The working principle of transmission electron microscope
Transmission electron microscope (Transmission Electron Microscope, TEM for short) can see the microstructures smaller than 0.2um that cannot be seen clearly under the optical microscope. These structures are called submicrostructures or ultrastructures. To see these structures clearly, a light source with a shorter wavelength must be chosen to increase the resolution of the microscope. In 1932, Ruska invented a transmission electron microscope with an electron beam as the light source. The wavelength of the electron beam is much shorter than that of visible light and ultraviolet light, and the wavelength of the electron beam is inversely proportional to the square root of the voltage of the emitted electron beam, that is, the higher the voltage. the shorter the wavelength. At present, the resolution of TEM can reach 0.2 nm.
The working principle of the transmission electron microscope is that the electron beam emitted by the electron gun passes through the condenser along the optical axis of the mirror body in the vacuum channel, and is condensed into a sharp, bright and uniform light spot by the condenser, and illuminates the sample in the sample chamber. On; the electron beam after passing through the sample carries the structural information inside the sample, the amount of electrons passing through the dense part of the sample is small, and the amount of electrons passing through the sparse part is more; after the focusing and primary magnification of the objective lens, the electron beam The intermediate lens entering the lower stage and the first and second projection mirrors carry out comprehensive magnification imaging, and finally the magnified electronic image is projected on the fluorescent screen in the observation room; the fluorescent screen converts the electronic image into a visible light image for users to observe. This section will introduce the main structure and principle of each system respectively.
Transmission Electron Microscope Imaging Principles
The imaging principle of transmission electron microscope can be divided into three situations:
1. Absorption image: When electrons hit a sample with high mass and density, the main phase-forming effect is scattering. Where the mass and thickness of the sample are larger, the scattering angle of electrons is larger, and fewer electrons pass through, and the brightness of the image is darker. Early transmission electron microscopes were based on this principle.
2. Diffraction image: After the electron beam is diffracted by the sample, the diffracted wave amplitude distribution at different positions of the sample corresponds to the different diffraction power of each part of the crystal in the sample. The amplitude distribution of diffracted waves is not uniform, reflecting the distribution of crystal defects.
3. Phase image: When the sample is thinner than 100Å, electrons can pass through the sample, and the wave amplitude change can be ignored, and the imaging comes from the phase change.
Uses of Transmission Electron Microscopy
Transmission electron microscopy is widely used in materials science and biology. Since electrons are easily scattered or absorbed by objects, the penetration is low, and the density and thickness of the sample will affect the final imaging quality. Thinner ultrathin sections must be prepared, usually 50-100 nm. Therefore, the sample for observation by transmission electron microscope needs to be processed very thinly. Commonly used methods are: ultra-thin sectioning, frozen ultra-thin sectioning, freeze-etching, freeze-fracture and so on. For liquid samples, it is usually observed by hanging on a pretreated copper grid.
