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There are high-resolution TEM (HRTEM) which have been built. In fact the resolution is sufficient to show carbon atoms in diamond separated by only 89 picometers and atoms in silicon at 78 picometers. This is at magnifications of 50 million times. The ability to determine the positions of atoms within materials has made the HRTEM a very useful tool for nano-technologies research. It is also very important for the development of semiconductor devices for electronics and photonics.
Transmission electron microscopes produce two-dimensional images.
The Scanning Electron Microscope (SEM) produces images by hitting the target with a primary electron beam which then excites the surface of the target. This causes secondary electrons to be emitted from the surface which are then detected. So the electron beam in the SEM is moved (or scanned) across the sample, while detectors build an image from the secondary electrons.
Generally, the transmission electron microscope's 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 (unlike optical microscopes and TEM which require the samples to be thin) 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.
Electron microscopes are expensive to buy and maintain. They are also very sensitive to vibration and external magnetic fields. This means that special facilities are required to house microscopes aimed at achieving high resolutions. Also the targets have to be viewed in vacuum, as the electrons would scatter off the molecules that make up air.
Scanning electron microscopes usually image conductive or semi-conductive materials best. A common preparation technique is to coat the target with a several-nanometer layer of conductive material, such as gold, from a sputtering machine; however this process has the potential to disturb delicate samples.
The targets have to be prepared in many ways to give proper detail. This may result in artifacts purely as a result of the treatment. This gives the problem of distinguishing artifacts from material, particularly in biological samples. 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.
The first electron microscope prototype was built in 1931 by the German engineers Ernst Ruska and Max Knoll. It was based on the ideas and discoveries of Louis de Broglie. Although it was primitive and was not ideal for practical use, the instrument was still capable of magnifying objects by four hundred times. 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. Researchers use it to examine biological materials (such as microorganisms and cells), a variety of large molecules, medical biopsy samples, metals and crystalline structures, and the characteristics of various surfaces.
Electron microscopes can be used to study:
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