二維過渡金屬二硫族化物之對稱相關光電特性

Abstract

本論文旨在研究二維過渡金屬二硫族化物之對稱相關光電特性，其中包含了雙層二硫化鉬之層間耦合效應與單層二硒化鎢之自旋動力學。第一部分中，我們量測了人工堆疊雙層過渡金屬二硫族化物之光學二次諧波產生，發現了二階極化率的角度相依性。我們論證了雙層過渡金屬二硫族化物之光學二次諧波產生來自於電場的同調疊加，其中的相位差則決定於堆疊角度。以上結果讓具備高效率、靈敏與非破壞性的光學二次諧波產生成為探測堆疊方向、晶體極性與邊界之好方法。接著，我們透過帶間光學躍遷與聲子振動模態研究雙層二硫化鉬之層間耦合效應，發現能帶結構因層間耦合效應產生了兩種變化。首先，堆疊角度將改變平衡的層間間距，造成第一布里淵區中Γ點的能量改變與伴隨而來的間接能隙改變。接著，具有自旋與對稱相依性的層間跳躍則影響了K點的價電帶自旋分裂與能帶次序。

第二部分中，我們利用超快時間解析光譜探測了單層二硒化鎢之自旋動力學。首先，我們證實了可以透過圓偏振光產生固有電洞的能谷極化。利用時間解析柯爾旋轉光譜，我們觀測到正激子的自旋生命期約為700皮秒，遠長於正激子的復合生命期(~10-20皮秒)。此長時間的能谷極化來自於能谷贗自旋從光激載子到固有電洞的傳遞現象，證實了電洞的谷間散射生命期約為1.4奈秒。接著，我們利用變溫時間解析柯爾旋轉光譜探測電洞自旋的谷間去極化機制。我們發現在溫度100K以上時，電洞自旋的衰減速率有隨溫度指數上升的情況。此點驗證了在高溫下，電洞自旋的衰減主要來自於受熱激活的聲子散射過程。

This dissertation is devoted to the symmetry-dependent electronic and optical properties of two-dimensional layered transition metal dichalcogenides (TMDs). The main focuses include the interlayer coupling of twisted bilayer MoS2 and the spin dynamics in monolayer WSe2. In the first part, we report the optical second harmonic generation (SHG) from homo- and heterostructural TMD bilayers formed by artificial stacking with an arbitrary stacking angle, showing the twist-angle dependence of second-order susceptibility. We demonstrate the SHG from the twisted bilayers is a coherent superposition of the electric fields from the individual layers, with a phase difference depending on the stacking angle. We show here that the SHG is an efficient, sensitive and nondestructive characterization for the stacking orientation, crystal polarity and domain boundary of van der Waals heterostructures made of noncentrosymmetric layered materials.

Then, the interlayer electronic couplings in chemically grown MoS2 twisted bilayers are investigated by the measurements of interband optical transitions and phonon vibration modes. The interlayer twist is found to affect the bilayer band structure in two different ways. First, the stacking orientations affect the equilibrium interlayer spacing, which in turn largely alters the bilayer band structure at the Γ points of the first Brillouin zone, leading to a remarkable change in the indirect optical transition with the interlayer twist angle. On the other hand, the stacking orientation also impacts the valence band spin splitting at K points via the spin and symmetry dependent interlayer hopping. This results in a stacking-orientation dependent spin splitting and band ordering of the valence band near the K points in bilayer MoS2. Band structure calculations based on the density functional theory further confirm our experimental findings. Bilayer with an interlayer twist thus provides a new platform for exploring coupled spin-valley physics other than those already found in conventional 2H-stacked bilayers.

In the second part, the carrier and spin dynamics in chemically grown WSe2 monolayers are investigated by ultrafast optical spectroscopy. Here we demonstrate that a robust valley polarization of holes in monolayer WSe2 can be initialized by circularly polarized light. Using time-resolved Kerr rotation (TRKR) spectroscopy, we have observed a long-lived valley polarization for positive trion with a lifetime up to ~700 ps, which is much longer than the trion recombination lifetime (~10-20 ps). The long-lived valley polarization arises from the transfer of valley pseudospin from photocarriers to resident holes in a specific valley, demonstrating the intervalley scattering time of hole to be ~1.4 ns. The depolarization mechanism of hole spin at elevated temperatures was further investigated by the temperature-dependent TRKR spectroscopy. The spin relaxation rate shows an exponential increase after T > 100K, indicating the intervalley scatterings of holes are mediated by thermally activated phonon modes. The long-lived valley pseudospin of holes remains robust up to T = 280K and opens up the opportunity to realize TMD-based valleytronic devices operating at room temperature.