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Electron microscopes use a beam of electrons to magnify samples by up to 2 million times. But whatever they look at takes a beating, a problem when examining delicate biological specimens.

Similar magnification should be possible using a much lower-energy, gentler beam of helium ions and recording how they are scattered by a sample. But developing a mirrored surface able to effectively reflect and focus helium atoms into a tight beam has proven difficult.

Two become one

The best results until now have come from research groups experimenting with thin silicon surfaces, which can easily be bent into a convex shape able to focus helium ions. But silicon is not particularly reflective. Only about 1% of incoming helium atoms are accurately focused, resulting in fuzzy magnified images.

Metal materials reflect helium ions much better but are harder to bend precisely into the right shape. Now materials scientists from the Autonomous University of Madrid led by Amadeo Vázquez de Parga have combined silicon and metal to make what they say is the smoothest surface ever made.

They made the near-perfect mirror by coating a thin layer of lead onto a silicon surface. This is not straightforward, because when a very thin metal layer is deposited onto a flat silicon surface it usually forms an uneven coating of differently sized bumps that perform badly as a mirror for helium ions.

Sub zero

Vázquez de Parga and his team found they could avoid surface bumps by depositing the lead onto the silicon surface at low temperatures between -173 and -133°C. This appears to alter the quantum properties of electrons in the lead film, altering its growth so that a "magic" super-smooth layer of uniform thickness forms.

"The idea is to grow one of these magic thicknesses at low temperature and then heat up the sample slowly to room temperature," Vázquez de Parga told New Scientist.

The end result is a perfectly smooth lead film that can act as an almost flawless mirror. The surface is atomically flat, more than 90% of the film is exactly the same thickness, down to the level of individual lead atoms. It can focus more than 15% of incoming helium atoms into a tight beam, and Vázquez de Parga hopes to increase this proportion to 40%.

Bill Allison at Cambridge University, UK, leads a team experimenting with thin silicon mirrors to focus helium ions. "[This work] represents a key step forward in producing a device to focus helium atoms," he says.

"The remaining step is to combine the high reflectivity with a carefully deformed surface in order to create a focused atomic spot. That is still quite a challenge."