science-fiction fantasy of nanotechnology building
novel structures, devices, and materials at the atomic
or molecular scale is becoming a reality. For the
great potential of nanoscience and nanotechnology to be
fully realized, however, research efforts must cross many
disciplines, from electrical engineering, mechanical engineering,
materials science, and computer science to bioengineering,
chemistry, and physics.
is this cross-disciplinary approach fostered more than
at UC Berkeley. Each month, Lab Notes is proud
to present the work of nanotechnology researchers from
the College of Engineering and our collaborators across
Nanocrystals, Quantum Dots, and Nature's Own Assembly Line
by David Pescovitz
Alivisatos's pioneering research into tiny nanocrystals and nanorods
is paying off in big ways. Chemically-pure clusters of anywhere
from 100 to 100,000 atoms, Alivisatos's nanocrystals and nanorods
have myriad applications that impact the macroworld from
tagging biological samples for genetic analysis and drug discovery
to the creation of plastic solar cells that can be painted onto
Alivisatos, Janke Dittmer, and Wendy Huynh with one of their
nanorod-based solar cells in the upper-right foreground.
The beauty of these nanomaterials, Alivisatos explains, is that
their unusual properties predicted by quantum mechanics
can be tuned by controlling the crystal's size and surface. For
example, the materials' ability to emit or absorb different colors
of light or conduct electricity can be altered.
Alivisatos's latest small tech innovation nanotechnology is a potentially
giant leap for solar energy. Several months ago, the group reported
a technique to make flexible solar cells that could someday provide
power for next-generation mobile phones, handheld computers, and
wearable electronics. The first prototypes boast efficiencies of
1.7 percent. This means that they can only convert 1.7 percent of
the energy they receive from the sun into electricity, far less
than the 10 percent efficiency of today's commercial photovoltaics.
"Our efficiency is not good enough yet by a factor of 10, but this
technology has the potential to do a lot better," says Alivisatos,
who is also part of the Materials Science Division of Lawrence Berkeley
National Laboratory. "There is a pretty clear path for us to take
to make this perform much better."
The solar cells were born from a previous breakthrough by the Alivisatos
Research Group the growth of two-dimensional rod-shapped
nanocrystals made from cadmium selenide, a semiconducting material.
Previously, all nanocrystals were dot-like spherical structures.
A panel of eight plastic solar cells based on inorganic nanorods and semiconducting polymers. The shiny ovals are the aluminum back electrodes of the individual solar cells.
The solar cells
consist of nanorods with (relatively speaking) large surface areas
dispersed in an organic polymer and sandwiched between electrodes.
Unlike traditional solar cells which are manufactured in expensive
clean rooms, plastic solar cells are created in beakers.
Alivisatos and his colleagues hope that the efficiency of the solar
cells can be increased by developing methods to align them perpendicular
to the electrodes rather than haphazardly mixing them in
the polymer so the loss of electrical current can be minimized.
They also hope to tune the nanorods to absorb varying colors of
sunlight to further increase the efficiency.
"This opens up all sorts of new applications, like putting solar
cells on clothing to power LEDs, radios, or small computer processors,"
says post-doctoral fellow Janke Dittmer, who with Alivisatos and
graduate student Wendy Huynh co-authored a paper reporting their
development in Science magazine.
In October, Nanosys Incorporated a Palo Alto-based firm co-founded
by Berkeley scientists signed an exclusive licensing agreement
in October for a broad set of Alivisatos's nanotechnology patents.
One potential application might be to grow nanorods into large plates
of Light-Emitting Diodes (LEDs) for lightweight computer displays.
this cross-section of mouse cells labeled with two different
sizes of semiconductor nanocrystals, or quantum dots, nuclei
show up as green and actin fibers show up as red under the
The solar cells are only the most recent in a long line of breakthroughs
from the Alivisatos group. In 1997, Alivisatos and UC Berkeley physicist
Paul McEuen built a single-electron nanocrystal transistor. And
in 1999, the work of Alivisatos and LBNL colleague Shimon Weiss
led to the development of quantum dots, nanocrystals that emit different
colors of light when a laser shines on them. The use of these nanocrystals
as barcode-like tags to detect and trace biological materials
proteins or DNA, for instance led to Alivisatos, Weiss, and
several others founding a company, Quantum Dot, in 1998.
Alivisatos's latest merging of nano and bio is to employ nature's
own assembly lines to arrange nanocrystals into more complex structures.
One technique, Alivisatos explains, is to piggyback nanocrystals
on single strands of DNA. When single strands of DNA recognize complementary
sequences on other strands, they pair off to form a familiar double
helix. The DNA, he explains, acts a template for creating desired
nanocrystal molecules. Alivisatos believes that this directed DNA
assembly technique could result in nanoscale devices as complex
as today's semiconductor circuits, potentially enabling the creation
of ultra-powerful and tiny nanocomputers.
"We've shown how organic chemistry can direct the assembly of inorganic
nanocrystals," Alivisatos says. "That's the first step in bringing
DNA from the biological world to the material world."
Alivisatos Group Homepage
UC Berkeley Press Release
Quantum Dot Corporation
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