Transmission Electron Microscope Working Principle and Usage
Transmission Electron Microscope (TEM), can see in the optical microscope can not see less than 0.2 um of fine structure, these structures are called sub-microscopic structure or ultramicrostructure. To see these structures, it is necessary to choose a shorter wavelength of the light source, in order to improve the resolution of the microscope. 1932 Ruska invented the electron beam as the light source of the transmission electron microscope, the wavelength of the electron beam is much shorter than the wavelength of visible light and ultraviolet light, and the wavelength of the electron beam and the emission of the electron beam of the square root of the voltage inversely proportional to that is to say that the higher the voltage of the wavelength of the shorter. Currently the resolving power of TEM is up to 0.2nm.
Transmission electron microscope working principle is the electron beam emitted by the electron gun, in the vacuum channel along the optical axis of the mirror body through the condenser mirror, through the condenser mirror will be converged into a beam of sharp, bright and uniform spot, irradiation of samples in the sample chamber on the samples; through the samples after the beam of electrons carrying samples with internal structural information, samples in the dense through the amount of electrons is small, the amount of electrons transmitted through the sparser place more electrons; after the convergence of the objective lens focusing and After the objective lens convergence focusing and primary magnification, the electron beam into the lower level of the intermediate lens and the first, second projection mirror for integrated magnification imaging, and ultimately the enlarged electronic image projected on the observation room of the fluorescent screen board; fluorescent screen will be converted into a visible image of the electronic image for the user to observe. In this section, the main structures and principles of each system are described.
The imaging principle of transmission electron microscope can be divided into three cases:
1. Absorption like: when the electron shot to the mass, density of the sample, the main phase-forming effect is the scattering effect. Sample on the mass thickness of the place on the scattering angle of the electron is large, through the electron is less, like the brightness of the darker. Early transmission electron microscopes were based on this principle.
2. Diffraction image: After the electron beam is diffracted by the sample, the amplitude distribution of the diffraction wave at different positions of the sample corresponds to the different diffractive ability of each part of the crystal in the sample. When there is a crystal defect, the diffractive ability of the defective part is different from that of the intact area, thus making the amplitude distribution of the diffraction wave uneven and reflecting the distribution of the crystal defect.
3. Phase image: When the sample is as thin as 100Å or less, the electrons can pass through the sample, and the amplitude change of the wave can be neglected, and the imaging comes from the phase change.
