(00l)-oriented Bi2Sr2Co2Oy and Ca3Co4O9 films: Self-assembly orientation and growth mechanism by chemical solution deposition

Abstract In this study, two typical cobaltate-based thermoelectric films, Bi 2 Sr 2 Co 2 O y (BSC) and Ca 3 Co 4 O 9 (CCO), with structures of [Bi 2 Sr 2 O 4 ][CoO 2 ] 2 and [Ca 2 CoO 3 ] RS [CoO 2 ], respectively, are prepared by a simple chemical solution deposition on SrTiO 3 (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 SrTiO 3 (1 1 0), and the c -axis stripes of CCO on SrTiO 3 (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.