Erwin Wilhelm Müller invented the field emission microscope, the first instrument to depict the crystal structure of metals and the forerunner of the more powerful field ion microscope.
Erwin Wilhelm Müller began to study the physical process that would constitute the basis of the field emission microscope while he was a research physicist in Berlin from 1935 to 1937. Field emission is the emission of electrons, the negatively charged subatomic constituents of all matter, from a metal electrode under the influence of a strong electrical field into a vacuum. The strong field induces the electrons to travel in the direction of the field. In 1876, Eugen Goldstein had demonstrated this effect experimentally. Goldstein projected the image of a small coin onto the fluorescent wall of a vacuum tube by using the coin as the electron-donating electrode, or cathode, of an electrical circuit, and the fluorescent wall of the tube as the electron-receiving electrode, or anode. The resulting electrical field around the metal coin induced electrons to traverse the vacuum tube and strike the fluorescent wall. When the moving electrons struck the fluorescent coating of the tube wall, a blurred, glowing image of the coin appeared.
Müller’s research involved electron emission from point sources, not flat sources such as coins. One particular source consisted of a sharply edged tungsten needle. At first, Müller was simply interested in the paths taken by the field emitted electrons. Because Müller was familiar with the work of Goldstein and others, he set out to construct a similar apparatus, replacing the flat coin with his convex metal needles in order to stimulate the electron flow.
Müller’s vacuum tubes were more like lightbulbs than tubes, with the hemisphere opposite the needlelike cathode coated with a fluorescent material. In 1936, when Müller applied an electric field of approximately 40 million volts per centimeter to this apparatus, he did not view an image of the needle point analogous to that of Goldstein’s coin face. Instead, his images were single, unstable blotches of light and dark that were at first nearly impossible to interpret. He soon recognized that the images of the convex needles produced by the instrument were highly magnified, whereas Goldstein’s images offered no magnification at all; they were mere representations. The apparatus was a crude version of the field emission microscope.
In order to overcome the poor image quality, Müller worked to improve the quality of the needle tip through the chemical and electrolytic preparation of fine, heat-treated metal wires. Such procedures allowed Müller to manufacture needles with tips that were no larger than 0.00002 centimeter in diameter. At this size, many of the needles were composed of one nearly perfect crystal of the metal. With such improvements, Müller was able to publish emitted electron images of the surface crystallographic structure of tungsten in the German journal Zeitschrift für Physik in 1937. The magnification of these images was so great that it offered the first glimpse of the metallic crystal structure at the level of the atoms making up the crystal lattice. These were the first applications of the new invention, dubbed the field emission, or field electron, microscope. Soon thereafter, Müller obtained similar micrographs of the crystal structures of molybdenum, platinum, nickel, and copper.
Müller realized the importance such an instrument would have on the study of metallic surfaces. Such an instrument could be used to study the adsorption, or the physics of adherence, of contaminating materials onto the very structure of the metal. He also recognized that the instrument would be useless without a firm understanding of the physical process on which it depended—the field emission of electrons. The power of the field emission microscope would depend on the limits that the physical process of field emission imposed on it, for any scientific instrument is limited by the physical system on which it depends in order to operate.
Two of the most important parameters in assessing the power of any microscope are magnification and resolution. The former is a measure of the size of the smallest objects that the microscope can make visible; the latter concerns the size of the objects that it can distinguish, and is thus a measure of accuracy. For example, a microscope that has a magnification of one hundred times will make objects appear one hundred times larger than they are in reality. A microscope with a resolution of two millimeters can distinguish objects that are separated by a distance of two millimeters. Objects that are separated by less than two millimeters will not be seen as distinct objects. The electron emission process influences both these parameters and would ultimately preclude the instrument from depicting the images of individual metal atoms within the crystal lattice.
Müller studied the effect of needle shape on the performance of the microscope throughout much of 1937. He discovered that the magnification of a field emission microscope depends on the ratio of the fluorescent screen radius to the metal emitter radius. This underscores the importance of having a needle-shaped electron emitter. The more precise the tip of a needle is, the smaller its radius will be. The smaller this measure becomes, the larger the ratio of the screen radius to emitter radius becomes, and hence the higher the magnification becomes. The reason Goldstein’s apparatus produced an unmagnified image of the coin was that the radius of the screen was approximately equal to that of the coin; because their ratio was approximately equal to one, the apparatus yielded no magnifying power at all. This should not be interpreted as meaning that the needle emitter should be fashioned into the sharpest point possible with a radius approaching zero. Indeed, when Müller showed the instrument to the German physicist Max von Laue
When the needles had been properly shaped, Müller was able to realize magnifications of up to one million times. This magnification allowed Müller to view what he called “maps” of the atomic crystal structure of metals. Although the magnification may have been great, the resolution of the instrument was severely limited by the physics of emitted electrons, which caused the images Müller obtained to be continually blurred.
