 
UC
Berkeley chemists find reliable way to grow quantum rods and
pack them into microscopic solar cells and LEDs
03
Mar 2000
By
Robert Sanders, Public Affairs
BERKELEY--
An assortment of microscopic crystals dubbed quantum dots and
quantum rods are proving to have properties that make for an
amazing variety of applications, from biological tracers to
electronic components.
A
report this week by chemists at the University of California,
Berkeley, details how to make quantum rods of a reliable size
and get them to pack together. The quantum rods can be used
as active elements in light-emitting diodes (LEDs) and solar
cells.
"This
is the first time anyone has gotten control of semiconductor
rod growth," said Paul Alivisatos, a professor of chemistry
at UC Berkeley and a member of the Materials Sciences Division
of Lawrence Berkeley National Laboratory. "These quantum
rods can be used as components in any number of devices. One
of our long-term projects is to make an effective and low-cost
photovoltaic device."
These
crystals, more properly known as nanocrystals because of their
nanometer or billionths-of-a-meter size, are chemically pure
clusters of from 100 to 100,000 atoms. Because of their small
size, they exhibit unusual properties predicted by quantum mechanics.
These
properties include emitting a single color of light when zapped
by a laser, with the color depending on the size of the nanocrystal.
A two-nanometer quantum dot flashes green; a five-nanometer
dot emits red. This property makes them ideal as markers or
tracers, like the dyes now used to stain cells or the tracers
used to follow processes in living cells.
Alivisatos
is part of UC Berkeley's Health Sciences Initiative, a research
effort that draws scientists from both the physical and biological
sciences into the search for solutions to today's major health
problems.
A
pioneer in the realm of nanocrystals, Alivisatos co-founded
a company last year - Quantum Dot Corp. - to develop nanocrystals
into biological markers for scientists and doctors alike.
"There
is a need for looking at many channels of information at once
so that biologists can follow many different proteins as they
move around a cell," said Alivisatos. "The advantage
of quantum dots is that you can label each protein with a different
quantum dot, shine a light on them and get all colors emitted
simultaneously - one input but different outputs."
Alivisatos
also has been experimenting with quantum dots and rods as photovoltaic
devices or solar cells. Instead of emitting colorful light when
illuminated by a laser or white light, they would produce electricity.
Three
years ago he and UC Berkeley physicist Paul McEuen created a
single-electron transistor using nanocrystals. In that electronic
circuit, a single nanocrystal served as a tunable bridge between
two leads of a transistor.
Now
Alivisatos and his UC Berkeley colleagues have found a way to
reliably stretch quantum dots into quantum rods with their own
unique properties.
In
a paper in the March 2 issue of Nature, they describe the chemical
manipulations necessary to grow rods of a given dimension, up
to 10 times longer than wide. The rods are made of cadmium selenide,
a semiconducting material from which Alivisatos also makes quantum
dots. The rods range in size up to about 10 nanometers (a millionth
of a centimeter) long and one nanometer thick.
"Once
we can do shape control, we can control the properties and get
homogeneous formation," he said. "As this field has
developed, research has centered around how to make and control
very small crystals and their fundamental properties."
Given
the right chemistry, nanorods even line up neatly, side by side,
into strips up to 25 nanometers long. This suggests that quantum
rods could be grown into large plates that emit light bright
enough to serve as light-emitting diodes (LED), which are found
today in many consumer electronics and appliances.
Similarly,
large quantities of quantum rods could be grown to make solar
cells. Alivisatos says the rods are 20 times better than dots
in converting light to electricity.
The
rods themselves could be useful as biological tracers too, Alivisatos
said. Unlike quantum dots, the rods emit polarized light, which
could provide information about the orientation as well as location
of the protein to which they are attached.
Alivisatos
has been experimenting with nanocrystals since 1985, when the
idea was new that clusters of several hundred atoms could exhibit
unique quantum properties not seen in larger crystals.
Experiments
by Alivisatos and colleague Shimon Weiss of the Materials Sciences
Division of LBNL led eventually to biological applications for
nanocrystals of different sizes, where each emits a different
color of light when hit with a laser.
The
idea of using these light-emitting quantum dots as biological
tracers was the nucleus of the start-up Quantum Dot Corp., which
he and Weiss founded with several others in February of 1999.
Their patent for the synthesis process was issued in January
of this year.
"Right
now we can get quantum dots that emit visible light at five
to 10 independent colors, but we can definitely extend this
further into the red," Alivisatos said.
The
dots are coated in shells of cadmium sulfide and glass, turning
them into glass beads that can easily be stuck onto biomolecules,
such as proteins or DNA. Unlike organic dyes, the separate colors
do not bleed together, and they last much longer than dyes,
which fade.
Alivisatos
hopes the quantum dots will find use in off-the-shelf medical
assays as well as in so-called "labs on a chip."
Coauthors
with Alivisatos on the Nature paper are Xiaogang Peng, now at
the University of Arkansas, Fayetteville, Department of Chemistry
and Biochemistry; UC Berkeley graduate students Liberato Manna,
Juanita Wickham, Erik Scher and Andreas Kadavanich; and postdoctoral
associate Weidong Yang.
The
work was funded by the Department of Energy and the National
Renewable Energy Laboratory.
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