Figure 4931a 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.
Figure 4931a. The positions of detectors in STEMs.
The schematics in Figure 4931b shows the electron optical column in a modern analytical electron microscope operated in STEM mode, indicating the projector lens controlling detector collection angle.
Figure 4931b. Schematics of the electron optical column in a modern
analytical electron microscope operated in STEM mode.
In EELS acquisition, in order to allow the electron beam to enter the GIF, only the annular dark field detector can be used. Therefore, we need to retract it if a BF (bright-field) detector is in.