Dark-field imaging, defined

Particle sizes and shapes were evaluated from dark-field images taken using a 100 superlattice reflection from thin foils in [001] orientation. Other diffraeting conditions were tried, but this one produced the best contrast. The sizes and shapes of the y precipitates were evaluated using image-analysis software. For non-equiaxed y precipitates, the radius , r, was defined as (a + b + c -H rf)/8, where a, b, c and d are the sides of the circumscribed polygon. 

Figure 5.150. An equatorial dark field image of PBO fiber heat treated at 600°C (A) shows well defined and localized boundaries between the kink and the deformed fiber. A low dose HREM 0.55 nm (200) micrograph (B) of a kink in PBO heat treated at 665°C shows (200) lattice fringes of spacing 0.55 nm. The kink band clearly contains very localized high angle bending or buckling of the stiff polymer chains. (From Martin and Thomas [706] used with permission.)...

Different types of interactions with the sample define different contrast images, which can be controlled via the lens system. Basically, two types of contrast are of most interest in TEM amplitude contrast (or diffraction contrast) and phase contrast. In the first case, electrons are elastically scattered by the atoms of an amorphous or crystalline or diffracted material as the Bragg condition. In this type of contrast include the thickness contrast/mass Z and the diffraction. In phase contrast, occurs the elastic scattering of electrons atoms of a crystal [12, 21]. In contrast range, bright-field or dark-field images are the most common types in TEM analysis, especially bright field. 

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. 

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