Electron microscopy

Electron microscopy uses a “bombardment” of electron particles rather than light beams to obtain magnified images of specimen material. The process depends on carefully placed electromagnetic fields to focus the negatively charged electron particles. Much greater detail in magnification is obtained because electron particle waves are approximately four times shorter than light waves.

Development of the Technology

Electron microscopy involves the substitution of a beam of electrons and a series of electromagnetic or electric fields in the place of light beams and optical glass lenses in the common microscope. Because electron particles exhibit the same—but much shorter—wavelike movements found in light, electron beam focusing allows a much higher degree of resolution, or detailed contrast, when images of the effects of their passage through specimens are recovered, after magnified refocusing, on the fluorescent screen of an electron microscope.

A first step in developing what would later become standard technology came in 1895, when German physicist Wilhelm Röntgen noted that solid materials released invisible rays in the first stage of what was then called a cathode-ray device (later known in the field of electron microscopy as an electron “gun”). In time, scientists discovered that these atomic-particle rays not only shared the electromagnetic characteristics of light but also moved in wavelengths that were about 100,000 times shorter than those of light waves.

Among the earliest scientists to pioneer the field of electron microscopy was Englishman Sir Joseph John Thomson. In 1897, Thomson performed an experiment with electron rays that would yield the basic technology for the first television tube and, with essential adaptations which were developed by German physicist Ernst Ruska in the 1930s, the first electron microscope. In essence, Thomson's cathode-ray device, which was not yet conceived of in terms of magnifying possibilities, created a beam of tiny negatively charged particles, or cathode rays—that is, electrons. This beam resulted from a glow discharge in a gas and an incandescent metallic filament in a vacuum. These rays (in reality, negatively charged electron particles) were caused to accelerate toward and pass through a tiny hole in a positively charged plate that served as an anode. The result was a beam of high-velocity rays, which could be recorded on a fluorescent screen. Thus was established the basic technology that would later produce the first television screen, when multiple electronic “pinpoints” would be used to create a complex image. Use of this process to obtain magnification of objects placed in the path of a ray, or electron beam, became possible only after the development of the “electronic lens.”

Stages of Magnification

Starting with the basic principle of a cathode-ray tube, the pioneers of electron microscopy experimented with several technological adaptations that aimed at focusing electron beams to obtain magnifying effects when focused rays are refocused and projected onto a fluorescent screen. In the simplest of terms, the electron lens magnifies by several stages. Each stage must take place in a vacuum.

After passing through the tiny aperture in the positively charged anode (the first phase in Thomson's 1897 cathode-ray experiment), the electron beam is acted upon by a first electronic lens, called the condenser lens. This process results from the effect of an electric or electromagnetic field, located within the tube device, which interacts with the “descending” electron beam. The effect of such an electronic lens is to “bend” electron paths toward an axis, just as converging glass lenses bend light rays toward a focal axis. Once focused by the condenser lens, the highly concentrated electron beam strikes, in a separate chamber sealed off by O-rings, a carefully prepared specimen. The beam actually passes through the object to be magnified. Because the beam is made up of particles, however, the physical effect of particle bombardment will vary according to varying levels of material density in the specimen itself. This is the key to the lighter/darker image produced in the final stage of image projection.

As the electron beam emerges (on the downward side of the specimen), two other electronic lenses (electromagnetic fields) influence its path. First, a minutely focused objective lens spreads the concentrated beam created by the condenser lens. Then, a projector lens “captures” the resultant image, which is recorded on a fluorescent screen as a picture. Details revealed in this image—as in images associated with common photography—appear lighter when highly exposed (that is, when large numbers of electrons passed directly through less dense areas of the specimen) and darker when underexposed (a substantial number of electrons were “scattered” upon impact with denser areas within the specimen). Because electron waves are approximately four times shorter than are light waves, the amount of minute detail recorded in this image (that is, the high degree of resolution obtained) promised to thrust electron microscopy to the forefront of laboratory research, especially following first stages of commercialization of the technology pioneered by Ernst Ruska in the 1930s. Ruska, along with two other scientists involved in the advanced technology of scanning tunneling microscopy, would be awarded the Nobel Prize in Physics in 1986.

Solution of Technical Problems

Eventual success in promoting electron microscopy originally hinged on the solution of several technical problems associated not with the phenomenon of electromagnetic lenses (which proved to be surprisingly easily adjustable, merely by varying the intensity of current) but with the effects of electron “bombardment” on different types of specimens. Effective solutions to several such problems were not found until several years after World War II, when not only German but also other firms producing scientific instruments would compete for commercialization of electron microscopy.

