Transmission electron microscopy (English): Transmission electron microscopy (TEM), referred to as transmission electron microscopy, projects an accelerated and concentrated electron beam onto a very thin sample. The electrons collide with atoms in the sample to change direction, resulting in a solid angle. scattering. The size of the scattering angle is related to the density and thickness of the sample, so that images with different brightness and darkness can be formed. Generally, the transmission electron microscope has a resolution of 0.1 to 0.2 nm and a magnification of tens of thousands to a million times. It is used to observe ultrastructure, that is, a structure that is less than 0.2 μm and cannot be seen under an optical microscope, and is also called "sub-display." microstructure".
Imaging principle
The imaging principle of transmission electron microscopy can be divided into three cases:
Absorption image: When electrons are injected into a sample of high mass and density, the main phase-forming effect is scattering. Where the mass thickness on the sample is large, the scattering angle of electrons is large, the electrons passing through are less, and the brightness of the image is dark. Early transmission electron microscopy was based on this principle.
Diffraction image: After the electron beam is diffracted by the sample, the diffraction wave amplitude distribution at different positions of the sample corresponds to the different diffraction ability of each part of the crystal in the sample. When a crystal defect occurs, the diffraction ability of the defect portion is different from the complete region, so that the diffraction é’µThe amplitude distribution is not uniform, reflecting the distribution of crystal defects.
Phase image: When the sample is as thin as 100Ã… or less, electrons can pass through the sample, the amplitude of the wave changes negligibly, and the imaging comes from the phase change.
Component
Electron gun: emits electrons, consisting of a cathode, a grid, and an anode. The electrons emitted from the cathode tube form a beam of rays through the small holes in the gate, and are accelerated by the anode voltage to be incident on the condensing mirror, thereby accelerating and pressurizing the electron beam.
Condenser: The electron beam is concentrated and the illumination intensity and aperture angle are controlled.
Sample chamber: Place the sample to be observed, and install a tilting table to change the angle of the sample, as well as equipment for heating and cooling.
Objective lens: A short-range lens with a high magnification, which is used to amplify an electronic image. The objective lens is the key to determining the resolution and imaging quality of the transmission electron microscope.
Intermediate mirror: a variable-magnification weak lens that acts to double-amplify an electronic image. By adjusting the current of the intermediate mirror, the image or electron diffraction pattern of the object can be selected for amplification.
Transmissive mirror: A high-powered strong lens that is used to magnify the intermediate image and image it on the screen.
There is also a secondary vacuum pump to evacuate the sample chamber and a camera to record images.
The transmission electron microscope structure consists of two major parts: the main body part is an illumination system, an imaging system, and an observation studio; the auxiliary part is a vacuum system and an electrical system.
1, lighting system
The system is divided into two parts: an electron gun and a converging mirror. The electron gun consists of a filament (cathode), a grid and an anode. The heating filament emits an electron beam. The voltage is applied to the anode and the electrons are accelerated. The potential difference between the anode and the cathode is the total acceleration voltage. Electrons with energy that are accelerated are emitted from the pores of the anode plate. The emitted electron beam energy is related to the acceleration voltage, and the gate functions to control the shape of the electron beam. The electron beam has a certain divergence angle. After adjustment by the convergence mirror, it is expected to obtain a divergence angle, which is a small or even zero parallel electron beam. The current density (beam current) of the electron beam can be adjusted by adjusting the current of the condenser mirror.
The size of the area on the sample that needs to be illuminated is related to the magnification. The higher the magnification, the smaller the illumination area, which in turn requires a finer electron beam to illuminate the sample. The electron beam directly emitted by the electron gun has a large beam spot size and poor coherence. In order to use these electrons more effectively, an illumination electron beam with high brightness and good coherence is obtained to meet the needs of the transmission electron microscope at different magnifications. The electron beam emitted by the electron gun still needs to be further concentrated to provide different beam spot sizes. , approximately parallel illumination beam. This task is usually done by two electromagnetic lenses called condensers. In the figure, C1 and C2 represent the first concentrating mirror and the second concentrating mirror, respectively. C1 usually remains unchanged, its role is to make the intersection of the electron gun into a reduced image, reducing its size by more than an order of magnitude. In addition, a beam tilting device is also installed in the illumination system, which makes it easy to tilt the electron beam in the range of 2° to 3° to illuminate the sample at certain specific tilt angles.
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