Knock-on/Elemental Displacement Threshold of
Atoms due to Electron Irradiation in Bulk
- Practical Electron Microscopy and Database -
- An Online Book -

http://www.globalsino.com/EM/  



 

This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.

 

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When an electron in an accelerating electron beam irradiates on materials, a Frenkel pair, consisting of an interstitial and a vacancy can be produced. This process is called ballistic knock-on process. In this process, the energy transferred in an elastic collision between the electron and the nucleus exceeds the displacement energy, that varies from ~ 0.1 eV for Van der Waals bonding to tens eV for ionics. That is, this process requires a minimum electron beam energy (called threshold energy) before it can occur. The value varies with the atomic number of the sample. For instance, Table 4663 lists the knock-on threshold energies of some typical bulk materials at room temperature. In a crystal, in addition to channelling effects, the threshold energy also depends on the direction of transferred momentum, being the highest in the direction normal to close-packed crystal planes and the smallest for atoms knocked-on into close-packed directions.

Table 4663. Knock-on threshold energies of typical bulk materials due to electron irradiation at room temperature.

Bulk material
Threshold energy
Bulk material
Threshold energy
Crystalline graphite (C)
54 ~ 80 keV
Crystalline silicon (Si)
145~220 keV
Crystalline copper (Cu)
400 keV
Crystalline molybdenum (Mo)
810 keV
Crystalline gold (Au)
1300 keV
 

Note that this is not currently a problem with SEMs due to low voltages (≤30 keV). However, the knock-on thresholds for thin films, e.g. in TEM specimens, can be much lower than that for bulk materials.

 

 

 

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