1, structural difference: mainly reflected in the different positions of the sample on the electron beam path. The transmission electron microscope sample is in the middle of the electron beam, the electron source emits electrons above the sample, passes through the condenser, and then penetrates the sample, and then the subsequent electromagnetic lens continues to amplify the electron beam and finally projects it on the fluorescent screen; The sample of SEM is at the end of the electron beam, and the electron beam emitted by the electron source above the sample reaches the sample after being reduced by several electromagnetic lenses. Of course, the structure of the subsequent signal detection and processing system will be different, but there is no substantial difference in basic physical principles. Similarities: they are all electric vacuum equipment, and the use principle of most parts is the same, such as electron gun, magnetic lens, various control principles, astigmatism, shaft closure and so on. 2. Basic working principle: transmission electron microscope: when the electron beam passes through the sample, it will scatter with the atoms in the sample, and at the same time, the direction of the electrons passing through a point on the sample is different, so that the point on the product is between 1-2 times the focal length of the objective lens, and these electrons are amplified by the objective lens and then converged again to form an enlarged real image of the point, which is the same as the convex lens imaging principle. Let's not talk about the theory of contrast formation mechanism here, but it is conceivable that if the inside of the sample is absolutely uniform and there is no grain boundary and atomic lattice structure, then the enlarged image will not have contrast. In fact, this substance does not exist, so there is a reason for the existence of this awesome instrument. The image amplified by the objective lens is further amplified by several intermediate magnetic lenses (the specific order is basically determined by the brightness of the electron beam, and if the brightness is infinite, it is finally determined by the resolution formula of Abelian optical instrument), and finally it is projected on the fluorescent screen for imaging. Due to the short focal length and small aberration coefficient of the objective lens of transmission electron microscope, the spatial resolution of transmission electron microscope is very high, which is 0. 1-0.2nm, but the depth of field is relatively small, which is insensitive to the surface morphology of the sample, and mainly observes the internal structure of the sample. Scanning electron microscope: the electron beam reaches the sample and excites the secondary electrons in the sample. The secondary electrons are received by the detector, and the pixels on the display are modulated to emit light through signal processing. Because the diameter of the electron beam spot is nanometer, and the pixels of the display are above 100 micron, the light emitted by the pixels above 100 micron represents the light emitted from the area excited by the electron beam on the sample. Realize the amplification of this object point on the sample. If the electron beam is raster scanned in a certain area of the sample, and the brightness of the display pixels is modulated according to the geometric arrangement, the enlarged imaging of the sample area can be realized. No specific image contrast formation mechanism is mentioned. Because the surface of the sample observed by SEM is very rough, it usually needs a long working distance, which requires the objective lens of SEM to have a long focal length and a large phase difference coefficient, which limits the brightness under the minimum beam spot size, and the spatial resolution of the system is generally much lower than that of transmission electron microscope 1-3 nm. However, due to the long focal length of the objective lens, the depth of field of the image is much higher than that of the transmission electron microscope. It is mainly used to observe the surface morphology of the sample, and the internal structure cannot be revealed from the surface. Unless the sample is destroyed, such as FIB-SEM of focused ion beam electron beam scanning electron microscope, the internal structure can be observed layer by layer. Transmission electron microscope and scanning electron microscope are fundamentally different in imaging principle. The electrons bombarded on the fluorescent screen by transmission electron microscope imaging are electrons in the electron beam passing through the sample, while the secondary electron signal pulse imaged by scanning electron microscope is only used as a signal to modulate the grid of CRT triode electron gun on traditional CTR display. Transmission electron microscope can be said to have seen electron optical imaging, while scanning electron microscope can't imagine electron optical imaging at all. 3. Sample preparation: TEM: The penetration ability of electrons is very weak. Transmission electron microscope often uses hundreds of thousands of volts of high-energy electron beams, but it is still the most basic requirement to grind the sample or ion thin or ultra-thin slice to the thickness of micro-nanometer. Transmission sample preparation is knowledge, and the quality of sample preparation often depends on luck. The sample preparation process specification is posted in the electron microscope laboratory and sample preparation room of Peking University Institute of Physics, and the conclusion is good luck! SEM: Almost no sample preparation, direct observation. Most non-conductors need to make conductive films, and most of them can be done in a few minutes. Water-containing biological samples need to be fixed, dehydrated and dried without deformation, which is troublesome. It takes several days to dry naturally. Both of them have the same requirements for samples: solid, as dry as possible, as free from oil pollution as possible, and the overall dimensions meet the requirements of the sample room.