What is tem?TEM,you can see the fine structure which are unable to see clearly under the optical microscope are smaller than 0.2um.These structures are called sub microstructure or ultrastructure. To see these structures, we have to choose a shorter wavelength light source to improve the resolution of the microscope. In 1932, Ruska invented the transmission electron microscope, the electron beam is the light source, the wavelength of the electron beam is much shorter than that of the visible light and ultraviolet light, and the wavelength of the electron beam is inversely proportional to the square root of the emitted electron beam. The TEM resolution is up to 0.2nm.
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What is tem---Large transmission electron microscope
Large transmission electron microscopy, the general use of 80-300kV electron beam acceleration voltage, different models corresponding to different electron beam acceleration voltage, its resolution and electron beam acceleration voltage, up to 0.2-0.1nm, high-end models can achieve atomic resolution.
What is tem---Low pressure transmission electron microscope
The electron beam accelerating voltage (5kV) used in the low pressure and small transmission electron microscope (TEM) is much lower than that of the large transmission electron microscope. The low acceleration voltage can enhance the effect of the electron beam and the sample, so that the image contrast enhancement, especially suitable for high molecular and biological samples, while the low pressure transmission electron microscopy, is less damage to the sample. 
Resolution is lower, 1-2nm. Due to the low voltage, the transmission electron microscopy, scanning electron microscopy and scanning transmission electron microscopy can be integrated in a single device.
What is tem---Cryo-microscopy
Cryo-microscopy is usually used in the ordinary transmission electron microscope, the sample cooling to liquid nitrogen temperature (77K), for the observation of protein, biological chip and other temperature sensitive samples. By freezing the samples, we can reduce the damage of the electron beam to the sample, reduce the deformation of the sample, and then get more real sample morphology.
The resolution of the electron microscope is far better than that of optical microscope, but the electron microscope can work under vacuum condition. Therefore, it is very difficult to observe the living creature. Other problems, such as the electron gun brightness and the improvement of the quality of the electron lens, also need to be further studied.