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Riku Tanaka1 Ryo Ishikawa1 Naoya Shibata1 2 Yuichi Ikuhara1 2

1, The University of Tokyo, Bunkyo, Tokyo, Japan
2, Japan Fine Ceramics Center, Nagoya, Aichi, Japan

The surface structures of metal oxides play important roles in a wide variety of phenomena such as catalytic reaction and epitaxial film growth, and have been extensively investigated in the past decades. Among the wide variety of metal oxide materials, SrTiO3 (STO) has an ABO3 type perovskite structure that is considered as one of the model in oxide materials. The free surface of STO is well known to form an atomically flat, step-terrace structure when annealed at high temperatures, and therefore is widely used as a substrate for thin film growths. Since the properties of the grown films strongly depend on the quality of the substrates, the control of surface atomic structures of the substrates is of great importance to obtain high quality films. To date, the surface atomic structures of annealed STO have been mainly observed by STM (Scanning Tunneling Microscopy). While STM enables imaging of atomic surface structures, its application is restricted to conducting materials, and moreover it is still challenging to identify atom species. On the other hand, ADF STEM (annular dark-field scanning transmission electron microscopy) is a versatile technique that provides atomic scale, element identifiable image of the bulk structure. In plane-view observations, STEM has not been employed to investigate surface structures due to the transmitting nature. However, it is expected that surface structures may be determined by taking a series of defocused ADF STEM images. In this study, we have investigated the (001) surface step structure of STO annealed in ambient air, using a defocus series of ADF STEM images.
The annealed STO single crystal was observed by both TEM and STEM. We confirmed that a step-terrace structure was formed by annealing at 1323 K, with steps faceted in either by {100} or {110}. In order to investigate the three-dimensional atomic structure of the faceted surface, we acquired a defocus series of ADF STEM images at exactly the same region. For each atomic column in the image, the standard deviation of the intensity was plotted as a function of focus depth. Then, column-by-column surface position was determined by the focus depth value with the largest standard deviation of the intensity, which was obtained by fitting the plotted standard deviation with a bell-shaped function. The annealed surface was consisted of three major atomic steps with the heights of 5 to 10 nm, and all of them were faceted in atomically flat planes oriented in either {100} or {110}.
Acknowledgement: This research was partly supported by Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING2) project of the New Energy and Industrial Technology Development Organization, Japan.

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