Field ion microscope (Inventions)

The invention: A microscope that uses ions formed in high-voltage electric fields to view atoms on metal surfaces.

The people behind the invention:

Erwin Wilhelm Muller (1911-1977), a physicist, engineer, and
research professor J. Robert Oppenheimer (1904-1967), an American physicist

To See Beneath the Surface

In the early twentieth century, developments in physics, especially quantum mechanics, paved the way for the application of new theoretical and experimental knowledge to the problem of viewing the atomic structure of metal surfaces. Of primary importance were American physicist George Gamow’s 1928 theoretical explanation of the field emission of electrons by quantum mechanical means and J. Robert Oppenheimer’s 1928 prediction of the quantum mechanical ionization of hydrogen in a strong electric field.
In 1936, Erwin Wilhelm Muller developed his field emission microscope, the first in a series of instruments that would exploit these developments. It was to be the first instrument to view atomic structures—although not the individual atoms themselves— directly. Muller’s subsequent field ion microscope utilized the same basic concepts used in the field emission microscope yet proved to be a much more powerful and versatile instrument. By 1956, Muller’s invention allowed him to view the crystal lattice structure of metals in atomic detail; it actually showed the constituent atoms.
The field emission and field ion microscopes make it possible to view the atomic surface structures of metals on fluorescent screens. The field ion microscope is the direct descendant of the field emission microscope. In the case of the field emission microscope, the images are projected by electrons emitted directly from the tip of a metal needle, which constitutes the specimen under investigation.
These electrons produce an image of the atomic lattice structure of the needle’s surface. The needle serves as the electron-donating electrode in a vacuum tube, also known as the “cathode.” A fluorescent screen that serves as the electron-receiving electrode, or “anode,” is placed opposite the needle. When sufficient electrical voltage is applied across the cathode and anode, the needle tip emits electrons, which strike the screen. The image produced on the screen is a projection of the electron source—the needle surface’s atomic lattice structure.
Muller studied the effect of needle shape on the performance of the microscope throughout much of 1937. When the needles had been properly shaped, Muller was able to realize magnifications of up to 1 million times. This magnification allowed Muller to view what he called “maps” of the atomic crystal structure of metals, since the needles were so small that they were often composed of only one simple crystal of the material. While the magnification may have been great, however, the resolution of the instrument was severely limited by the physics of emitted electrons, which caused the images Muller obtained to be blurred.


Improving the View

In 1943, while working in Berlin, Muller realized that the resolution of the field emission microscope was limited by two factors. The electron velocity, a particle property, was extremely high and uncontrollably random, causing the micrographic images to be blurred. In addition, the electrons had an unsatisfactorily high wavelength. When Muller combined these two factors, he was able to determine that the field emission microscope could never depict single atoms; it was a physical impossibility for it to distinguish one atom from another.
By 1951, this limitation led him to develop the technology behind the field ion microscope. In 1952, Muller moved to the United States and founded the Pennsylvania State University Field Emission Laboratory. He perfected the field ion microscope between 1952 and 1956.
The field ion microscope utilized positive ions instead of electrons to create the atomic surface images on the fluorescent screen.

Erwin Muller

Erwin Muller’s scientific goal was to see an individual atom, and to that purpose he invented ever more powerful microscopes. He was born in Berlin, Germany, in 1911 and attended the city’s Technische Hochschule, earning a diploma in engineering in 1935 and a doctorate in physics in 1936.
Following his studies he worked as an industrial researcher. Still a neophyte scientist, he discovered the principle of the field emission microscope and was able to produce an image of a structure only two nanometers in diameter on the surface of a cathode. In 1941 Muller discovered field desorption by reversing the polarity of the electron emitter at very low temperatures so that surface atoms evaporated in the electric field. In 1947 he left industry and began an academic career, teaching physical chemistry at the Altenburg Engineering School. The following year he was appointed a department head at the Fritz Haber Institute. While there, he found that by having a cathode absorb gas ions and then re-emit them he could produce greater magnification.
In 1952 Muller became a professor at Pennsylvania State University. Applying the new field-ion emission principle, he was able to achieve his goal, images of individual atoms, in 1956. Almost immediately chemists and physicists adopted the field-ion microscope to conduct basic research concerning the underlying behavior of field ionization and interactions among absorbed atoms. He further aided such research by coupling a field-ion microscope and mass spectrometer, calling the combination an atom-probe field-ion microscope; it could both magnify and chemically analyze atoms.
Muller died in 1977. He received the National Medal of Science posthumously, one of many honors for his contributions to microscopy.
When an easily ionized gas—at first hydrogen, but usually helium, neon, or argon—was introduced into the evacuated tube, the emitted electrons ionized the gas atoms, creating a stream of positively charged particles, much as Oppenheimer had predicted in 1928. Muller’s use of positive ions circumvented one of the resolution problems inherent in the use of imaging electrons. Like the electrons, however, the positive ions traversed the tube with unpredictably random velocities. Muller eliminated this problem by cryogenically cooling the needle tip with a super cooled liquefied gas such as nitrogen or hydrogen.
By 1956, Muller had perfected the means of supplying imaging positive ions by filling the vacuum tube with an extremely small quantity of an inert gas such as helium, neon, or argon. By using such a gas, Muller was assured that no chemical reaction would occur between the needle tip and the gas; any such reaction would alter the surface atomic structure of the needle and thus alter the resulting microscopic image. The imaging ions allowed the field ion microscope to image the emitter surface to a resolution of between two and three angstroms, making it ten times more accurate than its close relative, the field emission microscope.

Consequences

The immediate impact of the field ion microscope was its influence on the study of metallic surfaces. It is a well-known fact of materials science that the physical properties of metals are influenced by the imperfections in their constituent lattice structures. It was not possible to view the atomic structure of the lattice, and thus the finest detail of any imperfection, until the field ion microscope was developed. The field ion microscope is the only instrument powerful enough to view the structural flaws of metal specimens in atomic detail.
Although the instrument may be extremely powerful, the extremely large electrical fields required in the imaging process preclude the instrument’s application to all but the heartiest of metallic specimens. The field strength of 500 million volts per centimeter exerts an average stress on metal specimens in the range of almost 1 ton per square millimeter. Metals such as iron and platinum can withstand this strain because of the shape of the needles into which they are formed. Yet this limitation of the instrument makes it extremely difficult to examine biological materials, which cannot withstand the amount of stress that metals can. A practical by-product in the study of field ionization—field evaporation—eventually permitted scientists to view large biological molecules.
Field evaporation also allowed surface scientists to view the atomic structures of biological molecules. By embedding molecules such as phthalocyanine within the metal needle, scientists have been able to view the atomic structures of large biological molecules by field evaporating much of the surrounding metal until the biological material remains at the needle’s surface.
See also Cyclotron; Electron microscope; Mass spectrograph; Neutrino detector; Scanning tunneling microscope; Sonar; Synchrocyclotron; Tevatron accelerator; Ultra microscope.

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