Electron Microscopes

What is an electron microscope?

An electron microscope is a microscope that illuminates using a beam of accelerated electrons. They are used to study and identify the structures of very small objects. Since the wavelength of light is 100,000 times larger than an electron's wavelength, light microscopes can't be used to identify certain structures.

The infrastructure of a variety of biological and inorganic samples is examined using electron microscopes. These samples include cells, biopsy samples, crystals, metals, large molecules, microorganisms, etc.

Electron microscopes are used to create electron micrographs by capturing pictures with special digital cameras and frame grabbers.

The history of the electron microscope

The first prototype of an electron microscope (which was the first practical demonstration of how an electron microscope works) was developed by Ernst Ruska in 1931. Later in the same year, Reinhold Rudenberg obtained a patent for the electron microscope.

While the first images from a prototype electron microscope were achieved in 1932 by Ernst Ruska, using the concepts of Rudenberg's patent, the first electron microscope that with greater resolution than an optical microscope was built by Ernst Ruska in 1933.

The first commercial electron microscope, however, was produced by Siemens in 1938. Reinhold Rudenberg was the scientific director at that time.

Even though electron microscopes used today are able to create two million power magnification (a microscope's capacity to generate a picture of an item at a scale that is greater or smaller than its real size), the technology remains based on Ernst Ruska's prototype.

What are the types of electron microscope?

There are two types of electron microscopes currently used today, the transmission electron microscope and the scanning electron microscope. They both have their advantages and areas of use.

The transmission electron microscope (TEM)

The transmission electron microscope (TEM) is the first original form of the electron microscope. It creates a picture by illuminating the specimen with a high-voltage electron beam and is used in a variety of fields such as nanotechnology, medical, forensic analysis, industry, education, etc.

Let's see how a transmission electron microscope works step by step.

1. A high voltage electricity source gives power to the cathode of the microscope.
2. The cathode works similar to an electron gun in cathode-rays and consists of a heated filament. It creates a beam of electrons that finds its way in the microscope just like light does in optical microscopes.
3. The first lens, which is an electromagnetic coil, converts the electrons into a more powerful beam by concentration.
4. The second lens helps the beam to focus on a particular area of the specimen.
5. The specimen is placed on a copper grid in the microscope tube. When the electron beam passes through it, it picks up an image of the specimen.
6. The third lens helps magnify the image.
7. When the electron beam reaches a fluorescent screen at the machine's base, the image appears.
8. The image can be seen through binoculars at the side or a monitor connected to an image intensifier.

The scanning electron microscope (SEM)

The scanning electron microscope (SEM) probes the specimen with a concentrated electron beam that is scanned across a rectangular region of the specimen to obtain images, and is used in quality control, failure analysis, and materials science for research.

Let's see how a scanning electron microscope works step by step.

1. Electrons are sent into the microscope tube.
2. The tube of the microscope is held in a sealed vacuum chamber, since particular electron beams can't travel well through the air.
3. The anode pulls electrons towards itself and converts them into an energetic beam of electrons.
4. An electromagnetic coil acts similarly to a lens and focuses the beam to a precise region.
5. The electron beam is steered from side to side by a lower coil.
6. The beam spreads across the object in a controlled manner.
7. The electrons in the electron beam impact the object and are reflected from the surface of the object.
8. A detector detects the reflected electrons and turns them into an image.
9. The image of the object can be viewed on a monitor.

What are the advantages and disadvantages of electron microscopes?

You have learned about some of the advantages of electron microscopes, such as magnification and higher resolution, but there are also some disadvantages.

Advantages of electron microscopes

There are several advantages of electron microscopes compared with optical microscopes.

• Electron microscopes allow us to analyse small structures that can't be analysed with optical microscopes. This is because the wavelength of electrons is 100,000 times smaller than the wavelength of light and the resolution we can get using electron microscopes is in the range of up to 0.2 nm.

• Electron microscopes have a wide range of applications, in industry, biomedical science, and for analysing microorganisms, cells, etc.

• Electron microscopes can produce high-resolution images when used correctly allowing us to see complex structures that other microscopes may not provide.

Disadvantages of electron microscopes

There are also some disadvantages of using electron microscopes:

• Electron microscopes can only produce black and white images.

• Electron microscopes are usually expensive.

• Even though the technology is improving, electron microscopes are still larger in size compared to other microscopes.

• Since the electron microscope requires the samples to be analysed in a vacuum in order to avoid electrons scattering in the air by coming up against other molecules, living specimens can't be analysed using electron microscopes.