Aluminium oxide reveals its surface secrets

Amazing stuff!

"Determining the surface structure of an insulating material is a difficult task, but it is important for understanding its chemical and physical properties. A team of researchers in Austria has now succeeded in doing just this for the technologically important insulator aluminium oxide (Al2O3). The team’s new images – obtained using non-contact atomic force microscopy (AFM) – not only reveal the material’s surface structure but also explain why a simple cut through a crystal is not energetically favourable for the material and leads to a complex rearrangement of the surface.

Al2O3 is an excellent insulator and is routinely employed in many applications ...

The problem is that while non-contact AFM can identify where the atoms are located, it cannot distinguish between the different elements making up a compound. Balajka, Hütner and colleagues overcame this problem by modifying the tip and attaching a single oxygen atom to it. The oxygen atoms on the surface of the sample being studied repel this oxygen atom, while its aluminium atoms attract it. ..."

"Aluminum oxide (Al2O3), also known as alumina, corundum, sapphire, or ruby, is one of the best insulators used in a wide range of applications ...

The strongly insulating nature of alumina hindered experimental studies, and the surface structure evaded precise determination for more than half a century. ..."

From the editor's summary and abstract:

"Editor’s summary

Noncontact atomic force microscopy and density functional theory were used to determine the origin of the massive surface rearrangement of the (0001) surface of aluminum oxide. Previous studies had suggested that the surface loses oxygen atoms and has a metallic character, but Hütner et al. show that the complex (×)R ± 9° reconstruction is nearly stoichiometric  ... Imaging determined the lateral atomic positions, and theory shows that aluminum rehybridization allows bonding to subsurface oxygen atoms, which greatly stabilizes the reconstruction. ...

Abstract

Macroscopic properties of materials stem from fundamental atomic-scale details, yet for insulators, resolving surface structures remains a challenge. We imaged the basal (0001) plane of α–aluminum oxide (α-Al2O3) using noncontact atomic force microscopy with an atomically defined tip apex. The surface formed a complex (×)R±9° reconstruction. The lateral positions of the individual oxygen and aluminum surface atoms come directly from experiment; we determined with computational modeling how these connect to the underlying crystal bulk. Before the restructuring, the surface Al atoms assume an unfavorable, threefold planar coordination; the reconstruction allows a rehybridization with subsurface O that leads to a substantial energy gain. The reconstructed surface remains stoichiometric, Al2O3."

Aluminium oxide reveals its surface secrets – Physics World

The insulator unraveled (original news release) "Scientists at the TU Wien and the University of Vienna have uncovered the detailed structure of the aluminum oxide surface, a challenge that has baffled researchers for decades."

Stoichiometric reconstruction of the Al2O3(0001) surface (no public access)

The structure of the aluminum oxide surface was determined with noncontact atomic force microscopy and computational modeling