Transmission electron microscopy

:TEM redirects here; for other meanings, see TEM (disambiguation).

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Transmission electron microscopy (TEM) is an imaging technique whereby a beam of electrons is focused onto a specimen causing an enlarged version to appear on a fluorescent screen or layer of photographic film (see electron microscope), or can be detected by a CCD camera. The first practical transmission electron microscope was built by Albert Prebus and James Hillier at the University of Toronto in 1938 using concepts developed earlier by Max Knoll and Ernst Ruska.

Related Topics:
Electron - Fluorescent - Photographic film - Electron microscope - James Hillier - University of Toronto - Max Knoll - Ernst Ruska

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In the past, light microscopes have been used mostly for imaging due to their relative ease of use. However, the maximum resolution that one can image is determined by the wavelength of the photons that are being used to probe the sample; nothing smaller than the wavelength being used can be resolved. Visible light has wavelengths of 400-700 nanometers; larger than many objects of interest. Ultraviolet could be used, but soon runs into problems of absorption. Even shorter wavelengths, such as X-rays, exhibit a lack of interaction: both in focussing (nothing interacts strongly enough to act as a lens) and actually interacting with the sample.

Related Topics:
Wavelength - Photons - Nanometer

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Like all matter, electrons have both wave and particle properties (as demonstrated by Louis-Victor de Broglie), and their wave-like properties mean that a beam of electrons can in some circumstances be made to behave like a beam of radiation. The wavelength is dependent on their energy, and so can be tuned by adjustment of accelerating fields, and can be much smaller than that of light, yet they can still interact with the sample due to their electrical charge. Electrons are generated by a process known as thermionic discharge in the same manner as the at the cathode in a cathode ray tube, or by field emission; they are then accelerated by an electric field and focussed by electrical and magnetic fields onto the sample. The electrons can be focused onto the sample providing a resolution far better than is possible with light microscopes, and with improved depth of vision. Details of a sample can be enhanced in light microscopy by the use of stains; similarly with electron microscopy, compounds of heavy metals such as lead or uranium can be used to selectively deposit heavy atoms in the sample and enhance structural detail, the dense electron clouds of the heavy atoms interacting strongly with the electron beam. The electrons can be detected using a photographic film, or fluorescent screen among other technologies.

Related Topics:
Louis-Victor de Broglie - Thermionic discharge - Cathode ray tube - Field emission

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An additional class of these instruments is the electron cryomicroscope, which includes a specimen stage capable of maintaining the specimen at liquid nitrogen or liquid helium temperatures. This allows imaging specimens prepared in vitreous ice, the preferred preparation technique for imaging individual molecules or macromolecular assemblies.

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Another type of TEM is the scanning transmission electron microscope (STEM), where the beam can be rastered across the sample to form the image.

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In analytical TEMs the elemental composition of the specimen can be determined by analysing its X-ray spectrum or the energy-loss spectrum of the transmitted electrons.

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Modern research TEMs may include aberration correctors, to reduce the amount of distortion in the image, allowing information on features on the scale of 0.1nm to be obtained. Monochromators may also be used which reduce the energy spread of the incident electron beam to less than 0.15eV.

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