Principles, Structure and Disadvantages of Electron Microscope

Electron microscopy is based on the principle of electron optics, which replaces light beams and optical lenses with electron beams and electron lenses to make the fine structure of matter in the An instrument for imaging at very high magnification.

In recent years, the research and manufacture of electron microscopes have developed considerably: on the one hand, the resolution of electron microscopes has been increasing, and the point of transmission electron microscope.

The resolution has reached 0.2-0.3nm, the lattice resolution has reached about 0.1nm, and through electron microscope, one has been able to On the other hand, in addition to transmission electron microscope, a variety of electron microscopes have been developed, such as scanning electron microscope, analytical electron microscope, etc.

Although the ability of resolution of electron microscope has been far superior to optical microscope, electron microscope has to work under the vacuum, so it is difficult to use. Although the resolution ability of electron microscope is far better than optical microscope, but the electron microscope needs to work in a vacuum, so it is difficult to Observe living organisms and also the exposure of electron beam can damage biological samples by irradiation.

Hereunder, let's talk about electron microscope, including: electron microscope principles, structure, disadvantages, and applications. The difference between electron microscope and optical microscope, and its application in agriculture.

Electron Microscope's Principle

Currently, electron microscope has become an important means of studying the microstructure of the body. The commonly used ones are transmission electron microscope and scanning electron microscope. The following are the working principles of the two types of electron microscopes.

I. Transmission electron microscope

Transmission electron microscope, that is, transmission electron microscope is usually called electron microscope or electron microscope (EM), is the most widely used class of electron microscope.

1、Working principle: Under vacuum conditions, the electron beam is accelerated by high pressure to form scattering electrons and transmission electrons when it penetrates the sample. Imaging on a fluorescent screen under the action of an electromagnetic lens. When an electron beam is projected onto the sample, electron emission can occur with the density of the tissue constituents, as when the electron beam is projected onto a mass When the structure is large, the electrons are scattered more, so the electrons projected onto the fluorescent screen are few and dark, while the electronic photos are black.

2、Main advantages: high resolution, can be used to observe the internal ultrastructure of tissues and cells, as well as the overall picture of microorganisms and biomolecules. 

II. Scanning electron microscope
Scanning electron microscope, that is, scanning electron microscope, is mainly used to observe the surface morphology of the sample, the structure of the cut surface, the structure of the inner surface of the tube lumen.

1、Working principle: scanning electron microscope is the use of secondary electron signal imaging to observe the surface morphology of the sample. Scan with a very fine electron beam on the surface of the sample, the excitation of the sample surface to release secondary electrons, the secondary electrons generated by a special detector. Collected to form an electrical signal delivered to a cathode ray tube to display objects on a fluorescent screen. The three-dimensional conformation of the surface of the object (cells, tissues) can be photographed.

2、Main advantages: long depth of field, strong three-dimensional sense of the image obtained, can be used to observe various morphological features of biological samples.

Structure of electron microscope

The electron microscope consists of an electron optical system, a vacuum system and a power supply system, each of which is described below.

I. Electro-optical systems
1、Electro-optical systems mainly have components such as electron gun, electron lens, sample holder, fluorescent screen and camera mechanism, these components are usually Assembled into a column from top to bottom.
The electron gun is a component consisting of a tungsten hot cathode, a gate and a cathode. It can launch and form an electron beam with uniform velocity, so the stability of the accelerating voltage is required to be not less than one ten thousandth.

2The electron lens is the most important component of the electron microscope tube, which uses a space electric or magnetic field symmetrical to the barrel axis of the lens barrel so that the electron lens is the most important component of the electron microscope. The electron trajectory is bent towards the axis to form a focus, which is similar to the action of a convex glass lens to focus a beam of light, so it is called an electron lens. Most modern electron microscopes use electromagnetic lenses, where a strong magnetic field produced by a very stable DC excitation current through a coil with a polar shoe makes the electron beam focus. Electronic Focus.

II. Vacuum System
In order to ensure that the true only interacts with the specimen throughout the channel and does not collide with air molecules, so that the entire electron The channel from the electron gun to the camera base box must be placed in a vacuum system, typically 10-4 to 10-7 mm Hg. 

III. Electricity supply systems
The transmission electron microscope requires a two-part power supply: a high-voltage part that feeds the electron gun, and a low-voltage current regulator part that feeds the electromagnetic lens. The stability of the power supply is an extremely important indicator of the performance of the electron microscope. Therefore, the main requirement of the power supply system is to produce a high and stable acceleration voltage and the excitation current of the lens. In addition to the above-mentioned power supply, modern instruments still have automatic operating program control system and computer system for data processing.

