Laser detection atomic force microscope
The basic principle of atomic force microscopy is to fix one end of a microcantilever that is extremely sensitive to weak forces, and the other end has a tiny needle tip. The needle tip lightly contacts the surface of the sample. Due to the extremely weak repulsive force between the atoms at the tip of the needle and the atoms on the surface of the sample, the microcantilever with the needle tip will fluctuate and move in the direction perpendicular to the surface of the sample by controlling the constant force during scanning. By using optical detection or tunneling current detection methods, the position changes of the microcantilever corresponding to the scanning points can be measured, thereby obtaining information on the surface morphology of the sample. Next, we will take the Atomic Force Microscope (Laser AFM), a commonly used family of scanning probe microscopes, as an example to explain its working principle in detail.
The laser beam emitted by a laser diode is focused on the back of the cantilever through an optical system, and is reflected from the back of the cantilever to a spot position detector composed of photodiodes. During sample scanning, due to the interaction force between the atoms on the surface of the sample and the atoms at the tip of the microcantilever probe, the microcantilever will bend and fluctuate with the surface morphology of the sample, and the reflected beam will also shift accordingly. Therefore, by detecting changes in the position of the light spot through a photodiode, information on the surface morphology of the tested sample can be obtained.
Throughout the entire process of system detection and imaging, the distance between the probe and the tested sample is always maintained at the nanometer (10-9 meters) level. If the distance is too large, information on the surface of the sample cannot be obtained. If the distance is too small, it will damage the probe and the tested sample. The function of the feedback loop is to obtain the strength of the probe sample interaction from the probe during the working process, change the voltage applied in the vertical direction of the sample scanner, so as to make the sample expand and contract, adjust the distance between the probe and the tested sample, and in turn control the strength of the probe sample interaction, achieving feedback control. Therefore, feedback control is the core working mechanism of this system.
This system adopts a digital feedback control loop. Users can control the characteristics of the feedback loop by setting several parameters such as reference current, integral gain, and proportional gain in the parameter toolbar of the control software.
Atomic force microscopy is an analytical instrument used to study the surface structure of solid materials, including insulators. Mainly used to measure the surface morphology, surface potential, friction force, viscoelasticity, and I/V curve of materials, it is a powerful new instrument for characterizing the surface properties of materials. In addition, this instrument also has functions such as nano manipulation and electrochemical measurement.
