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研究生: 陳俊吟
Chen, Chun-Yin
論文名稱: 以理論計算探討單分子電子傳輸之量子干涉效應的機制與應用
A Theoretical Investigation on the Mechanism and Application of the Quantum Interference Effect in Single Molecule Electron Transport
指導教授: 李祐慈
Li, Yu-Tzu
學位類別: 碩士
Master
系所名稱: 化學系
Department of Chemistry
論文出版年: 2015
畢業學年度: 103
語文別: 中文
論文頁數: 123
中文關鍵詞: 量子干涉效應單分子電子傳輸
英文關鍵詞: Quantum Interference Effect, Single Molecule Electron Transport
論文種類: 學術論文
相關次數: 點閱:136下載:0
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  • 隨著導電材料製程越來越小,單分子導電研究已經是科學家們重要的課題之一,單分子導電過程中往往會伴隨著量子干涉效應(quantum interference)的發生,量子干涉效應又可分為兩種完全不同的效應,一種是建設性量子干涉(constructive quantum interference);另一種是破壞性量子干涉(destructive quantum interference),藉由這兩種量子干涉效應可改變單分子導電的特性及大小,本篇即將探討量子效應對分子的導電造成之影響,希望能加以利用來設計具備不同元件特性的分子。
    由先前文獻中得知已成功合成出phenyl-acetylene macrocycle (PAM)架構之分子旋轉柵門(molecular turnstile),透過外加電場可以使分子中間的轉軸(rotor)進行旋轉。中心轉軸與外環夾角不同則導電度也會不同。首先第三章將會探討轉軸旋轉角度進而改變分子導電度,在旋轉角度過程中意外發現:某些特殊角度下的電子傳輸會受到破壞性量子干涉效應影響,導致電子傳輸率下降,於是推斷出與量子干涉效應有關的兩種可能原因:分子軌域分佈與能量,並加以分析及討論。接著第四章是藉由另一篇文獻中發現:以PAM分子為參考,作者設計一種單分子電子旋轉門(single-molecule electric revolving doors,SMERDs)的分子開關元件,而且SMERDs一樣可透過電場改變本身的結構,進而影響導電度高低。在此章節中我們成功地改良出更好的2G-SMERDs,導電開關比例>104,所需的外加電場降低為1.0-1.5V/nm。上述章節的研究中也發現:不同系統所對應到的破壞性量子干涉電子傳輸曲線有不同的曲線特徵,所以最後第五章中我們探討分子之奇偶對稱性造成之特徵電子傳輸曲線,並延伸至cross-conjugation性質的討論。

    關鍵字:量子干涉效應,單分子電子傳輸

    As the size of the integrated circuits (IC) gets smaller, single-molecule electron transport becomes an important issue. Electron transport in single molecules is often accompanied and strongly influenced by quantum interference effect (QIE), which can be further divided into two parts: the constructive and the destructive quantum interference. Here we investigate QIE in single -molecule electron transport in the hope that this effect can be utilized to design different electric device component.
    Previous literatures have reported successful synthesis of a molecular turnstile based on the phenyl-acetylene macrocycle (PAM) architecture, which consists of a central rotor and an outer stator. The rotor, with electron donating and electron withdrawing substituents, can be rotated by an external electric field. The conductance through this molecule vary with different rotating angles. We observe a sudden transmission drop at some particular rotating angles, seemingly from the destructive QIE. The variations of the molecular orbital distribution and energy with rotation for this system are investigated in detail. In the second part of this thesis, inspired by a single-molecule electric revolving door device (SMERD) reported in a following literature, we propose an improved version for this device (2G-SMERD) which has a large on-off conductance ratio (>104) and that their open and closed states can be operated by a smaller external electric field (1.0-1.5V/nm). The above case studies raise another interesting issue. We notice that the destructive QIE can take a completely different characteristic transmission lineshapes in different molecular systems. In the last chapter, we examine the odd-even symmetry effect in the destructive QIE phenomenon, and extend this discussion to cross-conjugated systems.

    Keywords:Quantum Interference Effect,Single Molecule Electron Transport

    總目錄 2 圖目錄 6 中文摘要 10 Abstract 11 第一章 緒論 12 1-1前言 12 1-2單分子外在環境介紹 14 1-3單分子本身的構形 14 1-4分子導電的特性 15 1-5連接基團與金屬電極鍵結型式 17 1-6量子干涉效應(Quantum Interference Effect) 18 1-7研究目標 19 第二章 計算原理與方法 21 2-1計算化學原理 21 2-2密度泛函理論(Density Functional Theory) 23 2-2-1 Thomas-Fermi模型 23 2-2-2 Hohenberg-Kohn理論 24 2-2-3 Kohn-Sham方程式 28 2-2-4 局域密度近似法(Local Density Approximation,LDA) 31 2-2-5 廣度梯度近似法(generalized gradient approximation,GGA) 33 2-3非平衡格林函數理論(non-equilibrium Green’s functions,NEGF) 33 2-3-1薛丁格方程式與格林函數 35 2-3-2格林函數算符 37 2-3-3雙電極量子傳輸系統格林函數 39 2-4電子穿隧機率與Landauer Formula 41 2-5本篇所使用的計算方法 45 2-5-1 Hückel molecular orbital method (HMO) 45 2-5-2 maximally localized Wannier functions(MLWF) 49 2-5-3 Wide Band Limit approximation(WBLA) 49 2-6本篇所使用的計算軟體 51 第三章 分子對稱性與量子干涉現象Molecular Symmetry and Quantum Interference 53 3-1前言 53 3-2全環分子 58 3-3全環分子電子傳輸率探討 60 3-4全環分子能量與軌域分析 63 3-5小分子能量與軌域分析 66 3-6結論 71 第四章 單分子電子旋轉門之改良Single-Molecule Electric revolving doors 72 4-1前言 72 4-2以SMERDs添加連接基團 74 4-3新設計的兩種PAMs 83 4-3-1兩種PAMs的比較與分析 83 4-3-2 PAM-new5外加電場之效應 86 4-4全環半環 88 4-4-1半環分子之設計 88 4-4-2 PAM-naphthalene及PAM-Tnaphthalene分子外加電場之效應 96 4-5結論 98 第五章 量子干涉效應之特徵電子傳輸曲線Characteristic transmission lineshape of quantum interference 100 5-1前言 100 5-2量子干涉效應影響電子傳輸曲線 104 5-3設計具有cross-conjugation性質雙鍵導電分子 112 5-4結論 117 結論與未來展望 118 參考文獻 120

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