Composition of Electron Microscope Development History of Electron Microscope
Components of an electron microscope
Electron source: It is a cathode that releases free electrons, and a ring-shaped anode accelerates electrons. The voltage difference between the cathode and anode must be very high, typically between several thousand volts and three million volts.
Electrons: Used to focus electrons. Generally, magnetic lenses are used, and sometimes electrostatic lenses are also used. The function of the electron lens is the same as that of the optical lens in the optical microscope. The focus of the optical lens is fixed, but the focus of the electronic lens can be adjusted, so the electron microscope does not have a movable lens system like an optical microscope.
Vacuum device: The vacuum device is used to ensure the vacuum state inside the microscope, so that electrons will not be absorbed or deflected on their path.
Sample holder: Samples can be placed on the sample holder stably. In addition, there are often devices that can be used to change the sample (such as moving, rotating, heating, cooling, elongating, etc.).
Detector: A signal or secondary signal used to collect electrons. The projection of a sample can be obtained directly by using a transmission electron microscope (Transmission Electron Microscopy TEM). Electrons pass through the sample in this microscope, so the sample must be very thin. The atomic weight of the atoms making up the sample, the voltage at which the electrons are accelerated, and the desired resolution determine the thickness of the sample. The thickness of the sample can vary from a few nanometers to a few micrometers. The higher the atomic mass and the lower the voltage, the thinner the sample must be.
By changing the lens system of the objective, one can directly magnify the image at the focal point of the objective. From this one can obtain electron diffraction images. Using this image, the crystal structure of the sample can be analyzed.
The composition principle of electron microscope
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 the most 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 electron. 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 a component consisting of a tungsten filament hot cathode, a grid and a 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 The microspheres are formed 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 an electron gun. The electrons are emitted by the tungsten hot cathode, and the electron beams are focused by the first and second condensers. 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 dozens 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.
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. Scanning electron microscopy does not require very thin samples; 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 of electron beams and substances to analyze the composition of substances.
The electron gun and condenser lens 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 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.
Uses of Electron Microscopes
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 transmission 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.
