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 California. Visit our Web site at:
http://www.lbl.gov.
Lynn Yarris
lcyarris@lbl.gov
(510)
486-5375