Transmission electronic microscopy

As described in Sect. 12.2, the electron beam in electronic microscopy is generated in a cathode, heated by electric current. As shown, the electron beam is accelerated by an electrode system, the electron gun. The three cannon components are the filament (cathode), the cylinder Wehnelt, and the anode. 

As can be seen in Fig. 12.7, the cylinder Wehnelt causes the electrons that are ejected from the cathode to form a thin beam of electrons to the anode. 

In TEM, the acceleration voltage applied will determine the velocity of the electrons in the beam to be collimated by the condenser lenses, which also occurs in the SEM. In fact, the basic distinction between SEM and TEM is closely related to the electron beam. It is the intensity of the beam and how it is controlled by optical-electronic column that define much of what can be achieved in scanning analysis or transmission [12]. 

Unlike SEM, the inelastic scattered electrons, secondary electrons, for example, are not significant for image contrast in the TEM (if we disregard the imaging mode for STEM— scanning transmission electron microscopy ) and are excluded from analysis through the lens objective. 

In addition, the speed with which the electrons pass through the electron optic column in TEM is typically much larger than in SEM, since the former operates at higher accelerating voltages, which can vary depending on the analysis conditions between 80 and 200 kV in a conventional TEM. 

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Transmission electron microscopy (TEM) can resolve features down to about 1 nm and allows the use of electron diffraction to characterize the structure. Since electrons must pass through the sample however, the technique is limited to thin films. One cryoelectron microscopic study of fatty-acid Langmuir films on vitrified water [13] showed faceted crystals. The application of TEM to Langmuir-Blodgett films is discussed in Chapter XV. 

Thomas G and Goringe M J 1981 Transmission Electron Microscopy of Materials (New York Wiiey)... 

The history of EM (for an overview see table Bl.17,1) can be interpreted as the development of two concepts the electron beam either illuminates a large area of tire sample ( flood-beam illumination , as in the typical transmission electron microscope (TEM) imaging using a spread-out beam) or just one point, i.e. focused to the smallest spot possible, which is then scaimed across the sample (scaiming transmission electron microscopy (STEM) or scaiming electron microscopy (SEM)). In both situations the electron beam is considered as a matter wave interacting with the sample and microscopy simply studies the interaction of the scattered electrons. 

Williams D B and Carter C B 1996 Transmission Electron Microscopy, A Textbook for Material Science (New York Plenum)... 

Reimer L 1993 Transmission Electron Microscopy (Berlin Springer)... 

The spatial arrangement of atoms in two-dimensional protein arrays can be detennined using high-resolution transmission electron microscopy [20]. The measurements have to be carried out in high vacuum, but since tire metliod is used above all for investigating membrane proteins, it may be supposed tliat tire presence of tire lipid bilayer ensures tliat tire protein remains essentially in its native configuration. 

In many ways the nanocrystal characterization problem is an ideal one for transmission electron microscopy (TEM). Here, an electron beam is used to image a thin sample in transmission mode [119]. The resolution is a sensitive fimction of the beam voltage and electron optics a low-resolution microscope operating at 100 kV might... 

L. Reimer, Transmission Electron Microscopy, Springer Series in Optical Sciences, Vol. 36, 2nd ed., Springer-Vedag Berlin, 1989. 

The very high powers of magnification afforded by the electron microscope, either scanning electron microscopy (sem) or scanning transmission electron microscopy (stem), are used for identification of items such as wood species, in technological studies of ancient metals or ceramics, and especially in the study of deterioration processes taking place in various types of art objects. 

Fig. 6. Microstmcture of transparent P-quart2 soHd solution glass-ceramic as revealed by transmission electron microscopy (white bat = 0.1 jira).

Nylon-6. Nylon-6—clay nanometer composites using montmorillonite clay intercalated with 12-aminolauric acid have been produced (37,38). When mixed with S-caprolactam and polymerized at 100°C for 30 min, a nylon clay—hybrid (NCH) was produced. Transmission electron microscopy (tern) and x-ray diffraction of the NCH confirm both the intercalation and molecular level of mixing between the two phases. The benefits of such materials over ordinary nylon-6 or nonmolecularly mixed, clay-reinforced nylon-6 include increased heat distortion temperature, elastic modulus, tensile strength, and dynamic elastic modulus throughout the —150 to 250°C temperature range. 

Transmission electron microscopy (tern) is used to analyze the stmcture of crystals, such as distinguishing between amorphous siUcon dioxide and crystalline quartz. The technique is based on the phenomenon that crystalline materials are ordered arrays that scatter waves coherently. A crystalline material diffracts a beam in such a way that discrete spots can be detected on a photographic plate, whereas an amorphous substrate produces diffuse rings. Tern is also used in an imaging mode to produce images of substrate grain stmctures. Tern requires samples that are very thin (10—50 nm) sections, and is a destmctive as well as time-consuming method of analysis. 

The mechanism for coercivity in the Cr—Co—Fe alloys appears to be pinning of domain walls. The magnetic domains extend through particles of both phases. The evidence from transmission electron microscopy studies and measurement of JT, and anisotropy vs T is that the walls are trapped locally by fluctuations in saturation magnetization. 

Transmission electron microscopy is very widely used by biologists as well as materials scientists. The advantage of being able to resolve 0.2 nm outweighs the disadvantages of TEM. The disadvantages include the inabiUty of the common 100-kV electron beam to penetrate more than a few tenths of a micrometer (a 1000-kV beam, rarely used, penetrates specimens about 10 times thicker). Specimen preparation for the TEM is difficult because of the... 

An excellent historical account of the beginning of transmission electron microscopy in North America is available (22). 

The properties and performance of cemented carbide tools depend not only on the type and amount of carbide but also on carbide grain size and the amount of biader metal. Information on porosity, grain size and distribution of WC, soHd solution cubic carbides, and the metallic biader phase is obtained from metaHographicaHy poHshed samples. Optical microscopy and scanning and transmission electron microscopy are employed for microstmctural evaluation. Typical microstmctures of cemented carbides are shown ia Figure 3. 

A progressive etching technique (39,40), combined with x-ray diffraction analysis, revealed the presence of a number of a polytypes within a single crystal of sihcon carbide. Work using lattice imaging techniques via transmission electron microscopy has shown that a-siUcon carbide formed by transformation from the P-phase (cubic) can consist of a number of the a polytypes in a syntactic array (41). 

Occasionally, especially in the developmental phase of catalyst research, it is necessary to determine the oxidation state, exact location, and dispersion of various elements in the catalyst. Eor these studies, either transmission electron microscopy (TEM) or scanning electron microscopy (SEM) combined with various high vacuum x-ray, electron, and ion spectroscopies are used routinely. 

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