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The microscope with tunnel effect (in English STM, Scanning Tunneling Microscope ) was invented in 1981 by researchers of IBM, Gerd Binnig and Heinrich Rohrer, which accepted the Nobel Prize of physics for this invention in 1986. It is a microscope in close field. The microscope with tunnel effect uses a quantum phenomenon , the Tunnel effect, for to determine morphology and the Density of electronic states of conducting or semiconductor surfaces with a space resolution being able to be equal or lower than the size of the atoms.

Principle of operation

It acts, to simplify, of a feeler (a point) which follows the surface of the object. The point sweeps ( scanne ) surface to be represented. A computer adjusts (via a system of control) in real-time the swing-over bed to maintain current constant (running tunnel) and records this height which makes it possible to reconstitute surface.

For that, with a system of positioning of high degree of accuracy (carried out using piezoelectric S), one places a conducting point opposite surface to be studied and one measures the current resulting from the passage of electrons between the point and surface by Tunnel effect (the free electrons of metal leave a little surface, if one puts oneself very close without touching it, one can record an electric current). In the majority of the cases, this current depends very quickly (Exponentielle lies) on the distance separating the point from surface, with a distance characteristic of some tenth of Nanomètre S. Ainsi, one makes move the point with the top of the sample with a movement of sweeping and one adjusts the height of this one so as to preserve an intensity of the current tunnel constant, by means of a Boucle of feedback. One can then determine the profile of surface with a precision lower than the interatomic distances.

But let us remember that one has a Synthesized image, not a “photography” the atoms.

Technical details

Mechanical properties

To obtain a good resolution, it is necessary that the external disturbances cannot modify the distance point-surface (this one should not vary of more than few tenth of ångströms). For this reason, the microscopes small (a few centimetres) and are built in very rigid materials. Moreover, it is essential, in the majority of the cases, to use a system of damping to isolate it from the external vibrations.

System of positioning

The only means of reaching the precision sufficient for positioning is to use piezoelectric ceramics . In fact, it is this point which delayed the invention of the apparatus because the idea existed since the Sixties, but no system of adapted positioning existed then.

Positioning is done by application of potential difference on piezoelectric ceramics which has the property to become deformed in a way controlled under the effect of an electric field.

Electronics

The currents to be measured being of very low intensity (some Na, even some Pa), an electronic system of amplification is essential.

The point

The properties of the point are critical for the performances of the instrument. So various types of points are used according to the nature of studied surface and required information.

As long as surface is roughly plane on an atomic scale, the very fast variation of the current tunnel with the distance point-surface makes that only the atom of the point nearest to surface imports. In this case, the shape of the point does not have an influence on the resolution. On the other hand, if surface is broken, the shape of the point will limit the resolution and it is then essential to use a very fine point and thus to use a hard material like the Tungstène (W) or the iridic Platine (Pt/Ir).

Another difficulty comes owing to the fact that the majority of surfaces very quickly recover oxidized layer of a few tens of ångströms thickness, invisible in the everyday life but which prevents the passage of the current tunnel. There exist two means of circumventing this problem:

to use a noble metal which does not oxidize, like the Or or platino-iridium,
to make function the vacuum microscope or in an inert atmosphere (Diazote, Hélium,…) and to prepare the point in situ , i.e. in the same enclosure.

In addition, by using particular points, it is possible to reach information such as chemical nature or the magnetic properties of surface.

History

In 1990, the microscope with tunnel effect made it possible researchers of IBM to write the first letters of the history of the Nanotechnologie S by laying out 35 atoms of Xénon, on a surface of Nickel, these 35 atoms drawing the three IBM letters.

Limitations

Microscopy with tunnel effect requires to have a conducting sample of electricity. If the sample is insulating, one uses a close technique, the microscopy with atomic force. In addition the microscope with tunnel effect does not make it possible to see only the electronic clouds of the atoms. Thus an amorphous matter, (noncrystalline) cannot be observed with the atomic resolution.

Bond external

site of IBM - http://www.almaden.ibm.com/vis/stm/stm.html

http://perso.crans.org/~baffou/

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