STM Principle and AFM Working Principle of Microscopes
Working principle of STM
STM works by utilizing the quantum tunneling effect. If the metal needle tip is used as one electrode and the measured solid sample is used as another electrode, a tunneling effect will occur when the distance between them is about 1nm, and electrons will pass through the spatial potential barrier from one electrode to the other electrode to form a current. And Ub: bias voltage; k: Constant, approximately equal to 1, Φ 1/2: Average work function, S: Distance.
From the above equation, it can be seen that the tunnel current has a negative exponential relationship with the spacing S between the needle tip samples. Very sensitive to changes in spacing. Therefore, when the needle tip performs a planar scan on the surface of the tested sample, even if the surface has only atomic scale fluctuations, it will cause very significant, or even close to an order of magnitude, changes in the tunnel current. In this way, the fluctuation of atomic scale on the surface can be reflected by measuring the changes in current, as shown on the right side of the following figure. This is the basic working principle of STM, which is called constant height mode (keeping the needle tip height constant).
STM has another operating mode, called constant current mode, as shown on the left side of the figure. At this point, during the needle scanning process, the tunnel current is maintained constant through an electronic feedback loop. To maintain a constant current, the needle tip moves up and down with the fluctuation of the sample surface, thus recording that there is another working mode of STM on the needle tip, called constant current mode, as shown on the left side of the figure below. At this point, during the needle scanning process, the tunnel current is maintained constant through an electronic feedback loop. To maintain a constant current, the needle tip moves up and down with the fluctuation of the sample surface, thereby recording the trajectory of the needle tip's up and down movement, and providing the morphology of the sample surface.
The constant current mode is a commonly used working mode for STM, while the constant height mode is only suitable for imaging samples with small surface fluctuations. When the surface of the sample fluctuates significantly, due to the needle tip being very close to the sample surface, using constant height mode scanning can easily cause the needle tip to collide with the sample surface, leading to damage between the needle tip and the sample surface.
The working principle of AFM
The basic principle of AFM is similar to STM, in which a needle tip on an elastic cantilever that is very sensitive to weak forces is used to perform grating scanning on the surface of the sample. When the distance between the tip of the needle and the surface of the sample is very close, there is a very weak force (10-12-10-6N) between the atoms at the tip of the needle and the atoms on the surface of the sample. At this time, the microcantilever will undergo small elastic deformation. The force F between the needle 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. So, as long as the size of the micro cantilever deformation variable is measured, the magnitude of the force between the needle tip and the sample can be obtained. The force between the needle tip and the sample is strongly dependent on the distance, so a feedback loop is used during the scanning process to maintain a constant force between the needle tip and the sample, which is maintained as a cantilever shape variable. The needle tip will move up and down with the fluctuation of the sample surface, and the trajectory of the needle tip's movement up and down can be recorded to obtain information on the surface morphology of the sample. This working mode is called the 'Constant Force Mode' and is the most widely used scanning method.
AFM images can also be obtained using the "Constant Height Mode", which means that during the X, Y scanning process, no feedback loop is used to maintain a constant distance between the needle tip and the sample, and imaging is achieved by measuring the shape variable in the Z direction of the microcantilever. This method does not use a feedback loop and can adopt a higher scanning speed. It is usually used more frequently when observing atomic and molecular images, but is not suitable for samples with large surface fluctuations.
