Characteristics of Transmission Electron Microscopy
The imaging principle of electron microscope and optical microscope is basically the same, the difference is that the former uses electron beam as light source and electromagnetic field as lens. In addition, because the penetrating power of the electron beam is very weak, the specimen used for the electron microscope must be made into an ultra-thin section with a thickness of about 50nm. This slice needs to be made with an ultramicrotome. The magnification of the electron microscope can reach up to nearly one million times. It consists of five parts: illumination system, imaging system, vacuum system, recording system, and power supply system. If it is subdivided: the main part is the electronic lens and imaging recording system. Electron guns, condenser mirrors, sample chambers, objective lenses, diffraction mirrors, intermediate mirrors, projection mirrors, fluorescent screens and cameras in vacuum.
An electron microscope is a microscope that uses electrons to reveal the interior or surface of an object. The wavelength of high-speed electrons is shorter than that of visible light (wave-particle duality), and the resolution of the microscope is limited by the wavelength it uses. Therefore, the theoretical resolution of the electron microscope (about 0.1 nanometers) is much higher than that of the optical microscope. rate (about 200 nm).
Transmissionelectronmicroscope, abbreviated TEM, referred to as transmission electron microscope, is to project the accelerated and concentrated electron beam onto a very thin sample, and the electrons collide with the atoms in the sample to change the direction, thereby producing solid angle scattering. The size of the scattering angle is related to the density and thickness of the sample, so images with different brightness and darkness can be formed, and the images will be displayed on imaging devices (such as fluorescent screens, films, and photosensitive coupling components) after zooming in and focusing.
Due to the very short de Broglie wavelength of the electron, the resolution of the transmission electron microscope is much higher than that of the optical microscope, which can reach 0.1-0.2nm, and the magnification is tens of thousands to millions of times. Therefore, the use of transmission electron microscopy can be used to observe the fine structure of samples, even the structure of only a single column of atoms, which is tens of thousands of times smaller than the smallest structure that can be observed by optical microscopy. TEM is an important analytical method in many scientific fields related to physics and biology, such as cancer research, virology, materials science, as well as nanotechnology, semiconductor research, etc.
At low magnifications, the contrast in TEM imaging is mainly due to the different absorption of electrons due to the different thickness and composition of the material. When the magnification multiple is high, complex fluctuations will cause differences in the brightness of the image, so professional knowledge is required to analyze the obtained image. By using the different modes of TEM, it is possible to image a sample by its chemical properties, crystallographic orientation, electronic structure, electronic phase shift by the sample, and generally by absorption of electrons.
The first TEM was developed by Max Knorr and Ernst Ruska in 1931, this research group developed the first TEM with a resolution beyond visible light in 1933, and the first commercial TEM in 1939 success.
Large TEM
Conventional TEM generally adopts 80-300kV electron beam acceleration voltage. Different models correspond to different electron beam acceleration voltages. The resolution is related to the electron beam acceleration voltage and can reach 0.2-0.1nm. High-end models can achieve atomic-level resolution.
Low-voltage TEM
Low-Voltage electron microscope, the electron beam acceleration voltage (5kV) used by LVEM is much lower than that of large transmission electron microscope. A lower accelerating voltage will enhance the strength of the interaction between the electron beam and the sample, thereby improving the image contrast and contrast, especially suitable for samples such as polymers and biology; at the same time, the low-voltage transmission electron microscope will cause less damage to the sample.
The resolution is lower than that of the large electron microscope, 1-2nm. Due to the low voltage, TEM, SEM and STEM can be combined in one device
Cryo-EM
Cryo-microscopy is usually equipped with a sample freezing device on an ordinary transmission electron microscope to cool the sample to the temperature of liquid nitrogen (77K), which is used to observe temperature-sensitive samples such as proteins and biological slices. By freezing the sample, the damage to the sample by the electron beam can be reduced, the deformation of the sample can be reduced, and a more realistic sample shape can be obtained.
operating characteristics
1. Stability
The stability of the photomultiplier tube is determined by many factors such as the characteristics of the device itself, working status and environmental conditions. There are many situations where the output of the tube is unstable during the working process, mainly including:
a. Jumping instability caused by poor welding of electrodes in the tube, loose structure, poor contact of cathode shrapnel, tip discharge between electrodes, flashover, etc., and the signal is suddenly large and small.
b. Continuity and fatigue instability caused by too much anode output current.
c. Effect of Environmental Conditions on Stability. As the ambient temperature rises, the sensitivity of the tube decreases.
d. Humid environment causes leakage between pins, causing dark current to increase and become unstable.
e. Environmental electromagnetic field interference causes unstable work.
2. Limit working voltage
The ultimate working voltage refers to the upper limit of the voltage that the tube is allowed to apply. Above this voltage, the tube will discharge or even break down.
