Electron Localization Function for the Al(111)/Al2O3(0001) Interface |
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| This picture depicts two slices through
the Electron Localization Function (ELF)1 for an
interface between Al(111) and Al-terminated a-Al2O3(0001)
as calculated using Density Functional Theory2.
The dashed line indicates the position of the interface, with a 5 layer
slab of Al at the top and slighly more than 4 O layers from the alumina
below. In all, there are 55 atoms in the model. The magnitude
of the ELF is given by a color scheme in which high values correspond to
red, and low to blue. The locations of the Al ions bisected by the
slice planes are labelled, and are characterized by very low charge
densities, as this is a pseudopotential calculation. The O atoms are
identified by the large red annular regions, as alumina is a very ionic
material, with most of the charge pulled towards the anions. In
contrast, the ELF in the Al shows delocalized, free-electron-like
behavior.
The ELF is a superb tool for investigating bonding at metal/ceramic interfaces. It is capable of illustrating covalent, ionic, and metallic bonding in a concise graphical manner, and can give the location of bonded electron pairs, dangling bonds, and lone pairs. In the case above, it tells us that there is an ionic interaction between the metal's interfacial Al atom (just above the interface line in the left slice) and its nearest-neighbor O atoms. This bond is very similar to the Al-O bonds in the bulk oxide, as can be seen by comparing the shape of the ELF at the interface with that found in the bulk. However, there is an additional covalent interaction between the oxide's interfacial Al monolayer (just below the interface line) and another atom from the metal. For more information, download our preprint (in PDF format): 1A. Savin, R. Nesper, S. Wengert, and T. F.
Fassler, Agnew. Chem. Int. Ed. Engl. 36 1808 (1997) |
