Imaging in the TEM

The capabilities of the three instruments are quite different although the principles involved 

One remaining advantage of the STEM for radiation sensitive polymers is that only the scanned, imaged, area of the specimen is irradiated. In TEM it is difficult to limit the irradiation of adjacent areas. This is especially important for diffraction experiments. Microdiffraction can be readily conducted in a STEM with a probe that leaves adjacent regions undamaged. In polymers the microdiffraction area must be comparatively large, attainable by both instruments, but the STEM allows better control. Another possible advantage is that 

STEM imaging has higher resolution in very thick films of polymers (several micrometers thick). The advantage over the TEM is greatest for disordered, low atomic number materials [59] and least for single crystals of heavier atoms [60]. Advantages of STEM for polymer imaging are  

The C/STEM or AEM instrument has been used for several polymer studies [61-63]. 

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Overall platelet dimensions of mineral aurichalcite did not appear to change during calcination, but became polycrystalline and porous. By dark field Imaging in the TEM, the ZnO particles were observed to be uniformly and highly dispersed. The porosity can be accounted for by the approximately threefold increase in density of Zn atoms upon decomposition of aurichalcite to ZnO. For this density change to occur with a constant overall platelet volume, pores must form. 

Generally, images in the TEM are two-dimensional, because the depth-of-focus of the imaging lenses is greater than the thickness of most biological specimens. Tilting the specimen and viewing stereo pairs, especially of specimens prepared as replicas or that are shadowed, can provide three-dimen-... 

What firstly succeeded is electron spectroscopic imaging in the TEM, by means of focusing energy spectrometers. The basic idea here is to design an energy spectrometer that also has the properties of an axially symmetric (round) lens and so can take its place in the TEM column as an extra projector lens between the final and intermediate projectors. Such a device has several advantages and was available in certain Zeiss TEMs for sometime. 

After the radiation has passed through the specimen, the scattered radiation is collected by an objective lens. This lens is the most critical, and imperfections in it will affect the image quality directly it is adjusted to focus the image. In the TEM, an objective aperture at the back focal plane of the objective lens controls the angular divergence of radiation that contributes to the image. 

The preparation of very thin samples is normally conducted using a microtome. The samples to be microtomed are normally mounted in an epoxy resin to facilitate the slicing of the sample. For high-modulus polymers, this slicing can be done at room temperature. Many low-modulus and elastomeric samples require cryogenic cooling in order to obtain samples suitable for imaging in the TEM. The very thin and... 

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