The idea of tunneling can be applied to look at the surface profile of a material at the atomic scale. When a sharp metallic tip is brought very close to the surface of a conductor, an electric current can be detected due to the tunneling of electrons through the air gap. In this case, the air gap is considered as the barrier and is only several nanometers thick. Now, by moving the tip horizontally across the surface (Fig. 6), you will find the current varies exactly as the same pattern as the profile of the surface. In other words, the variation of current duplicates the profile of the surface and we can "view" the surface simply by measuring the tunneling current. This forms the basic operation of a STM. The scanning tip (Fig. 7) of a STM is so "sharp" that it only consists of a few atoms, and the tunneling current is as small as a few nanoamperes.

The following animations will help you understand the working principle of STM:

Animation: The working principle of Scanning Tunneling Microscope

Animation: Tunneling current and thickness of air gap


 

The components of a STM include scanning tip, piezoelectric controlled scanner, distance control and scanning unit, vibration isolation system, and computer (Fig. 8).

1.

Scanning tip:
Electrons tunnel from the scanning tip to the sample, creating the tunnelling current.

2.

Piezoelectric controlled scanner:
Piezoelectric crystals expand and contract very slightly depending on the voltage applied to them and this principle is used to control the horizontal position x, y, and the height z of the scanning tip.

3.

Distance control and scanning unit:
Position control using piezoelectric means is extremely fine, so a coarse control is needed to position the tip close enough to the sample before the piezoelectric control can take over.

4.

Vibration isolation system:
STM deals with extremely fine position measurements so the isolation of any vibrations is very important.

5.

Computer:
The computer records the tunneling current and controls the voltage to the piezoelectric tubes to produce a 3-dimensional map of the sample surface.


The website http://www.virlab.virginia.edu/VL/easyScan_STM.htm is a very good virtual STM lab from an American university. It contains the experimental aspect of how to operate a STM.

 
 
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