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5 December 2013 11:26 am ,
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Snake venoms are remarkably complex mixtures that can stun or kill prey within minutes. But more and more researchers...
At age 30, Dutch biologist Freek Vonk has built up a respectable career as a snake scientist. But in his home country,...
Since arriving on the island of Guam in the 1940s, the brown tree snake ( Boiga irregularis ) has extirpated native...
An animal rights group known as the Nonhuman Rights Project filed lawsuits in three New York courts this week in an...
Researchers have been hot on the trail of the elusive Denisovans, a type of ancient human known only by their DNA and...
Thousands of scientists in the Russian Academy of Sciences (RAS) are about to lose their jobs as a result of the...
Dyslexia, a learning disability that hinders reading, hasn't been associated with deficits in vision, hearing, or...
Exotic, elusive, and dangerous, snakes have fascinated humankind for millennia. They can be hard to find, yet their...
- 5 December 2013 11:26 am , Vol. 342 , #6163
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Telling Who's Who in Atom Images
9 October 1997 9:00 pm
Scanning tunneling microscopy sketches exquisite atomic-level landscapes of material surfaces, but reveals little information about the identity of those atoms. Now, two researchers have found a way to pluck small groups of atoms from a surface and tell what they are. The technique should interest the semiconductor industry, which often wants to identify the impurities that can hamper the performance of devices.
The probe of a scanning tunneling microscope has a tip, sharpened to only a few atoms, that hovers several angstroms above the surface to be imaged. Electrons slip or "tunnel" across the gap, and as the probe slides across the surface, the changing current reveals the ridges and valleys of individual atoms. Sometimes a particular atom can be identified from context, but usually it's terra incognita. "There are always these mystery blobs you can't interpret," says John Spence, a physicist at Arizona State University in Tempe.
So Spence and colleague Uwe Weierstall designed a way to identify the anonymous atoms. Their strategy was to apply a 5-volt pulse to the tip to pull the suspect atoms off the surface. They then moved the sample to one side and applied a much stronger pulse--10 kilovolts--which has the opposite effect, flinging the atoms to a detector 20 centimeters away. The flight time from tip to detector reveals the ratio of the atom's charge to its mass, and with a little analysis, the identity of the atoms. (A heavy atom moves more slowly than a light one, as does one with a small charge, which receives a smaller kick from the electric pulse.)
The researchers tested their technique on a silicon surface dotted with islands of germanium. They selectively grabbed the 20-atom islands, and found that most of the travel times indeed corresponded to germanium. But it didn't always work. Some atoms they tested refused to stay on the tip. Spence expects that by cooling the tip with liquid nitrogen, they should be able to reduce thermal noise enough to make the more mobile ones stick.
Mike Miller, a microscopy expert at Oak Ridge National Laboratory in Tennessee is impressed with the technique. "Currently this is the only way to identify atoms on a surface," he says. Miller predicts the Arizona State device could go commercial in 5 to 10 years.