Working principle and application of Transmission electron microscopy
Transmission electron microscopy (TEM for short) can see fine structures less than 0.2um that cannot be seen clearly under the optical microscope. These structures are called Ultrastructure or ultrastructure. To see these structures clearly, it is necessary to choose a shorter wavelength light source to improve the resolution of the microscope. Ruska invented the Transmission electron microscopy with the electron beam as the light source in 1932. The wavelength of the electron beam is much shorter than 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 0.2nm.
The working principle of the Transmission electron microscopy 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 then converges it into a sharp, bright and uniform light spot through the condenser, which shines on the sample in the sample room; The electron beam passing through the sample carries the structural information inside the sample, with less electrons passing through the dense areas and more electrons passing through the sparse areas; After focusing and primary magnification of the objective lens, the electron beam enters the intermediate lens and the first and second projection mirrors of the lower level for comprehensive magnification imaging. The amplified electron image is finally projected onto the fluorescent screen plate in the observation room; A fluorescent screen converts electronic images into visible light images for users to observe. This section will introduce the main structures and principles of each system separately.
Imaging principle of Transmission electron microscopy
The imaging principle of Transmission electron microscopy can be divided into three cases:
1. Absorption image: When electrons are emitted onto samples with high mass and density, the main phase formation is scattering. The areas with large mass and thickness on the sample have a larger scattering angle of electrons, fewer electrons pass through, and the brightness of the image is darker. The early Transmission electron microscopy was based on this principle.
2. Diffraction image: after the electron beam is diffracted by the sample, the amplitude distribution of the diffracted wave at different positions of the sample corresponds to the different diffractive ability of each part of the crystal in the sample. When Crystallographic defect appear, the diffractive ability of the defect part is different from the complete area, so that the amplitude distribution of the diffracted wave is uneven, reflecting the distribution of Crystallographic defect.
3. Phase image: When the sample is thinner than 100 Å, electrons can pass through the sample, and the amplitude change of the wave can be ignored. The imaging comes from the change in phase.
Use of Transmission electron microscopy
Transmission electron microscopy is widely used in material science and biology. Due to the susceptibility of electrons to scattering or absorption by objects, the penetration force is low, and the density, thickness, and other factors of the sample can affect the final imaging quality. Therefore, it is necessary to prepare thinner ultra-thin slices, usually 50-100nm. Therefore, the samples observed by Transmission electron microscopy need to be treated very thin. The commonly used methods include: ultra-thin sectioning method, frozen ultra-thin sectioning method, frozen etching method, frozen fracture method, etc. For liquid samples, it is usually observed by hanging pre-treated copper wire mesh.
