過渡金屬二硫族化物單層與異質結構之柯爾旋轉光譜

Abstract

近年來，二維層狀材料如石墨烯、六方氮化硼以及過渡金屬二硫族化物等開啟了新的異質接面材料領域。此類以凡得瓦力組成的層狀材料可藉由控制其堆疊結構或是更換組成的材料來調控元件的電子及光學特性。而單層過渡金屬二硫族化物中新興起的自旋-能谷學，更使得此種材料非常有潛力應用於未來的能谷電子元件。 本論文中，首先透過時間解析柯爾旋轉光譜量測單層二硒化鎢樣品中的能谷去極化過程。接著，我們透過變激發功率實驗，檢測柯爾訊號與背景電洞濃度間的關係。另外，我們發現螢光強度與柯爾旋轉角在空間分布上有強烈負相關性，透過理論分析證實了此為缺陷與背景電洞濃度所致。接著，我們以柯爾光譜測量旋轉角與橢圓率隨能量的變化，發現柯爾訊號主要來自於受激後的振子強度下降。第二部分，主要量測二硒化鎢與二硒化鉬垂直堆疊的異質結構。我們以雙色時間解析柯爾旋轉光譜測量樣品的自旋轉移以及自旋-能谷極化動力學，發現兩者皆決定於堆疊結構的對稱性。

Two-dimension layered materials, such as graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDCs), have opened up a new field of heterojunction material. These kinds of van der Waals hybrid materials' electronic and optical properties could be tuned by changing the constituent layered materials or controlling the stacking structures. The novel spin-valley physics in monolayer TMDCs further makes them very promising for developing the future valleytronic device. In this work, we have first measured the process of valley depolarization by time-resolved Kerr rotation (TRKR) spectroscopy in tungsten diselenide (WSe2) monolayers. The correlation between Kerr signal and resident hole density has been investigated by the power dependence of pump beam. Moreover, the anticorrelation between strong PL intensity and large Kerr rotation reveal the importance of defect and resident hole density in the sample. The energy dependence of Kerr signal, including Kerr rotation and Kerr ellipticity, are further studied by Kerr spectra. We found the Kerr signals are mainly originated from the reduced oscillator strength by pump laser. Second, the spin transfer and dynamics of spin-valley polarization in WSe2-MoSe2 vertical heterostrutures have been investigated by two-color TRKR. We found both of them are determined by the stacking symmetry.