Advantages of electron microscopy over optical microscopy
An electron microscope is an instrument that uses electron beams and electron lenses instead of light beams and optical lenses based on the principles of electron optics to image the fine structure of matter at very high magnification.
The resolving power of an electron microscope is expressed by the small distance between two adjacent points that it can resolve. In the 1970s, the resolution of transmission electron microscopes was about 0.3 nanometers (the resolving power of the human eye is about 0.1 millimeters). Nowadays, the maximum magnification of electron microscopes exceeds 3 million times, while the maximum magnification of optical microscopes is about 2,000 times. Therefore, atoms of certain heavy metals and the neatly arranged atomic lattice in crystals can be directly observed through electron microscopes.
Although the resolving power of electron microscopes is far better than that of optical microscopes, electron microscopes need to work under vacuum conditions, so it is difficult to observe living organisms, and the irradiation of electron beams can also cause radiation damage to biological samples. Other issues, such as the improvement of the brightness of the electron gun and the quality of the electron lens, also need to be continued to be studied.
The resolving power is an important indicator of an electron microscope, which is related to the incident cone angle and wavelength of the electron beam passing through the sample. The wavelength of visible light is about 300 to 700 nanometers, and the wavelength of the electron beam is related to the accelerating voltage. When the accelerating voltage is 50 to 100 kilovolts, the electron beam wavelength is approximately 0.0053 to 0.0037 nanometers. Since the wavelength of the electron beam is much smaller than the wavelength of visible light, even if the cone angle of the electron beam is only 1% of that of the optical microscope, the resolving power of the electron microscope is still far superior to that of the optical microscope.
The electron microscope consists of three parts: lens tube, vacuum system and power cabinet. The lens barrel mainly includes components such as an electron gun, an electron lens, a sample holder, a fluorescent screen, and a camera mechanism. These components are usually assembled into a cylinder from top to bottom; the vacuum system consists of a mechanical vacuum pump, a diffusion pump, a vacuum valve, etc., and is pumped through The gas pipeline is connected to the lens tube; the power cabinet is composed of a high-voltage generator, an excitation current stabilizer and various adjustment control units.
The electron lens is an important component of the electron microscope barrel. It uses a spatial electric field or magnetic field that is symmetrical to the axis of the barrel to bend the electron trajectory toward the axis to form a focus. Its function is similar to that of a glass convex lens to focus the beam, so it is called an electron lens. . Most modern electron microscopes use electromagnetic lenses. The strong magnetic field generated by a very stable DC excitation current passing through a coil with pole shoes focuses the electrons.
The electron gun is a component composed of a tungsten filament hot cathode, a grid and a cathode. It can emit and form electron beams with uniform speed, so the stability of the accelerating voltage is required to be no less than one ten thousandth.
Electron microscopes can be divided into transmission electron microscopes, scanning electron microscopes, reflection electron microscopes and emission electron microscopes according to their structures and uses. Transmission electron microscopes are often used to observe minute material structures that cannot be distinguished with ordinary microscopes; scanning electron microscopes are mainly used to observe the morphology of solid surfaces, and can also be combined with X-ray diffractometers or electron energy spectrometers to form electron Microprobes are used for material composition analysis; emission electron microscopes are used for the study of self-emitting electron surfaces.
