Degradation effect on optical properties of NaxCoO2 (x = 0.68 and 0.75) thin films

[[abstract]]We report the degradation effects on optical properties of NaxCoO2 (x = 0.68 and 0.75, thickness 100 nm, 280 nm, and 500 nm) thin films. Our primary techniques are Raman-scattering and ellipsometric spectroscopy. Previous Raman-scattering spectra of NaxCoO2 single crystals (x = 0.68 and 0.75) show two significant Raman-active phonons at about 465 cm-1 and 580 cm-1 [22, 33, 49, 56, 61].Our fresh x = 0.75 thin film with thickness 280 nm and 500 nm exhibit similar results. Interestingly, other fresh NaxCoO2 thin films (x = 0.68, thickness 100 nm, 280 nm, and 500 nm；x = 075, thickness 100 nm) demonstrate additional phonon mode at about 445 cm-1, corresponding to sodium ion’s E2g (Na) vibrations. Notably, this phonon peak gradually diminishes with time in that sensitively gauges the Na ion’s order-disorder effects. Moreover, we observe lots of black dots with dimesion 40 ?m on the surface of fresh x = 0.68 thin film. Raman-scattering spectra indicate dramatic Na ion’s diffusion occurs toward the center area. Finally, optical conductivity obtain from the ellipsometric measurements in both x = 0.68 and 0.75 thin films displays complicated varations with time, suggesting degradation effects also induce changes in the electronic structure of CoO2 layer.