No room is left at the bottom. A team of physicists has shown how a common type of electron microscope can spot single hydrogen atoms — the smallest atoms of them all.
Previously, electron microscopes had trouble imaging single atoms lighter than carbon.
The University of California, Berkeley team visualized defects and impurities — including atoms of hydrogen— on graphene, the one-atom-thick, chicken-wire nets of carbon that normally stack up to form graphite. “Think of it sort of as a spider web,” says study coauthor Alex Zettl of the graphene, “and the atoms you want to view are flies on the spider web.”
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The researchers say that using graphene could enable scientists to understand the structure of molecules that have been difficult to image with other techniques capable of resolving single atoms, such as X-ray diffraction. Thanks to graphene’s sturdiness, single-molecule motions and chemical reactions could be filmed as they happen, the team suggests in the July 17 Nature.
And the technique would be available to any lab that has a transmission electron microscope. Researchers would just have to use graphene as a petri dish. “It opens up a whole new world for conventional-TEM users,” Zettl says.
Michel Bosman, a research engineer at the Institute of Microelectronics in Singapore, agrees. “Many TEM users can in principle reproduce these results,” he says.
A TEM scans microscopic objects with a thin beam of electrons, essentially running a current though the objects. Typically, researchers place samples on films of carbon. In the TEM, lighter elements tend to give lower contrast. Any atoms lighter than carbon — itself one of the lightest elements — are hard to see, even when the carbon films are just nanometers thick, or a few tens of atoms deep.
The Berkeley team suspended graphene sheets across nanometer-sized holes in a conventional carbon sheet. The team reasoned that the uniform arrangement of atoms in graphene also makes it appear as a uniform shade of gray in TEM images, which is easy to remove from the data.
In graphene, carbon atoms bind to one another even more strongly than they do in diamond. That makes the material extremely stable under the deluge of electrons of a TEM, despite being literally as thin as it gets.
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Even so, when the researchers tested their technique by imaging defects on their graphene sheets, they didn’t expect to see individual hydrogen atoms. “We were definitely surprised,” says study coauthor Jannik Meyer, now at the University of Ulm in Germany. “Those individual signals clearly had a strength that is only consistent with hydrogen atoms.”
Meyer says it could still take some work for scientists to figure out how to deposit molecules on graphene one at a time — as opposed to just looking for preexisting impurities — and to make the molecules stick so they can be imaged.
The team achieved “a real breakthrough for transmission electron microscopy,” comments Stephen Pennycook, who heads the electron microscopy group at the Oak Ridge National Laboratory in Tennessee. Previously, he says, scientists suspected that lighter atoms were invisible because of an ultimate limitation of the resolution of TEMs. But the results show that the culprit was actually the “speckle pattern” of conventional support films.