'Tissue Paper' Could Stop Bullets, Harness Solar Energy

THE GIST:

• A new fabric made from germanium is nearly as strong Kevlar.
• The material is as soft and flexible as tissue paper.
• The fabric could lead to solar powered, bulletproof clothing.

A soft "tissue paper" made from normally brittle germanium and silicon contains individual fibers as strong as bulletproof Kevlar. Woven into traditional fabric or embedded in hard plastics, the new nanowires could stop bullets, harvest solar energy or perform dozens of other tasks.

"Paper is made of wood fibers compressed together," said Brian Korgel, a scientist at the University of Texas at Austin and co-author of a new paper in ACS Nano that describes the germanium nanowires. "In this case, we took bulk semiconductors, turned them into nanowires and compressed them together to make a material with a tissue paper consistency."

Germanium is usually quite hard and brittle. "When I handle a block of the bulk material, I have to handle it very carefully so it doesn't break," said Korgel.

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Unlike bulk germanium, however, germanium tissue paper is flexible and won't break when bent. The individual nanowires that make up the tissue paper are also incredibly strong, having a similar strength-to-weight ratio as Kevlar. They can even absorb blows that would ordinarily shatter a block of germanium.

The germanium wires have the strength of Kevlar, but that doesn't mean they will make ordinary clothing bulletproof -- at least not immediately.

Kevlar stops bullets because not only are individual fibers strong, but so are the bonds between the fibers. However, individual germanium nanowires are not there yet. Korgel compares the bonds between his strong germanium nanowires with the proteins spiders use to make drag line silk as strong as steel.

"Scientists know how to make the proteins spiders use in test tubes," said Korgel. "But you have to take those proteins and spin them into a web to match their mechanical properties."

Even after decades of research, scientists still can't match the spider's silk exactly. It will likely be years before scientists can bond germanium nanowires tightly enough to make next generation body armor.

In the meantime, however, Korgel and his colleagues plan to use the new fabric's semiconducting properties for another use: solar power.

Germanium absorbs light that the human eye can see, as well as infrared light that are invisible. Woven into a soft shirt or surrounded by hard plastic, germanium nanowires could then turn solar power into electrical energy to power embedded sensors or other electrical devices.

A germanium-based photovoltaic won't absorb light as well as its more widely used cousin, silicon. Luckily, the University of Texas scientists have also developed a similar tissue-like fabric made from silicon nanowires.

Besides turning more sunlight into electricity, individual silicon nanowires are about 35 percent stronger than the germanium nanowires. They are also more resistant to corrosion.

Korgel and his colleagues are currently preparing a more detailed paper describing their silicon nanowires for peer review.

Jillian Buriak, a scientist from the University of Alberta, said that Korgel's work bridges the gap between nanotechnology research and practical applications.

"(Korgel and his team) have really brought nanotechnology to the level where other people can use it," said Buriak. Electrically conductive paper, ultra thin LED displays, flexible batteries, biomedical implants and any number of other devices could result from the germanium and silicon tissue paper.

"It's so neat when people do what nanoscience is supposed to do: cramming these well known materials into very small dimensions to got entirely new properties out of them," said Buriak. "It's like teaching an old dog new tricks."