Selected Area Diffraction Characteristics

Scattering Nature Scattering by shell electrons Scattering by atom nucleus 

Wave paths Reflection (XRD) Transmission (Laue) Transmission 

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A diffraction pattern is formed on the back-focal plane of the objective lens when an electron beam passes through a crystalline specimen in a TEM. In the diffraction mode, a pattern of selected area diffraction (SAD) can be further enlarged on the screen or recorded by a camera as illustrated in Figure 3.16. Electron diffraction is not only useful to generate images of diffraction contrast, but also for crystal structure analysis, similar to X-ray diffraction methods. SAD in a TEM, however, shows its special characteristics compared with X-ray diffraction, as summarized in Table 3.4. More detailed SAD characteristics are introduced in the following section. 

The selected area diffraction (SAD) pattern shows characteristic rings with discontinuity. The phases can also be idetified as seen from fig.ll. 

Fig. 22 Characteristics of the gelatin/30% HAp nanocomposites, (a) FUR analysis. Data on gelatin/30% HAp conventional composite and pure gelatin are also presented, (bl, b2) SEM and (b3) TEM images of the gelatin/30 wt% HAp nanocomposite. Insets in (b3) are enlargements showing elongated HA nanocrystals lower, arrowed) and the selected area diffraction pattern of the crystal upper). (b4, bS) Images of conventional composite. Scale bars 500 ftm (bl), 300 nm (b2), 100 nm (b3), 20 nm (b3, inset), 500 fm (b4) and 30 pm (bS). Reprinted from [242] with permission from Elsevier...

Figure 5. Electron selected area diffraction diagram produced by two layers (mutually disoriented) of nontronite (Pfaffenreuth). Of the group (11) (02), only (02) spots are observed (arrows on the figure). This diagram is characteristic of the centrosymmetrical arrangement of octahedral atoms (group c2 mm Figure 1).

The details of the structural characteristics of individual constituents in the various carbon deposits were obtained by examination of a number of specimens from each experiment in a JEOL 100 CX transmission electron microscope that was fitted with a high resolution pole piece, capable of 0.18 nm lattice resolution. Suitable transmission specimens were prepared by applying a drop of an ultrasonic dispersion of the deposit in iso-butanol to a carbon support film. In many cases the solid carbon product was found to consist entirely of filamentous structures. Variations in the width of the filaments as a function of both catalyst composition and growth conditions were determined from the measurements of over 300 such structures in each specimen. In certain samples evidence was found for the existence of another type of ca naceous solid, a shell-like deposit in which metal particles appeared to be encapsulated by graphitic platelet structures. Selected area electron diffraction studies were performed to ascertain the overall crystalline order of the carbon filaments and the shell-like materials produced from the various catalyst systems. 

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