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研究生(外文):Chia-Jung Lin
論文名稱(外文):Anchoring Effect on Spin Transport in Amine-Ended Single-Molecule Magnetic Junctions: A First-Principles Study
指導教授(外文):Yu-Hui Tang
外文關鍵詞:Single-Molecule JunctionsSpintronicsMolecular ElectronicsAnchoring EffectSpin Transport
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近年來,將微小尺寸的分子使用在電子元件中,其特殊的電子傳輸特性使得單分子接面吸引相當多的研究;但是透過磁性金屬作為電極引入自旋極化效果,進而探討單分子磁性接面中的自旋傳輸特性,目前仍然相當缺乏實驗和理論的研究。在實驗上,STM-BJ和MCBJ兩項技術都能成功地建構所謂的單分子通道。但是對於精準度還是有很大的改善空間,所以需要重複這些實驗好幾百次來匯集成長條圖,以便降低實驗之間的差異性。而在理論上,我們已成功的利用第一原理計算方法以及非平衡態格林函數方法來研究單分子接面。足夠的實驗研究告訴我們,在如此微小的尺度,介面的影響力對於單分子通道是非常重要的,像是橋接分子就是其中一個切入點。經驗上,傳統的非磁性單分子通道認為以金作為電極,並以硫(thiol)作為橋接分子,可以得到有效的charge transfer和穩定的鍵結。但對於磁性電極(鈷),哪一個橋接分子才能得到有效的spin transfer?

Two novel disciplines: spintronics and molecular electronics are significant revolutions in electronics applications. A specific topic between these two fields is single-molecule magnetic junction which combines spin effect and molecule devices. To thoroughly understand and control the single-molecule magnetic junction, we focus on the surface effect by replacing the anchoring group. Among different experimental techniques, mechanically controllable break junction (MCBJ) and STM break junction (STM-BJ) are extensively used in investigating the transport properties of single-molecule junction. Based on experimental processes, strain effect is necessarily included by stretching simulation. Moreover, to clarify the anchoring effect, this study considers two different anchoring ions forming two molecules, 1,4-benzenediamine (BDA) and 1,4-benzenedithol (BDT), sandwiched by two Co hcp[0001] nanowires. First-principle calculation with non-equilibrium green’s function gives adequate investigations of strain and anchoring effect on the spin transport properties. These two effects crucially impact on spin transport in single-molecule magnetic junctions. Our research demonstrates that the strain-enhanced spin injection efficiency of amine-ended single-molecule magnetic junction (BDA-based junction) involving a sign reversal and nearly perfect spin injection could be tuned by mechanical control. Besides, comparing BDA- and BDT-based junctions reveals the superior spin transfer in the amine-ended magnetic junctions (Co/Amine-ended/Co) is in sharp contrast to better charge transfer in the thiol-ended non-magnetic junctions (Au/thiol-ended/Au). Our interesting findings may pave a way for promising and potential tunability in spin injection efficiency for amine-ended single-molecule magnetic junction, simply under mechanical stimulus of break junction technique.

Chapter 1 Introduction 1
Chapter 2 Theory 8
 2.1  Density Function Theory 8
  2.1.1   Born-Oppenheimer Approximation 9
  2.1.2   Hartree-Fock Method 11
  2.1.3   Hohenberg-Kohn Theorem 13
  2.1.4   Kohn-Sham Equation 16
  2.1.5   Exchange-Correlation Energy Functional 17
  2.1.6   Pseudopotential Method 19
2.2   Non-Equilibrium Green’s Function Method 21
  2.2.1   Self-Consistent DFT+NEGF Calculation 21
  2.2.2   Spin-Transport Properties Calculation 25
Chapter 3 Computational Details 29
 3.1  Structural Geometry 29
 3.2  Parameter for Structural Relaxation 31
 3.3  Parameter for Spin-Transport Properties 33

Chapter 4 Results and Discussion 35
 4.1  Structural Relaxation under Stretching 35
 4.2  Spin Transport Properties of Co/BDA/Co 40
 4.3  Spin Transport Properties of Co/BDT/Co 44
 4.4  Spin Injection Factor and Spin Current 47
Chapter 5 Conclusion 50
References 51

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