摘要
尋找奈米主動電子元件為奈米科技上重要的議題。此研究以第一原理計算方法以及新的使用局部區域熱平衡近似法的電流計算方法去計算金-二硫化苯分子-金、金-硫氧化苯分子-金接點以及極性半導體金-氮化鋁(0001)-金接點的電子與傳輸性質。由金-二硫化苯分子-金接點計算出的電流-電壓以及電導-電壓特性曲線與實驗值在合理的範圍內可說是吻合的。此研究鑑定出將金的5d電子當做核心電子以及將二硫化苯分子的硫原子直接吸附於金電極的表面時，都會高估電流。計算結果顯示非對稱金-硫氧化苯-金接點的電流電壓特性曲線呈現脈波形狀的雙電導狀態，此計算結果與實驗觀察也非常相像。由電子結構的分析顯示此種雙電導狀態來自於電極與分子接點間微妙的電荷轉移現象。計算得到的金-氮化鋁(0001)-金接點的電流密度-電壓特性曲線同時具有歐姆特性、開關效應以及負微分電導的行為。電子結構的計算結果顯示由於(0001)氮化鋁薄膜的極性特性所造成自有的能帶傾斜效應導致非對稱的傳輸性質並引起在氮化鋁薄膜中氮表面層這邊的電洞態以及鋁表面層這邊的界面狀態的存在。偏壓引起的電洞態、界面態以及在中間層內氮離子與鋁離子平均化學位能附近的狀態的變化，造成金-氮化鋁(0001)-金接點有趣的傳輸特性。

Abstract
The search for nanoscale active electronic devices has been an important objective in nanoscience and nanotechnology. In this study, the electronic and transport properties of the benzene-1,4-dithiol-molecule (BDT) and Au-atom-S-benzene-ring-O-(SBO)-Au-atom junctions and the Au-AlN(0001)-Au polar semiconductor junction have been calculated using the first-principles calculation method and a new integrated piecewise thermal equilibrium approach for the current. The current-voltage (I-V) and conductance-voltage (C-V) characteristic curves obtained for the Au-BDT-Au molecular junction agreed reasonably well with experimental ones. The study of Au-BDT-Au identifies that treating Au 5d electrons as core electrons and letting the S end of BDT be bonded to the Au surface directly overestimated the current. Calculated I-V characteristic curve revealed that the asymmetric Au-SBO-Au molecular junction has a pulse-like I-V characteristic curve with dual differential conductance, which resembled well the one observed experimentally. The analysis of the electronic structures showed that this dual differential conductance transport property was due to a subtle charge transfer at the electrode-molecule contacts. The calculated J-V characteristic curve of the Au-Al(0001)-Au junction shows coexistence of ohmic, switching effect and negative differential conductance. The electronic structure calculations show the existence of an intrinsic band tilt due to the polar nature of the AlN(0001) film, which gives rise to an asymmetric transport property of the junction and the presence of hole states at the N-surface side and interface states at the Al-surface side of the AlN film. The bias induced changes of the hole states, interface states and the states of the Al and N ions in central layers in the vicinity of the local chemical potential give rise to the interesting transport property of the Au-AlN(0001)-Au junction.

Content
Chapter 1 / Introduction 6
Chapter 2 / The calculation method 12
2-1 The density functional theory (DFT) with the local-density approximation (LDA) 12
2-2 The pseudofunction (PSF) method 15
2-3 The molecular dynamics (MD) method 21
2-4. The calculation of current by the integrated piecewise thermal equilibrium approach 29
Chapter 3 / Electronic and transport properties of the Au-benzene-1,4-dithiol-Au (Au-BDT-Au) molecular junctions 34
3-1 Computation details 34
3-2 Electronic structure of the Au-BDT-Au molecular junction 35
3-3 The transport properties of the Au(111)-BDT-Au(111) tri-layer system 37
Chapter 4 / Electronic and transport properties of the Au-benzene-1-olate-4-thiolate-Au (Au-SBO-Au) model asymmetric molecular junction system 40
4-1 Computation details 40
4-2 Dual-conductance transport property 41
4-3 Electronic structures of high- and low-current states 42
4-4 Special properties of the asymmetric molecular junction considered in this study 43
Chapter 5 / Electronic and transport properties of the Au-AlN(0001)-Au polar semiconductor junction 46
5-1 Computation details 46
5-2 The transport property of the Au-AlN(0001)-Au junction 47
5-3 Electronic structure of the Au-AlN(0001)-Au junction 48
Chapter 6 / Conclusion 51
References 53
Table captions 58
Figure captions 58
Tables 63
Figures 64

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