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Nano
Research...Nano-Forschung
Nano Onderzoek
www.nano-Tsunami.com
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TRADING
PLACES NANOSTYLE:
Nanocrystals Show a Quick Route to Change
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Paul
Alivisatos is director of the Molecular Foundry,
a U.S. Department of Energy
national user facility aimed at the development
of nanotechnology.
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| BERKELEY,
CA -- Just as the Microtechnology Age was built upon
the introduction of impurities into crystals of semiconductor
materials, so, too, will crystalline doping be the bedrock
upon which the Nanotechnology Age is built. To advance
the arrival of this next technological era at a faster
pace, however, scientists need a better understanding
of what happens to nano-sized crystals under the various
forms of doping. Researchers
with the U.S. Department of Energy’s Lawrence Berkeley
National Laboratory (Berkeley Lab) and the University
of California at Berkeley have good news for the burgeoning
nanotechnology industry.
They’ve shown that for nanocrystals, the doping process
in which one type of positively charged atom, or cation,
is exchanged for another, take place at a much faster
rate than for crystals of extended size, and is fully
reversible, something that is virtually forbidden
in micro-sized crystals under the same environmental
conditions.
“Our
findings show that the cation exchange reaction offers
a versatile route for expanding the range of nanoscale
materials with diverse compositions, structures, and
shapes without having to develop new synthetic methods
to produce each individual nanostructure,” says chemist
Paul Alivisatos, the principal author of a paper reporting
this research which appears in the November 5, 2004
edition of the journal /Science./
Alivisatos
holds a joint appointment with Berkeley Lab, as director
of its Materials Sciences Division, and UC Berkeley,
where he’s the Chancellor's Professor of Chemistry
and Materials Science. He’s also director of the Molecular
Foundry, a U.S. Department of Energy national user
facility aimed at the development of nanotechnology.
Other
co-authors of the /Science/ paper were chemists Dong
Hee Son, Steven Hughes, and Yadong Yin, all of whom
hold appointments with either Berkeley Lab’s Materials
Science Division, UC Berkeley’s Chemistry Department,
or both.
Says
Dong Hee Son, “Another important result from this
study is that ionic nanocrystals apparently can be
transformed into other materials with different physical
and chemical properties but without altering their
original shape, simply through an exchange of cations.”
Doping
a crystal to transform it into another type of material
with different physical and chemical properties is
a long-established practice in extended solids. This
practice is being been carried over into the transformation
of nanocrystals grown from inorganic materials, including
metals and semiconductors. However, because nanocrystals
have a high surface-to-volume ratio (meaning they
are virtually all surface and no interior), their
reactions to the various forms of doping can be quite
different from the reactions of extended solids. For
example, in extended solids, chemical reactions run
very slowly because of the high activation energies
required to diffuse atoms and ions. These transformations
are also not reversible.
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In
these transmission electron microscope images, extremely
thin rod-shaped nanocrystals of cadmium selenide
in (A), are immediately transformed into sphere-shaped
nanocrystals of silver selenide (B)through a cation
exchange. Had the original rods been thicker, the
transformed crystals would also have been rod-shaped.
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| “In
our study with cation exchange reactions, the nanocrystals
acted more like molecules in chemical reactions than
extended solids,” says Son. “The speed and reversibility
of the reactions demonstrates that inorganic nanocrystals
are far more chemically dynamic than previously realized.”
Under
the leadership of Alivisatos,
Son and the other authors of the /Science/ paper worked
with nanocrystals of the semiconductor cadmium-selenide
(CdSe), which offer a high degree of control over
size and shape. They mixed a solution of CdSe nanocrystals
together with a small amount of silver nitrate at
room temperatures. In less than one second, the silver
cations reacted with the CdSe spheres to produce spheres
of silver-selenide (Ag_2 Se). When these Ag_2 Se spheres
were mixed with a solution containing an excessive
amount of cadmium cations, the reaction was reversed.
Though the reverse reaction took about a minute to
complete, the final product was CdSe spheres.
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(From left) Yadong Yin,
Dong Hee Son and Steven Hughes were co-authors of
a paper in Science which showed that doping semiconductor
nanocrystals with cations can transform them into
a different material much faster, by orders of magnitude,
than similar transformations in micro-sized crystals.
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| The
Berkeley researchers performed similar tests to transform
hollow spheres of cadmium-sulfide into hollow spheres
of silver-sulfide, and crystals of cadmium-telluride
in the shape of tetrapods into tetrapod crystals of
silver-telluride. Again, the transformation reactions
were fast, complete, and fully reversible.
“The cation exchange reaction in the nanocrystals
we investigated in this study, can easily be extended
to exchange with other cations,”
Alivisatos says. “On the other hand, attempts to induce
anion (negatively charged ions) exchange have not
been successful under similar experimental conditions,
possibly because the much larger size of the anions,
relative to the cations, makes diffusion more difficult.”
Berkeley
Lab is a U.S. Department of Energy national laboratory
located in Berkeley, California. It conducts unclassified
scientific research and is managed by the University
of California. Visit our Website at www.lbl.gov/ <http://www.lbl.gov/>.
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story has been adapted from a news release -
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