Microsketching an underwater surface - scanning tunneling microscope

Science News, April 19, 1986 by Ivars Peterson

Microsketching an underwater surface

Surface details on the scale of typical atoms are notoriously difficult to detect, especially when the sample is immersed in water. Now a team of researchers reports the design and construction of a special "scanning tunneling" microscope that can pick out the atomic-scale bumps and hollows on a water-covered graphite surface.

This effort represents the first successful attempt to resolve such fine details on a wet surface. At best, an optical microscope is restricted to features 1,000 or more times larger. An electron microscope operates only when the sample is in a vacuum.

In the April 11 SCIENCE, Richard Sonnenfeld and Paul K. Hansma of the University of California at Santa Barbara also report that the new instrument can be operated in salt solutions. This opens up the possibility of imaging proteins and other biological materials in their active states. The microscope may also be useful in electrochemistry for detecting surface changes that occur at electrodes.

In a tunneling microscope, an extremely sharp metal needle is brought within

a few angstroms of the sample's surface. This distance is small enough for electrons to leak or tunnel across the gap and generate a minute current. As the gap between the tip and the sample increases, the current decreases. A scanning mechanism pulls the needle across the sample's surface, constantly adjusting the tip's height to keep the current constant. The result is a microscopic sketch of the surface's contours (SN: 4/6/85, p.215).

Getting such a microscope to work in water was a challenge because water conducts electricity. The resulting electrical current could swamp the tunneling current. The answer was to minimize the area of the needle that could conduct current through water and into the sample surface. The researchers did this by coating a platinum-iridium needle with glass insulation, leaving only its tip bare.

Says Sonnenfeld, "Because the electrical current through the water remains constant, we could pretty much ignore it. It didn't have any effect on the images."

The microscope took 20 seconds to produce an image of a clean graphite surface immersed in deionized water. The image revealed rounded peaks and valleys that fell into the hexagonal pattern of graphite's characteristic honeycomb lattice. The researchers also obtained lower-magnification images of a gold film immersed in a sodium chloride solution.

Meanwhile, IBM and Stanford University scientists have modified a scanning tunneling microscope to map forces on the surfaces of both conducting and insulating materials. Conventional tunneling microscopes work best if the sample is an electrical conductor.

This new device, called an atomic force microscope, has a diamond tip mounted on a tiny gold-foil spring, which is sandwiched between a sample and a microscope needle's tip. Fluctuating forces between atoms in the sample and on the end of the diamond tip cause the tip to waver slightly.

As the diamond tip scans a surface, the changes in the tunneling current reflect the arrangement of atoms on the surface. A prototype instrument has mapped the surfaces of insulators to a resolution of 30 angstroms, which is getting near the atomic-scale resolution possible for conductors like graphite. -- I. Peterson

Photo: Images of an underwater graphite surface showing features smaller than 3 angstroms (above) are possible with a special scanning tunneling microscope (right) designed to work with immersed samples.