Epitaxial layered cobaltite Na_xCoO₂ thin films: X = 0.5, 0.7

Epitaxial Na_xCoO₂ thin films were deposited on (0 0 1) sapphire by the pulsed laser deposition method. Epitaxial Na _0.5CoO₂ thin film with a high crystallinity was achieved resulting from Na deintercalation of epitaxial Na_0.7CoO₂ with the solution of iodine-dissolved acetonitrile. Based on the x-ray diffraction data along the out-of-plane and the in-plane, there was only elongation of the c-lattice constant of Na_0.5CoO₂ after sodium deintercalation. The Na_0.7CoO₂ and Na _0.5CoO₂ thin films show layer-by-layer growth closely following an ideal step flow growth mode. The metallic behaviour of the Na _0.7CoO₂ thin film and the charge-ordered insulator property of the Na_0.5CoO₂ thin film were confirmed.