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Electron microscopy gets twisted

Spiralling electron beams have the potential to measure and manipulate the properties of single atoms.

By Zeeya Merali

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.

Platinum mask

Verbeeck and his colleagues used a different approach, based on computer-generated holograms. They created a "mask"--a thin sheet of platinum, with a specific grid-like pattern etched out--that was placed in the path of the beam. The researchers ran computer simulations to work out the precise grid configuration that would cause the electron beam to split as it passed through the mask and recombine as a spiral on the other side. "The first time that we tried the mask, it created the vortex beam just as predicted. I was surprised by how easy it was," says Verbeeck. "That is the power of our method: It will be easy for others to make and use."

"This is not the first demonstration of an electron vortex, but it is the first time it has been demonstrated in a way that can easily be implemented in many electron microscopes," says Javier García de Abajo, an electron microscopy expert at the Institute of Optics in Madrid. "This will have far-reaching consequences."

The most immediate application of the vortex beam will be to measure the magnetic properties of nanoparticles. When spiraling electron beams pass magnetic particles, in theory, their degree of rotation should change, depending on the strength of the magnetism. Verbeeck and his team have confirmed that this is the case by shining the spiraling beam onto a ferromagnetic film of iron.

Uchida welcomes the alternative method for generating a vortex beam. The technique is "very important," he says, because it will extend the capabilities of conventional electron microscopes and should allow "atom-by-atom magnetic analysis in the near future."

Verbeeck and his colleagues are testing whether the beam can not only probe but also manipulate single atoms. If successful, says García de Abajo, the vortex beam will be an important resource for those working to reduce electronic chips to just a few atoms. "For those applications, you need tools to be able to fabricate the chips and keep a check on what the atoms are doing," he says.

The technique could also help physicists to understand more about exotic materials, such as superfluids and superconductors, which often contain vortices that are generated when currents pass through them, says Verbeeck.

Natural vortices that are generated spontaneously within these materials are difficult to study, as it is hard to isolate their behavior within the exotic material, where other unrelated phenomena are also occurring. Verbeeck says, however, that his team's electron vortex beam could be set up to mimic the behavior of these natural vortices, acting as an easily controllable model for materials scientists to investigate. "But that is a far longer-term goal," he says.

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