What is an Electron Microscope?

An electron microscope uses an accelerated electron beam to illuminate and is called an electron microscope. It is a particular style of microscope with very sharp pictures that can magnify things down to the nanometer scale. Electrons are precisely guided to form the pictures in a vacuum, which are then captured on a phosphorescent screen. The ideas underpinning Ernst Ruska’s prototype, in 1931, a German scientist and engineer, invented the first electron microscope, still apply to current EMs.

The Working Principle of an Electron Microscope

In order to learn more about a sample’s structure, morphology, and composition, electron microscopes employ signals created when an electron beam interacts with it.

1. Electrons are produced by the electron cannon.
2. The specimen is in the electron beam’s field of focus and then into a narrow, tight beam using two sets of condenser lenses.
3. An accelerating voltage is provided between the tungsten filament and the anode to accelerate the movement of electrons down the column (often between 100 kV and 1000 kV).
4. The specimen being studied is manufactured to be very thin, 200 times or more thinner than those used in optical microscopes. Sections that are cut and put on the specimen holder vary in thickness from 20 to 100 nm.
5. Depending on the thickness or refractive index of various specimen components, electrons are scattered by the electrical beam as it travels through the object.
6. Since fewer electrons hit that area of the screen, the specimen’s denser areas disperse more electrons, making them seem darker in the image. Transparent sections, however, are brighter.
7. The high-powered objective lens receives the electron beam leaving the specimen and creates the image with intermediate magnification.
8. The ocular lenses then produce the final, much magnified image.

Types of Electron microscope

There are two varieties of electron microscope, each having a unique way of working:

1. Transmission Electron Microscope (TEM)

• Thin specimens that allow electrons to flow through and produce a picture are seen using a transmission electron microscope.
• The standard (compound) light microscope and the TEM are similar in many aspects.
• TEM has been used, among other things, to see the inside of cells in thin layers; the structure of protein molecules in comparison to metal shadowing; the positioning of protein molecules in cell membranes; the structure of molecules in viruses as well as cytoskeletal filaments in order to prepare for negative staining; and the structure of molecules in viruses (by freeze-fracture).
• 2. Scanning Electron Microscope (SEM)

• For traditional scanning electron microscopy to work, secondary electrons must be emitted from a specimen’s surface.
• Due to its improved depth of focus, a scanning electron microscope is the EM counterpart of a stereo light microscope.
• It provides detailed images of the surfaces of cells and entire organisms, something that TEM lacks. It can also be used for counting, particle size analysis, and process control.
• Because the image is produced by rastering an electron beam with a focused field of view over the specimen’s surface, it is known as a scanning electron microscope. 

Parts of an Electron Microscope

The electron microscope is placed vertically and has the shape of a tall vacuum column. It consists of the following elements:

1. Electron gun

• A heated tungsten filament that produces electrons makes up the electron cannon.

• 2. Electromagnetic lenses

• The condenser lens directs the electron beam to the specimen. A second condenser lens changes the electrons into a narrowed, focused beam.
• When the electron beam leaving the specimen goes down the second of two magnetic coils, called the objective lens, the image is intermediately amplified.
• The last further enlarged picture is created by the third pair of magnetic lenses, the projector (ocular) lenses.
• Every one of those lenses enlarges the image while retaining an amazing degree of clarity and definition.

• 3. Specimen Holder

• The specimen holder is a carbon or collodion film kept in place by a metal grid.

• 4. Image viewing and Recording System

• On a fluorescent screen is the finished picture presented.
• Below the luminous display is a camera for picture capture.

Applications of the electron microscope

• Microorganisms, cells, big molecules, biopsy samples, metals, and crystals are just a few of the biological and inorganic specimens that may have their ultrastructure examined with an electron microscope.
• For quality assurance and failure analysis in the workplace, electron microscopes are often employed.
• The pictures are captured using specialised digital cameras and frame grabbers by modern electron microscopes to create electron micrographs.
• The development of the discipline of microbiology is due to the electron microscope.The efficacy of sick treatment has significantly increased as a result of research on bacteria, viruses, and other pathogens.

Advantages of Electron microscope

• Extra-large magnification
• High resolution to the extreme
• Rarely is material deformed during preparation.
• A deeper depth of field may be investigated.
• Various applications

Limitations of the electron microscope

• The live specimen cannot be seen.
• Since the penetrating strength of the electron beam is so low, the object should be very thin. The material is dried and cut into very small pieces before examination.
• The specimen must be completely dry, since the EM functions in a vacuum.
• Costly to construct and maintain.
• We need training for researchers.
• Image artefacts brought on by the processing of the material.
• This kind of microscope is very sensitive to external magnetic fields and vibration.

References

1. http://www.biologydiscussion.com/microscope/electron-microscope/electron-microscope-principle-components-specimen-preparation-and-uses/16595
2. https://en.wikipedia.org/wiki/Electron_microscope
3. https://www.wikilectures.eu/w/Electron_microscopy/principle
4. http://www.yourarticlelibrary.com/microeconomics/working-principle-of-a-electron-microscopes-with-diagram/26479
5. https://www.slideshare.net/gangahuvin/electron-microscopy-16995175
6. https://www.umassmed.edu/cemf/whatisem/
7. https://www.microscopemaster.com/electron-microscope.html
8. https://getrevising.co.uk/grids/electron_microscopes_2