[22] Enhanced power factor of epitaxial layered cobaltite Na_xCoO₂ thin film induced by strain: x=0.5, 0.7

Epitaxial Na_0.5CoO₂ thin films were fabricated by the sodium deintercalation of the epitaxial Na_0.7CoO₂ thin films in the solution of iodine-dissolved acetonitrile, in which the epitaxial Na_0.7CoO₂ thin films were deposited on (001) sapphire substrates by pulsed laser deposition. The Na_0.7CoO₂ thin films show small resistivity, which is probably due to the high crystallinity as well as the compressive strain of 1.1% along the 𝑎-axis. From the spectroscopic ellipsometry analysis, we obtained the optical constants as well as the optical conductivities for the Na_0.5CoO₂ and Na_0.7CoO₂ thin films and the increased energy splitting between e_g and a_1g results from the structural strain of the Na_0.7CoO₂ thin film. We suggest that the structural strain should be the source of the lower resistivity and the preservation of the strongly correlated system up to 200 K for the Na_0.7CoO₂ thin film. At 300 K, the Na_0.7CoO₂ thin film exhibits the enhanced power factor (S²/rho) of 94𝜇W/K²cm with the low electrical resistivity of 177𝜇𝛺cm and the high thermoelectric power of 128.9𝜇V/K.