A simple approach of fabricating thermoelectric γ-Na_xCoO₂ and superconductive Na_x(H₂O)_yCoO_2-δ films using the sol-gel spin-coating method

We report a simple approach of fabricating thermoelectric γ-Na_xCoO₂ film with the c-axis orientation using the sol-gel spin-coating method. The inferred sodium content is x = 0.65 according to the correlation between the c-axis lattice constant and x. Temperature dependence of both the resistivity and thermopower resembles that of the γ-Na_0.68CoO₂ film grown by the reactive solid-phase epitaxy. The fitted thermopower data show that the bandwidth of γ-Na_xCoO₂ is found to be ~ 101 meV, being close to the quasi-particle band (70-100 meV) derived from an angle-resolved photoemission study of γ-Na_0.7CoO₂. These results enable the possibility of low-cost fabrication of γ-Na_xCoO₂-based thermoelectric film devices. Furthermore, we have also topotactically transformed the of γ-Na_xCoO₂ film to a superconducting Na_x(H₂O)_yCoO_2-δ film with T_{c, onset} = 4.12 K.