The
Weizmann Institute of Science have announced that a
research group headed by Dr. Ernesto Joselevich has
developed a new approach to create patterns of carbon
nanotubes by formation along atomic steps on sapphire
surfaces. Carbon nanotubes are excellent candidates
for the production of nanoelectronic circuits, but their
assembly into ordered arrays remains a major obstacle
toward this application.
The team was initially researching in a different direction:
they were trying to give carbon nanotubes (structures
reminiscent of rolled-up sheets of graphite) a preferred
orientation on a wafer by applying an electrical field
as the tubes were being formed. This works very well
with silicon dioxide wafers. On a sapphire support (sapphire
is a form of aluminum oxide), on the other hand, it
didn't work: the nanotubes were beautifully arranged
in parallel, but with an orientation that was completely
independent of the electrical field – even when no field
was applied at all.
Closer examination
of the sapphire surface solved the mystery: commercial
sapphire wafers are generally not cut exactly along
the plane of the crystal. Their surface is thus not
completely smooth; instead, it has parallel steps
– of atomic dimensions – between the different planes
of the crystal. The nanotubes wind up lying along
these steps. The researchers explain it like this:
the nanotubes form from a catalyst of iron nanoparticles
and are attracted to a local field created by the
steps. It is clear that these iron particles don't
like "climbing stairs;" instead, they "glide"
along the inner edge of the step, as though on a track.
Thus they remain continuously in contact with two
surfaces, rather than just one, which seems to stabilize
the catalyst. Just as an airplane leaves behind a
condensation trail, the iron particles leave the newly
formed nanotubes lying along their "tracks."
The nanotubes even follow kinks in the steps, which
are caused by defects in the crystal. This results
in either straight or zigzag-shaped tubes, which are
expected to have particularly interesting electronic
properties.
"The orientation
and form of the atomic steps on a crystal surface
can be controlled by the cutting process, and defects
can be created artificially," says Joselevich.
"It should thus be possible to produce different
nanowire arrangements in a controlled fashion."
Dr. Joselevich's findings appeared as the cover story
of Angewandte Chemie. Dr. Ernesto Joselevich can be
reached at ernesto.joselevich@weizmann.ac.il or 972-8-934-2350.
Dr. Joselvich's research
is supported by the Asher and Jeannette Alhadeff Research
Award, the Ilse Katz Institute for Material Sciences
and Magnetic Resonance Research, the Philip M. Klutznick
Fund for Research, Sir Harry A.S. Djanogly, CBE, UK
and Sylvia and Henry Legrain, Spain. He is the incumbent
of the Dr. Victor L. Ehrlich Career Development Chair.
The Weizmann Institute
of Science in Rehovot, Israel, is one of the world's
top-ranking multidisciplinary research institutions.
Noted for its wide-ranging exploration of the natural
and exact sciences, the Institute is home to 2,500
scientists, students, technicians and supporting staff.
Institute research efforts include the search for
new ways of fighting disease and hunger, examining
leading questions in mathematics and computer science,
probing the physics of matter and the universe, creating
novel materials and developing new strategies for
protecting the environment.
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