奈米線的各種性質在尺度小於數十奈米時，會有強烈的尺寸效應，也就是說，其各種性質會和奈米線半徑(假設奈米線長度相對於半徑為無限長)有關係存在。此一效應源自於在奈米結構中，表面對體積的比例(surface to volume ratio:SVR)急遽的上升，使得表面會影響整體結構的性質。
我們的模擬是針對NiSi2的奈米線，使用第一原理計算預測其可能穩定的結構，楊氏係數，以及相關電性行為。在我們的結果中顯示，[111]軸向奈米線相對於[100]奈米線，在直徑小於1奈米時具有相對較好的穩定性，這可供成長超細奈米線參考。在機械行為方面，NiSi2奈米線在我們所計算的範圍(直徑0.8-1.3nm)僅具有原本塊材12.5%的楊氏系數，這樣的改變，我們相信來自於奈米線表面應力，此表面應力隨著奈米線尺寸上升會跟著慢慢下降，而表面應力和奈米線側面的方向有密切的關係，奈米線各種側面對機械行為的影響是值得繼續探討的問題。
在電性方面也深受各種奈米線側面的影響，NiSi2在製程上的應用主要用在歐姆接面(ohmic contact), 而幾乎所有我們進行計算的NiSi2奈米線都具有半金屬(semi-metal)的性質，且它們的能帶結構卻有很大的差異。[100]和[110]為典型半金屬能帶，而[111]則較接近半導體的能帶型態，這樣的差異我們認為主要來自於側面(facet)的不同。而側面對電性造成的影響，是來自於表面低鍵結度(low coordination number)的原子，從投影態密度(PDOS)中可發現，表面低鍵結度的原子可貢獻許多額外的態密度，造成與塊材迥異的能帶結構。

Physical properties at the nanoscale are known to be different from those in macro-dimensions. It was indicated that both structural and electronic properties are dependent on size effect or surface effect according to the high surface-to-volume ratio (SVR), which will exaggerate the surface status.
Our research examined the small diameter NiSi2 NWs with first principle calculations via CAmbridge Serial Total Energy Package (CASTEP). Numerical convergence has been checked with different k-point sets and cut-off energy from 300 to 380 eV. We have grown self-assembled NiSi2 NWs with triangular cross section on (100) Si substrate and these kinds of wires have been observed having very low resistivity. On the other hand, free-standing NiSi2 NWs have been fabricated by silicidation of Si NWs produced by etching Si substrates. Hence, we attempted to simulate the properties of low index axis NiSi2 NWs.
(1) Structural Properties
All the structures were carried out with total energy minimization to reach the most stable state and tensile and compressive strain were applied to analyze the Young’s modulus. Both types of NWs were found to have shorter bond length for edge (surface) bonding compared to bulk NiSi2 but longer bond length for inner bonding. The Young’s modulus was found to be much softer than that of the bulk. The difference of the bond length can be considered as “surface stress” which accounts for the softening.
(2) Electronic Properties
As to know the electronic properties in such low dimensions, we examined the band structure and density of states (DOS). The band structure was computed along the direction from □ (0,0,0) to Z (0,0,0.5). All the NWs simulated are calculated to be semimetal. Ni and Si atoms both contributed to the low resistivity because half of the DOS near the Fermi level comes from Si and half from Ni. Further analyzing the Partial Density of States (PDOS), the surface Si atoms with low coordination number make greater contribution to the total Si DOS.

CHAPTER 1. INTRODUCTION 1
1.1 PROPERTIES DOWN TO NANOSCALE 1
1.1 NICKEL SILICIDE NANOWIRES 3
1.2 FRAMEWORK OF RESEARCH 3
CHAPTER 2. LITERATURE SURVEY 4
2.1 EXPERIMENTAL RESULTS 4
2.1.1 Metal NWs 4
2.1.2 Si and Ge NWs 5
2.2 SIMULATION RESULTS 6
2.2.1 Molecular Dynamics 6
2.2.2 First Principle calculation 7
2.2.2.1 Cu nanoplate 7
2.2.2.2 Si NWs 9
2.2.2.3 GaN NWs 10
CHAPTER 3. FIRST PRINCIPAL CALCULATION 13
3.1 MANY-BODY QUANTUM MECHANICS 13
3.2 DENSITY FUNCTIONAL THEORY 14
3.3 KOHN-SHAM THEORY22 15
3.4 APPROXIMATION METHOD 16
3.4.1 Local Density Approximation23 16
3.4.2 Generalized Gradient Approximation 16
3.4.3 Psudopotential 17
CHAPTER 4. COMPUTATION PROCEDURE 18
4.1 CASTEP 18
Energy 19
Geometry Optimization 19
Band structure and DOS calculation 20
4.2 SIMULATION CELL 20
[111] 21
[110] 21
[100] 22
4.3 SIMULATION STEPS 23
CHAPTER 5. RESULTS AND DISCUSSION 27
5.1 [111] NW 27
Structure Analysis & Young’s Modulus 27
Convergence Test for Young’s Modulus 31
5.2 [110] NW 32
Structure Analysis & Young’s Modulus 32
5.3 [100] NW 35
Structure Analysis 35
5.4 DISCUSSIONS 36
5.4.1 Structure Stability 36
5.4.2 Young’s Modulus 37
5.4.3 Electronic properties 38
Band Structure 38
Density of State 39
CHAPTER 6. CONCLUSIONS 41
REFERENCE 42
ACKNOWLEDGE 44
附錄-使用國家高速網路中心大型主機進行計算 45

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