Principle of operation and application of transmission electron microscopy
Transmission Electron Microscope (TEM) can observe fine structures smaller than 0.2um that cannot be clearly seen under an optical microscope. These structures are called submicroscopic structures or ultrastructures. To see these structures clearly, it is necessary to choose a shorter wavelength light source to improve the resolution of the microscope. In 1932, Ruska invented a transmission electron microscope using an electron beam as the light source. The wavelength of the electron beam is much shorter than visible 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, meaning that the higher the voltage, the shorter the wavelength. At present, the resolution of TEM can reach 0.2nm.
The working principle of a transmission electron microscope is that an electron beam emitted by an electron gun passes through a condenser along the optical axis of the mirror body in a vacuum channel, and converges into a sharp, bright, and uniform beam of light through the condenser, which is irradiated on the sample inside the sample chamber; The electron beam passing through the sample carries the internal structural information of the sample. The amount of electrons passing through the dense part of the sample is less, while the amount of electrons passing through the sparse part is more; After focusing and primary magnification through the objective lens, the electron beam enters the lower intermediate lens and the first and second projection mirrors for comprehensive magnification imaging. Finally, the magnified electron image is projected onto the fluorescent screen 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.
The uses of transmission electron microscopy
Transmission electron microscopy is widely used in materials science and biology. Due to the easy scattering or absorption of electrons by objects, the penetration force is low, and the density, thickness, and other factors of the sample can affect the final imaging quality. Therefore, thinner ultra-thin slices, usually 50-100nm, must be prepared. So when observing with a transmission electron microscope, the sample needs to be processed very thinly. 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 a pre treated copper mesh.
