  Recent direct measurements of the growth kinetics of individual carbon
nanotubes revealed abrupt changes in the growth rate of nanotubes maintaining
the same crystal structure. These stochastic switches call into question the
possibility of chirality selection based on growth kinetics. Here, we show that
a similar average ratio between fast and slow rates of around 1.7 is observed
largely independent of the catalyst and growth conditions. A simple model,
supported by computer simulations, shows that these switches are caused by
tilts of the growing nanotube edge between two main orientations,
close-armchair or close-zigzag, inducing different growth mechanisms. The rate
ratio of around 1.7 then simply results from an averaging of the number of
growth sites and edge configurations in each orientation. Beyond providing new
insights on nanotube growth based on classical crystal growth theory, these
results point to ways to control the dynamics of nanotube edges, a key
requirement for stabilizing growth kinetics and producing arrays of long
structurally-selected nanotubes.
