Scanning Tunneling Electron Microscopy Principle
Scanning tunneling microscope (STM) is an instrument that uses the tunneling effect in quantum theory to detect the surface structure of materials. It uses the quantum tunneling effect of electrons between atoms to convert the arrangement of atoms on the surface of materials into image information. of.
Introduction
The transmission electron microscope is very useful in observing the overall structure of the substance, but it is more difficult in the analysis of the surface structure, because the transmission electron microscope obtains information through the high-energy electricity through the sample, reflecting the sample substance. inside information. Although scanning electron microscopy (SEM) can reveal certain surface conditions, since the incident electrons always have a certain energy and will penetrate into the sample, the so-called "surface" analyzed is always at a certain depth, and the splitting rate is also greatly affected. limit. Although Field Emission Electron Microscope (FEM) and Field Ion Microscope (FIM) can be well used for surface research, the sample must be specially prepared and can only be placed on a very thin needle tip, and the sample must also be able to withstand high-intensity electric fields, so that It limits its scope of application.
Scanning Tunneling Electron Microscope (STM) works on a completely different principle, it does not obtain information about the substance of the sample by acting on the sample with an electron beam (such as transmission and scanning electron microscopes), nor does it use a high electric field to make the electrons in the sample gain more than come out The emission current imaging (such as field emission electron microscope) formed by the energy of work can be used to study the sample material. It is imaged by detecting the tunnel current on the surface of the sample, so as to study the surface of the sample.
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.
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 make three-dimensional movements 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 ups and downs 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 is recorded by the computer and converted into The image signal is displayed, that is, a scanning tunneling electron microscope micrograph is obtained. This way of working is only suitable for samples with relatively flat surfaces and single components.
