Surface reconstruction

A surface reconstruction of the arrangement of the atoms of a solid surface, if this is different than ( gedanklichem ) cutting of the crystal lattice of the volume. This does not take minor shifts of atoms ( surface relaxation ) ignored if they do not change the symmetry of the surface.

Reconstructed surfaces usually have a larger unit cell as a (sometimes hypothetical ) unrekonstruierte surface of the same crystal lattice. In this case, one also speaks of a superstructure. Often reconstructions are marked by the indication of the superlattice cell; However, due to the fact not the atomic arrangement of the reconstructed surface.

Reason for the formation of Oberflächenrelaxationen is to reduce the free energy. In most cases, the reconstructed surface and the lowest total energy ( surface energy ), and is also at the absolute zero point the lowest surface structure.

Semiconductor surfaces

The surface reconstructions of semiconductor surfaces can usually be explained by the fact that the number of " cut-off " bonds ( dangling bonds, Eng. Dangling bonds ) are minimized.

Si (100): When ( mental ) cutting a silicon crystal ( diamond lattice ) along the ( 100) plane are broken two bonds per silicon atom. Pairs of respectively adjacent silicon atoms bond with each of the cut-off linkages to each other and form so-called dimer. It remains only a unabgesättigte bond per silicon atom. In this case, the lattice is distorted so that each an atom in the dimer is higher, one lower in the list (English buckled dimer); immediately adjacent dimers are oriented in opposite directions. At room temperature, the dimers change but by thermal excitations rapidly their orientation and appear in the scanning tunneling microscope symmetrical.

The stable surface of the Si ( 111) surface has a complex (7 x 7) Reconstruction. The structure, the dimer - adatom - stacking fault model (English dimer adatom stacking fault DAS) was proposed in 1985 by K. Takayanagi and co-workers, using measurements by Gerd Binnig and co-workers with the scanning tunneling microscope constituted an important basis.

Metal surfaces

At the surfaces of pure metals reconstructions are less frequent than with semiconductors; except for the face-centered cubic metals of the sixth period, Ir, Pt and Au. In these three metals, the ( 100) surfaces reconstruct and form instead of the square lattice of the face-centered cubic structure, a hexagonal close-packed atomic layer at the surface. The ( 110) surfaces of these metals show a "missing row" reconstruction; thereby missing every second close-packed atomic row. Gold in the ( 111) surface is reconstructed; the topmost atomic layer is contracted ( herringbone reconstruction, Eng. herringbone reconstruction ). These reconstructions are caused by the particularly low surface energy dense hexagonal surfaces of these metals and a high tensile stress in the surface.

Non-conductors

In non-conductors, in particular ionic crystals and oxides most frequent reason for the formation of surface reconstructions is that surfaces of macroscopic objects substantially electrically neutral ( " charge- compensated " ) need to be. For example, a unrekonstruierte ( 111) surface of NaCl lattice would be a polar surface, which means that the topmost atomic layer would either only positive or only negative ions, which would lead to an extremely high electric field. This can be avoided by a reconstruction, in which a part of the ions in the uppermost atom layer is missing.

Adsorbate -induced reconstruction and deconstruction

Reconstructions can be caused by adsorbate atoms or molecules. Most of the surface atoms arrange themselves in such a way in order that a favorable binding geometry for the adsorbate is created.

Many reconstructions pure surfaces are, however, repealed by adsorbates. For example, the dangling bonds are saturated by the silicon surface by hydrogen, so that the reason for the reconstruction of omitted.