Physics: tiny world garners grand laurels.

Physics: Tiny world garners grand laurels

Modern microscopy has brought scientists within sight of the very bonds that hold together the atoms of matter. For their innovations in this field, three Europeans have won the 1986 Nobel Prize in physics.

Cited for designing, between 1931 and 1933, the first electron microscope and for doing "fundamental work in electron optics,' West German scientist Ernst Ruska of the Fritz-Haber Institute of the Max Planck Society in West Berlin will receive half of the $290,000 prize. Sharing the other half for their 1981 design of the scanning tunneling microscope are Gerd Binning of West Germany and Heinrich Rohrer of Switzerland. Both work at IBM Corp.'s research laboratory in Zurich, Switzerland.

Before the 1930s, the resolution or "defining power' of microscopes was limited by the wavelength of light, which is roughly 2,000 times the diameter of a typical atom. "Trying to probe atomic structures with visible light is like trying to find hairline cracks on a tennis court by bouncing tennis balls off its surface,' wrote Binnig and Rohrer in the August 1985 SCIENTIFIC AMERICAN.

By switching from visible light to a beam of high-energy electrons, whose wavelengths can be roughly 100 times smaller than an atom, Ruska was tossing the tennis balls away in favor of balls smaller than a grain of sand. In 1931, Ruska used two simple magnetic coils to focus this electron beam, and the electron microscope was born.

Modern electron microscopes can resolve down to about 1 angstrom or 10(-10) meters, which is smaller than the typical atomic diameter.

Unlike the electron microscopes and their visible-light predecessors, the scanning tunneling microscope does not produce an image by focusing beams of wave/particles. Instead, it works like the stylus of a record player, albeit on a much smaller scale.

With a tip so fine it consists of a single atom, the microscope's stylus moves across the surface of a sample and traces its topography. To prevent the stylus from scratching the surface, Binnig and Rohrer kept the two apart by 5 to 10 angstroms. A potential difference across the gap induces electrons to flow from the stylus to the sample, and the stylus rides along on this blanket layer of electrons.

The key to the sensitivity of the scanning tunneling microscope is a quantum mechanical effect known as tunneling (SN: 4/6/85, p.215). To allow the stylus to ride within 2 atomic diameters of the surface, the voltage across the gap between the two must be kept very low. And according to classical mechanics, there would not be enough energy to excite the electrons to jump across the gap. This is analogous to trying to throw a ball over a mountain. In the quantum mechanical world, however, the ball has a certain probability of tunneling through the mountain, if the mountain is very thin.

The scanning tunneling microscope has reached a horizontal resolution of 2 angstroms and a vertical resolution of a few hundredths of an angstrom, opening up new dimensions in the study of surfaces. Scientists are eager to define the arrangement and electronic states of surface atoms. This knowledge could lead to a better understanding of subject ranging from integrated circuits to the details of electrochemical reactions on surfaces.

Photo: Above: Heinrich Roher (left) and Gerd Binnig. At right: Ernst Ruska.