熱電材料NaxCoO2(x=0.68, 0.75, and 0.84)之光譜性質研究

Abstract

我們研究不同濃度NaxCoO2樣品(其中包含了兩個x = 0.68和0.75的單晶薄膜,以及一個x = 0.84的單晶)的拉曼光譜。在新鮮的x = 0.84單晶樣品中,我們發現隨著觀測區域的不同,出現三個名為α-、β-和γ-型的拉曼光譜,其分別含有2、3和7個拉曼振動膜。我們認為α-型的數據代表(除了鈉離子濃度為0.5之外)NaxCoO2的拉曼光譜;而β-和γ-型的拉曼光譜則是由於Co3O4雜質所造成的。我們也發現Co3O4雜質與實驗時間和過量雷射功率有密切的關連性。有趣地是,在另一個放置了兩年半的Na0.84CoO2單晶樣品中,由Electron Probe for Microanalysis (EPMA)量測發現其鈉離子濃度會由樣品中間向邊緣呈現由高至低遞減的分布,拉曼區域影像光譜顯示在距離樣品邊緣11微米的地方發現了Na0.5CoO2的存在,這和EPMA的結果相吻合,證實了鈉離子濃度分布的現象。 此外,我們對量測了NaxCoO2薄膜(x = 0.68與0.75)的變溫光譜。我們發現在低溫時前者比後者具有更長的電荷載子平均自由路徑,顯示Na0.68CoO2有較強的金屬性。在光學電導率光譜中,我們發現x = 0.75的波峰A(~25000 cm-1)和波峰B (~12000 cm-1) 其中心頻率差值比x = 0.68的來得大,顯示出Na0.75CoO2有較強的自旋擾動行為,和其具有較佳的熱電能力相關。這兩個薄膜的光學電導率在頻率約為150 cm-1時,都出現了壓抑的情形,造成了”異常的居德行為”,這與虛能隙的狀態有關,我們也進一步藉由散射率在此頻率附近急速下降的趨勢來驗證此狀態的存在。這顯示出NaxCoO2在這兩個濃度下,不是一個簡單的金屬氧化物,呼應了NaxCoO2系統具有複雜x-T相圖的物理特性。
We present the Raman-scattering studies of two single-crystalline thin films NaxCoO2 (x = 0.68 and 0.75) and one single crystal Na0.84CoO2. We observe three different Raman-scattering spectra (α-, β-, and γ-type Raman spectra, containing two, three, and seven Raman phonon modes, respectively) on different areas of freshly cleaved Na0.84CoO2 single crystal. We suggest that α-type spectrum stands for NaxCoO2 (beside x = 0.5), while β- and γ-type spectra are caused by the appearance of Co3O4 rather than the other structures of NaxCoO2. We also find that the presence of Co3O4 impurities has a correlation with ageing and excess laser power. Interestingly, for another aged Na0.84CoO2 single crystal, Electron Probe for Microanalysis (EPMA) shows Na content is the lowest close to edge, and then enhances with increasing distance from edge up to impregnate. We find that the area of x = 0.5 content of this single crystal is located on ~11 µm from edge by Raman mapping experiments, similar to the EPMA result, verifying such Na ions distribution. The temperature-dependent optical reflectance of NaxCoO2 (x = 0.68 and 0.75) thin films have also been measured. The conductivity spectra display a metallic character for both thin films. The carrier mean free path of x = 0.68 at 20 K is about 250 Å, which is much larger than that of x = 0.75 (12 Å) at 10 K, revealing stronger metallic behavior of x = 0.68. Two E1u phonon modes at about 505 and 535 cm-1 are identified, while the other three phonon peaks are due to the sapphire substrate. The difference of center frequency between A (~25000 cm-1) and B (~12000 cm-1) bands of x = 0.75 is larger than that of x = 0.68, indicating stronger spin fluctuation in x = 0.75, consistent with the results of thermopower measurements. In addition, there is a suppression of the optical conductivity at around 150 cm-1 for both thin films, resulting in an “anomalous Drude” behavior. Such phenomenon is suggested to be the infrared signature of the pseudogap state, while the scattering rate 1/τ(ω) drops rapidly at this frequency. Such phenomenon reveals NaxCoO2 (x = 0.68 and 0.75) are not a simple metal oxide, which closely correlates with the characteristics of its phase diagram.

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Keywords

熱電材料, 鈷氧化物, thermoelectric material, NaxCoO2, cobalt oxide

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