2 - Surface crystallography and diffraction

Summary

Surface symmetry

The classification and description of symmetry properties and structures of bulk (three-dimensional) crystalline materials require a reasonable understanding of crystallography; notably of the restricted number of types of translational symmetry which crystals can possess (characterised by their associated unit cell which must be one of the 14 Bravais lattices) and the finite number of point and space groups which can define the additional symmetry properties of all possible crystals. Many properties of solids are intimately related to the special symmetry properties of these materials. While a solid surface is intrinsically an imperfection of a crystalline solid, destroying the three-dimensional periodicity of the structure, this region of the solid retains two-dimensional periodicity (parallel to the surface) and this periodicity is an important factor in determining some of the properties of the surface. In particular, it plays a dominant role in allowing electron, X-ray and atom diffraction techniques to provide information on the structure of the surface, as well as strongly influencing the electronic properties of the surface. For these reasons a proper understanding of surface crystallography is important for a general understanding of many surface effects and is critical for an understanding of the electron diffraction techniques, Low Energy Electron Diffraction (LEED) and Reflection High Energy Electron Diffraction (RHEED), of surface X-ray diffraction and of He atom diffraction (see chapter 8).