The Scanning Transmission Electron Microscope STEM

Most modern AEMs are based on a STEM, which scans a fine probe of electrons across the specimen. The so-called dedicated STEM (DSTEM) only operates in scanning mode, whereas modified TEM (TEM/STEM) instruments are more numerous. Both types of instrument employ a FEG source. 

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The scanning transmission electron microscope (STEM) was used to directly observe nm size crystallites of supported platinum, palladium and first row transition metals. The objective of these studies was to determine the uniformity of size and mass of these crystallites and when feasible structural features. STEM analysis and temperature programmed desorption (TPD) of hydrogen Indicate that the 2 nm platinum crystallites supported on alumina are uniform In size and mass while platinum crystallites 3 to 4 nm in size vary by a factor of three-fold In mass. Analysis by STEM of platinum-palladium dn alumina established the segregation of platinum and palladium for the majority of crystallites analyzed even after exposure to elevated temperatures. Direct observation of nickel, cobalt, or iron crystallites on alumina was very difficult, however, the use of direct elemental analysis of 4-6 nm areas and real time Imaging capabilities of up to 20 Mx enabled direct analyses of these transition metals to be made. Additional analyses by TPD of hydrogen and photoacoustic spectroscopy (PAS) were made to support the STEM observations. 

The scanning transmission electron microscope (STEM) combines the two modes of operation. Here, the scanning coils are used to illuminate a small area of... 

Analytical electron microscopy of individual catalyst particles provides much more information than just particle size and shape. The scanning transmission electron microscope (STEM) with analytical facilities allows chemical analysis and electron diffraction patterns to be obtained from areas on the order of lOnm in diameter. In this paper, examples of high spatial resolution chemical analysis by x-ray emission spectroscopy are drawn from supported Pd, bismuth and ferric molybdates, and ZSM-5 zeolite. 

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). 

This paper describes chemical analyses at points across individual zeolite crystals in the size range 0.1-2.0pm. The technique employed was x-ray emission spectroscopy in the scanning transmission electron microscope (STEM). Two ZSM-5 preparations were made with Si Al ratios about 10 and 40. Many particles were examined carefully to detect chemical segregation. To check the analysis procedure, particles of NaA zeolite were examined as a control. 

The principle of the scanning transmission electron microscope (STEM) is, at first glance, very different from that of the transmission electron microscope the electrons are focused on a probe scanned on a sample and the transmitted electrons are detected on a scintillator via a collection aperture. There is, however, a so-called reciprocity relationship between transmission electron microscopy and the STEM that can be used to describe image formation using the same formalism and facilitates the understanding of contrast. 

Among the different types of electron microscopes such as the SEM, the TEM, the scanning transmission electron microscope (STEM), and the field emission... 

Also the scanning transmission electron microscope (STEM) can be regarded as a peripheral instrument (64) that until now has had little impact on polymer research. This form of microscopy may become more imgortant with greater availability. Resolution of better than 15 A is already attainable in the STEM mode. 

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