It uses quantum mechanical concept according to which wave nature of electron allow them to tunnel through a very thin electrically insulating layer of vacuum or air.

STM scans an electric probe over a surface to be imaged to detect a weak electric current flowing between the tip and the surface. It employs principle of electron tunneling of quantum mechanics.

A very sharp needle is brought close to the surface to be imaged. The distance is of the order of few A0. At such a distance, electron from the surface tunnel across the gap and set up a measurable tunneling current in the needle.

This current is a precise indicator of distance between tip and the surface. the needle can now be moved across the surface, at each point height being adjusted to maintain tunneling current at a constant value.

Thus, a vertical position of a needle becomes equivalent to a record of surface topography and can be converted into an observable picture.

Advantages :

1.Provide 3D profile of surface

2.Useful for charactering surface roughness

Applications :-

1. Observing surface defect

2. Determining size of molecule

3. STM tips are used to rotate individual bonds within molecules

4. High quality STM can reach significant resolution to show single atom

Disadvantage :

Required extremely clean, stable surface and conducting specimen.