What does a scanning electron microscope do?

I have not heard of the term "scanning electron microscope". At present, the common microscopes are mainly divided into optical microscopes, electron microscopes and scanning tunneling microscopes in principle.

Optical microscopes use two-sided convex lenses to image using the principle of refraction of light, which can reach a magnification of 1000-1500.

In the thirties of the last century, on the basis of de Broglie's theory of matter waves, using the influence of magnetic fields on the flow of electrons to form a convex lens effect, mankind created an electron microscope with a resolution of 30 angstroms, so that humans can observe macromolecules and even some atoms.

In the 80s of the last century, the Americans built a scanning tunneling microscope, which used the current changes caused by the tunneling effect in quantum physics to draw a three-dimensional image through computer processing. This new microscope has a magnification of up to 300 million times, and the minimum resolving distance between two points is 1 10 of the diameter of the atom, which means that its resolution is as high as angstrom.

Thus, a light microscope can see biological cells, microorganisms. Viruses can be seen with an electron microscope. Scanning tunneling microscopy can be used to get to all the atoms.

A scanning electron microscope is a type of electron microscope.

Electron microscopy is divided into: scanning electron microscope and transmission electron microscope.

The former only looks at the surface morphology of the substance; The latter electrons pass through the sample, are stereoscopic, and have a higher resolution.

Electron microscopy is generally better at metals and ceramics.

The principle of scanning electron microscope is that hundreds of electron beams are emitted by the top electron gun, which are focused by the gate, and under the action of accelerating voltage, through the electron-optical system composed of two to three electromagnetic lenses, the electron beams converge into a thin electron beam focused on the surface of the sample. A scanning coil is installed on the top of the final lens, and the electron beam is scanned on the surface of the sample under its action.

Due to the interaction of the high-energy electron beam with the sample substance, a variety of information is produced as a result: secondary electrons, back-reflection electrons, absorption electrons, X-rays, Auger electrons, cathodoluminescence, and transmitted electrons, among others. These signals are received by the corresponding receivers, amplified and sent to the gate of the picture tube, which modulates the brightness of the picture tube.

Since the current on the scanning coil corresponds to the brightness of the picture tube, that is, when the electron beam hits the sample, a bright spot appears on the fluorescent screen of the picture tube. Scanning electron microscopy is such a point-by-point imaging method to convert different features on the surface of the sample into ** signals in proportion to the order of response, so as to complete a frame of image, so that we can observe the sample on the phosphor screen. **：

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Mitochondria, endoplasmic reticulum, centrosomes, chloroplasts, Golgi apparatus, ribosomes and other organelles can be seen under the electron microscope, and plastids and vacuoles can be seen under the light microscope.

Organelles are usually divided into: mitochondria; Chloroplast; Endoplasmic reticulum; Golgi apparatus; lysosomes; Vacuoles, ribosomes, centrosomes. Among them, chloroplasts are only found in plant cells, vacuoles are only found in plant cells and lower animals, and centrosomes are only found in lower plant cells and animal cells.

At the secondary level, the nucleus is not recognized as an organelle, whereas at the college level, the nucleus is considered to be the largest and most important organelle in the cell.

In addition, in cells, cytosol accounts for about 55% of the total volume of cells, in which thousands of enzymes are present. Most intermediate metabolism, including glycolysis, gluconeogenesis, and the synthesis of sugars, fatty acids, nucleotides, and amino acids, takes place in the cytosol.

The cytoplasmic matrix is essentially a highly organized system at different levels, rather than a simple solution. However, the tangible structures within the cytoplasmic matrix are not visible under ordinary transmission electron microscopy.

The principle of scanning electron microscopy is to use the differences in the characteristics of the micro-area on the surface of the material (such as morphology, atomic number, chemical composition, or crystal structure, etc.) to produce different brightness differences in different areas of the sample under the action of electron beams, so as to obtain images with a certain contrast.

Simple sample preparation, wide adjustable magnification, high resolution of the image, large depth of field, high fidelity, real three-dimensional effect, etc., for conductive materials, can be directly put into the sample chamber for analysis, for poor conductivity or insulation of the sample, it is necessary to spray the conductive layer to make noise.

Scanning electron microscopy (SEM) is an observation method that is intermediate between transmission electron microscopy and optical microscopy. It uses a focused and narrow high-energy electron beam to scan the sample, and excites various physical information through the interaction between the beam and the substance, and collects, amplifies and re-images this information to achieve the purpose of microscopic morphology characterization of the substance.

The resolution of the new scanning electron microscope can reach 1 nm; The magnification can reach 300,000 times and above and can be continuously adjusted; And the depth of field is large, the field of view is large, and the imaging stereo effect is good. In addition, the combination of scanning electron microscope and other analytical instruments can be used to observe the microscopic morphology and analyze the microstructure of the material at the same time.

Scanning electron microscopy is widely used in the study of geotechnical, graphite, ceramics and nanomaterials. Therefore, scanning electron microscopy plays a significant role in the field of scientific research.

When an electron beam hits a sample, the electrons react with the sample in a variety of ways. Some of these electrons can pass directly through the sample; Some of the electrons are scattered by the sample; The other part of the electrons is reflected off the surface of the sample. By collecting all these different types of electrons and imaging them, you can make up different types of electron microscopes.

In particular, the electrons reflected from the surface of the sample are collected and imaged, and this electron microscope is called a scanning electron microscope.

