Atomically engineered oxide films are widely studied because their physical properties can be finely tuned for applications in sensors and electronics. However, the films are rarely analyzed during formation because complex multiple techniques and a controlled environment are required. Instead, thin cross-sections of the samples are removed once the oxide layers have formed and these sections are analysed by electron microscopy. Although such studies provide a vast amount of information, it is clear that they do not present the full picture of what is happening at the interface, while the oxide is building up on the surface.
In this latest study [Shin et al., ACS Nano. (2010) doi: 10.1021/nn1008337], the team from Tennessee, Louisiana and Wyoming has grown ferroelectric BaTiO3 on conducting SrRuO3 electrodes by pulsed laser deposition then used scanning tunnelling microscopy (STM) and low energy electron diffraction (LEED) to examine the oxide layers at different stages of growth. By combining these in situ measurements with ex situ observations and first-principles simulations, they reveal that contrary to belief, the atomic structure of the SrRuO3 surface is restructured at the start of the process due to the presence of excess oxygen. In fact, the studies reveal that this oxygen-induced renconstruction produces SrO rows along the (100) or (010) crystallographic directions which are spaced at twice the bulk periodicity. Consequently, there is a modification in structure of the first layers of BaTiO3 layers grown on the electrode surface, with an interface of mixed SrO and BaO composition in between.
“These observations provide insight into origins of the cation intermixing at these interfaces, and hence will allow us to formulate strategies for their optimization,” Sergei Kalinin tells Materials Today. This includes tailoring charge transfer, superconductivity, and magnetism; properties which are linked to atomic structure and oxygen concentration in functional materials.
“Overall, this is probably the first time when the surface of a conventional perovskite such as SrRuO3 that has been grown by pulsed laser deposition by hundreds of groups worldwide is observed with atomic resolution, and as often the case for SPM, what we see is totally unexpected,” says Kalinin.
Caption:
Investigation of an oxide surface with a buried interface indicates surface intermixing and interface reconstruction. Z contrast STEM reveals well-ordered columns with some intermixing (a), while interface reconstruction can be seen using LEED (b). Atomic resolution STM provides an image of the row structure (c). (© ACS Nano 2010)