标题: | 二维过渡金属二硫族化物之对称相关光电特性 Symmetry-Dependent Electronic and Optical Properties of Two-Dimensional Transition Metal Dichalcogenides |
作者: | 徐玮廷 Hsu, Wei-Ting 张文豪 Chang,Wen-Hao 电子物理系所 |
关键字: | 二维层状材料;过渡金属二硫族化物;层间耦合效应;自旋动力学;光激萤光光谱;二次谐波产生;时间解析柯尔旋转光谱;Two-Dimensional Layered Material;Transition Metal Dichalcogenides;Interlayer Coupling;Spin Dynamics;Photoluminescence;Second Harmonic Generation;Time-Resolved Kerr Rotation |
公开日期: | 2015 |
摘要: | 本论文旨在研究二维过渡金属二硫族化物之对称相关光电特性,其中包含了双层二硫化钼之层间耦合效应与单层二硒化钨之自旋动力学。第一部分中,我们量测了人工堆叠双层过渡金属二硫族化物之光学二次谐波产生,发现了二阶极化率的角度相依性。我们论证了双层过渡金属二硫族化物之光学二次谐波产生来自于电场的同调叠加,其中的相位差则决定于堆叠角度。以上结果让具备高效率、灵敏与非破坏性的光学二次谐波产生成为探测堆叠方向、晶体极性与边界之好方法。接着,我们透过带间光学跃迁与声子振动模态研究双层二硫化钼之层间耦合效应,发现能带结构因层间耦合效应产生了两种变化。首先,堆叠角度将改变平衡的层间间距,造成第一布里渊区中Γ点的能量改变与伴随而来的间接能隙改变。接着,具有自旋与对称相依性的层间跳跃则影响了K点的价电带自旋分裂与能带次序。 第二部分中,我们利用超快时间解析光谱探测了单层二硒化钨之自旋动力学。首先,我们证实了可以透过圆偏振光产生固有电洞的能谷极化。利用时间解析柯尔旋转光谱,我们观测到正激子的自旋生命期约为700皮秒,远长于正激子的复合生命期(~10-20皮秒)。此长时间的能谷极化来自于能谷贗自旋从光激载子到固有电洞的传递现象,证实了电洞的谷间散射生命期约为1.4奈秒。接着,我们利用变温时间解析柯尔旋转光谱探测电洞自旋的谷间去极化机制。我们发现在温度100K以上时,电洞自旋的衰减速率有随温度指数上升的情况。此点验证了在高温下,电洞自旋的衰减主要来自于受热激活的声子散射过程。 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. |
URI: | http://140.113.39.130/cdrfb3/record/nctu/#GT079821814 http://hdl.handle.net/11536/125862 |
显示于类别: | Thesis |