Microstructure evolution of Ca_0.33CoO₂ thin films investigated by high-angle annular dark-field scanning transmissionelectron microscopy

Microstructures of epitaxial Ca_0.33CoO₂ 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 Ca_0.33CoO₂ 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 Na_xCoO₂ film while forming Na–Al–O amorphous layer at the interface.