Electronic microscope

1964 electronic microscope.

An electron microscope uses electrons instead of photons or visible light to form images of tiny objects. Electron microscopes allow higher magnifications to be achieved. before the best light microscopes, because the wavelength of the electrons is much shorter than that of the photons.

The first electron microscope was designed by Ernst Ruska and Max Knoll between 1925 and 1932, who were based on Louis-Victor de Broglie's studies of the wave properties of electrons.

A scanning transmission electron microscope has achieved resolution greater than 50 pm in dark field ring imaging mode and magnification up to approximately 10,000,000× while most light microscopes are diffraction limited to a resolution of approximately 200 nm and useful magnifications below 2000×. Electron microscopes use shaped magnetic fields to form electronic optical lens systems that are analogous to the glass lenses of an optical light microscope.

Electron microscopes are used to investigate the ultrastructure of a wide range of biological and inorganic specimens, including microorganisms, cells, large molecules, biopsy samples, metals, and crystals. Industrially, electron microscopes are often used for quality control and failure analysis. Modern electron microscopes produce electron micrographs using specialized digital cameras and frame grabbers to capture the images.

Limitations of the Electron Microscope

• The limited opening does not allow detailed information to reach the image, thus limiting the resolution.
• The contrast of amplitude (which lies in the corpuscular nature of electrons) is due to the contrast of diffraction, caused by the loss of electrons of lightning. It is a dominant contrast in thick specimens.
• The phase contrast (which lies in the undulating nature of the electrons) is due to the contrast of interference caused by the displacements in the relative phases of the portions of lightning. It is a dominant contrast in fine specimens.
• There are also different aberrations produced by lenses: astigmatic, spherical and chromatic.
• The problem of the contrast transfer function (CTF): the FTC describes the response of an optical system to a broken image in quadratic waves.

Biological material presents two fundamental problems: the vacuum environment and energy transfer. To solve them, different techniques are used depending on the size of the sample:

• for large samples such as organs, tissues or cells, three techniques are used:

1. chemical fixing or Cryophylation;
2. inclusion in resins (criosustitution);
3. the metal replica.

• for small samples such as macromolecule complexes are used the following techniques:

1. the negative stain: the most used tintion agents are amonic molybdate, sodium phosphotungstat and uranium salts as acetate and formiate. All of them have the following properties: they interact minimally with the sample and are stable in interaction with electrons, are highly soluble in water, present a high density that favors the contrast, have a high melting point, have a small grain size;
2. the Metallic mirror: to build the metal replica the metal (size) is evaporated, which is deposited on the sample at the same time it is dissolved by the vacuum;
3. the cryomicroscopy. Since the 1980s, scientists have also increasingly used cryoprojection analysis and vitrified samples, which further confirms the validity of this technique.

Types of electron microscopes

(1) housing, (2) electron emitter, (3) electrons, (4) cathode, (5) anode, (6) Condensing lens, (7) sample analyzed, (8) Target lens, (9) Lente projectr, (10) Detector (sensor or photographic film).

There are two main types of electron microscopes: the transmission electron microscope and the scanning electron microscope.

Transmission Electron Microscope (TEM)

The transmission electron microscope (TEM) emits a beam of electrons directed at the object whose image is to be enlarged. A portion of the electrons bounce off the sample, thus forming a magnified image. To use a transmission electron microscope, the sample must be cut into thin layers, no larger than about 2000 angstroms. Transmission electron microscopes can magnify an object's image up to a million times.

Scanning Electron Microscope (SEM)

Image of an ant taken with a MEB (electronic sweep microscope).

In the scanning electron microscope (SEM) the sample is coated with a thin layer of metal, and is scanned with electrons sent from a gun. A detector measures the amount of electrons sent that shows the intensity of the sample area, being able to show figures in three dimensions, projected on a TV image. Its resolution is between 3 and 20 nm, depending on the microscope. It allows to obtain high resolution images in stone, metallic and organic materials. The light is replaced by an electron beam, the lenses by electromagnets, and the samples are made conductive by metallizing the surface. Relying on the work of Max Knoll from the 1930s, it was Manfred von Ardenne who managed to invent the SEM in 1937, which consisted of a beam of electrons that swept the surface of the sample to be analyzed, which, in response, re-emitted some particles. These particles are analyzed by different sensors that make it possible to reconstruct a three-dimensional image of the surface.

Other types of electron microscopes

• Electronic Reflection Microscope (REM)
• Tunnel effect microscope (STM)
• Barrido probe microscope (SPM)
• High-resolution electronic microscopy (HRTEM)

Applications in different areas

In the study of integrated circuits, the electron microscope is usually used due to a curious property: As the electric field modifies the trajectory of the electrons, in an integrated circuit in operation, seen under the electron microscope, you can appreciate the potential at which each element of the circuit is.

Electron crystallography is a method used to determine the arrangement of atoms in solids through a transmission electron microscope. This method is used in many situations where X-ray crystallography cannot be used and was invented by Aaron Klug.