LiMPO4 (M = Fe, Mn) 奈米顆粒之光譜性質研究

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我們量測LiMPO4 (M = Fe, Mn)單晶、奈米顆粒以及粉末壓錠樣品的x光繞射能譜、拉曼散射光譜及橢圓偏光光譜,探究不同顆粒大小對LiMPO4樣品的晶格結構及電子結構之影響。另一方面,我們為了解不同鋰含量LiFePO4對於晶格穩定性之影響,測量了Li0.5FePO4之拉曼散射光譜。 我們發現LiMPO4奈米顆粒的單位晶胞體積有些微變大。LiFePO4和LiMnPO4室溫拉曼散射光譜分別顯示11個與7個拉曼特徵峰,LiFePO4拉曼峰的頻率位置分別為145 cm-1、158 cm-1、197 cm-1、444 cm-1、584 cm-1、630 cm-1、658 cm-1、950 cm-1、996 cm-1、1068 cm-1及1140 cm-1,LiMnPO4部分則分別為93 cm-1、142 cm-1、437 cm-1、585 cm-1、948 cm-1、1005 cm-1及1066 cm-1。相較於單晶樣品,奈米顆粒的拉曼峰值皆紅移了1~2 cm-1,我們推測奈米顆粒表面非晶相層引起舒張應力或聲子侷限效應導致紅移現象。此外,LiFePO4單晶與奈米顆粒的低溫拉曼峰參數,在尼爾溫度附近,沒有異常的溫度效應。在Li0.5FePO4奈米顆粒的拉曼散射光譜分析部分,我們發現146 cm-1、242 cm-1、443 cm-1、998 cm-1及1069 cm-1拉曼峰強度有所改變。 分析LiFePO4與LiMnPO4的橢圓偏光光譜,吸收能譜顯示在3.8 eV到6.4 eV區間內存在數個吸收峰,我們比較第一原理理論計算,了解每一個吸收峰所對應的電子能階結構。另我們估算LiFePO4單晶與奈米顆粒的能隙值約為3.8 ± 0.1 eV與3.45 ± 0.1 eV,LiMnPO4粉末壓錠與奈米顆粒的能隙值約為3.75 ± 0.1 eV與4.8 ± 0.1 eV。
We present x-ray powder diffraction, Raman-scattering, and spectroscopic ellipsometry measurements of LiMPO4 (M = Fe, Mn) nanoparticles. Our goal is to explore the influence of finite-size effects on the lattice dynamics and electronic structures in these materials by optical spectroscopy. At room temperature, x-ray powder diffraction data show that the lattice constants of nanoparticles are slightly larger than those of bulk samples. Raman-scattering spectra of LiFePO4 and LiMnPO4 nanoparticles show eleven and seven phonon modes. The phonon modes are observed at about 145 cm-1, 158 cm-1, 197 cm-1, 444 cm-1, 584 cm-1, 630 cm-1, 658 cm-1, 950 cm-1, 996 cm-1, 1068 cm-1, and 1140 cm-1 for LiFePO4 nanoparticles. Similarly, the phonon modes appear at about 93 cm-1, 142 cm-1, 437 cm-1, 585 cm-1, 948 cm-1, 1005 cm-1, and 1066 cm-1 for LiMnPO4 nanoparticles. They are red shifted in frequency by 1 ~ 2 cm-1 compared with that of bulk counterpart. This slight redshift observed in the Raman phonon modes of nanoparticles can be attributed to the combination effect of strain induced by amorphous layer and v phonon confinement. Furthermore, with decreasing temperature, no anomaly of phonon parameters was observed near the antiferromagnetic ordering temperature in both LiFePO4 single crystal and nanoparticles. Additionally, the frequencies of Raman phonon modes in Li0.5FePO4 and LiFePO4 nanoparticles are close, however, their intensities differ. The absorption spectra determined from spectroscopic ellipsometry analysis of LiFePO4 and LiMnPO4 show several absorption bands in the spectral range from 3.8 to 6.4 eV. Their assignments are based on the predictions of first-principles calculations. Finally, the values of direct band gap of LiFePO4 single crystal and nanoparticles are estimated to be about 3.80 ± 0.1 eV and 3.45 ± 0.1 eV. The 3.75 ± 0.1 eV and 4.80 ± 0.1 eV band gap are obtained for LiMnPO4 bulk and nanoparticles.



磷酸鋰鐵, 磷酸鋰錳, 拉曼散射光譜, 橢圓偏振光譜, LiFePO4, LiMnPO4, Raman-scattering spectra, spectroscopic ellipsometry