R-SPE NaxCoO2 & related epitaxial films
[Cryst. Growth Des. 2005/ Inorg. Chem. 2006/ Adv. Mater. 2006/ Appl. Phys. Lett. 2006/ J. Phys. Soc. Jpn 2007]
High-quality epitaxial films of layered cobaltite Na0.83CoO2 were successfully grown on (0001)-oriented alpha-Al2O3 substrate by reactive solid-phase epitaxy using CoO epitaxial layer and NaHCO3 powder. Thermal oxidation of the CoO film into Co3O4 occurs at lower temperature (300 ~ 500 oC). Then, NaxCoO2 epitaxial layer forms via lateral diffusion of Na+ ions together with oxide ions into Co3O4 layers, which take place at 600 ~ 700 oC. Epitaxial relationship between Na0.83CoO2 and alpha-Al2O3 were (0001)[11-20] Na0.83CoO2 || (0001)[1-100] alpha-Al2O3. Thermoelectric properties such as electrical conductivity (sigma), carrier hole concentration, Hall mobility and Seebeck coefficient (S), and power factor (S2sigma) of NaxCoO2 epitaxial layer were 1,160 Scm-1, 3.8 x 1021 cm-3, 1.9 cm2V-1s-1, +117 microVK-1, and 1.6 x 10-3 Wm-1K-2, respectively, which were comparable to those of bulk single-crystalline NaxCoO2.