Advantages of Electron Microscopes vs. Light Microscopes
An electron microscope is an instrument that uses electron beams and electron lenses instead of light beams and optical lenses to image the fine structures of substances at very high magnifications based on the principle of electron optics.
The resolving power of an electron microscope is represented by the small distance between two adjacent points that it can resolve. In the 1970s, transmission electron microscopes had a resolution of about 0.3 nanometers (the resolving power of the human eye is about 0.1 millimeters). Now the maximum magnification of the electron microscope exceeds 3 million times, while the maximum magnification of the optical microscope is about 2000 times, so the atoms of some heavy metals and the neatly arranged atomic lattices in the crystal can be directly observed through the electron microscope.
In 1931, Knorr-Bremse and Ruska of Germany refitted a high-voltage oscilloscope with a cold cathode discharge electron source and three electron lenses, and obtained an image magnified more than ten times, which confirmed the possibility of electron microscope magnified imaging . In 1932, after Ruska's improvement, the resolving power of the electron microscope reached 50 nanometers, about ten times the resolving power of the optical microscope at that time, so the electron microscope began to receive people's attention.
In the 1940s, Hill in the United States used an astigmatizer to compensate the rotational asymmetry of the electron lens, which made a new breakthrough in the resolving power of the electron microscope and gradually reached the modern level. In China, a transmission electron microscope was successfully developed in 1958 with a resolution of 3 nanometers, and in 1979 a large electron microscope with a resolution of 0.3 nanometers was manufactured.
Although the resolving power of the electron microscope is far superior to that of the optical microscope, it is difficult to observe living organisms because the electron microscope needs to work under vacuum conditions, and the irradiation of electron beams will 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 further studied.
Resolving power is an important index of 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 300-700 nanometers, while the wavelength of electron beams is related to the accelerating voltage. When the accelerating voltage is 50-100 kV, the electron beam wavelength is about 0.0053-0.0037 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 barrel, vacuum system and power supply cabinet. The lens barrel mainly includes electron guns, electron lenses, sample holders, fluorescent screens, and camera mechanisms. These components are usually assembled into a column from top to bottom; the vacuum system is composed of mechanical vacuum pumps, diffusion pumps, and vacuum valves. The gas pipeline is connected with the lens barrel; 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 part of the electron microscope lens barrel. It uses a space electric field or magnetic field symmetrical to the axis of the lens barrel to bend the electron track to 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 electronic lens. . Most modern electron microscopes use electromagnetic lenses, which focus electrons through a strong magnetic field generated by a very stable DC excitation current passing through a coil with pole shoes.
The electron gun is composed of tungsten hot cathode, grid and cathode. It can emit and form an electron beam with a 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 the fine material structures that cannot be resolved by 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 electronic Microprobes for material composition analysis; emission electron microscopy for the study of self-emitting electron surfaces.
The projection electron microscope is named after the electron beam penetrates the sample and then magnifies the image with the electron lens. Its optical path is similar to that of an optical microscope. In this type of electron microscope, the contrast in image detail is created by the scattering of the electron beam by the atoms of the sample. The thinner or lower-density part of the sample has less electron beam scattering, so that more electrons pass through the objective diaphragm and participate in imaging, and appear brighter in the image. Conversely, thicker or denser parts of the sample appear darker in the image. If the sample is too thick or too dense, the contrast of the image will deteriorate, or even be damaged or destroyed by absorbing the energy of the electron beam.
The top of the transmission electron microscope lens barrel is the electron gun. The electrons are emitted by the tungsten hot cathode and pass through the *, and the second two condensers focus the electron beam. After passing through the sample, the electron beam is imaged on the intermediate mirror by the objective lens, and then enlarged step by step through the intermediate mirror and projection mirror, and then imaged on the fluorescent screen or the photocoherent plate.
The magnification of the intermediate mirror can be continuously changed from tens of times to hundreds of thousands of times mainly through the adjustment of the excitation current; by changing the focal length of the intermediate mirror, electron microscopic images and electron diffraction images can be obtained on the tiny parts of the same sample . In order to study thicker metal slice samples, the French Dulos Electron Optics Laboratory developed an ultra-high voltage electron microscope with an accelerating voltage of 3500 kV. Schematic diagram of scanning electron microscope structure
The electron beam of the scanning electron microscope does not pass through the sample, but only scans and excites secondary electrons on the surface of the sample. The scintillation crystal placed next to the sample receives these secondary electrons, amplifies and modulates the electron beam intensity of the picture tube, thereby changing the brightness on the screen of the picture tube. The deflection coil of the picture tube keeps synchronous scanning with the electron beam on the surface of the sample, so that the fluorescent screen of the picture tube displays the topographic image of the sample surface, which is similar to the working principle of an industrial TV.
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 picture tube to the scanning amplitude on the sample, which can be continuously changed from tens of times to hundreds of thousands of times. The scanning electron microscope does not require a very thin sample; the image has a strong three-dimensional effect; it can use information such as secondary electrons, absorbed electrons, and X-rays generated by the interaction between the electron beam and the substance to analyze the composition of the substance.
The electron gun and condenser lens of a scanning electron microscope are roughly the same as those of a transmission electron microscope, but in order to make the electron beam thinner, an objective lens and an astigmatizer are added under the condenser lens, and two sets of mutually perpendicular scanning beams are installed inside the objective lens. coil. The sample chamber below the objective lens is equipped with a sample stage that can move, rotate and tilt.
