Improved field-emission microscope images electron orbitals, confirming their theoretical shapes
Chemistry textbooks typically include illustrations of atoms, but with caveats. The drawings depict atomic nuclei surrounded by electron orbitals—fuzzy spheres, barbells, tripods, and so on—but those figures represent the probability of finding an electron at a certain place around the nucleus rather than an actual “shape.” Researchers have now managed to image the electron orbitals and show for the first time that, in a sense, atoms really look like those textbook images.
Specifically, Igor Mikhailovskij and his collaborators at the Kharkov Institute of Physics and Technology in Ukraine have imaged the shapes of those orbitals in carbon atoms by improving an old imaging technique called field-emission microscopy.
The researchers fashioned a chain of carbon atoms, dangled it from a graphite tip, and then placed it in front of a detection screen. When they applied an electric field of thousands of volts between the graphite and the screen, electrons flowed one by one through the graphite and along the carbon chain, until the electric field pulled them off the last atom in the chain. From the places where the electrons landed on the screen, the investigators could trace back the points where they left their orbital on the last atom. The “denser” parts of the probability clouds had a higher chance of emitting an electron, and the information from many electrons combined into an image of the clouds. “We really have an image of single atoms,” Mikhailovskij says.
The pictures look, well, textbook, although only the outermost orbitals appear, which shroud the inner orbitals and the nuclei. By changing the intensity of the current, the team could switch the energy of the last atom’s outermost electron from a lower level to a higher level. Correspondingly, the shape of the orbital changed from spherical to barbell, as theory predicts. The group also observed electrons switching spontaneously from one state to another—for reasons that are unclear, Mikhailovskij says—and stranger shapes that may result from the presence of impurities, in the form of other atoms such as hydrogen. The results are in the October Physical Review B.
Scientists have imaged single atoms before, using tools such as transmission electron microscopes (which shoot electrons through an object and measure how they get deflected) or scanning tunneling microscopes (which “feel” the sample’s shape with a microscopic tip).
But the atoms typically appeared as little more than blobs. Field-emission microscopy, on the other hand, pulls the electrons off the very object that is being imaged. This difference, says Alex Zettl of the University of California, Berkeley, may mean a lower chance of distortions and misinterpretations of the signal. “It is like hearing the spoken word directly from the original storyteller, not from a translator or interpreter,” he says.
Beyond confirming textbook artwork, the technique could elucidate the properties of chains of carbon atoms, which are still largely unknown. Physicists suspect that they may be excellent conductors and mechanically strong and could become useful in future atomic-scale computers.