二維材料因為其具有獨特的物理特性和應用潛能吸引了大量的研究關注。層
狀的過渡金屬二硫屬化物(Layered transition metal dichalcogenides)是最受歡迎的二維材料，這類材料通常會結合其他二維材料形成凡德瓦異質結構(van
der Waals heterostructure)，這些異質結構需要優化才有可能成為各種不同
原件的基石， 我們的實驗專注於二硫化鉬(MoS2)在高定向熱解石墨(Highly
Oriented Pyrolytic Graphite)上的電子結構。我們的研究主要可以分成兩個部份
第一個部份是缺陷(defect)對MoS2電性的影響，我們研究了MoS2的晶界(grain
boundary)對其帶隙(band gap)的影響。我們還研究了不同種類的缺陷像是點缺
陷(point defect), 硫空穴(sulfur vacancies)和點缺陷上帶有吸附物。
第二個部份和MoS2當氫演化反應(Hydrogen evolution reaction)的電催化劑相
關。氫演化反應可以用於製造氫氣，製造的氫氣可以當氫燃料電池的燃料。白
金也可以當氫演化反應的催化劑。但是，白金在實際使用中成本太高。MoS2 有
潛力取代白金的角色，但是問題是催化的效率。為了要增加其催化效率，必須
用電漿處理去製造硫空穴。在測試完MoS2的氫演化反應催化效率以後，我們會
使用掃描穿隧式顯微鏡(Scanning Tunneling Microscopy)研究，我們的實驗目的是要驗證硫空穴的存在。我們有觀測到不同形式的硫空穴。
除了上述的兩個部份，我們還做了很多MoS2定性的測量像是厚度和帶隙的
關係、大範圍地形圖、原子結構、莫列波紋。

2D Materials have attracted lots of interests due to their unique physical properties and potential applications in electronics. Layered transition metal dichalcogenides(TMDs) are one of the most popular 2D materials. TMDs are often combined with other 2D materials to make van der Waals heterostructure. These heterostructures need to be optimized to be possibly used as building blocks for many different devices. Our experiments focus on molybdenum disulfide on highly oriented pyrolytic graphite. Our research on MoS2 on HOPG can mainly be divided into two parts.
One is the defect influence on the electronic properties of MoS2. We've studied the influence of the grain boundary on the band gap of MoS2. We also investigated different kinds of defects such as point defects, sulfur vacancies, point defects with adsorbates.
The other is related to the MoS2 as electrocatalyst of hydrogen evolution reaction (HER). HER can be used to produce elemental hydrogen. The produced hydrogen can serve as fuel for hydrogen oxygen fuel cells. Platinum can be used as a catalyst for HER. However, it is too expensive in practical use. MoS2 has the potential to replace the role of platinum, but the problem is the efficiency of the catalysis. In order to increase the efficiency of catalysis, sulfur vacancies are created by plasma treatment on MoS2. After testing the HER catalysis efficiency, samples are investigated by scaning tunneling microscopy (STM). Our experiment is to identify the existence of sulfur vacancies. As a result, we "observed" many different types of sulfur vacancies.
Besides these two parts, we've also done lots of characterization of MoS2 on HOPG such as the thickness dependence of the band gap, large area topography, atomic structure and morie patterns.

1 Introduction 4
2 Theory 6
2.1 Principle of STM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Principle of STS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3 Quantum confinement . . . . . . . . . . . . . . . . . . . . . . . . . 8
3 Instrument 10
3.1 Rotary pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2 Turbomolecular pump . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3 Bakeout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.4 Ion pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.5 Ion gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.6 Low temperature environment . . . . . . . . . . . . . . . . . . . . . 13
3.7 Thermal diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4 STM Repair 14
4.1 STM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2 X-Y table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2.1 Problem description . . . . . . . . . . . . . . . . . . . . . . 16
4.2.2 Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.3 Current cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.3.1 Problem description . . . . . . . . . . . . . . . . . . . . . . 18
4.3.2 Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.4 Tip holder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.4.1 Problem description . . . . . . . . . . . . . . . . . . . . . . 20
4.4.2 Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.5 Sample holder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.5.1 Problem description . . . . . . . . . . . . . . . . . . . . . . 23
4.5.2 Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.6 Z-shift controll . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.6.1 Problem description . . . . . . . . . . . . . . . . . . . . . . 25
4.6.2 Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.7 New equipments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.7.1 Heater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.7.2 Drift correction program . . . . . . . . . . . . . . . . . . . . 26
4.8 Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.8.1 Plugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.8.2 Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
5 Literature review 31
5.1 Point Defects and Grain Boundaries . . . . . . . . . . . . . . . . . . 31
5.2 Charging effect at grain boundaries of MoS2 . . . . . . . . . . . . . 34
5.3 Temperature Triggered Sulfur Vacancy Evolution in Monolayer MoS2/Graphene
Heterostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.4 Hydrogen evolution reaction . . . . . . . . . . . . . . . . . . . . . . 39
5.5 Electrocatalysts for hydrogen evolution reaction . . . . . . . . . . . 40
6 Experiment 42
6.1 Molybdenum disulfide on highly oriented pyrolytic graphite . . . . . 42
6.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
6.1.2 Atomic Resolution . . . . . . . . . . . . . . . . . . . . . . . 46
6.1.3 Point defects . . . . . . . . . . . . . . . . . . . . . . . . . . 47
6.1.4 Sulfur Vacancies . . . . . . . . . . . . . . . . . . . . . . . . 49
6.1.5 Band gap of different layers of MoS2 on HOPG . . . . . . . 50
6.1.6 Grain boundaries . . . . . . . . . . . . . . . . . . . . . . . . 52
6.2 Zinc oxide on n-Si . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
6.2.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
6.2.2 Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
6.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Appendix 58
A Daily record 59
B Drift correction program source code 64
Reference 73

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