扭轉雙層二硫化鉬之層間耦合的電子及光學特性

Abstract

近年來，石墨烯、六方氮化硼和過渡金屬二硫族化物等二維材料開啟了一個新的異質接面材料領域。這類以凡德瓦力組成的材料可藉由控制其堆疊結構或更換其組成之層狀材料來調控其各種電子及光學特性，也因此展現了未來在應用方面的潛力。所以對於這類層狀材料之層間交互作用的了解，將會是未來設計凡德瓦同質及異質結構時的關鍵因素。 本論文中，我們首先展現了一種有效、靈敏、非破壞性的光學量測技術─非線性光學之二次諧波產生，其可以用來偵測過渡金屬二硫族化物晶體之晶軸方向及決定其人工堆疊混和材料之層間扭角。接著我們利用二次諧波產生，分析以化學氣象沉積法成長的扭轉雙層二硫化鉬之層間扭角。我們發現其晶體結構與具有反對稱性的理想柏納爾堆疊之天然晶體相當不同。在進一步分析其拉曼震動模態和間接能隙發光後，我們證明了影響其層間耦合的電子及光學特性之關鍵因素為其層間間距的改變。

Two-dimensional (2D) layered materials like graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDCs) have opened up a new field of heterojunction materials. The tunability of the electronic and optical properties by changing the constituent layered material or controlling the stacking structure shows the great application potentials for these van der Waals hybrid materials. Hence, understanding the interlayer interactions is the crucial point for future designing of van der Waals homo- and heterostructures. In this work, we first present that second harmonic generation (SHG) is a sensitive, efficient, non-destructive optical technique to determine the crystal orientation and interlayer twist angle of artificially stacked noncentrosymmetric layered TMDCs. Next, we use the SHG to resolve the interlayer twist angle of the CVD-grown twisted bilayer molybdenum disulfide.We found their structures are largely deviated from the ideal Bernal (2H) stacking order in the natural crystal with inversion symmetry. Furthermore, the unusual evolutions of phonon vibration and indirect transition demonstrate that the interlayer spacing plays a key role in the interlayer couplings of both electronic and optical properties.