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

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