Structures and Mechanics of 2D nanomaterials : A study of Graphene, Silicene and platinum Silicide

現今科技的發展，製成技術不斷的進步，使得元件尺寸進入了奈米等級，低維度科學成為半導體科技中的一環。為了解決低維度科學所面臨問題，發展出了一套二維結構的元件，其中常見的二維材料包含了單一原子與過度金屬二硫屬化物兩種為主，單一原子種類當中，受到大家屬矚目的就是石墨烯與矽烯。因此本文將以二維結構為主，利用分子動力學理論計算與實驗觀察二為結構。 本文我們成功利用分子動力學理論計算二維結構，包含石墨烯與矽為主基板，觀察到其力學性質與物理特性，同時利用超高真空掃描穿隧顯微鏡觀察到矽化鉑島狀結構，此島狀結構為蜂窩式的結構，排列形狀與石墨烯、矽烯非常近似，詳細結論為下列結果。 首先利用分子動力學理論針對石墨烯的模擬計算，石墨烯在拉伸的過程中，所能承受的應力跟拉伸方向的關係。當沿著armchair方向拉伸時，拉伸應力皆集中在C-C鍵的鍵結上，往zigzag方向拉伸時，拉伸方向主要應力是處在鍵結夾角的部分，與armchair方向拉伸相比會具有較佳的延展性，因此能夠承受較大的應變承受力。石墨烯加入了 'Stone-wales defect' 之後，計算結果發現缺陷排列方向與拉伸方向相同，缺陷數目增多時石墨烯能承受的應力增加。要是缺陷排列的方向與拉伸方向垂直，則是缺陷數目增多時，石墨烯能承受的應力減少。缺陷受力的形變形式也會隨著拉伸的方向不同而有兩種形式。變溫理論計算下，我們可以從100K到500K的結果中發現，拉armchair方向因為有熱擾動的影響，使得部分的C-C鍵會產生錯位旋轉，讓缺陷的5-7環能回到穩定的六環結構。而往zig-zag方向拉伸時因鍵與拉伸方向相同，不易有錯位的的情況產生，使得結果與低溫環境結果較相同。 將以矽原子取代石墨烯的結構稱為矽烯（Silicene），使用矽原子來取代碳原子的位置，形成以矽原子為主的二維矽膜，在此我們針對下面三種主要以矽組成的表面進行分子動力學理論計算，包含有矽稀的二維結構，矽最穩定的結構的√3x√3 geometry，以及矽(111)的7x7重構表面等三種表面進行物理性質的分析。透過分子動力學理論模擬來計算出三種結構的力學性質。 三種結構抗拉強度依序為： Silicene結構 ≥ √3x√3 geometry結構 > Si (111)-7x7 重構結構 我們觀察到Silicene結構所能承受的應力最大，意味著也能承受較大來自於基板作用力，因此Silicene結構在不同基板上大面積生長時，能夠維持完整的Silicene結構。此外也可以從楊示係數看出Si (111)-7x7 重構結構為三者中最易產生形變的結構。 最後使用超高真空掃描穿隧顯微鏡，觀察高蒸鍍量蒸鍍在Si(111)-7x7重構表面上，經由高溫退火後有三種不同位置類型的島狀結構，分別為(A) 在平台上單獨的島狀結構(B)在平台上的島狀結構伴隨著一凹陷結構 (C)在階梯邊緣的島狀結構等三種。我們推論島狀結構的形成，主要是經由擴散的矽原子和蒸鍍上的鉑原子交互作用產生的島狀結構，藉由高溫退火的熱處理，可以觀察到形成矽化鉑的島狀結構，此島狀結構為六角形的Honeycomb的結構，排列形狀與石墨烯、矽烯非常近似，因此我們將利用超高真空掃描穿隧顯微鏡，針對觀察不同蒸鍍量的鉑原子在矽(111)-7x7重構表面上的行為，觀察鉑原子成核結構，來研究其表面結構。Nowadays the development of science and technology, make progress of technology continuously, making the component size into the nano-level, low-dimensional science has become a part of semiconductor technology. In order to solve the problems faced by low-dimensional science, we developed a set of two-dimensional structure of the device. Among them, the common two-dimensional material contains two kinds, single atoms and excessive metal dichalcogenides. The single atomic species, graphene and silicene have attracted much attention. This work will be based on two-dimensional structure, the use of molecular dynamics theory calculations and experimental observation on the two-dimensional structure. In this paper, we successfully use molecular dynamics theory to calculate two-dimensional structure, including graphene and silicene, observe the mechanical properties and physical characteristics. At the same time, the island structure of platinum silicide was observed by ultra-high vacuum scanning tunneling microscopy. The island structure is honeycomb structure. The arrangement shape is very similar to graphene and silicene. The detailed conclusion is as follows. First of all, using molecular dynamics theory for graphene simulation, graphene in the process of stretching, the stress can withstand the relationship with the stretching direction. We can observe that the armchair direction in the strain of about 0.18 to achieve this configuration rupture yield point, yield strength is about 116.86 (GPa). The Young's modulus of the structure is 1005.45 (GPa). And that zigzag direction of graphene in the strain of about 0.26 to achieve this configuration rupture yield point, yield strength is about 148.5 (GPa). The Young's modulus of the graphene is 1714.28 (GPa). When the stretching direction along the armchair, tensile stresses are concentrated at the C-C bond; zigzag direction to stretching, the stretching direction is the principal stresses in the bond angle portion, compared with armchair direction stretching will have better ductility, it can withstand greater strain tolerance. When graphene joined the 'Stone-wales defect'. The calculated results show that the defects are arranged in the same direction as the tensile direction, and the stress that graphene can withstand will increase with the number of defects increase. If the orientation of the defect is perpendicular to the direction of the stretching, the stress that graphene can withstand decreases as the number of defects increases. There are two forms