First, the composition of the electron microscope;
The electron microscope consists of a lens barrel, a vacuum device and a power cabinet.
The lens barrel mainly has components such as an electron source, an electron lens, a sample holder, a fluorescent screen, and a detector. These components are usually assembled into a cylinder from top to bottom.
The electron lens is used to focus electrons and is the most important component in an electron microscope tube. Generally used is a magnetic lens with fibers under an electron microscope
There are also electrostatic lenses used. It uses a spatial electric field or magnetic field symmetrical to the axis of the barrel to bend the electron trajectory toward the axis to form a focus. The effect is the same as that of the optical lens (convex lens) in the optical microscope, so it is called an electron lens. The focus of the optical lens is fixed, and the focus of the electron lens can be adjusted, so the electron microscope does not have a movable lens system like the optical microscope. Most modern electron microscopes use an electromagnetic lens that focuses the electrons by a strong magnetic field generated by a coil with a very stable DC excitation current. The electron source is a cathode that releases free electrons, a grid, and an anode that accelerates electrons. The voltage difference between the cathode and the anode must be very high, typically between several thousand volts and 3 million volts. It can emit and form a uniform beam of electrons, so the stability of the acceleration voltage is required to be no less than one in ten thousand.
The sample can be placed stably on the sample holder. In addition, there are often electron microscopes that can be used to change samples (such as moving, rotating, heating, and cooling).
, elongated, etc.).
The detector is used to collect electronic or secondary signals.
The vacuum device is used to ensure the vacuum state in the microscope, so that the electrons are not absorbed or deflected in the path, and are composed of a mechanical vacuum pump, a diffusion pump and a vacuum valve, and are connected to the lens barrel through an air suction pipe.
The power cabinet is composed of a high voltage generator, an excitation current stabilizer and various adjustment control units. [
2, the type of electron microscope:
Electron microscopes can be classified into transmission electron microscopes, scanning electron microscopes, reflection electron microscopes, and emission electron microscopes according to their structures and uses.
Transmission electron microscopy is often used to observe the structure of fine materials 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 spectrometers to form electrons. Microprobes for material composition analysis; emission electron microscopy for self-emission electron surface studies. [1]
Transmission electron microscope
It is named after the electron beam penetrates the sample and then magnified by electron lens imaging. Its optical path is similar to that of an optical microscope, allowing direct projection of a sample. By changing the lens system of the objective lens, one can directly magnify the image of the focus of the objective lens. Thus, an electron diffraction image can be obtained. Use this image to analyze the crystal structure of the sample. In this electron microscope, the contrast of the image detail is formed by the scattering of the electrons from the atoms of the sample. Because the electrons need to pass through the sample, the sample must be very thin. Atomic electron microscope observation of the atoms constituting the sample
The amount, the voltage of the accelerated electrons, and the desired resolution are determined by the thickness of the sample. The thickness of the sample can vary from a few nanometers to a few microns. The higher the atomic weight and the lower the voltage, the thinner the sample must be. The thinner or lower density part of the sample has less electron beam scattering, so that more electrons pass through the objective lens and participate in imaging, which is 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 and even be damaged or destroyed by the energy of the electron beam.
The resolution of the transmission electron microscope is 0.1 to 0.2 nm, and the magnification is tens of thousands to hundreds of thousands of times. Since electrons are easily scattered or absorbed by an object, penetration is low, and thinner ultrathin sections (usually 50 to 100 nm) must be prepared.
The top of the transmissive electron microscope barrel is an electron gun. The electrons are emitted by the tungsten hot cathode, and the electron beam is focused by the first and second condensing mirrors. The electron beam passes through the sample and is imaged by the objective lens on the intermediate mirror, and then amplified step by step through the intermediate mirror and the projection mirror, and imaged on a fluorescent screen or a photodrying plate. The intermediate mirror mainly adjusts the excitation current, and the magnification can be continuously changed from several tens of times to several hundred thousand times. By changing the focal length of the intermediate mirror, electron micrographs and electron diffraction images can be obtained on the tiny parts of the same sample. . [2]
scanning electron microscope
The electron beam of the scanning electron microscope does not pass through the sample, only the electron beam is focused as much as possible on a small portion of the sample, and then the sample is scanned line by line. The incident electrons cause the surface of the sample to be excited out of the secondary electrons. Microscope electron microscope image 3
Observing the electrons scattered at each of these points, the scintillation crystal placed next to the sample receives these secondary electrons, and by modulating the intensity of the electron beam of the picture tube, thereby changing the brightness of the picture tube phosphor screen. The image is a three-dimensional image that reflects the surface structure of the specimen. The deflection coil of the picture tube is scanned synchronously with the electron beam on the surface of the sample, so that the screen of the picture tube shows a topographical image of the surface of the sample, which is similar to the working principle of an industrial television. Since electrons in such a microscope do not have to transmit a sample, the voltage of their electron acceleration does not have to be very high.
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 kinescope to the scanning amplitude on the sample, which can vary continuously 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 analyze the material composition by using secondary electrons, electron absorption and X-rays generated by the interaction of electron beams and substances. [3-4]
Scanning electron microscopy is based on the interaction of electrons and matter. When a beam of high-energy electrons strikes the surface of a substance, the excited region will produce secondary electrons, Auger electrons, characteristic x-rays and continuum X-rays, backscattered electrons, transmitted electrons, and visible, ultraviolet, and infrared. Electromagnetic radiation generated in the light region. At the same time, electron-hole pairs, lattice vibrations (phonons), and electron oscillations (plasma) can also be generated. [4]
3. Development history of electron microscope:
In 1926, Hans Bush developed the first magnetic electron lens.
In 1931 Ernst Luca and Max Knorr developed the first fluoroscopy electron microscope. The microscope used to display this microscope is not a perspective sample, but a metal grid. In 1986, Lucca won the Nobel Prize in Physics for this. The world's first electron microscope
Tannin was proposed in 1934 to enhance the contrast of the image.
In 1937, the first scanning transmission electron microscope was introduced. The primary purpose of developing an electron microscope at the outset was to show pathogens such as viruses that could not be resolved in an optical microscope.
In 1938 he developed the first commercial electron microscope at Siemens.
After the emergence of metal foils that could be projected in 1949, material science has become more interested in electron microscopy.
In the 1960s, the accelerating voltage of the projection electron microscope was getting higher and higher to see the thicker and smaller electron microscope.
Substance. Electron microscopy at this time reached the ability to resolve atoms.
In the 1980s, people were able to observe wet samples using a scanning electron microscope.
In the mid-1990s, computers were increasingly used to analyze electron microscope images, and computers could also control increasingly complex lens systems, while electron microscopes became easier to operate. [1][5]
4, 4, the shortcomings of electron microscope
In an electron microscope, the sample must be observed in a vacuum, so the live sample cannot be observed. With the advancement of technology, environmental scanning electron microscopy will gradually realize the observation of live samples directly.
When the sample is processed, it may produce a structure that the sample does not have, which exacerbates the difficulty of analyzing the image thereafter;
Due to the strong electron scattering ability, secondary diffraction is easy to occur;
Since the image is projected for a two-dimensional plane of a three-dimensional object, sometimes it is not unique;
Since the transmission electron microscope can only observe very thin samples, it is possible that the structure of the surface of the substance is different from the structure inside the substance;
Ultra-thin samples (below 100 nm), the sample preparation process is complicated and difficult, and the sample preparation is damaged;
The electron beam may damage the sample by collision and heating;
In addition, the price of electron microscope purchase and maintenance is relatively high.
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