Advantages of Electron Microscopes vs. Optical Microscopes
Electron microscope is an instrument based on the principle of electron optics, which uses electron beams and electron lenses instead of beams and optical lenses to image the fine structures of matter at very high magnification.
The resolution of an electron microscope is represented by the small distance between adjacent points it can distinguish. In the 1970s, the resolution of transmission electron microscopes was about 0.3 nanometers (the human eye's resolution is about 0.1 millimeters). Nowadays, electron microscopes have a magnification of over 3 million times, while optical microscopes have a magnification of about 2000 times, so it is possible to directly observe the atoms of certain heavy metals and the neatly arranged atomic lattice in crystals through electron microscopes.
In 1931, Knorr and Ruska from Germany modified a high-voltage oscilloscope with a cold cathode discharge electron source and three electron lenses, and obtained images magnified more than ten times, confirming the possibility of electron microscopy for magnification imaging. In 1932, with the improvement of Ruska, the resolution of electron microscopes reached 50 nanometers, which was about ten times the resolution of optical microscopes at that time. Therefore, electron microscopes began to receive attention from people.
In the 1940s, Hill in the United States used a defogger to compensate for the rotational asymmetry of electron lenses, which led to a new breakthrough in the resolution of electron microscopes and gradually reached modern levels. In China, a transmission electron microscope with a resolution of 3 nanometers was successfully developed in 1958, and a large electron microscope with a resolution of 0.3 nanometers was developed in 1979.
Although the resolution of electron microscopes has far exceeded that of optical microscopes, they are difficult to observe living organisms due to the need to work under vacuum conditions, and the irradiation of electron beams can also cause radiation damage to biological samples. Other issues, such as improving the brightness of the electron gun and the quality of the electron lens, also need further research.
Resolution is an important indicator of electron microscopy, 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 300-700 nanometers, while the wavelength of the electron beam is related to the acceleration voltage. When the acceleration voltage is between 50-100 kV, the wavelength of the electron beam is approximately 0.0053-0.0037 nm. Due to the fact that the wavelength of the electron beam is much smaller than that of visible light, even if the cone angle of the electron beam is only 1% of that of an optical microscope, the resolution of the electron microscope is still far superior to that of an optical microscope.
An electron microscope consists of three parts: a tube, a vacuum system, and a power cabinet. The main components of the lens barrel include an electron gun, an electron lens, a sample holder, a fluorescent screen, and a camera mechanism, which are usually assembled into a cylindrical body from top to bottom; The vacuum system consists of a mechanical vacuum pump, a diffusion pump, and a vacuum valve, and is connected to the cylinder through an exhaust pipeline; The power cabinet consists of a high-voltage generator, an excitation current stabilizer, and various regulating and control units.
