Brief introduction of transmission electron microscope
brief introduction
The imaging principle of electron microscope and optical microscope is basically the same, but the difference is that the former uses electron beam as light source and electromagnetic field as lens. In addition, because the penetration of electron beam is very weak, the specimen used for electron microscope must be made into ultra-thin sections with a thickness of about 50nm. This kind of slice needs to be made with an ultramicrotome. The magnification of an electron microscope can be up to nearly one million times, and it consists of five parts: illumination system, imaging system, vacuum system, recording system and power supply system. If subdivided, the main parts are electronic lens and imaging recording system, which are composed of electron gun, condenser, sample room, objective lens, diffraction mirror, intermediate mirror, projection mirror, fluorescent screen and camera placed in vacuum.
An electron microscope is a microscope that uses electrons to show the inside 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 microscope is limited by the wavelength used, so the theoretical resolution of electron microscope (about 0.1 nm) is much higher than that of optical microscope (about 200 nm).
Transmissionelectronmicroscope (TEM), referred to as transmission electron microscope [1], projects the accelerated and concentrated electron beam onto a very thin sample, and the electrons collide with the atoms in the sample to change direction, thus producing solid angular scattering. The scattering angle is related to the density and thickness of the sample, so images with different brightness can be formed, and the images will be displayed on imaging devices (such as fluorescent screens, films and photosensitive coupling components) after amplification and focusing.
Because the de Broglie wavelength of electrons is very short, the resolution of transmission electron microscope is much higher than that of optical microscope, which can reach 0.1 ~ 0.2 nm and the magnification is tens of thousands ~ millions of times. Therefore, the transmission electron microscope can be used to observe the fine structure of the sample, even the structure of only one column of atoms, which is tens of thousands of times smaller than the smallest structure that can be observed by the optical microscope. TEM is an important analytical method in many scientific fields related to physics and biology, such as cancer research, virology, materials science, nanotechnology, semiconductor research and so on.
When the magnification is low, the contrast of TEM imaging is mainly caused by the different absorption of electrons caused by different thickness and composition of materials. However, when the magnification is high, the complex fluctuation will cause different brightness of the image, so professional knowledge is needed to analyze the obtained image. By using different modes of TEM, the samples can be imaged by the chemical characteristics, crystal orientation, electronic structure, electron phase shift caused by the samples and the usual absorption of electrons.
