Question

Distinguish between SEM and TEM. What is the principal difference in sample preparation and detection?

Answer 1

TEM might be used in preference to an SEM because: Transmission Electron Microscopes can produce images that havethan images produced by Scanning Electron Microscopes. Also: Higher Magnification and Greater Resolution

An SEM might be used in preference to a TEM because: Scanning Electron Microscopes produce three-dimensional (3D) images while Transmission Electron Microscopes only produce flat (2D) images. 3D images provide more information about the shape of features and also about the location of features relative to each other. In some situations this information is very useful and more important than the higher resolution and magnification that a TEM would provide.

In some cases it is useful to study both transmission electron micrographs and scanning electron micrographs of the same tissue or material. In that case it would be necessary to prepare appropriate samples for each of these types of electron microscope.

NOTE: here are some additional data:

• List of features electron microscopes generally have in common with each other:
• Size: Large - much larger than light microscopes.
• Radiation Type: Use beams of electrons (approx equivalent wavelength 1 nm).
• Control of image formation : Beams of electrons are focused using electromagnets due to negative charge on electrons.
• Resolution: Electron microscopes have much higher resolution than light microscopes due to the shorter equivalent wavelength of electron beams compared with visible light.
• Magnification: Electron microscopes have much higher magnification than light microscopes. This is due to the shorter equivalent wavelength of electron beams compared with visible light.
• Colour Images: Electron microscopes produce greyscale images. However, "false-colour" electron micrographs are common - and can be very beautiful!
• Preparation of specimens : Generally involves several steps and often harsh processes, e.g. using corrosive chemicals. Appropriate skill is needed both to prepare specimens and to interpret electron micrograph images due to "artefacts" in images. (Artefacts are features in images that are not present in the specimen but rather are due to the processes used to prepare the specimen and produce the image.)
• Image Formation : Unlike light microscope images, which can be viewed directly, electron microscopes including TEMs and SEMS require use of an interface such as a fluorescent screen, photographic plate or electronic display. This is because electrons cannot be observed directly by the human eye.
• Usage Limitations : Living specimens cannot be viewed using electron microscopes because electron microscopes require there to be a vacuum in the tube - otherwise the electrons would be absorbed by air molecules.