摻雜不同離子對Cs2Nb4O11光譜性質之影響
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2013
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我們量測單晶、粉末壓錠、及摻雜不同離子Cs2Nb4O11(CNO)樣品的x光繞射能譜、拉曼散射光譜以及橢圓偏光光譜，探究摻雜不同離子對CNO樣品的晶格常數、反鐵電-順電相變溫度、及電子結構之影響。
我們發現摻雜離子半徑較小者(例如：摻雜V離子、Ta離子及Rb離子)，CNO的單位晶胞體積變小；而摻雜離子半徑較大者(例如：摻雜S離子)，CNO的單位晶胞體積增大。CNO室溫拉曼散射光譜顯示12個拉曼特徵峰，頻率位置分別為157 cm-1、171 cm-1、185 cm-1、201 cm-1、255 cm-1、538 cm-1、620 cm-1、668 cm-1、717 cm-1、847 cm-1、868 cm-1及877 cm-1，我們發現摻雜不同離子對620 cm-1拉曼峰之影響最為顯著，其對應鈮氧八面體之氧離子伸張振動，當摻雜V離子與Ta離子時，620 cm-1拉曼峰展現紅移，我們推測八面體因之鍵長伸長，使得鍵能下降。此外，隨著樣品溫度升高，620 cm-1拉曼峰的頻率位置或半高寬顯現異常溫度效應，這暗指反鐵電-順電相變溫度對晶格動力學的影響，不同摻雜樣品之相變溫度比之未摻雜CNO皆有下降的趨勢。
橢圓偏光光譜分析未摻雜CNO樣品的能隙值約為3.23 eV，而摻雜V離子、Ta離子、Rb離子、及S離子的C28及C29樣品之能隙值分別約為2.17 eV、2.09 eV、3.26 eV、2.6 eV、及3.2 eV，我們發現除了摻雜Rb離子樣品之能隙值未下降，其餘摻雜不同離子樣品的能隙值皆變小，此與第一原理理論計算結果相符，由於摻雜陽離子取代Nb離子，導致能隙值下降最多，故建議以此作為調變CNO樣品能隙值的基礎。
We present x-ray powder diffraction, Raman-scattering, and spectroscopic ellipsometry measurements of Cs2Nb4O11 (CNO) with different dopants. Our goal is to unveil the effects of dopants on lattice dynamics and electronic structures as well as antiferroelectric to paraelectric phase transition temperatures. At room temperature, x-ray powder diffraction data show that the replacement of O ions with larger S ions causes an expansion of unit cell volume. In contrast, the unit cell volume decreases when doped with Ta and V on Nb, and Rb on Cs sites. Furthermore, Raman-scattering spectrum of undoped CNO shows twelve phonon modes at about 157, 171, 185, 201, 255, 538, 620, 668, 717, 847, 868, and 877 cm-1. One strong vibration observed at about 620 cm-1 corresponding to the stretching modes of NbO6 octahedra exhibits a redshift trend in S-doped and Ta-, V-, and Rb-doped samples. Additionally, antiferroelectric to paraelectric phase transition temperature is decreasing in all doping samples. Finally, the absorption spectra determined from spectroscopic ellipsometry analysis of undoped CNO, S-, Ta-, V-, and Rb-doped ones show a direct gap of about 3.23 eV, 2.60 eV, 2.09 eV, 2.17 eV, and 3.26 eV, which are in good agreement with the predictions of first-principles calculations. Most importantly, these studies demonstrate band gap tunability in the CNO system.
We present x-ray powder diffraction, Raman-scattering, and spectroscopic ellipsometry measurements of Cs2Nb4O11 (CNO) with different dopants. Our goal is to unveil the effects of dopants on lattice dynamics and electronic structures as well as antiferroelectric to paraelectric phase transition temperatures. At room temperature, x-ray powder diffraction data show that the replacement of O ions with larger S ions causes an expansion of unit cell volume. In contrast, the unit cell volume decreases when doped with Ta and V on Nb, and Rb on Cs sites. Furthermore, Raman-scattering spectrum of undoped CNO shows twelve phonon modes at about 157, 171, 185, 201, 255, 538, 620, 668, 717, 847, 868, and 877 cm-1. One strong vibration observed at about 620 cm-1 corresponding to the stretching modes of NbO6 octahedra exhibits a redshift trend in S-doped and Ta-, V-, and Rb-doped samples. Additionally, antiferroelectric to paraelectric phase transition temperature is decreasing in all doping samples. Finally, the absorption spectra determined from spectroscopic ellipsometry analysis of undoped CNO, S-, Ta-, V-, and Rb-doped ones show a direct gap of about 3.23 eV, 2.60 eV, 2.09 eV, 2.17 eV, and 3.26 eV, which are in good agreement with the predictions of first-principles calculations. Most importantly, these studies demonstrate band gap tunability in the CNO system.
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鈮酸銫, 拉曼散射光譜, 橢圓儀, x光繞射能譜, 能隙, Cs2Nb4O11, Raman-scattering spectra, ellipsometry, X-ray, energy gap