Dark field contrast

For a specimen of B-N which has been annealed for only 5 s, strong dark field contrast is observed in E[W] images (Fig. 9(a)). Even after such a short anneal, the structure is very different from the banded structure of as-sheared specimens domains of constant orientation are no longer elongated perpendicular to the shear direction on the contrary they tend to lie parallel to it. Only a low level of contrast is observed in the flat BF image (Fig. 9(b)) but upon tilting through 42 ° about an axis perpendicular to the... 

Another specialized application of EM image contrast is mass measurement. Using the elastic dark-field image in the STEM or the inelastic image in the EETEM, a direct measurement of the scattering mass can be performed. Eor reviews on this teclmique see [60.61]. 

At this point it is worth comparing the different techniques of contrast enliancements discussed so far. They represent spatial filtering teclmiques which mostly affect the zeroth order dark field microscopy, which eliminates the zeroth order, the Schlieren method (not discussed here), which suppresses the zerotii order and one side band and, finally, phase contrast microscopy, where the phase of the zeroth order is shifted by nil and its intensity is attenuated. 

Tdrdk P, Sheppard C J R and LaczikZ 1996 Dark field and differential phase contrast imaging modes in confocal microscopy using a half aperture stop Optik 103 101-6... 

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.

Figure 5 Images of a thin region of an epitaxial film of Ge on Si grown by oxidation of Ge-implanted Si (a) conventional TEM phase contrast image with no compositional information and b) high-angle dark-field STEM image showing atomically sharp interface between Si and Ge. (Courtesy of S.J. Pennycook)...

The conventional hand of a particular isochiral cluster of tubes can be deduced from dark field diffraction contrast tilting experiments [26]. 

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).

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. 

Figure 7.5 shows images of a Pd/AI2CF catalyst in bright and dark field mode. In the latter, the geometry was chosen such to selectively detect the (111) and (200) diffraction rings of Pd. Features indicated by arrows clearly demonstrate the better contrast and resolution of the dark field image [14],... 

Figure 7.5 Transmission electron micrographs of a Pd/AFC), catalyst in bright (above) and dark field (below). The latter shows enhanced contrast for the Pd particles as well as better resolution (from Freeman et al. [14]).

Fig. 3 shows a topographic image of a Pt/y-A s catalyst. Contrast from particles is clearly separated from the substrate topography. On the other hand pores on the substrate are well defined. If the aperture includes some portion of the dark field spot then the resolution for small particles is improved. Fig. 4 shows an image of a 100% dispersed catalyst (as measur ed by chemisorption methods) in which particles of about 5 A can be seen. 

TIRF is easy to set up on a conventional upright or inverted microscope with a laser light source or, in a special configuration, with a conventional arc source. TIRF is completely compatible with standard epi-fluorescence, bright-field, dark-field, or phase contrast illumination so that these methods of illumination can be switched back and forth readily. Some practical optical arrangements for observing TIRF through a microscope are described in Section 7.4. 

TIRF is an experimentally simple technique for selective excitation of fluorophores on or near a surface. It can be set up on a standard upright or inverted microscope, preferably but not necessarily with a laser source, or in a nonmicroscopic custom setup or commercial spectrofluorimeter. In a microscope, the TIRF setup is compatible and rapidly interchangeable with bright-field, dark-field, phase contrast, and epi-illumination and accommodates a wide variety of common microscope objectives without alteration. 

Figure 4. Bright - and annular dark field image of a CdSe tetrapod and an adjacent gold particle recorded in a TEM. The line profiles demonstrate Z-contrast in the dark field image.

Video microscopy has permitted direct observation of microtubule assembly/disassembly dynamics in vitro. Horio and HotanP first used dark-field optics to observe the growth and shrinkage phases, but so-called Allen video-enhanced contrast microscopy has become most convenient. 

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