有機自旋閥的磁阻效能與介面性質關連之研究

Abstract

摘要� 我們嘗試針對苝四甲酸二酐(PTCDA)蒸鍍於不同的材料基板上的 分子間排列整齊度以及電子結構進行實驗了解。 本實驗室先前成功的開發出以苝四甲酸二酐作為穿隧層的有機自旋閥 (spin-valve)(//FM/AlOx/PTCA/AlOx/FM)，並在室溫下可有穩定的磁阻表現達12%。但此有機自旋閥的表現對氧化鋁 (AlOx)穿隧層有很強的依賴性，亦即若不在層狀結構中加入氧化鋁，則無法得到可工作的有機自旋閥。此研究的目的即在於嘗試著想要了解此氧化層所扮演的角色。在這個系統內有兩個可能的因素：苝四甲酸二酐的排列整齊度，與苝四甲酸二酐和磁性金屬間的電荷轉移現象所造成 的電子結構改變。 首先我們進行了偏振入射解析X光吸收光譜術 (polarizationdependence X-ray absorption spectroscopy)量測，並確認了苝四甲酸二酐分子不論是在鈷或氧化鋁層上都是沿著較平行於底層基板的方向排列，甚至在苝四甲酸二酐上方再鍍了一層金屬鈷，情況也是一樣。這點也透過了角度解析X光吸收光譜 (angle-dependent X-ray absorption spectroscopy)量測更進一步的確認。 接下來我們進行了X光光電子能譜術 (X-ray photoelectron spectroscopy)量測，確認了在沒有氧化鋁的情況下，苝四甲酸二酐在醚基 (ether)上的氧將會與下方基底的金屬鈷發生電子作用，造成該氧原子化學環境的改變。這點也在羰基 (carbonyl) 上的碳訊號得到確認：在沒有氧化鋁夾層的情況下，羰基上的碳原子訊號也會相對變弱。 我們的研究成果顯示，氧化鋁在這個系統中的角色並非影響了苝四甲酸二酐的分子間排列狀況，而是在於防止苝四甲酸二酐分子與金屬鈷之間的電荷轉移。

We have done polarization-dependent X-ray absorption spectroscopy (XAS), angle-resolved XAS, and X-ray photoelectron spectroscopy (XPS) to probe the interfaces between PTCDA and 2 di erent materials, Co and AlOx, in order to figure out the reason on the dependence of AlOx layer in the organic spin-valve with the structure of //FM/AlOx/PTCDA/AlOx/FM, as suggested and studied by our previous group members. There are two candidates, the intramolecular { overlapping, due to intramolecular orientations of PTCDA, and/or the possibility to have di erent electronic structures of either the PTCDA molecule or the FM layer underneath, for with or without the AlOx layer. First we did the measurement of the polarization-dependent XAS. We con firmed that the molecular orientation of PTCDA showed some specific orientation, much more parallel to the surface of the underlying substrate, no matter to be either AlOx or Co layer. And this was also further con rmed by the angle-dependent XAS. Also, the deposition of another top Co capping layer onto the PTCDA layer would merely react the structure of the PTCDA molecules. Secondly, XPS was performed. The data showed that the oxygen atoms of the ether group tends to vanish for the PTCDA molecules deposited on bare Co, accordance to the comparison with the molecules deposited on the AlOx layer. The data suggests while the PTCDA molecules would all have the same molecular orientation on either AlOx, Co, or even with another Co capping layer on the top of PTCDA layer, their electronic structures are di erent. The AlOx seems to have the ability to prevent the charge-transfer (the formation of chemical bonds), and this may be correlated to the dependance of the AlOx layer in OSVs. More studies have to be carried out for further confirmation.