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Electron microscopy is a class of high-powered microscopy techniques which use electrons rather than photons to image a surface. They are often referred to as super resolution microscopies as their resolution, and ability to image small objects and materials, far outstrips that of optical microscopy techniques. While all electron microscopy techniques use electrons, the internal workings differ from technique to technique. In this article, we look at how the most common electron microscopy techniques work.
Scanning Electron Microscopy (SEM)
SEM is one of the most widely used electron microscopy techniques and its use is growing across many scientific fields. In SEM, an electron gun fires a beam of high energy electrons towards a sample. The electrons are controlled, directed and accelerated using a series of optical components, including lenses, scanning coils, deflector plates and an aperture.
The electrons then interact with the surface of the sample, causing the sample to emit secondary Auger electrons and X-rays. The primary electrons which interact with the sample are also backscattered. When the electrons interact with the sample, they quickly lose energy through elastic scattering and absorption mechanisms and this energy exchange causes the primary electrons to backscatter off the surface. Detectors are used to measure both types of electrons and the X-rays to build up an image of the sample.
The image of the sample is constructed by measuring the intensity of the signal produced by these surface interactions. The emitted X-rays have characteristics specific to each element, and this enables the elemental composition of the surface to be determined. Cathodoluminescence can also be integrated into an SEM imaging mode to determine multiple structural (at the atomic level) and electrical properties of the sample, as well as whether any contamination is present on the surface of the material.
Transmission Electron Microscopy (TEM)
TEM is similar to SEM in many ways, especially with respect to firing high energy electrons at a sample using an electron gun using a series of optical components to accelerate and focus the electrons. TEM is another widely used electron microscopy technique and has also gained a lot of traction in recent years as an effective imaging technique.
However, there are some differences between SEM and TEM. Where SEM is concerned with firing electrons and measuring how they scatter when they interact with a sample, the electrons in TEM pass through the material and are not scatted by the sample. Some electrons are still scattered but are not measured in a regular TEM imaging mode (see the below REM mode for more details).
This operating principle makes TEM a useful imaging technique for thinner materials. The actual working principles are very similar to optical microscopy, where photons pass through a sample to generate an image. However, in the case of TEM, it is the transmission of the electrons through the material that relays information about the sample, which are then magnified by an objective lens to produce the image. Overall, as well as an image of the sample, TEM can be used to provide information on a sample’s crystal structure, morphology and surface stress state.
Reflection Electron Microscopy (REM)
REM is a variation of TEM where, rather than the transmitted beam, the reflected beam of electrons is measured. While TEM measures the electrons that pass through the sample, some of the electrons are backscattered, and this mode can be used to measure these electrons. It is an imaging mode that is used with a TEM but is not as widely used in other electron microscopy methods, and when it is it is often combined with various spectroscopy techniques to fully characterize a sample. Nevertheless, it has a use for measuring flat faces of a crystalline sample and can be used to give a high resolution of a sample’s surface, as only the first few atomic layers of a sample are involved with these backscattering interactions.
Sources and Further Reading
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