Page 20 - Carbon Nanotubes

ELECTRIC EFFECTS IN NANOTUBE GROWTH

DANIEL T. COLBERT and RICHARD E. SMALLEY
Rice Quantum Institute and Departments of Chemistry and Physics, MS 100,
Rice University, Houston, TX 77251-1892, U.S.A.

(Received 3 April 1995; accepted 7 April 1995)
Abstract-We present experimental evidence that strongly supports the hypothesis that the electric field
of the arc plasma is essential for nanotube growth in the arc by stabilizing the open tip structure against
closure. By controlling the temperature and bias voltage applied to a single nanotube mounted on a mac-
roscopic electrode, we find that the nanotube tip closes when heated to a temperature similar to that in
the arc unless an electric field is applied. We have also developed a more refined awareness of “open”
tips in which adatoms bridge between edge atoms of adjacent layers, thereby lowering the exothermicity
in going from the open to the perfect dome-closed tip. Whereas realistic fields appear to be insufficient
by themselves to stabilize an open tip with its edges completely exposed, the field-induced energy lower-
ing of a tip having adatom spot-welds can, and indeed in the arc does, make the open tip stable relative
to the closed one.
Key Words-Nanotubes, electric field, arc plasma

1. INTRODUCTION the electric field concentrates, and never in the soots
condensed from the carbon vapor exiting the arcing
As recounted throughout this special issue, significant
advances in illuminating various aspects of nanotube region, suggest a vital role for the electric field. Fur-
growth have been made[l,2] since Iijima’s eventful thermore, the field strength at the nanotube tips is very
discovery in 1991;[3] these advances are crucial to large, due both to the way the plasma concentrates
gaining control over nanotube synthesis, yield, and most of the potential drop in a very short distance
properties such as length, number of layers, and he- above the cathode, and to the concentrating effects of
licity. The carbon arc method Iijima used remains the the field at the tips of objects as small as nanotubes.
principle method of producing bulk amounts of qual- The field may be on the order of the strength required
ity nanotubes, and provides key clues for their growth to break carbon-carbon bonds, and could thus dra-
there and elsewhere. The bounty of nanotubes depos- matically effect the tip structure.
ited on the cathode (Ebbesen and Ajayan have found In the remaining sections of this paper, we describe
that up to 50% of the deposited carbon is tubular[4]) the experimental results leading to confirmation of the
is particularly puzzling when one confronts the evi- stabilizing role of the electric field in arc nanotube
dence of UgarteI.51 that tubular objects are energeti- growth. These include: relating the plasma structure
cally less stable than spheroidal onions. to the morphology of the cathode deposit, which re-
It is largely accepted that nanotube growth occurs vealed that the integral role of nanotubes in sustain-
at an appreciable rate only at open tips. With this con- ing the arc plasma is their field emission of electrons
straint, the mystery over tube growth in the arc redou- into the plasma; studying the field emission character-
bles when one realizes that the cathode temperature istics of isolated, individual arc-grown nanotubes; and
(-3000°C) is well above that required to anneal car- the discovery of a novel production of nanotubes that
bon vapor to spheroidal closed shells (fullerenes and significantly alters the image of the “open” tip that the
onions) with great efficiency. The impetus to close is, arc electric field keeps from closing.
just as for spheroidal fullerenes, elimination of the
dangling bonds that unavoidably exist in any open 2. NANOTUBES AS FIELD EMITTERS
structure by incorporation of pentagons into the hex-
agonal lattice. Thus, a central question in the growth Defects in arc-grown nanotubes place limitations
of nanotubes in the arc is: How do they stay open? on their utility. Since defects appear to arise predom-
One of us (RES) suggested over two years ago161 inantly due to sintering of adjacent nanotubes in the
that the resolution to this question lies in the electric high temperature of the arc, it seemed sensible to try
field inherent to the arc plasma. As argued then, nei- to reduce the extent of sintering by cooling the cath-
ther thermal nor concentration gradients are close to ode better[2]. The most vivid assay for the extent of
the magnitudes required to influence tip annealing, sintering is the oxidative heat purification treatment
and trace impurities such as hydrogen, which might of Ebbesen and coworkers[7], in which amorphous
keep the tip open, should have almost no chemisorption carbon and shorter nanoparticles are etched away be-
residence time at 3000°C. The fact that well-formed fore nanotubes are substantially shortened. Since, as
nanotubes are found only in the cathode deposit, where we proposed, most of the nanoparticle impurities orig-
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