Surface reconstruction causes structural variations in nanometric amorphous Al₂O₃

The physical properties of nanocrystalline materials are known to be size dependent, owing to surface effects. Theoretically, a similar effect should also exist in amorphous materials. To examine this possibility, we carried out a study in which amorphous thin films of aluminum oxide were produced by atomic layer deposition (ALD) and studied by X-ray Absorption Near Edge Structure Spectroscopy (XANES) in grazing incidence geometry, as a function of the grazing angle. This allowed us to probe the Al local environment as a function of depth from the surface. The fraction of Al₆ sites was found to be substantially lower at the surface than deeper in the film, meaning that the surface is relatively rich in Al₄ sites. These results are in line with previous theoretical and experimental findings, shed further light on the structure and properties of amorphous nanometric materials and indeed indicate that size effects exist in amorphous nanometric materials.