人工堆疊雙層過渡金屬二硫族化物的層間耦合作用

Abstract

本研究主要目標為以人工方式製造過渡金屬二硫族化物雙層結構，並以拉曼與光激螢光光譜探討人造雙層中的聲子震動與電子耦合現象。論文首先利用E2g與A1g聲子的位移來判斷在製程中產生的應力與電荷參雜效應。我們發現拉曼模態的耦合與組成材料並沒有直接關係，只要疊上另一層材料，拉曼模態的耦合即會與天然雙層一致。緊接著我們利用光激螢光光譜探測人造雙層中的電子耦合。在同質MoS2/MoS2雙層中可觀察到明顯的Κ-Γ間接能隙發光，證明兩層間的電子耦合已接近化學氣相沉積與天然形成的雙層樣品。最重要的是，我們發現人造樣品中存在層間旋角的空間重組(spatial reconstruction)現象，讓間接能隙的能量變化不如化學氣相沉積樣品般的陡峭。在異質MoS2/WSe2雙層中我們則觀察到明顯的第二型Κ-K直接能隙發光，證明了MoS2/WSe2介面為第二型能帶排列。透過實驗，我們可因此決定此第二型能帶的導電帶與價電帶能帶差異。最後一部分我們則利用穿透式電子顯微鏡探測化學氣相法成長的單(雙)層MoS2之晶格排列。我們發現顯微鏡下常見的單一正三角形區域是完全的單晶，並且有Zigzag方向沿著三角形邊緣的特性。進一步我們以穿透式電子顯微鏡分析雙層MoS2的層間旋角與原子結構。與天然晶體中以2H結構為主的情況相當不同，我們發現化學氣相法成長出的雙層主要為3R與2H結構。

The main target of this research is fabricating the bilayer transition metal dichalcogenides (TMDs) artificially, in which the effect of interlayer coupling was investigated by Raman and photoluminescence (PL) spectroscopy. In the first part, the effects of strain and charge doping in fabrication process were studied by the shifts of E2g and A1g phonon modes. We found the coupling of phonon mode is not directly correlated to the constitute materials, and the coupling will become as same as natural crystal if stacking another material. Furthermore, the interlayer electronic coupling was studied by PL spectroscopy. In homo-stacked MoS2/MoS2 bilayers, the clear Κ-Γ indirect PL demonstrates the electronic coupling is close to the bilayers formed in nature or grown by chemical vapor deposition (CVD). Most importantly, we observe the spatial reconstruction of twist angle into commensurate conditions, leading to the smooth evolution of indirect energy than CVD-grown twisted bilayers. In hetero-stacked MoS2/WSe2 bilayers, we observed a type-II K-K direct PL, demonstrating the type-II band alignment at the MoS2/WSe2 interface. The conduction and valence band offset were then determined by the PL measurements. In the last part, the crystal structures of CVD-grown monolayer and bilayer MoS2 were investigated by transmission electron microscopy (TEM). From the selected-area electron pattern, we found the triangle region is a single-crystal region with the Zigzag direction along the edge of triangle. Furthermore, the interlayer twist angle and atomic structure were studied by the high-resolution TEM image. Unlike the domination of 2H-bilayers formed in nature, we found the main stacking orders in CVD-grown bilayers are in 3R and 2H structures.