Synthesis and characterization of self-assembled c-axis oriented Bi2Sr3Co2Oy thin films by the sol-gel method
Bi2Sr3Co2Oy thin films are prepared on SrTiO3 (100), (110) and (111) single crystal substrates using the sol–gel method. All the thin films are c-axis oriented regardless of the orientation of the substrate suggesting self-assembled c-axis orientation, and X-ray photoelectron spectroscopy results give evidence of coexistence of Co3+ and Co2+ ions in the derived films. Transmission electronic microscopy observations reveal that all samples are c-axis oriented with no obvious differences for different samples, and the c-axis lattice constant is determined as 15 Å suggesting the misfit structure. A phenomenological thermodynamic phase diagram for self-assembled c-axis orientation is established for misfit cobaltate-based films using chemical solution deposition. All samples behave like semiconductors due to the coexistence of Co3+/Co2+ ions, and the resistivity at 350 K is 47, 39 and 17 mΩ cm for the thin films on SrTiO3 (100), (110) and (111), respectively, whereas the Seebeck coefficient at 300 K is 97, 89 and 77 μV K−1. The successful attainment of Bi2Sr3Co2Oy thin films with self-assembled c-axis orientation will provide an effective prototype for investigation of growth mechanisms in complex oxide thin films with a misfit structure.