Editing Electron microscope (section)

=== Electron diffraction ===
{{Main| Electron diffraction}}
Transmission electron microscopes can be used in [[electron diffraction]] mode where a map of the angles of the electrons leaving the sample is produced. The advantages of electron diffraction over [[X-ray crystallography]] are primarily in the size of the crystals. In X-ray crystallography, crystals are commonly visible by the naked eye and are generally in the hundreds of micrometers in length. In comparison, crystals for electron diffraction must be less than a few hundred nanometers in thickness, and have no lower boundary of size. Additionally, electron diffraction is done on a TEM, which can also be used to obtain many other types of information, rather than requiring a separate instrument.<ref>{{Cite book |last=Cowley |first=J. M. |author-link=John M. Cowley |title=Diffraction physics |date=1995 |publisher=Elsevier |isbn=978-0-444-82218-5 |edition=3rd |series=North Holland personal library |location=Amsterdam}}</ref><ref name="Saha-2022">{{cite journal | vauthors = Saha A, Nia SS, Rodríguez JA | title = Electron Diffraction of 3D Molecular Crystals | journal = Chemical Reviews | volume = 122 | issue = 17 | pages = 13883–13914 | date = September 2022 | pmid = 35970513 | pmc = 9479085 | doi = 10.1021/acs.chemrev.1c00879 }}</ref>
[[File:CBEDThickness.png|thumb|Variations in CBED with thickness for Si (001)]]
{{Main|Convergent beam electron diffraction|Precession electron diffraction}}

There are many variants on electron diffraction, depending upon exactly what type of illumination conditions are used. If a parallel beam is used with an aperture to limit the region exposed to the electrons then sharp diffraction features are normally observed, a technique called [[Electron diffraction#Selected area electron diffraction|selected area electron diffraction]]. This is often the main technique used. Another uses conical illumination and is called [[convergent beam electron diffraction]] (CBED). This approach is good for determining the symmetry of materials. A third is [[precession electron diffraction]] where a parallel beam is spun around a large angle, producing a type of average diffraction pattern.<ref name="Original">{{cite journal |last1=Vincent |first1=R. |last2=Midgley |first2=P.A. |year=1994 |title=Double conical beam-rocking system for measurement of integrated electron diffraction intensities |journal=Ultramicroscopy |volume=53 |issue=3 |pages=271–82 |doi=10.1016/0304-3991(94)90039-6}}</ref> These often have less multiple scattering.<ref name="thesis">Own, C. S.: PhD thesis, System Design and Verification of the Precession Electron Diffraction Technique, Northwestern University, 2005,http://www.numis.northwestern.edu/Research/Current/precession.shtml</ref>