What did we do ?

The silicon sample was « flashed », which means that it was heated briefly in order to evacuate contaminants like oxide. His color changes, because the wavelength of the emitted light depends on the temperature.

 

 

 

Then, the warmed atoms are cooled slowly; they have enough time to reorganize differently: there is still a hexagonal symmetry, with a new order.

We can verify if the atoms are well reorganized thanks to the LEED (low energy electron diffraction).

How does it work?

Electrons are emitted thanks to a filament. The electrons head for the surface. They are reflected at angles corresponding to the surface order.

 

The interaction between the emitted electrons and the electronic cloud of the atoms is very important: so the incoming electrons are strongly diffracted by the first layer of atoms. That's why it allows the experimentalist to study the surface crystallography.

In the LEED picture, we can see a hexagonal periodic structure. There are also extraspots which correspond to the reorganization of the surface atoms. Indeed, the atoms on the surface of silicon spontaneously reorganize in a structure which is seven times bigger than the crystal lattice. It's called the 7x7 reconstruction of silicon.

 

Before the STM experiment, we have outgassed the tungsten tip to remove the insulating oxide covering the tip.