Accelerated electrons in an SEM carry significant amounts of kinetic energy, and this energy is dissipated as a variety of signals produced by electron-sample interactions when the incident electrons are decelerated in the solid sample. These signals include secondary electrons that produce SEM images , backscattered electrons BSE , diffracted backscattered electrons EBSD that are used to determine crystal structures and orientations of minerals , photons characteristic X-rays that are used for elemental analysis and continuum X-rays , visible light cathodoluminescence--CL , and heat.
Secondary electrons and backscattered electrons are commonly used for imaging samples: secondary electrons are most valuable for showing morphology and topography on samples and backscattered electrons are most valuable for illustrating contrasts in composition in multiphase samples i. X-ray generation is produced by inelastic collisions of the incident electrons with electrons in discrete ortitals shells of atoms in the sample. As the excited electrons return to lower energy states, they yield X-rays that are of a fixed wavelength that is related to the difference in energy levels of electrons in different shells for a given element.
Thus, characteristic X-rays are produced for each element in a mineral that is "excited" by the electron beam. SEM analysis is considered to be "non-destructive"; that is, x-rays generated by electron interactions do not lead to volume loss of the sample, so it is possible to analyze the same materials repeatedly. The specific capabilities of a particular instrument are critically dependent on which detectors it accommodates. The SEM is routinely used to generate high-resolution images of shapes of objects SEI and to show spatial variations in chemical compositions: 1 acquiring elemental maps or spot chemical analyses using EDS , 2 discrimination of phases based on mean atomic number commonly related to relative density using BSE , and 3 compositional maps based on differences in trace element "activitors" typically transition metal and Rare Earth elements using CL.
Precise measurement of very small features and objects down to 50 nm in size is also accomplished using the SEM. Backescattered electron images BSE can be used for rapid discrimination of phases in multiphase samples.
SEMs equipped with diffracted backscattered electron detectors EBSD can be used to examine microfabric and crystallographic orientation in many materials. There is arguably no other instrument with the breadth of applications in the study of solid materials that compares with the SEM.
The SEM is critical in all fields that require characterization of solid materials. While this contribution is most concerned with geological applications, it is important to note that these applications are a very small subset of the scientific and industrial applications that exist for this instrumentation. Most SEM's are comparatively easy to operate, with user-friendly "intuitive" interfaces. Many applications require minimal sample preparation. Modern SEMs generate data in digital formats, which are highly portable.
Samples must be solid and they must fit into the microscope chamber. A schematic representation of an SEM is shown in Figure 1. Electrons are generated at the top of the column by the electron source. They are then accelerated down the column that is under vacuum, which helps to prevent any atoms and molecules present in the column from interacting with the electron beam and ensures good quality imaging.
Electromagnetic lenses are used to control the path of the electrons. Scanning coils are used to raster the beam onto the sample. In many cases, apertures are combined with the lenses in order to control the size of the beam. Different types of electrons are emitted from samples upon interacting with the electron beam.
Images show contrast information between areas with different chemical compositions as heavier elements high atomic number will appear brighter. A Secondary Electron SE detector is placed at the side of the electron chamber, at an angle, in order to increase the efficiency of detecting secondary electrons which can provide more detailed surface information.
The key difference between electron and optical microscopy is right there in the name. SEMs use a beam of electrons rather than a beam of light. An electron source located at the top of the microscope emits a beam of highly concentrated electrons. In SEMs, there are three different types of electron sources :. Looking for the perfect analytics instrument for YOUR next big discovery? Speak with the ATA Scientific team today to get expert advice on the right instruments for your research.
In modern materials science, investigations into nanotubes and nanofibres, high temperature superconductors, mesoporous architectures and alloy strength, all rely heavily on the use of SEMs for research and investigation.
The average wavelength is nm which results in a theoretical limit of resolution not visibility of the light microscope in white light of about — nm. The figure below shows two points at the limits of detection and the two individual spots can still be distinguished. The right image shows the two points so close together that the central spots overlap. The electron microscope was developed when the wavelength became the limiting factor in light microscopes.
Electrons have much shorter wavelengths, enabling better resolution. As dimensions are shrinking for materials and devices, many structures can no longer be characterized by light microscopy. For example, to determine the integrity of a nanofiber layer for filtration, as shown here, electron microscopy is required to characterize the sample.
The main SEM components include:. Electrons are produced at the top of the column, accelerated down and passed through a combination of lenses and apertures to produce a focused beam of electrons which hits the surface of the sample.
The sample is mounted on a stage in the chamber area and, unless the microscope is designed to operate at low vacuums, both the column and the chamber are evacuated by a combination of pumps. The level of the vacuum will depend on the design of the microscope.
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