FEATURE ARTICLE
Fullerene Nanotubes: C1,000,000 and Beyond
Some unusual new molecules—long, hollow fibers with tantalizing electronic and mechanical properties—have joined diamonds and graphite in the carbon family
Boris Yakobson, Richard Smalley
Applications
Potential applications of carbon nanotubes abound, together forming
a highly diversified technology portfolio. The first group includes
macro-applications, where armies of nanotube molecules might line up
to form a light, strong wire or a composite that could be unbeatable
as a material for making lightweight vehicles for space, air and
ground. If the costs ever permit, these materials might be used in
the elements of bridges, or of tall, earthquake-resistant buildings
or towers. Light ammunition and bulletproof vests can be envisaged.
All these applications rely on mechanical strength, a property that
is essentially straightforward but that requires volume production
of the crucial components, defect-free nanotubes of greater length.
The hollow structure of nanotubes, in particular of the single-wall
and wider variety, apparently gives them their ability to collapse
under compression and then to restore the volume. Such a property is
required for a product such as heavy-duty shock absorbers. The
outstanding thermal conductivity along the tubes, combined with the
relatively low rate of heat transport in the perpendicular
direction, may be of interest for microelectronics, where
progressing miniaturization demands better heat sinks. Development
of some of the next-generation processors for our computers is
currently arrested by the simple problem of overheating. Further,
the similarity in structure and mechanical properties of carbon- and
boron-nitride nanotubes suggests a perfect marriage, where a
conducting carbon tubule is coated by an insulating and more stable
to oxidation boron-nitride.
On the scale of the very small, we encounter an even broader
spectrum of possibilities for the use of single nanotubes. The use
of crash-proof nanoprobes in scanning microscopy, already
demonstrated, exploits the mechanical resilience and conductivity of
carbon nanotubes. Open-ended "nanostraws" could penetrate
into a cellular structure for chemical probing or could be used as
ultrasmall pipettes to inject molecules into living cells with
almost no damage to the latter. The yet-unexplored possibility of
excitonic transport through semiconducting nanotubes—where
energy would travel without charge flow—may lead to novel
probes for near-field optical microscopy.
Nanotubes with a wide range of electrical properties likely will
serve in smaller and faster computing machines in the future. A
pure-carbon metal-semiconductor heterojunction (based on embedding a
pentagon-heptagon pair between nanotube segments of different
helicity) has been recently analyzed. At low temperatures, quantum
ballistic transport gives us nanotube quantum wires, the core of a
"single-electron transistor."
The probable coupling of external mechanical stimuli with
conductance gives nanotubes entry into the family of future
submicroelectromechanical systems. Indeed, a C60-based
electromechanical amplifier has recently been reported. One can
anticipate even better performance from a nanotube, prone as it is
to mechanical distortions. Sharp, conducting nanotube tips could
serve as electron guns, lighting up the phosphor layer on flat-panel
displays. Furthermore, a buckyball encapsulated in a nanotube
segment of proper diameter glides freely from end to end, trapped
weakly in each end-cap by van der Waals forces, and an external
voltage could move a charged C60- molecule back and
forth. One bit of information read or written into such a two-state
trigger gives a computer equipped with a two-dimensional array of
such "buckyshuttles" an astonishing RAM capacity .
All these applications face a common problem: how to properly
implant a nanotube with desirable properties into a larger device or
a circuit. To become practical, furthermore, this has to be done
multiply and reproducibly. The issues of multiplicity and batch
fabrication arise as soon as experimental feasibility is demonstrated.
Making nanotubes of high quality and in volume is crucial if their
properties are to be exploited. Recent breakthroughs are
significant, but better ways of nanotube making are needed. They may
come as a gradual extension of the current methods, or the solution
may lie in processes entirely different and unexpected. To challenge
biotechnologists, Rod Ruoff from Washington University once tossed
out the idea of breeding new spider species that would spin
nanotubes on the cheap. The energetics of carbon bonds and the known
methods of synthesis suggest that such a useful arachnid would
probably be made of metal and enjoy a very hot climate! But nature
sometimes does offer a way; there may be some enzyme on the shelf
that would reduce the high-temperature process to the soft chemistry
of a fermenter.
Vanguard laboratories around the world seek better solutions, both
collaborating and competing with each other. Nature doesn't compete
with anybody; it just takes its time and surprises us once and
again. Who could expect a beautifully shaped pure-carbon torus
(Figure 17) to persistently appear as a spinoff of
nanotube growth, looking just like a "crop circle" viewed
from the air? The future of nanotube science is full of surprises,
some of them peculiar, some with the actual promise to improve our lot.
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