What Is an Electron Microscope?
An electron microscope is a microscope that illuminates samples with an electron beam. Due to the extremely short wavelength of the electron beam, it is possible to visualize ultrafine structures that cannot be observed with an optical microscope. There are two main types of electron microscopes: those that output the transmittance of the electron beam as an image, and those that render the signal produced by the interaction between the electron beam and the sample.
Most electron microscopes sold on the market are optimized for industrial materials and for observing biological specimens. The term electron microscope is often abbreviated as “EM.”
Uses of Electron Microscope
In the industrial field, electron microscopes are used to analyze the fractured surface of damaged metal parts to determine the cause of the damage, or to observe the surface of a processed part to check its quality. They are also used to examine the instrumental properties of macromolecular polymers by observing their networks, and to evaluate the presence of impurities. In the life science field, they are used to visualize the microstructure of intracellular organelles and to map the connections between neurons by observing intricately entangled nerve cells. The electron microscope was also awarded the 2017 Nobel Prize in chemistry for its potential application in the structural analysis of proteins by performing simple pretreatments on samples.
Principles of Electron Microscopes
Electron microscopes comprise a source, a lens, and a detector, and have a configuration very similar to that of an optical microscope. However, each of these elements is very different in principle from that of an optical microscope.
First of all, electron beams are immediately attenuated and annihilated when they collide with molecules and other substances in the air. Therefore, electron beams must be generated and emitted in a vacuum.
Second, since glass lenses such as those used in general optical systems are transparent, magnetic lenses that converge by applying a magnetic field must be used to refract the electron beams.
Such lenses have large optical aberrations, so they are designed with a small numerical aperture to mitigate this issue. This allows electron microscopes to have a deep depth of focus and to observe objects in three dimensions with great depth.
Standard electron microscopes are classified into two categories:
1. Transmission Electron Microscopes (TEM)
In these microscopes, an electron beam is transmitted through the sample, and contrast is obtained based on its attenuation. The sample must be very thin for the electron beam to be able to penetrate it. The strength of the electron beam is called the acceleration voltage. At an acceleration voltage of 300 kV, the wavelength is 0.00197 nm, which is extremely short, and the resolution is 0.1 nm, which is on the order of the size of the original material. The resolution is 0.1 nm, which is on the order of the size of the original material. This is 800,000 times higher than the resolution of an optical microscope. Transmission electron microscopes are excellent for observing the internal structure of a sample, such as a crystalline structure in a very compact area, because they observe the electrons transmitted through the sample.
Scanning Electron Microscope (SEM)
When materials are illuminated with electron beams in a vacuum, secondary electrons, reflected electrons, and characteristic X-rays are emitted. Scanning electron microscope images are formed from secondary electrons and reflected electron signals by scanning a spatially focused electron beam. Secondary electrons are generated near the surface of the specimen, making the secondary electron image suitable for viewing microscopic irregularities in the specimen. The number of reflected electrons depends on the composition of the sample (atomic number, crystal orientation, etc.), making the reflected electron image suitable for evaluating the compositional distribution of the sample surface.
When an electron beam strikes a sample, the atoms that make up the surface are excited and emit electrons. Other emissions include reflected electrons and characteristic X-rays, which are called secondary electrons, and are obtained by point-scanning the intensity of the emitted secondary electrons.
Things That Can Only Be Observed With Electron Microscopes
Electron microscopes have extremely high resolution compared to ordinary optical microscopes, facilitating, for example, the observation of microscopic tissue structures such as cells and metal crystals on the order of atomic size.
Taking cells as an example, optical microscopes cannot observe in detail the minute structures other than the nucleus, but electron microscopes can. This makes it possible to investigate in detail the function of enzymes in the cell, reactions of cellular structures, and various other functions.