Erwin Wilhelm Müller achieved atomic resolution with the field ion microscope, an improved version of his field emission microscope, allowing scientists to visualize individual atoms for the first time.
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 among these developments were George Gamow’s
Erwin Wilhelm Müller holds his field ion microscope in front of a model of tungsten crystal atoms.
In 1936, Erwin Wilhelm Müller developed his field emission microscope
Field emission and field ion microscopes examine the atomic surface structures of metals by viewing their projections on fluorescent screens. Since the field ion microscope is the direct descendant of the field emission microscope, it is essential to gain a basic understanding of the field emission microscope in order to understand the field ion 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 serves as the electron-receiving electrode, or the anode, and is placed opposite the needle. When a 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.
Two of the most important parameters in assessing the power of any microscope are magnification and resolution. While the former is a measure of the size of the smallest objects the microscope can make visible, the latter measures the accuracy of the instrument. 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 2 millimeters can distinguish objects that are separated by a distance of 2 millimeters. Objects separated by less than 2 millimeters will not be seen as distinct objects. The electron-emission process influences both of these parameters and ultimately precludes the field emission microscope from depicting the images of individual metal atoms within the 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 upon the ratio of the fluorescent screen radius to the metal emitter’s radius. 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 screen radius to emitter radius becomes, and hence, the higher the magnification becomes.
When the needles had been properly shaped, Müller was able to realize magnifications of up to 1 million times. This magnification allowed Müller 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 Müller obtained to be continually blurred.
In 1943, while working in Berlin, Müller realized that the resolution of the field emission microscope was limited by two factors: the velocity of the electrons and their associated de Broglie wavelengths. The latter was a development of contemporary quantum mechanics that noted that entities such as electrons behave both as particles and as waves.
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 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 meters. 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.
By 1951, this limitation led Müller to conclude that new technology was needed. In 1952, Müller moved to the United States and founded the Pennsylvania State University Field Emission Laboratory
The field ion microscope utilized positive ions instead of electrons to create the atomic surface images on the fluorescent screen. 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.
Müller admitted that the use of positive ions rather than electrons to produce the image of the emitter lattice seemed to be unconventional. It involved more than switching the polarity of the field emission microscope by making the emitter cathode the anode and the screen anode the cathode to produce a stream of positive particles. Given the limitations imposed by the quantum mechanical properties of electrons, it seemed that the larger and more massive ions would only compound the electron’s inherent resolution problems, but larger and more massive objects have much smaller wavelengths than less massive ones. Therefore, Müller’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. Müller eliminated this problem by cryogenically cooling the needle tip with a supercooled liquefied gas such as nitrogen or hydrogen.
An essential difference between the field emission microscope and other microscopes is that the imaging medium—the electrons—actually comes from the specimen under examination, the metal needle. Positive ions will not travel through a metal crystal like the negatively charged electrons, because in the presence of an electric field, electrons are the conductors of electricity. Thus, the problem that presented itself to Müller was how to utilize the superior imaging properties of positive ions, which, unlike the electrons of the field emission microscope, did not emit directly from the metal sample.
By 1956, Müller 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, Müller was assured that no chemical reaction would occur between the needle tip and the gas, for any reaction would alter the surface atomic structure of the needle and thus alter the resulting microscopic image.
The imaging process worked largely because of the extremely high positive electrical potential present at the needle tip. Whereas the field emission microscope generated negative potentials at the emitter tip of approximately 40 million volts per centimeter, the field ion microscope generated positive potentials exceeding ten times that amount, approaching 500 million volts per centimeter. At this field strength, an atom of the inert gas would transfer one of its electrons to the needle. When this occured, the atoms giving up an electron became positive ions and were accelerated by the electrical field toward the screen, where they created images of their place of origin on the emitter tip. 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.
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. In the early twentieth century, Max von Laue
While the instrument may be extremely powerful, however, the very 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. This limitation of the instrument makes it extremely difficult to use for the examination of biological materials, which cannot withstand the amount of stress that metals can.
It was a practical by-product in the study of field ionization—field evaporation—that eventually permitted scientists to view large biological molecules. When the electrical field is increased beyond the limit required for the ionization of the imaging gas, individual atoms begin to shed, or “field evaporate,” from the needle tip. When metal specimens are induced to field evaporate, the needle sheds the constituent atoms that are nearest the surface and, consequently, are often combined with other materials. In the absence of these impurities, a field evaporated specimen remains, offering the scientist the best opportunity to view the pure metal’s crystal structure.
In addition to the ability to view uncontaminated crystal structures, 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 were able to view the atomic structures of large biological molecules by field evaporating much of the surrounding metal until the biological material remained at the needle surface.
-
Miller, M. K. Atom Probe Tomography: Analysis at the Atomic Level. New York: Kluwer Academic/Plenum, 2000. Survey of the methods for imaging and analyzing atomic structure, including the use of field ion microscopy. Bibliographic references and index. -
Müller, Erwin W. “The Field Ion Microscope.” American Scientist 49 (March, 1961): 88-98. The most easily accessible of Müller’s writings on the field ion microscope. It presents some introductory material on the physical limitations of the field emission microscope which eventually led to the field ion microscope’s development. In addition, the lay reader should have no difficulty with the details of the field ion imaging process. Amply illustrated and contains a concise description of the uses and achievements of the instrument. -
_______. “Field Ion Microscopy.” Science 149 (August, 1965): 591-600. Presents the technical details of the technology and the physics behind the field ion microscope. For the reader interested in a more technical introduction to the subject, this article contains much information on the theoretical and practical limitations and abilities of the field ion microscope. It also contains an important section on artifacts created by the imaging process. -
_______. “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, Ohio: CRC Press, 1977. Müller traces the history of the field emission and field ion microscopes and includes illustrations and micrographs from his original German publications. For those interested in the technical details of the applications of the field ion microscope, this is undoubtedly the finest source available. -
Müller, Erwin W., 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. The introductory chapter gives a concise account of the historical events leading to the development of both instruments. Useful bibliography. -
Oudar, Jacques. “Recent Methods in the Study of Adsorption.” In Physics and Chemistry of Surfaces. Glasgow: Blackie and Son, 1975. In an article devoted exclusively to the study of adsorption, Oudar details the use of both the field emission and the field ion microscopes in surface studies. Adequately illustrated, although sparse in technical content.
Physicists Develop the First Synchrocyclotron
Hofstadter Discovers That Protons and Neutrons Have Structure
Gell-Mann and Zweig Advance Quark Theory
Friedman, Kendall, and Taylor Discover Quarks