History of TEM

** The primary reason for using a transmission electron microscope (TEM) as opposed to a light microscope is the significant increase in resolving capabilities of the TEM. This is due to the wavelength of illumination that is produced by an energized beam of electrons.

** It was known in the 1920's that electrons traveling as an energized beam behave in a wave-like fashion similar to light waves. Taking advantage of this two German scientists, Ernst Ruska and Max Knoll, developed the first working electron microscope in 1932. In their paper the term electron microscope?was first used, and the idea of electron lenses and electron images was first developed into the practical reality. Thirty-four years later, they were awarded the Nobel Prize in Physics in 1986 for this outstanding accomplishment.

** TEMs were developed by commercial UK companies in 1936.

** Initially it was thought that biological specimens could not be examined with this unique instrument. This was because the extreme conditions inside the TEM (high vacuum, intense heat generated by the beam of electrons, depth to which electrons can penetrate a specimen, etc.) were thought to be incompatible with wet, thick, biological specimens.

** It was Ernst Ruska's brother Helmut (a medical student) who was most encouraging in pursuing the development of the TEM for the study of biological specimens. Some of the earliest TEMs were of the wing of a house fly (a reasonably dry and electron transparent specimen), diatom frustules, and bacteria. By 1941 the first electron micrographs of viruses were being produced and funding for further development of the TEM came from medical researchers not physicists.

** Before the 1950's, the electron microscope was mainly used by biologists. For materials scientists a most important development took place in the late 1940s when Heidenreich (1949) first thinned metal foils to electron transparency. This work was followed up by Bollman in Switzerland and Hirsch and co-workers in Cambridge.

** TEMs became widely available from several major companies such as Hitachi, JEOL, FEI/Philips and RCA, inter alia after the end of World War II.

** Because so much of the early TEM work examined metal specimens, the word foil has come to be synonymous with specimen?

** The Cambridge group also developed the theory of electron diffraction contrast with which we can now identify, often in a quantitative manner, all known line and planar crystal defects in TEM images.

** Practical applications of the TEM for the solution of materials problems were pioneered in the U.S. by Thomas. Other materials-oriented texts followed, e.g., Edington (1976) and Thomas and Goringe (1979).

** Ultrahigh voltage TEM instruments (up to 3 MeV at CEMES-LOE/CNRS in Toulouse, France, and at Hitachi in Tokyo, Japan), in the 1960s and 1970s gave electrons higher energy to penetrate more deeply into thick samples.

** In later years, the evolution and incorporation of other detectors (electron microprobes, electron energy loss spectroscopy (EELS), energy-dispersive x-ray spectrum (EDS), etc.) made the TEM into a true analytical electron microscope (AEM). The development of brighter electron sources, such as the lanthanum hexaboride filament (LAB6) and the field emission gun in the 1960s, and their commercialization in the 1970s brought researchers a brighter source of electrons and with it better imaging and resolution. Tilting specimen stages permitting examination of the specimen from different angles aided significantly in the determination of crystal structure.

** In the last 60 years, additional quantitative information generated from the electron/specimen interactions has been used to analyze materials. The first quantitative information routinely obtained was due to electron diffraction in thin crystalline specimens. The newest generation of microscopes can generate electron-diffraction patterns from small volumes (<50 nm in diameter) in the specimen by electron
microdiffraction.

** Currently, A transmission electron microscope can appear in several different forms, including conventional transmission electron microscope (CTEM), high resolution transmission electron microscope (HRTEM), scanning transmission electron microscope (STEM), and
analytical electron microscope (AEM). It is usual to divide the TEM into three components: the illumination system, the objective lens/stage, and the imaging system. The illumination system comprises the gun and the condenser lenses and its role is to extract the electrons from the source and transfer them to the specimen. There are two kinds of electron guns available: thermionic gun (using tungsten or LaB6 filaments) and field-emission gun (FEG). The electrons emitted from the electron gun are condensed by magnetic lenses to form the electron beam. Two principal modes of the electron beam can be obtained by adjusting the condenser lenses: parallel beam and convergent beam. The first mode is used for conventional TEM imaging, diffraction and HRTEM imaging, while the second is used for STEM imaging, microanalysis and convergent-beam electron diffraction (CBED). The objective lens/stage system is the heart of the TEM.

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