Disadvantages of Electron Microscope

1、In the electron microscope the sample must be observed in a vacuum, so it is not possible to observe the live sample, with the advancement of technology, the environment Scanning electron microscope will gradually enable direct observation of live specimens.

2. the processing of the sample may produce structure that the sample would not have had, which exacerbates the difficulty of analyzing the image thereafter.

3. Due to the strong electron scattering ability, it is easy for secondary diffraction to occur, etc..

4. Sometimes the image is not unique because it is a two-dimensional projected image of a three-dimensional object.

5、As the transmission electron microscope can only observe a very thin sample, it is possible that the structure of the surface of the substance is different from the structure of the interior of the substance. (a).

6. Ultra-thin samples (below 100 nm), complex, difficult and damaging sample preparation.

7. The electron beam may destroy the sample through collision and heating.

8、In addition, the purchase and maintenance of electron microscopes are more expensive.

Application fields of electron microscope

I. Industrial sector.
1. Industrial inspection, such as circuit boards, precision machinery, etc..
2. Print inspection, SMT welding inspection.
3. Textile inspection.
4. IC surface inspection.
II. Beauty.
1. Skin examination.
2. Root inspection.
3. Infrared physiotherapy (specific products).
III. Biological applications.
1. Microbiological observations.
2. Observation of animal sections.
3. Plant disease and insect observation.
IV. Other.
1. A visual amplifier to assist persons with intellectual disabilities in reading.
2. Gem identification.
3. Identification of antiques, paintings, jade artifacts, etc..
4. some other areas of video image analysis.

Difference between an electron microscope and an optical microscope

1. Different lighting sources.
The source of illumination used in an electron microscope is the stream of electrons emitted by an electron gun, while the source of illumination in a light microscope is visible light (daylight or light), due to the electron stream. The wavelength of light waves is much shorter than the wavelength of light waves, so the magnification and resolution of the electron microscope is significantly higher than the light microscope.

2, the lens is different.
The objective in an electron microscope that acts as a magnifying lens is an electromagnetic lens (a toroidal electromagnetic coil that generates a magnetic field in the central part), while the objective in an optical mirror is It is an optical lens made of ground glass. There are three groups of electromagnetic lenses in the electron microscope, which are functionally equivalent to the concentrating lens, objective lens and eyepiece in the light microscope.

3, the imaging principle is different.
In an electron microscope, the electron beam applied to the sample is amplified by an electromagnetic lens and imaged on a fluorescent screen or on a photographic film. . The mechanism of the difference in electron intensity is that when the electron beam is applied to the sample, the incident electron collides with the atom of the substance to produce the electron beam. Scattering, due to the different parts of the sample have different degrees of scattering to the electrons, so the electron image of the sample is presented in shades. And the object image of the sample in the light microscope to the brightness difference, it is caused by the different structures of the sample to be examined by the amount of light absorption of different.

Because of the interference and diffraction of light, the resolution of optical microscope is limited to 02-05um. Because electron microscope uses electron beam as light source, its resolution can reach between 1-3nm, so the tissue of optical microscope can be easily resolved. Observations are micron-scale analysis, and electron microscope tissue observations are nanoscale analysis.

5. depth of field.
The depth of field of an ordinary optical microscope is between 2-3um, so the surface smoothness of the sample is very high, so the preparation of the sample is very important. The process is relatively complex. Scanning electron microscope can be as high as a few millimeters, so there is no requirement for a smooth surface geometry and the sample preparation process is relatively complex. It is relatively simple, and some sample geometries do not require sample preparation. Body microscopes also have a greater depth of field, but their resolution is very low. Magnification: 1000X for optical microscope, 1000X for electron microscope.

