The typical electron dose (De) inducing atomic displacement in conducting bulk is from 103 and 104 C/cm2 (6.25 x 105~6.25 x 106 e-/Å2) . Here, 1 electron = 1.6 x10-19 C. Some materials are able to withstand the >105 e-/Å2 dose that a typical aberration-corrected high-voltage STEM probe provides, but beam damage is still one of the main issues for most materials in both biology and material science. Especially, point defects are mobile, and thus the defect and interface structures are less stable than the bulk material.
Table 4412 lists typical electron doses in different conditions and for different applications with sufficient contrast. The damage of TEM specimens of non-thermally-conductive materials (e.g. PMMA) depends not only on the electron dose but also on the beam size. However, for thermally conductive materials, the damage depends only on the dose. The total electron dose for STEM is remarkably less than in TEM imaging and thus, STEM imaging reduces possible radiation damage to beam-sensitive specimens.
Table 4412. Typical electron doses in different conditions or for different applications.
|Electron dose (e-/Å2)
||Material: poly(methyl methacrylate) (PMMA) polymer
changes in the EEL spectra at liquid nitrogen temperature when a large probe
size was used
|> 1.6 x 104
changes in the EEL spectra at liquid nitrogen temperature when a small probe
size (in nm range) was used
||Damage specimens in some biological cases
changes in the EEL spectra of poly(ethylene terephthalate) (PET) at liquid nitrogen temperature when a large probe
size was used
||Low-dose electron microscopy for tissues at room temperature
||With specimens kept at
liquid nitrogen temperature for biological samples
||For biomaterials in vitreous ice to prevent hydrogen gas evolution
changes in the EEL spectra of polystyrene (PS) polymer at liquid nitrogen temperature when a large probe
size was used
||Sugar-embedded biological specimens that are at liquid
||Low-dose cryo-electron microscopy for tissues
||For cryo-biological TEM analysis
|Causes damage of catalysts and zeolites
|102 ~ 5 x 103
||Typical dose in HRTEM imaging
||For a single atom imaging in TEM
|3.9 x 103
||Induces the growth of the nanocrystals at 300 kV
|Does not induce the growth of the nanocrystals at 80 kV
||Some materials are able to withstand
|6.25 x 105 ~ 6.25 x 106
||Causes atomic displacement in conducting bulk
| ~1.0 x 108
||Typical high-dose STEM imaging conditions
In many cases, the electron-beam-induced damage is proportional to the electron dose, which is the current density in the probe multiplied by the exposure time).As shown in Table 4412, in such cases the electron doses are so high that severe beam damage to the TEM specimen is inevitable, especially because taking a image consists of three steps, namely searching, focusing, and recording, and the first two normally need a larger dose than the third. Therefore, the exposure time and thus the doses should be lowered to avoid beam damage through different ways:
i) Take images at low magnifications if higher magnifications are not necessary.
ii) Do searching and focusing processes in an area, and then move the sample to the area of interest for recording.
iii) Using averaging technique by averaging over a large number of identical structures.
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