Complex Functional Oxides

Complex functional oxides play a crucial role in many areas of advanced technologies. We are interested in investigating structural defects and their electronic impact on the functionality of epitaxial oxide thin films for device applications. In the case of multiferroic materials characterized by simultaneous magnetic and ferroelectric order, we have investigated the effect of substitutional doping in BiFeO3 thin films and revealed the spontaneous formation of a layered structure consisting of nonpolar Ca-doped rich regions and dopant-depleted ferroelectric regions. This alternation of layers with different ferroelectric states gives rise to a vertical polar structure exhibiting giant polarization gradients as large as 70 μC/cm2 across 30 Å thick domains. This novel pattern has been investigated combining several advanced scanning transmission electron microscopy (STEM) techniques at the atomic level, that is, HAADF/ABF and DPC. For details see Campanini et al., Nano Letters (2018), 10.1021/acs.nanolett.7b03817.

We are also interested in various kinds of crystallographic defects (domain walls, antiphase boundaries, misfit dislocations and oxygen vacancies) as symmetry breaking at these positions may induce emergent behavior with properties that significantly differ from the bulk. For this purpose we employ atomic resolution imaging, mostly in STEM mode, combined with local electron energy-loss spectroscopy (EELS), energy-dispersive x-ray (EDX) spectroscopy, as well as off-axis electron holography. Additionally, the electronic structure information contained in the electron energy-loss spectrum is analyzed by comparing the experimental results with first-principle electronic structure calculations. This approach enables us to directly relate the atomic structure information with the electronic properties and thus to provide important information about the structure-property relation.

This research topic is funded by the Swiss National Science Foundation.
Analysis of the polar structure in a Ca-doped BiFeO3 thin film by HAADF- and DPC-STEM. The polarization map is superimposed to the high-resolution HAADF-STEM image showing the nonpolar Ca-doped rich regions acting as spacers between consecutive dopant-depleted ferroelectric regions displaying coupled ferroelectric states. A magnified view of the structure is presented together with the vector plot of the atomic electric fields, where the hue gives the field direction and the saturation is proportional to the vector modulus. The scheme shows the doping-induced distortion of the O-octahedron tilting pattern and the structural phase transformation from the Pnma to the R3c space group as the doping concentration decreases. (Marco Campanini, Rolf Erni, Marta D. Rossell. For details: Campanini et al., Nano Letters (2018), 10.1021/acs.nanolett.7b03817)