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Conventional methods to convert silica (sand) into silicon use temperatures that are well above silicon's melting point (1,414 °C). Ken Sandhage, a ceramist at the Georgia Institute of Technology in Atlanta, and his colleagues have developed a method that works at a much lower temperature (650 °C) by converting intricate silica microshells of diatoms (planktonic algae) into silicon nanostructures.(see page 172)

How does a materials scientist get the idea to convert diatom microshells into silicon replicas?

Through luck and coincidence. I was a Humboldt fellow on sabbatical in Germany several years ago. While on a Humboldt-sponsored bus tour of Germany, I met marine biologist Monica Schoenwaelder from the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven. She told me about diatoms, these amazing single-celled algae that form intricate microscopic silica structures. My lab had been working on shape-preserving chemical reactions with macroscopic ceramics, so I started thinking about using diatom microshells as templates. I never would have thought of that on my own.

How does your process circumvent the previous constraints to making silicon?

We react silica with magnesium gas at 650 °C to create silicon and magnesia — then we selectively dissolve the magnesia. We can convert microscale silica assemblies into silicon and preserve the intricate shapes and nanoscale features — in part because the silicon is intertwined with magnesia during the reaction, inhibiting coarsening of the silicon.

What could these structures be used for?

One example is a highly sensitive and rapidly responding miniature gas sensor to detect nitric oxide or other gas pollutants. But that's really the tip of the iceberg. Silicon replicas of diatom microshells could be used as stiff, highly porous particles in high-pressure liquid chromatography to speed up purification of drugs or proteins. Alternatively, our reaction method could be applied to synthetic silica of higher purity.

Could these ancient algal organisms become the latest semiconducting craze?

A challenge with using diatom-derived silicon as a semiconductor is controlled doping. Silicon used in transistors is carefully doped to become an n- or p-type semiconductor. The silica in diatom microshells contains impurities derived from minerals in sea or lake water. Further work is needed to achieve controlled doping of diatom-derived silicon.