Superconducting
nanotubes may lie on the technology horizon, suggests a
theoretical study recently published by researchers from
the Commerce Department's National Institute of Standards
and Technology (NIST), the University of Pennsylvania, and
Bilkent University in Turkey.
The
intriguing possibility is the team's most recent finding
in a spate of studies showing how changing the shape of
tiny single-walled tubes of carbon may open a potential
mother lode of technologically useful properties. The theoretical
investigations are pointing out productive paths for other
researchers to follow in experiments that pursue opportunities
to make new materials and technologies with nanotubes.
Although
formidable obstacles remain, nanotubes were discovered only
about a decade ago, and initial product offerings are beginning
to edge onto the market.
"Carbon
nanotubes are now considered to be building blocks of future
electronic and mechanical devices," explains Taner
Yildirim, a physicist at the NIST Center for Neutron Research.
"We'll get there quicker if we have a good understanding
of the properties of the materials and the interactions
among them."
The
new calculations by Yildirim and his colleagues indicate
that strategically placing hydrogen on the exterior of so-called
zigzag nanotubes leads to dense concentrations of charge-carrying
electrons just below the material's conduction band.
In fact,
the structure of the molecules-initially resembling cylindrical
rolls of chicken wire-becomes rectangular, with a carbon
atom at each corner. During the structural makeover, the
nanotubes become diamond-like and are transformed from insulators
to metals.
The
result, says Yildirim, is a "four wire nanocable."
Because of the high density of electrons in this particular
configuration, he adds, it may be possible to chemically
engineer nanotube wires that are superconducting.
Depending
on the initial geometry of the nanotubes and on the pattern
of hydrogen coverage on tube walls, electronic structures
will vary greatly among the resultant materials, as will
their properties. The team's calculations indicate that
selective bonding of hydrogen to nanotubes can give rise
to a number of potentially useful applications in the emerging
field of molecular electronics.
In an
earlier study, team members and another collaborator predicted
that, when exposed to external pressure, nanotubes will
bind tightly and form stable ropelike networks. Published
in late 2000, the prediction was later verified in experiments
by other researchers.
Subsequent
studies published by the team indicate that the chemical
and electrical properties of a single-walled carbon nanotubes
can be controlled through a reversible process called mechanical
deformation. Flattening the radius of a nanotube so that
it becomes elliptical, says Yildirim, alters the arrangement
of electrons, suggesting an approach to engineering the
gap between different bands of electrons within the materials.
"Our
calculations indicate that, with radial deformation, it
is possible to close the band gap and make an insulating
nanotube metallic and vice versa," Yildirim explains.
If verified in experiments, this predicted capability could
yield new types of carbon-based materials and a host of
novel devices built using nanotubes with properties optimized
for specific applications.
The
work was partially supported by grants from the National
Science Foundation and the Scientific and Technical Research
Council of Turkey.
As a
non-regulatory agency of the U.S. Department of Commerce's
Technology Administration, NIST develops and promotes measurements,
standards and technology to enhance productivity, facilitate
trade and improve the quality of life.
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NOTE:
"Effects of hydrogen adsorption on single-wall carbon
nanotubes: Metallic hydrogen decoration," by O. Gulseren,
T. Yildirim, and S. Ciraci, was published in Physical Review
B, Vol. 66, Article121401. A copy of the paper, in Adobe
Acrobat PDF format, is available from Mark Bello at mark.bello@nist.gov.
For more information on the team's nanotube research, go
to www.ncnr.nist.gov/staff/taner/nanotube/