The advantages of electron microscopy and optical microscopy
A sub microscope is an instrument that uses electron beams and lenses instead of light beams and optical lenses based on the principles of electron optics, to image the fine structures of matter at very high magnifications.
The resolution of an electron microscope is represented by the small distance between adjacent two points that it can distinguish. In the 1970s, the resolution of transmission electron microscopy was about 0.3 nanometers (the human eye's resolution was 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. Therefore, electron microscopes can directly observe the neatly arranged atomic lattice in the atoms and crystals of certain heavy metals.
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 by more than ten times, confirming the possibility of electron microscope 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. As a result, electron microscopes began to receive attention.
In the 1940s, Hill in the United States compensated for the rotational asymmetry of electron lenses with an astigmatizer, resulting in a new breakthrough in the resolution of electron microscopes and gradually reaching modern levels. In China, a transmission electron microscope with a resolution of 3 nanometers was successfully developed in 1958. In 1979, a large electron microscope with a resolution of 0.3 nanometers was also developed.
Although the resolution of electron microscopes is far superior to that of optical microscopes, they are difficult to observe living organisms due to the need to work under vacuum conditions, and electron beam irradiation can also cause radiation damage to biological samples. Other issues, such as the improvement of electron gun brightness and electron lens quality, also require 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 50-100 kV, the wavelength of the electron beam is about 0.0053-0.0037 nanometers. 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 much better than that of an optical microscope.
The electron microscope consists of three parts: a tube, a vacuum system, and a power cabinet. The mirror barrel mainly consists of components such as an electron gun, electron lens, sample holder, fluorescent screen, and photography mechanism, which are usually assembled into a cylinder from top to bottom; The vacuum system consists of a mechanical vacuum pump, a diffusion pump, and a vacuum valve, which are connected to the mirror tube through an extraction pipeline; The power cabinet consists of a high-voltage generator, excitation current stabilizer, and various regulating and control units.
A sub lens is an important component in the tube of an electron microscope. It uses a spatial electric or magnetic field symmetrical to the axis of the tube to bend the electron trajectory towards the axis, forming a focus. Its function is similar to that of a glass convex lens to focus the light beam, so it is called an electron lens. Most modern electron microscopes use electromagnetic lenses, which focus electrons with a strong magnetic field generated by a stable DC excitation current passing through a coil with pole shoes.
An electron gun is a component composed of a tungsten wire hot cathode, gate, and cathode. It can emit and form electron beams with uniform velocity, so the stability of the acceleration voltage is required to be no less than one thousandth.
