PRINCIPLE:The Scanning Tunneling Microscope works on the principle of tunneling, i.e. the quantum physical phenomenon which makes electrons go through regions where they are not allowed to go classically.
WORKING:The Scanning Tunneling Microscope consists of a platinum-rhodium or tungsten needle. This is the probe. This needle type stylus (or the probe) is scanned through the surface to be observed. The surface should be conducting. When the probe is brought close to the surface, electrons can tunnel through the gap. This is observed as a current.
There are two modes of working of the Scanning Tunneling Microscope - the constant current mode and the constant-z mode.
In the constant current mode of working of the scanning tunneling microscope, a needle (stylus or the probe) moves through the surface of the sample to be scanned, and moves up and down according to the structure of the surface. For example, at places where there is a rise at the surface, the scanning stylus moves up and at depressions, moves down.
This moving up and down of the probe is achieved by fixing it to a piezoelectric cylinder, which expands or contracts according to the potential difference applied to it. In this way, the surface of the given sample can be observed. The tip of the probe can be made as sharp as just one atom thick. So, the surface can be observed very precisely, down to the atomic scale.
Moving the probe up and down according to the rises and falls of the sample surface keeps the distance between the stylus and the surface constant. This in turn keeps the tunneling current constant as tunneling current depends on the distance between the stylus and the surface.
In the constant-z mode of operation, the vertical tip of the probe is held constant, and the current through it is monitored. The tunneling probability is very sensitive to the size of the gap. So, atomic-scale variations in height of the surface can be observed.
Atomic Force Microscope
Scanning Electron Microscope