Friday, 20 September 2019

Scanning Electron Microscope

Scanning Electron Microscope

Scanning Electron Microscope


Scanning electron microscope (SEM) permits the direct study of biological ultrastructure owing to higher magnification and resolving power much greater than that of the light microscope. It is used when greatest resolution is required. In scanning electron microscope the image of the specimen’s surface is produced by secondary electrons reflected back from the specimen. SEM can be used in the field of material science, agriculture, medicine, forensic science, etc.

Scanning Electron Microscope
Fig.: Scanning Electron Microscope

Electron Gun

It is equivalent to the source of light as it thermionically emits a beam of electrons. Tungsten filament is normally used for this process. Tungsten emits negatively charged electrons when a high accelerating voltage (40,000 – 100,000 V) is passed between cathode and anode, that passes through a hole in anode forming a beam.


Anode


It is a metal plate which has a positive charge. It attracts the negatively charged electrons towards itself to form an electron beam.

Magnetic Lens System 

The beam of electrons passes through a stack of electromagnetic lenses constituting its lens system. It is made up of a roll of wire that carries electricity and creates a magnetic force that directs the path of the electron beam.

Deflector Coils

These are the roles of wire that face each other to make a magnetic field that pushes beam from side to side. This is the scanning part of SEM.

Electron Detector

It attracts and collects the electrons that bounce off the sample and turns them into an electrical current.

Sample Stage

This is the place where the sample is placed.

Sample Chamber

It refers to the container at the end of the column. This evacuated chamber must be very sturdy and insulated. It may also include tilt and rotation devices, feed through to the outside, temperature stages, optical cameras, and various other devices to assist in imaging the sample.

Controller

Knobs, joysticks, tracker balls, and keyboards are used to move the sample to achieve the best image.

Computer

Electron detector is attached to a computer that measures the electrical current from the detector and turns them into an image. Depending on the height and shape of the sample, at each step, we get black, grey and white dots that create an image on the screen. Black happens when no electrons hit the detector. White is when all the electrons hit the detector and grey is in between black and white.

Sample Preparation


It is one of the most important steps in SEM. The sample is fixed in glutaraldehyde, and dehydrated through a series of solvents and dried completely to avoid surface tension during the process.

Biological samples are made conductive by covering them with a thin layer of a conductive material (usually gold) using a process called sputter coating.

It is finally mounted onto a special metal holder or stub in the sample stage.

Working Mechanism


Cloud of the electron is emitted from an electron gun. Anode pulls them away from the wire and speeds them into a stream (electron beam). The magnetic lens bends the beam of electrons over the sample in the same way as the glass lens bends a beam of light.

A beam of electrons hits the sample while some of the electrons bounce back. But since the sample is made up of atoms, some new electrons are also emitted. Deflectors push the beam back and forth across the surface of the sample. These emitted electrons are attracted by detectors that build up an image by mapping the detected signals with beam position and turns current into an image on a screen.


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