High electrical conductivity of layered cobalt oxide Ca3Co4O9 epitaxial films grown by topotactic ion-exchange method

Epitaxial film of a layered cobalt oxide, Ca3Co4O9, was fabricated on a (0001) face of an α-Al2O3 substrate by a topotactic ion-exchange method using a γ-Na0.8CoO2 epitaxial film as a precursor. High-resolution x-ray diffraction and atomic force microscope measurements revealed that the film was high-quality (001)-oriented Ca3Co4O9 with stepped and terraced surface morphology. The film exhibits a high electrical conductivity of 2.95×102Scm−1 and a large Seebeck coefficient of ∼+125μVK−1, which leads to the thermoelectric power factor (TPF) of 4.5×10−4Wm−1K−2 at 300K, potentially usable as a building block of the multilayered film structure with an enhanced TPF value.