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
Quantum Wires
Often the nanoworld plays by different rules. Although the
electrical conductance of nanotubes is an important emulation of
big-world materials, their small size and perfect structure lead to
something utterly novel. They behave like waveguides for electrons,
permitting only a few propagating modes—a property more common
in fiber-optic communication.
Instead of changing smoothly with applied voltages, for instance,
currents in nanotubes increase and decrease in a stepwise fashion,
revealing the grainy nature of such quantum wires. This phenomenon
was first noticed in bundles of tubes, where it was thought that
perhaps a single nanotube was throttling the current. It was then
explicitly measured on a seventh-of-a-micron-long section of a
1.4-nanometer-wide armchair tubule. Except for the minuscule size
(Figure 16), the setup resembles a field-effect
transistor in your computer: The current through the tube depends on
the bias voltage between the ends and the potential in the middle of
the tube (gate).
A symmetrical and stiff nanotube allows no defects and almost no
vibrations (phonons), so nothing scatters an electron as it
travels almost freely from end to end. This makes its motion
ballistic. It behaves like a "particle in a box," and the
box is so tiny that the electron motion is quantized, so only a few
energy levels are possible. In addition, the capacitance,
C, of this box is so small that adding or removing just one
electron is energetically costly, e 2/C
being greater than the thermal energy. Overall these factors create
visible spacing between the energy levels involved in a conductance
event. The electron can only glide smoothly from source to drain if
a nice overall slope is in place. This happens only at certain gate
voltages that adjust the ladder of energy levels up or down, and is
indeed observed as a sequence of sharp peaks in the current.
Similarly, a gradual change in the bias causes a stepwise rather
than smooth growth of current, demonstrating again quantum behavior
in a nanotube wire. To suppress all thermal noise, studies of such
behavior require very low temperatures, from 10 degrees Kelvin down
to millikelvins, just above absolute zero. Electrical properties of
this nature are also sensitive to the perfection of the tube: Even a
minor twist or bend can shift those energy levels and result in a
sharp electrical signal in the tiny circuit. If these results
predict their real-world behavior, nanotubes may open up fascinating
opportunities for the developers of microelectromechanical systems
of the future.
The conductivity in molecular wires brings attention again to the
way that carbon achieves the itineracy necessary for metallic
behavior in an extended lattice. It is the same property that makes
benzene aromatic. Here the π electrons are completely
itinerant around each carbon ring without at the same time being
chemically reactive. No normal metal has that property. Lengths of
(n,n) carbon nanotubes will be true molecules that are also
true metals, something chemistry has never had before. There have
been conducting molecules, but they were never good conductors. When
doped they became pretty good conductors but pretty bad molecules,
destroyed by contact with air or water. The (10,10) buckytubes are
the first in a potentially infinite new class of objects that are
great molecules and great metallic conductors.
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