Electron microscope principle

Electron microscope. Principle of the transmission electron microscope. 

Historically, EELS is one of the oldest spectroscopic techniques based ancillary to the transmission electron microscope. In the early 1940s the principle of atomic level excitation for light element detection capability was demonstrated by using EELS to measure C, N, and O. Unfortunately, at that time the instruments were limited by detection capabilities (film) and extremely poor vacuum levels, which caused severe contamination of the specimens. Twenty-five years later the experimental technique was revived with the advent of modern instrumentation. The basis for quantification and its development as an analytical tool followed in the mid 1970s. Recent reviews can be found in the works by Joy, Maher and Silcox " Colliex and the excellent books by Raether and Egerton. ... 

R. E Egerton. Electron Energy Loss Spectroscopy in the Electron Microscope. Plenum, New York, 1986. The principle textbook on EELS. 

The alternative approach to detection and analysis incorporates a solid state detector and a multichannel pulse height analysis system. The crystals used are of silicon (of the highly pure intrinsic type), or the lithium drift principle (p. 463 etseq.) is utilized. All emitted radiations are presented to the detector simultaneously and a spectrum is generated from an electronic analysis of the mixture of voltage pulses produced. Chapter 10 contains a more detailed account of pulse height analysis and solid state detectors. Production of an X-ray spectrum in this way is sometimes known as energy dispersive analysis ofX-rays (EDAX) and where an electron microscope is employed as SEM-EDAX. 

Figure 7.23 Ordering of adsorbates on a surface into islands gives rise to regions of different work function, which can be imaged because of the associated differences in photoelectron intensity. The principle forms the basis of photoemission electron microscopy (PEEM). The same principle underlies the imaging of single molecules in the field electron microscope (FEM) (see also Fig. 7.9).

Immunohistochemical labeling for electron microscopy is based on the same principles as immunohistochemistry for light microscopy. The differences are that specimen sections must be much thinner (50 100 nm) and the label must be electron-dense. The first electron-dense labels used for immunolabeling at the electron microscope level were ferritin and peroxidase. Peroxidase label can be visualized using DAB reaction product which becomes electron-dense after osmi-cation. With the advent of colloidal gold particles as markers in immunocytochemical... 

In contrast to X-rays, electrons can be foeussed by magnetic lenses to give images of the investigated objeets. This is the basic principle behind every transmission electron microscope (TEM). As shown by the sketch in figure 9 the central part of every TEM is the objeetive lens. This lens eolleets all diffracted electron beams from the erystal and sorts them in the baek foeal... 

The convenience of using X-rays for stmcture determination stems from the nature of their interactions with matter the wavelengths of radiation in the X-ray region of the electromagnetic spectmm are comparable to the sizes of atoms and interatomic distances that are to be analyzed. Although, in principle, interatomic distances can be determined by electron microscopy, unlike electron microscopic... 

Electron microscopy is widely used in the characterization of solids to study structure, morphology, and crystallite size, to examine defects and to determine the distribution of elements. An electron microscope is similar in principle to an optical microscope. The electron beam is produced by heating a tungsten filament, and focused by magnetic fields in a high vacuum (the vacuum prevents interaction of the beam with any extraneous particles in the atmosphere). The very short wavelength of the electrons allows resolution down to 0.1 nm. 

Figure 2.5 Operational principles of a transmission electron microscope. S, specimen OL, objective lens BF, back focal plane Imj, intermediate image IL, intermediate lens Im2, second intermediate image PL, projector lens FIm, final image DP, diffraction pattern.

The first electron microscope was built in Germany in 1931 by Knoll and Ruska (Ref 2). Its principles were based on previous works of L. de Broglie (1924), Busch(1926) and others. The first electron microscopes gave images inferior to those obtained by optical microscopes, but by 1934 a quite satisfactory instrument was obtained by B. von Borries, E. Ruska and M. Knoll. Commercial production of electron microscopes was begun in 1939 by Si emens and Halske, AG, Berlin. These instruments (the total number built was about 30) used electromagnetic.lenses... 

This type of electron microscope is completely different in principle and application from the conventional transmission-type electron microscope. In the scanning instrument, the surface of a solid sample is bombarded with a fine probe of electrons, generally less than 100 A in diameter. The sample emits secondary electrons that are generated by the action of the primary beam. These secondary electrons are collected and amplified by the instrument. Since the beam strikes only one point on the sample at a lime, the beam must be scanned over the sample surface in a raster pattern to generate a picture of the surface sample. The picture is displayed on a cathode ray tube from which it can be photographed. 

Microscopes. There are two basic modes of operation for X-ray analysis in a modern-day AEMs with a static (or flood) beam and with a rastered beam. This instrument is essentially a conventional TEM with either (a) scanning coils to raster and focus the beam or (b) an extra NminiN (or objective pre-field) condenser lens to provide a small (nm-sized) cross-over of a static beam at the objective plane. Some AEM configurations contain both scanning coils and a third condenser lens whilst others may have only one of these. In either condition, a small-sized electron probe can be obtained as a static or a rastered beam. The basic electron-optical principles which provide nanometer-sized beams for microanalysis are similar to those for electron microdiffraction which are well described by Spence and Carpenter [19]. 

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