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Microstructure evolution of Ca0.33CoO2 thin films investigated by high-angle annular dark-field scanning transmissionelectron microscopy

  • Rong Huang (a1), Teruyasu Mizoguchi (a2), Kenji Sugiura (a3), Shin-ichi Nakagawa (a3), Hiromichi Ohta (a3), Tomohiro Saito (a1), Kunihito Koumoto (a3), Tsukasa Hirayama (a1) and Yuichi Ikuhara (a4)...

Microstructures of epitaxial Ca0.33CoO2 thin films, which were grown on m plane and c(0001) plane of α–Al2O3 by the reactive solid-phase epitaxy (R-SPE) method and the subsequent ion-exchange treatment, were investigated in detail by using selected-area electron diffraction, high-resolution transmission electron microcopy, spherical-aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (Cs-corrected HAADF-STEM), and electron energy-loss spectroscopy (EELS). Detailed electron diffraction analyses reveal that the orientation relationships between Ca0.33CoO2 thin film and substrate are and , having an angle of about 43° with for the film deposited on m plane, and and for the film deposited on c(0001) plane though a Ca–Al–O amorphous layer formed between them. CoO seed layer near the interface and residual Co3O4 phase inside the films were observed and identified by HAADF-STEM and EELS in both samples. Such microstructural configuration indicates that the processes of film growth during R-SPE are (i) oxidation of CoO into Co3O4 with residual CoO layer near the interface and (ii) intercalation of Na+ layer into Co3O4 to achieve the layered NaxCoO2 film while forming Na–Al–O amorphous layer at the interface.

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Journal of Materials Research
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