Aluminum oxide (Al2O3), also known as corundum, is used in a variety of applications, for example as an insulator in electronic components or as a carrier material for catalysts. Inside the material, the atoms follow a fixed order. On the surface, however, the structure differs from that inside the crystal. Researchers at the Vienna University of Technology and the University of Vienna have now solved the mystery of the structure of the Al2O3 surface. The research group led by Jan Balajka and Ulrike Diebold recently published their results in the renowned journal Science.
High-resolution microscopy identifies surface atoms
The research team used atomic force microscopy to analyze the surface structure. In this method, the surface is scanned at close range using a sharp tip mounted on a quartz tuning fork. The frequency of the tuning fork changes when the tip interacts with the atoms on the surface without touching the material. This produces an image of the surface. The researchers attached a single oxygen atom to the tip so that they could distinguish between oxygen and aluminum atoms on the surface. The local repulsion and attraction on the Al2O3 surface made it possible to directly visualize the chemical identity of the individual surface atoms together with their position.
Restructuring stabilizes the surface
In this way, the team found that the surface restructures in such a way that the aluminum atoms can penetrate the surface and form chemical bonds with the oxygen atoms in the deeper layers. This rearrangement of the first two atomic layers significantly reduces the energy and stabilizes the structure, while the numerical ratio of aluminium to oxygen atoms remains unchanged. The 3D model of the aluminium oxide surface was optimized using machine learning methods.
Mystery of the atomic structure solved
"By combining experimental and computational research, we have not only solved the long-standing puzzle of the insulator's atomic structure, but also discovered principles for structure formation that apply to a whole class of materials. Our results pave the way for advances in catalysis, materials science and other fields," says Jan Balajka, who led the research. Parts of the setup in which the contactless atomic force microscope is embedded have also been registered as a patent.