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  5. Researchers gain new understanding of layered oxide growth

Researchers gain new understanding of layered oxide growth

Molecular beam epitaxy (MBE) is a powerful tool widely used for growing thin films with atomic precision. It is often believed that if a film is grown in a layer-by-layer fashion, the layer growth order will be exactly the same as the deposition order from the molecular beams. 

However, in a paper published August 3, 2014, in the journal Nature Materials, a team from Argonne National Laboratory and University of Wisconsin-Madison discovered that during the MBE growth of strontium titanate ((SrO)(SrTiO3)) layers, the actual growth order of strontium oxide (SrO) and titanium dioxide (TiO2) layers is different from the deposition order. Using in-situ synchrotron X-ray scattering experiments, which were led by Dillon Fong and John Freeland at Argonne National Laboratory, the researchers directly observed this unexpected reordering, or layer "swapping."

UW-Madison materials science and engineering postdoctoral scholar Guangfu Luo and Professor Dane Morgan worked with modelers at Argonne and used molecular simulations to show that such "swapping" is highly energetically favorable. Their simulations also suggested a kinetic pathway for the swapping process, as shown in the animated figure. 

Insertion of a titanium dioxide (TiO2) molecule into a strontium oxide (SrO) bilayer and lift-up of a fragment of the top strontium oxide (SrO) layer, which is perhaps the first step of a complete layer swapping. This insertion is predicted to be quite rapid at the growth temperature in the presence of other titanium dioxide (TiO2) or vacancies on the surface.

The pair's calculations also suggested that such swapping is energetically favorable in many other similar oxides with the formula of (AO)m(ABO3)n. This class of materials, generally called Ruddlesden-Popper phases, possesses a wide range of unusual properties for potential application as catalysts, magneto-resistors, superconductors, ion conductors, and other technological materials. With its swapping prediction, the team then proposed a way to redesign the deposition order to obtain the target Ruddlesden-Popper phases. Using this new growth strategy, the experimental team obtained its desired strontium titanate ((SrO)(SrTiO3)) compound and achieved the first successful synthesis of high-quality single crystal lanthanum nickelate ((LaO)(LaNiO3)2) layered material.

The research was funded by grants from the U.S. Department of Energy and the National Science Foundation. Additional authors on the paper include: J.H. Lee, I.C. Tung, S.H. Chang, Z. Luo, M. Malshe, A. Bhattacharya, J.A. Eastman, H. Hong, and J. Jellinek, all from Argonne National Laboratory; S.M. Nakhmanson, from the University of Connecticut; and M. Gadre, from UW-Madison.

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