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Electron microscopy gets twisted

Whirling electron vortices could help materials scientists to map the properties of nanomaterials in new detail. A technique, detailed in the September 16 issue of Nature, could be used in electron microscopes as part of the continuing quest to scale down the size of electronic chips.

Optical physicists have been using spiraling laser beams, in which light waves are twisted into vortices, for almost 30 years, says Jo Verbeeck, a materials scientist at the University of Antwerp in Belgium and first author on the Nature paper. These vortex beams are routinely used to trap and move microscopic biological particles for study in the lab. "It's as though they are trapped in the eye of a cyclone," he says.

Materials scientists would like to be able to twist the electron beams used in electron microscopes in a similar way. "An electron vortex beam could manipulate nanomaterials just a few atoms thick," says Verbeeck, which would help efforts to build ever-smaller electronic chips.

Earlier this year, Masaya Uchida and Akira Tonomura at the Advanced Science Institute in Wako, part of Japan's network of research labs known as RIKEN, showed that electron beams can be twisted. They passed an electron beam through a stack of thin-film graphite, which was layered to mimic the first few steps of a spiral staircase.

As the beam passed through the stack, different parts of it were staggered by varying amounts, depending on how many graphite "steps" it had to travel through. So when the beam emerged it had taken on a spiral form.

The disadvantage of this method is that it is hard to build and maintain a "staircase" out of such fragile material, says Verbeeck. Just passing an electron beam through it once would contaminate it, destroying the precise step-pattern needed. "This was a very exciting piece of work, but even if you create a twisted electron beam one day, you can't easily go back and do it again the next day, using the same equipment," he says.

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