A microscope using a beam of electrons instead of light, and magnetic or electrostatic fields as lenses. If considered as a wave system, the electron beam has a much higher frequency than visible light, and so provides a much higher resolution. In the transmission electron microscope, the direct passage of the beam through the specimen produces an image on a fluorescent screen. The specimen must be very thin, but the resolution is high: c.0·20·5 nm. In the scanning electron microscope, the specimen is scanned by the beam, which produces secondary electron emission. The consequent current produced can be amplified and the signal fed to a cathode ray screen to give the image. The specimen can be thicker, and an image of some depth produced, but resolution is limited to c.1020 nm. The scanning tunnelling microscope, invented by Gerd Binnig and Heinrich Rohrer in 1982, has a resolution of a few Ångströms and can image down to atomic scales. In this technique, electrons migrate between the sample surface and the microscope tip via the process of quantum tunnelling. Tunnelling current is very sensitive to tipsurface distance; an image is constructed using the control signal passed to the tip positioning system which maintains a constant tipsample distance. Binnig and Rohrer were awarded the 1986 Nobel Prize for Physics.
The electron microscope is a type of microscope that uses electrons to create an image of the target.
History
The first electron microscope prototype was built in 1931 by the German engineers Ernst Ruska and Max Knoll. The first practical electron microscope was built at the University of Toronto in 1938, by Eli Franklin Burton and students Cecil Hall, James Hillier and Albert Prebus.
Although modern electron microscopes can magnify objects up to two million times, they are still based upon Ruska's prototype and his correlation between wavelength and resolution. The electron
microscope is an integral part of many laboratories.
Types
Transmission Electron Microscope (TEM)
The original form of electron microscopy, Transmission electron microscopy (TEM) involves a high voltage electron beam emitted by a cathode and formed by magnetic lenses. The electron beam that has been partially transmitted through the very thin (and so semitransparent for electrons) specimen carries information about the inner structure of the specimen. The ability to determine the positions of atoms within materials has made the HRTEM an indispensable tool for nano-technologies research and development in many fields, including heterogeneous catalysis and the development of semiconductor devices for electronics and photonics.
Transmission electron microscopes produce two-dimensional images.
Scanning Electron Microscope (SEM)
Unlike the TEM, where electrons are detected by beam transmission, the Scanning Electron Microscope (SEM) produces images by detecting secondary electrons which are emitted from the surface due to excitation by the primary electron beam. In the SEM, the electron beam is rastered across the sample, with detectors building up an image by mapping the detected signals with beam position.
Generally, the TEM resolution is about an order of magnitude better than the SEM resolution, however, because the SEM image relies on surface processes rather than transmission it is able to image bulk samples and has a much greater depth of view, and so can produce images that are a good representation of the 3D structure of the sample.
Scanning Transmission Electron Microscope (STEM)
In an STEM, an electron transparent sample is studied in much the same way as in TEM. However, instead of contiunously illuminating the sample, a small electron probe is scanned over the area to be studied. The transmitted electrons are then collected by an annular detector mounted a long way from the specimen. Another important advantage of STEM is that any analytical signal, such as X-ray fluorescence spectroscopy and electron energy loss spectroscopy (EELS), can also be obtained at high resolution (0.1 nm in the very best, aberration-corrected STEMs).
Reflection Electron Microscope (REM)
In addition there is a Reflection Electron Microscope (REM). Like TEM, this technique involves electron beams incident on a surface, but instead of using the transmission (TEM) or secondary electrons (SEM), the reflected beam is detected. This technique is typically coupled with Reflection High Energy Electron Diffraction and Reflection high-energy loss spectrum (REELS). Another variation is Spin-Polarized Low-Energy Electron Microscopy (SPLEEM), which is used for looking at the microstructure of magnetic domains .
Scanning Tunneling Microscope (STM) A Scanning Tunneling Microscope (STM) is considered as a type of electron microscope, but it is recommended to be studied under the heading of scanning probe microscope.
Sample Preparation
Samples viewed under an electron microscope may be treated in many ways:
Cryofixation - freezing a specimen so rapidly, to liquid nitrogen or even liquid helium temperatures, that the water forms vitreous (non-crystalline) ice. Sectioning - produces thin slices of specimen, semitransparent to electrons. Staining - uses heavy metals such as lead, uranium or tungsten to block electrons to give contrast between different structures, since many (especially biological) materials are nearly "transparent" to electrons (weak phase objects). Ion Beam Milling - thins samples until they are transparent to electrons by firing ions (typically argon) at the surface from an angle and sputtering material from the surface. A subclass of this is Focused ion beam milling, where gallium ions are used to produce an electron transparent membrane in a specific region of the sample, for example through a device within a microprocessor. Conductive Coating Evaporation, Thin-film deposition, or sputtering of carbon, gold, gold/palladium, platinum or other conductive material to avoid charging of non conductive specimens in a scanning electron microscope.Disadvantages
Electron microscopes are expensive to buy and maintain.
The samples have to be viewed in vacuum, as the molecules that make up air would scatter the electrons. Recent advances have allowed hydrated samples to be imaged using an environmental scanning electron microscope.
Scanning electron microscopes usually image conductive or semi-conductive materials best. Non-conductive materials can be imaged by an environmental scanning electron microscope.
The samples have to be prepared in many ways to give proper detail, which may result in artifacts purely the result of treatment. Scientists maintain that the results from various preparation techniques have been compared, and as there is no reason that they should all produce similar artifacts, it is therefore reasonable to believe that electron microscopy features correlate with living cells.
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