Classification and Maintenance of Electron Microscopes
High-Voltage Electron Microscope (HVEM) is a type of Transmission Electron Microscope (TEM). Conventional TEM usually operates at an acceleration voltage of 100 kV and can only penetrate samples with a thickness of several thousand angstroms. Electron penetration increases with β² = v²/c² (the square of the ratio of electron velocity to the speed of light). Due to relativistic effects, the growth of β² slows down significantly above 500 kV. Commercially available models include 200 kV, 300 kV and 1000 kV versions. Specially built 3000 kV units exist in France and Japan. Apart from the accelerating column, an HVEM is similar to a standard TEM but built on a larger scale. A 1000 kV HVEM can be as tall as a two‑story building. The enlarged dimensions create extra space around the specimen holder for installing various sample manipulation accessories, such as stretching, heating, cooling and reaction cells, which can be combined with a tiltable specimen stage. Dynamic in‑situ observations can be performed, and changes during sample treatment can be recorded by video. High‑energy electrons induce radiation damage inside specimens, which provides great convenience for investigating the microscopic mechanisms of radiation damage in materials.
High-Resolution Electron Microscope (HREM) Raising the acceleration voltage shortens the electron wavelength and improves resolving power. Restricted by technical difficulties, ultrahigh-voltage electron microscopes before the 1970s mainly aimed to increase penetration. Technical breakthroughs from the late 1970s to the early 1980s brought commercial 200 kV and 300 kV high‑resolution microscopes, plus a small number of 500 kV, 600 kV and 1000 kV HREM instruments. Their resolution reaches approximately 2 Å, and 1.5 Å is expected to be achieved soon. Biological molecules are extremely susceptible to radiation damage, so HREM is currently mainly used to observe atomic arrangements in inorganic materials.
Scanning Electron Microscope (SEM) It is primarily used for direct observation of surface morphology of solid samples, following the principle illustrated in the schematic diagram of the scanning electron microscope (Figure 2). An electron lens first focuses the electron beam into a spot of tens of angstroms. A deflection system scans the electron beam across the specimen surface in a raster pattern. Where the electron beam strikes the sample, secondary electrons are emitted. A detector collects these secondary electrons to form a signal that modulates the brightness of a synchronously scanned display tube for image formation. Surface irregularities cause some local areas to face the secondary-electron detector while others face away. Regions oriented toward the detector collect more secondary electrons and appear brighter, while the others appear darker, forming images with contrast and strong stereoscopic perception. The magnification equals the ratio of the scanning range on the display tube to the scanning range of the electron beam on the specimen surface. The resolution of an SEM is slightly larger than the diameter of the electron probe spot. Modern SEMs can achieve a resolution of up to 60 Å.
Scanning Transmission Electron Microscope (STEM) Its imaging mode resembles that of an SEM, but it detects transmitted electrons (including some low-angle scattered electrons) instead of secondary electrons. The specimen must also be an ultrathin film. The resolution of a STEM is comparable to the diameter of its electron probe. Dedicated STEM instruments adopt high-brightness field-emission electron guns, which require an ultrahigh vacuum of 10⁻¹⁰ torr, and can reach a resolution of 3 Å. Such STEMs have been used to image single heavy atoms on light-element support films. Particularly valuable for practical work, its fine electron probe enables crystallographic analysis (via electron diffraction) and compositional analysis (via characteristic X-rays excited by the electron beam or electron energy loss spectroscopy) within tiny microregions of several tens of angstroms. Commercial TEM systems can be equipped with STEM attachments. Without a high-brightness field-emission gun, however, the probe can only be focused down to tens of angstroms, supporting structural and compositional analysis over a range of roughly 100 Å. An electron microscope capable of carrying out such localized analytical measurements while acquiring microscopic images is known as an analytical electron microscope.
