Selected-area electron diffraction polycrystalline

Fig. 6.7 a TEM revealing the nanotubular character and polycrystallinity of the double-gyroid-structured NiO. b Magnified view of (a) resolving nanocrystals with dimensions of 5-lOnm. c Corresponding selected area electron diffraction pattern of polycrystalline NiO. 

Selected-area electron diffraction (SAD) is a basic TEM technique to obtain diffraction information from a part of the specimen. A selected-area aperture is inserted below the sample holder and in the image plane of the objective lens. Only the area selected by the aperture on the screen contributes to the SAD pattern. In case of polycrystalline specimens, if more than one crystal contributes to the SAD pattern, it can be difficult or impossible to analyze. As such, it is useful to select a single crystalline region for analysis at a time. It may also be useful to select two crystals at a time, in order to examine the crystallographic orientation between them. 

Figure 4.20 Dark-field electron micrograph and selected area electron diffraction pattern recorded on a tribological film particle obtained in the presence of Sr octanoate micelles. The film is made of polycrystalline strontianite and the EDX spectrum does not evidence the presence of iron, underlining the high antiwear properties. The EEL spectrum shows that hydrocarbon chains are no longer present in the film...

For all compositions, the polyacetylene domains are crystalline as is revealed by X-ray diffraction. This is further confirmed by electron diffraction from thin microtomed sections of blends with polyacetylene compositions of 40% or higher. Micro-toming of samples with less than 40% polyacetylene without sample cooling is difficult due to the rubbery nature of the composite. The polycrystalline nature of the polyacetylene domains is established from the observation that even selected area aperture of a few hundred angstroms produces Debye ring patterns. 

For polycrystalline materials, electron methods can be used to supplement and clarify X-ray results. For example, with X-ray powder patterns, it is often difficult or even impossible to index lines in a diffraction pattern if mixtures are present, whereas, using electron diffraction, the problem can be solved by obtaining unit cell data for each phase from selected-area diffraction patterns. Likewise, with complex powders of new materials, the indexing of X-ray patterns is especially difficult if the unit cell is large and of low symmetry. Electron diffraction patterns from small crystals reveal the reciprocal lattice, giving information on the principal axes and crystallographic symmetry. Many new materials are prepared as small samples of tiny crystals suitable only for electrons microscopy, so that electron diffraction is often the best way to obtain crystallographic data on the material. 

Figure 3.34 Images of polycrystalline specimen of thallium chloride (a) SAD ring pattern and (b) bright-held image in which the dark circle (50 //m diameter) indicates the area selected for diffraction. (Reproduced with permission from M. von Heimandahl, Electron Microscopy of Materials, Academic Press, New York. 1980 Elsevier B. V.)...

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