Interpretation of HRTEM images

The simulation of HRTEM images has been reviewed recently by Self and O Keefe (1988), and an excellent summary has been given by Veb-len (1985a). 

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The recognition of the significance of the YZ projections (and thus also the five MDO groups given in Table 7), which can be derived also directly from the full polytype symbols (Durovic et al. 1984), is very important also for the interpretation of HRTEM images (Kogure, this volume). 

In this chapter, a brief introductory discussion is given of (i) the experimental variables that influence the nature of HRTEM images, (ii) the experimental techniques involved, and (iii) the basis of image interpretation. More detailed accounts have been given by Spence (1981) and Veb-len (1985a). 

Equation 10 can be interpreted as the aberrations of the objective lens multiplying the intensities of the diffracted beams by a phase factor sin[2(g)], which depends on the spatial frequency. Thus, in the WPOA, the observed image is proportional to the projected potential, but is modulated by the phase factor. Without the phase shift, j, due to the lens aberrations, a weak phase object would not be visible in HRTEM (this is analogous to the interpretation of equation 6). 

The point resolution of a TEM, which only depends on the spherical aberration, C, and the electron wavelength. A, (which is determined by the accelerating voltage) sets the limit for a straightforward interpretation of a HRTEM image of a thin object. However, this is different from the information limit, which defines the highest frequencies that can be transferred in a microscope. 

Figure 4. Influence of three-fold astigmatism, A3, on HRTEM images of diamond in [110] orientation (left A3 = 2250 nm right A3 < 50 nm). When the 3-fold astigmatism is corrected, the HRTEM image shows the dumbbell-structure of diamond, whereas strong 3-fold astigmatism results in an image that cannot be directly interpreted in terms of the atomic...

As described earlier, many EM techniques including in situ ETEM, STEM, HRTEM, image analysis and EEELS are being pushed to their limits by researchers to study surfaces and ultrafine particles (or clusters) which are thought to be active species. Eor catalysis, metal particles are, in general, supported on ceramic substrates. Detection of supported particles less than 10 A (1 nm) in size is possible by HRTEM however, caution must be exercised in interpreting the size and surface structure of the metal particles since the surface structure is obscured by the support contrast, as demonstrated by calculations. 

The previous section showed that interpretable HRTEM images are not obtained unless quite stringent experimental conditions are fulfilled. The important questions - What instrumentation does one need, and what does one actually do, to obtain an interpretable HRTEM image - have recently been considered by Veblen (1985a) who has described in some detail the experimental techniques that he has found essential for successful high-resolution microscopy. Because descriptions of tricks-of-the-trade are relatively rare in the literature, his main points are summarized in the following subsections. 

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