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messy biology. The sponge s secret, they discovered, was
that amine and hydroxyl chemical groups in the enzyme
produce the silicon oxide and assemble it in the required
way. That meant that all the chemicals a new synthesis tech-
nique would require could be found in ammonia and water.
The researchers found that by mixing molecules containing
the metal oxides precursors into water, and then exposing
the mixture to ammonia gas, they could create thin lms
of highly crystalline semiconductors---materials useful for
electronics. "This is the breakthrough that gets us into the
domain of practical usefulness," Morse says.
Moreover, the crystals have a complex nanostructure that
could improve the performance of photovoltaic devices.
Near the surface of the water, the concentration of ammo-
nia gas is relatively strong, so this is where the semiconduc-
tor crystal starts to form. As the ammonia slowly di uses
deeper into the water, however, it causes crystals to grow
down into the mixture, producing a thin lm that is not
uniform but rather comprises a network of needles or at
plates each merely a few billionths of a meter thick. That
network could be the basis for a more e cient solar cell.
The crystalline-silicon solar cells that currently dominate
the photovoltaic market are expensive---so expensive that the
energy they produce costs several times as much as energy
generated by fossil fuels. One reason is the high price of
their raw materials. Silicon is extremely abundant on earth,
but it doesn t exist as a pure element; instead, it s bound
up with oxygen and other elements---in sand, for example.
Making pure silicon requires a lot of energy.
To lower the costs of solar cells, researchers have looked
for ways to cut down on the amount of silicon they use.
Some have turned to less expensive thin lms made from
cadmium telluride or copper indium diselenide. Extremely
thin layers of these new semiconductors can absorb the
same amount of light as thicker slabs of crystalline silicon.
Morse s fabrication technique could be an inexpensive way
to make such thin lms; in addition, the nanostr ucture that
his method produces is particularly well suited for absorb-
ing light and converting it into power.
A challenge in designing solar cells is making sure that
the electrons dislodged when light hits a semiconductor
create a current. When a photon strikes a solar-cell mate-
rial, the result is both a free electron and its positive coun-
terpart, called a hole. If these can be pulled apart quickly to
opposite electrodes, an electrical current results. However,
the di culty of separating them before they recombine and
dissipate energy as heat is "one of the major roadblocks for
higher-e ciency solar cells," says Aravinda Kini, program
manager for biomolecular materials research at the U.S.
Department of Energy.
Morse s structures could sur mount this roadblock. The
network of crystalline projections could be immersed in
a transparent solid or liquid electrode. Light would pass
through the electrode, where it would be absorbed by the
crystal. Because the surface area of the structured thin lm
is high (in one material, 90 to 100 times that of a traditional
thin lm), many of the electron-hole pairs generated by the
light would be near the electrode interface; as a result, they
could quickly separate, with one charge carrier moving into
the transparent electrode and the other carrier traveling
through the crystal to exit at the opposite electrode.
Already, Morse and colleagues have made more than
30 types of semiconductor thin lms and tested their pho-
tovoltaic properties. They are now working to incorporate
the semiconductors into functional solar cells. At the same
time, Morse continues to develop new biologically inspired
methods for assembling materials, with an eye to addi-
tional applications, including semiconductor devices for
safer, higher-power-density batteries and smaller memory
chips; he is also interested in creating laminated bers for
ultrastrong building materials.
But excited though he is by the potential applications of
his work, Morse remains at heart a molecular biologist.
Even as he talks about how his research could lead to better
solar cells, he gazes out the window at the dolphins frolick-
ing in the harbor. And he s still devoted to understanding
the mechanism behind the complexity of the sponge. Once
again he examines the exquisite skeleton of the Venus s
ower basket, though he s no doubt seen it thousands of
times. "This was made of glass, by a living creature," he
exclaims. "It s incredible!"
Kevin Bullis is Technology Review s nanotechnology and materials
Others in Bio-Inspired Materials
Murray Hill, NJ
building materials and
and better biosensors Investigating sea-
shells and other
Better batteries and
advanced materials for
electronics, energy, and
viruses to assemble
Samuel I. Stupp,
Better sensors and
to direct the forma-
tion of inorganic
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