Editing Electron microscope (section)

== Main operating modes ==
[[File:Ant SEM.jpg|thumb|right|An image of an [[ant]] in an SEM]]The most common methods of obtaining images in an electron microscope involve selecting different directions for the electrons that have been transmitted through a sample, and/or electrons of different energies. There are a very large number of methods, although not all are very common.

=== Secondary electrons ===
[[File:Electron-matter_interaction_volume_and_various_types_of_signal_generated_-_v2.svg|thumb|Electron–matter interaction volume and types of signal generated in a SEM]]
In a SEM the signals result from interactions of the electron beam with atoms within the sample. The most common mode is to use the [[secondary electrons]] (SE). Secondary electrons have very low energies on the order of 50 [[Electronvolt|eV]], which limits their [[Inelastic mean free path|mean free path]] in solid matter, a few [[nanometer]]s below the sample surface.<ref name=":1" />  The electrons are detected by an [[Everhart–Thornley detector]],<ref name="Everhart-1960">{{cite journal |last=Everhart |first=T. E. |author2=Thornley, R. F. M. |year=1960 |title=Wide-band detector for micro-microampere low-energy electron currents |url=http://authors.library.caltech.edu/12086/1/EVEjsi60.pdf |journal=Journal of Scientific Instruments |volume=37 |issue=7 |pages=246–248 |bibcode=1960JScI...37..246E |doi=10.1088/0950-7671/37/7/307}}</ref> which is a type of collector-[[scintillator]]-[[photomultiplier]] system. The signal from secondary electrons tends to be highly localized at the point of impact of the primary electron beam, making it possible to collect images of the sample surface with a resolution of below 1 [[Nanometre|nm]], and with specialized instruments at the atomic scale.<ref>{{Cite journal |last=Ciston |first=J. |last2=Brown |first2=H. G. |last3=D'Alfonso |first3=A. J. |last4=Koirala |first4=P. |last5=Ophus |first5=C. |last6=Lin |first6=Y. |last7=Suzuki |first7=Y. |last8=Inada |first8=H. |last9=Zhu |first9=Y. |last10=Allen |first10=L. J. |last11=Marks |first11=L. D. |date=2015-06-17 |title=Surface determination through atomically resolved secondary-electron imaging |url=https://www.nature.com/articles/ncomms8358 |journal=Nature Communications |language=en |volume=6 |issue=1 |doi=10.1038/ncomms8358 |issn=2041-1723 |pmc=4557350 |pmid=26082275}}</ref>

The brightness of the signal depends on the number of secondary electrons reaching the [[Sensor|detector]]. If the beam enters the sample perpendicular to the surface, then the activated region is uniform about the axis of the beam and a certain number of electrons "escape" from within the sample. As the angle of incidence increases, the interaction volume increases and the "escape" distance of one side of the beam decreases, resulting in more secondary electrons being emitted from the sample. Thus steep surfaces and edges tend to be brighter than flat surfaces, which results in images with a well-defined, three-dimensional appearance.<ref name=":1" />
=== Backscattered electrons ===
Backscattered electrons (BSE) are those emitted back out from the specimen due to beam-specimen interactions where the electrons undergo [[Elastic scattering|elastic]] and [[Inelastic scattering|inelastic]] scattering. They have energies from 50 eV up to the energy of the primary beam by conventional definition. Backscattered electrons can be used for both imaging and to form an [[electron backscatter diffraction]] (EBSD) image that can be used to determine the crystallographic structure of the specimen.<ref name=":1" />[[File:EBSD_(001)_Si.png|thumb|Electron backscatter diffraction pattern for (001) single crystal silicon crystals taken at 20kV using Oxford S2 detector]]

Since heavy elements (high atomic number) backscatter electrons more strongly than light elements (low atomic number), and thus appear brighter in the image, BSEs are used to detect contrast between areas with different chemical compositions.<ref name=":1" />  Dedicated backscattered electron detectors are positioned above the sample in a "doughnut" type arrangement, concentric with the electron beam, maximizing the solid angle of collection. BSE detectors are usually either of scintillator or of semiconductor types. When all parts of the detector are used to collect electrons symmetrically about the beam, atomic number contrast is produced. However, strong topographic contrast is produced by collecting back-scattered electrons from one side above the specimen using an asymmetrical, directional BSE detector; the resulting contrast appears as illumination of the topography from that side. Semiconductor detectors can be made in radial segments that can be switched in or out to control the type of contrast produced and its directionality.<ref name=":1" />

=== Diffraction contrast imaging ===
Diffraction contrast uses the variation in either or both the direction of diffracted electrons or their amplitude as a function of position as the contrast mechanism. It is one of the simplest ways to image in a transmission electron microscope, and widely used.

