金屬及有機材料蒸鍍之表面電性與磁性調變

Modifications on Surface Electronic and Magnetic Properties via Metal and Organic Molecule Deposition

朱宇軒(Yu-Hsun Chu)

指導教授 ： 林敏聰

國立臺灣大學/理學院/物理研究所/博士(2015年)

https://doi.org/10.6342/NTU.2015.10479

摘要

於電子學及自旋電子學之發展中，表面與界面之電、磁性之調控為一重要研究領域。近數十年來掃描穿隧電子顯微鏡之成熟發展大幅拓展基礎科學與奈米科技之範疇，由晶體結構、能態量測至單一原子操控、人造奈米結構盡皆可行；藉此技術，我們嘗試解析奈米尺度下有機及金屬材料蒸鍍之成長模式及所引起之基板表面、界面電性與磁性變化，期以為未來異質結構與表面奈米結構之基石。

自旋極化掃描穿隧電子顯微鏡展示錳酞花菁及五聯苯與鈷奈米島基底交換耦合後之磁性行為：錳原子透過d-d直接交換耦合與鈷基底形成鐵磁耦合，五聯苯與鈷之p-d鍵結則形成反鐵磁耦合，並使五聯苯於鈷上具自旋選擇之幾何對稱性；此結果顯示選擇適當有機材料可局域調整、設計有機─鐵磁系統之磁性方向甚至自旋極化分布。

晶體表面之準粒子干涉可改變電子與自旋極化之空間分布，或帶有複雜自旋能帶系統如Rashba材料中之自旋相關散射資訊。傳統與自旋極化掃描穿隧電子顯微鏡結果之比較顯示鈷奈米島上自旋極化與電子密度分布之相關性，並符合二維無限位能阱之描述、預測。強Rashba晶體BiTeI上之準粒子干涉則提供可能之電子散射途徑及其自旋選擇性。

考量BiTeI於實際裝置之應用，鐵磁材料於BiTeI表面之交互作用應予以討論。藉蒸鍍少量鐵原子於BiTeI表面，我們發現可能因鐵置換缺陷引起之能帶上移，但未測得額外散射途徑。此外我們亦發現有機材料如苝四甲酸二酐與五聯苯於BiTeI表面具截面材料之選擇性，提供異質結構製作與設計之方法及基礎。

關鍵字

自旋極化掃描穿隧電子顯微鏡 ； 有機分子 ； 自旋界面 ； Rashba 晶體 ； 準粒子干涉

並列摘要

During development of electronics and spintronics, modifications on the surface electronic and magnetic properties have drawn broad interest. One of the most sophisticated surface-sensitive instrument, scanning tunneling microscopy (STM), has largely expanded the horizon of fundamental physics as well as nanotechnology, from observations of crystalline structures and electronic states to control of individual adatoms for artificial and novel constructions. In this dissertation, deposition of metals and organic molecules and the consequent effects on exotic surfaces are revealed by STM and spin-polarized STM (SP-STM). From morphology in topography to variance of electronic and magnetic structures in space and energy, the studies function as the cornerstone of surface engineering of heterostructure and its applications.

On single ferromagnetic domains from Co nanoisland on Cu(111), investigations of manganese-phthalocyanine(MnPc)-Co and pentacene(PEN)-Co hybrid systems are achieved using STM, SP-STM, and first-principle calculations. While MnPc follows the Co spin-polarization near the Fermi level, PEN exhibits an opposite one. We conclude that different orbitals hybridizing with Co and different exchange mechanism result in the inverse spin polarization between MnPc and PEN. In addition, symmetry reduction of PEN, which clarifies the Co stacking methods, is found recovered in SP-STM spin-dependently, which brings out the significance of bonding and antibonding states in organic-ferromagnetic interfaces for designs of spin-distribution.

On the other hand, spatial variations of electron and spin-polarization distributions induced by quasiparticle interference in Co islands are revealed by SP-STM. Tip manipulations allow us to compare interference patterns with and without spin information, which correspond to each other closely and are described by the particle-in-a-box model. Quasiparticle interference also reveals spin-dependent scattering processes in the Rashba spin-split surface states. STM measurements on the strong bulk Rashba crystal BiTeI resolve both the morphology of different surface terminations as well as scattering events from an interference dispersion.

To put BiTeI in practical use, submonolayer Fe is deposited on it to unveil influences from magnetic impurities on Rashba spin-split states and electron scattering. Without appearance of new scattering channels and distortions in the intrinsic interference dispersion, Fe induces band shift which can be attributed to creations of Fe-substitution Bi defects. Organic molecules PTCDA (perylene-3,4,9,10-tetracarboxylic dianhydride) and PEN are also deposited and termination-selected growths are observed. Intramolecular dipoles in PTCDA induce molecule self-assembly as well as possible preference to the Te-termination.

The dissertation covers issues in surface science from measurements of electronic and magnetic properties using STM to state modifications and creations via deposition of metals and organic materials. The combination of in-situ sample fabrications and delicate STM measurements with first-principles calculations resolves the complicated interactions among different elements and systems. Each of them contributes to development of spintronics from different angles, and in total, they depict a vision of future devices based on organic molecules and emergent crystals with all kinds of versatility.

並列關鍵字

spin-polarized scanning tunneling microscopy ； organic molecule ； spinterface ； bulk Rashba crsytal ； quasiparticle interference

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