Schematic diagram of microscope imaging

Schematic diagram of microscope imaging

I know that the eyepiece is equivalent to a magnifying glass, but the magnifying glass is the same side of the object, and the object lens in the microscope magnifies the object, and the image formed should be in the microscope tube. If the principle of the eyepiece is the same as that of the magnifying glass, then its image is not magnified in the opposite direction to the human eye (the same side of the object), then how to see the secondary magnified image? The imaging principle of the microscope is shown in the figure. The objective lens has a short focal length and the eyepiece has a long focal length. The object is formed into an inverted real image AB through the objective lens. This image is located within the eyepiece focus (inside the lens tube), which can also be regarded as the object of the eyepiece. After passing the eyepiece, it becomes an upright virtual image It is the same as the magnifying glass, and the object image is on the same side).

This is the result of double refraction, not just the function of an eyepiece.

Operating principle of STM

STM works by using quantum tunneling effect. If the metal tip is used as an electrode and the measured solid sample is used as another electrode, when the distance between them is about 1nm, the tunneling effect will occur, and electrons will pass through the space barrier from one electrode to the other to form a current. And Ub: bias voltage; k: Constant, approximately equal to 1, Φ 1/2: average work function, S: distance.

It can be seen from the above formula that the tunnel current has a negative exponential relationship with the tip sample spacing S. Very sensitive to changes in spacing. Therefore, when the tip makes plane scanning on the surface of the measured sample, even if the surface has only atomic scale fluctuations, it will lead to a very significant, even close to the order of magnitude, change in the tunnel current. In this way, the fluctuation of atomic scale on the surface can be reflected by measuring the change of current, as shown on the right side of the figure below. This is the basic working principle of STM. This operation mode is called constant height mode (keeping the needle tip height constant).

STM also has another working mode, called constant current mode, as shown on the left of the figure below. At this time, the tunnel current is kept constant through the electronic feedback loop during tip scanning. In order to maintain a constant current, the needle tip moves up and down with the fluctuation of the sample surface, so as to record the trajectory of the needle tip moving up and down, which can give the appearance of the sample surface.

The constant current mode is the common working mode of STM, while the constant height mode is only suitable for imaging samples with small surface fluctuation. When the sample surface fluctuates greatly, because the tip is very close to the sample surface, constant height mode scanning is easy to cause the tip to collide with the sample surface, resulting in damage to the tip and the sample surface.

STM schematic diagram

Working principle of AFM The basic principle of AFM is similar to that of STM. In AFM, the needle tip on the elastic cantilever, which is very sensitive to weak force, is used to conduct grating scanning on the sample surface. When the distance between the tip and the sample surface is very close, there is a very weak force (10-12~10-6N) between the atoms at the tip and the atoms on the sample surface. At this time, the micro cantilever will undergo small elastic deformation. The force F between the tip and the sample and the deformation of the microcantilever follow Hooke's law: F=- k * x, where k is the force constant of the microcantilever. Therefore, as long as the micro cantilever deformation is measured, the force between the tip and the sample can be obtained. The force between the tip and the sample is strongly dependent on the distance, so during the scanning process, the feedback loop is used to keep the force between the tip and the sample constant, that is, to keep the shape variable of the cantilever unchanged, the tip will move up and down with the fluctuation of the sample surface, and the information of the sample surface morphology can be obtained by recording the trajectory of the tip's movement up and down. This working mode is called "Constant Force Mode" and is the most widely used scanning mode. The image of AFM can also be obtained by using the "ConstantHeightMode", that is, in the X and Y scanning process, the feedback loop is not used, the distance between the tip and the sample is kept constant, and the image is formed by measuring the deformation of the micro cantilever in the Z direction. This method does not use a feedback loop and can use a higher scanning speed. It is usually used more when observing atomic and molecular images, but is not applicable to samples with large surface fluctuations.

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