Figure 3

(a, b) High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) image of a CuCrO₂ thin film grown on an Al₂O₃ (0001) substrate seen along the (a) [\({\overline{1}}100\)] and (b) [1000] zone axis. (c)–(f) Electron energy loss spectroscopy (EELS) spectrum imaging of the CuCrO₂/Al₂O₃ interface seen along the [1000] zone axis. The monolayers (MLs) for CuCrO₂ thin film and Al₂O₃ substrate in the (0001) direction were defined as a set of Cu and CrO₂ sublayers and a single Al₂O₃ layer, respectively. (c) Simultaneously acquired HAADF–STEM image. (d) Color-coded composite elemental map with Al in blue, Cr in green, and Cu in red. (e) Integrated line profile of Al, Cr, and Cu signals in (d) across the interface. The dotted lines in (c)–(e) indicate the position of the CuCrO₂/Al₂O₃ interface. (f) Layer-resolved integrated EELS spectra of Al–L_2,3, Cr–M_4,5, O–K, Cr–L_2,3, and Cu–L_2,3 edges. The position of the atomic layer corresponding to each EELS spectrum is indicated by the numerical index between (c) and (d). (g) EELS O–K edge spectra of the Al₂O₃ substrate, CuCrO₂ thin film, and Cr_1−xAl_xO₂ interface layer with an X-ray absorption spectroscopy (XAS) O–K edge reference spectrum of CuAlO₂³⁰. It is worth noting that no discernible vacancy-related features could be detected from the integrated line-profile spectra.