Electron Microscopy

An electron microscope used a beam of electrons to create an image of the specimen. It is capable of higher magnification and greater resolving power allowing it to see much smaller objects with finer details.

In the case of an electron microscope, the beam of electrons is used instead of light rays to produce a magnified image of the object. The source of illumination for the light microscope is either a light bulb or natural light while in the case of electron microscopy the use of tungsten filament of approximately 80kw is commonly seen.

The electron microscope is very useful for understanding the ultrastructure of the virus, various plant and animals cells. In the case of an electron microscope, the image which is formed is not observed by the eye but is projected on a screen.

Electron microscopy can be divided into two main categories:

1.) Scanning Electron Microscopy (SEM).

2.) Transmission Electron Microscopy (TEM).

Scanning Electron Microscope [SEM]

All microscopes described till now use light coming from the specimen to produce a magnified image. The SEM is different from another microscope. SEM is a type of microscope that uses electrons instead of light to form an image.

Components of SEM

• Electron gun
• Electron lenses
• Sample stage
• Detectors for all signals
• Display/data output devices
• Infrastructure requirement
• Power supply
• Vacuum system
• Cooling system
• Vibration-free floor
• A room free of the electrical and magnified field

The SEM always has at least one detector and mostly has an additional detector also. The specific capabilities of a particular instrument are critically dependent upon which type of detector it accommodates.

Working Principle

• The image formed in the case of a SEM is done by aiming a beam of electron onto the specimens and studying these electron emissions on a closed television circuit.

• The beam of an electron is emitted from a hot tungsten filament and is focused by electromagnets onto the surface of the specimen.

• This primary electron beam causes an emission of electrons known as secondary electrons, from the elements that make up the upper layer of the specimen.

• Moreover, about 20-30% of the primary electrons rebound on the surface, these electrons are known as backscattered electrons.

• The emitted electron is collected and the amplified signal is displayed on a cathode ray tube.

• By scanning the primary electron across the specimen surface in a synchronization cathode ray tube it is possible to convert the emitted electron into the form of an image of the specimen for display on the cathode ray tube.

A SEM can magnify an image up to approximately 200,000times. Some of the SEM also has the capability of measuring the x-rays that are emitted from the sample. This technique is called energy dispersive x-ray.

Uses

• This microscope has high magnification, high resolution, and great depth of focus.

• SEM has also been applied for the use of x-ray production to determine the elemental composition of a particular specimen.

• SEM is to determine whether a suspect has recently fired using a gun and an attempt is made to remove any gun shot particles.

Transmission Electron Microscope [TEM]

It is a technique where an electron beam interacts and passes through the specimen.

The various components of the TEM include:

• A detection source
• Electron beam
• Electromagnetic lenses
• Vacuum chamber
• Condenser
• Sample stage
• Fluorescent or phosphor screen
• A computer system or a digital system.

Working

• The electron is emitted by an electron source and is focused and magnified by a system of magnetic lenses.

• The electron beam is confined by two condenser lenses which control the brightness of the beam.

• This beam passes the condenser apparatus and hits the sample surface.

• The electron that is elastically scattered consists of various transmitted beams which pass through the objective lens.

• The objective lens forms the image and the following two aperture that is the objective aperture and the selected area aperture are used for choosing the elastically scattered electrons that will form the image for the microscope.

• Finally, the beam goes to the magnifying system consisting of three lenses.

• The first and the second intermediate lens and the projector lens.

• The final image formed is shown either on a fluorescent screen or a monitor or both and it is printed on a photographic film.

Uses

• It provides topographical, morphological, compositional, and crystalline information.

• It helps to view samples on a molecular level making it possible to study the structure and texture.

• It is useful in analyzing and studying the crystal and metal structures.

• It can be applied to semi-conductor analysis to even micro sized objects.

• It can be used by various educational institutions for research and training.