A schematic illustrating the microscope's image system
The function of the eyepiece is equivalent to that of a magnifying glass, but the image of the magnifying glass is on the same side as the object. After the objective lens in the microscope magnifies the object, the resulting image should be in the microscope tube. If the principle of the eyepiece is the same as that of the magnifying glass, isn’t its image facing The human eye zooms in the opposite direction (the same side of the object), so how do you know how to see the double-magnified image? The imaging principle of the microscope is shown in the figure. The focal length of the objective lens is short, and the focal length of the eyepiece is long. The object forms an inverted real image A"B through the objective lens ", the image is located within the focal point of the eyepiece (inside the lens barrel), and it can also be regarded as the object of the eyepiece, which becomes an upright virtual image after passing through the eyepiece; it is still the same as the magnifying glass, and the object image is on the same side).
How STMs work
STM works by utilizing the quantum tunneling effect. If the metal needle tip is used as one electrode, and the solid sample to be measured is used as the other electrode, when the distance between them is as small as about 1nm, a tunnel effect will appear, and electrons will pass through the space barrier from one electrode to the other electrode to form a current. . And where Ub: bias voltage; k: constant, approximately equal to 1, Φ1/2: average work function, S: distance.
It can be seen from the above formula that the tunneling current has a negative exponential relationship with the tip-sample distance S. Very sensitive to changes in spacing. Therefore, when the needle tip scans the surface of the sample to be tested, even if the surface has only atomic-scale fluctuations, it will cause very significant changes in the tunnel current, even close to the order of magnitude. This allows atomic-scale fluctuations in the surface to be reflected by measuring changes in electrical current, as shown on the right in the image below. This is the basic working principle of STM, and this mode of operation is called constant height mode (keep the tip height constant).
STM also has another working mode, called constant current mode, as shown on the left side of the figure below. At this time, during the tip scanning process, the tunnel current is kept constant through the electronic feedback loop. In order to maintain a constant current, the needle tip moves up and down with the ups and downs of the sample surface, so as to record the trajectory of the needle tip's up and down movement, and then the topography of the sample surface can be given.
Constant current mode is a commonly used working mode of STM, while constant height mode is only suitable for imaging samples with little surface fluctuation. When the sample surface fluctuates greatly, since the needle tip is very close to the sample surface, scanning in constant height mode may easily cause the needle tip to collide with the sample surface, resulting in damage to the needle tip and the sample surface.
How AFMs work
The basic principle of AFM is similar to that of STM. In AFM, a needle tip on an elastic cantilever that is very sensitive to weak forces is used to scan the sample surface in a raster manner. When the distance between the needle tip and the sample surface is very close, there is a very weak force (10-12~10-6N) between the atoms at the tip of the needle tip and the atoms on the sample surface. At this time, the micro-cantilever will undergo a small elastic deformation . The force F between the tip and the sample and the deformation of the cantilever follow Hooke's law: F=-k*x, where k is the force constant of the cantilever. Therefore, as long as the deformation of the micro-cantilever is measured, the force between the tip and the sample can be obtained. The force between the needle tip and the sample has a strong dependence on the distance, so the feedback loop is used to keep the force between the needle tip and the sample constant during the scanning process, that is, the deformation of the cantilever is kept constant, and the needle tip will follow the sample. The ups and downs of the surface move up and down, and the trajectory of the needle tip’s up and down movement can be recorded to obtain the information of the surface topography of the sample. This working mode is called "Constant Force Mode" and is the most widely used scanning method.
AFM images can also be obtained using the "Constant Height Mode", that is, during X, Y scanning, without using a feedback loop, keeping the distance between the needle tip and the sample constant, by measuring the Z direction of the microcantilever The amount of deformation to image. This method does not use a feedback loop and can adopt a higher scanning speed. It is usually used more when observing atoms and molecules, but it is not suitable for samples with relatively large surface fluctuations.