6、The specimen preparation methods used are different.
Electron microscope The procedures for preparing tissue cell specimens for observation are complex, technically difficult and costly, with a high degree of difficulty in obtaining, fixing, dehydrating and Special reagents and operations are required for embedding, and finally, the embedded tissue block should be put into an ultra-thin microtome and cut into 50~50 mm pieces. 100 nm thick ultra-thin specimen slides. Specimens for light microscope are usually placed on slides, such as plain tissue section specimens, cell smear specimens, tissue impression specimens, and tissue specimens. Cell drop specimen. Up to 1,000,000X

7. applications.
Optical microscope is mainly used for the observation and measurement of micron-sized tissues on smooth surfaces. The tissue on the surface of the sample but within a certain range below the surface can also be observed, and the optical microscope has a very good understanding of color. The identification is very sensitive and accurate. Electron microscope is mainly used to observe the surface topography of samples at the nanoscale, since SEM relies on the intensity of the physical signal to distinguish between tissues. Scanning electron microscope (SEM) images are black and white, therefore SEM is not able to recognize color images. Scanning electron microscope (SEM) is not only able to visualize the topography of the sample surface, but also, through the use of different accessories such as EDS, WDS, EBSD, etc., it is possible to identify the sample's color image. equipment, the scanning electron microscope can be further extended for use. By using EDS, WDS auxiliary equipment, scanning electron microscope can analyze the chemical composition of micro-areas, which is essential for failure analysis. The research area is by far the most important. Using EBSD, scanning electron microscope allows the lattice orientation of materials to be studied.

Applications of electron microscope in agriculture

Application and progress of electron microscope in agriculture

1、Scanning electron microscope application in agricultural field
Scanning Electron Microscope (SEM) has a large depth of field, stereoscopic image, high resolution, large image range, and sample preparation process. The advantages of scanning electron microscope, such as its comparative simplicity, have attracted great attention and favor from agricultural researchers. Scanning electron microscope is mainly used in agricultural research to study the surface morphology of different tissues and microscopic organs of plants and animals, microorganisms and insects, as well as Internal structure, thus deepening the understanding of their application in physiological functions and exploring the rules of living of organisms. For example, in insects, the main purpose is to improve the ability to distinguish and classify their microscopic organs, as well as to further clarify the role of organs. To characterize and compare the external morphology of insects and to study their shape changes and the characteristics of their patterns and structures in order to gain a deeper understanding of their characteristics. . In plants, the study of characteristics such as pollen, pericarp, seed coat surface pattern and internal seed structure of crops, which are taxonomically Significance. In microbiology, it plays a greater role in studying the classification of fungi, actinomycetes, and bacteria, identifying families, and determining the source of disease, especially the Satisfactory results can be obtained on the activities of germs, spore germination, invasion of hosts, etc. by scanning electron microscope.

2、Transmission electron microscope applications in agriculture

A transmission electron microscope (TEM) consists of an electron optical system, a vacuum system, a power supply system, and an auxiliary system. The principle of transmission electron microscope imaging is based on the use of electron rays without information, which interact with the sample as they pass through it. The other side of the sample reappears already with information about the sample, which is then magnified so that the microscopic inside can be seen. information for interpretation. When an electron beam interacts with a sample material, it can generate a lot of information with the sample, such as transmitted electrons, scattered electrons, secondary electrons etc. The image contrast of transmission electron microscope is determined by the scattering absorption difference, diffraction difference and aberration that occurs when the incident electron passes through the sample. In the observation of agricultural biological samples, as the resolution of the electron microscope has increased, the clarity of the electron microscope image has been imperfectly It depends on the resolution of the electron microscope and to a large extent on the techniques used for sample preparation. The commonly used techniques for the preparation of biological samples for transmission electron microscope in agriculture are: ultra-thin sectioning, immunoelectron microscope, negative staining, biological Macromolecular electron microscope, etc. Plant viruses as an important group of pathogens attacking angiosperms, nudibranches and ferns, causing crop failure worldwide. Diseases of fruit trees, flowers, pasture and medicinal plants cause a decline in yield and quality, seriously affecting human production and life. The application of electron microscope technology is useful in determining the morphological structure, gene structure and function, virus replication process, virus and host, and the function of the virus. The in-depth understanding of the interrelationships between cellular ultrastructure and the observation of cellular ultrastructural lesions has an irreplaceable role in other methods, providing a step-by-step basis for the development of Reveal the nature of the virus, and ultimately solve the problem of viruses and diseases to lay the foundation.

The above is a small introduction to the electron microscope principle, structure, disadvantages, applications, and the difference between optical microscope, in agriculture On the application. With the continuous development of modern science and technology. Besides, the application technological of electron microscope is also becoming more and more extensive, as the observation of the microscopic world is called " the eye of science" - the electron microscope has the characteristics of high resolution and intuitiveness which cannot be replaced by any other scientific instruments. Electron microscope has played an outstanding role in the development of the disciplines of medicine, biology, physics, chemistry, metallurgy, and materials science and it has become an indispensable and routine instrument in the research work of many disciplines.



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