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研究生： 徐偉豪
Hsu, Wei-Hao
論文名稱： 以低能量電子點投影顯微術/繞射顯微術研究懸浮之石墨烯
Low-energy Electron Point Projection Microscopy/Diffraction Study of Suspended Graphene
指導教授： 黃英碩
Hwang, Ing-Shouh
口試委員: 張嘉升
蘇維彬
楊志文
朱明文
陳健群
莊程豪
學位類別： 博士
Doctor
系所名稱： 工學院 - 材料科學工程學系
Materials Science and Engineering
論文出版年： 2018
畢業學年度： 106
語文別： 英文
論文頁數： 86
中文關鍵詞： 石墨烯 、石墨烯撓曲 、低能量電子束 、點投影顯微術 、點投影繞射成像術 、吸附物 、散焦電子束繞射
外文關鍵詞： graphene, graphene ripples, low-energy electron beams, point projection microscopy, point projection diffractive imaging, adsorbates, divergent beam electron diffraction
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    • 在這本論文中，我們用低能量電子點投影顯微術/繞射顯微術觀察懸浮的石墨烯樣品。在傳統的低能量電子點投影顯微術中，用來擷取影像的屏幕是固定於一個位置。我們做了一項改變：在我們的儀器中，屏幕可以在光軸上前後移動。這樣的改變使我們可以收集到高角度的繞射訊息，這是在過去的低能量電子點投影顯微術無法獲得的資訊。如此一來，二維材料如石墨烯上的奈米級吸附物、表面乾淨度等可藉由投影影像觀察，而繞射影像可提供石墨烯不同層間的方向性改變及表面起伏等訊息。我們準備了兩種樣品，分別是物理性剝離的石墨烯以及以化學氣相沉積法製備的石墨烯，兩種樣品都懸浮在樣品載台並取得電子穿透影像。實驗結果顯示，石墨烯上的奈米級吸附物在低能量電子束長時間照射之下會隨著時間聚集形成更大的結構。再者，奈米級吸附物會使石墨烯表面產生形變形成奈米級撓曲。這些奈米級撓曲會隨著奈米級吸附物的聚集而產生改變。根據實驗結果搭配模擬計算，我們實驗得到的繞射圖形可以用”散焦電子束繞射術”分析。分析顯示實驗的繞射圖形可以觀察二維材料表面的形變。根據理論計算，使用50到250 eV的低能量的電子束最小可觀察到1埃的形變。最後我們也討論了這個新架構會面臨的問題，以及運用低能量電子點投影顯微術/繞射顯微術發展同調電子繞射成像術的潛力，以達成觀測超薄二維材料、生物分子和其他奈米材料原子結構的目標。

    In this dissertation, I present our study of suspended graphene with low-energy electrons based on a new electron microscopy, point projection microscopic/diffractive imaging technique. In contrast to traditional point projection microscopy (PPM) imaging, our setup allows adjustment of the sample-screen distance to record high-angle diffraction patterns. This technique allows imaging of individual adsorbates on graphene at the nanometer scale, determining the quality of the graphene with projection images, and characterizing the graphene thickness, graphene lattice orientations, and ripples on graphene with diffraction patterns. Both exfoliated and chemical vapor deposition (CVD) graphene samples were studied in an ultra-high vacuum chamber. We found that long-duration exposure to low-energy electrons induced the accumulation of adsorbates on the graphene when the electron dose rate was above a certain level. In addition, the adsorbates may induce ripples on the graphene, and the ripples may change during electron illumination due to the aggregation of the adsorbates. The patterns recorded using our new technique can be viewed as “divergent beam electron diffraction (DBED)”. Theoretical simulations of the DBED patterns confirm observations of adsorbates and strain distribution at a lateral resolution of several nanometers in our experiments. The simulations also indicate that three-dimensional displacement of atoms as small as 1 angstrom from a perfect graphene lattice can be detected with low-energy electrons (50 – 250 eV). We also discuss the challenges of PPM and the potential of the low-energy electron point projection microscopic/diffractive imaging technique to conduct coherent diffractive imaging for determining the atomic structures of biological molecules deposited on suspended graphene.

    致謝 I 中文摘要 II Abstract III Chapter 1 Introduction 1 Chapter 2 Literature review 5 2.1 Brief introduction to PPM 5 2.2 PPM imaging of one-dimensional objects 7 2.2.1 PPM imaging of carbon fibers 7 2.2.2 PPM imaging of single macromolecule 8 2.2.3 PPM imaging of individual bio-molecules 9 2.2.4 Charge effect in PPM imaging 10 2.3 Observations of graphene with PPM 11 2.4 Single-atom tip (SAT) 19 Chapter 3 Experimental details 25 3.1 Instrumentation 25 3.2 Microscope stage 25 3.3 Image detection system 26 3.4 SAT preparation 27 3.5 Sample preparation 29 3.5.1 Exfoliated graphene samples 30 3.5.2 CVD graphene samples 32 Chapter 4 Low-energy electron point projection microscopic/diffractive imaging 34 4.1 Traditional PPM 34 4.2 Retractable screen 35 Chapter 5 Initial characterization of graphene samples using PPM/diffractive imaging technique 38 5.1 Exfoliated Graphene Sample 38 5.2 Exfoliated graphene sample without heat treatment 43 5.3 Cleanliness of CVD graphene samples 44 Chapter 6 PPM imaging of adsorbates on graphene surface 49 6.1 Accumulation of adsorbates 49 6.2 Imaging of mobile adsorbates 52 6.3 Discussion 54 Chapter 7 Study of suspended graphene based on low-energy electron point projection microscopic/diffractive imaging 56 7.1 Diffractive imaging of CVD graphene 57 7.2 Changes in diffraction patterns caused by aggregation of adsorbates 59 7.3 Theory of DBED and simulated DBED pattern of ripples in graphene 62 7.4 Changes in the diffraction patterns during the sample heating 67 7.5 Study of graphene sample with nanometer-scale broken holes 69 7.6 Orientations in double-layer region of CVD graphene 72 Chapter 8 Challenges and further development 74 Chapter 9 Conclusions 78 References 79

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