Sizing up atoms with electron holograms.

In their ongoing quest to see the all-but-invisible, physicists have developed a method for using patterns of scattered electrons to observe the three-dimensional atomic textures of materials. In this emerging technology, called electron holography, investigators exploit the wave-like properties of electrons to make their observations (SN: 10/15/88, p.252).

Now scientists have used a lensless electron projector to discern the arrangement of atoms in several types of materials. In the Sept. 16 PHYSICAL REVIEW LETTERS, Hans-Werner Fink and Heinz Schmid of IBM's Zurich (Switzerland) Research Laboratory and their colleagues describe the technique and the calculations that helped them create holograms of carbon fibers and of thin gold films. Since doing that work, they have produced a holographic image of DNA--published here for the first time--demonstrating that the approach also works with biological materials.

Fink and Schmid illuminate their samples with a beam of electrons emitted from an ultrathin tungsten tip similar to that used in scanning tunneling microscopy. By sharpening this tip to a width of one atom, they create a beam coherent enough to make atomic-scale holograms.

"Holograms are not direct pictures of objects; they are sort of smeared-out representations," explains Hans Jurgen Kreuzer, a physicist at Dalhousie University in Halifax, Nova Scotia. He and Dalhousie colleague Andrzej Wierzbicki worked with the IBM scientists to make a model for predicting the scattering patterns and to interpret experimetnal results. "We now have a theory that eliminates the guesswork," Kruezer says.

But these researchers and others agree that the images still need work. The pictures display wavy patterns as well as hints of the real structure. "It's somewhere in between what you can see directly and a true hologram," says Dilano K. Saldin, a physicist at the University of Wisconsin-Milwaukee. "They claim that you can see atomic detail, but [the images] are not very clear."

Saldin and others rely on photoelectron and Auger spectroscopy data to reconstruct three-dimensional images (SN: 9/1/90, p.134). These images represent what most people think of as true holograms. Saldin expects Finn's group to develop comparable reconstruction schemes for the new technique.

One advantage of the electron projector, says Kreuzer, is that "you can fine-tune it depending on what you want to see." It can shoot out low-energy electrons for seeing overall shape and structure, and then high-energy electrons to resolve fiber details. In addition, the scientists can adjust the distance between the tip and the sample.

The electrons emitted by the projector pack far less power than those used in transmission or scanning electron microscopy, two common imaging methods involving electrons. "Therefore, the amount of beam damage is much smaller," says Saldin. The new technique thus enables researchers to make holograms of biological molecules, such as DNA, without having to replace carbon atoms with larger atoms like gold.

"We are quite convinced that within a few years there will be thousands of these instruments in materials science and biomedical laboratories," says Kreuzer. "It is very powerful.