of deformation that are subject to imperfections with different directions of stretching. The structure that replaces graphene with silicon atoms is called 'Silicene', the use of silicon atoms to replace the carbon atoms to form the silicon-based two-dimension. Here we focus on the following three kinds of silicon surface composed of molecular dynamics theoretical calculations, including the Silicene, two-dimensional structure of silicon, the √3x√3 geometry, the most stable structure of silicon, and the Si (111)- 7x7 reconstructed structure are analyzed. Through the molecular dynamics simulation to calculate the mechanical properties of the three structures. Tensile strength of the three structures as follow: Silicene structure ≥ √ 3x√3 geometry structure> Si (111)-7 × 7 structure We can deduce that due to Silicene honeycomb structure can withstand the most stress, the Silicene structure maintains a complete Silicene structure when growth on the different substrate can withstand force from the substrate, and the Si (111)- 7x7 reconstructed structure can be seen from the Young's coefficient as the most easily deformable structure among the three. Finally, the ultra-high vacuum scanning tunneling microscope was used to observe that the high evaporation amount was deposited on the Si (111) -7x7 reconstructed structure, after annealing at high temperature there are three different types of island-shaped structures. (A) Alone island structures on the terrace. (B) The island structure on the terrace is accompanied by a concave structure. (C) Island structure at the edge of the step. We infer that the formation of island-shaped structure, mainly by high-temperature annealing heat treatment, making the proliferation of silicon atoms and deposition of platinum atoms on the interaction of the island-like structure, we observed the formation of the island structure of platinum silicide, the island structure hexagonal honeycomb structure, arrangement of the shape with graphene and silicene is similar. Therefore, we used ultra-high vacuum scanning tunneling microscopy to observe the different amounts with platinum atoms on the Si (111) -7x7 reconstructed structure behavior observed platinum nucleation structure to study its surface structure.摘要 i Abstract iii List: v Figure List: vii Table List: x Chapter.1 Introduction 1 1.1. Foreword 1 1.2. Motivation 3 1.3. Organization of the dissertation 5 Chapter.2 Molecular dynamics 6 2.1. Introduction to Molecular Dynamics 6 2.2. Basic assumptions of molecular dynamics 6 2.3. Motion equation 7 2.4. Velocity Verlet algorithm 7 2.5. Periodic boundary 8 2.6. Potential energy function 8 2.7. Ensemble system 9 2.8. Molecular dynamics basic process architecture 9 Chapter.3 Scanning tunneling microscope 11 3.1. Introduction to STM 11 3.2. Tunneling effect 12 3.3. Imaging methods 13 3.4. Dimer-adatom-stacking-fault model 15 3.5. Experimental device 17 3.6. Experimental methods 19 3.6.1 Probe production 19 3.6.2 Specifications and sample processing method 19 3.6.3 Vapor deposition method 20 Chapter.4 Calculating two-dimensional structure of graphene 22 4.1. Theoretical simulation model introduction 22 4.2. Tensile graphene 24 4.3. Initial conditions 28 4.4. Tensile experiment at different temperatures 30 4.5. Summary 44 Chapter.5 Calculating two-dimensional structure of Si based 45 5.1. Si based introduction 45 5.2. Mechanical properties analysis 48 5.2.1 Silicene structure 48 5.2.2 √3x√3 geometry structure 51 5.2.3 7x7 reconstruction structure 54 5.2.4 Structure comparison 57 5.3. Summary 59 Chapter.6 Pt on the Si substrate 61 6.1. Pt atoms on Si(111)-7x7 62 6.2. PtSi island structures 64 6.2.1 Alone island structures on the terrace 65 6.2.2 The island structure on the terrace is accompanied by a concave structure 68 6.2.3 Island structure at the edge of the step 71 6.3. Formation of Island Structure 73 6.4. Analysis of Island Structure 75 6.5. PtSi island structures on disorder 78 6.6. Summary 83 Chapter.7 Conclusion 84 Reference 8