Electron irradiation effects in transmission electron microscopy: Random displacements and collective migrations

Electron beam damage in transmission electron microscopy (TEM) is complicated because the damage phenomena can be the result of random atomic displacements or collective migrations. The former is categorized as the primary beam effects and the latter is the secondary beam effects. The mechanisms for these two distinguishing atomic processes of damage are different. The primary beam effects can be caused by the mechanisms of knock-on and/or radiolysis, while the secondary effects must be driven by a field that is induced by electron irradiation. One such field has been identified to be the electric field produced by the accumulated charges due to the ejection of secondary and Auger electrons from the irradiated region. One convincing example is the electron irradiation-induced domain switch in ferroelectric materials, in which the collective cation displacements are driven by the induced electric field. A detailed interpretation is given in this review. The sintering of metal NPs under electron irradiation is a secondary beam effect and is most likely also caused by the induced electric fields. The interactions between the charged NP and substrate, and between charged NPs, result in NP motion. Interchanging atoms between NPs during the sintering may also be driven by the electric fields. Although many beam-damage phenomena in C nanotubes and layered materials, such as graphene, BN, and transition metal dichalcogenides, are caused by the primary beam effects and have been well studied experimentally and theoretically in the literature, some phenomena from the secondary beam effects have also been identified in this review. These phenomena are sensitive to electron current density, the shape and orientation of the specimen, and even the illumination mode (i.e., TEM or STEM). Unfortunately, the mechanisms responsible for these phenomena still need to be clarified.