyst a mi
xture of two metals (Co and Pt) in the anode, we observe a higher dens
ity of single-shell carbon nanotubes than that found with cobalt alone
. Using other mixtures (Co and Fe), the nanotube yield is always lower
, and their formation is even hindered (Co and Ru). First attempts of
vacuum annealing at high temperature on samples containing cobalt-cata
lyzed single-shell carbon nanotubes suggest that this might be a good
way to eliminate the catalyst particles. We present high-resolution tr
ansmission electron microscopy images of samples obtained by such proc
edures.