Friday, July 11, 2014

Peeling back the layers of thin film structure and chemistry

Nanowerk News reports: Perovskites — any material with the same structure as calcium titanium oxide (CaTiO3) —continue to entice materials scientists with their ferroelectricity, ferromagnetism, catalytic activity, and oxygen-ion conductivity. In recent years, scientists realized that they could vastly improve the properties of perovskites by assembling them into thin films. The problem was that no one understood why thin films beat out bulk materials.Researchers gained new insight into thin-film superiority by probing the structure of perovskites at the X-ray Science Division 33-ID-D,E x-ray beamline at the U.S. Department of Energy's Advanced Photon Source (APS), Argonne National Laboratory. They used a groundbreaking approach to tease apart the thin-film structure and chemistry layer-by-layer

Read more: Peeling back the layers of thin film structure and chemistry 

                                              Graphical abstract: Revealing the atomic structure and strontium distribution in nanometer-thick La0.8Sr0.2CoO3−δ grown on (001)-oriented SrTiO3

Zhenxing Feng, Yizhak Yacoby, Wesley T. Hong, Hua Zhou, Michael D. Biegalski, Hans M. Christen and Yang Shao-Horn

Surface segregation in metal oxides can greatly influence the oxygen transport and surface oxygen exchange kinetics critical to the performance of solid-state devices such as oxygen permeation membranes and solid oxide fuel/electrolytic cell electrodes. Unfortunately detecting elemental distributions at the atomic scale near the surface remains challenging, which hampers the understanding of underpinning mechanisms and control of surface segregation for the design of high-performance materials. Using the coherent Bragg rod analysis (COBRA) method, we report the first direct 3D atomic imaging of a 4 nm-thick “La0.8Sr0.2CoO3–δ”/SrTiO3epitaxial film. Of significance, energy differential COBRA revealed pronounced Sr segregation (La1−xSrxCoO3−δ, x 0.4) in the four unit cells from the top surface while complete Sr depletion was detected in the five unit cells from the “La0.8Sr0.2CoO3−δ”/SrTiO3 interface. The drastic strontium compositional changes in the film were associated with large changes in the atomic positions of apical oxygen sites in the perovskite structure. Such Sr segregation tendencies toward the surface were also found in nominal “La0.6Sr0.4CoO3−δ” thin films, which can greatly enhance the surface oxygen exchange properties of oxides. The results presented here show that COBRA and the differential COBRA methods can be used to investigate a variety of electrochemically active systems providing atomic scale structural and chemical information that can help understand the physical and chemical properties of these systems and serve as a basis for comparison with DFT calculations.

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