Advantages of Electron Microscope and Optical Microscope
Electron microscope is an instrument based on the principles of electronic optics, which uses electron beams and lenses instead of light beams and optical lenses to image the fine structures of matter at very high magnification.
The resolution ability of an electron microscope is represented by the small distance between adjacent two points it can distinguish. In the 1970s, the resolution of a transmission electron microscope was about 0.3 nanometers (the resolution of the human eye 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 of certain heavy metal atoms and crystals.
Although electron microscopes have much better resolution than optical microscopes, they are difficult to observe living organisms due to their 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 need to be further studied.
Resolution is an important indicator of electron microscopy, which is related to the cone angle and wavelength of the electron beam passing through the sample. The wavelength of visible light is approximately 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 approximately 0.0053-0.0037 nanometers. Due to the fact that 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 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 mirror tube, a vacuum system, and a power cabinet. The lens barrel mainly consists of electronic guns, electronic lenses, sample racks, fluorescent screens, and camera mechanisms, which are usually assembled into a column from top to bottom; The vacuum system is composed of mechanical vacuum pumps, diffusion pumps, and vacuum valves, and is connected to the mirror tube through an air extraction pipeline; The power cabinet is composed of a high-voltage generator, excitation current stabilizer, and various regulating and control units.
An electron 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 beam of light, so it is called an electron lens. Most modern electron microscopes use electromagnetic lenses, which focus electrons through 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, a gate, and a 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.
The electron beam of a scanning electron microscope does not pass through the sample and only scans and excites secondary electrons on the surface of the sample. The scintillation crystal placed beside the sample receives these secondary electrons and modulates the electron beam intensity of the picture tube after amplification, thereby changing the brightness of the picture tube fluorescent screen. The deflection coil of the picture tube is synchronized with the electron beam on the sample surface for scanning, so that the fluorescent screen of the picture tube displays the morphology image of the sample surface, which is similar to the working principle of industrial television.
The resolution of a scanning electron microscope mainly depends on the diameter of the electron beam on the surface of the sample. The magnification is the ratio of the scanning amplitude on the imaging tube to the scanning amplitude on the sample, which can continuously change from tens of times to hundreds of thousands of times. Scanning electron microscopy does not require very thin samples; Images have a strong sense of stereoscopy; It can analyze the composition of substances using information such as secondary electrons, absorbed electrons, and X-rays generated by the interaction between electron beams and substances.
The electron gun and condenser of a scanning electron microscope are roughly the same as those of a transmission electron microscope, but in order to make the electron beam finer, an objective lens and an astigmatizer are added under the condenser, and two sets of mutually perpendicular scanning coils are also installed inside the objective lens. The sample chamber below the objective lens is equipped with a sample stage that can be moved, rotated, and tilted.
