TEM Diffraction Contrast

TECHNIQUES OF HIGH RESOLUTION TRANSMISSION ELECTRON MICROSCOPY (HREM), TRANSMISSION EM (TEM) DIFFRACTION CONTRAST, AND ANALYTICAL EM (AEM)... 

Figure 4 (a) Schematic of the perfect crystal structure of 123. (b) Schematic of a twin, (c) Low magnification TEM (diffraction contrast) image of twins observed on (110) planes, (d) HREM structure image of 123 in [010] at a defocus of -70 nm. Atoms are white and individual atom columns are shown. The corresponding selected area... 

F re 3.9 (a) BF-TEM diffraction contrast images, (b) HREM phase contrast... 

TEM diffraction contrast imaging is by far the most important technique for studying defects in great detail. From the diffraction contrast theory [3], defects introduce an extra phase to the electron wave, a = 2jtg-R, where g is the diffraction... 

The im< e mode produces an image of the illuminated sample area, as in Figure 2. The imj e can contain contrast brought about by several mechanisms mass contrast, due to spatial separations between distinct atomic constituents thickness contrast, due to nonuniformity in sample thickness diffraction contrast, which in the case of crystalline materials results from scattering of the incident electron wave by structural defects and phase contrast (see discussion later in this article). Alternating between imj e and diffraction mode on a TEM involves nothing more than the flick of a switch. The reasons for this simplicity are buried in the intricate electron optics technology that makes the practice of TEM possible. 

There are three main formation mechanisms of TEM image contrast massthickness contrast, diffraction contrast and phase contrast. When electrons pass... 

In accord with the optical examination, both specimens show under the TEM piogressivc zones of shock damage, reflecting the changing shock conditions in the depth of the sample. The laser-shocked specimen displays the largest variation in microstructures. Just below the excavated zone of this specimen, i.e., within a depth of -50 pm below the original preshock surface, we observe a spotty diffraction contrast (Fig. 1.11a). The spots represent tiny dislocation loops that may result from decomposition of CaCOs into CaO and CO2 [41,42]. 

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. 

Pikunic et al. also developed a simple method to simulate TEM of simulated structures. They assume that the material is quasi-amorphous. Diffraction contrast due to crystalline registry is therefore negligible compared to mass-thickness... 

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