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In this article we will discuss about:- 1. Principle of Transmission Electron Microscopy 2. Components of Transmission Electron Microscopy 3. Applications.
Principle of Transmission Electron Microscopy:
The transmission electron microscope can be compared with a slide projector. In a slide projector, light from a light source is made into a parallel beam by the condenser lens; this passes through the slide (object) and-is then focused as an enlarged image onto the screen by the objective lens.
In the electron microscope, the light source is replaced by an electron source, the glass lenses are replaced by magnetic lenses, and the projection screen is replaced by a fluorescent screen that emits light when struck by electrons or more frequently in modern instruments, an electronic imaging device such as a CCD (charge-coupled device) camera.
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The whole trajectory from source to screen is under vacuum and the specimen (object) has to be very thin to allow the electrons to travel through it. Not all specimens can be made thin enough for the TEM. A scanning electron microscope may be used to look at the surface of the specimen, rather than a projection through it.
Components of Transmission Electron Microscopy:
A TEM is composed of the following:
1. Electron Gun:
The TEM consists of an electron emission source, which may be a tungsten filament or a lanthanum hexaboride (LaB6) source, known as electron gun. By connecting this gun to a high-voltage source of about 100 – 300 kV, the gun begins to emit electrons by either thermionic or field electron emission into the vacuum.
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2. Vacuum System:
Electrons behave like light only when they are manipulated in vacuum. Different vacuum pumps are used to obtain and maintain vacuum of pressure as low as 10–8 Pa. A number of airlocks and separation valves are used to avoid need to evacuate the whole column every time a specimen or photographic material or a filament is exchanged.
3. Electromagnetic Lenses:
The electromagnetic lenses typically consist of coils, and when an electric current is passed through these coils, an electromagnetic field is created between the pole pieces creating a gap in the magnetic circuit. By varying the current through the coils, the strength of the field, and thereby the power of the lens, can be varied.
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Typically, a TEM consists of three stages of lensing – condenser lenses, objective lenses, and projector lenses. The condenser lenses are responsible for primary beam formation, whereas the objective lenses focus the beam that comes through the sample itself.
The projector lenses are used to expand the beam onto the phosphor screen or other imaging device such as film. The magnification of the TEM is due to the ratio of the distances between the specimen and the objective lens’ image plane.
4. Specimen Stage:
The specimen is placed on a mesh having a diameter of about 2.5 mm and is held by a specimen holder. The mesh or grid is made of copper, molybdenum, gold, or platinum. The specimen stage also includes airlocks to allow for the insertion of the specimen holder into the vacuum with a minimal increase in pressure in other areas of the microscope.
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Sample preparation includes the preparation of thin sections of the maximum thickness of a few tens or hundreds of nanometers. As TEM specimen holders permit the rotation of a sample by a desired angle, multiple views of the same specimen can be obtained by rotating the angle of the sample along an axis perpendicular to the beam. By taking multiple images of a single TEM sample at differing angles, typically in 1° increments, a set of images known as a “tilt series” can be collected.
5. Imaging Devices:
Originally, TEMs used a fluorescent screen that emitted light when impacted by the transmitted electrons, for real-time imaging and adjustments; and a film camera to record permanent, high-resolution images. Modern instruments rely primarily on solid-state imaging devices, such as a CCD (charge-coupled device) camera, for image capture. They may still include a fluorescent screen, but it may be observed by a video camera.
Applications of Transmission Electron Microscopy:
A TEM can be used in any branch of science and technology where it is desired to study the internal structure of specimens down to the atomic level.
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