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Chemists
find reliable way to grow quantum rods and pack them into microscopic
solar cells and LEDs
03
Mar 2000
IN THIS STORY:
Potential
applications for the health sciences 
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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 UC 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.
Potential applications for the health sciences
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."
Source:
Robert Sanders, Public Affairs
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