石墨烯中無能隙的能帶對於調節電子設備的電導率至關重要，也使得科學研究和工業應 用變得越來越複雜。接下來的目標是發展一種類似石墨稀，由矽或鍺原子組成具有蜂巢狀的 結構，為了能有相仿的能隙以利更加廣泛的電子應用，以及擁有絕佳的電子移動率和熱穩定 性。過去在銀 (111) 上已經成功長出兩個不同相的鍺烯:條紋相 (SP) 和準獨立相 (QP)。前 者在 0.72 ML 時形成均勻的蜂巢狀晶格，沿著條紋方向的部分與基底銀 相稱，而後者在 1.03 ML 時形成凌亂的蜂巢結構，與銀 完全不相稱。
在這篇論文裡我透過 LEED、STM、STS 和 ARPES，研究在形成 SP 鍺烯之前的階段。 在剛開始鍺原子蒸鍍沉積到 1/3 ML 時，Ag2Ge 合金相形成，而對應的光電子能譜呈現一條 清楚的表面態能帶在以 MAg(111) (Malloy) 為中心處分裂，這個能帶分裂是由於 Ag2Ge 表面合 金和下面的銀 (111) 之間不相稱，界面處表面態電子的 umklapp 散射. 鍺原子比銀原子小很 多，因此 Ag2Ge 表面合金相對於銀 (111) 在鬆弛時收縮，而從能帶分裂大小所取得的晶格不 匹配和銀 相比約為-5%。此外，當鍺的厚度開始超過 1/3ML，我們發現 另一個以 Γ 區域為中心的能帶分裂，這個分裂在不同對稱方向有差異，在 alloy 方向分裂 最大而在 alloy 方向最小，此分裂是來自兩個不同的合金相，兩個表面態能帶疊加的結果; 一個 (相 1) 是鬆弛的 Ag2Ge 表面合金，與銀 的整體晶格不匹配為 -5%， 另一個(相2)是Ag2Ge表面合金，沿著alloy 單一方向拉伸應變。我們提出的模型與STM 圖像上呈現的大規模具有週期性三角形陣列一致;三角形的三個邊由相 2 的條紋狀合金構成， 而相 1 的合金填充在三角形內較大的區域。隨著鍺的覆蓋率增加到 0.72 ML，新的條紋相在 相 2 條紋狀合金的 30◦ 方向上生長並最終主宰整個區域。根據不同厚度下的 STS 量測結果， 新的條紋相是 SP 鍺烯，說明經歷獨特的去合金化程序而形成鍺烯的過程。

The gapless band in graphene, essential for regulating the conductivity of electronic de- vices, makes scientific research and industrial applications increasingly complex. The next goal is to develop an analogue of graphene with a honeycomb structure made of Si or Ge atoms in order to have a comparable band gap for more extended electronic applications in addition to excellent mobility and thermal stability. Germanene has been successfully grown on Ag(111) in two phases: the striped phase (SP) and the quasi-freestanding phase (QP). The former forms at 0.72 ML with a uniform honeycomb lattice partially commensurate with underlying along the stripe direction, while the latter forms at 1.03 ML with disordered honeycombs, completely incommensurate with.

My research work presented herein in this thesis is investigating the prequel stage before
the formation of SP germanene via four different techniques such as LEED, STM, STS, and
ARPES. Upon initial Ge deposition to 1/3 ML, Ag2Ge alloy phase forms, and the corresponding
photoemission spectra show a distinct surface-state band splitting centered at Mbar of Ag(111) (Mbar of alloy). This band splitting is due to umklapp scattering of the surface state electrons at the incommen- surate interface between the Ag2Ge surface alloy and the underlying Ag(111). Ge atom has a much smaller size than that of Ag atom, so Ag2Ge surface alloy tends to relax in a contracting manner with respect to Ag(111), and the resulting lattice mismatch extracted from the band splitting size is ∼ −5% compared to the lattice of root3 of Ag(111). Moreover, as the coverage started exceeding 1/3 ML, we found another band splitting centered at the zone center gamma. The splitting is anisotropic in that it maximizes at the symmetry direction Gamma-Mbar of alloy alloy and minimizes at the symmetry direction gamma -Mbar alloy . Such splitting results from the superposition of two surface-state bands derived from two different alloy phases; one (phase 1) is the relaxed Ag2Ge surface alloy with an overall -5% lattice mismatch with root3 of Ag(111), and the other (phase 2) is the Ag2Ge surface alloy with uniaxial tensile strain along the gamma-Mbar of alloy direction. The proposed model is consistent with the STM image showing the large-scale periodic triangular arrays; the three sides of the triangles are made of phase-2 alloy stripes, and the phase-1 alloys fill in the flat areas inside the triangles. As the Ge coverage increases toward 0.72 ML, new-type stripes grow in the direction 30 degree of the phase-2 alloy stripes and finally dominate. Based on the coverage-dependent STS measurement, the new stripe is SP germanene, which manifests that germanene is going through a unique process of de-alloying.

1 Introduction to thesis topic and Literature Survey 1
1.1 2D Dirac Materials 1
1.2 Introduction to Xenene family 4
1.3 Germanium on Ag(111) 5
1.4 Literature survey of alloy phase 7
Bibliography 16


2 Surface characterization techniques 19
2.1 Low Energy Electron Diffraction 19
2.2 Photon Light Source 22
2.2.1 VUV Source Lamp 24
2.2.2 Synchrotron Light Source 26
2.3 Photoemission Spectroscopy 28
2.3.1 Theory & Working Principle 29
2.4 Scanning Tunneling Microscopy 33
2.4.1 Theory & Working Principle 34
2.5 Vacuum Technology 36
2.6 First Principle Calculation 37
2.6.1 Theory of DFT (ab initio) 37
Bibliography 39


3 Thin-Film Growth and Characterization 41
3.1 Characterization in LT-STM 41
3.1.1 STM Tip preparation 41
3.1.2 Thin-film deposition and characterization 44
3.2 Characterization in ARPES 46
3.2.1 Thin-film characterization 46
3.2.2 Work function characterization 47
3.3 Theoretical calculation & simulation 48
3.3.1 DFT calculation using unfolding technique, (VASP) 48
3.3.2 LEED calculation & simulation 50
3.3.2 Bibliography 54


4 Ag2Ge alloy surface state band splitting 55
4.1 Ag2Ge alloy surface state band splitting at surface zone boundary 56
4.2 Coverage-dependent Ag2Ge alloy surface state band splitting 66
4.3 Conclusion 69
Bibliography 71


5 Isotropically relaxed and uniaxially relaxed Ag2Ge alloy phases coexist 73
5.1 Evolution of the Ag2Ge alloy electronic band structures at the surface
zone centre Γ 73
5.2 Conclusion 84
Bibliography 85


6 Summary 87
6.1 Summary of band-splitting like electronic feature near ΓAg(111) and
MAg(111) 87
6.2 Future work in the Ag2Ge alloy surface on Ag(111) 91
Bibliography 92

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