Scanning Tunneling Microscope
The latter half of the twentieth century has opened to scientists an entirely new world: the world of atomic and subatomic particles. With the inventions of the electron microscope and the field ion microscope, scientists have been able to observe the microcosm as never before.
Until the early 1980s, however, one mystery that eluded researches was the nature of the surfaces of substances. Since the arrangement of atoms in the surface of a substance differs greatly from that of its bulk, it requires other methods of analysis. Scientists had lacked a mechanism for studying the intricacies of surfaces until 1981, when German physicists Gerd Binnig (1947-) and Heinrich Rohrer (1933-) invented the scanning tunneling microscope (STM).
The word tunneling used here describes an effect of quantum mechanics theorized upon for years and first verified in the laboratory in 1960. It was known that an electron orbits about the nucleus of an atom, its motion random and diffuse, behaving as a cloud. When an element is placed very close to another, the cloud-orbits of the surface atoms will overlap slightly. The resulting diffusion of electrons is called tunneling.
When Binnig and Rohrer met in 1978 they were both working at IBM research laboratories in different cities, each studying the atomic structures of surfaces. They decided to combine their efforts toward using tunneling to explore these structures. By 1980 they had constructed a prototype STM, and in the spring of 1981 they succeeded in obtaining microscopic images using electron tunneling.
The STM that Binnig and Rohrer had built was actually based upon the field ion microscope invented by Erwin Wilhelm Müller. The field ion microscope uses a tiny sharpened needle placed within a cathode-ray tube; as an electrical field is applied, metal ions are emitted from the tip of the needle, creating an image of the metal's atomic structure upon the cathode screen.
In Binnig and Rohrer's device, a similar needle is placed in a vacuum, above a specimen to be scanned at a height of less than one nanometer. The sharper the needle, the more precise is the STM reading; the best needle tips are only one or two atoms wide. A very low voltage is applied, causing the overlapping clouds of electrons to tunnel from the needle to the specimen. This electron flow is called a tunneling current, and by measuring this flow irregularities in the surface of the specimen can be determined. The scanning tip is swept over the sample, so that the entire surface may be mapped.
Even the very first tests of the STM showed it to be extremely powerful. When scanning crystals of calcium-iridium, Binnig and Rohrer resolved surface hills only one atom high. Maintaining the very small distance between the needle tip and the specimen proved to be difficult, however, since noises as unobtrusive as a footstep would jar the instrument. Sinnig and Rohrer used magnets to suspend the microscope over a table equipped with shock absorbers to solve this problem. Other improvements increased the magnification of the STM, and today's tunneling microscopes can resolve features as small as one hundred-billionth of a meter, or about one-tenth the width of a hydrogen atom. It has also been discovered that STMs are equally useful in air, water, and cryogenic fluid media.
The incredibly precise three-dimensional images provided by STMs have found varied applications in a number of industries. They are used for quality control in manufacturing digital recording heads as well as in the construction of compact audio disk stampers. In 1991 the STM was used to move and place 35 atoms of xenon in a predetermined pattern; this ability to manipulate matter at the level of a single atom may allow scientists to customize molecules, possibly creating ultramicroscopic data storage chips.
Because it is effective in many media and uses a very low voltage, the STM can be used to study the atomic structure of living and biologic matter if that matter readily conducts electrons. Most biologic matter does not, however, and must be coated with a thin layer of a conducting substance. The STM is standard equipment in most atomic research laboratories. It is undeniably the most powerful optical tool yet invented, and for its invention Gerd Binnig and Heinrich Rohrer shared the 1986 Nobel Prize for Physics, along with Ernst Ruska, the inventor of the electron microscope.
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