Scanning tunneling microscope (Inventions)

The invention: A major advance on the field ion microscope, the scanning tunneling microscope has pointed toward new directions in the visualization and control of matter at the atomic
level.

The people behind the invention:

Gerd Binnig (1947- ), a West German physicist who was a
cowinner of the 1986 Nobel Prize in Physics Heinrich Rohrer (1933- ), a Swiss physicist who was a
cowinner of the 1986 Nobel Prize in Physics Ernst Ruska (1906-1988), a West German engineer who was a
cowinner of the 1986 Nobel Prize in Physics Antoni van Leeuwenhoek (1632-1723), a Dutch naturalist

The Limit Of Light

The field of microscopy began at the end of the seventeenth century, when Antoni van Leeuwenhoek developed the first optical microscope. In this type of microscope, a magnified image of a sample is obtained by directing light onto it and then taking the light through a lens system. Van Leeuwenhoek’s microscope allowed him to observe the existence of life on a scale that is invisible to the naked eye. Since then, developments in the optical microscope have revealed the existence of single cells, pathogenic agents, and bacteria.
There is a limit, however, to the resolving power of optical microscopes. Known as “Abbe’s barrier,” after the German physicist and lens maker Ernst Abbe, this limit means that objects smaller than about 400 nanometers (about a millionth of a millimeter) cannot be viewed by conventional microscopes.
In 1925, the physicist Louis de Broglie predicted that electrons would exhibit wave behavior as well as particle behavior. This prediction was confirmed by Clinton J. Davisson and Lester H. Germer of Bell Telephone Laboratories in 1927. It was found that high-energy electrons have shorter wavelengths than low-energy electrons and that electrons with sufficient energies exhibit wavelengths comparable to the diameter of the atom. In 1927, Hans Busch showed in a mathematical analysis that current-carrying coils behave like electron lenses and that they obey the same lens equation that governs optical lenses. Using these findings, Ernst Ruska developed the electron microscope in the early 1930′s.
By 1944, the German corporation of Siemens and Halske had manufactured electron microscopes with a resolution of 7 nanometers; modern instruments are capable of resolving objects as small as 0.5 nanometer. This development made it possible to view structures as small as a few atoms across as well as large atoms and large molecules.
The electron beam used in this type of microscope limits the usefulness of the device. First, to avoid the scattering of the electrons, the samples must be put in a vacuum, which limits the applicability of the microscope to samples that can sustain such an environment. Most important, some fragile samples, such as organic molecules, are inevitably destroyed by the high-energy beams required for high resolutions.


Viewing Atoms

From 1936 to 1955, Erwin Wilhelm Muller developed the field ion microscope (FIM), which used an extremely sharp needle to hold the sample. This was the first microscope to make possible the direct viewing of atomic structures, but it was limited to samples capable of sustaining the high electric fields necessary for its operation.
In the early 1970′s, Russel D. Young and Clayton Teague of the National Bureau of Standards (NBS) developed the “topografiner,” a new kind of FIM. In this microscope, the sample is placed at a large distance from the tip of the needle. The tip is scanned across the surface of the sample with a precision of about a nanometer. The precision in the three-dimensional motion of the tip was obtained by using three legs made of piezoelectric crystals. These materials change shape in a reproducible manner when subjected to a voltage. The extent of expansion or contraction of the crystal depends on the amount of voltage that is applied. Thus, the operator can control the motion of the probe by varying the voltage acting on the three legs. The resolution of the topografiner is limited by the size of the probe.

Gerd Binnig and Heinrich Rohrer

Both Gerd Binnig and Heinrich Rohrer believe an early and pleasurable introduction to teamwork led to their later success in inventing the scanning tunneling microscope, for which they shared the 1986 Nobel Prize in Physics with Ernst Ruska.
Binnig was born in Frankfurt, Germany, in 1947. He acquired an early interest in physics but was always deeply influenced by classical music, introduced to him by his mother, and the rock music that his younger brother played for him. Binnig played in rock bands as a teenager and learned to enjoy the creative interplay of teamwork. At J. W. Goethe University in Frankfurt he earned a bachelor’s degree (1973) and doctorate (1978) in physics and then took a position at International Business Machine’s Zurich Research Laboratory. There he recaptured the pleasures of working with a talented team after joining Rohrer in research.
Rohrer had been at the Zurich facility since just after it opened in 1963. He was born in Buch, Switzerland, in 1933, and educated at the Swiss Federal Institute of Technology in Zurich, where he completed his doctorate in 1960. After post-doctoral work at Rutgers University, he joined the IBM research team, a time that he describes as among the most enjoyable passages of his career.
In addition to the Nobel Prize, the pair also received the German Physics Prize, Otto Klung Prize, Hewlett Packard Prize, and King Faisal Prize. Rohrer became an IBM Fellow in 1986 and was selected to manage the physical sciences department at the Zurich Research Laboratory. He retired from IBM in July 1997. Binnig became an IBM Fellow in 1987.
The idea for the scanning tunneling microscope (STM) arose when Heinrich Rohrer of the International Business Machines (IBM) Corporation’s Zurich research laboratory met Gerd Binnig in Frankfurt in 1978. The STM is very similar to the topografiner. In the STM, however, the tip is kept at a height of less than a nanometer away from the surface, and the voltage that is applied between the specimen and the probe is low. Under these conditions, the electron cloud of atoms at the end of the tip overlaps with the electron cloud of atoms at the surface of the specimen. This overlapping results in a measurable electrical current flowing through the vacuum or insulating material existing between the tip and the sample. When the probe is moved across the surface and the voltage between the probe and sample is kept constant, the change in the distance between the probe and the surface (caused by surface irregularities) results in a change of the tunneling current.
Two methods are used to translate these changes into an image of the surface. The first method involves changing the height of the probe to keep the tunneling current constant; the voltage used to change the height is translated by a computer into an image of the surface. The second method scans the probe at a constant height away from the sample; the voltage across the probe and sample is changed to keep the tunneling current constant. These changes in voltage are translated into the image of the surface. The main limitation of the technique is that it is applicable only to conducting samples or to samples with some surface treatment.

CONSEQUENCES

In October, 1989, the STM was successfully used in the manipulation of matter at the atomic level. By letting the probe sink into the surface of a metal-oxide crystal, researchers at Rutgers University were able to dig a square hole about 250 atoms across and 10 atoms deep. A more impressive feat was reported in the April 5,1990, issue of Nature; M. Eigler and Erhard K. Schweiser of IBM’s Almaden Research Center spelled out their employer’s three-letter acronym using thirty-five atoms of xenon. This ability to move and place individual atoms precisely raises several possibilities, which include the creation of custom-made molecules, atomic-scale data storage, and ultrasmall electrical logic circuits.
The success of the STM has led to the development of several new microscopes that are designed to study other features of sample surfaces. Although they all use the scanning probe technique to make measurements, they use different techniques for the actual detection. The most popular of these new devices is the atomic force microscope (AFM). This device measures the tiny electric forces that exist between the electrons of the probe and the electrons of the sample without the need for electron flow, which makes the technique particularly useful in imaging nonconducting surfaces. Other scanned probe microscopes use physical properties such as temperature and magnetism to probe the surfaces.
See also Cyclotron; Electron microscope; Ion field microscope; Mass spectrograph; Neutrino detector; Sonar; Synchrocyclotron; Tevatron accelerator; Ultra microscope.

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