Axial bright-field imaging

The transmission electron microscope is now well established as a useful tool for the characterization of supported heterogeneous catalysts(l). Axial bright-field imaging in the conventional transmission electron microscope (CTEM) is routinely used to provide the catalyst chemist with details concerning particle size distributions, 3), particle disposition over the support material(2-6) as well as particle morphology(7). Internal crystal structure(8-10), and elemental compositions(ll) may be inferred by direct structure imaging. 

Figure 1 Diagrams showing the essential electron-optical configurations used for various imaging modes in CTEM and STEM as seen by two points A and B on the sample, (a) CTEM axial bright field, (b) CTEM tilted dark field, (c) CTEM hollow cone dark field, and (d) STEM with bright field and annular dark field detectors.

Microdiffraction.—Perhaps more important than SAD techniques, particularly in the context of catalyst research, microdiffraction allows the user to benefit from the small probe size generated in STEM in the structural analysis of small particles and localized areas in thin foils. If the small probe is stopped on a particle, then clearly a transmission diffraction pattern will be observable after the beam has traversed the sample, provided we have the means available for its display. In CTEM such a pattern will, of course, be formed by the imaging system in a manner identical to SAD, but in STEM the pattern must be scanned across the detector. This is accomplished by means of a set of post-specimen scan coils which once more scan the diffracted beams across the axial bright-field detector. Such a pattern is shown in Figure 13 where a beam of approximately 10 A FWHM was stopped on a small second-phase particle during the omega-phase transformation in a Zr-Nb alloy. The relatively poor definition of the reflection is a consequence of both the convergent nature in the probe (necessary in order to obtain the smallest probe sizes) and a S/N limited by the available current in the probe. Nevertheless, weak reflections with half-order indices are clearly visible between the main alloy reflections and it is therefore possible to attempt structural... 

Fig. 5.107 Axial bright field lattice images are shown for a heat treated PBZT fiber prepared by detachment replication. Note that the lattice fringes are most easily visible when the figure is viewed obliquely down the parallel lines drawn on the figure. The polymer is translationally disordered and does not form a true three dimensional crystal. This is why the fringes marked m are wavy. (From Shimamura et al [493] reproduced with permission.)...

Fig. 2a-c. High resolution axial bright field transmission electron micrograph taken near Sherzer focus, for which the point-to-point resolution is = 2.5 A. The sam e consists of dispersed CdS crystallites on a thin carbon support film a an area containing several crystallites and demonstrating the size variational b, c magnified images of individual crystallites [7]... 

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