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In general, a low voltage STEM is a hybrid instrument with the features of SEM and TEM with a convergent probe, while a high voltage TEM is a hybrid instrument with the features of TEM with both a parallel beam (for TEM function) and a convergent probe (for STEM function).
STEM was invented by von Ardenne, German in 1937. [1] However, the application of STEM was not successful until several decades later, first performed by Albert Crewe in 1970.[2] Figure 4538a shows the positions of the detectors which can be installed in a STEM system. Depending on the scattering angle of the transmitted electrons, various signals can be detected as a function of the position of the scanning probe: BF (bright-field)-STEM, DF (dark-field)-STEM or HAADF (high angle annular dark field)-STEM. The DF detectors are annularly shaped to maximize the collection efficiency and the range of the collected scattering angles can be adjusted through the magnification of the intermediate lenses.
![The positions of detectors in STEMs](image1/4538.jpg)
Figure 4538a. The positions of detectors in STEMs.
The schematics in Figure 4538b shows the electron optical column in a modern analytical electron microscope operated in STEM mode.
![Schematics of the electron optical column in a modern analytical electron microscope operated in STEM mode](image1/4537.gif)
Figure 4538b. Schematics of the electron optical column in a modern
analytical electron microscope operated in STEM mode.
[1] Von Ardenne M, Beischer DZ. Electrochemie 1940;46:270–7.
[2] Crewe AV, Wall J, Langmore J. Science 1970;168:1338–40.
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