The idea is to use an objective aperture below the sample and select only one or a range of different diffracted directions, then use these to form an image. When the aperture includes the incident beam direction the images are called ''bright field'', since in the absence of any sample the field of view would be uniformly bright. When the aperture excludes the incident beam the images are called ''dark field'', since similarly without a sample the image would be uniformly dark.<ref name="HirschEtAl">{{Cite book |last1=Hirsch |first1=P. B. |url=https://www.worldcat.org/oclc/2365578 |title=Electron microscopy of thin crystals |last2=Howie |first2=A. |last3=Nicholson |first3=R. B. |last4=Pashley |first4=D. W. |last5=Whelan |first5=M. J. |date=1965 |publisher=Butterworths |isbn=0-408-18550-3 |location=London |oclc=2365578}}</ref><ref>{{Cite journal |last=Reimer |first=Ludwig |date=1997 |title=Transmission Electron Microscopy |url=https://doi.org/10.1007/978-3-662-14824-2 |journal=Springer Series in Optical Sciences |volume=36 |doi=10.1007/978-3-662-14824-2 |isbn=978-3-662-14826-6 |issn=0342-4111}}</ref> One variant of this is called [[weak-beam dark-field microscopy]], and can be used to obtain high resolution images of defects such as dislocations.<ref>{{Cite journal |last1=Cockayne |first1=D. J. H. |last2=Ray |first2=I. L. F. |last3=Whelan |first3=M. J. |date=1969-12-01 |title=Investigations of dislocation strain fields using weak beams |url=https://www.tandfonline.com/doi/abs/10.1080/14786436908228210 |journal=The Philosophical Magazine |volume=20 |issue=168 |pages=1265–1270 |doi=10.1080/14786436908228210 |bibcode=1969PMag...20.1265C |issn=0031-8086}}</ref>

=== High resolution imaging ===
{{Main|High-resolution transmission electron microscopy|Annular dark-field imaging}}
[[File:CuTe-HRTEM.jpg|thumb|CuTe High resolution image]]
In high-resolution transmission electron microscopy (also sometimes called high-resolution electron microscopy) a number of different diffracted beams are allowed through the objective aperture. These interfere, leading to images which represent the atomic structure of the material. These can include the incident beam direction, or with scanning transmission electron microscopes they typically are for a range of diffracted beams excluding the incident beam.<ref name=":2" /> Depending upon how thick the samples are and the [[Optical aberration|aberrations]] of the microscope these images can either be directly interpreted in terms of the positions of columns of atoms, or require a more careful analysis using calculations of the [[Multislice|multiple scattering]] of the electrons<ref>{{Cite journal |last=Ishizuka |first=Kazuo |date=2004-02-01 |title=FFT Multislice Method—The Silver Anniversary |url=https://academic.oup.com/mam/article-abstract/10/1/34/6912350?redirectedFrom=fulltext |journal=Microscopy and Microanalysis |volume=10 |issue=1 |pages=34–40 |doi=10.1017/S1431927604040292 |issn=1431-9276}}</ref> and the effect of the [[contrast transfer function]] of the microscope.<ref>{{Cite journal |last=Wade |first=R. H. |date=1992-10-01 |title=A brief look at imaging and contrast transfer |url=https://linkinghub.elsevier.com/retrieve/pii/0304399192900118 |journal=Ultramicroscopy |volume=46 |issue=1 |pages=145–156 |doi=10.1016/0304-3991(92)90011-8 |issn=0304-3991}}</ref>{{Main|Electron holography|4D scanning transmission electron microscopy|l2 = 4D STEM}}

