The lack of binding partners on one side causes the atoms on the surface to adopt an energetically more favorable state by changing their bond length to deeper layers (relaxation) or by rearranging their positions and saturating open bonds (recombination). Surface structures can therefore have a different periodicity than deeper layers.
The band structure is disturbed or interrupted near the surface . Surface states allow electrons in the band gaps to adopt atypical energy states. Surface effects lead to special localized states in real crystals. Charge imbalances occur on surfaces , the electrostatic effects of which influence the function of semiconductors and non-conductors .
Geometry and surface
The surface of a crystalline solid is understood as the area of the interface in which the geometric and electronic structure differs significantly from that of the bulk solid , i.e. essentially a few atomic layers counted from the surface.
The geometry of the surface is described with two-dimensional crystallography . Instead of the 14 Bravais grids in three dimensions, there are only five Bravais grids in two dimensions , the parallelogram , square , rectangle , hexagonal and the square face-centered grid .
Elementary processes on the surface
Adsorption of a substance on a surface means that atoms or molecules from the gas phase accumulate on the surface and areboundthere by van der Waals forces ( physisorption ) or chemical bonds ( chemisorption ). Therefore all solids in air are covered by at least one whole layer of molecules or atoms. Even individual foreign atoms can change the properties of surfaces. To avoid this, most experiments are carried out under a strong vacuum , mostly ultra-high vacuum.
- Physisorbates are usually very weakly bound, so the solid body has to be cooled at least with liquid nitrogen , often even with liquid helium , in order to examine physisorbates. They can be desorbed by heating to relatively low temperatures , i.e. i.e., evaporated from the surface.
- Chemisorbates are usually more strongly bound and some can be examined at room temperature, for more weakly bound chemisorbates cooling with liquid nitrogen is sufficient.
Desorption is the opposite of adsorption, in which attached atoms or moleculesovercomethe binding energy of the surface and leave the solid. Due to the different kinetics of adsorption and desorption, complex heterostructures can be generated in non-equilibrium.
Diffusion occurs both within a layer and between several layers. Diffusion barriers arise between the individual grid positions, in particular above the step of two layers. This position is particularly unfavorable from an energetic point of view, which is why the Ehrlich-Schwöbel barrier develops . Below the level, the binding force is stronger.
In addition, complex molecular systems produce thermodynamic degrees of freedom .
Methods of surface physics
The following spectroscopic methods are used in surface physics: Atomic spectroscopy :
- Auger Electron Spectroscopy (AES)
- Photoelectron Spectroscopy (PES)
- Ion scattering spectroscopy
- Optical scanning near field microscopy (SNOM)
- Scanning Transmission Electron Microscopy (STEM)
- Scanning tunnel microscopy (STM)
- Atomic Force Microscopy (AFM)
- Diffraction of low energy electrons (LEED)
- Diffraction of high energy electrons (RHEED)
- Electron Beam Microanalysis (EPMA)
- Field electron microscope (FEM)
- Field ion microscope (FIM)
- Helium atom beam scattering (HAS)
- Helium Ion Microscopy (SHIM)
- Metastable Impact Electron Spectroscopy (MIES)
- Low energy electron microscope (LEEM)
- Photoacoustic Spectroscopy (PAS)
- Scanning electron microscope (SEM)
- X-ray absorption spectroscopy : near edge x-ray absorption fine structure (NEXAFS) and SEXAFS
- Temperature-programmed desorption (TPD), also known as thermal desorption spectroscopy (TDS)
- Surface Sensitive X-ray Diffraction (SXRD)
- H. Lüth: Solid Surfaces, Interfaces and Thin Films . Springer Verlag; Munich, Berlin 2001, ISBN 3-540-42331-1 .
- K. Oura, VG Lifshits, AA Saranin, AV Zotov, M. Katayama: Surface Science . Springer Verlag, Heidelberg 2003, ISBN 3-540-00545-5 .
- S. Kowarik, K. Broch, F. Schreiber: Watching growth. In: Physik Journal 13 (2014) No. 12 Wiley-VCH Verlag, Weinheim 2014, .