Four generations of high-voltage electron microscopes

High-voltage electron microscopes (i.e. electron microscopes with maximum o perating voltages of nor less than 0.5 MV) have been in use for almost half a century. Originally the main incentives for designing and constructing h igh-voltage electron microscopes came from cell biology. The further develo pment of high-voltage electron microscopy from about 1960 onwards was stron gly motivated by problems in material science. The present overview emphasi zes those areas of material science in which high-voltage electron microsco py has become the technique of the choice or in which it offers distinct ad vantages over 'conventional' electron microscopy. These advantages are rela ted to the possibility to investigate thicker specimens because of better p enetration of the electrons, to larger energy transfers in electron-atom co llisions, and to the larger separation of the pole pieces of the objective lens, which allows the instalment of a 'mini-laboratory' for in situ experi ments 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 u ltrahigh-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 genera tions, namely laboratory-built, early or advanced commercial, and atomic-re solution instruments. The point-to-point resolution of the most recent atom ic-resolution HVEMs is now very close to their theoretical resolution of ab out 0.1 nm. In spite of the shorter electron wavelengths, up to date the re solution of the ultrahigh-voltage electron microscopes is distinctly poorer , their strength lying in their capability to allow the implementation of i n situ experiments that are difficult or even impossible to perform in HVEM s. In order to illustrate the power of high-voltage electron microscopy exampl es of in situ studies of self-organization processes during the irradiation with energetic electrons are summarized. From the viewpoint of thermodynam ics, the specimens are open systems from which during the electron irradiat ion 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 graphi te under ambient external pressure. It is pointed out that even if the defe ct density generated by the electron irradiation is too high for the self-o rganized patterns to be resolved by microscopy, electron-diffraction studie s of the so-called critical voltages may still furnish information on these patterns that is not obtainable by other techniques.