Scanning Tunneling Electron Microscopy Applications
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.
