Introduction to Scanning Tunneling Electron Microscopy
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.
application
The principle of the tunneling microscope is to cleverly use the physical tunneling effect and tunneling current. There are a large number of "free" electrons in the metal body, and the energy distribution of these "free" electrons in the metal body is concentrated near the Fermi level, and there is a potential barrier with energy higher than the Fermi level on the metal boundary. Therefore, from the perspective of classical physics, "free" electrons in a metal, only those electrons whose energy is higher than the boundary barrier, can escape from the inside of the metal to the outside. However, according to the principles of quantum mechanics, free electrons in metals also have wave properties, and when this electron wave propagates to the metal boundary and encounters a surface barrier, part of it will be transmitted. That is to say, some electrons with energy lower than the surface potential barrier can penetrate the metal surface potential barrier and form an "electron cloud" on the metal surface. This effect is called tunneling. So, when two metals are in close proximity (less than a few nanometers), the electron clouds of the two metals will penetrate each other. When an appropriate voltage is applied, even if the two metals are not really in contact, a current will flow from one metal to another. This current is called tunnel current.
Tunnel current and tunnel resistance are very sensitive to changes in the tunnel gap. Even a change of 0.01nm in the tunnel gap can cause significant changes in the tunnel current.
If a very sharp probe (such as a tungsten needle) is used to scan parallel to the surface in the x and y directions at a height of a few tenths of nanometers away from the smooth sample surface, since each atom has a certain size, the The middle tunnel gap will vary with x and y, and the tunnel current flowing through the probe will also be different. Even height variations of a few hundredths of a nanometer can be reflected in tunneling currents. A recorder synchronized with the scanning probe is used to record the changes of the tunneling current, and a scanning tunneling electron microscope image with a resolution of a few hundredths of nanometers can be obtained.
