Taking apart a single molecule.

Taking apart a single molecule

The scanning tunneling microscope, invented only a few years ago, is already an important tool for obtaining images that reveal the locations of individual atoms on surfaces (SN:10/25/86, p.262). Lately, scientists have also been investigating the use of the same apparatus for altering surface structures and for manipulating matter at the atomic level. Now a team of researchers at the IBM Almaden Research Center in San Jose, Calif., reports success in pinning small organic molecules to a graphite surface. The researchers also have evidence indicating that they were able to split a pinned molecule into smaller pieces - perhaps the most delicate chemical surgery yet performed.

"We've all learned about molecules in school," says John S. Foster, who led the research. "We'd like to see them firsthand, and we'd like to try to take them apart and put them back together firsthand. That's the ultimate goal."

The key component of the scanning tunneling microscope is an extremely sharp needle that rides just above the surface being scanned. As the needle skims the surface, electrons flow between surface atoms and the needle. By continually adjusting the needle's height to keep the current constant, researchers obtain an image of the surface.

Foster and his colleagues found that by applying a brief electrical pulse-about 4 volts for 100 nanoseconds-through a needle dipped into a small liquid droplet lying on a graphite surface, they could pin individual organic molecules such as phthalates (see molecular model shown in top photograph) to the surface. Then, using the microscope (shown below the molecular model), they could obtain an image of the organic molecule against a graphite background (bottom photograph).

"We're using short pulses to manipulate matter, and we think we can undo what we've done," says Foster. "But it's not a fully reliable process." The result of applying an electrical pulse is somewhat unpredictable. Often, a second pulse applied near a pinned molecule erases the molecule from the surface. Sometimes, however, only a piece of the molecule disappears. This suggests that the molecule had been cleaved.

What causes certain organic molecules to stick to graphite also isn't known yet. The process works with some types of molecules and not with others. "Clearly, more theoretical and experimental work is necessary to determine the mechanics of the process," the researchers report in the Jan. 28 NATURE. "Although much work remains to be done, we believe that tunneling microscopy is on the threshold of a revolution in manipulating atoms and molecules for a variety of purposes."

Previous surface-alteration studies by other researchers had focused, for example, on depositing small groups of germanium atoms on a germanium surface. In other words, they used the apparatus to "write," the "read," an atomic message. "The work of Foster (and his colleagues)," says J.B. Pethica of Oxford University in England, "represents a significant attempt at the much more important and difficult problem of the direct manipulation of the structure of biological materials."