Electron dark-field

Dark-field methods Dark field images are obtained by admitting only diffracted electrons and excluding directly transmitted electrons. Dark-field imaging selectively detects crystallites with crystallographic plane spacings within a relatively narrow range. 

In SEM and STEM, all detectors record the electron current signal of tire selected interacting electrons (elastic scattering, secondary electrons) in real time. Such detectors can be designed as simple metal-plate detectors, such as the elastic dark-field detector in STEM, or as electron-sensitive PMT. For a rigorous discussion of SEM detectors see [3],... 

Figure Bl.17.3. STEM detectors (a) conventional bright and dark-field detectors, electrons are detected according to their different scattering angles, all other positional infonnation is lost (b) positional detector as developed by Haider and coworkers (Haider etal 1994).

In dark-field electron microscopy it is not the transmitted beam which is used to construct an image but, rather, a beam diffracted from one facet of the object under investigation. One method for doing this is to shift the aperture of the microscope so that most of the beam is blocked and only those electrons... 

Figure 4.11 Electron micrographs of polyethylene crystals, (a) Dark-field illumination shows crystals to have a hollow pyramid structure. (Reprinted with permission from P. H. Geil, Polymer Single Crystals, Interscience, New York, 1963.) (b) Transmission micrograph in which contrast is enhanced by shadow casting [Reprinted with permission from D. H. Reneker and P. H. Geil, /. Appl. Phys. 31 1916 (I960).]...

Figures 4.1 la and b, respectively, are examples of dark-field and direct transmission electron micrographs of polyethylene crystals. The ability of dark-field imaging to distinguish between features of the object which differ in orientation is apparent in Fig. 4.11a. The effect of shadowing is evident in Fig. 4.11b, where those edges of the crystal which cast the shadows display sharper contrast.

Fig. 10. A dark field (DF) transmission electron micrograph showing interface in a continuous fiber (F) a-Al202 (F)/Mg alloy (ZE41A) matrix (M) within...

There are three primary image modes that are used in conventional TEM work, bright-field microscopy, dark-field microscopy, and high-resolution electron microscopy. In practice, the three image modes differ in the way in which an objective diaphragm is used as a filter in the back focal plane. 

Figure 3 Bright-field (a) and dark-field (b) STEM images of crushed ceramic particles dispersed on a "holey" carbon film supported on an electron microscope grid (shown at the right).

S. J. Pennycook. EMSA Bulletin. 19, 67, 1989. A summary of compositional imaging using a high-angle annular dark-field detector in a field emission STEM instrument published by the Electron Microscopy Society of America, Box EMSA Woods Hole, MA 02543. 

Figure 48-6. Dark field electron micrograph of a proteoglycan aggregate in which the proteoglycan subunits and filamentous backbone are particularly well extended. (Reproduced, with permission, from Rosenberg L, Heilman W, Kleinschmidt AK Electron microscopic studies of proteoglycan aggregates from bovine articular cartilage. J Biol Chem 1975 250 1877.)...

Characterization methods. The 100 kV Vacuum Generator HB-5 STEM was used to mlcroanalyze samples. The HB-5 has a KEVEX SI(LI) energy dispersive X-ray spectrometer (EDS) and micro area electron diffraction (MAED) capabilities In conjunction with simultaneous bright and dark field Imaging capabilities. A more detailed explanation of the Instrumental operation can be obtained In a publication by C. Lyman(12). 

The structure of the metal particles dispersed on a silica powder support ( Aerosil 380, 70 A average silica particle diameter) has been studied by Avery and Sanders (47) using electron microscopy in both bright and dark field, to determine the extent to which the metal particles were multiply twinned or of ideal structure. Platinum, palladium, and gold were examined. These catalysts were prepared by impregnation using an aqueous solution of metal halide derivatives, were dried at 100°-150°C, and were hydrogen... 

Figure 4. Electron diffraction pattern (bottom left) of a typical disordered wol-lastonite specimen. Evidence of the disorder comes from the streaked (Tc odd) diffraction spots. The fringes shown in the dark-field image (a) are 7 A apart. Dark-field image b, taken from the streaked diffraction spots, shows a one-dimensional image of the disordered wollastonite. Dark-field image c shows a two-dimensional image which shows the haphazard stacking of the triclinic and monoclinic...

Figure 12. A dark-field transmission electron micrograph of a (lll)Ag platelet grown on single-crystal MoS2. The regions with different contrast differ in thickness by one monatomic step from one another. The larger the number marked on each region, the thicker the crystal (3, 71).

Figure 9.3. Characterization of mesoporous Ti02 films templated by Pluronics block copolymers using diverse characterization techniques XRD pattern (a), transmission electron microscope (TEM) image (b), dark-field TEM image (c), and isotherms of Kr adsorption (d).The Pluronic-templated Ti02 films were calcined at 400°C (solid points) and 600°C (open points). The films were prepared according to Alberius et al. (Ref. 14).

Dark field Visualization technique for ashes produced by microincineration and fluorescence microscopy useful for low-contrast subjects Electron systems imaging EM shadowing Detection, localization, and quantitation of light elements Structural information from ordered arrays of macromolecules... 

Finally, the array of modern LMs, including nuclear magnetic resonance, confocal laser, dark-field, phase-contrast fluorescence (Chapter 1), continues to be extended. The array offers the electron microsco-pist many opportunities for correlative LM and EM possibilities. 

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