Page 22 - Carbon Nanotubes

Electric effects in nanotube growth 13

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Fig. 1. Field emission data from a mounted nanotube. An activated nanotube emits a higher current when
heated by the laser than when the laser beam is blocked (a). When activated by exposing the nanotube
to oxygen while heating the tip, this behavior is reversed, and the emission current increases dramatically
when the laser is blocked. The activated state can also be achieved by laser heating while maintaining a
bias voltage of -75 V. Note that the scale of the two plots is different; the activated current is always higher
than the inactivated current. As discussed in the text, these data led to the conclusion that the emitting
feature is a chain of carbon atoms pulled from a single layer of the nanotube-an atomic wire.

per adatom. We also knew at this time that the electric
field of the arc was not by itself sufficient to stabilize
an open tip having no spot-welds against closure[l2],
so we now regard these adatom "spot-welds'' as a nec-
essary ingredient to explain growth of nanotubes in the
oven laser-vaporization method as well as in the arc,
and probably other existing methods of nanotube
production.

5. ELECTRIC FIELD STABILIZATION
OF AN OPEN NANOTUBE TIP
The proposal that the essential feature of arc
growth was the high electric field that concentrates at
the growing nanotube tip prompted ab initio structure
calculations[ 12,131 to assess this hypothesis quantita-
tively. These calculations, which were performed for
single-walled nanotubes in high applied electric fields,
showed that field-induced lowering of the open tip en-
ergy is not sufficient to make the open conformation
more stable than the closed tip at any field less than
10 V/A. Whereas single-walled objects certainly an-
t'
Fig. 2. A graphic of a nanotube showing a pulled-out atomic neal and at 12000c form 'pheroidal
wire and several stabilizing spot-welds. Only two layers have fullerenes[l4,151, open multiwalled species have other
been shown for clarity, although typical multiwalled nano- alternatives, and thus may be auite different in this
tubes have 10-15 layers. The spot-weld adatoms shown be- respect. In particular, for multiwalled species, adatom
tween layers stabilize the open tip conformation against spot-welds may be sufficiently stabilizing to allow
closure. The atomic wire shown was previously part of the
hexagonal lattice of the inner layer. It is prevented from pull- growth and before succumbing to the
ing out further by the spot-weld at its base. conformation.