Damage mechanisms in electron microscopy of insulating materials

With a probe of atomic dimensions, the aberration-corrected scanning transmission electron microscope (STEM) has become one of the most advanced analytical tools for materials research. However, electron-beam damage limits its applications to materials that are insensitive to damage, and these are a small fraction of the materials that are important to the future of material science. One important group of materials which are susceptible to the electron beam is electrically insulating inorganic materials, including both crystals and glasses. In this paper, damage phenomena for these materials under the illumination of a finely focused electron beam in STEM are summarized. The leading mechanism for damage is massive atom displacement driven by an electric field, which is induced by the excitations and ionizations of atomic electrons. The electric field induced by the STEM probe, and thus the damage, is spatially delocalized and dependent on beam current density (dose rate) and exposure time but approximately independent of specimen thickness. This mechanism causes either phase separation, including precipitation and formation of gas bubbles, or phase transformation. From a microscopic analysis point of view, higher beam currents and longer exposures should therefore be avoided if the induced electric field is the main cause for damage. Methods of minimizing damage are reviewed.