Electron Beam Convergence Angle and Coherence
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By changing the angle of illumination upon the TEM sample, the coherence of the electron beam can be varied. The diameter of the specimen over which the illumination is coherent is given by,
          d = λ/α --------------------------- [1708]
where,
          λ -- The wavelength of the incident electron beam (e.g. = 0.0025 nm for 200-keV electrons).
          α -- The semiangle subtended at the specimen by the electron illumination.

Table 1708 lists some examples of α (mrad) versus d (nm) which are in the range of normal BF (bright field)-TEM and -STEM conditions. Therefore, theoretically, due to the large d, parallel-beam TEM with α ≈ 0 mrad can be used to record lattice fringes with diffraction contrast (so-called HRTEM). However, the lattice fringes in BF-STEM mode are not as good as that in TEM mode. In principle, DF STEM almost doesn't present diffraction contrast due to the negligible d.

Table 1708. Some examples of α (mrad) versus d (nm).

Imaging mode α (mrad) d (nm)
BF TEM 0.001 2500.00
0.005 500.00
0.01 250.00
0.05 50.00
0.1 25.00
0.5 5.00
BF STEM 1 2.50
5 0.50
10 0.25
20 0.13
DF STEM 30 0.08
40 0.06
50 0.05
100 0.03

 

 

 

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