Berkeley Researchers Make First
Rod-Shaped Semiconductor Nanocrystals MEDIA ADVISORY
BERKELEY, CA -- Size matters a lot in the world of
electronics and will matter even more in the upcoming age of
nanotechnology where devices may be a thousand times smaller than the
microchips of today. But shape matters too. To date, experimental
nanocrystals fashioned from semiconductors have all been shaped like
dots or spheres. No longer. Researchers with the U.S. Department of
Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and the
University of California at Berkeley have learned to make
semiconductor nanocrystals that are shaped like rods.
"We have demonstrated that controlling the kinetics of
semiconductor nanocrystal growth can be used to vary the shapes of the
resulting particles from a nearly spherical morphology to a rod-like
one," says Paul Alivisatos, the leader of the experimental team who
holds a joint appointment with Berkeley Lab's Materials Sciences
Division, and with the UC Berkeley Chemistry Department. "These
rod-like semiconductor nanocrystals may prove advantageous in
biological labeling experiments and as chromophores in light-emitting
diodes."
An earlier discovery by Alivisatos and his research
group that nanometer-sized crystal dots (spheres a few billions of a
meter in size) made from semiconductors such as cadmium selenide can
emit multiple colors of light depending upon the size of the crystal
opened the door to a number of potential applications including their
use as fluorescent probes for the study of biological materials.
However, optical and other properties of nanocrystals are also
dependent upon shape.
Until now, all non-metal nanocrystals have been
dot-shaped, meaning they are essentially one-dimensional. No
techniques had been reported for making two-dimensional or rod-shaped
semiconductor nanocrystals that would also be of uniform size.
However, in a paper that appeared in the March 2 issue of the journal
Nature, Alivisatos and his colleagues reported on
techniques they used to select the size but vary the shapes of
the cadmium-selenide nanocrystals they produced.
By carefully maintaining a relatively fast rate of
growth in the right mix of surfactant, the Berkeley researchers could
induce crystals of a selected size to assume an elongated rod-like
faceted shape that maximized crystal surface area. Subsequent tests
showed that these rod-shaped nanocrystals emit light that is polarized
along their long-axis in contrast to the
non-polarized light fluoresced by cadmium-selenide nanocrystal
dots.
"Polarized emission along the long axis of these rods
should be helpful in biological tagging experiments where the
orientation of the tag needs to be determined," says Alivisatos.
Other tests showed that the gap between emission and
absorption energies is larger for nanocrystal rods than for
nanocrystal dots which Alivisatos says should be an advantage in
applications such as Light-Emitting Diodes (LEDs) where the
re-absorption of light can be a problem. It was also shown that the
multiple rods could be packed and aligned, another advantage for both
LEDs and for the use of these rods in photovoltaic cells.
Co-authoring the Nature paper with Alivisatos were
Xiaogang Peng, Liberato Manna, Weidong Yang, Juanita Wickham, Erik
Scher, and Andreas Kadavanich.
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 Califoria.
Visit our Web site at: http://www.lbl.gov.
Lynn Yarris
lcyarris@lbl.gov
(510) 486-5375