NCAFM2023 Programme Booklet

Thursday 1140 - 1200

THE RECONSTRUCTED Al 2 O 3 (0001) SURFACE RESOLVED BY NON CONTACT AFM

Johanna Hütner , Andrea Conti, David Kugler, Florian Mittendorfer, Ulrike Diebold, Michael Schmid, Jan Balajka

Institute of Applied Physics, TU Wien, Austria Email: huetner@iap.tuwien.ac.at

Alumina α -Al 2 O 3 is used in a broad range of technological applications and has attracted interest from different fields of research. It has a wide bandgap (more than 8 eV) and is used as a substrate for thin-film growth or as a catalyst support. Although it is widely used, its insulating nature limits available methods allowing to identify the surface atomic structure. Years ago, the (√31 × √31)R±9° surface reconstruction of Al 2 O 3 (0001) which forms upon heating to high temperatures in ultrahigh vacuum had been successfully imaged with AFM [1,2]. Based on high-resolution AFM data and DFT calculations, Lauritsen et al. [2] proposed a model for this reconstruction, which consists of a hexagonal metallic Al layer covering the Al-terminated surface, forming a Moiré pattern. This model is questionable because the high vapor pressure of metallic Al would lead to rapid evaporation of such a metallic layer at the temperatures required to form this surface reconstruction. To resolve existing controversies on the model of the reconstructed Al 2 O 3 (0001) surface, we conducted non-contact AFM measurements with a copper oxide terminated tip, as advocated by Mönig et al. [3]. This tip functionalization yields enhanced contrast and facilitates chemical identification of atoms on the reconstructed Al 2 O 3 (0001) surface (see Fig. 1), allowing us to directly determine the positions of the Al and O atoms in the topmost layer. These results clearly demonstrate that the surface reconstruction does not contain a metallic Al layer but consists of both, Al and O atoms in the topmost layer. We develop a detailed structure model based on the Al and O positions derived from the experimental AFM images, the understanding of alumina surfaces derived earlier [4] and DFT calculations.

Fig. 1 Non-contact AFM image of the reconstructed Al 2 O 3 (0001) surface. Imaged at 4 K with a copper oxide terminated tip. References

[1] C. Barth and M. Reichling, Nature, 2001, 414 , 54–57. [2] J. V. Lauritsen et al., Phys. Rev. Lett., 2009, 103 , 076103. [3] H. Mönig et al., ACS Nano, 2016, 10 , 1201–1209. [4] G. Kresse, M. Schmid, E. Napetschnig, M. Shishkin, L. Köhler, P. Varga, Science, 2005, 308 , 1440.

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