Published online 24 September 2008 | Nature | doi:10.1038/news.2008.1130

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Ultrasmooth mirror could herald birth of a new microscope

Helium atoms could probe the smallest structures with a light touch.

mirrorThis artist's impression shows helium atoms bouncing off an ultrasmooth mirror.D. BARREDO ET AL./ADV. MATER.

A microscope that studies the most delicate materials by bouncing helium atoms off their surfaces could be made within a year, thanks to the development of the world's smoothest mirror.

That's the claim from Rodolfo Miranda of the Autonomous University of Madrid, Spain, whose research team has created the mirror by depositing a few atom-thick layers of lead onto an almost perfectly smooth silicon surface at 114 kelvin (-159 °C).

"We are aiming to provide an essential optics device for a new type of microscope," says Miranda. That device is the atomic microscope, which would allow the surface of biological samples, for example, to be probed in a way that is impossible with other state-of-the-art microscopes.

Electron microscopes can produce highly magnified images, but they have serious drawbacks. The samples must conduct electricity; electrons penetrate into the sample, leading to an image that doesn't accurately represent the surface; and, worst of all, the very-high-energy electron beams can obliterate the precious samples.

An atomic microscope with a low-energy beam of helium atoms could get around these problems. Neutral helium can bounce off any surface, conducting or insulating, and the beam would be deflected by the electrons at the very edge of the sample, giving a true image of the surface. But an atomic microscope demands a focused beam of helium atoms — and that requires a mirror that reflects the beam with very little scattering of the atoms.

Super-smooth

Miranda's mirror builds on previous work1 in which a curved silicon surface was used to reflect and focus a beam of helium atoms. But this mirror focused fewer than 1% of the helium atoms that hit the silicon surface to a spot 210 micrometres across.

A metal coating would reflect and focus helium much more tightly, but metals grown on silicon tend to clump into islands that scatter incoming atoms in all directions. "The point is to try to reproduce the perfection of the underlying surface," says Miranda.

Miranda managed this by exploiting a quantum stabilization effect previously seen in ultrathin lead films. For certain thicknesses of lead, the metal's electrons can sit in very stable energy levels so that, under the right conditions, the surface simply smooths itself out to a continuous thickness.

The resulting 2-centimetre-wide flat mirror is atomically smooth, even after it has warmed up to room temperature, and it can reflect 15% of incoming helium atoms. The research is published in Advanced Materials2.

It is a major breakthrough in efforts to make an atomic microscope, says Bodil Holst from the University of Bergen, Norway, who is hoping to put Miranda's mirror into a working microscope.

Tiny beam

The next major hurdle is to use the same technique to make a curved mirror. That's not trivial, says Bill Allison of the University of Cambridge, UK, who is working with Holst to build an atomic microscope. Although Allison is cautious about progress, he expects the first images to be produced from an atomic microscope within two years.

The beam size determines the size of features that the microscope can detect. Holst hopes that the lead mirror will eventually focus the helium beam to an area 20 nanometres across. "We are making progress on that front," she says. "Our ultimate aim is to be able to look at the electron density of individual proteins."

Peter Toennies, an expert in helium scattering at the Max Planck Institute for Dynamics and Self-Organization in Göttingen, Germany, agrees that an atomic microscope would revolutionize microscopy, providing a crucial alternative to 'messy' electron microscopy.

Atomic microscopy could even rival atomic-force microscopy, adds Holst. Because atomic-force microscopes makes their measurements by dragging a tiny cantilever across the sample, the surface has to be very flat, and "it's a very slow process," Holst says.

Toennies is unsure about whether an ultrasmooth, curved lead surface could ever be made. But Miranda is very confident, and predicts that by next summer he will have a focused beam of helium 100 nanometres wide — narrow enough to start taking some seriously small snapshots. 

  • References

    1. Barredo, D. et al. Adv. Mater. 20, 3492–3497 (2008).
    2. Holst, B. & Allison, W. Nature 390, 244 (1997). | Article | ChemPort |
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