Electron diffraction lattice imaging

We shall now show some examples of studies on shear compounds performed using electron microscope (lattice image and structure image) and electron diffraction techniques. Figure 2.13 shows the lattice image of a slightly reduced WO3 ( the bundle of black lines (lattice image)... 

The question of composition can be addressed in another manner by electron microscopy. Lattice imaging techniques, which involve the reconstruction of the direct image from the diffraction pattern of a particle, can allow for the measurement of lattice spadngs characteristic of the constituent phases, and thus provide constituent analysis based on the structures. [3, 194]... 

If the scattered beam is a sharp spot diffracted from a single crystal, the phase contrast image when it is recombined is an image of the crystal lattice. This specialized phase contrast technique is applied to the study of atomic scale structure in crystalline specimens of metals and ceramics. It has only rarely been applied to the study of polymer materials due primarily to their instability in the electron beam. Lattice images have been obtained from radiation stable aromatic molecules, such as the liquid crystalline polymers (Section 5.6). They have shown important information regarding the ordered structure. 

A progressive etching technique (39,40), combined with x-ray diffraction analysis, revealed the presence of a number of a polytypes within a single crystal of sihcon carbide. Work using lattice imaging techniques via transmission electron microscopy has shown that a-siUcon carbide formed by transformation from the P-phase (cubic) can consist of a number of the a polytypes in a syntactic array (41). 

It is noteworthy that the HRTEM cannot distinguish core and shell even by combining X-ray or electron diffraction techniques for some small nanoparticles. If the shell epitaxially grows on the core in the case of two kinds of metals with same crystal type and little difference of lattice constant, the precise structure of the bimetallic nanoparticles cannot be well characterized by the present technique. Hodak et al. [153] investigated Au-core/Ag-shell or Ag-core/Au-shell bimetallic nanoparticles. They confirmed that Au shell forms on Ag core by the epitaxial growth. In the TEM observations, the core/shell structures of Ag/Au nanoparticles are not clear even in the HRTEM images in this case (Figure 7). 

Electron diffraction analysis for these carbon tubes revealed that the tube wall consists of cylindrically stacked carbon layers. The lattice image for the carbon tubes with a diameter of 30 nm is shown in Figure 10.1.7, where at least four tubes cross each other. The thickness of the walls is about 10 nm, and consequently the carbon has a hollow with a diameter as small as 10 nm. Many small lines, which correspond to 002 lattice planes, are observed in the cross section of the walls for each tube. 

If there is alignment, contrast in TEM images is strong, because of the periodic strain field in the crystal. Selected-area diffraction shows evidence of such alignment by the location of satellite intensities around the Bragg peaks arising from the modulation of atomic scattering factors, lattice constant, or both [19]. In Fig. 18.10, the electron diffraction effects, expected from an f.c.c. crystal with (100) composition waves, are depicted with a [001] beam direction. 

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