Advantages of Electron Microscopy vs. Light Microscopy
Electron microscope optical microscope imaging principle similarities and differences
Electron microscope is an instrument that replaces light beam and optical lens with electron beam and electron lens according to the principle of electron optics, so that the fine structure of matter can be imaged under very high magnification.
The resolving power of an electron microscope is expressed 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 human eye has a resolving power of about 0.1 millimeters). Now the maximum magnification of electron microscope is more than 3 million times, and the maximum magnification of optical microscope is about 2000 times, so the atoms of certain heavy metals and the neatly arranged atomic lattice in crystals can be directly observed through electron microscope.
In 1931, Knorr-Bremse and Ruska in Germany modified a high-voltage oscilloscope with a cold-cathode discharge electron source and three electron lenses, and obtained a magnified image more than ten times, which confirmed the possibility of magnifying imaging by an electron microscope. . In 1932, after Ruska's improvement, the resolving power of the electron microscope reached 50 nanometers, which was about ten times the resolving power of the optical microscope at that time, so the electron microscope began to attract people's attention.
In the 1940s, Hill in the United States compensated for the rotational asymmetry of the electron lens with an astigmatist, 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 with a resolution of 3 nanometers was successfully developed in 1958, and a large-scale electron microscope with a resolution of 0.3 nanometers was made in 1979.
Although the resolving power of electron microscopes is far better than that of optical microscopes, it is difficult to observe living organisms because electron microscopes need 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.
The resolving power is an important indicator of the 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 to 700 nanometers, while the wavelength of the electron beam is related to the accelerating voltage. When the accelerating voltage is 50-100 kV, the electron beam wavelength is about 0.0053-0.0037 nm. 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 an optical microscope, the resolving power of an electron microscope is still far superior to that of an optical microscope.
The electron microscope consists of three parts: the lens tube, the vacuum system and the power supply cabinet. The lens barrel mainly includes electron gun, electron lens, sample holder, fluorescent screen and camera mechanism, which are usually assembled into a cylinder from top to bottom; the vacuum system is composed of mechanical vacuum pump, diffusion pump and vacuum valve, etc. The gas pipeline is connected with the lens barrel; the power supply cabinet is composed of a high-voltage generator, an excitation current stabilizer and various adjustment and control units.
The electron lens is an important part of the electron microscope barrel. It uses a spatial electric field or magnetic field symmetrical to the axis of the barrel to bend the electron trajectory to the axis to form focus. Its function is similar to that of a glass convex lens to focus the beam, so it is called an electron lens. . Most modern electron microscopes use electromagnetic lenses, which focus the electrons by a strong magnetic field generated by a very stable DC excitation current through a coil with a pole shoe.
The electron gun is a component composed of a tungsten filament hot cathode, a grid and a cathode. It can emit and form an electron beam with uniform speed, so the stability of the accelerating voltage is not less than 1/10,000.
Electron microscopes can be divided into transmission electron microscopes, scanning electron microscopes, reflection electron microscopes and emission electron microscopes according to their structure and use. Transmission electron microscopes are often used to observe those fine material structures that cannot be distinguished 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 electrons. Microprobes for the analysis of material composition; 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 uses the electron lens to image and magnify. Its optical path is similar to that of an optical microscope. In this electron microscope, the contrast of image details is created by the scattering of the electron beam by the atoms of the sample. Thinner or less dense parts of the sample, the electron beam scatters less, so more electrons pass through the objective aperture, participate in the 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 tube is the electron gun, the electrons are emitted by the tungsten filament hot cathode, pass through the laser, and the second two condenser lenses 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 the projection mirror, and then imaged on the fluorescent screen or photographic dry plate.
The intermediate mirror mainly adjusts the excitation current, and the magnification can be continuously changed from tens of times to hundreds of thousands of times; by changing the focal length of the intermediate mirror, electron microscope images and electron diffraction images can be obtained on tiny parts of the same sample. . In order to study thicker metal slice samples, the French Dulos Electron Optics Laboratory has developed an ultra-high voltage electron microscope with an accelerating voltage of 3500 kV. Scanning Electron Microscope Structure Schematic
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 and modulates the intensity of the electron beam of the picture tube after amplification, thereby changing the brightness on the screen of the picture tube. The deflection yoke of the picture tube keeps scanning synchronously with the electron beam on the sample surface, 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 industrial television.
The resolution of a scanning electron microscope is mainly determined by the diameter of the electron beam on the surface of the sample. 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. Scanning electron microscope does not require very thin samples; the image has a strong three-dimensional effect; it can analyze the composition of matter using information such as secondary electrons, absorbed electrons and X-rays generated by the interaction of electron beams with matter.
The electron gun and condenser of the scanning electron microscope are roughly the same as those of the transmission electron microscope, but in order to make the electron beam thinner, an objective lens and an astigmatist are added under the condenser, and two sets of scanning electrons that are perpendicular to each other are installed inside the objective lens. coil. The sample chamber under the objective lens houses the sample stage which can be moved, rotated and tilted.
