Transmission Electron Microscopy Operating Characteristics
Introduction
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).
Transmission electron microscope (TEM for short), referred to as transmission electron microscope [1], 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
Large-scale transmission electron microscopes (conventional TEM) generally use 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, which can reach 0.2-0.1nm. High-end models can achieve atomic-level distinguish.
Low-voltage TEM
The electron beam acceleration voltage (5kV) used in the low-voltage small TEM (Low-Voltage electron microscope, LVEM) is much lower than that of the large TEM. 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 sample freezing equipment on the 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 of the electron beam to the sample can be reduced, the deformation of the sample can be reduced, and a more realistic sample shape can be obtained.