There are many variants that can also to lead to images with atomic level information. [[Electron holography]] uses the interference of electrons which have been through the sample and a reference beam.<ref>{{Cite journal |last=Cowley |first=J. M. |date=1992-06-01 |title=Twenty forms of electron holography |url=https://linkinghub.elsevier.com/retrieve/pii/0304399192902134 |journal=Ultramicroscopy |volume=41 |issue=4 |pages=335–348 |doi=10.1016/0304-3991(92)90213-4 |issn=0304-3991}}</ref> [[4D scanning transmission electron microscopy|4D STEM]] collects diffraction data at each point using a scanning instrument, then processes them to produce different types of images.<ref>{{Cite journal |last=Ophus |first=Colin |date=2019-06-01 |title=Four-Dimensional Scanning Transmission Electron Microscopy (4D-STEM): From Scanning Nanodiffraction to Ptychography and Beyond |url=https://academic.oup.com/mam/article/25/3/563/6887544 |journal=Microscopy and Microanalysis |volume=25 |issue=3 |pages=563–582 |doi=10.1017/S1431927619000497 |issn=1431-9276|doi-access=free }}</ref>

=== X-ray microanalysis ===
{{Main|Energy-dispersive X-ray spectroscopy}}
[[File:EDS_-_Rimicaris_exoculata.png|thumb|class=skin-invert-image|EDS spectrum of the mineral crust of the vent shrimp ''[[Rimicaris exoculata]]''<ref>{{cite journal |author=Corbari, L |display-authors=etal |year=2008 |title=Iron oxide deposits associated with the ectosymbiotic bacteria in the hydrothermal vent shrimp Rimicaris exoculata |journal=Biogeosciences |volume=5 |issue=5 |pages=1295–1310 |bibcode=2008BGeo....5.1295C |doi=10.5194/bg-5-1295-2008 |doi-access=free}}</ref> Most of these peaks are [[K-alpha]] and [[K-beta]] lines. One peak is from the L shell of iron.]]
X-ray microanalysis is a method of obtaining local chemical information within electron microscopes or all types, although it is most commonly used in scanning instruments. When high energy electrons interact with atoms they can knock out electrons, particularly those in the [[inner shell]]s and [[core electrons]]. These are then filled by [[valence electron]], and the energy difference between the valence and core states can be converted into an [[x-ray]] which is detected by a spectrometer. The energies of these x-rays is somewhat specific to the atomic species, so local chemistry can be probed.<ref name=":1">{{Cite book |last=Goldstein |first=Joseph |url=https://books.google.com/books?id=ruF9DQxCDLQC |title=Scanning Electron Microscopy and X-Ray Microanalysis: Third Edition |date=2003-01-31 |publisher=Springer US |isbn=978-0-306-47292-3 |language=en}}</ref>

=== EELS ===
{{Main|Electron energy loss spectroscopy}}
[[File:Electron_energy_loss_spectrum_feature_overview.svg|thumb|class=skin-invert-image|Experimental electron energy loss spectrum, showing the major features: zero-loss peak, plasmon peaks and core loss edge.]]
Similar to X-ray microanalysis, the energies of electrons which have transmitted through a sample can be analyzed and yield information ranging from details of the local electronic structure to chemical information.<ref>{{Citation |last=Egerton |first=Ray F. |title=An Introduction to Electron Energy-Loss Spectroscopy |date=1986 |work=Electron Energy-Loss Spectroscopy in the Electron Microscope |pages=1–25 |url=https://doi.org/10.1007/978-1-4615-6887-2_1 |access-date=2025-02-25 |place=Boston, MA |publisher=Springer US |doi=10.1007/978-1-4615-6887-2_1 |isbn=978-1-4615-6889-6}}</ref>

=== 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>

=== Other electron microscope techniques ===

* [[Cathodoluminescence]] - analysing photons due to the electron beam
* [[Charge contrast imaging]] - using how charging varies with position to produce images
* [[Cryogenic electron microscopy|CryoEM]] - using frozen samples, almost always biological samples
* [[Electron backscatter diffraction|EBSD]] - using the back-scattered electrons, typically in a SEM
** [[Transmission Kikuchi diffraction|TKD]] - using the [[Kikuchi lines (physics)|Kikuchi lines]] in diffraction patterns
* [[Electron channelling contrast imaging|ECCI]] - using contrast due to electron channelling
* [[Electron beam-induced current|EBIC]] - measuring the current produced as a function of the position of a small beam
* [[Electron tomography]] - methods to produce 3D information by combining images
* [[Fluctuation electron microscopy|FEM]] - technique to interrogate nanocrystalline materials
* [[Immune electron microscopy]] - the use of electron microscopy in immunology
* [[Geometric phase analysis]] - a method to analyze high-resolution images
* [[Serial block-face scanning electron microscopy]] - a way to produce 3D information from many images
* [[Wavelength-dispersive X-ray spectroscopy|WDXS]] - higher precision detection of x-rays to analyze local chemistry