The #ScanningElectronMicroscope (SEM) uses a focused beam of high energy #electrons to generate signals from the surface of specially prepared specimens. It has various components including #ElectronGun, #ElectronLens to #EDSDetectors and data output devices. #SEM has revolutionized #microscopy and #CellBiology in the last couple of years.
Principle
Accelerated electrons in a #SEM carry significant amounts of #KineticEnergy, 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.
The signals include secondary electrons, back-scattered electrons (#BSE), diffracted back-scattered electrons (#DBSE, 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) and heat.
Photons are produced by inelastic collisions of the incident electrons with electrons in discrete orbitals of atoms in the sample.
Secondary electrons are mainly used for showing #morphology and #topography of samples and #BSE are valuable for illustrating contrast in composition of #multiphase samples (for rapid phase discrimination).
#SEM analysis is "non-destructive" as photons generated by electron interactions do not lead to volume loss of the sample, so it is possible to analyze the same sample repeatedly with #reproducibility.
Merits
It is easier to operate than #TEM and has user-friendly "intuitive" interfaces.
Areas ranging from 1cm to 5microns width can be imaged with #SEM. Magnification range 20X to ~30,000X. Spatial resolution 50 to 100nm.
#Minimal sample preparation required.
#rapid data acquisition: <5 minutes per image for #SEI, #BSE, spot #EDS analysis.
Data generated in digital formats which are #portable.
Demerits
Samples must be #solid and fit into the microscope #chamber.
For most machines, samples must be #stable in a #vacuum on the order of 10^-5 to 10^-6 torr.
Samples likely to outgas at low pressures and to decrepitate at low pressure are #unsuitable.
Samples need to be electrically insulated with an electrically conductive coating.
Applications
It is routinely used for generation of high resolution images of shapes of objects and showing #spatial variations in #chemical compositions.
It is also used to identify phases based on qualitative chemical analysis and/or #crystalline structure using back-scattered electrons (#BSE).
#CompositionalMaps can be prepared based on differences in trace element "activators" using #Cathodoluminescence.
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