Advantages of electron microscopy in comparison to optical microscopy
Although the resolution of electron microscope is far better than that of optical microscope, it is difficult to observe living organisms because it needs to work under vacuum condition, and the irradiation of electron beam will make the biological samples suffer from irradiation damage. Other problems, such as the brightness of the electron gun and the improvement of the quality of the electron lens also need to continue to study.
The resolving power is an important index of the electron microscope, which is related to the cone angle of incidence and wavelength of the electron beam 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 kV, the wavelength of the electron beam is about 0.0053 to 0.0037 nanometers. As the wavelength of the electron beam is much smaller than the wavelength of visible light, so even if the cone angle of the electron beam is only 1% of the optical microscope, the resolution capability of the electron microscope is still far better than the optical microscope.
Electron microscope consists of three parts: mirror tube, vacuum system and power supply cabinet. The barrel mainly has an electron gun, electron lens, sample holder, fluorescent screen and camera mechanism and other components, these components are usually assembled from top to bottom into a column; vacuum system consists of a mechanical vacuum pump, diffusion pumps and vacuum valves, etc., and through the pumping pipeline connected to the barrel of the mirror; power supply cabinet consists of a high-voltage generator, the excitation current stabilizer and a variety of regulatory control units.
Electron lens is an important part of the electron microscope barrel, it is symmetrical to the axis of the barrel of the space electric field or magnetic field so that the electron track to the axis of the formation of the focusing of the role of the glass convex lens to make the role of the beam of light focusing is similar to the role of the lens, so it is called an electron lens. Most modern electron microscopes use electromagnetic lenses, by a very stable DC excitation current through the coil with a pole shoe generated by the strong magnetic field to focus the electrons.
The electron gun is a component consisting of a tungsten hot cathode, a gate and a cathode. It emits and forms an electron beam with uniform velocity, so the stability of the accelerating voltage is required to be not less than one part in ten thousand.
Electron microscopes can be categorized into transmission electron microscopes, scanning electron microscopes, reflection electron microscopes and emission electron microscopes according to their structure and use. Transmission electron microscope is often used to observe those with ordinary microscopes can not distinguish the fine structure of the material; scanning electron microscope is mainly used to observe the morphology of solid surfaces, but also with the X-ray diffractometer or electron spectrometer combined to form an electron microprobe, used for the analysis of the material composition; emission electron microscope for the study of the surface of the self-emission of electrons.
The electron beam of a scanning electron microscope does not pass through the sample, but only scans the surface of the sample to excite secondary electrons. A scintillation crystal placed next to the sample receives these secondary electrons, which are amplified to modulate the intensity of the electron beam of the CRT, thus changing the brightness on the CRT's fluorescent screen. The deflection coil of the CRT is synchronized with the electron beam on the surface of the sample, so that the CRT's fluorescent screen displays a topographical image of the sample surface, which is similar to the working principle of industrial television sets.
The resolution of a scanning electron microscope is mainly determined by the diameter of the electron beam on the sample surface. The magnification is the ratio of the scanning amplitude on the tube to the scanning amplitude on the sample, which can be varied continuously from tens to hundreds of thousands of times. Scanning electron microscopes do not require very thin samples; the image has a strong sense of three-dimensionality; and can analyze the composition of a substance by using information such as secondary electrons, absorbed electrons, and X-rays generated by the interaction of the electron beam with the substance.
Scanning electron microscope electron gun and spotting mirror and transmission electron microscope is more or less the same, but in order to make the electron beam is more fine, in the spotting mirror under the objective lens is added and the disperser, in the objective lens is also equipped with two sets of perpendicular to each other inside the scanning coil. The sample chamber under the objective lens is equipped with a sample stage that can be moved, rotated and tilted.
