How a scanning tunneling electron microscope works
Way of working
Although the configurations of scanning tunneling electron microscopes are different, they all include the following three main parts: a mechanical system (mirror body) that drives the probe to move three-dimensionally relative to the surface of the conductive sample, and is used to control and monitor the probe. The electronic system for the distance from the sample and the display system for converting the measured data into images. It has two working modes: constant current mode and constant high mode.
Constant current mode
The tunneling current is controlled and kept constant by an electronic feedback circuit. Then the computer system controls the needle tip to scan on the sample surface, that is to make the needle tip move two-dimensionally along the x and y directions. Since the tunnel current needs to be controlled to be constant, the local height between the needle tip and the sample surface will also remain constant, so the needle tip will perform the same undulating movement with the ups and downs of the sample surface, and the height information will be reflected accordingly. come out. That is to say, the scanning tunneling electron microscope obtains the three-dimensional information of the sample surface. This working method obtains comprehensive image information, high-quality microscopic images, and is widely used.
Constant height mode
Keep the absolute height of the needle tip constant during the scanning process of the sample; then the local distance between the needle tip and the sample surface will change, and the size of the tunnel current I will also change accordingly; the change of the tunnel current I will be recorded by the computer and converted into an image The signal is shown, and a scanning tunneling electron microscope micrograph is obtained. This way of working is only suitable for samples with relatively flat surfaces and single components.
principle
Scanning tunneling microscope is a new type of microscopic device to distinguish the surface morphology of solids by detecting the tunneling current of electrons in atoms on the solid surface according to the principle of tunneling effect in quantum mechanics.
Due to the tunneling effect of electrons, the electrons in the metal are not completely confined within the surface boundary, that is, the density of electrons does not suddenly drop to zero at the surface boundary, but decays exponentially outside the surface; the decay length is about 1nm, which is A measure of the surface barrier for electrons to escape. If two metals are very close to each other, their electron clouds may overlap; if a small voltage is applied between the two metals, an electric current (called tunneling current) between them can be observed.
