NaxCoO2 (x = 0.68 and 0.75) 薄膜劣質化效應之光譜性質研究

Abstract

我們量測 NaxCoO2 (x = 0.68 與 0.75；膜厚 100 nm、280 nm 及 500 nm) 薄膜系統的雷射拉曼散射光譜與橢圓偏光光譜，藉由拉曼活性振動模與電子結構隨時間的變化，探討 NaxCoO2 薄膜的劣質化效應。 先前許多參考文獻指出 [22, 33, 49, 56, 61]，NaxCoO2 (x = 0.68 與 0.75) 單晶樣品的拉曼散射光譜展現二個顯著的特徵峰，其頻率位置約 465 cm-1 與 580 cm-1，我們的新鮮薄膜樣品 (x = 0.75；膜厚 280 nm 與 500 nm) 也有相似的結果。有趣地是，我們觀察到所有新鮮之 NaxCoO2 薄膜 (x = 0.68、膜厚 100 nm、280 nm 及 500 nm 與 x = 075、膜厚 100 nm) 多顯示了一個拉曼散射峰，其頻率位置約為 445 cm-1，此特徵峰對應鈉離子的 E2g (Na) 振動模，且隨著樣品老化而消逝。因此，445 cm-1 之拉曼峰便成為我們判別鈉離子由有序性排列轉為無序性排列的重要指標。另外，我們發現新鮮之 x = 0.68 薄膜樣品表面上散佈大小約為 40 m 的黑點，藉由量測其外圍、邊緣及中心三區域之拉曼散射光譜隨著時間的變化，得知在八天時間內，外圍與邊緣之鈉離子即明顯地向中心擴散。 最後，不論是 x = 0.68 或是 0.75 薄膜樣品，我們皆觀察到其高頻光學電導率隨著劣質化效應影響而呈現出不規則的變化，暗示因為鈉離子的無序性排列，改變了鈷氧層的電子結構。

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.