The most obvious technical problem was connected with the heat created in the process of concentrating a stream of electrons. This heat enters the specimen even if the time of exposure is very limited. Absorption of externally induced heat into the specimen caused automatic image distortion because of increased molecular agitation—a state that would not have existed prior to exposure to an electron beam. One way this problem was reduced was by use of condenser lenses to create what is called “small region radiation” techniques. Double-stage focusing reduced the area of electron bombardment to an absolute minimum and, therefore, reduced the number of electrons (that is, the intensity of the bombardment and thus the origin of heat) that were needed to obtain a detailed image. These developments increased the possibilities of using electron microscopy in research involving organic specimens.

A second technical problem involved the condensation of minor residual gases, mainly hydrocarbons, inside the chambers within which the various stages of electromagnetic focusing take place. Such condensation was particularly bothersome if it gathered on the specimen itself, causing a general darkening, and even distorting of the image produced. Research directed by the electron microscope's original inventor in Berlin, Ernst Ruska, solved this problem by “bathing” all surfaces surrounding the specimen with superchilled liquid air at the time of each experiment, which kept the specimen warmer than any of the other elements in its environment. Thus, any condensation that might occur gathers on surrounding elements rather than on the specimen itself.

Resolutions limited by aberration problems have been partially resolved with lens correctors. These have increased resolution to achieve image production at atomic dimensions, with magnifications above 50 million times.

Scanning Electron Microscopy

What has been described up to this point applies to what is known as transmission electron microscopy (TEM), in which all elements of electron activity involved in contact with the specimen are transferred simultaneously to the image. In addition, researchers, again working mainly in Germany beginning in the mid-1930s, extended the technology of electron microscopy into a somewhat more complex domain—that of scanning electron microscopy (SEM). SEM technology relates to an entire subfield known as surface studies. Here, the image is built up in time sequence as a far tinier electron beam than that used in TEM moves across (“scans”) the specimen. The image is derived from secondary electron currents that are released from a very thin surface layer in the atomic structure of the specimen being examined. The technology needed to detect, and then to “capture,” the image of such surface-layer emissions accurately took a number of years to develop. Even following a breakthrough in secondary electron retrieval by British researchers at the University of Cambridge in 1948, the first commercial production of an effective SEM did not come until 1965. Since that date, SEMs have been used primarily to create images that resemble three-dimensional photography of the “topography” (meaning, cross sections of molecular structures) at the surface of specimen materials.

Scientific and Industrial Applications

A number of special fields of research relating to the earth sciences depend on diverse applications either of basic transmission electron microscopy or of scanning electron microscopy. Among them should be mentioned dark-field electron microscopy, in which the angle of the bombarding electron beam is “tilted” to produce bright spots in a darker field—a method that is particularly useful in determining whether there are crystalline structures in the specimen. The importance of scanning electron microscopy, together with computerized simulation in three dimensions, is particularly apparent in the field of crystallography and in the detailed analysis of minerals.

By far the most widely recognized application of electron microscopy in earth sciences relates to the petroleum industry and its by-products, either petrochemicals or related synthetic materials. In such areas, use of analytical electron microscopic techniques enables researchers to observe variations in molecular linkages that characterize “families” of synthetic products derived from petroleum. The results of such findings are critical in the search for new synthetic materials that are often much better suited to the specialized needs of modern industry and aerospace science than are natural metals and their alloys.

It is important to note that electron microscopy involves applied uses as well as investigative uses. Some of the former enable alteration of the molecular or atomic structures of natural substances, especially metals and their alloys, in ways that can make them more useful for technological purposes. An example is to be found in amorphization technology, which utilizes the electron irradiation process in the electron microscope to cause a break in crystalline periodicity (regular, latticelike chains in the atomic structure of substances), creating what specialists call a “zigzag” atomic chain that alters the basic electronic and/or chemical nature of the substance in question. Without necessarily being conscious of the fact, humans are surrounded by materials, many of them originally of natural mineral or metallic origin (especially those necessary for use as conductors in high-energy-intensity situations), that have been “transformed” for a number of special functions by processes connected with electron microscope technology.

Principal Terms

anode: in early cathode-ray tubes, this positively charged plate attracted negatively charged electrons that, passing through a tiny aperture in the plate, formed an electron beam

cathode-ray tube: a tubular device, the interior chambers of which are vacuum-sealed, through which an electron beam passes, recording a dot-like image on a fluorescent screen

condenser lens: the first stage of electromagnetic focusing, which “bends” the electron beam into a tightly concentrated focal point before it passes through the specimen

electronic lens: electromagnetic fields inside the electron microscope that interact with the electron beam, bending the trajectory of its particles

objective lens: the second, magnifying stage of electronic lens focusing, which occurs after the electron beam passes through the specimen

scanning electron microscopy (SEM): a later (mid-1960s) development in electron microscopy, in which surface structure images are obtained by a process that records patterns of electron emissions off the surface of the specimen as it is subjected to the impact of the microscope beam

transmission electron microscopy (TEM): electron microscopy in which all elements of electron activity involved in contact with the specimen are transferred simultaneously to the image

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