Selected area diffraction
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Selected area diffraction, abbreviated as SAD, is a crystallographic experimental technique that can be performed inside a transmission electron microscope (TEM).
In a TEM, a thin crystalline specimen is subjected to a parallel beam of high-energy electrons. As TEM specimens are typically ~100nm thick, and the electrons typically have an energy of 100-400 kiloelectron volts, the electrons pass through the sample easily. In this case, electrons are treated as wave-like, rather than particle-like (see wave-particle duality). Because the wavelength of high-energy electrons is a fraction of a nanometer, and the spacings between atoms in a solid is only slightly larger, the atoms act as a diffraction grating to the electrons, which are diffracted. That is, some fraction of them will be scattered to particular angles, determined by the crystal structure of the sample, while others continue to pass through the sample without deflection.
As a result, the image on the screen of the TEM will be a series of spots -- the selected area diffraction pattern, SADP, each spot corresponding to a satisfied diffraction condition of the sample's crystal structure. If the sample is moved under the beam, bringing different sections of it under illumination, the arrangement of the spots in the diffraction pattern will change. If the sample is tilted, the same crystal will stay under illumination, but different diffraction conditions will be activated, and different diffraction spots will appear or disappear.
SAD is referred to as "selected" because the user can easily choose which part of the specimen to obtain the diffraction pattern from. Located below the sample holder on the TEM column is a selected area aperture, which can be inserted into the beam path. This is a thin strip of metal that will block the beam. It contains several different sized holes, and can be moved by the user. The effect is to block all of the electron beam except for the small fraction passing through one of the holes; by moving the aperture hole to the section of the sample the user wishes to examine, this particular area is selected by the aperture, and only this section will contribute to the SADP on the screen. This is important, for example, in polycrystalline specimens. If more than one crystal contributes to the SADP, it can be difficult or impossible to analyze. As such, it is useful to select a single crystal for analysis at a time. It may also be useful to select two crystals at a time, in order to examine the crystallographic orientation between them.
As a diffraction technique, SAD can be used to identify crystal structures and examine crystal defects. It is similar to x-ray diffraction, but unique in that areas as small as several hundred nanometers in size can be examined, whereas x-ray diffraction typically samples areas several centimeters in size.
SAD is used primarily in material science and solid state physics, and is one of the most commonly used experimental techniques in those fields.
