Introduction:
An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination.
Ernst Ruska, a German engineer and academic professor, built the first Electron Microscope in 1931.
It is a special type of microscope having a high resolution of images.
Working Principle:
The electron gun generates electrons.
Two sets of condenser lenses focus the electron beam on the specimen and then into a thin tight beam.
To move electrons down the column, an accelerating voltage (mostly between 100 kV-1000 kV) is applied between tungsten filament and anode.
The specimen to be examined is made extremely thin, at least 200 times thinner than those used in the optical microscope.
The electronic beam passes through the specimen and electrons are scattered depending upon the thickness or refractive index of different parts of the specimen.
The denser regions in the specimen scatter more electrons and therefore appear darker in the image since fewer electrons strike that area of the screen. In contrast, transparent regions are brighter.
The electron beam coming out of the specimen passes to the objective lens, which has high power and forms the intermediate magnified image.
The ocular lenses then produce the final further magnified image.
Types of Electron microscope:
There are two types of electron microscopes,
The transmission electron microscope (TEM)
The scanning electron microscope (SEM).
The transmission electron microscope (TEM):
The transmission electron microscope is used to view thin specimens through which electrons can pass generating a projection image.
The TEM is analogous in many ways to the conventional (compound) light microscope.
TEM is used, among other things, to image the interior of cells (in thin sections), the structure of protein molecules, the organization of molecules in viruses etc.
The scanning electron microscope (SEM):
Conventional scanning electron microscopy depends on the emission of secondary electrons from the surface of a specimen.
It provides detailed images of the surfaces of cells and whole organisms that are not possible by TEM.
It can also be used for particle counting and size determination, and for process control.
It is termed a scanning electron microscope because the image is formed by scanning a focused electron beam onto the surface of the specimen in a raster pattern.
Parts of Electron microscope:
EM is in the form of a tall vacuum column which is vertically mounted. It has the following components:
Electron gun:
Heated tungsten filament which releases electrons.
Electromagnetic lenses:
Condenser lens focuses the electron beam on the specimen. A second condenser lens forms the electrons into a thin tight beam.
The electron beam coming out of the specimen passes down the second of magnetic coils called the objective lens, which has high power and forms the intermediate magnified image.
The third set of magnetic lenses called projector (ocular) lenses produce the final further magnified image.
Each of these lenses acts as an image magnifier all the while maintaining an incredible level of detail and resolution.
Specimen Holder:
The specimen holder is an extremely thin film of carbon or collodion held by a metal grid.
Image viewing and Recording System:
The final image is projected on a fluorescent screen.
Below the fluorescent screen is a camera for recording the image.
Applications:
Very high magnification
Incredibly high resolution
Material rarely distorted by preparation
It is possible to investigate a greater depth of field
Limitations:
The live specimen cannot be observed.
As the penetration power of the electron beam is very low, the object should be ultra-thin.
As the EM works in a vacuum, the specimen should be completely dry.
Expensive to build and maintain
Requiring researcher training.
This type of microscope is a large, extremely sensitive to vibration and external magnetic fields.