Müller was well aware of the contemporary developments in quantum mechanics, especially those resulting from the work of Ralph H. Fowler, which applied these theories to field electron emission. Quantum mechanics is the set of physical theories that describes phenomena at the subatomic level, which states that entities such as electrons behave both as particles and as waves. Given that the electrons have particle-like properties, they would have a particular velocity, like any other moving particle. Given that they also behaved like waves, they would have a particular wavelength, as do sound waves or light waves. The electron velocity was extremely high and uncontrollably random, which caused the blurred micrographic images. In addition, the electrons had an unsatisfactorily high wavelength. When Müller combined these two factors, he was able to determine that, theoretically, the resolution of the field emission microscope would never reach below 20 angstroms. This may seem quite small. In fact, a resolution of 20 angstroms is equal to a resolution of 0.000000002 meter. Müller noted, however, that the atoms in an atomic crystal lattice are separated by only 4 to 5 angstroms. Thus the field emission microscope could never depict single atoms, for it was a physical impossibility for it to distinguish one atom from another.
Even with its inherent limitations, the field emission microscope had an enormous impact on two fronts: the field of surface science and Müller’s development of the field ion microscope in the early to mid-1950’s.
Robert Gomer, an American chemist and physicist, was among the first scientists to put Müller’s invention to use in actual scientific investigation. Most other microscopes and magnifying instruments consist of an independent system of lenses that process the image of the specimen under investigation by focusing the reflection of some wavelength of radiation (visible light, X rays, and the like). The field emission microscope does not require such an apparatus, however. The specimen under study—the needle—is an integral part of the instrument, for it is also the electron emitting cathode. Whatever happens physically or chemically to the specimen will affect directly the resulting image. Gomer was attracted to the device because of this simplicity, and it was this simplicity that permitted the types of experiments he conducted.
As size requirements dictated that the needle often consists of only one individual crystal of the metal, Müller realized that the properties of individual crystals could be examined. The most important of these properties was the adsorption of materials onto the needle surface. The instrument offered such a precise map of the crystal structure that a layer as thin as one one-hundredth of an atom would affect the emission quality and hence the resulting image. Gomer used the field emission microscope to study the adsorption and desorption of materials such as barium.
The field of surface science
Despite these advances, Müller’s studies of the limitations of the microscope eventually led to his development of the more powerful field ion microscope between 1951 and 1956. Rather than imaging the needle tip with emitted electrons, Müller used emitted positive ions. The ions had smaller velocities and shorter wavelengths than the much lighter electrons, and hence the inherent resolution problems of the field emission microscope were overcome at last. By 1956, Müller and his colleagues at the Pennsylvania State University’s Field Emission Laboratory reported the first images of individual atoms in metal lattices.
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Müller, Erwin Wilhelm. “The Field Ion Microscope.” American Scientist 49 (March, 1961): 88-98. Largely devoted to Müller’s work on the field ion microscope, but presents some introductory material concerning the physical limitations of the field emission microscope that eventually led to the later microscope’s development. -
_______. “Field Ion Microscopy.” Science 149 (August, 1965): 591-600. Focuses on the later and more powerful field ion microscope, but discusses Müller’s realization of the resolving problems of the field ion microscope. Also addresses the ramifications of the fact that the imaging medium and sample are one and the same in both the field emission and field ion microscopes. -
_______. “The Imaging Process in Field Ion Microscopy from the FEM to the Atom Probe.” In Chemistry and Physics of Solid Surfaces, edited by Ralf Vanselow and S. Y. Tong. Cleveland: CRC Press, 1977. Technically concise and accessible account of the development of the field emission microscope. Like most of Müller’s work, presented in the context of the later field ion microscope. Traces the history of the field emission microscope and includes illustrations and micrographs from Müller’s original German publications, including micrographs of the tungsten crystal and barium adsorbed onto tungsten. -
Müller, Erwin Wilhelm, and Tien Tzou Tsong. “Fundamentals of Field Ion Microscopy.” In Field Ion Microscopy: Principles and Applications. New York: Elsevier, 1969. Müller’s most comprehensive account of the application of his inventions in the field of surface science. Focuses mostly on the later field ion microscope, but the introductory chapter gives a concise account of the historical events leading to the development of both instruments. -
Oudar, Jacques. “Recent Methods in the Study of Adsorption.” In Physics and Chemistry of Surfaces. Glasgow: Blackie & Son, 1975. Details the use of both the field emission and the field ion microscopes in surface studies. Adequately illustrated, although sparse in technical content.
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