Editing Electron diffraction (section)

==== Convergent beam electron diffraction ====
{{main|Convergent-beam electron diffraction}}
{{anchor|Figure 17}}[[File:CBED sketch.png|thumb|Figure 17: Schematic of CBED technique. Adapted from W. Kossel and G. Möllenstedt.<ref name=KM>{{Cite journal |last1=Kossel |first1=W. |last2=Möllenstedt |first2=G. |date=1939 |title=Elektroneninterferenzen im konvergenten Bündel |url=https://onlinelibrary.wiley.com/doi/10.1002/andp.19394280204 |journal=Annalen der Physik |language=de |volume=428 |issue=2 |pages=113–140 |doi=10.1002/andp.19394280204|bibcode=1939AnP...428..113K |url-access=subscription }}</ref>|alt=Experimental setup for convergent beam electron diffraction.]]
In convergent beam electron diffraction (CBED),<ref name=":4" /><ref name=":5" /><ref name=":6" /> the incident electrons are normally focused in a converging cone-shaped beam with a crossover located at the sample, e.g. [[#Figure 17|Figure 17]], although other methods exist. Unlike the parallel beam, the convergent beam is able to carry information from the sample volume, not just a two-dimensional projection available in SAED. With convergent beam there is also no need for the selected area aperture, as it is inherently site-selective since the beam crossover is positioned at the object plane where the sample is located.<ref name="Morniroli 2004"/>

{{anchor|Figure 18}}[[File:CBEDThickness.png|thumb|Figure 18: Variations in CBED due to dynamical diffraction, with thickness increasing from a)-d) for Si [110]|left|alt=Changes in CBED patterns for different thicknesses of the sample, showing that they get more complicated with thicker samples.]]

A CBED pattern consists of disks arranged similar to the spots in SAED. Intensity within the disks represents dynamical diffraction effects and symmetries of the sample structure, see [[#Figure 7|Figure 7]] and [[#Figure 18|18]]. Even though the zone axis and lattice parameter analysis based on disk positions does not significantly differ from SAED, the analysis of disks content is more complex and simulations based on dynamical diffraction theory is often required.<ref>{{Cite journal |last1=Chuvilin |first1=A. |last2=Kaiser |first2=U. |date=2005 |title=On the peculiarities of CBED pattern formation revealed by multislice simulation |url=https://linkinghub.elsevier.com/retrieve/pii/S0304399105000483 |journal=Ultramicroscopy |language=en |volume=104 |issue=1 |pages=73–82 |doi=10.1016/j.ultramic.2005.03.003|pmid=15935917 |url-access=subscription }}</ref> As illustrated in [[#Figure 18|Figure 18]], the details within the disk change with sample thickness, as does the inelastic background. With appropriate analysis CBED patterns can be used for indexation of the crystal point group, space group identification, measurement of lattice parameters, thickness or strain.<ref name="Morniroli 2004"/>

The disk diameter can be controlled using the microscope optics and apertures.<ref name=":8" /> The larger is the angle, the broader the disks are with more features. If the angle is increased to significantly, the disks begin to overlap.<ref name="KM" /> This is avoided in large angle convergent electron beam diffraction (LACBED) where the sample is moved upwards or downwards. There are applications, however, where the overlapping disks are beneficial, for instance with a [[ronchigram]]. It is a CBED pattern, often but not always of an amorphous material, with many intentionally overlapping disks providing information about the [[optical aberrations]] of the electron optical system.<ref>{{Cite journal |last1=Schnitzer |first1=Noah |last2=Sung |first2=Suk Hyun |last3=Hovden |first3=Robert |date=2019 |title=Introduction to the Ronchigram and its Calculation with Ronchigram.com |journal=Microscopy Today |volume=27 |issue=3 |pages=12–15 |doi=10.1017/s1551929519000427 |s2cid=155224415 |issn=1551-9295|doi-access=free }}</ref>