Four generations of high-voltage electron microscopes

High-voltage electron microscopes (i.e. electron microscopes with maximum operating voltages of not less than 0.5 MV) have been in use for almost half a century. Originally the main incentives for designing and constructing high-voltage electron microscopes came from cell biology. The further development of high-voltage electron microscopy from about 1960 onwards was strongly motivated by problems in material science. The present overview emphasizes those areas of material science in which high-voltage electron microscopy has become the technique of the choice or in which it offers distinct advantages over ‘conventional’ electron microscopy. These advantages are related to the possibility to investigate thicker specimens because of better penetration of the electrons, to larger energy transfers in electron-atom collisions, and to the larger separation of the pole pieces of the objective lens, which allows the instalment of a ‘mini-laboratory’ for in situ experiments inside the specimen chambers of high-voltage electron microscopes.A distinction is made between ‘ordinary’ high-voltage electron microscopes (HVEMs), with maximum operating voltages reaching up to about 1.5 MV, and ultrahigh-voltage electron microscopes, which cover the voltage range 2.0–3.5 MV. The evolution of the first group is described in terms of four generations, namely laboratory-built, early or advanced commercial, and atomic-resolution instruments. The point-to-point resolution of the most recent atomic-resolution HVEMs is now very close to their theoretical resolution of about 0.1 nm. In spite of the shorter electron wavelengths, up to date the resolution of the ultrahigh-voltage electron microscopes is distinctly poorer, their strength lying in their capability to allow the implementation of in situ experiments that are difficult or even impossible to perform in HVEMs.In order to illustrate the power of high-voltage electron microscopy, examples of in situ studies of self-organization processes during the irradiation with energetic electrons are summarized. From the viewpoint of thermodynamics, the specimens are open systems from which during the electron irradiation more entropy is exported to the environment than is internally produced. This permits the emergence of ordered defect patterns such as lattices of stacking-fault tetrahedra or the formation of diamond crystals from graphite under ambient external pressure. It is pointed out that even if the defect density generated by the electron irradiation is too high for the self-organized patterns to be resolved by microscopy, electron-diffraction studies of the so-called critical voltages may still furnish information on these patterns that is not obtainable by other techniques.