(0 0 l)-oriented Bi2Sr2Co2Oy and Ca3Co4O9 films: Self-assembly orientation and growth mechanism by chemical solution deposition

In this study, two typical cobaltate-based thermoelectric films, Bi2Sr2Co2Oy (BSC) and Ca3Co4O9 (CCO), with structures of [Bi2Sr2O4][CoO2]2 and [Ca2CoO3]RS[CoO2], respectively, are prepared by a simple chemical solution deposition on SrTiO3 (1 0 0), (1 1 0), and (1 1 1) single crystal substrates. X-ray results reveal that all films are c-axis oriented regardless of the orientation of the substrate, suggesting self-assembly orientation. Transmission electron microscopy reveals amorphous/delamination regions for BSC film on SrTiO3 (1 1 0), and the c-axis stripes of CCO on SrTiO3 (1 1 1) are inclined at 30° to the interface, whereas the c-axis stripes are parallel to the interfaces for other films. The growth mechanism is established, and the driving force for self-assembly c-axis orientation is attributed to the syneresis stress due to solvent evaporation. The microstructures and properties are also studied and discussed, with the conclusion that self-assembly c-axis oriented layered cobaltates films are good candidates for thermoelectric applications.