장소: 제1과학관 31214  e+첨단강의실 

초록: Multivalent transition metal oxide SrCoOx (SCO) has rich oxygen content (x)–dependent electronic, magnetic and optical phases, ranging from antiferromagnetic insulator to ferromagnetic metal. The modification of x changes the valence state of Co, which governs the overall material’s property. The redox reaction of the material and the resultant changes in the physical properties are physically interesting and are essential ingredients in applications such as solid oxide fuel cells, gas sensors, and many other devices that exploits ionic (oxygen) transport property.

Using real-time x-ray diffraction and optical spectroscopy complemented by first principles calculation, we studied lattice structures, optical properties, and electronic structures of SCO epitaxial thin films. In particular, brownmillerite SrCoO2.5 (BM-SCO) and perovskite SrCoO3 (PV-SCO) thin films have been studied in detail, where they have distinct crystal structures and valence states. BM-SCO has a one-dimensional oxygen vacancy ordered structure with a common Co3+ valence state. On the contrary, PV-SCO has a typical perovskite structure. Both experimental and theoretical results agreed that these two films have drastically different electronic ground states as well. While BM-SCO showed an insulating ground state with an optical band gap of ~0.5 eV, PV-SCO showed a clear Drude response suggesting a metallic ground state. Despite such large discrepancy in the physical properties, however, we found that a topotactic transformation between two structurally distinct phases could be readily achieved. The temperature dependent, ambient controlled real-time ellipsometry conspicuously showed that these two topotactic phases could be reversibly obtained at relatively low temperatures. Our study suggests that the electronic structure of SCO can be switched reversibly through oxygen insertion and extraction, simultaneously with the crystal lattice structure and Co valence state. Thus, it provides an invaluable insight in studying the link between the fundamental physical properties and its technological applications of transition metal oxide epitaxial thin films.