CTEM

Langmore, and M. S. Isaacson in Physical Aspects of Electron Microscopy and Microbeam Analysis , ed. B. M. Siegel and D. R. Beaman, Wiley, New York, 1975, p. 47. 

STEM and CTEM images will be equivalent, presuming, of course, equal brightnesses, lens quality, and acceleration voltage for the respective imaging systems. Since CTEM instruments typically operate with p small, this requires for an equivalent STEM image that j /a 1, i.e., that the collector aperture is as small as possible. Under such circumstances, the equivalence can be experimentally demonstrated. Such equivalence is an essential prerequisite for the practical materials scientist whose interpretive background will almost certainly derive from CTEM work. 

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Giaz. Ctem Hal Gazz-Chtm Hal J Org Chem Angew Chem Int Ed... 

Supported transition metals. As mentioned previously, detection of transition metal oxides <4 nm on alumina by CTEM Is virtually Impossible because of the fundamental difficulty of providing sufficient contrast between support and crystallite (14, 15). 

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.

Over the past decade increasing use has been made of the scanning transmission electron microscope (STEM) for examining catalysts(20). Owing to the increased efficiency with which scattered electrons and associated signals such as X-rays, secondary and Auger electrons may be collected, the STEM offers greater analytical flexibility compared with the CTEM (21). 

It should be noted that the various CTEM imaging modes are acquired simultaneously in the STEM. Furthermore, because samples are examined on a point-by-point basis, microanalytical information is potentially available with higher spatial resolution than is normally permissible in the CTEM. Thus the great attraction of the STEM over the CTEM is that it increases the information available, particularly from heterogeneous specimens. 

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