Editing Electron microscope (section)

== Types of electron microscopes ==

[[File:Transmission Electron Microscope operating principle.ogg|thumb|upright=1.5|Operating principle of a transmission electron microscope]]

=== Transmission electron microscope (TEM) ===
{{Main|Transmission electron microscope}}
[[File:Electron_Microscope.png|right|frameless]]
The original form of the electron microscope, the [[transmission electron microscope]] (TEM), uses a [[high voltage]] [[electron beam]] to illuminate the specimen and create an image. An electron beam is produced by an [[electron gun]], with the electrons typically having energies in the range 20 to 400 keV, focused by [[Electromagnetism|electromagnetic]] lenses, and transmitted through the specimen. When it emerges from the specimen, the electron beam carries information about the structure of the specimen that is magnified by lenses of the microscope. The spatial variation in this information (the "image") may be viewed by projecting the magnified electron image onto a [[Detectors for transmission electron microscopy|detector]]. For example, the image may be viewed directly by an operator using a fluorescent viewing screen coated with a [[phosphor]] or [[scintillator]] material such as [[zinc sulfide]]. A high-resolution phosphor may also be coupled by means of a lens optical system or a [[fibre optic]] light-guide to the sensor of a [[digital camera]]. [[Detectors for transmission electron microscopy#Direct electron detectors|Direct electron detectors]] have no scintillator and are directly exposed to the electron beam, which addresses some of the limitations of scintillator-coupled cameras.<ref>{{cite journal | vauthors = Cheng Y, Grigorieff N, Penczek PA, Walz T | title = A primer to single-particle cryo-electron microscopy | journal = Cell | volume = 161 | issue = 3 | pages = 438–449 | date = April 2015 | pmid = 25910204 | pmc = 4409659 | doi = 10.1016/j.cell.2015.03.050 }}</ref>

The resolution of TEMs is limited primarily by [[spherical aberration]], but a new generation of hardware correctors can reduce spherical aberration to increase the resolution in [[high-resolution transmission electron microscopy]] (HRTEM) to below 0.5 [[angstrom]] (50 [[picometre]]s),<ref name="Erni-2009">{{cite journal | vauthors = Erni R, Rossell MD, Kisielowski C, Dahmen U | title = Atomic-resolution imaging with a sub-50-pm electron probe | journal = Physical Review Letters | volume = 102 | issue = 9 | pages = 096101 | date = March 2009 | pmid = 19392535 | doi = 10.1103/PhysRevLett.102.096101 | bibcode = 2009PhRvL.102i6101E | osti = 960283 | url = https://www.escholarship.org/uc/item/3cs0m4vr }}</ref> enabling magnifications above 50 million times.<ref>{{cite web|url=http://www.sc.doe.gov/bes/scale_of_things.html |title=The Scale of Things |date=2006-05-26 |publisher=Office of Basic Energy Sciences, U.S. Department of Energy |access-date=2010-01-31 |url-status=dead |archive-url=https://web.archive.org/web/20100201175106/http://www.sc.doe.gov/bes/scale_of_things.html |archive-date=2010-02-01 }}</ref> The ability of HRTEM to determine the positions of atoms within materials is useful for nano-technologies research and development.<ref>{{cite web| vauthors = O'Keefe MA, Allard LF |title = Sub-Ångstrom Electron Microscopy for Sub-Ångstrom Nano-Metrology |url=http://www.osti.gov/bridge/servlets/purl/821768-E3YVgN/native/821768.pdf |publisher=Information Bridge: DOE Scientific and Technical Information – Sponsored by OSTI |date=2004-01-18}}</ref>

=== Scanning electron microscope (SEM) ===
[[File:Scanning Electron Microscope.ogv|thumb|upright=1.5|right|Operating principle of a scanning electron microscope]]
{{Main|Scanning electron microscope}}
[[File:Bacillus subtilis image.jpg|thumb|Image of ''[[Bacillus subtilis]]'' taken with a 1960s electron microscope]]
The SEM produces images by probing the specimen with a focused electron beam that is scanned across the specimen ([[raster scan]]ning). When the electron beam interacts with the specimen, it loses energy by a variety of mechanisms. These interactions lead to, among other events, emission of [[secondary emission|low-energy secondary electrons]] and high-energy backscattered electrons, light emission ([[cathodoluminescence]]) or [[X-ray]] emission, all of which provide signals carrying information about the properties of the specimen surface, such as its topography and composition. The image displayed by SEM represents the varying intensity of any of these signals into the image in a position corresponding to the position of the beam on the specimen when the signal was generated.<ref name=":0" />{{RP|pages=1-15}}

SEMs are different from TEMs in that they use electrons with much lower energy, generally below 20 keV,<ref>{{Cite journal | vauthors = Dusevich V, Purk J, Eick J |date= January 2010 |title=Choosing the Right Accelerating Voltage for SEM (An Introduction for Beginners) |journal=Microscopy Today |volume=18 |issue=1 |pages=48–52 |doi=10.1017/s1551929510991190 }}</ref> while TEMs generally use electrons with energies in the range of 80-300 keV.<ref name="Saha-2022" /> Thus, the electron sources and optics of the two microscopes have different designs, and they are normally separate instruments.<ref>{{Cite web |date=2022-04-07 |title=Electron Microscopy {{!}} Thermo Fisher Scientific - US |url=https://www.thermofisher.com/us/en/home/electron-microscopy.html |access-date=2024-07-13 |archive-url=https://web.archive.org/web/20220407190819/https://www.thermofisher.com/us/en/home/electron-microscopy.html |archive-date=2022-04-07 }}</ref>

=== Scanning transmission electron microscope (STEM) ===
{{Main|Scanning transmission electron microscopy}}
A STEM combines features of both a TEM and a SEM by rastering a focused incident probe across a specimen. Many types of imaging are common to both TEM and STEM, but some such as T [[annular dark-field imaging]] and other analytical techniques are much easier to perform with higher spatial resolutions in a STEM instrument. One drawback is that image data is acquired in serial rather than in parallel fashion.<ref name=":0">{{cite book |last=Kohl |first=Helmut |title=Transmission Electron Microscopy |last2=Reimer |first2=Ludwig |date=2008 |publisher=Springer |isbn=978-0-387-40093-8 |series=Springer Series in Optical Sciences |volume=36 |pages= |chapter=Elements of a Transmission Electron Microscope |doi=10.1007/978-0-387-40093-8_4}}</ref>{{Rp|pages=75–138}}