Definition of Electron Microscope
- An electron microscope is a microscope, which uses electron beams as a primary source of illumination.
- An EM uses the same principles of an optical microscope but instead of photons or particles of light, it concentrates electrons, charged particles located on the outside of atoms, onto an object.
- Electron microscope is used to obtain high-resolution images of biological and non-biological specimens.
- It is used to study the structure of tissues, cells, organelles, and macromolecular complexes in detail and often used for quality control and failure analysis.
- These microscopes use electrons of up to 100,000 times shorter wavelength as compared to the visible light photons.
- Electron microscope a higher resolving power than light microscopes.
- Instead of glass lenses, EM uses shaped magnetic fields to form an electron-optical lens.
- In 1931, a physicist and electrical engineer, Ernst Ruska, and Max Knoll discovered the first transmission electron microscope.
- All electron microscopes use a coil of wire wrapped around the outside of a tube (commonly referred to as a solenoid) as an electromagnetic and/or electrostatic lenses.
- The resolution in Em depends on the wavelength of radiation, Smaller the radiation, greater will be the resolution.
Working Principle of Electron Microscope
Electrons are referred to as subatomic particles orbit around the atomic nucleus. When atoms of a metal are excited by the external heat energy, electrons started to fly off from the atom.
In the electron microscopes, a tungsten metal is heated with the help of a high voltage current, electrons form a continuous stream, which is used like a light beam. The lenses used in EM are magnetic coils capable of focusing the electron beam on the specimen and illuminating it.
The strength of the magnetic lens depends upon the current that flows through it. Greater the flow of the current, the greater will be the strength of the magnetic field. The electron beam cannot pass through the glass lens.
Mechanism of Electron Microscope
- The specimen is prepared.
- Now place the prepared specimen in the vacuum chamber.
- Heat the electron gun to release beams of electron.
- Use of coiled electromagnets instead of glass lenses to point the electron beam over the specimen.
- After passing through the specimen the electron beam creates an image or electron micrograph, which is then viewed on a fluorescent screen rather than an eyepiece.
Parts of an Electron Microscope
Electron Microscopes contain these following parts;
1. Electron Source
- EM consist of an electron gun as a main source of electron.
- Electron gun is a heated tungsten filament, which emitted electron beams.
2. Electromagnetic lense System
This system allows electrons within a smell energy range to pass through, so that the electrons in the electron beam will have a well-defined energy. In an electron microscope, different types of lenses are used such as;
- Condenser lens: It focuses the electron beam on the specimen.
- objective lens: Electron beam after coming out from the specimen is passed through a second of magnetic coils called the objective lens. It helps in the formation of an intermediate magnified image.
- Projector (ocular) lenses: It helps in the formation of a final further magnified image.
3. Sample Holder or Specimen Holder
It is a platform equipped with a mechanical arm for holding the specimen and controlling its position.
4. Image viewing and Recording System
The final image of the specimen is projected on a fluorescent screen. This image is recorded by a camera, which is located below the screen.
Classification of Electron Microscope
Electron Microscopes are divided into three classes;
1. Transmission Electron Microscope (TEM)
- Transmission Electron Microscopes uses a high voltage electron beam to illuminate the specimen and create an image.
- These electron beams are produced by an electron gun, which is consists of a tungsten filament called Cathode.
- Then the electron beam is accelerated by an anode typically at +100 keV (40 to 400 keV).
- The electron beam is focused on the specimen by using an electrostatic and electromagnetic lense.
- Transmitted through the specimen that is in part transparent to electrons and in part scatters them out of the beam.
- The transmitted electron beams are now magnified by the objective lens system of the microscope.
- Now the magnified electron image is projected onto a fluorescent viewing screen to see the clear and magnified image of the specimen.
- The screen is coated with a phosphor or scintillator material such as zinc sulfide.
- The image is recorded with a digital camera.
2. Scanning electron microscope
- The heated electron gun releases a narrow electron beam.
- Now, the Specimen is exposed to these narrow electron beams.
- These narrow electron beams will rapidly move over or scans the surface of the specimen.
- As a result, this will emit a shower of secondary electrons and other types of radiations from the outer surface of specimen.
- The intensity of these secondary electrons depends upon the shape and the chemical composition of the irradiated object.
- These secondary electrons are collected by a detector, which helps to generate electronic signals.
- These electronic signals are now scanned in the manner of a television system to produce an image on a cathode ray tube (CRT).
- The image is recorded by capturing it from the CRT.
- The Modern variants of Scanning electron microscopes have the facility to record the photograph by digital camera. This microscope is used to observe the surface structure of microscopic objects.
3. Scanning and Transmission Electron Microscope (STEM)
- This electron microscope has both functions of a Scanning electron microscopes and Transmission Electron Microscopes.
4. Other types of Electron Microscope
Except, these two types of Electron Microscopes, there are also present;
a. Reflection electron microscope (REM)
In this microscope, an electron beam is incident on a surface of specimen, and the reflected beam of elastically scattered electrons is detected.
This technique is typically coupled with reflection high energy electron diffraction (RHEED) and reflection high-energy loss spectroscopy (RHELS).
b. Scanning transmission electron microscope (STEM)
The STEM rasters a focused incident probe across a specimen that (as with the TEM) has been thinned to facilitate the detection of electrons scattered through the specimen.
c. Scanning tunneling microscopy (STM)
In this microscope, a conductive tip held at a voltage is brought near a surface, and a profile can be obtained based on the tunneling probability of an electron from the tip to the sample since it is a function of distance.
Application of Electron Microscopes
- In industry, it is used for quality control and failure analysis.
- Used to study the structure of different living and non-ling materials.
- In microbiology, it is used to study of microorganisms like bacteria, viruses, etc.
- Used to detect the fault or weakness of a building material.
- To study the Crystal structure of material.
- Adsorption of particle on surfaces.
Advantages of Electron Microscopes
- Electron Microscopes can Magnifies objects over 500,000x times.
- Material rarely distorted by preparation.
- EM allows us to investigate a greater depth of field.
- It Can resolve objects up to 200nm.
Disadvantages of Electron Microscopes
- Electron Microscopes are expensive.
- Live specimens cannot be observed.
- EMs are very large in size and must be operated in special rooms.
- It is Expensive to operate.
- As the penetration power of the electron beam is very low, the specimen should be ultra-thin.
- All images forms in EM, are in black and white.
- A high vacuum is required for viewing the living material.
- Required expertise to handle it.
- Affected by magnetic fields.