1. The working principle of the scanning electron microscope Under the action of high voltage, because the electrons emitted by the hot cathode are focused and accelerated by the electric field between the cathode, the gate and the anode, an electron beam spot with high energy is formed between the gate and the anode, which is called an electron source. This electron beam spot is then compressed by the condenser, converging into an extremely fine electron beam and focusing on the surface of the sample, this high-energy finely focused electron beam scans on the surface of the sample under the action of the scanning coil, interacts with the sample, and excites various physical signals. The intensity of various physical signals is related to the surface characteristics of the sample, which can be detected, amplified, and imaged with corresponding detectors for various microscopic analyses.

The main signals collected by scanning electron microscopy are secondary electrons and double electrons.

2. The structure of the SEM Because the working characteristics of the SEM are different from those of the TEM, their structures are also very different. The scanning electron microscope is generally composed of an electronic photowriting system, a scanning system, a signal detection and amplification system, an image display and recording system, a vacuum system and a power supply system. Among them, the electron-optical system is mainly composed of an electron gun, an electromagnetic condenser, an optical diaphragm, and a sample chamber.

Unlike transmission electron microscopy, it is not used for imaging, but simply to obtain a high-energy finely focused electron beam, which is the excitation source that causes the sample to produce various signals. The function of the scanning system is to make the incident electron beam sweep regularly on the sample surface, and the cathode ray tube electron beam can be synchronized with the scanning screen to change the scanning amplitude of the incident electron beam on the sample surface to obtain the required magnification of the image. The scanning system is mainly composed of a scanning generator, a scanning coil, and a magnification converter.

The system performs the process of detecting various physical signals generated on the surface of the sample under the action of incident electrons and converting them into signals that can be used to modulate images or perform other analyses. Different detectors are used for different physical signals. At present, the commonly used detectors for SEM are mainly electronic detectors and X-ray detectors.

3. Sample preparation of SEM A prominent feature of SEM is that it has great adaptability to samples, and all solid samples, whether block, powder, metallic, non-metallic, organic, or inorganic, can be observed. Moreover, the preparation of the sample is relatively simple, but it still requires certain techniques and requirements, otherwise it will not be able to achieve satisfactory results. The general SEM requirements for samples are mainly as follows:

Proper size and good electrical conductivity. The longer the depth of field of the SEM image, the more stereoscopic the image.

Scanning electron microscopy does not rely on the step-by-step magnification of the imaging system to achieve microscopic functions like projection electron microscopes and ordinary microscopes, but provides sufficiently high resolution by zooming out to the spot. Scanning electron microscopy has the following characteristics.

1) The scanning electron microscope is mainly used to observe the surface structure of the sample, and there is no limit to the thickness of the sample, and the three-dimensional structure of the sample surface can be directly observed. Although the projection electron microscope has a high resolution ability, it can generally only obtain a two-dimensional image of the sample.

2) When the magnification of optical microscope and transmission electron microscope increases, the focal length and depth of field decrease. When the magnification of the SEM increases, the focal length remains the same and the depth of field does not decrease, so it is convenient to observe and take pictures.

3) The magnification range of the SEM is very wide, from the level of the magnifying glass (several times) to the level of the optical microscope (hundreds of times) to the horizontal front of the transmission electron microscope (hundreds of thousands of times), so it can be considered that the scanning electron microscope fills the gap between the optical microscope and the transmission electron microscope.

4) In the scanning electron microscope, because the image is not formed by the lens, but is recorded in sequence according to the signal sequence, it not only avoids the influence of lens defects on the image resolution of the key pants, but also is easy to record the image on the storage medium for further processing.

5) Scanning electron microscope can be combined with various analysis techniques to form an analytical electron microscope (also known as electron probe microscope analyzer), which can realize the comprehensive analysis of samples.

6) It has an extremely high resolution of the atomic order, and its resolution in the direction of perpendicular and parallel to the surface is and respectively, that is, it is able to distinguish individual atoms. Therefore, STM can directly observe the local structure of the surface of the single atomic layer, such as surface defects, surface reconstruction, and the morphology and position of surface adsorbents.

7) STM can give a three-dimensional image of the surface in real time, and can measure the surface structure with or without periodicity.

8) STM can work under different environmental conditions, including vacuum, atmosphere, low temperature, and even immersion of the specimen in water or electrolyte, so it is very suitable for studying the influence of environmental factors on the surface of the specimen.

9) The molecular structure of nanofilms can be studied.

However, STM also has its limitations, and its disadvantages are mainly manifested in: Because STM is designed by the action of tunnel current, this instrument can only be used for the surface morphology measurement of conductors and semiconductors, and for non-conductors, the sample must be coated with a conductive film, which conceals the authenticity of the sample surface and reduces the accuracy of STM. Even if there is a non-single electronic state on the surface of the conductive material sample, the scanning tunneling microscope does not observe the real surface morphology image, but the comprehensive performance of the surface morphology and surface electronic properties.

Scanning electron microscopes are manufactured on the basis of electron-matter interactions.

In principle, SEM uses a very finely focused high-energy electron beam to scan a specimen and excite various physical information. By receiving, magnifying, and displaying this information, an observation of the surface topography of the test specimen is obtained.

When a very fine beam of high-energy incident electrons bombards the surface of a scanned sample, the excited region produces secondary electrons, Auger electrons, characteristic X-rays and continuum X-rays, backscattered electrons, transmitted electrons, and electromagnetic radiation in the visible, ultraviolet, and infrared regions. At the same time, electron-hole pairs, lattice vibrations (phonons), and electron oscillations (plasma) can be generated.