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TEM Transmission Electron Microscope (TEM) uses a beam of high-energy electrons to project a magnified image of a sample onto a fluorescent screen. Its optical configuration resembles a 35-mm slide projector with the sample taking the place of the photographic slide. A TEM uses electrons instead of photons, and the sample removes energy from the beam due to electron scattering rather than light absorption. A TEM illuminates an entire sample and uses electromagnetic lenses to focus the transmitted electrons into a highly magnified image. In contrast, a scanning electron microscope (SEM) scans a finely focused probe of electrons over the imaged area, simultaneously monitoring an induced signal. It maps the signal variations to form 2D and 3D representations of the sample surface or object. TEMs will never replace SEMs because the depth of focus allows the microscopist to examine real, intact and 3D samples. TEMs require thin samples, typically 100 nm or less, that can transmit most of the incident electrons. TEMs offer a resolution of 0.8 to 1 A, an order of magnitude better than the resolution of SEMs. Because TEMs can use the atomic lattice of the silicon substrate as an internal calibration standard, they can make very accurate dimensional measurements. The interactions between electrons and sample atoms generate a number of secondary signals that find use in both TEMs and SEMs. Transmitted electrons are the primary imaging signal in TEMs. Secondary electrons (sample electrons ejected by beam electrons) and backscattered electrons (beam electrons scattered backward by collisions with sample nuclei) provide the primary imaging signals in SEMs. The TEM can analyze transmitted electrons to determine the energy they lost when scattered in the sample. The energy loss indirectly characterized the state and type of the scattering atom. Electron diffraction analysis can provide information about the crystalline structure of the sample.
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