Heinrich Rohrer

(b. 1933)

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(1933–) Swiss physicist

Born at Buchs in Switzerland, Rohrer was educated at the Federal Institute of Technology, Zurich, where he obtained his PhD in 1960. After two years' postdoctoral work at Rutgers, New Jersey, he returned to Zurich in 1963 to join the staff of the IBM Research Laboratory, Roschliken.

The conventional electron microscope developed by Ruska in the 1930s could present a two-dimensional image only. Further, while atoms have a diameter of 1–2 angstroms (1 angstrom = 10–10 meter), electron microscopes could not resolve images below 5 angstroms. Consequently, the surface structure at the atomic level was beyond the range of any existing microscope. To overcome this limitation Rohrer, in collaboration with his IBM colleague, Gerd Binnig, began work in 1978 on a scanning tunneling microscope (STM).

In the STM a fine probe passes within a few angstroms of the surface of the sample. If a positive voltage is applied to the probe, electrons can move from the sample to the probe by the tunnel effect, and a current can be detected. This current is sensitive to distance from the surface; a slight change in distance will produce a significant change in current. Consequently, in theory at least, a feedback mechanism should be able to keep the probe at a constant distance from the surface, or, in other words, trace the surface's contours. If the tip is allowed to scan the surface by sweeping through a path of parallel lines, a three-dimensional image of the surface can be constructed.

Inevitably practice proved less straightforward than theory. A major difficulty was to eliminate vibration. As the magnification required was of the order of 100 million, any interference would grossly distort the image produced. The microscope was suspended on springs and placed in a vacuum, and further vibrations were dampened by resting the microscope on copper plates positioned between magnets. If the copper plates begin to move an eddy current will be induced by the magnetic field and the interaction between current and field will, in turn, damp the motion of the plates. Vibration was so reduced as to allow a vertical resolution of 0.1 Å; the lateral resolution, depending upon the sharpness of the probe, was initially no better than 6 Å.

The STM has proved useful in the study of the surfaces of semiconductors and metals and has also been applied to biological samples such as viruses. For their work in this field Rohrer and Bining shared the 1986 Nobel Prize for physics with Ruska.

Subjects: Science and Mathematics.

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